B.A.R.C.-638 "Ba^spjU' GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION SPECTROPHOTOMETRIC DETERMINATION OF URANIUM IN COMMERCIAL WET PROCESS PHOSPHORIC ACID AND PHOSPHATE ROCK USING THIOCYANATE by R. A. Nagle and T. K. S. Murthy Chemical Engineering Division BHABHA ATOMIC RESEARCH CENTRE BOMBAY, INDIA 1972 We regret that some of the pages in the microfiche copy of this report may not be up to the proper legibility standards, evea though the best possible copy was used for preparing the master fiche. B.A .B.C.-630 GOVERNMENT OF INDIA co ATOMIC ENERGY COMMISSION to o « « < SPECTROBiOKMETRIC DETERMINATION OF URANIUM IN COMMERCIAL WET PROCESS PHOSPHORIC? ACID AND PHOSPHATE RUCK USING TRIOCYANATE by R.A. Nagle and V.K.S. Murthy Chemical Engineerirg Division 1-JHABHA ATOMIC RESEARCH CENTRE BOMBAY, INDIA 1972 SPECTROIHCTOMETRIC DETERMINATION OP URANIUM IN COMMERCIAL WET PROCESS PHOSPHORIC ACID AND PHOSPHATE ROCK USING THIOCYANATE by R.A. Nagle and T.K.S. Murthy 1 . INTRODUCTION The thiocyanate method1 for the spectrophotometry determination 2 of uranium has been studied by several authors. Nietzel and DeSe3a reviewed the modifications introduced in the working of this method to make it applicable fo•r 4differen t types of samples. Clinch and Guy , Koppiker and Coworkers extracted the uranyl thiocyanate complex with tri-n-butyl phosphate (TBI?) and measured the absorbance in the organic layer. Moderate amounts of other ions could be tolerated in these procedures., During the course of studies on recovery of uranium from commercial wet process phosphoric acid the present authors felt the need for a quick and reliable spectrophotometry method. Phosphate in moderately high concen- tration is one of the serious interferences in most of the above mentioned methods. It was, therefore, felt a preliminary separation of uranium from large amounts (0,5 g and above) of phosphate and other ions would be necessary before the final determination. Based on erlier experience it was felt that this could best be done by extraction of uranium with TBP from an aqueous solution with high aluminium nitrate concentration. By contacting the uranium bearing extract with an aqueous solution containing thiocyanate, the uranyl ion in the organic phase can be converted into the thiocyanate complex and this can be used for absorbance measurement. A methoO was developed on these lines and used for the determination of uranium in varying concentrations. This report summarises the results of this study. -2- 2. APPARATUS AND REAGENTS a) Spectrophotometry measurements were made on Beckman Model-B Spectrophotometer using 10 ram. rectangular glass cells. b) Extraction with solvent was cai-ried out in 50 ml. glass stoppered separatory funnels. c) Petroleum ether, 60°-80°C boiling range.. d) Tri butyl phosphate (TBP)t 100 ml* B.D.H. Laboratory reagent grade TBP was diluted, to one litre with petroleum ether and washed thrice with 250 ml portion of 10% w/v sodium hydroxide and then with distilled water till the washings were free from alkali. The solvent was filtered and stored. (e) Aluminium Nitrates Saturated (at room temperature) aluminium nitrate solutions was prepared from laboratory reagent grade Al(NO^)^ 9H20. (f) Ammonium Nitratex Saturated (at room temperature) ammonium nitrate solution was prepared from laboratory reagent grade NH^NO^. (g) Ammonium Thiocyanate: 2.5 I solution was prepared and filtered. (h) Standard uranium solution* A stock solution containirg about 5 gnj. Per litre was prepared from pure UOgfNO^g-SHgO. Uranium content was determined by gravimetric method using carbon dioxide free ammonia. A tandard solution containing.o.10 rag. per ml. was prepared by dilution of stock solution. J. GENERAL PROCEDURE The method is intended for application to three types of samplesi (a) Commercial phosphoric acid eontainirg 0.05-0.2 gm. U^O0 per litre. (b) Commercial acid containing less than 0.05 gm. U,0„ per litre or raffinates obtained in the solvent extraction tests which are of the general chemical composition as the commercial phosphoric acid but depleted with respect to uranium (as low as 0.01 gm. per litre). (c) Phosphate rock which is the raw material for the manufacture of phosphoric acid, bearing 0,005 to 0.02 percent In the case of phosphoric acid samples containing more than 0.05 gm U^Og per litre, 0.5 to 2 ml. aliquot was added to 15 ml. of saturated aluminium nitrate and the uranium was extracted with 10 ml TBP petroleum ether solvent. After separation of the aqueous layer the organic extract was contacted with jan aqueous solution containing anmonium thiocyanate and ascorbic acid. The uranyl thiocyanate complex was formed in the organic layer. The absorbance of this solution was measured. In the case of phosphoric acid samples containing less than 0.05 gm U^Og per litre, an aliquot of 2 to 5 ml was taken and uranium was concen- trated by coprecipitation with aluminium phosphate. " This step reduced the amount of phosphate accompanying the uranium. The precipitate was dissolved and uranium extracted with TBP and determined spectrophotometrically. In the case of phosphate rock samples 2 to 5 gm of the sample was dissolved in nitric acid and uranLum concentration, extraction, and determination carried out as in the case of low grade phosphoric acid samples. 4. EXPERIMENTAL 4.1. Uranium extraction from Aluminium Nitrate When 1 to 3 ml aliquot of uranium solution containing 0.1 to 0.5 mg tJ-OL was added to 15 ml saturated aluminium nitrate and extracted with 10 ml TBP, it was found that the extraction of uranium was more than 99$. (5) This was verified by fluorimetric determination of the remaining uranium in the equeous phase. The effect of adding phosphoric acid to the aqueous solution on uranium extraction was studied by adding varying amounts of phosphoric acid to a known aliquot of uranium and processing through TBP extraction and complex formation. In. each case the percent recovery of uranium was calculated by comparing the optical density of the final organlo layers, with that of the sample not containing phosphate. The results given in TABLE - 1 show that upto about 1.5 gm H^PO^ can be present without affecting uranium extraction. 4.2. Colour Development in the Organic Layer The TBP layer carrying uranium was contacted with an aqueous solution containing ammonium thiocyanate, ascorbic acid and adjusted to pH 1.5 - 2.0. The presence of ascorbic acid reduces traces of iron (ill) and prevents the formation of ferric thiocyanate complex in the organic phase. It was found that a solution containing 5 ml ascorbic acid and 2.5 ml ammonium thiocyanate in a total volume of 10 - 15 ml would lead to reproducible colour formation in the organic layer. The absorption spectrum of the thiocyanate complex in TBP layer was similar to that obtained by Clinch and Guy^. The optical density of the organic layer was measured at 365 m JL, against a reagent blank extracted and treated in the same way as uranium containing solutions. 4.3. Interferences Many common metal ions like Cu (ll), Pb (il), Ni (ll), Co (il), alkali and alkaline earth metals do not interfere in the extraction or colour . development when present in moderate amounts (20 mg). Iron (III), v(v), tf" interfere in the colour development by either forming coloured thiocyanate complexes extractable by TBP or by ccmplexing uranium (VI) and inhibiting the formation of thiocyanate complex. However, when 50 zag iron (ill), 5 mg V(V) and 50 mg F were added to known aliquots of uranium and taken through the combined extraction and colour development procedure only slight error was noted in the final absorption value. It was suspected that the error was due to the aqueous droplets clinging to the separatory funnel or suspended in the organic layer. To eliminate this effect the TBP extract was scrubbed twice with 10 ml lots of saturated ammonium nitre,te• 4.4. Uranyl Thiocyanate Complex in tho Organic Layer In its absorption spectrum, range of concentration over which Beer'ei law is valid and stability of colour, the uranyl thiocyanate complex in the organic' phase behaved in the same way as the TBP - carbon tetrachloride (7v ) extracted c omplex '. 5. PROCEDURE FOR THE DETERMINATION OF URANIUM IN PHOSPHORIC ACID (0.05 gm/l AND ABOVE) 0.5 to 2 ml aliquot of phosphoric acid (5 - 6 M in H^PO^) was added to 15 ml saturated aluminium nitrate in a separatory funnel and 10 ml TBP- petroleum ether mixture was added and the contents agitated for 2-3 minutes. After the two layers separated, the aqueous was run down and discarded. 10 ml saturated ammonium nitrate was added to the organic layer and contents were agitated for 1 minute. The aqueous was discarded. The washing was repeated once more. A mixture containing 2.5 ml ammonium thiocyanate, 5 ml ascorbic acid diluted to 10 ml and adjusted to pH 1.5 - 2.0 was added to the organic layer in the separatory funnel. _ The c ontents were shaken for 1 -2 minutes, the phases allowed to separate and the aqueous layer discarded. The optical density of the organic layer was measured at 365 mp. against a reagent blank prepared by extracting 15 ml aluminium nitrate and taken -6- through the complete procedure. A calibration graph showing uranium concentration against optical density was prepared by adding known aliquots of uranium to aluminium nitrate and carrying out the procedure mentioned above. The uranium content of the sample was found by reference to this calibration graph (Figc 1.) The method was tested using samples of commercial phosphoric acids from three different manufacturing plants and the results were compared with those obtained by the fluorimetric method.