Патент USA US3071533код для вставки
Jan. 1, 1963 J. B. ZADRA ETAL 3,071,523 METHOD FOR ELECTROWINNING MOLYBDENUM FROM MOLTEN ELECTROLYTES Filed Feb. 6, 1961 2/ INVENTORS JOHN B. ZADRA HÂROLD J HE?NE?V JOHN M. GOMES BY (EM Ö. ÇM W WORNEYS ”iee &071523 Fatentecl Jan. i, 1953 2 such as molybdenum oxide, with a mixture of alkali or alkaline earth metal pyrophosphates, borates and halides 34371523 METHOD FOR ELECTROWINNIN G MOLYBDE NUM FRGM MOLTEN ELECTROLYTES in 'a crucible. Passage of a direct current of proper our- rent density therethrough results in the deposition of high John B. Zadra, Harold J. Heinen, and John M. Gomes, CT ?purity crystalline molybdenum on the cathode. Reno, Nev., assignors to the United States of America While the exact theoretical explanation for the success as represented by the Secretary of the Interior ful production of molybdenum is not known at present, Filed Feb. 6, 1961, Ser. No. 87,502 it `Would appear to be due to the particular electrolyte 6 Claims. (Ci. 204-64) employed. The essential Components of the electrolyte (Granted under Title 35, U.S. Code (1952), sec. 266) li) are a molybdenum compound, a pyrophosphate 'and .a The invention herein described and claimed may be manufactured and used ?by or for the Government of the United States of America for government-al purposes without the payment of royalties vthereon or therefor. The present invention relates to the production of pure molybdenum in massive form by the electrolysis of ia fused electrolyte. Currently, the most important commercial procedure for producing molybdenum metal is by the hydrogen re duction of high purity molybdic oxide or ammonium molybdate in an electric furnace. Molybdenum powder produced thereby contains approximately 0.25 to 0.30 percent oxygen. It is then converted to massive metal and reduced in oxygen content vby sintering in a hydro gen atmosphere, or by are melting. These methods are costly, and in addition, -are accompanied by excessive fume losses and other technical dif?cultíes. As shown in Bureau of Mínes Information Circular 7723 by Campbell and Jones, issued July 1955, entitled “A Survey of the Literature on the Electrodeposition of Molybdenum,” it has been proposed to electrodeposit molybdenum from many aqueous or fused salt electro lytic systems. Substantially all of the methods disclosed sutfer from the serious disadvantage that the metal de posits are extremely thin and are usually contaminated : with the lower oxides and hydroxides of the molybdenum. The Senderoif and Brenner Patent 2,715,093 describes halide. Molybdic oxide MoOa is usually the most con venient molybdenum compound to use. However, other molylbdenum compounds may be employed, such as the molybdenum chloride for example. The purity of the compound is not signi?cant in this process, and in fact one of the advantages claimed is that impure molyb denum compounds may be employed. Sodium pyrophosphaite is the most readily obta?'nable pyrophosphate, and is therefore .the one ordinarily used. Similarly, sodiurn chloride is the chloride usually em pioyed. However, the sodiurn in these compounds may be replaced by lithium, potassium, rubidium, cesiurn, calcium, -barium, or strontium, and the halogen in the halide may be ñuorine, bromine, or iodine, as well as chlorine. The addition of sodium tetraboraite to the elec trolyte makes'the ratio of sodium chloride to sodium py rophosphate less critical for the production of molyb denum essentially free of phosphorus. The sodíurn tet raborate also facilitates the ñuxing of silicious impurities that may be present in the molybdenum compounds ern ployed. Sodium metaborate may be substituted for the tetraborate, and the sodium may be replaced by other -alkalí metals. In percent, the molybdenum content of the electrolyte may range from 2.0 to 15.0; pyrophosphate from 15 to However, this 85; tetraborate from 0 to 60; and halide from 15 to 85. The preferred ranges are: Molybdenum 4 to 8; pyro phosphate 30 to 50; tetraborate O to 20; and halide 30 to SO. All the foregoing and subsequent percentages re process requires rigid reaction conditions and control, such as special high purity alkali metal hexahalomolyb fer to percent by weight. The following table lists four compositions, having date, inert atmosphere and the exclusion of all traces of oxygen and moisture, resulting necessaríly in a relatively high cost process. Components within the -above ranges, found satisfactory in the process: TABLE 1 the formation of massive molybdenum 'by electrolysis of a fused bath of a salt such as K3MoCls. Patent 2,960,4S 1, issued November 15, 1960 to Slatin, recites the production of molybdenum by the electrolysis of a CaC12-CaO-Mo03 electrolyte. The presence of -al kali metal compounds is stated by the patentee to be harmful. Also, MoO3 is stated as not being as compat ible with his CaCIZ-CaO bath as WO3 (col. 6, line 61). . It is the main object of this invention to provide -a simple, practical method for electrowinning high purity molybdenum from its compounds. A further object of this invention is to produce high purity molybdenum from its impure compounds by fused salt electrolysis in an open cell under atmospheric pres MoOs Na4P2O7 NagBioç NaBOz Eutectic of NaCl-NaF H NFPW menus› The temperature of the electrowinning reaction may range from 800° to l200° C. A preferred range is from 950° C. to 1050° C., at which the bath is very ?uid. Lower temperatures result in electrodeposits consisting largely of molybdenum dioxide and a phosphorus con A further object of this invention is to produce high taining compound, presumably a phosphide, with little, purity molybdenum metal substantially free from oxygen if any, crystals of metallic molybdenurn. and phosphorus. 60 Current density on the cathode may range from 20 A 'still further object of the invention is to produce to 280 amperes per square decirneter (a./dm.2), based on massive deposits of pure crystalline molybdenum metal. the original dimensions of the electrode. Since the cath A further object of this invention is to provide a meth sures. od for the electrolytic recovery of molybdenum from a ode 'surface area changes during the electrolysis, the ac molybdenum in a fused salt electrolytic process. density on the `anode is not critical, and is determined by denum electrolyte by fusing a molybdenum compound over the whole range. tual current density during operation is diñicult to de bath containing sodium. 65 termine. However, that based on the original dimen A further object of this invention is to provide for ef sions is adequate for the present purposes. The current ?cient spacíng of the electrodes to produce highpurity its surface area once the current density of the cathodc Further objects and advantages will become appa?'ent is selected. from the rest of the speci?cation. 70 Current e?iciencies of around 90 percent are obtained In brief, our method consists in making a molyb In general, this factor appears to a,a71,523 'Ë have less in?uence on the results achieved than either bath temperature or composition of the electrolyte. 4. from. This shows the high purity of the product ob tained. FEG. l illustrates a cross-sectional View of the electro TABLE 2 lytic cell showing crucible l, furnace 2, graphite block 3 and cathode 4. The electrolytic cell or crucible may be made of graphite MoOa feed, percent Element silicon nitride or silieon carbide. However, the preferred material of Construction is graphite. Usually the graphite cell is made the anode. The cathode may be made of any suitable material, such as iron, molybdenum or carbon. Graphite is very Satis factory for the purpose. Any material that is not attacked may be employed as the anode in electro-Winning. In the following example the graphite electrolysis cell was em~ ployed as the anode. Although an all-graphite cell is used, carbon contamina tion of the electrodeposited molybdenum may be prac tically eliminated by a proper spacing of the electrodes, use of optimurn coneentration of molybdenum for the particular electrolyte employed, and by the hydraulic 0.08 (1) .07 . 00075-0. 0025 .05 .90 404 .00075- .0025 (1) Electrowon molybdenum, perserit <0. 001 . 00075-0. 0025 .00075- . 0025 <. 001 <.001 . 00075- .0025 <.001 0025 - .0075 - .05 .OS <.001 1. 73 <.05 (I) (1) 034 .066 i Not determined. A comparison of molybdenum produced by the instant process with commercially available hydrogen reduced classiñcation or washing of the processed crystalline metal. molybdenum powder is given in Table 3. For example, an electrolyte of the second composition in Table l, contains molybdenum within the optin?um TABLE 3 range for excellent over-all electrolytic performance. A Analysis of Feed Material and Electrolyt?'c Molybdenum 25 l-inch diameter graphite cathode was centrally positioned in a 3-inch diameter graphite cell and lowered in the bath to within two inches from the bottom of the cell. The P?'od??ced Therefrom and Compamz?'ve Analysis of Hydi'ogen Redz?ced Molybdenum Powder resulting electrowon molybdenum metal averaged 0.01 percent carbon. in other runs carbon contamination was 30 lowered to 20 parts per million. Elements The following example illustrates one speci?c embodi ment of the method, and the invention is not to be con strued as being limited thereto. Feed Material percent Hydrogen Eleetrolytic Reduced, C.P. Molybdenum Molybdenum percent Powder, percent A graphite crucible 7 inches deep and 3 inches inside diameter preheated to a temperature of about 750° C., about the melting point of the mixture and then charged with 900 grams of a salt-molybdic oXide electrolyte mix~ ture consisting of 41.6% sodium pyrophosphate, 333% sodium chloride, 16.7% sodium tetraborate, and &4% molybdic trioxide. The crucible, open to the atmosphere, was placed in a vfurnace under atmospheric pressure and connected to the positive terminal or' a DC. power source to serve as the anode. After heating for about 30 min utes at a selected temperature of between 950° to l050° C. to stabilize the conditions, a graphite cathode 1 inch in diameter, connected to the negative terminal of the D.C. power source, was centrally positioned and lowered into the molten bath to within 1% to 2 inches from the bottom of the cell and electrolysis was started. Proper spacing of 50 the cathode in the cell is important in order to eliminate codeposition of crystalline MoO2 and to prevent short circuiting. It has been found that the spacíng between 1 Not determined. From the table, it can be seen that electrolytic molyb denum compares favorably with commercial hydrogen reduced molybdenum. A portion of each of the two the immersed tip of the cathode and the bottom of the cell must be greater than 1% inches because of the down 55 metals are melted into ingots and tested for hardness. The. ward crystal growth of the molybdenum up to one inch Vickers hardness number (VHN) for electrolytic mo~ beyond the cathode tip. A shorter spacing than 1% lybdenum was 186 as Compared to 170-172 for the inches usually .results in some codeposition of crystalliue commercial molybdenum metal. This is believed at MoOz. Current density on the cathode was maintained tributable to the phosphorus content in the metal. Sub-‹ at between 80 and 90 a./clm.2 and the bath temperatura 60 sequent experiments have produced phosphorus free, was about 1000° C. Two hours of current passage Wás metals by a more careful control of conditions. required to complete the electrolysis. Various modi?cations of the process may be made' The cathode was then removed from the bath, allowed within the scope of the invention, as de?ned by the fol to cool in the air, and then immersed in water. Adhering lowing claims. solidi?ed electrolyte dissolved in the water and was re 65 We claim: moved from the deposited metal. The molybdenum metal 1. A process for obtaining massive molybdenum which was then removed from the cathode and further treated comprises: fusing a mixture of about 41.6% sodium with boiling dilute hydrochloric acid to dissolve any adher_ pyrophosphate, about 333% sodium chloride, about ing calcium or phosphate compounds. After water wash 16.7% sodium tetraborate, and about &4% molybdic tri ing to remove the acid, ultra ?ne particles of carbon de 70 oxide to form an electrolyte bath, heating the electrolyte rived from the electrodes were removed by hydraulic to a temperature of from about 950° C. to about 1050° classi?cation, leaving high purity dendritic crystals of mo C., electrolyzing the said electrolyte at a current density lybdenum metal. on the cathode of from 20 a./dm.2 to about 280 a./dm.2. The following table gives a comparative analysis of the 2. ,The method of claim 1, whereín the temperature of Mo03 feed and the molybdenum metal produced there 75 the bath is about 1000° C. and the current density on 3,o71,523 6 Ll the cathode is between about 80 a./dm.2 to about 90 a./dm.2. 3. A process for obtaining massive molybdenum which comprises: fusing a mixture of a molybdenum compound, a metal pyrophosphate, a metal tetraborate, and a metal halide, said metal being a member of the class consisting of lithium, sodium, potassium, rubidium, cesium, calcium, 6. A process for obtaining massive molybdenum which comprises: fusing a mixture of molybdenum oxide, sodi~ um pyrophosphate, sodium tetraborate and sodium chlo ride to form an electrolyte bath, heating the said elec trolyte to a temperature of from about 950° C. to about 1050° C. and electrolyzíng the electrolyte at a current density on the cathode of from about 20 a./dm.2 to about 280 a./dm.2, the amount of the molybdenum oxide being barium, and strontium, in a vessel serving as an anode to su?âcient to provide a bath containing about 2 to 15% form an electrolyte bath, heating the said electrolyte to a temperature of from about 950° C. to about 1050° C., 10 of molybdenum. inserting a cathode rod in the said bath, and electrolyzing References Cšted in the ?le of this patent the electrolyte while maintaining the lower end of the said cathode rod a minimum distance of about 1% inches from UNITED STATES PATENTS the bottom of the anode vessel, the amount of said mo 2,554,527 Fink et al. __________ __ May 29, 1951 lybdenum compound being su?icient to provide a bath con 15 taining about 2 to 15% of molybdenum. FOREIGN PATENTS 4. An electrolyte for producing molybdenum metal by electrolysis consisting of the product obtained by fusing 203,614 together a mixture of a molybdenum compound, a metal pyrophosphate, a metal tetraborate, and a metal halide, 20 `said metal being a member of the class consisting of Leo et al.: Transactíons of the Electro-Chemical So Australia ____________ __ Sept. 13, 1956 OTHER REFERENCES ciety, Volume 66, pages 461, 469 (1934). lithium, sodium, potassium, rubidium, cesium, calcium, Fink et al.: “The Electro Chemical Society,” Preprint barium, and strontium, the amount of said molybdenum 84-20, October 14, 1943, pages 197-226. compound being su?'icient to provide a bath containing 25 Pink et al.: “Chem. Trade Journal and Chemical En about 2 to 15% of molybdenum. gineer,” January 14, 1944, pages 33-34. 5. An electrolyte for producing molybdenum metal by Bureau of Mines, Report No. 5554, “Electrowinning electrolysis consisting of the product obtained by fusing Tungsten and Associated Molybdenum From Scheelite," together a mixture of about 416% sodium py?rophosphate, Zadra et al. (1959). about 333% sodium chloride, about 16.7% sodium tetra 30 borate, and about &4% molybdic trioxide.