Manufacture of rare-earth magnetic materials from rare-earth salts

Abstract

A method for the production of magnetic materials of the form Re(Fe,Co)B. The method includes mixing metal oxalate powders with boric acid and reacting the mixture to form rare earth borates and metallic iron and cobalt. These components are then combined with calcium hydride and are further reacted at elevated temperature and pressure to form crystals of the Re(Fe,Co)B magnetic phase.

Description

MANUFACTURE OF RARE-EARTH MAGNETIC MATERIALS FROM RARE-EARTH SALTSFIELD

[0001] The present disclosure relates to the field of rare earth magnetic materials, particularly rare earth magnetic materials of the general form ReFeB.SUMMARY

[0002] In one embodiment, a method for the manufacture of a rare earth magnetic material of the form ReFeB, where Re is a rare earth metal, is disclosed. The method includes the steps of:

[0003] - first mixing at least a first rare earth oxalate powder comprising a rare earth metal Re with iron oxalate powder, cobalt oxalate powder and boric acid powder to form a first powder mixture;

[0004] - heating the first powder mixture to a first reaction temperature that is sufficient to react the rare earth oxalate with the boric acid to form a second powder mixture comprising at least a first rare earth borate compound, metallic iron and a metallic iron-cobalt alloy;

[0005] - second mixing the second powder mixture with calcium hydride (CaH2) to form a third powder mixture comprising the rare earth borate compounds and calcium hydride;

[0006] - compacting the third powder mixture at a compaction pressure of at least about 15 MPa to form a compacted powder body;

[0007] - heating the compacted powder body to a second reaction temperature of at least about 730°C in a vacuum to form a reacted powder body comprising ReFeB.

[0008] The foregoing method is subject to a number of refinements, characterizations and implementations, which may be applied to the foregoing method individually or in any combination.

[0009] In one refinement, the first rare earth oxalate powder has an average particle size of at least about 15 pm. In another refinement, the first rare earth oxalate powder has an average particle size of at least about 50 pm. In another refinement, the first rare earth oxalate powder has an average particle size of not greater than about 250 pm. In another refinement, the first rare earth oxalate powder has an average particle size of not greater than about 200 pm. In yet another refinement, the boric acid powder has an average particle size of not greater than about 10 pm. In a further refinement, the boric acid powder has an average particle size of at least about 0.5 pm.

[0010] In one refinement, the first reaction temperature is at least about 750°C. In another refinement, the first reaction temperature is at least about 800°C. In yet another refinement, the first reaction temperature is not greater than about 915°C.

[0011] In one refinement, the second mixing step is carried out in a V-blender.

[0012] In one refinement, the compaction pressure is at least about 20 MPa. In another refinement, the compaction pressure is at least about 24 MPa. In yet another refinement, the compaction pressure is not greater than about 35 MPa.

[0013] In one refinement, the second reaction temperature is at least about 750°C. In yet another refinement, the second reaction temperature is at least about 800°C. In yet another refinement, the second reaction temperature is not greater than about 1050°C.

[0014] In one refinement, the step of heating the compacted powder body comprises heating the compacted powder body for at least about 2 hours. In another refinement, the step of heating the compacted powder body comprises heating the compacted powder body for not greater than about 8 hours.

[0015] In one refinement, the method includes the step of milling the reacted powder body to disperse the ReFeB powder. In one implementation, the step of milling the reacted powder body comprises jet milling the reacted powder body.

[0016] In another refinement, the method includes the step of removing CaO from the dispersed ReFeB powder.

[0017] In another refinement, the ReFeB powder has a purity of at least about 85%. In another refinement, the ReFeB powder comprises magnetic alloy crystals selectedfrom the group consisting of La2Fei4B, Ce2Fei4B, Gd2Fei4B, Tb2Fei4B, Pr2Fei4B, Dy2Fei4B, Nd2Fei4B, (Nd,Pr)2Fei4B, and (Nd,Pr,Dy)2Fei4B.

[0018] In one refinement, the rare earth element Re is selected from neodymium, praseodymium and dysprosium. In one implementation, the rare earth element Re is neodymium.DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a flowsheet illustrating one embodiment of the present disclosure for the production of a magnetic material Nd2(Fe,CO)i4B.DESCRIPTION

[0020] According to the present disclosure, a method is disclosed for the manufacture of a rare earth magnetic material of the form Rex(Fe,Co)yBz, where Re is a rare earth metal and where the concentration of iron (Fe) is greater than the concentration of cobalt (Co). The method includes the steps of first combining (e.g., mixing) one or more rare earth oxalate powders, e.g., of the form Re(C2O4)3, iron oxalate powder and cobalt oxalate powder with boric acid (HBOs) powder to form a powder mixture, e.g., a first powder mixture. The first powder mixture is heated to a reaction temperature, e.g., a first reaction temperature, that is sufficient to react the rare earth oxalate powder with the boric acid to form a powder mixture, e.g., a second powder mixture, comprising at least a first rare earth borate compound, metallic iron and a metallic iron-cobalt alloy. The second powder mixture is then combined (e.g., mixed) with calcium hydride (CaFh) to form a third powder mixture comprising the rare earth borate compound(s), iron metal and iron-cobalt alloy and calcium hydride. The third powder mixture is then compacted to form a compacted powder body, which is then heated in a vacuum to a reaction temperature (e.g., a second reaction temperature) to form a reacted powder body comprising a magnetic material of the general form Rex(Fe,Co)yBz.

[0021] The rare earth metal may be selected from any rare earth metal, i.e., may be selected from scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. In one characterization, the rare earth metal Re is a rare earth metal selected from neodymium (Nd), praseodymium (Pr) and dysprosium (Dy). Combinations of rare earth elements may also be utilized, e.g., in the form of (Nd,Pr)x(Fe,Co)yBz and the like. In one particular characterization, the rare earth metal Re is neodymium.

[0022] The rare earth oxalate powder may be of the form Re(C2C )3 xH2O for example. The water of hydration will be removed during the heating step with boric acid or may be removed by heating the oxalate powders, e.g., at a relatively low temperature, before heating to the first reaction temperature with the boric acid. Water of hydration associated the iron oxalate and cobalt oxalate may be removed in a similar manner. In one characterization, the average particle size of the rare earth oxalate, iron oxalate and cobalt oxalate powders is not greater than about 250 pm, such as not greater than about 200 pm. Typically, the average particle size of the oxalate powders will be at least about 15 pm, such as at least about 50 pm.

[0023] The method includes mixing the oxalate powders with boric acid (H3BO3) powder. The boric acid powder may have a relatively small average particle size. In one characterization, the average particle size of boric acid powder is not greater than about 30 pm, such as not greater than about 25 pm, such as not greater than about 20 pm, such as not greater than about 15 pm, such as not greater than about 10 pm or even not greater than about 6 pm. Typically, the average particle size of the boric acid powder will be at least about 0.5 pm, such as at least about 1 pm. The oxalate powders and boric acid powder may be mixed in a dry state, e.g., e.g., without the addition of water or other solvents, using known mixing techniques and apparatus and may be mixed with or without comminution of the powders. In one implementation, the oxalate powders and the boric acid powder are mixed in a V-blender.

[0024] Thereafter, the first powder mixture comprising the oxalate powders and the boric acid powder are heated, e.g., in a reactor, to a reaction temperature, e.g., a first reaction temperature, that is sufficient to react the rare earth oxalate powders with the boric acid to form at least a first rare earth borate compound, such as NdBOs, PrBOs,DyBC , and the like. In one implementation, the heating in the reactor may be carried out in a reducing atmosphere. In one characterization, the reducing atmosphere includes hydrogen gas (H2), such as at least about 10% H2, such as at least about 20% H2 and up to about 100% H2, e.g., about 50% H2. In another implementation, the heating in the reactor is carried out, e.g., in a sealed reactor, at an elevated pressure such as at least about 2 bar and not greater than about 7 bar, such as from about 3 bar to about 5 bar. In another implementation, the first reaction temperature is at least about 750°C, such as at least about 800°C. Typically, the reaction temperature will be not greater than about 915°C. While not wishing to be limited by any theory, it is believed that under such conditions the rare earth oxalates will decompose to form rare earth oxides and carbon dioxide (CO2), and the boric acid will decompose to form boron oxide (B2O3) and water (H2O). The rare earth oxides and boron oxide will subsequently react to form rare earth borates. Exemplary reactions are illustrated as follows:Re2(C2O4)3-> Re2Os + CO2H3BO3 -> B2O3 + H2ORe2Os + B2O3 2ReBOs

[0025] At the same time, the iron oxalate and cobalt oxalate will decompose to their metallic forms and oxalic acid:FeC2O4 + H2 -> Fe + H2C2O4COC2O4 + H2 -> Co + H2C2O4

[0026] Thus, the resulting powder mixture (e.g., a second powder mixture) comprises rare earth borate(s) powder, iron powder and cobalt powder. This second powder mixture is then combined (e.g., mixed) with calcium hydride (CaH2) powder to form a third powder mixture comprising the ReBOs, CaH2, Fe and Co. This third powder mixture is then compacted to form a compacted powder body. For example, the third powder mixture may be compacted at a sufficient pressure such that the compacted powder body may be handled without a substantial risk of crumbling. In one characterization, the third powder mixture is compacted in a uniaxial manner after being placed in a mold. In oneimplementation, the compaction pressure is at least about 10 MPa, such as at least about 15 MPa, such as at least about 20 MPa or even at least about 24 MPa. Typically, the compaction pressure will not be greater than about 35 MPa, such as not greater than about 30 MPa.

[0027] After formation of the compacted powder body, the compacted powder body is heated to a reaction temperature, e.g., to decompose the calcium hydride. In one characterization, the reaction temperature is at least about 730°C, such as at least about 750°C. The reaction temperature may also be at least about 850°C, such as at least about 900°C. Typically, the reaction temperature will be not greater than about 1050°C, such as not greater than about 1000°C or even not greater than about 950°C. In one particular characterization, the reaction temperature is at least about 750°C and is not greater than about 915°C. In one implementation, the heating step may be carried out under a reduced pressure, e.g., under a substantial vacuum, e.g., at a pressure of not greater than about 100 Pa, such as not greater than about 60 Pa, such as not greater than about 50 Pa.

[0028] While not wishing to be bound by theory, it is believed that the calcium hydride decomposes and the liberated calcium reacts with the rare earth borate(s) to form an ReB metal alloy, e.g., an NdB metal alloy. For example:NdBOs + 3Ca -> NdB + 3CaO

[0029] The rare earth boron alloy is then available to react with the iron and / or cobalt metal to form a magnetic alloy phase, e.g., of the form Rex(Fe,Co)yBz.

[0030] The compacted powder body may be heated to the reaction temperature for a sufficient amount of time to convert at least about 85% of the Re, Fe, Co and B to the Rex(Fe,Co)yBz magnetic material, such as at least about 90%, at least about 95% and even at least about 99%. Typically, compacted powder body will be kept at the reaction temperature for at least about 1 hour and for not more than about 3 hours, although longer heating times may be utilized if necessary to achieve the desired completeness of the reactions.

[0031] While the dominant product in the reacted powder body with be the Rex(Fe,Co)yBz magnetic alloy, it is believed that some Fe2B and / or C02B may also form in the reacted powder body, particularly if the stochiometric concentrations of Fe and B are not maintained, e.g., to form a Nd2(Fe,Co)i?B magnetic alloy. Thus, it is desirable that the Fe and Co be used in substantially stoichiometric ratios to reduce the formation of these phases in the compacted powder body and in the final magnetic powder.

[0032] In one embodiment, the reacted powder body may then be comminuted (e.g., milled) to break-up the reacted powder body into a powder, e.g., a dispersed powder that flows freely. In this regard, the reacted powder body may be milled, such as in a jet mill, to form the dispersed magnetic powder comprising magnetic Rex(Fe,Co)yBzalloy. In one characterization, the dispersed magnetic powder may have an average particle size of not greater than about 30 pm, such as not greater than about 25 pm, such as not greater than about 20 pm, such as not greater than about 15 pm, such as not greater than about 10 pm or even not greater than about 6 pm. Typically, the average particle size of the dispersed magnetic powder will be at least about 0.5 pm, such as at least about 1 pm.

[0033] As can be seen from the foregoing reactions, the dispersed magnetic powder may include small quantities of calcium oxide (CaO) that forms from the calcium source. In one implementation, the method includes the step of separating the CaO from the Rex(Fe,Co)yBz magnetic material, e.g., to form a high purity Rex(Fe,Co)yBzmagnetic powder. For example, the CaO, which is less dense than the magnetic powder, may be separated by gravity separation techniques, e.g., in a cyclone. Other methods for the separation of CaO from particles such as ReFeB magnetic materials are known to those skilled in the art, such as selective dissolution of the CaO.

[0034] The magnetic powder may comprise at least about 98% Rex(Fe,Co)yBz, such as at least about 99% Rex(Fe,Co)yBz, at least about 99.5% Rex(Fe,Co)yBzor even at least about 99.9% Rex(Fe,Co)yBz. In one characterization, the rare earth metal Re comprises neodymium and the Rex(Fe,Co)yBzmagnetic material is predominantly Nd2(Fe,Co)i4B, e.g., predominantly Nd2(Fe,Co)i4B crystals where x=2, y=14 and z=1. It is an advantage of the method disclosed herein that the resulting rare earth magnetic powder may have a very low concentration of oxygen. In one characterization, the rare earth magneticpowder comprises not greater than about 0.4 wt.% elemental oxygen, such as not greater than about 0.2 wt.% elemental oxygen, or even not greater than about 0.1 wt.% elemental oxygen.

[0035] The foregoing description refers to the formation of a magnetic alloy in the generic form Rex(Fe,Co)yBz, i.e., containing a rare earth, iron, cobalt and boron. A highly desirable magnetic alloy of this form is Re2(Fe,Co)i4B and is referred to as such above. However, the actual alloy will have different forms depending upon the oxalate compounds input to the process and the purity of those compounds. For example, although the alloy represented as Re2(Fe,Co)i4B is known to those of skill in the art, the actual alloy will typically include Pr and Dy in lesser concentrations than Nd so that the alloy may be more formally written as (Nd,Pr,Dy)2(Fe,Co)i4B. Further, Co is included in lesser concentrations than Fe such that the Co substitutes for a portion of the Fe, as in Feii.2Co2.8. It is to be understood that the present disclosure is directed to the formation of these various permutations of rare earth magnetic alloys.PROPHETIC EXAMPLE

[0036] FIG. 1 is a flowsheet illustrating an embodiment of the present disclosure for the production of magnetic powder Nd2(Fe,Co)i4B from rare earth metal oxalates. While FIG. 1 illustrates a particular method for the production of magnetic powder that is useful for understanding the methods of the present disclosure, it will be appreciated that the present disclosure is not limited to the reactants, process specifications and end products illustrated therein.

[0037] Powders of Nd2(C2O4)3, FeC2O4, COC2O4 and HBO3 are mixed 102 to form a first powder mixture. The oxalate powders have a mean average particle size of about 100 pm and the boric acid powder has a mean average particle size of about 8 pm. The powders are added at about 65 to 67 wt.% iron oxalate, about 0.91 to 1.4 wt.% cobalt oxalate, about 0.94 to 1 .34% wt.% boric acid, with the balance being neodymium oxalate.

[0038] The powder mixture is then heated 104 in a reactor to a reaction temperature of about 810°C under an inert gas comprising about 50% H2 to convert the neodymium oxalate to neodymium borate and reduce the iron oxalate and cobalt oxalate to the respective metals.

[0039] The resulting powder mixture is mixed 106 with calcium hydride (CaFh) and is then compacted 108 at a compaction pressure of about 26 MPa to form a compacted powder body. The compacted powder body is then heated 110 to a second reaction temperature of about 850°C and under a substantial vacuum of not greater than about 60 Pa. The resulting reacted powder body is milled 112 and then CaO is separated 114 to form a high purity powder of Nd2(Fe,Co)i4B

[0040] While various embodiments of a method for the manufacture of a rare earth magnetic material have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.

Claims

What is Claimed is:1 . A method for the manufacture of a rare earth magnetic material of the form Rex(Fe,Co)yBz, where Re is a rare earth metal, comprising the steps of: first mixing at least a first rare earth oxalate powder comprising a rare earth metal Re with iron oxalate powder, cobalt oxalate powder and boric acid powder to form a first powder mixture; heating the first powder mixture to a first reaction temperature that is sufficient to react the rare earth oxalate with the boric acid to form a second powder mixture comprising at least a first rare earth borate compound, metallic iron and a metallic iron-cobalt alloy; second mixing the second powder mixture with calcium hydride (CaH2) to form a third powder mixture comprising the rare earth borate compounds and calcium hydride; compacting the third powder mixture at a compaction pressure of at least about 15 MPa to form a compacted powder body; heating the compacted powder body to a second reaction temperature of at least about 730°C in a vacuum to form a reacted powder body comprising Rex(Fe,Co)yBz.

2. The method recited in Claim 1 , wherein the first rare earth oxalate powder has an average particle size of at least about 15 pm.

3. The method recited in any one of the previous claims, wherein the first rare earth oxalate powder has an average particle size of at least about 50 pm.

4. The method recited in any one of the previous claims, wherein the first rare earth oxalate powder has an average particle size of not greater than about 250 pm.

5. The method recited in any one of the previous claims, wherein the first rare earth oxalate powder has an average particle size of not greater than about 200 pm.

6. The method recited in any one of the previous claims, wherein the boric acid powder has an average particle size of not greater than about 10 pm.

7. The method recited in any of the previous claims, wherein the boric acid powder has an average particle size of at least about 0.5 pm.

8. The method recited in Claim 1 , wherein the first reaction temperature is at least about 750°C.

9. The method recited in any of the previous claims, wherein the first reaction temperature is at least about 800°C.

10. The method recited in any of the previous claims, wherein the first reaction temperature is not greater than about 915°C.11 . The method recited in any of the previous claims, wherein the second mixing step is carried out in a V-blender.

12. The method recited in Claim 1 , wherein the compaction pressure is at least about 20 MPa.

13. The method recited in any of the previous claims, wherein the compaction pressure is at least about 24 MPa.

14. The method recited in any of the previous claims, wherein the compaction pressure is not greater than about 35 MPa.

15. The method recited in Claim 1 , wherein the second reaction temperature is at least about 750°C.

16. The method recited in any of the previous claims, wherein the second reaction temperature is at least about 800°C.

17. The method recited in any of the previous claims, wherein the second reaction temperature is not greater than about 1050°C.

18. The method recited in any of the previous claims, wherein the step of heating the compacted powder body comprises heating the compacted powder body for at least about 2 hours.

19. The method recited in any of the previous claims, wherein the step of heating the compacted powder body comprises heating the compacted powder body for not greater than about 8 hours.

20. The method recited in any of the previous claims, further comprising the step of milling the reacted powder body to disperse the ReFeB powder.

21. The method recited in Claim 20, wherein the step of milling the reacted powder body comprises jet milling the reacted powder body.

22. The method recited in any one of Claims 20 or 21 , further comprising the step of removing CaO from the dispersed Rex(Fe,Co)yBzpowder.

23. The method recited in Claim 22, wherein the ReFeB powder has a purity of at least about 90%.

24. The method recited in any one of the previous claims, wherein the Rex(Fe,Co)yBz powder comprises magnetic alloy crystals selected from the group consisting of La2(Fe,Co)i4B, Ce2(Fe,Co)i4B, Gd2(Fe,Co)i4B, Tb2(Fe,Co)i4B, Pr2(Fe,Co)i4B, Dy2(Fe,Co)i4B, Nd2(Fe,Co)i4B, (Nd,Pr)2(Fe,Co)i4B, and (Nd,Pr,Dy)2(Fe,Co)i4B.

25. The method recited in any of the previous claims, wherein the rare earth element Re is selected from neodymium, praseodymium and dysprosium.

26. The method recited in Claim 25, wherein the rare earth element Re is neodymium.

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