A ball mill is operated in closed circuit with sieve bend under steady state conditions as shown in the attached diagram.The % solids in each stream are indicated.The water addition to the sump is 100cubic metres per hr and to the mill feed is 67cubic metres per hr.
The sump has a feed of 60.1% solids,water addition to sump is 100 cubic meters per hr and outflow from sump constitutes 49.1 % solids.This feeds onto a sieve band which has an under-screen flow of 42.9% solids. The overflow from the sieve bend(56.1% ) feeds into the ball mill which has a water addition of 67 cubic meters per hr.The ball mill outflow consists of 49.6% solids and this goes into the sump to complete the closed circuit.
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In the previous article we considered the method for determining the bulk weigh of new grinding media. Determination the grinding balls bulk weigh directly operating in a ball mill becomes necessary on practice. It is done in order to accurately definition the grinding ball mass during measuring in a ball mill and exclude the mill overloading with grinding balls possibility
In this article, we will consider the technique for determining the grinding balls bulk weight in fully unloaded mill. This method used in the mills repair (armor plates replacement). The grinding balls unload from the mill into a special pit (needs to open hatches and pour the grinding balls from the drum during mill scroll). Then, need to definition maximum and minimum grinding balls diameter located in the mill. Unloaded grinding balls sorted by classes gradation by diameters. The gradation scale selected in steps of 10 mm. Sorting can be done manually (samples measured by a caliper in diameter and visually sorted by classes comparing with other balls size) or by the screen use.
The grinding balls bulk weight determined by using tabular data. The grinding balls bulk weight corresponds to the calculated average grinding balls diameter in the mill. Calculated (tabular) data of the steel grinding balls bulk weight present in the tables of the following specialized printed publications:
The calculated steel grinding balls bulk weight shown below in the table. Please note the calculated bulk weight may differ from the actual weight. This depends on several factors: the material of the grinding media, the range by geometric dimensions.
Thus, during calculating the grinding balls mass in ball mill (after measuring the mill filling degree with grinding media) needs to use the grinding balls bulk weight whose diameter was determined earlier. In this case, it may be differ from the grinding balls bulk weight loaded to the mill. The formula for calculating the grinding balls mass in ball mill is given below (we will consider the measurements process and calculations in more detail in our next articles).
The correct determination of the grinding balls bulk weight in mill allows accurately determination the mill balls feed weight. The mill balls feed weight is necessary for calculating the grinding media specific consumption and avoid mill overloading, thereby eliminating the motor load increasing possibility.
CompactMix offers you fast, efficient mixing and consistent homogenizing for the production of your high quality chocolate and compound masses. It includes the mixing tank, holding tank and piping in one easy-to-install system.
Mixers, piping and pumps are all made from stainless steel, making it easy to clean between batches. The piping is designed to prevent sugar and solid sedimentation to deliver a stable process and ensure continuous production.
Mixers, piping and pumps are all made from stainless steel, making it easy to clean between batches. The piping is designed to prevent sugar and solid sedimentation to deliver a stable process and ensure continuous production.
The CompactMix enables you to produce a wide range of chocolate and compound masses from white to dark. You can save up to 100 recipes in the control system for fast and repeatable production. Using the Nova S ball mill downstream, you can further grind your mixed masses to your desired fineness. CompactMix and Nova S together are capable of producing 400 to 3400 kg of chocolate mass per hour.
The hygienic design ensures that only minimal product residues are left over in the system, making recipe changes fast and simple. Efficient tank discharge, a short piping length and the small grinding chamber volume of the Nova S make it easy to clean the system with minimal product waste.
Using our global experience across chocolate manufacturing, we can customize your solution to meet your precise requirements. Our experienced engineers can help you plan, implement, and support new technology to improve the quality, efficiency, and safety for your plant.
Billions of people come into contact with Bhler technologies to cover their basic needs for food and mobility every day. Our motto is creating innovations for a better world. Find out more about our key topics.
A lot of time has passed since the first refiner conches were built to make chocolate. At that stage all necessary processing steps were done in the same machine, which sometimes took a week to get the final product. This paper is not intended to summarise all the technical developments since then as such information is available in textbooks1. Instead it aims to briefly introduce the different systems for chocolate mass production offered by various companies in order to give readers an overview on what is currently available on the market.
Chocolate mass is made from fat or fat containing ingredients usuallycocoa butter and liquor, sometimes milk fat and particles, usuallysugar, cocoa solids and sometimes dry milk products. Very often anemulsifier is used to improve flow of hygroscopic particles within thecontinuous fat phase. During production several incidents occur:
Coming from the old refiner conches,where all this happened simultaneously andwas hard to control, the majority of latertechnologies perform the grinding stepseparately. Only few mill types are able tohandle chocolate preparations, as it is initiallya very sticky mass, which can transform to asticky powder during milling, when specificsurface of particles increases. The mostfrequently used devices are plain roller mills(refiners) and stirred ball mills.
Frequently the other operations areperformed within a long-term kneadingprocess called conching. Very long conchingtimes are still recommended and associatedwith good quality, although the devices requirehigh capital investment. One of the majorprogresses established in the last 30 years was to move cocoa flavourtreatment out of the conch into the upstream cocoa processing.Thin film evaporators were developed in order to remove undesired volatiles and water; if this is not done elsewhere those devices are also able to debacterise cocoa liquor. Unfortunately the very popular Petzomat is not built any more, but alternatives from other companies are available. Nowadays chocolate producers can strongly reduce conching times if they insist on using pre-treated cocoa liquor of high flavour quality. Untreated cocoa is also still used, which then requires extra conching, like in former times.
Similar principles are followed for milk chocolates by developing milk powder pre-treatment procedures. For example it was proposed to dry skimmed milk powder to below one per cent water and to coat it with fat, which allows us to perform a very short liquefaction process instead of classical conching2.
Crumb is an ingredient made by drying milk together with sugar and cocoa liquor. Originally this was done for preservation of the milk, but nowadays it is performed in order to create the strong caramel flavour preferred in some countries. For downstream mass production the same technologies can be used, as with other chocolate types.
If cocoa butter is replaced by another fat, the product is usually called compound and not chocolate. Technologically most compounds are close to chocolate mass and similar equipment can be used to make it. The largest difference is rather an economical one, as very expensive cocoa butter is replaced by relatively inexpensive alternative fats.
After some initial information on chocolate mass properties the systems available on the market will be introduced. For that purpose information was obtained from various manufacturers, followed by questions and discussions on aspects such as:
Of course not all questions could be answered. In particular the last point, as process equipment is usually designed individually by machine manufacturers for their clients. So in practice, chocolate makers will always have to negotiate individually with suppliers. This paper will provide an introduction to the possibilities on the market.
Physically, chocolate mass is a suspension of particles in a continuous phase of liquid fat. Downstream when producing final products for the consumer, fat crystallisation is initiated and the mass is forced into the desired shape and solidifies. These steps are not considered here, although many properties of the final product can be predicted by measurable properties of the still liquid chocolate mass. Therefore flow properties are usually measured at a temperature of 40C, which is close to the temperature that chocolate melts in our mouths. So texture sensations like a smooth melt or a sticky behaviour are usually correlated to flow properties.
As chocolate mass is a non-Newtonian fluid we have to measure its shear stress at different shear rates, which results in a flow curve. Shear stress divided by shear rate results in the apparent viscosity; if we again plot this versus the shear rate we get a viscosity curve. Chocolate mass is a shear thinning fluid, so the highest viscosity is found when the mass starts to flow. Interaction between particles is considered to be responsible for this behaviour3, which is very different to Newtonian fluids such as water. So one important part of the flow curve is at very low shear. The yield value defines the shear stress, when the mass starts to move. As a minimum shear rate is necessary for the measurement, usually the yield value has to be extrapolated from the flow curve according to model equations, like the ones developed by Casson and Windhab1. Yield values or measurements at low shear stress also have a great practical importance, as many industrial operations are carried out with masses flowing slowly, for example the equal distribution of still liquid mass in a mould.
On the other hand side some processing is done under high shear, e.g. when pumping or spraying masses. This is best described by the other end of the flow curve. So usually it is extrapolated to infinite shear, the result is then called Casson or Windhab infinite viscosity. Naturally, fat content, emulsifiers and ingredient properties have the largest influence on viscosity. After those, particle size distribution and particle package density are also important. Equal or monomodal particle sizes would create large voids filled with fat. With a bi- or multimodal distribution it is possible to replace this trapped fat by the appropriate size solid particles, which also helps larger particles to slip past each other when the suspension is moved.
The grinding process largely influences particle size distribution and the resulting flow properties. Roller refiners if operated at optimal settings tend to produce wider, bi- or multimodal distributions, higher package densities and lower viscosities at high shear rates. In contrast, ball mills result in narrower distributions, less specific surface and lower yield values4. An example is shown in Figure 1.
Physically measurable properties of chocolate masses, like flow attributes or hardness, are correlated to sensory perceptions such as snap, hardness, melting and the like. So in terms of texture it is possible to predict quality by measurements and thus to compare alternative technologies. This is much more difficult in terms of flavour. Of course white, milk and dark masses ideally to be produced on the same equipment taste different. This means there are a lot more varieties in each category up to the specific house tastes that are aimed at by individual chocolate manufacturers. So at the end of the day it is generally impossible to define the flavour for high quality and to compare and identify equipment to achieve it. If considering processing alternatives it will always be necessary to adapt recipes and technology to each other in order to get the desired result.
This technology is used by the majority of chocolate producers in Europe. A typical line consists of mixer, 2-roll-refiner, 5-roll-refiner and conch. In the mixer the largest part of the recipe is blended, although some fat is left out, as otherwise the mix would be too fluid for the refiners. The 2-roll-refiner crushes sugar crystals to sizes below 100m. Alternatively, sugar can be ground separately by a sugar mill, which was common practice some decades ago. Although sometimes this set-up can still be found, most companies nowadays prefer the 2-roll-refiner due to the danger of dust explosions in sugar mills. The following 5-roll-refiner is a sophisticated machine, not very easy to operate, but essential for final product quality. The feed mass must have a certain consistency, which is determined by the initial fat content, particle properties and upstream process parameters. Here the particles are ground to their final size, usually below 30m in order to avoid a sandy texture in the mouth in the final product. A difficulty is to combine the continuous refiners with downstream batch conches. Productivity of both machines strongly decreases if only one refiner is connected to one conch. Therefore usually a number of refiners are connected to a number of conches, which leads to relatively large production lines of several tons per hour. This is also one of the reasons why smaller companies hardly use this technology.
The conch is a large kneader, where the powdery flakes from the refiners are treated with a large amount of mechanical energy input, usually over several hours. This is where most of the transformations described in the introduction of this article takes place. During the process the remaining fat and emulsifier are added. Conches are built in various forms and can be equipped with one, two or three mixing shafts. More detailed descriptions of the process can be found in1.
The Swiss company Bhler is market leader in this technology and looks back to a long experience in building and installing complete production lines8. In order to also meet the needs of smaller producers, recently the MicroFactory line was launched with a capacity of 300-600 kg/h, where the 2+5-roll-refiners are replaced by two three-rollers, see Figure 2.
Since the Dutch company DuyvisWiener joined with F.B.Lehmann and Thouet, they are also in a position to supply complete lines consisting of refiners and Thouet-conches. Interesting for smaller companies is the F.B.Lehmann 5-roll-refiner with integrated micro-2-roller9. Nevertheless also here one refiner would need several hours to fill a large 6-t-conch, which can only be solved by always having one machine idle or by using at least two smaller conches. For very small scale or test production the company also builds a pilot scale 5RR with 50cm rolls and 3-rollers.
Another solution for smaller companies or for niche products is offered by BSA-Schneider, an established conch builder, who since recently also builds refiners. Their CHOCompact system combines a small 5-roll-refiner with a conch10 (see Figure 3). Only one machine is operating at the time, so the conch has to wait for the refiner and vice versa. There are several other companies building refiners, e.g. Carle&Montanari-OPM11, HDM-Petzholdt-Heidenauer12 and conches such as Thouet13 and Lipp Mischtechnik14.
Petzholdt-Heidenauer, now part of the Probat group, carries forward the long experience on continuous conching dating back to the 1970s. The solution currently offered is based on using conventional 5-roll-refiners. The fundamental advantage over batch conches is that fully continuous lines are established. On the other hand side a minimum throughput of 1,250kg/h is required over a longer time, so the process is not suitable for frequent recipe change or smaller companies.
The process is shown in Figure 4. Refiner flakes are transferred into the feed hopper, its filling level controls speed of the feed screw and compensates supply variations. While some cocoa butter is added, the screw feeds the pasting columns. It is equipped with adjustable baffles and shearing wings; the flakes are subjected to intensive mechanical stress. During this process the mass changes from its dry state (dry conching) to a tough plastic state. Cleaned conditioned air is supplied by fan. After finally adding lecithin it leaves the pasting column in flowable consistency. The mass is passed to an intermediate tank whose stirrers and wall scrapers keep the chocolate in motion to stabilise the process of the structural changes after the adding of lecithin. Process air, loaded with volatile and undesired flavour is separated. In the weighing station the recipe is completed by liquid components. The wall scraper of the vessel prepares already a pre-mixture. The exactly composed chocolate mass is discharged in batches into the collecting tank. There it is further mixed and cooled. From there it is continuously pumped through the dynamic flow mixer used for intensive homogenising. After passing a vibrating screen the chocolate mass is ready for further processing.
The device holds 450 to 500kg, which results in residence times of 15-20minutes in the conch and 4-5minutes in the column. Energy density is up to 1200 kW/t and energy input 70 to 90 kWh/t. The modular structure allows us to extend the plant step by step.
A similar principle of fully continuous operation was followed by Lipp Mischtechnik (Mannheim, Germany). Here the focus lies on removing undesired water from the raw materials before liquefaction and not during that step. This is possible through pre-drying refiner flakes5 or milk powder6. Downstream, the liquefaction can be done in a very rapid batch process or continuously using a high-speed in-line mixer14.
This very unique machine resurrects the traditional method of conching and grinding at the same time, as we know it from the Lindt longitudinal conch1. It consists of a double-jacket cylinder with serrated internal surface. Spring-loaded scrapers break the particles during rotation; volatile water and flavours are removed by ventilation and heating.
There has been some discussion about the optimisation of flow properties and flavour in those machines and it has also been tried to combine it with other systems, e.g. refiners1. It is also known, that operation is relatively noisy. An advantage is that batch sizes between 45kg and 5t are possible, which means a lot of flexibility for smaller companies.
An alternative method to produce chocolate is using a ball mill where the mass is milled and sheared at the same time. Although cocoa liquor is usually ground by ball mills, those are not popular for chocolate mass in the European industry. Nevertheless those systems are commonly used worldwide. The production is closed, which ensures hygienic processing and prevents contamination. Industrial-scale ball mills work continuously. Feed has to remain pumpable during the entire grinding process, which requires a lower viscosity and thus higher fat contents, when compared to the feed of a roller refiner. Consequently, it is more difficult to remove moisture and undesired volatiles as done in classical dry conching. The fact is ignored by some ball mill manufacturers, who sell all-in-one solutions. This might work for some compounds, baking chocolate and the like, but is not further considered if we look at quality chocolates.
An early approach to include the removal of volatiles into a recirculating ball mill system was made by DuyvisWiener which included a taste changer; a rotating disk where hot air is blown over the chocolate layer formed by rotation1,15. These devices are still sold for small scale applications. F.B.Lehmann, now part of DuyvisWiener, has a long experience in building thin film evaporators and horizontal ball mills for cocoa processing and had also offered systems for chocolate mass production. This is continued after the merger and further processing alternatives have been designed using devices from both subsidiaries. So for larger continuous lines, thin film flavour treatment can be combined either with horizontal or vertical ball mills15. Together with the traditional refining conching solutions (see above) the company now can offer a large variety of processing alternatives to their clients.
Recently, Bhler seems to have followed a similar strategy. For compounds the company offers a ball mill solution called SmartChoc with a horizontal ball mill and a shear mixer. After adding a single-shaft conch for flavour treatment (light conching) the system for small scale production (60-300g/h) is now called SmartChoc Plus and allows manufacturing a variety of chocolate and compound masses16 (Figure 5).
Another system is offered by Netzsch17; it comes in variations for smaller companies (batch size 25-300 kg/h, called ChocoEasy) and also for larger production (batch size 750-6000kg, then called Rumba). After pre-grinding sugar by an impact mill the raw materials are mixed in a conch, where hot air is applied for flavour treatment during dry conching. After that the mass is liquefied by adding cocoa butter and then ground by circulation through a horizontal ball mill. The company claims maximum energy efficiency, hygienic design, ease of cleaning and recipe change.
After building highly reputed conches, batch and in-line mixers for a long time, Lipp Mischtechnik has now developed a complete chocolate line called Eco2choc (Figure 6). It is based on the coarse conching processing concept. Development and optimisation are described in7; research has also shown that milk chocolate of good flow properties and taste can be produced. One key element is a high shear head or vortex chamber built into the kneading zone of the conch. It intensifies mass and energy transfer, but also reduces particle size of crystal sugar to approximately 300m thus no pre-grinding device is necessary. Coarse conching time can be short if just drying is needed, e.g. for white chocolate or milk chocolate with small quantities or high quality cocoa mass. If a stronger treatment is necessary, e.g. for flavour development of dark chocolate, this can be achieved by increasing energy input and time. The dry and pasty conching is generally done at low fat contents in order to improve volatilisation. Fat and other ingredients are added then and grinding can be performed from a buffer mixer by two vertical ball mills with an intermediate cooler. The latter helps to keep temperature of sensitive products below the desired level, e.g. when recipes contain lactose and glass transition during milling must be avoided. The process can be downsized for small production scale, then it consists of a conch with vortex chamber, a ball mill and a pump for circulation.
One of the first things a chocolate producer has to consider are the influences of recipe, ingredients and particles on chocolate mass properties as discussed above. First of all, if raw material cost is less important, e.g. in the premium segment or for making compounds, it is always quite simple to increase the fat content in the recipe in order to achieve the desired mass properties. Also the taste can be largely influenced by choosing the right ingredients. In those cases, processing technology becomes less important and most of the systems on the market will be able to produce the desired quality.
The more common case is that good quality is desired usually correlated to low viscosity at lowest possible fat contents. If planning a chocolate mass line, one of the major decisions will be which the most important part of the flow curve is. If low shear downstream applications like moulding are in the focus, low yield values are important; here ball milling could be an advantage. On contrary, if the mass has to move fast, for example, if pumped or sprayed infinite viscosity is more important and roller refiners might be preferential.
Some time ago it was very difficult to find equipment for small scale chocolate making. This has changed; now there are a number of ball mill-based systems on the market and also smaller scale roll refiners have been developed. Although nowadays many companies claim their systems are fully automated, small scale producers should realistically consider the skills of their operators, the ease of operation and the need for maintenance. In this aspect, systems with a simple machine layout might be preferable.
For medium- and large-scale producers there are a wide range of technical options. The varying needs of chocolate producers and the various advantages and disadvantages of the systems on the market make it impossible to give a general recommendation. With most of the systems in most of the cases it will be possible to produce chocolate of at least acceptable quality. Fine-tuning and final choice has to be made in every single case; it is always both recipe and process that influences final quality and there is no out-of-the-box solution. So the best possible advice might be:
Prof. Dr. Siegfried Bolenz studied food engineering in Stuttgart-Hohenheim and started his career 1989 in a fruit juice company while studying for his PhD. From 1992-1997 he worked with Kraft-Jacobs-Suchard (now Mondelz) in R&D on various dairy, food and chocolate process development projects. Since 1997 he has been professor of food technology at the Neubrandenburg University of Applied Sciences, where he teaches dairy, confectionery and beverage technology, product and process development. One research focus is chocolate processing, where he cooperates with various companies and has published a number of papers and patents. For further information visit: www.hs-nb.de/ppages/bolenz-siegfried.
Thanks for the comprehensive article on Chocolate making. I found it a very interesting read. If one was looking for a flexible system that can make a variety of chocolates using different fats which needs different change overs but still produces acceptable product quality, which technology would you consider? Has triple stone mill ever been considered for particle size reduction like they do for Cocoa Liquor?
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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).
Our automatic production line for the grinding cylpebs is the unique. With stable quality, high production efficiency, high hardness, wear-resistant, the volumetric hardness of the grinding cylpebs is between 60-63HRC,the breakage is less than 0.5%. The organization of the grinding cylpebs is compact, the hardness is constant from the inner to the surface. Now has extensively used in the cement industry, the wear rate is about 30g-60g per Ton cement.
Grinding Cylpebs are made from low-alloy chilled cast iron. The molten metal leaves the furnace at approximately 1500 C and is transferred to a continuous casting machine where the selected size Cylpebs are created; by changing the moulds the full range of cylindrical media can be manufactured via one simple process. The Cylpebs are demoulded while still red hot and placed in a cooling section for several hours to relieve internal stress. Solidification takes place in seconds and is formed from the external surface inward to the centre of the media. It has been claimed that this manufacturing process contributes to the cost effectiveness of the media, by being more efficient and requiring less energy than the conventional forging method.
Because of their cylindrical geometry, Cylpebs have greater surface area and higher bulk density compared with balls of similar mass and size. Cylpebs of equal diameter and length have 14.5% greater surface area than balls of the same mass, and 9% higher bulk density than steel balls, or 12% higher than cast balls. As a result, for a given charge volume, about 25% more grinding media surface area is available for size reduction when charged with Cylpebs, but the mill would also draw more power.