All Grinding Mill & Ball Mill Manufacturers understand the object of the grinding process is a mechanical reduction in size of crushable material. Grinding can be undertaken in many ways. The most common way for high capacity industrial purposes is to use a tumbling charge of grinding media in a rotating cylinder or drum. The fragmentation of the material in that charge occurs through pressure, impact, and abrasion.
The choice of mill design depends on the particle size distribution in the feed and in the product wanted. Often the grinding is more economic when executed in a primary step, followed by a secondary step, giving a fine size product.
C=central trunnion discharge P=peripheral discharge R=spherical roller trunnion bearing, feed end H=hydrostatic shoe bearing, feed end R=spherical roller trunnion bearing, discharge end K=ring gear and pinion drive
Type CHRK is designed for primary autogenous grinding, where the large feed opening requires a hydrostatic trunnion shoe bearing. Small and batch grinding mills, with a diameter of 700 mm and more, are available. These mills are of a special design and described on special request by allBall Mill Manufacturers.
The different types of grinding mills are based on the different types of tumbling media that can be used: steel rods (rod mills), steel balls (ball mills), and rock material (autogenous mills, pebble mills).
The grinding charge in a rod mill consists of straight steel rods with an initial diameter of 50-100 mm. The length of the rods is equal to the shell length inside the head linings minus about 150 mm. The rods are fed through the discharge trunnion opening. On bigger mills, which need heavy rods, the rod charging is made with a pneumatic or manual operated rod charging device. The mill must be stopped every day or every second day for a few minutes in order to add new rods and at the same time pick out broken rod pieces.
As the heavy rod charge transmits a considerable force to each rod, a rod mill can not be built too big. A shell length above 6100 mm can not be recommended. As the length to diameter ratio of the mill should be in the range of 1,2-1,5, the biggest rod mill will convert maximum 1500 kW.
Rod mills are used for primary grinding of materials with a top size of 20-30 mm (somewhat higher for soft materials). The production of fines is low and consequently a rod mill is the right machine when a steep particle size distribution curve is desired. A product with 80% minus 500 microns can be obtained in an economical manner.
The grinding charge in a ball mill consist of cast or forged steel balls. These balls are fed together with the feed and consequently ball mills can be in operation for months without stopping. The ball size is often in the diameter range of 20-75 mm.
The biggest size is chosen when the mill is used as a primary grinding mill. For fine grinding of e.g. sands, balls can be replaced by cylpebs, which are heat treated steel cylinders with a diameter of 12-40 mm and with the same length as the diameter.
Ball mills are often used as secondary grinding mills and for regrinding of middlings in concentrators. Ball mills can be of the overflow or of the grate discharge type. Overflow discharge mills are used when a product with high specific surface is wanted, without any respect to the particle size distribution curve. Overflow discharge mills give a final product in an open circuit. Grate discharge mills are used when the grinding energy shall be concentrated to the coarse particles without production of slimes. In order to get a steep particle size distribution curve, the mill is used in closed circuit with some kind of classifier and the coarse particles known as classifier underflow are recycled. Furthermore, it should be observed that a grate discharge ball mill converts about 20% more energy than an overflow discharge mill with the same shell dimensions.
Ball mill shells are often furnished with two manholes. Ball mills with small balls or cylpebs can produce the finest product of all tumbling mills. 80% minus 74 microns is a normal requirement from the concentrators.The CRRK series of wet grinding ball mills are tabulatedbelow.
No steel grinding media is used in a fully autogenous mill. When choosing primary autogenous grinding, run of mine ore up to 200-300 mm in size is fed to the mill. When using a crushing step before the grinding, the crusher setting should be 150-200 mm. The feed trunnion opening must be large enough to avoid plugging. The biggest pieces in the mill are important for the size reduction of middle size pieces, which in their turn are important for the finer grinding. Thus the tendency of the material to be reduced in size by pressure, impact, and abrasion is a very important question when primary autogenous grinding is proposed.
When autogenous grinding is used in the second grinding step, the grinding media is size-controlled and often in the range of 30-70 mm. This size is called pebbles and screened out in the crushing station and fed to the mill in controlled proportion to the mill power. The pebble weight is 5-25% of the total feed to the plant, depending on the strength of the pebbles. Sometimes waste rock of high strength is used as pebbles.
Pebble mills should always be of the grate discharge type. The energy that can be converted in a mill depends on the total weight of the grinding charge. Consequently, pebble mills convert less power per mill volume unit than rod and ball mills.
High quality steel rods and balls are a considerable part of the operating costs. Autogenous grinding should, therefore, be considered and tested when a new plant shall be designed. As a grinding mill is built to last for decades, it is more important to watch the operation costs than the price of the mill installation. The CRRK series of wet grinding pebble mills are tabulated below.
Wet grinding is definitely the most usual method of grinding minerals as it incorporates many advantages compared to dry grinding. A requirement is, however, that water is available and that waste water, that can not be recirculated, can be removed from the plant without any environmental problems. Generally, the choice depends on whether the following processing is wet or dry.
When grinding to a certain specific surface area, wet grinding has a lower power demand than dry grinding. On the other hand, the wear of mill lining and grinding media is lower in dry grinding. Thus dry grinding can be less costly.
The feed to a dry grinding system must be dried if the moisture content is high. A ball mill is more sensitive to clogging than a rod mill. An air stream through the mill can reduce the moisture content and thus make a dry grinding possible in certain applications.
Due to the hindering effect that the ball charge gives to the material flow in dry grinding, the ball charge is not more than 28-35% of the mill volume. This should be compared with 40-45% in wet grinding. The expression used for this phenomenon is that the charge in a dry grinding mill is swollen.
Big dry grinding ball mills are often two-compartment mills, with big balls in the first compartment and small balls or cylpebs in the second one. An extra grate wall is used to separate the two charges.
The efficiency of wet grinding is affected by the percentage of solids. If the pulp is too thick, the grinding media becomes covered by too thick a layer of material, which hinders grinding. The opposite effect may be obtained if the dilution is too high, and this may also reduce the grinding efficiency. A high degree of dilution may sometimes be desirable in order to suppress excessive slime formation.
The specific power required for a certain grinding operation, usually expressed in kWh/ton, is a function of both the increase in the specific surface of the material (expressed in cm/cm or cm/g) and of the grinding resistance of the material. This can be expressed by the formula
where c is a material constant representing the grinding resistance, and So and S are the specific surfaces of the material before and after the grinding operation respectively. The formula is an expression of Rittingers Law which is shown by tests to be reasonably accurate up to a specific surface of 10,000 cm/cm.
When the grinding resistance c has been determined by trial grinding to laboratory scale, the net power E required for each grinding stage desired may be determined by the formula, at least as long as Rittingers Law is valid. If grinding is to be carried out not to a certain specific surface S but to a certain particle size k, the correlation between S and k must be determined. The particle size is often expressed in terms of particle size at e.g. 95, 90 or 80% quantity passing and is denoted k95, k90 or k80.
where E =the specific power consumption expressed in kWh/short ton. Eo = a proportionality and work factor called work index k80p = particle size of the product at 80% passage (micron) k80f =the corresponding value for the raw material (micron)
The value of Eo is a function of the physical properties of the raw material, the screen analyses of the product and raw material respectively, and the size of the mill. The value for easily-ground materials is around 7, while for materials that have a high grinding resistance the value is around 17.
Eo is correlated to a certain reduction ratio, mill diameter etc. Corrections must be made for each case. The simplest method of calculating the specific power consumption is test grinding in a laboratory mill, and comparison of the results with a known reference material. The sample is ground in batches for 3, 6,12 minutes, a screen analysis is carried out after each period, after which the specific surface is determined. A good estimate of the grinding characteristics of the sample can be obtained by comparison of the specific surfaces with corresponding values for the reference material.
When the net power required has been determined, an allowance is made for mechanical losses. The gross power requirement thus arrived at, should with a satisfactory margin be utilised by the mill selected.
The critical speed of a rotating mill is the RPM at which a grinding medium will begin to centrifuge, namely will start rotating with the mill and therefore cease to carry out useful work. This will occur at an RPM of ncr, which may be determined by the formula
where D is the inside diameter in meters of the mill. Mills are driven in practice at a speed corresponding to 60-80% of the critical speed, the choice of speed being influenced by economical considerations. Within that range the power is nearly proportional to the speed.
The charge volume in the case of rod and ball mills is a measure of the proportion of the mill body that is filled by rods or balls. When the mill is stationary, raw material and liquid should fill the voids between the grinding media, in order that these should be fully utilized.
Maximum mill efficiency is reached at a charge volume of approximately 55%, but for a number of reasons 45-50% is seldom exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, while there is a greater choice in the case of grate discharge mills.
For coarse grinding in rod mills, the rods used have a diameter of 50-100 mm and their lengths are approx. 150 mm below the effective inside shell length. Rods will break when they have been worn down to about 20 mm and broken rods must from time to time be taken out of the mill since otherwise they will reduce the mill capacity and may cause blockage through piling up. The first rod charge should also contain a number of rods of smaller diameter.
It may be necessary to charge the mill with rods of smaller diameter when fine grinding is to be carried out in a rod mill. Experience shows that the size of the grinding media should bear a definite relationship to the size of both the raw material and the finished product in order that optimum grinding may be achieved. The largest grinding media must be able to crush and grind the largest pieces of rock, while on the other hand the grinding media should be as small as possible since the total active surface increases in inverse proportion to the diameter.
A crushed mineral whose largest particles pass a screen with 25 x 25 mm apertures shall be ground to approx. 95% passing 0.1 mm in a 2.9 x 3.2 m ball mill of 35 ton charge weight. In accordance with Olewskis formula
Grinding media wear away because of the attrition they are subjected to in the course of the grinding operation, and in addition a continuous reduction in weight takes place owing to corrosion. The rate of wear will in the first place depend on the abrasive properties of the mineral being ground and naturally also on the hardness of the grinding media themselves.
The wear of rods and balls is usually quoted in grammes per ton of material processed (dry weight) and normal values may lie between 100 and 1500 g/ton. Considerably higher wear figures may however be experienced in fine wet grinding of e.g. very hard siliceous sand.
A somewhat more accurate way of expressing wear is to state the amount of gross kWh of grinding power required to consume 1 kg of grinding media. A normal value in wet grinding is 15 kWh/kg.The wear figures in dry grinding are only 10-30 % of the above.
where c is a constant which, inter alia, takes into consideration the mean slope a of the charge, W is the weight in kp of the charge n is the RPM Rg is the distance in metres of the centre of gravity from the mill centre
W for rod and ball mills shall be taken as the weight of the rod or ball charge, i.e. the weight of the pulp is to be ignored. For pebble mills therefore W is to be calculated on the basis of the bulk weight of the pebbles.
It should be pointed out that factor c in the formula is a function of both the shape of the inner lining (lifter height etc.) and the RPM. The formula is however valid with sufficient accuracy for normal speeds and types of lining.
The diagram gives the values of the quantity Rg/d as a function of the charge volume, the assumption being that the charge has a plane surface and is homogeneous, d is the inside diameter of the mill in metres. The variation of the quantity a/d, where a is the distance between the surface of the charge and the mill centre, is also shown in the same figure.
In order to keep manufacturing costs at a minimum level, Morgardshammar has a series of standard mill diameters up to and including 6.5 m. Shell length, however, can be varied and tailor made for each application. The sizes selected are shown on the tables on page 12-13 and cover the power range of 200-5000 kW.
Shells with a diameter of up to about 4 m are made in one piece. Above this dimension, the shell is divided into a number of identical pieces, bolted together at site, in order to facilitate the transport. The shell is rolled and welded from steel plate and is fitted with welded flanges of the same material. The flanges are machined in order to provide them with locating surfaces fitting into the respective heads. The shells of ball and pebble mills are provided with 2 manholes with closely fitting covers. The shells have drilled holes for different types of linings.
Heads with a diameter of up to about 4 m are integral cast with the trunnion in one piece. Above this diameter the trunnion is made as a separate part bolted to the head. The head can then be divided in 2 or 4 pieces for easy transport and the pieces are bolted together at site. The material is cast steel or nodular iron. The heads and the trunnions have drilled holes for the lining.
Spherical roller (antifriction) bearings are normally used. They offer the most modern and reliable technology and have been used for many years. They are delivered with housings in a new design with ample labyrinth seals.
For very large trunnions or heavy mills, i.e. for primary autogenous grinding mills. Morgardshammar uses hydrostatic shoe bearings. They have many of the same advantages as roller bearings. They work with circulating oil under pressure.
The spherical roller bearing and the hydrostatic shoe bearing take a very limited axial space compared to a conventional sleeve bearing. This means that the lever of the bearing load is short. Furthermore, the bending moment on the head is small and as a result of this, the stress and deformation of the head are reduced. Ask Morgardshammar for special literature on trunnion bearings.
Ring gears are often supplied with spur gears. They are always split in 2 or 4 pieces in order to facilitate the assembly. Furthermore, they are symmetrical and can be turned round in order to make use of both tooth flanks. The material is cast steel or nodular iron. They are designed in accordance with AGMA.The ring gear may be mounted on either the feed or the discharge head. It is fitted with a welded plate guard.
The pinion and the counter shaft are integral forged and heat treated of high quality steel. For mill power exceeding about 2500 kW two pinions are used, one on each side of the mill (double-drive). The pinion is supported on two spherical roller bearings.
The trunnion bearings are lubricated by means of a small motor- driven grease lubricator. The gear ring is lubricated through a spray lubricating system, connected to the electric and pneumatic lines. The spray nozzles are mounted on a panel on the gear ring guard.
In order to protect the parts of the mill that come into contact with the material being ground, a replaceable lining of wear-resistant material is fitted. This may take the form of unalloyed or alloyed rolled or cast steel, heat treated if required, or rubber of the appropriate wear resistant quality. White cast iron, unalloyed or alloyed with nickel (Ni-hard), may also be used.
The shape of the mill lining is often of Lorain-type, consisting of plates held in place between lifter bars (or key bars) of suitable height bolted on to the shell. This system is used i.e. of all well-known manufacturers of rubber linings. Ball mills and autogenous mills with metal lining also can be provided with single or double waved plates without lifter bars.
In grate discharge mills the grate and the discharge lifters are a part of the lining. The grate plates with tapered slots or holes are of metal or rubber design. The discharge lifters are fabricated steel with thick rubber coating. Rubber layer for metal linings and heavy corner pieces of rubber are included in a Morgardshammar delivery as well as attaching bolts, washers, seal rings, and self-locking nuts. A Morgardshammar overflow mill can be converted into a grate discharge mill only by changing some liner parts and without any change of the mill. Trunnion liners are rubber coated fabricated steel or cast steel. In grate discharge mills the center cone and the trunnion liner form one piece.
Scoop feeders in combination with drum feeders are used when retaining oversize from a spiral or rake classifier. As hydrocyclones are used in most closed grinding circuits the spout feeders are used most frequently.
Vibrating feeders or screw feeders are used when charging feed to dry grinding mills. Trommel screens are used to protect slurry pumps and other transport equipment from tramp iron. Screens can have perforated rubber sheets or wire mesh. The trommel screens are bolted to the discharge trunnion lining.
Inching units for slow rotation of the mills are also furnished. Rods to the rod mills are charged by means of manual or automatic rod charges. Erection cradles on hydraulic jacks are used when erecting medium or big size mills at site.
A symbol of dependable quality ore milling machinery manufacturing, industrial and mining equipment, ball mills and rod mills as well as supplies created for your specific needs. During this period thousands of operators have experienced continuous economical and unequalled service through their use.As anindustrial ball mill manufacturer and supplier, we havecontinuously accumulated knowledge on grinding applications. It has contributed greatly to the grinding process through the development and improvement of such equipment.
Just what is grinding? It is the reduction of lump solid materials to smaller particles by the application of shearing forces, pressure, attrition, impact and abrasion. The primary consideration, then, has been to develop some mechanical means for applying these forces. The modern grinding mill applies power to rotate the mill shell and thus transmits energy to some form of media which, in turn, fractures individual particles.
Through constant and extensive research, in the field of grinding as well as in the field of manufacturing. Constantly changing conditions provide a challenge for the future. Meeting this challenge keeps our company young and progressive. This progressive spirit, with the knowledge gained through the years, assures top quality equipment for the users of our mills.
You are urged to study the following pages which present a detailed picture of our facilities and discuss the technical aspects of grinding. You will find this data helpful when considering the selection of the grinding equipment.
It is quite understandable that wetakes pride in the quality of our mills.Complementing the human craftsmanship built into these mills, our plants are equipped with modern machines of advanced design which permit accurate manufacturing of each constituent part. Competent supervision encourages close inspection of each mill both as to quality and proper fabrication. Each mill produced is assured of meeting the high required standards. New and higher speed machines have replaced former pieces of equipment to provide up-to-date procedures. The use of high speed cutting and drilling tools has stepped up production, thereby reducing costs and permitting us to add other refinements and pass these savings on to you, the consumer.
Each foundry heat is checked metallurgically prior to pouring. All first castings of any new design are carefully examined by the use of an X-ray machine to be certain of uniformity of structure. The X -ray is also used to check welding work, mill heads, and other castings.
Each Mills, regardless of size, is designed to meet the specific grinding conditions under which it will be used. The speed of the mill type of liner, discharge arrangement, size of feeder, size of bearings, mill diameter and length, and other factors are all considered to take care of the size of feed, tonnage, circulating sand load, selection of balls or rods, and the final size of grind.
All Mills are built with jigs and templates so that any part may be duplicated. A full set of detailed drawings is made for each mill and its parts. This record is kept up to date during the life of the mill. This assures accurate duplication for the replacement of wearing parts during the future years.
As a part of our service our staff includes experienced engineers, trained in the field of metallurgy with special emphasis on grinding work. This knowledge, as well as a background gained from intimate contact with various operating companies throughout the world, provides a sound basis for consultation on your grinding problems. We take pride in manufacturing rod mills and ball millsfor the metallurgical, rock products, cement, process, and chemical industries.
As an additional service we offer our testing laboratories to check your material for grindability. Since all grinding problems are different some basis must be established for recommending the size and type of grinding equipment required. Experience plays a great part in this phase however, to establish more direct relationships it is often essential to conduct individual grindability tests on the specific material involved. To do this we have established certain definite procedures of laboratory grinding work to correlate data obtained on any new specific material for comparison against certain standards. Such standards have been established from conducting similar work on material which is actually being ground in Mills throughout the world. The correlation between the results we obtain in our laboratory against these standards, coupled with the broad experience and our companys background, insures the proper selection and recommendation of the required grinding equipment.
When selecting a grinding mill there are many factors to be taken into consideration. First let us consider just what constitutes a grinding mill. Essentially it is a revolving, cylindrical shaded machine, the internal volume of which is approximately one-half filled with some form of grinding media such as steel balls, rods or non-ferrous pebbles.
Feed may be classified as hard, average or soft. It may be tough, brittle, spongy, or ductile. It may have a high specific gravity or a low specific gravity. The desired product from a mill may range in size from a 4 mesh down to 200 mesh, or into the fine micron sizes. For each of these properties a different mill would be indicated.
The Mill has been designed to carry out specific grinding work requirements with emphasis on economic factors. Consideration has been given to minimizing shut-down time and to provide long, dependable trouble-free operation. Wherever wear takes place renewable parts have been designed to provide maximum life. A Mill, given proper care, will last indefinitely.
Mills have been manufactured in a wide variety of sizes ranging from laboratory units to mills 12 in diameter, with any suitable length. Each of these mills, based on the principles of grinding, provides the most economical grinding apparatus.
For a number of years ball mill grinding was the only step in size reduction between crushing and subsequent treatment. Subsequently smaller rod mills have altered this situation, providing in some instances a more economical means of size reduction in the coarser fractions. The principal field of rod mill usage is the preparation of products in the 4-mesh to 35-mesh range. Under some conditions it may be recommended for grinding to about 48 mesh. Within these limits a rod mill is often superior to and more efficient than a ball mill. It is frequently used for such size reduction followed by ball milling to produce a finished fine grind. It makes a product uniform in size with only a minimum amount of tramp oversize.
The basic principle by which grinding is done is reduction by line contact between rods extending the full length of the mill. Such line contact results in selective grinding carried out on the largest particle sizes. As a result of this selective grinding work the inherent tendency is to make size reduction with the minimum production of extreme fines or slimes.
The small rod mill has been found advantageous for use as a fine crusher on damp or sticky materials. Under wet grinding conditions this feed characteristic has no drawback for rod milling whereas under crushing conditions those characteristics do cause difficulty. This asset is of particular importance in the manufacture of sand, brick, or lime where such material is ground and mixed with just sufficient water to dampen, but not to produce a pulp. The rod mill has been extensively used for the reduction of coke breeze in the 8-mesh to 20-mesh size range containing about 10% moisture to be used for sintering ores.
Grinding by use of nearly spherical shaped grinding media is termed ball milling. Strictly speaking, such media are made of steel or iron. When iron contamination is detrimental, porcelain or natural non-metallic materials are used and are referred to as pebbles. When ore particles are used as grinding media this is known as autogenous grinding.
Other shapes of media such as short cylinders, cubes, cones, or irregular shapes have been used for grinding work but today the nearly true spherical shape is predominant and has been found to provide the most economic form.
In contrast to rod milling the grinding action results from point contact rather than line contact. Such point contacts take place between the balls and the shell liners, and between the individual balls themselves. The material at those points of contact is ground to extremely fine sizes. The present day practice in ball milling is generally to reduce material to 35 mesh or finer. Grinding in a ball mill is not selective as it is in a rod mill and as a result more extreme fines and tramp oversize are produced.
Small Ball mills are generally recommended not only for single stage fine grinding but also have wide application in regrind work. The Small Ball millwith its low pulp level is especially adapted to single stage grinding as evidenced by hundreds of installations throughout the world. There are many applications in specialized industrial work for either continuous or batch grinding.
Wet grinding may be considered as the grinding of material in the presence of water or other liquids in sufficient quantity to produce a fluid pulp (generally 60% to 80% solids). Dry grinding on the other hand is carried out where moisture is restricted to a very limited amount (generally less than 5%). Most materials may be ground by use of either method in either ball mills or rod mills. Selection is determined by the condition of feed to the mill and the requirements of the ground product for subsequent treatment. When grinding dry some provision must be made to permit material to flow through the mill. Mills provide this necessary gradient from the point of feeding to point of discharge and thereby expedites flow.
The fineness to which material must be ground is determined by the individual material and the subsequent treatment of that ground material Where actual physical separation of constituent particles is to be realized grinding must be carried to the fineness where the individual components are separated. Some materials are liberated in coarse sizes whereas others are not liberated until extremely fine sizes are reached.
Occasionally a sufficient amount of valuable particles are liberated in coarser sizes to justify separate treatment at that grind. This treatment is usually followed by regrinding for further liberation. Where chemical treatment is involved, the reaction between a solid and a liquid, or a solid and a gas, will generally proceed more rapidly as the particle sizes are reduced. The point of most rapid and economical change would determine the fineness of grind required.
Laboratory examinations and grinding tests on specific materials should be conducted to determine not only the fineness of grind required, but also to indicate the size of commercial equipment to handle any specific problem.
In Grinding, selecting (calculate)the correct or optimum ball sizethat allows for the best and optimum/ideal or target grind size to be achieved by your ball mill is an important thing for a Mineral Processing Engineer AKA Metallurgist to do. Often, the ball used in ball mills is oversize just in case. Well, this safety factor can cost you much in recovery and/or mill liner wear and tear.
Mill ball charging systems have undergone little change during the past two decades. Most operations utilize a locomotive/ball car combination to load and haul charges from the ball storage bin to the mill charging boxes or, alternatively, transport charges to the charging boxes by means of a ball bucket handled by bridge crane.
The grinding section at Island Copper is a semi-autogenous, two-stage circuit comprising six 9.75 m diameter by 4.27 m primary mills with 6000 HP drives and three 5.03 m diameter by 6.71 m secondary mills with 3000 HP drives.
At the charging station the top strand of the belt rides from the transition idlers onto a flat steel plate set at the same elevation as the last transition idler in front of the station. Belt hold-down rolls are provided at each corner of the flat plate to stabilize transit of the belt through the station.
Rod mill charges usually occupy about 45% of the internal volume of the mill. A closely packed charge of single sized rods will have a porosity of 9.3%. With a mixed charge of small and large diameter rods, the porosity of a static load could be reduced even further. However, close packing of the charge rarely occurs and an operating bed porosity of 40% is common. Overcharging results in poor grinding and losses due to abrasion of rods and liners. Undercharging also promotes more abrasion of the rods. The height (or depth) of charge is measured in the same manner as for ball mill. The size of feed particles to a rod mill is coarser than for a ball mill. The usual feed size ranges from 6 to 25mm.
For the efficient use of rods it is necessary that they operate parallel to the central axis and the body of the mill. This is not always possible as in practice, parallel alignment is usually hampered by the accumulation of ore at the feed end where the charge tends to swell. Abrasion of rods occurs more in this area resulting in rods becoming pointed at one end. With this continuous change in shape of the grinding charge, the grinding characteristics are impaired.
The bulk density of a new rod charge is about 6.25t/m3. With time due to wear the bulk density drops. The larger the mill diameter the greater is the lowering of the bulk density. For example, the bulk density of worn rods after a specific time of grinding would be 5.8t/m3 for a 0.91m diameter mill. Under the same conditions of operation, the bulk density would be 5.4t/m3 for a 4.6m diameter mill.
During normal operation the mill speed tends to vary with mill charge. According to available literature, the operating speeds of AG mills are much higher than conventional tumbling mills and are in the range of 8085% of the critical speed. SAG mills of comparable size but containing say 10% ball charge (in addition to the rocks), normally, operate between 70 and 75% of the critical speed. Dry Aerofall mills are run at about 85% of the critical speed.
The breakage of particles depends on the speed of rotation. Working with a 7.32m diameter and 3.66m long mill, Napier-Munn etal.  observed that the breakage rate for the finer size fractions of ore (say 0.1mm) at lower speeds (e.g., 55% of the critical speed) was higher than that observed at higher speeds (e.g., 70% of the critical speed). For larger sizes of ore (in excess of 10mm), the breakage rate was lower for mills rotating at 55% of the critical speed than for mills running at 70% of the critical speed. For a particular intermediate particle sizerange, indications are that the breakage rate was independent of speed. The breakage ratesize relation at two different speeds is reproduced in Figure9.7.
The blending of different ore types is a common practice to provide a consistent feed to a process in terms of uniform hardness or assay. When several different ore deposits of varying grindabilities are blended prior to closed circuit grinding, the work index of the ore is not an average or even a weighted average of the work indices of the components. The reason for this is that the circulating load will consist predominantly of the harder component and if the circulating load is high then the mill charge will also consist of mostly the harder components. Thus, the work index of the blend will be weighted towards the harder components . Figure3.16 shows the Bond work index of a blend of hard and soft ores as a function of the volume fraction of the softer ore in the blend. The dotted line between the two extremes indicates the weighted average work index based on volume fraction. The work index values of the Magdalinovic method agree with this average Bond work index because the method does not simulate the recycling of harder components into the mill charge. On the other hand, the work index obtained using the standard Bond test shows the weighting of the work index towards the harder component as a result of the circulating load.
Yan and Eaton  also measured the breakage rates and breakage distribution functions of the different ore blends in order to predict the work index of the blend by simulation of the Bond batch grinding test. Qualitative analysis of the breakage properties suggests that there is an interaction between the components of the blend that affect their individual breakage rates. The breakage properties of the harder material appear to have a greater influence on the overall breakage properties and the Bond work index of the blend than the softer material.
Whereas most of the ball-milled systems usually prepared with using ball-to-powder weight ratio (Wb:Wp) in the range between 10:1 and 20:1, the effect Wb:Wp on the amorphization reaction of Al50Ta50 alloy powders in a low-energy ball mill was investigated in 1991 by El-Eskandarany etal. They have used 90, 30, 20, 10, and 3g of powders to obtain Wb:Wp ratios of 12:1, 36:1, 54:1, 108:1, and 324:1, respectively.
The XRD patterns of mechanically alloyed Al50Ta50 powders as ball-milled for 1440ks (400h) as a function of the Wb:Wp ratio is presented in Fig.4.32. Single phase of amorphous alloys is obtained when ratios 36:1 and 108:1 were used. The Bragg peaks of elemental Al and Ta crystals still appear when the Wb:Wp ratio is 12:1, indicating that the amorphization reaction is not completed. In contrast, when the Wb:Wp ratio is 324:1, the amorphous phase coexists with the crystalline phases of AlTa, AlTa2, and AlTaFe.
Based on their results, it is concluded that the rate of amorphization depends strongly on the kinetic energy of the ball mill charge and this depends on the number of opportunities for the powder particles to be reacted and interdiffused. Increasing the Wb:Wp ratio accelerates the rate of amorphization, which is explained by the increase in the kinetic energy of the ball mill charge per unit mass of powders. It has been shown in this study that the volume fraction of the amorphous phase in the mechanically alloyed ball-milled powders increases during the early stage of milling, 86173ks (48h) with increasing Wb:Wp ratio. It is noted that further increasing this weight ratio leads to the formation of crystalline phases and this might be related to the high kinetic energy of the ball mill charge which is transformed into heat. When the Wb:Wp ratio was reduced to 12:1, however, the amorphization reaction was not completed. This indicates that the kinetic energy of the mill charge is insufficient for complete transition from the crystalline to the amorphous phase.
It is worth noting that powder particles reached the minimum of extreme fineness when using a high Wb:Wp ratio. One disadvantage of using such a high weight ratio is being the high concentration of iron contamination which is introduced to the milled powders during the MA process, as presented in Fig.4.33.
Romankova etal. have applied the vibration ball milling for coating of stainless steel balls during milling of TiAl powders. They examined metallographically the development of the TiAl coating structure after milling for 60min as a function of the ball-to-powder weight ratio for 6mm balls (Fig.4.34).
The results showed that the milling energy increased with increasing the number of balls. When the weight ratio was 3:1, the substrate could be covered with a thin Al layer (Fig.4.34A). For this case, only small Ti particles were embedded into the Al matrix. It should be noted that the substrate underwent plastic deformation under the ball impacts and its surface became slightly bent. When the weight ratio was increased to 4:1, the energy was sufficient to embed larger Ti particles in the Al layer than at ratio 3:1 (Fig.4.34B). Al bound these Ti particles to the substrate. They notified that, at the 4:1 ratio, the growth for the TiAl coating across the substrate was clustered; this resulted in a hillock-like morphology and increased the surface roughness. Upon further increasing the ball-to-powder weight ratio from 6:1 to 14:1, the coating roughness gradually decreased. They also reported that the lamellar structure was refined when the ball-to-powder weight ratio was 14:1, as presented in Fig.4.34E.
More recently, Waje etal. have studied the effect of the ball-to-powder weight ratio (BPR) on the crystallite size of ball-milled CoFe2O4 nanoparticles, using XRD (Fig.4.35). From their results it can be seen that the particle size decreases linearly from 15.3 to 11.4nm when used BPR of 8:1 and 30:1, respectively.
The mass-size balance models as written above are in the time-domain. To be more practical they need to be converted to the energy-domain. One way is by arguing that the specific rate of breakage parameter is proportional to the net specific power input to the mill charge (Herbst and Fuerstenau, 1980; King, 2012). For a batch mill this becomes:
where SiE is the energy-specific rate of breakage parameter, P the net power drawn by the mill, and M the mass of charge in the mill excluding grinding media (i.e., just the ore). The energy-specific breakage rate is commonly given in t kWh1. For a continuous mill, the relationship is:
where is the mean retention time, and F the solids mass flow rate through the mill. Assuming plug flow, Eq. (5.17) can be substituted into Eq. (5.15) to apply to a grinding mill in closed circuit (where t=).
The distinctive feature of tumbling mills is the use of loose crushing bodies, which are large, hard, and heavy in relation to the ore particles, but small in relation to the volume of the mill, and which occupy (including voids) slightly less than half the volume of the mill.
Due to the rotation and friction of the mill shell, the grinding medium is lifted along the rising side of the mill until a position of dynamic equilibrium is reached (the shoulder), when the bodies cascade and cataract down the free surface of the other bodies, about a dead zone where little movement occurs, down to the toe of the mill charge (Figure 7.3).
The driving force of the mill is transmitted via the liner to the charge. The speed at which a mill is run and the liner design governs the motion and thus nature of the product and the amount of wear on the shell liners. For instance, a practical knowledge of the trajectories followed by the steel balls in a mill determines the speed at which it must be run in order that the descending balls shall fall on to the toe of the charge, and not on to the liner, which could lead to liner damage. Simulation of charge motion can be used to identify such potential problems (Powell et al., 2011), and acoustic monitoring can give indication of where ball impact is occurring (Pax, 2012).
At relatively low speeds, or with smooth liners, the medium tends to roll down to the toe of the mill and essentially abrasive comminution occurs. This cascading leads to finer grinding and increased liner wear. At higher speeds the medium is projected clear of the charge to describe a series of parabolas before landing on the toe of the charge. This cataracting leads to comminution by impact and a coarser end product with reduced liner wear. At the critical speed of the mill centrifuging occurs and the medium is carried around in an essentially fixed position against the shell.
In traveling around inside the mill, the medium (and the large ore pieces) follows a path which has two parts: the lifting section near to the shell liners, which is circular, and the drop back to the toe of the mill charge, which is parabolic (Figure 7.4(a)).
Consider a ball (or rod) of radius r meters, which is lifted up the shell of a mill of radius R meters, revolving at N rev min1. The ball abandons its circular path for a parabolic path at point P (Figure 7.4(b)), when the weight of the ball is just balanced by the centrifugal force, that is when:
Mills are driven, in practice, at speeds of 5090% of critical speed. The speed of rotation of the mill influences the power draw through two effects: the value of N and the shift in the center of gravity with speed. The center of gravity first starts to shift away from the center of the mill (to the right in Figure 7.4(a)) as the speed of rotation increases, causing the torque exerted by the charge to increase and draw more power (see Section 7.2.2). But, as critical speed is reached, the center of gravity moves toward the center of the mill as more and more of the material is held against the shell throughout the cycle, causing power draw to decrease. Since grinding effort is related to grinding energy, there is little increase in efficiency (i.e., delivered kWh t1) above about 4050% of the critical speed. It is also essential that the cataracting medium should fall well inside the mill charge and not directly onto the liner, thus excessively increasing steel consumption.
At the toe of the load the descending liner continuously underruns the churning mass, and moves some of it into the main mill charge. The medium and ore particles in contact with the liners are held with more firmness than the rest of the charge due to the extra weight bearing down on them. The larger the ore particle, rod, or ball, the less likely it is to be carried to the breakaway point by the liners. The cataracting effect should thus be applied in terms of the medium of largest diameter.
As already discussed, this control loop is provided to maintain the PA header pressure before the mixing of hot and cold PA duly controlled for temperature. FigureVIII/4-2 is also applicable for this type of mill when the PA is common to all the mills. The control loop is of course different for individual PA fan systems, as the above is applicable for the common PA system only. For control loop description, see Section 22.214.171.124 of this chapter. Common PA fans are provided with suction normally from the atmosphere or it may be from the FD discharge header. Header pressure control is performed through various types of final control elements.
As the fuel/load control is solely done by position adjustments to the PA damper near the mill, this control loop assists smooth and bumpless control of the fuel flow transported by the PA flow to the mill as the upstream PA header pressure control takes responsibility for providing an adequate quantity of air at any environmental condition without sacrificing the required downstream pressure,
FigureVIII/5.3-3 later in the chapter depicts the simple control loop. Any of the mill DP transmitters or level (sound-detector) transmitters is selected and the selected signal is connected to the controller as the process or measured variables against a fixed-level set point. Sufficient redundancy in measurement may vary according to the plants operating philosophy. The controller output is utilized for adjustment of feeder speed with the help of a VFD or SCR control for the gravimetric feeder/feeder speed variator.
At the higher load the charge level inside the drum decreases and the feeder speed should increase accordingly to replenish the material. For a decreasing load, the reverse action takes place. To take care of the sudden load change, the deviation between characterized PA flow and DP acrossthe mill is used to modify the controller output to achieve the desired mill charge level.
Mill load or fuel flow control follows the fuel demand from the boiler master demand control signal and is achieved by regulating the quantity of PA that is transporting agent only. Figures VIII/5.2-4 and VIII/5.2-4 depict the functioning of the control loop, which is similar to that of other mill types. For other mills the fuel demand signal from the boiler master demand is first taken care of by the mill-wise PA flow control system if the demand is less than the prevailing air flow control system. The characterized PA flow is then construed as the feeder speed demand. The ball-and-tube mill control system, on the other hand, uses feeder speed control for maintaining mill level control only and so the fuel flow control is achieved through control of the feeder-wise PA flow to mill itself.
However, the feeder-wise PA flow as measured after redundant transmitters voting selection and density compensated through temperature correction is again determined to get equivalent fuel flow. The total fuel flow is then computed by summing all the fuel (PF) flow of the running feeders and the supporting fuel (oil or gas) if any are being utilized at that time with proper weightage, taking consideration of their thermal or calorific value. The higher selection of this total equivalent fuel flow signal and the air flow demand signal from the boiler master demand (FigureVIII/2.1) is then taken as actual air demand just as in other type of mills.
As already discussed in Section 5.2.1, there is another feeder-wise control system associated with fuel flow control known as a bypass damper control. This feeder-wise damper is provided for each mill end for preheating the raw feed, which is an essential requirement during startup. No process measurement signal is utilized in this subloop. The same fuel demand from the boiler master demand (FigureVIII/ 2-1) is taken as the set point for the position demand of the bypass damper after due characterization, as shown (refer to Figures VIII/5.2-4 and VIII/5.2-5) in the control strategy and the graphical representation of approximate positions of the two final control elements. The previously mentioned two-position demands operate in opposite directions. After being in a fully open position for a certain load, ensuring elimination of initial moisture, this bypass damper begins to close gradually as the load increases.
There are two main types of fuel flow controls achieved through the proportionate PA flow only: (1) common PA fans with individual PA dampers and (2) individual PA fans with vane or speed control. There is also one known as a mill-wise PA flow control that is common to both sides.
FigureVIII/5.2-4 may be referred to for this type of control along with FigureVIII/5.2-2. Here the mill PA flow and bypass PA flows are combined to form the total mill-wise PA flow to the furnace. The boiler master demand acts as a set point here, where the mill-wise PA flow is the measured value as this air flow is only responsible for transporting the fuel to the furnace. The controller output is the demand signal for the individual PA damper. For bypass dampers, the boiler master demand generates the set point while the actual position of this damper acts as the measured value for the controller output, which is the demand signal for the bypass damper.
For any load change, the two flows readjust their positions to deliver the required PA flow. For higher load the bypass damper tends to close to allow less flow for preheating of raw feed and the PA damper to the mill opens more to take care of the load demand.
FigureVIII/5.2-5 may be referred to for this type of control along with FigureVIII/5.2-2(a). Here bypass PA flows need to be subtracted from the total mill-wise PA flow for the fuel flow control, and the total mill-wise PA flow to the furnace is required for air flow control. The reason for this is that the final control element and the flow element are both located in the common primary air path to the individual mill. The boiler master demand acts as a set point here, where the PA flow to the mill is the measured value. The controller output is the demand signal for the individual PA vane or variable speed drive as the case may be.
This type of mill design vis--vis operation is somewhat different from other types, as discussed earlier. FigureVIII/5.2-2(b), which is mainly followed by manufacturers, such as the Foster Wheeler Energy AG corporation, may be referred to for information. Here the boiler combustion control signal regulates the output of the mill by PA flow control dampers placed in the common line to both the ends or sides. The predrying of coal feed is done at the entry of each side before it enters the drum, unlike what is done by the bypass PA damper in many types of tube mills.
Another significant difference is the provision of an auxiliary air and purge air supply line taken from the cold PA for each side of the mill drum. The same is designed to the required minimum velocities of the PA/fuel mixture for maintaining proper flow inside the coal duct and to prevent fuel settling during startup or in extreme low-load operation. This feature extends the individual mill load range without encountering drifting or pulsating fuel flow to the burners. The other purpose is to purge the coal air line automatically when burners are taken out of service.
The feed level control in the drum, classifier outlet temperature control, and seal air DP control are very much similar to those in the other type of mills with the exception of the source of the seal air. Here the seal air supply is taken from the cold PA without any provision of a seal air fan.
Selecting dispersion equipment for a specific application is a complex task. Dispersion of the mixture must be complete and the process and equipment must meet economic constraints. But much more is involved. In practice, such simple criteria are complicated by a variety of parameters related to fillers and to the materials in which they are dispersed. These parameters complicate the problem to the degree that it is not easy to formulate general guidelines. In this discussion we will consider the available equipment types most frequently used for filler dispersion and illustrate their applicability with some examples.
A ball mill is an effective means of dispersing solid materials in solids or liquids.8,9 Ball mills have several advantages which include versatility, low cost of labor and maintenance, the possibility of unsupervised running, no loss of volatiles, and a clean process. The disadvantages are related to discharging viscous and thixotropic mixtures, and considerably lower efficiency when compared with other mixing equipment. The millbase viscosity is usually restricted to about 15-20 Poise, and therefore ball mills are most frequently found in production applications such as paints, flexographic, publication gravure, and letterpress news inks, and carbon paper inks which are dispersed at elevated temperatures.
The mill should rotate at 50-65% of the theoretical centrifugal speed in order to allow balls to cascade, since the cascading balls grind most effectively and do not cause an excessive loss of ball material
Viscosity, the order of filler addition, and the quantity of material should be chosen so as not to cause a viscosity increase above the specified range, since the milling efficiency drastically decreases at that point
The degree of dispersion and jetness achieved when grinding carbon black depends on the wetting properties of the dispersing material and to some degree on the filler form. For instance, pelletized carbon black is easier to disperse than a fluffy type
The sandmill has some drawbacks. It is a two stage process (premixing followed by milling). Milling develops high temperatures in the mixture which causes loss of volatiles and requires cooling. If the millbase is high in viscosity or dilatant, the sandmill process may not work at all. Agglomerated or extremely hard pigments are difficult or impossible to disperse
Both ball and sand mills operate based on a viscous shear principle, thus the viscosity of the millbase is a critical factor in achieving dispersion. The size of filler particles is critical, especially in sandmills. It was found that the shearing force is inversely proportional to the square of the linear size of filler agglomerate. An agglomerate of diameter of 7 m attains 100 times the shear stress of an agglomerate of 70 m diameter. The difference between the ball mill and the sand mill is in the size and density of the grinding media, which is reflected in their performance. Sandmilling uses small particles of low density, and therefore, there is no noticeable reduction in the size of the sand particle, whereas the balls in ballmills are very much larger and may have a high density (steel), which results in a more complex mechanism of grinding including shattering and impacting which cause this mill to be more effective in disintegrating hard particles and agglomerates containing sintered particles.
There is another mill type called an attritor, which is similar to both the ball mill and the sandmill. In construction, it is similar to a sandmill. It also has a vertical shaft, but in the attritor the agitator bars replace the milling discs of the sandmill. It is also similar to a ball mill because it uses balls, usually ceramic ones having 5-15 mm in diameter. Because the motion of the balls is independent of gravity, an attritor can handle thixotropic materials and slightly higher viscosity of millbases, but the principle of action and type of forces operating are similar to those of the ball mill. An attritor applied to pigment dispersion gives several advantages. These include rapid dispersion, the possibility of either a continuous or batch process, low power consumption, small floor space, and easy cleaning and maintenance. Their main disadvantage is high heat generation. Attritors are equipped with a cooling water jacket which can control the heat flow to some extent, but conditions are often too severe for some resins, which may degrade during the process.
Three-roll, one-roll, and stone mills constitute a more mature dispersion technology still in use with medium viscosity millbases. A three-roll mill consists of the feed, center, and apron rolls. In roll mill operation:
The speeds of feed and apron rolls are adjustable, and each roll rotates with a different speed in order to induce shear in the material at the nip and facilitate the material transfer from one roll to the other
For mechanical reasons the gap between rolls cannot be less than 10 m and it usually ranges from 40 to 50 m.7 Small particles will not be affected as they pass through the nip, but agglomerates smaller than the distance between rolls will be disintegrated due to the shear stress imposed on the material
The one-roll mill works on a similar principle but the nip is regulated by a pressure bar. Shearing takes place between the roller and the shearing bar. Stone mills have similar principles of operation. The rotor turns on a stator to achieve shearing
With current raw materials, both the primary particles and agglomerates are very small, and if any positive action can be achieved during the milling process, it can only be done by affecting these small particles. It is thus necessary to operate these machines at very tight gaps which causes abrasion of the mechanical elements, rapid deterioration of equipment, and contamination of the product by the abraded material. This affects the properties of the millbase and the color of the product
The high-speed impeller or shear mixer is the most common equipment to prepare dispersions of solids in liquid. High speed shear mills and kinetic shear mills have retained their usefulness because of their ability to deagglomerate material that is not adequately dispersed in the premixing step. A high-speed shear mill is composed of two elements a container and an impeller. These factors are important in the design:
In the first stage, the viscosity changes from low to high as fillers are incorporated; in the second stage, viscosity remains constantly high because of the disintegration of particles which occurs during the application of the highest shear stress
Long mixing increases temperature and decreases viscosity. This does not provide the conditions for the best filler dispersion. By extending mixing over, for example, a 15 min period, the degree of dispersion is not improved, but the resin may actually be degraded
If the quality of dispersion is not satisfactory, the parameters of mixing should be changed. If the expected result cannot be attained, the range of conditions available is not adequate in this particular mill
In the third stage, the viscosity changes from high to low due to the addition of diluent. The viscosity range which can be handled by high speed mixers is similar to the range of a three-roll mill, i.e., up to about 200 Poise
The range of shear rates available in high-speed mixers is not broad. The flow rate of fluid in motion decreases as viscosity increases and is inversely proportional to the width of the flow passage which, in this case, is the distance between the disperser and the container which is very large in a high speed mixer. It is not so much due to an improvement in mixing equipment that high-speed mixers have become so popular, it is mostly because of the high quality raw materials (pigments, fillers) which are available now. High structure carbon blacks can be more easily dispersed. But with the increased structure, the size of the primary particles decreases, inhibiting dispersion. Because of the interrelation between both parameters, only the medium structure, coarser particles of carbon blacks can be dispersed by high-speed mixers. Other carbon black types demand further treatment. It should be noted that this is only true of a few fillers which are known to possess strongly bonded, small sized particles. In most cases, fillers can be successfully dispersed in high-speed mixers. However, care should be taken that the filler is selected with an appropriate particle size.
High-speed mixers have several important advantages over other existing equipment including the possibility of processing a batch in the same vessel, easy cleaning, and flexibility in color changes. The main disadvantage is that the final dispersion depends greatly on the chosen composition and technology, and these are sometimes limiting factors. Frequently, the proper conditions for quality dispersion cannot be achieved at all.
The basic construction of a high-speed mixer can easily be modified to one's special requirements. For example, a change from impeller to turbine rotor changes both the principle of dispersion and the range of application. The tangential velocities of filler particles can be as high as 500 m/sec. Such particles have a very high kinetic energy, sufficient to cause size reduction. Size reduction is due to particle-particle or particle-wall collisions, and this in turn, is related in efficiency to the relative velocities at the moment of collision. Relative velocity can be increased by decreasing the viscosity of the millbase. The upper limit of millbase viscosity is somewhere around 3 to 4 Poise. It is not viscosity alone which is important but the entire rheological character of the millbase. The best results are obtained when the millbase is nearly Newtonian. For this reason, the dispersion process is best performed in a diluted millbase. As is the case with high-speed mixers, a proper dispersion should be achieved in a matter of 10-20 min. If such is not the case, the conditions of processing should be modified. Once dispersion has been achieved, it should be stabilized, with the mixer continuously running, by the addition of more resin to increase the viscosity in order to prevent sedimentation or flocculation of the pigment.
The other possible modification to such a mixer can be achieved by a substantial lowering of the speed and a change in the motion of the mixing element to planetary. This configuration can process material of a much higher viscosity, up to several thousand Poise. The high speed mixer can be modified in various ways to match its capabilities to the process requirements. Stationary baffles may be added to increase the shear rate. The distance between the rotating and stationary elements can be decreased again increasing the shear rate. The mixer may be designed to work under both pressure and vacuum and with inert gas blanketing which permits deaeration and processing of volatile or moisture sensitive materials.
The other group includes heavy-duty mixers, such as the Banbury mixer and double-arm kneading mixers. The Banbury mixer with a power input of up to 6000 kW/m3 is the strongest and the most powerful mixing unit used by industry. Nearly solid materials are mixed by a rotor which is a heavy shaft with stubby blades rotating at up to 40 rpm. The clearance between the walls and rotor is very small, which induces a very high shear in the material. The high shear generates a great amount of heat which melts the polymer rapidly and allows for quick incorporation of filler. After the filler is incorporated, the dispersion process begins, with rapid distributive mixing along and between two rotors and between the chamber walls and rotor tips. Within 2-3 min, mixing is normally completed and the compound discharged into a pelletizing extruder or a two-roll mill which converts it to a sheet form.8 Carbon black, which is most frequently processed in a Banbury mixer, is usually placed between two layers of polymeric material in order to reduce dusting.
Double-arm kneading mixers are very popular in some industries. They consist of two counter-rotating blades in a rectangular trough carved at the bottom to form two longitudinal half cylinders and a saddle section. A variety of blade shapes are used, with a clearance between them and the blades and the side walls of up to 1 mm. The most popular blade shapes include: sigma, dispersion, multiwiping overlapping, single-curve, and double-naben blades. It is important for filler dispersion in this mixer that the viscosity of the millbase be kept high enough to create the required shearing force to disperse the material. The strong construction of the mixer and its high power allow one to work with concentrated compositions of pigments which could not be processed by any other method.
High volume production is done by mixing in an extruder.11 This method offers several advantages such as a continuous process, material uniformity, a clean environment, high output, and low labor. The biggest disadvantage of this method is a high investment cost. The twin-screw extruder is the most flexible type of extruder and most appropriate for compounding. Their screw designs can be varied as can the method of dosing and the output rate. The abrasiveness of the filler may affect the life-span of the equipment, and particle size and its distribution may influence the quality of filler dispersion and material uniformity. But in general, there is adequate machinery available for almost all requirements. For instance, glass-fiber reinforced materials can be produced by this technique with little change to the initial structure and dimensions of the glass fibers, which shows the versatility of the technology. The production rate of this method is comparable to the Banbury mixer, and an additional advantage comes from the fact that the material can be completely processed in one pass through the machinery.
The importance of the proper dispersion of fillers and the complexity of techniques for measuring the degree of dispersion are reflected in numerous publications. Further information on the mixing of fillers is included in Chapter 18.
The renewable power sources are being explored due to possibility of lack in availability of conventional resources in future. The major drawback of Renewable energy resources are dependency on geographical locations and environmental conditions however, the high initial cost, increased maintenance cost, and different rates of depreciation are the main challenges associated with these hybrid systems. The irregular pattern of natural resources necessitates developing a hybrid system which can generate maximum conceivable energy for continuous and reliable operations . The design of hybrid system is influenced by various factors such as condition of sites, energy availability, efficiency of energy sources as well as technical and social limitation In this specific situation, a combination of optimal sizing method is an indispensable factor to accomplish higher reliability quality with least expense [21,79,87,149]. The fundamental parts of the hybrid energy systems are renewable power source, nonrenewable generators, control unit, storage system, load or grid some times, sources and load may be AC/DC .
An arrangement of the renewable power generation with appropriate storage and feasible amalgamation with conventional generation system is considered as hybrid energy system or some time referred as a micro grid . This system may be any probable combination of Photovoltaic, wind, micro turbines, micro hydro, conventional diesel generation, battery storage, hydrogen storage and Fuel Cell in grid-connected or off grid arrangement,
An assembly of interconnected loads, conventional distributed energy resources like distributed generators (DG), renewable resources and energy storage systems in a specified boundary as a controllable single entity referred as micro grid. It may be eternally connected to grid, or isolated by grid. There are worldwide numerous remote communities those are not directly connected to grid, and fulfill electricity demand from distributed generators based on fossil fuel in isolated Microgrids[97,165]. In this paper a assimilated arrangement of solar PV and wind renewable energy resources is discussed which is slightly different from the concept of microgrid.
Solar Photovoltaic /Wind based Hybrid Energy System shows its adequacy to provide the essential electrical demand for off grid utilization. The at most imperative feature of a Solar Photovoltaic (PV) and Wind based Hybrid Energy System is that it uses at least two sustainable power sources which enhances reliability, efficiency and financial restrictions emerges from single energy resources of renewable nature [18,89,133]. Solar Photovoltaic and Wind based Hybrid Energy System is considered as amalgamation of solar PV panel, Wind mills, charge controller, storage system, power conditioning units, diesel based generator set and load . The assessment of performance of Hybrid system can be done by recreating their models at Simulink platform for the accessible insulation, speed of wind, electrical load and various components .
The essential objective for evaluation of Hybrid System are building up the suitable models for various components and their simulation in a sequential manner as firstly availability of speed wind, accessibility of sunlight and the demand of load models are simulated after that model of battery storage and diesel generator can be Simulated. Last strides in the entire procedure of assessment is deciding the coveted criteria and exploring the optimum structure of system. . The optimal hybrid system arrangement should satisfy and compromise the objectives of power reliability and cost of system. The load demand frequently considered as limitation of the optimization issue and ought to be totally satisfied . The solar PV/wind hybrid system is mostly reliant on execution of individual segments. To estimate the performance of solar PV/wind hybrid system, individual components are modeled initially after that entire system evaluated to meet the demand . In general key aspects to analyze a hybrid system are hybrid system configuration with respect to the available resources, the optimization of the available renewable resources exploitation and the optimization of the output power quality .
Solar energy and wind energy are analogous to each other in nature and both are well appropriate to develop a hybrid system . Availability of solar radiations are relatively greater in summer, winds are more accessible in the evening times of winters. This hybrid renewable energy systems give a more reliable output throughout the year can be planned to fulfill craved qualities on more decreased possible cost . The constraints of Photo voltaic system, the assessed energy of wind energy system and the battery storage are the majorly considered parameters for evaluation of solar and wind based hybrid energy system. In addition, the precise angular attitude of Photo voltaic panels and the tower height of wind turbines are considered for achieving the minimum levelised cost of energy. Ribeiro  proposed multi-criteria based analytical decision scheme abbreviated as MCDA which consider several issues like economic, quality of life, technical and environmental issues of local populations.
Metrological data based on technological, economical, socio-political and environmental factors having major impact for estimation and selection of various components of Solar Photovoltaic and Wind based Hybrid Energy System . Hourly climate information as sun oriented radiation, wind speed and temperature are raw information illustrates the inconstancy of the parameter input. Place to place data is hard to obtain for designing purpose at remote location [3,73]. Statistical metrological climatic information can be delivered by the average of month to month meteorological information. The information of climate can be anticipated from an adjacent site or synthetically can be produced . Simulation for performance of Solar PV/Wind Hybrid Energy System required climate data including solar radiation, speed of wind and temperature which can be find from web sources and also from local meteorological station, it is best to find realistic solution preference should be given to the specified location based weather data . To optimize solar photovoltaic and wind based hybrid energy system are hourly or day by day climate information of solar and wind energy are considered as required significant inputs . Meteorological data determined the receptiveness and amount of sunlight based radiation and wind energy sources at a particular region. An investigation of characteristics of sun based emission and availability of wind at a specific location ought to be concluded before starting . Bianchini A et al. gives stress on the metrological data in the form of solar irradiance and wind distribution and considered hybrid renewable energy system as a amalgamation of PV panel of rated power, horizontal axis wind turbine of rated power, a diesel generator of precise nominal power able to manage peak load and a battery bank of specific storage capacity . Hall et al.  proposed the well-known engineered climate information term Typical meteorological year (TMY) utilized in simulation of solar energy model is first time. It is observational technique picking particular months from different years using the Fleckenstein Schafer accurate system .
load demand play a very important role in establishment of solar PV/wind hybrid renewable energy system provides more reliable power for off-grid and standalone applications compared to individual systems  The most of the reviewed studies are about the alone Solar Photovoltaic /Wind based Hybrid Energy System and few studies are available for grid connected system. The unsatisfied load request is procured from the grid. Along this way the hybrid system became noticeably trustworthy. The stand-alone systems with storage infused surplus energy to the grid at a prime cost. Along these lines, the grid connected system becomes more financially acceptable.
A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.
By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.
The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.
where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles as well as collision energy. These forces are derived from the rotational motion of the balls and the movement of particles within the mill and contact zones of colliding balls.
By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).
With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls remain attached to the wall with the aid of centrifugal force is:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).
The degree of filling the mill with balls also influences the productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 30%35% of its volume.
The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.
where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption. A mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation .
Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.
Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.
Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.
For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.
With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.
The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.
Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.
When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.
In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).
Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15
Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.
Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.
For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.
In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.
Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.
Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.
For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.
The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.
Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.
Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.
Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.
As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.
Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.
The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.
The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.
The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.
Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.
Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.
Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.
More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling . However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.
In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.
A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.
It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C .
Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles . Fig. 12 shows the SEM image of the iron nanoparticles.
The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).
The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere. Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.
In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal. and El-Eskandarany etal. proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.
In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.
Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation, using for example cold-rolling approach, as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.
Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial and the effective centrifugal force reaches up to 20 times gravitational acceleration. The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed.
In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).
Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.
Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).
The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.
Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).
Although it was developed nearly 50 years ago, Bonds method is still useful for calculating necessary mill sizes and power consumption for ball and rod mills. This paper discusses the basic development of the Bond method, the determination of the efficiency correction factors based on mill dimensions and feed characteristics, and the application of the results to designing grinding circuits.
The development of the ball mill during the twentieth century has been described as the most significant development in the machinery for performing the grinding of ores (Lynch and Rowland 2006). A key part of the implementation of ball mills was the development of the ability to predict their performance in the plant based upon grindability data from standardized tests performed in small-diameter laboratory mills.
In 1930, Allis-Chalmers hired Fred Bond to design and build a laboratory for testing ores and grains, for minerals processing and flour milling, and to conduct research for processes for the treatment of ores and grains. Bonds first developments for grinding ores and rocks are now known as the Bond rod milling and ball milling grindability tests. The grindability results from these tests are still reported as net grams produced per revolution of the test mill. Bond carried out two studies using his grindability tests:
1. The first study was to determine if either of the two existing theories of comminution the Rittenger theory or the Kick theory were correct. Bond concluded that neither was correct. He developed a theory that the energy required for comminution was a function of the difference in the square root of the size of theparticles in the feed and in the product of the material being comminuted. This is known as Bonds third theory of comminution.
2. The second study was used to develop a correlation between ball mill operating data and grindability test data. This was based on the Work Index concept. From this concept came two equations: (a) the equation to determine the Work Index from Bond grindability tests, and (b) the Bond equation, which uses the Work Index to determine the energy needed for grinding.
When Bond introduced the Work Index concept (Bond 1952), he introduced a new method for determining the energy required for grinding ores and outlined a mathematical method for using the Work Index to design grinding circuits. Though it is an empirical procedure, even at this time, there is little prospect that the Work Index will be replaced as a tool for determining the energy required to grind a mineral or ore. Numerical examples of the use of the Work Index are given in this paper. The calculations are based upon measurements giving the amount of the size reduction by the difference in the size distributions of the feed (F) and the product (P).
Early in the twentieth century, ball mill manufacturers and other research laboratories developed proprietary batch-grinding tests to measure the resistance of metallic mineral ores, industrial mineral ores, cement raw materials, cement clinkers, and related materials being ground to fine sizes in rod and ball mills. These tests, called grindability tests, were needed to help determine the energy required to grind these materials from a coarse-sized feed to the desired product size. During the first half of the century, none of these test methods could be used to directly determine the energy needed for grinding at plant size capacity. If grinding tests for direct determination of the energy required were needed, pilot-plant tests in small-diameter continuous operating ball mills were run. Pilot-plant grinding data always had to be adjusted for scale-up factors to larger-diameter mills.
As the developer and manufacturer of industry-leading particle size reduction equipment, including Attritors (internally agitated ball mills) and DMQX horizontal media mills, Union Process is uniquely positioned to help you identify and source the correct grinding media for your application.
Union Process customers know they can rely on our extensive technical expertise and years of experience to ensure they get the right grinding media at the right time and the right price for their specific needs. Working in close consultation with our customers, our skilled technical service representatives reviewcustomer requirements like final particle size, physical compatibility and contamination concernsand then recommend media with the right characteristics, including:
metallic grinding medialike carbon steel, forged steel, stainless steel or chrome steel grinding balls are best for some applications, while others requirenon-metallic mediamade of alumina, ceramics, glass, silicon carbide, zirconium oxide or other specialized materials
Union Process is the source for the most up-to-date information on grinding balls and other media. Download our Grinding Media Literature (PDF) to view a detailed sheet, outlining factors to consider when selecting grinding media, along with specifications on the most common types of media.
Offering the optimal combination of grinding media knowledge and manufacturing expertise, Union Process takes your entire operation into account to identify the best grinding media to consistently generate the final particle size and shape required by your application, optimizing the cost effectiveness of your process and extending the life of your mills.
Backed by our long-standing commitment to customer satisfaction, we ensure quality manufacturing and reliable supply of grinding balls and media specifically engineered to meet the requirements of your most demanding milling applications.
Through-Hardened Carbon Steel Balls are magnetic and can be used in the food industry along with 440C stainless steel media. They are a low-cost media that are superior and recommended over case-hardened carbon steel media which have a soft core. They are packaged with no oil finishalways dry packedas they will rust in water.
Chrome Balls (steel type 52100) are through-hardened and tempered steel balls designed to achieve maximum strength and quality. Ball hardness is in the 6067 HRC range. They wear better than 440C stainless steel and through-hardened carbon steel. They are also recommended for applications where a through-hardened steel ball is needed in larger sizes ( and larger). They are sometimes packaged with a very light oil finish to reduce rust due to humidity.
440C Stainless Steel Balls are through-hardened and tempered throughout for maximum strength and quality. They are magnetic, and corrosion-resistant (generally rust-resistant). They are recommended for food applications and lighter colored slurries.
Forged Steel Balls are used for gold mining, cement factories, oil processing and large scale industrial applications. They are made by machine (standard) sizes 20mm75mm. They are manually made (hit by air hammer) sizes 75mm125mm. They have hardness 5563 HRC. All forged balls are through-hardened, and shipped in 55-gallon steel drums. They range in sizes from 20mm125mm. Standard lead time is 68 weeks FOB Akron, Ohio USA.
High Chrome Steel Balls can be used for many different applications. They are available in two grades1013% chrome (surface hardness 60 HRC, core hardness 58 HRC), and 1418% chrome (surface hardness 62 HRC, core hardness 60 HRC). They have a very rough black finish which quickly wears off during initial milling. After that, they have an excellent wear rate. They are available in sizes 6mm120mm. They are shipped in 55-gallon steel drums with standard lead time of 68 weeks FOB Akron, Ohio USA.
Union Process is the one source for 90%, 94%, 99.5% and 99.9% alumina media. 90% alumina is available in satellites and rod/cylinders. 94% alumina balls have excellent wear resistance with higher impact strength to save running costs with less contamination. They have great wear rate generally better than 90% or 99.5%. 99.5% alumina balls have the highest alumina content for a moderate price. The .5% impurity is MgO that is added to inhibit grain growth during sintering in the kiln. There is less than 0.1% silica in the media. 99.9% alumina balls are made of very pure and reactive (expensive) raw materials. They are for high purity alumina applications where contamination is a factor.
Alumina Beads are specially formulated to be used in high-energy mills in which a high degree of fineness is required. They are used in various industrial fields such as inks, paints, advanced ceramics, mining, cosmetics and pharmaceutical industries. They are a perfectly spherical shape with high mechanical properties and high wear resistance at a moderate price.
Silicon Carbide Balls are very high-cost grinding media that are used for milling same materials (silicon carbide ball to mill silicon carbide materials) to avoid contamination. They are only available in 5mm,10mm, 15mm and 20mm sizes. They are a special order item.
Silicon Nitride Balls are very high-cost grinding media that are used for milling same materials (silicon nitride ball to mill silicon nitride materials) to avoid contamination. They are now available in 2mm and 3mm and sizes up to 25mm. They are a special order item.
Tungsten Carbide media is the hardest and densest (highest specific gravity) media and is available in both satellites and balls upon request. They are available in sizes ranging from 3/32 to 1 in diameter. They are a high-cost media and are a special order item.
Zirconium Oxide Balls (95% ZrO2) are the strongest, best wearing ceramic media for metal-free, pharmaceutical and food processing grinding. These balls have a white, shiny appearance. They are also available in 38 and cylinders. This 95% grade is high-cost.
Zirconium Oxide SatellitesCeria stabilized (rare earth)are a cheaper zirconium oxide alternative for metal-free applications. They are a brown, shiny ball media that come in size ranges in the smallest sizes (ex: 0.4 0.6mm), then at 6mm come in uniform sizes (6mm, 8mm, etc.) up to 31mm.
Zirconium Silicate Beads are available in fused 68% ZrO2 beads which are a standard reliable media at low cost, and sintered 58% ZrO2 beads which have high breakage resistance, are durable and cost effective. They are used to microgrind paints, inks, dyes, magnetic coatings, minerals, agrochemicals and ceramics.
NOTE: Grinding balls and media are sold on a per pound basis, but ATTRITORS and DMQX-Mills are loaded by volume. Therefore, the more dense the media, the more pounds of media required. For instance, a machine requiring only 40 lbs. of stainless steel may require up to 80 lbs. of tungsten carbide. Information contained herein is accurate and reliable to the best of our knowledge, but our suggestions and recommendations cannot be guaranteed because the conditions of use are beyond our control.
Our skilled technical service representatives also are available for consultation and are always ready to assist you with the selection of grinding media that best answer your particular needs. For assistance, contact Media Sales by calling (330) 929-3333 x228 or byemailing us.