Ball Mill free essay sample

CHAPTER 1- INTRODUCTION Ball Mill plays an important role in paint industry. The manufacture of pigmented paint involves the dispersion of the pigments into binder and solvent components. This is an important part in the manufacturing process of paints. For this purpose grinding devices like ball mill, bead mill, sand grinder, roll mill or high-speed grinders are used. Though compared to ball mill the other devices provide better dispersion, they require constant attention, semi-skilled labor, high power requirements and have low rate of production, making the ball mill an important and significant part of paint industry. This report comprises of basic information related to paints, principles of grinding, the use of ball mill in paint industry, factors governing the efficiency of mill and various technological aspects regarding the use of ball mill in paint industry. CHAPTER 2- PAINT 2. 1 Paint It is a coating consisting mainly of resin, a solvent, additives, and pigments. We will write a custom essay sample on Ball Mill or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page It made its earliest appearance about 30,000 years ago. Cave dwellers used crude paints to leave behind the graphic representations of their lives that even today decorate the walls of their ancient rock dwellings [1]. 2. 2 Components of paint Paint comprises of various components. They are as follows: Pigments- Pigments are finely ground particles or powders that are dispersed in paints. They impart qualities such as color and opacity and influence properties such as gloss, film flow, and protective abilities. They can be categorized into two main types: 1. Prime or hiding pigments- They provide whiteness and color. They are also a source of hiding capacity. Eg- TiO2. They also protect the substrate from the harmful effects of ultraviolet light. 2. Inert or extender pigments-Extender pigments or extenders provide bulk at relatively low cost. They impact many properties like sheen, scrub resistance, exterior color retention [2]. Binder- It is the actual film forming component of the paint. It imparts adhesion, binds the pigments together, and strongly influences properties such as gloss, exterior durability, flexibility, and toughness. Binders include synthetic or natural resins such as acrylics, polyurethanes, polyesters, melamine resins, epoxy or oils [2]. Solvent Vehicle- Solvent is used to adjust the viscosity of the paint. Vehicle acts as a carrier for paint application. Usually solvent and vehile are same. The vehile or the solvent is volatile and does not become part of the paint film. It can also control flow and application properties. Its main function is as the carrier for the non volatile components. Eg- water, alcohols, ketones, esters, glycols and ethers [2]. Additives- They impart specific properties like pigment stability, anti-freeze properties and foam control [2]. Misc. Fillers are used to thicken the film, support its structure and simply increase the volume of the paint. Eg-talc, lime. Catalysts, Thickeners, Stabilizers, Emulsifiers, Texturizers, Adhesion promoters, Flatteners (de-glossing agents) are also added [2]. . 3 Important characteristics of paint The important characteristics of paint are as follows [3] :- 1. Hiding or covering power 2. Required color 3. Required sheen or gloss 4. Weather resistance 5. Dirt resistance 6. Blister resistance 7. Resistance to peeling 8. Anti-corrosive properties 9. Proper consistency or viscosity 10. Non-Toxicity 11. Low cost 2. 4 Paint Applications The main purpose of use of paints is to protect products from environmental corrosion and to improve their consumer appeal. The various applications of paint are [1]:- 1. Paints are used as protective coating for equipments and structures in industries. . Paints are used as marine coatings, mainly as coatings to protect new and existing commercial ships or offshore oil and gas rigs and equipments. 3. Paints are used for highway or traffic markings. 4. Paints are used as architectural coatings. 5. Paints are used to protect food and beverages in metal cans from spoilage and contamination. 2. 5 Paint Manufacturing Process The various steps of paint manufacturing process are as follows [4] :- 1. The pigment is mixed with sufficient binder and solvent or vehicle to make a paste, which has the correct consistency for grinding. . The paste is grinded in a mill until the aggregates are broken down, as indicated by the ‘fineness of grind’ test and that the p igment gets dispersed in the paste. 3. After the required dispersion the paste is thinned to the required consistency. 4. Performance enhancers and preservatives are added to improve the properties. 5. Tinting of batch is done to get the required color. 6. Testing is done to determine physical properties and performance requirements. 7. Filling and packing of paints is done. Figure 1 gives a detailed description of the various steps of paint anufacture. Figure 1: Process Flow Diagram of Paint Production[4] CHAPTER 3- GRINDING 3. 1 Grinding Grinding is a unit operation designed to break a solid material into smaller pieces. There are many different types of grinding mills and many types of materials processed in them. Historically mills were powered by hand (mortar and pestle), working animal, wind (windmill) or water (watermill). Today they are also powered by electricity. The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcomin g of the interior bonding forces. After the grinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape. As represented in figure 2, grinding, in which size reduction of the particles takes place, can occur in two ways, either by collision or compressive forces between the particles or by rubbing or attrition forces. Grinding may serve the following purposes in engineering: 1. Magnification of the surface area of a solid 2. Manufacturing of a solid with a desired grain size 3. Pulping of resources [5] [pic] Figure 2: Schematic representation of compressive and attrition forces in grinding [5] . 2 Energy and power requirements in grinding During size reduction the particles of feed material are first distorted and strained. The work necessary to strain them is temporarily stored in the solid as mechanical energy of stress. As additional force is applied to the stressed particles they are distorted beyond their ultimate strength and suddenly rupture into fragments and ne w surfaces are created. Every unit area of solid has a definite amount of surface energy, the creation of new surface requires work which is supplied by the release of energy of stress when the particles break [6]. 3. 3 Grinding laws In spite of a great number of studies in the field of fracture schemes there is no formula known, which connects the technical grinding work with grinding results. To calculate the needed grinding work against the grain size changing three half-empirical models are used: 1. KICK for d 50 mm [pic] Equation- 1 According to Kick’s law (in 1885, based on stress analysis of plastic deformation within elastic limits) work required for grinding a given mass of material is constant for the same reduction ratio, that is the ratio of initial particle size to final particle size. 2. BOND for 50 mm d 0. 5 mm [pic] Equation- 2 According to Bond’s law (in 1952) the work required is inversely proportional to the square root of the diameter of the particles. 3. RITTINGER for d 0. 05 mm [pic] Equation- 3 According to Rittinger (in 1867) work required is proportional to new surface created. It also means that for a given material and a given machine the grinding efficiency is indepe ndent of size of feed and product. In all these postulates W is the grinding work in kJ/kg, c is the grinding coefficient, dA is the grain size of the source material and dE is the grain size of the ground material. A reliable value for the grain sizes dA and dE is d80. This value signifies that 80% (mass) of the solid matter has a smaller grain size. To calculate the KICKs and RITTINGERs coefficients following formulas can be used : [pic] Equation- 4 [pic] Equation- 5 with the limits of BONDs range : upper dBU = 50 mm and lower dBL = 0. 05 mm [5] [6]. 3. 4 Grinding degree To evaluate the grinding results the grain size of the source material (1) and of the ground material (2) is needed. Grinding degree can be expressed by various parameters: 1. Grinding degree referring to grain size d80 pic] Equation 6 Instead of the value of d80 also d50 or other grain diameter can be used. Grinding degree referring to specific surface [pic] Equation 7 The specific surface area referring to volume Sv and the specific surface area referring to mass Sm can be found out through experiments [5]. CHAPTER 4- MILLS USED FOR GRINDING 4. 1 Ball mill A typical type of fine grinder is the ball mill. A slightly inc lined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls. The feed is at one end of the cylinder and the discharge is at the other [5]. 4. 2 Pebble mill A rotating drum causes friction and attrition between rock pebbles and ore particles. It can be used where product contamination by iron from steel balls is to be avoided [5]. 4. 3 Rod mill A rotating drum causes friction and attrition between steel rods and ore particles. The rods range from 25 mm to 125 mm in diameter [5]. 4. 4 Tube mill It is a continuous mill with a long cylindrical shell. Tube mills are excellent for grinding to very fine powders in a single pass when the amount of energy consumed is not of primary importance [5]. . 5 Compartment mill When slotted transverse partitions are used in a tube mill then it is called a compartment mill. The compartments have balls of different sizes. One having large heavy balls, the other small balls and another having pebbles. This segregation of grinding media in the various compartments helps in avoiding waste work as heavy balls break on ly the large particles. The smaller ones fall only on the small particles and not on large lumps, which they cannot work [5]. 4. 6 Conical ball mill The feed enters through a 60o cone into the grinding zone where the diameter of the shell is maximum. Product leaves through the 30o cone. Such a mill contains balls of different sizes all of which wear and become smaller as the mill is operated. New balls are added periodically. As the shell rotates the large balls move toward the point of maximum diameter and small balls migrate to the discharge [5]. Figure 3 gives the detailed description of a conical ball mill. Figure 3: Conical Ball Mill [6] 4. 7 SAG mill SAG is an acronym for Semi-Autogenous Grinding, and applies to mills that utilize steel balls in addition to large rocks for grinding. The SAG mills use a minimal ball charge of 6 to 15%. SAG mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries [5]. [pic] Figure 4: Principle of SAG Mill Operation [5] Principle of SAG Mill operation As shown in figure 5, in SAG mill a rotating drum throws large rocks and steel balls in a cascading motion, which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. SAG mills are characterized by their large diameter and short length. The inside of the mill is lined with lifting plates to lift the material inside up and around the inside of the mill, where it then falls of the plates and falls back down. 4. 8 Autogenous mill A rotating drum throws large rocks in a cascading motion, which causes impact breakage of larger rocks and compressive grinding of finer particles. It is similar in operation to a SAG mill but does not use steel balls in the mill. Attrition in the charge causes grinding of finer particles. It is also known as ROM or Run Of Mine grinding [5]. CHAPTER 5- USE OF BALL MILL IN PAINT INDUSTRY 5. 1 Role of ball mill in paint industry The manufacture of pigmented paint involves the dispersion of the pigments into part of the binder and solvent components. This is the key component of the manufacturing process. For this various grinding devices like balling mill, bead mill, sand grinder, rolling mill or high-speed grinder are used. As compared to ball mill though the other devices provide better dispersion but they require constant attention, require semi-skilled labor, require high power and have low rate of production [7]. 5. 2 Ball Mill The ball mill is a horizontal cylindrical vessel, which is used for size reduction. It can reduce size of a particle upto one-tenth of a micron. It rotates about a horizontal axis. It is partially filled with the material to be grounded and the grinding medium. The various advantages of a ball mill are: 1. It is a well-established technology. 2. It is a simple technology not requiring many moving parts. Hence it is a robust construction. 3. It is possible to recirculate the ground material and achieve finer grinding. It is also possible to regulate the amount of recirculated product to control the final product. 4. Apart from grinding ball mills are also used for mixing. . By using different media in the same drum we can process wide variety of feed [8]. 5. 3 Applications of Ball Mill in paint industry The applications of ball mill in paint industry are as follows [9]: 1. Ball mills are used for reduction of size of pigments. 2. They are used to make pigments uniform in the finished product. 3. They are used for grinding inexpensive coarse extender pigments in the batch, which gives more hiding power. 4. In case of agglomerated pigments resulting from storage and handling, ball mills are used. Steel ball mills and pebble mills are sed in the paint industry. CHAPTER 6- STEEL MILL PEBBLE MILL 6. 1 Pebble Mill The pebble mill can be obtained in various diameters and lengths with almost any capacity desired. The ends or heads of the mill are usually cast iron and the shell is steel. The inside of the cylinder is lined with either porcelain or stone (Silex or French Buhrstone). A shaft extends from center of each head. These shafts ride in very heavy bearings supported on trunions or legs, which can be mounted either on the floor or from the ceiling. The machine is suspended in a horizontal plane. A charging gate is located on one side of the cylinder with a grilled discharge valve on the opposite side. Removable plug is provided in one end for taking the samples. A large ring gear of the same diameter as the mill is bolted to one end of the cylinder for rotating. The grinding charge consists of porcelain balls for the porcelain-lined mills and flint pebbles for stone lined mills. These mills can be used for grinding white gloss and semi-gloss enamels or house paints. The advantage of pebble mill is that there is no discoloration of products because the components do not contact any metal surface. The diameter of the cylinder should be large enough to allow proper cascading of pebbles or balls of proper size when the mill is rotated [7]. Figure 5 shows a photograph of pebble mill, provided by Paul O Abbe Inc. 6. 2 Steel Ball Mills Steel ball mills operate on the same principle as pebble mills but they are jacketed for water-cooling. The shell is made of chrome-manganese steel. The grinding charge is usually composed of chrome-manganese steel balls that are 5/8 in. in diameter. This tough and hard metal is used to reduce wear and minimize discoloration. Water cooling jackets are necessary to remove heat generated by contact of metallic surfaces when the mill is in operation. Fine grinds and better dispersion are usually obtained with ball mills in a much shorter time than with the pebble mills, as the steel balls are heavier and of higher density as compared to flint pebbles or porcelain balls. Hence small size balls can be used thereby providing more grinding contacts for each revolution of the cylinder. However they cannot be used for white or light colored paints as abrasive action of steel on steel can produce free iron to discolor the light colored batches. The discoloration is not significant in case of darker colors. Apart from this steel mills can’t handle materials affected by metallic contamination [7]. Figure 6 shows a photograph of steel mill, provided by Paul O Abbe Inc. Figure 5: Pebble Mill [10] Figure 6: Steel Mill [10] 6. 3 Essential features of Ball and Pebble Mill The essential features of ball and pebble mill are as follows [7]: 1. They are simple to load and unload. 2. They can be operated by unskilled labor. 3. These mills do not require premixing of paste. 4. They can be run during off-peak loads and hence they are economical to operate. 5. They are well suited to continuous batch grinding of the same pigment, hence excessive cleaning between the batches is avoided. 6. They require no attention during operation, hence can be loaded in the afternoon and run overnight and can be unloaded in the morning. 7. However in 24-hour operation, four hours are required to load and unload a mill. 8. If the mill is run for more than one day, it is run for even days plus the original 20-hour interval. 9. They are well suited to continuous batch grinding, hence avoiding excessive cleaning between the batches. . 4 Types of grinding media used in Ball or Pebble Mill There are three types of grinding media that are most commonly used: 1. Flint Pebbles 2. Porcelain Balls regular and high density 3. Steel and other metal Balls Flint Pebbles These are the oldest type of grinding media in use. They can be used with all types of lining and even in the chrome manganese mills. They are exceptionally tough and longwearing and last for many years regardless of the kind of service. Porcelain Balls This is a pure white ceramic material with a dense, highly vitrified body that does not chip or crack in service. High Density Media This is another grinding media developed for ball and pebble Mills. They are made with a high alumina oxide content and have a density 40 to 50% greater than the regular porcelain balls. They are also fired at higher temperature making them harder and more abrasion resistant. High-density media are available in various shapes including spheres, cylinders and ovals resembling the natural flint pebbles. It is advantageous to use high density grinding media when the product is hard to grind and requires all the energy available to break it down or where higher viscosities can be developed as in the case of paint industry. Steel and other Metal Balls Steel balls are used for faster grinding job than any of the other commercially available media. They are especially valuable in the paint industry. The following metals are used to make balls, which are commonly used in ball mills: 1. High carbon high manganese steel with alloying elements or molybdenum, chromium or nickel. 2. Cast Nickel Alloy 3. Stainless Steel 4. Chilled Iron 5. Forged Low Carbon Steel 6. Other more special types include bronze or brass, aluminum, tungsten or carbide [9]. 6. Linings used in Ball or Pebble Mill The various linings, which are used in ball or pebble mill are as follows: Burrstone lining- It is a pure flint lining, noted for its exceptional durability. It is one of the toughest lining materials available for Pebble Mills. Burrstone lined mills can run for a continuous production schedule for more than 25 years without repairs. Apart from its exceptional wearing quality, Burrstone has a natural rough surface, which help s prevent the charge from sliding or slipping thereby insuring a more efficient grinding action. Porcelain lining- This is a pure white ceramic lining which has a dense, tough structure especially developed for Pebble Mill service. It can stand up for many years under the most severe kind of service and is highly recommended where requirements call for a white lining. High density porcelain lining– It is made with a high alumina content, it is the toughest and most abrasion resistant of the synthetic linings and under normal conditions, it will outlast the standard porcelain lining several times. Metal linings- The cylinders of batch type steel ball mills are usually made of abrasion resistant alloy steel like chrome manganese. Apart from this, linings of other metals like chilled iron, manganese, high carbon, stainless steel, bronze can be also used [10]. 6. 6 Operations of Pebble or Ball Mill The various operations of pebble or ball mill are as follows [7]: 1. The pigment and a predetermined portion of the vehicle are placed in the mill. 2. First the liquid or the vehicle is loaded followed with the pigment as it gives better initial wetting and there is less danger of ball formation. If the pigment is loaded first it prevents liquids from penetrating through the pebble voids and makes the mill more difficult to load and an extra session of spinning of mill might be required. Sufficient vehicle should be put into the mill to come just to the top of the ball level. 3. After loading, the mill is then revolved. Loading and discharge closures are checked to ensure that they are tight and do not leak. At the end of the run time (predetermined by number of hours or revolutions required) a sample of paste is checked for fineness of grind. Grinding gauges are used to test the degree of fineness of grind. Removable plug is used to take the sample. If the fineness is not satisfactory the mill is operated for longer time until the correct degree of dispersion is obtained. 4. The mill is then stopped and enough of the remaining vehicle is added to produce a consistency suitable for discharging the ground paste from the mill. 5. The mill is closed again, rotated for 15 to 30 minutes to ensure uniform mixing and then the slurry of paste is discharged either by gravity or by means of pumping device. When pumping is used, the suction line may be attached directly to the discharge valve or cover. A safety screen is provided to prevent damage to the pump by a pebble or ball chip, which might be sucked through the discharge grate by the pump. The vent plug should always be removed so that the pump may do its work without being hindered by the vacuum, which would build up in the mill if the vent were left closed. When blowing, the air line is connected to the vent opening and the air pushes on top of the material forcing it out the discharge opening. 6. When the charge has been removed a portion of the thinning vehicle is put in the mill, which is then closed and rotated again. This is used to wash the mill. 7. The pebble or ball charge is to be periodically checked to ascertain if the correct volume of grinding medium is present and to check for the wear of the pebbles. At least once a year the pebble charge is unloaded from the mill and the pebble or the balls are screened and those who are badly worn, broken, defective or are undersized are replaced. In order to have daily production of 2000 gallons of paste product the following equipments are used: 1. Two 13 x 32 in. high-speed mills each equipped with a 25 hp explosion proof motor. 2. One 13 x 32 in. roller mill equipped with a 20 hp explosion proof motor. 3. One 4 x 5 ft Buhrstone lined pebble mill equipped with a 10 hp explosion proof motor. One 3. 5 x 4 ft chrome manganese steel ball mill equipped with a 10 hp explosion proof motor [7]. 6. 7 Grinding Mechanism The working of pebble and steel ball mill depends on the movement of stone or porcelain pebbles or steel balls within revolving cylinders for dispersion of dry pigments in liquid vehicles. Each mill rotates at such a rate of speed that the pebbles or balls are carried up on the side of the cylinder by centrifugal action, but at a speed that enables the force of gravity or the centripetal force to overcome the centrifugal force and allow the pebbles to fall and cascade over each other. The balls or the pebbles break contact from the wall at the â€Å"angle of break†. Thus the material, which is being grounded, is subjected to crushing by impact of pebbles on pebbles and by slipping or rolling of pebbles against each other and the lining of the cylinder. This process is continued as long as is necessary to obtain the desired degree of dispersion. The speed of rotation should be less than critical speed. At critical speed centrifugal force is so high that balls are carried over. No grinding takes place the mill is said to be centrifuging [7]. Figure 7 gives the pictorial representation of the various forces and the directions in which their components are acting on a particle in a ball mill operation Figure 7: Forces acting on a particle in a ball mill operation [6] 6. 8 Slip in Ball or Pebble Mill The term â€Å"slip† means relative motion between the layer of grinding media nearest the lining and the surface of the lining itself. Slip may be caused by several factors: 1. A smooth inside surface on the lining 2. A low viscosity batch of material 3. Material with a very low co-efficient of friction 4. A very light weight charge of grinding media and material. The effects of slip are detrimental to efficient and economical grinding. The angle of break drops sharply when excessive slip occurs and as a result, the grinding media break away from the periphery of the mill too soon, thereby developing less grinding energy. The grinding time and power costs increase considerably. Some grinding does occur between the shell and the grinding media, but this action is considerably less than that which accompanies the rolling, cascading, sliding, and relative motion in the center of the charge which occurs when the correct angle of break is maintained. Hence it is important to have a correct angle of break to have proper dispersion. The most detrimental effect of slip is that the grinding media in contact with the lining slide around the entire weight of the charge forcing them down onto the lining. As a result the mill interior becomes full of ruts and ridges and wears away very rapidly. Then the lining has to be replaced which is costly, time consuming operation. Apart from this the wear on the lining results in excessive batch contamination. Slip is most prevalent in steel ball mills but it can also create a problem in porcelain-lined mills. To prevent it burrstone lining, with rough surface can be used to prevent the slip problem. In some cases, where slip occurs, it can be removed by increasing the viscosity of the batch. However mechanical means of controlling the slip known as baffle bars are usually used. In steel ball mills baffle bars are made of special, chip -proof, hardened steel. In porcelain -lined mills the baffle bars are made of high-density material. The bars are spaced equally in a horizontal position around the inner circumference of the lining. The quantity used is dependent upon the diameter of the mill and the type of operation involved. They project above the inside lining surface just enough to prevent the first layer of grinding media in direct contact with the lining from slipping backwards on the smooth lining while the mill is turning. Baffle bars with relatively, flat sides are more effective than any other design. The flat sides secure the maximum locking effect along the cylinder wall with the first layer of grinding media held stationary relative to the lining surface until they reach the angle of break. Backward slipping of the media and material is completely eliminated. The outer edges of the baffle bars in contact with the media charge should be smoothly rounded to prevent excessive wear and chipping [10]. The figure number 8 shows the principle of slip, angle of break and the use of baffle bars in a ball mill. pic] Figure 8: Angle of Break, Principle of slip baffle bars [7] 6. 9 Jackets The use of a jacket on a ball or pebble Mill makes possible the control of the batch temperature while the mill is in operation. If mill temperatures are too low, many products will run at much higher viscosities than are recommended. This would lead to slow grinding time. If mill temperatures are too high, lower viscositi es will usually result. This causes excessive wear on linings and grinding media and therefore batch contamination. Another danger of grinding at high temperatures is that of pressure buildup. The jacket is usually fitted around the cylindrical shell only, but it can be also extended to enclose both ends of the mill according to the situation. Fluids as hot water, steam, or hot oil pass through the jacket and maintain the fluid consistency. Cold water or brine passing through the jacket, maintain cool temperatures while grinding. Cooling is necessary for paste production in the mill in paint industry [10]. 6. 10 Venting and pressure in mills The vent plug in the head of the mill has two primary functions: 1. It aids in the loading and discharging of the mill. The use of a vent helps in loading of the liquids in the batch. During unloading, the vent plug should be removed before the cover or discharge valve is opened. The vent permits more rapid discharging. The vent plug is removed so that the pump can discharge the material from the mill without being hindered by the vacuum, which would build up in the mill if the vent were left closed 2. The second use of the vent is for reducing pressure, which may build up in a mill during operation due to volatile liquids or heat being generated in the mill. Ball and Pebble Mills are not normally built for pressure operation. Therefore it is important to make certain there is no excessive pressure built up during the grinding cycle, to avoid damage to the mill end and lining. Special care should be exercised when grinding volatile solvents, as these are most apt to cause high pressures if heat develops while grinding. The mill is vented about one -half hour after starting a new batch and it is followed with such periodic ventings. Periodic venting to prevent pressure buildup has resulted in increased grinding efficiency. It is possible in many cases to withdraw air from the interior of the mill, causing a vacuum. This draws air from the bubbles in the batch, thus removing them. As a result it helps in easier and quicker grinding [10]. 6. 11 Discharge procedure Built-in discharge valves are generally located in the mill directly opposite the manhole opening. When the mill is spotted with the valve on the bottom to discharge, the manhole opening will be at the top. After the finished material is unloaded the discharge valve is closed, the manhole cover is removed, and a new batch of material may be dumped into the cylinder without having to turn the mill [10]. 6. 12 Cleaning of mill Good cleaning procedure is one of the most important factors in avoiding batch contamination. Where cleaning is necessary, the established procedure is to run the mill with a solvent or other liquid so that any of the products remaining around the media or sidewalls of the mill may be picked up and subsequently discharged. A good method of cleaning is to dump a small quantity of solvent into the mill, run it for 15 to 30 revolutions and dump it immediately. If necessary, the cycle can be repeated two or three times. In this way thorough cleaning is accomplished and batch contamination is avoided. The mill should not be run during cleaning for longer period of time. When no protective film is around the grinding media they will wear down very quickly. During a long cleaning run, a considerable amount of contamination is picked upon the surface of the grinding media. When the cleaning material is dumped out, a lot of this contamination remains in the mills. When a new batch of material is loaded, this dirt is absorbed and will come up in the finished product. At the same time the grinding media wear away, needing more frequent replenishing or dumping and sorting and excessive wear can also occur on the lining. Therefore, when cleaning, running time should definitely be kept to a minimum [10]. 6. 13 Maintenance of mill A well-made ball or pebble Mill will give many years of service with a minimum of maintenance costs and trouble. However, as with any equipment having moving parts, periodic maintenance checks are required for smooth performance and for low operating costs. These checkups take very little time or effort and are suggested to prevent any trouble from developing. Some of these checks include: 1. Material should not be allowed to accumulate on the outside of the cylinder. This adds to the weight and throws mill off balance. 2. Threads on the tightening bolts of the cover should be kept clean and occasionally oiled. 3. Lubricants recommended by the manufacturer should be strictly adhered to. 4. Lubricating periods are best determined by the operator and are generally controlled by existing conditions. 5. The level of the grinding media should be frequently checked. If it falls below the correct operating level, the required grinding media should be added. At longer intervals the charge should be dumped and inspected. All grinding media, which are excessively worn or damaged, should be removed and replaced with new media. 6. Even though a Mill may be perfectly aligned when new, occasionally it will settle after being in operation a while, due to the floor sag or settling of foundations. For this reason the alignment should be checked occasionally. 7. The foundation bolts should be examined periodically to see that they are secure [10]. 6. 14 Mill efficiency The controlling factors that govern the grinding efficiency of cylindrical mills are as follows [7]: 1. Speed of mill affects capacity, also liner and ball wear up to 65–75 percent of critical speed. 2. Ball charge equal to 35–50 percent of the mill volume gives the maximum capacity. 3. Minimum-size balls capable of grinding the feed give maximum efficiency. 4. Bar-type lifters are essential for smooth operation. 5. Higher circulating loads tend to increase production and decrease the amount of unwanted fine material. 6. Ratio of solids to liquids in the mill must be considered on the basis of slurry characteristics. 6. 15 Cares during mill operation The various cares to be taken during mill operation are [10]: 1. There should be enough material in the batch to cover the grinding media. 2. Grinding time is to be watched carefully to avoid excessive grinding. 3. Excessive buildup of heat should be avoided. In paint grinding, this may lower the operating viscosity beyond the critical point. A reduction in mill speed may help to avoid overheating, but it is more desirable to circulate a cooling medium around the cylinder. If the mill is not jacketed, a water spray can be used. 4. The smallest possible grinding media should be used. This reduces the danger of overheating. They also provide faster and better results. 5. When using extenders, their abrasive nature may cause excessive wear. To avoid this, extenders are held out until the grinding is almost complete and then they are added for the final operation. CHAPTER 7- FACTORS AFFECTING EFFICIENCY OF MILL 7. 1 Factors affecting the efficiency of the mill The various factors affecting the efficiency of the mill are as follows [7]: 1. Speed of Rotation 2. Relative volume of grinding medium and batch. 3. Size and density of grinding medium. 4. Consistency of batch. 7. 2 Speed of rotation There is a specific operating speed for most efficient grinding. At a certain point, controlled by the mill speed, the load nearest the wall of the cylinder breaks free and forms a cascading, sliding stream containing several layers of balls. The top layers in the stream travel at a faster speed than the lower layers thus causing a grinding action between them. There is also some action caused by the gyration of individual balls or pebbles. A pebble mill of the same size as that of ball mill usually runs at a slightly faster speed. This is due to the smaller inside diameter of the pebble mill with its lining, which is lacking in the ball mill. There is a point where the charge, as it is carried upward, breaks away from the periphery of the mill. This is called the â€Å"break point† or â€Å"angle of break† as it is measured in degrees. It is measured up the periphery of the mill from the horizontal. Usually the angle of break ranges from 50 to 60 degrees from the horizontal with the lower range recommended for wet grinding operations like paint manufacture and the higher break point (which provides a more severe grinding action) for dry materials. Angle of break should be lower for larger mills as compared to smaller mills because of severe grinding action in the mill. While in the old days operating speeds were determined by trial and error, now a days correlations are used to determine the critical speed, which is the speed at which the grinding media, without material, begin to centrifuge. The speed of rotation ranges from 50-60% of critical speed. According to Fischer [7] S=K/(D) ? Equation- 8 Where S is critical speed, K is constant and D is the diameter of the mill in ft. According to Coghill and De Vaney [7] S=54. 18/(R) ? Equation- 9 Where S is critical speed and R is the radius of the mill in ft. The smaller the mill the faster in RPM it has to run to attain critical speed. For example, 4. 5† diameter mill has a critical speed of 125 RPM, and 90† diameter ball mill has 28 RPM [7][9]. 7. 3 Relative volume of grinding medium and batch The volume of grinding medium and the volume of the paste in a mill affect the grinding time. For fastest grinding a pebble mill should contain 50-55% by volume of pebbles and a ball mill 45-50% of steel balls and in each case the paste should be sufficient to cover the grinding media slightly. Ball mills can contain as low as 33. 3% by volume of steel balls. This reduces the cost of balls and permits a larger loading space in the mill. However the grinding time is longer than with a 50% charge of balls. The grinding efficiency of the one -half charge is considerably greater than for the one-third, hence power consumption per gallon output will be less. Apart from this, the mills containing a low ball or pebble charge are harder to clean than when they are half full because paste tends to adhere to the ends of the mill and is difficult to remove when the charge is low. When the mill is half filled the grinding medium is continuously cleaning this area. The volume of voids between the spherical grinding media does not change with the size of the media. When a mill is charged with pebbles to 50% of its volume the voids among the pebbles are about 20% of the total volume of the mill. The minimum charge of paste is 25% of mill volume, which would give the fastest dispersion to the required fineness. The void volume between the grinding media, with a one-third charge of grinding media is 13 ? %. Fastest grinding occurs where there is just sufficient material in a batch to fill all voids and slightly cover the grinding media. This gives highest ratio of medium to paste. This equals approximately 25% of the total volume with a half ball charge and 18% with a one -third ball charge. The material should never be allowed to drop below the surface of the grinding media because when this happens excessive wear occurs to the mill and grinding media and contaminates the material itself. The largest size batches should not exceed 60% of total mill volume. Usually the minimum load of paste is 40-45% of mill volume and it can increase up to 60% but the grinding time also gets increased considerably. A 40% load requires twice the grinding time of the minimum 25% and the 60% load requires four times as long. It is not always necessary that the shortest grinding time is the most desirable. The mill can operate overnight without any attention therefore the loading can be increased to correspond to the overnight period and a larger yield of paste will be obtained. The size of load can be adjusted to conform to the specific working hours or the production schedules but in no case the load should be below the level of grinding medium, as it would result in excessive wear of the medium [7] [9]. 7. 4 Size and density of grinding medium The most common cause for faulty operation in a ball mill is regarding the size of grinding media. Usually smallest feasible grinding media are used. The optimum size of media should not change with mill size. If the laboratory pebbles or small balls successfully grind a sample batch in a lab ill, the same size grinding media can do the best job in a production mill irrespective of the size of the mill. The criteria is that the size and density of grinding media should be great enough to prevent them from floating in the material and to obtain the maximum amount of grinding surfaces. For steel balls the optimum size of balls recommended are ? † and 5/8†. However, balls of size as small as ? † in p roduction mills can be used and they are found to be extremely advantageous when exceptionally fine grinds are required. Figure 9 shows ? † and ? † sized steel balls. Porcelain balls range from 1-1. 5 in.. The heavy balls exert more force between them than light weight balls. However as the size of ball increases, to give it more weight, the surface area per weight decreases. To overcome this situation grinding medium of maximum possible density should be used, hence steel balls are preferred over pebbles in most of the cases. Balls of mixed size can provide greater area of contact between the balls. However there are chances of increased wear and more uneven wear of the balls with the possibility of large pebbles cracking or chipping the smaller ones. The advantages of small size grinding media are as follows: 1. They provide many more grinding contacts per revolution than larger media. This results in much quicker grinding action. One-inch balls have approximately 38 contacts per pound whereas 5/8 in. balls have 144. 2. They provide smaller voids, limiting the size of particles or agglomerates, which can exist there. 3. They do not create excessive energy, which cannot be utilized. Oversized grinding media frequently develop more grinding energy than is needed for the job. This excess builds up heat and wears down the media and the lining, introducing contamination in the batch. The disadvantages of small size grinding media are as follows: 1. Smaller balls tend to float or rise in the paste as the mill rotates. This can be overcome by lowering the pigment concentration to reduce the paste viscosity. However a stage reaches where reduced the grinding time is overbalanced by the smaller quantity of pigment ground. 2. Smaller balls or pebbles make a mill harder to discharge because of smaller channels in the mass through which the paste can flow, hence increased surface tension in the smaller voids. However, the reduced grinding time by using smaller media offsets this disadvantage. Slight air pressure can be used to assist in more rapid discharge. 3. Balls of too small a size will not be held back by the screen or grate in the discharge opening of the mill. These can cause serious trouble if they lodge in valves or in the pumping mechanism. 4. Small pebbles because of relative small size are less effective in breaking up large agglomerates of paste or pigment and may tend to promote formation of pigment balls in a mill. The attempts to increase the area of contact between the grinding media have produced tubular or rod shaped porcelain media. They have outside diameters of 13/16 in. nd 1 ? in. and lengths of the same dimensions. Porcelain balls of higher than normal density are also used. High density balls are used when the material is hard to grind or when the paste is of high viscosity [7] [9]. Figure 9: Steel Balls of ? † and ? † [5] 7. 5 Consistency of batch The consistency should be such so as to permit the particular size and density of grin ding media to move through the paste smoothly. The correct paste consistency is difficult to determine because it depends on the ball size, diameter of the mill, speed of the mill and density of the paste. If the consistency is too thick the entire charge moves as a mass and there will be no individual movement of balls and no grinding action takes place. If the paste is too thin there will be excessive wear on balls and mill lining, resulting in discoloration of the batch by abraded material, contamination and heat build-up. If the low viscosities cannot be avoided then small grinding media should be used. The range of the viscosities of the paste, handled by the various grinding media is shown in Table 1: Grinding Media |Size |Viscosity of paste (cp) | |Flint Pebbles |1 ? † |600-1100 | |Porcelain Balls |1- ? † |600-1100 | |High Density Balls |- |1100-2100 | |Steel Balls |? ? † |1100-2400 | Table 1: Viscosity of paste handled according to grinding media and size Small size balls are used for low viscosity paste while large size balls are used for high viscosity pastes. Consistency changes with difference in ratio of pigment to vehicle and with variation in solid con tent of the vehicle. To develop maximum shear the paste should be a viscous fluid however ball mill operation does not permit high consistency. Therefore the vehicles of relatively low solid content are used with high ratio of pigment to vehicle. The sound of the mill in operation is used to determine whether the consistency is correct or not. An experienced operator is able to tell if the sound volume is too high or too low for best grinding conditions. Though a noise meter can be also used but usually it is used in grinding ore or cement and not in paint industry. The consistency is affected by temperature hence it is important to know the operating temperature and it has to be maintained uniformly from batch to batch. Many mills like steel ball mills are jacketed for water cooling and temperature control [7] [9]. CHAPTER 8- TECHNOLOGICAL ASPECTS 8. 1 Technological Aspects The various technological aspects, which have to be taken care of during ball mill operation process, are: 1. When unloading materials that do not flow well some of the vehicle is held out until most of the batch has been removed from the mill. This vehicle is then added to the mill to help in removing as much of the batch as possible [7]. 2. Small size mills are used to produce small batches of paints [7]. . Ball formation in a mill is a major problem. In this situation a mass of pigment, balls or pebbles and some vehicle form a large round ball that will roll in the mill for hours and would not be broken. It is caused due to too high a paste level, too low a pebble level and too thin a paste. Increasing the consistency of a too thin paste will eliminate the ball formation because the more viscous paste tends to pull the round mass down below the surface of the grinding medium where all the dispersing action takes place [7]. 4. One of the most successful techniques employed in the dispersion of pigment in vehicle and solvent is known as low solids grinding [9]. The advantages of this technique are: a. Dispersion is accomplished quickly. b. A greater pigment quantity can be dispersed in a mill batch. 5. There are a few pigments on which it is desirable to avoid direct impact and attrition and dispersion of pigment relies mostly on shear, such as toluidine red. Excessive grinding through impact can destroy the pigment structure thereby reducing its hiding power. To avoid excessive grinding by impact the consistency should be heavier than for normal operation and the size batch should be sufficient to induce spreading of the grinding media in order to prevent direct contact and merely induce a shearing action [9]. 6. Step loading is more advantageous than tightly packing a bulky pigment to try and get it all in the mill in one loading as excessive packing can cause reaggregation of pigment particles [9]. 7. Periodic venting of the Mill relieves internal pressures and also helps in grinding [9]. 8. The use of wetting agents has greatly increased the capacity of ball mills and pebble mills without altering the viscosity during the grind. The capacity of the mill can increase from 50% (prior to the addition of wetting agent) upto 85% of solid content by using wetting agents. The wetting agents help in reducing the surface tensions of the aggregated particles, therefore grinding operation can occur in faster time and the finished product has greater stability [9]. 9. It is important to ensure that the batch size is consistent with the allowable running time. For example: if a 25% batch takes 9 hours, this would be too long for an 8-hour shift. In that case the batch size is increased and the mill is run to the next working day. Now if a 40% batch takes 9 hours, then a slight cutback can make it possible to turn out a batch within an 8-hour working day. Hence it is better to do a little experimenting with the batch size and to try to develop a system that will work out best as per the grinding conditions. However the grinding media must be covered with paste [9]. There are occasions where additional thinning of the batch after grinding may be done to increase the yield of the Mill. For example: in case of a pebble mill of volume of 450 gallons, a minimum material charge of 25% for this Mill would be 112 gallons and the maximum charge of 60% would be 270 gallons. After grinding, if the mill were loaded to the extreme top with thinner, the yield produced would be 315 gallons or 70% of the total volume of the mill [9]. 8. 2 Technological upgradation An example of technological upgradation is the use of adjustable timers in ball mills in liquid paint processing in Asian Paints Ltd. , Kasna Plant (UP). In Kasna Plant the ball mills in liquid paint processing used to run idly after the completion of grinding time in absence of any control switchgear. Hence adjustable timers were used. Table 2(a) shows the motor capacity, power consumption and running cost before the installation of the timers, while Table 2(b) gives the motor capacity, power consumption and running cost details after the installation of the timers. It has been observed that by the use of timers an annual saving of 0. 46 lacs has been achieved. [11] |Motor Capacity |55. kW | |Power consumption |3. 64 lacs kWh/annum | |Running Cost |15. 30 lacs/annum | Table 2(a): Kasna Plant details before installation |Motor Capacity |55. 2 kW | |Power Consumption |3. 3 lacs kWh/annum | |Running Cost |14. 83 lacs/annum | Table 2(b): Kasna Plant details after installation CHAPTER 9- CONCLUSIONS The followings conclusions can be made out regarding the use of ball mill in paint industry: 1. Paint manufacture involves dispersio n of pigment into binder and solvent components. 2. Steel ball mills and pebble mills are used for this purpose 3. Ball or pebble mills are used to make pigments uniform in the finished product. 4. They are used for grinding inexpensive coarse extender pigments in the batch to give more hiding power to the paint. 5. They are used in case of agglomerated pigments resulting from storage and handling. 6. Speed of rotation, volume of grinding medium and that of batch, size and density of grinding medium and consistency of the batch affect the efficiency of ball and pebble mills. CHAPTER 10- REFERENCES 1. Paint industry information, facts and figures by National Paint Coatings Association, Washington, US, www. aint. org 2. Paint from wikipedia, www. wikipedia. org 3. Gopala Rao. M and Sittig. Marshall: â€Å"DRYDEN’S OUTLINES OF CHEMICAL TECHNOLOGY†, Affiliated Eat West Press Pvt. Ltd. , 3rd Edition 2004 Reprint, pp 307-312. 4. Chemical fact sheet on paints by Orica Limited. Copyriht 1992-98. ACN 004 145 868. 5. Grinding from wikipedia, www. wikipedia. org 6. McCabe W. L, Smith J. C. , Harriott Peter: â€Å"UNIT OPERATIONS OF CHEMICAL ENGINEERING†, McGraw Hill Book Company, 6th Edition, International Edition 2001,pp 965-977. 7. Biblac V. C. , Edgar W. S. : â€Å"PAINTS WARNISH PRODUCTION MANUAL†, 1991, John Wiley and Sons, NY, pp 40-46, 132-137, 996-1004. 8. Presentation given by P. Poornima, R. Sreenivasan, Ankit Bhatnagar in IIT-Chennai on ball mill operation. 9. Handbook of grinding by Paul O Abbe, A division of Aaoron Engineered Process Equipment, Inc. , 1994 10. Ball mill handbook by Paul O Abbe, A division of Aaoron Engineered Process Equipment, Inc. , 1994. , www. pauloabee. com. 11. Asian Paints Ltd. , Kasna Plant, UP, Unit Profile, www. asianpaints. com