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Purification of synthetic laterite leach solution by solvent extraction using D2EHPA
Hydrometallurgy 56 Ž2000. 369–386 www.elsevier.nlrlocaterhydromet
Purification of synthetic laterite leach solution by solvent extraction using D2EHPA Chu Yong Cheng AJ Parker Centre for Hydrometallurgy & CSIRO Minerals, PO Box 90, Benteley, WA 6982, Australia Received 2 June 1999; received in revised form 21 March 2000; accepted 27 March 2000
Abstract The world mineral industry is experiencing an unprecedented interest in nickel–cobalt extraction from laterite ores through acid pressure leach and SX-EW processes. The recovery of cobalt and nickel from the leach solution through direct solvent extraction is of great interest as this would result in significant capital and operating cost savings. In the direct solvent extraction approach, the separation of zinc, calcium, copper and, in particular, manganese from cobalt and nickel is highly important. A series of shakeout tests was undertaken to investigate the fundamentals of the separation of the above impurities from cobalt and nickel using di-2-ethylhexyl phosphoric acid ŽD2EHPA. in kerosene. D2EHPA pH-extraction isotherms from solutions each containing a single element showed that the extraction order for the seven elements of interest as a function of pH 50 was Zn2q) Ca2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q. This confirmed that manganese would be extracted from sulfate solution ahead of cobalt and nickel. Extraction isotherms from solutions containing Zn, Ca, Mn, Cu, Co, Ni and Mg showed that the separation of zinc and calcium from the other elements was not difficult and the separation of copper and manganese from cobalt and nickel was possible. The separation of manganese from cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature Ž238C.. At pH 3.5, better separation of manganese from cobalt was achieved at room temperature Ž238C.. However, better separation of manganese from nickel could be obtained at elevated temperatures Ž40–608C.. The McCabe–Thiele diagram for the system showed that at pH 3.5 and 408C, two theoretical extraction stages at ArO ratio 1:1 were needed to extract 99.9% manganese from the aqueous solution and to reduce the manganese concentration from 2.0 grL to 3 ppm. Multiple stage extraction with fresh aqueous solution showed that cobalt and nickel were crowded out by zinc and manganese. Multiple stage extraction with fresh organic solution showed that manganese and copper in the aqueous solution were eliminated. Multiple stage scrubbing of
E-mail address: [email protected] ŽC.Y. Cheng.. 0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 9 5 - 5
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the loaded organic solution with manganese solution indicated that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution. q 2000 Elsevier Science B.V. All rights reserved. Keywords: D2EHPA; McCabe–Thiele diagram; Laterite; Nickel; Cobalt; Manganese; Solvent extraction
1. Introduction Around one third of the world’s nickel is produced from laterite ores, which account for two thirds of the world’s nickel resources. It is therefore likely that increasing amounts of nickel will be produced from laterites. There are two processes to extract nickel and cobalt from laterite ores. One is the Nicaro or Caron process in which nickel and cobalt oxides are reduced to metallic form in a roasting stage and the metals are then leached in ammoniacal solution w1x. The other is the Moa Bay or Sherritt process in which the nickel and cobalt oxides are subjected to high pressure acid leach ŽHPAL. followed by sulfide precipitation. The major advantage of the latter over the former is the much higher metal recovery. The nickel recovery in the Moa Bay process can reach 95% and the cobalt recovery 93% compared with 70% and 50%, respectively, in the Nicaro process. The world mineral industry is experiencing an unprecedented interest in nickel–cobalt extraction from laterite ores through HPAL and SX-EW processes. In Western Australia, three nickel laterite projects are underway. These are the Cawse project of Centaur Mining, the Bulong project of Resolute Žnow Preston Resources. and the Murrin Murrin project of Anaconda Nickel. The HPAL process for the three projects is very similar, however, the down stream processes differ substantially. In particular, quite different strategies for the separation of impurities such as zinc, copper and, in particular, manganese from nickel and cobalt are used by each project. In the Cawse project, a hydroxide precipitation-re-leach approach is used to eliminate the manganese after iron neutralisation w2x. After precipitation, part of the manganese remains in the leach solution and part of it reports to the precipitate. After re-leaching the precipitate with ammoniacal carbonate solution, the manganese remains in the residue. Other projects which may use the same processing route are the PNG Ramu project w3x, the Marlborough project w4x and the Indonesian Weda Bay project w5x. In the Bulong project, a direct solvent extraction approach is proposed after iron neutralisation w6,7x. The manganese is extracted together with cobalt while the nickel remains in the solution. Nickel is subsequently solvent extracted and electrowon. A sulfide precipitation is used to precipitate the cobalt and to keep the manganese in the leach solution. Cobalt can be electrowon or precipitated. In the Murrin Murrin project, a sulfide precipitation is used to separate the nickel and cobalt from manganese, which remains in the solution w8x. The cobalt and nickel are then re-leached and the cobalt is separated from nickel by solvent extraction. The sulfide precipitation route may also be used by the Calliope project w9x. Sulfide precipitation, acid or ammoniacal re-leach and solid–liquid separation are capital and operating cost intensive. If the manganese and other impurities such as zinc
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and copper could be separated by solvent extraction before nickel–cobalt separation, a complete solvent extraction process without sulfide or hydroxide precipitation and re-leach would be realized. This process would be much simpler and there would be a significant saving on capital and operating costs. The current work investigated the potential of the cationic extractant, di-2-ethylhexyl phosphoric acid ŽD2EHPA. for the separation of manganese and other impurities from nickel and cobalt in laterite leach solutions. D2EHPA has been used for the separation of cobalt from nickel and can also be used for the separation of zinc, beryllium, copper, vanadium, indium, gallium and rare earth elements w10x. Its use for the separation of manganese from cobalt and nickel is also reported by a few authors. The separation of manganese from cobalt in sulfate solution using D2EHPA in kerosene was reported by Hoh et al. w11x. pH and extraction isotherms were determined, batch tests were conducted and a continuous trial was proposed. However, extraction was plotted against organic pH, which was unusual. The extraction order of manganese and cobalt was also not consistent. A complete separation of manganese from cobalt in three extraction stages without scrubbing was reported. However, only two elements were considered while in laterite leach solutions, zinc, copper, nickel and magnesium coexist with manganese and cobalt. Cook and Szmokaluk w12x reported that cobalt sulfate solution containing nickel was refined by extraction of iron, zinc, copper and manganese with D2EHPA in Shellsol 140. It was found that manganese was extracted ahead of cobalt. The order of extraction as a function of pH 50 value was Fe 3q) Zn2q) Fe 2q) Cu2q) Mn2q) Co 2q. The solution contained 10 grL zinc, 0.05 grL manganese, 10 grL cobalt and 0.8 grL nickel. After 13 stages of operation, the raffinate contained 0.1 grL zinc, 0.008 grL manganese, 10 grL cobalt and 0.8 grL nickel. Though the manganese concentration in laterite leach solution is much higher Žabout 2 grL., the data indicated that the separation of manganese from cobalt and nickel was possible. Ritcey and Ashbrook w10x reported that cobalt and nickel were extracted ahead of manganese in sulfate solution. The order of extraction as a function of pH 50 was reported to be Fe 3q) Zn2q) Cu2q) Co 2q) Ni 2q) Mn2q) Mg 2q) Ca2q. Ritcey w13x reported the extraction order of D2EHPA was Fe ) Cu ) Zn ) Ca ) Mg ) Co, Mn ) Ni. Clark et al. w14x reported the purification of nickel sulfate solution using D2EHPA. The solution contained 90–100 grL nickel, 0.6 grL calcium and 0.25–2.5 grL magnesium. Shellsol AB was used as diluent. The extraction order obtained was Ca2q) Mg 2q) Ni 2q. A continuous trial of a full-scale pilot plant was conducted. The concentration of magnesium and calcium in the purified nickel sulfate solution was reduced to 100 and 5 ppm, respectively. Yao et al. w15x investigated the solvent extraction of metals by organophosphorus acids including DP-8R, the Japanese brand of D2EHPA. It was found that the extraction order as a function of pH 50 was Fe 3q) Zn2q) Mn2q) Cd 2q) Cu2q) Mg 2q) Co 2q) Ni 2q in a sulphate system. The metal concentration was 0.03 M, and Solvessoa150 was used as the diluent. The pH 50 value of manganese and cobalt with 0.6 M DP-8R was 2.81 and 4.13, respectively. The separation of manganese from cobalt using D2EHPA was recently reported by Feather et al. w16x. The original work was reported by Dry et al. in 1998 w17x; however,
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the reagent used to separate manganese from cobalt was undisclosed at that time. The bulk manganese has to be removed before cobalt electrowinning to avoid the manganese build-up. The manganese removal circuit consisted of three extraction stages, one scrubbing stage, two stripping stages and one re-strip stage. In a pilot plant operation, some 75–80% of the manganese was extracted with a cobalt loss less than 0.5%. The aim of the current work was to clarify the fundamentals of the separation of manganese and other impurities such as zinc, calcium, copper and magnesium from cobalt and nickel using D2EHPA. Possibilities for the separation of zinc, calcium, copper and manganese from cobalt and nickel in synthetic laterite leach solution were then explored.
2. Experimental 2.1. Organic solution Industrial grade D2EHPA was supplied by Albright & Wilson Australia and was used without purification. Short cut kerosene ŽShellsol 2046. supplied by Shell Australia was used as diluent. Tri-butyl phosphate ŽTBP. was used as modifier. Organic solution was prepared by dissolving 10% vrv D2EHPA and 5% vrv TBP in 85% vrv Shellsol 2046. 2.2. Aqueous solution For the determination of pH-extraction isotherms, aqueous solution containing a single element Ž3.0 grL. was used. Synthetic leach solution containing Ni, Co, Mn, Mg, Zn and Cu was prepared by dissolving AR grade NiSO4 P 6H 2 O, CoSO4 P 7H 2 O, MnSO4 P H 2 O, MgSO4 P 7H 2 O, ZnSO4 P 6H 2 O and CuSO4 P 5H 2 O in distilled water. Calcium solution was prepared by dissolving lime in water overnight and neutralizing the slurry to pH 4.5. The slurry was filtered and the filtrate was used to make aqueous solution for extraction tests. To simulate laterite leach solution after iron precipitation, the aqueous solution contained 3.0 grL nickel, 0.3 grL cobalt, 2.0 grL manganese, 3.0 grL magnesium, 0.3 grL zinc, 0.1 grL copper and 0.5 grL calcium. The solution pH was adjusted to 4.5 to simulate the pH of the leach solution after iron precipitation. 2.3. Shake-out tests Stainless steel rectangular boxes Ž80 = 80 = 150ŽH. mm3 . were used for mixing the organic and aqueous solutions using Eurostar digital overhead stirrers and 40-mm diameter disc impellers with a stirring speed of 1200 rpm. Sulfuric acid Ž100 grL. and ammonia solution Ž25%. were used to adjust the solution pH as required. After mixing for 10 min, the mixture was allowed to stand for 5 min to separate. A syringe with a small tube on the tip was used to take about 20-mL samples of aqueous solution. The pH of the aqueous solution was measured using a Hannan portable pH meter. If the pH was higher or lower than the pH required, the 20-mL sample was returned back to the box.
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One hundred grams per liter sulfuric acid and 25% ammonia solutions were used to adjust the pH. After mixing for another 10 min, the mixture was allowed to separate and the pH was measured and adjusted again. The test was continued until the solution equilibrium pH was obtained. The aqueous and organic solutions were then separated using separatory funnels. Shake-out tests were carried out at room temperature Ž238C., at 408C and 608C with mixing boxes immersed in a temperature-controlled water bath. 2.4. Chemical analysis For each test, about 10-mL aqueous solution was taken for chemical analysis. About 25–50 mL organic solution, depending on the availability, was taken for stripping using 100 grL sulfuric acid solution with ArO ratio 5:1. Where calcium was involved, 10% HCl solution was used as a strip solution. After stripping, about 10 mL of strip solution was taken for chemical analysis. All chemical analyses were conducted by SGS Australia.
3. Results and discussion 3.1. pH-extraction isotherms pH-extraction isotherms of nickel, cobalt, manganese, magnesium, copper, calcium and zinc were determined by shakeout tests at different equilibrium pH values with an ArO ratio of 1:1. The distribution coefficient and extraction at different pH values were determined. 3.1.1. pH-extraction isotherms at room temperature (238C) The pH-extraction isotherms of the seven elements of interest at room temperature Ž238C. are shown in Fig. 1. In the pH range of 2.0–2.5, the zinc extraction was between 83% and 93% and the calcium extraction between 82% and 100%, while the manganese extraction was between 5% and 30% and the copper extraction between 3% and 23%. Cobalt and nickel extraction in this pH range was negligible. This means that zinc and calcium can be easily separated from the other five elements in the pH range of 2.0–2.5. In subsequent investigations, zinc and calcium were not included in some cases since the separation of zinc and calcium using this organic extractant should be straightforward. In the pH range of 3.0–3.5, the manganese extraction ranged from 74% to 92% and copper from 68% to 90%, while the cobalt extraction ranged from 12% to 41% and nickel from 10% to 32%. Magnesium extraction was not tested across the whole range but the estimated extraction was between 15% and 25%. This indicates that the separation of manganese and copper from cobalt, nickel and magnesium should be carried out in the pH range of 3.0–3.5. This also indicates that good separation of the two groups of elements would require scrubbing as more than 10% of the nickel, cobalt and magnesium were extracted in this pH range.
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Fig. 1. D2EHPA pH-extraction isotherms for seven elements at 238C.
3.1.2. pH-extraction isotherms at eleÕated temperature (408C) The pH-extraction isotherms of copper, manganese, cobalt and nickel at 408C were determined to investigate the effect of temperature. The results are shown in Fig. 2. It is probable that the increase in temperature made the separation of manganese and copper from cobalt and nickel more difficult. At pH 3.0 and 408C, the manganese and copper
Fig. 2. D2EHPA pH-extraction isotherms for four elements at 408C.
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extractions were 75% and 64%, respectively, while the cobalt and nickel extractions were 47% and 29%, respectively. In comparison, at 238C and the same pH, the manganese and copper extractions were 73% and 68%, respectively, while the cobalt and nickel extractions were 12% and 10%, respectively. The effect of temperature on the pH-extraction isotherms of manganese and cobalt is further shown in Fig. 3. At higher temperatures, the cobalt extraction isotherm was shifted towards lower pH while the isotherm shape remained unchanged. However, the manganese pH–extraction curve flattened off at higher temperatures though it also shifted towards lower pH. 3.1.3. Extraction order of metals by D2EHPA The pH 50 value is the pH value at 50% metal extraction and the DŽpH 50 . value denotes the pH 50 difference of two metals from which the separation of the two metals can be judged to some degree. The extraction order of metals can be expressed as the function of the pH 50 . The extraction order obtained in this work was Zn2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q which is consistent with that reported by Cook and Szmokaluk w12x and Yao et al. w15x as far as the extraction order of manganese and cobalt is concerned. All three researchers reported that manganese was extracted ahead of cobalt. The DŽpH 50 . of cobalt over manganese was 0.98, 1.30 and 1.32 for this work, Cook and Szmokaluk, and Yao et al., respectively. This means that it is possible to use D2EHPA for the separation of manganese from cobalt since the DŽpH 50 . of cobalt over manganese is reasonably large. The DŽpH 50 . of cobalt over manganese obtained by this work was much smaller than that reported by Cook and Szmokaluk and Yao et al., which may be attributed to the method used to determine the pH 50 value, the diluent used in the test work and other factors. The present author has been working on the
Fig. 3. Effect of temperature on the D2EHPA pH-extraction isotherms of cobalt and manganese.
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effect of diluent on the pH 50 value of metals, which will be reported later in a separate paper. The pH 50 value of metals and the extraction order data in the literature have to be treated very carefully as a variety of factors may affect the test results. These factors include the aqueous solution composition Žtype of medium and concentration of metals., the composition of the organic solution Žconcentration of the extractant, synergist, modifier and diluent., temperature, ArO ratio, etc. Some authors determine the pH 50 value using solutions containing multiple elements while others may use solutions containing a single element. Although the pH 50 values of metals depend on such a large range of factors, the extraction order of metals for a particular extractant depends largely on the use of synergists and the type of aqueous medium. Preston and du Preez w18x reported that nickel was extracted ahead of calcium by D2EHPA when a synergist Ž2-ethylhexyl 4-pyridinecarboxylate ester. was used and calcium was extracted ahead of nickel by D2EHPA when no synergist was used. 3.2. Extraction isotherms Extraction isotherms were generated at selected pH values at ArO ratios of 1:2, 1:1, 2:1, 5:1 and 10:1 with aqueous solution containing multiple elements. Distribution coefficients and separation factors were determined. Theoretical extraction stages were established using McCabe–Thiele diagrams. 3.2.1. Effect of pH on extraction isotherms In this series of tests, the aqueous solution contained six elements: Mn, Zn, Mg, Cu, Co and Ni. The extraction isotherms of zinc and manganese at pH 2.0, 3.0 and 3.5 are shown in Fig. 4. At these pH values, the extraction of copper, cobalt and nickel was too
Fig. 4. Manganese and zinc extraction isotherms at different pH values Ž238C..
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low to form a normal isotherm. Zinc gave a very steep and straight extraction isotherm. The shape of the manganese isotherms at pH 3.0 and 3.5 is interesting. Manganese extraction decreased when ArO ratios were 5:1 andror 10:1. This was caused by the increased extraction competition by zinc over manganese when the ArO ratio was increased. At an ArO ratio of 10:1, the available zinc in the system was 10 times that at an ArO ratio of 1:1. Therefore, at the high ArO ratios, the zinc displaced the extracted manganese from the organic, causing the lower manganese extraction at high ArO ratios. The separation factors of manganese over cobalt and nickel reached hundreds at pH 3.5 ŽTable 1., indicating a good separation. The separation factors decreased with the increasing of ArO ratios indicating an increased competition of zinc over manganese. The separation factors increased with the increasing of pH indicating better separation of manganese over the other five elements at higher pH.
Table 1 Separation factors of manganese over other elements at different pH values Ž238C. Elements
ArO ratio
pH 2.0
pH 3.0
pH 3.5
pH 3.0 Žwith Ca.
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
12.2 12.8 14.8 11.0 8.0 3.5 4.6 4.9 3.9 4.2 14.9 20.5 22.9 19.7 11.6 49.7 20.5 56.8 30.4 50.3 0.05 0.05 0.04 0.03 0.04
29.6 35.9 33.7 34.9 20.3 4.3 4.5 2.6 3.8 2.6 90.2 98.3 99.2 111.0 86.5 309.1 307.6 262.7 289.7 198.1 0.29 0.14 0.05 0.02 0.01
70.1 44.6 52.4 31.9 25.5 6.7 17.9 24.8 14.6 10.9 198.7 142.7 81.0 67.1 26.2 690.4 446.0 571.8 167.0 114.3 1.26 0.23 0.08 0.02 0.01
27.1 27.4 24.2 15.8 15.2 4.1 3.8 3.4 2.5 2.0 89.3 97.7 84.3 15.8 7.1 197.8 229.0 198.1 19.8 8.2 0.27 0.08 0.03 0.01 0.01 0.07 0.08 0.09 0.10 0.11
Cu
Co
Ni
Zn
Ca
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Fig. 5 shows the extraction isotherms of seven elements: Mn, Mg, Cu, Ca, Zn, Co and Ni at pH 3.0 and 238C. Again, zinc gave a very steep and straight extraction isotherm. The calcium isotherm showed that calcium extraction decreased gradually with the increasing of ArO ratio. The manganese isotherm showed that manganese extraction started to decrease substantially from ArO ratio 2:1. This was caused by the increased extraction competition by zinc over calcium and manganese when the ArO ratio was increased. The separation factors of manganese over the other six elements at pH 3.0 and 238C are also shown in Table 1. Compared with the results when six elements were in the aqueous solution, the separation factors decreased when calcium was present indicating an increased competition of zinc and calcium over manganese. 3.2.2. Effect of temperature on extraction isotherms In this series of tests, the aqueous solution contained five elements: Mn, Mg, Cu, Co and Ni. Zinc and calcium were not included. Fig. 6 shows the manganese extraction isotherms for pH 3.0 at 238C, 408C and 608C. The manganese extraction was much higher at 238C and decreased with increasing temperature. The effect of temperature on the extraction isotherms of the other elements was negligible. The separation factors of manganese over the other four elements at pH 3.0 are shown in Table 2. In most cases, the highest separation factor was also obtained at room temperature. Fig. 7 shows the manganese extraction isotherms for pH 3.5 at 238C, 408C and 608C. Manganese extraction increased with increasing temperature. The extraction of copper, cobalt and nickel was too low to form a normal isotherm. The magnesium extraction increased slightly with increasing temperature. The separation factors of manganese over
Fig. 5. Extraction isotherms of seven elements.
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Fig. 6. Manganese extraction isotherms at different temperatures ŽpH 3.0..
cobalt decreased with the increase in temperature while the separation factors of manganese over nickel increased with the increase in temperature except at 608C with
Table 2 Separation factors of manganese over other elements at different temperatures ŽpH 3.0. Name of elements
ArO ratio
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
Cu
Co
Ni
Separation factor Ž b Mn rM . 238C
408C
608C
29.4 32.5 37.5 35.7 36.5 5.6 5.0 5.7 4.4 4.5 119.3 133.4 132.9 193.9 340.6 361.4 434.2 329.4 225.0 307.2
18.6 22.5 25.4 24.1 24.8 1.4 2.1 2.0 1.8 2.5 36.9 44.6 33.0 39.7 37.2 368.7 430.5 471.3 738.7 672.0
22.4 14.5 18.9 18.8 20.2 2.7 1.3 1.5 2.0 1.3 78.1 44.1 37.6 58.1 56.6 296.1 205.9 346.6 277.1 264.0
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Fig. 7. Manganese extraction isotherms at different temperatures ŽpH 3.5..
ArO ratio 10:1 ŽTable 3.. The separation factors of manganese over magnesium, copper and cobalt were the highest at room temperature. Table 3 Separation factors of manganese over other elements at different temperatures ŽpH 3.5. Name of elements
ArO ratio
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
Cu
Co
Ni
Separation factor Ž b Mn rM . 238C
408C
608C
30.6 35.6 59.5 47.0 44.0 5.5 4.6 4.2 2.7 2.5 116.4 128.8 147.0 98.4 90.4 372.6 475.7 596.8 475.0 354.8
33.6 35.0 41.6 42.7 39.5 4.5 2.5 2.4 2.2 1.7 104.2 67.3 48.7 42.7 38.3 515.9 585.8 663.5 661.9 408.7
27.6 29.6 32.1 30.4 27.8 1.8 2.8 1.9 1.3 1.4 32.9 25.2 20.9 16.9 15.7 557.6 620.0 729.8 794.0 278.0
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3.2.3. Extraction stages estimated by McCabe–Thiele method Fig. 8 shows a McCabe–Thiele diagram constructed for manganese to determine the theoretical extraction stages for the separation of manganese from cobalt and nickel at pH 3.5 and 408C. Two theoretical extraction stages would suffice to reduce the manganese concentration in the aqueous solution from 2 grL to less than 3 ppm using an ArO ratio 1:1. Of course, this is also dependent on the extraction of other elements by the organic solution. At pH 3.5, the cobalt extraction would be much higher than at pH 3.0. Therefore, more scrubbing stages would have to be used to reduce the cobalt extraction in the organic solution. 3.3. Multiple stage extraction 3.3.1. Multiple extraction with fresh aqueous solutions Multiple stage extraction tests with fresh aqueous solution were conducted to simulate the first stages of counter-current extraction Žfresh aqueous solution to meet loaded organic solution. and to investigate the ‘‘crowding effect’’ during the separation. In these tests, the same organic was contacted with fresh aqueous solution five times with ArO ratio 1:1 at pH 3.5. Table 4 shows the metal concentration in organic solution after each stage of extraction at pH 3.5. The concentration of cobalt, magnesium and nickel decreased with the increase in stage number due to the crowding effect of manganese and zinc. Again, the copper concentration was basically unchanged while both zinc and manganese concentration increased with the contact stages. The multiple extraction tests showed that the extracted nickel and cobalt in the organic solution were crowded out by zinc and manganese. At pH 3.5, the cobalt
Fig. 8. Manganese McCabe–Thiele diagram at pH 3.5, 408C.
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Table 4 Metal concentration in organic or aqueous solutions during multiple stage extraction and scrubbing at pH 3.5 Name of elements
Stage no.
Multiple extraction with fresh aqueous Concentration in organic ŽgrL.
Multiple extraction with fresh organic Concentration in aqueous ŽgrL.
Multiple scrubbing with Mn solution Concentration in organic ŽgrL.
Mg
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5
0.000 0.475 0.200 0.145 0.095 0.100 0.000 0.065 0.080 0.075 0.060 0.055 0.000 0.020 0.007 0.007 0.005 0.004 0.000 2.120 3.110 3.325 3.300 3.340 0.000 0.070 0.015 0.025 0.005 0.010 0.000 0.315 0.650 0.960 1.175 1.585
3.460 2.960 2.050 1.590
0.160 0.015 0.006 0.006
0.064 0.025 0.007 0.002
0.080 0.050 0.027 0.016
0.338 0.343 0.323 0.280
0.013 0.003 0.002 0.003
1.970 0.286 0.022 0.002
2.640 2.715 3.180 2.785
3.420 3.340 3.070 3.320
0.0400 0.0025 0.0025 0.0025
Cu
Co
Mn
Ni
Zn
0.975 1.010 0.925 1.000
concentration in the organic solution dropped from 20 to 4 ppm after five stages of contact while nickel concentration dropped from 70 to 10 ppm. The extracted magnesium was also crowed out from the organic. 3.3.2. Multiple extraction with fresh organic solutions Multiple stage extraction tests with fresh organic solution were conducted to simulate the last stages of counter-current extraction Žfresh organic solution mixing with nearly
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barren aqueous solution. and to reduce the target elements Žzinc, manganese and copper in this case. in the aqueous solution. In these tests, the same aqueous solution was contacted with fresh organic solution three times at pH 3.5. The ArO ratio was 1:1. The initial aqueous solution contained 3.46 grL Mg, 0.064 grL Cu, 0.338 grL Co, 1.97 grL Mn and 3.42 grL Ni. Table 4 shows the metal concentration in aqueous solution after each stage of extraction at pH 3.5. The concentration of copper and manganese decreased dramatically to about 2 ppm. The cobalt concentration was slightly decreased while nickel concentration was almost unchanged. The magnesium concentration decreased to approximately half of the original value. This is due to too many extraction sites available in the organic solution, which resulted in less competition. The multiple extraction tests with fresh organic solution showed that the manganese and copper concentration in the aqueous solution dropped to 2 ppm at pH 3.5 after three stages of contact. 3.4. Multiple stage scrubbing The aim of scrubbing the organic solution was to wash out any cobalt and nickel that were co-extracted with manganese. Nickel and cobalt could then be recovered by recycling the scrub solution to the feed. The loaded organic solution was prepared by contacting the same organic solution three times with fresh aqueous solution ŽArO ratio 1:1. at pH 3.5. The organic solution prepared contained 160 ppm magnesium, 80 ppm copper, 13 ppm cobalt, 40 ppm nickel, 2.64 grL manganese and 0.975 grL zinc. Three-stage scrubbing tests were carried out by contacting the same loaded organic solution three times with fresh aqueous solution containing 5.0 grL manganese. The ArO ratio was 1:5 to simulate the very low ArO ratio used in practice. The pH values of the scrub solution was 3.5 and no pH adjustment was conducted during the scrubbing tests. The metal concentration in organic solution after each stage of scrubbing is shown in Table 4. The concentrations of magnesium, copper, cobalt and nickel in the organic solution decreased with the increase in stage number due to the crowding effect of manganese. The concentrations of cobalt and nickel were reduced to about 3 ppm, and the concentrations of copper to 16 ppm and magnesium to 6 ppm. The zinc concentration was basically unchanged while the manganese concentration increased. Nearly all the nickel and cobalt co-extracted were crowded out from the organic solution after one stage of scrubbing. At pH 3.5, the cobalt concentration dropped from 13 to 3 ppm and the nickel concentration dropped from 40 to 3 ppm after one stage of contact. Co-extracted magnesium and copper were also crowded out from the organic solution by manganese, suggesting a much weaker manganese solution should be used to keep the extracted copper and magnesium in the organic solution. 3.5. Calcium stripping and iron poisoning Calcium was completely extracted by D2EHPA at pH 2.5 ŽFig. 1. and iron was completely extracted at 1.5 w10x. The laterite leach solution after iron precipitation is
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saturated with calcium for limestone is usually used as a reagent. Residual iron may be present in the aqueous solution and enter the D2EHPA solvent extraction circuit, especially when the iron precipitation circuit is malfunctioned. If sulphuric acid is used as the stripping solution, nasty gypsum precipitation may occur and iron remains in the organic solution causing poisoning of the extractant. Therefore, a two-strip strategy has to be used. In the first set of strip stages, sulphuric acid is used as the strip solution to selectively strip manganese and copper from the organic solution at pH 2.3–2.5. The strip raffinate solution will be used as scrub solution after making-up and pH adjustment. In the second set of strip stages, hydrochloric acid solution has to be used to strip calcium and iron if any.
4. Conclusions and recommendations A series of shake-out tests was undertaken to investigate the fundamentals of the separation of manganese from cobalt and nickel using D2EHPA in kerosene and to explore the possibility of separation of zinc, copper, manganese and magnesium from cobalt and nickel in synthetic laterite leach solution. Ž1. D2EHPA pH-extraction isotherms generated for single element solutions showed that the extraction order for the six elements tested as a function of pH 50 was Zn2q) Ca2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q. This order is significantly different from commonly referenced isotherms for this system. Ž2. D2EHPA extraction isotherms generated for the group of elements, Zn, Ca, Mn, Cu, Co, Ni and Mg, showed that the separation of zinc and calcium from the other elements was easy and the separation of manganese and copper from cobalt and nickel was possible. Ž3. The separation of manganese from solutions containing cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature Ž238C.. At pH 3.5, better separation of manganese from cobalt was achieved at room temperature Ž238C., while better separation of manganese from nickel could be obtained at elevated temperatures Ž40–608C.. Ž4. The McCabe–Thiele diagram for the system showed that at pH 3.5 and 408C, two theoretical extraction stages were needed to extract 99.9% of the manganese from the solution and to reduce the manganese concentration from 2.0 grL to 3 ppm. Ž5. Multiple stage extraction with fresh aqueous solution showed that after two stages of extraction at pH 3.5, only about 7 ppm cobalt and 15 ppm nickel were present in the organic solution due to the crowding effect of zinc and manganese. Ž6.Multiple stage extraction with fresh organic solution showed that after three stages of extraction at pH 3.5, only about 2 ppm manganese and copper were present in the aqueous solution. Ž7. Multiple stage scrubbing of the loaded organic solution with manganese solution showed that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution due to the crowding effect of the manganese.
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Ž8. Magnesium was extracted after nickel and cobalt in terms of the pH 50 order. Therefore, magnesium will remain in the aqueous solution when D2EHPA is used to separate manganese from nickel and cobalt. Ž9. A hydrochloric acid stripping is necessary to prevent gypsum precipitation and iron poisoning of D2EHPA. Ž10. D2EHPA is a cheap and stable extractant and as demonstrated in the current work, impurities such as zinc, calcium, manganese and copper could be eliminated from the laterite leach solution before nickel–cobalt separation. A continuous trial using synthetic solution and real leach solution should be conducted to verify this.
Acknowledgements The author thanks the AJ Parker Centre for Hydrometallurgy for funding this project and the permission to publish this paper. The project was supported by the Department of Minerals and Energy, Western Australia, and CSIRO Minerals. The test work was carried out at the former Mineral Processing Laboratory. The support from the staff and management of both organisations is gratefully acknowledged. Mr. Michael Davies carried out the test work. Thanks are extended to Ms. Caroline Hughes and Dr. Martin Houchin for reviewing the draft paper and their valuable comments.
References w1x J.R. Boldt Jr., The Winning of Nickel, Its Geology, Mining and Extractive Metallurgy, Methuen & Co., 1967, pp. 425–453. w2x P.G. Manson, J.V. Groutsch, R.S. Mayze, D. White, Process development and plant design for the Cawse nickel project, in: Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, Alta Metallurgical Service, 1997. w3x E. Anderson, Ramu nickel project — post pre-feasibility, in: Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, Alta Metallurgical Service, 1997. w4x A. Griffin, The Marlborough laterites project, in: Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, Alta Metallurgical Service, 1998. w5x M.G. Baillie, G.C. Cock, Weda Bay Laterite project, Indonesia, in: Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, ALTA Metallurgical Service, 1998. w6x A. Taylor, D. Cairns, Technical development of the Bulong laterite treatment project, in: ALTA Metallurgical Service, Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia 1997. w7x K. Soldenhoff, N. Hayward, D. Wilkins, Direct solvent extraction of cobalt and nickel from laterite-acid pressure leach liquors, EPD Congress 1998, The Minerals, Metals and Materials Society,1998, pp. 153–165. w8x G. Motteram, M. Ryan, R. Berezowsky, R. Raudsepp, Murrin Murrin nickel and cobalt project: project development overview, Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, ALTA Metallurgical Service, 1996. w9x P.J. Matheson, G.A. Becker, G.A. Lennard, The Calliope nickel cobalt project, Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, ALTA Metallurgical Service, 1997. w10x G.M. Ritcey, A.W. Ashbrook, Solvent Extraction: Principles and Applications to Process Metallurgy Vol. 1 Elsevier, 1984, pp. 106–107.
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w11x Y.-C. Hoh, W.-S. Chuang, B.-D. Lee, C.-C. Chang, The separation of manganese from cobalt by D2EHPA, Hydrometallurgy 12 Ž1984. 375–386. w12x L.F. Cook, W.W. Szmokaluk, Refining of cobalt and nickel sulfate solution by solvent extraction using di-Ž2-ethyl hexyl. phosphoric acid, Proceedings ISEC’ 71, Soc. Chem. Ind. London 1 Ž1971. 451–462. w13x G.M. Ritcey, Process options using solvent extraction for the processing of laterites, in: Nickel ’96, Kalgorlie, September,1996, pp. 251–258. w14x P.D.A. Clark, P.M. Cole, M.H. Fox, Purification of nickel sulphate using di-Ž2-ethyl hexyl. phosphoric acid, in: D.H. Lonsdail, M.J. Slater ŽEds.., ISEC’93, Solvent Extraction in the Process Industry, London,1993, pp. 175–182. w15x B.H. Yao, N. Yukio, S. Masatada, N. Akihiko, H. Kiyoshi, Solvent extraction of metal ions and separation of nickelŽII. from other metal ions by organophosphorus acids, Solvent Extr. Ion Exch. 14 Ž5. Ž1996. 849–970. w16x A. Feather, K.C. Sole, D.B. Dreisinger, Pilot-plant investigation of manganese removal and cobalt purification by solvent extraction, in: ISEC’ 99, Barcelona, Spain, 1999. w17x M.J. Dry, G. Irio, D.F. Jacobs, P.M. Cole, A.M. Feather, K.C. Sole, J. Engelbrecht, K.C. Matchett, CurCo tailings treatment project, Democratic Republic of Congo, in: Nickel and Cobalt Pressure Leaching and Hydrometallurgy Forum, Perth, Western Australia, ALTA Metallurgical Service, 1998. w18x J.S. Preston, A.C. du Preez, Solvent extraction of nickel from acidic solutions using synergistic mixtures containing pyridinecarboxylate esters. Part 1: Systems based on organophosphorus acids, J. Chem. Tech. Biotechnol. 66 Ž1996. 86–94.
Purification of synthetic laterite leach solution by solvent extraction using D2EHPA Chu Yong Cheng AJ Parker Centre for Hydrometallurgy & CSIRO Minerals, PO Box 90, Benteley, WA 6982, Australia Received 2 June 1999; received in revised form 21 March 2000; accepted 27 March 2000
Abstract The world mineral industry is experiencing an unprecedented interest in nickel–cobalt extraction from laterite ores through acid pressure leach and SX-EW processes. The recovery of cobalt and nickel from the leach solution through direct solvent extraction is of great interest as this would result in significant capital and operating cost savings. In the direct solvent extraction approach, the separation of zinc, calcium, copper and, in particular, manganese from cobalt and nickel is highly important. A series of shakeout tests was undertaken to investigate the fundamentals of the separation of the above impurities from cobalt and nickel using di-2-ethylhexyl phosphoric acid ŽD2EHPA. in kerosene. D2EHPA pH-extraction isotherms from solutions each containing a single element showed that the extraction order for the seven elements of interest as a function of pH 50 was Zn2q) Ca2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q. This confirmed that manganese would be extracted from sulfate solution ahead of cobalt and nickel. Extraction isotherms from solutions containing Zn, Ca, Mn, Cu, Co, Ni and Mg showed that the separation of zinc and calcium from the other elements was not difficult and the separation of copper and manganese from cobalt and nickel was possible. The separation of manganese from cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature Ž238C.. At pH 3.5, better separation of manganese from cobalt was achieved at room temperature Ž238C.. However, better separation of manganese from nickel could be obtained at elevated temperatures Ž40–608C.. The McCabe–Thiele diagram for the system showed that at pH 3.5 and 408C, two theoretical extraction stages at ArO ratio 1:1 were needed to extract 99.9% manganese from the aqueous solution and to reduce the manganese concentration from 2.0 grL to 3 ppm. Multiple stage extraction with fresh aqueous solution showed that cobalt and nickel were crowded out by zinc and manganese. Multiple stage extraction with fresh organic solution showed that manganese and copper in the aqueous solution were eliminated. Multiple stage scrubbing of
E-mail address: [email protected] ŽC.Y. Cheng.. 0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 9 5 - 5
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the loaded organic solution with manganese solution indicated that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution. q 2000 Elsevier Science B.V. All rights reserved. Keywords: D2EHPA; McCabe–Thiele diagram; Laterite; Nickel; Cobalt; Manganese; Solvent extraction
1. Introduction Around one third of the world’s nickel is produced from laterite ores, which account for two thirds of the world’s nickel resources. It is therefore likely that increasing amounts of nickel will be produced from laterites. There are two processes to extract nickel and cobalt from laterite ores. One is the Nicaro or Caron process in which nickel and cobalt oxides are reduced to metallic form in a roasting stage and the metals are then leached in ammoniacal solution w1x. The other is the Moa Bay or Sherritt process in which the nickel and cobalt oxides are subjected to high pressure acid leach ŽHPAL. followed by sulfide precipitation. The major advantage of the latter over the former is the much higher metal recovery. The nickel recovery in the Moa Bay process can reach 95% and the cobalt recovery 93% compared with 70% and 50%, respectively, in the Nicaro process. The world mineral industry is experiencing an unprecedented interest in nickel–cobalt extraction from laterite ores through HPAL and SX-EW processes. In Western Australia, three nickel laterite projects are underway. These are the Cawse project of Centaur Mining, the Bulong project of Resolute Žnow Preston Resources. and the Murrin Murrin project of Anaconda Nickel. The HPAL process for the three projects is very similar, however, the down stream processes differ substantially. In particular, quite different strategies for the separation of impurities such as zinc, copper and, in particular, manganese from nickel and cobalt are used by each project. In the Cawse project, a hydroxide precipitation-re-leach approach is used to eliminate the manganese after iron neutralisation w2x. After precipitation, part of the manganese remains in the leach solution and part of it reports to the precipitate. After re-leaching the precipitate with ammoniacal carbonate solution, the manganese remains in the residue. Other projects which may use the same processing route are the PNG Ramu project w3x, the Marlborough project w4x and the Indonesian Weda Bay project w5x. In the Bulong project, a direct solvent extraction approach is proposed after iron neutralisation w6,7x. The manganese is extracted together with cobalt while the nickel remains in the solution. Nickel is subsequently solvent extracted and electrowon. A sulfide precipitation is used to precipitate the cobalt and to keep the manganese in the leach solution. Cobalt can be electrowon or precipitated. In the Murrin Murrin project, a sulfide precipitation is used to separate the nickel and cobalt from manganese, which remains in the solution w8x. The cobalt and nickel are then re-leached and the cobalt is separated from nickel by solvent extraction. The sulfide precipitation route may also be used by the Calliope project w9x. Sulfide precipitation, acid or ammoniacal re-leach and solid–liquid separation are capital and operating cost intensive. If the manganese and other impurities such as zinc
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and copper could be separated by solvent extraction before nickel–cobalt separation, a complete solvent extraction process without sulfide or hydroxide precipitation and re-leach would be realized. This process would be much simpler and there would be a significant saving on capital and operating costs. The current work investigated the potential of the cationic extractant, di-2-ethylhexyl phosphoric acid ŽD2EHPA. for the separation of manganese and other impurities from nickel and cobalt in laterite leach solutions. D2EHPA has been used for the separation of cobalt from nickel and can also be used for the separation of zinc, beryllium, copper, vanadium, indium, gallium and rare earth elements w10x. Its use for the separation of manganese from cobalt and nickel is also reported by a few authors. The separation of manganese from cobalt in sulfate solution using D2EHPA in kerosene was reported by Hoh et al. w11x. pH and extraction isotherms were determined, batch tests were conducted and a continuous trial was proposed. However, extraction was plotted against organic pH, which was unusual. The extraction order of manganese and cobalt was also not consistent. A complete separation of manganese from cobalt in three extraction stages without scrubbing was reported. However, only two elements were considered while in laterite leach solutions, zinc, copper, nickel and magnesium coexist with manganese and cobalt. Cook and Szmokaluk w12x reported that cobalt sulfate solution containing nickel was refined by extraction of iron, zinc, copper and manganese with D2EHPA in Shellsol 140. It was found that manganese was extracted ahead of cobalt. The order of extraction as a function of pH 50 value was Fe 3q) Zn2q) Fe 2q) Cu2q) Mn2q) Co 2q. The solution contained 10 grL zinc, 0.05 grL manganese, 10 grL cobalt and 0.8 grL nickel. After 13 stages of operation, the raffinate contained 0.1 grL zinc, 0.008 grL manganese, 10 grL cobalt and 0.8 grL nickel. Though the manganese concentration in laterite leach solution is much higher Žabout 2 grL., the data indicated that the separation of manganese from cobalt and nickel was possible. Ritcey and Ashbrook w10x reported that cobalt and nickel were extracted ahead of manganese in sulfate solution. The order of extraction as a function of pH 50 was reported to be Fe 3q) Zn2q) Cu2q) Co 2q) Ni 2q) Mn2q) Mg 2q) Ca2q. Ritcey w13x reported the extraction order of D2EHPA was Fe ) Cu ) Zn ) Ca ) Mg ) Co, Mn ) Ni. Clark et al. w14x reported the purification of nickel sulfate solution using D2EHPA. The solution contained 90–100 grL nickel, 0.6 grL calcium and 0.25–2.5 grL magnesium. Shellsol AB was used as diluent. The extraction order obtained was Ca2q) Mg 2q) Ni 2q. A continuous trial of a full-scale pilot plant was conducted. The concentration of magnesium and calcium in the purified nickel sulfate solution was reduced to 100 and 5 ppm, respectively. Yao et al. w15x investigated the solvent extraction of metals by organophosphorus acids including DP-8R, the Japanese brand of D2EHPA. It was found that the extraction order as a function of pH 50 was Fe 3q) Zn2q) Mn2q) Cd 2q) Cu2q) Mg 2q) Co 2q) Ni 2q in a sulphate system. The metal concentration was 0.03 M, and Solvessoa150 was used as the diluent. The pH 50 value of manganese and cobalt with 0.6 M DP-8R was 2.81 and 4.13, respectively. The separation of manganese from cobalt using D2EHPA was recently reported by Feather et al. w16x. The original work was reported by Dry et al. in 1998 w17x; however,
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the reagent used to separate manganese from cobalt was undisclosed at that time. The bulk manganese has to be removed before cobalt electrowinning to avoid the manganese build-up. The manganese removal circuit consisted of three extraction stages, one scrubbing stage, two stripping stages and one re-strip stage. In a pilot plant operation, some 75–80% of the manganese was extracted with a cobalt loss less than 0.5%. The aim of the current work was to clarify the fundamentals of the separation of manganese and other impurities such as zinc, calcium, copper and magnesium from cobalt and nickel using D2EHPA. Possibilities for the separation of zinc, calcium, copper and manganese from cobalt and nickel in synthetic laterite leach solution were then explored.
2. Experimental 2.1. Organic solution Industrial grade D2EHPA was supplied by Albright & Wilson Australia and was used without purification. Short cut kerosene ŽShellsol 2046. supplied by Shell Australia was used as diluent. Tri-butyl phosphate ŽTBP. was used as modifier. Organic solution was prepared by dissolving 10% vrv D2EHPA and 5% vrv TBP in 85% vrv Shellsol 2046. 2.2. Aqueous solution For the determination of pH-extraction isotherms, aqueous solution containing a single element Ž3.0 grL. was used. Synthetic leach solution containing Ni, Co, Mn, Mg, Zn and Cu was prepared by dissolving AR grade NiSO4 P 6H 2 O, CoSO4 P 7H 2 O, MnSO4 P H 2 O, MgSO4 P 7H 2 O, ZnSO4 P 6H 2 O and CuSO4 P 5H 2 O in distilled water. Calcium solution was prepared by dissolving lime in water overnight and neutralizing the slurry to pH 4.5. The slurry was filtered and the filtrate was used to make aqueous solution for extraction tests. To simulate laterite leach solution after iron precipitation, the aqueous solution contained 3.0 grL nickel, 0.3 grL cobalt, 2.0 grL manganese, 3.0 grL magnesium, 0.3 grL zinc, 0.1 grL copper and 0.5 grL calcium. The solution pH was adjusted to 4.5 to simulate the pH of the leach solution after iron precipitation. 2.3. Shake-out tests Stainless steel rectangular boxes Ž80 = 80 = 150ŽH. mm3 . were used for mixing the organic and aqueous solutions using Eurostar digital overhead stirrers and 40-mm diameter disc impellers with a stirring speed of 1200 rpm. Sulfuric acid Ž100 grL. and ammonia solution Ž25%. were used to adjust the solution pH as required. After mixing for 10 min, the mixture was allowed to stand for 5 min to separate. A syringe with a small tube on the tip was used to take about 20-mL samples of aqueous solution. The pH of the aqueous solution was measured using a Hannan portable pH meter. If the pH was higher or lower than the pH required, the 20-mL sample was returned back to the box.
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One hundred grams per liter sulfuric acid and 25% ammonia solutions were used to adjust the pH. After mixing for another 10 min, the mixture was allowed to separate and the pH was measured and adjusted again. The test was continued until the solution equilibrium pH was obtained. The aqueous and organic solutions were then separated using separatory funnels. Shake-out tests were carried out at room temperature Ž238C., at 408C and 608C with mixing boxes immersed in a temperature-controlled water bath. 2.4. Chemical analysis For each test, about 10-mL aqueous solution was taken for chemical analysis. About 25–50 mL organic solution, depending on the availability, was taken for stripping using 100 grL sulfuric acid solution with ArO ratio 5:1. Where calcium was involved, 10% HCl solution was used as a strip solution. After stripping, about 10 mL of strip solution was taken for chemical analysis. All chemical analyses were conducted by SGS Australia.
3. Results and discussion 3.1. pH-extraction isotherms pH-extraction isotherms of nickel, cobalt, manganese, magnesium, copper, calcium and zinc were determined by shakeout tests at different equilibrium pH values with an ArO ratio of 1:1. The distribution coefficient and extraction at different pH values were determined. 3.1.1. pH-extraction isotherms at room temperature (238C) The pH-extraction isotherms of the seven elements of interest at room temperature Ž238C. are shown in Fig. 1. In the pH range of 2.0–2.5, the zinc extraction was between 83% and 93% and the calcium extraction between 82% and 100%, while the manganese extraction was between 5% and 30% and the copper extraction between 3% and 23%. Cobalt and nickel extraction in this pH range was negligible. This means that zinc and calcium can be easily separated from the other five elements in the pH range of 2.0–2.5. In subsequent investigations, zinc and calcium were not included in some cases since the separation of zinc and calcium using this organic extractant should be straightforward. In the pH range of 3.0–3.5, the manganese extraction ranged from 74% to 92% and copper from 68% to 90%, while the cobalt extraction ranged from 12% to 41% and nickel from 10% to 32%. Magnesium extraction was not tested across the whole range but the estimated extraction was between 15% and 25%. This indicates that the separation of manganese and copper from cobalt, nickel and magnesium should be carried out in the pH range of 3.0–3.5. This also indicates that good separation of the two groups of elements would require scrubbing as more than 10% of the nickel, cobalt and magnesium were extracted in this pH range.
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Fig. 1. D2EHPA pH-extraction isotherms for seven elements at 238C.
3.1.2. pH-extraction isotherms at eleÕated temperature (408C) The pH-extraction isotherms of copper, manganese, cobalt and nickel at 408C were determined to investigate the effect of temperature. The results are shown in Fig. 2. It is probable that the increase in temperature made the separation of manganese and copper from cobalt and nickel more difficult. At pH 3.0 and 408C, the manganese and copper
Fig. 2. D2EHPA pH-extraction isotherms for four elements at 408C.
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extractions were 75% and 64%, respectively, while the cobalt and nickel extractions were 47% and 29%, respectively. In comparison, at 238C and the same pH, the manganese and copper extractions were 73% and 68%, respectively, while the cobalt and nickel extractions were 12% and 10%, respectively. The effect of temperature on the pH-extraction isotherms of manganese and cobalt is further shown in Fig. 3. At higher temperatures, the cobalt extraction isotherm was shifted towards lower pH while the isotherm shape remained unchanged. However, the manganese pH–extraction curve flattened off at higher temperatures though it also shifted towards lower pH. 3.1.3. Extraction order of metals by D2EHPA The pH 50 value is the pH value at 50% metal extraction and the DŽpH 50 . value denotes the pH 50 difference of two metals from which the separation of the two metals can be judged to some degree. The extraction order of metals can be expressed as the function of the pH 50 . The extraction order obtained in this work was Zn2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q which is consistent with that reported by Cook and Szmokaluk w12x and Yao et al. w15x as far as the extraction order of manganese and cobalt is concerned. All three researchers reported that manganese was extracted ahead of cobalt. The DŽpH 50 . of cobalt over manganese was 0.98, 1.30 and 1.32 for this work, Cook and Szmokaluk, and Yao et al., respectively. This means that it is possible to use D2EHPA for the separation of manganese from cobalt since the DŽpH 50 . of cobalt over manganese is reasonably large. The DŽpH 50 . of cobalt over manganese obtained by this work was much smaller than that reported by Cook and Szmokaluk and Yao et al., which may be attributed to the method used to determine the pH 50 value, the diluent used in the test work and other factors. The present author has been working on the
Fig. 3. Effect of temperature on the D2EHPA pH-extraction isotherms of cobalt and manganese.
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effect of diluent on the pH 50 value of metals, which will be reported later in a separate paper. The pH 50 value of metals and the extraction order data in the literature have to be treated very carefully as a variety of factors may affect the test results. These factors include the aqueous solution composition Žtype of medium and concentration of metals., the composition of the organic solution Žconcentration of the extractant, synergist, modifier and diluent., temperature, ArO ratio, etc. Some authors determine the pH 50 value using solutions containing multiple elements while others may use solutions containing a single element. Although the pH 50 values of metals depend on such a large range of factors, the extraction order of metals for a particular extractant depends largely on the use of synergists and the type of aqueous medium. Preston and du Preez w18x reported that nickel was extracted ahead of calcium by D2EHPA when a synergist Ž2-ethylhexyl 4-pyridinecarboxylate ester. was used and calcium was extracted ahead of nickel by D2EHPA when no synergist was used. 3.2. Extraction isotherms Extraction isotherms were generated at selected pH values at ArO ratios of 1:2, 1:1, 2:1, 5:1 and 10:1 with aqueous solution containing multiple elements. Distribution coefficients and separation factors were determined. Theoretical extraction stages were established using McCabe–Thiele diagrams. 3.2.1. Effect of pH on extraction isotherms In this series of tests, the aqueous solution contained six elements: Mn, Zn, Mg, Cu, Co and Ni. The extraction isotherms of zinc and manganese at pH 2.0, 3.0 and 3.5 are shown in Fig. 4. At these pH values, the extraction of copper, cobalt and nickel was too
Fig. 4. Manganese and zinc extraction isotherms at different pH values Ž238C..
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low to form a normal isotherm. Zinc gave a very steep and straight extraction isotherm. The shape of the manganese isotherms at pH 3.0 and 3.5 is interesting. Manganese extraction decreased when ArO ratios were 5:1 andror 10:1. This was caused by the increased extraction competition by zinc over manganese when the ArO ratio was increased. At an ArO ratio of 10:1, the available zinc in the system was 10 times that at an ArO ratio of 1:1. Therefore, at the high ArO ratios, the zinc displaced the extracted manganese from the organic, causing the lower manganese extraction at high ArO ratios. The separation factors of manganese over cobalt and nickel reached hundreds at pH 3.5 ŽTable 1., indicating a good separation. The separation factors decreased with the increasing of ArO ratios indicating an increased competition of zinc over manganese. The separation factors increased with the increasing of pH indicating better separation of manganese over the other five elements at higher pH.
Table 1 Separation factors of manganese over other elements at different pH values Ž238C. Elements
ArO ratio
pH 2.0
pH 3.0
pH 3.5
pH 3.0 Žwith Ca.
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
12.2 12.8 14.8 11.0 8.0 3.5 4.6 4.9 3.9 4.2 14.9 20.5 22.9 19.7 11.6 49.7 20.5 56.8 30.4 50.3 0.05 0.05 0.04 0.03 0.04
29.6 35.9 33.7 34.9 20.3 4.3 4.5 2.6 3.8 2.6 90.2 98.3 99.2 111.0 86.5 309.1 307.6 262.7 289.7 198.1 0.29 0.14 0.05 0.02 0.01
70.1 44.6 52.4 31.9 25.5 6.7 17.9 24.8 14.6 10.9 198.7 142.7 81.0 67.1 26.2 690.4 446.0 571.8 167.0 114.3 1.26 0.23 0.08 0.02 0.01
27.1 27.4 24.2 15.8 15.2 4.1 3.8 3.4 2.5 2.0 89.3 97.7 84.3 15.8 7.1 197.8 229.0 198.1 19.8 8.2 0.27 0.08 0.03 0.01 0.01 0.07 0.08 0.09 0.10 0.11
Cu
Co
Ni
Zn
Ca
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Fig. 5 shows the extraction isotherms of seven elements: Mn, Mg, Cu, Ca, Zn, Co and Ni at pH 3.0 and 238C. Again, zinc gave a very steep and straight extraction isotherm. The calcium isotherm showed that calcium extraction decreased gradually with the increasing of ArO ratio. The manganese isotherm showed that manganese extraction started to decrease substantially from ArO ratio 2:1. This was caused by the increased extraction competition by zinc over calcium and manganese when the ArO ratio was increased. The separation factors of manganese over the other six elements at pH 3.0 and 238C are also shown in Table 1. Compared with the results when six elements were in the aqueous solution, the separation factors decreased when calcium was present indicating an increased competition of zinc and calcium over manganese. 3.2.2. Effect of temperature on extraction isotherms In this series of tests, the aqueous solution contained five elements: Mn, Mg, Cu, Co and Ni. Zinc and calcium were not included. Fig. 6 shows the manganese extraction isotherms for pH 3.0 at 238C, 408C and 608C. The manganese extraction was much higher at 238C and decreased with increasing temperature. The effect of temperature on the extraction isotherms of the other elements was negligible. The separation factors of manganese over the other four elements at pH 3.0 are shown in Table 2. In most cases, the highest separation factor was also obtained at room temperature. Fig. 7 shows the manganese extraction isotherms for pH 3.5 at 238C, 408C and 608C. Manganese extraction increased with increasing temperature. The extraction of copper, cobalt and nickel was too low to form a normal isotherm. The magnesium extraction increased slightly with increasing temperature. The separation factors of manganese over
Fig. 5. Extraction isotherms of seven elements.
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Fig. 6. Manganese extraction isotherms at different temperatures ŽpH 3.0..
cobalt decreased with the increase in temperature while the separation factors of manganese over nickel increased with the increase in temperature except at 608C with
Table 2 Separation factors of manganese over other elements at different temperatures ŽpH 3.0. Name of elements
ArO ratio
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
Cu
Co
Ni
Separation factor Ž b Mn rM . 238C
408C
608C
29.4 32.5 37.5 35.7 36.5 5.6 5.0 5.7 4.4 4.5 119.3 133.4 132.9 193.9 340.6 361.4 434.2 329.4 225.0 307.2
18.6 22.5 25.4 24.1 24.8 1.4 2.1 2.0 1.8 2.5 36.9 44.6 33.0 39.7 37.2 368.7 430.5 471.3 738.7 672.0
22.4 14.5 18.9 18.8 20.2 2.7 1.3 1.5 2.0 1.3 78.1 44.1 37.6 58.1 56.6 296.1 205.9 346.6 277.1 264.0
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Fig. 7. Manganese extraction isotherms at different temperatures ŽpH 3.5..
ArO ratio 10:1 ŽTable 3.. The separation factors of manganese over magnesium, copper and cobalt were the highest at room temperature. Table 3 Separation factors of manganese over other elements at different temperatures ŽpH 3.5. Name of elements
ArO ratio
Mg
1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1 1:2 1:1 2:1 5:1 10:1
Cu
Co
Ni
Separation factor Ž b Mn rM . 238C
408C
608C
30.6 35.6 59.5 47.0 44.0 5.5 4.6 4.2 2.7 2.5 116.4 128.8 147.0 98.4 90.4 372.6 475.7 596.8 475.0 354.8
33.6 35.0 41.6 42.7 39.5 4.5 2.5 2.4 2.2 1.7 104.2 67.3 48.7 42.7 38.3 515.9 585.8 663.5 661.9 408.7
27.6 29.6 32.1 30.4 27.8 1.8 2.8 1.9 1.3 1.4 32.9 25.2 20.9 16.9 15.7 557.6 620.0 729.8 794.0 278.0
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3.2.3. Extraction stages estimated by McCabe–Thiele method Fig. 8 shows a McCabe–Thiele diagram constructed for manganese to determine the theoretical extraction stages for the separation of manganese from cobalt and nickel at pH 3.5 and 408C. Two theoretical extraction stages would suffice to reduce the manganese concentration in the aqueous solution from 2 grL to less than 3 ppm using an ArO ratio 1:1. Of course, this is also dependent on the extraction of other elements by the organic solution. At pH 3.5, the cobalt extraction would be much higher than at pH 3.0. Therefore, more scrubbing stages would have to be used to reduce the cobalt extraction in the organic solution. 3.3. Multiple stage extraction 3.3.1. Multiple extraction with fresh aqueous solutions Multiple stage extraction tests with fresh aqueous solution were conducted to simulate the first stages of counter-current extraction Žfresh aqueous solution to meet loaded organic solution. and to investigate the ‘‘crowding effect’’ during the separation. In these tests, the same organic was contacted with fresh aqueous solution five times with ArO ratio 1:1 at pH 3.5. Table 4 shows the metal concentration in organic solution after each stage of extraction at pH 3.5. The concentration of cobalt, magnesium and nickel decreased with the increase in stage number due to the crowding effect of manganese and zinc. Again, the copper concentration was basically unchanged while both zinc and manganese concentration increased with the contact stages. The multiple extraction tests showed that the extracted nickel and cobalt in the organic solution were crowded out by zinc and manganese. At pH 3.5, the cobalt
Fig. 8. Manganese McCabe–Thiele diagram at pH 3.5, 408C.
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Table 4 Metal concentration in organic or aqueous solutions during multiple stage extraction and scrubbing at pH 3.5 Name of elements
Stage no.
Multiple extraction with fresh aqueous Concentration in organic ŽgrL.
Multiple extraction with fresh organic Concentration in aqueous ŽgrL.
Multiple scrubbing with Mn solution Concentration in organic ŽgrL.
Mg
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5
0.000 0.475 0.200 0.145 0.095 0.100 0.000 0.065 0.080 0.075 0.060 0.055 0.000 0.020 0.007 0.007 0.005 0.004 0.000 2.120 3.110 3.325 3.300 3.340 0.000 0.070 0.015 0.025 0.005 0.010 0.000 0.315 0.650 0.960 1.175 1.585
3.460 2.960 2.050 1.590
0.160 0.015 0.006 0.006
0.064 0.025 0.007 0.002
0.080 0.050 0.027 0.016
0.338 0.343 0.323 0.280
0.013 0.003 0.002 0.003
1.970 0.286 0.022 0.002
2.640 2.715 3.180 2.785
3.420 3.340 3.070 3.320
0.0400 0.0025 0.0025 0.0025
Cu
Co
Mn
Ni
Zn
0.975 1.010 0.925 1.000
concentration in the organic solution dropped from 20 to 4 ppm after five stages of contact while nickel concentration dropped from 70 to 10 ppm. The extracted magnesium was also crowed out from the organic. 3.3.2. Multiple extraction with fresh organic solutions Multiple stage extraction tests with fresh organic solution were conducted to simulate the last stages of counter-current extraction Žfresh organic solution mixing with nearly
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barren aqueous solution. and to reduce the target elements Žzinc, manganese and copper in this case. in the aqueous solution. In these tests, the same aqueous solution was contacted with fresh organic solution three times at pH 3.5. The ArO ratio was 1:1. The initial aqueous solution contained 3.46 grL Mg, 0.064 grL Cu, 0.338 grL Co, 1.97 grL Mn and 3.42 grL Ni. Table 4 shows the metal concentration in aqueous solution after each stage of extraction at pH 3.5. The concentration of copper and manganese decreased dramatically to about 2 ppm. The cobalt concentration was slightly decreased while nickel concentration was almost unchanged. The magnesium concentration decreased to approximately half of the original value. This is due to too many extraction sites available in the organic solution, which resulted in less competition. The multiple extraction tests with fresh organic solution showed that the manganese and copper concentration in the aqueous solution dropped to 2 ppm at pH 3.5 after three stages of contact. 3.4. Multiple stage scrubbing The aim of scrubbing the organic solution was to wash out any cobalt and nickel that were co-extracted with manganese. Nickel and cobalt could then be recovered by recycling the scrub solution to the feed. The loaded organic solution was prepared by contacting the same organic solution three times with fresh aqueous solution ŽArO ratio 1:1. at pH 3.5. The organic solution prepared contained 160 ppm magnesium, 80 ppm copper, 13 ppm cobalt, 40 ppm nickel, 2.64 grL manganese and 0.975 grL zinc. Three-stage scrubbing tests were carried out by contacting the same loaded organic solution three times with fresh aqueous solution containing 5.0 grL manganese. The ArO ratio was 1:5 to simulate the very low ArO ratio used in practice. The pH values of the scrub solution was 3.5 and no pH adjustment was conducted during the scrubbing tests. The metal concentration in organic solution after each stage of scrubbing is shown in Table 4. The concentrations of magnesium, copper, cobalt and nickel in the organic solution decreased with the increase in stage number due to the crowding effect of manganese. The concentrations of cobalt and nickel were reduced to about 3 ppm, and the concentrations of copper to 16 ppm and magnesium to 6 ppm. The zinc concentration was basically unchanged while the manganese concentration increased. Nearly all the nickel and cobalt co-extracted were crowded out from the organic solution after one stage of scrubbing. At pH 3.5, the cobalt concentration dropped from 13 to 3 ppm and the nickel concentration dropped from 40 to 3 ppm after one stage of contact. Co-extracted magnesium and copper were also crowded out from the organic solution by manganese, suggesting a much weaker manganese solution should be used to keep the extracted copper and magnesium in the organic solution. 3.5. Calcium stripping and iron poisoning Calcium was completely extracted by D2EHPA at pH 2.5 ŽFig. 1. and iron was completely extracted at 1.5 w10x. The laterite leach solution after iron precipitation is
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saturated with calcium for limestone is usually used as a reagent. Residual iron may be present in the aqueous solution and enter the D2EHPA solvent extraction circuit, especially when the iron precipitation circuit is malfunctioned. If sulphuric acid is used as the stripping solution, nasty gypsum precipitation may occur and iron remains in the organic solution causing poisoning of the extractant. Therefore, a two-strip strategy has to be used. In the first set of strip stages, sulphuric acid is used as the strip solution to selectively strip manganese and copper from the organic solution at pH 2.3–2.5. The strip raffinate solution will be used as scrub solution after making-up and pH adjustment. In the second set of strip stages, hydrochloric acid solution has to be used to strip calcium and iron if any.
4. Conclusions and recommendations A series of shake-out tests was undertaken to investigate the fundamentals of the separation of manganese from cobalt and nickel using D2EHPA in kerosene and to explore the possibility of separation of zinc, copper, manganese and magnesium from cobalt and nickel in synthetic laterite leach solution. Ž1. D2EHPA pH-extraction isotherms generated for single element solutions showed that the extraction order for the six elements tested as a function of pH 50 was Zn2q) Ca2q) Mn2q) Cu2q) Co 2q) Ni 2q) Mg 2q. This order is significantly different from commonly referenced isotherms for this system. Ž2. D2EHPA extraction isotherms generated for the group of elements, Zn, Ca, Mn, Cu, Co, Ni and Mg, showed that the separation of zinc and calcium from the other elements was easy and the separation of manganese and copper from cobalt and nickel was possible. Ž3. The separation of manganese from solutions containing cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature Ž238C.. At pH 3.5, better separation of manganese from cobalt was achieved at room temperature Ž238C., while better separation of manganese from nickel could be obtained at elevated temperatures Ž40–608C.. Ž4. The McCabe–Thiele diagram for the system showed that at pH 3.5 and 408C, two theoretical extraction stages were needed to extract 99.9% of the manganese from the solution and to reduce the manganese concentration from 2.0 grL to 3 ppm. Ž5. Multiple stage extraction with fresh aqueous solution showed that after two stages of extraction at pH 3.5, only about 7 ppm cobalt and 15 ppm nickel were present in the organic solution due to the crowding effect of zinc and manganese. Ž6.Multiple stage extraction with fresh organic solution showed that after three stages of extraction at pH 3.5, only about 2 ppm manganese and copper were present in the aqueous solution. Ž7. Multiple stage scrubbing of the loaded organic solution with manganese solution showed that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution due to the crowding effect of the manganese.
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Ž8. Magnesium was extracted after nickel and cobalt in terms of the pH 50 order. Therefore, magnesium will remain in the aqueous solution when D2EHPA is used to separate manganese from nickel and cobalt. Ž9. A hydrochloric acid stripping is necessary to prevent gypsum precipitation and iron poisoning of D2EHPA. Ž10. D2EHPA is a cheap and stable extractant and as demonstrated in the current work, impurities such as zinc, calcium, manganese and copper could be eliminated from the laterite leach solution before nickel–cobalt separation. A continuous trial using synthetic solution and real leach solution should be conducted to verify this.
Acknowledgements The author thanks the AJ Parker Centre for Hydrometallurgy for funding this project and the permission to publish this paper. The project was supported by the Department of Minerals and Energy, Western Australia, and CSIRO Minerals. The test work was carried out at the former Mineral Processing Laboratory. The support from the staff and management of both organisations is gratefully acknowledged. Mr. Michael Davies carried out the test work. Thanks are extended to Ms. Caroline Hughes and Dr. Martin Houchin for reviewing the draft paper and their valuable comments.
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