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Recovery of scandium from magnesium, aluminium and iron scrap
hydrometallurgy ELSEVIER
Hydrometallurgy
44 (1997) 179- 184
Recovery of scandium from magnesium, aluminium and iron scrap A. Ditze a9*, K. Kongolo b a Department of Nonferrous Metals, Technical University of Clausthal, D-38678 Clausthal-Zellerfeld, Germany b Polytechnic Faculty University, Lubumbashi, Zaire Received 8 March 1996; accepted 25 April 1996
Abstract A hydrometallurgical
route for recovering
of scandium
from magnesium,
aluminium
and iron
dross or scrap was developed and the extraction isotherm for scandium with HDEHE established. After a single stage of leaching, extraction and stripping, nearly 100% recovery of scandium as scandium oxide resulted, while the separation of magnesium 100%. About 10% of the iron input was co-extracted.
from scandium
was also nearly
1. Introduction The rare earth element scandium is increasingly used in the metallurgy of magnesium [l], aluminium [2] and iron. Scandium improves the precipitation of spheroidal graphite in cast iron and gives significant improvements to high strength and creep resistant Al-Mg, Al-Li and Mg alloys at high temperatures. Because of its high reactivity with oxygen, chlorine and fluorine, there are considerable scandium losses during the production of these alloys. Smelting with and without salt yields salty wastes or oxidic/metallic dross containing the valuable rare earth metal scandium. The hydrometallurgical route leaching-solvent extraction-precipitation offers the chance of selective recycling of this metal - especially in combination with a remelting process for the alloys. For magnesium and aluminium alloys containing high amounts of scandium, even the recycling of the used products for scandium recovery using the following method may be of interest.
* Corresponding
author. E-mail: Andrc.Ditze@TU-ClausthaLde
0304-386X/97/$17.00 Copyright PII SO304-386X(96)00041-2
0 1997 Elsevier Science B.V. All rights reserved.
A. Dim.
180
2. Extraction
K. Kon~olo/Hydrontrttrllur~y
44 (IYY7J 179-184
of scandium
Synthetic leach solutions were made of ScCI, 6H,O, M&l, .6H,O, hydrochloric acid and water. This system was chosen, because leaching of scrap, dross or waste with hydrochloric acid should give no significant problems and in smelting and remelting chloride salts are often used. With this combination the whole concept remains in the chloride system. One solution contained 2.5 g/l SC and 0.5 mol of HCl, the other solution investigated had 2.5 g/l SC, 25 g/l Mg and 0.5 mol of HCl, assuming scrap with about 10% SC and 90% Mg. The solvent extraction using these solutions was carried out using the well known [3] and cheap extractant di-2-ethylhexyl phosphoric acid (HDEHP) in kerosene as the diluent and tributyl phosphate (TBP) as the modifier at room temperature. The best mixture for these solutions was found to be 20 ~01% HDEHP, 15 ~01% TBP and 65 ~01% kerosene with no significant volume change after mixing of the single components. TBP stabilizes the organic phase and inhibits third phase formation. More than 15 ~01% of TBP resulted in slow separation of the liquid phases. The extractant prior to use was equilibrated with hydrochloric acid (6.25 M), the SC and Sc-Mg solution, sodium hydroxide solution (5 M) and finally with hydrochloric acid again. Scandium is very extensively extracted from acid solutions and the minimum hydrochloric acid necessary was found to 0.5 M. If there was less hydrochloric acid, the organic phase was destroyed. The extraction isotherm was obtained with single phase contacts of the aqueous feed with different volumes of organics. Mixing time was 15 min. The total volume was 200 cm’. Fig. 1 shows the extraction isotherm for the partition of scandium between the organic and aqueous solutions. Assuming the simple reaction mechanism: SC3+ + 3HDEHP = Sc( DEHP) 3 + 3H + the maximum load of the organic phase under these conditions is about 9.55 g/l SC. However, all the values in Fig. 1 to the right of the abscissa are only of scientific
0
0,5
1 I,5 g/l Sc aqu
2
a Extraction Sc-Mg :>Extraction SC Fig. 1. The extraction
23
I
isotherm for the partition of scandium between the organic and aqueous solutions.
A. Dim,
K. Kongolo/Hydromercrllur~y
44 (1997) 179-184
181
0
UI
60
20 t 0’
0 Fig. 2. Scandium
1
2
extraction
3 O/A
4
5
as a function of the O/A
6 ratio.
interest, because the organic phases are very viscous and the phase separation is poor. O/A ratios greater than 0.4 gave nearly 100% scandium extraction from the investigated solutions, that is, the scandium in raffinate was smaller than 1 mg/l. Scandium extraction as a function of the O/A ratio is shown in Fig. 2. The average magnesium co-extraction was 1.5%.
3. Stripping
of scandium
Stripping of the loaded solvent was carried out with sodium hydroxide solution after equilibrating the organic phase with the 2.5 g/l SC, 25 g/l Mg and 0.5 M HCl solution and scrubbing the organic phase with 4 M sodium chloride solution free of magnesium. The advantage is that solid and pure scandium hydroxide are obtained directly. Stripping of scandium with hydrochloric acid at any concentration was not possible. First there has to be enough aqueous solution to collect the total amount of the hydroxide so the maximum O/A ratio was 2; otherwise the separation of the scandium hydroxide from the organic phase was not complete. Therefore no stripping isotherm was established. Stripping with 2 M sodium hydroxide gave good settling but considerable solubility of the organic phase loaded with scandium in the aqueous phase. Scandium in the aqueous phase then exists both as a hydroxide (about 60%) and as a dissolved organic scandium compound (about 40%). In this case the solvent must be renewed after about five loading cycles. At higher sodium hydroxide concentrations the solubility of the organic phase was reduced but the grain size of the scandium hydroxide was also smaller and direct separation of the hydroxide particles was no longer possible. Nevertheless. 5 M sodium hydroxide was used for stripping, because the solubility of the solvent is then very small. For solid-liquid separation, dilution with water was necessary and for settling after dilution the anionic flocculant Supeffloc A 100 (Cytec Industries) gave good results. Scandium extraction from the organics was nearly 100% in one stripping
182
A. Dim, K. Kangolo/Hydrometallur~y
44 (1997) 179-184
stage. The remaining scandium (and magnesium) in the organic phase was less than 1 mg/l.
4. Aluminium-scandium
solutions
Due to the importance of scandium-containing aluminium alloys, some experiments with a synthetic 2.5 g/l SC, 25 g/l Al and 0.5 M HCl solution were carried out. Extraction, scrubbing and stripping operations were comparable with the scandium magnesium solutions. Scandium recovery was nearly 100%
5. Treatment
of scandium-containing
magnesium
dross
With real dross, supplied from MEL (Magnesium Electron Ltd.), the extraction of scandium was checked and the complete process at a laboratory scale was developed. The reason for smelting high scandium-magnesium alloys at MEL is the establishment of a research program for magnesium alloys at the universities of Clausthal and Hannover, financed by the Deutsche Forschungsgemeinschaft (DFG). The flowsheet of the process is shown in Fig. 3. The average composition of the dross was 12-23% SC, 64-77% Mg, l-1.6% Fe, and O-4% leaching residue. The leaching residue consisted mainly of carbon and silica. In each case 30 g of dross were leached with hydrochloric acid to give 1 1 of solution for extraction. The total amount of dross leached was 90 g. Fig. 3 shows the amounts of liquids and the distribution of scandium, magnesium and iron. Extraction, washing, stripping and acidification of the solvent are single-stage operations. The mixing time was 15 min in each case. Phase separation at the extraction, washing and acidification stages presented no problem. However, phase separation after stripping has to be done carefully. Gently stirring the organic phase with a special stirrer at 25 rotations per minute allowed the very fine grained scandium hydroxide to settle. The amount of hydrochloric acid fed to the acidification stage was sufficient for conversion of the solvent to the hydrogen form, washing out the impurities in the washing stage and leaching the scandium-containing dross. At the stripping and acidification stages, in particular, there were considerable volume changes in the organic and aqueous phases. However, the sum of organic and aqueous volume input and output did not change much, as can be seen in Fig. 3. The ‘dilution’ and ‘solid-liquid separation’ operations have not yet been fully optimized. The flocculant used, Superfloc A 100, worked only after feeding a considerable amount of water. To minimize the waste water stream some flocculants used in alumina production should be tested. Without flocculation no filtration of the fine grained hydroxide was possible. Calcination had to be carried out at temperatures of at least 700°C and gave Sc,O, with a composition of 64.5% SC, 0.5% Mg and 0.4% Fe. It can be seen that about 10% of the iron input is co-extracted. It is known that trivalent iron is extracted with HDEHP at high acid concentrations [4,5] and stripping with high acid concentrations is then difficult or even impossible. Thus, nearly
A. Ditze, K. Kongolo/Hydrometallurgy
44 (1997) 179-184
183
l.lOMg 3.90 Fe
99.9g 90.0
Fe
Flocculant
Solid - liquid Separation
-
Waste
ScPh
* Drying Calcino’ion 100.0 SC 0.1 Mg 10.0 Fe
sc,o,
1
Fig. 3. Flow sheet for scandium extraction
from Mg-Sc
dross.
64.5% 0.5% 0.4% 34.6%
SC Mg Fe 0
Water
b
184
A. D&e, K. Kongolo/
Hydmmetullur~y 44 (1997) 179-184
all of the iron which is extracted together with scandium will be found in the product after stripping with sodium hydroxide. This can be avoided by reduction of ferric iron to ferrous iron.
6. Conclusion Scandium-containing magnesium and aluminium scrap or dross can be processed by a hydrometallurgical route. Starting from synthetic Mg-Sc and AI-SC solutions it has been shown that extraction of scandium from chloride solutions with HDEHP and stripping with sodium hydroxide in a single stage respectively resulted in nearly 100% scandium recovery. An extraction isotherm has been established. A complete process was developed, with leaching with hydrochloric acid, solvent extraction, washing, stripping with sodium hydroxide, solid-liquid separation and calcination. The product of the process was Sc,O,.
References [l] Drits, M.E., Sviderskaya, Z.A. and Nikitina, N.I., New Magnesium alloys for operation at elevated temperature. In: Co11 Nonferreous Metal Alloys. Nauka, Moscow (1972), pp. 193- 197. [2] Anon, Abstracts of reports. Int. Conf. Scandium and Prospects of its Use (Moscow, 18-19 October, 1994). [3] Ritcey, G.M. and Ashbrook, A.W., Solvent Extraction. Principles and Applications to Process Metallurgy. Elsevier, Amsterdam, Part I (19841, Part II (1979). [4] Yu, S. and Chen, J., Stripping of Fe(III) extracted by di-2.ethylhexyl phosphoric acid from sulfate solutions with sulfuric acid. Hydrometallurgy, 22 (1989): 267-272. [5] Green, G.K. and Harbuck, D.D., Miner. Metall. Process., Febr. (19%): l-3.
Hydrometallurgy
44 (1997) 179- 184
Recovery of scandium from magnesium, aluminium and iron scrap A. Ditze a9*, K. Kongolo b a Department of Nonferrous Metals, Technical University of Clausthal, D-38678 Clausthal-Zellerfeld, Germany b Polytechnic Faculty University, Lubumbashi, Zaire Received 8 March 1996; accepted 25 April 1996
Abstract A hydrometallurgical
route for recovering
of scandium
from magnesium,
aluminium
and iron
dross or scrap was developed and the extraction isotherm for scandium with HDEHE established. After a single stage of leaching, extraction and stripping, nearly 100% recovery of scandium as scandium oxide resulted, while the separation of magnesium 100%. About 10% of the iron input was co-extracted.
from scandium
was also nearly
1. Introduction The rare earth element scandium is increasingly used in the metallurgy of magnesium [l], aluminium [2] and iron. Scandium improves the precipitation of spheroidal graphite in cast iron and gives significant improvements to high strength and creep resistant Al-Mg, Al-Li and Mg alloys at high temperatures. Because of its high reactivity with oxygen, chlorine and fluorine, there are considerable scandium losses during the production of these alloys. Smelting with and without salt yields salty wastes or oxidic/metallic dross containing the valuable rare earth metal scandium. The hydrometallurgical route leaching-solvent extraction-precipitation offers the chance of selective recycling of this metal - especially in combination with a remelting process for the alloys. For magnesium and aluminium alloys containing high amounts of scandium, even the recycling of the used products for scandium recovery using the following method may be of interest.
* Corresponding
author. E-mail: Andrc.Ditze@TU-ClausthaLde
0304-386X/97/$17.00 Copyright PII SO304-386X(96)00041-2
0 1997 Elsevier Science B.V. All rights reserved.
A. Dim.
180
2. Extraction
K. Kon~olo/Hydrontrttrllur~y
44 (IYY7J 179-184
of scandium
Synthetic leach solutions were made of ScCI, 6H,O, M&l, .6H,O, hydrochloric acid and water. This system was chosen, because leaching of scrap, dross or waste with hydrochloric acid should give no significant problems and in smelting and remelting chloride salts are often used. With this combination the whole concept remains in the chloride system. One solution contained 2.5 g/l SC and 0.5 mol of HCl, the other solution investigated had 2.5 g/l SC, 25 g/l Mg and 0.5 mol of HCl, assuming scrap with about 10% SC and 90% Mg. The solvent extraction using these solutions was carried out using the well known [3] and cheap extractant di-2-ethylhexyl phosphoric acid (HDEHP) in kerosene as the diluent and tributyl phosphate (TBP) as the modifier at room temperature. The best mixture for these solutions was found to be 20 ~01% HDEHP, 15 ~01% TBP and 65 ~01% kerosene with no significant volume change after mixing of the single components. TBP stabilizes the organic phase and inhibits third phase formation. More than 15 ~01% of TBP resulted in slow separation of the liquid phases. The extractant prior to use was equilibrated with hydrochloric acid (6.25 M), the SC and Sc-Mg solution, sodium hydroxide solution (5 M) and finally with hydrochloric acid again. Scandium is very extensively extracted from acid solutions and the minimum hydrochloric acid necessary was found to 0.5 M. If there was less hydrochloric acid, the organic phase was destroyed. The extraction isotherm was obtained with single phase contacts of the aqueous feed with different volumes of organics. Mixing time was 15 min. The total volume was 200 cm’. Fig. 1 shows the extraction isotherm for the partition of scandium between the organic and aqueous solutions. Assuming the simple reaction mechanism: SC3+ + 3HDEHP = Sc( DEHP) 3 + 3H + the maximum load of the organic phase under these conditions is about 9.55 g/l SC. However, all the values in Fig. 1 to the right of the abscissa are only of scientific
0
0,5
1 I,5 g/l Sc aqu
2
a Extraction Sc-Mg :>Extraction SC Fig. 1. The extraction
23
I
isotherm for the partition of scandium between the organic and aqueous solutions.
A. Dim,
K. Kongolo/Hydromercrllur~y
44 (1997) 179-184
181
0
UI
60
20 t 0’
0 Fig. 2. Scandium
1
2
extraction
3 O/A
4
5
as a function of the O/A
6 ratio.
interest, because the organic phases are very viscous and the phase separation is poor. O/A ratios greater than 0.4 gave nearly 100% scandium extraction from the investigated solutions, that is, the scandium in raffinate was smaller than 1 mg/l. Scandium extraction as a function of the O/A ratio is shown in Fig. 2. The average magnesium co-extraction was 1.5%.
3. Stripping
of scandium
Stripping of the loaded solvent was carried out with sodium hydroxide solution after equilibrating the organic phase with the 2.5 g/l SC, 25 g/l Mg and 0.5 M HCl solution and scrubbing the organic phase with 4 M sodium chloride solution free of magnesium. The advantage is that solid and pure scandium hydroxide are obtained directly. Stripping of scandium with hydrochloric acid at any concentration was not possible. First there has to be enough aqueous solution to collect the total amount of the hydroxide so the maximum O/A ratio was 2; otherwise the separation of the scandium hydroxide from the organic phase was not complete. Therefore no stripping isotherm was established. Stripping with 2 M sodium hydroxide gave good settling but considerable solubility of the organic phase loaded with scandium in the aqueous phase. Scandium in the aqueous phase then exists both as a hydroxide (about 60%) and as a dissolved organic scandium compound (about 40%). In this case the solvent must be renewed after about five loading cycles. At higher sodium hydroxide concentrations the solubility of the organic phase was reduced but the grain size of the scandium hydroxide was also smaller and direct separation of the hydroxide particles was no longer possible. Nevertheless. 5 M sodium hydroxide was used for stripping, because the solubility of the solvent is then very small. For solid-liquid separation, dilution with water was necessary and for settling after dilution the anionic flocculant Supeffloc A 100 (Cytec Industries) gave good results. Scandium extraction from the organics was nearly 100% in one stripping
182
A. Dim, K. Kangolo/Hydrometallur~y
44 (1997) 179-184
stage. The remaining scandium (and magnesium) in the organic phase was less than 1 mg/l.
4. Aluminium-scandium
solutions
Due to the importance of scandium-containing aluminium alloys, some experiments with a synthetic 2.5 g/l SC, 25 g/l Al and 0.5 M HCl solution were carried out. Extraction, scrubbing and stripping operations were comparable with the scandium magnesium solutions. Scandium recovery was nearly 100%
5. Treatment
of scandium-containing
magnesium
dross
With real dross, supplied from MEL (Magnesium Electron Ltd.), the extraction of scandium was checked and the complete process at a laboratory scale was developed. The reason for smelting high scandium-magnesium alloys at MEL is the establishment of a research program for magnesium alloys at the universities of Clausthal and Hannover, financed by the Deutsche Forschungsgemeinschaft (DFG). The flowsheet of the process is shown in Fig. 3. The average composition of the dross was 12-23% SC, 64-77% Mg, l-1.6% Fe, and O-4% leaching residue. The leaching residue consisted mainly of carbon and silica. In each case 30 g of dross were leached with hydrochloric acid to give 1 1 of solution for extraction. The total amount of dross leached was 90 g. Fig. 3 shows the amounts of liquids and the distribution of scandium, magnesium and iron. Extraction, washing, stripping and acidification of the solvent are single-stage operations. The mixing time was 15 min in each case. Phase separation at the extraction, washing and acidification stages presented no problem. However, phase separation after stripping has to be done carefully. Gently stirring the organic phase with a special stirrer at 25 rotations per minute allowed the very fine grained scandium hydroxide to settle. The amount of hydrochloric acid fed to the acidification stage was sufficient for conversion of the solvent to the hydrogen form, washing out the impurities in the washing stage and leaching the scandium-containing dross. At the stripping and acidification stages, in particular, there were considerable volume changes in the organic and aqueous phases. However, the sum of organic and aqueous volume input and output did not change much, as can be seen in Fig. 3. The ‘dilution’ and ‘solid-liquid separation’ operations have not yet been fully optimized. The flocculant used, Superfloc A 100, worked only after feeding a considerable amount of water. To minimize the waste water stream some flocculants used in alumina production should be tested. Without flocculation no filtration of the fine grained hydroxide was possible. Calcination had to be carried out at temperatures of at least 700°C and gave Sc,O, with a composition of 64.5% SC, 0.5% Mg and 0.4% Fe. It can be seen that about 10% of the iron input is co-extracted. It is known that trivalent iron is extracted with HDEHP at high acid concentrations [4,5] and stripping with high acid concentrations is then difficult or even impossible. Thus, nearly
A. Ditze, K. Kongolo/Hydrometallurgy
44 (1997) 179-184
183
l.lOMg 3.90 Fe
99.9g 90.0
Fe
Flocculant
Solid - liquid Separation
-
Waste
ScPh
* Drying Calcino’ion 100.0 SC 0.1 Mg 10.0 Fe
sc,o,
1
Fig. 3. Flow sheet for scandium extraction
from Mg-Sc
dross.
64.5% 0.5% 0.4% 34.6%
SC Mg Fe 0
Water
b
184
A. D&e, K. Kongolo/
Hydmmetullur~y 44 (1997) 179-184
all of the iron which is extracted together with scandium will be found in the product after stripping with sodium hydroxide. This can be avoided by reduction of ferric iron to ferrous iron.
6. Conclusion Scandium-containing magnesium and aluminium scrap or dross can be processed by a hydrometallurgical route. Starting from synthetic Mg-Sc and AI-SC solutions it has been shown that extraction of scandium from chloride solutions with HDEHP and stripping with sodium hydroxide in a single stage respectively resulted in nearly 100% scandium recovery. An extraction isotherm has been established. A complete process was developed, with leaching with hydrochloric acid, solvent extraction, washing, stripping with sodium hydroxide, solid-liquid separation and calcination. The product of the process was Sc,O,.
References [l] Drits, M.E., Sviderskaya, Z.A. and Nikitina, N.I., New Magnesium alloys for operation at elevated temperature. In: Co11 Nonferreous Metal Alloys. Nauka, Moscow (1972), pp. 193- 197. [2] Anon, Abstracts of reports. Int. Conf. Scandium and Prospects of its Use (Moscow, 18-19 October, 1994). [3] Ritcey, G.M. and Ashbrook, A.W., Solvent Extraction. Principles and Applications to Process Metallurgy. Elsevier, Amsterdam, Part I (19841, Part II (1979). [4] Yu, S. and Chen, J., Stripping of Fe(III) extracted by di-2.ethylhexyl phosphoric acid from sulfate solutions with sulfuric acid. Hydrometallurgy, 22 (1989): 267-272. [5] Green, G.K. and Harbuck, D.D., Miner. Metall. Process., Febr. (19%): l-3.