INDUSTRIAL AND ENGINEERING CHEMISTRY
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a 10.8 per cent nitrogen material of the same viscosity, while the latter might have advantages of solubility and miscibility. These data also show that the well-known brittleness of very lorn-viscosity types extend over the whole range of nitrogen content, and that it is not probable that this characteristic can be altered by a mere change in nitrogen of the material.
Acknowledgment The writer wishes to thank R. L. Stern of Union Plant for many of the materials tested, and H. M. Spurlin for the temperature-change tester used. The interest of D. R. Wiggam
Columbium and Tantalum CLARENCE W. BALKE Fansteel Products Company, Inc., North Chicago, Ill.
T
HE present state of the columbium and tantalum industry in the United States is the result of nearly twenty years of research, investigation, and technological effort. As a result of this activity and many practical tests of the metals continued over a long period of time, there is a constantly increasing interest in the metals, particularly in tantalum, and numerous unforeseen applications are constantly being made. Tantalum was first developed in Germany for lamp filaments, and when this use of the metal was discontinued, because of its replacement by tungsten, there seems t o have been no tantalum industry of consequence anywhere in the world. However, a t the present time the wide variety of application.. of this metal insures the permanency of an industry devoted to the manufacture of tantalum and columbium and the fabrication of these metals into useful forms. Treatment of Ore Tantalum and columbium are always found associated with each other in their minerals. There are but two minerals of consequence from the commercial standpoint as sources for these metals-namely, tantalite and columbite. While these minerals are found in limited quantities in many localities, they are, for the most part, accessory minerals and can be
VOL. 27, NO. 10
and W. M. Billing in this subject led to permission to publish the results.
Literature Cited (1) Buchner and Samwell, Trans. Faraday Soc., 29, 40 (1933). ( 2 ) Carothers and Van Natta, J . Am. Chem. Soc., 5 5 , 4717 (1933). (3) Nelson and Rundle, Proc. Am. Soc. Testing .Ifateriala, 21, 1111
(1921). ( 4 ) Staudinger, ‘:Die hochmolekularen organischen Verbindungen,”
p. 5 6 , Berlin, Julius Springer, 1932. ( 5 ) Ibid.: pp. 506-7. RECEIYED April 19, 1935. Presented before the Division of Paint and Varnish Chemistry a t the 89th Meeting of the American Chemical Society, New York, X. Y.,April 22 t o 26, 1935.
Tantalum and columbium, from the minerals tantalite and columbite, are separated from each other by the recrystallization of their double fluorides with potassium. The electrolysis of the fused double fiuorides yields the respective metals in the form of finely crystalline powder. These powders are hydraulically pressed into bars which are heat-treated in vacuum furnaces to produce ingots of the metals which are then capable of being rolled into sheet or drawn into wire a t room temperature. The sheet may be spun, drawn, or converted into seamless tubing. Improved processes for the fabrication of apparatus and equipment of tantalum have greatly increased the range of its usefulness and application in the chemical and rayon industries. Methods of hardening have recently been developed by w-hich the degree of hardness and depth of penetration can he varied through wide ranges and duplicated with fidelity. Both columbium and tantalum are finding increased use in the construction of powTer and other vacuum tubes. Their carbides have been incorporated into a series of hard carbide compositions for use as tools, dies, and abrasionresisting surfaces.
extracted a t a profit only in connection with mining for other minerals. Only in western Australia has tantalite been found in sufficient concentration t o warrant mining operations for this mineral alone. Australian tantalite contains more than 60 per cent of tantalum oxide with about one-fourth that percentage of columbium oxide. This mineral is also unusually free from objectionable impurities such as titanium, tin, and tungsten, and is relatively free from gangue material which is u.ually mica, silica, or feldspar. While ferrotantalum or socalled ferrocolumbium can be produced directly from the ore, the preparation of these metals in the pure condition involves a somewhat elaborate chemical and metallurgical process. The pulverized ore is fused with caustic soda which converts the tantalum and columbium content of the mineral
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INDUSTRIAL AND ENGINEERING CHEMISTRY
partly into the form of sodium tantalate and columbate, and partly into iron tantalate and columbate. Unlike the minera1 itself, iron tantalate and columbate are readily decomposed by mineral acids. Sodium tantalate and columbate are practically insoluble in water containing free alkali so that, when the fusion is treated with water, a large portion of the free alkali remaining is removed by decantation. The mixed iron and sodium tantalates and columbates are finally removed from the fusion extract by filtration and are digested with hot hydrochloric acid which leaves a fairly white mixture of tantallic and columbic acids. This precipitate is thoroughly washed by decantation with water t o remove all iron and other soluble impurities. These earth acids are next dissolved in hydrofluoric acid and filtered. The sohtion is then treated with sufficient potassium fluoride to convert the tantalum and columbium into their respective double fluorides which have the formulas K2TaF7 and K2CbOFS.H20. In other words, the classical method of separating tantalum from columbium is used on the commercial scale, and no better method has yet been found. If the solutions are not too concentrated, the double fluoride of potassium and tantalum will precipitate, whereas the columbium salt remains in solution, the solubility of the columbium salt being about twelve times that of the tantalum salt. The tantalum double fluoride is removed by filtration and can be purified by recrystallization from water containing a small amount of hydrofluoric acid. The hydrofluoric acid is necessary in the case of tantalum since the use of pure water would result in sufficient hydrolysis to produce an insoluble oxyfluoride. On the other hand, the columbium double fluoride can be recrystallized repeatedly from pure water. After being dried, the recrystallized and pure potassium tantalum double fluoride is ready for the preparation of the metal. The filtrate from the original tantalum double fluoride precipitate contains all of the columbium with some tantalum and the residual impurities from the ore. This solution is evaporated to remove the excess of hydrofluoric acid, and the impurities, consisting mainly of tantalum, are removed by partial neutralization of the solution to render the tantalum fluoride basic and insoluble. In case it is necessary to remove tungsten, this can be accomplished by precipitating the columbium from the fluoride solution with ammonia, or by pouring the columbium salt solution into a strong caustic soda solution which forms the insoluble sodium columbate. This material can then be reworked into the potassium double fluoride for use in the preparation of the metal. Because of its greater solubility and the fact that impurities in the original mineral tend to concentrate with the columbium, the purification of this material is far more difficult than the purification of the tantalum material, especially when this is done on a commercial scale.
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fluoride used during the operation. After the furnace is cool the mass is readily removed from the furnace pot and is pulverized, preferably with an impact pulverizer with air separation. The lighter furnace salt is blown out of the chamber of the pulverizer, whereas the heavy powdered metal remains and must be removed from time to time from the chamber of the pulverizer itself. Grinding by impact is preferable to other types of grinding bince this tends to break the salt away from the metal grains. The metallic powder, which will contain some of the salt, is next cleaned by any suitable method. A portion of the remaining salt may be removed by passing the metal powder over a Wilfley table. The final cleaning is done chemically by alternate treatments with dilute sulfuric acid and dilute potassium hydroxide solution, and completed with dilute hydrofluoric acid. After the cleaning operations hare been carried out carefully, the powder is remarkably pure, the main impurities being a very small amount of carbon resulting from slight carburization of the metal from carbon originating in the graphite anode, and a certain amount of dyssolved hydrogen. Hydrogen is not present in the original electrolytic powder, but, if this powder is treated wit>halkalies, or particularly with dilute hydrofluoric acid, some hydrogen will be produced and will be absorbed by the metal. I n the process used in the United States, this tantalum metal is never fused, the powder being converted into a workable ingot by pressing it into bars and heat-treating the bars in vacuum furnaces. The tantalum powder produced as outlined is relatively coarse-grained. As a consequence, the bars do not shrink in the same proportion as in the case of molybdenum or tungsten but are, after the first heat treatment, somewhat porous and consist of a mass of grains welded to each other. In this state the bars are compacted under the blows of a 200-pound hammer and subjected to a second heat treatment in vacuum, which process completes the welding of the particles and produces ingots which are capable of almost every type of mechanical operation. All of the work is done a t room temperatures. The bars may be rolled into sheet, or swaged and drawn into wire. As both tantalum and columbium combine readily with all common gases a t elevated temperatures, it is essential that the ingots be prepared sufficiently well to be workable without the application of heat.
e
Preparation of the Metals The following description applies to the process used for making metallic tantalum, but this process, with but few variations, is also applicable to the preparation of columbium. The tantalum metal is produced by the electrolysis of a fused bath of the double fluoride, K2TaF7, using a cast iron pot as cathode and a rod of graphite as anode. In addition to the double fluoride, an oxygen-carrying compound, such as tantalum oxide, must be added to the bath to prevent the anode effect which would otherwise result and prevent the electrolysis. The tantalum metal is deposited on the walls of the pot as a crystalline metallic powder; as these crystals form, the salt surrounding them solidifies, with the result that after many hours of operation the pot becomes full of an electrolytic mass through which crystals of metallic tantalum are disseminated. The weight of the tantalum metal will usually be about 15 per cent of the weight of the tantalum double
Properties of Tantalum and Columbium The scope of this paper limits the discussion of the properties of these two metals to those characteristics which result in their useful application. I n nearly all of the applications to which tantalum and columbium are put, tantalum is a t present to be preferred because of its lower cost and to the fact that it is slightly more resistant to chemical corrosion than is columbium. Tantalum is finding extensive use for the manufacture of the internal parts of vacuum tubes where its use can be economically justified by the advantages obtained, particularly for the manufacture of grids and for plates in high-power tubes. Most tube engineers recognize the superiority of tantalum for such use over all other materials. The ease of fabrication, the fact that the radiation coefficient of tantalum is higher than nickel, molybdenum, or tungsten, the fact that the metal is a good absorber of ionized gases, that the tubes are more easily evacuated, that the metal does not contain pockets of gas, and that the tube elements do not become brittle regardless of time or temperature of operation-all of these properties are contributory to the satisfactory use of tantalum in this connection. Columbium also bids fair to prove itself highly valuable for the construction of vacuum tubes for high-power duty. It has the lowest work function of any pure refractory metal; that is, less energy is required to remove an electron from its surface than from that
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of any other refractory metal.' There seems to be worldwide interest in columbium, and, if present indications continue t o hold true, thi- metal may permit the construction of tubes handling thouqands instead of tens of kiloTVatts a. a t present. By far the greatest interest in tantalum results from its remarkable resistance t o acid corrosion. It is attacked neither by hydrochloric nor nitric acids, by aqua regia, nor by wet or dry chlorine at ordinary temperature.. If we limit the use of tantalum to ordinary temperatures (not above 200" C.) and exclude hydrofluoric acid, concentrated sulfuric acid, and strong alkalies, we can say that tantalum is chemically inert. Under certain conditions, however, it may he deteriorated a t ordinary temperatures by hydrogen gas liberated in the nascent condition in contact with the metal, as in an electrolytic procesq. Tantalum can absorb as much as 740 times it. own volume of hydrogen, producing a brittle hydride. -kt devated temperatures it also combines readily with nitrogen, oxygen, or carbon. Tantalum cannot be used as an anode in electrolysis, on-ing to the formation of an oxide film. This property of the metal accounts for its use in the manufacture of electrolytic rectifiers which were in great demand during the days when direct current was used in radio reception. Tantalum electrolytic rectifiers are still used for certain industrial purposes where reliability is of paramount importance. There is a small but continued demand for tantalum condensers which operate also because of the formation of an electrolytic film. While gas absorption in some connections is undesirable in the case of tantalum, it makes possible a process for hardening the metal which is now extensively used in the manufacture of small parts for special purposes. This hardening process is a recent development which has been perfected and extended to regular commercial application. This process of gas-hardening tantalum can now be controlled so that any degree of hardness from 100 to 600 Brinell may be reproduced a t will. The degree of penetration may be varied from a thin case to complete and uniform penetration. Also, certain articles may be hardened a t some portions and left unhardened a t others, The hardened objects are not warped or changed in dimension, and polished surfaces are conipletely undisturbed. Thus full advantage of the ductile and working properties of soft annealed tantalum may be realized in the manufacture of complicated or delicate parts which may then be hardened t o the proper degree without suffering any damage. This hardening process changes the color of the tantalum from bluish gray to platinum-white. There is every indication that the hardened tantalum possesses the full resistance to corrosion that characterizes the unhardened metal. The metal columbium is susceptible t o hardening in the same manner. In addition to its resistance to chemical corrosion, tantalum in its soft form, and especially in the hardened condition, is exceptionally resistant to the erosion produced by rapidly flowing liquids or gases. This property accounts for a number of practical applications to which the metal is now being put.
have proved satisfactory and succebsful in actual service are as folloTvs: Tantalum agitators have been constructed TVlth blades u p to 4 inches in length, the blades being of solid tantalum and the shafts of tantalum-covered steel. These agitators are operated a t speeds up to 1200 r. p. m., and notwithstanding the vibration of the relatively thin blades during such operation, the metal does not become strain-hardened and brittle and no breakage has been experienced. In general, annealetl tantalum can be worked to an unusual extent without strainhardening. Stills have been produced for the manufacture of chemicalllpure acids. Tllese stills are produced from relatively thin sheet supported by suitable base-metal frames and are heated by direct oil heating or other suitable means. Heat exchangers have been constructed of tantalum in thc, form of steam coils in glass-lined or ceramic casings, or the convelltional tubular types with steel shells and straight tantalum tubes and tantalum-covered tube sheets. One unit containing four sections of the double pipe type exchanger has been in operation for nearly five years. Tantalum-lined steel pipes are produced f o r c o n veying hydrochloric acid to and into r e a c t i o n equipment. Connections are made through flanged jointand sealed by t a n t a l u m - c o v e r e d asbestos or rubber gaskets. Such pipes are essential where the acid must be kept free from contamination, or where the acid temperature is high enough to destroy base metal pipes in a few days. A number of such installations have proved to be absolutely permanent. Tantalum tubing has been produced for bleach tank duty wherever chlorine or hypochlorites must be fed into hot solutions. Tantalum is completely satisfactory. It does not color or contaminate the product and is not corroded. In some instances the pipes have been fitted with diffusers. These may be made to produce bubbles of any desired degree of fineness. The presence of sulfuric or nitric acids in the bleach liquors does not affect the tantalum metal. Numerous parts have been made for chlorination work in general, such as needle valves, nozzles, etc., for use in water chlorination. These have been in use for several years. Diaphragms for chlorine regulators are also used in many places and apparently will never wear out. As stated before, tantalum is completely resistant to wet or dry chlorine a t temperatures below 150" C. Keedle valves may be made with hardened tantalum needles and soft tantalum seat.. These have proved to be fully as resistant as platinum under the most extreme conditions. Parts for fountain pens are being made of tantalum since it resists the corrosive action of all inks under conditions of partial or intermittent immersion. Tantalum has been used for the manufacture of spinneret> for rayon spinning in Canada and Europe for many years. The hardening process referred t o renders the finished jets a i hard as glass, and this property has created considerable 111terest in this type of spinneret. Tantalum is used for the construction of thermometer wells. Tantalum-covered steel wells have been employed for five years. The recent development of seamless tantalum tubing permits the construction of heavy-walled wells which n ill withstand up to 200 atmospheres external pressure without a steel liner, thus eliminating the lag introduced by the air which is present in 15.ells containing steel reinforcing tubes, Where pressure-type recording thermometers are required, a complete unit of bulb and capillary tubing may be lnade of turbines Successful tests have been conducted &h in which the blades in the high-pressure stages are faced with
0
Commercial Applications of Tantalum The construction of tantalum equipment necessarily involves a technic of joining tantalum to tantalum or to base metals such as nickel. Tantalum can now be joined to itself by roller welding, butt welding, and arc welding; a special technic for carrying out these operations has been developed. 1-i number of actual installations of tantalum equipment which 1
Wahlin, H. B., and Sordahl, L O.,P h ~ sReu., 46. No. 12 (1934).
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INDUSTRIAL AND ENGIKEERIXG CHEMISTRY
hardened tantalum. This use of the metal 1- due to the resistance of tantalum to the erosion of gases under high velocities. Calorimeters of complicated design for special purposeb have been constructed entirely of tantalum.
Tantalum Carbide in Hard-Carbide Compositions In addition to the many uses of tantalum in the pure metallic condition, tantalum carbide is now being employed in considerable quantities in the manufacture of hard carbide compositions for cutting tools, wire dies, and abrasion-resisting wrfaces. These compositions in general consist of particle>of a refractory carbide cemented with a binder metal, usually
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either iron, cobalt, or nickel. Such compositions, consisting almost entirely of tantalum carbide, are especially raluable for the turning of steel since they do not develop t,ool deterioration, known as cratering, on the t'op of the tool; this defect' rendered the earlier known tungsten carbide compositions impracticable for use on steel. Other compositions containing mixtures of tantalum carbide and tungsten carbide in varying proportions are also manufactured for use in niachining metals of various types and under varying conditions. RECEIVED April 27, 1935. Presented as part of the Syniposium on Recent .Idrances in the Chemistry of the Rarer Elements before t h e Division of Physical and Inorganic Chemistry at the 89th Meeting of t,he ;\nierirnn Chemical Society, New York, N. Y . , April 22 t o 26, 1935.
Lacquer Solvents in Commercial Use ARTHUR IC. DOOLITTLE Carbide and Carbon Chemicals Corporation, South Charleston. W. Va.
The technology of lacquer formulation is slowly emerging from the trial-and-error stage wherein the necessary information can be gained only by recourse to empirical methods, to a level which approaches the dignity of a science. The immense amount of data amassed both in the laboratories of the lacquer manufacturers and those of the raw material suppliers is now becoming digested and assimilated, and efforts are being made both here and abroad ( 6 , 1 3 ) to regiment the important facts and theories in the orderly arrangement of an exact science. I t is in the hope of contributing to this effort that this article is offered.
S
0 MUCH has been published in recent years on the subject of lacquers sndlacquer solvents that the author ventures with considerable reluctance to offer still another r6sum6 of lacquer holvent technology. The consideration which prompted this htudy, however, is the fact that a wide divergence exists among the published values of the constants of lacquer solvents, and in no cases have the relative properties of all of the commonly used solvents been grouped together in a convenient and accessible form for the use of the lacquer formulator. Many of the data presented here were collected in the regular course of the work of this laboratory in evaluating new products and making comparisons with the commercially available products of other manufacturers. I n some cases, where the published methods of making certain determinations failed to provide the degree of reliability desired, modifications were worked out to improve the precision of the measurements. Lacquer solvents may be roughly defined as the volatile liquid ingredients of lacquers and as such include not only solvents for the nitrocellulose but also solvents for the gums, oils, and resins that enter into the composition of the modern pyroxylin finish.
Of particular interest, however, are the organic liquids that serve to disperse the nitrocellulose to form the sols commonly referred to as nitrocellulose solutions. These are, of cour-e, the so-called nitrocellulose solvents, and some insight into the cause of their solvent action may be gained by considering the structure of nitrocellulose itself. Calvert (3) has pointed out that the lacquer grade of pyroxylin contains approximately 58 per cent of oxygen as compared with only 30 per cent of carbon and hydrogen together. I t is not suprising to find, therefore, that practically all solvent\ for nitrocellulose are oxygen-bearing compounds. Several investigators hare attempted to correlate the solvent strength of nitrocellulose solvents with the proportion of oxygen-bearing groups in the molecule (5), or more simply, as Calrert ha\ done, with the percentage of oxygen present. il proportionate relationship holds accurately in the case of the normal 2ketones (Figure 1) but fails in may other cases, as has been pointed out by Davidson and Reid (4) and other..
-0 5 k
a z 4
0 k 3
2 3 n
6
8
10
I2
16 18 2 0 2 2 2 4 E 6 2 8 3 0 3 1 34 YO O X Y G E N - BY W E I G H T
14
'/.
A C E T O N E - 99% MLTHYL LTHYL KLTONL - 9 9 % MLTHYL PROPYL KETONE. 99% MCTHYLn-BUTYL K L T O N L - 9 9 %
METHYL n.AMYL KLTONL-98% MCTHYL nHCXYL K L T O N L
.9 9 %
TOLULNL F*MULAKIL'WT. 0 DILUTION RATIO C,H,O 5 8 . 1 27.6 4.5 C 4 H 8 0 7 2 . 1 22.2 4.3 C s H , o O 8 6 . I 18.6 4.3 C 6 H , r 0 100. I 16.0 4.0 C,H,,O I 14. I 14.0 3.3 C8H,bC I 2 8 . I I 2.5 3.6
FIGURE 1. SOLVENT STREXGTH us. OXYGEXCOYTESTOF N O R M Z-KETONES ~L
.4n explanation of the degree of solvent ability is, therefore, not to be found in such a simple hypothesis as that "like dissolves like" in proportion to the percentages of the corresponding active groups or atoms present.
