Calculating a grinding circuits circulating loads based on Screen Analysis of its slurries. Compared to %Solids or Density basedCirculating load equations, amore precise method of determining grinding circuit tonnages uses the screen size distributions of the pulps instead of the dilution ratios. Pulp samples collected around the ball mill or rod mill and hydrocyclones, screenor classifier (classification system) are screened and the cumulative weight percentage retained is calculated for several mesh sizes to obtain the Ball Mill Circulating Load.
Although the functions of the classifying device and the mill in a grinding circuit are quite different, the performance of each is interrelated and should be viewed as a single unit operation. The performance of the mill is influenced by the quantity and quality of the circulating load being returned to the mill. The performance of the cyclone is influenced by the particle size distribution of the mill discharge. The net result of the unit operation is a reduction in size of the new feed tonnage from a prescribed top size to a desired product size.
It is the above interrelationship that governs both the capacity of the grinding circuit and the desired grind. The capacity of the grinding circuit is dictated by the number of tons of feed coarser than the desired product size that the mill is capable of grinding finer than the desired product size. A the mill produces a diminishing percentage of finished material as the new feed rate increases. To maintain the same product distribution, the coarser mill discharge requires a larger circulating load to return the coarse fraction back to the mill. Therefore, to increase circuit capacity or produce a finer grind, higher circulating loads are required.
As the circulating load increases, typically the cyclone underflow density increases, causing the density and viscosity in the mill to also increase. This can lead to excessive mill viscosity, causing the balls to float, leading to a sharp drop in the power draw. The operator can be misled to conclude that the mill is overloaded due to the higher circulating load. The operator will usually sharply reduce the feed rate, when actually an increase in dilution water at the feed end of the mill will lower the mill viscosity and allow operation at the higher circulating load.
As the circulating load increases to higher levels, it is also not uncommon for the pump or the cyclone to become the capacity bottleneck due to volume constraints. These volume constraints are not always readily apparent, leading to the assumption that the increased circulating load has limited the ball mill capacity. Therefore, changes to the pump and cyclone may be required to handle the higher circulating load caused by an increased new feed rate.
The circulating loads generated in a typical ball mill cyclone circuit contain a small fraction of bypassed fines. The concept that high circulating loads will result in overgrinding can be refuted by regarding increases in circulating load in the same vein as multistage grinding. That is, for every incremental increase in circulating load of 100%, an additional stage of grinding and classification occurs. With high circulating loads, the particle size reduction per pass is proportionately less and the generation of slimes considerably less than with low circulating loads or open circuit grinding. This is due to the lower residence time per pass.
In a properly designed circuit, each finely ground particle of the required product size has ample opportunity to exit the grinding circuit after each pass through the mill. As a result, few -10 micron fines are generated in the grinding circuit. The presence of these fines are likely attributed to the inherent fines in the new mill feed.
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).
Am sure your BallMill is considered the finest possible grinding mill available. As such you will find it is designed and constructed according to heavy duty specifications. It is designed along sound engineering principles with quality workmanship and materials used in the construction of the component parts. YourBallMill reflects years of advancement in grinding principles, materials, and manufacturing techniques. It has been designed with both the operators and the erectors viewpoints in mind. Long uninterrupted performance can be expected from it if the instructions covering installation and maintenance of the mill are carried out. You may be familiar with installing mills of other designs and manufacture much lighter in construction. YourBallis heavy and rugged. It should, therefore, be treated accordingly with due respect for its heavier construction.
The purpose of this manual is to assist you in the proper installation and to acquaint you a bit further with the assembly and care of this equipment. We suggest that these instructions be read carefully and reviewed by everyone whenever involved in the actual installation and operation of the mill. In reading these general instructions, you may at times feel that they cover items which are elementary and perhaps not worthy of mention; however in studying hundreds of installations, it has been found that very often minor points are overlooked due to pressure being exerted by outside influences to get the job done in a hurry. The erection phase of this mill is actually no place to attempt cost savings by taking short cuts, or by-passing some of the work. A good installation will pay dividends for many years to come by reduced maintenance cost.With the modern practice of specialized skills and trades, there is often a line drawn between responsibilities of one crew of erectors and another. Actually the responsibility of installation does not cease with the completion of one phase nor does it begin with the starting of another. Perhaps a simple rule to adopt would be DO NOT TAKE ANYTHING FOR GRANTED. This policy of rechecking previously done work will help guarantee each step of the erection and it will carefully coordinate and tie it into subsequent erection work. To clarify or illustrate this point, take the example of concrete workers completing their job and turning it over to the machinist or millwright. The latter group should carefully check the foundation for soundness and correctness prior to starting their work.
Sound planning and good judgement will, to a great extent, be instrumental in avoiding many of the troublesome occurrences especially at the beginning of operations. While it is virtually impossible to anticipate every eventuality, nevertheless it is the intention of this manual to outline a general procedure to follow in erecting the mill, and at the same time, point out some of the pitfalls which should be avoided.
Before starting the erection of the mill, adequate handling facilities should be provided or made available, bearing in mind the weights and proportions of the various parts and sub-assemblies. This information can be ascertained from the drawings and shipping papers.
The gearing, bearings, and other machined surfaces have been coated with a protective compound, and should be cleaned thoroughly with a solvent, such as Chlorothene, (made by Dow Chemical). Judgement should be exercised as to the correct time and place for cleaning the various parts. Do not permit solvents, oil or grease to come in contact with the roughened top surfaces of the concrete foundation where grouting is to be applied; otherwise proper bonding will not result.
After cleaning the various parts, the gear and pinion teeth, trunnion journals and bearings, shafting and such, should be protected against rusting or pitting as well as against damage from falling objects or weld splatter. All burrs should be carefully removed by filing or honing.
Unless otherwise arranged for, the mill has been completely assembled in our shop. Before dismantling, the closely fitted parts were match marked, and it will greatly facilitate field assembly to adhere to these match marks.
The surfaces of all connecting joints or fits, such as shell and head flanges, trunnion flanges, trunnion liner and feeder connecting joints, should be coated with a NON-SETTING elastic compound, such as Quigley O-Seal, or Permatex to insure against leakage and to assist in drawing them up tight. DO NOT USE WHITE LEAD OR GREASE.
Parts which are affected by the hand of the mill are easily identified by referring to the parts list. In general they include the feeder, feed trunnion liner, discharge trunnion liner if it is equipped with a spiral, spiral type helical splitter, and in some cases the pan liners when they are of the spiral type. When both right and left hand mills are being assembled, it is imperative that these parts which involve hand be assembled in the correct mill.
Adequate foundations for any heavy equipment, and in particular grinding mills, are extremely important to assure proper operation. The foundation should preferably be in one piece, that is, with a reinforced slab footing (so called mat) extending under both trunnion bearing foundations as well as the pinion bearing foundation. If possible or practical, it should be extended to include also the motor and drive. With this design, in the event of some movement, the mill and foundation will tend to move as a unit. ANY SLIGHT SETTLING OF FOUNDATIONS WILL CAUSE BEARING AND GEAR MISALIGNMENT, resulting in excessive wear and higher maintenance costs. It has been found that concrete foundations on a weight basis should be at least 1 times the total weight of the grinding mill with its grinding media.
Allowable bearing pressure between concrete footings and the soil upon which the foundation rests should first be considered. The center of pressure must always pass through the center of the footing. Foundations subject to shock should be designed with less unit pressure than foundations for stationary loads. High moisture content in soils reduces the amount of allowable specific pressure that the ground can support. The following figures may be used for preliminary foundation calculations.
Portland cement mixed with sand and aggregate in the proper proportions has come to be standard practice in making concrete. For general reference cement is usually shipped in sacks containing one cubic foot of material. A barrel usually holds 4 cubic feet. Cement will deteriorate with age and will quickly absorb moisture so it should be stored in a dry place. For best results the sand and gravel used should be carefully cleaned free of humus, clay, vegetal matter, etc.
Concrete may be made up in different mixtures having different proportions of sand and aggregate. These are expressed in parts for example a 1:2:4 mixture indicates one bag of cement, 2 cubic feet of sand, and 4 cubic feet of gravel. We recommend a mixture of 1:2:3 for ball mill and rod mill foundations. The proper water to sand ratio should be carefully regulated since excess water increases the shrinkage in the concrete and lends to weaken it even more than a corresponding increase in the aggregate. Between 5 to 8 gallons of water to a sack of cement is usually recommended, the lower amount to be used where higher strength is required or where the concrete will be subject to severe weathering conditions.
Detailed dimensions for the concrete foundation are covered by the foundation plan drawing submitted separately. The drawing also carries special instructions as to the allowance for grouting, steel reinforcements, pier batter, foundation bolts and pipes. During concrete work, care should be taken to prevent concrete entering the pipes, surrounding the foundations bolts, which would limit the positioning of the bolts when erecting the various assemblies. Forms should be adequately constructed and reinforced to prevent swell, particularly where clearance is critical such as at the drive end where the gear should clear the trunnion bearing and pinion bearing piers.
For convenience in maintenance, the mill foundations should be equipped with jacking piers. These will allow the lifting of one end of the mill by use of jacks in the event maintenance must be carried out under these conditions.
Adequate foundations for any heavy equipment, and in particular Marcy grinding mills, are extremely important to assure proper operation of that equipment. Any slight settling of foundations will cause bearing and gear misalignment, resulting in excessive wear and higher maintenance costs. It has been found that concrete foundations on a weight basis should be approximately 1 times the total weight of the grinding mill with its grinding media.
Allowable bearing pressure between concrete footings and the soil upon which the foundation rests should first be considered. The center of pressure must always pass through the center of the footing. Foundations subject to shock should be designed with less unit pressures than foundations for stationary loads. High moisture content in soils reduces the amount of allowable pressure that that material can support. The following figures may be used for quick foundation calculations:
Portland cement mixed with sand and aggregate in the proper proportions has come to be standard practice in making concrete. For general reference cement is usually shipped in sacks containing one cubic foot of material. A barrel usually consists of 4 cubic feet. Cement will deteriorate with age and will quickly absorb moisture so it should be stored in a cool, dry place. The sand and gravel used should be carefully cleaned for best results to be sure of minimizing the amount of sedimentation in that material.
Concrete may be made up in different mixtures having different proportions of sand and aggregate. These are expressed in parts for example a 1:2:4 mixture indicates one bag of cement, 2 cubic feet of sand, and 4 cubic feet of gravel. We recommend a mixture of 1:2:3 for ball mill and rod mill foundations. The proper water to sand ratio should be carefully regulated since excess water will tend to weaken the concrete even more than corresponding variations in other material ratios. Between 5 to 8 gallons of water to a sack of cement is usually recommended, the lower amount to be used where higher strength is required or where the concrete will be subject to severe weathering conditions.
We recommend the use of a non-shrinking grout, and preferably of the pre-mixed type, such as Embeco, made by the Master Builders Company of Cleveland, Ohio. Thoroughly clean the top surfaces of the concrete piers, and comply with the instructions of the grouting supplier.
1. Establish vertical and horizontal centerline of mill and pinion shaftagainst the effects of this, we recommend that the trunnion bearing sole plate be crowned so as to be higher at the center line of the mill. This is done by using a higher shim at the center than at the endsand tightening the foundation bolts of both ends.
After all shimming is completed, the sole plate and bases should be grouted in position. Grouting should be well tamped and should completely fill the underside of the sole plate and bases. DO NOT REMOVE THE SHIMS AFTER OR DURING GROUTING. When the grout has hardened sufficiently it is advisable to paint the top surfaces of the concrete so as to protect it against disintegration due to the absorption of oil or grease.
If it is felt that sufficient accuracy in level between trunnion bearing piers cannot be maintained, we recommend that the grouting of the sole plate under the trunnion bearing opposite the gear end be delayed until after the mill is in place. In this way, the adjustment by shimming at this end can be made later to correct for any errors in elevation. Depending on local climatic conditions, two to seven days should he allowed for the grouting to dry and set, before painting or applying further loads to the piers.
Pinion bearings are provided of either the sleeve type or anti-friction type. Twin bearing construction may use either individual sole plates or a cast common sole plate. The unit with a common sole plate is completely assembled in our shop and is ready for installation. Normal inspection and cleaning procedure should be followed. Refer to the parts list for general assembly. These units are to be permanently grouted in position and, therefore, care should be taken to assure correct alignment.
The trunnion bearing assemblies can now be mounted on their sole plates. If the bearings are of the swivel type, a heavy industrial water-proof grease should be applied to the spherical surfaces of both the swivels and the bases. Move the trunnion bearings to their approximate position by adjustment of the set screws provided for this purpose.
In the case of ball mills, all internal wearing parts will pass through the manhole, whereas in the case of open end rod mills they will pass through the discharge trunnion opening. When lining the shell, start with the odd shaped pieces around the manhole opening if manholes are furnished. Rubber shell liner backing should be used with all cast type rod mills shell liners. If the shell liners are of the step type, they should be assembled with the thin portion, or toe, as the leading edge with respect to rotation of the mill.
Lorain liners for the shell are provided with special round head bolts, with a waterproof washer and nut. All other cast type liners for the head and shell are provided with oval head bolts with a cut washer and nuts. Except when water proof washers are used, it is advisable to wrap four or five turns of candle wicking around the shank of the bolt under the cut washer. Dip the candle wicking in white lead. All liner bolt threads should be dipped in graphite and oil before assembly. All liner bolt cuts should be firmly tightened by use of a pipe extension on a wrench, or better yet, by use of a torque wrench. The bolt heads should be driven firmly into the bolt holes with a hammer.
In order to minimise the effect of pulp race, we recommend that the spaces between the ends of the shell liners and the head liners or grates be filled with suitable packing. This packing may be in the form of rubber belting, hose, rope or wood.
If adequate overhead crane facilities are available, the heads can be assembled to the shell with the flange connecting bolts drawn tightly. Furthermore, the liners can be in place, as stated above, and the gear can be mounted, as covered by separate instructions. Then the mill can be taken to its location and set in place in the trunnion bearings.
If on the other hand the handling facilities are limited it is recommended that the bare shell and heads be assembled together in a slightly higher position than normal. After the flange bolts are tightened, the mill proper should be lowered into position. Other intermediate methods may be used, depending on local conditions.
In any event, just prior to the lowering of the mill into the bearings the trunnion journal and bushing and bases should be thoroughly cleaned and greased. Care should be taken not to foul the teeth in the gear or pinion. Trunnion bearing caps should be immediately installed, although not necessarily tightened, to prevent dirt settling on the trunnions. The gear should be at least tentatively covered for protection.
IMPORTANT. Unless the millwright or operator is familiar with this type of seal, there is a tendency to assume that the oil seal is too long because of its appearance when held firmly around the trunnion. It is not the function of the brass oil seal band to provide tension for effective sealing. This is accomplished by the garter spring which is provided with the oil seal.
Assemble the oil seal with the spring in place, and with the split at the top. Encircle the oil seal with the band, keeping the blocks on the side of the bearing at or near the horizontal center line so that when in place they will fit between the two dowel pins on the bearing, which are used to prevent rotation of the seal.
Moderately tighten up the cap screws at the blocks, pulling them together to thus hold the seal with its spring in place. If the blocks cannot be pulled snuggly together, then the oil seal may be cut accordingly. Oil the trunnion surface and slide the entire seal assembly back into place against the shoulder of the bearing and finish tightening. Install the retainer ring and splash ring as shown.
In most cases the trunnion liners are already mounted in the trunnions of the mills. If not, they should be assembled with attention being given to match marks or in some cases to dowel pins which are used to locate the trunnion liners in their proper relation to other parts.
If a scoop feeder, combination drum scoop feeder or drum feeder is supplied with the mill, it should be mounted on the extended flange of the feed trunnion liner, matching the dowel pin with its respective hole. The dowel pin arrangement is provided only where there is a spiral in the feed trunnion liner. This matching is important as it fixes the relationship between the discharge from the scoop and the internal spiral of the trunnion liner. Tighten the bolts attaching the feeder to the trunnion liner evenly, all around the circle, seating the feeder tightly and squarely on its bevelled seat. Check the bolts holding the lips and other bolts that may require tightening. The beveled seat design is used primarily where a feeder is provided for the trunnion to trunnion liner connection, and the trunnion liner to feeder connection. When a feeder is not used these connecting joints are usually provided by a simple cylindrical or male and female joint.
If a spout feeder is to be used, it is generally supplied by the user, and should be mounted independently of the mill. The spout should project inside the feed trunnion liner, but must not touch the liner or spiral.
Ordinarily the feed box for a scoop tender is designed and supplied by the user. The feed box should be so constructed that it has at least 6 clearance on both sides and at the bottom of the scoop. This clearance is measured from the outside of the feed scoop.
The feed box may be constructed of 2 wood, but more often is made of 3/16 or plate steel reinforced with angles. In the larger size mills, the lower portion is sometimes made of concrete. Necessary openings should be provided for the original feed and the sand returns from the classifiers when in closed circuit.
A plate steel gear guard is furnished with the mill for safety in operation and to protect the gear and pinion from dirt or grit. As soon as the gear and pinion have been cleaned and coated with the proper lubricant, the gear guard should be assembled and set on its foundation.
Most Rod Mills are provided with a discharge housing mechanism mounted independently of the mill. This unit consists of the housing proper, plug door, plug shaft, arm, and various hinge pins and pivot and lock pins. The door mechanism is extra heavy throughout and is subject to adjustment as regard location. Place the housing proper on the foundation, level with steel shims and tighten the foundation bolts. The various parts may now be assembled to the housing proper and the door plug can be swung into place, securing it with the necessary lock pins.
After the mill has been completely assembled and aligned, the door mechanism centered and adjusted, and all clearances checked, the housing base can be grouted. The unit should be so located both vertically and horizontally so as to provide a uniform annular opening between the discharge plug door and the head liners.
In some cases because of space limitation, economy reasons, etc., the mill is not equipped with separate discharge housing. In such a case, the open end low discharge principal is accomplished by means of the same size opening through the discharge trunnion but with the plug door attached to lugs on the head liner segments or lugs on the discharge trunnion liner proper. In still other cases, it is sometimes effected by means of an arm holding the plug and mounted on a cross member which is attached to the bell of the discharge trunnion liner. In such cases as those, a light weight sheet steel discharge housing is supplied by the user to accommodate the local plant layout in conjunction with the discharge launder.
TRUNNION BEARING LUBRICATION. For the larger mills with trunnion bearings provided with oil seals, we recommend flood oil lubrication. This can be accomplished by a centralized system for two or more mills, or by an individual system for each mill. We recommend the individual system for each mill, except where six or more mills are involved, or when economy reasons may dictate otherwise.
In any event oil flow to each trunnion bearing should be between 3 to 5 gallons per minute. The oil should be adequately filtered and heaters may be used to maintain a temperature which will provide proper filtration and maintain the necessary viscosity for adequate flow. The lines leading from the filter to the bearing should be of copper tubing or pickled piping. The drain line leading from the bearings to the storage or sump tank should be of adequate size for proper flow, and they should be set at a minimum slope of per foot, perferably per foot. Avoid unnecessary elbows and fittings wherever possible. Avoid bends which create traps and which might accumulate impurities. All lines should be thoroughly cleaned and flushed with a solvent, and then blown free with air, before oil is added.
It is advisable to interlock the oil pump motor with the mill motor in such a way that the mill cannot be started until after the oil pump is operating. We recommend the use of a non-adjuslable valve at each bearing to prevent tampering.
When using the drip oil system it is advisable to place wool yarn or waste inside a canvas porous bag to prevent small pieces of the wool being drawn down into the trunnion journal. If brick grease is used, care should be taken in its selection with regard to the range of its effective temperature. In other words, it should be pointed out that brick grease is generally designed for a specific temperature range. Where the bearing temperature does not come up to the minimum temperature rating of the brick grease, the oil will not flow from it, and on the other hand if the temperature of the bearing exceeds the maximum temperature rating of the brick grease, the brick is subject to glazing; therefore, blinding off of the oil. This brick should be trimmed so that it rests freely on the trunnion journal, and does not hang up, or bind on the sides of the grease box.
When replacing the brick grease, remove the old grease completely. Due to the extended running time of brick grease, there is usually an accumulation of impurities and foreign matter on the top surface, which is detrimental to the bearing.
Where anti-friction bearings are supplied, they are adequately sealed for either grease or oil lubrication. If a flood system is used for the trunnion bearings and it is adequately filtered, it can then be used for pinion bearings with the same precautions taken as mentioned above, with a flow of to 1 gallons per minute to each bearing.
These lubricants can be applied by hand, but we highly recommend some type of spray system, whether it be automatic, semi-automatic or manually operated. It has been found that it is best to lubricate gears frequently with small quantities.
Start the lubrication system and run it for about ten minutes, adjusting the oil flow at each bearing. Check all of the bolts and nuts on the mill for tightness and remove all ladders, tools and other obstructions prior to starting the mill.
Before starting the mill, even though it is empty, we recommend that it be jogged one or two revolutions for a check as to clearance of the gear and its guard, splash rings, etc. The trunnion journal should also be checked for uniform oil film and for any evidence of foreign material which might manifest itself through the appearance of scratches on the journal. If there are any scratches, it is very possible that some foreign material such as weld splatter may have been drawn down into the bushing, and can be found imbedded there. These particles should be removed before proceeding further.
If everything is found to be satisfactory, then the mill should be run for ten to fifteen minutes, and stopped. The trunnion bearings should be checked for any undue temperature and the gear grease pattern can be observed for uniformity which would indicate correct alignment.
It should be noted that with an empty mill the reactions and operating characteristics of the bearings and gearing at this point are somewhat different than when operating with a ball or rod charge. Gear noises will be prominent and some vibration will occur due to no load and normal back-lash. Furthermore, it will be found that the mill will continue to rotate for some time after the power is shut off. Safety precautions should therefore he observed, and no work should be done on the mill until it has come to a complete stop.
We have now reached the point where a half ball or rod charge can be added, and the mill run for another six to eight hours, feeding approximately half the anticipated tonnage. The mill should now be stopped, end the gear grease pattern checked, and gear and pinion mesh corrected, if necessary, according to separate instructions.
The full charge of balls or rods can now be added, as well as the full amount of feed, and after a run of about four to six days, ALL BOLTS SHOULD AGAIN BE RETIGHTENED, and the gear and pinion checked again, and adjusted if necessary.
Where starting jacks are provided for the trunnion bearings of the larger sized mills, they should be filled with the same oil that is used for the lubrication of the trunnion bearings. Before starting the mill they should be pumped so as to insure having an oil film between the journal and the bushing.
When relining any part of the mill, clean away all sand from the parts to be relined before putting in the new liners. For the head liners and shell liners you may then proceed in the same manner used at the time of the initial assembly.
Before relining the grate type discharge head, it is advisable to refer to the assembly drawings and the parts list.Because of such limitations as the size of the manhole opening, and for various other reasons, it will be found that the center discharge liner and cone designs vary. The cone may be a separate piece or integral with either the trunnion liner, or the router discharge liner. Furthermore, it will be found in some mills that the center discharge liner is held by bolts through the discharge head, whereas in other cases it depends upon the clamping effect of grates to hold it in position. In any event, the primary thing to remember in assembling the discharge grate head parts is the fact that the grate should be first drawn up tightly towards the center discharge liner by adjusting the grate set screws located at the periphery of the discharge head. This adjustment should be carried out in progressive steps, alternating at about 180 if possible and in such a manner that, the center discharge liner does not become dislodged from its proper position at the center of the mill. These grate set screws should be adjusted with the side clamp bar bolts loosened. After the grates have been completely tightened with the set screws, check for correct and uniform position of each grate section. The side clamp bar bolts may now be lightened, again using an alternate process. This should result in the side clamp bars firmly bearing against the beveled sides of the grates. The side clamp bars should not hear against the lifter liners.
When new pan liners are installed, they should be grouted in position so as to prevent pulp race in the void space between the discharge head and the pan liner. Another good method of preventing this pulp race is the use of the sponge rubber which can be cemented in place.
After the mill is erected, in order to avoid overlooking both obvious and obscure installation details, we recommend the use of a check list. This is particularly recommended for multiple mill installations where it is difficult to control the different phases of installation for each and every mill. Such a check list can be modeled after the following:
No. 1 Connecting Bolts drawn tight. A. Head and Shell flange bolts. B. Gear Connecting, bolts. No. 2 Trunnion studs or bolts drawn up tight. No. 3 Trunnion liner and feeder connecting bolts or studs drawn up tight. No. 4 Feeder lip bolts tightened. No. 5 Liner bolts drawn up tight. No. 6 Gear. A. Concentric B. Backlash C. Runout D. Joint bolts drawn up tight. No. 7 Coupling and Drive alignment and lubrication. No. 8 Bearings and Gearing cleaned and lubricated. No. 9 Lubrication system in working order with automatic devices including alarms and interlocking systems.
We further recommend that during the first thirty to sixty days of operation, particular attention be given to bolt tightness, foundation settlement and condition of the grouting. We suggest any unusual occurrence be recorded so that should trouble develop later there may be a clue which would simplify diagnosing and rectifying the situation.
As a safety precaution, and in many cases in order to comply with local safety regulations, guards should be used to protect the operators and mechanics from contact with moving parts. However, these guards should not be of such a design that will prevent or hinder the close inspection of the vital parts. Frequent inspection should be made at regular intervals with particular attention being given to the condition of the wearing parts in the mill. In this way, you will be better able to anticipate your needs for liners and other parts in time to comply with the current delivery schedules.
When ordering repair or replacement parts for your mill, be sure to identify the parts with the number and description as shown on the repair parts list, and specify the hand and serial number of the mill.
By following the instructions outlined in this manual, mechanical malfunctions will be eliminated. However, inadvertent errors may occur even under, the most careful supervision. With this in mind, it is possible that some difficulties may arise. Whenever any abnormal mechanical reactions are found, invariably they can be attributed to causes which though sometimes obvious are often hidden. We sight herewith the most common problems, with their solutions.
Cause A GROUT DISINTEGRATION. Very often when the grouting is not up to specification the vibration from the mill tends to disintegrate the grouting. In most instances the disintegration starts between the sole plate and the top surface of the grouting near or at the vertical centerline of the mill. As this continues, the weight of the mill causes the sole plate and trunnion bearing base to bend with a resultant pinching action at the side of the bearing near the horizontal center line of the mill. This pinching will cut off and wipe the oil film from the journal and will manifest itself in the same manner as if the lubrication supply had been cut off. If the grout disintegration is limited to about . 050 and does not appear to be progressing further, the situation can be corrected by applying a corresponding amount of shimming between the trunnion bearing base and the sole plate near the centerline of the mill in such a fashion that the trunnion bearing base has been returned to its normal dimensional position. If, on the other hand, the grouting is in excess of . 050 and appears to be progressing further, it is advisable to shut down operations until the sole plate has been re grouted.
Cause B HIGH SPOT ON THE BUSHING. While all BallMill bushings are scraped in the shop to fit either a jig mandrel or the head proper to which it is to be fitted, nevertheless there is a certain amount of seasoning and dimensional change which goes on in the type of metals used. Therefore if high spots are found, the mill should be raised, the bushings removed and rescraped. Bluing may be used to assist in detecting high spots.
Cause C INSUFFICIENT OIL FLOW. Increase the oil supply if it is a flood oil system. If brick grease is used, it is possible that the particular grade of brick may not be applicable to the actual bearing temperature. Refer to the remarks in this manual under the paragraph entitled Lubrication.
Cause E EXCESSIVE RUBBING ON THE SIDE OF THE BUSHING. This comes about due to the improper setting of the bearings in the longitudinal plane. In some cases, particularly on dry grinding or hot clinker grinding mills, the expansion of the mills proper may account for this condition. In any event, it can be remedied by re-adjusting the bearing base on the sole plate longitudinally at the end opposite the drive.
There are many more lubricant suppliers, such as E. F. Houghton and Co. , or Lubriplate Division of Fiske Bros. Refining Co. In making your final selection of lubricants, you should consider the actual plant conditions as well as the standardization of lubricants. New and improved lubricants are being marketed, and we, therefore, suggest that you consult your local suppliers.
In this article, alternative forms of optimizing the milling efficiency of a laboratory scale ball mill by varying the grinding media size distribution and the feed material particle size distribution were investigated. Silica ore was used as the test material. The experimental parameters that were kept constant in this investigation was the grinding media filling, powder filling and the mill rotational speed. The data obtained from these batch tests was then analyzed using a model free technique called the Attainable Region method. This analysis technique showed that the required product fineness is a function of grinding media and feed material size distributions. It was also observed from the experimental results that in order to increase the milling efficiency of a ball mill, towards optimum production of material in the desired size class, there is a need to correlate the ball size and the feed size distributions.
Starch damage will have a strong influence on most dough and baking processes. Therefore it will affect the quality of most finished product. Better knowledge of levels of damaged starch in flours is essential for better screening of flour and breeding lines.
INTRODUCTION The recovery of flour ingredients from wheat during processing is not without deleterious effects. High speed rollers and mechanical disruption of the wheat kernel bring about some damage to starch granules. While milling procedures are designed for maximum recovery of starch and the minimum inclusion of bran, they invariably result in a small but significant amount of starch damage. Regardless of what type of milling is used 5 to 12% of the starch granules are damaged (Viot 1992). This in turn changes flour characteristics in dough mixing and bread baking. This phenomenon is also true for the production of noodles and tortilla which are also sensitive to small changes in starch chemistry.
Starch is the chief storage form of carbohydrate in plants and the most important source of carbohydrate in human nutrition. A starch molecule is a polysaccharide assembled from the simple sugar glucose; chemically, starch is composed of two different molecules, amylose and amylopectin.
WHAT IS DAMAGED STARCH? It is a starch granule that is broken up into pieces. Not only does it increase water absorption and affect dough rheology, it increases food supply to the yeast and is more susceptible to fungal alpha amylase. Starch represents 67-68% of whole grain wheat and between 78-82% of the flour produced from milling. The semi crystalline structure of the starch granule in the grain kernel can be damaged by mechanical operations, particularly the milling process. Damaged starch (DS) is important in bread making: it absorbs 4 times its weight in water as compared to 0.4 for native starch.
Damaged starch granules are also subject to preferential attack by specific enzymes ( and -amylases). Some of these enzymes are incapable of attacking an intact granule because of the protective coating on the granules. The term Damaged starch is somewhat of a misnomer as the word damaged has a negative connotation implying something to be avoided.
THE IMPORTANCE OF DAMAGED STARCH It increases water absorption and provides extra nutrition for the yeast. A high level of damaged starch would result in sticky dough that produces a weak side wall and a sticky crumb (if enough amylolytic enzymes are available). The level of starch damage directly affects the water absorption and the dough mixing properties of the flour and is of technological significance. Damaged starch absorbs 2 to 4 times more water than regular starch granules. Sticky doughs, high water absorption, longer proofing times, and red bread crust color are just some of the effects of damaged starch. Damaged starch granules are susceptible to enzymatic degradation in comparison to native starches.
Starch damage will have a strong influence on most dough and baking processes. Therefore it will affect the quality of most finished product. Better knowledge of levels of damaged starch in flours is essential for better screening of flour and breeding lines.
EFFECTS ON BREAD QUALITY In fact, damaged starch (DS) should be optimized as it has both positive and negative effects on bread quality. Increasing damaged starch increases the water retention capacity of the flour; however, too much DS leads to sticky dough, strong proofing, and undesirable browning of crust. The optimum DS value varies with the use of the flour and is greatly dependent upon the flour protein content, the alpha amylase activity, and the type of bread to be made from the flour. Most baked products around the world have specifications in terms of quality and functionality of flour used, and DS is one of these specifications. Flour with high DS cannot be used for the same purpose as the one with a low DS content.
Millers can manipulate damaged starch (DS) content of flours through wheat choice, grain preparation and mill setup and adjustments. The wheat choice is based on the impact of the grain hardness: the more resistant to milling, the greater the DS. This hardness can be partly modified when preparing the wheat for milling. At milling particular attention is given to the moisture conditioning and tempering time for the grain to be milled. From a proper conditioning or selection of the wheat, it is possible to increase or decrease the DS at the mill. Furthermore, hardness is higher when the protein content is higher; thus, a direct correlation between the protein content and DS. Nevertheless, the mill set-up and adjustments are the major ways of influencing the end flour DS. This study focuses on those aspects.
QUANTITY OF DAMAGED STARCHPRODUCED IN MILL STREAMS We cannot get into details of the milling diagram and the codification, but let us take a 150t/d mill with 4 breaking streams (BK), 1 sizing (SIZ) and 7 middling and second quality streams (converting/reduction) (MID) and ancillary equipment for an all-purpose flour (US Grade, French type 55). 51%, about half of the damaged starch produced into this milling diagram is on 1st and 2nd middling (1M and 2M), 1rst sizing (S1) and 3rd middling. They are the main streams where we should focus our attention.
The modern flours mills are all taking advantage of features such as computerized, Grinding gap adjustment, and, as seen, this can be particularly efficient in terms of damaged starch creation when applied to the head reduction streams (1st and 2nd middling (1M and 2M), 1st Sizing (S1) and 3rd middling (3M), representing 50% of the DS produced. Those streams must be considered as central to the final quality of the flour and their variation may have the greater impact in the way of predicting the DS.
EFFECTS OF DAMAGED STARCHON THE FINAL PRODUCT Water absorption by starch that becomes damaged can improve baking properties up to a critical level above which properties of flour are negatively affected. Alongside with the action already determined on hydration, starch damage has an action on dough plastic characteristics, Proofing and Bread crust color. Effects on proofing characteristics can be shown if we know that amylases cant attack a native starch granule. More damage more attacks are possible. Breaking the granule molecules liberates water, Simple sugar are present and create: Intense yeast activity (a lot of CO2) Coloration possibility higher.
Higher input of water allows keeping the loaf fresh longer. But Simple sugar release provokes a very red crust. If intense, damaged starch can be responsible for: Sticky crumb, No volume bread and too red bread.
Procedures for controlling the quantity of damaged starch: If I do not have enough starch damage I can set the mill differently, choose a harder type of wheat or both of them. If I have too many starch damage I can Take care of amylases content (falling number), add gluten to increase rheological properties. Set differently the mill, and we can also Change wheat for a softer one.
The impact of the starch damage onthe rheological behavior of doughs: The damage Starch has an impact on the alveo graph curve and the rheological properties. The Mixolab analyses carried out on the flour show that an increase in the damaged starch content results in an increase in the water absorption capacity (approximately 0.5% hydration for each additional UCD); a decrease of the viscosity of the starch paste obtained during the gelatinization process and reduced stability under heat (increased liquefaction) ;indicating higher amylasic activity. Decrease of the starch retrogradation indicating better shelf life.
There is a clear explanation. The damaged starch presents a water absorption capacity ten times greater than the native starch, and greater sensitivity to enzymes (the amylases in particular). The action of the amylases occurs more quickly and in a more intensive manner. The damaged starch action impacts the whole bread-making process. It is essential to adapt and quantify damaged starch content in accordance with the desired end use. The SDmatic / Mixolab couple is perfectly suited to this challenge.
For good quality breads, there has to be a balance between the amounts of water used in the kneading, protein content of the flour, the amount of damaged starch and -amylase activity. These values also differ in different bread making methods. In fast bread processes, with short resting time, the effect of damaged starch in providing substrate is minimal, but with long fermentation processes the effect is substantial. It has been determined that the level of damaged starch is less important in whole meal bread than in white bread. Except for some biscuit and cake types, wheat with low DS is preferred in cake making (POMERANZ 1988).
Relationship between DS, protein content and desirable tolerances for the final product: End products are classified according to the DS and protein content. Knowing that information, a flour mill laboratory can use the DS according to the purpose of their flours for, as an example: Baking target:
This study was carried out to determine the effect of Pb exposure on the status of platelet indices in workers exposed to Pb during lead-acid battery plant process. Platelet indices and blood lead levels (BLLs) were determined in 429 male workers. BLLs were determined by using an atomic absorption spectrophotometer. Platelet indices in the samples were quantified by using the Sysmex KX-21 hematology analyzer. The levels of platelet count (PLT), plateletcrit (PCT) and mean platelet mass (MPM) were significantly decreased and platelet distribution width (PDW), platelet large cell ratio (P-LCR) and mean platelet volume were increased with an increase in BLLs. The results of linear multiple regression analysis showed that the platelet count ( 0.143, P=0.005), PCT ( 0.115, P=0.023) and MPM ( 0.110, P=0.030) were negatively associated with BLLs and P-LCR ( 0.122, P=0.016) was positively associated with BLLs. The variable of body mass index showed a positive association with PCT ( 0.105, P=0.032) and MPM ( 0.101, P=0.039). The results of the study may indicate that lead exposure may impair coagulation function through endothelial tissue injury and reduction of nitric oxide.
Lead is a major environmental and occupational pollutant, and its toxicity continues to be a major public health problem.1 Pb exposure can cause damages to hematopoietic, nervous and renal systems.2 Workers are exposed to Pb in many occupations including battery manufacturing and recovery, soldering, lead mining and smelting, lead alloy production and in the paint, glass, plastic, printing, ceramic industries, construction and fiber optic technologies.3 Lead is absorbed through inhalation, ingestion and dermal contact. Inhalation is the primary route of occupational exposure to lead. After absorption into the blood, 99% of Pb is associated with red blood cells.4 Platelet is small size blood components and they are formed in the bone marrow from large cells called megakaryocytes. These cells are multifunctional and are necessary for homeostasis, thrombosis, clot retraction, vessel constriction, endothelial repair and promotion of atherosclerosis.5 The normal platelet count in human beings ranges from 150 109/l to 400 109/l. If platelet counts are less than 150 109/l considered as thrombocytopenia and greater than 400 109/l considered as thrombocytosis.
Epidemiological studies have reported a positive association between mean platelet volume (MPV) and risk of cardiovascular disease.6 Occupational exposure, studies have reported higher platelet count (PLT) in pesticide workers,7, 8 firefighters,9 cement factory workers,10 taxi drivers11 and metallic mercury workers,12 as well as those exposed to noise13 and diesel exhaust inhalation.14 Low platelet counts were reported in gasoline filling workers,15, 16 and those exposed to gas flares17 and benzene,18 as well as beedi rollers.19 Lead affects the blood coagulation actions through endothelial tissue injury, reduced nitric oxide, tissue plasminogen activator and an increased production of plasminogen activator inhibitor-1.20 The chronic Pb intoxication of animals has reported higher levels of PLT, platelet distribution width (PDW) and MPV.21, 22, 23 In lead battery workers, only PLT count was assessed and other platelet indices such as plateletcrit (PCT), MPV, PDW, platelet large cell ratio (P-LCR) and mean platelet mass (MPM) were not determined.24, 25 Therefore, this study was undertaken to investigate the effect of lead exposure on the status of platelet indices in workers exposed to Pb from lead-acid battery plant with lifestyle confounding factors.
Platelets are special blood cells that plug up damaged blood vessels with the help of a blood clot to stop bleeding. The MPV is a measurement of the average size of platelet cells in the blood, and it is used as an indicator of inflammation and thrombosis.26 PDW quantifies the variability in the platelet size. The increased level of platelet distribution width indicates platelet anisocytosis. It is a specific marker of platelet activation.27 PCT reflects the proportion % of whole blood occupied by platelets. It is associated with the PLT count and the size of platelets. P-LCR is the ratio of large platelets. It is used to evaluate the differential diagnosis of conditions associated with abnormal platelet counts.28 MPM is a better predictor of thrombopoietin production and regulation of megakaryocytic cells.
This study was a cross-sectional study carried out in 429 male subjects working in a lead-acid battery manufacturing plant located in Tamil Nadu, India. Status of platelet indices among lead-exposed workers was compared with age, body mass index (BMI), experience, alcohol consumption, smoking, blood lead level (BLL) and job categories. Before including them in the study, an informed consent was obtained from each one of them.
Demographic details, occupational history and habits of subjects (smoking and alcohol consumption) were collected by using pre-designed questionnaire. BMI was calculated by using subjective weight (kg) and height (m) and expressed as kg/m2.
Three milliliters of venous whole blood was collected in a heparinized vacuette from the workers and stored at 20C until it was taken for analysis. Two milliliters of blood was digested by using an ETHOS-D milestone microwave system (Italy) with 2ml of nitric acid (HNO3) and 0.2ml of hydrogen peroxide (H2O2) by maintaining power, temperature and duration of time. The digested samples were made up to 5ml using triple-distilled water and centrifuged. The concentration of lead in blood was measured using an atomic absorption spectrophotometer (GBC-Avanta, Australia). 20g/dl of the standard solution was prepared from the lead standard solution of Merck (1.19776.0500) and added to the lowest concentration of the sample, and the analysis found 100% recovery with % relative standard deviation at <0.5 for three replicates. BLLs were expressed as g/dl.
Two milliliters of venous whole blood was collected from each subject in a lavender-colored top vacutainer tube that contains K3-EDTA. The platelet indices such as PLT, PDW, MPV and P-LCR were quantified within 30min after collection of samples by using the Sysmex KX-21 hematology analyzer. It is a three-part differential and eighteen-parameter analyzer that works on the principle of volumetric impedance. PCT is a product of platelet count multiplied with MPV divided by 10,000. MPM is a product of MPV and PLT count. The instrument was calibrated by using Bio-Rad QC before analysis of the samples.
Statistical Package for the Social Sciences (SPPS) version 7.5 for Windows, was used for the statistical analysis of the data. The data presented in proportions and adjusted mean with standard error. One-way analysis of variance (ANOVA) was used to assess the effect of categorical variables on platelet indices. One-way ANOVA with post hoc test was used to compare various categories within the categorical variables on platelet indices. The Spearman correlation coefficient test was used to find out the correlation between BLLs and platelet indices. Multiple linear regression analysis with the above-mentioned method was used to evaluate the association between platelet indices and BLLs by controlling for the influence of age, BMI, smoking, alcohol consumption and experience. Probability <0.05 was considered significant.
Demographic characteristics of the study population are presented in Table 1. The age distribution reveals that the highest percentage of workers were in the age group of 3439 years and the lowest percentage of workers were in the age group of 2227 years. BMI of subject indicates that 56.6% of subjects fall within the normal BMI, 38.5% of subjects correspond to overweight and 3.3% of subjects were obese. Only 1.6% of subjects were underweight. Maximum percentage of lead exposed workers had 1115 years of exposure. Alcohol consumption and smoking habits were recorded in 39.6% and 19.8%, respectively. The frequency distribution of BLL among subjects was 15.2%, 28.4%, 33.6%, 18.6% and 4.2% in 1520, 2130, 3140, 4150 and >51g/dl, respectively. The highest percentage of BLL was noticed in the range of 3140g/dl. The job categories involved in the manufacturing process are casting, pasting, ball mill, plate cutting, formation, acid filling, charging and assembly. The highest percentage of workers was occupied in the assembly.
Table 2 shows univariate analysis of variables that affect on the status of platelet indices among lead-exposed workers. One-way ANOVA was used to find out the effect of categorical variables of age, BMI, experience, alcohol consumption, smoking, BLLs and job category on platelet indices (PLT, PCT, PDW, MPV, P-LCR and MPM) among lead-exposed workers. The results of the model were indicating that the age group category was significantly associated with P-LCR (P=0.012) and job category groups were associated with PLT (P=0.003), PDW (P=0.006), MPV (P=0.010) and P-LCR (P=0.002). The one-way ANOVA post hoc comparison method was used to assess the effect of various categories within the categorical variables on the status of platelet indices. The level of P-LCR was significantly reduced in the age group of 3439 years as compared to age group of 2227 years. The measurements of PCT and MPM significantly decreased in the experience category of 610 and 1115 years as compared with 5 years. PLT count reduced in the BLL category from 41 to 50 and >51g/dl as a contrast to BLL 1520g/dl. PCT and MPM noticeably reduced in BLL categories from 2030, 3140 and 4150g/dl, as evaluated with a BLL of 1520g/dl. The levels of PLT, PCT and MPM were decreased significantly in operators, who are working in assembly, casting, pasting, charging, plate cutting and acid filling, as compared with the executives. The variables of BMI, alcohol consumption and smoking habits shows no significant effect on platelet indices.
The results of Spearmans correlation coefficients (r) between BLL and platelet indices among lead-exposed workers are presented in Table 3. A negative correlation coefficient was noticed between BLL and PLT (r=0.117; P=0.015), PCT (r=0.080; P=0.097) and MPM (r=0.076; P=0.114). A significant correlation was noticed between BLL and PLT. The positive correlation coefficient was noticed between BLL and PDW (r=0.077; P=0.113), MPV (r=0.087; P=0.071) and P-LCR (r=0.097; P=0.045). The correlation coefficient between BLL and P-LCR was significant. Positive correlation coefficients (r) were found between PLT and PCT (r=0.884; P<0.01) and MPM (r=0.889; P<0.01). Negative correlation coefficients (r) were found between PLT and PDW (r=0.485; P<0.01), MPV (r=0.515; P<0.01) and P-LCR (r=0.524; P<0.01).
Linear multiple regression analysis of variables that affect platelet indices among lead-exposed workers were presented in Table 4. In this model, each of the parameters of platelet indices (PLT, PCT, PDW, MPV, P-LCR and MPM) were used as dependent variables, and BLL, age, BMI, alcohol consumption (yes=1 and no=0), smoking habits (yes=1 and no=0) and experience (years) were used as independent variables. The results of linear multiple regression analysis showed that the platelet count ( 0.143, P=0.005), PCT ( 0.115, P=0.023) and MPM ( 0.110, P=0.030) were negatively associated with BLL and the P-LCR was positively associated with BLL. The variable of BMI showed a positive and significant association with PCT ( 0.105, P=0.032) and MPM ( 0.101, P=0.039). The variables of age, smoking, alcohol consumption and experience were not predicted significantly on platelet indices.
The present study assessed the effect of Pb exposure on the status of platelet indices in subjects exposed to Pb during lead-acid battery plant process. Biino et al.29 has reported decreasing PLT by 35% in male subjects and 25% in female subjects from infancy to adulthood. In this study, age groups showed a significant association with P-LCR. Samocha-Bonet et al.30 and Farhangi et al.31 have reported a significant association between obesity and platelet counts in female subjects. During this study, the comparison of univariate analysis did not find any association. However, linear multiple regression analysis indicates that the BMI significantly associated with PCT and MPM. Alcohol consumption causes low platelet counts, impaired platelet function and diminishes fibrinolysis.32 In the current study, we observed decreased platelet count, PCT and MPM and increased PDW, MPV and P-LCR in alcoholics as compared with nonalcoholics. Akkani et al.33 reported significantly decreased platelet count in heavy alcohol consumers as compared with nonalcoholics. In this study, both univariate and linear multiple regression analysis showed no significant association between alcohol consumption and platelet indices. This was because subjects were using minimal to moderate amount of alcohol. Gitte34 reported significantly increased platelet count in male smokers, who smoke 20 or more cigarettes per day in 20 years of duration. In the current study, the univariate and linear multiple regression analysis showed no significant association between smokers and platelet indices. The findings of this study were similar to the study of Suwanskasri et al.35
The effect of Pb intoxication of animal has reported higher levels of PLT, MPV and PDW as compared with controls.21, 22, 23 Shaik and Jamil24, 25 reported no significant differences in PLT of lead battery workers as compared with controls. These studies assessed only platelet counts among lead battery workers. In the current study, we assessed the effect of Pb exposure on platelet indices among lead-exposed workers. The reduced platelet count was reported in acute lead intoxicated in female battery workers,36 lead exposed to workers in traditional tile factories37 and fish exposed to lead.38 The univariate analysis indicated that the levels of PLT, PCT and MPM were significantly decreased with an increase of BLL and PDW, and the MPV and P-LCR were increased with an increase of BLL. The levels of PLT, PCT and MPM were significantly reduced in operators, who were occupied in assembly, casting, pasting, charging, plate cutting and acid filling, as compared with executives. The results of linear multiple regression analysis showed that the platelet count, PCT and MPM were inversely associated and P-LCR was positively associated with BLLs.
Adeyemo et al.39 and Kianoush et al.40 reported a negative association between BLL and PLT in fish exposed to Pb and car battery industry workers exposed to Pb. In the current study, we found a negative association between BLL and PLT, and PCT and MPM, and a positive association between BLL and PDW, and MPV and P-LCR. The significant association was noted between BLL and PLT and P-LCR.
The alteration of platelet indices in lead-exposed workers may be the effect of intracellular Ca2+ homeostasis, either by mimicking Ca2+ action or antagonizing Ca2+-dependent cellular functions and activation of protein kinase C41 and also its actions on endothelial tissue injury, decreased levels of nitric oxide, tissue plasminogen activator and an increased level of plasminogen activator inhibitor-1.20
The levels of PLT, PCT and MPM were significantly decreased and PDW, P-LCR and MPV were increased with an increase of BLL. The levels of PLT, PCT and MPM were negatively associated with BLL, and the levels of PDW, MPV and P-LCR were positively associated with BLL. The results of the study may indicate that lead exposure may impair coagulation function through endothelial tissue injury and reduction of nitric oxide.
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Barman, T., Kalahasthi, R. & Rajmohan, H. Effects of lead exposure on the status of platelet indices in workers involved in a lead-acid battery manufacturing plant. J Expo Sci Environ Epidemiol 24, 629633 (2014). https://doi.org/10.1038/jes.2014.4