20 Ball-milling

S.S. islam

epgp books

 

 

 

Ball-milling

 

Ball milling is an economic and facile technique to produce nanosized materials. It is a top-down approach of nanoparticle synthesis which includes mechanical breakdown of large substances into smaller one. It is used in producing metallic as well as ceramic nanomaterials. In this module, the readers will learn the working principle and applications of Ball-mill technique in nanomaterials synthesis.

 

The basic principle of a ball mill is very ancient. However, the machine itself could be produced only after the industrial revolution. Typically, ball mill is a grinder and is often employed blend materials by grinding/crushing them, for potential applications in mineral dressing processes, paints, pyrotechnics, ceramics as well as selective laser sintering. The working of a ball mill is based on impact and attrition: the impact, caused by the balls dropping from top of the shell, breaks down the particles, thereby resulting in reduction in size. Ball mill includes a hollow cylindrical shell which is rotated about its own axis. The axis of the shell is either horizontal or slightly inclined, thereby making a small angle with the horizontal. The shell is partially filled with balls. The balls form the grinding medium of the ball mill. These balls are usually made up of steel, ceramic, flint pebbles, or hard rubber. The inner wall of the shell is generally contains a coating of abrasion resistant material, e.g., manganese steel or rubber. Rubber lined (or coated) ball mills cause less wear to the products. The mill is nearly equal in length and diameter.

 

In continuously operated mills, precursor material is fed from left side at an angle of 60°, and the end product is removed from right side at 30°. When the shell is rotated, the balls lift upwards on the rising side of the mill. After reaching near the top of the shell, the balls fall down, causing an impact on the particles trapped between these balls and the shell surface. This impact reduced the size of these particles.

 

Large and medium sized mills are rotated mechanically on their axes. Small mills usually come with a cylindrical capped vessel sitting on two drive shafts which uses pulley and belt system to transfer rotary motion. Ball-milling finds wide applications in pyrotechnics and for producing black powder. However, some pyrotechnic applications such as flash powder do not use ball-mill due to their sensitivity to impact. Particle size can be reduced to as low as 5nm in high energy mills, though they are quite expensive. Such fine particles have enormous surface area and therefore the reaction rates are greatly enhanced. Further, ball milling is widely employed in mechanical alloy manufacturing wherein they are employed for grinding and cold welding. The rationale behind cold welding is production of alloys from powders.

 

There exists a critical speed at which grinding occurs. This speed is described as the minimum speed required to rotate the balls.

 

The working of a ball mill can be understood as follows: A powder mix is positioned in ball mill and is subjected to high energy collision by the balls (Figure 1). The ball milling process can be summed up as:

 

a. It consists of stainless steel chamber and several small iron, silicon carbide, hardened steel, or tungsten carbide balls to rotate inside the mill.

 

b. Powder of material is put in the steel chamber. The powder is reduced to nanosize using ball mill. A magnet is positioned outside the chamber to apply pulling force on the material. This force raises milling energy as the milling chamber or container rotates the metallic balls.

 

c. The ball and material – mass ratio is generally kept at 2:1.

 

d. These metallic balls impart very high energy to the powder resulting in crushing of the powder. The ball milling process generally takes 100 to 150 hrs to give uniformly crushed fine powder.

 

e. It is mechanical processing technique; consequently the structural as well as chemical changes are caused by the mechanical energy.

 

The size of the nanopowders produced by this technique depends on the speed of rotation of the balls and the dimensions of 2 to 20 nm can be achieved.

 

This method holds the advantage of low temperature synthesis as against the traditional methods employing high temperature synthesis for producing materials. Apart from synthesis of materials, this is a method of altering the circumstances which give rise to occurrence of chemical reactions occurring by varying the reactivity of the as-milled solids (by mechanical activation — growing reaction rates, dropping reaction temperature — of crushed powders) or by means of boosting chemical activities during the milling process (mechanochemistry). This is an approach of encouraging phase alterations in starting powders or reactants whose elements have similar chemical composition: viz. amorphization or polymorphic changes of compounds, disarranging of ordered alloys, etc.

 

The method was devised by Benjamin in the late 1960s when he produced fine uniformly dispersed particles of various oxides, such as Al2O3, Y2O3, etc. in Ni-base super alloys which could not be produced by conventional powder metallurgy. This method has revolutionized traditional synthesis techniques which mandated elevated temperature processings. In addition to synthesizing materials, this technique can modify reaction conditions. As-milled solids readily undergo the chemical reactions owing to the process called mechanical activation where the reaction rate is increased by lowering the reaction temperature of the ground powders. Additionally, chemical reactions can also occur during milling process, termed as mechanochemistry. High energy ball-mill can also induce phase transformations in precursors such as: amorphization or polymorphic transformations of compounds, disordering of ordered alloys, etc., however, all these have similar chemical compositions.

 

Alloys can be prepared by using different equipments such as, attritor, planetary mill or horizontal ball mill. The working principle for all these techniques is same. During alloy formation using ball-mill, two processes simultaneously occur: (a) fracturing, and (b) cold welding of powders. Thus, it becomes imperative to create a balance between these processes for successful alloying.

 

In mechanical alloying, planetary ball mill is commonly employed because it involves very small quantities of precursor powders. Thus, this system is most popular for use in research laboratories. Planetary mill includes one turn disc or table and around four bowls. The turn disc and the bowls rotate in opposite directions to each other. The rotation of bowl on its axis plus the rotation of turn disc, create centrifugal forces. These centrifugal forces operate on the powder mixture and the balls, resulting in fracturing and cold welding of the powder mixture. The milling balls can attain impact energies of upto 40 times more than that caused by the gravitational acceleration, in normal directions. Thus, planetary ball milling can be employed for high speed/energy milling. Figure 1 shows the schematics of ball milling.

 

 

Figure 1 Schematics showing nanoparticle synthesis via ball milling method.

 

In high-energy milling, the powder mixture is subjected to highly energetic impact. Microstructurally, mechanical alloying has four stages:

 

a. Initial Stage: Initially, compressive forces from collisions of balls flatten the powder particles. Micro-forging causes variations in the shapes of individual particles, or cluster of particles, owing to repeated impact of high energy (kinetic energy) milling balls. In spite of this, such deformations of the powder cause do not effectively change the mass.

 

b. Intermediate stage: In this stage, considerable variations occur in contrast to the first stage. Powders experience considerable cold welding. The fine blending of the constituents of powder reduces the diffusion distance to few microns. The dominating processes at this stage are fracturing and cold welding. Though dissolution may occur to a certain extent, yet the alloyed powder does not have homogeneous chemical composition.

 

c. Final stage: This stage exhibits marked refinement and size reduction. Microstructurally, particles appear more homogenous at this stage. This stage marks the formation of true alloys.

 

d. Completion stage: The structure of the powder particles is tremendously deformed and is metastable. The lamellae are not resolvable via optical microscope. Alloying beyond this stage does not cause any improvement in the constituents’ distribution. True alloy is formed which has same composition as the precursor.

 

Factors affecting the milling performance include mill geometry, ratio of angular velocities of the planetary to the system wheel, temperature, grinding atmosphere, chemical composition of powder mixtures, chemistry of grinding tools, etc.

 

Surface and interfacial contamination is the main problem with nanoparticles produced by high-energy milling. This contamination may be caused by the milling tools (milling balls), ambient gases (traces of oxygen, nitrogen, and other rare gases present in the environment). Nonetheless, the milling speed and processing time can be optimized to alleviate the problem of contamination. Additionally, by coating the grinding tools with ductile materials, the chances of contamination can be reduced further. To minimize contamination from ambient gases, the powder mixture is loaded in inert glove box and then the contained is properly sealed by an “O-ring”. Aside from these concerns, other drawbacks of ball-milling include long processing times, no control over particle morphology, possibilities of particle agglomeration, residual strain in the crystallized phases.

 

Despite these limitations, high energy ball milling is extensively used due to simple design, working, and use to provide finely ground particles. Most popular application of ball milling is production of oxides of various metals for applications in sensor devices such as gas sensing.

 

Ball mill is crucial to numerous industries as an equipment for producing extremely crushed materials, e.g. cement, refractory materials, fertilizers, glass ceramic, ore dressing of ferrous as well as non-ferrous metals, etc. Ball mill can be used to grind ores and other materials which can be both wet and dry. On the basis of removal of end products, ball mill can be of two types: (a) grate type, and (b) overfall type. Additionally, the grinding media can be of different types as well. The critical attributes of a grinding medium include size, density, hardness, and composition. We will discuss these properties in detail:

 

a. Size: The size of the grinding media directly influences the dimensions of the produced particles. As the size of milling balls is reduced, the size of final products also decreases. However, the grinding media cannot be made infinitely small. A limitation on the size of grinding media is imposed by the size of the largest pieces of material to be crushed. Thus, the milling balls must be considerably larger than the largest particle in the powder mixture required to be ground.

 

b. Density: The density of the grinding media should more than that of the material being crushed.

 

c. Hardness: The hardness of the grinding media should be such that it is durable enough to grind the material, and it should not be very hard. Very hard media can damage the cylindrical shell at a faster pace.

 

d. Composition: Different grinding applications have different requirements. The possible presence of grinding media in the finished product is also considered, and some applications make use of this grinding media in the end product. Other applications take into consideration the reactions between the grinding media with the material being crushed. These applications include:

  • If the color of produced material is important, the color as well as the material of the grinding media must be considered.
  • If highly pure form of product is required, the grinding media must be selected such that it can be easily separated from the finished product (for example, steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). Alternatively, the media of same material as the material being ground can be used.
  • In case of flammable products being ground, steel media can cause ignition and lead to explosion. In such cases, either wet-grinding, or non-sparking media, e.g. ceramics or lead balls can be used.
  • Certain grinding media (e.g., iron) can react with corrosive materials. In such cases, stainless steel, ceramic, and flint grinding media are often employed.

    To avoid oxidation and explosion, the reactor can be filled with an inert gas which does not react with the material being ground.

 

Applications

 

It is extensively employed to grind materials like coal, pigments, and feldspar for pottery. Both wet and dry grindings are possible, however wet grinding is usually done at low speeds. Ball-milling increases the solid-state chemical reactivity in multiple components systems. Furthermore, it can also be used to produce amorphous materials.

 

Advantages

 

The advantages of ball-mill over conventional synthesis techniques are:

– low installation cost

– low cost of grinding medium

– suitability for both batch and continuous operation

– capable of both open and closed circuit grinding

– can be used on materials of practically any degree of hardness.

 

you can view video on Ball Milling

 

References

  1. http://www.understandingnano.com/nanomaterial-synthesis-ball-milling.html.
  2. https://en.wikipedia.org/wiki/Ball_mill.
  3. Lynch, A.; Rowland C (2005). The history of grinding. SME. ISBN 0-87335-238-6.
  4. US Army, Department of the Army technical manual: military explosives (TM 9-1300-214), p. 10-8.
  5. Takacs, Laszlo (January 2002). “Self-sustaining reactions induced by ball milling”. Progress in Materials Science. 47 (4): 355–414. doi:10.1016/S0079-6425(01)00002-0.
  6. Florez-Zamora, M. I.; et al. (2008). “Comparative study of Al-Ni-Mo alloys obtained by mechanical alloying in different ball mills” (PDF). Rev. Adv. Mater. Sci. 18: 301.
  7. Mechanical Alloying Technology, Institute of Materials Processing.

    Review Questions

  1. Can planetary ball mill grind particles to the size smaller than 0.1 micron?
  2. How to choose the right jar material?
  3. Will particles tend to agglomerate when they gets to 1um range? How to prevent agglomeration?
  4. How to tell the size distribution of the particles?