US3625779A

US3625779A - Reduction-fusion process for the production of rare earth intermetallic compounds

Info

Publication number: US3625779A
Application number: US852100A
Authority: US
Inventors: Robert E Cech
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-08-21
Filing date: 1969-08-21
Publication date: 1971-12-07
Anticipated expiration: 1988-12-07
Also published as: NL168272C; DE2041094B2; FR2059611B1; NL7012002A; JPS497297B1; DE2041094C3; GB1319142A; DE2041094A1; NL168272B; FR2059611A1

Abstract

A reduction-fusion process for producing novel rare earth intermetallic compounds, for example, cobalt rare earth intermetallic compounds, especially compounds useful in preparing permanent magnets. A particular mixture of rare earth metal oxide and calcium hydride is heated to affect reduction of the rare earth metal oxide. The resulting rare earth metal-containing mixture is fused with cobalt or other ferromagnetic metal to form the rare earth intermetallic compound and allowed to solidify. The solid is pulverized and then treated to recover the rare earth intermetallic compound.

Description

United States Patent [72] Inventor Robert E. Cech Scotia, N.Y. [211 App]. No. 852,100 [22] Filed Aug. 21, 1969 [45] Patented Dec. 7, 1971 [73] Assignee General Electric Company [54] REDUCTION-FUSION PROCESS FOR THE PRODUCTION OF RARE EARTH INTERMETALLIC COMPOUNDS 5 Claims, No Drawings [52] US Cl 148/101, 75/84. 148/105 [5 1] Int. Cl H011 1/04, C22b 59/00 [50] FieldolSearch 148/101, 102,103, 105; 75/84, 152, 170, 31.57

[56] References Cited UNITED STATES PATENTS 2,038,402 4/1936 Alexander 75/84 2,584,411 2/1952 Alexander..... 75/84 X 3,104,970 9/1963 Downing et a1 75/152 X 3,424,578 1/1969 Strnat et a1. 75/170 UX 3,463,678 8/1969 Becker 148/105 3,524,800 8/1970 Morrice et al, 75/152 X 2,043,363 6/1936 Alexander 75/84 FOREIGN PATENTS 1,488,054 7/1967 France 75/152 6,608,335 12/1967 Netherlands 148/3157 Primary Examiner-L. Dewayne Rutledge Assislan! Examiner-G. K. White Attorneys-Charles T. Watts, Paul A. Frank, Jane M.

Binkowski, Frank L. Neuhauser, Oscar B. Waddel and Joseph B. Forman ABSTRACT: A reduction-fusion process for producing novel rare earth intermetallic compounds, for example, cobalt rare earth intermetallic compounds, especially compounds useful in preparing permanent magnets. A particular mixture of rare earth metal oxide and calcium hydride is heated to affect reduction of the rare earth metal oxide. The resulting rare earth metal-containing mixture is fused with cobalt or other ferromagnetic metal to form the rare earth intermetallic ep npound and allowed to solidify. The solid is pulverized and then treated to recover the rare earth intermetallic compound.

REDUCTION-FUSION PROCESS FOR THE PRODUCTION OF RARE EARTH INTERMETALLIC COMPOUNDS RARE EARTH INTERMETALLIC COMPOUNDS PRODUCED BY A REDUCTION-FUSION PROCESS The present invention relates to rare earth intermetallic compounds, permanent magnets and a reduction-fusion process for preparing the compounds.

Permanent magnets, i.e. hard" magnetic materials, are of technological importance because they can maintain a high, constant magnetic flux in the absence of an exciting magnetic field or electrical current to bring about such a field. A number of cobalt rare earth intermetallic compounds, as for example C0,Sm, can be made into permanent magnets. However, these intermetallic compounds are not widely used in forming permanent magnets because the methods of preparing these compounds are lengthy, time-consuming and costly. For example, a conventional process for preparing a cobaltsamarium intermetallic compound useful as a permanent magnet comprises reduction of the samarium oxide by a number of techniques such as heating the oxide with lanthanum metal chips in a high-temperature vacuum retort. On heating in vacuum the samarium oxide is reduced and, being more volatile than lanthanum, is vaporized from the retort and condensed in the cold zone where it later must be chipped ofi the walls of the retort. This recovered bulk samarium metal is suitable only for melting stock which is admixed with molten cobalt in proper amount and cast into an ingot. The ingot is then ground to a fine particle size, ordinarily finer than one micron, to develop its permanent magnet properties. The ground material may then be compressed in a magnetizing field and sintered to form a solid magnet. A flexible magnet may be formed by incorporating the ground material in a magnetizing field in a matrix of an elastomer or polymer.

It is an object of the present invention to produce rare earth intermetallic compounds without the time-consuming, costly steps of conventional processes. For example, the present invention would eliminate the necessity of the separate steps of past processes of forming the rare earth bulk metal, the use of costly lanthanum metal as a reducing agent, and of recovering the rare earth metal itself. This object is shared in US. Pat. application, Ser. No. 849,875, filed Aug. 13, 1969, in the name of Robert E. Cech and assigned to the assignee hereof wherein another method of producing novel rare earth intermetallic compounds is disclosed.

Briefly stated, the process of the present invention comprises heating a particulate mixture of a rare earth metal oxide and calcium hydride to effect reduction of the oxide. The resulting rare-earth-metal-containing material is fused with cobalt to form the intermetallic compound and then allowed to solidify. The solid is pulverized and treated to recover the cobalt rare earth intermetallic compound.

The oxides of the rare earth metals useful in the present process are those of the rare earth metals which are the 15 elements of the lanthanide series having atomic numbers 57 to 7l inclusive. The element yttrium (atomic number 39) is commonly found with and included in this group of metals and, in this disclosure, is considered a rare earth metal. Mixtures of rare earth metal oxides can also be used. Representative of the oxides useful in the present invention are samarium oxide sm,0, yttrium oxide v o and misch metal oxides (M misch metal being the most common alloy of the rare earth metals which contains the metals in the approximate ratio in which they occur in their most common naturally occurring ores.

The rare earth metal oxide can vary in particle size. It is usually available in commerce in precipitated form, which is the preferred form herein since this form is a very fine particle size, i.e. of the order of 0.1 micron and highly pure. The smaller the particle size, the faster the oxide is reduced.

Since the calcium hydride decomposes in the present process, it may vary widely in particle size and may be as coarse as l2-mesh or coarser. Generally, a pulverized powder is preferred so that an intimate mixture of the active constituents can be produced. Commercially available calcium hydride always contains some calcium oxide. This will not interfere with proper operation of the process so long as there is a sufiicient amount of calcium hydride to reduce the rare earth metal oxide. The necessary excess amount of commercial calcium hydride needed is determinable empirically.

ln carrying out the process of the present invention, the rare earth metal oxide is admixed with the calcium hydride to affect reduction of the oxide. Specifically, using samarium oxide as an example, the samarium oxide powder is admixed with calcium hydride, and the mixture is then heated to affect reduction of the rare earth metal constituent. The stoichiometric reaction is as follows:

sm.0.,+ 3c;.1H A 2sm+3cao+3n,.

Although stoichiometric amounts of the rare earth metal oxide and calcium hydride are satisfactory, it is preferable to use an excess amount of calcium hydride to insure that all of the rare earth metal oxide is reduced to the rare earth metal. The most suitable amount of excess calcium hydride can be determined empirically. The product would then contain excess calcium in addition to the rare earth metal and calcium oxide.

A number of conventional techniques can be used to carry out the instant process. Preferably, the calcium hydride and rare earth metal oxide are thoroughly mixed so that in carrying out the reaction, the calcium hydride, which is the reducing agent, can act on the oxide effectively. ln grinding calcium hydride, if any grinding is required, and in handling the powder mixture, it is essential to use protective enclosures so that the atmosphere may be maintained completely free of moisture. While calcium hydride is substantially inert in completely dry air, the powder or dust is highly explosive under conditions where an electrostatic discharge might occur. Therefore, for safety considerations a protective atmosphere such as a nitrogen atmosphere is preferable to air for mixing and handling the powder. To prevent contamination, the loose powder mixture is preferably placed in a metal foil bag, e.g., molybdenum or iron metal foil, or a self-supporting metal pan having a close-fitting cover. Alternatively, the loose powder can first be pressed into bricks to decrease the volume per unit weight of material, thereby increasing the furnace throughput.

The mixture of calcium hydride and rare earth metal oxide is heated to decompose the calcium hydride and reduce the rare earth metal oxide. Such initial heating should be carried out in an inert atmosphere such as, for example, argon or helium or a partial vacuum. It can also be carried out in an atmosphere of hydrogen since hydrogen is evolved at this time. In addition, since hydrogen gas is evolved, this heating can be carried out at atmospheric pressure. Specifically, at about atmospheric pressure when -a temperature of about 850 C. is attained, the reduction process begins as indicated by the evolution of hydrogen and it continues to evolve up to a temperature of about l,0O0 C. Substantially all the rare earth metal oxide is reduced under these conditions. The product is a fused cake comprised of the rare earth metal, calcium oxide and usually excess calcium. The rare earth metal-containing cake, while still hot, may be fused with cobalt, or it may be cooled in an inert atmosphere prior to fusion. If desired, this rare earth metal-containing cake can be crushed prior to the fusion procedure.

The cobalt used in the process can take a number of forms. Preferably, cobalt of melt stock or electrolytic cobalt is used. The fusion of cobalt or other ferromagnetic metal with the rare earth metal can be carried out by a number of techniques. One technique comprises melting the cobalt, then mechanically pushing the rare-earth-containing material in cake or crushed form under the surface of the molten cobalt to fuse the cobalt and rare earth metal to form the intermetallic compound. The period of time necessary to carry out the fusion can be determined empirically. The amount of cobalt or other ferromagnetic metal used depends on the intermetallic compound to formed, therefore, a stoichiometric amount of cobalt or other ferromagnetic metal should be used. For example, for the formation of Co R where R is a rare earth metal the stoichiometric reaction is as follows:

The fusion temperature may vary but it should be such as to maintain the cobalt in a molten form. Fusion should be carried out under an inert atmosphere such as argon or helium. It may be carried out at atmospheric pressure if a minor amount of calcium metal is present. For best results, however, it should be carried out at a pressure higher than atmospheric because excess calcium metal, which is usually present in a significant amount, has a high vapor pressure. To carry out the fusior a pressure induction melt furnance is preferred. Upon completion of the fusion, the rare earth intermetallic compoundcontaining product can be allowed to solidify, preferably in an inert atmosphere to prevent oxidation of the rare earth metal. This product can also be cast in a conventi o n al manner to the desired form such as an ingot.

The solid rare earth intermetallic compound-containing product is then pulverized. This may be done by a number of conventional techniques such as, for example, crushing the solid with a jaw crusher or a Diamond mortar. The crushed product may then be ground in a conventional manner such as by a ball mill or ajet mill.

To recover the cobalt rare earth intermetallic compound particles, a variety of separation techniques can be employed. In one technique a magnetic separator can be used to attract the cobalt intermetallic compound particles, leaving the calcium oxide. In another method, water is added to the particulate product to convert the calcium oxide to calcium hydroxide which is a flocculate precipitate that can be effectively decanted off with repeated washings with water. A preferred final cleanup treatment comprises admixing dilute acetic acid with the recovered cobalt intermetallic compound particles to leach away traces of remaining calcium hydroxide. The cobalt rare earth intermetallic compound particles can then be rinsed with water and dried in a conventional manner.

In the present process, if desired, the calcium hydride can be formed in situ by a number of methods. One method comprises admixing calcium carbide with the rare earth metal oxide and heating the mixture in the presence of hydrogen to form the calcium hydride. In another method magnesium chips or powder are admixed with calcium oxide and heated in hydrogen to form calcium hydride and magnesium oxide which can remain in the mixture until completion of the process. Once the calcium hydride is formed in situ, the process then can proceed in the same manner as if calcium hydride had been added initially.

The present process is useful in forming cobalt rare earth intermetallic compounds, and particularly Co R compounds, which are useful in preparing permanent magnets. In the present process, alloys of cobalt with other ferromagnetic metals may be used instead of cobalt alone to produce rare earth intermetallic compounds which are particularly useful in forming magnets. Representative of such alloys are those of cobalt and iron, alloys of cobalt, iron and manganese, and alloys of cobalt and manganese. In addition, in this invention, iron can be used instead of cobalt to produce the iron-rare earth intermetallic compound desired. Likewise, alloys of iron with other ferromagnetic metals, can also be used such as, for example, alloys of iron and manganese. Again, the resulting rare earth intermetallic compounds are particularly useful in forming magnets.

All parts and percentages used herein are by weight unless otherwise noted and where screen size is referred to, it is the U.S. Standard screen size.

The invention is further illustrated by the following examples.

EXAMPLE in this example, in preparing the formulation, a quantity adjustment factor of 0.1445 was used. The formulation was as follows:

Sm O (precipitated, 99.9% pure) 348.86 (mol.wt.Sm,0,-,

0. l445=grams. Calcium Hydride (l4-mesh) =42.l (mol.wt.CaH F3 g.

moles X 1.8 (L8 X stoichiometric req.) 0.l445=32.84 g.

The constituents of the formulation were placed in a plastic bag under a nitrogen atmosphere and blended manually until a thoroughly blended mixture was obtained. The mixture was then placed in a molybdenum foil lined iron foil envelope. The envelope was placed in a closed end clear fused silica tube attached to a vacuum system. The system was evacuated to remove air and refilled with hydrogen gas. The tube, still attached to the vacuum system, was placed in an air atmosphere tube furnace and the temperature was raised from room temperature to l,l00 C. in 40 minutes. Hydrogen gas began to evolve when a temperature of about 850 C. was reached and continued until the final temperature of l,l00 C. in hydrogen to insure the complete reduction of samarium oxide. The system was then evacuated and refilled with helium before withdrawing the tube from the furnace and allowing it to cool to room temperature.

The product of this reduction process was a fused cake comprised of samarium metal, calcium oxide, and excess calcium metal. Assuming complete reduction of samarium oxide to metal the cake would contain 43.45 g. of samarium.

The cake was then placed in the feeding tray of an inert atmosphere pressure induction melting furnace. 80.7 grams of cobalt-melting stock were placed in an alumina crucible and placed in the furnace which was maintained at 9 atmospheres of argon pressure and a temperature of about l,500 C. When the cobalt was completely melted, the cake was dropped into the melt and pushed under the melt surface to facilitate dissolution of the cake in the melt.

When dissolution of the cake was complete, as evidenced by its disappearance from the surface of the melt, the resulting melt product was allowed to cool in the crucible in the furnace and solidify under an inert atmosphere. The crucible was then broken, and the solid product removed therefrom. The product was crushed with a hammer to -20-mesh and further pulverized by ball milling for 2 hours under mineral spirits. The powder was then stored under the same mineral spirits.

A portion of the stored powder was washed with mineral spirits to remove the calcium oxide that was entrained with the cobalt-samarium intermetallic compound powder. Since calcium oxide was of lower density and a finer particle size than the cobalt-samarium powder, it remained suspended in the mineral spirits after stirring and was decanted off. The mineral spirits-wet cobalt-samarium intermetallic powder was recovered and a portion of it was washed with hexane, dried and examined by X-ray diffraction and found to contain Co Sm phase, Co -,sm phase and a trace of unreacted cobalt. Another portion of the mineral spirits-wet cobalt-samarium powder was pressed into a green body at 120,000 p.s.i. in a magnetic field of 18 kiloersteds. The green body was in the form of a cylinder about five-sixteenth inch in diameter and about one-half inch in length. The green body was placed in a protective molybdenum foil tube and heated for 30 minutes at a temperature of l,l00 C. in a calcium-gettered helium atmosphere furnace. The body was partially sintered by this treatment. It was then magnetized in a magnetizing field of 30 kiloersteds. The resulting magnet was found to have an open circuit flux density of 2,720 gauss and an intrinsic coercive force H, of 6.5 kiloersteds. From these values a minimum energy product was determined and found to be 3.75Xl0 gauss-oersted.

Another portion of the stored powder was washed with acetone to remove the mineral, spirits then washed with water to convert calcium oxide to a fiocculant precipitate of Ca(OH) The Ca(OH) was then removed by repeated washes in water and the residual calcium hydroxide dissolved by a final wash with dilute acetic acid. The powder was then washed with water, alcohol and acetone, and placed under hexane.

The hexane-wet powder was pressed into a green body at l20,00 p.s.i. in an aligning magnetic field of 18 kiloersteds. The resulting cylindrical compact, weighing about 3 grams and having a diameter five-sixteenth inch and a length of fivesixteenth inch, was placed in a close fitting molybdenum foil capsule together with 0.1 gram of calcium metal and it was heated to a temperature of l,lO C. for 5 minutes in a calcium-gettered helium atmosphere. The calcium-infiltrated compact was electroplated with copper in a cynanide bath to protect it from oxidation. It was then magnetized in a field of 30 kilogauss. The resulting magnet was found to have an intrinsic coercive force H, of 7.2 kiloersteds, and open circuit flux of 3517 gauss and an estimated energy product Bl-i,,,,,, =6 l0 gauss-oersteds.

The estimated energy product was arrived at from measurements of the open circuit flux of the sample, its intrinsic coercive force, and its dimensions. On a plot of B vs H, the open circuit flux point was plotted on a load line corresponding to the sample shape, and the intrinsic coercive force point was drawn on the line B =H in the third quadrant. This point and the open circuit point were connected with a straight line. The rest of the demagnetization curve was approximated by a straight line from the open circuit point to the H=O axis with a slope of 45. The maximum energy product on this line-segment demagnetization curve is the estimated energy product.

What is claimed is:

l. A process for preparing a rare earth intermetallic compound comprising heating an intimate particulate mixture of a rare earth metal oxide and calcium hydride to decompose said calcium hydride and thereby efiect reduction of said rare earth metal constituent, contacting the resulting solid rare earth metal-containing material with a metallic material in molten from selected from the group consisting of cobalt. iron, alloys of cobalt and iron, alloys of cobalt, iron and manganese, alloys of iron and manganese, and alloys of cobalt and manganese in an inert to form the rare earth intermetallic compound of said rare earth metal and said metallic material, allowing the resulting rare earth intermetallic compound-containing product to solidify, pulverizing the resulting solid product and recovering the rare earth intermetallic compound therefrom.

2. A process according to claim 1 wherein said metal group member is cobalt.

3. A process according to claim 1 wherein the rare earth metal oxide is samarium oxide.

4. A process according to claim 1 wherein calcium hydride is used in an amount in excess of stoichiometric.

5. A process according to claim 1 wherein said calcium hydride is formed in situ in said mixture.

it i t i i

Claims

2. A process according to claim 1 wherein said metal group member is cobalt.
3. A process according to claim 1 wherein the rare earth metal oxide is samarium oxide.
4. A process according to claim 1 wherein calcium hydride is used in an amount in excess of stoichiometric.
5. A process according to claim 1 wherein said calcium hydride is formed in situ in said mixture.