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Buna, which is being made in increasing quantities in Germany, is a butadiene polymer produced by synthesis from acetylene in the following steps: (a) acetylene to acetaldehyde (b) acetaldehyde to acetaldol, (c) acetaldol to butyleneglycol, (d) butyleneglycol to butadiene, and (e) butadiene to synthetic rubber polymers. The basic raw material is, therefore, available in large quantity and the synthesis is not difficult, although the yields in some of the steps are not high. Until recently all the Buna rubber produced in Germany has been retained by the government for its own use. Neoprene is also derived from acetylene by converting it into vinylacetylene, formed by passing acetylene gas into an acid aqueous solution containing powdered cuprous chloride; it is then converted to chloroprene by the addition of hydrochloric acid and polymerized to a stage which is suitable as a rubber substitute. Xeoprene rubber has already reached a production rate of about 2,000,000 pounds in 1937, and the 1938 production is expected to be several times this amount. Thiokol is an entirely different type of rubber substitute; it is not a polymerized diolefin but a product of the interaction of ethylene dichloride and sodium polysulfide. There is little likelihood of shortage of raw materials for this type because both ethylene dichloride and sodium polysulfide are cheap and available in practically unlimited quantity if the demand arises, Thiokol is exceptionally resistant to hydrocarbon solvents, printing inks, etc., and by compounding with natural rubber markedly increases the latter’s aging properties.
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Resins Derived from Rubber Derivatives which use rubber as the base material and have recently come on the market are Tornesit, a chlorinated rubber used in paint and protective coatings, and Plioform, which is rubber hydrochloride or a similar derivative. These materials are of a plastic nature, but only Plioform is used in molding and then to a limited degree. Plioform can also be sheeted to give Pliofilm, which may be used as a thin foil for wrapping and other purposes. At present rubber is a commodity of unstable price. Consequently, the general adoption of derivatives from it may depend a great deal on the price of natural rubber. There is a trend towards world stabilization of the price of rubber, but this utopia has not yet been reached. Since the synthetic resin industry is fundamentally based on ultimate raw materials which are among the most common and most widely distributed throughout the world, it is apparent that the only serious impediments to its orderly and continued growth are temporary limitations of processing the ultimate crude materials. None of these seems a t all likely to impose more than a passing resistance to the expansion of the plastics industry. Rather, from every point of view continued acceleration of growth seems assured as more resins of greater utility are produced. This growth is importantly stimulating all branches of American chemical industry. RECEIYED October 11, 1937. Presented before the Division of Paint and Varnish Chemistry at the 94th Meeting of the Ameriaan Chemical Society, Rochester, N. Y., September 6 to 10, 1937.
DEVELOPMENTS IN GRINDING---
T
HE story of crushing and pulverizing has been one of
slow and steady development dating from prehistoric times, but it received a special impetus with the advent of power generation equipment. During the past decade or two the field has become recognized as important to chemical industries. Although it was well established in the mineral and milling industries, it was barely recognized in the earliest lists of unit operations in chemical engineering. Gradually the rate-controlling feature of surface has become important in the physical and chemical behavior of heterogeneous sys-
LINCOLN T. WORK Columbia University, New York, N. Y.
terns involving a t least one solid phase, and grinding to fine size has taken on new importance as a means of creating surface. The tools of measurement have been advanced materially during this time. Fineness over a range of microns to meters is now subject to quantitative evaluation. With this means of measurement available, the grindability of a great diversity of raw mateGals has been escablished with a satisfactory degree of quantitative precision. The basic problems of energy consumption, mill maintenance, and size distribution of product are now open to solution. This paper is an effort to give the present status of crushing and pulverizing and to indicate possible future trends. The few striking developments of recent years are specifically designated by name. As a rule, the developments have been gradual and have consisted in utilizing new materials of construction and in varying the combination of drives, feeders, mills, classifiers, and collectors. No mention is made of several of the well-established mill types, whereas in other cases reference may be made to special features without indicating the specific name of the mill. Only time will SIDEVIEW OF A BARTLETT & SNOW24 X 24 INCH SINGLE-ROLL COAL tell the wisdom of the author’s selection of C R U S H E R WITH PORTION O F S I D E SECTION B R O K E N AWAY T O SHOW THE examples. INTERIOR CONSTRUCTION
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Direct-Pressure Mills D i r e c t - p r es s u r e mills, represented by the jaw and g y r a t or y crushers, have been largely restricted to the coarse crushing field because reduction ratios are limited by limitations in their discharge openings. As a type, however, these mills are attractive because of their low maintenance cost. Within recent years a number of changes have been made in the gyratory crusher in an effort to overcome the difficulties in finer g r i n d i n g . Increasing the diameter at the bottom of the mill has helped t o increase the discharge area. T h a t t r e n d was carried further i n t h e Symons crusher in which two disks are rotated together; one of the disk shafts is moved eccentrically to bring direct C o u r t e s y , American Pulverizer Company pressure to bear in the same CROSSSECTION OF RINGCRUSHER SHOWING ADJUSTABLE GRINDING PLATEAND METAL TRAP manner as in the gyratory crusher. The rotation of the disks introduced the between the disks and has accelerated the discharge ol fines additional factor of centrifugal action which has helped to a t the periphery. This type of mill has been used to produce force the load into the outer and more restricted spaces somewhat smaller sizes than the standard gyratory crusher. Recently the Gervais crusher has been under development. In form it resembles the usual gyratory crusher, except that the grinding frustum on the center spindle is divided into Recent developments in crushing and layers, each section of which is actuated separately as to cycle and eccentricity. One section is crushing its si5e of feed pulverizing mill construction are reviewed on one side of the mill while another is effecting its reduction critically, and a description of the more along another element of the external crushing wall, thus loaddistinctive new mills is presented. The ing the shaft more evenly. Large reduction ratios would apnormal improvements of the standard pear possible, but the question of clogging and wear are permill types have consisted in the utilizatinent. Both difficulties appear to be obviated by the use tion of newer materials of construction of water flowing through all crevices from the center spindle. This washes the material through the mill, and products of and in varying combinations of drives, the order of 20-40 mesh can be obtained. feeders, mills, classifiers, and collectors. Usually the gyratory crusher is considered to operate by Problems of reducing energy consumpthe slow application of pressure and not by impact. The tion by more effective use of grinding McNally-Pittsburgh Manufacturing Corporation offers the Centriflex pulverizer which has the form of a gyratory crusher forces, of lowering maintenance charges but is operated a t high speeds with a direct-connected motor. by minimizing wear on costly parts, and The claim is that this is really an impact mill because of its of classifying the materials to obtain the greater speed. Its application appears to lie in the finer desired size reduction well into subsieve meshes, generally less than 20.
sizes by control in design and operation, continue as dominant factors in the development of new types of mills. Adjuncts to the milling process include preconditioning of the material for grindability, the use of addition agents to avoid mill packing and to give desired qualities to the product, and the use of flotation in connection with grinding and classification of nonmetallic ores.
Roller Mills The principle of the roller, either in the form of crushing rolls or of a roller traveling on a path, is one of the old principles of grinding; it has continued to be attractive for a wide variety of uses ranging from crushing rolls for quarry stone to rolls for paint manufacture, and ranging from the Chilean mill to modern coal pulverizers. The diversity of its application is due to the fact that this structure can be operated in different ways to meet the divergent demands. Crushing rolls grip the piece by friction and apply a direct compressive force upon it. Differential rolls emphasize shear and rubbing actions. The use of rolls in fine grinding intensifies the prob-
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lem of maintenance. Obviously the production of new surface in such fine materials as powdered coal and cement would exact a heavy toll if the fracture had to be accomplished by breaking each piece between the wearing faces of the mill; every effort is made in such devices to cause particles to act
FIGURE 1. ROLLGRANULATOR
upon each other in order to make a stated amount of roller action transmit its force through an optimum layer of material. In coarse crushing there has been a tendency to emphasize the single-roll crusher in contrast to double rolls, and to construct projections on the face of the rolI which would grip the material more positively. The localized overloading caused by this construction helps to fracture the material and to produce less splinters and less dust than is the case with smooth rolls. However, the localized loading of the projections also intensifies the wear a t these points so that provision is made for replaceable projections. The breaking action which can be developed with irregular forms is utilized in a German device which consists of a double roller mill for crushing filter cakes and other fragile substances to a uniform granular size (Figure 1). The device consists of a pair of rolls with square circumferential recesses placed with their projections opposite the recesses. Doctor knives keep the recesses clear. Ring roller and ball roller mills have been standard for a number of years and have been used for pulverization of coal and in some steps in the pulverization of cement and raw mixtures. These mills depended upon centrifugal force to swing the rollers or balls outward against a grinding ring. The Bethlehem pulverizer introduced a new driving mechanism. The rollers were placed in fixed positions around the mill and were pressed onto the grinding plate by spring action. The grinding plate was a flattened cone, and the rollers were conical; both had their apexes a t the same point. Thus, in this construction the old status of the ring roller mill was maintained in which there was a minimum of sliding action between the roller and the plate. The grinding plate was driven from beneath, and its rotation caused the rollers to revolve. The Raymond bowl mill (Figure 2) is a new development along similar lines. The
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rollers operate against the inner face of a rotating bowl. Material is fed to the bowl and is thrown outward against its grinding face by centrifugal action where it is subjected to the crushing action of the rollers. There is a spring loading which permits the rollers to move inward if noncrushable objects get into the mill, and there is a stoa to
and the vertical-shaft Fuller-Lehigh mill in this country. The latter has been displaced by the new Babcock & Wilcox mill (Figure 3),which is really a specially designed ball thrust bearing used for grinding. Pressures are controlled by springs, and the path of flow is controlled by the main construction and auxiliary baffles. In this new type of construction, which is not dependent upon centrifugal force, slow speeds and high pressures may be used. When compared with the roller mills, it has the advantage that it requires no axles and hence no lubrication in the immediate zone of grinding. I n the roller mill i t is noted that the straight line of contact may be so controlled with respect to speed that there is little sliding between roller and ring a t any point in the line of contact. This is not possible with the ball construction. However, when the action of each is viewed with the charge in place, it is a question as t o whether the same argument will apply. The charge in a roller mill tends to flow outward a t the sides of the rollers, leaving a denser mass in the center, and this produces grooving. In this ball mill the balls appear to rotate spirally; if they wear down uniformly, they will continue to fit the race throughout their life. Thus, the elimination of the pusher arms, as in the old type Fuller-Lehigh mill, gives improved wear characteristics to the balls in the Babcock & Wilcox mill.
Courtesy, Raymond Brothers I m p a c t Pulverizer Company
FIGURE 2. BOWLMILL
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High-speed Mills The high-speed mechanical mill has been capable of a wide usefulness from the breaking of coarse rocks to sizes suitable for use as concrete aggregate to the fine grinding of bituminous coal for coal dust firing. It is useful as a dispersing mill for breaking down agglomerates such as filter cakes to yield a soft bulky product. As a coarse grinder, it has been accepted as an impact mill where the beaters strike the pieces of feed and drive them against breaker plates or bars, I n fine grinding there have been advocates of many theories such as air action, attrition, impact, etc. I n the coarse-crushing types, portable crushers have been devised for use on trucks; others have been made with ribbed cast-steel housing and sectional liners to permit easy assembly and removal. They have been used in the production of aggregates where shape factors preclude the use of splintered and elongated pieces commonly produced in some other types of mills. Special forms of beaters are used for reducing materials such as wood. I n the field of pulverization the problem of wear is important, and these mills are usually limited to grinding the softer materials. The ring roll pulverizer is one of the newer types where striking, rolling, and crushing actions
Courtesy, Babcock: & Wilcox C o m p a n y
FIGURE 3. TYPEB PULVERIZER
inner one. Material fed to the inner basket, builds up a bed to conform to the normal angle of repose uiider centrifugal force. Additional feed moves down this slope and is cast off the edge of the basket to build up a layer or to slide along the slope also created in the outer basket. A scraper, the only structure to come into contact with rapidly moving material, returns the unground particles to the feed inlet. Fines are removed by air flow through the mill.
Ball Mills Courtesy, Patterson F o u n d r y &! M a c h i n e C o m p a n y
PORCELAIN BALLAND PEBBLE(UNDER FYIXG GLASS)
MAQNI-
are all encountered. The squirrel-cage or knife-type beaters may be included in this class as high-speed mills. The Unimax mill, offered by Zerkleinerungs-Maschinen, has an interesting arrangement of disks and knife blades between which disintegration takes place. The Kolloplex mill is a disk-and-pin type with interchangeable pins; in spite of the effect of wear on rotor balance a t high speeds, this mill is said to operate a t 12,000 r. p. m. I n this country the Mikropulverizer is operated a t speeds above normal for hammer mills. Wear has been such a problem with high-speed mills that many attempts have been made to transmit grinding energy to and from surfaces of materials. Bucky (3) has developed a mill consisting of concentric baskets driven in opposite directions. The outer basket surrounds the open ends of the
The question of liners has assumed special importance because the slip of balls along the walls has impaired proper action in their cascading. Corrugated, ship-lap, step liners, and the El-Oro type of Newell in England have their advocates. The latter is cited in particular because it is claimed that the rectangular groove holds some of the balls rigidly and really gives a ball liner with less wear upon the mill wall. The case of steel balls us. pebbles usually leads to the use of steel. Porcelain balls may be used instead of pebbles. The uniformity of the porcelain appears to be an advantage, particularly in color mills for batch operation. The use of central peripheral discharge has been shown t o have its place for different materials. The Allis-Chalmers Manufacturing Company indicates that the pulp level should be a t a point sufficiently high to interpose a bed of pulp to cushion the impact of the balls partially, and thus maximum crushing effect with a minimum wear of steel results. An extreme in the type of peripheral discharge is the German Alpine mill which has peripheral discharge slots along the entire length of the mill. Other features are the end-plate construction of Newell in England, in which reinforcing members are welded on the in-
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Grinding hid8 Factors separate from grinding devices must at all times be considered in connection with this operation. They may aid grinding or they may become substitutes for it. Conditioning of material is one of these factors. Heating expansive materials and quenching may produce the shock to disintegrate them. The careful washing of filter cakes may remove the soluble materials which cause the dried cake to be hard and grind to a gritty product. There are many other specific examples of conditioning, but these will serve as illustrations. In the grinding operation itself certain auxiliary actions may be carried out. IIydrates may be calcined to lower or anhydrous forms by the control of air temperatures in the mill during the griiiding operation. The prodnction of plaster of Paris is one example, and the steaming of bones and the chilling of sensitive organic materials are other BRIT IS^ REMATUBEM ~ L L FITTED WITH PATENTED VACUUMSYSTEM OF C~assrrrexamples of the effect of controlled CATION AND DRYING, WORKING ON THE CLOSED CIRCUIT, BIN, OR STORAGE SYSTEM environments within the niiils. Breyer (8) showed that the use of side arid relieve wear on the enclosing plate, the use of the addition agents in dry grinding permits the attainment of finer sizes ordinarily inhibited by coating of the grinding spiral outlet liner for the return of balls and oversize aocidenelements. The Devrey and Almy Chemical Company hai tally carried into the outlet ducts of air-swept mills, and the use of insulation to reduce sound. applied such agents in the final grinding of cement and the The IIardinge Company and Rema Manufacturing Company dry the feed by dropping i t through warm air which also removes fines and carries them to the separator without subjecting them to mill action. The use of lesser amounts of air or none at all for sweeping t,he mill, and the rise of external aeration and classification. are novelties since the development of the air-swept ball mill of Kennedy. One entirely novel contribution to this field is the Vibratom developed in Germany (Figure 4). It consists of a basketlike casing practically filled with balls and suspended on opposite sides by a driving mechanism which shakes the entire chamber. The shock energy is absorbed in springs and reused. The balls apparently vibrate through the entire mill. Few actual data are available with respect to power and fineness, and these factors will need study. The advertising matter indicates large reductions and a fine size of product. There does not appear to be a classifying action in the mill, and further evidence is needed to establish its place i n the field. Jet
Mills
Air or steam jet mills have recently taken on a new status since the earlier developments along this line. Dean has .worked with the jet-and-anvil type and has revived an inter.estin this kind of impact action. The International Pulverizing Corporation offers the Mieronizer (f) which consists of a shallow chamber with grinding jets a t or near the periphery. When the material is ground under the action of the jets, i t moves inward to a central collector or a n outlet pipe against strong centrifugal classification. The mill can separate at small maximum size, in some eases as low as 3 or 4 microns. Jet devices are comparatively expensive in energy cost. Opportunity lies in lowering this cost or io producing an unusual quality of material.
result is improved flow and working properties of the concrete. Such agents may also be useful in wet grinding to disperse the particles and create colloidal suspensions rather than agglomerates. Sddit,ion agents have heen used in the final grinding of pigments in order that they may more readily be dispersed in oil.
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The use of flotation nloiig wit11 grinding is well k n o w n in the metallic mineral industries. I t has been applied in the nonmetallic field by Breerwood and the Dorr Company PO that the argillaceous, silica, and lime-bearing ingredients of a cement may first be separated and then recombined as desired. It obviates the step of proportioning a t the quarry and appears to justify itself in improved operating costs and in quality and uniformity of product.
Auxiliaries Auxiliary equipment associated with grinding would constitute a paper in itself. The developments in classification have been extensive, and many of these classifiers are used as adjuncts to grinding mills. There h a s b e e n a p r o nounced tendency to utilize closedcircuit grinding and air-classifying s y s t e m s i n s t e a d of air-swept mills. Apparently the air currents in certain mills scour the grinding zones and remove more material than is desirable, leaving a zone of ineffectual grinding or inducing unnecessary wear. The question of controls in Courtesy, Allis-Chalmers Manufacturing C o m p a n y grinding is of considerable imporCROSS AND LONGITUDINAL SECTIONS OF BALLMILLFLOW LEVELS tance. since the overloading of the Diapk tragm set for high discharge (above) and for low discharge (below) mill may impair the effectiveness of the grinding action and the underloading may increase maintenance cost. Various devices have been used, such as control of operator can hear or what he thinks is right. The subject feeder from the power line or the control of the feeder in of feeders operated either independently or in conjunction with the mill, would also constitute a paper in itself, and terms of circulating air friction. Savings of the order of 20 to 40 per cent are said to result from the use of such conno effort will be made here to do more than refer to them trols over manual operation, which depends upon what the as part of the complete grinding cycle. Belt drives have been used in certain types of mills in order that slip may occur and the mill be not d a m a g e d i n c a s e irreducible objects or too large a charge of 'eed overloads the mill. Several directc o n n e c t e d types are now in use, with shear pins to be b r o k e n if the mill is too heavily overloaded.
Literature Cited
I
A
Courtesy. Ernest Newel1 &. Co., L t d .
(1) Andrews, N. H. (to International Pulverizing Corp.), U. S. Patent 2,032,827 (March 3, 1936). (2) Breyer, U. S. Patent 1,985,076 (Deo.. 1934). (3) Bucky ( t o University Patents, Ino.), German Patent 645,522 ( M a y 28, 1937).
TYPES OF BALLMILLLINERS A.
Wavy or undulating
F.
C.
Clamping bar Ship lap Step
H.
B. Cascade D. E.
El-Oro
U. Block
Wedge I. Komata L. Osborne
RECEIWIDNovember 12, 1937. Presented before the 30th Meeting of the American Institute of Chemical Engineers, St. Louis, Mo., November 15 and 16, 1937.
