Radiochemical Determination of Radium in Uranium Milling Process Samples ,
HENRY G. PETROW,’ OSCAR A. NIETZEL,2 and MICHAEL A. DeSESA3 Raw Materials Development laboratory, Nafional l e a d
b A carrier-free method is described for the determination of radium-226 in mill tailings solutions and other liquid and solid samples from the uranium milling process. Radium is carried on lead sulfate to effect a separation from the bulk of the sample. Purification from other naturally radioactive nuclides is achieved by conversion of the lead sulfate to the carbonate, dissolution of the carbonate in nitric acid, and selective precipitation of lead and radium from fuming nitric acid. The radium is separated from the lead by adsorption of the lead on Dowex 1-X8 anion exchange resin from 1.8M hydrochloric acid. Radium is then determined by conventional alpha counting techniques.
R
ADIUM-226has been the isotope of greatest interest in the problem of radiochemical pollution of uranium mill waste streams. Several procedures for the determination of radium-226 have been published (1-4). However, none of these procedures seemed applicable because the equipment required was too elaborate or not commercially available, the waiting period before an answer is available was too long, or the method was not capable of the desired sensitivity and accuracy. Therefore, a new method for radium was developed and evaluated. The radiochemical method described gives satisfactory results on aqueous samples from carbonate and sulfuric acid leach liquors, as well as solubilized ore and residue samples. Interferences are removed for the most part by a lead sulfate precipitation; and those alphaemitting isotopes which are carried along with the radium are eliminated b y the selective coprecipitation of radium with lead nitrate. An anion exchange separation from hydrochloric acid provides additional purification of radium from isotopes such as uranium, polonium, bismuth, and protactinium. Alpha Present address, Ionics Inc., Cambridge 42, Mass. Present address, M & C Kuclear Inc., Attleboro, M a s s .
’
Present address, National Lead Co. of Ohio, Cincinnati 39, Ohio. 3
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ANALYTICAL CHEMISTRY
Co.,Inc., Winchesfer,
Mass.
pulse height analysis of typical samples verified the radioactive purity of the radium. The radium solution obtained b y this method is free of dissolved salts which, if present, would deposit on the planchet and cause serious error through alpha particle absorption. If significant amounts of strontium and barium were present in solid samples, these elements would accompany radium and possibly cause errors through alpha absorption. This is not a problem with liquid samples because of the insolubility of these elements in carbonate or sulfuric acid solutions, which are the only lixiviants used for uraniferous ores. Fortunately, none of the ores encountered t o date contained enough barium or strontium to interfere.
Pb(Ra)SOc
Pb(Ra)C03
PROCEDURE
This procedure is suitable for carbonate or sulfuric acid solutions and phosphoric acid solutions resulting from the dissolution of solid samples. Adjust 500 ml. of the clarified sample to p H 1 h-ith concentrated nitric acid and heat to 70’ C. Xhile stirring with a Teflon-coated magnetic stirring bar, add 5 ml. of the lead carrier solution; then add, very slowly, 60 ml. of 9111 sulfuric acid (1 to 1). Stir continuously for 1 hour, allow the precipitate to settle, and discard the supernatant solution by careful decantation. P b + 2 SOa+ sample a t pH 1 + Pb(Ra)SO,
+
+
Transfer the precipitate to a 50-ml. centrifuge tube using 1JI sulfuric acid. Centrifuge and discard the supernatant liquid. Add 5 ml. of 6 J 1 ammonium acetate to dissolve the lead sulfate. Add 25 ml. of 25% sodium carbonate solution and stir occasionally for 15 minutes.
+ S01-2
Centrifuge, discard the supernate, and add 3-11 nitric acid until effervescence ceases. Add water and heat to dissolve the salts. The total volume after addition of acid and mater should be approximately 5 to 7 ml. If a t this point of the procedure all the salts are not in solution after heating, add 25 ml. of 25% sodium carbonate solution; stir for 15 minutes; centrifuge and discard the supernate; and redissolve in nitric acid. When the salts are dissolved, place the centrifuge tube in a n ice bath; add 30 ml. of red fuming nitric acid and stir. Cool, centrifuge, and discard the supernatant liquid.
+ + +
-
Pb(Ra)CO, 2Hxo3 + Pb(Ra)+2 2x03COZ Pb(Ra)+Z 2XO3- red fuming
APPARATUS AND REAGENTS
The counting equipment can be a gas-flow counter with accessory equipment such as proportional counting gas, a lead shield, and a suitable scaler and amplifier. A scintillation counter with a crystal sensitive to alpha radiation and a suitable scaler may also be used. Lead carrier, 0.49X lead nitrate solution (100 mg. of P b +2 per ml.).
+ C03-2-,
+
+ H20
”02
Pb(Ra )(NO1)Z
Place the centrifuge tube in a water bath and spread the solid material to prevent splattering. Heat to drive off excess acid, add 30 ml. of water, and heat, if necessary, to dissolve the salts. Add 5 ml. of concentrated hydrochloric acid dropwise (stirring constantly) to precipitate lead chloride, and cool in a n ice bath. Centrifuge and pass the supernatant solution over a column of 15 ml. of Dowex 1-X8, 100- to 200mesh anion exchange resin, suspended in 1.8X hydrochloric acid (1 to 6). Use a flow rate of 1 ml. per minute and collect the effluent in a 150-ml. beaker. The column is most conveniently constructed from a standard 25-ml. buret. PbC1,2-z
-
+ Ra+2 + resin 1.8M HC1 resin - (PbClZ2-”)+ R a f 2
Add 35 ml. of water to the lead chloride precipitate and heat in a water bath with stirring until the solid dissolves. 4 d d 6 ml. of concentrated hydrochloric acid dropwise (stirring constantly). Cool in an ice bath and centrifuge. Pass the supernatant solution over the same resin bed, and Fash the resin bed 6-ith an additional 15 ml. of 1.8M hydrochloric acid. Collect and combine all resin bed effluents and discard the lead chloride precipitate. Evaporate the effluent to dryness, but do not bake. Add 10 ml. of nitric acid and evaporate to dryness to destroy organic matter from the resin.
Wash the beaker with small volumes of 1JI nitric acid and transfer to a 10-ml. volumetric flask. Evaporate a 0.2-ml. aliquot on a stainless steel planchet and count for alpha activity. Count the samples immediately. If this is not possible, boil the sample for 2 to 3 hours before diluting to final volume. The radium yield for this procedure is
777, when starting with either aqueous samples or solid samples. Alpha counting rates should be corrected accordingly. The total time elapsed for duplicate samples is 6 to 8 hours. The method for liquid samples has been checked b y analyzing samples which were also sent to four other laboratories foI check analyses. Very good agreement was obtained with all the other laboratories as indicated by the results (Table I). The check analyses were performed b y analytical techniques different from that described in this paper. I n connection with mill survey work, i t became necessary to determine radium in solid samples from various points in the mill. The approach taken in developing a method of analysis for the solid samples was to devise a means of dissolving the sample completely and then using the carrier-free method to analyze the solution of the sample. Of various techniques tested for dissolving an ore sample prior to radium determination, the following procedure, based on the technique of Wamser et al. ( 6 ) ,was found to be most suitable: Treat a 1- to 3-gram sample with hydrofluoric and nitric acids, and evaporate dry to eliminate the bulk of the silica. Take u p the residue in 5 ml. of hydrofluoric acid and 20 ml. of phosphoric acid. Heat the mixture strongly on a hot plate until dense white fumes appear and then heat in a n open oven for 15 minutes or until the mixture forms a clear, viscous liquid. Cool the concentrated phosphoric acid solution slightly and dissolve in 500 ml. of hot distilled nater. Adjust the p H of the solution to 1.0 n-ith a few drops of
Table
I.
Comparison of Radium-226 Analyses on Two Tailing Pond Samples
Laboratory Raw Materials Development Laboratory National Lead Co. of Ohio AEC New Brunswick Laboratory AEC Health and Safety, KYOO Public Health Service, Salt Lake City
Radium Concentration Sample 1 Sample 2 pc.,’ml. pc./ml. D.p.m./l. X io9 D.p.m.jl. X l o 9 209 240
...
194
...
294
132
...
304
137
...
312
141
94 110
...
...
88
...
...
~
Table 11. Analyses of Standard Samples Sample 1. Pitchblende ore sample, theoretical radium content of 58,000 d.p.m. Ea/g. 1 2 3 4 Av. Analyses, d.p m. Ra/ g. 45,800 47,050 41 800 44 700 44,840 Chemical yield, yo 79 0 81.1 72 1 77 1 77.3 Sample 2. Pitchblende ore sample, theoretical radium content of 8,750 d p.m. Ra/g. 1 2 3 4 Av . Analyses, d p.m. Ra,’g. 6,350 6,850 6,480 6,850 6,630 Chemical yield, mc 72 6 78 3 74 1 78 3 i5 8 ~
ammonium hydroxide, and analyze the solution for radium.
Tno standard samples were analyzed in quadruplicate using this phosphoric acid method of dissolution. -4per cent chemical yield was calculated from the results listed in Table 11. On the basis of the results obtained on these standard samples, a yield factor of 77% was assigned t o the procedure. T o date, several uranium ore samples have been analyzed for radium. I n each case, the radium content was determined to be equal n-ithin experimental error t o the theoretical secular equilibrium value calculated from the uranium content. This procedure should prove useful where the routine analysis of a large number of samples is necessary, as long waiting periods before counting are eliminated. Furthermore, elaborate equipment such as is required for the emanation method is not needed. The method is sensitive to about 5 t o 10
d.p.m. of radium per liter (depending on the background of the counting equipment), and increased sensitivity may be achieved by taking a larger aliquot of the solids-free solution for counting. LITERATURE CITED
(1’ Ames, D. P., Sedlet, J., -hderson, H. H., Kohman, T. P., U. P. Atomic Energy Comm., Rept. AECD-2696 (September 1949). (2) Barker, F. B , Thatcher, L. L., ASAL. CHEW29, l 5 i 3 (1957). (3) Bryant, J., Michaelis, AI., Brit. Radiochemical Center, Rept. RCC/R-26 (September 1951). (4) Hudgens, J. E , Benzing, R. O., Cali, J. P., AIeyer, R. C., Nelson, L. C., Nucleonics 9 , So. 2, 14 (1951). ( 5 ) Wamser, C. A., Bernsohn, E., Keeler, R. A., Oneoki, J.. Ax.4~.CHEBI.2 5 , 827 (1953).
RECEIVEDfor reviex January 25, 1960. Accepted May 4, 1960. Division of Analytical Chemistry, 136th Meeting, SCS, Atlantic City, K. J., September 1959. Work supported by the United States Atomic Energy Commission under Contract AT-(49-6)-924.
VOL. 32, NO. 8, JULY 1960
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