US2463367A - Method of producing tungsten - Google Patents

Description

Patented Mar. 1, 1949 F F I C E METHOD OF PRODUCING TUNGSTEN Colin G. Fink and Ohuk Ching Ma,

New York, N. Y.

No Drawing. Application February 19, 1944, Serial No. 523,092

This invention relates to the recovery of metallic tungsten from tungsten compounds, tungsten ores and concentrates of tungsten ores. More specifically, the invention provides an electrolytic method for the direct production of crystalline grains of substantially pure metallic tungsten from compounds, ores, and ore concentrates of tungsten.

Tungsten occupies a position of major importance among industrial metals, owing to its widespread use not only as lamp filaments but also as a constituent of alloy steels, of cutting tools, of dies, and other articles which must resist abrasion or be strong at high temperatures.

The commercially important ores of tungsten are scheelite (calcium tungstate) and Wolframite, an iron-manganese tungstate of which two common variants are the iron-rich ferberite and the manganese-rich huebnerite. These ores are usually concentrated by hand or mechanically, although recently chemical extraction methods have been developed for leaching lean ores of tungsten and producing concentrates in the form of pure or nearly pure calcium or sodium tungstate. The concentrates, however produced, are ordinarily either smelted in the electric furnace to produce a ferrotungsten alloy, or purified and converted to tungstic oxide which is then reduced with carbon or hydrogen to pure or nearly pure tungsten metal.

It has been proposed to produce metallic tungsten by electrolytic reduction of tungsten compounds. in 1867 Zettnow (Pogg. Ann, vol. 130, pages 16 and 241) described the production of an impure tungsten upon electrolysis of sodium tungstate. In 1919 Keyes patented (Patent 1,293,117) a method of producing finely divided tungsten metal by electrolyzing at 1200 to 1400 C. a solution of tungsten oxide in boric acid. In 1924 Kahlenberg and Kahlenberg (Trans. Electrochem. Soc, vol. 46, page 181) described the preparation of a smooth plating of tungsten by electrolyzing at low current densities an electrolyte formed by heating tungstic oxide with fused sodium chloride. In 1925 Van Liempt (Zeitschr. Elektrochem., vol. 31, page 249) disclosed the production of amorphous tungsten powder by electrolyzing alkali metal tungstate. In 1929 Andrieux (Ann. Chim., vol. 12, page 495; Comptes Rend, vol. 184, page 91), having found that fused mixtures of tungstic and boric anhydrides form very viscous baths havin poor conductivity, described the production of a crystalline tungsten powder by electrolyzing a fused mixture of tungstic oxide, borates, and fluorides. Andrieux considered the further addition of zinc oxide to be necessary to obtain pure tungsten at a high current efiiciency. In 1931, Hartmann et a1. (Zeitschr. Anorg. Chem., vol. .198, p. 116) described a method of produc- 3 Claims. (01. 204-) fused mixture of sodium pyrophosphate and metaphosphate with tungstic acid, at 650-700 C., and also from a solution of tungstic oxide in sodium pyrophosphate. This process was further investigated by Leo and Shen (Trans. Electrochem. Soc., vol. 66, p. 461).

It will be noted that all of these prior investigators used either tungstic oxide or sodium tungstate, a water-soluble salt. Apparently, none considered his method applicable to impure tungsten materials such as ores or concentrates. Inasmuch as relatively expensive procedures are required to produce pure tungstic oxide or sodium tungstate from tungsten ores, these electrolytic reduction methods offer little or no cost advantage over other reduction methods in producing tungsten, particularly when it is considered that prior workers were able to operate only at low current efiiciencies and were unable to prevent the simultaneous production of large amounts of compounds of lower oxides of tungsten or tungsten bronze. Largely for these reasons, production of tungsten metal by electrolysis has not attained commercial acceptance.

It has now been discovered that tungsten ores and concentrates, such as wolframite, ferberite,

. 6 ing metallic tungsten electrolytically from a huebnerite, and scheelite, preferably in finely granular form, may be readily dissolved in molten alkali tetraborates, and that under certain conditions described hereinafter the solutions may be electrolyzed to produce substantially pure, crystalline tungsten metal grains.

More specifically, the invention comprises mixing a tungsten ore or a tungsten ore concentrate with molten sodium tetraborate (NazB4O7), maintaining the solution so obtained at a temperature between 950 and 1300" C., and electrolyzing said hot solution between an anode and a cathode at a cathode current density between 2 and 4 amperes per square inch. Neither fluorides nor zinc oxides are used in the salt bath.

It is preferred that the initial proportions of ingredients be such that there are between 1.5 and 3.5 parts of solvent to each part by weight of tungstic oxide (although as much as '7 parts of solvent to 1 part of tungstic oxide may be used), and it is also preferred that the proportions be maintained within these limits throughout the electrolysis. Gangue constituents may becloud the solution, but such cloudiness does not interfere with the process.

Tests have established that the solvent sodium salts may be replaced wholly or in part by the corresponding potassium salts without substantially changing the characteristics of the process. It is also preferred that the temperature of the solution during electrolysis be maintained between 1050 C. and 1250 C.

The preferred current density range is from 2.5 to 3.5 amperes per square inch ofv cathode.

current density at the anode is not critical but is preferably of about the same order as that at the cathode.

The cathode may be made of any suitable conductive material, such as iron or graphite; but is preferably of molybdenum or tungsten. Graphite is the preferred anode material.

The process steps and conditions justdescribe'd are critical if a pure product is to be obtained under efficient operating conditions. Thus, baths initially containing less than about 28%, or more than about 67% by weight of tungstic oxide are electrolyzed only at very low current efficiency. The temperature and the current density exert strong effects not only on the current efficiency but also on the purity of the product. It seems most probable, on the basis of extensive data accumulated in the course of investigation of this process, that a film of sodium vapor is formed on the cathode, that for optimum results in' respect to current efficiency and purity of product there is a narrow range of optimum thicknesses of such film, and that the most si nificant of the factors controlling such film thickness are thetemperature of the electrolyte and the current density at the cathode.

There is a very sharp enhancement of current efficiency as the electrolyte temperature is raised from 950 to 1025 C. A further increase in temperature is accompanied by a much less rapid increase in current efficiency, until a maximum is reached in the range of 1050 to 1150 C., after which, at higher temperatures, the efliciency decreases, again reaching a low value at about I300'-1350 CL Similarly,current efliciency is at a maximum when the current density at the cathode is in the neighborhood of 3 amperes per square inch, and reaches a low value at densities below 1.5 and above 4' amperes per square inch. The greatest purity of product, i. e. greatest freedom from tungsten boride and tungsten bronzes, is achieved at current'densities above 2.5 amperes per square inch.

In" the course of a series of experiments on the method of this invention, using wolframite concentrates dissolved in fused borax, best results were obtained under the following conditions: one part of weight of ore concentrate (60%-'70% W03) in each 1.5 to 0.75 part of fused borax as the initial electrolyte; electrolyte temperature between 1050" and 1300 0.; current density at the cathode, about 50 amperes per square decimeter (3.23 amp/sq. in). Under these conditions, current efficiencies in the neighborhood of 78%, yields of about 0.92 pound per kilowatt-hour, and a product analyzing 99.57% tungsten, have been attained.

The tungsten metal, as it is formed, usually drops to the bottom of the electrolyte bath where it forms a sludge which is readily separated from the main body of the electrolyte by decantation. Ifa hollow molybdenum cathtode is used, tungsten crystals will adhere to it. The sludge also contains various impurities such as compounds of lower oxides of tungsten. After the sludge has'been cooled and solidified, the pure tungsten is easily separated by leaching. The metal itself is'substantially free from those impurities such as phosphorus, arsenic, sulfur, and tin, which are commonly'present in tungsten ores but which impair the value of tungsten metal for use in steels. The impurities making up'the greater part of the Y tungsten metal are iron matures.

All

The process may be applied not only to high grade ore concentrates but also to low rade ores and concentrates, with some decrease in current efficiency. Somewhat better current efficiencies have been attained with the use of wolframite, ferberite, and huebnerite than with scheelite, but very good results are achieved even with scheelite.

In general, within the ranges of conditions specified herein, raising the current density tends to yield smaller crystals of tungsten, while raising the temperature tends to yield larger crystals. Insoluble impurities, such as iron oxide, also effect the crystal size, an increased concentration of such impurities tending to decrease the crystal size.

What is claimed is:

1. Method of producing substantially pure tungsten crystalline grains directly from oxidic tungsten ores and concentrates thereof which comprises mixing said tungsten-containing material with molten sodium tetraborate maintaining the solution so obtained at a temperature between 950" and 1300 C.; and electrolyzing, at such temperatures, said hot solution as an electrolyte between an anode and a cathode at a cathode current density between 2 and 4 amperes per square inch.

2. Method of producing substantially pure tungsten crystalline grains directly from oxidic tungsten ores and concentrates thereof which comprises mixing said tungsten-containing ma"- terial with molten borax in the proportions of 1.5 to parts by weight of borax' to each part by weight of tungstic oxide; maintaining the solution so formed at a temperature between 950 and 13=30 C.; electrolyzing, at such temperature, said hot solution as an electrolyte between an anode and cathode at a cathode current density between 2.5 and 3.5 ampers per square inch, thereby forming a tungsten and salt sludge; separating and cooling said sludge; and leaching the cooled sludge to recover substantially pure tungsten metal grains.

3. Method of producing substantially pure tungsten crystalline grains directly from oxidic tungsten ores and concentrates thereof which comprises mixing said tungsten-containing material with molten sodium tetraborate maintaining the solution so obtained at a temperature between 1050 and 1250 C.; and electrolyzing, at such temperatures, said hot solution as an electrolite between an anode and a cathode at a cathode current density between 2.5 and 3.5 amperes per square inch.

COLIN G. FINK. CHUK CHING MA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,545,582 Cobb July 14, 1925 1,874,090 Driggs Aug. 30, 1932 FOREIGN PATENTS Number Country Date 552,669 France Jan. 25, 1923 5,132 Norway Jan. 4, 1897 OTHER REFERENCES Comptes Rendus, vol. 53, p. 727 (1861). Transactions of the Electrochemical Society, vol. 66, pp461-469 (1934)