In Grinding, selecting (calculate)the correct or optimum ball sizethat allows for the best and optimum/ideal or target grind size to be achieved by your ball mill is an important thing for a Mineral Processing Engineer AKA Metallurgist to do. Often, the ball used in ball mills is oversize just in case. Well, this safety factor can cost you much in recovery and/or mill liner wear and tear.
A) Total Apparent Volumetric Charge Filling including balls and excess slurry on top of the ball charge, plus the interstitial voids in between the balls expressed as a percentage of the net internal mill volume (inside liners).
B) Overflow Discharge Mills operating at low ball fillings slurry may accumulate on top of the ball charge; causing, the Total Charge Filling Level to be higher than the Ball Filling Level. Grate Discharge mills will not face this issue.
C) This value represents the Volumetric Fractional Filling of the Voids in between the balls by the retained slurry in the mill charge. As defined, this value should never exceed 100%, but in some cases particularly in Grate Discharge Mills it could be lower than 100%. Note that this interstitial slurry does not include the overfilling slurry derived from the difference between the Charge and Ball Filling.
D) Represents the so-called Dynamic Angle of Repose (or Lift Angle) adopted during steady operation by the top surface of the mill charge (the kidney) with respect to the horizontal. A reasonable default value for this angle is 32, but may be easily tuned to specific applications against any available actual power data.
The first step in mill design is to determine the power needed to produce the desired grind in the chosen ore. The most used equation, for this purpose, is the empirical Bond equation (Bond, 1960, 1961; Rowland and Kjos, 1978).
In this equation, E is the specific energy required for the grind, and F80 and P80 are the sizes in micrometers that 80% of the weight passes of the mill feed and product respectively. The parameter Wi, known as the work index of the ore, is obtained from batch bench tests first devised by Bond (1961). The power calculated on using equation 1, (Bond, 1961; Rowland and Kjos, 1978), relates to:
1) Rod milling a rod mill with a diameter of 2.44 meters, inside new liners, grinding wet in open circuit. 2) Ball milling a ball mill with a diameter of 2.44 meters, inside new liners, grinding wet in open circuit.
When the grinding conditions differ from these specified conditions, efficiency factors (Rowland and Kjos, 1978) have to be used in conjunction with equation 1. In general, therefore, the required mill power is calculated using the following equation
where n is the number of efficiency factors, EFi, used and fo is the feed rate of new ore to the mill. The power calculated from equation 2 can be looked up in published tables (Rowland and Kjos, 1978) and the correct mill size and type can be selected.
The philosophy in the development of the MRRC grinding simulation package was to build interactive software that could be used as an inexpensive means of providing a semi-quantitative check on a grinding mill design. In addition the software is designed to slot in to a general mineral processing package now undergoing development at the MRRC.
A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.
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.
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 the movement of particles within the mill and contact zones of colliding balls.
By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).
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 remain attached to the wall with the aid of centrifugal force is:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).
The degree of filling the mill with balls also influences the 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 30%35% of its volume.
The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.
where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 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, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation .
Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.
Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.
Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.
For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.
With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.
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).
Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15
Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.
Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.
For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.
In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.
Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.
Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.
For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.
The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.
Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.
Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.
Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.
As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.
Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.
The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.
The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.
The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.
Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.
Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.
Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.
More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling . However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.
In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.
A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.
It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C .
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 . Fig. 12 shows the SEM image of the iron nanoparticles.
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.
In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal. and El-Eskandarany etal. proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.
In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.
Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation, using for example cold-rolling approach, as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.
Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the 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.
In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).
Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.
Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).
The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.
Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).
We study the effect of stress modes from three mills on structure of gamma-alumina.Extent of size reduction and mechanochemical effects are analysed.Jet milling is effective in size reduction and does not initiate mechanochemistry.Shear-induced phase transformation is observed in planetary ball mill.Transformation is by slip on alternate close packed oxygen layers from ccp to hcp.
The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; -Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d90 of 37m to 2.9m compared to planetary ball milling (30.2m) and single ball milling (10.5m). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired -Alumina resulting in a notable decrease in surface area from 136.6m2/g to 82.5m2/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure.
MACSALAB Drive Rolls for Rod / Ball Mills are Rubber coated and manufactured in Double and Triple Roll models. The Rolls are 120 mm diameter x 1200 mm long and powered by a 0.37 KW 220 Volt Motor with a variable speed controller.
The Ball/Rod mills are meant for producing fine particle size reduction through attrition and compressive forces at the grain size level. They are the most effective laboratory mills for batch-wise, rapid grinding of medium-hard to very hard samples down to finest particle sizes.
A horizontal rotating cylinder (vessel) is partially filled with balls/rods (grinding media), usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls/rods. A rotating drum throws material and balls/rods in a counteracting motion which causes impact breakage of larger particles and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles.
Try to limit the size of the batch to 25% of the total vessel volume which is sufficient to fill the voids and slightly cover the grinding Media. Any larger batches cause the balls to spread out throughout the mass of solids so they cannot make effective contact with each other, because of the layers of material between them. This greatly reduces the grinding efficiency of the mill and in some cases makes it impossible to attain the desired results.
The Feed size should preferable be 8 mesh or smaller, although many operations start with much larger pieces. Having the feed material as fine as possible enables the use of smaller sizes of grinding media, which are always best for fine uniform grinding and dispersions. For hard material it is especially advantageous to start with a fairly fine product.
In comminution we have a number of types of machines, to fit nearly all needs, including size of the original particles, size of the desired final product powder, desired throughput, etc. Although there are machines that nearly mimic what comes to mind when breaking something, that is, hitting it with a big enough hammer, if we consider which machines make up for the greatest part of the effort in comminution, tumbling mills come to mind.
These mills are characterized by using loose pieces that act as middle men from the machine motion to stressing single particles. It is a simple, yet clever solution to the problem one has when manually breaking fine particles: it becomes awfully tedious to hit particles one by one with a hammer! The grinding media do a great job of spreading the mechanical energy into collisions which, if all goes well, will result in particles being broken.
Anyone who has been to a mining or chemical engineering conference in the last 20 years or so probably has a good chance to have already seen an animation of a tumbling mill simulated using DEM. In the early 1990s, when the first steps towards the application of DEM in the minerals industry were taken, this was one of the first applications that demonstrated direct value of the technique in the field, crossing the border from being just cool animations to being of real use to make engineering decisions. By simulating industrial tumbling mills, it became possible to find out if a proposed configuration of the mill internals (liners) would result in excessive projection of the grinding media or not.
Why would this be relevant?Imagine if a mill with a given liner started to throw 100 mm (4 kg) steel balls directly against the liner! A few of these collisions (which can be as powerful as 500 J in kinetic energy) could break liners that would, otherwise, last at least 6 to 9 months, costing as much as a million dollars!It is no wonder why in the late 1990s a number of DEM packages were already being applied in this case.
Another reason for the early success of DEM in tumbling mill simulation lies in the fact that the most common grinding media are balls, typically made of steel and other ferrous alloys. As such, the common simplifying approach of using spheres to describe particles that makes life simpler in DEM is not a problem, since this is basically their shape, thus contributing to the success of the technique in this application. To this date, a number of mining companies and liner manufacturers rely on DEM to assist in liner selection for each application. It is certainly a valid and worthwhile application of the technique, although some would argue that almost as much about the influence of liner configuration on projection of media could be learned by using much simpler and faster one-particle codes, but this is another topic of discussion
As exciting as it is to be able to analyze whether or not a given set of grinding conditions would lead to ultraprojection of media, any engineer who is responsible for the operation of a mill would say that they are more interested in figuring out how they can improve the performance of their machine, be it increase of throughput or fineness of the product!
In the early 1990s people were already aware of that potential application of DEM and were hoping the technique would also allow leapfrogging over the empirical and phenomenological black box models that have been used to answer this question for the last 70 years or so. Unfortunately, this proved to be one of those questions that are easy to pose, but hard to answer properly.
The key problem here is that we can very quickly set up a simulation of a tumbling mill: the geometry and the motion are simple and the media are spheres! As soon as simulations start running, they look very lifelike. The problem arises when we attempt to predict the productivity or throughput of a mill. In order to do that, we have to deal with a part that is not generally simulated explicitly in tumbling mills: the ore or powder charge! This is the material that has to be broken.
One nave approach would be to include particles and proper routines that mimic breakage inside DEM environment, replacing particles with their progeny every time they break. This is indeed the required solution for a number of comminution machines, mainly crushers and some types of mills. That is, unless we can describe explicitly particles breaking simultaneously as the DEM simulation evolves, we stand little chance to move towards predicting performance of these machines. In the case of ball mills, this is certainly not straightforward.
Lets consider a simple example: the most common tumbling mill test used in the lab is the Bond ball mill grindability test, used for nearly 70 years to determine the Bond work index, which is a number used to estimate how hard it will be to grind the material of interest. The mill (with 30 cm in diameter and 30 cm in length) and the test conditions are standardized, with a maximum ball size of 40 mm and an ore top size of 3.35 mm. With less than 300 balls, it poses no challenge in running the simulations, if only grinding media are included. For such a small mill, one could also include the ore or powder charge. For a typical powder size distribution, we would be able to include particles as fine as about 0.25 mm before running out of memory, since it would take as much as 10 GB of computer memory to run them (please see Wei Dengs blog posting on the subject of memory and number of particles in DEM simulations). This certainly does not look too bad!
However, if we now look at industrial mills, which could have as much as 8 m in diameter, for instance, we would run out of memory (>10 GB) if we include particles with a minimum size of 2.8 mm, which are just about the coarsest size present in the mill, if we use the same ball and particle size distributions! Moreover, this is only considering a 30 cm slice of the mill, which could have as much as 3 billion particles of sizes above 0.25 mm, and a trillion particles if we go to particles as fine as 40 microns!
What alternative do we have in this case? Should we wait until computers become powerful enough so that we can put the full size distribution of the powder in the DEM simulations? Well, a number of researchers do not think so.
We believe that a clever alternative to this is to use DEM the way it can do its best job: describe the collisions between grinding media, tracking down their frequency and magnitude. This information, loosely called collision energy spectrum, can then be used, along with some description of the resulting breakage that would result from each of these events, to predict the size distribution in the mill. In order to do that, authors have used some special formulations of the population balance model, a tool that has been used in chemical engineering for over 50 years to design crystallizers and dissolvers.
At UFRJ we are particularly fond of our solution to the problem, which we call UFRJ mechanistic ball mill model. What is particular about it is that we have a suite of sub-models that describe breakage according to the different mechanisms (volume and surface breakage) (see figure), besides accounting for the fact that particles in a mill, if they manage to survive the collisions, will inevitably become weaker because of them. This made the model much more complex, but quite capable of predicting the performance of mills on the basis of information from breakage of single particles and DEM simulations. It is essentially a post-processing tool, but that can be as compute intensive as the DEM simulations that are required to run it!
Does it mean we nailed the problem to such an extent that we should put a rock on top of the subject and move on? I do not think so! There is still much to do until these advanced techniques become sufficiently mature. The description of the fluid flow (air or water) through the mill, and its interaction with the discharge mechanisms is still in its infancy, and should evolve to the level that it can couple properly with DEM and the microscale population balance model descriptions seamlessly. The same can be said about the simulation of the wear of liners, another relevant area that is still under early stages of development.
We must acknowledge that the industry is very adverse to risk, and that any error in sizing a mill at the design stage could lead to losses that can reach a fraction of a billion dollars over time. It is no wonder why it will take time and effort until these become accepted engineering tools. I believe we are a step behind other areas such as chute design, where DEM is already recognized as a mature engineering technology. I am confident we are following now quickly on its footsteps and should reach the same status in less than a decade.
Of course, much remains to be done in comminution modeling. Other machines that have existed for even longer than ball mills, and others that have been patented a decade ago offer great opportunities for DEM to unravel their mysteries. We certainly have no shortage of good opportunities for research in applying DEM to comminution in the next few decades! I am sure all researchers in the field are more than eager to take advantage of improved tools that EDEM and other state-of-the-art DEM codes can offer! We then hope to demonstrate that our tools saved the industry millions of dollars, making them more competitive when facing future challenges.
Marcelo Tavares, Ph.D., is Professor of Mineral Processing at the Universidade Federal do Rio de Janeiro since 1998. He works mainly in advanced modeling of comminution and particle breakage fundamentals, besides other topics in mineral processing. He is the head of the Laboratrio de Tecnologia Mineral, which has been a service partner of DEM Solutions since 2010 in the fields of DEM simulation and lab/pilot-scale testing for mineral processing. He has supervised or co-supervised over 30 masters dissertations and PhD thesis and published over 60 papers in peer-reviewed journals.
1. Which of the following is an advantage of size reduction?a) Enhanced heat/mass transferb) Intimate contact with certain food itemsc) Enhanced heat/mass transfer & intimate contact with certain food itemsd) None of the mentionedAnswer: cClarification: Increased surface area due to size reduction enhances heat/mass transfer. Small sized particles have more intimate contact with certain food items.
2. Statement 1: Dispersion of one immiscible liquid into another also involves size reduction.Statement 2: Leaching requires size reduction.a) True, Falseb) True, Truec) False, Falsed) False, TrueAnswer: bClarification: Dispersion of one immiscible liquid into another also involves size reduction so that they mix well. Leaching requires size reduction as it is desirable to access the interior parts of the particles which are enhanced by reducing the size of the particles.
3. Which of the following is NOT true with respect to size reduction?a) Size reduction is an energy inefficient process as the energy required for grinding is very highb) Some of the energy liberated in the formation of new small surfaces is the grinding energy required by food material per unit surface area to form new surface areas and the rest is generally just heatc) The crushing efficiency is inversely proportional to the surface createdd) None of the mentionedAnswer: cClarification: The crushing efficiency is directly proportional to the surface created
5. Statement 1: Grinding laws are based on the energy required for the creation of new surface area.Statement 2: In general, all the laws are based on the principle that the energy required per unit area of the surface is directly proportional to the size of the particle.a) True, Falseb) True, Truec) False, Falsed) False, TrueAnswer: bClarification: Grinding laws are based on the energy required for the creation of new surface area. In general, all the laws are based on the principle that the energy required per unit area of the surface is directly proportional to the size of the particle.
6. Statement 1: A hammer mill uses an impact to break particles.Statement 2: A chakki uses compression to break particles.a) True, Falseb) True, Truec) False, Falsed) False, TrueAnswer: aClarification: A chakki uses attrition to break particles.
9. A ball mill uses _____a) Impactb) Attritionc) Impact & Attritiond) None of the mentionedAnswer: cClarification: A ball mill uses both impacts when the solids fall from the top to the bottom and attrition when the particles rub against each other and reduce in size.
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Size reduction is a process of reducing large unit masses into small unit masses like the coarse or fine particles. Size reduction is also known as comminution or diminution or pulverization. Generally this process is done by two methods:
3. What happens in attrition mode? A. Blades are used for size reduction B. Forceful object strikes the stationary object C. Force is applied by the means of hammer D. Rubbing the material between two surfaces
5. Match the following mill which is not used for following material- A. Cutter mill 1. Soft material B. Hammer mill 2. Sticky material C. Ball mill 3. Friable material D. End runner mill 4. Abrasive material
9. Which of the following statement is NOT true? A. Penetration becomes slow when particles are large B. Wet grinding is used for production of tablets C. Colloid mill is not used for dry milling D. Impact is done in 2 ways
ANSWERS:- 1. A 2. Mercuric oxide 3. Rubbing the material between two surfaces 4. None of the above 5. A 3 B 4 C 1 D 2 6. All of the above 7. Ball mill 8. Ball mill 9. Wet grinding is used for production of tablets 10. Very fine particles