Extraction of Uranium from Canadian Pitchblende' ALICE KUEBEL2 University of Pittsburgh, Pittsburgh, Pennsylvania
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HIS paper contains a description of a laboratory procedure for the extraction of uranium from Canadian pitchblende. It is subdivided into two parts to include: (1) the removal of uranium in the form of crude uraninm sulfate from pitchblende concentrate, and (2) the steps for the purification of the crude uranium sulfate. The pitchblende concentrate which was used as the starting material was purchased from the Radium Refinery of the Eldorado Gold Mines Limited a t Port Hope, Ontario. It came originally from mines in the Great Bear Lake region of the Northwest Territories. In laboratory work under the direction of Dr. L. A. Goldblatt a t the Erie Center of the University of Pittsburgh, 1050 g. of pitchblende concentrate were treated for the extraction of uranium as crude uraninm sulfate, of radium as a mixed radium-barium bromide salt, and of silver as the sulfide. The process followed for the removal of these constituents was, in essential details, the same process which is in use in the radium Refinery a t Port Hope, Ontario. The 1050 g. of pitchblende were divided into three batches consisting of 100 g., 500 g., and 450 g.; each of these was treated separately for the removal of crude uranium, radium, and silver compounds. Because all of the uranium which was finally prepared originated in the 500-g. batch of pitchblende concentrate, this batch will be described in some detail-primarily with respect to the extraction of its uraninm. The ore was placed in a porcelain evaporating dish and heated in a muffle furnace a t a cherry red for a total of one and one-half hours. During this time, there was considerable smoking over the surface of the hot ore and a t the end of the roast, it was found that there was a total loss in weight of 25.8 g., or 5.16 per cent of the weight of the original material. The purpose of this preliminary roast was to decompose any organic matter which might have been present and also to decompose metallic carbonates and sulfides. These last two constituents, if left in the ore, would cause mechanical difficulties (i. e., effervescence) in the sulfuric acid leach which is the first step in wet treatment of the ore. A second roast was then carried out for the same length of time. In this roast, the ore was heated with approximately 38 g. of sodium chloride to convert any silver present in the free or combined
Presented before the Division of Chemical Education of the American Chemical Society, lOlst meeting. St. Louis, Missouri, April 10, 1941. Present address: National Bureau of Standards, Washington. D. C.
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states to silver chloride. (The chloride was later removed by a sodium thiosulfate leach.) During the salt roast, the ore gained 22.5 g., in addition to the weight of the salt. For the removal of uranium which was present in the ore mainly as U308,the product of the two roasts was treated with 185 cc. of sulfuric acid (1 :1). A solution of 25 g. of sodium nitrate in GO cc. of water was added to oxidize the uranium. The mixture was boiled for about an hour and then was permitted to settle overnight before filtering by suction through a Biichner funnel. After the residue had been thoroughly washed, it was spread out over glazed paper to dry. The loss in weight through the sulfuric acid leach was 107 g.-leaving a residue of 427 g. Although the sulfuric acid leach was for the extraction of uranium, the lossin weight could not be taken as indicative of the true quantity of uraninm in the ore because impurities such as iron, aluminum, etc., were also removed from the ore by the acid leach. A second sulfuric acid leach was carried out to insure a complete removal of uranium. Two hundred and fifty cc. of 1 : l sulfuric acid were used in the second leach. The filtrates and washes from both leaches were combined and partially decomposed with sodium carbonate to precipitate some of the metallic impurities present and to form a solution of sodium uranyl carbonate. The entire mixture of metallic impurities, nranium sulfate, and sodium nranyl carbonate was set aside for approximately one and one-half years before the uranium salt was finally purified. Because the solntion containing uranium had been permitted to settle over so long a time, it could be assumed that the solution was sufficiently homogeneous that a sample of it might be purified and the yield of uranium measured quantitatively. Then the entire solution could be refined more easily by knowing in advance the approximate quantities of reagents which would be required, the mechanical difficulties which might be expected, and the probable yield of uranium. Altogether, there were about 1800 cc. of the crude sodium uranyl carbonate solution produced from the 500-g. batch of ore. For the preliminary analysis, 100 cc. of the solution were carried through steps for purification; and the final greenish black product of U308 amounted to 1.88 g. The procedure used for purification is described as it was carried out with the main analysis. Fifteen hundred cc. of crude uranium sulfate were placed in a 2-liter beaker. Although the solution bad
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been partially decomposed with sodium carbonate, its pH was approximately 6 indicating that most of the uranium was present as uranium sulfate rather than as the desired compound of sodium uranyl carbonate. Forty g. of anhydrous sodium carbonate were dissolved in water and then added to the mixture in the beaker to produce a thick pale green precipitate and effervescence (COz). Upon continued additions of sodium carbonate, the precipitate became a reddish brown color. The pale green precipitate contained uranium and was soluble in excess sodium carbonate. Metallic impurities such as iron, copper, manganese, and nickel precipitated when the excess carbonate was added. These last impurities were removed by filtration through a Biichner suction funnel. The filtrate of sodium uranyl carbonate was decomposed with approximately 120 cc. of concentrated sulfuric acid and the solution was warmed to drive off all carbon dioxide. To precipitate the uranium as sodium diuranate (NaaUa07),sodium hydroxide pellets were added. Because both the sulfuric acid and the sodium hydroxide which were added were in a concentrated form, sodium sulfate crystallized in undesirably large quantities throughout the orange uranium precipitate. The supernatant liquor in the beaker was decanted and replaced by adding distilled water in hopes of dissolving the sodium sulfate crystals away from the uranium precipitate. The sodium diuranate was centrifuged and washed several times before being dissolved out of the centrifuge cups with 1:3 sulfuric acid. To the yellow solution of nranyl sulfate was added a solution of sodium sulfide which produced effervescence (HzS) and a reddish brown precipitate of impurities. At this point, the pH of the solution was kept below 7. Otherwise, uranium would have been carried down with the precipitate as uranyl sulfide. After filtration to remove the suliide impurities 260 g. of NaOH pellets were dissolved in water and added to the yellow solution of nranyl sulfate to reprecipitate orange sodium diuranate. The mixture was centrifuged, the precipitate was washed and then dissolved out of the centrifuge cups with 1:3 sulfuric acid. Sodium diuranate was precipitated once more, centrifuged, washed, and redissolved in sulfuric acid. Instead of being clear yellow (which is the normal color of a solution of uranyl sulfate) the solution contained a finely divided precipitate which gave it a reddish brown color. (It is possible that the HzS which was formed when Na2S was used to precipitate impurities reduced some of the uranium from a valence of +6 to a valence of +4; and that the reddish brown ~ r e c i ~ i t a twas e the red or brown oxide of uranium represented by the formula UOz.) The sulfate solution
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was neutralized with ammonium hydroxide and an excess of reagent was added to precipitate ammonium dinranate. This ammonium compound and the corresponding sodium compound are similar in nature. They may be varying shades of yellow or orange depending upon the amount of base used in precipitation. (The greater the amount of reagent, the darker the color of the precipitate.) They settle out of solution readily, leaving a clear, colorless supernatant liquor, but they are very difficult to iilter and to wash because of their compactness. It is even more difficult to wash them thoroughly when they have been centrifuged, because in this case the precipitate is packed together more tightly. For this reason, the ammonium dinranate was washed by decanting the supernatant liquor and replacing i t with distilled water a total of six times. The precipitate was stirred thoroughly each time. By a titration with standardized hydrochloric acid, it was found that the normality of the fifth wash (with respect to NKOH) was 0.0025. After the sixth wash, the precipitate was dissolved in sulfuric acid. Again the solution contained an undesirable reddish brown precipitate, and so was filtered to yield a clear yellow filtrate of uranyl sulfate. The residue was ignited and after ignition, i t had the color and texture of UaOs. Ammonium hydroxide was added to the filtrate to precipitate ammonium diuranate a second time. This precipitate was washed eight times by decanting the supernatant liquid and then stirring the precipitate and washing down the sides of the beaker with distilled water. In order to dry the precipitate it was placed in a large evaporating dish over a hot-water bath. When it was nearly dry it was put on a clock glass in an oven a t 100' and brought to constant weight. The yield of ammonium diuranate from 1500 cc. of crude uranium sulfate was 42.3 g. Uranosic oxide (U30s) was prepared from the ammonium compound by ignition in a mnfle furnace. The yield of oxide was 34.9 g. This yield of oxide (UsOs) from 500 g. of pitchblende is probably low for several reasons. It is reasonable to assume that some uranium was carried down when metallic impurities were precipitated with sodium carbonate and with sodium sulfide. Some uranium was probably lost when the mixture of uranyl sulfate and the red precipitate was filtered, because in this case uranium may have been left behind in the residue. Also, because sodium hydroxide and ammonium hydroxide form corresponding carbonates on exposure to air, and because sodium uranyl carbonate is water soluble, some uranium was lost in decanting the supernatant liquid from above the precipitates of sodium and ammonium diuranates
