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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 22, No. 5
Preparation o f Metal Powders bs - Electrolysis of Fused Salts' I-Ductile Uranium F. H. Driggs and W. C. Lilliendahl WESTINGHOUSE LAMPCOMPANY, BLOOMFIELD, N. J.
A survey of the literature reveals the fact that meager a spongy mass c o n t a i n i n g H E first attempt t o data exist concerning the properties of uranium of a small metallic crystals. prepare metallic urahigh degree of purity. Uranium metal powder was Fere6 (d), by electrolysis of nium was made in 1842 prepared by electrolysis of potassium uranous fluoride an aqueous solution of uraby Peligot ( I d ) , who reduced in fused calcium and sodium chlorides. The powder nium chloride using a mercury uranium chloride by means was fused to a coherent mass in an evacuated highcathode, obtained an amalof sodium in a glazed porfrequency induction furnace. A chemical analysis gam from which he distilled celain crucible. The sodium of the metal showed carbon (0.06 per cent), iron (0.05 off the mercury in uuczto. was covered with layers of per cent), and silicon (0.01 per cent) to be the only imThis product, owing to its dry potassium chloride and purities present. extremely fine particle size, uranium chloride and the reChemical properties of the metal are listed, together burned spontaneously in the action was started by heatwith its physical constants such as melting point, air. Pierle and Kahlenberg ing the crucible externally. density, hardness, and working properties, electrical (1.5) investigated the possiZimmerman's (16) method conductivity, and temperature coefficient of resistance. bility of the electrolysis of was similar e x c e p t t h a t Samples of this metal in the form of wire and sheet uranium salts in non-aqueous sodium chloride was substiare illustrated. solvents. Solutions of uranyl t u t e d f o r t h e potassium c h l o r i d e , acetate, sulfate, chloride in covering the mixture. According to Moissan (10) this method yields a and oxalate, as well as the anhydrous UC14,were electrolyzed powder which is iontaminated with small amounts of sodium in pyridine, ether, amyl alcohol, and ethyl acetate, but no as well as combined nitrogen. Moissan used the more stable satisfactory yields of metal were obtained. These authors double salt UC14.2KaC1, which was heated with sodium in found the electrolysis of fused salts, such as potassium sodium uranate, potassium nitrate and uranyl nitrate, silver a closed iron tube. Moissan (11) also attempted the reduction of uranium nitrate and uranyl nitrate and the green salt, potassium oxide (U308) with sugar charcoal in an electric furnace and uranous fluoride, unsatisfactory for the production of the obtained a metal containing some carbon and other im- metal. purities. Aloy (1) reduced the oxide UOz with sugar charElectrolytic Preparation of Uranium Metal Powder coal, the resulting product being contaminated n-ith carbon. It was found that a very satisfactory uranium powder The reduction of UOz by means of aluminum has also been attempted (?). Burger (3) claims that U03 may be reduced could be produced by electrolysis of fused baths. Many to metallic uranium by means of calcium vapor in vacuo. bath mixtures of varying composition were investigated t o Fischer and Rideal ( 5 ) , using the methods of Peligot and get some general idea of their effect upon the yield and Zimmerman, obtained a brown or black powder, which they purity of the metal. Since it was desirable that the elecstate showed no conductivity after being pressed and heat- trolytic preparation of the metal should be carried out a t a treated in hydrogen. Lely and Hamburger ( 8 ) , with addi- comparatively low temperature, mixtures of salts were used tional refinements, such as the production of a dense uranium and the principal portion of the baths consisted of the halides chloride and careful sublimation of sodium in vacuo,obtained of the alkali or alkaline earths with only a small proportion a powder which, when pressed and heated to 1400' C., gave of the uranium salt added. The following salts or mixtures a brown metal easily attacked by hydrochloric or nitric acid. of two or sometimes three of them in various proportions Moore (13), using the same method, obtained a brown were tried as electrolytes: sodium chloride, sodium fluoride, powder which he fused in an arc furnace in an atmosphere potassium chloride, potassium fluoride, and calcium chloride. of argon. The molten metal ran out of the unfused portion The following uranium compounds were added after the and solidified on a water-cooled table of iron or monel metal. bath had become molten and the electrolysis was started: Fogg and James (6) prepared uranium by reduction of the uranium trioxide (UOs), potassium uranous fluoride (KUF5), chloride with calcium in an alundum-lined bomb t o prevent uranyl chloride (U02C12),and uranous chloride (UCld. I n contamination of the metal with iron. An analysis showed all baths where uranium trioxide or uranyl chloride was used the metal to contain 0.57 per cent iron and 0.09 per cent as the source of uranium, the principal product deposited a t carbon. The metal was obtained in the form of large fused the cathode consisted of black shining crystals of the dioxide globules, which were said to be "somewhat brittle and hard (UOz), while with potassium uranous fluoride or uranous and not easily worked." A reduction using sublimed calcium chloride the deposit, which was loosely adhering, consisted lowered the iron content to 0.01 per cent, but no statement of metallic uranium or a mixture of uranium and the brown oxide which mas apparently formed by oxidation of the was made concerning its other impurities or workability. Moissan (12) tried to produce uranium metal by elec- deposited metal. These results seem to indicate that uranyl trolysis of the double sodium uranium chloride in an atmos- salts are unsatisfactory for electrolytic reduction, since they phere of hydrogen using carbon electrodes. He obtained appear to ionize as the uranyl (UOz"-) ion and are deposited upon the cathode as the dioxide. On the other hand, salts Presented before the Division of Industrial 1 Received March 3 , 1930. of quadrivalent uranium, such as uranous chloride and and Engineering Chemistry at the 79th Meeting of the American Chemical potassium uranous fluoride, apparently yield the uranous Society, Atlanta, Ga , April 7 to 11, 1930.
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I,VDUSTRISL B Y D ENGIl%ERISG CHEMISTRY
(I?’+’+) ion in a molten bath and as a result may be electrolyzed to metallic uranium. The most satisfactory results were obtained using a bath consisting of equal parts by weight of sodium chloride and calcium chloride to which \vas added the green salt (KUFj). The apparatus for this electrolysis consisted of a graphite crucible about 6 cni. in diameter and 15 cm. in depth, inside measurements, with a wall thickness of 1 to 2 cm. The
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crucible as a n anode prevented the appearance of the troublesome “anode effect.” Treatment of Metal Powder The deposit adhering to the molybdenum cathode consisted of a gray spongy mass interspersed with solidified salt which served to protect the metal from oxidation while it cooled. After cooling, the mass was broken off the cathode and dissolved in water. KO corrosion or alloying effect could be observed on the molybdenum cathode and, as the chemical analysis will show later, no molybdenum was present in the adhering metal. Horn-ever, with nickel cathodes the product was contaminated, analyses showing as high as 17 per cent nickel a t times. The alloying effect with iron was even more pronounced. Owing t o the high density of the uranium powder, any traces of calcium fluoride or other insoluble salts, as well as the finer grained portions of the pom-der, could be washed away, leaving a fairly pure, coarse gray product. This was washed with 5 per cent acetic acid to remove traces of calcium carbonate and oxide and was then filtered, washed with alcohol and ether, and dried. This powder, when examined under the microscope, showed no traces of oxide or other foreign material and the individual particles were silver-white in appearance. The powder was either pressed and heat-treated immediately or preserved in tightly stoppered flasks until ready for use. This product was not markedly pyrophoric, but it was found that when the powder contained a small amount of finely divided metal it was likely to ignite spontaneously after being thoroughly dried; thus no effort was macle t o recover the finer portions of the Figure 1-Electrolysis Cell metal powder owing to this tendency toward rapid oxidation. A--hIolybdenum cathode B-Carbon crucible Fusion of t h e Metal C-Sheet-nickel anode connector D-Sil-0-Cel Cylindrical pellets were pressed from the uranium powder E-Cast-iron electric furnace core F-Sichrome furnace windine in a hardened tool-steel mold using a pressure of about G-Sheet iron 12,000 kg. per sq. cm. It was found that the pressed pellet, crucible served as the anode, electrical connection being upon removal from the mold, was likely to ignite spontamade by means of a nickel strip around the outside of the neously upon exposure to the air. This was probably due crucible. The cathode consisted of a molybdenum strip either to the heat developed by friction in pressing or, possi0.05 cm. thick and 1.0 em. wide suspended in the center of bly, to the fact that fresh surfaces of the metal were broken the crucible and extending to within 2.5 cm. of the bottom. up and exposed when the pellet mas pressed. By removing The accompanying diagram (Figure 1) s h o w a (TOSS section the pellet from the mold immediately after being pressed of the electrolysis cell and heating unit. and plunging it into ether, this tmdency to take fire mas The sodium chloride-calcium chloride mixture was added prevented and it was kept in this manner until ready to be t o the crucible and brought to fusion by means cif a n electric placed in the heat-treating bottle. furnace surrounding the crucible. The melting point of this The heat treatment of the pellet was carried out in an bath was approximately 625” C. An effort v a s made to induction heat-treating bottle of exactly the same type as maintain the temperature of the bath within a few degrees used for the treatment of thorium buttons in this laboratory of 775’ C. during a run. When the bath was thoroughly and described in previous publications ( 9 ) . The pellet fused, the cathode was loivered into the melt and a current was supported in a thoria-lined dish, and upon heating of 30 amperes was used, with a potential drop between the it abo1.e its melting point the fused metal flowed out and electrodes of about 5 volts. The uranium salt (KUFS) collected in a ring surrounding the bottom of the unfused was then added t o the bath in small amounts, as there was portion. Although the metal powder was very pure, some some difficulty due to formation of brown oxide unless the oxidation always takes place during handling and treating, salt was quickly fused. The bath now consisted of approxi- especially if very slow and careful degasification is not mately 250 grams of sodium chloride, 250 grams calcium employed. A shell consisting of a small amount of slag chloride, and 30 grams of potassium uranium fluoride. As and unfused metal always remained and was discarded. soon as the uranium salt was added, metallic uranium began Chemical Analysis of Fused Metal t o appear upon the cathode in a treelike deposit, which The fused metal was analyzed for the following constitucontinued to grow until it had reached a thickness of approximately 2.5 cm. in about 45 minutes. At this point the ents: uranium, silicon, carbon, iron, vanadium, and molybcathode was slowly lifted from the bath, a new one inserted, denum. URawunf-Total uranium was determined gravimetrically and the electrolysis was continued. Fresh additions of the salts xere made a s the bath became depleted. The cathode by converting the metal to U308 and calculating the percurrent density corresponded to approximately 150 amperes centage uranium, making corrections for impurities found. per square decimeter. Current densities as high as 3000 URAUIUTY SAMPLE u308 amperes per square decimeter were tried, but no pronounced Gram Grams Gram Per cent 0 940 1 1071 0 9389 99 88 change was noticed in the appearance or particle size of the 0 8929 1 0521 0.8923 99.92 metal powder. The use of the entire inner surface of the Av. 99 90
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Literature Cited
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(9) hfarden and Rentschler, I X D . ENO.CHEM,,19, 97 (1927). (10) Moissan, Compf. rend., 113, 2 (1891). (11) Moissau, IDzcf., 116, 347 (1893). (12) l l o i s s a n , I b i d . , 122, 1088 (1896). (13) Moore, Trans. A m . Elecfrochem. Soc., 43, 317 (1923). (14) Peligot, Ann. chim. phys., [3]. 6, 5 (1842). (15) Pierle and Kahlenberg, J . P h y s . Chem., 23, 517 (1919). (16) Zimmerman, A n n . , 213, 290 (1882); 216, 1 (1883). (17) Zimmerman, Ber., 17, 2739 (1884).
(1) Aloy, Bull. SOC. chim., [3] 26, 344 (1901). (2) Aloy, A n n . chim. phys., 171 24, 412 (1901). (3) Burger, Dissertation Basel, p. 19 (1907). (4) Feree, Bull. SOC. chim., [3] 25, 622 (1901). ( 5 ) Fischer and Rideal, Z . anorg. Chem., 81, 170 (1913). (6) Fogg and James, I N D . ENG.CHEM., 18, 114 0 9 2 6 ) . (7) Giolitti and Tavanti, G a m chim. i f a l , , 38, 239 (1908). (8) Lely and Hamburger, Z . anorg. Chem., 87, 209 (1914).
Properties of Strontium-Tin Alloys' K. W. Ray DEPARTMENT OF CHEMISTRY, UNIVERSITY OF Iowa, IOWACITY,IOWA
E V E M L strontium-tin alloys have been prepared (3, 5 ) , but no systematic study of the properties of these alloys has been made. The thermal diagram of the magnesium-tin system ( 2 ) as well as the thermal diagram of a,,portion of each of the calcium-tin ( 1 ) and the barium-tin (G) systems have been worked out, but there is no record of any such work for the strontium-tin system. According to Tammann's rule ( 7 ) that an element forms a compound with all the elements of a natural group or with none of the members of the group, tin should form compounds with strontium, since it does with magnesium, calcium, and barium. However, the crystal structures of barium and of strontium are different ( 4 ) , and crystal form seems to be somewhat related to compound formation.
S
Figure 1-Current
The electrolysis was carried out a t the lowest temperature a t which the contents of the crucible could be kept liquid. The salt bath was usually entirely liquid a t 600" C., but in some cases the temperature had to be kept much higher than this to maintain the alloy in a molten condition. The alloys prepared in this way contained appreciable amounts of sodium, which was largely removed from all alloys containing less than 25 per cent strontium by remelting the crude alloy under a bath of pure strontium chloride. The extent of the purification is shown in the following case: An alloy as first made contained 20.04 per cent strontium and 0.21 per cent sodium, while after its fusion with strontium chloride it contained 18.95 per cent strontium and 0.04 per cent sodium. This remelting was done in a fire-clay crucible heated in a gas-fired furnace. The thermal diagram JTas based on the data obtained by methods of thermal analysis as well as by microscopic study of the alloys. I n taking a cooling curve, the metal was placed in a Pyrex test tube which was wrapped with asbestos paper and then placed in a vertical electric tube-furnace. The temperature was raised to about 100" C. above the melting point of the alloy and the current shut off. The cooling rate was recorded automatically by a Brown electric recording pyrometer. A few drops of a paraffin oil were placed in the tube before heating t o preyent oxidization of the sample.
Efficiency
Even though strontium-tin alloys have no industrial use these alloys should have as much interest from the point of view of theoretical metallurgy as any other alloy system. They might also have sufficient bearing on problems of both theoretical and practical interest to justify their investigation. The purpose of this investigation, therefore, was to prepare a large number of strontium-tin alloys, to study their properties, and to construct as much of the thermal diagram as possible from the data obtained. Experimental Methods
The strontium-tin alloys were prepared and tested by methods similar to those used by Ray and Thompson (G) for the barium-tin alloys. The alloys were produced by the electrolysis of a mixture of fused sodium and strontium chlorides over molten tin, in a chromium-plated iron crucible. The molten tin was made the cathode and a carbon rod which extended into the salt bath served as the anode. Chlorine was evolved a t the anode while strontium was deposited in the liquid cathode metal. The composition of the bath varied from 60 to 90 per cent strontium chloride by weight, the remainder being sodium chloride. The crucible and contents were heated by one or more gas burners. 1
Received March 6, 1930.
Figure 2-Thermal
Diagram
hlost of the alloy samples were studied microscopically (1) as chill-cast; ( 2 ) after annealing several hours a t 240" C.; and (3) after annealing a few hours near the melting point followed by rapid cooling. It was found necessary to seal the samples in Pyrex tubes before annealing to prevent oxidation. The specific gravity was determined only on the chill-cast alloys. A section of the ingot prepared by pouring the molten metal into a steel mold was smooth-polished on a fine emery wheel. The piece mas then weighed in air, and
