When I decided I wanted to make my own BP, I was inspired by Skylighter's article on making high-powered black powder. I wanted to make hotter powder than the commercial product we could have once got here, but now even that is unobtanium. I wanted it to be like Swiss 3Fg.
Verge collections of large rubbish are a big thing here. We go sharking, cruising the piles for garden furniture and toys for grandchildren. From an earlier one I had slabs of steel 19mm and 25mm thick for a Skylighter-style hydraulic press! Hmm, what has drive shafts and motor I could loot for a tumbler? Oooooh, a tumble dryer would also have a giant pulley for reduction!
So I thought a moment more and said why build another frame and motor mount? Just mount the white-pipe tumbler barrel inside the dryer tumbler. It took a while to think that out. MDF profiles bolted against the three ribs in the dryer drum now make up a spider to hold the barrel in the centre. I used high density foam scraps from an Ikea kids play mat to cushion the barrel in position.
Sandbags for safety? I stacked up bags of pool salt. Same price as sacks of plasterers sand, dont have to fill, and can use in the pool later. After seeing a youTube of someone letting off small explosive charges in a dryer,I found a 3' square aluminum expanded mesh for confining door explosions.
A cargo strap to hold it in axially didn't work; the strap tended to fall off around the roundness of the barrel lid even with duct tape trying to retain it. Yesterday I set bolts through the MDF profiles as lugs to run cord lashing around. I will add hooks and rubber strap next.
The dryer stops and reverses direction every minute. By my calculation this drops about 5 seconds or 8% of milling time in the toilet. Instructables.com has a couple dryer motor salvage articles, and its clearly possible to wire it differently.
I stripped the dryer and traced out the wiring, trying to understand all the switching. Having found everything I expected (and a lot I didn't expect -clever guys these appliance makers), I sketched an original circuit diagram for the machine, and thought out what I wanted. Now it has:
And for a few $ I got a 24hr time switch for the wall socket, which lets me set start and stop times for the run, at a distance from the machine. For single-ingredient milling it could be left unattended to start and end.
On this Simpson dryer the pulleys at the back had nothing to do with the drum. They run an extractor fan which is like the old blower for a forge - just radial vanes pulling air in past the heater element and through the drum, to exhaust at the top rear.
There is no ball or roller bearing setup for the drum; instead, the front and back of the drum have a 1/2" axial length cylinder formed around the opening and around the mesh of the closed end, about 200mm dia at the closed end, running in formed sheet metal rings, with a strong fabric tape between the drum and frame to separate the metal from rubbing.
The barrels I made are 150mmx150mm long sewer pipe, from the guidelines in the ball milling post, intended to mill 500g/1lb of powder at once. Balls are .690 cast, and I plan to make a cylinder instead of ball and re-cast in wheel-weights or some other harder lead alloy. The screw caps seem like a good idea but since I have to tape them I might as well have gone to end caps both ends and saved a lot of time modifying the fitting to make it short enough.
Pro tip 2: If the tumbler barrel leaks, that fan will evenly distribute a half-pound of meal across every horizontal surface in the workshop despite the best efforts of the sandbags and mesh to keep trouble enclosed.
Pro tip 2: If the tumbler barrel leaks, that fan will evenly distribute a half-pound of meal across every horizontal surface in the workshop despite the best efforts of the sandbags and mesh to keep trouble enclosed.
Pro tip 2: If the tumbler barrel leaks, that fan will evenly distribute a half-pound of meal across every horizontal surface in the workshop despite the best efforts of the sandbags and mesh to keep trouble enclosed.
But the lid was completely unscrewed, despite the duct tape I put around it to seal. Obviously it was not clean enough when I taped it shut and maybe not tightened down, but I didn't think that should happen!
My assessment of the cause: the tumbler barrel itself was loose in the mounting frame, and only the lid was supported by the foam packing. So the tumbler barrel was relatively free, and it just unwound from the lid under the large weight, and rotating hammering, of the ball media charge.
Chris - Just salvage the motor, purchase some pillow block bearings and shafting to match, and make something more aligned with the purpose. Dicking about with modifying a dryer drum is silly. Google "Bigg Dawg wet tumbler" for a real simple design that works.
The problem as I see it, is that I didn't use the kind if barrel and lid that Bigg dawg is using. Yes his entire design will do a great job but I am not modding the dryer drum at all, its just a cheap way to rotate a sewer-pipe tumbler barrel.
The dust was left over from the last run, in hidey holes of the MDF barrel holder that my cleaning didn't find. Frank Rizzo's recommendation looks even better, because the Bigg Dawg style tumbler is far easier to vacuum.
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).
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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Ball-milled biochars (BM-biochars) were produced through ball milling of pristine biochars derived from different biomass at three pyrolysis temperatures (300, 450, and 600C). The results of scanning electron microscopic (SEM), surface area, hydrodynamic diameter test, and Fourier transform infrared spectroscopy (FTIR) revealed that BM-biochars had smaller particle size (140250nm compared to 0.51mm for unmilled biochar), greater stability, and more oxygen-containing functional groups (2.24.4mmol/g compared to 0.82.9 for unmilled biochar) than the pristine biochars. With these changes, all the BM-biochar-modified glassy carbon electrodes (BM-biochar/GCEs) exhibited prominent electrochemical properties (e.g., Ep of 119254mV compared to 850mV for bare GCE). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) show that ball-milled 600C biochar/GCE (BMBB600/GCE and BMBG600/GCE) had the smallest peak-to-peak separation (Ep=119 and 132mV, respectively), series resistance (RS=88.7 and 89.5, respectively), and charge transfer resistance (RCT=1224 and 1382, respectively), implying its best electrocatalytic activity for the reduction of Fe(CN)63. It is supposed that the special structure (i.e., internal surface area, pore volume, oxygen-containing functional groups, and graphitic structure) facilitates the electron transfer and reduces interface resistance. Economic cost of BM-biochar/GCE was 1.97107 USD/cm2, much lower than that of a low-cost platinum electrode (0.03 USD/cm2). The results indicate potential application of the novel BM-biochar for low cost and high efficient electrodes.
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This work was partially supported by the Key Laboratory of Original Agro-Environmental Pollution Prevention and Control, Ministry of Agriculture/Tianjin Key Laboratory of Agro-environment and Safe-product [18nybcdhj-5 and 18nybcdhj-1], the National Natural Science Foundation of China (41807363), Guangxi Natural Science Foundation (Nos.AD17195058), the Key Research and Development Project of the Ministry of Science and Technology (2018YFB0605101), and the Key Project Natural Science Foundation of Tianjin (18JCZDJC39800).
Key Laboratory of Original Agro-Environmental Pollution Prevention and Control, Ministry of Agriculture/Tianjin Key Laboratory of Agro-environment and Safe-product, School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, 300401, China
Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin Engineering Center of Environmental Diagnosis and Contamination Remediation, College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, China
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Ball mills are a special instrument used to break up hard solids into a fine powder. They are similar to rock tumblers in that the instrument is a rotating container filled with heavy balls to grind the substance into powder. Ceramic material, crystalline compounds, and even some metals can be ground up using a ball mill. Using a motor, container, belt, caster wheels, and some basic building supplies, you can make your own ball mill. X Research source
To make a ball mill, start by building a wooden platform and attaching a motor underneath it. Then, cut a slit into the wooden platform for the belt to pass through and attach casters to the platform for the container to sit on. Next, thread the belt through the slit and position the container so the belt is pulled tight. Finish by connecting the motor to the power supply, and filling the cylinder with metal balls and the substance you want to grind. For tips on how to operate your ball mill, read on! Did this summary help you?YesNo
Mechanochemical activation can be defined as a process able to induce structural disorder through intensive grinding. In certain conditions, it may increase the chemical reactivity of the processed material. The process is extensively utilized in extractive metallurgy, synthesis of nanocomposites or pharmacology. It is also considered an environmentally friendly alternative to activate kaolinitic clay avoiding high calcination temperature. This paper aims to give a comprehensive overview of the process, its evolution, process parameters and applications. The paper focuses on the mechanochemical treatment of natural clay minerals, aiming at their transformation into cementitious or pozzolanic materials. It provides a summarized review of the theories related to the mechanochemistry and discusses commonly used models. The paper also analyzes various key factors and parameters controlling the mechanochemical activation process. The optimization and control of the several factors, as the filling ratio, the grinding media, the velocity, the time of grinding, etc., can promote developments and new research opportunities on different fields of application. Examples of applications, with a special focus on mechanochemically activated clay minerals and their use as cementitious binders, are listed as well.
Mechanochemical activation (MCA) is a process able to induce structural disorder, amorphisation and increased chemical reactivity in the material treated by intensive grinding, (McNaught and Wilkinson 1997; Bal et al. 2013). The MCA is a simple method to apply, especially when done by milling. The entire process is characterized by small energy requirements, low processing temperatures, and thus reduced costs, and increased environmental friendliness, (Bal, 2000, 2008; Boldyrev 2006; Rescic et al. 2011).
In the book De Lapidus (On Stones), Theophrastus of Eresus (371286B.C.) did the first documented description of a process, which could relate to the MCA, (Takacs 2000). In 1820, Michael Faraday reduced AgCl to Ag with a pestle and a mortar, which also simulated to a certain extent the MCA process, (Fox 1975; Bal 2008; James et al. 2012; Bal et al. 2013). At the end of the nineteenth century, Mathew Carey Lea discovered that application of mechanical forces and heat treatment decomposed to halogen and metal the halides of gold, silver, platinum, and mercury. The same components melt or sublime under heat action, (Fox 1975; Bal 2000, 2008; Takacs 2004; Bal et al. 2013).
Later, Clark and Rowan discovered, that milling in the solid state or applying a high pressure on certain compounds could produce the same effects in their structure, (Bal 2000, 2008). Walter Kelley investigated grinding of bentonite and kaolinite, and formulated a theory including only a physical disintegration of the minerals followed by a decrease in the particle size. Alfred Perkins stated, that grinding induces also disorders in the crystal lattice structure, (Gregg et al. 1953; Garcia et al. 1991).
In 1938, Bowden and Tabor published results of their studies in tribology, focusing on the contact area between moving surfaces. Temperatures of over 700C were observed at the contact of solid substances exposed to friction, (Fox 1975; Bowden and Tabor 1939, 1966; Bal 2000, 2008; Bal et al. 2013). Httig (1943) determined that activated solid produced by grinding had a thermodynamically and structurally unstable arrangement of the lattice. Furthermore, the value of enthalpy and the Gibbs energy also increased in the activated solid, (Httig 1943; Tkcov 1989; Tkov et al. 1993).
In 1942, for the first time, Adolf Smkal used the term mechanical activation. Otherwise, Wilhelm Ostwald was credited by some authors to classify mechanochemistry as one of the four sub-disciplines of chemistry, among thermochemistry, electrochemistry and photochemistry, each of which is based on a different type of the input energy, (Rose and Sullivan 1958; Juhsz 1998a; James et al. 2012).
Leonard Austin determined that the MCA process occurs when quantitative changes related to the particle size of the material cause qualitative changes in the nature of the material, (Boldyrev et al. 1996; Hennart et al. 2009).
In 1959, Hiroshi Takahashi published results of his studies focusing on the dry grinding of pure kaolinite and natural clay. Disordered crystalline structures and increased amorphous phases were detected in the processed materials. The crystalline structure tended to be amorphous at different times of dry grinding, depending on the structural order of the original mineral unit layers, (Takahashi 1959a, 1959b).
Effects produced by high energy ball milling and mechanochemical activation could be important for different technological applications including for example materials engineering, waste treatment, mineral processing, agriculture, extractive metallurgy, construction technology, pharmacy, or the coal industry, (Bal 2008).
The clay itself, which is the main scope of this review, is a commonly occurring and sustainable natural mineral used in several applications. For example, calcined kaolinitic clays are widely used to manufacture pozzolanic cement, (Ambroise et al. 1994; Sabir et al. 2001; Vizcayno et al. 2010; Souri et al. 2015; Sandstrm 2016). Metakaolin, which is produced by thermal calcination of kaolinitic clays, is used as a supplementary cementitious material in the production of concrete. It has good pozzolanic activity, can increase the concrete strength and improve the durability. Furthermore, the MCA showed to enhance pozzolanic activity of the natural kaolinitic clays, (Ambroise et al. 1994; Sabir et al. 2001; Vizcayno et al. 2010; Souri et al. 2015).
The main aim of this paper is to review and analyze various factors and process parameters affecting the efficiency of the MCA process especially when applied on clay minerals. The output of this analysis should enable to define a set of initial parameters leading to an effective activation of natural clay minerals.
The MCA process can be defined as a sequence of four stress-related events including: compression, shear (attrition), impact and collision. Each type of milling equipment use different combination of these various stresses, leading to different efficiency of the process, (Bal 2000, 2008; Bal et al. 2013; Delogu and Takacs 2018). The main objective of the MCA process is to transfer as much energy as possible, from the milling balls to the ground material. The accumulated energy will enhance the chemical reactivity of the processed materials through dislocations and induced defects, (Boldyrev 2006; Bal et al. 2013). A number of various types of milling equipment can be used for the MCA including ball mill, planetary ball mills or ring mills, Fig.1.
The planetary ball mills showed short processing times, safe handling and good reproducibility, (Bal 2000, 2008; Bal et al. 2013). In the planetary ball mill, the jar is attached to a disk that rotates around the central axis. At the same time, the jar rotates around its own axis. During grinding, the movement of the balls is dependent on several process parameters. For example, the increase of the revolution speed and/or the filling ratio of the jar are able to change the movement of the balls from a cascading regime to a cataracting one and finally to a rolling regime, Fig.2, (Burmeister and Kwade 2013).
The repetitive occurrence of structural changes can lead to the co-crystallization of the nearby particles by decreasing the specific surface area and the surface free energy, as well as inducing shrinkage. Juhsz and Opoczky (1990) described the so-called melt-bridges formation because of the co-crystallization initiated by the interpenetration of particles and by the consecutive creation of a thin liquid film on their surface. During fine grinding, mechanical effects, friction and plastic deformation provide the necessary energy to form the melt-bridges, similar to the high temperature sintering process.
Another phenomenon observed during the grinding process is the chemi-hesion, which could be defined as a chemical bond between two solid surfaces being sufficiently close to each other. The chemi-hesion occurs only due to the developing capillary pressure, which could partly explain why these forces act even between particles of the same crystallinity and composition. Combined chemi-hesion and surface dislocations promote chemical reactions and intergrowth between particles of different compositions. Polymorphic transformations can occur as well, (Juhsz 1998a, 1998b; Bal et al. 2013).
Concerning kaolinitic clay, which is the focus of this paper, the MCA process is able to split the hydrogen bonds between the adjacent kaolinite layers, (Frost et al. 2001, 2001). It causes structural modifications of the crystal structure by the rupture of O-H, Al-OH, Al-O-Si and Si-O bonds. The presence of quartz appeared to accelerate the amorphisation. The higher hardness of quartz particles allow them to act as a grinding media for the kaolinite particles, thus accelerating the grinding process, (Mak et al. 2001; Rescic et al. 2011; Hamzaoui et al. 2015). Results obtained by Snchez et al. (1999) showed that mechanical milling of the kaolinite can generate a significant amount of Al surface in a tetrahedral coordination and can thus lead to a 20% decrease of the Si/Al atomic ratio on the kaolinite surface. Structural amorphisation of kaolinite during repetitive grinding process has been detected through TEM and SEM analysis, respectively Figs.3 and 4, and through X-ray diffraction analysis, Fig.5. TEM and SEM micrographs show how the order of the structure is altered and replaced by amorphous regions.
TEM micrographs of the kaolinite/talc mixture with well-developed pseudo-hexagonal symmetry of kaolinite particles shown in Fig. 3a (unground) is replaced by amorphous, shapeless rounded aggregates after 60min grinding in the ring mill in Fig. 3b, (Zbik and Smart 2005). Reprinted from Minerals Engineering, 18/9, Zbik M., Smart, R., Influence of dry grinding on talc and kaolinite morphology: inhibition of nano-bubble formation and improved dispersion, Pages 969976, Copyright (2018), with permission from Elsevier
SEM photomicrographs of kaolinite particles after HEBM for a 0, b 16 and (c, d) 256min, (Vdovic et al. 2010). Reprinted from Applied Clay Science, 48/4, Vdovi, N., Jurina, I., kapin, S. D., Sondi, I., The surface properties of clay minerals modified by intensive dry milling revisited, Pages 575580, Copyright (2018), with permission from Elsevier
XRD patterns of untreated kaolin a and of MA samples using 500rpm, and 15min grinding time as well as b 1:8, c 1:11, and d 1:14ms:mgb values. (k: kaolinite PDF 14164; m: muscovite PDF725; q: quartz PDF331161), (Balczr et al. 2016). Reprinted from Ceramics International, 42/14, Balczr, I., Korim, T., Kovcs, A., Mak, ., Mechanochemical and thermal activation of kaolin for manufacturing geopolymer mortars Comparative study, Pages 1536715375, Copyright (2018), with permission from Elsevier
Aglietti et al. (1986) observed physico-chemical changes while grinding kaolinite in an oscillating mill. The results showed that, at a certain stage, the ground material tends to agglomerate due to the high surface energy produced in the grinding process.
The adherence of the ground particles to the surface of the jar and of the grinding balls is defined as caking, or simply an adherence, and it interferes with the grinding process. The deposition of the first layer is followed by an accumulation of successive layers of material, which doesnt take part anymore in the grinding process, (Avvakumov et al. 2001).
Other studies showed, that the dehydroxylation temperature changes with the process duration, (Frost et al. 2003; Ding et al. 2012). The thermal analysis, Thermogravimetric analysis - TGA and Differential thermal analysis - DTA, showed a loss of hydroxyl groups from the kaolinite surface and their replacement with water molecules, (Frost et al. 2001, 2001). Amorphisation also appeared to destroy the bonds between the tetrahedral and the octahedral layers, (Takahashi 1959c). Grinding of kaolinite for up to 10h reduced the crystallinity and increased the specific surface area, (Frost et al. 2001, 2001).
The X-ray Diffraction (XRD), the infrared spectroscopy (IR) and the DTA of kaolinite samples, performed after grinding, showed significantly altered structure, where the OH groups were removed from their positions and the tetrahedral and octahedral layers were distorted, (Kristof et al. 1993).
The MCA affected also other natural clay minerals, including for example, illite, (Yang et al. 2005), muscovite, montmorillonite, (Hrachova et al. 2007), or pyrophyllite, (Pi et al., 1988). Fragmentation of the clay minerals (e.g. montmorillonite, ripidolite or kaolinite) due to MCA caused the formation of new more reactive surfaces, (Vdovic et al. 2010).
A similar tendency related to structural changes was observed during grinding of talc and pyrophyllite. In these cases, dry grinding in a planetary ball mill deteriorated their structure, which became highly disordered, (Sugiyama et al. 1994). Fine grinding of talc has produced an altered and amorphous structure after less than 30min. As for the clay minerals, a particle size reduction associated with an increase in the specific surface area is obtained for MC activated talc, (Sugiyama et al. 1993; Sanchez-Soto et al. 1997).
MC processing of montmorillonite or kaolinite, in addition to decreased crystallinity and particle size, has shown interesting changes as e.g. creation of amorphous oxides of Mg, increased content of Fe3+ cations compared to Fe2+ in processed montmorillonite, improvement of the DC conductivity in both kaolinite and montmorillonite, (Bekri and Sasra 2016; Ondruka et al. 2018).
Processing of bentonite by MCA led to the damage of the crystal structure and generated almost an amorphous material, especially when using long grinding times. The loss of crystallinity led to the formation of a solid silica gel, (Volzone et al. 1987).
One of the first models describing the MCA is the so-called hot-spot model proposed by Bowden and Tabor. It assumed that MCA occurs in micro-size areas of the contact zone due to friction phenomena. In these small areas, a local temperature could exceed 1300K and last for 104103seconds, causing the mechanically initiated reactions, (Bowden and Tabor 1939, 1966; Tkcov 1989; Bal 2008).
Peter Thiesen developed the magma-plasma model, which considered the direct impact between two rough surfaces as the main process occurring during mechanochemical activation, Fig.6, (Weichert and Schnert 1973).
The model assumed that local temperatures, higher than103K, can be generated at the impact points and can induce an excited state of the material. The high amount of energy can bring the processed solid to a plasmatic state, characterized by the formation and ejection of electrons, photons and excited fragments, (Tkcov 1989; Juhsz 1998a, 1998b; James et al. 2012; Bal et al. 2013).
Miller and Oulton observed that long and repeated processes of dry grinding creates greater stresses and structural changes in kaolin crystals in comparison with the thermal treatment. They also developed the theory of prototropy, which assumed a possible migration of protons during grinding processes, thus enabling the interaction with OH groups and the consequently generation of water molecules. These molecules can be lost from the structure already at lower temperatures than required by the dehydroxylation process, (Miller and Oulton 1970; Mishirky et al. 1974; Garcia et al. 1991). The dehydroxylation process is defined as the elimination of water from the OH groups of minerals or inorganic compounds structures, and, in clay structures, it usually occurs at high temperatures between 400 and 800C, (Vaughan 1955; Stoch and Wacawska 1981).
In 1974, Emmanuel Gutman showed another approach to explain the MCA process, through the so-called dislocation theory. The theory assumed that mechanical properties of a solid are related to the movement and interaction of dislocations. Increasing dislocations due to the mechanical action will enable their movement to the surface and transformation into areas with higher chemical activity, (Juhsz and Opoczky 1990; Bal, 2000, 2008; Bal et al. 2013).
Boldyrev and Urakaev developed the kinetic model, which combines kinetics of the activation process with the kinetics of grinding. According to this approach, the MCA occurs due to the impulsive nature of temperature and pressure. The kinetic equation, which was also experimentally verified, described the evolution of the MCA process and effects of various parameters, as for example the rotation velocity, the number and size of the balls, mechanical properties of the balls and of the processed material, (Boldyrev et al. 1996; Urakaev and Boldyrev 2000a, 2000b).
Pavel Butyagin showed that during mechanochemical processes, the absorption of the elastic energy, high pressure and high temperature could supply a sufficient amount of energy for chemical reactions to occur, (Butyagin 1971; Boldyrev 2006; Bal 2008).
All the described models considered only one of the various energies related to MCA, while the phenomena are multiple and complex. For example, the hot-spot and the magma-plasma model consider only high temperatures generated during the impact or in the contact zone, the dislocation theory considers that evolution of the process is only related to the dislocation movement. The kinetic model takes into account only the time of the process, (Juhasz and Opoczky 1990; Bal 2008; Delogu and Cocco 2008).
One first attempt to unify the models and to better explain the MCA reactions was done by Tkov et al. (1989). The main conclusion was that the energy concentrates at the tip of the propagating crack, which results in creation of extreme conditions favorable for the MCA. These reactions were also assumed to be initiated by an impulsive temperature and pressure increase at the contact zones between the particles. The theory enables to explain the MCA process as being controlled by different effects: the dislocations movement, the contact zones rising temperature, high temperature and pressure with a pulse-type nature, and kinetics, (Tkaova et al. 1989).
Another approach illustrated by the so-called comminution theory was developed by Juhsz and Opozcky (1990). The comminution is identified as a repetitive breakage occurring during grinding, (Juhasz and Opoczky 1990; Juhsz 1998a, 1998b). The MCA process develops in two distinctive stages. The first stage, the comminution, is characterized by an increase of the specific surface area, higher surface energy and accumulated micro deformations. The second stage is the actual mechanochemical activation, where the specific surface area and the size of the crystallites remain constant but the mechanical energy is acquired in the form of the excess energy, which induces defects of the crystal lattice, (Tkaova et al. 1989; Juhsz 1998a, 1998b; Prokofev and Gordina 2012). According to Rittingers law, the energy required for the comminution to occur is proportional to the produced new surface and it is determined by the energy required to stress the particle until its failure, (Fuerstenau and Abouzeid 2002). Results showed, that up to 50% of the input energy could be transformed into strain, (Rumpf 1973; Tkov et al. 1993). According to the comminution theory, the rupture of the particles starts from the weakest points such as existing defects, dislocations or micro-cracks on the surface of the original material. During grinding and with the increasing dispersion degree, the particles become smaller. Moreover, the weak points have the tendency to move to the surface of the solid processed substance, (Rumpf 1973; Juhsz 1998a, 1998b). The accumulation of defects and dislocation on the boundary layer at the particle surface can lead to the annihilation of dislocations.
This could act as an energy barrier with either increased or decreased load. The increased load would block further movements. Eventually, the developed high energy can cause plastic deformation, polymorphic transformations and amorphisation, (Juhsz 1998a, 1998b).
The effects of the milling balls during the MCA are considerably different from those in the simple comminution process. Which is due to the effect of the reaction threshold energy, (Shinozaki and Senna 1981). The threshold effect is directly dependent on the initial impact stresses transmitted to the particles from the grinding device. Consequently, occurrence and development of the MCA process is strongly controlled by the process parameters of the grinding device, (Tkaova et al. 1989; V. Boldyrev et al. 1996).
Kinetics of reactions are rather complex due to the variability of the process conditions. The process is not homogeneous in terms of time and space, due to the pulse-type interaction between the grinding balls and the ground material. A model created by Avvakumov et al. (2001) distinguishes three main stages of the MCA process, Fig.7.
The first stage implicates the fragmentation of the material and the gradual increase of the specific surface area. In the second the so-called dispersion stage, the formation of secondary particles or secondary aggregates dominates. The secondary aggregates are formed during the grinding process, through the agglomeration of fine particles, (Tkcov 1989). The third stage is characterized by the amorphisation and the simultaneous crystallization of new phases. This third stage continues until a stationary state is reached. The stationary state is directly dependent on the amount of mechanical energy. At a low energy level, the process can only continue until the first process stage. The second and the third stages can be reached only if the energy is sufficiently high, (Tkcov 1989; Avvakumov et al. 2001).
The most important parameters controlling the amorphisation process are the rate of stress and the efficiency of the energy transfer in the mill, (Tkov et al. 1993). MC-activated kaolinite in both planetary ball mill and high power ring mill has showed structural disorder during dry grinding process, Fig.8.
SEM micrographs. (a and b) Unground kaolinite crystals; (c and d) amorphous, pulverized aggregates of nano-sized particles after 60min grinding in the ring mill. (Souri et al. 2015). Reprinted from Cement and Concrete Research, 77, Souri, A., Kazemi-Kamyab, H., Snellings, R., Naghizadeh, R., Golestani-Fard, F., Scrivener, K., Pozzolanic activity of mechanochemically and thermally activated kaolins in cement, Pages 4759, Copyright (2018), with permission from Elsevier
The choice of process parameters determined the final structural order and influenced the duration of grinding. The duration is a crucial factor affecting the amorphisation of the structure. Ring and ball mills are also used to produce fine powders where the amorphisation of the structure should be avoided. For example, the preparation of samples for quantitative analysis of minerals with XRD technique. For this technique, wet grinding for very short duration, around 5min, can be used. The grinding medium is usually water, methanol or hexane. (rodo 2006).
In the case of ball mills, the degree of mechanochemical transformation, , is a function of the rotational frequencies k, the number of milling balls N, the ratio (R/lm)of the ball size Rto the diameter of the jar lm, the material properties of milling ball and compounds to be treated X, and the time of mechanical treatment, (Urakaev and Boldyrev 2000a, 2000b):
The initial particle size distribution of the processed material tends to influence the development of the mechanochemical reactions and the kinetics of the process. Delogu and Takacs (2018) detected an exponential grain size reduction during grinding of powders having particle size distribution in the micrometer range. A gradual increase was also observed when grinding nanosized powders. Creation of melt-bridges and interpenetration of particles are considered to cause the grain growth. Consequently, a high particle size distribution may require more time for the MCA to occur, while the nanosize particle distribution can lead to agglomeration and mechanical alloying, (Juhsz and Opoczky 1990; Delogu and Takacs 2018).
The Agglomeration zone, where the specific surface area decreases and the particles are chemically bonded together. The agglomeration can be also induced by moisture originating from the processed material, which can be counteracted by the addition of various types of dispersive agents, (Orumwense and Forssberg 1992). Agglomerates can be dispersed by a brief peptisation re-grinding which consists of a short-term wet re-grinding, (Tkcov and Stevulova 1987; Tkov et al. 1993). The grinding process tends to be easier if done in a wet environment where water or alcohol are commonly used. The wet grinding can reduce the agglomeration but can also increase the shrinkage of the material during the drying, and can promote cracks and deformations, (Rose and Sullivan 1958; Bond 1975; El-Shall and Somasundaran 1984; Juhsz and Opoczky 1990). In general, the dry grinding appeared to be more effective than the wet grinding to impose significant structural amorphisation, (Takahashi 1959c; Juhsz and Opoczky 1990).
The total impact energy tends to increase at higher rotational speeds, but at the same time, the grinding balls tend to attach to the inner walls of the grinding jar and to move in a rolling pattern. It results in a disappearance of the impact forces, Fig. 2, (Suryanarayana 2001; Ashrafizadeh and Ashrafizaadeh 2012).
A high grinding speed can also lead to increased temperature in the grinding jar. Therefore, the determination of the optimum rotational speed is crucial to enable the development of the maximum collision energy, (Suryanarayana 2001). Another parameter, which seems to affect the efficiency of the grinding process, is the grinding ball to powder ratio. Increasing of this ratio tends to increase the mass of the system and its kinematic energy. The collision probability will be higher but will also result in a greater amount of abrasion and contamination of the final product, (Ashrafizadeh and Ashrafizaadeh 2012). The free volume value of the grinding jar has to provide a sufficient space for the material to move during the grinding process.
The efficiency of mechanochemical activation is also affected by the used grinding media, material density, particle shape and size, as well as by the overall system viscosity. In general, the grinding media should have a higher hardness of at least 3 Mohs than the ground material. They should be also denser than the processed material and chemically inert, (Orumwense and Forssberg 1992).
Milling balls with a higher density or large diameter produce higher impact energy during the collision, which results in a more effective surface activation. Smaller milling balls provide higher frictional forces and thus are useful for amorphisation or the formation of metastable phases, (Suryanarayana 2001). Typically, grinding balls are made of stainless steel, special metals, corundum, zirconia, carbides, etc., (Suryanarayana et al. 2001). A short list of various used ball sizes is shown in Table 2.
Using water as a grinding medium enabled viscosity changes during the mechanochemical process in a ball mill, (Orumwense and Forssberg 1992). This effect can significantly influence the efficiency of the MCA. Diluted suspension can hinder the movement of the balls and thus decrease the impact forces transmitted to the processed material. On the other hand, an insufficient amount of water can enable to obtain increased viscosity of the system during grinding. The powders became finer and the viscous system can literally block the movement of the grinding media in the jar, (Tole et al. 2018).
Polymer Nanostructured Composites (PNC) can be produced by delamination of inorganic minerals through the mechanochemical treatment (MC). Clay minerals have been widely used as reinforcing fillers for polymeric matrixes to enhance their certain properties, (Lu et al. 2004; Yoshimoto et al. 2005; Perrin-Sarazin et al. 2009). MC treatments can modify the surface of clay powders and ameliorate their dispersion in the solid composite. The addition of MC treated clay minerals in the polymeric matrix showed lower gas permeability, higher thermal stability and improved mechanical properties of the PNC, (Yoshimoto et al. 2004, 2005; Hasegawa et al. 2007).
The simultaneous dry grinding of kaolin and solid urea mixtures can lead to the mechanochemical intercalation. The intercalation process allowed creation of guest urea molecules in the unfilled space of the kaolinite structure. The formed nanostructured complexes can be suitable to be uses as reinforcement in clay-polymer nanocomposites, (Mak et al. 2009).
MCA of phosphates together with kaolinite is used to produce slow-release fertilizers. As mentioned before, fine grinding of clay minerals is able to produce nanosized reactive materials. These processed minerals are capable to retain certain elements such as for example potassium followed by their subsequent slow release, (Zhang et al. 2011; Borges et al. 2015).
Toxic and non-biodegradable heavy metals present in wastewaters are usually produced by mining and metal industries. The adsorption process is considered a simple and effective treatment for their removal, (Jiang et al. 2010; Fu and Wang 2011; Kumri et al. 2013). Bentonites and kaolins were found to be efficient in removing fluoride or divalent heavy metals by sorption, (Meenakshi et al. 2008; Kumri et al. 2013). The MC treatment appeared to decrease the structural order and to increase the specific surface area of clays. The present phenomena including fragmentation of the particles, abrasion, and amorphisation, can lead to a higher cation exchange capacity and thus to an improved sorption capability, (Jiang et al. 2010; Fu and Wang 2011; Djuki et al. 2013).
Cement and concrete are considered to be irreplaceable construction materials. A very high carbon dioxide footprint of Portland cement puts an increasing pressure on the industry to mitigate this negative feature. Increasing the availability of concrete is at the present strongly connected with an effort to reduce its negative environmental impact due to a high carbon dioxide footprint, (Scrivener 2014). Now, two approaches are used, modernization of the production technology of the Portland cement and limiting of its content in a typical concrete. The cement content can be reduced by optimizing the mix design, increasing its reactiveness (Elfgren et al. 2004) or replacing by pozzolanic materials. Typically used pozzolanic materials include fly ash, slag and natural pozzolans, are considered a fast short-term countermeasure to reduce CO2 emission, (Scrivener 2014). On the other hand, the research aiming to develop new cements or SCM is limited by the availability of different elements constituting the earths crust, (Scrivener 2014). Clay is a commonly occurring material, and the use of limestone and calcined clay to replace up to 50% of clinker in concrete has shown good results in reducing the CO2 emission, (Fernandez et al. 2011; Scrivener 2014; Joseph et al. 2015; Vizcano et al. 2015). However, high carbon dioxide emitting thermal activation method through calcination of kaolin is by far the main way to obtain metakaolin characterized by a sufficient pozzolanic activity. An alternative solution is to use MCA, (Vizcayno et al. 2010; Mitrovi and Zduji 2012, 2014; Souri et al. 2015; Balczr et al. 2016; Ili et al. 2016). Souri et al. (2015) observed that both treatments, thermal and mechanochemical, led to the formation of pozzolanic materials, but are characterized by different physico-chemical properties.
Results showed that the increased specific surface area of the mechanochemically processed kaolin enhanced the early age hydration speed and the strength development of blended cements, (Souri et al. 2015). The intensive grinding produced a strong structural alteration of the silicates. It increased the specific surface area and decreased the particle size. Furthermore, grinding showed a progressive amorphisation and formation of agglomerates, (Sanchez-Soto et al. 1994, 2000).
Mechanochemical treatment of kaolinitic clay with a high quartz content produced materials having a very high pozzolanic activity, especially at an early stage due to the amorphisation, (Vizcayno et al. 2010). Using the MCA on clays rich in montmorillonite or illite also lead to the formation of a reactive material. Despite, their lower 28-days strength these materials could be still used in certain engineering applications, (Murat and Comel 1983; Murat 1983a, 1983b; Ambroise et al. 1985; Yang et al. 2005). The MCA activated kaolinite was also successfully used to produce geopolymers, (Davidovits 1991; Provis and Bernal 2014; Balczr et al. 2016).
The MCA can be considered an ecologically friendly technique, which can contribute to reduce the CO2 emissions by replacing some currently thermally intensive processes. Processing of commonly available raw materials, including natural materials as well as industrial wastes, with MCA should enable the development of a new generation of environmentally friendly and construction sustainable materials. The tailor optimized MCA process could be a valuable instrument to produce active natural clay, which could replace partly or completely the Portland cement in the production of concrete.
However, the literature study performed in this review showed a number of areas, which must be further investigated, including the MCA process itself, effects of various process parameters on produced materials, and its behavior in hydration or geopolymerisation processes.
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Tole, I., Habermehl-Cwirzen, K. & Cwirzen, A. Mechanochemical activation of natural clay minerals: an alternative to produce sustainable cementitious binders review. Miner Petrol 113, 449462 (2019). https://doi.org/10.1007/s00710-019-00666-y