Reduction-melting process to form rare earth-transition...

Details Reduction-melting process to form rare earth-transition... Reduction-melting process to form rare earth-transition...

1999-04-16

2001-10-30

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Electrothermic processes

C075S610000, C420S083000, C420S581000, C420S590000

Reexamination Certificate

active

06309441

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a process for making rare earth-transition metal alloys. In particular, the invention relates to a process using electroslag refining to melt an electrode and reduce metal oxides in a calcium-containing slag to form rare earth-transition metal alloys used in permanent magnets.
BACKGROUND OF THE INVENTION
Medical imaging systems presently are committed to using rare earth permanent magnets made from alloys of neodymium iron boron (Nd-Fe-B), such as Fe
14
Nd
2
B. The main polarizing field in this system is provided by two very large neodymium iron boron (NdFeB) permanent magnets, with an iron yoke used for the return path.
The standard techniques for preparation of the highest performance NdFeB permanent magnets build on the process developed for samarium cobalt magnets, such as SmCo
5
magnets. The alloy is prepared, crushed to a fine powder, oriented in a magnetic field, pressed, sintered, annealed, machined, magnetized, and used. This is done with powders and may require no melting. While there are many steps in this process, the overall cost is dominated by the first step, preparation of the alloy. For SmCo
5
magnets, the Reduction-Diffusion (R-D) process has become the most economical approach used for preparation of the alloy. In this process cobalt powder, calcium granules, and rare earth oxide powder are blended together and reacted under hydrogen at 1150° C. The calcium reduces the samarium oxide, and the samarium metal diffuses into the cobalt. After cooling, the excess calcium and calcium oxide are removed from the reacted product by hydrating with wet nitrogen, followed by washing with water and dilute acid. The principal cost advantage for this approach is realized by starting samarium as an oxide rather than as a pure metal.
Variations of the Reduction-Diffusion process have been applied to of neodymium iron boron (NdFeB) permanent magnets. It was found that alloy composition control and the leaching step are more difficult and expensive with of neodymium iron boron (NdFeB) than samarium cobalt (SmCo
5
) magnets, limiting somewhat the commercial usefulness of these variations for of neodymium iron boron (NdFeB). As a result, separate reduction and melting steps are primarily used for preparation of the of neodymium iron boron (NdFeB) alloy commercially. This approach requires very expensive high performance vacuum melting furnaces. Thus, a need is created for a lower cost method to make the of neodymium iron boron (NdFeB) alloy needed for the permanent magnet.
SUMMARY OF THE INVENTION
To satisfy this need, a method has been invented to make rare earth alloy compositions, such as the neodymium iron boron (NdFeB) alloy by a Reduction-Melting process. The Reduction-Melting process of this invention comprises the steps of: preparing a primary electrode containing at least one compound or metal to be reduced to form a refined metal or metal alloy ingot; placing said electrode in an electroslag refining furnace; passing a current through said electrode into a molten flux or slag to melt said electrode; reducing the metal or compound in the slag while forming an oxide by-product; collecting melted metal or metal alloy droplets falling through the slag; forming an ingot of said metal or metal alloy from said melted metal metal alloy droplets in a cooled crucible; and solidifying the slag containing oxide byproducts.
The inventive process is calciothermic and uses a reducing agent, such as pure calcium, magnesium, aluminum or a reducing compound such as, but not limited to, calcium hydride or calcium carbide. The reducing agent may be present in the electrode composition and may be further fed into the crucible of slag during the electroslag refining operation. The slag composition is any suitable material that becomes molten when heated and is capable of refining and reducing the electrode material. Halide compositions are often used as slag materials in electroslag refining. For instance, a slag may comprise a metal halide, such as calcium halide, a reducing agent, such as calcium, and additional metals and compounds that will form the refined metal or metal alloy ingot.
Melting of the electrode is accomplished by electroslag refining. The method further comprises the melting of electrodes in neutral or reducing atmospheres. The reduction of alloying elements or compounds that are contained in the electrode or that are added to the slag, takes place in the molten slag by refining the melted electrode droplets. Generally, the electrodes contain compounds of the alloying elements that are desired in the final refined ingot. For example, if neodymium, lanthanum, or zirconium are the alloying elements of interest, they may be present in the electrode as oxides, fluourides, or chlorides. They usually are present in quantities of about 50% or less in the electrode. Metal compounds, containing the desired element or elements in the refined ingot, can also be fed directly into the slag during the electroslag refining operation.
The process of this invention could be used for all rare earth-transition metal alloys of interest, such as alloys of neodymium iron boron (NdFeB), samarium cobalt such as Sm
2
Co
17
or SmCo
5
, lanthanum nitride, such as LaNi
5
, for hydrogen storage media, and for other elements which need to be reduced, such as chromium, vanadium, cobalt, boron, manganese, scandium, and beryllium.
Advantages of the Reduction-Melting process for rare earth-transition metal alloys include a cost reduction in traditional manufacturing of neodymium iron boron NdFeB permanent magnets by using neodymium as an oxide starting material rather than a pure metal. Also, the electroslag refining furnace is less expensive than high performance vacuum melting furnaces. A further advantage of the inventive process is the easy separation of oxide byproducts, such as calcium oxide, and the metal alloy. Also, the leaching step is eliminated when recovering the alloy.


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patent: 5332197 (1994-07-01), Benz et al.
patent: 5472525 (1995-12-01), Tokunaga et al.
Ellis et al, “Methods and Opportunities in the Recycling of Rare Earth Based Materials”, Metals and Materials Waste Reduction, Recovery and Remediation, 1994, pp. 199-206.*
Abstract—Ellis, T.W. et al., Methods and Opportunities in the Recycling of Rare Earth Based Materials, Minerals, Metals and Meterials Scoeity/AIME, USA, Oct. 1994, pp. 199-206, 12 Ref. Accession No.: 19(95):3—2-00.
Japanese Abstract 59023811, Jul. 1984 Naoyuki, Yamauchi (inventor).
International Search Report PCT/US97/18367 Feb. 12, 1998.

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