Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1997-06-20
1999-08-03
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C25C 300, C25C 334
Patent
active
059320842
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND ART
The present invention relates to a process for producing rare earth metals including rare earth-containing alloys which can be used for rare earth-containing alloy magnets, hydrogen storage alloys for anodes of nickel-hydrogen rechargeable batteries, and the like.
Rare earth metals are used in a wide variety of usage such as lighter flints or steel refining additives. It is known that such rare earth metals can be produced by a molten salt electrolysis of rare earth chlorides. Recently, rare earth element-transition metal alloys have been developed for permanent magnets providing high performance, and samarium-cobalt magnets, neodymium-iron-boron magnets and the like have been put into practice. Alternatively, hydrogen storage alloys providing high performance such as a lanthanum-nickel alloy and a misch metal (mixed rare earth metals)-nickel alloy have been put to use in a large amount as anode materials for nickel-hydrogen rechargeable batteries. Rare earth metals used in these alloys are required to have high quality, but the rare earth metals produced by the molten salt electrolysis of rare earth chlorides contain a quantity of impurities such as chlorine, oxygen, or the like, so that performance of such rare earth metals cannot be improved sufficiently.
In order to overcome this problem, electrolysis in a fluoride molten salt bath with a charge of oxides have been developed (E. S. Shedd, J. D. Marchant, M. M. Wong: U.S. Bureau of Mines RI 7398 P.3 (1970)), and performed in an industrial scale (Electrochemistry Handbook, Fourth Edition, Edited by The Electrochemical Society of Japan, Published by Maruzen Co., Ltd., P.399 (1985)) for producing a large amount of misch metals. According to this method, a mixed salt consisting of 50 to 75% by weight of rare earth fluorides, 15 to 30% by weight of lithium fluoride, and 10 to 20% by weight of barium fluoride, is charged into an electrolytic cell made of a refractory material, and heated to 850 to 1000.degree. C. for melting the mixed salt. Then, while bastnasite ore previously calcined and refined or refined rare earth oxides are charged into the heated and molten mixed salt, electrolysis is performed at a voltage of 6 to 12 V, an anodic current density of 0.5 to 1 A/cm.sup.2, and a cathodic current density of 1 to 10 A/cm.sup.2, using a graphite anode and a molybdenum cathode, to thereby electrodeposit and recover misch metal. In this electrolytic reaction, the oxides dissolved in the fluoride molten salt is electrolyzed in accordance with a reaction formula 2 Mm.sub.2 O.sub.3 .fwdarw.4 Mm+3O.sub.2 to form misch metal (Mm). The oxygen in the oxides reacts with graphite in the anode in accordance with a reaction formula 3O.sub.2 +3C (anode).fwdarw.3CO.sub.2 .uparw. to become carbonic acid gas, and exits the reaction system.
Alternatively, if the electrolysis in a fluoride molten salt bath with a charge of oxides is applied for producing a neodymium metal used as a neodymium-iron-boron magnet material or the like, neodymium will precipitate in a solid state at the electrolytic temperature of the misch metal since the melting point of the neodymium metal is as high as 1050.degree. C., and recovery of neodymium metal will be difficult. Thus, the electrolysis should be performed at an elevated temperature. The electrolytic reaction proceeds in accordance with a reaction formula 2Nd.sub.2 O.sub.3 .fwdarw.4Nd+3O.sub.3, and oxygen in the oxides reacts with graphite in the anode as in the case using the misch metal, to become carbonic acid gas, and exits the system. The neodymium metal can be produced in an electrolytic cell equipped with a consumable cathode. In particular, if the neodymium metal is to be produced in the form of an alloy of neodymium and iron, using an iron cathode as the consumable cathode, under the conditions to set the iron content of the alloy to be 10 to 20% by weight, the melting point of the alloy is as low as 750 to 850.degree. C. Therefore, in this case, the neodymium metal can be recovered as a molten alloy even at a temp
REFERENCES:
patent: 3383294 (1968-05-01), Wood
patent: 4966662 (1990-10-01), Tamamura
patent: 5258103 (1993-11-01), Endoh et al.
Santoku Metal Industry Co. Ltd.
Valentine Donald R.
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