Design and Fabrication of Mini Ball Mill [PDF]

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DESIGN AND FABRICATION OF MINI BALL MILL



INTRODUCTION



Ball mills are widely used in comminuting process in mineral industry. The comminuting in the ball mill takes place by impact, friction, and abrasion between rocks and balls inside the mill during rotation Steel balls are charged into a cylinder, along with the material to be ground, and rotated, allowing the balls to crush material which travels between them. Without the balls in the cylinder, or some other media to crush the material we wish to grind, little grinding will take place. To ensure the stability of the material in the ball mill, many parameters of the equipment should be adjusted often , such as, the speed of the scatter machine, the quantity of the feeding material, the speed of the dust collecting machine, the speed of the powder selecting machine, etc .There are balls of different diameter in the mill cavity, which will mill different kinds of material. In mill cavity, big balls will be drive to 1



top of the cavity by centripetal force, then they will fall to bottom of the ball mill cavity, and the balls will rap materials. Small balls will grind material at the bottom of the cavity. Nearly all materials can be processed, including metals, organics and pharmaceutical, as well as composites or low dimensional structures. Ceramic materials can be produced either indirectly or directly via ball milling. Solid solution can also be considerably supersaturated compared to the thermodynamic equilibrium. The optimum ball size distribution is thus determined by the amount and combination of grinding balls of different diameters in the make-up charge. (1) There are three types of grinding media that are commonly used in ball mills, steel and other metal balls, metal cylindrical bodies called cylpebs, ceramic balls with regular or high density. Steel and other metal balls are the most frequently used grinding media with sizes of the balls ranging from 10 to 150 mm in diameter. Cylpebs are slightly tapered cylindrical grinding media with rounded edges and equal length and diameter with sizes varying from 8x8 to 45x45 mm. Their shape is developed to maximize the grinding efficiency due to their high density and specific surface area Ceramic balls with regular density are usually porcelain balls and the high-density balls are made with a high alumina oxide content and they are more abrasion resistant. The basic properties of the milling bodies are their mass and size, ware rate, influence on the particle breakage rate and energy efficiency of the grinding process. The basic condition, which must be met while grinding the material in a mill is that the ball, while breaking the material grain, causes in it stress which is higher than the grain hardness. Therefore, for the biggest grain size, it is necessary to have a definite number of the biggest balls in the charge, and with the decreased grain size, the necessary ball size also decreases (For each grain size there is an optimal ball size. Particles migrate slowly along the length of the mill while circulating rapidly in the plane orthogonal to the mill axis. The radial and azimuthal particle motion is assisted by regularly lifter bars attached to the mill shell. Replaceable liner plates are bolted to the shell between the lifters to the reduce wear in this hostile environment. A typical 5 m diameter ball mill consumes around 3 to 4 MW of power and has a very low energy efficiency. Significant economic and environmental benefits can be obtained by improving the efficiency even modestly. 2



Whenever two grinding balls collide, a small amount of powder is trapped in between them. Typically, around 1000 particles with an aggregate weight of about 0.2 mg are trapped during each collision. The product particle size and circulating load are sensitive to various disturbances which can be divided into external and internal ones. The external disturbances include the variations of ore hardness and feed particle size, and so on, while the internal disturbances are generally caused by model mismatches and coupling effects. Variations of the ore hardness and feed particle size may cause continuous fluctuations of the product particle size. Model mismatches and coupling effects may affect the dynamic features and even result in unstable control of the grinding circuit. Mechanical alloying is a complex process and hence involves optimization of a number of variables to achieve the desired product phase and/or microstructure. Some of the important parameters that have an effect on the constitution of the powder are, type of mill, milling container, milling speed, milling time, type, size, and size distribution of the grinding medium, ball-to-powder weight ratio, extent of milling the vial, milling atmosphere, process controf agent, and temperature of milling. All these process variables are not completely independent. For example, the optimum milling time depends on the type of mill, size of the grinding medium, temperature of milling, ball-to-powder ratio, etc. An increase in the number of balls had a minimal impact on the milling energy during BPR increment for two reasons: First, the weight loss of the ball led to a decrease in its kinetic energy and a consequent reduction in milling efficiency. Second, the degree of filling of the mill was raised so the balls mobility became more difficult and as a result the kinetic energy of the balls was reduced .The ball mill with double inlets and outlets is a complicated object with multivariable coupling, nonlinear and time-delay, and the mathematical mode is difficult to be set up due to its complicated dynamic characteristic, and no way can be found to understand the dynamic characteristic of the ball mill through generally experiments. The milling efficiency decreases with increased milling time as the particles become smaller. It is assumed to subside completely when the grain size reaches a critical value. This is a consequence of the force applied to the slurry as two milling balls approach one another, causing a slurry flow away from the balls prior to collision. The smaller the particle, the more likely it is to be caught in the slurry flow. The change of mechanical 3



properties of the powder particles as they change their microstructure: Questions that frequently will be asked is how do the mechanical properties change while forming composites forming solid solutions, or forming new phases? The energy balance of the mill: how much energy is consumed to achieve a particular particle microstructure? Where does the energy go? The energy determined is transferred from the milling balls to the powder particles as many times as the balls hit the wall of the vial. The impact frequency is the number of times the ball hits the vial in one second. Corrosion may also occur in ball milling due to high metal environment. Presumably, a ball mill employs a comminution mechanism combining the compressional and torsional stresses, which enables a product in the submicron size range to be produced. Many researchers have attempted to describe the relationship between energy input and size reduction in this process and find a theory that applies. (2) The degree of milling in a ball mill is influenced by; 1) Residence time of the material in the mill chamber. 2) The size, density and number of the balls. 3) The nature of the balls (hardness of the grinding material) 4) Feed rate and feed level in the vessel. 5) Rotation speed of the cylinder.



Objectives Our objectives for this project are: 1. To design and fabricate a low-cost mini ball mill that capable to work efficiently. 2. To fabricate mini ball mill that can grind various type of materials into powder form.



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LITERATURE REVIEW In metallurgy, ball mill machine is a type of grinder used to grind and blend many materials into fine powder [1]. It is used to grind many kinds of raw materials. There are two ways of grinding: first the dry way and the second is the wet way. The mills break-up the grains in different ways, according to their operation principles, so the kinetics of the procedure is different too [2]. Beside that there is another type of ball mill machine called mini ball mill or planetary ball mill [3]. Planetary ball mill are smaller than common ball mills and mainly used in the laboratories for grinding sample material like ceramic or other raw material to a very small size/ powder. Usually planetary ball mill consist at least one grinding jar and must be operated in a closed system. For the more efficient grinding there is some specific operating speed that needed. When it is, controlled by the speed, the load nearest the wall of the cylinders will break and it quickly followed by other particle in the top curves and form a sliding stream containing several layers of balls separated by material of varying thickness [4] . There is some action caused by the turning of individual balls or pebbles and secondary movements having the nature of rubbing or rolling contacts occur inside the cylinder. Moreover, in this type of mill, it has been considered as high energy. It is because, the milling stock and balls come off the wall and the effective centrifugal force reaches up to twenty times gravitational acceleration [5]. 6xxx Al-Mg-Si alloys have been widely used in automotive body panels as main substitute materials for weight reduction. These alloys have a combination of good formability, good corrosion resistance and remarkable strengthening potential due to the formation of a large number of nano-sized [6]. In ball milling, the transmission of grinding energy from the steel balls to the particles in the slurry is closely linked to the way the media and the slurry are mixed. One of the key parameters that influence this mixing is related to the pulp rheological properties, and more precisely its viscosity [7,8,9]. The most important point is that the choice of ball size or ball size mixes to be used is dictated by some form of experience [10]. Once the material has reached the required fineness, it is sent to the next processing stage. The downstream process therefore determines what the optimum mill product fineness should be [11,12,13]. The performance of a ball mill is measured with reference to the quantity of undersize or fines (amount of grounded material passing through 120 mesh screen) per revolution of the mill, collected for variation of each of the individual parameters [14 & 15]. However, the first direct photo- and high-speed video-recording of the ball motion led to the conclusion that the process should be primarily described in terms of attrition and wear and not in terms of impact [16]. An approximated linear 5



transfer function model was reported in. Mill modelling using system identification method was reported by Corti et al. in 1984 [17-19]. Ball mill speed is a factor which effects directly capacity, product size, energy and material costs [20-26]. A wide variety of powders can be processed using ball milling to achieve nanostructured materials [27-31]. One possible route for the production of nanometric pow-der to produce these new composites is the reactive milling that is a synthesis process in which high-energy milling promotes the reaction in a mixture of reactive powders [32-34]. One of the most simple and effective methods of powder preparation is the mechanical dispersion by means of ball mills which have found considerably wider application in the last 20 years [35].



Factors Used for Proper Grinding



A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless-steel balls. An intemal cascading effect reduces the material to a fine powder. Industrial ball mills 6



can operate continuously, fed at one end and discharged at the other end. Large to mediumsized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulley sand belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. Highquality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on the principle of critical speed. Critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus, causing no further grinding. Ball mills are used extensively in the mechanical alloying process in which they are not only used for grinding but for cold welding as well, with the purpose of producing alloys from powders.



Fig 2 Grinding Balls The ball mill is a key piece of equipment for grinding crushed materials, and it is 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 and non-ferrous metals. The ball mill can grind various ores and other materials either wet or dry. There are two kinds of ball mill, grate type and overfall type due to different ways of discharging material. Many types of grinding media are suitable for use in a ball mill, each material having its own specific properties and advantages. Key properties of grinding media are size, density, hardness, and composition.



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Size: The smaller the media particles, the smaller the particle size of the final product. At the same time, the grinding media particles should be substantially larger than the largest pieces of material to be ground.



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Density: The media should be denser than the material being ground. It becomes a problem if the grinding media floats on top of the material to be ground.



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Hardness: The grinding media needs to be durable enough to grind the material, but where possible should not be so tough that it also wears down the tumbler at a fast pace.



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Composition: Various grinding applications have special requirements. Some of these requirements are based on the fact that some of the grinding media will be in the finished product. Others are based in how the media will react with the material being ground.



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Where the color of the finished product is important, the color and material of the grinding media must be considered.



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Where low contamination is important, the grinding media may be selected for ease of separation from the finished product (i.e.: steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). An alternative to separation is to use media of the same material as the product being ground.



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Flammable products have a tendency to become explosive in powder form. Steel media may spark, becoming an ignition source for these products. Either wetgrinding, or non-sparking media such as ceramic or lead must be selected.



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Some media, such as iron, may react with corrosive materials. For this reason, stainless steel, ceramic, and flint grinding media may each be used when corrosive substances are present during grinding. The grinding chamber can also be filled with an inert shield gas that does not react with the material being ground, to prevent oxidation or explosive reactions that could occur with ambient air inside the mill. (3)



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3.1 Grinding Media Charge Motion Inside Mill The motion of the charge, that is the grinding media and the material undergoing grinding, within a mill is of considerable theoretical interest and practical importance, and for these reasons, has been the subject of considerable study by a number of workers, but, even so, no rigid and complete theory, covering all the aspects of the dynamics of the mill charge, has yet been produced. The practical importance of this subject clearly resides in the possibility of the prediction of the grinding behaviour, and other such characteristics, of a mill from the knowledge of the trajectories of the elements of the mill charge. The theoretical interest lies in the study of the dynamics of the system and in the derivation of equations to define the millions of the elements of the mill charge in terms of fundamental quantities such as the size and the speed of rotation of the mill. A simple example of the practical importance of this information is in the use of the knowledge of the trajectories followed by the balls in a mill to determine the speed at which the mill must run in order that the descending balls shall fall on the toe of the charge, and not upon the mill liner. The impact of the balls upon the liner plates can lead to unduly rapid wear of the latter, and so to high maintenance costs.



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Fig 3 Placement of rods A study, from first principles, of the behavior of a mill charge is much simplified if the charge is imagined to be composed of rods, instead of balls, since by this means the complication of any axial motion of the balls is eliminated and the problem is reduced to one in two dimensions. Consider first the motion of a single rod, of diameter d, within a smooth shell of internal diameter D when the shell rotates about a horizontal axis with an angular velocity η radians per second. In such a case the rod will lie near the lowest point of the mill, as in Fig. 2.1 and will rotate at such speed that the peripheral speed of the rod is the same as that of the shell. Furthermore, the displacement of the radius vector joining the Centre of the mill and that of the rod would be such that the work done by reason of the couple formed by this displacement is equal to the energy dissipated in the distortion of the rod and shell at the line of contact. If now two rods are placed within the mill, as in Fig. 2.2, the motions of the surfaces of the rods at the point of contact are in opposite senses and free motion inside the shell is eliminated. In this case the angle between the vertical and the radius vector joining the centre of the mill and the centre of gravity of the pair of rods is much greater than that for a single rod. But, again, equality exists between the work done to rotate the shell and that dissipated in friction at the contact points and in distortion of the metal surfaces. A further increase in the number of rods would enhance this effect 10



until relative rotation between the rods is largely precluded and, in this respect, the charge behaves almost as a solid body. (4) If the speed of rotation of the mill is so low that the effects of centripetal acceleration may be neglected, then the displacement of the center of gravity of the charge will increase.



Fig 4 Rotation of balls in mill If the speed of rotation of the mill is so low that the effects of centripetal acceleration may be neglected, then the displacement of the center of gravity of the charge will increase until either of two possible limiting conditions is reached; these conditions being: 11



1. The tangential force at the shell, necessary to maintain the displacement of the mass center of the charge, becomes equal to the frictional force. When this state is reached the whole charge slips back. 2. The angle of displacement, θ, reaches 30° when, since the centers of the rods in each layer then fall on vertical lines, the heap of rods collapses. In fact, this is a trivial case, since the effects of centripetal acceleration profoundly affect the motion of the elements of the charge, but for slow speeds of rotation the motion of the charge in a practical mill approximates to that given by case (2) above. In this motion the balls (or rods) will travel on circular arcs, concentric with the shell of the mill, until the point of instability is reached, after which they roll down the surface, which is inclined at about 30° to the horizontal, in a series of parallel layers. This motion is shown on a photograph taken through the transparent end of a model mill in Fig. 2.4. It will be noticed that a small “vortex” exists towards the middle of the charge. At higher speeds of rotation, the balls no longer roll down the surface of the charge but, at a certain point, are projected into space and thereafter describe approximately parabolic paths before again meeting the ball mass; these ball paths being as shown in Fig. 2.5. There appear to be no universally adopted names for these two types of motion of the charge, but the evidence appears to be in favor of “cascading” for the first type and “cataracting” for the second type. These names will be adopted for the present work. 



Cascade mode motion – speed mode with a rolling of grinding balls, but without they flight.



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Mixed mode motion – speed mode with a partial rolling and a partial flight of grinding balls.



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Waterfall mode motion – speed mode with flying of grinding balls.



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Fig 5 Types of motion in ball mill 1)Cascade mode motion of grinding balls carried out at low drum speed. At start-up of a mill, the grinding material rotated by a certain angle and grinding balls start to move by closed path. The curved surface of natural slope is close to the plane inclined at some angle to the horizontal. This angle is equal to a limit angle of rotation. In this mode, the ground material remains in this position, but the grinding balls continuously circulate, rise on circular trajectory and “cascade” roll to the reference point. There is a zone or “core” in the central trajectory of the grinding material. This zone is inactive. In cascade mode grinding occurs as a result of crushing and abrasive actions by grinding balls. This mode used in the ball mill with a central discharge. 2)Mixed mode motion of grinding balls participate balls positioned between the outer layers and inactive “core”. The circulation of grinding balls occurs around an inactive “core”. 3)Waterfall mode motion of grinding media in the mill carried out by the drum rotation speed, ensures the transfer all of the grinding balls layers from a circular to a parabolic trajectory. In this mode, grinding balls rise on circular trajectory and at certain points deviate from it and make a free flight by a parabolic curve.



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3.2 Formula Used for Ball Mill It has been commonly accepted that the critical rotation speed is a function of a ball radius and a jar diameter. However, show that the critical rotation speed significantly depends on ball-containing fraction in jars, and approaches a value asymptotically as the ball-containing fraction approaches to one. The critical speed needed for jar is given by Eq. (1)



Where C.S is critical speed, I.D is the internal diameter of jar and d is the size of media. All the units are in inch and speed in revolution per minute (RPM). To determine the desired jar RPM, usually 55% to 75% of critical speed is required. In our experiment, we are taking 75% of the critical speed to maximize our impact energy. To determine how many balls to be put in the jar, we have to find it out using the formula below:



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Due to stainless steel balls have to fill up 25% of the jar volume and we can get the mass of balls needed. Using digital scales, we can measure the mass of balls we calculated. Impact energy on each particle is crucial in our system, insufficient impact energy might cause the failure of the grinding process. To determine the energy acting on the specimens, we assume that no energy loss during the rotational motion:



Where m is mass of balls, v is velocity of the balls, I is the moment of inertia, is the rotational speed. After obtained the total kinetic energy of the system, we divide it by the mass of the balls to get the force per kg on the balls.



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3.3 Critical Speed CRITICAL SPEED OF THE BALL MILL



Let a grinding ball of mass m is in motion in a mill of diameter D meters It is at a position making an angle a at the center (called the angle of repose). (5)



The force acting on the ball are described in the figure; 1. The centripetal force acting along C. 2. The force of gravity due to its weight along G 16



3. The resultant force along P The centripetal force is C



The force of gravity is P



To maintain the position of the ball in the mill, the condition C ≥ P should be satisfied,



if the angle becomes 90o



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where D is in meters Generally, the mill speed is maintained at 70 to 75% of the critical speed.



3.4 Action of Charge at Slow Speed



Any loose charge piled up will assume a certain definite critical angle, usually called the angle of repose. If more of the charge is added to the top of the cone, this will be increased in size but the same critical angle will be maintained. This is what happens inside a mill revolving at very low speeds. The charge is tilted until the critical angle is reached, after which the balls simply roll down the slope to the lower side of the mill. This critical angle is affected but slightly by a change in the speed of the mill, up to a certain point; the increase in speed simply increases the rapidity with which the charge israised to the top of the incline. In this condition the balls are in contact with one another except as they may bounce in rolling down the slope of the charge; also, the balls must roll down the incline at the same rate, pounds per hour, at which they are raised to the top. Then, with a mill half full of balls, any particular ball will roll down the incline something less than twice per revolution of the mill. As the speed of the mill is slowly increased, the time required to bring the ball back to the top of the pile is diminished, but the time required by it in rolling down remains practically the same. It would seem then that the whole problem of crushing would resolve itself into getting the balls to the top of the heap fast enough. This would be true if it were not for centrifugal force and inertia. As the speed of the mill is increased these two forces grow very rapidly in importance. 18



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EXPERIMENTAL SETUP In this experiment, several materials were purchased and several materials were fabricated. All the materials were then assembled into one system. The base of the whole system was design and fabricated. The motor, gear pulley system, stainless steel balls and mill jar were purchased. The setup consisted of following apparatus and materials: Table 1: List of apparatus and materials used to build ball mill. Amount (units)



Description 324 mm in diameter mild steel (AISI4340) 2 kg of grinded material 25 mm inner diameter ball bearing. Set of 10mm,20mm,30mm and 40mm high-density stainless-steel balls.



1 1 6 60



1



1HP of AC Motor 50hz



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Belt for pulleys (75inches,65inches,60inches)



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Pulleys (3 inches (1), 10inches (2) ,12 inches (1) ,4inches (2)) 20



2



25 mm shaft for pulley system



The present study aims to associate the time taken for the ball mill to grind the materials into nanometre particles with the optimum Speed of the ball mill jar. The main control objectives are as follows. At the first place, it is necessary to maintain a stable operation of the ball mill. The ball mill should operate at a constant and optimum speed which is in the range of 50 rpm to 60 rpm. Within this objective, it is most important to maintain a stable product size distribution measured as a percentage of the output material with size of particles less than gm. Alternatively, the application of forces on the particles in different size of grinding media balls causes the breakages of the particles into nanoparticles.



4.1 Design Typically, the mini ball milling device consists of a cylindrical vessel mounted on an appropriate basis at both ends which allows rotation of the vessel around the centre axis. The mill is driven by an AC motor mounted to the base of the ball mill. A-gear pulley system is used in our model whereby one is attached to the motor the main power supply and the other one is attached to the shaft that is rotates the cylindrical vessel. Using the pulleys, the reduction of the velocity of the motor is take place because a desirable spend is needed to grind the mill. After the mill is charged with the starting material (ore, rock, etc.) and the grinding media (balls), the milling process takes place during rotation as a result of the transfer of kinetic energy of the moving grinding media into the grinding product. The grinding media used is with sizes of the balls ranging from 6 to 30mm in diameter. As in the speed of rotation of the mill, centrifugation 21



characteristic which means very fast rotation is used to grind micro metal powder into nano powders. Furthermore, the mass, volume, hardness, density and size distribution of the material charged in the mill is considered alongside with the mass, density, ball size distribution of the grinding media balls. The influence of grinding conditions on the production of fine particles and the width of the particle size distribution produced during ball mill grinding. As a result, measurement is taken to check whether nano powder is produce or not.



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Fig 6 Section View of the ball mill drum



4.2 AC Motor An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings. We are using AC single phase 1 HP motor for our project. The rpm of the motor will be 1440.



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Fig 7 Single Phase AC Motor



4.3 Pulley System The rpm of the motor is 1440, but we have to rotate the ball mill cylinder at 75% of the critical speed. We have to rotate the cylinder at 57 rpm. So, we have to design a pulley gearing ratio to decrease our rpm. We have to reduce the speed to 1:24. So we will design a compound pullet system for our ball mill machine. We will be using 6 pulleys in our speed reduction pulley system. We will use one 3 inches pulley, two 10 inches pulley, two 4 inches pulley, one 12 inches pulley. N1=1440 RPM D1=3 inches



N2=? D2=10 inches 𝑁1 𝐷1 = 𝑁2 𝐷2 𝑁1 𝐷1 = 𝑁2 𝐷2 N2= 432 RPM



N2=N3=432 RPM N4=? 24



D3=4 inches



D4= 10 inches



𝑁3 𝐷3 = 𝑁4 𝐷4 N4= 172 RPM N4=N5= 172 RPM N6=? D5= 4 inches



D6= 12 inches 𝑁5 𝐷5 = 𝑁6 𝐷6 N6=57 RPM



So, we will get our desired RPM with the help of the Pulley system.



Fig 8 Pulley System Arrangement 25



4.4 Critical Speed Calculation The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shell's inside surface. This is the rotational speed where balls will not fall away from the mill's shell. Critical speed for our ball mill will be-



D= 324mm=0.324m



C.S =



42.3



√0.324



C.S= 74.318 RPM Generally, the mill speed is maintained at 70 to 75% of the critical speed. Which will be equal to approx. 57 RPM.



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EXPERIMENTAL PROCEDURE A laboratory size ball mill of diameter 32.4 cm was used with ball media of sizes 10 mm, 20 mm, 30 mm and 40 mm respectively as shown in Figure 8. Glass was the material used to run experiments. A mill run having a mixture of the 4 ball diameter sizes was also conducted.



Figure 9 Ball Mill Drum



The ball filling level was kept constant at 40% by volume for all ball diameters. The rotational speed of the mill was kept at 85% of the critical speed. The sample ore charge, Q, was such that the glass completely filled the voids or interstitial 27



spaces between the balls, meaning that the mass of the charge varied with the ball diameters. The residence time was material dependent and thus depended on the grindability or hardness of the glass. The products from the four milling processes were screened using a range of different aperture screens to determine the extent of size reduction (comminution ratio) as well as the size distribution of the product samples after each of the four milling processes. For the milling process, the steel balls were first used, the mill was loaded with 14 Kg of the balls as calculated and then 2 kg of a glass sample in the size class. The mill was first run for 30 minutes and then the entire mill load was removed and then a 100gram sample was taken from the glass sample. The remaining sample was then loaded into the mill along with the steel balls and the mill was run for an additional 1 hour, thus making up the second mill run interval of 1 hour 30 minutes. A 100-gram sample was once again taken for this particular milling interval. Once again, the remaining sample was reloaded into the mill and the mill was run for 2 hours. The sampling procedure was once again repeated with a 100-gram sample being taken from the sample. The 20 mm and 10 mm diameter balls were also used during the milling following the above-mentioned procedure. Another mill run was performed using a combination of the four ball diameters and the same milling and sample process was followed.



Fig 10 Glass after milling 28



Uses of Ball Mill  The small and average capacity Ball mills are used for the final grinding of drugs or for grinding suspensions.  Milling of hard materials such as minerals, glass, advanced ceramics, metal oxides, solar cell and semiconductor materials.



Advantages of Ball Mills  



   



It produces very fine powder (particle size less than or equal to 10 microns). It is suitable for milling toxic materials since it can be used in a completely enclosed form. Has a wide application. It can be used for continuous operation. It is used in milling highly abrasive materials. Ball mills cab be used equally well for wet or dry grinding.



Limitations 



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Contamination of product may occur as a result of wear and tear which occurs principally from the balls and partially from the casing. High machine noise level especially if the hollow cylinder is mode of metal, but much less if rubber is used. Relatively long time of milling.



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CONCLUSION



For our objectives, we are able to achieve the first objective which is to fabricate a mini ball mill that can grind specimen into powder form. From our first trial, we were able to gain some product that is not ‘powder’ enough after one hours of grinding. We will improve it and continue with the next trial and achieve our second objective which is determine RPM, time needed and fineness of powder. Throughout our project, we found out that determining all the variables before fabricating is essential. By knowing the required parameter, fabricating become easier as no huge changing of design is required. The mini ball mill should able to grind the material to Nano-powder with the optimum speed and optimum amount of media ball. The rotation will be calculated to ensure a correct grinding occurs.



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REFERENCES



1 Longhurst, D. Economics and methodology of ball mill media maintenance. in Cement Industry Technical Conference, 2010 IEEE-IAS/PCA 52nd. 2010. IEEE. 2 Chattopadhyay, P., et al., A mathematical analysis of milling mechanics in a planetary ball mill. Materials Chemistry and Physics, 2001. 68(1): p. 85-94. 3 https://en.wikipedia.org/wiki/Ball_mill 4https://www.911metallurgist.com/blog/grinding-media-charge-motion-insidemill 5 https://vdchari.com/critical-speed-of-the-ball-mill/



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