Hydrometallurgy2 by Sushanta K Sahu

Hydrometallurgy 103 (2010) 45–53

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Prospects for solvent extraction processes in the Indian context for the recovery of base metals. A review Vinay Kumar ⁎, S.K. Sahu, B.D. Pandey Metal Extraction and Forming Division, National Metallurgical Laboratory, Jamshedpur-831 007, India

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Article history: Received 30 October 2009 Received in revised form 18 January 2010 Accepted 19 February 2010 Available online 25 February 2010 Keywords: Solvent extraction Base metal processing Metal extraction and separation Waste solutions Efﬂuents Copper Zinc Cobalt Manganese Nickel Chloride solutions Sulfate solutions

a b s t r a c t Details of the research and development work carried out in India using SX for the treatment of the leach/ waste solutions and efﬂuents containing base metals are reviewed, focusing on solutions obtained from lowgrade or complex ores, secondary resources or industrial wastes. Commercially available organic extractants, such as hydroxyl-oximes, hydroxyl-quinoline, tertiary amines, substituted phosphoric acid and other phosphate based extractants, have been used for the recovery of metals from acidic, alkaline and mixed sulfate–chloride solutions containing copper, nickel, cobalt, zinc and associated metals. The applicability of commercially available tailor-made extractants is described for the puriﬁcation and enrichment of the metal contents of lean and complex solutions in an Indian context. Research efforts for the separation and recovery of valuable metals by SX are also highlighted. Treatment of solutions for producing metals by crystallization or electrowinning is described. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Solvent extraction (SX) is an important tool for the processing of lean, complex and secondary resources of non-ferrous metals due to its versatile applicability in metal separation, purification, enrichment and analysis. The process is used on commercial scale for the extraction and separation of copper, nickel, cobalt, zinc, tungsten, vanadium, gallium, uranium and rare earths from the leach solutions, industrial waste streams and effluents. With the development of improved SX equipment design and materials of construction and newer organic extractants, it is possible to recover metals as valueadded products from solutions in presence of complexing anionic species. Usually, commercially available reagents such as LIX 84, LIX 621, LIX 87QN, ACORGA, di-(2-ethylhexyl) phosphoric acid (DEHPA), Cyanex 272, Alamine 336, tri-n-butyl phosphate (TBP) and PC 88A, which may be termed anionic, cationic, solvating or mixed reagents are employed for the recovery of different non-ferrous metals. The first non-nuclear application of SX was reported for the production of copper from dilute acidic solutions in the presence of iron using the substituted hydroxy-oxime, LIX 64N (Ritcey and

⁎ Corresponding author. Tel. +91 6572 345272; fax: +91 6572 345245. E-mail addresses: vkumar@nmlindia.org, 7.vinay.kumar@gmail.com (V. Kumar). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.02.016

Ashbrook, 1979). In ammoniacal medium, LIX 64N was employed by the SEC Corporation at El Paso Texas to extract copper first followed by pH adjustment to 9–10 using ammonia for nickel extraction (Eliason and Edmunds, 1974). Subsequently several large copper SX plants were set up in different parts of the world. The process is usually operated in a closed loop with leaching and electrolysis (EW) to produce metal while recycling the reagents in the system. The oxime-type reagents such as ketoximes, aldoximes and mixtures of these, as awell as β-diketones, are usually employed for copper extraction and have undergone constant improvement to suit different leach solutions (Kordosky, 1992; Koppiker, 2002). Furthermore, SX has found extensive applications in different plants for the extraction and separation of cobalt and nickel from chloride, ammoniacal and sulfate solutions. As cobalt forms anionic 2− chloro-complexes (CoCl− 3 and CoCl4 ) in strong chloride solutions, tri-octylamine (such as Alamine 336) selectively extracts cobalt leaving nickel in the aqueous raffinate. The Yabulu refinery of Queensland Nickel (QNI) has recently switched over to a SX route for nickel recovery, after removing cobalt as the sulfide. In this process, LIX 87QN, developed by Cognis is employed for nickel extraction from ammonia solution (Sandhibigraha and Sarma, 1997). The loaded nickel is stripped with a high concentration of ammonia– ammonium carbonate solution and the stripped solution is used to produce basic nickel carbonate.

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Since sulfuric acid is the predominant medium for hydrometallurgical metal extraction, SX processes for the separation of Ni–Co from dilute sulphuric acid solutions is often reported. Cation-exchange type reagents such as DEHPA, PC88A (2-ethylhexylphosphonic acid mono-2ethylhexyl ester), Cyanex 272 (bis (2,4,4-trimethyl-pentyl) phosphinic acid), Cyanex 301 (di-isooctyl-dithiophosphinic acid) or Versatic-10 (R1R2(C)CH3COOH, where R1, R2 = C6) have been employed for the extraction and separation of Ni–Co on an industrial scale from different solutions. As regards SX for zinc, DEHPA has been used for zinc extraction in the ZINCEX (Thorsen, 1983), the modified ZINCEX (Diaz and Martin, 1994) and the CENIM-LNETI (Amer et al., 1995) processes. Recently, a SX process has been exploited to produce zinc on a commercial scale in Namibia from oxide/silicate ores using DEHPA which selectively extracts zinc leaving chloride and fluoride ions in the raffinate (Sole et al., 2005). Besides the above commercial applications of SX processes, a number of research and review papers are appearing in the open literature for the extraction and separation of different metals from complex leach solutions and industrial effluents (Bhattacharya et al., 1999; Bacon and Mihaylov, 2002; Kumar et al., 1999; Jha et al., 2002). Basic studies have been reported for the extraction and separation of metals in presence of anionic and cationic species and for the formation of complexes in the organic phase from different aqueous solutions using various extractants. The properties of the reagent have also been modified with mixed extractants to improve the separation of the metals. In the present paper, new trends and prospects for the application of SX processes in the Indian context are highlighted for the recovery of base metals from lean and complex ores, including un-exploited materials and industrial wastes. 2. Commercial processes in India In India, an SX process is used on a commercial scale in nuclear and rare earth industries (Maharana and Nair, 2005; Singh and Gupta. 2001; Singh et al., 2004). For example, the extraction and separation of zirconium and hafnium from a leach solution at the Nuclear Fuel Complex, Hyderabad and the separation and purification of individual rare earths at Indian Rare Earths Ltd, Cochin. However, very little expertise was available in the private sector to be used for the metal extraction industry. Hindustan Zinc Ltd (HZL), Udaipur, was the first to enter this field and has been operating a small plant to recover cobalt from their process wastes since the early 1990's. Later on, they planned to establish an SX operation to recover copper values from their lead dross at the Chittorgarh Imperial Smelter plant, but this operation was never realised. 2.1. Zinc Only a few plants were set up in the country utilising SX processes for the production of base metals from different low-grade ores and secondary materials or wastes. These plants are of small capacities (less than 2 t metal/day). The largest among them has been set up by Sunrise Zinc Ltd. (SZL) located at Cuncolim, Goa. Although, the plant has stopped its production since 2001, some details are given. In this plant, the basic raw materials were zinc ash, brass ash, die casting ash, etc. A simplified flow sheet of this plant is shown in Fig 1. The reagents used were DEHPA for zinc, LIX 984 for copper and Alamine 336 for iron extraction (Shah, 1998). In this process, zinc from the leach liquor purified with respect to iron, was extracted with DEHPA leaving other impurities in the solution. Any iron remaining in the solution was extracted by DEHPA in preference to zinc. In the case of brass ash or similar raw materials, the raffinate contained copper, which was recovered in a second SX circuit using LIX 984 as the extractant. Copper and zinc metals were produced as cathodes by EW. The iron extracted in the DEHPA was stripped with

6 M HCl to regenerate the solvent for recycle. The plant utilised mixersettlers of three different designs. 2.2. Cobalt and nickel The first commercial cobalt refining plant in India to produce cobalt cathodes was set up by Nicomet Industries Ltd (NICCO), Cuncolim, Goa. This cobalt refinery was commissioned in April 1997 (capacity 180 t/y) and increased capacity to 1000 t/y in 2006 (Cobalt News, 2006). It now has separate SX circuits for copper, cobalt and nickel, and is in a position to produce high quality cobalt metal (99.98% pure). It is the only plant in India which recovers nickel from basic raw material and produces NiSO4 crystals (almost 99.99% purity. The simplified flow sheet followed at this refinery is shown in Fig 2. Two more cobalt refineries based on SX processes (Koppiker, 2001) were set up in 1998 — one at Baroda (Rubamin Ltd) with a current production of 500 t/d (Cobalt News, 2006) and another at Taloja, near Mumbai (Conic Metals Ltd). These plants produce cobalt cathode and various metal salts. Rubamin Ltd initially used acid (sulfuric acid with and without SO2 or HCl–HNO3) to leach the cobalt sludge or super-alloy scrap and dissolve the metallic values (Sarma et al., 2002). After removal of iron(III) as hydroxide, nickel and cobalt are separated using PC 88A. More recently, the process has been modified for the recovery of metallic values from different resources such as ores, concentrates or secondary materials following reduction leaching. The process then uses SX to separate nickel and copper from cobalt, EW to produce cobalt and nickel metal (99.8% purity) and evaporates/precipitates metal sulphate salts. (www.rubamin.com/ cobalt_processes_tech.php, 2009). In addition to this production, cobalt metal powder is recovered from cemented carbide scrap by Sandvik Asia Ltd at a pilot plant in Pune. Spent catalysts from plants producing terephthalic acid are reprocessed by more than a dozen small cobalt chemical processors in un-organised sectors. Another cobalt-containing waste product, known as beta-cake, is generated during the production of zinc at Hindustan Zinc Ltd, Udaipur (Sarma et al., 2002). As discussed earlier, a process was developed and run at HZL for the recovery of cobalt from such a waste involving roasting of the beta cake, sulfuric acid leaching of the calcine, iron precipitation and SX. Again, DEHPA was used for the extraction and separation of zinc and cobalt. The purified cobalt solution obtained from stripping was used to produce cobalt metal by EW. 2.3. Copper Two copper SX-EW plants have also been set up — one near Chennai and another near Hyderabad. They produce about 2–3 t/d copper metal. Raw material availability is a one of the reasons for the low productivity in these plants. Looking into the progress of SX technology over the past few years, one can optimistically expect several base metal SX plants to be operational in India in coming years. The success of SX plants depends on the proper economical design of the equipment. Usually, different designs of the mixer settler are employed, depending upon the volumes and flow-rates used, for efficient operation of the process so that minimum loss of organic and low crud formation takes place. In India, the expertise is available in different industries to fabricate the equipment and supply to various SX industries producing metals such as uranium, rare earth and base metals. The details are available on internet (www.mixersettlers.com, 2009). 3. SX processes under development in India Metal separation, enrichment and recovery by SX for the treatment of acidic and ammoniacal solutions obtained from the

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Fig. 1. Simpliﬁed ﬂow sheet for recovery of zinc and copper from brass and zinc ash.

leaching of lean grade and unexploited ores and wastes have been extensively studied in India, mainly at the National Metallurgical Laboratory (NML) (Jamshedpur), the Institute of Minerals and Materials Technology (IMMT — formerly the Regional Research Laboratory (RRL) (Bhubaneswar)), the National Institute of Interdisciplinary Science and Technology (NIIST — formerly RRL (Trivan-

drum)), Bhabha Atomic Research Centre (BARC) (Mumbai), Hindustan Zinc Ltd. (HZL) (Udaipur) and Nuclear Fuel Complex (NFC) (Hyderabad). The efﬂuents generated from the process industries also contain heavy and toxic metallic ions along with anion complexing ions. The presence of these ions creates pollution and the treatment of such

Fig. 2. SX process for the recovery of metals/salts from secondary resources.

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solutions by conventional methods is not very effective in meeting the strict environmental regulations on heavy metal concentrations. In order to treat such effluents, SX is considered an alternate approach, which has the ability to recover or recycle the metallic constituents. A brief account of these developments is discussed in the following sections. 3.1. Ammoniacal solutions Ammoniacal solutions have often been employed in the hydrometallurgical extraction of base metals because iron and manganese are rejected to the residue. The solutions free from such impurities can be readily processed for metal separation by SX. The investigation of SX processes has involved the recovery of valuable constituents from leach solutions obtained in the treatment of low grade ores or wastes, such as poly-metallic sea nodules, chromite ore overburden, lateritic nickel ore, sulfide concentrates and super-alloy scrap as discussed below. 3.1.1. Ocean nodules At NML, extensive studies have been carried out on the extraction and separation of copper, nickel and cobalt from ammonia–ammonium carbonate leach solutions obtained from the processing of poly-metallic sea nodules (Jana et al., 1999; Pandey et al., 1989, 1994; Pandey and Kumar, 1991; Kumar et al., 1991). The co-extraction and selective stripping of nickel and copper have been used for extraction and separation of copper, nickel and cobalt from the ammoniacal solutions. The solvent, LIX 64N was found to be excellent for the treatment of ammoniacal solutions obtained from the ocean nodules in bench and continuous operations. With the phasing out of LIX 64N and marketing of the superior reagent, LIX 84 (5-nonyl 2-hydroxyacetophenoneoxime) for copper extraction by Cognis, the process was modified, while utilizing this reagent. The simplified flowsheet adopted for the metal separation by SX-EW is shown in Fig. 3. In this process, both copper and nickel were co-extracted from the ammoniacal carbonate leach liquor (containing (g/L), 2.40 Cu, 2.41 Ni,

0.163 Co, 0.008 Mn, 0.002 Fe, 70 NH3 and 60 CO2), followed by scrubbing of loaded ammonia (Kumar et al., 1999). Nickel was then stripped from the loaded 20% LIX 84 using nickel spent electrolyte (NiSE). However, about 0.045 g/L copper was also stripped and reported to the nickel pregnant solution (NiPS) which was unsuitable for the electrowinning of nickel. The purification of NiPS was achieved by copper removal with 5% LIX 84 in kerosene. The copper level dropped to 0.005 g/L in the final nickel pregnant electrolyte (NiPE) in four stages. Copper was similarly stripped from 5% and 20% LIX 84. The copper spent electrolyte (CuSE) from the electrowinning cell, containing 180 g/L sulfuric acid and 40.0 g/L copper, was first used to recover copper from 5% LIX 84 in two stages. The copper pregnant solution (CuPS) was then utilized to strip copper from 20% LIX 84. Stage optimisation studies also showed that copper stripping was effectively achieved in two stages. Electrowinning of nickel and copper was carried out in a closed-loop operation with SX. Before electrowinning from the pregnant electrolyte solution, entrained organic matter was removed using activated carbon as it adversely affected the purity and deposit character of the metal. About the same time, Indian Ocean Manganese Nodules developed at IMMT-Bhubaneswar a process for the extraction and separation of copper, nickel and cobalt from a ammoniacal ammonium sulfate leach solution (1.72 g/L Cu and Ni each, 200 g/L (NH4)2SO4) of using LIX64N (Nathsarma and Sarma, 1993). Copper and nickel were co-extracted in two counter-current stages at an A/O phase ratio of 1:2 using 40% LIX 64N. From the loaded organic, the co-extracted ammonia was removed using dilute sulfuric acid at an equilibrium pH of 6.0. Subsequently, nickel from the organic phase was selectively stripped using the spent nickel electrolyte at pH 1.80 with an A;O phase ratio of 1:1 in three counter-current stages. Similarly, copper was stripped with its spent electrolyte in three stages at an A:O ratio of 2:5. The process was developed on a bench scale and was tested in a demonstration plant for the future exploitation of the technology to recover metals from sea nodules (Mittal and Sen, 2003). Tangri and Suri (1999) also reported laboratory studies on the extraction and separation of cobalt, nickel, copper and zinc using SX processes from ocean nodules. 3.1.2. Nickel laterites Ammonia–ammonium carbonate solutions containing nickel and cobalt have been obtained from the leaching of low grade lateritic nickel ore from the Sukhinda Valley as well as from the leaching of chromite ore overburden. The leach solutions were processed using LIX 84 (Kumar et al., 1997a) and LIX 64N (Sarma et al., 1987) for the selective extraction of nickel, leaving cobalt(III) in the raffinate. Bench scale studies were initially carried out to optimise the process parameters required for the continuous extraction of the metals. The results showed the feasibility of extracting nickel with 30%LIX 84 in kerosene in three stages and the stripping of the loaded nickel with electrolyte solution at pH 1.1 in four stages. The nickel pregnant electrolyte obtained was found to be suitable for producing metal by electrowinning (EW) in a closed-loop SX-EW operation. The process was also evaluated on a pilot scale following the reduction roastammonia leach-SX route to recover nickel and cobalt from lateritic nickel ore.

Fig. 3. Separation of metals from leach liquor by solvent extraction-electrowinning (SXEW).

3.1.3. Sulﬁde concentrates A sulﬁde concentrate containing copper, nickel and iron is generated in small quantities at Uranium Corporation of India Ltd (UCIL), Jaduguda. In order to utilise such materials, ammonia– ammonium sulfate leaching was used, followed by SX with LIX 84. A process of co-extraction and selective stripping was used for the separation of metals from a leach liquor containing 13.8 g/L Cu and 10.7 g/L Ni (Sarma and Reddy, 2002; Sarma et al., 2002). A similar byproduct containing copper, nickel and cobalt was also generated in

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the UCIL plant. Pressure leaching followed by SX has been developed as a processing option to treat such materials (Sahu et al., 2004). Ammonia–ammonium sulfate leach solutions containing copper and zinc were also generated during pressure leaching of a complex sulfide concentrate at Ambamata, Gujarat (Rao et al., 1984). LIX 64N and Hostarex DK-16 (Hoechst AG) selectively extracted copper and zinc respectively from the metal ammine leach solution (containing 3.5 g/L Cu, 13.0 g/L Zn and 52.5 g/L (NH4)2SO4 at pH 9.7). The loaded metals were stripped with sulfuric acid to recover metals by EW. Further studies on the effect of ammonium salts on the extraction of zinc showed a decrease in extraction with increasing pH (Rao et al., 1990, 1992). This was attributed to the formation of zinc ammine complexes. Rao and Sahoo (1993) also studied the extraction of copper using Hostarex DK-16 from solutions containing varying amounts of ammonium sulfate (1–4 M) over a pH range of 2 to 10. Extraction was found to be low at higher pH due to the formation of free ammonia, which subsequently gave rise to non-extractable copper ammine complexes. 3.1.4. Super-alloy scrap and other waste materials Nickel obtained from the leaching of a super-alloy scrap (25.5 g/L Ni and 18.1 g/L ammonium sulfate) was extracted with PC 88A (Reddy et al., 1999). Parija et al. (1998) reported the extraction of nickel from a similar ammonium sulfate solution (20.5 g/L Ni, 0.2 g/L Co and 23.6 g/L ammonium sulfate) during the processing of industrial wastes and secondaries, such as cobalt sludge, using LIX 84I (2-hydroxy-5-nonyl-acetophenone-oxime). The nickel was stripped with sulfuric acid to produce a solution suitable for EW. Parija and Sarma (2000) also reported the separation of nickel and copper from ammoniacal solutions through co-extraction and selective stripping using LIX 84I. Furthermore, different authors reported the influence of increasing ammonium ion concentration on extraction efficiency (Sarma, 1986; Nathsarma and Sarma, 1996; Sandhibigraha and Sarma, 1996). 3.2. Sulfate solutions The extraction and separation of different metals such as copper, nickel, cobalt and zinc by SX have also been studied for the processing of sulfate leach solutions of various low grade materials or wastes, such as brass ash, converter slag and complex sulfide ores using different organic extractants (Murthy and Mishra, 1986; Kumar et al., 1989). Depending on the metallic constituents present in the solution, a specific processing scheme was developed using different extractants. 3.2.1. Copper Reddy et al. (2007) compared the performance of oxime-based extractants such as LIX 84 and LIX 973N for the recovery of copper from sulfate liquors of synthetic Cu–Ni–Co–Fe matte. Higher extraction with LIX 973N was found in comparison to LIX 84. Extraction isotherm data with 40 vol.% LIX extractants at A:O phase ratios of 1:1, indicated that three stages of extraction with LIX 84 and two stages of extraction with LIX 973N were necessary to achieve N95–99% Cu extraction from 10.68 g/L Cu in the aqueous feed. The co-extraction of nickel, cobalt and iron was b2 mg/L. Stripping of copper from the loaded organic phase at A: O = 1:1 with a synthetic spent electrolyte (SE) solution containing 35 g/L Cu and 180 g/L H2SO4 was 81% for LIX 84; whereas, it was ∼10% for LIX 973N indicating the non-suitability of this extractant under these conditions. The bench-scale continuous mixer settler experiments further confirmed the laboratory-scale data with small variations in percentage extraction and stripping. Mishra and Roy Choudhury (1995) reported the selective extraction of copper in two stages from a feed solution at pH 1.8 with 0.54 g/L Cu and 4.0 g/L Fe using 7.5% LIX 84. Another oxime extractant, MOC-45 (2-hydroxy-5-nonyl-acetophenone oxime sup-

plied by Allco Chemical, USA), was employed for the extraction of copper from the sulfate solution obtained in the leaching of converter slag (Rao et al., 2000). Copper extraction (N99%) was achieved for an aqueous feed of 0.005 M Cu and 0.1 M sodium sulfate solution at an equilibrium pH of 1.82. Total loaded metal was stripped with 2.0 M H2SO4. Recently, Sarangi and Pattanaik (2007) used a hybrid process comprising of reverse osmosis (RO) and SX for the enrichment of copper from a dilute (100 mg/L) feed solution. The copper (5 g/L) from the enriched solution was extracted using 50 vol.% LIX 84I in three counter-current stages (A:O = 1:1) with an extraction efficiency of 99.4%. The copper was stripped up to 99.4 and 99.2% at A:O phase ratios of 1:3 and 1:4, respectively, from the loaded organic phase in three counter-current stages using 178 g/L H2SO4. The spent organic retained 67 mg/L copper and the strip solution contained 30–40 g/L copper suitable for crystallization. Thus, it was possible to recover copper from a dilute solution (100 mg/L Cu) through a hybrid process (RO-SX) techniques and to regenerate treated water for re-use or safe disposal. The bleed solutions generated in copper smelters and from the SXEW processing of poly-metallic sea nodules contain high concentrations of acid along with copper and nickel. With a view to developing an efficient process avoiding evaporation and crystallisation for metal recovery, an SX-based approach was investigated at NML. Sulfuric acid was extracted by 25% Alamine 336 (tri-iso-octylamine) and 10% isodecanol in kerosene (Agarwal et al., 1996). The results indicated that 90% acid was extracted at an O: A ratio of 4:1. The kinetics of extraction was also quite fast, allowing the use of the solvent for continuous operation in a mixer settler. The copper and nickel were separated from the synthetic solution using 25% naphthenic acid and 10% isodecanol. The results showed that 92.1% copper was extracted from the 4.86 g/L Cu feed at an equilibrium pH of 4.0. Nickel could be recovered at higher pH. This showed the possibility of using SX for the extraction of acid and separation of metals from copper bleed solutions. An alternate approach for the treatment of copper bleed solutions from a copper smelter was developed to recover the high value products such as copper and nickel powders. The process consists of partial de-copperisation of the bleed stream followed by crystallisation of a mixed salt of copper and nickel sulfate. The mixed salt is leached, purified with respect to iron and then copper and nickel were separated using LIX 84 (Agarwal et al., 2002). The pure solutions of nickel and copper were electrolysed to produce pure powders for powder metallurgical applications (Agarwal et al., 2007). 3.2.2. Copper–zinc The separation and recovery of copper and zinc from the leach solution of a complex sulfide concentrate were studied using SX at NML (Kumar et al., 1989; Pandey et al., 1986). The results showed that LIX 64N selectively extracted copper from the leach liquor generated in the sulfation roast — leach of the complex sulfide concentrate from Ambamata, Gujrat, leaving zinc in the raffinate. The loaded copper was stripped with strong sulfuric acid and metal was produced by EW in a closed-loop SX-EW operation. The raffinate containing zinc was purified and the metal was produced by EW. Further studies were carried out to compare the performance of LIX 84 with LIX 64 N for the separation of copper and zinc from sulfate solutions (Kumar et al., 1997b). As the leach liquor often contained impurities like iron and manganese, the performance of both solvents was compared in presence of impurities under identical conditions for an aqueous system such as Cu–Zn–Fe–Mn. In the pH range 0.7–3.9, LIX 84 showed higher copper loading compared to LIX 64N. Improvements in the separation factor for Cu–Zn were observed with the former extractant in the above pH range. LIX 84 also exhibited better selectivity for the extraction of copper in the presence of iron and manganese at levels of 0.21 and 1.0 g/L, respectively.

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Similarly, Reddy and Priya (2004, 2005) developed a process for extraction and separation of Cu(II), Ni(II) and Zn(II) using LIX 84I as an extractant from sulfate solution. Extraction of each metal depended upon the equilibrium pH and temperature had no effect on the extraction of metal. From the difference in the extraction behaviour of these metals as a function of pH, it was possible to separate and recover these metals. 3.2.3. Zinc–cobalt A SX process for the extraction of zinc from the sulfate leach liquor of a rayon plant sludge was developed at NML using thio-phosphinic extractants (Jha et al., 2005). However, the most widely used extractants are phosphoric acid-based alkyl phosphorus reagents such as DEHPA, PC88A, Cyanex 272 or their equivalents for separation of zinc and zinc and cobalt from sulfate solutions. Nathsarma and Sarma (2003) compared their performance using 0.04 M Na.DEHPA, Na.PC 88A and Na.Cyanex 272 at constant pH. Of the three extractants studied, Na.DEHPA was found to be the most suitable extractant for the separation of these metals. The separation factor for cobalt and zinc increased with increasing equilibrium pH and was the highest (SF = 3394) at pH 5.65. Quantitative extraction of zinc at equal phase ratio could be achieved in two counter-current stages. Sulfuric acid solution was found to be a better stripping agent for zinc than zinc spent electrolyte (containing the same concentration of sulfuric acid). Extraction of cobalt (N99%) could be achieved in two counter currentcurrent stages at equal phase ratio and metal was stripped quantitatively in two counter-current stages from the loaded solvent with 2.0 g/L sulfuric acid solution. At NML, DEHPA has been employed for the extraction and separation of cobalt and zinc from the sulfate leach solution of a cobalt cake obtained in the processing of sea nodules (Kumar et al., 2006). A scheme was developed to simulate the extraction of the metals in a counter-current scheme. With an aqueous feed of 17.5 g/L Zn, 20.0 g/L Co at 2 pH, a raffinate containing 0.047 g/L Zn and 19.03 g/ L Co was obtained in four stages with partially saponified DEHPA (0.64 M) containing 4% isodecanol in kerosene. The conditions for cobalt extraction from the model solution of cobalt cake were also optimised. With completely saponified DEHPA (0.64 M) along with an additive in kerosene, a high degree of cobalt extraction (99.8%) was achieved at an aqueous feed pH of 5.0 and O:A of 2:1. The behaviour of impurities, such as Fe, Mn, Cu and Ni, was also examined during the extraction and it was found that these could be effectively discarded to minimise the contamination of zinc/cobalt sulfate solutions. A complete flow sheet has been proposed to recover Zn and Co from the leach liquor of cobalt cake (Kumar et al., 2006). 3.2.4. Zinc–manganese The application of phosphoric acid–based extractants has been studied for the extraction of zinc in the presence of manganese from sulfate solutions (Devi et al., 1995, 1997a,b). Devi et al. (1995) investigated the extraction of zinc with sodium salts of DEHPA, PC88A and Cyanex 272 while varying the pH, extractant concentration and degree of saponification. The study showed the extraction of zinc in the following order: Na.DEHPA N Na.PC88A N Na.Cyanex 272. Subsequently, the extraction of manganese(II) from sulfate solutions was carried out with binary mixtures of Na.DEHPA, Na.PC 88A and Na. Cyanex 272 (Devi et al., 1997a). The best results were obtained with Na.Cyanex 272 as the extractant and Na.PC 88A and Na.Cyanex 272 as the synergists. The system involving NaPC 88A as the extractant and NaDEHPA as the synergist proved to be the least effective. Further studies on the extraction of manganese(II) and zinc(II) from sulfate solution using sodium salts of Cyanex 272 showed an increase in separation factor of zinc over manganese with decreasing equilibrium pH, which decreased with a rise in NaCl, NaSCN and Na2SO4 concentrations in solution (Devi et al., 1997b). In a similar later study by Nathsarma and Devi (2006), the extraction and

separation of zinc and manganese(II) from a sulfate solution was carried out using sodium salts of DEHPA, PC 88A and Cyanex 272. Not surprisingly, extraction of metal ions increased with an increase in equilibrium pH of the aqueous solution and extractant concentration. The separation factor of zinc over manganese was maximized according to the equilibrium pH and extractant concentration and was highest with 0.05 M Na.PC88A. Extraction of zinc from the Zn–Mn solution and that of manganese from the Zn-free solution were carried out with 0.05 M Na.PC88A in two stages at 1:1 phase ratio. Fairly good stripping efficiencies of zinc and manganese from their respective loaded organic phases were achieved with 0.03 M H2SO4 and 0.02 M H2SO4 in two stages at 1:1 phase ratios. 3.2.5. Cobalt–nickel The alkyl phosphorous acid–based extractants, although they extract both cobalt and nickel, exhibit selectivity for cobalt over nickel and other metals (Devi et al., 1994; Koladkar and Dhadke, 2001; Reddy and Sarma, 2001; Sarma and Reddy, 2002; Thakur, 1998). Devi et al. (1998) investigated the separation and recovery of 0.01 M Co(II) and Ni(II) ions from sulfate solutions containing 0.1 M sodium sulfate using 0.03–0.06 M sodium salts of DEHPA, PC 88A and Cyanex 272 in kerosene. Preferential extraction of cobalt over nickel for the desired separation was found with 0.05 M PC88A and Cyanex 272 and the highest separation factor was obtained with Cyanex 272 at an equilibrium pH of 6.85. The separation factor for Co over Ni was also high with increased loading of cobalt in PC 88A (Thakur, 1998). Recently, Reddy et al. (2009) reported the SX separation of 1.06 g/L Co and 1.19 g/L Ni from sulfate solutions using a mixture of TOPS 99 (Talcher Organo Phosphorus Solvent 99 — an equivalent of DEHPA from Heavy Water Plant, Talcher, India) and TIBPS (CH3–CH2–(CH3)–CH2–)3 P = P) from Cytec, Canada. The highest separation factor of 12,245 was obtained with the synergistic mixture of 0.1 M TOPS 99 and 0.05 M TIBPS at pH 1.1. McCabe–Thiele plots for cobalt extraction indicated the necessity of three theoretical stages for N99% Co extraction at an A:O phase ratio of 2:1. The extraction behaviour of Co(II) and Ni from sulfate solutions with bis (2-ethylhexyl) phosphinic acid (PIA-8) in toluene has been studied by Koladkar and Dhadke (2001). Quantitative extraction of Co(II) was observed at pH 5.0–5.9, while Ni(II) was extracted at pH 6.8–7.0 with 0.03 M PIA-8. The difference in pH0.5 for Co(II) and Ni was 1.9. Cobalt(II) was separated from nickel even at a 1: 20 (Co: Ni) ratio in solution. The stoichiometry of the extracted species was determined by the slope analysis method. The extraction reaction involved a cation exchange process with Co.R2(HR)2 and Ni.R2. 2(HR)2 being the extracted species. The separation of cobalt(II) from nickel was favoured with an increase in temperature. Electrolytic cobalt was produced from a solution containing nickel and other metallic impurities employing SX-EW (Rao et al., 1997). The solution was generated in batches by leaching super-alloy scrap. The purified solution contained 7–12 g/L Co, 35–50 g/L Ni, and Zn and Cu in small quantities. The organic extractant was a partially neutralised PC88A. Cobalt from the metal-loaded organic phase was stripped by dilute H2SO4 or acidic CoSO4 solution. The resulting cobalt sulfate solution, enriched to 45 g/L Co, was used as the electrolyte for EW. Data from the laboratory-scale studies were validated in a continuous pilot plant with minor deviations. More than 98% Co with about 0.2% Ni was transferred from the leach solution to the electrolyte, from which cobalt cathode of 99.3% purity was produced at a current efficiency of 65%. At BARC, Thakur (1998) reported an increase in the separation factor for copper–cobalt with an increase in copper loading in PC88A, while the separation factor decreased for manganese–cobalt with the loading of manganese. The effect of loading of the better extracted metal in a binary mixture was explained on the basis of the various extraction equilibria involved. Further, Thakur and Mishra (1998) determined the distribution co-efficient (D) against initial aqueous

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acidity at various initial concentrations of base metals such as copper, nickel, cobalt and manganese using PC 88A. A mathematical model was developed to calculate the concentration of metals in the aqueous and organic phases in counter-current cascades. The process parameters such as the phase ratio, acidity of the feed and scrub solution, number of extraction and scrubbing stages were optimised to obtain the desired purity and recovery of cobalt and nickel. Process parameters were optimised by Singh et al. (1999) for the extraction of nickel in the presence of Cr(III), Fe(III), Mn(II), Co(II), Cu (II) and Zn from sulfuric acid media employing the Cyanex 301toluene system. On the basis of the distribution data, the composition of extracted species was proposed. The practical utility of this extractant was demonstrated for recovery of nickel from spent catalyst and electroplating bath residue. Senapati et al. (2004) reported the puriﬁcation of nickel sulfate solutions containing Fe, Cu, Co, Zn and Mn using 0.2 M Cyanex 272 (partially neutralised). Iron(III) was removed from the solution by SX and lime precipitation methods. From the iron-free solution, other impurities were extracted in a single stage. The metal ions from the loaded organic were stripped with 0.5%v/v sulfuric acid solution in a single stage. The entire operation needed only seven stages with b0.5% Ni loss: two stages for iron extraction, three for iron stripping from the organic; and one stage each for extraction and stripping of other impurities. Gupta et al. (2002) also reported the extraction of Cu(II), Ni(II), Co (II) and Zn(II), along with other 3d transition metal ions, from acid sulfate, chloride and nitrate media using Cyanex 923. The investigation showed the potential of Cyanex 923 for conveniently achieving the mutual binary separations of some 3d transition metals. 3.3. Chloride solutions SX has also been employed for the recovery and recycle of metals from chloride solutions. Sarangi et al. (1999) reported the extraction and separation of cobalt and nickel from 1 M chloride solutions using Na.Cyanex 272 as extractant and TBP as a phase modiﬁer. An increase in metal extraction was observed with an increase in aqueous phase pH and a difference in pH0.5 values of 1.25 for the two metals with Na. Cyanex 272 indicated the possible separation of cobalt and nickel. An increase of temperature increased the extraction of nickel; while for cobalt, the extraction increased up to 303 K, and then decreased. The separation factor for Co/Ni was found to be 5.6 times higher with NaPC 88A as extractant and Na–Cyanex 272 as synergist than that with Na.Cyanex 272 alone. Selective extraction of cobalt from acidic chloride leach solutions of Sukinda lateritic nickel ores was achieved using TBP (Kanta Rao et al., 1975). Pure cobalt solution was also obtained using PC88A with a chloride solution containing cobalt and nickel from super-alloy scrap material, which allowed the production of cobalt metal by EW (Rao et al., 1997). A SX process for the separation and recovery of pure Co(II), Ni(II) and Cu(II) from the hydrochloric acid leaching of poly-metallic sea nodules has been described by Gupta et al. (2003). The Co(II) and Cu (II) were extracted from the leach liquor with Cyanex 923 (a mixture of four tri-alkyl phosphine oxides, namely R3′P = O, R2R′P = O, RR2P = O, and R3′P = O (where R is n-octyl and R′ is an n-hexyl chain, Cytec, Canada)). Whereas Cyanex 301 extracted Ni(II). The Co (II) and Cu(II) were partitioned into the organic phase as H2CoCl4.2Cyanex 923 and CuCl2.2Cyanex 923 species; whereas Ni(II) formed species such as NiR2 (HR = Cyanex 301). A solution of 0.001 M H2SO4 was used for the stripping of Co(II) and Cu(II); while stripping Ni(II) with 5% NH4Cl in 75% NH3. Both extractants were found to be stable towards prolonged contact with HCl and showed negligible loss in their extraction capacity even after 20 cycles. The partition data was adequately utilised in developing conditions for the separation of Co (II), Ni(II), and Cu(II) mutually and from other metal ions, namely Ti

(IV), Al(III), Fe(III), Mn(II), and Zn(II). The purity of the metal ions thus obtained was found to be around 99% (Gupta et al., 2003). More recently, Reddy et al. (2005) reported the extraction and separation of Cd(II), Ni(II) and Co(II) from the chloride leach solution of Ni–Cd batteries. Cadmium(II) was selectively extracted with Cyanex 923, leaving nickel and cobalt in the raffinate which were extracted with Cyanex 272 and TOPS 99, respectively. 3.4. Mixed sulfate–chloride solutions During the processing of sulfide ores or secondaries in chloride media, leach solutions containing mixed sulfate and chloride ions are often generated. A leach liquor containing 24.8 g/L Cu, 0.23 g/L Zn, 3.8 g/L Co, 35.2 g/L Ni, 11.8 g/L Fe, 176.3 g/L Cl−and 48.9 g/L SO2− 4 was processed for extraction and separation of metals by TBP, LIX 84I and Cyanex 923 in kerosene (Sarangi et al., 2007). Iron from the leach liquor was extracted in two counter-current stages at equal phase ratio using 1 M TBP and 2% MIBK (methyl isobutyl ketone) leaving 0.004 g/L iron in the raffinate. Stripping was carried out with distilled water at O:A of 2:1. Copper from the iron-free raffinate was separated in three stages at an A:O ratio of 1:2 using 70% LIX 84I in kerosene followed by zinc extraction with 0.05 M Cyanex 923 at A:O of 2:1. The stripping of iron and zinc from the loaded TBP and Cyanex 923 was quantitative with distilled water, whereas copper stripping from LIX 84I was carried out with sulfuric acid solution. 4. Conclusions Based on the foregoing discussion on the application of SX for base metals in India, the following conclusions can be drawn. Firstly, SX processes are employed in India on a commercial scale to separate and recover base metals such as copper, nickel, cobalt and zinc from secondary resources or waste materials from different industries. These plants are normally of smaller size and produce electrolytic grade metals or metal salts using organic extractants such as DEHPA, LIX 984, LIX 984N PC 88A and Alamine 336. Secondly, various R&D studies are currently underway to develop processes for the extraction and separation of metals from acidic, alkaline (including ammoniacal) and mixed sulfate–chloride leach liquors generated in the treatment of complex or lean ores or industrial wastes which have not been exploited to date. Some of the processes have been tested on pilot scale in continuous mode while recycling the reagents and salts in the system. Large pilot-scale developments that are worth mentioning are the separation and recovery of copper, nickel and cobalt from an ammoniacal carbonate leach liquor (NH3–CO2/NH3–SO2 system) derived from Indian ocean nodules; nickel and cobalt recovery from the sulfate leach liquor of super-alloys and nickel and cobalt recovery from sulfate leach liquors of cobalt cake from ocean nodules or beta cake of zinc plants. Finally, several organic extractants are reported for the selective extraction and separation of metals and the potential exists for further exploitation of SX-based processes. Development of newer organic extractants, mixed extractants and synergists may be more selective for the recovery of specific metals from the acidic or alkaline leach solutions in the presence of complexing agents. Acknowledgements The authors are thankful to the Director, National Metallurgical Laboratory, Jamshedpur (Council of Scientific & Industrial Research, New Delhi) for kind permission to publish this paper. References Agarwal, A., Pandey, B.D., Kumar, V., Premchand, 1996. Solvent extraction in copper metallurgy, recovery of acid and metals from copper bleed stream. Proceedings of

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