A molten salt process for producing neodymium and neodymium-iron alloys
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I.
INTRODUCTION
G E N E R A L Motors has developed a process known as M A G N E Q U E N C H * for producing powerful Nd-Fe-B *MAGNEQUENCH is a tradename of General Motors Corporation. The product is a hard magnetic material of general formula Ndo.~4Feo.TaBo.08.
alloy magnets, m In support of this activity, a process for producing low-cost neodymium is undergoing development. r2-61 This process uses the least expensive and most easily available reactants such as neodymium oxide (Nd203) and sodium, and salts, calcium chloride (CaC12) and sodium chloride (NaC1), which are also plentiful and inexpensive. The actual process consists of suspending fine Nd203 powder in the CaC12-based molten salt at - 7 5 0 ~ by mechanical mixing. The suspended NdeO3 is reduced by calcium (either produced in situ by the reaction of sodium with CaC12 or added directly) to produce fine neodymium powder and calcium oxide (CaO) in the stirred reactor. The neodymium produced can be extracted by a neodymium-zinc (Nd-Zn) or neodymiumiron (Nd-Fe) pool at the bottom of the reactor. Pure neodymium metal can be recovered by vacuum distillation of the Nd-Zn pool, or the Nd-Fe pool may be used directly in making Nd-Fe-B alloy for the MAGNEQUENCH process. The results of experiments in which calcium was used directly as the reductant have already been reported. I4,51 The present study was undertaken to establish the parameters for reduction of Nd203 with sodium in a CaC12based molten salt using a Nd-Zn or Nd-Fe extraction pool, which would give high yields. In particular, some of the parameters studied were salt composition, the ratio of RAM A. SHARMA, Senior Staff Research Scientist, and RANDALL N. SEEFURTH, Staff Research Scientist, are with the Physical Chemistry Department, General Motors Research Laboratories, 30500 Mound Road, Warren, MI 48090-9055. Manuscript submitted November 15, 1988. METALLURGICAL TRANSACTIONS B
the grams of salt used in the process to the neodymium that could be theoretically produced, and the quantity of excess sodium required over the stoichiometric amount. This report describes the findings of this study.
II.
THEORETICAL CONSIDERATIONS
On the basis of the standard free energies of formation of sodium oxide (Na20) and Nd203,17'81 neodymium metal cannot be produced by the reduction of Nd203 with sodium by the following reaction: Nd203 + 6Na ~ 2Nd + 3Na20
[ 1]
However, on the basis of the standard free energies of formation of CaO and Nd203,17'81 neodymium metal can be produced by the following reaction: Nd203 + 3Ca ~ 2Nd + 3CaO
[2]
This has been confirmed experimentally, r4'51 Calcium required for Reaction [2] may be generated by the reaction of CaC12 with sodium by the following reaction: CaC12 + 2Na ~ Ca + 2NaC1
[3]
The standard free energy change of this reaction is negative below 900 K (Figure 1). Above this temperature, Reaction [3], because of the positive value of the standard free energy change, becomes doubtful. The data presented in Figure 6 of Reference 3 indicate that melts of calcium and sodium coe
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