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high-energy ball milling - an overview | sciencedirect topics

high-energy ball milling - an overview | sciencedirect topics

High-energy ball milling is a ball milling process in which a powder mixture placed in a ball mill is subjected to high-energy collisions from the balls. High-energy ball milling, also called mechanical alloying, can successfully produce fine, uniform dispersions of oxide particles in nickel-base super alloys that cannot be made by conventional powder metallurgy methods. High-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place, either by changing the reactivity of as-milled solids or by inducing chemical reactions during milling [20].

High-energy ball milling is a mechanical deformation process that is frequently used for producing nanocrystalline metals or alloys in powder form. This technique belongs to the comminution or attrition approach introduced in Chapter 1. In the high-energy ball milling process, coarse-grained structures undergo disassociation as the result of severe cyclic deformation induced by milling with stiff balls in a high-energy shaker mill [8,9]. This process has been successfully used to produce metals with minimum particle sizes from 4 to 26nm. The high-energy ball milling technique is simple and has high potential to scale up to produce tonnage quantities of materials [8]. However, a serious problem of this technique is the contamination from milling media (balls and vial) and/or atmosphere. Therefore, a number of improvements, including the usages of surfactants, alloy-coated milling media, and protective atmospheres, have been developed to alleviate the contamination problem [8].

The fine powder (in nano or submicron sizes) produced from ball milling can be consolidated to bulk form for large-scale applications such as hip implants and bone screws. Usually, the fine powders are compacted and sintered together via methods like hot isostatic pressing and explosive compaction under the temperatures or conditions that suppress grain growth and maintain nanocrystalline microstructure [8,10]. Bulk metallic materials produced by this approach have achieved the theoretical densities of nanocrystalline materials and greatly improved mechanical properties compared to their conventional, micron-grained counterparts.

High-energy ball milling is effective in getting well-dispersed slurry.79 The preparation procedure is summarized in Fig.24.2. First, commercially available PZT powders (APC 850) were high-energy ball milled to get the desired particle size. Secondly, a selected dispersant was added to the milled powders to get the surface-modified powders. The smaller the powder, the more important this procedure. Afterwards, PZT precursor solution was added to these surface-modified powders and mixed by further ball milling. Finally, the resultant uniform slurry was ready for further processing, such as spin coating, tape casting, screen printing and molding. The recipe for the slurry, including the concentration of xerogel solution and powder to solution mass ratio, depends on the further processing method employed. For our convenience, the recipes for the slurry were given four numbers with regard to the above two important parameters. For example, in 3025, the first two numbers represent the concentration of the xerogel solution9 in weight percent, i.e. 30wt%, and the last two numbers represent the mass ratio of the added PZT powder to xerogel solution, namely 2 to 5.

High-energy ball milling, also called mechanical attrition, can be used to reduce the grain size of materials from many micrometers to 220nm (see Mechanical Alloying). This is a result of the cold-working process creating large-angle grain boundaries. Most of the reduction in grain size occurs rapidly, but the process slows, and long times are required to reach the smallest sizes. This process has the advantage of being relatively inexpensive and can be easily scaled up to produce large quantities of material. Usually, to maximize the energy of collision, high-mass hard-steel or WC balls are used. Contamination by materials removed by the balls is a major concern. Severe mechanical deformation and plastic deformation at high strain rates (103104s1) occurs during the process. Initially, shear bands are formed consisting of a high density of dislocations. Later these dislocations annihilate and recombine as small-angle grain boundaries forming nanometer-sized grains. Finally, the orientation of these nanometer-sized grains is randomized.

The range of solubility of multicomponent systems is greatly increased by mechanical attrition. Mechanical attrition can also produce metastable materials. If the milling is done in the presence of O2 or N2, oxides or nitrides can be formed.

High-energy ball milling, a predominantly mechanical process, nevertheless results in significant structural and chemical changes in the material. Nonequilibrium synthesis of materials at low temperatures via ball milling is possible through a combination of multiple processes, which occur during milling. These processes include thermal shock, high-speed plastic deformation, mechanical grinding and fracturing, cold welding, and intimate mixing [9].

BNNTs were typically synthesized by the prolonged (approximately 150 h) high-energy milling of pure boron or h-BN powder using stainless-steel milling vessels and hardened steel balls in a pressurized (2.3 103 Torr) NH3 atmosphere. The milled material was then annealed at high temperature (>1000 C) in an N2 atmosphere for 10 h. It was found that large quantities of BNNTs can be synthesized using this method. The yield of the BNNTs depended on the duration of the milling treatment [11]. It was proposed that nanotube formation by this method was caused by two different mechanisms. The first mechanism being the nitridation of B nanoparticles in the NH3 atmosphere, which in turn served as nucleation sites for the formation of BNNTs. The second mechanism proposed was that the Fe (and other metals such as Cr and Ni) from the milling process was incorporated into the B powder during high-energy milling and that the metal particles then served as catalysts for BNNT growth [11]. In order for these two mechanisms to operate effectively, it is necessary that both the ball milling and annealing steps be carried out for long times. Other variations of this technique have been reported including the use of tungsten carbide (WC) balls, and a mixture of NiB and alumina [5,12]. Even though the yield of BNNTs can be very high using this method, the resultant nanotubes can suffer from contamination and structural defects. Figure 8.6 shows a micrograph of BNNTs synthesized using this technique.

Figure 8.6. Transmission electron microscope image of BNNTs synthesized by the ball milling process. The growth of the nanotubes from the milled material is clearly evident. The largest nanotube imaged has a bamboo-like morphology.

High energy ball milling can lead to glass formation from elemental powder mixtures as well as by amorphization of intermetallic compound powders. Solid state amorphization by high energy milling has been demonstrated in a number of Ti- and Zr-based and other alloy systems such as NiTi, CuTi, AlGeNb, SnNb, NiZr, CuZr, CoZr and FeZr. The process of ball milling is illustrated in Figure 3.56. Powder particles are severely deformed, fractured and mutually cold welded during collisions of the balls. The repeated fracturing and cold welding of powder particles result in the formation of a layered structure in which the layer thickness keeps decreasing with milling time. A part of the mechanical energy accumulates within these powder particles in the form of excess lattice defects which facilitate interdiffusion between the layers. The continuous reduction in the diffusion distance and the enhancement in the diffusivity with increasing milling time tend to bring about chemical homogeneity of the powder particles by enriching each layer with the other species being milled together. The sequence of the events that occur during milling can be followed by taking out samples from the ball mill at several intervals and by analysing these powder samples in respect of their chemical composition and structure. Let us describe one such experiment in which elemental powders of Zr and Al were milled in an attritor under an Ar atmosphere.

Elemental powders of Zr and Al of 99.5 purity, when milled in an attritor using 5 mm diameter balls of zirconia as the milling media and keeping the ball to powder weight ratio at 10:1, showed a progressive structural change as revealed in XRD patterns (Figure 3.57(a) and (b)). Diffraction peaks associated with the individual elemental species remained distinct upto 5 h of milling at a constant milling speed of 550 rpm. All particles and the balls appeared very shiny in the initial stages. With increasing milling time, the particles lost their lustre, the 111 and 200 peaks of fcc Al gradually shrunk and the three adjacent low-angle peaks of hcp -Zr, corresponding to1010, 0002 and1011, became broader. After about 15 h of milling, XRD showed only -Zr peaks which shifted towards the high angle side, implying a decrease in the lattice parameters resulting from the enrichment of the -Zr phase with Al. After 20 h of milling, all Bragg peaks except one broad peak close to the{1010} peak disappeared. Powders milled for 25 h showed an extra reflection corresponding to a lattice spacing of 5.4 nm, which matches closely to a superlattice reflection of a metastable D019 (Zr3Al) phase. On further milling, the powders transformed into an amorphous phase. The sequence of structural evolution could be described as -Zr + Al -Zr (Al) solid solution + Al nanocrystalline solid solution + localized amorphous phase Zr3Al (D019) + -Zr (Al) solid solution + amorphous phase bulk amorphous phase.

Figure 3.57. XRD patterns showing a progressive structural change for different times when elemental powders of Zr and Al of 99.5 purity were milled in an attritor using 5 mm diameter balls of zirconia with a ball to powder weight ratio of 10:1.

The mechanism of solid state amorphization during mechanical alloying has been studied on the basis of experimental observations made on several alloy systems. One of the probable mechanisms, based on local melting followed by rapid solidification, has not found acceptance as evidence of melting could not be seen in experiments. The example of ball milling of elemental Zr and Al powders has demonstrated that the amorphisation process is preceded by the enrichment of the -Zr phase to a level of approximately 15 at.% Al. The solute concentration progressively changes during milling. The various stages encountered in the course of amorphization can be explained in terms of schematic free energy versus concentration plots for the , the metastable D019, and the amorphous phases (Figure 3.58). With increasing degrees of Al enrichment, the free energy of the interface region gradually moves along the path 1-2 (Figure 3.58). Once the concentration crosses the point 2, it becomes thermodynamically feasible to nucleate the Zr3Al phase which has the metastable D019 structure. Although the equilibrium Zr3Al phase has the L12 structure, it has been shown (Mukhopadhyay et al. 1979) that the metastable D019 structure is kinetically favoured during the early stages of precipitation from the -phase. This is not unexpected as the hcp structure and the D019 structure (which is an ordered derivative of the former) follow a one-to-one lattice correspondence and exhibit perfect lattice registry.

Figure 3.58. Schematic free energy concentration plots in ZrAl system for the , the metastable D019 and the amorphous phases illustrating the various stages encountered in the course of amorphization.

With further Al enrichment, as the concentration crosses the point 3, nucleation of the amorphous phase becomes possible. It is to be emphasized that the change in composition occurs gradually from the interface to the core of the particles, with the result that the amorphous phase starts appearing at interfaces while the core remains crystalline. As the Al concentration in the powder particles crosses point 4, each particle can turn amorphous by a polymorphic process. The observed sequence of solid state amorphization in the case of ball milling of elemental Zr and Al powders suggests the occurrence of amorphization by a lattice instability mechanism which is brought about by solute enrichment of the -phase beyond a certain limit (point 4 in Figure 3.58).

The synthesis of materials by high-energy ball milling of powders was first developed by John Benjamin (1970) and his coworkers at the International Nickel Company in the late 1960s [42,43]. It was found that this method, called mechanical alloying, could successfully produce fine and uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys which could not be made by conventional powder metallurgy methods.

It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. Fig. I.7 shows the motions of the balls and the powder. Since the rotation directions of the bowl and balls are opposite, the centrifugal forces are alternately synchronized. Thus, friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling [44].

During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (1) initial stage, (2) intermediate stage, (3) final stage, and (4) completion stage [44].

At the initial stage of ball milling, the powder particles are flattened by the compressive forces caused by the impact of the balls. Microforging leads to changes in the shapes of individual particles, or clusters of particles being repeatedly impacted by the balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.

At the intermediate stage of the mechanical alloying process, a significant change occurs as compared to the initial stage. Cold welding becomes significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.

At the final stage of the mechanical alloying process, more refinement and reduction in particle size becomes evident. The microstructure of the particle also appears to be more homogeneous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.

At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with a composition similar to the starting constituents is thus formed [44].

MA in high-energy ball milling equipment is accomplished by processing an initial powder charge usually comprising a mixture of elemental, ceramic (e.g., yttria for ODS alloys), and master alloy powders, all supplied in strictly controlled size ranges. Master alloy powders are used in order to reduce in situ oxidation of highly reactive species, such as aluminum or titanium alloy additions during processing. The milling medium normally used in commercial systems is a charge of hardened steel balls, typically 2cm in diameter. The ball-to-powder weight ratio is chosen carefully for each mill and powder charge combination, but is typically around 10:1 for commercial systems. Given the enormous surface area, both of the initial powders and the fresh powder surfaces generated during MA processing, control of the milling atmosphere and its purity is essential to avoid undue alloy contamination. The principal protective atmospheres employed during commercial milling of MA powders are usually either argon or hydrogen and this protection generally extends both to pre- and post-MA powder handling. Both the purity of these gas atmospheres and the integrity of gas seals on the milling equipment are essential to control contamination, particularly when processing reactive species. For example, levels of oxide contamination in Ni3Al can double with just a few hours of milling in impure argon. Occasionally, however, deliberate doping of the milling environment has been used to facilitate alloying additions during processing.

The central event during MA is the ballpowderball collision within the milling medium during processing. It is repetition of these high-energy collisions which leads eventually to MA of the powder charge. Intimate mixing and eventual MA of the powder charge occurs in a series of identifiable, more or less discrete stages during processing (e.g., Gilman and Benjamin 1983). For ductileductile or ductilebrittle combinations of starting powders, MA initially proceeds by the flattening and work hardening of ductile powders and fragmenting of brittle constituents, which is followed by extensive cold welding between powder particles, formation of lamellar structures, and coarsening of the powder particle size distribution. Brittle powder fragments are trapped at cold weld interfaces between the evolving lamellas of the ductile constituents and thus, while continuing to comminute, become dispersed. With continued milling a balance, which is dependent on processing parameters and the composition of the constituents, is established between further cold welding and powder particle fracture, leading to relatively stable powder particle sizes.

This balance between welding and fracture is accompanied both by further decreases in lamella spacings and by folding and mixing-in of lamella fragments to produce microstructures typical of MA (Fig. 1). For ODS alloys, powder constituents are milled to the stage where light microscopy examination reveals that lamella spacings have decreased to below the resolution limit (1m). For typical levels of oxide addition (e.g., 0.5wt.% yttria) this criterion ensures average dispersoid interparticle spacings of <0.5m (Fig. 2). In other systems, milling can progress until true alloying occurs. Surprisingly, MA can also be achieved between essentially brittle powder constituents. The mechanisms by which this occurs are less well understood than in systems incorporating at least one ductile powder component. Nevertheless, granular as opposed to interlamellar mixtures of brittle powder constituents do evolve, typically with smaller, harder fragments progressively incorporated to a very fine scale within the less hard constituents, e.g., aluminanickel oxide. Moreover, MA of these brittle constituents can progress to true alloying, as has been demonstrated using lattice parameter measurements on Si28 at.% Ge progressively milled from constituent powders (Davis and Koch 1987).

Figure 1. Polished and etched metallographic section of ODM 751 FeCrAl alloy powders in the fully MA condition, showing the folded lamellar structures typical of material processed by high-energy ball milling (courtesy of D.M. Jaeger).

Figure 2. Transmission electron microscope image showing alignment of a fine-scale dispersion of oxide particles in extruded ODS alloy PM2000. The arrow shows the extrusion direction (courtesy of Y.L. Chen).

Milling of very ductile metals such as aluminum and tin has to be carefully controlled to avoid complete agglomeration of the ductile phase rather than the balance between cold welding and fracture that leads to MA. This is normally achieved by adding precise amounts of organic compounds termed process control agents (PCAs) to the milling environment. Typically waxes or solvents, these compounds that interfere with cold welding progressively break down during milling to become incorporated within the final MA powders (e.g., in aluminum alloys) as fine-scale distributions of carbides or oxides. Similar restrictions to the proclivity for cold welding in ductile powders can be achieved without use of PCAs by milling at low temperatures, e.g., below 100C for aluminum.

The processing equipment used to effect MA by high-energy ball milling of powders originated in mining and conventional powder metallurgy industries. The range of high-energy ball milling equipment divides, approximately, into two categories: small, high-energy laboratory devices, and larger facilities capable of milling commercial quantities of powder. The former category includes SPEX shaker mills and planetary ball mills. Both devices are capable of rapidly effecting MA, but in quantities of powders up to no more than a few tens of grams. SPEX mills vibrate at up to 1200rpm in three orthogonal directions to achieve ball velocities approaching 5ms1. Planetary mills incorporate a rotating base plate upon which are mounted counter-rotating, smaller-radius vials containing the ball/powder charge. The kinetic energy imparted to the ball charge in the planetary mill depends on the base plate and vial radii and angular velocities. Attritor or Szigvari ball mills, depending on their size, can be used either for laboratory or commercial ball milling applications and incorporate a rotating vertical shaft with attached horizontal impellors which stirs a container housing the ball and powder charge. These devices can process batches of up to several kilograms or more of powder through the significant differential movement the impellors generate between the ball and powder charge. Balls can either cascade or tumble when leaving the mill wall during attritor processing, depending on the ball charge and impellor velocity.

The largest commercial devices applied to MA are horizontal ball mills. When these devices exceed several meters in diameter they impart sufficient kinetic energy through ball impacts to effect MA and can process over 1000kg of powder per batch in larger units. Balls either cascade or tumble during processing in these mills depending on rotational speed (see Fig. 3). The time taken to achieve MA scales approximately inversely to the size of the milling equipment used. Hence, milling which might take minutes to accomplish in a SPEX mill could take hours in an attritor or days in a horizontal ball mill. All of these processing routes, however, have very low energy conversion efficiency, in that only a small fraction of the milling energy expended effects microstructural change contributing to the MA process.

Figure 3. Configuration of a horizontal ball mill, showing the release of the powder and ball charge (at angular position ) from the inner wall of the mill rotating with angular velocity (after Lu et al. 1995).

It is worth noting that during MA, powder particles also coat (condition) the ball milling medium, which means that, to avoid cross-contamination of commercial alloys, the repeat use of ball charges is restricted to compositionally similar batches of raw materials.

Mechanical means, such as high-energy ball milling, ultrasonic or jet milling, and others, can have powder prepared into nanoparticles. This is an example of a top-down approach, which is suitable for refractory metals or materials beyond the use of chemical reactions. The disadvantages include the difficulties in classification according to the particle size and serious surface contamination.

Bombarding a metal surface with high-energy balls makes it possible to turn the surface structure into nanoscale; this can improve the abrasion and corrosion resistance of the processed material. Meanwhile, the surface is identical to the bulk material, and thus it does not peel off like nanocoating material. The main mechanism of this method is to produce a large number of defects and dislocations, which further develop into dislocation walls, and thus cut the large crystals into nanocrystalline grains (Figure 5.11).

MCP is normally a dry, high-energy ball milling technique and has been employed to produce a variety of commercially useful and scientifically interesting materials. The formation of an amorphous phase by mechanical grinding of a Y-Co intermetallic compound in 1981 (Ermakov et al., 1981) and its formation in the Ni-Nb system by ball milling of blended elemental powder mixtures (Koch et al., 1983) brought about the recognition that this technique is a potential non-equilibrium processing technique. Beginning in the mid-1980s, a number of investigations have been carried out to synthesize a variety of equilibrium and non-equilibrium phases including supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and amorphous alloys. Additionally, it has been recognized that powder mixtures can be mechanically activated to induce chemical reactions, at room temperature or at least at much lower temperatures than normally required, to produce pure metals, nanocomposites and a variety of commercially useful materials. Efforts have also been under way since the early 1990s to understand the process fundamentals of MA through modeling studies. Because of all these special attributes, this simple but effective processing technique has been applied to metals, ceramics, polymers and composite materials. The attributes of mechanochemical processing are listed below. However, in the present chapter, the focus will be on the synthesis of nanocrystalline metal particles.

Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Waste processing, (b) Metals refining, (c) Combustion reactions, and (d) Production of discrete ultrafine particles

Nanocrystalline materials are single- or multi-phase polycrystalline solids with a grain size of the order of a few nanometers (1nm=109m=10), typically 1100nm in at least one dimension. Since the grain sizes are so small, a significant volume of the microstructure in nanocrystalline materials is composed of interfaces, mainly grain boundaries. That is, a large volume fraction of the atoms resides in the grain boundaries. Consequently, nanocrystalline materials exhibit properties that are significantly different from, and often an improvement on, their conventional coarse-grained polycrystalline counterparts. Compared to the material with a more conventional grain size, that is, larger than a few micrometers, nanocrystalline materials show increased strength, high hardness, extremely high diffusion rates and consequently reduced sintering times for powder compaction, and improved deformation characteristics. Several excellent reviews are available giving details on different aspects of processings, properties, and applications of these materials (Gleiter, 1989; Suryanarayana, 1995a, 2005).

high energy ball mills instead of planetary ball mills

high energy ball mills instead of planetary ball mills

Grinding materials to a nano-scale is an important aspect of quality control and R&D. AZoM speaks to Dr. Tanja Butt from RETSCH GmbH about the benefits of using High Energy Ball Mills instead of Planetary Ball Mills.

In laboratories worldwide, Planetary Ball Mills are frequently used, for both, quality control and R&D. They have a reputation to be the best mills to grind semi-hard to brittle samples to very fine particles in the nanometer range < 100 m, always depending on the samples properties. However, such grinding processes can easily take several hours, again, depending on the sample material. With increasing speed, which means also increasing energy input, the chance to obtain actual nano-sized particles increases, but also the risk of warming and rising pressure inside the grinding jars. A practical solution to avoid that problem especially for temperature-sensitive materials - is to interrupt the grinding process and work with cool down breaks. This, however, increases the total process time usually by factor 2-4. Furthermore, the energy input of Planetary Ball Mills is limited, again due to the warming effects and forces inside the machines. Consequently, not all materials can be ground to nanometer size. To offer a solution, RETSCH developed the High Energy Ball Mill Emax which is able to grind many different substances to particle sizes < 100 m and usually in a much shorter time than required by other ball mills. Ideally, the grinding breaks can be skipped altogether, which results in enormous time savings.

The Emax is an entirely new type of ball mill for high energy milling. The unique combination of high friction and impact results in extremely fine particles within a very short process time. The high energy input is a result of the extreme speed of 2000 min-1 and the optimized jar design. Thanks to the revolutionary cooling system with water, the high energy input is effectively used for the grinding process without overheating the sample. Due to the special grinding jar geometry in combination with the circular movement of the jar with fixed orientation - the sample is thoroughly mixed which results in a narrow particle size distribution. The grinding jar supports are mounted on two discs respectively which turn in the same direction. As a result, the jars move on a circular course without changing their orientation. The interplay of jar geometry and movement causes strong friction between grinding balls, sample material and jar walls as well as a rapid acceleration which lets the balls impact with great force on the sample at the rounded ends of the jars. This significantly improves the mixing of the particles resulting in smaller grind sizes and a narrower particle size distribution than has been possible to achieve in ball mills so far. The time required to obtain a specific particle size is often less than in a Planetary Ball Mill, even in processes where no interruption in the grinding times are required in Planetary Ball Mills, thanks to the more efficient grinding mechanism.

The greatest challenge when developing a high energy ball mill is keeping the temperature under control as the enormous size reduction energy leads to considerable heat built-up inside the grinding jar. RETSCH solved this problem with an innovative integrated water cooling system. Hence, the Emax usually doesnt require cooling breaks which are typical for long-term processes in conventional ball mills, even at low speed. This dramatically reduces the grinding time. The cooling system cools the grinding jars via the jar brackets. This is very effective because heat is more easily discharged into water than into air. The user can choose between 3 cooling modes: in addition to the internal cooling, the mill can be connected to a chiller or the tap to further reduce the temperature. For temperature-sensitive samples, the Emax has a great benefit over Planetary Ball Mills. The software allows the user to carry out the grinding process within a defined temperature range, i. e. it is possible to define a minimum and a maximum temperature. On reaching the maximum temperature, the mill automatically interrupts the grinding process and resumes it when the jar has cooled down to the minimum temperature.

When using conventional ball mills the adequate cycles of grinding and cooling need to be ascertained by empirical trials. This may lead to degeneration of the sample or to unnecessarily long processing times. The Emax, in contrast, allows for variable cycles of grinding and cooling within the defined temperature limits. Thus, the entire size reduction process remains reproducible and is carried out in the shortest possible time.

The final size of a certain material always depends on the samples chemical and physical properties. Hard and brittle samples like quartz or some types of pigments can truly be ground to a nanometer size which means D90 < 100 nm! For example, barium titanate or titanium dioxide can be pulverized to D90 < 90 nm. Other, more difficult materials, like the lubricant graphite, can be ground to smaller sizes (D90 = 2.8 m) than in a Planetary Ball Mill. In principle, the Emax reaches final sizes similar to or better than those which are obtained in Planetary Ball Mills.

Special focus was placed on operating convenience and safety when developing the Emax. The grinding jar lids with integrated safety closure, which are simply screwed onto the jars, ensure absolute tightness for wet grinding processes or in cases of pressure increase inside the jar. The grinding jars are quickly and easily placed in the mill and are safely clamped with the ergonomic hand wheel. A sensor monitors the correct position of the jars before starting the machine. Possible imbalances are permanently monitored; if they become too strong the mill stops automatically and the remaining grinding time is displayed. Grinding parameters such as speed, time, interval operation or temperature control are quickly and conveniently set via the color touchscreen. The temperature is displayed during the entire grinding process. The user can store up to 10 grinding programs for routine operations.

Yes, definitely! A huge application field for the Emax is mechanical alloying mainly for R&D. For materials which cannot be alloyed by fusion, mechanical alloying is carried out in ball mills which provide high energy input through impact and friction. Trials have shown that the alloying process in the Emax takes considerably less time than in, for example, a Planetary Ball Mill. Further advantages include a better transformation rate as well as less amorphous particles and less caking of sample material in the jar.

A wide selection of accessories makes the High Energy Ball Mill a versatile instrument.There are grinding jars of three different materials stainless steel, zirconium oxide and tungsten carbide - ensuring contamination-free sample preparation. The first two materials are available in 50 ml jars and in 125 ml jars, tungsten carbide is available 50 ml jar. RETSCH offers a special aeration cover for the grinding jars designed for applications where a special atmosphere is to be maintained in the jar. The grinding balls are available in stainless steel, tungsten carbide and zirconium oxide. Sizes range from 0.1 mm to 15 mm, depending on the material. By selecting the adequate ball numbers and sizes, a wealth of applications can be covered. By continuously measuring pressure and temperature the processes and reactions which take place inside the grinding jar during grinding can be monitored and recorded.

The Emax is not a Planetary Ball Mill it is much more. Higher speed, less warming, finer particles, faster grinding procedures. The high energy input in combination with the unique cooling system provides perfect conditions for effective mechanical alloying or grinding down to the nanometer range. To sum it up, the Emax is faster - finer -cooler!

With more than 100 years of experience RETSCH is the leading solution provider for size reduction and particle sizing technology worldwide. Our philosophy is based on customer orientation and leading-edge technology. This is reflected in instruments whose high-quality components are designed for perfect interaction. Our products not only guarantee representative and reproducible results for size reduction and particle analysis but also allow for easy and comfortable operation. With RETSCH you get: First class product quality thanks to advanced manufacturing methods; Comprehensive application support including free test grindings and product trainings; Excellent sales and service network throughout the world.

The RETSCH website www.retsch.com provides all the details, including a product video of the Emax and an application database. We can also be found at many different trade shows all around the world or people may visit one of our end-user workshops or seminars. To keep our customers up to date with the latest dates and news, we send out a newsletter on a regular basis.

Dr. Tanja Butt studied Biology at the University of Duesseldorf, Germany, and graduated with a Diploma in Biology. After that she earned her PhD in biotechnology at the Forschungszentrum Juelich, Germany. Later, she worked in R&D and project management for different institutions. She joined Retsch in 2013 as Product Manager for the whole milling and sieving product range.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

RETSCH GmbH. (2019, October 30). Nano-Range Grinding with High Energy Ball Mills Instead of Planetary Ball Mills. AZoM. Retrieved on July 08, 2021 from https://www.azom.com/article.aspx?ArticleID=15254.

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ball mills - an overview | sciencedirect topics

ball mills - an overview | sciencedirect topics

A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.

By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.

The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.

where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles as well as collision energy. These forces are derived from the rotational motion of the balls and the movement of particles within the mill and contact zones of colliding balls.

By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).

With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls remain attached to the wall with the aid of centrifugal force is:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).

The degree of filling the mill with balls also influences the productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 30%35% of its volume.

The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.

where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption. A mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation [23].

Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.

Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.

Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.

For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.

With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.

The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.

Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.

When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.

In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).

Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15

Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.

Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.

For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.

In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.

Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.

Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.

For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.

The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.

Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.

Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.

Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.

As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.

Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.

The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.

The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.

The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.

Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.

Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.

Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.

More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling [70]. However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.

In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.

A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.

It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C [71].

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

The 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.[44] Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.

In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal.[58] and El-Eskandarany etal.[59] proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.

In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media.[60] The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.

Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation,[61] using for example cold-rolling approach,[62] as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.[8]

Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial and the effective centrifugal force reaches up to 20 times gravitational acceleration. The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed.

In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).

Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.

Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).

The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.

Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).

planetary milling equipment | planetary ball mills mse supplies llc

planetary milling equipment | planetary ball mills mse supplies llc

Planetary ball mills are much smaller in comparison to common ball mills and are largely used in laboratories to grind sample materials to very small sizes. For this purpose, there are specific types of equipment as can be seen on our website. They are used across several industry fields, including chemicals, ceramics, environmental protection, medicine, mines, and geology.

The noise produced in these ball mills, which make them ideal for laboratory use. Using them, one can grind powder samples in vacuum state as well, if vacuum mill jars are present in the ball mill. One gets to see excellent workmanship on all the equipment available here.

Quality of the equipment available on our website is guaranteed to be excellent. We attach immense importance to customer satisfaction and strive for high levels each time. You can send feedback about products as well, and we will continue to improve them accordingly.

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