EP0170372B1

EP0170372B1 - Metallothermic reduction of rare earth oxides with calcium metal

Info

Publication number: EP0170372B1
Application number: EP85304046A
Authority: EP
Inventors: Ram Autar Sharma
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1984-07-03
Filing date: 1985-06-07
Publication date: 1988-08-17
Also published as: AU4448885A; ES8702508A1; ATE36560T1; JPS6130639A; KR910001581B1; BR8503140A; EP0170372A1; DE3564451D1; MX173880B; KR860001203A; JPS6137341B2; AU575965B2; ES544799A0; ZA854474B

Abstract

Rare earth oxides can be reduced to rare earth metals by, a novel, high yield, metallothermic process. The oxides are dispersed in a suitable, molten, calcium chloride-based bath (44) along with calcium metal. The bath (44) is agitated and calcium metal reduces the rare earth oxides to rare earth metals. The metals collect in a discrete layer (43) in the reaction vessel (22).

Description

This inveniton relates to a novel metallothermic process for the direct reduction of rare earth oxide, particularly neodymium oxide, to rare earth metal. The method has particular application to low cost production of neodymium metal for use in neodymium-iron-boron magnets.

Background

In the past, the strongest commercially produced permanent magnets were made from sintered powders of an alloy of samarium and cobalt (SmCo_s). Recently, even stronger magnets have been made from alloys of the light rare earth elements, preferably neodymium and praseodymium, iron and boron. These alloys and methods of processing them to make magnets are described in European patent application Nos. 0 108 474, 0 125 752, 0 133 758 and 0 144 112.
Sources of the rare earth (RE) elements, atomic nos. 57 to 71 of the periodic table as well as yttrium, atomic no. 39, are bastnaesite and monazite ores. Mixtures of the rear earths can be extracted from the ores by several well known beneficiating techniques. The rear earths can then be separated from one another by such conventional processes as elution and liquid-liquid extraction.
Once the rare earth metals are separated from one another, they must be reduced from the oxides to the respective metals in relatively pure form (95 atomic percent or purer depending on the contaminants) to be useful for permanent magnets. In the past, this final reduction was both complicated and expensive, adding substantially to the cost of rare earth metals. Both electrolytic and metallothermic (non-electrolytic) processes have been used to reduce rare earths. The electrolytic processes include (1) decomposition of anhydrous rare earth chlorides dissolved in molten alkali or alkaline earth salts, and (2) decomposition of rare earth oxides dissolved in molten rare earth fluoride salts.
Disadvantages of both electrolytic processes include the use of expensive electrodes which are eventually consumed, the use of anhydrous chloride or fluoride salts to prevent the formation of undesirable RE-oxy salts (NdOCI, e.g.), high temperature cell operation (generally greater than 1000°C), low current efficiences resulting in high power costs, and low yield of metal from the salt (40% or less of the metal can be recovered). The RE-chloride reduction process release corrosive chlorine gas while the fluoride process requires careful control of a temperature gradient in the electrolytic salt cell to cause solidification of rare earth metal nodules. An advantage of electrolytic processes is that they can be made to run continuously if provision is made to tap the reduced metal and to refortify the salt bath.
The metallothermic (non-electrolytic) processes include (1) reduction of RE-fluorides with calcium metal powder (the calciothermic process), and (2) reduction-diffusion of RE-oxide with calcium hydride (CaH₂) or calcium metal (Ca). Disadvantages are that both are batch processes, they must be conducted in a non-oxidizing atmosphere, they are energy intensive and the product in the case of reduction-diffusion is a powder which must be hydrated to purify it before use. Both processes involve many steps. One advantage of the metallothermic reduction processes is that the yield of metal from the oxide or fluoride is generally better than ninety percent.
Processes involving RE fluoride or chloride require pretreatment of the RE-oxide to create the halide. This additional step adds to the end cost of rare earth metals.
With the invention of light rare earth-iron permanent magnets, the demand for low cost, relatively pure, rare earth metals rose substantially. However, none of the existing methods of reducing rare earth compounds showed much promise for reducing the cost or increasing the availability of magnet-grade metals. Accordingly, it is an object of this invention to provide a new, efficient and less costly method of producing rare earth metals.

Brief summary

A metallothermic, non-electrolytic method of reducing rare earth oxide to rare earth metal according to the present invention consists in that the reduction takes place in a molten salt bath comprised predominantly of calcium chloride, a volume of rare earth oxide that is less than the salt bath volume is dispersed in the bath, a stoichiometric excess of calcium metal with respect to the amount of rare earth metal ion present is added to the bath, and said bath is agitated so that the oxide is reduced to rare earth metal in accordance with the formula
where RE represents one or more rare earth elements having a valency of 2,3 or 4,0 represents oxygen, Ca represents calcium, CaO represents calcium oxide, and n and m are integers such that the valency of the rare earth element multiplied by n equals the valency of oxygen multiplied by m.
A preferred embodiment of the invention is as follows:
A reaction vessel is provided which can be heated to desired temperatures by electrical resistance heaters or some other heating means. The vessel body is preferably made of a metal or refractory material that is either substantially inert or innocuous to the reaction constituents.
A predetermined amount of RE-oxide is charged into the reaction vessel containing a salt mixture of about 70 weight percent calcium chloride or greater and about 5 to 30 weight percent sodium chloride (NaCI). The salt serves as a medium for the reduction reaction. A stoichiometric excess of calcium metal, based on the amount of rear earth oxide, is added. It may be advantageous to add an amount of another metal such as iron or zinc to form a eutectic alloy with the reduced rare earth metal to enable the reaction to be carried out at lower temperatures and to obtain the RE metal product in a liquid state.
To run the reaction, the vessel is heated to a temperature above the melting point of the constituents (about 675°C). The molten constituents are rapidly stirred in the vessel to keep them in contact with one another as the reaction progresses. The bath is replenished with calcium chloride (CaCl₂) as necessary to maintain a weight percent of 70% of the combined weights of CaC1₂ and NaCI. While the reaction runs at CaCI₂ concentrations lower than 70%, the yield falls of rapidly.
Several different and competing chemical reactions occur in the vessel, however the reduction of the RE-oxide is believed to be accomplished in accordance with the empirical reaction formula
where "n" and "m" are the number of moles of constituent and where the relation of n and m is determined by the oxidation state of the rare earth element.
The reduced metal has a density of about 7 grams/cc while that of the salt bath is about 1.9 grams/cc. When stirring is stopped, the reduced metal forms a clean layer at the bottom of the reaction vessel. This layer may be tapped while molten or separated from the salt layer after it solidifies.
Thus, the method of the invention provides many advantages over prior art methods. It is carried out at a relatively low temperature of about 700°C, particularly where the rare earth metal is recovered as a zinc or iron eutectic alloy. It uses relatively inexpensive RE-oxide, CaCl₂, NaCI and Ca metal reactants. It does not require pretransformation of RE-oxide to chloride or fluoride, nor the use of expensive pure Ca or CaH₂ reducing agent. Energy consumption is low because the method is not electrolytic and it is preferably carried out at atmospheric pressure at temperatures at about 700°C. The method can be practiced as either a batch or a continuous process, and the by-products of CaC1₂, NaCI and calcium oxide (CaO) are easily disposed of. Moreover, the rare earth metals may be alloyed with additional iron in the reaction vessel if they are made as a RE-Fe eutectic alloy or may be alloyed later for use in RE-Fe magnets without further expensive purification treatments.

Detailed description

The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which:

Figure 1 is a side view in cross-section of an apparatus suitable for carrying out a method of reducing RE-oxides to RE metals according to the present invention;
Figure 2 is a flow chart for the reduction of neodymium oxide (Nd₂0₃) with calcium to yield a neodymium-eutectic alloy; and
Figure 3 is a plot of neodymium (Nd) metal yield from Nd₂0₃ as a function of the percentage of CaC1₂ in a flux bath used in the invention.

This invention relates to an improved method of reducing compounds of rare earth elements to the corresponding elemental metals. The rare earth metals include elements 57 to 71 of the periodic table (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium) and atomic number 39, yttrium. The oxides of the rare earths are generally coloured powders produced in the metals separation process. Herein, the term "light rare earth" refers to the elements lanthanum (La), cerium (ce), praseodymium (Pr) and neodymium (Nd).
In the method of this invention, the RE-oxides can generally be used as received from the separator but may be calcined to remove excess absorbed moisture or carbon dioxide. In the following examples, the RE-oxides were oven-dried for about two hours at 1000°C prior to use. The CaCI₂ and NaCI for the salt baths were reagent grade and dried for about two hours at 500°C prior to use.
In the initial work, care was taken to make sure that no moisture was introduced into the reaction vessel to prevent any hazardous reaction with Na or Ca. When calcium is added to a NaCI-containing bath, some sodium metal may form by the reaction
When Nd₂0₃ is mixed with CaC1₂ in a molten salt bath, oxychloride is formed by the reaction
The presence of such RE-oxychlorides was known to reduce yield in prior art electrolytic processes. However, in the present invention both RE-oxides and RE-oxychlorides are readily reduced by calcuim metal. In fact, the formation of RE-oxychlorides is advantageous because they float on molten layers of reduced RE metals. RE-oxides, on the other hand, have densities close to the reduced RE metals so they may be retained as contaminants in the molten layers of reduced RE metals, and may make the RE metals unsuited for use in magnets. The RE metals reduced by the method according to the present invention have been substantially oxide-free.
Unalloyed Nd metal has a melting temperature of about 1025°C. The other rare earth metals also have high melting points. If one wanted to run the subject reaction at such temperatures, it would be possible to do so and obtain pure metal at high yields. However, it is preferred to add amounts of other metals such as iron, zinc, or other non-rare earth metals to the reduction vessel in order to form an alloy with the recovered rare earth metal that melts at a lower temperature. For example, iron forms a low melting eutectic alloy with neodymium (11.5 weight percent Fe; m.p. about 640°C) as does zinc (11.9 weight percent Zn, m.p. about 630°C). If sufficient iron is added to a Nd₂0₃ reduction system, the reduced metal will form a liquid pool at about 640°C. A Nd-Fe eutectic alloy may be directly alloyed with additional iron and boron to make magnets having the optimum Nd₂Fe₁₄B magnetic phase described in the aforementioned European patent applications.
If it is preferred to lower the melting point of the recovered rare earth metal but not to retain the metal added to do so, a metal with a boiling point much lower than the boiling point of the recovered rare earth can be added to the reaction vessel. For example, Zn boils at 907°C, and Nd boils at 3150°C. The low-melting metal can then be readily separated from the rare earth metal by simple distillation.
Materials used for reaction vessels or liners thereof should be chosen carefully because of the corrosive nature of molten rare earth metals, particularly rare earth metals retained in a salt flux environment. Yttria-lined alumina and boron nitride are non-reactive, refractory materials generally acceptable. It is also possible to use a refractory liner made of a substantially inert metal such as tantalum or a consumable but innocuous metal such as iron. An iron liner could be used to contain.reduced RE metal and then be alloyed with the RE for use in magnets.
In accordance with this invention, a new method has been discovered of using calcium metal to reduce rare earth oxides. The method entails bringing together molten calcium and RE-oxide to cause the reaction
Unless the reaction vessel is pressurized, it is desirable to keep the temperature at below 910°C to avoid the excessive loss of Na formed by the reaction of Ca with NaCI. It is preferred to run the reactions at atmospheric pressure. The most preferred range of operating temperatures is between about 650°C and 750°C. At such temperatures wear on the reaction vessel is not excessive. This temperature range is suitable for reducing Nd₂0₃to Nd metal because the Nd-Re and Nd-Zn eutectic melting-point temperatures are below 700°C. Moreover, at about 700°C the solubility of Ca metal in the salt bath is about 1.3 molecular percent. This is sufficient to rapidly reduce RE-oxide to RE metal.
Where good separation of reduced RE metal from the flux is needed, the reaction temperature must be above the melting point of the reduced RE metal or the melting point of the reduced RE metal alloyed or co-reduced with another metal. These relatively dense RE metals and alloys collect at the bottom of the reaction vessel when allowed to settle. There they can be tapped while molten or removed after solidification. Table I shows the molecular weight (m.w.), density (p) at 25°C, melting point (m.p.) and boiling point (b.p.) for elements and compounds used in the present invention.
Figure 1 shows an apparatus suitable for the practice of the invention in which the experiments set out in the several examples were conducted.
All experiments were carried out in a deep furnace wall 20 having an inside diameter of 12.7 cm and a depth of 54.6 cm mounted to the floor 4 of a dry box with bolts 6. A helium atmosphere containing less than one part per million each of oxygen (0₂), nitrogen (N₂) and water (H20) was maintained in the box during experimentation.
The furnace was heated by means of three tubular, electric, clamshell heating elements 8, 10 and 12 having an inside diameter of 13.3 cm and a total length of 45.7 cm. The side and bottom of the furnace well were surrounded with refractory insulation 14. Thermocouples 15 were mounted on an outer wall 16 of furnace wall 20 at various locations along its length. One of the centrally located thermocouples was used in conjunction with a proportional band temperature controller (not shown) to automatically control centre clamshell heater 10. The other three thermocouples were monitored with a digital temperature readout system and top and bottom clamshell heaters 8 and 12 were manually controlled with transformers to maintain a fairly uniform temperature throughout the furnace.
The reduction reactions were carried out in a reaction vessel 22 retained in a stainless steel crucible 18 having a 10.2 cm outer diameter, 12.7 cm deep and 0.15 cm thick retained in stainless steel furnace well 20. Reaction vessel 22 was made of tantalum metal unless otherwise noted in the examples.
A tantalum stirrer 24 was used to agitate the melt during the reduction process. It had a shaft 48.32 cm long and a welded blade 26. The stirrer was powered by a 100 W variable speed motor 28 capable of operating at speeds up to 700 revolutions per minute. The motor was mounted on a bracket 30 so that the depth of stirrer blade 26 in reaction vessel 22 could be adjusted. The shaft was journalled in a bushing 32 carried in an annular support bracket 34. The bracket is retained by collar 35 to which furnace well 20 is fastened by bolts 37. Chill water coils 36 were located near the top of well 20 to promote condensation and prevent escape of volatile reaction constituents. Cone-shaped stainless steel baffles 38 were used to reflux Na vapors. Reflux products drop through tube 40 on bottom baffle 42.
When the constituents in the furnace are not stirred at the end of the run, they separate into layers with a rare earth alloy pool 43 on the bottom, an RE-oxychloride, calcium/sodium chloride salt bath 44 above that and any unreacted calcium metal 45 above that.
Figure 2 is an idealized flow chart for the reduction of Nd₂0₃ to Nd metal in accordance with this invention. The Nd₂0₃ is added to the reaction vessel along with calcium chloride and sodium chloride in suitable proportions. Calcium metal and enough of a eutectic forming metal such as iron or zinc to form a near eutectic Nd alloy are added. The reaction is run, with rapid stirring at about 300-700 revolutions per minute at a temperature of about 700°C for at least one hour. Preferably, a blanket of an inert gas such as helium is maintained over the reaction vessel. After the Nd₂0₃ has been reduced, stirring is continued at a lower speed of 100 revolutions per minute for one hour and then stirring is stopped to allow the various liquids in the vessel to stratify. The reduced Nd eutectic alloy collects at the bottom because it has the highest density. The remaining salts and any unreacted Ca collect above the Nd alloy and can be readily broken away after the vessel has cooled and the constituents have solidified. Nd-Fe alloys so produced can be alloyed with additional elements to produce permanent magnet compositions. These magnet alloys may be processed by melt-spinning or they can be ground and processed by powder metallurgy to make magnets.

Example I

265 grams of 99% pure Nd metal chunks and 50 grams of 99.9% purity Zn metal were placed in a tantalum reaction vessel to make 315 grams of near eutectic alloy. The vessel was lowered into the furnace and heated to 800°C to alloy the Nd and Zn.
The furnace temperature was lowered to about 720°C. 150 grams of NaCl and 350 grams of CaC1₂ were added to create a salt bath of 70 weight percent CaCl₂. 234 grams (0.7 moles) Nd₂0₃ were added. 104 grams of Ca (2.6 moles) metal were added to the crucible and it was stirred at a rate of 300 revolutions per minute for about two hours and then for another hour at a stirring rate of 60 revolutions per minute. The crucible was removed from the furnace and cooled on the floor of the drybox.
189 grams of Nd metal of purity greater than 99% was recovered (not including the 265 grams of Nd metal from the original seed pool) by distilling the Nd-Zn alloy collected at the bottom of the vessel. The yield of Nd metal from the oxide was about 94%.

Example II

350 grams of 99% pure Nd metal chunks and 64 grams of electrolytic iron were placed in a 6 mm thick mild steel reaction vessel to make 414 grams of near eutectic alloy. The vessel was lowered into the furnace and heated to 800°C to alloy the Nd and iron.
The furnace temperature was lowered to about 720°C. 300 grams of NaCl and 700 grams of CaCI₂ were added to create a salt bath of 70 weight percent CaCl₂. 117 grams (0.35 moles) of Nd₂0₃ were added. 46 grams (1.15 moles) of Ca metal and 10.8 grams (0.47 moles) of Na were added to the crucible and they were stirred at a rate of 300 revolutions per minute for about 135 minutes. At this point an additional 117 grams (0.35 moles) of Nd₂0₃, 46 grams (1.15 moles) of Ca metal and 10.8 grams (0.47 moles) of Na were added. The reactants were stirred for another 114 minutes at 300 rpm and then for another hour at a stirring rate of 60 rpm. The reaction vessel was removed from the furnace and cooled on the floor of the drybox. A layer of unreacted Ca-Na alloy formed on top of the salt layer.
594 grams of 97% purity Nd-Fe alloy were recovered. Such alloy could be combined directly as recovered with additional iron and boron to make the ideal Nd-Fe-B alloy for permanent magnet manufacture. 180 grams of Nd metal of purity greater than 99% was recovered as Nd-Fe alloy. This example shows that a calcium and sodium melt is capable of reducing a rare earth oxide in a CaCl₂-NaCl flux bath.

Example III

Table II sets out the amounts of various constituents used in the metallothermic reduction of about 234 grams Nd₂O₃ with Ca metal using the process set out in Example except that the reactants were stirred for four hours at 300 revolutions per minute followed by an additional hour of stirring at 60 rpm.
At a salt bath ratio of 65.5 wt% CaCI₂ and 34.5 wt% NaCl, the yield increases to 65.2%. At 70 wt% CaCl₂ or more, the Nd yield in each case is greater than 85% and generally over 95%. Figure 3 is a plot of Nd metal yield from Nd₂0₃ as a function of the weight percent CaC1₂ in a two component NaCI-CaCI₂ starting salt bath with a Ca metal reductant. Referring to Table II and Figure 3, it has been found that, to obtain high yields, it is necessary to maintain the amount of CaC1₂ in the salt bath above about 70 weight percent. It is also desirable to have a salt to RE-oxide volume ratio of at least 2:1 to provide adequate flux for the dispersion of RE-oxide. It has been observed that, as the volume ratio of the salt bath to RE-oxide increases, the rate of stirring may be decreased to obtain similar yields in a given period of time. The CaC1₂-containing bath is a significant feature of this invention.
Several of the samples were combined and the Zn metal was removed by vacuum distillation. The resultant alloy was analyzed and was found to be of greater than 99% purity with 0.4% aluminium, 0.1 % silicon, 0.01 % calcium and traces of zinc, magnesium and iron contamination. The Nd metal so produced was melted in a vacuum furnace with electrolytic iron and ferroboron to produce an alloy having the nominal composition Nd_0.15B_0.05Fe_0.80. The alloy was melt-spun, as described in European patent application No. 0108474 cited above, to produce very finely crystalline ribbon with an as-quenched coercivity of about 10 megaGaussOersteds.
While the invention has been described in detail for the reduction of Nd₂0₃, it has equal applicability to reducing other single rare earth element oxides or combinations of rare earth oxides. This is due to the fact the CaO is more stable than the oxides of any of the rare earths. While one skilled in the art could have made a determination of the relative free energies of RE-oxides and CaO in the past, before this invention it was not known that RE-oxides could be reduced by Ca metal in a non-electrolytic, liquid phase process. Oxides of transition metals such as Fd and Co can be co-reduced with RE-oxides by the process of the present invention if desired.
In summary, a new, efficient and less costly method of reducing rare earth oxides to rare earth metals has been developed. It entails the formation of a suitable, molten, CaCl₂-based bath in which rare earth oxide is stirred with an amount of calcium metal sufficient to reduce it. When stirring is stoppped, the components settle into discrete layers which can be broken apart when they cool and solidify. In the alternative, the reduced liquid rare earth metal can be tapped from the bottom of the reaction vessel. After the RE metal is tapped, the bath can be refortified to run another batch, making the process a substantially continuous one.

Claims

1. A metallothermic, non-electrolytic method of reducing rare earth oxide to rare earth metal, consisting in that the reduction takes place in a molten salt bath (44) comprised predominantly of calcium chloride, a volume of rare earth oxide that is less than the salt bath volume is dispersed in the bath, a stoichiometric excesss of calcium metal with respect to the amount of rare earth metal ion present is added to the bath, and said bath is agitated so that the oxide is reduced to rare earth metal in accordance with the formula

where RE represents one or more rare earth elements having a valency of 2,3 or 4,0 represents oxygen, Ca represents calcium, CaO represents calcium oxide, and n and m are integers such that the valency of the rare earth element multiplied by n equals the valency of oxygen multiplied by m.

2. A metallothermic, non-electrolytic method of reduction according to Claim 1, characterised in that neodymium oxide is reduced to neodymium metal in accordance with the reaction formula

3. A metallothermic, non-electrolytic method of reduction according to Claim 1, characterised in that the molten salt bath (44) contains at least 70 weight percent calcium chloride; said bath (44) is maintained in a molten state and agitated until the calcium metal reduces the rare earth oxides to rare earth metal; and then agitation is stopped so that a discrete layer (43) containing the rare earth metal is formed in the bath (44).

4. A metallothermic, non-electrolytic method of reduction according to Claim 3, characterised in that the rare earth oxide is one or more rare earth oxides taken from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide and neodymium oxide.

5. A metallothermic, non-electrolytic method of reduction according to Claim 3, characterised in that the molten bath (44) contains at least 70 weight percent calcium chloride and the balance sodium chloride; a volume of neodymium oxide Nd₂0₃ is added to the bath (44) which is less than 50% of the volume of the molten bath (44); said bath (44) is stirred so that the constituents are mixed with one another and until a substantial portion of the neodymium oxide is reduced to neodymium metal; and then stirring is discontinued whilst maintaining the constituents in a molten state so that said discrete layer (43) containing the reduced neodymium metal, substantially free of neodymium oxide inclusions, is formed in the bath (44).

6. A metallothermic, non-electrolytic method of reduction according to Claim 1, characterised in that the molten bath (44) contains at least 70 weight percent calcium chloride and the balance sodium chloride, a volume of rare earth oxide is added to the bath (44) which is less than 50% of the volume of the molten bath (44); the bath (44) is stirred so that the constituents are mixed with one another and until a substantial portion of the rare earth oxide is reduced to rare earth metal; and then stirring is discontinued whilst maintaining the constituents in a molten state, so that a discrete layer (43) containing the reduced rare earth metal is formed in the bath (44).

7. A metallothermic, non-electrolytic method of reduction according to Claim 1, characterised in that the molten salt bath (44) comprises at least 70 weight percent calcium chloride and from 0 to 30 weight percent sodium chloride; an amount of non-rare earth metal is added to said bath (44) sufficient to form a rare earth/non-rare earth metal alloy with a melting temperature substantially lower than the melting temperature of the rare earth metal; and then agitation is stopped so that the rare earth/non-rare earth metal alloy collects in a discrete layer (43) in said bath (44).

8. A metallothermic, non-electrolytic method of reduction according to Claim 7 characterised in that the rare earth oxide is one or more rare earth oxides taken from the group consisitng of lanthanum oxide, cerium oxide, praseodymium oxide and neodymium oxide.

9. A metallothermic, non-electrolytic method of reduction according to Claim 7 characterised in that the rare earth oxide is neodymium oxide.

10. A metallothermic, non-electrolytic method of reduction according to any one of Claims 7 to 9, characterised in that the non-rare earth metal is iron.

11. A metallothermic, non-electrolytic method of reduction according to any one of Claims 7 to 9, characterised in that the non-rare earth metal is zinc.