Pulverizers with
Two types are used -the stationarv or rigid hammer on a disk or plate, and the swing hammer attached between two plates on a pin. The latter type is most used today, as i t gives g r e a t e r crushing force with a given speed and has the further advantage of relieving itself when it strikes hard portions in the material or tramp iron. Hammer mills for the fine powderW. A. KOREN ing of material are Raymond Pulverizer Division, usually constructed Combustion Engineering Company, Inc., Chicago, Ill. w i t h v e r y close clearance between the ends of the revolving hammers and t h e grinding chamber; when heavy loads are present, the result unPrinciples of Fine Grinding doubtedly is not only an impact action but some attrition of the particles being rubbed between the ends of the hammers First let us consider the principles used in the reduction of and the grinding chamber. materials to powdered form. Attrition is one of the oldest Two other types of equipment utilize combinations of the methods used. It involves the breaking down of the mathree grinding principles or actions. One of these is the ball or terial between two surfaces by a rubbing action, the surfaces tube mill, with its charge of steel balls or flint pebbles which revolving in opposite directions. This is best illustrated by reduces material by the action of impact or dropping of the the common buhrstone or stone mill, which has been used for ball on the product and by attrition or sliding of the ball on hundreds of years. I n the case of the stone mill, one stone the product. Generally this type of mill is most applicable was usually held stationary while the other was revolved. to the harder materials. The other type of equipment is also Modern mills of this type use hard metal disks placed in a a ball mill in which the balls are of large diameter and roll in vertical plane and driven in opposite directions, so that as the a race or ring, as exemplified by the old Fuller-Lehigh mill material i s fed to the center, it works out between the two and the present B & W mill. Here both a rolling and atdisks and is reduced to fine powdered form. The attrition trition action take place on the material. principle is particularly applicable to the handling of fibrous materials where a tearing action is necessary to reduce the Settling Chamber fiber to powder. Until the commercial development of air separation, maThe rolling principle employed in fine crushing and pulterials were used as delivered direct from the pulverizer ; verizing involves either a heavy rolling member or the use of if finer classification was wanted, screens and bolting reels centrifugal force on a roll to crush and pulverize a bed of mawere used. As a result, materials were not pulverized to any terial. The common dry pan is an example of the use of this great fineness. A product testing 95 per cent passing a 100principle, but it has been applicable only to the coarser pulmesh screen was considered an extremely fine material. verizing of materials; the Huntington or roller type mill, to There had also been a certain amount of development on the which air separation is applied for classifying the powder made, has had a much wider application in the fine pulverizing use of settling bins, where the product from the mill was blown into a series of bins and the velocity of the air carrying the of dry materials. I n this latter type of mill a roll is carried material was gradually reduced so that in each succeeding on a journal which, in turn, is hung on the periphery of a revolving spider; as a result the roll is thrown out against the settling chamber or bin a finer product would be obtained. This settling chamber idea was the forerunner of the present inside surface of a grinding ring by centrifugal force. The air separation principle. speed of the spider then determines the amount of force George and Albert Raymond, in their first invention during exerted. At the same time an inclined plow brings up material the middle eighties, used this principle of a settling chamber from the bottom of the mill and throws it between the face of the roll and ring. Theoretically this principle involves a rollto classify the material. They started with the impact or hammer type mill. An exhaust fan carried the fines from ing crushing action only, but in actual practice some slight atthe grinding chamber through a large settling chamber, and trition takes place between the face of the roll and the ring; beyond that a cyclone collector was used; the air was then if the design is such that the face of the roll is other than in returned to the grinding chamber. This was a crude method the vertical plane, the attrition action, in addition to the of classifying a powdered material. rolling action, can be materially increased. In the early nineties they developed a large-diameter cone The third principle used in the pulverizing of dried material attached to the top of a hammer type grinding mill; it conis the impact action or hammer blow. This involves the operation of hammers a t a high speed; they strike the lumps tained an inner cone, a t the top of which they used what of material and throw them against a stationary surface. were then known as curtain deflectors. These were nothing
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HE subject of pulverizing covers an extrernely wide field, and the purpose of t h i s article is t o take in only one branch of that field, confining it to the fine pulverizing of dried materials, principally t h e softer materials with a hardness less than four to five. This also includes the pulverizing to a fineness of not less than 10 to 20 mesh, up to 325 mesh and finer, w h i c h f a l l s within the scope of air separation as the classifying medium.
Air Separation
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VOL. 30, NO. 8
Where formerly materials pulverized t o a fineness of 95 to 99 per cent passing a 100-mesh screen was considered fine, the development of air separation made it economically pcssible to produce materials from 99 to 99.5 per cent passing a ZOO-mesh screen. From 1905 to about 1920 automatic pulverizers and roller mills became standard equipment for the pulverizing of all nonabrasive nonmetallic minerals and of manufactured products. Air separation was not only used as a built-in unit with the pulverizer but as a separate unit to be employed in closed circuit with other types of pulverizers. This was known as the vacuum separating plant but was built on exactly the same principle as the separation on the pulverizers. It involved the double-cone separator with the adjustable deflector principle, where the fineness could be obtained by the adjustable deflectors; and it incorporated the exhaust fan to generate the volume and velocity of air required, as well as the cyclone and cloth dust-collector system, with return of the air to the separator. The return air system was a great advantage. Any dust left in the air could be returned through the equipment; only a small proportion of the air used for separation, which was drawn in with the material as fed and through leaks on the suction side of the equipment, was vented to the atmosphere or to cloth collectors.
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FIGURE1. SEPARATIXG CHAMBER
more than baffle plates, so that, as the material was drawn from the grinding chamber by air, the material and air changed direction sharply, creating an inertia on the larger particles. They then dropped into the inner cone and passed back to the grinding chamber through valves, while the fine powder was taken off the top of the separator through the exhaust fan to a cyclone collector. This principle was the forerunner of the present-day double-cone air separator.
Double-ConeAir Separator The next outstanding development in air separation was the double-cone air separator, where an inside cone was placed in an outer cone. At the top of this inner cone a series of openings were cut, and deflector doors were placed on each side of the openings. The doors were made both rigid and adjustable, the latter being the most used. The adjustable doors were attached to rods extending through the top of the separator with handles and indicators on them so that all of the doors could be set a t the same angle. As the material, drawn up by the air from the grinding chamber, passed through these doors, it was given a tangential motion, setting up centrifugal force or a cyclonic action, which threw the oversize particles out of the air current and concentrated them along the inner surface of the inner cone. The fines were carried out of the top of this inner cone chamber through an exhaust fan to a cyclone collector, and the air was returned to the mill. The oversize particles concentrated on the inner surface of the inner cone and flowed by gravity back through valves at the bottom of the inner cone into the grinding chamber for further reduction. This development took place about 1905 and has been the basic principle upon which all air separation equipment, for the classifying of fine products, is based today. From 1905 to 1906 this type of air separation was incorporated in the impact or hammer type mill and in the Huntington or roller type mill, and was the basis for the development of fine grinding and classification of all dry materials. It raised the degree of fineness economically obtainable on all filler and nonmetallic materials.
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FIGURE2. CURTAIN SEPARATOR
During this time and somewhat previous to 1920, other manufacturers of pulverizers began to develop air separation according to the same principle. Today air separation is common practice with most pulverizing mills for the production of finely graded materials. It is now being used with ball and tube mills for the more abrasive products with a hardness beyond four. Mechanical Air Separators Another type of air separator, commonly called today a “mechanical air separator,” was in development over approximately the same period. It consisted of a large outer cone with a smaller inner one. A vertical shaft on the top of the separator, usually driven through gears by a horizontal shaft,
INDUSTRIAL AND ENGINEERING CHEMISTRY
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carried a distributor plate in the inner cone and a fan consisting of arms and fan blades; the fan operated in the space between the top of the inner cone and the top of the outer one. The fan generated an air volume which passed down between the outer and inner cones to a point low on the inner cone, where deflector doors were provided for air returning to the inner cone. Material, fed down along the vertical shaft, struck on the distributor plate and was thrown out across the space between the edge of the distributor plate and the inner side of the inner cone. Air passing up through this material picked up the fines and carried them up through the fan to the outer cone. Centrifugal force concentrated the fines in the outer cone, and they dropped out at the bottom by gravity. The oversize material in the inner cone dropped out of a spout extending through the outer cone. The separation depended almost entirely upon the velocity of the air to pick out the proper size particles and carry them into the outer cone where they were concentrated and discharged. Some centrifugal action occurred by setting the deflectors on an angle where the air returned to the inner cone; centrifugal force was set up a t this point, which had a tendency to whirl the air and drop out the larger oversize particles. This type of separator was developed in the United States by Emerick and applied extensively to the classification of material ground on some type of pulverizer. It was used principally in the cement plants, but did not prove effective since cement was not pulverized very fine in the early days. The gain a t that time in the use of this type of air separator was not great enough, and too many mechanical difficulties were encountered to warrant its extensive use. Then, in the early twenties, Rupert M. Gay improved on this principle to the extent of using auxiliary fan blades in the inner cone above the distributor disk; they set up a greater centrifugal action and made it possible to classify a product more closely by varying the number and position of these auxiliary fan blades. From that time to the present the centrifugal or mechanical air separator has been developed still further until nom it is a commonly used unit in many pulverizing operations.
Wh'izzers I n this connection a type of equipment was developed which is known today as whizzers. These whizzers are attached to a plate of approximately the same diameter as the distributing plate and are set above the distributing plate, between it and the inlet to the fan or the top of the inner cone. The whizzer blades are beveled a t the end and extend out to a whizzer cone attached to the inner side of the inner cone and covering the end of the whizzer blade. The return deflectors or doors a t the opening where the air enters the inner cone are set radially, theoretically to straighten out the air current so that it rises vertically through the material as it is thrown off the distributor plate. By using high volume and velocity, the air then picks up not only the fines wanted but a considerable part of the medium coarse material. The whizzer blades above the distributor disk set up a centrifugal action of the right amount to throw out the oversize particles carried by the air. These are concentrated in the corner of the inner cone above the whizzers, allowing the fines to be carried out by the fan to the inner cone and discharged. As the oversize material piles up above the whizzer cone, it gradually works back in concentrated form over the whizzer cone where the air has a second chance to recover any fines that have been eliminated. By this method much higher recoveries of the available fines take place. As a result, when a mechanical air separator is used in closed circuit with a grinding mill, such as n ball or tube mill, it is possible to obtain much higher recovery with closer classification to the required
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fineness. In thi$ way circulating loads have been materially reduced. With the olde type of centrifugal or mechanical air separator, it was necessary to use circulating loads of 300-700 per cent to raise the fineness of a feed containing 60-70 per cent of fines by 25-35 points. Today it is possible to cut the circulating load as low as 100-150 per cent. This greatly improves such operations, making them fat more economical
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FIGURE3. POOLEDEFLECTOR SEPARATOR from a power and repair standpoint. With the improved separators it is also possible today to obtain materials of extremely high fineness. This depends to some extent upon the fineness or percentage of very fine powder in the material as fed, but with reasonably fine grinding these mechanical air separators are capable of producing materials as high as 99.99 per cent passing a 400-mesh screen. The greatest fineness that could be obtained with this type of equipment, previous to the development of the positive means of classification, was about 95 per cent passing 325 mesh; and that could be accomplished only by having a tremendous circulating load and creating a high saturation of material in the circulated air. The old type separators were most effective in making products to a fineness between 85 and 95 per cent passing 200 mesh. As a result of decreasing the circulating load and a more effective recovery of the available fines in the feed, these improved separators are able to produce a much finer and more uniform material a t considerably greater capacity. As a general illustration, a number of the old type separators were converted to the use of whizzers, and in many instances the average increase in capacity a t the same fineness with these whizzers was 33 per cent. The application of mechanical air separators can generally be made with any grinding mill, as long as the mill is capable of reducing the fine tailings that are rejected by the separator. The main application, however, is in connection with ball and tube mills, and cement plants are probably the largest users of this equipment. The reason is that ball and tube mills do not give a uniform finished product except under exact feed control, and such variations as hardness of the material from hour to hour and as moisture content in the
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material will vary the kind of product obtained. This also results in limiting the capacity of the mill to an appreciable extent. By placing a mechanical air separator in closed circuit with such a mill, close control of the feed and any minor variations in the character of the material or moisture present can easily be controlled by the separator when classifying to a given fineness. The immediate result is a much larger volume of fines; increases brought about by the addition of mechanical air separators in closed circuit with ball and tube mills will vary from a minimum of 15-20 per cent to a maximum of 100 per cent, depending on the character of product being pulverized. The development of whizzers in the centrifugal or mechanical type air separator has led to the application of the whizzer separator to roller mills and automatic pulverizers. They have taken the place of the old double-cone air separator with adjustable deflectors and have shown probably the greatest increase in capacity, uniformity of finished product, and ability of a mill with this type of separator to produce extreme fineness. The principle of this whizzer separator is the same as that of the mechanical air separator. It is built as a single cone with very much the same shape as the outer cone on the double cone type of air separator, and the diameter used is based principally on the kind of material to be handled and the range of fineness wanted. Inside of this single cone a gear housing is mounted with cut steel gears and a horizontal and vertical shaft. The vertical shaft carries the whizzer disk to which the whizzer blades are attached, and on the in-
u FIGURE 4. DOUBLE-CONE SEPARATOR
VOL. 30, NO. 8
side of the cone a whizzer cone or cap is attached to extend over the end of these whizzer blades. The top of the separator is flat or in a horizontal plane, and a sufficient space is left between the whizzers and this top to obtain the necessary separation zone where the cyclonic action set u p by the whizzers throws the oversize material out and concentrates it so that it will flow down the inside surface of the cone and back into the mill for further grinding. By employing a variable-speed device in the operation of this whizzer separator, it is possible to have a range of fineness from 80 per cent passing 100 mesh to 99.9 per cent passing 325 mesh. This has increased the range of fineness obtainable from a given mill over a much wider range than ever before. Since this whizzer method of creating cyclonic action for concentration of oversize and removing it from the air stream is so much more effective than the older method, and since it has considerably less pressure loss across the separator, the capacity from a given mill has been greatly increased by its use. This has been due not only to the effectiveness of this method of classification, but also to the fact that former air separating equipment did not have as much capacity as the grinding portion of the mill; now, with more effective separation, full advantage is taken of the mill grinding capacity.
Applications of Whizzer Separators In general, the addition of whizzer separators to roller mills has given a minimum increase a t the same fineness of 25 per cent and a maximum of 100 per cent. A great many actual performance figures can be cited, but only a few will be mentioned. A plant grinding limestone, with three four-roller mills, equipped with double-cone air separators, gave an average of 4.75 tons per hour or 3160 pounds per mill hour a t a fineness of 99 per cent passing a 325-mesh screen. Two of these mills were equipped with whizzer type air separators, and no further changes were made in the setup. The result was a total capacity from two mills of 6.25 tons per hour or 6250 pounds per mill hour a t the same fineness. This reduced the power cost alone by $500 per month. A plant equipped with a four-roller mill for grinding talc produced, with the old double-cone separator, 3700 pounds per hour a t 99.9 per cent passing 200 mesh or 1600 pounds per hour a t 99.8 per cent passing 325 mesh. With the application of a whizzer separator and no further changes, the capacity was 6200 pounds per hour a t 99.92 per cent passing 200 mesh, or 3200 pounds per hour with a trace left on a 325mesh screen. A plant equipped with a four-roller mill for pulverizing bentonite clay, and with the old double-cone separator, produced 3 tons per hour a t a fineness of 93 to 94 per cent passing 200 mesh. After installation of a whizzer separator, the capacity was 4.5 tons per hour a t the same fineness. The grinding of English chalk is difficult because of its extremely sticky nature and the presence of flint. A plant with a five-roller mill gave 1500 to 2000 pounds per hour a t 99.5 per cent passing 325 mesh, using the old double-cone air separator. This was changed to a whiazer separator, and capacity went to 4000 pounds per hour a t 99.7 per cent passi n i 325-mesh. With ~b double-cone air separator, a five-roller mill installed for pulverizing sulfur produced 6000 pounds per hour a t 95 per cent passing a 325-mesh screen. With a whizaer separator in place of the double cone, the capacity went up to 7500 pounds per hour a t 98 per cent passing a 325-mesh screen. To illustrate the extreme fineness that a whizzer separator can produce, another four-roller mill for grinding sulfur was equipped with a whizzer separator of special design. It pro-
AUGUST, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
913
FIGURE 5 . DOUBLE-WHIZZER SEPARATOKL
duced 400 pounds per hour a t a fineness of 99.9 per cent under 10 microns in size with an average particle size of 2 to 3 microns, 1000 pounds per hour a t 99.2 per cent under 10 microns with an average particle size of 4 microns, and 1300 pounds per hour a t 99.92 per cent passing 325 mesh. Whimer separators have also been applied to the automatic or impact type of pulverizers with great success. These mills are used principally on disintegrating operations where the particle size of material is already established but it is a matter of breaking up lumps and classifying to a given fineness. The first application was to a No. 1 automatic pulverizer disintegrating and classifying hydrated lime with the elimination of impurities. This particular installation, with the old type double-cone air separator, was capable of only a maximum of 1.75 tons per hour at a fineness of 98 per cent passing 325 mesh. With the whizzer separator the capacity went up to 4 tons per hour a t 98.4 per cent passing 325 mesh.
Another application was on a unit handling lime made from forkings or the fines left after removing pebble lime. Before the installation of the whiaaer separator, the best performance after this lime was hydrated was 3000 pounds per hour a t 99 per cent passing a 200-mesh screen, With the whizzer separator the performance was 6600 pounds per hour a t 98 per cent passing 400 mesh, 6000 pounds per hour a t 99 per cent passing 400 mesh, and 5000 pounde per hour at 99.36 per cent passing 400 mesh. At the 6000-pound rate of capacity, the throwout on this mill eliminated tailings or impurities from the line a t the rate of 868 pounds per hour.
Low-Side Mills Considerable development has also been done in the past fifteen years on what is known as the low-side type of mill for coarser pulverizing. These machines are used for grades of material from 60 per cent passing 100 mesh to about 80 per
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INDUSTRIAL AND ENGINEERING CHEMISTRY
4==b
FIGURE 6. MECHANICAL SEPARATOR
cent passing 200 mesh. The principal improvements have been a better feed device with an automatic control; the latter is also used on the high-side type of mills and it feeds the mills automatically. Other improvements have been in the width of the grinding ring and the weight and width of the grinding rolls. Application of auxiliary blades, similar to whizzer blades, has been made to these mills, but they are attached to the mill spider and do not require a separate drive. The result of these improvements can be illustrated by three examples: A plant pulverizing Florida pebble rock for acidulation on a five-roller mill of the old type produced 5 tons per hour a t 90 per cent passing 100 mesh with 26.5 horsepower per ton. With the new improvements a four-roller mill gave 6.2 tons per hour a t 90 per cent passing 100 mesh, with the expenditure of 18.6 horsepower per ton. I n a gypsum plant an old style mill gave 6.6 tons per hour a t 95 per cent passing 100 mesh or 7.5 tons per hour a t 88 per cent passing 100 mesh. A new mill, with all up-to-date improvements, produced 11 tons per hour a t 95 per cent passing 100 mesh or 14.5 tons per hour a t 82 per cent passing 100 mesh. I n another plant grinding raw gypsum, an old five-roller mill produced 12 tons per hour at 70 per cent passing a 100-mesh screen, and a new four-roller mill gave 17 tons per hour a t the same fineness. Total power requirements of each mill are approximately the same.
Mill Drying The development of mill d r y i n e t h a t is, introducing heat in the form of high-temperature gases to a mill while it is pulverizing material containing moisture-is an interesting de-
VOL. 30, NO. 8
velopment. As early as 1916 engineers noticed in the grinding of certain materials, particularly coal which came from a rotary dryer and had the same temperature as it did when it left the dryer, that as much as 1 per cent of the moisture was removed by the time the powdered material was delivered to a storage bin. For example, a coal, which came from a rotary dryer with a temperature of 175-200" F. and with as much as 2 per cent surface moisture remaining in it, would be delivered from the mill cyclone a t not over 1 per cent moisture. No particular attention was paid to this phenomenon for a number of years, but finally in 1925 this indication of drying in the mill, due to the presence of heat, was put into practical use. A limestone-grinding plant equipped with a roller mill handling the stone following a rotary dryer was selected for the first attempt. The rotary dryer was cut out of the system, and the furnace ahead of the rotary dryer was changed slightly by taking a hot gas duct from the top of the furnace and introducing it into the return air line of the mill. The furnace was hand-fired with coke. Raw limestone screenings from the crushing plant, containing a t times as high as 5 to 6 per cent surface moisture, were fed directly to the Raymond mill; by taking in gases from the coke-fired furnace a t 600-700' F., the excess moisture was eliminated with the excess air from the top of the system, and it was easily possible to deliver a bonedry pulverized limestone from the system. It was also definitely established that the outlet temperature in the cyclone collector controlled the amount of residual moisture left in the finished product. With a temperature of about 175" F. in the cyclone, the limestone was delivered bone-dry ; with 150" F. it still retained 0.2-0.4 per cent moisture. With lower delivering temperatures more moisture could be left in the finished product. In all cases the temperature of the hot gases entering the mill were maintained approximately a t the same point, the only change being in the volume of heat or the gases entering the system. Pulverizers, equipped with air separation, lend themselves to this method because the air for separation is carried through the mill by an exhaust fan and discharged to a collector where material is removed from the air, and then the air is returned through the system. It is a closed-circuit system, so that there is no difficulty about introducing heat in the form of either hot air or products of combustion from a furnace; and by taking the excess air and gases from the system through the vent, the excess steam is carried away from the material. The first systems, called 'Lkilnmills," were applied principally to low-moisture-carrying materials such as coal, limestone, talc, various clays, barytes, etc.-in other words, the nonmetallic minerals which, when mined, always carried some surface moisture. This surface moisture had previously been removed by the use of rotary dryers and, by changing a mill system to a kiln mill, it was possible to eliminate rotary dryers entirely. The next development in mill drying came from an attempt to handle materials with a much higher moisture content. Usually these materials of higher moisture content were not in the form of a solid lump but were more like a plastic mass, so that the roller mill principle did not lend itself to the breaking up of the particles fine enough for the hot air to do instantaneous drying. Mill drying depends upon the fact that the mill can break up the raw material and thus increase the exposed surface so that the heat immediately changes the moisture to steam and dries each particle. The time element in a mill system with air separation is not great enough to dry large particles. It can be used onIy on smaller particle sizes. The kiln mill, or roller mill with air drying, also had its limitations in that internal lubrication of the roller journals prevented the use of high initial temperatures and the air
AUGUST, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
volume was not great enough to obtain high evaporation rates. This resulted in the use of the “imp” mill for materials of higher moisture content. This imp mill is built on the ordinary hammer mill principle but was constructed initially for pulverizing coal to fire furnaces direct. The swing hammers operate in a circular grinding chamber, a t one end of which is located the feed device so that all material entering the mill chamber must pass the hammers. At the other end is a cone-shaped chamber ending in a built-in exhauster through which the material passes to a cyclone collector. This type of mill was particularly adaptable to removing high amounts of moisture, since gases from a furnace could enter the mill a t temperatures as high as 1500-1600” F. through a firebrick-lined duct. With very high initial temperatures it was possible to introduce a large amount of heat for evaporation purposes in a much smaller volume of air; as soon as the high temperatures came in contact with the wet material and the moisture was evaporated, the temperatures dropped to normal. The first commercial installation for material of high moisture content was on an acid-treated clay coming direct from an Olivcr continuous filter. The clay was in a filter cake or mud condition and contained over 40 per cent moisture. The installation gave excellent results with the one exception of the difficulty in feeding the filter cake to the system. In fact, the finished product made by this method had a much higher efficiency in the cJarifying of oils than had been obtained by previous drying methods. The difficulty in feeding the product was finally overcome by returning a portion of the dried material and mixing it with the wet to put the wet material in a better mechanical condition. Today that is the principle used on all milling and air-drying problems where the mechanical condition of the raw material does not lend itself to ready feeding into the system. Today this method of drying materials of high moisture content has been applied to many problems where it is necessary or advisable only to disintegrate the product and not to grind it fine. This method has been named “flash drying.” The filter cake or wet material is dropped into a low-speed disintegrator of the cage mill type instead of into the ordinary grinding mill. The cage mill principle lends itself to disintegration of such products; the agglomerates are broken up to expose surface so that the heat can remove the moisture from the small particle size. Applications of both the kiln mill and the flash dryer have been extremely successful in obtaining high heat efficiency and high-grade p r o d u c t s . The short time the material is in the system, varying from 15 to possibly 25 seconds, makes this type very useful where subjection to high temperature for a longer time will spoil the product. Although flash drying has its limitations, it has been applied successfully to a rather wide range of materials, such as acid-treated clay, sewage sludge, by-product gypsum or calcium sulfate, waste grains from breweries and distilleries, and similar operations. On acid-treated clay, for instance, the system takes filter cake from a continuous filter a t 60 per cent moisture and delivers a finished prod-
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uct of the required fineness in a free-flowing condition; t h e finished product has a much higher efficiency than was obtained previously with rotary dryers. On sewage sludge it takes the sludge a t 80 per cent or more moisture direct from the filter and reduces it to approximately 10 per cent moisture, and the powdered product is then burned in the pulverized form. One system was installed to3 handle a primary sludge from a packing house. The initial moisture in this sludge after it is cooked is 90 per cent, and the finished product from the flash drying system contains 10 per cent moisture.
Applications of Imp Type Mills Some of the applications of the imp type of mill for drying and grinding are interesting. An installation of this kind is handling a plastic fire clay used in the manufacture of tile and brick. At the rate of 4-4.5 tons per hour to a fineness of 90 per cent passing a 200-mesh screen, the clay comes to the system varying in moisture from 12 to 18 per cent, depending upon the season of the year; it is cut up in a pug mill with the addition of some dry clay before it enters the system. A stokerfired furnace, using ordinary coal, supplies the heat a t high temperatures. In another installation steamed bone from the cookers and following presses is fed to the system a t 35 per cent moisture, and a finished product containing less than 2 per cent moisture is delivered a t the rate of 1800 to 2000 pounds per hour. The finished product is coarsely ground and classified on a n outside 16-mesh screen, and the oversize goes back to the unit for finishing. The furnace used is automatically fired with oil to maintain constant inlet and outlet temperatures, and the heat efficiency is 65 per cent. Another application represents the removal of water of crystallization. An imp mill is handling copper sulfate crystals; by the addition of heat from an automatically controlled oil-fired furnace, the water of crystallization is removed so that the unit delivers a uniform copper sulfate monohydrate in finely powdered form. The outlet temperature in the cyclone must be closely controlled so as not to drive off the last molecule in the finished product. The system was guaranteed for 800 pounds of finished product per hour and produces, in regular operation, 1000 pounds per hour. As a result of the development of mill drying, practically all pulverizer manufacturers using air separators have applied the drying principle to their mills. It is used almost exclusively in the handling of the nonmetallic minerals with a hardness below four, except in cases where the raw material contains high percentages of moisture, as occurs in various kinds of clays, kaolins, etc. In these cases the volume of water to be removed is usually too great and beyond the evaporating capacity of a milling system. Rotary and similar dryers, particularly of the indirect type, are more suitable and give higher efficiencies in moisture removal under such conditions. In the pulverizing of coal, however, rotary and other types of dryers have been entirely eliminated, both for bin systems and for direct-fired systems. RECEIVED May 26, 1938.
