BALL MILLING MECHANISM OF GRAIN SIZE REDUCTION MECHANICAL ATTRITION SUBMITTED BY: ADITYA BHARDWAJ (12025101) M.TECH NANO 5 YRS SUBMITTED TO: DR. MANDEEP SINGH

CONTENTS Ball milling-Introduction, use , principle and construction, method, application, merits and de-merits and conclusion. Mechanism of grain size reduction-introduction, grain growth and its rules, mechanism, principle and various size reducing instruments. Mechanical attrition- introduction, principle and process, attrition devices and conclusion.

BALL MILLING

INTRODUCTION Ball milling is a method of production of nano materials. This process is used in producing metallic and ceramic nano materials. These mills are equipped with grinding media composed of wolfram carbide or steel. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium.

USE OF BALL MILLING Generation of curved or closed-shell carbon nanostructures , carbon scrolls by ball-milling of graphite Nanoporous carbon by ball milling Carbon nanotubes and carbon microspheres by high energy ball milling of graphite. Highly curved carbon nanostructures produced by ball-milling.

PRINCIPLE & CONSTRUCTION It works on the principle of impact, i.e., size reduction is done by impact as the balls drop from near the top of the shell. Construction A ball mill consists of a hollow cylindrical shell rotating about its axis. The axis of the shell may be either horizontal or at a small angle to the horizontal. it is partially filled with balls. The grinding media is the balls which may be made of steel (chrome steel), stainless steel or rubber. The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. less wear takes place in the rubber lined mills. The length of the mill is approximately equal to its diameter.

METHOD The method consist of loading graphite powder (99.8% purity) into a stainless steel container along with four hardened steel balls. The container is purged and argon gas (300 kPa) is introduced. The milling is carried out at room temperature for up to 150 hours. Following milling, the powder is annealed under a nitrogen (or argon) gas flow at temperatures of 1400 degree Celsius for six hours. The mechanism of this process is not known but it is thought that the ball milling process forms nanotube nuclei and the annealing process activates the nanotube growth. Multi-walled nanotubes are formed but single –walled nanotubes are more difficult to prepare .

ILLUSTRATIONS

APPLICATIONS The ball mill is used for grinding materials such as coal, pigments, and felspar for pottery. Widely used in production lines for powders such as cement, silicates, refractory material, fertilizer, glass ceramics, etc. as well as for ore dressing of both ferrous non-ferrous metals. 

MERITS AND DE-MERITS Advantages The cost of installation, Power required and grinding medium is low; it is suitable for both batch and continuous operation, similarly, it is suitable for open as well as closed circuit grinding and is applicable for materials of all degrees of hardness. Disadvantages Bulky size ,running a strong vibration and noise, there must be a solid foundation ,low efficiency, energy consumption is relatively large ,greater friction loss.

CONCLUSION The significant advantage of this method is that it can be readily implemented commercially. Ball milling can be used to make carbon nanotubes and boron nitride nanotubes. It is a preferred method for preparing metal oxide nano crystals like Cerium(CeO2) and Zinc Oxide (ZnO).

MECHANISM OF GRAIN SIZE REDUCTION

INTRODUCTION Grain- portion of material within which arrangement of atoms is identical, however, orientation of atom or crystal structure is different for each grain. Grain Boundary- surface that seperates individual grains, is a narrow zone in which atoms are not properly spaced.

GRAIN GROWTH Grain growth is the increase in size of grains (crystallites) in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved by reducing the total area of grain boundary. The term is commonly used in metallurgy but is also used in reference to ceramics and minerals.

RULES OF GRAIN GROWTH Grain growth occurs by the movement of grain boundaries and not by coalescence (i.e. like water droplets) Boundary movement is discontinuous and the direction of motion may change suddenly. One grain may grow into another grain whilst being consumed from the other side The rate of consumption often increases when the grain is nearly consumed A curved boundary typically migrates towards its centre of curvature When grain boundaries in a single phase meet at angles other than 120 degrees, the grain included by the more acute angle will be consumed so that the angles approach 120 degrees.

SIZE REDUCTION Smaller particle size and increased surface area leads to: More uniformity of weight and drug content More impalpability and spreading of dusting and cosmetic powders More stability of suspensions, increasing rates of reaction, drying, extraction, ..etc

DISADVANTAGE OF GRAIN SIZE REDUCTION Possible change in polymorphic form Possible degradation by heat Less flowability Static charge problems Air adsorption, hence, less wetting

PRINCIPLE Strengthening by grain size reduction is based on the fact that crystallographic orientation changes abruptly in passing from one grain to the next across the grain boundary. Thus it is difficult for a dislocation moving on a common slip plane in one crystal to pass over to a similar slip plane in another grain, especially if the orientation is very misaligned.

MECHANISM OF SIZE REDUCTION Compression: positive pressure, e.g., nut crusher Impact: material is stationary and hit by an object ,e.g., hammer mill Shear: cutting force, e.g., scissors Attrition: breaking the edges of the solid either by impact or particle collisions

SIZE REDUCTION INSTRUMENTS Hammer mill Ball mill Fluid energy mill Colloid mill

HAMMER MILL Intermediate crusher Feed P.S. 5-50mm, Product P.S. 0.1-5 mm Size control: Hammer speed, screen size, feed size Use of deflectors to present the particles into hammer impact Advantages: Ease of cleaning, Minimum scale-up problems Disadvantages: Clogging of screen, heat build up on milling, Mill and screen wear

FLUID ENERGY MILL Ultra-fine particle size production Feed P.S. :5OO microns Product P.S. :1-10 microns (Ultra-fine) Grinding medium: superheated steam or compressed air Mechanism of size reduction: shear and impact Built in classifier Advantages: 6000 kg feed milled per hour, no contamination. Disadvantages: quite expensive, not suitable for soft, tacky& fibrous materials.

COLLOID MILL Reduce size of a solid in suspension Reduce the droplet size of a liquid suspended in another liquid This is done by applying high levels of hydraulic shear. It is frequently used to increase the stability of suspensions and emulsions

COLLOID MILL CONTD......... The mill consists of a rotor and stator Clearance from zero to1.25mm Thin film of material is passed between working surfaces, subjected to high shear

GENERAL CHARACTERISTICS OF VARIOUS TYPES OF MILLS S.NO NAME OF MILL MECHANISM OF ACTION USES 1. HAMMER MILL IMPACT USED FOR ALMOST ALL DRUGS 2. ROLLER MILL ATTRITION AND COMPRESSION SOFT MATERIALS 3. COLLOID MILL IMPACT AND ATTRITION FOR ALL DRUGS AND BRITTLE MATERIALS 4. FLUID ENERGY MILL IMPACT AND ATTRIRION FOR ALL DRUGS 5. BALL MILL MODERATELY HARD AND FRIABLE MATERIALS

MECHANICAL ATTRITION

INTRODUCTION It is a Top-Down process. This process produces nanocrystalline nanoparticles and coarser particles that contain nanocrystals and used in fields, such as ,material classes, including metals, ceramics, and polymers. metallurgy industries for many years. The objectives of milling include : particle size reduction (comminution or grinding); shape changing (flaking); agglomeration; solid-state blending (incomplete alloying); modifying, changing, or altering properties of a material (density, flowability, or work hardening); and mixing or blending of two or more materials or mixed phases. However, the primary objective of milling is often purely particle size reduction.

PRINCIPLE & PROCESS The fundamental principle of size reduction in mechanical attrition devices lies in the energy imparted to the sample during impacts between the milling media. This model represents the moment of collision, during which particles are trapped between two colliding balls within a space occupied by a dense cloud, dispersion, or mass of powder particles. FIG 1 Model of impact event at a time of maximum impacting force, showing the formation of a microcompact. Reprinted with permission from [28], E. Kuhn, ASM Handbook, Vol. 7, Materials Park, OH, 1984. © 1984, ASM.

Figure 2 shows the process of trapping an incremental volume between two balls. Compaction begins with a powder mass that is characterized by large spaces between particles compared with the particle size. The first stage of compaction starts with the rearrangement and restacking of particles. Particles slide past one another with a minimum of deformation and fracture, producing some fine, irregularly shaped particles. The second stage of compaction involves elastic and plastic deformation of particles. The third stage of compaction, involving particle fracture, results in further deformation and/or fragmentation of the particles.

FIG 2 Process of trapping an incremental volume of powder between two balls in a randomly agitated charge of balls and powder. (a–c) Trapping and compaction of particles. (d) Agglomeration. (e) Release of agglomerate by elastic energy. Reprinted with permission from [28], E. Kuhn, ASM Handbook, Vol. 7, Materials Park, OH, 1984. © 1984, ASM.

ATTRITION DEVICES Attrition Mill Also known as the attritor or stirred ball mill. The balls are set in motion by rotation of the central shaft on which secondary arms are fixed. The cylinder itself is fixed. Horizontal Mill. The cylinder rotates around its horizontal axis. The combined effects of the centrifugal force induced by this rotation and gravity cause the balls to rise and fall onto the powder particles.

1D Vibratory Mill. Also known as a shaker mill 1D Vibratory Mill. Also known as a shaker mill. The vessel is set in vertical oscillatory motion. Under this action, the 1-kg ball rises then falls back onto the powder particles. Planetary Mill. The containers are fixed on a table which rotates until the centrifugal acceleration reaches 30 to 50 times the acceleration due to gravity. The containers themselves also rotate in modern mills, this rotation being either coupled or uncoupled with respect to the rotation of the table.

3D Vibratory Mill. Also known as a three-axis shaker 3D Vibratory Mill. Also known as a three-axis shaker. These operate according to the same principle as the 1D vibratory mill, but this time in a more complex way due to the 3 vibrational degrees of freedom. The balls collide with the side walls of the container (friction and impacts), but also with its floor and ceiling.

PROBLEMS OF MECHANICAL ATTRITION The problem of milling atmosphere is also serious and has been found to be a major cause of contamination. It has been observed that if the container is not properly sealed, the atmosphere surrounding the container, usually air, leaks into the container and contaminates the powder. This is particularly problematic for oxidation-sensitive materials, such as pure metals.

CONCLUSION It is a simple and useful processing technique that is being employed in production of nanomaterials or nanocrystals from all material classes. Despite the difficulties of contamination, MA is more widely used than ever and continues to be applied to the formation of nanoparticles and nanocrystalline structures in an ever-increasing variety of metals, ceramics, and polymers.

REFERENCES Wikipedia.org www.slideshare.net The science of engineering and materials book by Donald Askeland, Pradeep Fulay and Wendelin Wright. Callister’s material science book. Nanoparticles from Mechanical Attrition by Claudio L. De Castro, Brian S. Mitchell, Department of Chemical Engineering, Tulane University, New Orleans, Louisiana, USA

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