There are a lot of problems that most mineral processing plant meet when operating theball mill, such as low grinding efficiency, low processing capacity, high energy consumption, unstable product fineness of the ball mill. So how to effectively improve the grinding efficiency of ball mill is an important issue for the mineral processing plants. Here are ten ways to improve the grinding efficiency of ball mill.
The complexity of grindability is determined by ore hardness, toughness, dissociation and structural defects. Small grindability, the ore is easier to grind, the wear of lining plate and steel ball is lower, and the energy consumption is also lower.Therefore, the property of raw ore directly affects the productivity of the ball mill.
The larger feed size, the more work that the ball mill needs to do on the ore. To achieve the specified grinding fineness, the workload of ball mill will be increased inevitably, and then, the energy consumption and power consumption will be increased accordingly.
In order to reduce the feed size of ore, the particle size of crushing product must be small, that is, "morecrushingand less grinding". Moreover,the efficiency of crushing is obviously higher than that of grinding, and the energy consumption of crushing is low, which is about 12%~25% of the grinding.
In the case of a certain milling speed, larger filling rate, bigger grinding area, and stronger grinding effect. However, the power consumption is also large, and it is easy to change the motion state of the steel ball if the filling rate is too high, then the impact effect on large particle materials is reduced. Conversely,the smaller filling rate, the weaker grinding effect.
For many mineral processing plants, the filling rate is generally at 45%~50%. But the condition of mineral processing plants is different, just copying othersfilling rate cannot obtain the ideal grinding effect. The specific value should be decided by the mineral processing test.
Since the contact between steel ball in the ball mill and ore is point-to-point. If the diameter of steel ball is too large, the crushing force is also large, so the ore is crushed along the direction of the penetrating force, not the interface among different minerals. This crushing is not an option, which doesnt meet the purpose of grinding.
In addition, in the case of the same filling rate, too large diameter of the steel ball results in less steel balls, low crushing probability, serious over-crushing phenomenon and uneven product particle size. But if the steel ball is too small, the crushing force on the ore is small, and the grinding efficiency is low according. Therefore, it is very important for grinding efficiency to adopting accurate size of the steel ball and its proportion.
Obviously,the grinding action between steel ball and ore causes the wear of steel balls, which can change the proportion of steel balls, affect the grinding process and cause the fineness change of grinding products. Therefore, only by adopting reasonable steel ball supplementation system can the ball mill keep stable operation.
Low grinding density, fast pulp flow,the material is not easy to stick around the steel ball,so the impact and grinding effect of steel ball on materials is weak,theparticle size of ore discharging is unqualified, and the ideal grinding efficiency cannot be achieved;
High grinding density, the material is easy to stick around the steel ball, so the impact and grinding effect of steel ball on materials is good, but the pulp flows slow, which is not conducive to improve the processing capacity of the ball mill.
There are some measures taken to control the grinding density. For example, control the ore feed of the ball mill, control the water supply of the ball mill, adjust the classifying effect, and control the particle size composition and moisture of the sand return.
In actual production, the grinding process can be optimized according to the ore properties, such asthe disseminatedgrain size ofuseful minerals, monomer dissociation degree, the disseminated grain size of gangue minerals. For example, adopting pre-discarding tailings, pre-enrichment, stage grinding, pre-classification to optimize the grinding process, which not only reduces the amount of grinding, but also recovers the useful minerals as soon as possible.
Classifying efficiency plays an important role in grinding efficiency. High classifying efficiency means that those qualified grains can be discharged timely and efficiently,while low classifying efficiency means that most qualified grains are not discharged and sent to the ball mill for re-grinding, which is easy to cause over-grinding and thus affecting the separating effect.
Sand-returning ratio is the ratio between the amount of sand return of ball mill and the feeding capacity of raw ore, which can directly affect the productivity of the ball mill. One way to improve the sand-returning ratio is to increase the feeding capacity of raw ore, another way is to reduce the shaft height of the spiral classifier.
But there is also a limited value in the improvement of sand-returning ratio. When it is increased to a certain value, the increase amplitude of the productivity is very small,the total feeding capacity of the ball mill is very close to the maximum processing capacity of the ball mill, that may cause overage grind, so the sand-returning ratio should not be too large.
There are many variable parameters during the grinding operation, and one change will inevitably lead to the change of many factors one after another. Generally, the manual operation control may cause unstable production, while the automatic control can keep the grinding and classifying in a stable and suitable state, thus improving the grinding efficiency.
There are many factors affect the grinding efficiency in the grinding process. Many factors can be judged as qualitative analysis, which are difficult to make quantitative analysis. therefore, the ball mill operator must do a comprehensive analysis according to the actual production situation and the result of the qualitative analysis,thus drawing the reasonable parameters to decrease the production cost, saving energy and reduce the consumption.
DEAR EXPERTS PLEASE TELL ME HOW TO CALCULATE THE GRINDING EFFICIENCY OF A CLOSED CKT & OPEN CKT BALL MILL. IN LITERATURES IT IS WRITTEN THAT THE GRINDING EFFICIENCY OF BALL MILL IS VERY LESS [LESS THAN 10%]. PLEASE EXPALIN IN A N EXCEL SHEET TO CALCUALTE THE SAME. THANKS SIDHANT
Simply energy efficiency is about media size. Smaller media has more surface area and higher media / particle collision frequency making it more efficient. The high intensity in the IsaMill means small media can break coarse particles. The large scale of the IsaMill makes that efficiency available to mainstream grinding.
The high energy efficiency of stirred mills compared to ball mills is well understood. The use of tower mills as an energy efficient alternative to secondary and regrind ball milling became a common inclusion in the latter part of the previous century. Traditionally the higher energy efficiency was attributed to the difference between attrition grinding in tower mills and impact grinding in ball milling. However by far the most important factor for fine grinding is media size and therefore the breakage rate. Small media has a larger surface area which translates to better transfer of energy. Although Tower Mills are full attrition grinding, they are constrained to using relatively coarse media 12-25mm balls. In contrast the IsaMill can operate with much finer media (e.g. 1mm) and much higher intensity of power input.
Media selection has a major influence on mill parameters such as energy efficiency, internal wear and operating costs. With the introduction of economic ceramic media the energy efficiency of IsaMills increases dramatically.
" the problem of inefficient use of energy in comminution was so serious that new technology that makes possible comminution to minus 200 mesh (74micron) at half the energy requirement of conventional mills would justify a Nobel prize for service to mankind." Alban Lynch and Don McKee (1981)
The ball mill accepts the SAG or AG mill product. Ball mills give a controlled final grind and produce flotation feed of a uniform size. Ball mills tumble iron or steel balls with the ore. The balls are initially 510 cm diameter but gradually wear away as grinding of the ore proceeds. The feed to ball mills (dry basis) is typically 75 vol.-% ore and 25% steel.
The ball mill is operated in closed circuit with a particle-size measurement device and size-control cyclones. The cyclones send correct-size material on to flotation and direct oversize material back to the ball mill for further grinding.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.
By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.
The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.
where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.
Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.
When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.
In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).
Planetary ball mills. A planetary ball mill consists of at least one grinding jar, which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars according to a fixed ratio. The grinding balls in the grinding jars are subjected to superimposed rotational movements. The jars are moved around their own axis and, in the opposite direction, around the axis of the sun wheel at uniform speed and uniform rotation ratios. The result is that the superimposition of the centrifugal forces changes constantly (Coriolis motion). The grinding balls describe a semicircular movement, separate from the inside wall, and collide with the opposite surface at high impact energy. The difference in speeds produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill. Planetary ball mills are smaller than common ball mills, and are mainly used in laboratories for grinding sample material down to very small sizes.
Vibration mill. Twin- and three-tube vibrating mills are driven by an unbalanced drive. The entire filling of the grinding cylinders, which comprises the grinding media and the feed material, constantly receives impulses from the circular vibrations in the body of the mill. The grinding action itself is produced by the rotation of the grinding media in the opposite direction to the driving rotation and by continuous head-on collisions of the grinding media. The residence time of the material contained in the grinding cylinders is determined by the quantity of the flowing material. The residence time can also be influenced by using damming devices. The sample passes through the grinding cylinders in a helical curve and slides down from the inflow to the outflow. The high degree of fineness achieved is the result of this long grinding procedure. Continuous feeding is carried out by vibrating feeders, rotary valves, or conveyor screws. The product is subsequently conveyed either pneumatically or mechanically. They are basically used to homogenize food and feed.
CryoGrinder. As small samples (100 mg or <20 ml) are difficult to recover from a standard mortar and pestle, the CryoGrinder serves as an alternative. The CryoGrinder is a miniature mortar shaped as a small well and a tightly fitting pestle. The CryoGrinder is prechilled, then samples are added to the well and ground by a handheld cordless screwdriver. The homogenization and collection of the sample is highly efficient. In environmental analysis, this system is used when very small samples are available, such as small organisms or organs (brains, hepatopancreas, etc.).
The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).
The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere. Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.
A ball mill is a relatively simple apparatus in which the motion of the reactor, or of a part of it, induces a series of collisions of balls with each other and with the reactor walls (Suryanarayana, 2001). At each collision, a fraction of the powder inside the reactor is trapped between the colliding surfaces of the milling tools and submitted to a mechanical load at relatively high strain rates (Suryanarayana, 2001). This load generates a local nonhydrostatic mechanical stress at every point of contact between any pair of powder particles. The specific features of the deformation processes induced by these stresses depend on the intensity of the mechanical stresses themselves, on the details of the powder particle arrangement, that is on the topology of the contact network, and on the physical and chemical properties of powders (Martin et al., 2003; Delogu, 2008a). At the end of any given collision event, the powder that has been trapped is remixed with the powder that has not undergone this process. Correspondingly, at any instant in the mechanical processing, the whole powder charge includes fractions of powder that have undergone a different number of collisions.
The individual reactive processes at the perturbed interface between metallic elements are expected to occur on timescales that are, at most, comparable with the collision duration (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b). Therefore, unless the ball mill is characterized by unusually high rates of powder mixing and frequency of collisions, reactive events initiated by local deformation processes at a given collision are not affected by a successive collision. Indeed, the time interval between successive collisions is significantly longer than the time period required by local structural perturbations for full relaxation (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b).
These few considerations suffice to point out the two fundamental features of powder processing by ball milling, which in turn govern the MA processes in ball mills. First, mechanical processing by ball milling is a discrete processing method. Second, it has statistical character. All of this has important consequences for the study of the kinetics of MA processes. The fact that local deformation events are connected to individual collisions suggests that absolute time is not an appropriate reference quantity to describe mechanically induced phase transformations. Such a description should rather be made as a function of the number of collisions (Delogu et al., 2004). A satisfactory description of the MA kinetics must also account for the intrinsic statistical character of powder processing by ball milling. The amount of powder trapped in any given collision, at the end of collision is indeed substantially remixed with the other powder in the reactor. It follows that the same amount, or a fraction of it, could at least in principle be trapped again in the successive collision.
This is undoubtedly a difficult aspect to take into account in a mathematical description of MA kinetics. There are at least two extreme cases to consider. On the one hand, it could be assumed that the powder trapped in a given collision cannot be trapped in the successive one. On the other, it could be assumed that powder mixing is ideal and that the amount of powder trapped at a given collision has the same probability of being processed in the successive collision. Both these cases allow the development of a mathematical model able to describe the relationship between apparent kinetics and individual collision events. However, the latter assumption seems to be more reliable than the former one, at least for commercial mills characterized by relatively complex displacement in the reactor (Manai et al., 2001, 2004).
A further obvious condition for the successful development of a mathematical description of MA processes is the one related to the uniformity of collision regimes. More specifically, it is highly desirable that the powders trapped at impact always experience the same conditions. This requires the control of the ball dynamics inside the reactor, which can be approximately obtained by using a single milling ball and an amount of powder large enough to assure inelastic impact conditions (Manai et al., 2001, 2004; Delogu et al., 2004). In fact, the use of a single milling ball avoids impacts between balls, which have a remarkable disordering effect on the ball dynamics, whereas inelastic impact conditions permit the establishment of regular and periodic ball dynamics (Manai et al., 2001, 2004; Delogu et al., 2004).
All of the above assumptions and observations represent the basis and guidelines for the development of the mathematical model briefly outlined in the following. It has been successfully applied to the case of a Spex Mixer/ Mill mod. 8000, but the same approach can, in principle, be used for other ball mills.
The Planetary ball mills are the most popular mills used in MM, MA, and MD scientific researches for synthesizing almost all of the materials presented in Figure 1.1. In this type of mill, the milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial (milling bowl or vial) and the effective centrifugal force reaches up to 20 times gravitational acceleration.
The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed, as schematically presented in Figure 2.17.
However, there are some companies in the world who manufacture and sell number of planetary-type ball mills; Fritsch GmbH (www.fritsch-milling.com) and Retsch (http://www.retsch.com) are considered to be the oldest and principal companies in this area.
Fritsch produces different types of planetary ball mills with different capacities and rotation speeds. Perhaps, Fritsch Pulverisette P5 (Figure 2.18(a)) and Fritsch Pulverisette P6 (Figure 2.18(b)) are the most popular models of Fritsch planetary ball mills. A variety of vials and balls made of different materials with different capacities, starting from 80ml up to 500ml, are available for the Fritsch Pulverisette planetary ball mills; these include tempered steel, stainless steel, tungsten carbide, agate, sintered corundum, silicon nitride, and zirconium oxide. Figure 2.19 presents 80ml-tempered steel vial (a) and 500ml-agate vials (b) together with their milling media that are made of the same materials.
Figure 2.18. Photographs of Fritsch planetary-type high-energy ball mill of (a) Pulverisette P5 and (b) Pulverisette P6. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).
Figure 2.19. Photographs of the vials used for Fritsch planetary ball mills with capacity of (a) 80ml and (b) 500ml. The vials and the balls shown in (a) and (b) are made of tempered steel agate materials, respectively (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).
More recently and in year 2011, Fritsch GmbH (http://www.fritsch-milling.com) introduced a new high-speed and versatile planetary ball mill called Planetary Micro Mill PULVERISETTE 7 (Figure 2.20). The company claims this new ball mill will be helpful to enable extreme high-energy ball milling at rotational speed reaching to 1,100rpm. This allows the new mill to achieve sensational centrifugal accelerations up to 95 times Earth gravity. They also mentioned that the energy application resulted from this new machine is about 150% greater than the classic planetary mills. Accordingly, it is expected that this new milling machine will enable the researchers to get their milled powders in short ball-milling time with fine powder particle sizes that can reach to be less than 1m in diameter. The vials available for this new type of mill have sizes of 20, 45, and 80ml. Both the vials and balls can be made of the same materials, which are used in the manufacture of large vials used for the classic Fritsch planetary ball mills, as shown in the previous text.
Retsch has also produced a number of capable high-energy planetary ball mills with different capacities (http://www.retsch.com/products/milling/planetary-ball-mills/); namely Planetary Ball Mill PM 100 (Figure 2.21(a)), Planetary Ball Mill PM 100 CM, Planetary Ball Mill PM 200, and Planetary Ball Mill PM 400 (Figure 2.21(b)). Like Fritsch, Retsch offers high-quality ball-milling vials with different capacities (12, 25, 50, 50, 125, 250, and 500ml) and balls of different diameters (540mm), as exemplified in Figure 2.22. These milling tools can be made of hardened steel as well as other different materials such as carbides, nitrides, and oxides.
Figure 2.21. Photographs of Retsch planetary-type high-energy ball mill of (a) PM 100 and (b) PM 400. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).
Figure 2.22. Photographs of the vials used for Retsch planetary ball mills with capacity of (a) 80ml, (b) 250ml, and (c) 500ml. The vials and the balls shown are made of tempered steel (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).
Both Fritsch and Retsch companies have offered special types of vials that allow monitoring and measure the gas pressure and temperature inside the vial during the high-energy planetary ball-milling process. Moreover, these vials allow milling the powders under inert (e.g., argon or helium) or reactive gas (e.g., hydrogen or nitrogen) with a maximum gas pressure of 500kPa (5bar). It is worth mentioning here that such a development made on the vials design allows the users and researchers to monitor the progress tackled during the MA and MD processes by following up the phase transformations and heat realizing upon RBM, where the interaction of the gas used with the freshly created surfaces of the powders during milling (adsorption, absorption, desorption, and decomposition) can be monitored. Furthermore, the data of the temperature and pressure driven upon using this system is very helpful when the ball mills are used for the formation of stable (e.g., intermetallic compounds) and metastable (e.g., amorphous and nanocrystalline materials) phases. In addition, measuring the vial temperature during blank (without samples) high-energy ball mill can be used as an indication to realize the effects of friction, impact, and conversion processes.
More recently, Evico-magnetics (www.evico-magnetics.de) has manufactured an extraordinary high-pressure milling vial with gas-temperature-monitoring (GTM) system. Likewise both system produced by Fritsch and Retsch, the developed system produced by Evico-magnetics, allowing RBM but at very high gas pressure that can reach to 15,000kPa (150bar). In addition, it allows in situ monitoring of temperature and of pressure by incorporating GTM. The vials, which can be used with any planetary mills, are made of hardened steel with capacity up to 220ml. The manufacturer offers also two-channel system for simultaneous use of two milling vials.
Using different ball mills as examples, it has been shown that, on the basis of the theory of glancing collision of rigid bodies, the theoretical calculation of tPT conditions and the kinetics of mechanochemical processes are possible for the reactors that are intended to perform different physicochemical processes during mechanical treatment of solids. According to the calculations, the physicochemical effect of mechanochemical reactors is due to short-time impulses of pressure (P = ~ 10101011 dyn cm2) with shift, and temperature T(x, t). The highest temperature impulse T ~ 103 K are caused by the dry friction phenomenon.
Typical spatial and time parameters of the impactfriction interaction of the particles with a size R ~ 104 cm are as follows: localization region, x ~ 106 cm; time, t ~ 108 s. On the basis of the obtained theoretical results, the effect of short-time contact fusion of particles treated in various comminuting devices can play a key role in the mechanism of activation and chemical reactions for wide range of mechanochemical processes. This role involves several aspects, that is, the very fact of contact fusion transforms the solid phase process onto another qualitative level, judging from the mass transfer coefficients. The spatial and time characteristics of the fused zone are such that quenching of non-equilibrium defects and intermediate products of chemical reactions occurs; solidification of the fused zone near the contact point results in the formation of a nanocrystal or nanoamor- phous state. The calculation models considered above and the kinetic equations obtained using them allow quantitative ab initio estimates of rate constants to be performed for any specific processes of mechanical activation and chemical transformation of the substances in ball mills.
There are two classes of ball mills: planetary and mixer (also called swing) mill. The terms high-speed vibration milling (HSVM), high-speed ball milling (HSBM), and planetary ball mill (PBM) are often used. The commercial apparatus are PBMs Fritsch P-5 and Fritsch Pulverisettes 6 and 7 classic line, the Retsch shaker (or mixer) mills ZM1, MM200, MM400, AS200, the Spex 8000, 6750 freezer/mill SPEX CertiPrep, and the SWH-0.4 vibrational ball mill. In some instances temperature controlled apparatus were used (58MI1); freezer/mills were used in some rare cases (13MOP1824).
The balls are made of stainless steel, agate (SiO2), zirconium oxide (ZrO2), or silicon nitride (Si3N). The use of stainless steel will contaminate the samples with steel particles and this is a problem both for solid-state NMR and for drug purity.
However, there are many types of ball mills (see Chapter 2 for more details), such as drum ball mills, jet ball mills, bead-mills, roller ball mills, vibration ball mills, and planetary ball mills, they can be grouped or classified into two types according to their rotation speed, as follows: (i) high-energy ball mills and (ii) low-energy ball mills. Table 3.1 presents characteristics and comparison between three types of ball mills (attritors, vibratory mills, planetary ball mills and roller mills) that are intensively used on MA, MD, and MM techniques.
In fact, choosing the right ball mill depends on the objectives of the process and the sort of materials (hard, brittle, ductile, etc.) that will be subjecting to the ball-milling process. For example, the characteristics and properties of those ball mills used for reduction in the particle size of the starting materials via top-down approach, or so-called mechanical milling (MM process), or for mechanically induced solid-state mixing for fabrications of composite and nanocomposite powders may differ widely from those mills used for achieving mechanically induced solid-state reaction (MISSR) between the starting reactant materials of elemental powders (MA process), or for tackling dramatic phase transformation changes on the structure of the starting materials (MD). Most of the ball mills in the market can be employed for different purposes and for preparing of wide range of new materials.
Martinez-Sanchez et al.  have pointed out that employing of high-energy ball mills not only contaminates the milled amorphous powders with significant volume fractions of impurities that come from milling media that move at high velocity, but it also affects the stability and crystallization properties of the formed amorphous phase. They have proved that the properties of the formed amorphous phase (Mo53Ni47) powder depends on the type of the ball-mill equipment (SPEX 8000D Mixer/Mill and Zoz Simoloter mill) used in their important investigations. This was indicated by the high contamination content of oxygen on the amorphous powders prepared by SPEX 8000D Mixer/Mill, when compared with the corresponding amorphous powders prepared by Zoz Simoloter mill. Accordingly, they have attributed the poor stabilities, indexed by the crystallization temperature of the amorphous phase formed by SPEX 8000D Mixer/Mill to the presence of foreign matter (impurities).
Fine grinding, to P80 sizes as low as 7m, is becoming increasingly important as mines treat ores with smaller liberation sizes. This grinding is typically done using stirred mills such as the Isamill or Stirred Media Detritor. While fine grinding consumes less energy than primary grinding, it can still account for a substantial part of a mills energy budget. Overall energy use and media use are strongly related to stress intensity, as well as to media size and quality. Optimization of grinding media size and quality, as well as of other operational factors, can reduce energy use by a factor of two or more. The stirred mills used to perform fine grinding have additional process benefits, such as polishing the mineral surface, which can enhance recovery.
Fine grinding is becoming an increasingly common unit operation in mineral processing. While fine grinding can liberate ores that would otherwise be considered untreatable, it can entail high costs in terms of energy consumption and media use. These costs can be minimized by performing adequate test work and selecting appropriate operating conditions. This paper reviews fine grinding technology, research, and plant experience and seeks to shed light on ways in which operators can reduce both operating costs and the environmental footprint of their fine grinding circuit.
This paper will begin by giving an overview of fine grinding and the equipment used. It will then discuss energyproduct size relationships and modeling efforts for stirred mills in particular. The paper will go on to cover typical test work requirements, the effect of media size, and the contained energy in media. In closing, specific case studies will be reviewed.
Grinding activities in general (including coarse, intermediate, and fine grinding) account for 0.5pct of U.S. primary energy use, 3.8pct of total U.S. electricity consumption, and 40pct of total U.S. mining industry energy use. Large energy saving opportunities have been identified in grinding in particular.
TableI shows a very large disparity between the theoretical minimum energy used in grinding and the actual energy used. More interestingly, a fairly large difference remains even between Best Practice grinding energy use and current energy use. This suggests that large savings in grinding energy (and associated savings in maintenance, consumables, and capital equipment needed) could be obtained by improving grinding operations.
As fine grinding is typically used on regrind applications, the feed tonnages to fine grinding circuits are small compared to head tonnages, typically 10 to 30tph. However, the specific energies are often much larger than those encountered in intermediate milling and can be as high as 60kWh/t. Total installed power in a fine grinding circuit can range from several hundred kW to several MW; for example, the largest installed Isamill has 3MW installed power. This quantity is small compared to the power used by a semi-autogenous mill and a ball mill in a primary grinding circuit; a ball mill can have an installed power of up to 15MW, while installed power for a SAG mill can go up to 25MW. However, the energy used for fine grinding is still significant. Moreover, as this paper seeks to demonstrate, large energy reduction opportunities are frequently found in fine grinding.
Grinding can be classified into coarse, intermediate, and fine grinding processes. These differ in the equipment used, the product sizes attained, and the comminution mechanisms used. The boundaries between these size classes must always be drawn somewhat arbitrarily; for this paper, the boundaries are as given in TableII. As shown in the table, coarse grinding typically corresponds to using an AG or SAG mill, intermediate grinding to a ball mill or tower mill, and fine grinding to a stirred mill such as an Isamill or Stirred Media Detritor (SMD). Of course, various exceptions to these typical values can be found.
In fine grinding, a material with an F80 of less than 100m is comminuted to a P80 of 7 to 30m. (P80s of 2m are at least claimed by equipment manufacturers.) The feed is typically a flotation concentrate, which is reground to liberate fine particles of the value mineral.
The three modes of particle breakage are impact; abrasion, in which two particles shear against each other; and attrition, in which a small particle is sheared between two larger particles or media moving at different velocities. In fine grinding, breakage is dominated by attrition alone. In stirred mills, this is accomplished by creating a gradient in the angular velocity of the grinding media along the mills radius.
Fine grinding is usually performed in high-intensity stirred mills; several manufacturers of these stirred mills exist. Two frequently used stirred mills include the Isamill, produced by Xstrata Technology, and the SMD, produced by Metso (Figure1). A third mill, the KnelsonDeswik mill (now the FLS stirred mill), is a relative newcomer to the stirred milling scene, having been developed through the 1990s and the early 2000s. In all these mills, a bed of ceramic or sand is stirred at high speed. Ceramic media sizes in use range from 1 to 6.5mm.
The Isamill and the SMD have very similar grinding performance. Grinding the same feed using the same media, Nesset et al. found that the Isamill and SMD had very similar specific energy use. Gao et al. observed that an Isamill and SMD, grinding the same feed with the same media, produced very similar product particle size distributions (PSDs). This similarity in performance has also been observed in other operations.
Nevertheless, there are important differences. In the Isamill, the shaft is horizontal and the media are stirred by disks, while in the SMD, the stirring is performed by pins mounted on a vertical shaft. In an SMD, the product is separated from the media by a screen; the Isamill uses an internal centrifugation system. This means that the screens in an SMD constitute a wear part that must be replaced, while for the Isamill, the seals between the shaft and body constitute important wear parts. Liner changes and other maintenance are claimed by Xstrata Technology to be much easier than in an SMD: While an SMDs liner is removed in eight parts, the Isamills liner can be removed in two pieces, with the shell sliding off easily. The KnelsonDeswik mill is top stirred and can therefore be considered to be similar to an SMD.
An important difference among the Isamill, the SMD, and the KnelsonDeswik mill is that of scale. The largest Isamill installed at time of writing had 3MW of installed power; an 8MW Isamill is available, but appears not to have yet been installed. The largest SMD available has 1.1MW of installed power; one 1.1-MW SMD has been installed. The next largest size SMD has 355kW of installed power. Thus, several SMDs are often installed for a fine grinding circuit, while the same duty would be performed by a single Isamill. SMDs are typically arranged in series, with the product of one becoming the feed for the other. This has the advantage that each SMD in the line can have its media and operating conditions optimized to the particle size of its particular feed. The largest installed power in a KnelsonDeswik mill is 699kW; this places it in an intermediate position between the 355-kW and 1.1-MW SMDs.
In 2012, FLSmidth reported that it had acquired the KnelsonDeswik mill; the mill is now known as the FLSmidth stirred mill. An FLSmidth stirred mill will be installed to perform a copper concentrate regrind in Mongolia. It is speculated that the mill will continue to be scaled up under its new owners to allow it to effectively compete against the SMD and Isamill.
Gravity-induced stirred (GIS) mills include the Tower mill, produced by Nippon Eirich, and the Vertimill, produced by Metso. Grinding to below 40m in GIS mills or ball mills is usually not recommended. In their product literature, Metso give 40m as the lower end of the optimal P80 range for Vertimills. At lower product sizes, both tower mills and ball mills will overgrind fines. At Mt. Isa Mines, a GIS mill fed with material of F80 approximately 50m lowered the P80 size by only 5 to 10m, at the same time producing a large amount of fines. Similarly, in ball mills, it is known that grinding finer than approximately 40m will result in overgrinding of fines as well as high media consumption. However, it must be noted that the product size to which a mill can efficiently grind depends on the feed material, the F80, and media type and size. A Vertimill has been used to grind to sizes below 10m.
The phenomenon of overgrinding is largely the result of using media that are too large for the product size generated. The smallest ball size typically charged into ball mills and tower mills is inch (12.5mm), although media diameters as small as 6mm have been used industrially in Vertimills.
In a laboratory study by Nesset et al., a GIS mill charged with 5-mm steel shot, and with other operating conditions similarly optimized, achieved high energy efficiencies when grinding to less than 20m. This appears to qualitatively confirm the notion that fine grinding requires smaller media sizes. In the case of the Nesset study, the power intensity applied to the laboratory tower mill was lowthat is, the shaft was rotated slowly in order to obtain this high efficiency, leading to low throughput. This suggests that charging GIS mills with small media may not be practicable in plant operation.
Millpebs have been used as grinding media to achieve fine grinding in ball mills. These are 5- to 12-mm spherical or oblong cast steel pellets, charged into ball mills as a replacement of, or in addition to, balls. While Millpebs can give significantly lower energy use when grinding to finer sizes, they also can lead to high fines production and high media use.
Millpebs were tested for fine grinding at the Brunswick concentrator. The regrind ball mills at the concentrator used 25-mm slugs to produce a P80 of 28m. In one of the regrind mills, the slugs were replaced by Millpebs; these were able to consistently maintain a P80 of 22m while decreasing the power draw by 20pct. However, media use increased by 50pct and the production of fines of less than 16m diameter increased by a factor of 5. The observed drop in specific energy may be due to the fact that Millpebs had smaller average diameters than the slugs and so were more efficient at grinding to the relatively small product sizes required. It is therefore unclear whether the performance of Millpebs would be better than that of conventional 12-mm steel balls. To the best of the authors knowledge, no performance comparison between Millpebs and similarly sized balls has been performed.
A host of other technologies exist to produce fine grinding, including jet mills, vibrating mills, roller mills, etc. However, none of these technologies has reached the same unit installed power as stirred mills. For example, one of the largest vibrating mills has an installed power of 160kW. Therefore, these mills are considered as filling niche roles and are not treated further in this review. A fuller discussion of other fine grinding technologies can be found in a review by Orumwense and Forssberg.
Neese et al. subjected 50- to 150-m sand contaminated with oil to cleaning in a stirred mill in the laboratory. The mill operated at low stress intensities: A low speed and small-size media (200- to 400-m quartz or steel beads) were used. These conditions allowed the particles to be attrited without being broken. As a result, a large part of the oil contaminants was moved to the 5-m portion of the product. This treatment may hold promise as an alternative means of processing bituminous sands, for example, in northern Alberta.
The Albion process uses ultrafine grinding to enhance the oxidation of sulfide concentrates in treating refractory gold ores. In the process, the flotation concentrate is ground to a P80 of 10 to 12m. The product slurry is reacted with oxygen in a leach tank at atmospheric pressure; limestone is added to maintain the pH at 5 to 5.5. The leach reaction is autothermal and is maintained near the slurry boiling point. Without the fine grinding step, an autoclave would be required for the oxygen leaching process. It is hypothesized that the fine grinding enhances leach kinetics by increasing the surface area of the particles, as well as by deforming the crystal lattices of the particles.
Numerous researchers, for example, Buys et al., report that stirred milling increases downstream flotation recoveries by cleaning the surface of the particles. The grinding media used in stirred mills are inert, and therefore corrosion reactions, which occur with steel media in ball mills, are not encountered. Corrosion reactions change the surface chemistry of particles, especially with sulfide feeds, and hamper downstream flotation.
Further increases in flotation recoveries are obtained by limiting the amount of ultrafine particles formed; stirred mills can selectively grind the larger particles in the feed with little increase in ultrafines production. Ultrafine particles are difficult to recover in flotation.
In intermediate grinding to approximately 75m, the Bond equation (Eq. ) is used to relate feed size, product size, and mechanical energy applied. Below 75m, correction factors can be applied to extend its range of validity.
No general work index formula governing energy use over a range of conditions, like the Bond equation for intermediate grinding, has yet been found for the fine grinding regime. Instead, the work-to-P80 curve is determined in the laboratory for each case. The energy use usually fits an equation of the form
Signature plot (specific energy vs P80 curve) for Brunswick concentrator Zn circuit ball mill cyclone underflow; F80=63m. The plots give results for grinding the same feed using different mills and media. After Nesset et al.
Values for the exponent k have been found in the range 0.7 to 3.5, meaning that the work to grind increases more rapidly as grind size decreases than in intermediate grinding. The specific energy vs product size curve has a much steeper slope in this region than in intermediate grinding.
The values of k and A are specific to the grinding conditions used in the laboratory tests. Changes in feed size, media size distribution, and in other properties such as media sphericity and hardness can change both k and A, often by very large amounts. Media size and F80 appear to be the most important determinants of the signature plot equation.
The connections (if any) between k and A and various operating conditions remain unknown. Because of the relatively recent advent of stirred milling in mineral processing, fine grinding has not been studied to the same extent as grinding in ball mills (which of course entail much larger capital and energy expenditures in any case). One of the research priorities in the field of stirred milling should be the investigation of the effects of F80 and media size on the position of the signature plots. If analogous formulas to the Bond ball mill work formula and the Bond top ball size formula can be found, the amount of test work required for stirred milling would be greatly reduced.
Larson et al. found that when specific energy is plotted against the square of the percent particles in the product passing a given size (a proxy for particle surface area), a straight line is obtained. This is demonstrated in Figure3.
In contrast to the conventional signature plot, this function gives zero energy at the mill feed. It is therefore hypothesized that if a squared function plot is obtained by test work for one feed particle size, the plot for another feed particle size can be obtained simply by changing the intercept of the line while keeping the slope the same. Therefore, the squared function plot allows the effect of changes in both F80 and P80 to be modeled.
While the Squared Function Plot is intriguing, experimental validation of its applicability has not yet been published. It nevertheless remains an interesting topic for further investigation and if validated may be used in the future as an alternative measure of specific energy.
A similar analysis has been performed by Musa and Morrison, who developed a model to determine the surface area within each size fraction of mill product. They defined a marker size below which 70 to 80pct of the product surface area was contained; the marker size thus served as a proxy for surface area production. Specific energy use was then defined as kWh of power per the tonne of new material generated below the marker size. Musa and Morrison found that by defining specific energy in this way, it was possible to accurately predict the performance of full-scale Vertimills and Isamills from laboratory tests.
Blecher and coworkers[22,23] found that stress intensity combines the most important variables determining milling performance. Stress intensity for a horizontal stirred mill, with media much harder than the mineral to be ground, is defined as in Eq. .
Note that the stress intensity is strongly sensitive to changes in media diameter (to the third power), is less sensitive to stirrer tip speed (to the second power), and is relatively insensitive to media and slurry density.
For vertical stirred mills such as the SMD and tower mill, both SIs and SIg are non-zero. For horizontal stirred mills such as the Isamill, net gravitational SI is zero due to symmetry along the horizontal axis. Therefore, for horizontal stirred mills, only SIs need be taken into consideration.
Kwade and coworkers noted that, at a given specific energy input, the product P80 obtainable varies with stress intensity and passes through a minimum. Product size at a given energy input can be viewed as a measure of milling efficiency; therefore, milling efficiency reaches a maximum at a single given stress intensity. This idea was experimentally validated by Jankovic and Valery (Figure 4).
The stress intensity is defined by parameters that are independent of mill size or type. According to Jankovic and Valery, once the optimum SI has been determined in one mill for a given feed, the same SI should also be the point of optimum efficiency in any other mill treating that feed. Therefore, the optimum SI need only be determined in one mill (e.g., a small test mill); the operating parameters of a full-scale mill need only be adjusted to produce the optimum SI.
Stress frequency multiplied by stress intensity is equal to mill power; therefore, stress intensity could in theory be used to predict mill specific energy. However, to the authors knowledge, a comprehensive model linking stress intensity, stress frequency, and specific energy has not yet been developed. Therefore, there is not yet any direct link between stress intensity and specific energy.
The definition of SIs as given in Eq.  is valid only for cases where the grinding media are much harder than that of the material ground (for example, the grinding of limestone with glass beads). Becker and Schwedes determined that, in a collision between media and a mineral particle, the fraction of energy transferred to the product is given by Eq. :
To maintain high efficiency in milling, the media must be chosen so as to be much harder (higher Youngs modulus) than the product material, keeping E p,rel close to unity. Where the Youngs modulus of the product is similar to that of the media, much of the applied energy goes into deformation of the media instead of that of the particle to be ground. The energy used to deform the media is lost, lowering the amount of energy transferred to the product. This fact explains why steel media, with a relatively low Youngs modulus, tend to perform poorly in stirred milling, even though the media are much more dense than silica or alumina media.
The previous sections indicated that stress intensity is independent from individual millsi.e., the optimal stress intensity when using Mill A will also be the optimal stress intensity when using Mill B. However, this does not seem to be the case when actually scaling up mills.
Four-liter Isamills are commonly used for grindability test work. It can be assumed that operating parameters of the test mill (including media type, media size, and slurry density) are adjusted so far as possible to give the optimum SI. These parameters are then used in the full-scale mill as well. However, the 4-L test mills have a tip speed of approximately 8m/s, while full-scale Isamills have tip speeds close to 20m/s. If the same media size, media density, and slurry density are used in the test mill as in the full-scale mill, the stress intensity of the full-scale mill will be approximately 6.25 times larger than that of the test mill. This implies that the full-scale mill is operating outside of the optimum SI and will be grinding less efficiently. That is to say that the operating point of the full-scale mill will be above the signature plot determined by test work.
In reality, however, the operating points of full-scale stirred mills are generally found to lie on the signature plots generated in test work. Therefore, the full-scale mills and test mills have the same milling efficiency, even though the full-scale mill operates at a different stress intensity than the test mill.
This question remains unresolved. One possible answer arises from the observation that two of the P80 vs SI curves in Figure4 appear to have broad troughs, covering almost an order of magnitude change in SI. In this case, even a sixfold increase in SI might not create a noticeable difference in performance, considering experimental and measurement error.
Product size vs stress intensity at three different specific energies for a zinc regrind. Note optimum stress intensity at which the lowest product size is reached. Figure used with permission from Jankovic and Valery
The SMD test unit appears from photographs to have a bed depth of around 30cm, while the full-scale SMD355 has a bed depth of approximately one meter. This represents a change in the gravitational stress intensity of almost two orders of magnitude. As has been previously noted, however, laboratory and full-scale SMDs scale-up with a scale-up factor of approximately unity, with no apparent change in the optimum stress intensity. This observation suggests that the gravitational stress intensity, SIg, is unimportant in SMDs compared to the stirring stress intensity, SIs. By contrast, in GIS mills, where full-size units have bed depths of ten meters or more, gravitational stress intensity can be expected to be much more important in full-size units than in test units, adding a complicating factor to GIS mill scale-up.
Factorial design experiments were performed by Gao et al. and Tuzun and Loveday to determine the effect of various operating parameters on the power use of laboratory mills. Power models were determined giving the impact of different parameters as power equations with linear and nonlinear terms. The derived models did not appear to be applicable to mills other than the particular laboratory units being studied.
In ball milling, the Bond ball mill work index can be used to determine specific energy at a range of feed and product sizes. The Bond top size ball formula can be used to estimate the media size required. No such standard formulas exist in fine grinding. Energy and media parameters must instead be determined in the laboratory for every new combination of operating conditions such as feed size, media size, and media type.
For the Isamill, test work is usually performed with a 4-L bench-scale Isamill. Approximately 15kg of the material to be ground is slurried to 20pct solid density by volume. The slurry is then fed through the mill and mill power is measured. The products PSD is measured, additional water is added if needed, and the material is sent through the mill again. This continues until the target P80 is reached; typically, there will be 5 to 10 passes through the mill. The test work will produce a signature plot and media consumption data as the deliverables.
In contrast to laboratory-scale testing for ball mills and AG/SAG mills, test work results for stirred mills can be used for sizing full-size equipment with a scale-up factor close to one. Larson et al.[19,20] found a scale-up factor for the Isamill of exactly 1, while Gao et al. imply that the scale-up factor for SMDs is 1.25.
A common error in test work is using monosize media (e.g., fresh 2-mm media loaded into in the mill) as opposed to aged media with a distribution of particle sizes. The aged media will grind the smaller feed particles more efficiently. Therefore, using fresh media will give a higher specific energy than in reality.
Another pitfall is coarse holdup in the mill. If the mill is not sufficiently flushed, coarse particles will be kept inside the mill. The mill product then appears finer than it in reality is. This leads to lower estimates of specific energy than reality.
In ball milling, the product particle size distribution (PSD) can usually be modeled as being parallel to the feed PSD on a log-linear plot. When grinding to finer sizes in ball mills, the parallel PSDs mean that large amounts of ultrafine particles are produced. This consumes a large amount of grinding energy while producing particles which are difficult to recover in subsequent processing steps such as flotation.
As shown in the figure, at the left end of the graph, the product PSD is very close to the feed PSD; at the right, the two PSDs are widely spaced. This indicates that the mill is efficiently using its energy to break the top size particles and is spending very little energy on further grinding of fine particles. Thus, the overall energy efficiency of the fine grinding can be expected to be good. As a bonus, the tighter PSD makes control of downstream processes such as flotation easier.
In an experimental study, Jankovic and Sinclair subjected calcite and silica to fine grinding in a laboratory pin stirred mill, a Sala agitated mill (SAM), and a pilot tower mill. The authors found that for each mill, the PSD of the product was narrower (steeper) than that of the feed. In addition, when grinding to P80s below approximately 20m in any of the three mills tested, the PSD became more narrow (as measured by P80/P20 ratio) as the P80 decreased. (When the width of the PSD was calculated using an alternative formula, the PSD was only observed to narrow with decreasing P80 when using the pin stirred mill.) The authors concluded that the width of the PSD was strongly affected by the material properties of the feed, while not being significantly affected by the media size used.
In stirred milling, the most commonly used media are ceramic balls of 1 to 5mm diameter. The ceramic is usually composed of alumina, an alumina/zirconia blend, or zirconium silicate. Ceramic media exist over a wide range of quality and cost, with the lower quality/cost ceramic having a higher wear rate than higher quality/cost ceramic. Other operations have used sand as media, but at the time of writing, only two operations continue to use sand.[8,27,33] Mt Isa Mines has used lead smelter slag as media; however, it is now using sand media.[10,27] Mt Isa is an exception in its use of slag, as a vast majority of operations do not have a smelter on-site to provide a limitless supply of free grinding media. However, in locations where slag is available, it should be considered as another source of media.
Media use in fine grinding is considered to be proportional to the mechanical energy applied. Typical wear rates and costs are given in TableIII and Figure6; these figures can of course vary significantly from operation to operation.
Contained energy refers to the energy required to produce and transport the media, and is distinct from the mechanical (electrical) energy used to drive the mill. Hammond and Jones estimated the contained energy in household ceramics (not taking account of transportation). Hammond and Jones estimates range from 2.5 to 29.1MJ/kg, with 10MJ/kg for general ceramics and 29MJ/kg for sanitary ceramics. Given that ceramic grinding media require very good hardness and strength, especially compared to household ceramics, it is appropriate to estimate its contained energy at the top end of Hammond and Jones range, at 29MJ/kg.
Using 29MJ/kg for the contained energy of ceramic media and a wear rate of 35g/kWh of mechanical energy gives a contained energy consumption of 0.28kWh contained per kWh of mechanical energy applied. A wear rate of 7g/kWh gives a contained energy consumption of 0.06kWh contained per kWh of mechanical energy applied. Therefore, 6 to 20pct of the energy use in fine grinding using ceramic media can be represented by contained energy in the grinding media itself.
Sand media have much lower contained energy than ceramic media as the media must simply be mined or quarried rather than manufactured. Hammond and Jones report a contained energy of 0.1MJ/kg. Blake et al. reported that switching a stirred mills media from sand to ceramic results in a mechanical energy savings of 20pct. Therefore, using sand rather than ceramic media would produce savings in contained energy, but would cost more in mechanical energy. Likewise, Davey suggests that poor-quality media will increase mechanical energy use in stirred milling. It is speculated that this is due to the lower sphericity of sand media. On the other hand, the work of Nesset et al. suggests that the energy use between ceramic and sand media of the same size is the same. Slag media, where a smelter is on-site, would probably have the lowest contained energy consumption of the different media types. There is very little transportation, and for accounting purposes, almost no energy has gone into creating the media as the granulated slag is a by-product of smelter operation.
Becker and Schwedes point out that with poor-quality media, a significant part of the product will consist of broken pieces of media, which will affect the measured product PSD. Clearly, more information on the relationships between contained energy in media and media wear rates is desirable.
Of the different operating parameters for stirred mills, media size probably has the biggest influence on overall energy consumption. The appropriate media size for a mill appears to be a function of the F80 and P80 required. The grinding media must be large enough to break up the largest particles fed to the mill and small enough to grind the material to the product fineness desired. As demonstrated by the experience of Century mine, an inappropriate media size choice can result in energy consumption double that of optimum operation.
In their laboratory study, Nesset et al. varied a number of operating parameters for stirred mills and identified media size as having the largest impact on energy use. It was also noted that the trials which produced the sharpest product PSD were also the ones which resulted in the lowest specific energy use.
Gao et al. report that at Century mine, the grinding media in SMDs performing regrind duty were changed from 1 to 3mm. This resulted in a drop in energy use of approximately 50pct; the signature plot shifted significantly downward (Figure7).
Figure8 shows the product PSD for laboratory SMD tests using 1- and 3-mm media. The PSD for the test using 1-mm media shows that the SMD produced a significant amount of fines (20pct below 4m). The mill also had difficulty breaking the top size particlesthe 100pct passing size appears to be almost the same for both the feed and the product. In contrast, the PSD using 3-mm media shows less fines production (20pct below 9m) and effective top size breakage, with all the particles above 90m broken. This is in line with the observation of Nesset et al. that low energy use is associated with tight product size distributions.
Gao et al. tested copper reverberatory furnace slag (CRFS, SG 3.8) and heavy media plant rejects (HMPR, SG 2.4) in a laboratory stirred mill at two sizes: 0.8/+0.3mm, and 1.7/+0.4mm. For both CRFS and HMPR, the smaller size media gave a lower specific energy than the larger size media. At the same size, both CRFS and HMPR had similar specific energy use. However, the CRFS ground the material much faster than HMPR. Possibly, this was due to its higher density.
Data on F80, P80, and media size were compiled from the literature in order to allow benchmarking against existing operations. The sources are listed in Table IV. F80 and P80 were plotted against media size; the results are given in Figure9.
F80 plotted against media size (blue diamonds); P80 plotted against media size (red crosses). Century UFG=Century ultrafine grind; Century Regr.=Century regrind. Data are taken from Case studies table (Color figure online)
It can be seen from the figure that as the P80 achieved decreases, the media size does as well, from 3mm to achieve 45m to 1mm to achieve under 10m. The F80 decreases with media size in a similar way, from 90m at 3mm to 45m at 1mm. Dotted lines have been added to Figure7 to define the region of operation of mills; these delimit a zone in which the stirred mill can be expected to operate efficiently.
In general, for a particular media size, limits on both F80 and P80 must be respected. For example, the figure suggests that a mill operating with an F80 of 100m should use 3-mm media, while a mill grinding to below 10m would need to use 1-mm media. To reduce a feed of 90m F80 to 10m P80, Figure9 suggests that comminution be done in two stages (two Isamills or SMDs in series) for optimal efficiency. The first stage would grind the feed from 90m to perhaps 45m using 3-mm media, while the second would grind from 45 to 10m using 1- or 2-mm media.
A number of opportunities exist to reduce the energy footprint of fine grinding mills. There are no general formulas, such as the Bond work formula and Bond top size ball formula in ball milling, to describe the performance of stirred mills. Therefore, improvement opportunities must be quantified by performing appropriate test work.
In addition to obtaining the signature plot, the specific energy as a function of new surface area should be determined during test work. This could be done either by the method of Larsen or by that of Musa and Morrison. Defining specific energy as a function of new surface area may constitute a superior means of predicting the performance of full-scale mills, as opposed to defining specific energy as a function of feed tonnage.
Media size should be chosen with care. It is recommended that test work be done with several media sizes in order to locate the stress intensity optimum. Media size can be benchmarked against other operations using Figure9.
There are indications that lower-quality media, apart from degrading faster, require more mechanical energy for grinding due to factors such as lower sphericity. It is recommended to perform test work using media of different quality to determine the effect of media quality on energy use. Slag and sand media may also be considered. Subsequently, a trade-off study involving media cost, electricity cost, improvement in energy efficiency, and contained energy in media should be performed to identify the best media from an economic and energy footprint standpoint.
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S. Buys, C. Rule, and D. Curry: The Application of Large Scale Stirred Milling to the Retreatment of Merensky Platinum Tailings, Paper presented at the 37th Canadian Mineral Processors Meeting, Ottawa, 2005.
D. Curry, M. Cooper, J. Rubenstein, T. Shouldice, and M. Young: The Right Tools in the Right Place: How Xstrata Nickel Australasia Increased Ni Throughput at Its Cosmos Plant, Paper presented at the 42nd Canadian Mineral Processors conference, Ottawa, 2010.
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