EXTRACTION of RADIUM from CANADIAN PITCHBLENDE' ALICE KUEBEL Erie Center, University of Pittsburgh, Erie, Pennsylvania
T
HIS paper consists essentially of two parts: first, a description of the commercial process for the utilization of Canadian pitchblende; and second, a brief outline of some of the work we'have done in the laboratory of the Erie Center of the University of Pittsburgh in reproducing the commercial extraction of radium on a small laboratory scale. The largest known deposit of pitchblende in Canada is found in the Great Bear Lake district of the Northwest Territories. Ore is hand-selected a t the minesthe richest pitchblende being characterized by a thick, black, botryoidal formation. It is crushed and then concentrated by a gravity-water classification process in which the heaviest ore, also the richest, is separated from the lighter ores and metals. he concentrate is sent by boat and rail to the radium refinery a t Port Hope, -0ntario. Because approximately seven tons of chemicals are needed to treat a single ton of 'concentrate, the refinery is located closer to the supply of chemicals than to the source of the ore. The composition of the ore varies with diierent veins found. a t the mine; each batch of concentrate must be analyzed before i t is sent through the refinery to ascertain the exact amounts of chemicals which will be necessary a t each step in the treatment of the individual batch. Details of operation are changing constantly, and the procedure followed is not precisely the same from month to month. Besides radium, the concentrate always contains appreciable, but varying, quantities of uranium and silver. The extraction process, to be economically feasible, must include a separation
for these two elements. Dr. Marcel Pochon, who studied under the Curies, adapted their process for the extraction of radium to the special requirements for the commercial utilization of Canadian pitchblende. The first operation a t the Port Hope refinery is a thorough roasting of the ore in a tall Herreshoff furnace to decompose carbonates and sulfides. Carbonates and sulfides ate removed to'prevent excessive frothing in the sulfuric acid leach which is the first of the wet treatments. Two tons of bagged concentrate are fed each day into the top of the Herreshoff furnace a t 900°F.; then they fall slowly downward over approximately twelve revolving grates to the bottom of the furnace where the temperature- sometimes reaches 1400°F. Oil is used for fuel. The product of the
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Presented before the student meeting of the Division of Chemical Education at the ninety-ninth meeting of the A. C. S., Cincinnati, Ohio, April 10, 1940.
FIGURE1.-A
VAT IN
THE
URANIUMREFINERYBUILD-
ING. CRUDEURANIUMSULFATEIS STEAM-BOILED WITH
NA.CO~TO PRECIPITATE METALLIC IMPURITIES
Herreshoff furnace is reroasted with common salt to convert silver to silver chloride. In the form of the chloride, silver may be completely extracted from the ore with sodium thiosulfate. Two salt roast furnaces, also using oil as fuel, are used, but extremely high temperatures must be avoided to prevent loss of silver chloride by volatilization. After the two preliminary roasts have been com-
Air mart
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Sulfuric acid leach
Residue Crude inrolublc rulfates
Filtrate
Crude uranium sulfate
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1 Hypo leach
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+ NaaC01
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I
Ore Residve
Filtrate
1
Solution of Ag I
Metallic impllrities
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Caustic soda leach
Filtrate Sodium uranyl carbosate
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1
ore Residue
Filtrate
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of crude inof soluble Pb soluble sulfate. end hydroxides
Precipitate of emde sodium uranate
J. 7 Ruidue of insoluble carbooaten
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Filtrate of soluble sulfates and carbonates
J. Hydrochloric add I
Ore Residme
Filtrate of crude RaBa(C1.).
4
NaCOa
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Residue of crude RaBa(COa),
HBr
4
1
Filtrate of soluble earbonntcs
. . . . + Bas + Ba(OH)s
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Residue of metallic impurities
Filtkte of purified RaBa(Br&
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Fractional crystallization
pleted, the ore is sent through a series of wet treatments. The first of these is a sulfuric acid leach which extracts uranium as crude uranium sulfate. Approximately four hundred pounds of ore are placed in each of six tanks with a 1:l solution of commercial concentrated sulfuric acid; i. e., a solution which is 50 per cent by weight of concentrated acid. The leach takes six hours, and during this time a small amount of hydrated barium chloride is added. Barium must be added in some form to act as a carrier for the precipitation of the minute quantities of radium. The chemical properties of barium and radium are almost identical, and a radium salt is readily co-precipitated by the corresponding barium salt so that radium sulfate and barium sulfate precipitate together. More sodium chloride is also added here to precipitate any silver which might have escaped the salt roast and might still be in the form of a water-soluble compound. Sodium nitrate used to be added for the purpose of oxidizing the uranium, but it is no longer considered to be a necessary step. This sulfate mixture is filtered by suction through canvas. The filtrate of bright green uranium sulfate is made as nearly neutral as possible to minimize corrosion, and then is piped to the uranium refinery. From the crude uranium sulfate, a variety of compounds are prepared; these include the nitrate, the acetate, and the oxide, which are used chiefly in photography and in ceramics. The ore residue from the sulfate filtration contains the insoluble radium-barium sulfate and silver chloride, but also contains many impurities such as lead, calcium, silica, etc. The residue is taken from the filter bins in small hand carts to the acid-resistant Haveg tanks for the second step in the wet process.. This consists of the extraction of silver chloride tbith sodium thiosulfate. Sodium thiosulfate is added in proportion to the amount of silver the ore is known to contain. The leach is filtered, and the filtrate which contains the silver is treated with sodium sulfide to precipitate silver sulfide. Crude silver sulfide is sold to.silver refineries in the United States. Although i t used to average around 1500 ounces per ton, the ore now being used is much poorer in silver, and i t now averages only from 400 to 1000 ounces per ton. The residue from the hypo leach, which contains radium-barium sulfate, also contains enough lead sulfate to necessitate a separate step for its extraction. The lead is dissolved by boiling with a 15 per cent solution of sodium hydroxide for from one to two hours. The mixture is filtered, and the residue which still contains the radium-barium sulfate is washed thoroughly. The filtrate is discarded. The wet ore is next boiled for four hours with one and one-half times its weight in sodium carbonate to convert the radium-barium sulfate to the carbonate. From this point on, all steps are for the purification of the radium-barium compound, and finally for a separation of the radium salt from its barium partner. The carbonates are filtered, and the insoluble radium-barium carbonate residue is decomposed with hydrochloric acid to form a solution of crude radium-barium chloride. Now, for the first
FIGUREZ.-TANKS USED I N THE EXTRAC~ION OF RADIUM FROM THE ORERESIDUE WHICHIS LEFTAFTER REMOVAL OF URANIUMAND SILVER
time, the radium is kept in solution. That is, up to this stage, impurities have been dis~olved~away from the radium, and now radium is dissolved away from most of the remaining impurities. The succeeding steps in the purification of the crude radium-barium chloride in the refinery building involve a double precipitation as the carbonate, redissolving the carbonates in hydrochloric acid, and again a final precipitation as the carbonate. The insoluble residue of radium-barium carbonate is the product which is now transferred from the refinery to the laboratory. The carbonates are boiled with hydrobromic acid in open granite basins over coal-gas burners. The mixed bromides are purified further by additions of barium hydroxide and barium sulfide, which precipitate the last traces of accompanying impurities, chiefly iron and lead. Finally, the radium is separated from the barium (though not completely) by a long and systematic series of fractional crystallizations of the mixed bromides. Saturated radium-barium bromide liquor is put into large quartz evaporating dishes in a cooling
room just off the main laboratory. Because radium bromide is slightly less soluble than barium bromide, the first crystals contain more radium and less barium than the mother liquor. If these first crystals are separated from the liquor and are redissolved, the resulting solution, when saturated, will yield crystals which contain a higher percentage of radium than the first crop. The large quartz dishes (which, though expensive, are preferable to glass because of greater durability and purity) are arranged systematically on tables in the cooling room. The first crystals of mixed radium and barium bromides contain, very roughly, 500,000 parts of barium to one of radium. The final crystals contain approximately one part of barium to nine of radium; and these last crystals are sealed, 100 milligrams a t a time, into glass tubes each valued a t $3000. The tubes are stored separately in lead cylinders which fit into shelves inside a lead safe. Radon emanation is not removed from the tubes, but stagnant air above the safe is occasionally drawn out by suction pumps and bottled. Radium used in hospitals, or on the dials of watches and airplane instruments, or in the detection of internal flaws in metal parts, is not radium alone, but it is this radium-barium bromide combination containing enough selected impurities to produce a pronounced glow. The glow which is characteristic of a radium compound is caused mainly by the agitation of impurities present in the compound rather than by alpha, beta, and gamma rays alone. Throughout the entire process a t Port Hope, an electroscope is used to detect valuable amounts of radium which might otherwise be discarded with worthless residues or solutions. ..At the present time, a research division is working on the residue which remains after the second radium-barium carbonate is treated with hydrochloric acid. Heretofore, the residue had been dumped along the sides of an inlet of Lake Ontario. In laboratory work a t the Erie Center, University of Pittsburgh, we have separated the uranium, the silver, and a very small amount of radium as mixed radiumbarium bromide from 1050 grams of Canadian pitchblende. In essential details, we followed the process
FIGURE LARGE QUARTZ EVAPORATING DISHESI N T H E COOLINGROOM.THE DISHESCOST % . % APIECE, B U T ARE PREFERABLE TO GLASSBECAUSE or GREATERDURABILITY AND
PURITY
FIGURE '&-LEAD SAFE(LOWERRIGHT) I N WHICH GLASSTUBES,CONTAINING 100 MG. OF RABA(BR& EACH.A m KEFT IN INDIVIDUAL LEADCYLINDERS
occurred with the two later batches; the gain may possibly be explained by oxidation. The resulting 105.8 grams were transferred to a beaker and treated with a solution of 60 grams of concentrated sulfuric acid in 60 grams of water. This mixture, greenish brown in color, frothed considerably; the evolution of chlorine and hydrogen chloride gases were noted by their odors. Approximately one gram of barium chloride was added producing a chalky white precipitate of barium sulfate; and later five grams of sodium nitrate were stirred into the mixture with no noticeable reaction except a slight evolution of nitrogen dioxide. Upon settling, the supernatant liquid of crude uranium sulfate was a clear dark green; and the ore a t the bottom of the beaker was gray and muddy. We filtered this by suction through filter paper in a Biichner funnel. The filtrate of crudeuranium sulfate was put aside; the residue in the funnel was washed thoroughly to free it from soluble chlorides and sulfates. For the extraction of silver, we added 7.5 grams of sodium thiosulfate in approximately 25 cc. of water to the ore residue. After settling overnight, the hypo leach was suctionfiltered and washed. Silver was precipitated from the filtrate with sodium sulfide; and silver sulfide was separated from the solution by filtration in an ordinary funnel. Our yield of the dry sulfide amounted to 4.5 grams, of which 3.9 grams (87.06 per cent) were silver. The ore residue (46.5 grams) after filtration of the hypo leach was boiled for one and one-half hours with 200 cc. of a 10 per cent solution of sodium hydroxide. The hydroxide leach was suction-filtered; the filtrate containing lead and silica was put aside, and the residue of insoluble sulfates was dried in an oven at approximately 105°C. The dried residue containing crude radium-barium sulfate weighed 40.0 grams. To it were added 60 grams of sodium carbonate in 275 cc. of water, and this mixture was boiled for six hours under a reflux
desaibed by Dr. Pochon as in use at the Port Hope refinery.2 Considerable guessing had to be done with reference to quantities of chemicals we should use at each step throughout the process because we had no way of knowing the exact composition of our ore. There were three batches altogether: 100 grams, 500 grams, and 450 grams. The first batch consisting of 100 grams wilt be described here (partly, a t least) because it was the most successful. The preliminary roast and the salt roast were carried out in a large evaporating dish over a Bunsen burner. The ore was kept at a cherry-red for approximately one-half hour, and then was allowed to cool before being weighed and reroasted for two and a batf hours with 7.5 grams of salt. Loss in weight during the iirst roast amounted to 3.7 grams, but during the salt roast the ore gained two grams in addition to the weight of the salt. This increase in weight with the salt roast also 1 Pomo~, "Radium recovery from ores," Tramactions of the American Instifufeo j Chemical Engineers, Vol.33, No. 2 (1937).
FIGURE 5.-CONTRASI. PLATI: EXPOSED EIGHTDAYSTO CANADIAN "BOTRYOIDAL" PITCHBLENDE
