Production of Pure Rare Earth Metals a
FRANK H. SPEDDING, HARLEY A. WILHELM, WAYNE H. KELLERI, DONALD H. AIIMANN2, ADRIAN H. DAANE, CLIFFORD C. HACH3, AND ROBERT P. ERICSON4 Institute f o r Atomic Research and Department of Chemistry, Iowa State College, Ames, l a .
B
ECAUSE of their unique similarity, the rare earths have been the source of much fruitful work on the electronic structures of the elements. For the same reason, this group of elements should offer an excellent opportunity t o evaluate existing theories of metals and t o establish a more precise explanation for the relationships between the properties of metals and their electronic structures. With the advent of atomic piles, interest in the rare earths has been heightened as these elements make up an appreciable fraction of the fission products from such piles. The study of the rare earths being carried out in this laboratory has as its aim the preparation and study of pure rare earth salts and metals t o shed light on the above problems; the method of preparing pure rare earth metals described in this paper is one of t h e first steps in this program. The preparation of the rare earth metals was accomplished as early as 1827 by the reduction of cerous chloride with sodium or potassium by Mosander (15) who obtained powdered metal in poor yields, contaminated with excess reductant and reaction products. Wohler (24), de Marignac (la), and others (10,25) subsequently used similar procedures with the same results. Klemm and Bommer (9) prepared the powdered metals of all of the rare earths except promethium by reducing the chlorides with alkali metals and used these powders for x-ray and magnetic studies. Cerium fluoride was reduced by calcium or aluminum by Moldenhauer (14) who obtained alloys of the metal. More recently, Trombe and Mahn (23) reduced the chlorides of cerium, neodymium, and gadolinium b y molten magnesium t o obtain alloys of the rare earths in 50% yields; from these, they distilled the magnesium t o prepare metal of 99% punty. I n the past, most of the rare earth metal preparations were carried out by the electrolysis of the chlorides or fluorides as described b y Hillebrand and Norton (S),subsequently b y Muthmann and associates (16), Trombe (22), and Kremers (11). Hopkins and associates (4,7 , 13) have employed an electrolytic method to obtain rare earth amalgams, and the metals were obtained by removing the mercury by distillation. Work in this laboratory has shown the metallothermic reduction of rare earth halides t o be a very effective method of obtaining several of the rare earth metals in the massive state in a high degree of purity. The present paper is the first of a series on the preparation and properties of t h e rare earth metals and it establishes that cerium, lanthanum, neodymium, and praseodymium metals can be produced in a very pure state with high yields b y the reduction of the rare earth chloride with calcium, using iodine as an auxiliary oxidant. 1
The preparation of samarium and yttrium metals has been effected, although the yields and quality of the metal obtained were poor. MATERIALS, APPARATUS, AND GENERAL PROCEDURE
MATERIALS.The pure rare earth salts used for the preparation of the metal were obtained from several sources. The commercial grade of "pure" cerium (95 t o 98% cerium), as the oxalate and the chloride, and spectroscopically pure lanthanum, as the oxalate, were purchased from the Lindsay Light and Chemical Co. Spectroscopically pure cerium was supplied as the hydrated chloride or as cerium ammonium nitrate by the G. Frederick Smith Chemical Co. The commercial grade of cerium salts was used t o determine the conditions for optimum yields of cerium metal and the pure grade was used t o produce pure metal in high yields. Some of the commercial grade cerium chloride was purified by dissolving it in water and precipitating the thorium as the insoluble Th(IO&. Ceric ion present in the solution t o the extent of several grams per liter served as a carrier for the thorium by precipitating as Ce(IOd4. The other rare earths studied were obtained in the pure state by elution through Amberlite columns as the citrates (17-80). The various rare earth salts were analyzed for other rare earths and other impurities by spectroscopic methods developed in this laboratory ( I , 8). The anhydrous rare earth chlorides were prepared by a method similar t o that of Kleinheksel and Kremers (8). This consisted of dissolving the rare earth oxide in hydrochloric acid and evaporating this solution to a sirup which boiled at 128" C. This solution was poured into a large porcelain dish and stirred while it cooled, forming the hydrated chloride crystals. The hydrated chlorideswere heated slowly t o 400' C. in a stream of hydrogen chloride gas a t about 5 cm. pressure t o give the anhydrous chlorides. Anhydrous samarium triiodide was obtained by drying a n intimate mixture of hydrated samarium iodide and ammonium iodide under a n atmosphere of hydrogen iodide by the procedure of Jantsch and Skalla (6). Samarium trifluoride was prepared from samarium oxalate by passing dry hydrogen fluoride over the oxalate a t 300' t o 325' C. for 6 hours. Samarium dichloride was prepared by the method of Jantsch Ruping, and Kunse (5),which consisted of reducing t h e trichloride with a mixture of hydrogen and ammonia a t elevated temperatures. Analysis of these halides showed them t o contain the theoretical amounts of the two components, and consequently tKey were certain t o be free from the oxyhalides which are notoriously difficult t o reduce.
Present address, Mallinckrodt Chemical Co., St. Louis, Mc.
* Present address, Knolls Atomic Power Laboratory, General Electric Co., Schenectady, N. Y. 8 Present address, Hach Chemical Co., Ames, Ia. 4 Present address, Pittsburgh Plate Glass Co., Newark, N . J.
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T h e calcium metal used in this work was obtained from Metal Hydrides, Inc., and the Electrometallurgical Co. in the form of high purity aggregates which contained less than the indicated amounts of the following impurities: 5 p.p.m. of manganese; 5 p.p.m. of iron; 10 p.p.m. of aluminum; and 0.5y0of magnesium. These aggregates were first broken up by a punch press and then cut t o minus 10 mesh particles in a rotary knife mill. This material was sieved and only particles larger than 50 mesh were used. It was found that by this sieving, essentially all of the small amount of loose calcium oxide coating passed into the "fines," leaving behind a relatively stable but reactive reductant. The oxidants used were resubliined iodine, potassium chlorate, and sulfur which were supplied as reagent grade chemicals by the J. T. Baker Chemical Co. Zinc chloride, which was used as B, eo-reductant, was also obtained from t'hk company. APPARATUS.The reduction was carried out in bombs con&ructed from standard black st,eel pipe by welding a bottom 01 '/,-inch steel plate on one end. The top of the bomb Tvas t,hreaded to receive a standard steel or cast iron pipe cap. Four sizes of the bombs were used. The largest, 4 inches in diameter and 24 inches long, was utilized for large scale production; other X 8 inches, and I X 5 inches. sizes were 2l/2 x 12 inches, Contact of the reaction mixture with the steel Fall o€ the bomb was prevented by containing the reactant,s in a smooth-surfaaed refract'ory oxide liner, 3/le to 3/8 inch t,hick, depending on the size of the bomb. The liners u s u a l l y c o n s i s t e d of a sintered crucible inserted into the bomb and held in place hy filling the narrow annular **'IN I* ;3pace between the crucible and the bomb walls with 'Ip loose lime. A diagram of the apparatus is shown in Figure 1. The loose lime pre.vented leaks in case the crucible cracked during reaction. Figure 1. Loaded Bomb Loose packed liners were also used; these were prepared by placing a mandrel inside the bomb and jolting loose calcium oxide into the annular space between the bomb wall and mandrel by means of a pneumatic jolter. After thoroughly packing the liner by this means, the mandrel was carefully removed, leaving a firm, smooth-surfaced liner. This jolt-packed liner was relatively porous and was used only in the largest scale reductions (1600 grams of CeClS), but even on this scale the yields were reduced by several per cent by this practice. The sintered crucibles were made by jolting the lime or dolomit,ic oxide in a graphite crucible in a manner similar to that employed in preparing the loose packed liners, and then sintering by heating in an induction lurnace to 1750" C. Crucibles so prepared are dense, smooth-surfaced, mechanically strong, and fairly resistant to thermal shock. They absorb moisture from the atmosphere and hence must be stored in closed containers. To provide insulation on the top of the bomb, a l/z- to 3/4-inch lager of lime was tamped into the pipe-cap lid of the bomb, and .to prevent any of this lime from falling into the charge, the liner was covered with a sintered lime or graphite lid. Plumbers seal applied to the threads of the bomb effectively sealed it, preventing loss of volatile components and reducing losses by air oxidation. A few of the first bombs were heated in an induction furnace, b u t a gas-fired soaking furnace was found to be the most satisfactory method of heating the bomb to initiate the react>ion. The reaction mixture for the prepaGENERALPGOCEDURE. ration of the rare earth metals consisted of the rare earth halide, a reduetant-in this study, calcium-and in most cases .a"booster." In the first attempts t,o prepare a rare earth metal, $he reduction of cerous ehloride by calcium alone was used, but on the scale that these experiments were carried out, the heat af react'ion did not give a final bomb temperature that was high enough to allow good separation of slag and metal. An attempt wm m& t o obtain better separation of slag and metal by heating fhe b,omb in a furnace to a temperature above the
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melting points of both of these components, but this met with only indifferent success. An alternat,ive method of raising the final t,emperature of the bomb was to add materials to the bomb which would undergo an exothermic or booster reaction a t the same time the principal react'ion took place. Several important requirements of such a booster reaction are that it should be highly exothermic; it should not introduce impurities into the product; and it should be easily controlled-i.e., it should not fire prematurely. Since calcium was already a compdnent in the bomb as the reductant, one obvious choice for a series of boosters was oxidants which would react exothermically with calcium, such as iodine, sulfur, and potassium chlorate. Iodine was found t o be particularly successful as a booster, because its reaction with calcium was very exothermic and gave as a product calcium iodide which added t,o the calcium chloride from the primary reaction to give a low melting slag which greatly promoted the collection of the metal. Although iodine vapor tended t o diffuse through the packed bomb and slovcilg react. wit,h the calcium at rooni temperaturr, the major port,ion of the booster reacted a t the ignition temperature along with the primary reaction. ,4n example of a second type of booster which was used is anhydrous zinc chloride. This material, which has been used iii cerium and samarium reductions, provides an exothermic react,iorr xith calcium, forming zinc and calcium chloride, and in t,his case, the zinc reacts with the product metal t o give an alloy more easily agglomerated by virtue of its larger mass and possibly its lower melting point, although this last point has not been confirmed. I n cerium reductions, the zinc is not a seriouc: contaminant, for in vacuum casting the zinc distills out of the product metal completely. This is not the case for the several reductions that have been carried out with samarium salts. Other boosters in this same class would be salts of cadmium, antimony, etc. I n preparation for the reduction, a bomb was lined and then filled with a thoroughly mixed charge of rare earth halide, calcium! and oxidant. The charge' was tamped into the bomb and then covered by a refractory lid after m-hich the lined cap was screwed on. These operations were usually carried out in LL dry atmosphere; however, this precaution wae found t o be unnecessary for production runs in the large 4-inch bomb, provided that, the exposure of the anhydrous chloride t o the moisture of the atmosphere was limited. To initiate the reaction the loaded and sealed bomb is placed in a furnace held a t 650" t o 750" C. When the bomb reaches a temperature of about 400' C. the reaction suddenly goes t o completion in a matter of seconds, as evidenced by the sudden rise of the bomb t.emperature. The heat of reaction is sufficient to melt both the d a g and the metal, allowing the met.al t'o collect in the form of a massive cylinder in the bottom of the bomb. As soon as the bomb fires, it is removed from the furnace and allowed t o cool before being opened. EXPERIMENTAL
REDUCTION OF CERIUM. The first small scale reductions of cerium mere made by heatringin a homb a mixture of cerous chloride and 10% more calcium than the stoichiometric amount required for the reaction 3Ca
+ 2CeCL -++
3CaC1,
+ 2Ce
(1)
hfter the reaction subsided, heating was continued until the bomb reached a temperature above the melting point of cerium, on the assumption that such heating would melt the metal and slag and ensure separation and collection of the metal. It was found, however, that the prolonged heating produced a boiling action which resulted in the continued mixing of slag and metal as well as in greater wetting and penetration of t,he liner by the metal. These conditions resulted in poor ingot,s which were highly contaminated with slag, poorly formed, and covered with a tightly adhering crust of slag. To eliminate this prolonged post-reaction heating period with
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its resulting poor yields, a flux of potassium chloride was added to the reaction mixture so t h a t the final slag contained 20 mole % potassium chloride and 80 mole yo calcium chloride. This permitted separation of the slag and metal a t lower temperatures by lowering the melting point of the slag, and by this means, cleaner metal was obtained in yields of from 50 to SOYO, although there was still an appreciable amount of occludkd slag in the metal. Iodine was first investigated as a booster as described previously and for the initial set of reductions, charges of anhydrous cerous chloride and iodine in the ratio of 1 mole of iodine to 1 mole of the chloride were reduced by calcidm in a 5% excess over the stoichiometric amount. Approximately 200 grams of cerous chloride with corresponding amounts of iodine and calcium were reacted in each charge in 2l/2 X 8 inch bombs with sintered lime crucibles as liners by heating the bombs to 700' C. in a resistance furnace or a gas-heated soaking pit. Yields of 58 to 78% of fairly clean castable metal were obtained with an average yield of 65% for seven runs. A study of the effect of varying the ratio of iodine and calcium to cerous chloride was made, using a 312-gram charge of cerous chloride. Varying the iodine and calcium ratios independently, it was found t h a t opt,imum yields of cerium metal were obtained with an iodine-cerous chloride ratio of 0.625 to 1, with 15% exced calcium. Using this ratio, over 5 kg. of cerium were prepared with an average yield of over 93yO, Some work carried out on 1600-gram charges of cerous chloride showed that an iodinecerous chloride ratio of only 0.5 to 1, with 10% excess calcium was required t o obtain yields of over 94% on this scale; the higher thermal efficiency of the larger scale reaction required less booster heat to permit separation of metal from slag.
1
In an effort t o find a substitute for the iodine, which is expensive, other boosters were investigated. Sulfur was tried in the range of 1 / 2 t o 2 moles of sulfur per mole of cerous chloride. Although considerable heat was generated, as evidenced by the outside temperature of the bomb, no separation of the metal and slag occurred. Potassium chlorate was next tested as an auxiliary oxidant with more satisfactory results. The range from 0.10 t o 0.16 mole of potassium chlorate per mole of cerous chloride was investigated. An optimum yield of 83% was obtained with 0.11 mole of potassium chlorate per mole of cerous chloride. Even under t h e best conditions the use of potassium chlorate did not produce cerium metal in yields comparable t o those produced by use of iodine, either externally with respect t o adhering slag, or internally, with respect to inclusions. Since the small scale, 30-gram, reductions of cerium chloride with iodine booster gave yields of only about 70'%, zinc chloride was tried as a booster on this scale reduction, still employing calcium as the reductant, present t o the extent of 10% above the stoichiometric quantity. Zinc chloride was added in such proportions as to make the final product contain 2.8 and 5.5% zinc, respectively, assuming a 100% yield. These two runs gave yields of better than 98%, and on recasting in a vacuum, a n over-all yield of better than 90% was obtained in both cases, with the amount of residual zinc being below 20 p.p.m., the limit of detection by spectroscopic analysis. Iodine was chosen as the booster for large scale, 1600-gram, reductions of cerous chloride because it could be obtained in the requisite purity and condition and because i t was rather easy t o handle. The ratio of reactants described above was used in t h e preparation of over 1000 pounds of 95 to 99% cerium metal, which contained other rare earths and calcium as the impurities. For reductions in the 21/2-inch bomb as used for the preparation of 4 kg. of pure'cerium, the reactants were mixed in the ratio of 0.63 mole of iodine per mole of cerous chloride with a 15% excess of calcium. With this ratio, the metal was obtained as well-formed ingots weighing 150 t o 175 grams with an average yield of 93.5%. The ingots obtained were almost invariably very clean with smooth sides and top as illustrated in Figure 2. The cerium metal thus produced contained from 1 t o 5% calcium and 0.1 t o 1% magnesium. PREPARATION OF LANTHANUM, NEODYMIUM, AND PRASEODYMIUM METALS. The ratio of reactants used with optimum yields for the preparation of cerium was successfully applied for
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the reduction of lanthanum chloride, neodymium chloride, and praseodymium chloride t o metal. Several kilograms of lanthanum and 500 grams of neodymium were prepared with better than 90% yields. The praseodymium metal was produced on a smaller scale in the 11/2-inch bomb with a 76% yield for the 33gram ingot produced in the only reduction attempted. T h e results of the praseodymium reduction compare with the 75 t o 85% yields obtained for cerium reductions carried out on this scale and indicate t h a t when larger charges are reduced, yields equal t o those obtained for cerium will I:c obtained. With respect to yield?, contamination by calcium and magnesium, :+nd quality of ingoti, the results of the above red ue t i o n d d u p 1 ic:i t e d those obtained for cerium n..et:iI. PREPAR \ n o x OF S.111.i~it.11 AXD Y T T R I V M ~ I E : T . ~ I . ;k:stensioti ~. oI' Figure 2. 1'7O-Grani Cerium retluctioll metl,od
-
I11eot
.
.
..
using iodine or potassium chlorate as oxidants for the preparation of samarium metal was not successful. Attempted reductions of samarium trifluoride, samarium trichloride, and samarium triiodide by calcium with a n iodine booster in t h e ratio of 0.63 mole per mole of halide were failures, the dihalide being formed in each case. The reaction between the calcium and iodine and the trihalide proceeded quite vigorously as evidenced by the heat generated in the bomb a t the time of the reaction and the complete collection of reaction products in t h e bottom of the liner, but in no case was metal obtained. This result is in agreement with previous findings t h a t t h e rare earths having a stable divalent state arc extremely difficult t o reduce t o metal (9). Use of potassium chlorate as a booster was also tested in the attempted reduction of samarium trichloride t o metal. I n two runs in a 1-inch bomb using 25 grams of samarium trichloride with 0.1 and 0.167 mole of potassium chlorate per mole of chloride, the bombs were heated to a bright red heat by the reaction. The reaction products were completely fused, but only the dichloride was formed. A small amount of metal was prepared as a calcium alloy by the reduction of samarium dichloride by calcium; however, it could not be recovered for recasting. A zinc alloy of samarium was produced and separated in low yields by the co-reduction of samarium trichloride or trifluoride and zinc chloride by calcium. A small amount of this alloy was recast t o give 2 grams of metal containing greater than 50% samarium. The first experimental reduction of yttrium chloride was carried out on a sample which was 70% pure and contained 18% neodymium, 10% samarium, and 2% gadolinium. Using a ratio of 0.63 mole of iodine t o 1 mole of yttrium chloride with a 10% excess of calcium, 58 grams of this salt were reduced t o give 29 grams of metal for a 790/, yield. However, on 90% yttrium chloride, containing 6% dysprosium, 2% gadolinium, and 2% other rare earths, no ingot was obtained by two attempted reductions on the same scale. I n these latter reductions, the metallic yttrium produced by the reduction was intimately mixed with the slag, but there apparently was not enough heat generated t o fuse the metal, although the reaction proceeded vigorously enough t o permit complete slag collection. The high melting point which has been reported for this metal (81) may conceivably be a factor in this unsuccessful attempt t o obtain fused massive metal. Experimental work is continuing on the preparation of these metals. OF INGOT METAL. As mentioned previously, PURIFICATION
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INDUSTRIAL AND ENGINEERING CHEMISTRY ANALYSES b~ PURIFIEDMETALS TABLE I. TYPICAL
Impurities, P.P.M. Sample Other rare earths Ca Mg Fe Lanthanum Not detected 150
