low intensity magnetic separators high gradient magnetic

magnetic flux density - an overview | sciencedirect topics

The ELF magnetic field value at an arbitrary point can be assessed by assembling the contributions of all conductors divided in a certain number of straight segments. The kth straight segment carrying current ik in Cartesian three-dimensional coordinate system is shown in Fig. 3.18.

Total components of the magnetic flux density generated by N segments are assembled from the contributions of all segments. Therefore, the total value of the magnetic flux density at a given point of space can be expressed as

A computational example is related to the 110/10 kV/kV transmission substation of GIS (Gas-Insulated Substation) type. A simplified two-dimensional layout of the substation is shown in Fig. 3.15. The calculation domains 1 to 5, in which higher field values are expected, are assigned as in Fig. 3.15. The spatial distribution of the magnetic field over domain 3, where the highest field value is captured, is shown in Fig. 3.19.

Perhaps the most important electromechanical actuator in automobiles is an electric motor. Electric motors have long been used on automobiles beginning with the starter motor, which uses electric power supplied by a storage battery to rotate the engine at sufficient RPM that the engine can be made to start running. Motors have also been employed to raise or lower windows, position seats as well as for actuators on airflow control at idle (see Chapter 7). In recent times, electric motors have been used to provide the vehicle primary motive power in hybrid or electric vehicles.

There are a great number of electric motor types that are classified by the type of excitation (i.e., dc or ac), the physical structure (e.g., smooth air gap or salient pole), and by the type of magnet structure for the rotating element (rotor) which can be either a permanent magnet or an electromagnet. However, there are certain fundamental similarities between all electric motors, which are discussed below. Still another distinction between types of electric motors is based upon whether the rotor receives electrical excitation from sliding mechanical switch (i.e., commutator and brush) or by induction. Regardless of motor configuration, each is capable of producing mechanical power due to the torque applied to the rotor by the interaction of the magnetic fields between the rotor and the stationary structure (stator) that supports the rotor along its axis of rotation.

It is beyond the scope of this book to consider a detailed theory of all motor types. Rather, we introduce basic physical structure and develop analytical models that can be applied to all rotating electromechanical machines. Furthermore, we limit our discussion to linear, time-invariant models, which are sufficient to permit performance analysis appropriate for most automotive applications.

This motor has coils wound around both the stator (having N1 turns) and the rotor (having N2 turns), which are placed in slots around the periphery in an otherwise uniform gap machine. In this simplified drawing, only two coils are depicted. In practice, there are more than two with an equal number in both the stator and rotor. Each winding in either stator or rotor is termed a pole of the motor. Both stator and rotor are made from ferromagnetic material having a very high permeability (see discussion above on ferromagnetism). It is worthwhile to develop a model for this simplified idealized motor to provide the basis for an understanding of the relatively complex structure of a practical motor. In Figure 6.34, the stator is a cylinder of length and the rotor is a smaller cylinder supported coaxially with the stator such that it can rotate about the common axis. The angle between the planes of the two coils is denoted and the angular variable about the axis measured from the plane of the stator coil is denoted . The radial air gap between rotor and stator is denoted g. It is important in the design of any rotating electric machine (including motors) to maintain this air gap as small as is practically feasible since the strength of the associated magnetic fields varies inversely with g. The terminal voltages of these two coils are denoted v1 and v2. The currents are denoted i1 and i2 and the magnetic flux linkage for each is denoted 1 and 2, respectively. Assuming for simplification purposes that the slots carrying the coils are negligibly small, the magnetic field intensity H is directed radially and is positive when directed outward and negative when directed inward.

This magnetic flux density is continuous through the ferromagnetic structure, but because the permeability of the stator and rotor () is very large compared with that of air, the magnetic field intensity inside both the rotor and stator is negligibly small:

It is assumed in the integrals for 1 and 2 that the so-called fringing magnetic flux outside of the axial length of the rotor/stator is negligible. Using the concept of inductance for each coil as introduced in the discussion about solenoids, this flux linkage can be written as a linear combination of the contributions from i1 and i2:

The above formulas for these inductances provide a sufficient model to derive the terminal voltage/current relationships as well as the electromechanical models for motor performance calculations. The self-inductances for each coil are independent of , but the mutual inductance varies with such that Lm() is a symmetric function of . It can be formally expanded in a Fourier series in having only cosine terms in odd harmonics as given below:

For notational convenience, the subscript 1 on M1 is dropped. Any motor made up of multiple matching pairs of coils in the stator and rotor will have a set of terminal relations in the flux linkages for the stator and rotor s and r, respectively, given by

It is of interest to evaluate the motor performance by calculating the motor mechanical power Pm for a given excitation. Let the excitation of the stator and rotor be from ideal current sources such that

The above frequency conditions (Eqn (99)) are fundamental to all rotating machines and are required to be satisfied for any nonzero average mechanical output power. Each different type of motor has a unique way of satisfying the frequency conditions. We illustrate with a specific example, which has been employed in certain hybrid vehicles. This example is the induction motor. However, before proceeding with this example, it is important to consider an issue in motor performance. Normally, electric motors that are intended to produce substantial amounts of power (e.g., for hybrid vehicle application) are polyphase machines; that is, in addition to the windings associated with stator excitation, a polyphase machine will have one or more additional sets of windings that are excited by the same frequency but at different phases. Although three-phase motors are in common use, the analysis of a two-phase induction motor illustrates the basic principles of polyphase motors with a relatively simplified model and is assumed in the following discussion.

A two-phase motor has two sets of windings displaced at 90 in the direction and excited by currents with a 90 phase for both stator and rotor. A so-called balanced two-phase motor will have its coil excited by currents ias, ibs for phases a and b, respectively, where

A two-phase induction motor is one in which the stator windings are excited by currents given above (i.e., ias and ibs). The rotor circuits are short-circuited such that var=vbr=0, where var is the terminal voltage for windings of phase a and vbr is the terminal voltage for the b phase. The currents in the rotor are obtained by induction from the stator fields. By extension of the analysis of the single-phase excitation, the terminal flux linkages are given by

The current in phase b is identical except for a 90 phase shift. Substituting the currents for rotor and stator into the equation for torque Te yields the remarkable result that the this torque is independent of and is given by

The induction machine has three modes of operation as characterized by values of s. For 01, the induction machine acts like a brake with both electrical input and mechanical input power dissipated in rotor ir2Rr losses. Because of its versatility, the induction motor has great potential in hybrid/electric vehicle propulsion applications. However, it does require that the control system incorporates solid-state power switching electronics to be able to handle the necessary currents. Moreover, it requires precise control of the excitation current.

The application of an induction motor to provide the necessary torque to move a hybrid or electric vehicle is influenced by the variation in torque with rotor speed. Examination of Eqn (114) reveals that the motor produces zero torque at synchronous speed (i.e., ms). The torque of an induction motor initially increases from its value at m=0 reaches a maximum torque (Tmax) at a speed m>m when

Normally, an induction motor is operated in the negative slope region of Tm(m) (i.e., m>m

Next, we consider a relatively new type of electric motor known as a brushless DC motor. A brushless DC motor is not a DC motor at all in that the excitation for the stator is AC. However, it derives its name from physical and performance similarity to a shunt-connected DC motor with a constant field current. This type of motor incorporates a permanent magnet in the rotor and electromagnet poles in the stator as depicted in Figure 6.36. Traditionally, permanent magnet rotor motors were generally only useful in relatively low-power applications. Recent development of some relatively powerful rare earth magnets and the development of high-power switching solid-state devices have substantially raised the power capability of such machines.

The stator poles are excited such that they have magnetic N and S poles with polarity as shown in Figure 6.36 by currents Ia and Ib. These currents are alternately switched on and off from a DC source at a frequency that matches the speed of rotation. The switching is done electronically with a system that includes an angular position sensor attached to the rotor. This switching is done so that the magnetic field produced by the stator electromagnets always applies a torque on the rotor in the direction of its rotation.

The direction of this torque is such as to cause the permanent magnet to rotate toward parallel alignment with the driving field H (which is proportional to the excitation current). The magnitude of the torque Tm is given by

In a brushless DC motor, however, the excitation fields are alternately switched electronically such that a torque is continuously applied to the rotor magnet. In order for this motor to continue to have a nonzero torque applied, the stator windings must be continuously switched synchronous with rotor rotation. Although only two sets of stator windings are shown in Figure 6.36 (i.e., two-pole machine), normally there would be multiple sets of windings, each driven separately and synchronously with rotor rotation. In effect, the sequential application of stator currents creates a rotating magnetic field which rotates at rotor frequency (r).

A simplified block diagram of the two-pole motor control system for the motor of Figure 6.36a and b is shown in Figure 6.36c. A sensor S measures the angular position of the rotor relative to the axes of the magnetic poles of the stator. A controller determines the time for switching currents Ia and Ib on as well as the duration. The switching times are determined such that a torque is applied to the rotor in the direction of rotation.

At the appropriate time, transistor A is switched on, and electric power from the on-board DC source (e.g., battery pack) is supplied to the poles A of the motor. The duration of this current is regulated by controller C to produce the desired power (as commanded by the driver). After rotating approximately 90, current Ib is switched on by activating transistor B via a signal sent by controller C.

The rotor permanent magnet is equivalent to an electromagnet with d-c excitation (i.e., r=0). The frequency at which the currents to the stator coils are switched is always synchronous with the speed of rotation. Thus, the frequency condition for the motor is satisfied since s=m. This speed is determined by the mechanical load on the motor and the power commanded by the controller. As the power command is increased, the controller responds by increasing the duration of the current pulse supplied to each stator coil. The power delivered by the motor is proportional to the fraction of each cycle that the current is on (i.e., the so-called duty cycle).

The magnetic flux density B, which is relevant to the electromechanical power conversion process, is the effective or rms value of the radial component of B at the air gap. Except for special cases, such as superconducting field excitation and printed windings, this value is determined by the characteristics of the ferromagnetic structure into which the conductors are embedded. This consists of a core or yoke, which provides both physical integrity and a path for the magnetic flux, and a slotted portion adjacent to the air gap, which accommodates the active conductors. The slot dimensions represent a compromise between the conflicting requirements of conduction of current in the copper and of flux in the iron teeth. It turns out that there is a value of the ratio between the slot and tooth widths that minimizes the volume and weight of the ferromagnetic structure. This value is unity, so that the tooth width should be half the slot pitch. If one assumes that all the flux crossing the air gap passes through the iron teeth, the flux density Bt in the teeth is related to B as

When the tooth is driven too far into saturation, the magnetizing current and the iron loss rise steeply to unacceptable values and the wave shape of the flux distribution is deformed. Also, part of the flux is diverted to the slot, which causes an increase in the additional copper losses and the transfer of the force from the tooth to the conductor. As a result the electrical insulation is stressed mechanically. For these reasons rms values of Bt in excess of 1.4T are not recommended and B is practically limited to about 0.7T.

The magnetizing force, which induces the lines of force through a material, is called the field intensity, H (or H-field), and by convention has the units ampere per meter (Am1) (Bennett et al., 1978).

The intensity of magnetization or the magnetization (M, Am1) of a material relates to the magnetization induced in the material and can also be thought of as the volumetric density of induced magnetic dipoles in the material. The magnetic induction, B, field intensity, H, and magnetization, M, are related by the equation:

where 0 is the permeability of free space and has the value of 4107NA2. In a vacuum, M=0, and M is extremely low in air and water, such that for mineral processing purposes Eq. (13.1) may be simplified to:

so that the value of the field intensity, H, is directly proportional to the value of induced flux density, B (or B-field), and the term magnetic field intensity is then often loosely used for both the H-field and the B-field. However, when dealing with the magnetic field inside materials, particularly ferromagnetic materials that concentrate the lines of force, the value of the induced flux density will be much higher than the field intensity. This relationship is used in high-gradient magnetic separation (discussed further in Section 13.4.1). For clarity it must be specified which field is being referred to.

For paramagnetic materials, is a small positive constant, and for diamagnetic materials it is a much smaller negative constant. As examples, from Figure 13.1 the slope representing the magnetic susceptibility of the material, , is about 0.001 for chromite and 0.0001 for quartz.

The magnetic susceptibility of a ferromagnetic material is dependent on the magnetic field, decreasing with field strength as the material becomes saturated. Figure 13.2 shows a plot of M versus H for magnetite, showing that at an applied field of 80kAm1, or 0.1T, the magnetic susceptibility is about 1.7, and saturation occurs at an applied magnetic field strength of about 500kAm1 or 0.63T. Many high-intensity magnetic separators use iron cores and frames to produce the desired magnetic flux concentrations and field strengths. Iron saturates magnetically at about 22.5T, and its nonlinear ferromagnetic relationship between inducing field strength and magnetization intensity necessitates the use of very large currents in the energizing coils, sometimes up to hundreds of amperes.

The magnetic force felt by a mineral particle is dependent not only on the value of the field intensity, but also on the field gradient (the rate at which the field intensity increases across the particle toward the magnet surface). As paramagnetic minerals have higher (relative) magnetic permeabilities than the surrounding media, usually air or water, they concentrate the lines of force of an external magnetic field. The higher the magnetic susceptibility, the higher the induced field density in the particle and the greater is the attraction up the field gradient toward increasing field strength. Diamagnetic minerals have lower magnetic susceptibility than their surrounding medium and hence expel the lines of force of the external field. This causes their expulsion down the gradient of the field in the direction of the decreasing field strength.

The equation for the magnetic force on a particle in a magnetic separator depends on the magnetic susceptibility of the particle and fluid medium, the applied magnetic field and the magnetic field gradient. This equation, when considered in only the x-direction, may be expressed as (Oberteuffer, 1974):

where Fx is the magnetic force on the particle (N), V the particle volume (m3), p the magnetic susceptibility of the particle, m the magnetic susceptibility of the fluid medium, H the applied magnetic field strength (Am1), and dB/dx the magnetic field gradient (Tm1=NA1m2). The product of H and dB/dx is sometimes referred to as the force factor.

Production of a high field gradient as well as high intensity is therefore an important aspect of separator design. To generate a given attractive force, there are an infinite number of combinations of field and gradient which will give the same effect. Another important factor is the particle size, as the magnetic force experienced by a particle must compete with various other forces such as hydrodynamic drag (in wet magnetic separations) and the force of gravity. In one example, considering only these two competing forces, Oberteuffer (1974) has shown that the range of particle size where the magnetic force predominates is from about 5m to 1mm.

The operating magnetic flux density is the parameter that determines the loss in the magnetic core. Similarly, current density in the windings determines the loss in the windings. When the current density is increased, cross-sectional area of the windings is reduced and hence, the volume and in turn copper weight are reduced. On the other hand, copper loss, which varies as a square of current density, is increased causing efficiency to reduce. Moreover, temperature rise will increase and injure the insulation [7].

The choice of the current density must be done in such a way that the maximum temperature of the transformer due to losses is below the insulation class temperature. Current density chosen should guarantee the level of losses and cooling conditions required. However, a designer must compare the increased cost due to the improved cooling method required with the economy in material due to the choice of increased value of current density. In short, current density is governed by load losses, temperature class of insulation, and short circuit current withstanding ability. Maximum limit for current density is calculated as

B0 = external magnetic flux density; BD = detection field; BE = magnetic flux density, evolution interval; BP = magnetic flux density, preparation interval; Gi() = dipolar autocorrelation function; J(i)() = intensity function of the Larmor frequency; ME = Curie magnetization, evolution interval; MP = Curie magnetization, preparation interval; S/N = signal-to-noise ratio; T1 = spinlattice relaxation time; Td = dipolar-order relaxation time;T1 = rotating-frame relaxation time; T2 = transverse relaxation time; = gyromagnetic ratio; 0 = magnetic field constant.

B0 = magnetic flux density; D = dipolar coupling constant; D = effective dipolar coupling constant; h = Planck's constant; I = spin-12 nucleus; J = coupling constant; q = field gradient tensor; Q = nuclear quadrupole moment; s = doublet splitting; S = quadrupolar nucleus; = magnetogyric ratio; = angle between main tensor axes; = chemical shift; 1 = relaxation time; = angle between main axes of interaction tensors and sample spinning axis; = quadrapole coupling constant.

Table 7.7. Comparison of maximum magnetic flux density. From CVETKOVI, Mario; POLJAK, Dragan; HAUEISEN, Jens. Analysis of transcranial magnetic stimulation based on the surface integral equation formulation. IEEE Transactions on Biomedical Engineering, 2015, 62.6: 15351545 [28].

Fig. 7.6. Comparison of magnetic flux density in the human brain (coronal cross-section). The results on the left are obtained via analytical expressions for (A) circular, (B) 8-coil, and (C) butterfly coil, while the results on the right are obtained via proposed model for (D) circular, (E) 8-coil, and (F) butterfly coil. From [28].

The results from Table 7.7 and Fig. 7.6 indicate that the brain itself does not significantly disturb the magnetic field of the coil, although a lower maximum value of the magnetic flux density was obtained for the 8-coil and butterfly coil. The distribution of the magnetic flux density in the coronal cross-section obtained using the SIE model shows some discontinuities, which can be related to the interpolation method used. This numerical artifact could be overcome by calculating the field at more points before interpolating results in the neighboring area.

Fig. 7.8. Dependence of the induced electric field E and magnetic flux density B on the distance from the brain surface. The values given are on the points directly under the coil geometric center. From [28].

From Fig. 7.8 the rapid decrease of both E and B fields directly under the geometric center of the stimulation coil is clearly evident in all three cases. For the circular coil, the maximum value is much lower compared to the other two coils as the maximum field will be induced under the coil windings, as shown on Fig. 7.4.

A gaussmeter measures magnetic flux density (B) at a given point in space. Most gaussmeters employ Hall-effect sensor elements as the magnetic probe. In its simplest form, a gaussmeter is a linear Hall-effect sensor with a meter readout. Indeed, it is possible to build a simple gaussmeter from a linear Hall-effect sensor IC, a small amount of interface electronics, and a DMM, but the result would not provide anywhere near the capabilities of a modern gaussmeter. A few of the features to look for in a gaussmeter are:

Range is important because there are times when you will want to measure fields of a few gauss, and others where you will want to measure fields of several kilogauss. Low ranges are often important in sensor work. Even though most Hall-effect sensor ICs aren't useful for discriminating field differences much below 1 gauss, you will typically want an instrument with an order of magnitude finer resolution than what you need to measure.

The need for accuracy requires little if any elaboration. Inaccurate instruments can make your life vastly more difficult. Accurate instruments, regularly calibrated, can make development work go more smoothly, by reducing one potential source of errors. Note that accuracy is a key specification for gaussmeters and is often the only difference between two instrument models of differing price.

While interface options may not seem that important, they enable one to hook the gaussmeter to a PC and automate many simple tasks. Popular interface standards include RS-232, IEEE-488, and analog outputs.

lithium-ion battery fast charging: a review - sciencedirect

Literature on fast charging is reviewed from a multiscale perspective.Extreme temperatures and temperature/current inhomogeneities are considered.Alternative fast charging protocols are critically evaluated.No reliable onboard methods to detect lithium plating are currently available.The links between cell and pack level performance are still not well understood.

In the recent years, lithium-ion batteries have become the battery technology of choice for portable devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are introducing electrified models into their offering, range anxiety and the length of time required to recharge the batteries are still a common concern. The high currents needed to accelerate the charging process have been known to reduce energy efficiency and cause accelerated capacity and power fade. Fast charging is a multiscale problem, therefore insights from atomic to system level are required to understand and improve fast charging performance. The present paper reviews the literature on the physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly result from charging at high currents, and the approaches that have been proposed to address these issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are presented and critically assessed. Safety implications are explored, including the potential influence of fast charging on thermal runaway characteristics. Finally, knowledge gaps are identified and recommendations are made for the direction of future research. The need to develop reliable onboard methods to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging optimisation strategies are identified as key to enabling fast charging in all conditions. Thermal management strategies to both cool batteries during charging and preheat them in cold weather are acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and good temperature homogeneities.

intensity magnetic separator - an overview | sciencedirect topics

The Jones wet high-intensity magnetic separator (WHIMS) was developed in 1956. The structure of the Jones separator is shown in Figure 9.6 and consists mainly of an iron-core electromagnet, a vertical shaft with two (or more) separating rings, a driving system, and feeding and product collection devices. Grooved plates made of magnetic conductive iron or stainless steel serve as a magnetic matrix to enhance the field gradient of the electromagnet. The plates are vertically arranged in plate boxes that are placed around the periphery of the rotors. When the Jones magnetic separator is operating, its vertical shaft drives the separating rings with the matrix plates rotating on a horizontal plane.

When a direct electric current passes through the energizing coils, a high magnetic field with a high gradient is established in the separating zone located in the electromagnetic system, with the focused magnetic field at the teeth top of the grooved plates reaching 0.82T, which is adjustable. The slurry is gravity fed onto the matrix at the leading edge of the magnetic field where the magnetic particles are captured on the teeth top of the grooved plates, while the nonmagnetic fraction passes through and is collected in a trough below the magnet. When the plate boxes reach the demagnetized zone half-way between the two magnetic poles, where the magnetic field changes its polarity, the magnetic field is essentially zero and the adhering magnetic particles are washed out with high-pressure water sprays.

In the past, cross-belt and rotating disc high-intensity magnetic separators were used for concentration of relatively coarse weakly magnetic particles such as wolframite and ilmenite, etc., under dry conditions. In the operation of these two magnetic separators, material is distributed onto the moving conveyor belt in a very thin layer, through a vibrating feeder. Such magnetic separators are not effective even inapplicable for the treatment of fine materials.

With the increasing reduction in liberation size of valuable components in magnetic ores, the conventional cross-belt and rotating disc high-intensity magnetic separators are almost replaced by gravity and flotation, particularly by high-gradient magnetic separators, as a result of its effectiveness to fine materials and high solids throughput. In the recent years, however, a wet permanent disc high-intensity magnetic separator as shown in left Figure7 seems applicable in recovering fine magnetic particles from tailings. In this disc separator, slurry is fed across a round tank, in which vertically rotating discs with permanent magnet blocks pick up fine magnetic particles, and they are brought up and scraped down by rotating scrapers, near the top of discs. Nonmagnetic particles are discharged at the bottom of tank.

And, a dry high-intensity roll magnetic separator as shown in right Figure7 is replacing the conventional roll magnetic separators and is used for concentration of relatively coarse magnetic particles. The design of such a roll magnetic separator is similar to that of the conventional roll magnetic separator, but it achieves a higher magnetic induction and its installation requires a much smaller occupation for space.

Ferromagnetic solids of high magnetic permeability can be separated in a Low Intensity Magnetic Separator (LIMS) using permanent magnets of less than 2 T (see Figure 1.56). A typical unit operates continuously and comprises a rotating non-magnetic drum inside which four to six stationary magnets are placed. The wet or dry feed contacts the outer periphery of the drum and the magnetically susceptible particles are picked up and discharged leaving the weakly or non-magnetic material to pass by largely unaffected. Alternative designs include the disc separator and the cross-belt separator where dry solids are conveyed towards a cross-belt which moves across a series of permanent magnets.

The efficiency of magnetic separation is generally improved by maximising both the intensity and the gradient of an applied non-uniform field. By doing so paramagnetic material of low magnetic permeability can be separated in a High Intensity Magnetic Separator (HIMS). Electromagnets, with intensities in excess of 2 T, are used in continuous equipment such as the Jones rotating disc separator to affect separations of dry feeds down to 75 m and wet feeds to finer sizes. Very weakly paramagnetic material cannot usually be separated satisfactorily with a HIMS, and a High Gradient Magnetic Separator (HGMS) must be used (Figure 1.56). In these units a matrix of fine stainless steel wool is placed between the poles of either electromagnetic or superconducting magnets, the latter generating magnetic intensities up to 15 T. Very high magnetic gradients are produced adjacent to the wool fibres and this allows for the separation of very fine particulates. Although the capital cost of HGMS can be relatively high compared with more conventional equipment, commercial units are readily available.

Iron ore processors may also employ magnetic separation for beneficiation of classifier output streams. Wet high-intensity magnetic separators (WHIMS) may be used to extract high-grade fine particles from gangue, due to the greater attraction of the former to the applied magnetic field.

In addition to beneficiating the intermediate middlings streams from the classifier, WHIMS may be used as scavenger units for classifier overflow. This enables particles of sufficient grade to be recovered that would otherwise be sacrificed to tails.

Testwork has been performed on iron ore samples from various locations to validate the use of magnetic separation following classification (Horn and Wellsted, 2011). A key example was material sourced from the Orissa state in northeastern India, with a summary of results shown in Table 10.2. The allmineral allflux and gaustec units were used to provided classification and magnetic separation, respectively.

The starting grade of the sample was a low 42% Fe. It also contained significant ultrafines with 58% passing 20m. This is reflected in the low yield of allflux coarse concentrate; however, a notable 16% (abs) increase in iron grade was eventually achieved. The gaustec results for the middlings and overflow streams demonstrate the ability to recover additional high-grade material. With the three concentrate streams combined, an impressive yield of almost 64% was achieved with minimal decline in iron grade.

Various classification schemes exist by which magnetic separators can be subdivided into categories. Review of these schemes can be found in monographs by Svoboda (1987, 2004). The most illustrative classification is according to the magnitude of the magnetic field and its gradient.

Low-intensity magnetic separators (LIMS). They are used primarily for manipulation of ferromagnetic materials or paramagnetic of high magnetic susceptibility and/or of large particle size. These separators can operate either in dry or wet modes. Suspended magnets, magnetic pulleys, and magnetic drums are examples of these separators. Operation of a dry drum separator is shown in Fig. 3.

High-intensity magnetic separators. They are used for treatment of weakly magnetic materials, coarse or fine, in wet or dry modes. Induced magnetic rolls (IMR), permanent magnet rolls and drums, magnetic filters, open-gradient (OGMS) and wet high-intensity magnetic separators (WHIMS) are examples of this class of separators.

Weakly paramagnetic minerals can only be effectively recovered using high-intensity (B-fields of 2T or greater) magnetic separators (Svoboda, 1994). Until the 1960s, high-intensity separation was confined solely to dry ore, having been used commercially since about 1908. This is no longer the case, as many new technologies have been developed to treat slurried feeds.

Induced roll magnetic (IRM) separators (Figure 13.19) are widely used to treat beach sands, wolframite and tin ores, glass sands, and phosphate rock. They have also been used to treat weakly magnetic iron ores, principally in Europe. The roll, onto which the ore is fed, is composed of phosphated steel laminates compressed together on a nonmagnetic stainless steel shaft. By using two sizes of laminations, differing slightly in outer diameter, the roll is given a serrated profile, which promotes the high field intensity and gradient required. Field strengths of up to 2.2T are attainable in the gap between feed pole and roll. Nonmagnetic particles are thrown off the roll into the tailings compartment, whereas magnetics are held, carried out of the influence of the field and deposited into the magnetics compartment. The gap between the feed pole and rotor is adjustable and is usually decreased from pole to pole (to create a higher effective magnetic field strength) to take off successively more weakly magnetic products.

The primary variables affecting separation using an IRM separator are the magnetic susceptibility of the mineral particles, the applied magnetic field intensity, the size of the particles, and the speed of the roll (Singh et al., 2013). The setting of the splitter plates cutting into the trajectory of the discharged material is also of importance.

In most cases, IRM separators have been replaced by the more recently developed (circa 1980) rare earth drum and roll separators, which are capable of field intensities of up to 0.7 and 2.1T, respectively (Norrgran and Marin, 1994). The advantages of rare earth roll separators over IRM separators include: lower operating costs due to decreased energy requirements, less weight leading to lower construction and installation costs, higher throughput, fewer required stages, and increased flexibility in roll configuration which allows for improved separation at various size ranges (Dobbins and Sherrell, 2010).

Dry high-intensity separation is largely restricted to ores containing little, if any, material finer than about 75m. The effectiveness of separation on such fine material is severely reduced by the effects of air currents, particleparticle adhesion, and particlerotor adhesion.

Without doubt, the greatest advance in the field of magnetic separation was the development of continuous WHIMSs (Lawver and Hopstock, 1974). These devices have reduced the minimum particle size for efficient magnetic separation compared to dry high-intensity methods. In some flowsheets, expensive drying operations, necessary prior to a dry separation, can be eliminated by using an entirely wet concentration system.

Perhaps the most well-known WHIMS machine is the Jones separator, the design principle of which is utilized in many other types of wet separators found today. The machine has a strong main frame (Figure 13.20(a)) made of structural steel. The magnet yokes are welded to this frame, with the electromagnetic coils enclosed in air-cooled cases. The separation takes place in the plate boxes, which are on the periphery of the one or two rotors attached to the central roller shaft and carried into and out of the magnetic field in a carousel (Figure 13.20(b)). The feed, which is thoroughly mixed slurry, flows through the plate boxes via fitted pipes and launders into the plate boxes (Figure 13.21), which are grooved to concentrate the magnetic field at the tip of the ridges. Feeding is continuous due to the rotation of the plate boxes on the rotors and the feed points are at the leading edges of the magnetic fields (Figure 13.20(b)). Each rotor has two feed points diametrically opposed to one another.

The weakly magnetic particles are held by the plates, whereas the remaining nonmagnetic particle slurry passes through the plate boxes and is collected in a launder. Before leaving the field any entrained nonmagnetics are washed out by low-pressure water and are collected as a middlings product.

When the plate boxes reach a point midway between the two magnetic poles, where the magnetic field is essentially zero, the magnetic particles are washed out using high-pressure scour water sprays operating at up to 5bar. Field intensities of over 2T can be produced in these machines, although the applied magnetic field strength should be carefully selected depending on the application (see Section 13.4.2). The production of a 1.5T field requires electric power consumption in the coils of 16kW per pole.

There are currently two types of WHIMS machines, one that uses electromagnetic coils to generate the required field strength, the other that employs rare earth permanent magnets. They are used in different applications; the weaker magnetic field strength produced by rare earth permanent magnets may be insufficient to concentrate some weakly paramagnetic minerals. The variables to consider before installing a traditional horizontal carousel WHIMS include: the feed characteristics (slurry density, feed rate, particle size, magnetic susceptibility of the target magnetic mineral), the product requirements (volume of solids to be removed, required grade of products), and the cost of power (Eriez, 2008). From these considerations the design and operation of the separator can be tailored by changing the following: the magnetic field intensity and/or configuration, the speed of the carousel, the setting of the middling splitter, the pressure/volume of wash water, and the type of matrix material (Eriez, 2008). The selection of matrix type has a direct impact on the magnetic field gradient present in the separation chamber. As explained in Section 13.4.2, increasing magnetic field can in some applications actually cause decreased performance of the magnetic separation step and it is for this reason that improvements in the separation of paramagnetic materials focus largely on achieving a high magnetic field gradient. The Eriez model SSS-I WHIMS employs the basic principles of WHIMS with improvements in the matrix material (to generate a high field gradient) as well as the slurry feeding and washing steps (to improve separation efficiency) (Eriez and Gzrinm, 2014). While this separator is referred to as a WHIMS, it is in fact more similar to the SLon VPHGMS mentioned in Sections 13.4.1 and 13.5.3. Further discussion on high-gradient magnetic separation (HGMS) may be found in Section 13.5.3.

Wet high-intensity magnetic separation has its greatest use in the concentration of low-grade iron ores containing hematite, where they are an alternative to flotation or gravity methods. The decision to select magnetic separation for the concentration of hematite from iron ore must balance the relative ease with which hematite may be concentrated in such a separator against the high capital cost of such separators. It has been shown by White (1978) that the capital cost of flotation equipment for concentrating weakly magnetic ore is about 20% that of a Jones separator installation, although flotation operating costs are about three times higher (and may be even higher if water treatment is required). Total cost depends on terms for capital depreciation; over 10 years or longer the high-intensity magnetic separator may be more attractive than flotation.

In addition to recovery of hematite (and other iron oxides such as goethite), wet high-intensity separators are now in operation for a wide range of duties, including removal of magnetic impurities from cassiterite concentrates, removal of fine magnetic material from asbestos, removal of iron oxides and ferrosilicate minerals from industrial minerals such as quartz and clay, concentration of ilmenite, wolframite, and chromite, removal of magnetic impurities from scheelite concentrates, purification of talc, the recovery of non-sulfide molybdenum-bearing minerals from flotation tailings, and the removal of Fe-oxides and FeTi-oxides from zircon and rutile in heavy mineral beach sands (Corrans and Svoboda, 1985; Eriez, 2008). In the PGM-bearing Merensky Reef (South Africa), WHIMS has been used to remove much of the strongly paramagnetic orthopyroxene gangue from the PGM-containing chromite (Corrans and Svoboda, 1985). WHIMS has also been successfully used for the recovery of gold and uranium from cyanidation residues in South Africa (Corrans, 1984). Magnetic separation can be used to recover some of the free gold, and much of the silicate-locked gold, due to the presence of iron impurities and coatings. In the case of uranium leaching, small amounts of iron (from milling) may act as reducing agents and negatively affect the oxidation of U4+ to U6+; treatment via WHIMS can reduce the consumption of oxidizing agents by removing a large portion of this iron prior to leaching (Corrans and Svoboda, 1985).

At the CliffsWabush iron ore mine in Labrador, Canada (Figure 13.22), the cyclone overflow from the tailings of a rougher spiral bank is sent to a magnetic scavenger circuit utilizing both low-intensity drum separation and WHIMS. This circuit employs the low-intensity (0.07T) drum separators to remove fine magnetite particles lost during the spiral gravity concentration step, followed by a WHIMS step using 100th1 Jones separators which are operated at field strengths of 1T to concentrate fine hematite. Cleaning of only the gravity tailings by magnetic separation is preferred, as relatively small amounts of magnetic concentrate have to be handled, the bulk of the material being essentially unaffected by the magnetic field. The concentrate produced from this magnetic scavenging step is eventually recombined with the spiral concentrate before feeding to the pelletizing plant (Damjanovi and Goode, 2000).

The paramagnetic properties of some sulfide minerals, such as chalcopyrite and marmatite (high Fe form of sphalerite), have been exploited by applying wet high-intensity magnetic separation to augment differential flotation processes (Tawil and Morales, 1985). Testwork showed that a Chilean copper concentrate could be upgraded from 23.8% to 30.2% Cu, at 87% recovery.

By creating an environment comprising a magnetic force (Fm), a gravitational force (Fg), and a drag force (Fd), magnetic particles can be separated from nonmagnetic particles by MS. Magnetic separators exploit the differences in magnetic properties between particles. All materials are affected in some way when placed in a magnetic field.

where V: particle volume (determined by process); X: magnetic susceptibility; H: magnetic field (created by the magnet system design) in mT; GradH: magnetic field gradient (created by the magnet system design) in mT (mT: milli Tesla, 1kGauss=100mT=0.1T). Materials are classified into two broad groups according to whether they are attracted to or repelled by a magnet. Non/diamagnetics are repelled from and ferro/paramagnetics are attracted to magnets. Ferromagnetic substances are strongly magnetic and have a large and positive magnetism. Paramagnetic substances are weakly magnetic and have a small and positive magnetism. In diamagnetic materials, the magnetic field is opposite to the applied field. Magnetisms are small and negative. Nonmagnetic material has zero magnetism. Ferromagnetism is the basic mechanism by which certain materials (such as Fe) form permanent magnets, or are attracted to magnets. Ferromagnetic materials can be separated by low-intensity magnetic separators (LIMSs) at less than 2T magnetic intensity. Paramagnetic materials can be separated by dry or wet high-intensity magnetic separators (HIMSs) at 1020T magnetic intensities. Diamagnetic materials create an induced magnetic field in the direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field. Nonmagnetic substances have little reaction to magnetic fields and show net zero magnetic moment due to random alignment of the magnetic field of individual atoms. Induced roll separators, with field intensities up to 2.2T, and Permroll separators can be used for coarse and dry materials (>75m). Fine materials reduce the separation efficiency due to particlerotor and particleparticle agglomeration. For wet HIMS, Gill and Jones separators are used at a maximum field of 1.4 and 1.5T, at 150m size [80]. Dry LIMSs are used for coarse and strongly magnetic substances. The magnetic field gradient in the separation zone (approximately 50mm from the drum surface) ranges between 0.1 and 0.3T. Below 0.5cm, dry separation tends to be replaced by wet LIMS. Concurrent and countercurrent drum separators have a nonmagnetic drum containing three to six stationary magnets of alternating polarity. Separation depends on the pick up principles. Magnetic particles are lifted by magnets and pinned to the drum and then conveyed out of the field. Field intensities up to 0.7T at the pole surfaces can be used. Coarse particles up to 0.56mm can be tolerated. The drum diameter is 1200mm and the length 6003600mm. Concurrent operation is normally used as a primary separation (cobber) for large capacities and coarse feeds. Countercurrent operation is used as a rougher and finisher for multistage concentration.

Moderately magnetic dry substances on a conveyor/belt can be collected by overhead, cross-belt, or disc separators using magnetic field intensities between 0.8 and 1.5T. Very weakly paramagnetic substances can only be removed if field intensities are greater than 2.0T. At 5200mm size fractions, overhead permanent magnets are used to remove ferromagnetics. Magnetic separators, such as dry low-intensity drum types, are widely used for the recovery of ferromagnetic materials from nonferrous metals (Al and Cu) and other nonmagnetic materials (plastic and glass) at 5mm in size. The magnetic field may be generated by permanent magnets or electromagnets. There have been many advances in the design and operation of HIMS due mainly to the introduction of rare-earth alloy permanent magnets with the capability of providing high field strengths and gradients. There are, however, some problems associated with this method. One of the major issues is agglomeration of the particles, which results in the attraction of some nonferrous fractions attached to the ferrous fractions [81]. This leads to low efficiency of this method. Through the process of MS, it is possible to obtain two fractions: the magnetic fraction, which includes Fe, steel, Ni, etc., and the nonmagnetic fraction, which includes Cu [82]. For WEEE, MS systems utilize ferrite, rear-earth or electromagnets, with high-intensity electromagnet systems being used extensively. Veit et al. [81] employed a magnetic field of 0.60.65T to separate the ferromagnetic elements, such as Fe and Ni. The chemical concentration of the magnetic fraction was 43% Fe and 15.2% Ni on average. However, there was a considerable amount of Cu impurity in the magnetic fraction as well. Yoo et al. [83] used a two-stage MS for milled PCBs. The milled PCBs of particle size >5.0mm and the heavy fraction were separated from the <5.0mm PCB particles by gravity separation. In the first stage, a low magnetic field of 0.07T was applied, which led to the separation of 83% of Ni and Fe in the magnetic fraction and 92% of Cu in the nonmagnetic fraction. The second MS stage was conducted at 0.3T, which resulted in a reduction in the grade of the NiFe concentrate and an increase in the Cu concentrate grade.

Magnetic separations depend on a particle's magnetic susceptibility in a magnetic field. Based on magnetic susceptibility, materials can be one of two types: paramagnetic (those attracted by a magnetic field) and diamagnetic (those repelled by a magnetic field). It is usual to consider strongly magnetic materials as being in a separate category called ferromagnetic.

Magnetic separators are divided into low-intensity and high-intensity separators, the former being used for ferromagnetic minerals (and some paramagnetic minerals of high magnetic susceptibility) and the latter used for paramagnetic minerals of (lower) magnetic susceptibility. (In effect, a third category of separator exists: that used for removing tramp iron from process streams.) High- and low-intensity separation can be carried out wet or dry: tramp separators operate only on dry streams.

The most common separator, the wet low-intensity, consists of a revolving drum partly submerged in a suspension. An arc of magnets within the drum pulls the magnetically susceptible material against the drum, lifting it out of the slurry and over a discharge weir. Permanent ceramic magnets are now typical in these units.

Dry high-intensity separators use powerful electromagnets that induce a magnetic field in a comparatively small diametered roll, against which the magnetically susceptible particles are held until they pass a suitable discharge point.

where r=radial distance; V=particle volume; p, m=magnetic susceptibility of particle and medium, respectively. This shows that the force depends on both the strength and the gradient of the magnetic field. The latter component is especially significant in WHIMS, where high curvature ferromagnetic surfaces (e.g., wire, balls) are used to produce very high gradients.

An indication of the lower limits of particle size that can be treated in a magnetic separator can be obtained by balancing the magnetic force against the likely opposing forces (usually fluid drag and gravitation), but with the addition of centrifugal forces in drum separators. Mechanical considerations usually determine the upper particle size limit.

In principle, separability and performance curves can be used to predict separator performance. However, difficulties arise in determining properties independent of experimental conditions, so the approach has not been widely used.

Two simple models of wet low-intensity drum separators have been described. One uses a probability concept while the other empirically correlates losses of magnetic material near the drum take-up and discharge with feed rate and drum speed.

Nanomaterials have also been prepared by ball milling the parent materials. High-energy ball milling not only prepares nanoparticles quickly but it also uses little chemicals as compared to the sol-gel methods. However, it has low energy efficiency because it dissipate a lot of energy in form of heat.

Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles [11]. Fig. 12 shows the SEM image of the iron nanoparticles.

The next step involved the crushing of the pyrite particle by high-energy ball milling at a rate of 320rpm for various periods of time, that is, 2, 4, and 6h which ultimately resulted in the formation of pyrite nanoparticles.

The process of ball milling was employed under controlled parameters about 298K temperature and 760 torr pressure. Stainless steel made ball and bowl were utilized for the process. In the process, ball:pyrite ratio of 10:1 was selected and at varying time periods of 2h, the samples were removed. The method was named as interrupted milling. The synthesized nanoparticles were washed with ethyl alcohol thrice to eradicate contamination. The nanoparticles were dried in an oven for 4h at 50C [12,13]. Fig. 13 shows the SEM image of the nanoparticles.

Jaw and cone crushing was performed on the martite ore until they became within the size range of 0.52cm. The sample was further crushed by ball and rod milling until the particles size was reduced to 3050lm. Ultimately, the particles were subjected to interrupt high-energy planetary ball milling for different time durations, that is, 2, 4, and 6h to get the nanoparticles of martite. The ball:martite ratio of 10:1 was selected and a rotation speed of 320rpm was chosen [14]. Fig. 14 shows the SEM images of martite nanoparticles.

Caron onions preparation was carried out by employing graphite carbon having high purity. A reported method was used to synthesize AlCuFe quasicrystal. The synthesis of alloy was carried out under ambient environment. The percentage composition of alloy material was set to be Al64Cu24Fe12. The alloy was solidified under ambient conditions. Annealing of the synthesized alloy was performed under argon environment at 700C for 96h. The synthesized composite material is brittle and inclines to be fractured when subjected to ball milling process. In this typical procedure, the reaction of moisture with aluminum in the composite results in the formation of aluminum oxide film over the surface but simultaneously, the release of atomic hydrogen incites cleavage fracture of the composite material and occasionally it was observed that the whole material got converted into fine powder after a few days. Graphite and the composite materials were mixed in 1:1 ratio and then high-energy ball milling was performed on the mixture under ambient environment. Ball milling was performed for various time periods of 1.5, 3, 6, and 10h. The ball milling media was composed of hard steel vials and balls having a ratio of powder:ball to be 1:7. The mixture was grinded using a grinding medium size of 12.7mm. The synthesized nanocomposites were characterized using various techniques including XRD, Raman spectroscopy, TEM, and the size of the nanoparticles was observed to be within 412nm [15]. Fig. 15 shows the TEM image of the nanoparticles.

A modified ball milling device having assistance of ultrasonication was employed in the synthesis of zinc oxide nanoparticles. The synthesis of nanoparticles involved analytical grade zinc acetate dihydrated salt as the zinc precursor material. Ball milling medium of stainless steel with diameter of balls of 2mm was employed. The ratio of milling balls to the zinc precursor was set to be 1:100. The frequency and power of the microwave were 2450MHz and 0.8kW, respectively. The synthesized nanoparticles were characterized by UV-Visible spectroscopy, XRD, TEM, fluorescence measurements, and electroconductivity detections. The average size of the nanoparticles was observed to be 15nm [16]. Fig. 16 shows the TEM image of the nanoparticles.

The synthesis of Na3MnCO3PO4 nanoparticles involved dry ball milling of the precursors. The precursors of the nanoparticles were Mn(NO3)2.4H2O (A), Na2HPO4.2H2O (B), and Na2CO3.H2O (C). The concentrations optima were evaluated by doing extensive preliminary experiments and the amount of 8mmol of A and B and 12mmol of C. Planetary ball milling of the mixture was performed by keeping a ball: mixture ratio of 30:1. The mixture was ball milled for different time periods, that is, 15, 30, 60, and 180min at a rate of 300rpm. The synthesized billed nanoparticles were then added into deionized distilled water under continuous stirring so that the nanoparticles can be separated from impurities. The nanoparticles were separated and characterized by various techniques [17]. SEM image of the nanoparticles are provided in Fig. 17.

material handling equipment & conveyors | bunting

With over 60 years of experience handling just about any wholly or partially ferrous product or scrap, Bunting offers Americas most complete line of permanent magnetic equipment, conveyor components, and material handling equipment. Even plastic assemblies with minimal ferrous components in them, like circuit breakers, can easily be conveyed with the magnetic rails with positive control. You can also save valuable floor space by moving materials vertical or up and over a machine.

Bunting conveyors have been enhancing plant efficiency for decades. There is simply no better way to handle a wide range of ferrous parts. And for those who do not require magnetics but still want superior durability, Bunting manufactures non-magnetic models that have the same well designed features and engineer support.

We manufacture conveyors that can be used alone or in combinations to accomplish a variety of material handling tasks. Five factors should be considered when determining which conveyor will best serve your needs: Conveyor style, angle of incline, belt width, vertical height, and horizontal extension length.

Selecting the right conveyor or combinations of conveyors for your particular application is easy with our standard magnetic conveying equipment. Utilizing standard, pre-engineered components, Bunting offers conveyors to handle virtually any type of ferrous part or scrap by tailoring the physical dimensions to fit the application.

We can also custom design systems or tie several conveyors together for a total material handling solution.Our plant engineers can assist by providing you 3-D models and drawings to fit into your plant layout.

The Eddy Current Separator core is designed to provide a high level of gauss intensity to enhance separation on a wide range of nonferrous conductive materials and sizes. It is protected with the tough urethane belt and a fiber shell. From aluminum cans to aluminum flakes, our ECS units do an excellent job of throw and separation.

The Eddy Current Separator core is designed to provide a high level of gauss intensity to enhance separation on a wide range of nonferrous conductive materials and sizes. It is protected with the tough urethane belt and a fiber shell. From aluminum cans to aluminum flakes, our ECS units do an excellent job of throw and separation.

Grinder Feeder Conveyors can be used to protect your Shredder from being damaged by separating out the metal contaminant with a magnetic cross-belt or metal detector before the product is transferred to a shredder, and Discharge Conveyors can purify your product on the way out.

Grinder Feeder Conveyors can be used to protect your Shredder from being damaged by separating out the metal contaminant with a magnetic cross-belt or metal detector before the product is transferred to a shredder, and Discharge Conveyors can purify your product on the way out.

DragSlide conveyors are totally enclosed and use UHMW drag flights in a chain conveyor design to move and convey film & fiber materials within a sealed conveyor. This makes for a much cleaner facility allowing you to use your employees for production versus clean-up maintenance.

DragSlide conveyors are totally enclosed and use UHMW drag flights in a chain conveyor design to move and convey film & fiber materials within a sealed conveyor. This makes for a much cleaner facility allowing you to use your employees for production versus clean-up maintenance.

Bale Break Conveyors break up bales of recycled plastic bottle containers. Heavy-duty construction for rigorous applications. Designed to improve loading and separations efficiency with recycled materials.

Bale Break Conveyors break up bales of recycled plastic bottle containers. Heavy-duty construction for rigorous applications. Designed to improve loading and separations efficiency with recycled materials.

Most commonly used in the Municipal Recycling Industry, this Sorting Table Conveyor is built with short sideguards and UHMW skirting for safety. Complete with elevated working mezzanines, stairs, drop chutes and belly rests for the operator's comfort.

Most commonly used in the Municipal Recycling Industry, this Sorting Table Conveyor is built with short sideguards and UHMW skirting for safety. Complete with elevated working mezzanines, stairs, drop chutes and belly rests for the operator's comfort.

Horizontal and inclined regrind or pellet transfer conveyors in any size your application requires. Various belt styles including smooth flat flexed wall, cleated or cleat topped belted conveyors to transfer bulk material to a Granulator or storage facility.

Horizontal and inclined regrind or pellet transfer conveyors in any size your application requires. Various belt styles including smooth flat flexed wall, cleated or cleat topped belted conveyors to transfer bulk material to a Granulator or storage facility.

Used for conveying your ferrous parts such as bolts, nails, stamped parts or assemblies around the plant, from one machine to the next operation or to stabilize part in feeding applications and to convey larger ferrous scrap out of a press over to a dumpster.

Used for conveying your ferrous parts such as bolts, nails, stamped parts or assemblies around the plant, from one machine to the next operation or to stabilize part in feeding applications and to convey larger ferrous scrap out of a press over to a dumpster.

Usually designed for specific applications where the smooth top surface of a mattop belt has advantages over a fabric belted conveyor. Mattop belted conveyors are easier to slide parts across the belt and are better for accumulation applications.

Usually designed for specific applications where the smooth top surface of a mattop belt has advantages over a fabric belted conveyor. Mattop belted conveyors are easier to slide parts across the belt and are better for accumulation applications.

Our Permanent Magnetic Conveyor Components offer easy installation and years of trouble-free service in even the most demanding applications. Each component is available in a variety of sizes and magnetic strengths.

Our Permanent Magnetic Conveyor Components offer easy installation and years of trouble-free service in even the most demanding applications. Each component is available in a variety of sizes and magnetic strengths.

Used in horizontal and inclined fabric and mattop belted conveyors in wet wash-down applications. Built with the strongest magnetic rails in the industry. Our engineers are here to help you pick the right magnetic rail for your application.

Used in horizontal and inclined fabric and mattop belted conveyors in wet wash-down applications. Built with the strongest magnetic rails in the industry. Our engineers are here to help you pick the right magnetic rail for your application.

Our Magnetic Test Bath Rails are designed and manufactured for either a single or a dual lane chain for maximum hold down and are engineered and designed for your tank, your chain and your application.

Our Magnetic Test Bath Rails are designed and manufactured for either a single or a dual lane chain for maximum hold down and are engineered and designed for your tank, your chain and your application.

Induced roll magnetic separators are used for the continuous extraction of small magnetic particles from certain minerals to produce mineral purification for a wide range of mineral and ceramic processing industries.

Induced roll magnetic separators are used for the continuous extraction of small magnetic particles from certain minerals to produce mineral purification for a wide range of mineral and ceramic processing industries.

The Magnetic Drum Separator is normally installed at product discharge points and incorporates a 150 - 180 degree magnet system, encased in a stainless steel shell, or manganese wear plates for severe application.

The Magnetic Drum Separator is normally installed at product discharge points and incorporates a 150 - 180 degree magnet system, encased in a stainless steel shell, or manganese wear plates for severe application.

vertically pulsating high gradient magnetic separator - metso outotec

Effectively process fine, weakly-magnetic minerals with the SLon Vertically Pulsating High-gradient Magnetic Separator (VPHGMS). The unit is a wet, high-intensity magnetic separator that uses a combination of magnetic force, pulsating fluid, and gravity to process minerals. The advanced features are incorporated into a design utilizing a unique vertical configuration, jigging action, and special matrix materials to achieve the best results.

Slurry is introduced through slots in the upper yoke to the matrix, which is housed inside the vertical separating ring. The magnetic particles are attracted to the matrix and are then carried outside of the magnetic field where they are subsequently flushed to the magnetic concentrate trough. The non-magnetic or less magnetic particles pass through the matrix through slots in the lower yoke to the non-magnetic collection hoppers.

The ring is arranged in a vertical orientation as opposed to a traditional Jones-type WHIMS, which uses a horizontal carousel. The vertical nature of the carousel allows for reverse flushing, where magnetics flushing in the opposite direction of the feed allows strongly magnetic and/or coarse particles to be removed without having to pass through the full depth of the matrix volume. In addition, the magnetics flushing is accomplished near the top of rotation, a location with a low stray magnetic field to reduce any residual grip on the magnetic particles. The end result is high availability due to minimized matrix plugging.

An actuated diaphragm provides pulsation in the separation zone to assist the separation performance by agitating the slurry and keeping particles in a loose state, minimizing entrapment. This mechanism alsomaximizes the particle accumulation (trapping) on all sides of the rod matrix, creating more usable surfacearea for magnetics collection. A further benefit is to reduce particle momentum, which aids in particlecapture by the applied magnetic force. This leads to improved fine particle collection and separation.

The SLon VPHGMS utilizes a filamentary matrix constructed of steel rods to accommodate various size ranges of feeds. The rods are oriented perpendicular to the applied magnetic field to enable optimum magnetic force to be achieved while minimizing the risk for entrapment of particles, when compared to grooved plates, randomly positioned filaments (wool), or expanded metal sheets.

wet low intensity magnetic separators - metso corporation - pdf catalogs | technical documentation | brochure

Wet low intensity magnetic separators, LIMS Metso has been involved in magnetic separation for more than one hundred years. Metso has produced more than five thousand magnetic drums used in both dry and wet processing. Metso wet magnetic separators are continuously undergoing improvements to meet the everincreasing demands of our customers. Metso has been and is still the leader in the development of high capacity, high performance wet low intensity magnetic separators (LIMS) for several decades. 2 Wet low intensity magnetic separators Designs and sizes Metsos wet magnetic separators are...

e.g. glass sand and feldspar production. It is also commonly installed for removal of ferromagnetic matter ahead of WHIMS (Wet high intensity magnetic separators) or HGMS (High gradient magnetic separators) units. In dense media circuits LIMS is standard equipment; ask for our special brochure for dense media recovery. After market services Our greatest asset is the experience of our people which assures competent technical field services and parts supply for systems and equipment designed, manufactured and supplied by Metso. Our safety certified field service engineers have specialized...

Tank design CC and CR CC Concurrent The concurrent style of magnetic separator features: The counter-rotation style of magnetic separator features: Feed box with serrated weir overflow for even distribution of the feed slurry Feed box with feed tubes; Feed entry section to improve on feed pulp distribution thus ensuring full width feed to drum; Short pick-up zone, which reduces the risk of coarse material settling on tank bottom. Exchangeable outlet spigots in tank bottom to allow coarse material to discharge trouble free; Suited for processing of coarse ore up to 6 - 8 mm (3...

Tank design CTC and DWHG CTC Countercurrent DWHG Counter-rotation The countercurrent style of magnetic separator features: The DWHG style of magnetic separator features: Feed box with serrated weir overflow for even distribution of the feed; Basically counter-rotation tank design Extremely long pick-up zone; Feed entry section to improve on feed pulp distribution with full width feed to drum; Entry chamber designed to allow for entrapped air to escape and to improve concentrate drainage; Medium long pick-up zone; Longer magnet assembly arc to compensate for disturbances...

CR feed tubes are protected by polyurethane saddles against abrasion. Features and benefits Magnetic system The heart of the magnetic separator is the magnet assembly. Metso provides basically two different assemblies: high capacity and high gradient, (HG). The high capacity assembly is the standard magnetic system. The main differences between the two magnet assemblies are pole pitch, pole sizes, and number of poles. The magnet assemblies are similar in design with both having a number of main poles and a number of intermediate cross poles for flux control and enhancement of the magnetic...

Magnet positioning Direct gear motor drive Features and benefits Adjustment of magnet and drum position Feed boxes Drum drive system The magnetic drum and magnetic assembly can easily be adjusted to obtain the best process performance. The adjustment possibilities include The feed system for primary distribution of the pulp to the feed boxes that are supplied with the separators is normally not part of the equipment supply; however, Metso can optionally supply or advise on solutions for these systems. The drum on any Metso magnetic separator has a drive shaft, which can be adapted to any...

Adjustable concentrate overflow weir CR tank with pulp level bars Concentrate discharge and collection Effluent (tailings) discharge and collection An overflow weir is provided for the magnetic concentrate discharge. This weir, manufactured of HDPE or, optionally, in polyurethane, is adjustable to obtain optimum discharge conditions. The launders for collection of the concentrate that is discharged over the weir are made of a rubber-lined combination of mild and stainless steel and are bolted to the separator tank frame. Depending on the installation situation, standard launders or,...

Application guide lines Absolute guidelines for model selection and dimensions are not available due to the widely varying nature of iron ores; hence, the data shown in the table below are only indicative and, when in doubt of the properties of a specific ore, the lower feed rate should be used. Testing in a laboratory, followed by on site testing is always advisable especially when planning for larger installa- tions. Sizing of full-size machines using only laboratory data is normally not sufficient to determine the number of magnetic separators that are required. The capacities in the...

Metso Minerals Industries, Inc. 2715 Pleasent Valley Road, York, PA 17402, USA, Phone: +1 717 843 8671 etso Minerals (South Africa) (Pty) Ltd. M Metso Minerals (Australia) Ltd. Metso Minerals (India) Pvt Ltd 1th floor, DLF Building No. 10, Tower A, DLF Cyber City, Phase - III, Gurgaon - 122 002, India, Phone: +91 124 235 1541, Fax: +91 124 235 1601 Metso Minerals (Chile) S.A. Metso Brasil Indstria e Comrcio Ltda. Av. Independncia, 2500 den, 18087-101 Sorocaba-SP - Brazil, Phone: +55 15 2102 1300 www.metso.com E-mail: [email protected] Metso Corporation, Fabianinkatu 9...

magnetic separators

The science of magnetic separation has experienced extraordinary technological advancements over the past decade. As a consequence, new applications and design concepts in magnetic separation have evolved. This has resulted in a wide variety of highly effective and efficient magnetic separator designs.

In the past, a process engineer faced with a magnetic separation project had few alternatives. Magnetic separation was typically limited and only moderately effective. Magnetic separators that utilized permanent ferrite magnets, such as drum-type separators, generated relatively low magnetic field strengths. These separators worked well collecting ferrous material but were ineffective on fine paramagnetic particles. High intensity magnetic separators that were effective in collecting fine paramagnetic particles utilized electromagnetic circuits. These separators were large, heavy, low capacity machines that typically consumed an inordinate amount of power and required frequent maintenance. New developments in permanent magnetic separation technology now provide an efficient alternative for separation of paramagnetic materials.

Technological advances in the field of magnetic separation are the result of several recent developments. First, and perhaps most important, is the ability to precisely model magnetic circuits using sophisticated multi-dimensional finite element analysis (FEA). Although FEA is not a new tool, developments in computing speed over the last decade have made this tool readily accessible to the design engineer. In this technique, a scaled design of the magnetic circuit is created and the magnetic characteristics of the individual components quantified. The FEA model is then executed to determine the magnetic field intensity and gradient. Using this procedure, changes to the magnetic circuit design can be quickly evaluated to determine the optimum separator configuration. This technique can be applied to the design of both permanent and electromagnetic circuits. As a consequence, any type of magnetic separator can be developed (or redesigned) with a high level of confidence and predictability.

Equally important has been the recent development of rare-earth permanent magnets. Advances in rare-earth magnet materials have revolutionized the field of magnetic separation. The advent of rare-earth permanent magnets in the 1980s provided a magnetic energy product an order of magnitude greater than that of conventional ferrite magnets. Rare-earth magnetic circuits commonly exhibit a magnetic attractive force 20 to 30 times greater than that of conventional ferrite magnets. This development has provided for the design of high-intensity magnetic circuits that operate energy-free and surpass the strength and effectiveness of electromagnets.

Finally, the materials of construction used in the fabrication of magnetic separators have advanced to a point that significantly extends service life while decreasing maintenance. Advanced materials, such as fiber composites, kevlar, ultra high molecular weight polyester, and specialty steel alloys are now commonly used in contact areas of the separator. These materials are lightweight, abrasion resistant, and comparatively inexpensive resulting in significant design advantages as compared to previous construction materials.

The evolution of high strength permanent rare-earth magnets has led to the development of high-intensity separators that operate virtually energy free. The use of rare-earth magnetic separators for beneficiation of industrial minerals has become the industry standard with literally hundreds of separators placed in recent years. The following sections present an overview of the most widely used permanent magnetic separators: rare-earth drum and rare-earth roll-type separators.

Of the roll separators, there are at least fourteen manufacturers. Most of the different makes are based on the original Permroll design concept originated by this author. Various enhancements have been mainly focused on the belt tracking methods. New magnetic roll configurations and optimization of roll designs are relatively recent innovations. Additional optimization efforts are in progress.

At last count, seven manufacturers have commercially available drum separators, most based on magnet circuits derived from the use of conventional ferrite magnet. Two unique designs have been developed with one clearly offering advantages over older configurations.

Rare-earth elements have some unique properties that are used in many common applications, such as TV screens and lighters. In the 1970s, rare-earths began to be used in a new generation of magnetic materials, that have very unique characteristics. Not only were these stronger in the sense of attraction force between a magnet and mild steel (high induction, B), the coercivity (Hc) is extremely high. This property makes the magnetization of the magnet body composed of a rare-earth element alloy very stable, i.e., it cannot easily be demagnetized.

It was a well known fact that permanent magnets positioned on both sides of a flat steel body can magnetize the steel to a high level, if the magnet poles were the same on each side, i.e., the magnets would repel each other. However, in the past, large magnet volumes were required to achieve any substantial magnetization. With the new powerful magnets, the magnet volume could be relatively small to generate high steel magnetization. In 1981 this author determined the optimum ring size for samarium-cobalt magnets. Maximum steel magnetization (near saturation) could be obtained if the rings were stacked to make a roll using a 4:1 ratio of magnet to steel thickness, see Figure 1. Since magnetized particles are attracted to the magnetized steel surface on the roll periphery, this means that 20% of the exposed roll surface would collect such material. This collection area is an order of magnitude greater than what could be achieved with prior art magnets, making the magnetic roll useful for mineral separation.

Although one of the first prototype rare-earth magnetic rolls was calculated to have about 14,000 gauss steel magnetization, it was found in comparative testing with electromagnetic induced roll (IMR) separators operating at about 21,000 gauss, that similar performance was obtained in fine particle processing (smaller than 1 mm). When processing coarser particles an improved performance was established (e.g., less weakly magnetic contaminants remaining in the upgraded product and fewer separation passes to achieve high quality). The improvement results because the magnetic force acting on the particles is high, due to a high flux gradient. An electromagnetic induced magnetic roll separator has an air gap, which must be increased to accommodate the processing of larger particles. The rare-earth magnetic roll (REMR) magnetic separator has no such air gap. Consequently, the magnetic force does not decline in the manner of an IMR set with a large air gap.

As the name implies, suspended magnets are installed over conveyors to lift tramp iron out of the burden. Suspended magnets have been more frequently applied as conveyor speeds have increased. Suspended type magnets are capable of developing very deep magnetic fields and magnet suspension heights as high as 36 are possible.

Suspended magnets are of two basic types (1) circular and (2) rectangular. Because of cost considerations, the rectangular suspended magnet is nearly always used. Magnet selection requires careful analysis of the individual system to insure adequate tramp iron removal. Factors that must be considered include:

The position in which the magnet must be mounted will also influence the size of magnet required. The preferred position is at an angle over the head pulley of the conveyor where the load breaks open and the tramp iron is free to move easily to the magnet face. When the suspended magnet must be mounted back from the head pulley parallel to the conveyor, tramp iron removal is more difficult and a stronger magnet is required.

Magnetic drum separators come in many different styles. Tramp iron drum separators usually use a magnet design referred to as a radial type. In such a unit the magnet poles alternate across the width of the drum and are of the same polarity at any point along the drums circumference. The magnet assembly is held stationary by clamp bearings and the drum shell is driven around this magnet assembly.

Drum-separators lend themselves to installation in chutes or at the discharge point of bucket elevators or screen conveyors.The capacity and type of tramp iron to be removed will determine the size selection of a drum separator. They are available in both permanent and electro magnetic types.

Standard drum diameters are 30 and 36. General guide lines, in diameter selection, are based on (1) feed volume (2) magnetic loadings and (3) particle size. The 30 diameter drum guide lines are roughly maximum of 75 GPM per foot feed volume, 8 TPH per foot magnetic loading and 10 mesh particle size. The 36 guide lines are 125 GPM per foot feed volume, 15 TPH per foot magnetic loading and 3/8 inch particle size.

For many years, wet magnetic drum separator magnet rating has been on the basis of a specified gauss reading at 2 from the drum face. The gauss reading is an average of readings taken at the centerline of each pole and the center of the magnet gap measured 2 inches from the drum surface. This rating tends to ignore edge of pole readings and readings inside of the 2 inch distance, particularly surface readings which are highly important in effective magnetic performance.

We have previously discussed dry drum separators as used for tramp iron removal. A second variety of drum separator is the alternating polarity drum separator. This separator is designed to handle feeds having a high percentage of magnetics and to obtain a clean, high grade, magnetic concentrate product. The magnet assembly is made up of a series of poles that are uniform in polarity around the drum circumference. The magnet arc conventionally covers 210 degrees. The magnet assembly is held in fixed operating position by means of clamp bearings and the cylinder is driven around this assembly.

Two styles of magnet assemblies are made up in alternating polarity design. The old Ball-Norton type design has from 8 to 10 poles in the 210 arc and develops a relatively deep magnetic field. This design can effectively handle material as coarse as 1 inch while at the same time imparting enough agitation in traversing the magnetic arc to effectively reject non-magnetic material and produce a clean magnetic concentrate product. The 30 diameter alternating polarity drum is usually run in the 25 to 35 RPM speed range.

Application of the high intensity cross-belt is limited to material finer than 1/8 inch size with a minimum amount of minus 200 mesh material. The cost of this separator is relatively high per unit of capacity approaching $1000 per inch of feed width as compared to $200 per inch of feed width on the induced roll separator.

This investigation for an improved separator is a continuation of the previously reported pioneering research of the Bureau of Mines on the matrix-type magnetic separator. When operated with direct current. or a constant magnetic field, the matrix-type magnetic separator has several disadvantages, which include incomplete separation of magnetic and nonmagnetic components in one pass and the retention of some of the. magnetic fraction at the discharge quadrant. Since the particle agitation that results from pulsed magnetic fields may overcome these factors, operation with an alternating current would be an improvement. Another possibility is the separation of dry feeds, which may have applications where the use of water must be avoided.

The effects of an alternating field were first described by Mordey and later by others of whom Doan provides a bibliographical resume. The significant feature to note in the description by Mordey is the change from a repulsion in weak fields to an attraction in strong fields, in addition to a difference in response with different minerals. The application by Mordey was with wet feeds using launders and inclined surfaces, although applications by others are with both wet and dry feeds.

Except for occasional later references the interest in alternating current for magnetic separation has almost disappeared. Lack of interest is probably due to the apparent high power consumption required to generate sufficiently intense magnetic fields, a problem that warrants further consideration.

The matrix separator differed somewhat from the slotted pole type described in a previous report in that the flux passed into the matrix from only one side, the inverted U-shaped magnet cores 4 and 7 illustrated in figure 1. Figure 1 shows a front view, side view, and a bottom view of the matrix-type magnetic separator. By this arrangement, an upward thrust could be exerted on the matrix disk during each current peak; the resulting induced vibration would accelerate the passage of the feed as well as the separation of the magnetic particles from the nonmagnetic particles since the applied field during the upward thrust preferentially lifts

The matrix disk 5 rotates successively through field and field-free quadrants. Where a given point on the disk emerges into a field quadrant, feed is added from a vibrating feeder; nonmagnetic particles fall through the matrix, and magnetic particles are retained and finally discharged in the succeeding field-free quadrant.

Two types of disks were used, a sphere matrix illustrated in top and cross-sectional views in figure 2 and a grooved plate type similarly illustrated in figure 3. Both the spheres and grooved plates were mounted on a nonmagnetic support 1 of optimum thickness for vibration movement (figs. 2-3). The sphere matrix disk, similar to that of the earlier model, had a matrix diameter 8 of 8.5 inches and spokes 7 spaced 45 apart; the spheres were retained by brass screens 4 (fig. 2).

The grooved plate disk was an assemblage of grooved steel plates that tapered so that one edge 5 was thinner than the other 6 (fig. 4) to provide a stack in the form of a circle having an outside diameter 9 of 7.9 inches (fig. 3). The plates were retained by two split aluminum rings 8 and 3 clamped in two places 1 and 11. They were stacked so that the vertically oriented grooves of one plate touched the flat side of the second plate. As illustrated in figure 4, two slots 3 and 4 were added to reduce eddy current losses.

Both disks 5 illustrated in figure 1 were rotated by a pulley 1 through a steel shaft 8 held by two aluminum bars 2 and which in turn were fastened to aluminum bars 3 and steel bars 6. The magnetic cores 4 and 7 were machined from 10- by 12-inch E-shaped Orthosil transformer laminations. For wet feeds,

With the information derived from the performance of this separator, a cross-belt-type separator was also constructed as illustrated in figure 5, which shows a front view and a cross-sectional view through the center of the magnet core. The cross-belt separator mentioned here differs somewhat from the conventional cross-belt separator in that the belt 5 moves parallel to the feed direction instead of 90 with the feed direction. The magnetic core, composed of parts 17, 19, 21 and 22 that were machined from 7--by 9 inch E-shaped Orthosil transformer laminations, supplies a magnetic field between one magnetic pole 6, which has grooves running parallel to the feed direction, and the other magnetic pole 14. Owing to the higher intensity field at the projection from the grooves, magnetic particles are lifted from feeder 15 to the belt 5. By movement on flat-faced pulleys 3 supported by bearings 4 the belt 5 carries the particles to the discharge chute 7. Nonmagnetic particles fall from the feeder edge and are discharged on the chute 8. A special 0.035-inch-thick Macarco neoprene-dacron endless belt permits a close approach of the feeder surface to the magnet pole 6. The feeder 15 constructed of plexiglass to prevent vibration dampening by eddy currents, is fastened to a vibration drive at 16 derived from a small vibrating feeder used for granular materials. A constant distance between poles 6 and 14 was maintained by acrylic plastic plates 9 on each side of the poles 6 and 14 with a recessed portion 13 to provide room for the belt 5 and feeder 15. The structural support for the separator, which consisted of parts 1, 2, 11, 18, and 20, was constructed of 2- by 2- by -inch aluminum angle to form a rectangular frame, and part 10 was machined from angular stock to form a support for the magnet core.

Each U-shaped magnet core in figure 1 was supplied with two 266-turn coils and two 133-turn coils of No. 10 AWG (American wire gage) heavy polythermaleze-insulated copper wire. With alternating current excitation, the current and voltage are out of phase so that the kilovolt-ampere value is very high even though the actual kilowatt power is low. This difference may be corrected with either series capacitors to reduce the input voltage or parallel capacitors to reduce the input current. However, the circuit that was selected is illustrated in figure 6 in which the two 266-turn coils are connected in series with the capacitor 2. Power is supplied by the 133-turn drive coil 7 that is connected in series with the 133-turn drive coil 9 on the other U-shaped magnet core. Coils 4 and 6 and the capacitor 2 form a circuit that resonates at 60 hertz when the capacitor 2 has a value of 49 microfarads in accordance with the equation

For the capacitance in the power input circuit, the value is calculated on the basis of the equality of equations 2-3. When the input at point 10 is 10 amperes at 126 volts or 1.26 kilovolt-amperes, the current at point 3 and the voltage at

point 1 are 10 amperes and 550 volts, respectively, or a total of 11.0 kilovoIt-amperes for the two magnet cores, which provides a 5,320-ampere- turn magnetization current. The capacitors, a standard power factor correction type, had a maximum rating of 600 volts at 60 hertz.

Application of alternating current to the cross-belt separator is not successful. In contrast to the matrix-type separator in which the feed is deposited on the magnetized matrix, the feed for the cross belt is some distance below a magnet pole where the field is weaker and the force is a repulsion. Even though the magnetic force with the matrix-type separator may be a repulsion instead of an attraction, it would result in the retention of the magnetic fraction in the matrix. Replacement of the alternating current with an intermittent current eliminates the repulsion effect but still retains the particle vibration characteristics.

For an intermittent current the circuit shown in figure 7 is used. A diode 5 supplies the current to a coil 4, which can be the magnetizing coil for the cross-belt separator, or for one magnet core of the matrix-type separator that is connected in parallel or series with the coil for the other core. A coil 2 is supplied with half-wave-rectified current from a diode 6 but is out of phase with the other coil 4 and is only applicable to a second separator. However, the circuit illustrates the reduction of the kilovolt-ampere load of intermittent magnetizing currents. As an example, measurements were, made with the two magnet cores of figure 1; each core had 532 turns of wire. When the capacitor 9 has a value of 72 microfarads, the current at point 8 is 13 amperes, and the voltages at points 10, 1, and 7 are 75, 440, and 390 volts, respectively. The kilovoIt-ampere input at point 11 is therefore 0.98, and the kilovolt-amperes supplied to the coils is 5.07. This circuit is not a simple resonance circuit, as shown in figure 6, but a circuit in which the correct value of the capacitor 9 depends on the current. At currents lower than 13 amperes, the 72-microfarad value is too large.

However, separations with intermittent current were confined to a simple one-diode circuit. With the matrix-type separator, each magnet core carried 10.5 amperes at 240 volts through 399 wire turns or a total of 21 amperes since the two cores were connected in parallel. For the cross-

belt separator illustrated in figure 5, five 72-turn coils and one 96-turn coil wound with No. 6 AWG heavy polythermaleze-insulated square copper wire were used in series connection. Current-carrying capacity is approximately 40 amperes with an input of approximately 80 volts of half-wave-rectified 60-hertz current. At 40 amperes, the average number of ampere turns would be 18,240. Intermittent current and voltage were measured with the same dynamometer meters used for alternating current; these meters measure an average value.

It is possible to increase the magnetizing current for the matrix-type separator without excessive vibration by increasing the thickness of the plate 1 (figs. 2-3). Another alternative is a combination of intermittent and constant magnetic fields. Although a variety of circuits are possible, the combination of fields was accomplished with the simple adaptation of the stray field losses in a U-shaped magnet core using the circuit of figure 8. The power drawn is full-wave rectification, or half wave for each leg of the magnet core with the flux, from the coils 3 and 4 adding. Owing to magnetic leakage, the flux from the coil nearest to the magnet pole tested predominates. When the magnetic field is measured with a Bell model 300 gaussmeter and observed with a Tektronix type 547 oscilloscope with a type 1A1 amplifier, the results of figure 9 represent a pulsating magnetic field on top of a constant magnetic field plateau.

Although it is known that minerals in water suspension may be separated in the constant-field matrix-type separator at fine sizes, some tests were conducted to investigate if any beneficial effects exist with an intermittent field. One advantage that was found with a minus 325-mesh feed was an increase in the completeness of the discharge of the magnetic fraction with an intermittent field as illustrated in tables 1-2. Both tests had the same average current of 10.5 amperes through the magnetizing coils of each magnet core illustrated in figure 7. The matrix consisted of 1/16-inch-diameter steel spheres.

In the two short-period comparative tests, the wash water for removing the magnetic fraction was the same and was of a quantity that permitted complete discharge with the intermittent field and partial removal with the constant field. After the test was completed, magnetic particles retained with the constant field were determined by a large increase in the intensity of flow of wash water, a flow volume that would not be practical for normal operation. For separation efficiency, the intermittent field had no advantage over the constant field probably because of a lack of vibration response with minus 325-mesh particles at 60 hertz. This will be described later with dry feeds.

Dry magnetic separation at coarse sizes is not a problem because it may be accomplished with a variety of separator types. Difficulty at fine sizes is twofold. First, the feed rate capacity decreases in the separators with moving conveyor surfaces such as the induced roll and cross-belt separators in which the attracted magnetic particles would have to move at nominal feed rates through a thick layer of nonmagnetic particles; second, an agglomeration effect is present that increases with decrease in particle size.

Results of the separation of several mineral combinations in the size range of minus 200 plus 325 mesh are summarized in tables 3-5. Table 3 illustrates the separation of -Fe2O3 from quartz in an ore with one pass through a matrix of 1/8-inch-diameter steel spheres using the alternating current circuit of figure 6.

Application of an intermittent field with a matrix of 75 percent 1/16-inch-diameter steel spheres and 25 percent 1/8-inch-diameter steel spheres is illustrated in table 4 in a one-pass separation of pyrrhotite from quartz using the circuit of figure 7. Unlike table 3, no attempt was made to obtain an intermediate fraction, which would have resulted in raising and lowering the iron compositions of the magnetic and nonmagnetic fractions, respectively, and provided a fraction for repass with increased recovery.

Table 5 gives the results of the application of a partially modulated field using the circuit of figure 8 and the grooved plate matrix of figure 3 in a one-pass separation of ilmenite from quartz. The advantage of the grooved plate over the spheres is that the particles pass through the matrix in a shorter time. The high flow rate obtained using the grooved plate could be increased further, particularly if water is used, by attaching suction chambers under the disk in a manner similar to applications with continuous vacuum filters. Although the grade and recovery of ilmenite are very high, this need not necessarily be attributed to the grooved-plate matrix since the ampere turns are higher than in any of the other tests. Increased ampere turns is a prerequisite for successful application of alternating current separators and intermittent current separators.

When a minus 325-mesh fraction is tested, a separation sometimes occurs, but in most cases the feed passes through without separation. Response at higher frequencies was investigated with a smaller -inch-cross section U-shaped magnet core 1 (fig. 10). Separation was performed with a nonmagnetic nonconducting plane surface 3 moved manually across the magnet pole as illustrated by the direction arrow 4. When separation occurred, the nonmagnetic mineral 5 would move with the plane, and the magnetic mineral would separate from the nonmagnetic mineral by remaining attached to the magnet pole. When no separation occurred, the entire mixture of magnetic and nonmagnetic minerals would either move with the plane or adhere to the magnet pole.

Four magnetising coils of 119 turns each of No. 14 AWG copper wire were used; three were connected in series with a capacitor as in figure 6, and one was connected to a variable-frequency power supply. The current in the resonant circuit is approximately 5 amperes. When the capacitor has a value of 49 microfarads, the resonant frequency is 130 hertz, and no separation occurs. With the capacitor reduced to 10 microfarads to provide a resonant frequency of 300 hertz, a separation occurs. In the case of a minus 325-mesh -Fe2O3-quartz mixture, most of the quartz moves with the plane, and the -Fe2O3 remains attached to the magnet pole. Similar results are obtained with pyrrhotite-quartz. Indications are that the separation may be improved with preliminary treatment of the feed by dry grinding aids.

frequencies, the time per cycle is too short to permit initial magnetization; at very low frequencies, the magnetization is in phase with the field. The frequencies reported here are between these two extremes and probably near, and just above, the low frequency limit. Experimental values on particles in the size range of minus 35 plus 65 mesh were previously published. These data indicate that 0.16 second, the time required to traverse a magnetizing field distance of 0.9 inch at 5.5 inches per second, is adequate time for the magnetization of minerals, but 0.02 second, the time required to traverse approximately 0.1 inch at the same rate, is too short. Time lag has been reported in the literature for magnetic alloys and has been classified, to the exclusion of the eddy current lag, into a lag that is dependent on impurities and a Jordan lag that is independent of temperature.

From evidence derived from the Barkhausen effect, the magnetization does not proceed uniformly and simultaneously throughout a specimen but is initiated in a limited region from which it spreads in a direction parallel to the field direction at a finite velocity. In a changing magnetic field, the number of initiating nuclei is proportional to the cross-sectional area perpendicular to the direction of the field. For a specimen in the form of a cube, the rate of energy W transferred to the cube would therefore be proportional to the aforementioned cross-sectional area so that for a cube of side s,

Application of intermittent current to the cross-belt separator arose from the need for the dry separation of an iron composition material from the copper in a product submitted by personnel of a Bureau of Mines chalcopyrite vacuum decomposition project. Although this product was of a relatively coarse size, the matted mass resulting from the needle shape or fiber form of the copper and the magnetic field coagulation effects of the magnetic particles prevented use of commercial dry separators such as the induced roll separator and constant-field cross-belt separator. The pulsating magnetic field had a separation effect similar to the pulsations in a hydraulic jig; the pulsating magnetic field permits the nonmagnetic fibers to sink back to the vibrating feeder and allows the magnetic particles to rise to the belt. Other applications would include fibrous minerals such as tremolite, actinolite, and chrysolite, and matted and fibrous secondary materials.

Application of alternating and intermittent current to magnetic separation at a relatively high number of ampere turns was made possible by special electronic circuits. Actual power losses are low and include the IR loss, which is the same that occurs in direct-current magnetic separation, and the core loss, which has a magnitude corresponding to the IR loss. Minerals may be dry-separated close to the minus 325-mesh size at 60-hertz frequency and possibly at smaller particle sizes at higher frequency. In the wet separation of minus 325-mesh feeds, intermittent current provides for complete release of the magnetic fraction during the discharge cycle. For matted fibrous and magnetically coagulating feeds, a cross-belt separator with an intermittent magnetizing current provides efficient separations.

gradient magnetic separation - an overview | sciencedirect topics

High gradient magnetic separation (HGMS) (Trindale et al., 1974, and Liu, 1982) is based on coal being diamagnetic (repulsed by a magnet), whereas pyrite is paramagnetic (attracted to a magnet). The magnetic susceptibility of pyrite is 0.3 10-6 G/g (G = gauss), compared to coal at -0.4 10-6 to 0.8 10-4 G/g. If pyrite is not altered to the more magnetic pyrrhotite, perhaps via microwave heating, the separation can be enhanced through control of the particle size and/or through control of the field strength.

With the primary purpose of HGMS being the removal of pyrite from fine coal, experiments on high-S Illinois Basin coals by Murray (1977), Hise et al. (1979), Harris and Hise (1981), and Hower et al. (1984) were a logical test of the concept. Harris and Hise (1981) noted an increase in inertinite in the magnetic fraction, perhaps a function of coarse pyrite with fusain, while Hower et al. (1984) did not see much maceral difference between the clean and the refuse. Hower et al. (1984) noted that most of the pyrite in the clean coal was very fine (98% <10 m in one case).

High-gradient magnetic separation has achieved remarkable progress and wide applications since Jones in 1955 achieved a high-gradient magnetic field in a magnetized matrix (Svoboda, 2004). By using magnetic matrix, magnetic force upon magnetic particles is remarkably increased, resulting in the significantly improved recovery for fine weakly magnetic particles.

However, it has undergone a long period in the development of high-gradient magnetic separators, from cyclic to continuous operation, and from the horizontal design philosophy of separating ring to the vertical one. At the same time, magnetic matrix, as carrier for the capture of magnetic particles in a high-gradient magnetic separator, has evolved from the early used fibers and balls to meshes, and to the present rod poles. Now, the rod matrix made of rod poles has proven the most applicable medium in the magnetic capture of fine weakly magnetic particles from slurry in a high-gradient magnetic separator, due to its high operational reliability, simplified combinatorial optimization, and resistance to clogging (Chen etal., 2013a).

Physical processes (e.g., physical retrieval; over-packing/re-packaging/re-drumming; screening; soil washing; high gradient magnetic separation; solidification; vitrification/ceramics; incineration; filtration/ultra-filtration; reverse osmosis/membrane processes; solar evaporation); Fig. 8.5 shows a general view of the soil washing plant at Kurchatov Centre (Russia).

Conventional magnetic separators are largely confined to the separation or filtration of relatively large particles of strongly magnetic materials. They employ a single surface for separation or collection of magnetic particles. A variety of transport mechanisms are employed to carry the feed past the magnet and separate the magnetic products. The active separation volume for each of these separators is approximately the product of the area of the magnetised surface and the extent of the magnetic field. In order for the separators to have practical throughputs, the magnetic field must extend several centimetres. Such an extent implies a relatively low magnetic field gradient and weak magnetic forces.

To overcome these disadvantages HGMS has been developed. Matrices of ferromagnetic material are used to produce much stronger but shorter range magnetic forces over large surface areas. When the matrices are placed in a magnetic field, strong magnetic forces are developed adjacent to the filaments of the matrix in approximately inverse proportion to their diameter. Since the extent of the magnetic field is approximately equal to the diameter of the filaments the magnetic fields are relatively short range. However, the magnetic field produced is intense and permits the separation and trapping of very fine, weakly magnetic particles (Oberteuffer, 1979).

The transport medium for HGMS can be either liquid or gaseous. Dry HGMS processing has the advantage of a dry product although classification of the pulverised coal is required to ensure proper separation. Small particles tend to agglomerate and pass through the separator. It has been shown that individual particles of coal in the discharge of a power plant pulveriser flow freely and hence separate well only if the material below about 10 m is removed (Eissenberg et al., 1979). Even then drying of that part of run of mine coal to be treated by HGMS may be required to ensure good flow characteristics.

A schematic representation of a batch HGMS process is shown in Figure 11.5 (Hise, 1979, 1980; Hise et al., 1979). It consists of a solenoid, the core cavity of which is filled with an expanded metal mesh. Crushed coal is fed to the top of the separator. Clean coal passes through while much of the inorganic material is trapped to be released when the solenoid is later deactivated.

Data from a batch HGMS process of one size fraction of one coal are plotted in Figure 11.6 as weight per cent of material trapped in the magnetic matrix, the product sulphur and the product ash versus the independent variable of superficial transport velocity. At low superficial transport velocities the amount of material removed from the coal is high partly due to mechanical entrapment. As the velocity is increased the importance of this factor diminishes but hydrodynamic forces on the particles increase. These hydrodynamic forces oppose the magnetic force and the amount of material removed from the coal decreases (Hise, 1979).

For comparison, Figure 11.7 shows data from a specific gravity separation of the same size fraction of the same coal. While the sulphur contents of the products from the two separation processes are similar the ash content of the HGMS product is considerably higher than that of the specific gravity product. It should be emphasised that this comparison was made for one size fraction of one coal.

More recently dry HGMS has been demonstrated at a scale of 1 t/h on carousel type equipment which processes coal continuously (Figure 11.8; Hise et al., 1981). A metal mesh passes continuously through the magnetised cavity so that the product coal passes through while the trapped inorganics are carried out of the field and released separately.

Wet HGMS is able to treat a much wider range of coal particle sizes than dry HGMS. The efficiency of separation increases with decreasing particle size. However, depending on the end use a considerable quantity of energy may have to be expended in drying the wet, fine coal product. Wet HGMS may find particular application to the precleaning of coal for use in preparing coal water mixtures for subsequent combustion as both pulverising the coal to a fine particle size and transporting the coal in a water slurry are operations common to both processes.

Work at Bruceton, PA, USA has compared the pyrite reduction potential of froth flotation followed by wet HGMS with that of a two stage froth flotation process (Hucko and Miller, 1980). Typical results are shown in Figures 11.9 and 11.10. The reduction in pyritic sulphur is similar in each case although a greater reduction in ash content is achieved by froth flotation followed by HGMS than by two stage froth flotation. However, Hucko (1979) concludes that it is highly unlikely that HGMS would be used for coal preparation independently of other beneficiation processes. As with froth flotation there is considerable variation in the amenability of various coals to magnetic beneficiation.

Mssbauer spectroscopy was used to document the efficiency of pyrite and clay removal from raw coal by float-sink methods using organic solvents to simulate washing in coal-preparation plants. For coal from the Upper Freeport seam, it was found that pyrite could be removed more efficiently than clays by such procedures. This result is not surprising because clay minerals have densities considerably closer to that of coal than pyrite, and the efficiency of removal can be expected to be a function of the density difference unless the particle-size distributions of the minerals are quite different.

Similar tests of the efficiency of high-gradient magnetic separation (HGMS) were also made by Mssbauer spectroscopy and showed that the removal of siderite (FeCO3) was more efficient than removal of either pyrite or clay minerals. Detailed magnetic studies of this process have suggested that pyrrhotite formation may be responsible for the necessary enhancement of the magnetic properties of pyrite in order for pyrite to be removed efficiently by HGMS. High localized temperatures generated upon crushing of the coal were suggested to be responsible for converting a surface layer on pyrite particles to pyrrhotite.

One further application of Mssbauer spectroscopy, examined by Jacobs et al. (1978), was to use the technique to investigate mineral transformations during coal-liquefaction processes. The objective was to find the procedure for liquefaction that also generated the most highly magnetic form of pyrrhotite (Fe1-xS) for possible application of HGMS. Both Mssbauer and magnetic studies showed that most of the pyrite is converted during liquefaction of coal to pyrrhotite, which may be weakly or strongly ferrimagnetic, depending on the temperature-time pathways of the processes. By slow cooling from 225C, a highly ferrimagnetic form of pyrrhotite, monoclinic Fe7S8, can be obtained, which is the preferred form for removal by HGMS.

This process is being investigated in the separation of coal and pyrite. The difference in magnetic susceptibility of coal and pyrite is very small; therefore, the most promising application is for high-gradient magnetic separators (HGMS) or the pretreatment of coal, which could increase the susceptibility of coal mineral matter.

Because pyrite liberation usually requires very fine grinding (e.g., below 400 mesh), high-gradient magnetic separation, the process developed to upgrade kaolin clays by removing iron and titanium oxide impurities, seems to be particularly suitable for coal desulfurization. Wet magnetic separation processes offer better selectivity for the fine dry coal that tends to form agglomerates. Experiments on high-gradient wet magnetic separation have so far indicated better sulfur rejection at fine grinding.

Perhaps the most interesting studies are on the effect of coal pretreatment, which enhances the magnetic properties of the mineral matter, on the subsequent magnetic separation. It has, for example, been shown that microwave treatment heats selectively pyrite particles in coal without loss of coal volatiles, and converts pyrite to pyrrhotite, and as a result, facilitates the subsequent magnetic separation.

In the Magnex process, crushed coal is treated with vapors of iron carbonyl [Fe(CO)5], followed by the removal of pyrite and other high-ash impurities by dry magnetic separation. Iron carbonyl in this process decomposes on the surfaces of ash-forming minerals and forms strongly magnetic iron coatings; the reaction with pyrite leads to the formation of pyrrhotite-like material.

In 1970, Krukiewicz and Laskowski described a magnetizing alkali leaching process, which was applied to convert siderite particles into magnetic -Fe2O3 and Fe3O4. A similar process has recently been tested for high-sulfur coal, and it was reported that, after pretreatment of the coal in 0.5-M NaOH at 85C and 930kPa (135psi) air pressure for 25min, more than 50% of the coal sulfur was rejected under the same conditions in which the same high-gradient magnetic separation had removed only 5% sulfur without the pretreatment.

where 0 is the permeability constant of vacuum, Vp and Mp are the particle volume and magnetization, respectively and H is the strength of the magnetic field gradient at the location of the particle. Mp is given by Mp=H where is the magnetic volume susceptibility, which changes with particle size and shape, and H is the magnetic field strength.

High-gradient magnetic separation (HGMS) is an established concept where a magnetic field is typically applied across an array of magnetically soft metal wires to establish a high-field gradient.99,100 Magnetic nanoparticles travelling through such a field are attracted to the surfaces of the wires allowing effective collection. However, the operation of such a separator is strongly dependent on the size and magnetic properties of the particles involved and the strength of the magnetic field must be high enough to overcome possible competing colloidal forces acting on a flowing particle suspension arising from fluid drag, gravity, inertia and diffusion.99

The gravitational force may be expressed as Fg=(pg)Vpg where p and g are the spherical particle and fluid densities, respectively, g is the acceleration due to gravity and Vp is the particle volume. Hydrodynamic fluid drag results in a force according to Stokes Law: Fd=6b(vfvp) where is the dynamic viscosity of the fluid, b is a fluid-dependent constant and vf and vp are the fluid and particle velocities, respectively. In many of the laboratory-based ex situ demonstrations of removal of a contaminant from its suspension, a handheld magnet is usually strong enough to demonstrate magnetic separation over length scales of a few centimetres. This range may be sufficient for effective separation in, for example, bioseparation, but for industrial-scale in situ environmental remediation, a much greater separation distance is usually required necessitating the use of a stronger magnetic field as the dependence of field strength on distance follows an inversecube relationship (for a dipole source). Other significant factors acting against magnetic separation include buoyancy and random Brownian motion, both of which are enhanced as the particle becomes smaller. For Fe oxide, the critical diameter of a nanoparticle at which its volume becomes large enough for the magnetic force exerted on it to overcome Brownian motion is ~50nm, assuming large magnetic field strengths typically used in HGMS are applied.101

Yavuz et al. first demonstrated magnetic separation with very low fields (<100Tm1) using monodisperse 12nm Fe3O4 nanoparticles that had previously been applied to the removal of As from water.101 They demonstrated multiplexed separation of an initially bimodal mixture through the application of different magnetic fields and noted that aggregation, caused by high-field gradients at the surface of the nanoparticles, actually helped the separation process (Figure 7.12).

Figure 7.12. (A) Magnetic separation of 16-nm water-soluble Fe3O4 nanoparticles within several minutes using a low-field gradient of 23.3Tm1. (B) and (C) Adsorption isotherms comparing the removal of As3+ (A) and As5+ (B) using Fe3O4 nanoparticles of different diameters. Concentrations were measured using inductively coupled plasma-mass spectrometry (ICP-MS).

The magnetizing force, which induces the lines of force through a material, is called the field intensity, H (or H-field), and by convention has the units ampere per meter (Am1) (Bennett et al., 1978).

The intensity of magnetization or the magnetization (M, Am1) of a material relates to the magnetization induced in the material and can also be thought of as the volumetric density of induced magnetic dipoles in the material. The magnetic induction, B, field intensity, H, and magnetization, M, are related by the equation:

where 0 is the permeability of free space and has the value of 4107NA2. In a vacuum, M=0, and M is extremely low in air and water, such that for mineral processing purposes Eq. (13.1) may be simplified to:

so that the value of the field intensity, H, is directly proportional to the value of induced flux density, B (or B-field), and the term magnetic field intensity is then often loosely used for both the H-field and the B-field. However, when dealing with the magnetic field inside materials, particularly ferromagnetic materials that concentrate the lines of force, the value of the induced flux density will be much higher than the field intensity. This relationship is used in high-gradient magnetic separation (discussed further in Section 13.4.1). For clarity it must be specified which field is being referred to.

For paramagnetic materials, is a small positive constant, and for diamagnetic materials it is a much smaller negative constant. As examples, from Figure 13.1 the slope representing the magnetic susceptibility of the material, , is about 0.001 for chromite and 0.0001 for quartz.

The magnetic susceptibility of a ferromagnetic material is dependent on the magnetic field, decreasing with field strength as the material becomes saturated. Figure 13.2 shows a plot of M versus H for magnetite, showing that at an applied field of 80kAm1, or 0.1T, the magnetic susceptibility is about 1.7, and saturation occurs at an applied magnetic field strength of about 500kAm1 or 0.63T. Many high-intensity magnetic separators use iron cores and frames to produce the desired magnetic flux concentrations and field strengths. Iron saturates magnetically at about 22.5T, and its nonlinear ferromagnetic relationship between inducing field strength and magnetization intensity necessitates the use of very large currents in the energizing coils, sometimes up to hundreds of amperes.

The magnetic force felt by a mineral particle is dependent not only on the value of the field intensity, but also on the field gradient (the rate at which the field intensity increases across the particle toward the magnet surface). As paramagnetic minerals have higher (relative) magnetic permeabilities than the surrounding media, usually air or water, they concentrate the lines of force of an external magnetic field. The higher the magnetic susceptibility, the higher the induced field density in the particle and the greater is the attraction up the field gradient toward increasing field strength. Diamagnetic minerals have lower magnetic susceptibility than their surrounding medium and hence expel the lines of force of the external field. This causes their expulsion down the gradient of the field in the direction of the decreasing field strength.

The equation for the magnetic force on a particle in a magnetic separator depends on the magnetic susceptibility of the particle and fluid medium, the applied magnetic field and the magnetic field gradient. This equation, when considered in only the x-direction, may be expressed as (Oberteuffer, 1974):

where Fx is the magnetic force on the particle (N), V the particle volume (m3), p the magnetic susceptibility of the particle, m the magnetic susceptibility of the fluid medium, H the applied magnetic field strength (Am1), and dB/dx the magnetic field gradient (Tm1=NA1m2). The product of H and dB/dx is sometimes referred to as the force factor.

Production of a high field gradient as well as high intensity is therefore an important aspect of separator design. To generate a given attractive force, there are an infinite number of combinations of field and gradient which will give the same effect. Another important factor is the particle size, as the magnetic force experienced by a particle must compete with various other forces such as hydrodynamic drag (in wet magnetic separations) and the force of gravity. In one example, considering only these two competing forces, Oberteuffer (1974) has shown that the range of particle size where the magnetic force predominates is from about 5m to 1mm.

The normal particles of benign wear of sliding surfaces. Rubbing wear particles are platelets from the shear mixed layer which exhibits super-ductility. Opposing surfaces are roughly of the same hardness. Generally the maximum size of normal rubbing wear is 15m.

Break-in wear particles are typical of components having a ground or machined surface finish. During the break-in period the ridges on the wear surface are flattened and elongated platelets become detached from the surface often 50m long.

Wear particles which have been generated as a result of one surface penetrating another. The effect is to generate particles much as a lathe tool creates machining swarf. Abrasive particles which have become embedded in a soft surface, penetrate the opposing surface generating cutting wear particles. Alternatively a hard sharp edge or a hard component may penentrate the softer surface. Particles may range in size from 25m wide and 25 to 100m long.

Fatigue spall particles are released from the stressed surface as a pit is formed. Particles have a maximum size of 100m during the initial microspalling process. These flat platelets have a major dimension to thickness ratio greater than 10:1.

Laminar particles are very thin free metal particles between 2050m major dimension with a thickness ratio approximately 30:1. Laminar particles may be formed by their passage through the rolling contact region.

There is a large variation in both sliding and rolling velocities at the wear contacts; there are corresponding variations in the characteristics of the particles generated. Fatigue particles from the gear pitch line have similar characteristics to rolling bearing fatigue particles. The particles may have a major dimension to thickness ratio between 4:1 and 10:1. The chunkier particles result from tensile stresses on the gear surface causing fatigue cracks to propagate deeper into the gear tooth prior to pitting. A high ratio of large (20m) particles to small (2m) particles is usually evident.

Severe sliding wear particles range in size from 20m and larger. Some of these particles have surface striations as a result of sliding. They frequently have straight edges and their major dimension to thickness ratio is approximately 10:1.

The size and position of the particles after magnetic separation on a slide indicates their magnetic susceptibility. Ferromagnetic particles (Fe, Co, Ni) larger than 15m are always deposited at the entry or inner ring zone of the slide. Particles of low susceptibility such as aluminium, bronze, lead, etc, show little tendency to form strings and are deposited over the whole of the slide.

High-gradient magnetic separation (HGMS) with B>1000T/m [37] has been successfully used for the separation of microalgae from lakes for more than thirty years [42]. Recently, a lab-scale HGMS was applied for the magnetic separation of both freshwater algae and marine microalgae, and demonstrated the potential of HGMS for efficient microalgae harvesting using magnetic particles [50]. Toh et al., investigated the performance of both HGMS and low gradient magnetic separation (LGMS, B<80T/m) with varying dosages of magnetic particles. At a low particle dosage, the HGMS resulted in a higher RE compared with that of LGMS due to the high magnetophoresis kinetics under the high field gradient. However, the LGMS achieved an equal RE with that of HGMS when a high particle dosage was tested [37]. In HGMS, the high power consumption is necessary for magnetic power generation while the magnetic field can be simply generated by permanent magnet arrays in LGMS. Therefore, the LGMS system was more cost-effective due to its low energy consumption, and a system can be easily designed [37]. Although HGMS has been widely used in manufacturing, LGMS is more widely used in biotechnology processes [25,6062]. Furthermore, the LGMS has been used in magnetic harvesting of microalgae and shows good potential [46,49]. The magnetic field in LGMS can be generated without extra power and the separation time is even less than 3min using nanorod particles, and it may be concluded that the LGMS technique is more energy and time-efficient and is a better option for microalgae harvesting compared with HGMS technology [46].

magnetic techniques for mineral processing - sciencedirect

Magnetic separation, as a powerful technique for the manipulation of magnetic particles, has attracted worldwide attentions and achieved tremendous applications in the past several decades, due to its effectiveness, simple operation, low cost, and environmental sustainability. In the field of mineral processing, magnetic separation is mainly used for concentration of magnetic components and for removal of magnetic impurities, under dry or wet conditions. Basic theoretical principle of magnetic separations and three types of magnetic separators, i.e., low-intensity, high-intensity, and high-gradient magnetic separators, and their technological innovations and operational practices are reviewed. In particular, pulsating high-gradient magnetic separation, a landmark progress in the treatment of fine weakly magnetic minerals such as iron oxide ores and ilmenite, and its continual developments and operational variables are expatiated. Practical cases in the processing of typically magnetic ores, i.e., magnetite, hematite, limonite, ilmenite and nonmetallic ores where magnetic techniques are well embodied, are introduced. At the end of this chapter, the merits and limitations of current magnetic techniques and the future trends in the research and development of potential magnetic techniques are discussed.