Government of India Atomic Energy Commission

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

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).

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. Uranium recovery was also verified by spiking these samples with known amounts of uranium. The results

confirm the validity of the method. The general composition of the phosphoric

acid samples is: H^PO^: 5.5-5.8 M; SO^""; 30-50 gm/l; Fe^: 4-8 gm/l;

A12C5! 4-8 gm/l; Cr^: 0.3-0.5 gm/l; V^: 0.4-0.5 gm/l; U^: 0.07-0.15 gm/l.

6. APPLICATION 01? THE METHOD FOR PHOSPHORIC ACID SAMPLES LOW IN URANIUM (0.01 to 0.05 gm/l).

As large amount of phosphate interferes in the quantitative

extraction of uranium by TBP, it is not possible to analyse phosphoric acid V

samples low in uranium by taking suitably large aliquots. The method was,

therefore, modified to suit such samples. A preliminary concentration of

uranium by coprecipitati,on with aluninium phosphate and its separation from

much of the phosphate, is introduced for this purpose. The conditions for

quantitative precipitation of uranium were worked out. In preliminary

experiments known amounts of uranium were added to chemically pure phosphoric

acid and after dilution and addition of aluminium nitrate the acid was

neutralised to different PH. The precipitate was analysed for uranium by

dissolving it in nitric acid and carrying out the TBP extraction in presence

of saturated aluminium nitrate and then spectrophotometric determination as -7~

thiocyanate complex on lines indicated in p^ra 5 above,.

It was found that for a quantitative precipitation of aluminium and uranium the acid solution should be neutralised to about PH 4. At higher

PH the precipitate obtained was difficult to filter. The amount of aluminium to be added was also studied. When the phosphoric acid aliquot was 2-5 ml of

5-6.M acid about 100 mg. of aluminium gave reproducible and quantitative recovery of uranium. In the case of commercial phosphoric acid trivalent metal

ions like Fe (ill), A1 (ill) are precipitated at the PH used. As the concentration of these ions is, however, variable even in the case of these samples the addition of 100 mg aluminium was done.

6.1. Procedure

2-5 ml of commercial phosphoric acid was diluted to 500 ml after addition of 10 ml nitric acid (15 M) and 100 mg. aluminium as nitrate. The

solution was heated to boiling and kept on a water bath for 1 to 2 hours. It was neutralised with ammonia till some precipitate was formed. 15-2 0 gm of ammonium acetate was then added. The precipitate was filtered, washed with

ammonium nitrate solution and dissolved in minimum quantity of hot, dilute

nitric acid. This was evaporated to near dryness on the water bath. The

residue was dissolved in 15 ml saturated aluminium nitrate and transferred

into a separatory funnel. The extraction with TBP and spectrophotometry

determination were carried out as in para 5 above.

The method was tested by adding known aliquots of uranium to

2-5 gm of H^PO^ (as chemically pure acid) and also by spiking wLth known

amounts of uranium commercial phosphoric acid samples containing 0.01 to

0.05 gm ^JOQ P81" litre. The results of analysis of some phosphoric acid

samples are summarised in TABIE - 2. -8—

7. APPLICATION OF Tftti METHOD FOR THE DETEHfOMATIOIT OF URANIUM IN ROOK PHOSHJAl'E

Hock phosphate which is used as a feed for the manufacture of phosphoric a«id arid other industrial phosphates has a uranium content of about 0»01%. The general chemical composition of this material is given by

(s)

Leyshon . It was found that the thiocyanate method could be applied for the determination of uranium in rock phoBphate. For this purpose 1-2 gm of the ground sample was mixed with 10 ml of 15 M nitric acid and evaporated to dryness on a hot ple'fce. The residue was digested with 50 ml of 4 M nitric

acid and filtered. The filtrate was diluted to 50i 0 ml. Further determination of uranium was carried.out as in the case of phosphoric acid samples low in uranium (para 6.1). Samples of phosphate rock from Florida (u.S.A.)» Morroco, Algeria, » and Israel were analysed by this method and the results compared with those ' » obtained by radiometric method. TABLE - 3 shows that the present method gives satisfactory results.

8. CONCLUSION

A method has been worked out for the determination of uranium in

..commercial, wet process phosphoric acid and rock phosphate. It is based on the separation of uranium from interfering ions by its extraction from a sqluticm containing aluminium nitrate using TBP diluted with petroleum ether. By contacting the TBP extract with an aqueous'solution oontaining ammonium

thiocyanate., the uranium in the organic layer is converted into the coloured thiocyanate complex. Suitable modifications have been made in the procedure

% for analysis of samples low in uranium. The method is believed to bo simple and' reliable. -9-

BEEEHENCES

1. Currah, J.E. and Beamish, P.E., Ind. Eng. Chem., Anal. Ed., 19 (1947) 609.

2. Nietzel, O.A. and Desesa, M.A., Anal. Chem., 29 (1957) 756.

3. Clutch, J. and Guy, M.J., Analyst 82'(1957) 800.

4. Koppiker, ,K.S., Koregaonkar, V.G. and Murthy, T.K.S., Anal. Chim. Acta., 20 (1959) 366.

5. Centanni, F.A., Ross, A.M. and Desesa, M.A., Anal. Chem .j 28 (11) (1956) 1961 .

6. Clinch, J. and Guy, M.J., Analyst 82 (1957) 802.

7. Koppiker, K.S"., Koregaonkar, V.G. and Murthy, T.K.S., Anal. Chim. Acta., 20 (1959) 367.

8. Leyshon, D.W., Trans, soc. of mining Engineers, 238 (June 1967) 192. -10-

TABIE -1

EFFECT OF PHOSPHORIC ACID ON URANIUM RECOVERY

Uranium in aliquot 0.20 mg.

1 Phosphoric acid added 1 Uranium Recovery

g. h3po4. , percent

Nil 100

0.5 1 100 1.0 , 100

1.2 « 99 1.4 ' 98 1.8 « 96

2.0 J 93 2.5 1 90 i -11-

TABLE - 2

DETERMINATION OP URANIUM APTEH PRELIMINARY SEPARATION

Sample • U5°8 g/1' By fluorimetric By pre Bent method method

A 0.075 0.075

B 0.075 0.074

R1 0.015 0.016 0.021 0.020

R_j 0.031 0.02B

R. 0.040 0.043 4 R-. 0.050 0.051

A, B t Commercial phosphoric acid (5.8 - 6 M) from Gujarat Fertilisers.

R^-Rc : Raffinates from solvent extraction tests. In all canee 5 ml aliquot of sample was taken for analysis . TABLE -3

ANALYSIS OP ROCK PHOSPHATE SAMPLES

Percent (Mean of four Sample Origin determination)

By present By radiometric method assay

1. Algeria 0.0045 0.0045

2. Israel 0.014 0.0145

3. Florida (U.S.A.) 0.015 0.0155

4. Morrocco 0.0155 0.016 mg. U3p8

VOL. OF ORG. LAYER 10 ml. CELL LENGTH. 10mm. WAVE LENGTH 365 m/i. BLANK REAGENT BLANK.

FIG. 1. CALIBRATION GRAPH — URANIUM CONCENTRATION vs. OPTICAL DENSITY.