J Radioanal Nucl Chem (2017) 312:277–283 DOI 10.1007/s10967-017-5234-x
Study of scandium and thorium sorption from uranium leach liquors Alexey L. Smirnov1 • Svetlana M. Titova1 • Vladimir N. Rychkov1 • Grigory M. Bunkov1 • Vladimir S. Semenishchev2 • Eugeny V. Kirillov1 Nikolay N. Poponin3 • Ilya A. Svirsky1
•
Received: 5 December 2016 / Published online: 27 March 2017 Ó Akade´miai Kiado´, Budapest, Hungary 2017
Abstract Scandium and thorium sorption from simulated uranium leach liquors by phosphorous containing ion exchange resins was studied. Increase of thorium concentration resulted in a decrease of scandium sorption by 26–65%. Tulsion CH 93 resin was chosen for Sc separation from uranium leach liquors. It was shown that 180 g L-1 Na2CO3 allowed for elution 94.1% of Sc and 98.9% of Th in dynamic conditions. Using (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture for primary Sc/Th separation at the resin/eluent ratio of 1:5 resulted in thorium desorption degree as high as 66–69%, whereas scandium loss did not exceed 10%. Keywords Scandium/thorium separation Uranium leach liquors Ion exchange Desorption
Introduction A high diversity of scandium applications results in a high demand of this metal in the global market. Among the main scandium consumers there are metallurgy, ceramics and catalyst production, electronics, atomic energy. Production
& Vladimir S. Semenishchev
[email protected] 1
Department of Rare Metals and Nanomaterials, Ural Federal University, 19, Mira Street, Yekaterinburg 620002, Russia
2
Department of Radiochemistry and Applied Ecology, Ural Federal University, 19, Mira Street, Yekaterinburg 620002, Russia
3
Joint-Stock Company ‘‘Dalur’’, 42, Lenina Street, Uksyanskoe 641750, Russia
of oxide fuel cells is also prospective application for scandium [1–3]. Scandium is not rare, but very scattered element. The only six minerals are known, where scandium is the main component [4]; however, there are no commercially available deposits of these minerals. Isomorphous replacement of certain elements by scandium is possible, resulting in scandium presence in various ores and rocks. Therefore, certain kinds of industrial waste (e.g., coal ash, red mud, sulfuric acid after TiO2 production, uranium leach liquors, etc.) may be used as a potential scandium resource [5–11]. In spite of the fact of different charges of Sc3? and 4? Th , thorium shows some chemical properties similar to those for scandium; for example, works [12, 13] reported about very similar ion exchange behavior of these metals. Presence of thorium isotopes in uranium leach liquors may result in a high activity of scandium concentrates recovered from these liquors [14]. In our previous publication [15] we reported that activity of the primary scandium concentrate recovered from uranium leach liquors was as high as (1.8 ± 0.25) 9 108 Bq kg-1. A number of methods for scandium separation and concentration from mineral acids solutions is developed. Cation exchange resins Dowex 50 W 9 4, Dowex 50 W 9 8 and KU-2 with sulfonic groups, KB with carboxylic groups [13], Purolite D5041 with phosphate groups, chelating resin Purolite S 957 as well as aminomethyl phosphonic ampholytes Purolite S 950, Purolite S 940, Lewatit TP 260 Monoplus [16] may be used for scandium separation from sulfuric acid solutions. Anion exchange resins have also found a limited application for scandium sorption from acidic solutions; for example, Dowex 1 9 8 shows a high selectivity for scandium in a mixture of nitric acid and methanol that is used in analytical chemistry [17].
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Liquid extraction is also successfully used for scandium separation from sulfuric acid solutions and for scandium purification. Di(2-ethylhexyl)orthophosphoric acid (HDEHP) shows a very high selectivity for scandium. Use of 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester (as known as P-507, PC-88A or Ionquest 801) allows for scandium extraction degree as high as 98% from 1 to 5 M sulfuric acid. The extraction degree of Sc by trialkyl phosphine oxide (Cyanex 923) from strong acidic solution is reported to be 92% [11]. Selectivity of phosphinic acids (Cyanex 272, Cyanex 301, Cyanex 302) for scandium is lower as compared with selectivity of HDEHP and P 507; however, re-extraction from these extractants is easier and more effective [18]. Scandium extraction by various extraction chromatographic resins is also reported [17]. Liquid extraction processes may be used for thorium separation from sulfuric acid solutions. For example, neutral phosphorous organic extractant Cyanex 923 and some organic amines are effective for this aim [19]. Use of Cyanex 923 as an impregnate for Chromosorb 102 allows for 80% of Th extraction from 10 M sulfuric acid [20]. Use of 2-ethylhexyl phosphonic acid mono 2-ethylhexyl ester is also reported to be successful [21]. Sorption separation of thorium is performed mostly from nitric acid solutions by cation exchange resins. For example, sorption degree of Th by the Chellex-100 cation exchange resin from HNO3 solution at pH 2.0 is reported to be 80% [22]. The possibility of a selective thorium separation from aqueous solutions at pH 2.8–3.2 using the mixture of the cation exchange resin KU-23 in Na? form and the anion exchange resin AN-31 or AV-17 in OH– form is reported in the work [12]. Extraction chromatographic resins are also used for thorium separation from acidic solutions. Horwitz et al. TM [23] showed a high effectiveness of the TRU-Spec (the mixture of octyl(phenyl)-N,N-dibutyl carbamoylmethyl phosphine oxide and tributylphosphate impregnated onto the Amberchrom CG-71 ms inert support) for thorium separation from nitric and chlorine acidic solutions in a wide acid concentrations range. Industrial cycle at enterprises of uranium in situ leaching includes two main steps: in situ leaching process and further hydrometallurgical treatment of uranium containing solutions after leaching. Anion exchange resins used for uranium separation from leachates are not able to separate scandium; therefore, the uranium leach liquors after uranium separation can be used as a potential anthropogenic source of scandium. This paper deals with sorption recovery of Sc and Th from uranium leach liquors containing sulfuric acid. The possibility of Sc/Th partial separation at the desorption step was shown due to the use of a mixture of sulfates and carbonates of alkali metals solutions as an eluting agent.
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Experimental Sorbents pretreatment A number of commercially available phosphorus containing ion exchange resins was used in this study. Before testing, all resins were conditioned by 0.1 M sulfuric acid solution and by 100 g L-1 sodium sulfate solution in order to transfer the resin into Na? form. The Table 1 gives the main characteristics of studied resins. Study of thorium concentration influence on scandium sorption The influence of thorium concentration on scandium sorption was studied under batch conditions in order to choose the most suitable ion exchange resin for scandium recovery from uranium leach liquors. Simulated solutions containing 95.4 mg L-1 of scandium and 5 g L-1 of sulfuric acid were used in sorption experiments. Thorium concentration was varied from 1.6 to 184.2 mg L-1. Chemically pure reagent Sc2(SO4)35H2O was used in the experiment. Chemically pure Th(NO3)44H2O was converted to hydroxide and then dissolved in sulfuric acid. A 0.1 g sample of air dry ion exchange resin was contacted with 50 mL of the simulated solution and shaken for 24 h at 20–23 °C. After this, an aliquot of the solution was sampled and analyzed for scandium by ICP-MS using the mass spectrometer «Elan 9000» (Perkin Elmer, USA). Exchange capacity of the resin E was calculated in accordance with the following Eq. (1): E¼
ðC0 C Þ V m
ð1Þ
where C0 and C are scandium concentrations before and after sorption respectively (mg L-1), V is volume of the simulated solution (L), m is weight of the resin (g). Scandium and thorium distribution coefficients Kd were calculated according to the Eq. (2): Kd ¼
C0 C V C m
ð2Þ
Dynamic sorption experiments Sorption recovery of Sc, Th and some other elements was performed under dynamic conditions from real uranium leach liquors. The solution was passed through a column with 5 mL of an ion exchange resin at the flow rate of 25 mL h-1. Sulfuric acid concentration in the uranium leach liquor was 5 g L-1. Concentrations of the main elements in the uranium leach liquors are given in Table 2.
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Table 1 The main characteristics of studied ion exchange resins Ion exchange resin
Functional group
Resin type
Form
Moisture content in air dry resin (%)
Density (g cm-3) Air dry resin
Swollen in distilled water
Purolite D 5041
Phosphate
Cationite
Na?
5.39
1.9
3.0
Tulsion CH 93
Aminomethyl phosphonic
Ampholyte
Na?
5.10
2.0
2.7
Lewatit TP 260
Aminomethyl phosphonic
Ampholyte
Na?
5.11
1.6
2.4
Ampholyte
?
5.57
2.0
2.6
Purolite S 950
Aminomethyl phosphonic
Na
Study of desorption under batch conditions Oxalic acid (150 g L-1), hydrofluoric acid (100 g L-1) and sodium carbonate (180 g L-1) solutions were tested as potential eluting agents for scandium. 2 mL of a saturated resin was contacted with 10 mL of an eluent at 20–22 °C for 24 h. Then the resin was eliminated from the solution, the precipitate was filtered, washed and dissolved in concentrated nitric acid. The final solution was analyzed by ICP-MS. Concentrated nitric acid was also tested to be an eluting agent. Desorption by nitric acid was performed using the following method. 2 mL of a saturated resin was contacted with 10 mL of conc. nitric acid, heat to boiling and then cold to room temperature. Then the eluate was decanted to a 100 mL volumetric flask; a new portion of nitric acid was added to the resin and all steps described above were repeated. Both eluates were joined in one volumetric flask and then diluted by water to 100 mL. An aliquot of the final solution was analyzed by ICP-MS. Study of desorption under dynamic conditions Desorption was performed under dynamic conditions using 180 g L-1 sodium carbonate solution from a column with 5 mL of an ion exchange resin at the flow rate of 25 mL h-1. The precipitate from the eluate was dissolved in concentrated nitric acid and analyzed by ICP-MS. Selective thorium elimination from resin after thorium and scandium sorption The experiments on scandium/thorium separation in the desorption step were performed under batch conditions at the resin/eluent ratio of 1/5. Mixtures of Na2SO4 ? Na2CO3, K2SO4 ? K2CO3 and (NH4)2SO4 ? ACBM. ACBM is a Table 2 Chemical composition of uranium leach liquors after uranium separation by anion exchange Element
Sc
Th
Al
Fe
Ti
U
Concentration (mg L-1)
0.78
1.81
2086.5
1488.8
2.47
0.92
vendible reagent, a mixture of 30% (NH4)2CO3 ? 70% NH4HCO3. Time of phases contact was 24 h. After desorption the resin was separated from the eluate; the precipitate from the eluate was dissolved in concentrated nitric acid and analyzed by ICP-MS.
Results and discussion Study of thorium concentration influence on scandium sorption Scandium sorption by phosphorus containing ion exchange resins in acidic solutions occurs due to cation exchange mechanism; scandium is adsorbed mostly as complex cations [ScSO4]? [15]. Mechanism of thorium sorption is very similar to this of scandium [24]. Values of exchange capacities of various resins with respect to Sc and Th depending on thorium concentration are presented in Fig. 1. Values of distribution coefficients are given in Table 3. The decrease of resins capacities and distribution coefficients for Sc with the increase of thorium concentration shows to a competitive thorium influence on scandium sorption over the whole thorium concentrations range. In case of ion-exchangers Purolite D 5041 and Tulsion CH 93 the capacity decrease was the minimal among other resins being 26.6 and 44.5% respectively. The Purolite S 950 cation exchange resin showed the highest capacity decrease (65.6%). The Tulsion CH 93 ampholyte showed the highest ion exchange parameters: the capacities and scandium distribution coefficients are maximal over the whole thorium concentrations range being studied. Thus, this ion exchange resin was chosen for Sc recovery from uranium leach liquors and it was used in all following experiments. Dynamic sorption experiments Sorption curves of scandium, thorium and main constituents on Tulsion CH 93 resin from uranium leach liquors are shown at Fig. 2.
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Fig. 1 The dependences of exchange capacities of various ion exchange resins to scandium (a) and thorium (b) on thorium concentration
E , mg of metal per g of resin
(a)
45 Tulsion CH 93
Purolite D 5041
Lewatit TP 260
Purolite S 950
35
Sc 25
15
5 0
20
40
60
80
100
120
140
160
180
200
Inial thorium concentraon, mg L-1
(b)
35 Tulsion CH 93
Purolite D 5041
Lewatit TP 260
Purolite S 950
E , mg of metal per g of resin
30 25 20 15 10 5
Th
0 0
20
40
60
80
100
120
140
160
180
200
Initial thorium concentration, mg L-1
Table 3 Scandium and thorium distribution coefficients for various resins
C0(Th) (mg L-1)
1.6
Kd (mL g-1) Tulsion Ch 93
Purolite D 5041
Purolite S 950
Lewatit TP 260
Sc
Th
Sc
Th
Sc
Th
Sc
Th
11879.6
1269.0
1324.0
4355.8
1538.5
8288.1
748.71
500.18
26.2 41.8
1168.68 766.92
774.58 366.02
478.68 444.35
304.25 207.11
243.81 210.62
90.03 76.58
400.82 424.85
329.83 257.02
87.3
600.63
339.38
366.10
224.62
157.44
110.45
242.58
180.63
132.8
505.00
307.20
319.85
211.79
148.47
87.15
235.37
168.92
184.2
493.14
309.98
296.50
181.11
136.67
81.72
254.18
164.82
As it might be seen at Fig. 2, full saturation of the resin by scandium and thorium was achieved after pass of 700 and 645 bed volumes of the uranium leach liquor
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respectively. Values of Full dynamic exchange capacity of the Tulsion CH 93 resin with respect to studied elements are given in Table 4. Overall, the resin showed very similar
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281
Feeding concentration / outlet concentration
1
Table 5 Degree of scandium desorption from the saturated Tulsion CH 93 resin by various eluents (solid:liquid = 1:5)
0.8
Eluting agent
Degree of desorption (%)
0.6
0.4
Th
Al
Fe
Ti
U
Al
HNO3 conc.
55.5
11.0
9.2
75.0
4.0
56.0
Fe
H2C2O4 150 g L-1
1.6
1.2
10.0
91.7
1.0
14.0
Th
HF 100 g L-1 Na2CO3 180 g L-1
46.3 93.9
2.2 95.3
39.4 0.4
46.4 16.4
50.4 54.1
5.9 62.1
Sc
0.2
Sc
Ti U
0 200
400
600
800
1000
Volume, B.V.
700
Fig. 2 Sorption curves of certain elements on Tulsion CH 93 resin
selectivity for Sc and Th; therefore, we have tried to perform Sc/Th separation during further steps of leach liquors treatment. Study of desorption under batch conditions Alkali metals and ammonium carbonates, hydrofluoric acid and ammonium fluoride are known to be the most typical eluting agents for scandium desorption from phosphorus containing ion exchange resins. The effectiveness of agents for Sc desorption changes in accordance with the following series: Na2CO3 [ NaHCO3 [ (NH4)2CO3 [ NH4F [ H2SO4 [ Na2S2O3 [ HCl [ Na2SO4 [13]. We have used sodium carbonate, oxalic and hydrofluoric acids solution as well as concentrated nitric acid as potential eluting agents. Optimal concentration of Na2CO3 in the eluent was determined in our previous work [25]. The values of degree of scandium desorption from the saturated Tulsion CH 93 resin by various eluents under batch conditions are given in Table 5. As it might be seen, 180 g L-1 Na2CO3 solution was the most effective eluent for scandium; low cost of sodium carbonate is another one advantage of this reagent. However, sodium carbonate solution was not selective eluent for scandium; it also effectively eluted thorium preventing Sc/Th separation, as it might be seen at eluting curves (Fig. 3) and in values of degrees of desorption for these elements (Table 6). As it might be seen, sodium carbonate solution is an effective eluent for both scandium and thorium. Use of this reagent seems to be suitable for scandium stripping from
Outlet concentration, mg L-1
0
Al
600
Sc
500
Fe Th
400
U
300
Ti
200 100 0 0
5
10
15
20
25
Eluent volume, B.V.
Fig. 3 Eluting curves of certain elements from the Tulsion CH 93 resin by Na2CO3 (180 g L-1) solution
Table 6 Degrees desorption of certain elements from the Tulsion CH 93 resin by Na2CO3 (180 g L-1) solution under dynamic conditions Element
Sc
Th
Al
Fe
Ti
U
Degree of desorption (%)
94.1
98.9
21.3
28.0
18.7
64.1
the saturated resin; however, this will require additional separation step. Furthermore, precipitate formation is the main disadvantage of this eluent; therefore, desorption should be preferably performed under batch condition. A low desorption degree of other elements (Fe, Al, Ti, U) will require an additional step of the resin regeneration. Selective thorium elimination from resin after thorium and scandium sorption A significant quantity of thorium (especially 234Th and Th, daughters of 238U) results in a high radioactivity of the scandium concentrate [15]. Selective thorium
230
Table 4 Full dynamic exchange capacity of the Tulsion CH 93 resin for certain elements Element
Sc
Th
Al
Fe
Ti
U
E, mg of metal per g of resin
0.28
0.58
7.41
3.21
1.38
0.38
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Table 7 Desorption degrees of Sc and Th under batch conditions by various composite eluents
Composition of the eluent
Sc
Th
Na2SO4(50 g L-1) ? Na2CO3 (180 g L-1)
93.8
75.9
Na2SO4(50 u g L-1) ? Na2CO3 (200 g L-1)
94.6
70.9
Na2SO4(75 g L-1) ? Na2CO3 (100 g L-1)
59.7
57.5
Na2SO4(75 g L-1) ? Na2CO3 (150 g L-1)
74.7
98.8
-1
-1
Na2SO4(75 g L ) ? Na2CO3 (200 g L )
95.1
69.4
K2SO4(50 g L-1) ? K2CO3 (150 g L-1)
38.1
58.8
K2SO4(50 u g L-1) ? K2CO3 (180 g L-1)
56.8
61.8
K2SO4(50 g L-1) ? K2CO3 (200 g L-1)
62.5
64.1
K2SO4(75 g L-1) ? K2CO3 (150 g L-1)
36.3
56.7
K2SO4(75 g L-1) ? K2CO3 (180 g L-1)
58.4
69.3
K2SO4(75 g L ) ? K2CO3 (200 g L ) (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1)
86.3 10.1
99.1 69.5
(NH4)2SO4 (75 g L-1) ? ACBM (150 g L-1)
34.8
52.5
(NH4)2SO4 (75 g L-1) ? ACBM (180 g L-1)
47.1
91.6
-1
-1
Table 8 Desorption degrees of Sc and Th by the (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture versus resin/eluent ratio Resin/eluent ratio
Desorption degree (%) Sc
1:2.5
Th
2.5
26.5
1:5
9.7
66.3
1:10 1:20
35.3 57.9
98.3 99.3
elimination from resin after thorium and scandium sorption may be used for primary deactivation of this concentrate. A number of mixtures of alkali metals and ammonium sulfates and carbonates was tested for Sc/Th separation. The results of batch experiments are given in Table 7. The maximal Sc/Th separation was achieved by use of the (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture. In this case a high concentration of ammonium sulfate depresses scandium desorption from the resin; whereas, thorium elutes from the resin due to formation of very stable complex anions, such as [Th(CO3)4]4- and [Th(CO3)5]6- because of a high carbonate concentration [26]. The results of Sc and Th desorption by the (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture under batch conditions as a dependence on resin/eluent ratio are given in Table 8. As it might be seen in Table 8, the maximal Sc/Th separation was achieved at the resin/eluent ratio of 1:5. A lower resin/eluent ratio resulted in a weak thorium elimination, whereas a higher resin/eluent ratio resulted in unacceptably high loss of scandium (more than 10%).
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Desorption degree (%)
Furthermore, use of solutions containing alkali metals is not recommended for Sc/Th separation because scandium desorption degree was high, probably due to a strong competitive effect of sodium and potassium. Thus, use of the (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture as an eluting agent resulted in desorption of 2/3 of Th, whereas scandium desorption was less than 10%. This should result in a decrease of activity of the scandium concentrate and should allow for simplification of further treatment and decontamination of this concentrate [15].
Conclusions Scandium and thorium sorption from uranium leach liquors by various phosphorus containing ion exchange resins was studied. It was shown that competitive influence of thorium results in decrease of distribution coefficients and ion exchange capacity for scandium. The Tulsion CH 93 ampholyte showed the highest ion exchange capacity (22.6–40.0 mg of Sc per g of the resin) and distribution coefficient (493.1–11879.6 L kg-1) for scandium over the whole studied thorium concentrations range; this resin was chosen for further experiments. The value of full dynamic exchange capacity of the Tulsion CH 93 resin for Sc and Th was 0.27 and 0.58 mg g-1 respectively. It was shown that 180 g L-1 Na2CO3 was the best eluent for scandium, but it did not provide Sc/Th separation (Sc and Th desorption degrees in dynamic conditions were 94.1 and 98.9% respectively). Use of this eluent should result in a significant radioactivity of scandium concentrate. Also, sodium carbonate solution showed low degrees of
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desorption for other elements that should result in the necessity of the resin additional regeneration. The possibility of use of the (NH4)2SO4 (50 g L-1) ? ACBM (180 g L-1) mixture for primary Sc/ Th separation at the desorption step was shown. Then this process was performed under batch conditions at the resin/ eluent ratio of 1:5, thorium desorption degree was as high as 66–69%, whereas scandium loss did not exceed 10%. Acknowledgements Work was performed under contract with the Ministry of Education of the Russian Federation of April 27, 2016 No 02.G25.31.0210.
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