Abrasives—Technological Advances in an Essential I n d u s t r y WELLJAIMH. WAGGAIMAN and OLIVER C. RALSTON
Metallurgical Branch, Biiream of Mines, U. S. Department of the Interior, Washington 25, D. C. C O M M O D I T I E S are often defined in such broad t e r m s t h a t certain members of the same group have properties t h a t appear diametrically opposite—for instance, both filet mignon and hamburgers can be described as beef, but what a vast difference there is i n their availability, quality, and price; yet the economic gap that separates the common meatball from a choice steak is a narrow one compared to the spread between t h e lowest- and highest-priced materials clashed under t h e general heading of abrasives. T h e term " a b r a s i v e " means a substance used to r u b off, smooth, wear down, and polish. Kvery abrasive performs one or more of these functions, b u t the numerous materials used for abrasive purposes vary greatly i n physical- properties, chemical composition, q u a n t i t y , and cost (//, )j). Abrasives m a y be light or heavy and range all t h e w a y from t h e hardest to the softest minerals; some eonsist of coarse, sharp fragments and others a r e fine silky precipitates; they m a y eonsist of pure o r nearly p u r e elements, sucîa as t h e diamoriid and steel wool, or be m a d e up of somewhat complex inorganic compounds. Finally, they include materials occurring in inexhaustable quantities, wortli a few dollars per ton, a s well as those t h a t a r e relatively scarce a n d sold by t h e carat. An Immense but Vntipprecia ted Industry Since abrasives fade modestly into the background after their work i s done, few realize t h e immensity a n d importance of this industry. According to the latest statistics of t h e Bureau of Mines (#), the United States produced 1,328,340 tons of abrasives in 1945, valued a t the tidy sum of $31,694,809, a n d these figures do nol include certain metallic oxides and manufactured products used for polishing purposes. T h e far-reaching effeet of abrasives on industrial progress can hardly be overemphasized, for directly o r indirectly these materials play an essential role in nearly every peace- and w a r t i m e product; n o t only a r e t h e y responsible, in a large measure, for locating a n d developing our mineral wealth, b u t they are used in fabricating a n d finishing t h e metal and n o n metallic products d e r i v e d therefrom. W i t h o u t abrasives, mechanized equipm e n t would be crude a n d short-lived and! t h e manufacture of precision instruments* impossible. M a n y of o u r most beautiful a n d stately buildings would eventually b e -
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come soot-covered, weathered structures unless they were cleaned periodically with a sand blast. T h e home would be a pretty drab affair, lacking lustrous plate glass, burnished brass and silver, smooth finished woodwork, and polished marble. Grease and carbon would accumulate on pots and pans and junior's grimy hands often would be impossible to clean without t h e extra kick that abrasives give to soap a n d other detergents. Yet, these indispensable agents of modern civilization get lit tie credit for their invaluable services, because most of the products for which they are responsible must be free from every trace of abrasive material before they are displayed; marketed, a n d used. liroad
Classification
Abrasives may be divided into two broad classes—(1) natural minerals a n d rocks and (2) manufactured products obtained by metallurgical and chemical processes. Each h a s its own special properties and functions, and each is carefully selected or prepared by modern scientific methods
WD. T h e m a i n n a t u r a l abrasives i n the approximate order of their dollar value a r e as follows: diamond, corundum, garnet, diatomite, tripoli, talc, emery, pumice and puiinicite, whiting, a n d silica sand. T h e manufactured abrasives range from some of t h e hardest materials known, such Resistance-type ducing silicon
as silicon carbide, tungsten a n d boron carbides, to such mild compounds as ferric oxide (rouge), precipitated chalk (whiting), and calcium phosphate. Natural
Abrasives
Of ail the natural minerals, the diamond is the hardest, most effective abrasive and cutting tool known. T h e d e m a n d for industrial diamonds has grown steadily, and they perform m a n y arduous as well as the most exotic tasks. In t h e form of diamond drills, they bore through s t r a t a that few other substances can adequately pierce, and diamond dust is t h e most efficient, and in some cases the only practicable, abrasive to use for polishing gems, for making diamond wire-drawing dies, and for grinding down metallic alloys of exceptional hardness. Although we have been able to manufacture products having properties similar to those of a n u m b e r of natural minerals, t h e diamond has defied our efforts to produce it commercially by sj^nthetic means. Natural corundum or crystallized alumin u m oxide still holds i t s own in competition with fused alumina; in fact, it is preferred to t h e manufactured product for two main purposes where sharp, tough cutting edges are required (10). One of these uses is in snagging wheels where corundum in 14- to 16-mesh grains is required. Natural cor u n d u m in such wheels permits much faster cutting t h a n can be obtained with the best
electric furnace used in procarbide (Çrystolowi) abrasive
CHEMICAL
AND
ENGINEERING
NEWS
Aluminum polishing
oxide wheels
abrasive grains used for automatic bumper
artificial alumina wheels. T h e other important use for natural corundum is in grinding optical glass, where fine sizes from 60-mcsh down to impalpable powderare required. I t is estimated that during the late Avar, when the manpower shortage was so acute, lack of natural corundum would have slowed bj* 2 5 % the snagging of rough castings and the grinding of lenses and prisms required for optical inst ruments. As for garnet, feldspar, pumice, and silica sand, these are available in such large quantities, a n d at such low prices, that t h e y probably will always be the most economical abrasives to use for dressing stone, lumber, and rough metal castings, for cleaning t h e exterior of buildings, and for refinishing wood floors. Large quantifies of tripoli, diatomaceous earth, chalk, talc, and some other relatively soft natural abrasives will continue to be used, b u t these are encountering greater competition from manufactured compounds. Mannfaclured
Abrasives
Manufactured abrasives are playing an increasingly important p a r t in industry, and a number of them rival in hardness and durability the best natural abrasives, with t h e exception of the diamond. T h e carbides of silicon, tungsten, titanium, and boron a n d their binary and ternary mixtures lead the list of the harder and tougher manufactured products, b u t fused a l u m i n a is probably the most widely used of the synthetic abrasives. This product was first developed early in the present century but h a s been vastly improved and is now applicable to a wide variety of uses. Next t o fused alumina in respect to quantities consumed are the metallic abrasives, such a s chilled iron or steel shot, metallic granules, and steel wool. A t present these are chiefly used for relatively rough work in place of blasting sand, a n d for cleaning a n d scouring purposes, b u t advances in the manufacture of ferroalloys promise far greater possibilities for t h e metallic abrasives. Chemical precipitates, such as ferric oxide, calcium carbonate, and calcium phosphate, are among t h e mildest abraV O L U M E
2 4,
NO
1 9 .
oti
.set-up polishing
Resistance-type electric producing boron carbide
sives and a r e e m p l o y e d for p.dishini; and finishing products thîa-t mijçht be injured or marred b y m a n y o f the harsh and less uniform natural a b r a s i v e materials. A dealing with this general subject; moreover, t h e use of industrial diamonds hi. the abrasive industry is now so widespread t h a t this industry has its own technical magazine, Industrial Diamond Review. Advances in ore-dressing technique have made it possible to effect fairly clean separations of abrasive materials from ores containing small p r o p o r t i o n s of these ingredients (Γ, 13). T h e first application of dense-medium (sink-float) separation of a nomnetallic mineral w a s made on garnetbearing deposits of N e w York State, with beneficial a n d economical results. Where ores are low in a b r a s i v e ingredients but carry substantial q u a n t i t i e s of other sal able minerals, such a s chromite, zircon, ilmenite, a n d sillimanrte, their exploration m a y be e c o n o m i c a l l y - feasible and the abrasive recovered a s a by-product a t a reasonably low cost. The Oregon beach sands offer distinct possibilities for the r e covery of chromite, zircon, a n d garnet (6'), a n d an ore c o n t a i n i n g both kyanitc a n d garnet was found a m e n a b l e t o flotation (?). I n M o n t a n a , c o r u n d u m occurs associated with sillimanite, a n d i t has been demon strated that these two minerals can be sepa rated from a s o m e w h a t complex ore by flotation methods. Modern screening, grinding, and airseparation e q u i p m e n t permits far closer sizing and grading o f mineral products t h a n was possible in t h e past. These h. made it practicable t o u_se natural abrasi for finer and more delicate work b y elimi nating coarse or harsh foreign material.
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