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Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths
Hydrometallurgy 61 Ž2001. 75–80 www.elsevier.nlrlocaterhydromet
Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths R. Vijayalakshmi, S.L. Mishra, H. Singh, C.K. Gupta) Rare Earth DeÕelopment Section, Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India Received 26 July 2000; received in revised form 8 October 2000; accepted 8 October 2000
Abstract A process is described for the recovery of rare earths from xenotime concentrate by digestion with sulphuric acid. A low grade xenotime concentrate Žtypically assaying 57% light rare earths, 27% Y2 O 3 and 15.6% heavy rare earth. and a high grade xenotime Žassaying 38% light rare earths, 41.8% Y2 O 3 and 20.55% heavy rare earths. is routinely being produced in India. Both types of materials have been tested and ) 98% solubilisation of metal values has been achieved. Xenotime sulphate leach liquor has been processed for removal of thorium by selective precipitation with ammonia and sodium pyrophosphate. Although in both systems thorium removal was almost complete, the ammonia precipitation was found suitable since it facilitates the preferential precipitation of lighter rare earths along with thorium. Consequently, the heavy rare earths have been enriched in the filtrate. Other impurities such as iron, uranium, sulphate and phosphate have been effectively removed by precipitation of rare earths with oxalic acid. It has been shown possible to recover rare earths along with yttrium Ž; 99% recovery. by these processes. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Xenotime; Precipitation; Yttrium
1. Introduction Yttrium and heavy rare earths are finding increasing applications in the field of high technology such as nuclear reactors, lasers, super-conductors, phosphors and others. Potential sources of rare earths ŽRE. in India include monazite, xenotime and by-products of uranium and phosphate plants. The processing of monazite leads to generation of thorium concentrate, light RE and only a small amount of heavy rare earths ŽHRE. including yttrium
)
Corresponding author. E-mail address: [email protected] ŽC.K. Gupta..
Ž0.4%Y2 O 3rREO.. Recently xenotime, a phosphate mineral of HRE, has been found in different parts of the country and resource evaluation shows significant economic potential w1x. A typical heavy mineral concentrate assaying 18–20% Y2 O 3 has been recovered from sand using gravity and magnetic separation techniques. Hydrometallurgical processing of xenotime by sodium hydroxide digestion requires fine grinding of feed materials and presents difficulties in subsequent efficient separation of thorium and RE by controlled pH separation where 10–20% loss of high value yttrium in the thorium fraction is possible. In the case of monazite, the loss of low value light RE in thorium cake is tolerated. Studies involving alkali leaching, alkali pugging and alkali
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 1 5 9 - 6
76
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80
fusion have been carried out to recover REO from xenotime w2x. While it was possible to recover almost quantitative values of RE from pure xenotime concentrate, the recoveries from low and intermediate grade were relatively low. To increase the recovery of RE, the acid digestion route w3x was investigated. The parameters required for separation of other impurities such as thorium from RE were also determined.
2. Experimental
sured with a mechanical stirrer. The dissolution of product mass with water and subsequent purification of rare earths from other metals such as Th were carried out in beakers of different capacity. Chemicals such as sulphuric acid, ammonia, oxalic acid and sodium pyrophosphate were laboratory grade reagents. For determination of thorium, sample solution was extracted with tri-octyl phosphine oxide ŽTOPO. and separated from rare earths by selective stripping with dilute H 2 SO4 . Thorium was analysed in strip liquor spectrophotometrically using thoron reagent w4x.
2.1. Beneficiation of xenotime 3. Results and discussion A typical heavy mineral sand constituting about 1–2% by weight was separated at site from gangue minerals such as quartz, silicate, ferromagnetics and a xenotime concentrate containing 16% of Y2 O 3 was obtained as feedstock. Hot sand Ž1008C. was processed in a high tension separator ŽHTS. at a voltage of 21 kV for separating conducting and non-conducting products. The non-conducting stream was cleaned two to three times in the HTS. It essentially contained low grade xenotime Ž27–30% Y2 O 3 ., monazite, garnets and entrained opaque materials. Further processing was carried out in an induced roll magnetic separator ŽIRMS. at a magnetic field strength of 4.4 kG. The high grade xenotime concentrate Ž35–40% Y2 O 3 . was collected with 80–85% recovery. Both low and high grade xenotime materials have been used for finding suitable process parameters. The xenotime concentrate material was subjected to attrition grinding in a batch mill. The material was charged to a stainless steel vessel of size 180 mm height and 120 mm diameter along with stainless steel balls of diameters 6.5 mm each and attrited with a speed of 450rmin. A xenotime powder of 99% finer than 150 mesh size Ž100 mm. was obtained in 8 min and used for leaching tests. 2.2. Chemical processing Acid digestion of low and high grade xenotime was carried out in a glass beaker. The reaction was highly exothermic, the reactant mixture formed a thick slurry and slow homogeneous mixing was en-
3.1. Leaching Xenotime reacted with sulphuric acid according to the following overall reaction 2 REPO4 q 3 H 2 SO4 ™ RE 2 Ž SO4 . 3 q 2 H 3 PO4 Ž 1 . The leaching parameters such as sulphuric acid concentration, time and temperature were found using 25 g xenotime sand. In a typical experiment, 200 g ground xenotime was heated with 280 mL concentrated sulphuric acid at 2508C in acid to solid weight ratio ; 2.5 for 6 h. The mass was allowed to cool to room temperature and slurried with 800 mL water. The slurry was filtered and the residue washed with water until free of acid. Total oxide content was determined by precipitating a suitable aliquot of leach liquor with oxalic acid followed by calcination of oxalate to oxide. The total oxide recovered was 90 g, leaving 55.5 g residue Žoven dried.. Under similar Table 1 Composition of xenotime concentrate Element
Low grade xenotime Ž%.
Total oxide 46 ThO 2 2.0 U3 O 8 0.8 Rare earths Žweight % on REO basis. Light RE 57.2 Žup to Sm. Heavy RE 15.8 Yttrium 26.9
High grade xenotime Ž%. 55.7 1.1 0.8 38.0 20.5 41.8
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80 Table 2 Effect of feed concentration on separation of thorium Sample no.
Leach liquor Conc. Volume ŽgrL. ŽmL.
Loss of RE in ppt. Ž%.
Thorium separation Ž%.
1 2 3
7.25 14.4 28.9
5.8 6.0 9.1
95 95 95
200 100 50
conditions, the high-grade xenotime yielded 111.4 g oxide with 30.2 g residue. The residue weight proportion is reduced by a factor of 1.8 when high-grade xenotime was used. This is significant since the residue is a low-grade radioactive waste requiring careful disposal. The analysis of RE, Th and U in xenotime from the two sets is given in Table 1. It can be observed that high grade xenotime yielded a richer yttrium product for the same consumption of chemicals. The residue analysis ŽLRE—0.024%, HRE—0.08%, Y2 O 3 —0.18%, U and Th traces. shows that over 99% of yttrium in the low and high grade xenotime concentrate was solubilised.
77
ied for such separation w5–8x, the selective precipitation methods were studied in detail. 3.2.1. SelectiÕe precipitation of thorium with ammonia The stepwise neutralisation of diluted xenotime sulphate leach solution precipitates thorium, rare earths and uranium at a pH of 1, 3, and 6, respectively w6x. The effect of various parameters had been studied to obtain effective separation of thorium with minimum loss of rare earths.
3.2. Separation of impurities
3.2.1.1. Effect of feed concentration. Xenotime leach solution containing 28.9 g REOrL, 1.25 g ThO 2rL and 1 M sulphuric acid was diluted in different proportion with water and pH was adjusted to 1.5 with dilute ammonia Ž4 M.. After filtration, the total oxide concentration in the precipitate was determined by dissolving it in HCl, precipitating it as oxalate and calcining to oxide. The results recorded in Table 2 indicate that 4 = dilution Ži.e., 7.2 g REOrL. gave a minimum loss Ž5.8%. of oxide while recovery of thorium was almost constant Ž95%.. However, to avoid too much dilution, a leach solution containing 14–15 g REOrL was selected for further work.
The xenotime leach solution contains rare earths, uranium, thorium and ytrrium apart from phosphoric and unreacted sulphuric acid. Prior to purification of yttrium from this solution, the separation of impurities was necessary. Among the various methods stud-
3.2.1.2. Effect of pH. Xenotime leach solution containing 15 g REOrL and 0.5 M sulphuric acid was neutralised to different pH with dilute ammonia and digested for 30 min at ambient temperature. Results in Fig. 1 show that as the pH of neutralisation was
Fig. 1. Effect of pH on Th separation and RE loss.
78
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80
Fig. 2. Two-stage counter current precipitation of thorium with ammonia.
increased the thorium removal increased but loss of RE in the thorium fraction also increased. The optimum pH for thorium removal seems to be 1.8. It is important to maintain pH in a critically controlled way, which, however, may be difficult in a single stage plant operation. Therefore, a two-stage counter current precipitation was adopted to take care of fluctuations in the operating conditions and maximise efficiency of separation. 3.2.1.3. Counter current precipitation of thorium with ammonia. A two-stage counter current precipitation from a mixed leach solution containing 10 grL REO, 0.42 grL ThO 2 and 0.65 M H 2 SO4 was carried out using 4 M ammonia. The pH of the solution in the first stage was maintained around 1.5 while it was 1.9 in the second stage. After attaining the required pH, the slurry was digested for 30 min and then filtered. Precipitates and filtrates were moved in opposite direction ŽFig. 2.. After attaining steady state, it was found that about 80 and 8 mL ammonia are consumed in the first and second stage, respectively, for processing 300 mL of leach solution. The weight of thorium-bearing precipitate Ždried at 1108C., obtained in the first stage, was found to be 600 mg, which was equivalent to 300 mg of total oxide containing around 125 mg ThO 2 but no yttrium. The composition of oxide obtained from different fractions is recorded in Table 3, which indicates that although the leach solution contained 46.7% yttrium ŽY2 O 3rREO., the product solution contained 52% Y2 O 3 due to selective precipitation of lighter rare earths in the thorium fraction.
3.3. SelectiÕe precipitation of thorium with sodium pyrophosphate Thorium precipitated with sodium pyrophosphate according to the following reaction. Th Ž SO4 . 2 q Na 4 P2 O 7 ™ ThP2 O 7 q 2 Na 2 SO4
Ž 2.
The selectivity of thorium with respect to RE in pyrophosphate medium is high at low pH. Thorium
Table 3 Composition of oxide obtained in counter current precipitation of thorium Metal
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu ThO 2 a
Composition of oxide Žwt.% on REO basis. Leach solution
Product solution
Thorium-rich precipitate
8.0 15.0 2.0 6.0 1.5 0.5 2.3 0.56 5.3 1.3 46.7 4.5 0.7 4.9 0.7 4.34
7.5 11.06 1.85 5.3 1.5 0.5 2.1 0.57 5.8 1.4 52.0 5.0 0.77 5.3 0.8 n.d.
12.0 50.0 3.4 12.0 n.d.a n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 40.0
n.d.s not detected Ž - 0.1%..
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80 Table 4 Results of thorium precipitation with sodium pyrophosphate Sample no.
Volume of NH 4 OH Ž4 M. wmLx
Volume of sodium pyro phosphate solution wmLx
Thorium separation w%x
1 2 3 4 5
nil nil 24.6 20.0 18.8
3 10 0.5 5.0 10.0
8.0 9.0 90 )99 )99
Xenotime sulphate solution: 50 mL containing 20 g REOrL and 1.35 M H 2 SO4 . Precipitant: 7% Žwrv. sodium pyro phosphate solution.
was precipitated at ambient temperature with 7% Žwrv. sodium pyrophosphate solution from a xenotime sulphate solution containing 19.7 g REOrL and 1.35 M H 2 SO4 . Effects of various parameters such as amount of sodium pyrophosphate and ammonia had been studied and results recorded in Table 4 indicate that sodium pyrophosphate alone could not precipitate thorium quantitatively, even though it was added three times more than the stoichiometric amount required for thorium. Hence, for effective separation of thorium, pH of the solution was raised with ammonia in the presence of sodium pyrophosphate. It was observed that about 20 mL ammonia and 350 mg sodium pyrophosphate is required for 50 mL leach liquor. Thorium pyrophosphate precipitate contains 110 mg oxide including 8 mg Y2 O 3 . Yttrium recovery was 98.2%. Although this process was a single-step precipitation, the chemical consumption and Y loss were higher compared to ammonia precipitation. 3.4. Separation of other impurities The solution obtained after thorium removal either by ammonia or by sodium pyrophosphate precipitation, contained uranium, iron, sulphate and phosphate apart from RE and yttrium. These impurities were separated by treatment with oxalic acid where uranium and iron form soluble oxalate complexes and pass into the filtrate along with sulphate and phosphate. The rare earth oxide obtained after
79
calcination of oxalate was dissolved in HCl to form a feed for a solvent extraction process used for purification of yttrium. In the case of monazite, the recovery of yttrium was carried out essentially in three stages. The first stage was the fractionation of rare earths to obtain heavy rare earth concentrate Ž60% Y2 O 3 . along with other concentrates by adopting a solvent extraction process. The second stage was concerned with further enrichment to 93% Y2 O 3 by solvent extraction w9x. The final stage was also based on solvent extraction to further enrich the 93% grade to 99.9% Y2 O 3 w9x. The oxide obtained from xenotime, on the other hand, was taken as a feed for the subsequent two stages of solvent extraction to produce 99.9% pure Y2 O 3 .
4. Conclusion A process has been described for extraction of rare earth oxides from an intermediate grade xenotime concentrate by sulphuric acid digestion. The process resulted in more than 98% recovery of rare earth oxides and can be utilised on a large scale. Thorium removal from xenotime sulphate leach liquor had been studied by two-stage counter current precipitation with ammonia and single step precipitation with sodium pyrophosphate. Both processes resulted in more than 99% recovery of thorium. Rare earth oxides had been recovered and separated from other impurities by oxalate precipitation.
References w1x D.C. Banerjee, R.P. Sinha, K.K. Dwivedy, Xenotime resource in India—an overview, Proc. Recent Development In the Science and Technology of Rare Earths, Cochin, 1995, p. 14. w2x P. Alex, A.K. Suri, C.K. Gupta, Processing of xenotime concentrate, Hydrometallurgy 50 Ž1998. 331. w3x R.C. Vickery, The Chemistry of Yttrium and Scandium, Pergamon, New York, 1960, p. 34. w4x W.J. Ross, J.C. White, Extraction and determination of thorium from sulphate and phosphate with tri n-octyl phosphine oxide, Anal. Chem. 31 Ž1959. 1847. w5x F.L. Cuthbert, Thorium Production Technology, Wesley Publishing Company Inc., Reading, Massachusetts, USA, 1958, p. 72.
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R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80
w6x D.I. Ryabchikov, E.K. Golbraikh, Analytical Chemistry of Thorium, Pergamon Press, New York, USA, 1963. w7x A.N. Zelikman, E. Krien, G.V. Samsonov, Metallurgy of rare earths, Israel Program for Scientific Translation, 1966, p. 275. w8x D.K. Singh, S.L. Mishra, H. Singh, C.K. Gupta, Extraction studies of uranium with 2-Ethyl hexyl phosphonic acid mono-
2-ethyl hexyl ester ŽPC 88A., J. Radioanal. Chem. 245 Ž3. Ž2000.. w9x S.M. Deshpande, S.L. Mishra, R.B. Gajankush, N.V. Thakur, K.S. Koppiker, Recovery of high purity Y2 O 3 by solvent extraction route using organo phosphorus extractants, Miner. Process. Extr. Mtall. Rev. 10 Ž1992. 267.
Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths R. Vijayalakshmi, S.L. Mishra, H. Singh, C.K. Gupta) Rare Earth DeÕelopment Section, Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India Received 26 July 2000; received in revised form 8 October 2000; accepted 8 October 2000
Abstract A process is described for the recovery of rare earths from xenotime concentrate by digestion with sulphuric acid. A low grade xenotime concentrate Žtypically assaying 57% light rare earths, 27% Y2 O 3 and 15.6% heavy rare earth. and a high grade xenotime Žassaying 38% light rare earths, 41.8% Y2 O 3 and 20.55% heavy rare earths. is routinely being produced in India. Both types of materials have been tested and ) 98% solubilisation of metal values has been achieved. Xenotime sulphate leach liquor has been processed for removal of thorium by selective precipitation with ammonia and sodium pyrophosphate. Although in both systems thorium removal was almost complete, the ammonia precipitation was found suitable since it facilitates the preferential precipitation of lighter rare earths along with thorium. Consequently, the heavy rare earths have been enriched in the filtrate. Other impurities such as iron, uranium, sulphate and phosphate have been effectively removed by precipitation of rare earths with oxalic acid. It has been shown possible to recover rare earths along with yttrium Ž; 99% recovery. by these processes. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Xenotime; Precipitation; Yttrium
1. Introduction Yttrium and heavy rare earths are finding increasing applications in the field of high technology such as nuclear reactors, lasers, super-conductors, phosphors and others. Potential sources of rare earths ŽRE. in India include monazite, xenotime and by-products of uranium and phosphate plants. The processing of monazite leads to generation of thorium concentrate, light RE and only a small amount of heavy rare earths ŽHRE. including yttrium
)
Corresponding author. E-mail address: [email protected] ŽC.K. Gupta..
Ž0.4%Y2 O 3rREO.. Recently xenotime, a phosphate mineral of HRE, has been found in different parts of the country and resource evaluation shows significant economic potential w1x. A typical heavy mineral concentrate assaying 18–20% Y2 O 3 has been recovered from sand using gravity and magnetic separation techniques. Hydrometallurgical processing of xenotime by sodium hydroxide digestion requires fine grinding of feed materials and presents difficulties in subsequent efficient separation of thorium and RE by controlled pH separation where 10–20% loss of high value yttrium in the thorium fraction is possible. In the case of monazite, the loss of low value light RE in thorium cake is tolerated. Studies involving alkali leaching, alkali pugging and alkali
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 1 5 9 - 6
76
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80
fusion have been carried out to recover REO from xenotime w2x. While it was possible to recover almost quantitative values of RE from pure xenotime concentrate, the recoveries from low and intermediate grade were relatively low. To increase the recovery of RE, the acid digestion route w3x was investigated. The parameters required for separation of other impurities such as thorium from RE were also determined.
2. Experimental
sured with a mechanical stirrer. The dissolution of product mass with water and subsequent purification of rare earths from other metals such as Th were carried out in beakers of different capacity. Chemicals such as sulphuric acid, ammonia, oxalic acid and sodium pyrophosphate were laboratory grade reagents. For determination of thorium, sample solution was extracted with tri-octyl phosphine oxide ŽTOPO. and separated from rare earths by selective stripping with dilute H 2 SO4 . Thorium was analysed in strip liquor spectrophotometrically using thoron reagent w4x.
2.1. Beneficiation of xenotime 3. Results and discussion A typical heavy mineral sand constituting about 1–2% by weight was separated at site from gangue minerals such as quartz, silicate, ferromagnetics and a xenotime concentrate containing 16% of Y2 O 3 was obtained as feedstock. Hot sand Ž1008C. was processed in a high tension separator ŽHTS. at a voltage of 21 kV for separating conducting and non-conducting products. The non-conducting stream was cleaned two to three times in the HTS. It essentially contained low grade xenotime Ž27–30% Y2 O 3 ., monazite, garnets and entrained opaque materials. Further processing was carried out in an induced roll magnetic separator ŽIRMS. at a magnetic field strength of 4.4 kG. The high grade xenotime concentrate Ž35–40% Y2 O 3 . was collected with 80–85% recovery. Both low and high grade xenotime materials have been used for finding suitable process parameters. The xenotime concentrate material was subjected to attrition grinding in a batch mill. The material was charged to a stainless steel vessel of size 180 mm height and 120 mm diameter along with stainless steel balls of diameters 6.5 mm each and attrited with a speed of 450rmin. A xenotime powder of 99% finer than 150 mesh size Ž100 mm. was obtained in 8 min and used for leaching tests. 2.2. Chemical processing Acid digestion of low and high grade xenotime was carried out in a glass beaker. The reaction was highly exothermic, the reactant mixture formed a thick slurry and slow homogeneous mixing was en-
3.1. Leaching Xenotime reacted with sulphuric acid according to the following overall reaction 2 REPO4 q 3 H 2 SO4 ™ RE 2 Ž SO4 . 3 q 2 H 3 PO4 Ž 1 . The leaching parameters such as sulphuric acid concentration, time and temperature were found using 25 g xenotime sand. In a typical experiment, 200 g ground xenotime was heated with 280 mL concentrated sulphuric acid at 2508C in acid to solid weight ratio ; 2.5 for 6 h. The mass was allowed to cool to room temperature and slurried with 800 mL water. The slurry was filtered and the residue washed with water until free of acid. Total oxide content was determined by precipitating a suitable aliquot of leach liquor with oxalic acid followed by calcination of oxalate to oxide. The total oxide recovered was 90 g, leaving 55.5 g residue Žoven dried.. Under similar Table 1 Composition of xenotime concentrate Element
Low grade xenotime Ž%.
Total oxide 46 ThO 2 2.0 U3 O 8 0.8 Rare earths Žweight % on REO basis. Light RE 57.2 Žup to Sm. Heavy RE 15.8 Yttrium 26.9
High grade xenotime Ž%. 55.7 1.1 0.8 38.0 20.5 41.8
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80 Table 2 Effect of feed concentration on separation of thorium Sample no.
Leach liquor Conc. Volume ŽgrL. ŽmL.
Loss of RE in ppt. Ž%.
Thorium separation Ž%.
1 2 3
7.25 14.4 28.9
5.8 6.0 9.1
95 95 95
200 100 50
conditions, the high-grade xenotime yielded 111.4 g oxide with 30.2 g residue. The residue weight proportion is reduced by a factor of 1.8 when high-grade xenotime was used. This is significant since the residue is a low-grade radioactive waste requiring careful disposal. The analysis of RE, Th and U in xenotime from the two sets is given in Table 1. It can be observed that high grade xenotime yielded a richer yttrium product for the same consumption of chemicals. The residue analysis ŽLRE—0.024%, HRE—0.08%, Y2 O 3 —0.18%, U and Th traces. shows that over 99% of yttrium in the low and high grade xenotime concentrate was solubilised.
77
ied for such separation w5–8x, the selective precipitation methods were studied in detail. 3.2.1. SelectiÕe precipitation of thorium with ammonia The stepwise neutralisation of diluted xenotime sulphate leach solution precipitates thorium, rare earths and uranium at a pH of 1, 3, and 6, respectively w6x. The effect of various parameters had been studied to obtain effective separation of thorium with minimum loss of rare earths.
3.2. Separation of impurities
3.2.1.1. Effect of feed concentration. Xenotime leach solution containing 28.9 g REOrL, 1.25 g ThO 2rL and 1 M sulphuric acid was diluted in different proportion with water and pH was adjusted to 1.5 with dilute ammonia Ž4 M.. After filtration, the total oxide concentration in the precipitate was determined by dissolving it in HCl, precipitating it as oxalate and calcining to oxide. The results recorded in Table 2 indicate that 4 = dilution Ži.e., 7.2 g REOrL. gave a minimum loss Ž5.8%. of oxide while recovery of thorium was almost constant Ž95%.. However, to avoid too much dilution, a leach solution containing 14–15 g REOrL was selected for further work.
The xenotime leach solution contains rare earths, uranium, thorium and ytrrium apart from phosphoric and unreacted sulphuric acid. Prior to purification of yttrium from this solution, the separation of impurities was necessary. Among the various methods stud-
3.2.1.2. Effect of pH. Xenotime leach solution containing 15 g REOrL and 0.5 M sulphuric acid was neutralised to different pH with dilute ammonia and digested for 30 min at ambient temperature. Results in Fig. 1 show that as the pH of neutralisation was
Fig. 1. Effect of pH on Th separation and RE loss.
78
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80
Fig. 2. Two-stage counter current precipitation of thorium with ammonia.
increased the thorium removal increased but loss of RE in the thorium fraction also increased. The optimum pH for thorium removal seems to be 1.8. It is important to maintain pH in a critically controlled way, which, however, may be difficult in a single stage plant operation. Therefore, a two-stage counter current precipitation was adopted to take care of fluctuations in the operating conditions and maximise efficiency of separation. 3.2.1.3. Counter current precipitation of thorium with ammonia. A two-stage counter current precipitation from a mixed leach solution containing 10 grL REO, 0.42 grL ThO 2 and 0.65 M H 2 SO4 was carried out using 4 M ammonia. The pH of the solution in the first stage was maintained around 1.5 while it was 1.9 in the second stage. After attaining the required pH, the slurry was digested for 30 min and then filtered. Precipitates and filtrates were moved in opposite direction ŽFig. 2.. After attaining steady state, it was found that about 80 and 8 mL ammonia are consumed in the first and second stage, respectively, for processing 300 mL of leach solution. The weight of thorium-bearing precipitate Ždried at 1108C., obtained in the first stage, was found to be 600 mg, which was equivalent to 300 mg of total oxide containing around 125 mg ThO 2 but no yttrium. The composition of oxide obtained from different fractions is recorded in Table 3, which indicates that although the leach solution contained 46.7% yttrium ŽY2 O 3rREO., the product solution contained 52% Y2 O 3 due to selective precipitation of lighter rare earths in the thorium fraction.
3.3. SelectiÕe precipitation of thorium with sodium pyrophosphate Thorium precipitated with sodium pyrophosphate according to the following reaction. Th Ž SO4 . 2 q Na 4 P2 O 7 ™ ThP2 O 7 q 2 Na 2 SO4
Ž 2.
The selectivity of thorium with respect to RE in pyrophosphate medium is high at low pH. Thorium
Table 3 Composition of oxide obtained in counter current precipitation of thorium Metal
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu ThO 2 a
Composition of oxide Žwt.% on REO basis. Leach solution
Product solution
Thorium-rich precipitate
8.0 15.0 2.0 6.0 1.5 0.5 2.3 0.56 5.3 1.3 46.7 4.5 0.7 4.9 0.7 4.34
7.5 11.06 1.85 5.3 1.5 0.5 2.1 0.57 5.8 1.4 52.0 5.0 0.77 5.3 0.8 n.d.
12.0 50.0 3.4 12.0 n.d.a n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 40.0
n.d.s not detected Ž - 0.1%..
R. Vijayalakshmi et al.r Hydrometallurgy 61 (2001) 75–80 Table 4 Results of thorium precipitation with sodium pyrophosphate Sample no.
Volume of NH 4 OH Ž4 M. wmLx
Volume of sodium pyro phosphate solution wmLx
Thorium separation w%x
1 2 3 4 5
nil nil 24.6 20.0 18.8
3 10 0.5 5.0 10.0
8.0 9.0 90 )99 )99
Xenotime sulphate solution: 50 mL containing 20 g REOrL and 1.35 M H 2 SO4 . Precipitant: 7% Žwrv. sodium pyro phosphate solution.
was precipitated at ambient temperature with 7% Žwrv. sodium pyrophosphate solution from a xenotime sulphate solution containing 19.7 g REOrL and 1.35 M H 2 SO4 . Effects of various parameters such as amount of sodium pyrophosphate and ammonia had been studied and results recorded in Table 4 indicate that sodium pyrophosphate alone could not precipitate thorium quantitatively, even though it was added three times more than the stoichiometric amount required for thorium. Hence, for effective separation of thorium, pH of the solution was raised with ammonia in the presence of sodium pyrophosphate. It was observed that about 20 mL ammonia and 350 mg sodium pyrophosphate is required for 50 mL leach liquor. Thorium pyrophosphate precipitate contains 110 mg oxide including 8 mg Y2 O 3 . Yttrium recovery was 98.2%. Although this process was a single-step precipitation, the chemical consumption and Y loss were higher compared to ammonia precipitation. 3.4. Separation of other impurities The solution obtained after thorium removal either by ammonia or by sodium pyrophosphate precipitation, contained uranium, iron, sulphate and phosphate apart from RE and yttrium. These impurities were separated by treatment with oxalic acid where uranium and iron form soluble oxalate complexes and pass into the filtrate along with sulphate and phosphate. The rare earth oxide obtained after
79
calcination of oxalate was dissolved in HCl to form a feed for a solvent extraction process used for purification of yttrium. In the case of monazite, the recovery of yttrium was carried out essentially in three stages. The first stage was the fractionation of rare earths to obtain heavy rare earth concentrate Ž60% Y2 O 3 . along with other concentrates by adopting a solvent extraction process. The second stage was concerned with further enrichment to 93% Y2 O 3 by solvent extraction w9x. The final stage was also based on solvent extraction to further enrich the 93% grade to 99.9% Y2 O 3 w9x. The oxide obtained from xenotime, on the other hand, was taken as a feed for the subsequent two stages of solvent extraction to produce 99.9% pure Y2 O 3 .
4. Conclusion A process has been described for extraction of rare earth oxides from an intermediate grade xenotime concentrate by sulphuric acid digestion. The process resulted in more than 98% recovery of rare earth oxides and can be utilised on a large scale. Thorium removal from xenotime sulphate leach liquor had been studied by two-stage counter current precipitation with ammonia and single step precipitation with sodium pyrophosphate. Both processes resulted in more than 99% recovery of thorium. Rare earth oxides had been recovered and separated from other impurities by oxalate precipitation.
References w1x D.C. Banerjee, R.P. Sinha, K.K. Dwivedy, Xenotime resource in India—an overview, Proc. Recent Development In the Science and Technology of Rare Earths, Cochin, 1995, p. 14. w2x P. Alex, A.K. Suri, C.K. Gupta, Processing of xenotime concentrate, Hydrometallurgy 50 Ž1998. 331. w3x R.C. Vickery, The Chemistry of Yttrium and Scandium, Pergamon, New York, 1960, p. 34. w4x W.J. Ross, J.C. White, Extraction and determination of thorium from sulphate and phosphate with tri n-octyl phosphine oxide, Anal. Chem. 31 Ž1959. 1847. w5x F.L. Cuthbert, Thorium Production Technology, Wesley Publishing Company Inc., Reading, Massachusetts, USA, 1958, p. 72.
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w6x D.I. Ryabchikov, E.K. Golbraikh, Analytical Chemistry of Thorium, Pergamon Press, New York, USA, 1963. w7x A.N. Zelikman, E. Krien, G.V. Samsonov, Metallurgy of rare earths, Israel Program for Scientific Translation, 1966, p. 275. w8x D.K. Singh, S.L. Mishra, H. Singh, C.K. Gupta, Extraction studies of uranium with 2-Ethyl hexyl phosphonic acid mono-
2-ethyl hexyl ester ŽPC 88A., J. Radioanal. Chem. 245 Ž3. Ž2000.. w9x S.M. Deshpande, S.L. Mishra, R.B. Gajankush, N.V. Thakur, K.S. Koppiker, Recovery of high purity Y2 O 3 by solvent extraction route using organo phosphorus extractants, Miner. Process. Extr. Mtall. Rev. 10 Ž1992. 267.