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MARCH, 1938
molecular weight renders it rapidly soluble a t normal temperature with mechanical agitation. More rapid solution can be obtained by fusing the resin and then cutting back with the desired high-boiling solvent. This practice is useful in special cases. Heavy bodied solutions have been successfully used as aluminum paint vehicle, gloss oil, gum lacquer, leather dressing, and surface protective finishes. Most raw and refined vegetable oils are compatible with this new type of resin; blown oils do not always give satisfactory results. This heat polymer resin forms an excellent varnish material in bodied drying oils. Such varnish films possess quick through-drying and high luster, and have good grinding properties with asbestine, silica, or kaolin. It is compatible with such pigments as carbon black, chrome green, lithopone, titanium, and zinc oxides. It is recommended principally for inside use or where it is not exposed to direct sunlight. It does not react with other varnish components and is without effect upon driers.
25 1
The floor tile industry consumes large quantities of this new resin every year; where pale colors are not desired, it is without equal in the low-price resin field. Its properties render it most suitable for such use. The same resin of lower melting point has found widespread use in the adhesive and rubber industry. Printing inks have been made from it. Continued development of these hew cyclopentadiene and indene heat polymers is in progress and gives promise ,of extending the use of this resin into new and more diversified fields.
Literature Cited Alder and Stein, Ann., 485, 2 2 3 4 6 (1931). Kraemer and Spilker, Ber., 23, 3296 (1890). Staudinger and Bruson, Ann., 447,97 (1926). Whitby and Kata, Can. J . Research, 4, 344-60 (1931). (5) Whitby and Kata, J. Am. Chem. Soc., 50, 1160 (1928)
(1) (2) (3) (4)
RECEIVED August 27, 1937.
Recent Developments in
Tantalum and Columbium
T
HE various steps involved in producing tantalum and columbium powders from the minerals tantalite and columbite, and the methods by which these powders are converted into finished ingots, were described in a previous paper.' Since that time, work has been under way in the laboratory to improve and refine the various steps in the process. Engineering research work has resulted in new applications of the metals, primarily in the chemical industries, These efforts have resulted in a more uniform product and an increased volume of production. Occurrence High-grade ores of tantalum and columbium are scarce. Although tantalite and columbite occur in numerous localities, their concentration is usually too small to make mining operations profitable. Heretofore, most of the tantalite used in the production of tantalum has come from western Australia. Recently mining operations have been undertaken in the Black Hills near Tinton, S. Dak. A good ore is being obtained from this source, but its concentration in the matrix material is rather low. Two and one-half tons of rock must be mined, milled, concentrated, and processed to produce one pound of tantalum.
Metallurgy Tantalum and columbium are produced by the methods of powder metallurgy. At no point in the process are the metals actually melted. Workable ingots are made by the heat treatment of bars, produced by compression of the metals while in powder form. The results obtained by this type of metallurgy depend in large measure upon the chemical and physical characteristics of the initial powder. Tantalum and columbium powders have been subjected to a searching investigation in reference 1
IND.ENQ.CHEM.,27, 1166 (1935).
CLARENCE W. BALKE Fansteel Metallurgical Corporation, North Chicago, 111.
to the effects of small residual impurities such as carbon, iron, silica, alumina, and residual fluoride salts. Carbon is removed by means of a calculated amount of a suitable oxide admixed with the original powder. The carbon content of the finished metal should be below 0.01 per cent. Iron cannot be completely eliminated, but its presence in the order of 0.01 per cent is not detrimental. Materials such as silica, alumina, and residual salt are almost completely eliminated by the high temperature obtained during the heat treatment of the bars. The elimination of such impurities is further aided by the fact that this heat treatment is conducted in a vacuum, and some tantalum itself is volatilized during this operation. Hydrogen may be considered an impurity in the initial powder, and its presence is due to the chemical treatments used in the final purification of the powder. This hydrogen may vary between a few volumes to over 100 volumes. The presence of this gas affects the pressability of the powder and makes it necessary to modify the heat-treating schedules. The removal of this gas is one of the major objects to be accomplished in the heat treatment of tantalum and columbium. The furnace and pumping equipment have been greatly improved, with the result that the h a 1 gas pressure a t the completion of the heat treatment of a tantalum or columbium bar is of the order of one micron. Oxygen in any form is a detrimental impurity in these metals. Its presence produces abnormally high hardness:, increases the difficulty of annealing, and reduces the stretch of the material. A strip of pure tantalum sheet 0.010 inch in thickness will have an elongation of as much as 50 per cent. This is materially reduced by the presence of small amounts of impurities.
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HYDROCHLORIC ACID GAS ABSORPTION
UNIT
This unit has a rated capacity of 200 pounds of hydrochloric acid gas per hour, with ample reserve capacity for peak surges. I t occupies less than 12 square feat of floor space.
VOL. 30, NO. 3
Careful attention must also be given to the physical characteristics of the original powders. It has been found necessary to vary the heat-treating process in relation to the grain size and distribution of grain size in every lot or specimen of powder in order to obtain the same final results. The grain size of tantalum or columbium powder is coarse as compared to the powders used in the production of molybdenum or tungsten. Variations in the grain size affect the initial electrical conductivity of the bars and the heat gradient from the center to the sur-, face of the bar when these are heated in a vacuum by making them resistors to an electric current. It has therefore been found advisable to use a pyrometer control, as well as measurement of current, in order to obtain greater uniformity of the physical properties of the finished metal. The maximum permissible surface temperature of a pressed tantalum bar has been determined for tantalum powders over a wide range of mesh sizes. By making proper mixtures of various powders, a more uniform distribution of grain size is obtainable, and with proper regulation of the heat-treating schedules, bars are obtained of remarkable uniformity, hardness, workability, and other desirable properties which render them suitable for the fabrication of sheet, rod, and wire. The characteristics of finished tantalum sheet constitute a reliable indicator as to the previous history of the material. A combination of high chemical purity with carefully conducted heat treatments, interspersed with heavy mechanical working, are necessary to produce ingots from which sheet can be made with the
INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCH, 1938
proper softness, stretch, and freedom from surface defects. Failure in any of these respects will result in sheet which is too hard and deficient in stretch and will show surface defects due to porosity. As previously mentioned, these powders are produced by an electrolytic process. Other methods of making tantalum and columbium powders have been diligently sought and new methods seem possible. Columbium has always been more difficult to produce than tantalum, but with the improved technic which has been developed, larger quantities of columbium powder have been made. For the first time in the history of the industry seamless columbium tubes have been drawn and large quantities rolled to very thin sheet to meet the requirements of foreign consumers, At various stages during its manufacture, and especially in connection with the fabrication of sheet metal, appreciable quantities of clean scrap will accumulate. Formerly this material was utilized by dissolving the metal in a mixture of hydrofluoric and nitric acids and reconverting it into the double fluoride with potassium. A process has been perfected by which such scrap metal is converted directly into usable powder. When tantalum or columbium are heated in hydrogen under pressure a t relatively low temperatures, the gas enters the lattice and the material increases in volume and becomes sufficiently brittle so that it can be pulverized to any degree of fineness in an impact pulverizer. Powder produced by this method is treated with acids to remove any iron which may have been introduced from the pulverizing equipment and is then heated to a higher temperature in a vacuum. Even at relatively low temperatures as compared to the sintering temperature of the metal, all but two or three volumes of the hydrogen can be removed in this
FOUR TANTALUM COILSIN PARALLEL FOR THE RAPIDHEATINGOF A CORROSIVE SOLUTION IN A
DEEPKETTLE
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way. The resulting powder is suitable for the production of new tantalum ingots. However, there is a marked difference between the metal produced in this way and the original metal produced by electrolysis. The latter consists of crystals, or fused or semifused globules of solid metal, whereas the individual particles of the reclaimed powder are probably still aggregates and the actual particle sizes may be of molecular dimensions. Any differences which might otherwise exist in the properties of the finished ingots are eliminated by a proper adjustment of the heat-treating schedules.
Application of Tantalum The amount of tantalum used in the vacuum tube industry has increased about five times over its former value. The metal is used in the form of wire and thin sheet varying between 0.004 and 0.010 inch in thickness. This rapid increase in the amount of tantalum used in the electronic tube industry can be attributed to the rapid development of the use of ultra high-frequency radio power. The properties of tantalum which adapt it to this use are its high melting point, its low vapor pressure, and its getter characteristics. This last property permits the use of thoriated tungsten as a filament in tubes which can only operate efficiently with highplate voltages. Practically all other plate materiaIs either liberate some gas, or a t least do not absorb it when liberated by other parts of the tube. Tantalum is such an efficient absorber and keeper of gas, even a t pressures as low as 0.001 micron, that the cost of the metal is no bar to its application. Because of its remarkable resistance to chemical corrosion, it has long been recognized that tantalum equipment should find a place in the chemical industry where it would be economically advantageous. It now appears that its use in the chemical industries is largely in the form of various types of steam-heating equipment. Data covering the coefficient of heat transfer from steam through tantalum to a wide variety of liquids have not yet been accumulated. It has been determined, however, that in hydrochloric acid a t steam pressures above 30 pounds per square inch, the rate of heat transfer through tantalum is unusually great. Using a tubular tantalum heater in boiling dilute hydrochloric acid solution and with steam a t 50 pounds per square inch, this value has been found to be 3800 B. t. u. per square foot per F. per hour. In dilute sulfuric acid free from calcium salts or other constituents which might precipitate onto a hot metal, this coefficient is approximately 1500 or more. Various types of candle and bayonet heaters have been developed to take advantage of this property, which appears to be associated with the coefficient of wetting of various liquids on tantalum. After this coefficient is determined for a given liquid, small heater units can be produced which will do the work of other much larger types of heaters. For instance, in hydrochloric acid 1 square foot of tantalum can replace 18 square feet of lead. This comparison is not due entirely to the difference in the rates of heat transfer, although that through lead is poor enough. Measurements indicate that the over-all coefficient in the case of lead in acid solutions which do not precipitate any deposits varies between 200 and 300. However, tantalum is so much stronger than lead that thin-walled heaters which will withstand up to 150 pounds per square inch can be used, whereas the use of lead is usually restricted to a pressure of 30 pounds per square inch. It is this combination of ability to withstand high internal pressures, plus the high rate of heat transfer, which makes it possible to produce unusually efficient heater units of tantalum. In one test it was possible to evaporate 200 pounds of liquid per hour from dilute hydrochloric acid solution with a candle heater having only 0.5 square foot of submerged heating surface. It is often possible to produce heat transfer equipment which will not only last indefinitely in very cor-
2%
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
rosive solutions, but which in its Grst cost is comparable to any other material which might be applied. As further examples, the rate of heat transfer through tantaIum at equivalent steam pressures is approximately ten times as great EMthat through glass, or twenty times as great as that through stoneware. Tantalum heat transfer equipment is not limited to these candle or bayonet type heaters, but almost any type of heater which can be fabricated from tubing and sheet can be produced. An outstanding development in the field of chemical equipment is an absorption system for hydrochloric acid gas, which depends for its operation upon the use of a thinwalled tantalum tube. It now appears that this metal will stand supreme for this application. This absorber is classed in the falling-film type. It consists essentially of a vertical tantalum tube housed in a steel water jacket, together with upper and lower bonnets made of or produced with a suitable plastic material. This equipment results in a tremendous saving in space and, a t the same time, produces high-strength acid with unusually warm cooling water. It has been found possible to produce 36 per cent acid with cooling water a t 87”F. This absorber has the further advantage of flexibility of control and capacity, which means that it can be governed automatically to produce a constant strength of acid from a variable source of gas supply. The absorber, which has already been perfected, is intended for by-product hydrochloric acid gas-that is, hydrochloric acid gas which is generated as a result of chemical reactions in closed systems. A second type of absorber is under development for handling gases in which there is a relatively high percentage of noncondensables. I n the course of the development of this hydrochloric acid gas absorber very interesting results were obtained. The “make” water or acid and the gas to be absorbed are introduced to the top of a tantalum tube; that is, the gas and water flow in the same direction rather than countercurrently. Hydrochloric acid gas dissolves more readily in a dilute solution of hydrochloric acid than in pure water, and the success of the absorber depends upon the great rapidity with which the heat can be removed which is liberated during absorption. The layer of solution running down the inside of the tube is very thin, and the heat transfer through the tantalum tube is so rapid that not only is the solution itself cooled rapidly but the temperature of the interface between the solution and the gas can be kept sufficiently low to prevent decomposition of the solution. An absorber of this type rated a t 7 tons of 21” Baume hydrochloric acid per 24 hours occupies not more than 100 cubic feet of space. Conventional equipment for the same purpose wsuld occupy about 1600 cubic feet. I n addition to its use in the construction of heat exchangers, various typw of chemical equipment are being produced by covering the parts with thin layers of tantalum sheet. Considerable quantities of tantalum sheet are used in the manufacture of spinning jets used in the viscose process of rayon production. Another practical use of tantalum is in connection with the covering of pump shafts, either with seamless tubing or by spraying the shaft with tantalum, followed by grinding and polishing operations. It has long been recognized that tantalum is a superior film-forming valve metal, The dielectric characteristics of the film formed on tantalum are such that condensers made of tantalum have superior characteristics over other types of electrolytic condensers. I n the past, tantalum condenser elements were made of extremely thin sheet, rendering the cost of the electrolytic condenser high compared to other types. A new type of tantalum condenser element has been produced in whioh the tantalum is in a porous, spongy form.
VOL. 30, NO. 3
This element is produced by pressing tantalum powder of a special tested grain size a t a specific pressure. The element is then heat-treated by a special sintering schedule in a vacuum electric furnace. The variables involved in the construction of such a porous condenser element are grain size, the pressure used in forming the piece, the sintering schedule, and the amount of powder used. Unless other considerations are involved, these factors are adjusted to the end that the greatest capacity per unit weight of tantalum powder is attained. A capacity as high as 53 microfarads per gram a t 25 volts d. c. pressure can be obtained. Possible applications of this new type of tantalum condenser are for motor-starting capacitors, telephone filtering units, highgrade radio filter circuits, public address and sound systems, aviation equipment, any type of portable equipment where weight and space are a t a premium, lightning surge arrestors as applied to signal equipment on railroads, and similar applications where surge voltages are dangerous to equipment.
Hard Carbide Compositions Numerous papers have been published and many patents issued covering the use of tantalum and columbium in the form of their carbides as constituents of hard carbide materials used for cutting tools, wire-drawing dies, and, in general, wear-resisting surfaces. Many investigators have been active throughout the world in a comprehensive study of numerous types of hard alloys. The carbides of titanium, zirconium, vanadium, columbium, tantalum, molybdenum, and tungsten have all been investigated in connection with their possible beneficial use in these materials. Tools containing tantalum and columbium carbide have proved especially valuable in the machining of steel, since they have the property of retarding chip wear or “cratering,” which is detrimental to tools made entirely of tungsten carbide. Most producers of sintered carbide tools now make grades containing tantalum carbide or columbium carbide. A new cast alloy has been developed containing tantalum, tungsten, and other elements to supplement the sintered materials. The desirable properties of the known nonferrous hard cast alloys have been greatly enhanced by the addition of appreciable quantities of tantalum carbide. The hardness and resistance to chip wear which characterize tantalum carbide are present in this alloy to a marked degree. One type of this alloy has properties which lead us to believe that it may find a wide application in the chemical industries. It is characterized by high hardness together with admirable resistance to both abrasion and corrosion. Other types may find wide application in the mechanical fields. Tools of these cast alloys possess, in addition to hardness, a desirable toughness and resistance to wear. It has become apparent that these cast alloys, while not replacing the sintered hard carbide compositions, will be used in numerous ways.
Acknowledgment The author wishes to make the following acknowledgments for contributions by his associates: to Claire C . Balke in connection with his study of the metallurgy and metallography of tantalum and columbium, to E. G. Ramsey in connection with the development of the tantalum condenser, to F. L. Hunter who has engineered the development of heat interchangers and the hydrochloric acid gas absorber, and to Roy A. Haskell in connection with the development of the hard cast alloy. RECEIVED December 29, 1937. Presented as part of the Symposium on the Less Familiar Elements, the Second Annual Symposium of the Division of Physical and Inorganic Chemistry, American Chemical Society, held in Cleveland, Ohio, December 27 t o 29, 1937.
