ALBERT S. HESTER, Associate Editor in collaboration with
A. D. MITCHELL and R. W. REES, Hard Metals, Ltd., Springs, Transvaal, South Africa
Tungsten Carbide
T U N G S T E N CARBIDE is the most widely used of the so-called “hard metals.” These materials have properties of metals, but are actually compounds. Other carbides-for example, those of titanium, tantalum, molybdenum, and zirconium-are also used as hard metals. Besides carbides there are also metallic borides, nitrides, silicides, and other materials in this category. Hard metals are used for tips on rock drills in the mining industry, for metal cutting tools, for wire drawing dies, and for other applications where extreme hardness is required. I n war time they are used for armour piercing shells. Tungsten carbide is too brittle to be used alone, so it is cemented with a binder. Manufacture and use of hard metals (7, 2, 5, 7), and also tungsten (6) have been described. T h e Anglo American Corp. of South Africa has been interested in mining drill tips for many years. I t is one of the largest mining groups in the world, and uses a great many in its own operations. An associated company, De Beers, is the world’s largest diamond producer, and is also concerned with making drill tips. Previous articles in this series have described Anglo American’s gold ( 4 ) and uranium processes ( 3 ) . T h e company also mines and concentrates large quantities of copper, zinc, and other minerals. Hard Metals, Ltd., a subsidiary company, was set up in 1949 to make drill tips and other products from tungsten carbide. Murex Ltd. in Great Britain was a partner initially, but has since relinquished its interest. T h e plant is located in the Witwatersrand mining district a t Springs in the Transvaal, about 30 miles from Johannesburg. T h e factory is built in open country in the center of the gold-mining areas. I t stands on 6 3 acres of ground large areas of which are laid out as beautifully tended gardens and with the exception of the Assembly Plant the
94
INDUSTRIAL AND ENGINEERING CHEMISTRY
buildings are air-conditioned throughout. Plant operations are divided into three sections: chemical processing, powder metallurgy, and drill manufacture. I n the chemical section tungsten ores are digested and purified to get pure tungsten oxide, and an impure cobalt alloy is purified to get pure cobalt oxide. In the powder metallurgy section these two oxides are reduced to their metals. T h e tungsten is combined with carbon to form tungsten carbide, which is cemented with the cobalt and shaped into drill tips and other tools by powder metallurgy processes. In the drill manufacturing section the hard drill tips are brazed into place on the steel drills themselves. T h e last is not a chemical operation, and will be omitted from this article. T h e cemented tungsten carbide is made in about 24 different grades.
Some of the variables altered to make these grades are grain size, amount of cobalt, and presence or absence of other carbides, such as titanium or tantalum carbide. T h e majority of the sales are unique in that the rods are not sold according to weight or number but in terms of service. Thus a mine operator can be guaranteed a definite amount of drilling from the tools he buys. If a bit fails, the company replaces it. Company representatives are in close contact with their customers to administer this service. Raw materials are brought to the plant by train or truck. Tungsten ores come from Northern Transvaal, South West Africa along the Orange River and from the Rhodesias. The crude cobalt metal comes from the copper belt in Northern Rhodesia. I t is made by
Scheelite i s digested in hydrochloric acid in these vessels. They are placed on a mezzanine floor, so that they can be emptied into the vessels o f the next stage b y gravity
electrodeposition from a solution of a by-product from fire refining of copper at Rhokana Mines. Carbon, in the form of lampblack, is imported from outside the Union. Employees total about 250 Europeans 400 non-European operators and There are about 40 technically trained people on the staff.
Tungsten Is Extracted by a Chemical Process Scheelite, CaW04, is the usual tungsten ore used at Hard Metals. Wolframite, which is a collective term for iron or manganese tungstates, can also be used but it must first be converted into an artificial scheelite, which is then processed in the same way as the other ore. Ore concentrates arrive a t the plant from mines elsewhere in Southern Africa in small bags as a fine gravel-like material. Scheelite has a vitreous luster and may be white to pale yellow, or sometimes greenish, brownish, or reddish. T h e concentrate consists of a mixture of particles of scheelite and particles of impurities. These may be minerals containing tin, antimony, arsenic, bismuth, or other elements. They are separated on electrostatic rollers, by a process originally developed to separate diamonds from gangue. T h e ore concentrate is first classified on sieves so that the feed to the separator will be uniform. Adjustments are made for the various sizes. T h e separator (9E) separates particles according to electrical conductivity. I t consists of two pairs of rollers about 6 feet long and 6 inches in diameter and made of phenolic resin. A 25,000-volt current flows through a brush running along the length of one of the rollers of each set, and across to the second roller which is grounded. T h e first roller therefore gets a positive charge, the second a negative one. The feed moves a thin layer along a vibrating plate and onto the negative roller. T h e electrostatic charge causes the particles to adhere to the negative roller, but when the rotating roller brings the particles to the positively charged roller, the nonscheelite minerals move to the positive roller. Scrapers remove the particles from the two rollers-scheelite from one and impurities from the other. T h e separation is not perfect, and varies considerably among different lots of ore. The second set of rollers improve the separation. T h e scheelite is then ground in a continuous ball mill. A stream of air carries off the finer material through a cyclone separator. I t removes coarser particles which are recycled to the mill. T h e fines are carried to a bag filter, which removes them. Over 90% of the fines must pass a 200-mesh sieve. This material is made up into 770-pound
Ammonium tungstate is crystallized in these evaporator pans. scooped out from time to time by hand
batches, placed in drums, and transferred to the digesters in the chemical building. Although there is a large ground floor area in the chemical processing building, part of the building has four mezzanines so that gravity feed may be used in all the operations. The digesters are located on the top floor. Acids and demineralized water are stored in roof tanks. The digesters are closed vessels with a capacity of about 250 gallons. They are rubber-lined steel, and equipped with steam jackets and paddle stirrers. T o the empty digester is added 134 imperial gallons of concentrated hydrochloric acid. Then the 770 pounds of scheelite is fed in by hand, while heating is started. Temperature is brought up to 85" C., and once the entire batch has been fed in, the digester is sealed. Pressure is controlled a t 5 inches of water. After 12 hours the vessel is opened and concentrated nitric acid is added to the mixture (slowly at first, as the reaction is very vigorous). Acid is added until the reaction ceases. T h e total amount varies with the nature of the ore. The mixture is then boiled for 3 hours, with the digester open. This heating is by steam injection directly into the batch. During this process the original blue black product of the hydrochloric acid digestion is converted into yellow tungstic acid, H2W04, which is insoluble. T h e liquid is allowed to settle, and the liquor, which contains many of the impurities, is siphoned off. Then the solids are washed several times with water from the municipal mains and finally with demineralized water. These washes remove all of the acids. On the third mezzanine, below the digesters, are open stainless steel vats,
Crystals are
each of about 250-gallon capacity. The vat is filled with 80 imperial gallons of water. Into this is lowered a stainless steel sparging ring through which ammonia gas is blown into the water to make an ammonia solution. Any undissolved ammonia is carried o f f through a ventilator ring around the top edge of the vat. After 1.5 to 2 hours the ammonia is cut off, and the solution allowed to cool to 30" to 35" C. Then the crude tungstic acid slurry from the digester is discharged (through a rubber-lined steel pipe) into the ammonia solution. Sufficient crude tungstic acid is added to bring the density to 1.1. The tungstic acid dissolves, forming ammonium tungstate. Insoluble impurities form a sludge at the bottom of the vat. Other vats of similar size, but of rubber-lined steel can be used to hold portions of the digested batches when necessary. Stainless steel would be attacked by the acid. After the sludge has settled in the ammonium tungstate solution the clear liquid is pumped through a clarifying filter on the floor below and into the crystallizers. T h e clarifying filter consists of shallow stainless steel pans about 3 feet in diameter bolted together at their rims. Filter medium is stretched between them. This consists of two layers of filter paper between two layers of filter cloth. Pressure is between 5 and 10 pounds per square inch. The crystallizers are circular, shallow, stainless steel pans about 8 feet in diameter, heated by steam jackets. A conical cover with sliding doors for access and connecting to a fume duct at the top carries o f f the fumes. These include excess ammonia. Insoluble ammonium paratungstate VOL. 52, NO. 2
FEBRUARY 1960
95
J W Y
uz
W ln
W z
2 z
2
*?IF
zI I
LL
z 4
0
c LL 4
m >
Lz
W I-
* mW
2 n
W I
96
INDUSTRIAL AND ENGINEERING CHEMISTRY
TUNGSTEN CARBIDE crystals form in the crystallizer and drop t o the bottom. These are removed regularly by the operator and transferred to a washing vessel. If the crystals are left too long they settle to the bottom and become aggregated. After some time impurities build u p and the mother liquor becomes brownish a t which stage crystallization is discontinued. T h e crystallizer is then emptied and the material sent to the wolframite digesters, which will be described later. Crystals removed from the evaporator are placed in a stainless steel pan which is emptied on a simple vacuum filter ( 3 E ) . Crystals caught on the filter cloth are washed eight times with demineralized water and then partially dried by the air drawn through them by the vacuum pump for 30 minutes. T h e crystals are finally dried in stainless steel tray dryers at 120' C. for 4 to 6 hours. The dried white paratungstate powder is put through a small 100-mesh vibrating screen ( 5 E ) . Then it is calcined in air; ammonia and water are driven off, leaving tungsten trioxide, WOS. T h e ammonium paratungstate powder is fed into the calcining furnaces in stainless steel trays about 8 X 12 inches and 2 inches deep. These hold about 5 kg. of paratungstate. The furnace consists of an electrically heated tunnel about 20 feet long. A tray is fed in every 20 minutes and pushed ahead by a screw. This pushes the whole train of trays ahead, and the last tray is taken out. Air is blown through the furnace at a rate determined by previous operating experience. After cooling, the entire batch is remixed on a stainless steel table and placed in drums. As soon as it is assayed it is sent to the powder metallurgy section. Great care is taken in handling it to prevent contamination with impurities.
pounds per square inch pressure for 6 hours. T h e batch is then discharged into a rubber-lined 250-gallon vat on the third floor. Scrap liquors from the crystallizers may be added at this stage. A steam coil is used to boil the solution until it is of a specific gravity of 1.3 to 1.4. A stainless steel fume duct around the rim carries away the fumes. A filter press removes the sludge. The clear solution goes to a second vat similar to the first, but equipped with an agitator. I t is heated to boiling by steam injection, and calcium chloride is added producing insoluble calcium tungstate and sodium chloride. The calcium tungstate is recovered in a filter press and dewatered for 30 minutes with compressed air. A batch of 600 pounds of calcium tungstate is fed into the acid digesters and treated in the same way as the scheelite.
Cobalt Metal Powder Is Produced from Impure Cathode Metal T h e cathode metal used as raw material for cobalt is dissolved in concentrated hydrochloric acid in rubber-lined steel, steam-jacketed digesters. T h e impurities in the solution are then precipitated out in several steps, each one requiring an exact p H adjustment. The six pre-
cipitation vats are rubber-lined steel and each holds about 300 gallons. Filter presses plus a spare, serve all the vats. The first precipitation is to remove copper. This is done by passing hydrogen sulfide gas into the solution, and, as in each of these steps, adjusting the p H carefully. Samples for p H determination are tested in a laboratory meter set u p near the vessels. After filtering and pumping the batch into another precipitation vat, sodium chlorate and soda ash are added to precipitate out the iron. When this is removed, sodium hypochlorite is used in the next step to precipitate out the manganese. Nickel is removed with caustic soda and sodium hypochlorite in another precipitation. Finally, the cobalt itself is precipitated out as the carbonate, with sodium carbonate. This is filtered out in a different filter press. This pinkish purple material is then calcined in special steel boats to black oxide, C0304, or gray oxide, COO. The former requires a higher temperature than the latter. Both powders are then milled and washed until the wash water when tested shows no soluble impurities. The oxide is the drjed, milled, and carefully packed for transfer to the powder metallurgy department.
Wolframite Must Be Converted into Calcium Tungstate Scrap from used drills and scrap powder are subjected to the same treatment as wolframite. T h e drill tips are removed from the drills by an acid treatment which destroys the brazing. Further acid treatment removes the cobalt. T h e cobalt-free scrap is then oxidized. Wolframite or scrap is digested in caustic soda in autoclaves located on the fourth floor of the chemical building near the scheelite digesters. They are about the same size as the other digesters but are stainless steel pressure vessels. U p to 700 pounds of wolframite or oxidized scrap-the exact amount depending upon the tungsten content-is put into the autoclaves and caustic soda added. I t is heated to 145' C. and kept at 50
Cobalt solution i s purified by a series of precipitations for removing impurities. These are carried out in the tanks in the background and the precipitates removed by filter presses VOL. 52, NO. 2
FEBRUARY 1960
97
Furnaces, in powder metallurgy section, with auxiliary equipment, are the two boxlike pieces in the rear. other one is in right foreground
Cobalt compounds for coloring ceramics, agricultural uses, and for high temperature alloys are also produced.
Hydrogen I s Made by Electrolysis of Water Hydrogen is used for reducing tungsten and cobalt oxides to metal in the powder metallurgy section. As powder metals and carbides are all readily oxidized a t high temperatures, various other furnacing operations in the powder metallurgy section are also carried out in a n hydrogen atmosphere and the hydrogen plant to supply these needs has an output of several thousand cubic feet per hour. There are four banks of cells ( 4 E ) , with nickel plated and plain steel electrodes separated by an asbestos blanket. T h e electrolyte is water, containing sodium hydroxide to increase conductivity. Mercury arc rectifiers supply the direct current required : 2000 amperes a t 2.5 volts per cell. The hydrogen is dried by passing through alumina drying units (7E). For certain operations moisture may be left in the hydrogen. An automatic dew point controller, constructed in the plant’s instrument shop keeps this a t a set level.
Powder Metallurgical Techniques All materials for making hard metals must be kept in a highly pure condition. Otherwise, defects would occur in the finished products, causing too high a proportion of rejections at the numerous
98
INDUSTRIAL AND ENGINEERING CHEMISTRY
inspection points in the process. High standards of cleanliness are maintained throughout all parts of the plant. T h e entire building is air conditioned to keep out dust, and air locks are provided at the doors. Purity specifications are given in Table I.
Table 1.
Maximum Impurities in Metals
Tungsten Powder Tin, arsenic, and antimony (collectively) Calcium and magnesium (collectively) Silica Sodium and potassium (collectively) Oxygen Cobalt Powder Iron Manganese Copper Sulfur Silica Alkaline earths Chlorides Nickel Oxygen
% 0.05 0.02 0.02 0.01 0.05
0.02 0.02 0.005 0.002 0.03 0.03 0.025
0.02 0.6
The furnaces used for reducing the oxides to metal, carburizing, presintering, and sintering are all different from each other, but in general their construction and operation are quite similar. Each furnace consists of a tunnel, whose cross sectional shape and size varies. I t may be circular, rectangular, or semicircular, and vary from about 3 inches in diameter for the tube furnaces, to about 1 foot wide and 4 inches high.
Entrance to an-
T h e tunnel may be from about 12 to 20 feet long. Their mid-sections are heated by one or more electrical elements. A stream of hydrogen flows through the tunnel to provide a reducing atmosphere. The rate of flow must be constant for any one operation and consequently each furnace is fitted with an hydrogen flow meter. Excess hydrogen is usually burned as it leaves the tunnel. Boats, of graphite or stainless steel, are introduced, usually through a lock, at one end of the tunnel. T h e boats are pushed forward by a screw, which pushes the whole train ahead one boat length, and the last boat at the opposite end of the furnace is removed. Because hot material would oxidize in the air (cobalt would catch fire) the trays pass through a cooling section after leaving the heating section of the furnace. This usually takes the form of a water jacket, sometimes equipped with cooling fins. Tungsten and cobalt oxides are brought to the powder metallurgy building in stainless steel canisters. Two types of furnaces are used for reducing tungsten. The tunnels are stainless steel tubes about 3 inches in diameter and 10 feet long. The heating section is about 4 feet long, and heats the material in the boats to the required temperature. Each stainless steel boat contains enough oxide to produce 55 grams of metal. A new boat is introduced every 0.5 hour. Hydrogen flow is 1 5 cubic feet per hour. Excess is burned at the entrance. A simple door, held shut by a spring, closes the entrance. T h e tube furnaces are arranged in banks of 12 tubes (three tiers of four tubes each) with a common heating unit, controlled by a thermocouple de-
TUNGSTEN CARBIDE vice. A water jacket placed after the heaters cools the reduced metal to near room temperature. For other grades of tungsten, tray furnaces are used. These accommodate larger boats in the form of trays about 12 X 8 inches and 2 inches deep. They are introduced through a lock to keep air from entering the furnace. The tunnel enlarges as it approaches the heating section so that some of the heat can reach the boats and preheat them before they reach the main heating section. Trays are fed in at the rate of one every 20 minutes. Temperature reaches 800 ' to 850' C., and hydrogen flow is 800 cubic feet per hour. A finned cooling section cools the trays after heating. The metallic tungsten powder from the furnaces is crushed in stainless steel end runner mills, which are essentially mechanical mortars and pestles. The bowl is about 2 feet in diameter. An air duct just over the bowl draws off fine dust, which might otherwise contaminate the air in the building. The tungsten is then sieved to -100 or -200 mesh, depending upon the grade of the finished carbide. Carbon black is added to the sieved metal powder in atomic proportions. This is blended in a rotating blender and compressed into blocks of 6 X I'/z X 3/14 inches, for easy handling. These are fed into the carburizing furnaces a t 1500' to 1600' C. heating the tungsten carbon mixture t o form tungsten carbide. The blocks leaving the furnace are crushed in a jaw crusher (8E) and sieved
(5E). Powdered cobalt, which has been reduced in a manner similar to that for tungsten, is added at this point together with other additives, sometimes TIC, TaC, Cr3C2, NbC, etc. T h e amount of cobalt varies from 8 to 25%, but some experimental grades have been made using only 4%. Mixing is done in ball mills. Each canister for milling contains 6 kg. of powder. Balls are of tungsten carbide, to prevent contamination from any other substance. T h e mixture of powder and balls is covered with a milling fluid such as acerone or naphtha before the canister is sealed and placed on the vibrators, where they are kept from 36 to 72 hours. Wet milling is used to prevent oxidation, which would occur in air. Canisters removed from the vibrators are placed into receptacles in a steamheated table. T h e milling fluid is siphoned off, and the mixture dried by heating and the remaining vapor being drawn off through a vacuum line connected to the canister. The powder is then discharged and sieved. Milled powder then goes to the waxer (ZE). This device has an open stainless steel, cylindrical pan about 3 feet in
diameter and 1 foot deep. A 0.5 to 1% solution of paraffin wax or camphor in trichlorethylene solvent is added to the powdered metal in the pan. The slurry is heated to 95' C. and vigorously mixed by a paddle mixer. T h e heat drives off the solvent, leaving each metal particle covered with a thin layer of wax. This is necessary to make the particles adhere to each other when the powder is pressed into shape for presintering. Wax powder is sieved to - 60 mesh. Pill presses ( 7 E ) are used to press the powder into the approximately desired shapes for the finished product. A typical piece might be a drill tip 1 1 / 2 X "4 X '/4 inches. However, the variety of shapes is large. After pressing, each piece is checked for weight and dimensions. These pieces held together by wax are not strong enough to stand u p during the machining operations which give them final shape, so they are presintered to partially fuse the particles together-enough to withstand the machining operations, but not nearly enough to fuse the piece into the hard carbide metal. The pieces are placed on a bed of carbon black in a 5 X 7 X 2 inches deep graphite boat, and then completely covered with more carbon black. T h e boats are then run through the combination dewaxing and presintering furnace. Like the other furnaces, these have an hydrogen atmosphere, but the heating sections are divided into three zones: (200' C.) ; dewaxing preheating (400' C.); and presintering (800' C , ) . A water-jacketed section cools the boats before they are removed. Boats are fed in once every 20 minutes. In the dewaxing section the wax is driven out of
the piece, and in the presintering section the particles are partially cemented together. Machining operations on the presintered pieces are carried out to close tolerances, allowing for a 20% shrinkage during sintering. Presintered pieces are of the consistency of chalk, and can be used for writing on paper. Sintered pieces will cut glass. Ordinary drill bits are shaped on a series of grinding wheels, All this shaping is done by nonEuropean labor. Other pieces which are more complicated may be turned on lathes, or otherwise machined. After machining, the pieces are packed in alumina or carbon in graphite boats, and are sintered in furnaces a t 1400° to 1500" C. The exact temperature varies, depending upon the material, but it should be correct within f 5 O C. The automatic temperature control by thermocouple is checked with an optical pyrometer. Heating takes place in a very short hot zone in the center of the furnace. Feed rate is one boat every 0.5 hour. The cobalt fuses and forms a molten eutectic alloy which acts as the actual binder. I t wets the larger grains of tungsten carbide after some of the smaller ones have been absorbed in the solution. The larger particles of tungsten carbide coalesce and form a continuous skeleton of carbide ( 6 ) . The piece contracts 18 to 30% in s:ze, depending upon the pressure applied in forming the piece. Control is easier when high pressures are used, as they give less contraction. After sintering, each piece is etched with its grade and batch number. The finished drill tips are sent to the drill plant where they are brazed into place on the steel drills.
This typical mine drill has a tungsten carbide tip VOL. 52, NO. 2
FEBRUARY 1960
99
Drill tips are machined to shape before sintering
Testing and Control Are Important Chemical analysis of the raw materials and process samples from the chemical and powder metallurgy plants are carried out by a staff of about 20 chemists and technicians. Wet methods are generally used. For the finished product and many of the powder metallurgy samples, physical testing is most important. Particle size distribution is important for some of the powders. This measurement is made in a sedimentation balance ( 6 E ) in which a sample of powder is dropped through a liquid, such as glycerol solution, and allowed to fall onto a balance pan. T h e weight of material on the pan is plotted automatically against time. From this, a histogram of particle sizes can be constructed. Another test is powder permeability. This is measured by drawing air through a dry powder bed and measuring the flow. Hardness testing is important for the finished pieces. I n the testing machine the depth of the impression made by a blow from a diamond pyramid is measured. Hardness varies between 900 and 1800 V.P.N. (Vickers penetration number) on the Vickers hardness testing machine. A test is also made for transverse rupture strength. A polished cross section surface of a finished piece is examined a t 1 2 0 X magnification for porosity. The count is usually 3 to 5 pores per cross section: which is roughly ‘/4 X ‘/z inch. Another cross section is etched with alkaline ferricyanide or nitric acid solution and examined a t 15OOX. This shows u p micropores. Cobalt distribution can be seen as well as grain size. A ferromagnetic test is run to deter-
1 00
mine coercivity, a property interrelated with hardness and strength. The test piece is placed in a magnetic yoke, whose strong field magnetizes the cobalt to the point of saturation. The piece is then transferred into a field coil, and the magnetism measured by probes. Statistical methods are used to develop control charts to interpret test results to ensure uniformly high quality in all work.
Outlook Tungsten carbide drill tips have definitely come to stay in the mining industry. Since 1950 the footage sorld by Hard Metals, Ltd., has continuously increased and a t present is of the order of 180 million feet per year. I t seems unlikely that any other material will replace tungsten carbide in the mining industry in the foreseeable future. During the war there was a large market for tungsten carbide for armour piercing projectiles. Mining drill tips are Hard Metals most important product, but the company also produces a considerable amount of hard metal cutting tools for machine shops, and some for wood, together with most of the wire drawing pellets used in the Union. Indications are that the consumption of carbide for these and associated industries will increase steadily. Tungsten metal itself is used for making hard steels, for contact points, high melting point crucibles and for other uses. Tungsten filaments are widely used in electric bulbs, but 100 million bulbs can be made from less than two tons. Hard Metals, Ltd., sells to a large market in the mining industries in Southern Africa. It also exports ap-
INDUSTRIAL AND ENGINEERING CHEMISTRY
proximately SOY0 of its output. The company has a subsidiary which sells sizable quantities in Canada.
References (1) Dawiihl, W., “Handbuch der Hart-
metalle,” abridged translation, Department of Scientific and Industrial Research, H. M. Stationary Office, London, 1955. (2) Goetzel, Claus, G. “Treaties on Powder Metallurgy,” Interscience Publishers, London, 1949. (3) Hull, W. Q., and Pinkney, E. T., IND.END. CHEM.49, 1-10 (1957). (4) Hull, W. Q., and Stent, C., Ibid., 48, 2095-2106 (1956). (5) Kieffer, R., and Schwarzkopf, Paul, “Hardstoffe und Hartmetalle,” SpringerVerlag, Vienna, 1953. (6) Li, K. C., and Wang, Chung Yu, “Tungsten,” ACS Monograph No. 130, 3rd ed., Reinhold, New York, 1955. (7) Schwarzkopf, Paul, and Kieffer, Richard, “Refractory Hard Metals,” MacMillan, New York, 1953. (8) Smithells, Colin J., “Tungsten,” 3rd ed., Chapman & Hall, London, 1952.
Equipment (IE) Birlec Ltd., Birmingham, England. Electrodryer. (2E) Bonnet, Villefranche, Paris, France. Mixer and heater. (3E) Dorr-Oliver, Inc., 100 Barry Place, Stamford, Conn. Turnover vacuum filter. (4E) International Electrolytic Co., Sandycroft, Chester, England. Hydrogen cells. (5E) Russel Construction Ltd., London, England. Finex vibrating screen. (6E) Sartorius Werke, Goettingen, Germany. Sedimentation balance. (7E) F. J. Stokes, Machine Co., Philadelphia, 20, Pa. Dual pressure press. (8E) Sturtevant, London, England. Jaw crusher. (9E) The Switchgear and Erection Co. (Pty.) Ltd., Germiston, Transvaal, South Africa. Electrostatic separators.
-
