Simulated Small-Scale Pilot Plant Heap Leaching of Low-Grade Oxide Zinc Ore with Integrated Selective Extraction of Zinc

Minerals Engineering 20 (2007) 694–700 This article is also available online at: www.elsevier.com/locate/mineng

Simulated small-scale pilot plant heap leaching of low-grade oxide zinc ore with integrated selective extraction of zinc

Qin Wen-qing *, Li Wei-zhong, Lan Zhuo-yue, Qiu Guan-zhou

School of Resource Processing and Bioengineering, Central South University, Changsha 410083, China

Received 9 July 2006; accepted 5 January 2007 Available online 21 February 2007

Abstract

The leaching of low-grade oxide zinc ore and simultaneous integrated selective extraction of zinc were investigated using a small-scale leaching column and laboratory scale box mixer-settlers. Di-2-ethythexyl phosphoric acid (D2EHPA) dissolved in kerosene was used as an extractant. The results showed that it was possible to selectively leach zinc from the ores by heap leaching. The zinc concentration of the leach liquor in the ﬁrst leaching–extraction circuit was 32.57 g/L, and in the 16th cycle the zinc concentration was 8.27 g/L after the solvent extraction. The leach liquor was subjected to solvent extraction, scrubbing and selective stripping for the enrichment of zinc and the removal of impurities. The pregnant zinc sulfate solution produced from the stripping cycle was suitable for zinc electrowinning. 2007 Elsevier Ltd. All rights reserved.

Keywords: Zinc; Low-grade zinc oxide ores; Heap leaching; Solvent extraction; Di-2-ethythexyl phosphoric acid (D2EHPA)

1. Introduction such as Fe, Ca, Mg, and SiO2, etc. As a result, the acid consumption is high, and complex purification processes are Zinc oxide ore is an important source of zinc metal after required. The large quantity of silica may transform into zinc sulfide ores. With the escalating depletion of zinc sul- a gel and prevent the separation of zinc sulfate from the fide ores, the zinc-bearing minerals such as willemite slurry. Furthermore acidic leaching processes are uneco- (Zn2SiO4), hemimorphite [Zn4(Si2O7)(OH) Æ H2O] and nomic for treating low-grade zinc oxide ores. smithsonite (ZnCO3), etc., have become an important Zinc oxide ores can also be treated by pyrometallurgical source of zinc (Abdel-Aal, 2000). In China, zinc oxidized process (Chen and Qu, 1998). The high-grade zinc concen- ores are relatively abundant, and are mainly founded in trate is produced from low-grade ores by volatilization south-west and north-west China, in places such as Yun- techniques at high temperature in blast furnaces or nan, Sichuan, Guangxi and Gansu provinces, etc. (Duan Waelz-type kilns, and then subjected to a hydrometallurgi- and Luo, 2000). cal process. Though the pyrometallurgical process can treat Many studies have been done on concentration of zinc low-grade ores, it is not acceptable because of heavy pollu- oxide ores, yet very little progress has been made. Usually tion and high capital investment (Choi et al., 1993). zinc oxide ores are concentrated by flotation or gravity, Solvent extraction is regarded as a highly effective where the metal recovery is low and the operating cost is technique of separation and purification, which has high (Duan and Luo, 2000). Zinc oxide ores can be treated been used in the extraction of gold, copper, cobalt and by acidic leaching processes (Bodas, 1996). However, zinc nickel, etc. (Qiu et al., 2002, 2003; Hsu and Harrison, is dissolved following the dissolution of many other metals 1995). The extraction of zinc by various reagents including di-2-ethylhexyl phosphoric acid was studied in detail by Rice and Smith (1975). Conventional zinc extractants * Corresponding author. belong to the group of acidic reagents. Alkylphosphoric E-mail address: [email protected] (W.-q. Qin). acids have been used for many years, and among them

0892-6875/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.01.004 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 695 di-2-ethythexyl phosphoric acid (D2EHPA) is the extract- For zinc extraction with di-2-ethylhexyl phosphoric acid ant most used (Bart et al., 1992; Forrest and Hughes, (D2EHPA) as extractant dissolved in aliphatic diluents, the 1978; Sato et al., 1978). The recovery of zinc from the leach equilibrium is given by the following scheme reactions liquors of the CENIM-LNETI process by solvent extrac- (Mansur et al., 2002): tion with di-2-ethylhexyl phosphoric acid is performed by Zn2þðaqÞþ1:5ðRHÞ ðorgÞZnR RHðorgÞþ2HþðaqÞ Amer and Luis (1995). 2 2 Kongolo eﬃciently performed the removal of zinc and at the liquid–liquid interface manganese from industrial cobalt sulphate solutions by 2ZnR2RHðorgÞ2ZnR2ðorgÞþðRHÞ2ðorgÞ solvent extraction and they also investigated the recovery in the extract phase of Cobalt and zinc from copper sulphate solution by solvent extraction with D2EHPA, which has been successfully where RH represents the extractant species D2EHPA that used to separate cobalt and zinc into their respective solu- acts like a liquid cationic ion exchanger, and subscripts (aq) tions (Kongolo et al., 2000, 2003). and (org) refers to aqueous and organic species, respec- The solvent extraction of zinc and cadmium from phos- tively (Mellah and Benachour, 2006). phoric acid solutions using di-2-ethylhexyl phosphoric acid Solvent extraction may be used for the selective extrac- (D2EHPA) in treated kerosene as diluent has been investi- tion of impurities or valuable metal from process streams gated by Mellah and Benachour (2006). The dimerization into an organic phase. The loaded organic is often stripped constant of D2EHPA is about 106 in polar solvents and with an acid (aqueous) solution to back extract the Zn2+. the equation of the dimerized form can be written in the The organic is regenerated, reconditioned and recycled. following way: In this paper, the solvent extraction of zinc from leach 2ðHRÞ!ðHRÞ liquor using D2EHPA in treated kerosene as diluent has 2 been investigated. It presents the results of a study carried The mechanism of extraction by the D2EHPA and the out to determine if solvent extraction could be used to nature of the formed metal complexes depend on several selectively extract zinc from the leach liquor of low-grade factors such as concentration of the metal cations, the nat- zinc oxide ore, which was treated in a large diameter col- ure of organic solvent, the acidity of the aqueous phase and umn (4 m high · 0.2 m diameter) simulating a unit cell of the type of extracted cations (Pinipenko, 1974). a commercial operation. The organic extractant used was

Column Leaching of zinc oxidized ore

Leach liquor

Organic feed Solvent extraction Raffinate Organic recycling

Scrubbing water waste Scrubbing Scrubbing water

Stripping H2SO4

Organic Pregnant zinc electrolyte

HCl Stripping

Organic HCl

H2SO4 Stripping

H2SO4

Fig. 1. Schematic diagram of the leach-solvent extraction process. 696 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 di-2-ethylhexyl phosphoric acid (D2EHPA). The pregnant reduce the dissolved silica and other impurities or make zinc sulfate solution produced from the stripping cycle was them precipitate in the leach residues. Column leaching suitable for electrowinning. Fig. 1 shows the schematic dia- was adopted for the sulfuric acid leaching of zinc oxide gram for the leaching of low-grade zinc oxide ore, the zinc ores. solvent extraction process in this paper. The size distribution of the zinc oxide ore was 90% passing 20 mm and 42% passing 3 mm. The chemical analysis is shown in Table 1. The original concentration of H SO in 2. Experimental 2 4 the lixiviant was 0.306 mol/L. The leach liquor was recycled without addition of acid after solvent extraction of 2.1. Zinc oxide ore zinc. The zinc oxide ore was obtained from Gaofeng mine in Guangxi province in China, whose chemical composition is 2.2.1. Column construction given in Table 1. The Pb, Cd, Cu, Ni, Ag and Co content The experiments were carried out in a column reactor were analyzed by an atomic absorption spectrophotometer that was fabricated from 5 mm thick 304 L stainless steel. (Japan, Shimadzu AA-6800), and the Zn, Fe, MgO, CaO, The column was 4 m high with an internal diameter of 0.2 m, and it stood in a shallow tank with a capacity of Mn, Al2O3 were analyzed by titrimetric analysis. The S 3 was analyzed by combustion analysis. The As was analyzed 0.1 m , which collected the PLS solution draining from the column. The liquor level was maintained at a sufficient by colorimetric analysis and the SiO2 were analyzed by gravimetric analysis. Microscopic examination in thin height to provide a seal, forcing the air upwards through slides of the groundmass showed that the zinc minerals the column charge. Solution was applied to the surface of are mainly willemite (Zn SiO ), hemimorphite [Zn - the column charge using a simple garden sprinkler head 2 4 4 of the type used in drip irrigation systems. (Si2O7)(OH) Æ H2O] and smithsonite (ZnCO3), and gangue minerals are quartz, gypsum, dickite, and sericite (fine Support rock selection for use in column leaching pro- grained mica) in both crystalline and microcrystalline cess requires the rock be competent, low in carbonate forms. Table 2 shows the mineral composition. and other acid-consuming minerals, and sized at 30 + 10 mm. A granite rock was sourced from a local supplier. Mineralogical analysis showed that the major minerals 2.2. Column leaching were quartz. Sizing was 80% passing 25–30 mm with only 0.6% passing 10 mm. The rock contained <0.03% Zn, Leaching of zinc oxide ore by sulfuric acid can be illus- 1.7% Fe. trated as Fig. 2 shows the schematic of the heap reactor. ZnCO þ H SO ! ZnSO þ H O þ CO ð1Þ 3 2 4 4 2 2 2.2.2. Column operation Zn4Si2O7ðOHÞ H2O þ 4H2SO4 2 The original concentration of H2SO4 was 0.306 mol/L. 3 ! 4ZnSO4 þ 2SiðOHÞ4 þ 2H2O ð2Þ A tank with a capacity of 0.1 m was used to collect the PLS solution draining from the column. Leach liquor Zn2SiO4 þ 2H2SO4 ! 2ZnSO4 þ SiðOHÞ4 ð3Þ was sampled from this tank, and was analyzed to determine With high acidity both the zinc leach rate and the zinc solution concentrations, metal dissolution and the acid bal- extraction rate increases. However, iron and silica are also ance by an atomic absorption spectrophotometer. dissolved in large quantities (Tang and Ouyang, 1998; Ore was coated onto the support rock by tumbling Doepker and O’Connor, 1990). Silica forms a gel, which weighed batches. A layer of uncoated support rock, decreases the filtration rate or even results in the collapse of the operation. Controlling the acidity of leaching can

Table 1 Chemical composition of the zinc oxide ore

Element Zn Pb Cd Cu Fe MgO CaO SiO2 w/% 14.24 0.25 0.035 0.046 23.12 1.35 3.64 36.29

Element Mn Al2O3 Co Ni Ag S As w/% 1.42 8.52 Mim. 0.032 0.0028 2.55 0.23

Table 2 Minerals composition of the zinc oxide ore Mineral Willemite Hemimorphite Smithsonite Quartz Gypsum Composition 10.24 8.15 8.35 32.12 12.08 Fig. 2. Schematic of the heap reactor. (1) Peristaltic pump; (2) water bath; (%) (3) mixer; (4) feed vessel; (5) liquid distributor. W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 697

100 mm deep, was placed in the bottom of the column shakeout test at different equilibrium pH values. The metal before the coated ore was loaded. The leach liquor was concentration in aqueous solution was 3 g/L separately, passed through the ore sample by gravity and re-circulated which was prepared with AR grade sulfate, and was each through a side loop with a peristaltic pump. In the leaching studied separately. Extraction experiments were carried experiments, the column system was comprised of 100 kg out in mechanically agitated beakers. The solution was agi- of ore. For all of the tests the initial temperature and pH tated by a mechanical stirrer with a constant stirring rate. of the feed were 25 C and 1.2, respectively. After shaking beakers for 10 min, the organic phase was Solution and raffinate were fed to the top of the column separated from the aqueous phase. The pH of the aqueous from a feed container by means of a peristaltic pump at a solution was adjusted to the desired value by adding a rate of approximately 0.5 L/min, and collected at the base small amount of HNO3 or NaOH. After phase disengage- into the PLS stock tank by a pump controlled by a level ment, the aqueous phase was separated and its equilibrium sensor in the solution reservoir. The volume of liquid in pH was measured with a pH meter. The D2EHPA concen- the actual column was 80 L. From this and the flow rate, tration was 10%, the phase ratio (VO/VA) was 1:1, and the the apparent residence time was calculated to be about contact time was 10 min. 160 min. Fig. 3 shows the schematic diagram for the zinc solvent The zinc from the PLS was recovered by solvent extrac- extraction process. The extraction-stripping process of zinc tion with D2EHPA. Evaporation losses and sample sulphate with D2EHPA had been studied at laboratory removal were made up with water (pH 1.2) to a total circu- scale according to the following operating variables: pH lation volume of 80 L per column. After leaching, liquor in of the aqueous phase, concentration of D2EHPA, equili- the column was allowed to drain, and the column contents bration time and O/A volumetric ratio (Qin et al., 2003). were then rinsed to remove residual zinc and other soluble The D2EHPA concentration was about 30% v/v diluted species. The column charge was rinsed first with dilute sul- in a kerosene-type diluents, the phase ratio (VO/VA) was phuric acid solution (pH 1.2) followed by a water rinse. 1.5:1, and the contact time was 5 min. The concentration The column was then unloaded through the sampling of H2SO4 in the stripping aqueous feed is 1.53 mol/L. ports, and the residue was separated from the support rock The mixer-settler cascade used in this work was com- by wet screening. The residue was filtered, dried, and pre- posed of box-type mixer-settlers made of Teflon with simi- pared for final chemical analysis with similar chemical lar internal arrangement and dimension (width = 860 mm, analysis methods used for zinc ore (Section 2.1). depth = 250 mm and height = 500 mm). The active volume of one mixer-settler or stage was 6.0 L, while the ratio of the 2.3. Extraction and stripping processes mixer and settler volumes was 1:4 (1.2 and 4.8 L, respectively). The stirring speed used was 1200 rpm. The connec- Di-2-ethylhexyl phosphoric acid was obtained from tion between mixer and settler units was made by a central Tianjing Chemistry Reagent Factory in China. It had a hole. Each mixer unit was provided with a pump-mixer purity of 95%. Kerosene (260#) was obtained from Sinopec impeller made of Teflon. The mixer-settlers were tightly Shanghai Gpc Oil Refinery in China and distilled to collect connected into three sections of two stages each, one for the fraction distilling over 260 C. It was mostly aliphatic extraction, one for scrubbing and other for stripping of in nature. The pH-extraction isotherms of zinc, calcium, zinc. The residence time of each phase in the mixer was copper, cadmium, cobalt and nickel were determined by 5 min, so the flow rate for the feed and solvent streams

Organic feed

E1 E2 SB1 SB2

Leach liquor Scrubbing water waste Scrubbing water

Raffinate S1 S2 S3 S4 S5 S6

HCl

H2SO4 H2SO4 Pregnant zinc electrolyte H2SO4 HCl, FeCl3

Fig. 3. The schematic diagram for the zinc solvent extraction process. 698 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 was 120 mL/min in the extraction section, and 30 mL/min AA-6800). The results are shown in Tables 3 and 4 respec- for the stripping solution stream in the stripping section. tively, when the leach time is one or 16 days. The zinc con- When the zinc loaded organic was stripped by sulfuric tent was between 12 g/L and 33 g/L, and iron; calcium and acid, zinc transfers from organic phase to aqueous phase silica content were lower than 0.40, 1.20 and 3.00 g/L and iron(III) remains in the organic phase, which is directly respectively. The other impurities such as copper, cad- stripped by concentrated hydrochloric acid in two-stages. mium, nickel, antimony and arsenic, etc., which are delete- After the iron(III) is removed, the organic phase is again rious to zinc electrowinning, were at very low level. Hence, stripped by sulfuric acid in two-stages. A similar mixer-set- the leach liquor obtained from column leaching was suit- tler cascade was used for stripping. It was composed of able to the next process. box-type mixer-settlers that were connected into two sec- Compared with agitation leaching, the leaching time of tions of two stages each, one for hydrochloric acid strip- column (heap) leaching is longer. In this case the optimum ping and other for sulfuric acid stripping. The active recovery of zinc was observed after 10 days. The reasons volume of one mixer-settler was 6.0 L while the ratio of are, the particle size of ores is relatively larger and the acid- the mixer and settler volumes was 1:4. ity of lixiviant is moderate, which decreases the speed of leaching. However, heap leaching of zinc oxide ores has some advantages, such as low acid consumption and low 3. Results and discussion dissolved impurities. Above all, the filtration process is not needed in heap leaching, which is free from the negative 3.1. Column leaching of zinc oxidized ores effect of silica on solid liquid separation. In the leaching circuit, the accumulation of iron, silica and other impurities In these experiments, the column was set up initially was not observed. This is probably because of the absorp- with pH 1.2 medium, but no pH control was exercised dur- tion in the accumulated ores. In fact, the pH value of the ing the experiment. And the leach liquor was recycled with- solution soaked into the ores particles increases while the out adding compensating acid after the solvent extraction leaching reaction occurs, which results in iron and silica process. After about 16 days, solution pH level for the precipitating in the residues. experiment was typically about pH 1.6. Zinc extraction from the column was determined from solution assays, mass of concentrate filled and the initial head grade, as well as on the basis of assays of the residual solids. The extrac- Table 3 tions of zinc, iron and silica as a function of time are shown Chemical components of leach liquor (leach time 1 day) in Fig. 4a and b, respectively. The results of column leach- Composition Zn Fe SiO2 CaO MgO ing are shown in Fig. 4. In the 16th day (cycle) the recovery Content/(g/L) 32.57 0.38 2.84 1.17 0.60 Composition Cu Cd Ni Sb As of zinc was above 95% (Fig. 4a). Content/(mg/L) 84.34 0.85 0.02 0.16 3.01 It can be seen in the 16-cycle closed circuit leaching, that the pH of solution where less iron and silica were extracted was from pH 1 to 2. The recovery of iron and silica were 0.6% and 2.4%, respectively. Table 4 From Fig. 1, some leach liquor was recovered by solvent Chemical components of leach liquor (leach time 16 days) extraction, and the raffinate was recycled back to leaching. Composition Zn Fe SiO2 CaO MgO So the zinc extraction % values become lower. The chemi- Content/(g/L) 12.36 0.22 2.45 1.00 0.43 cal components of the leach liquor were analyzed by an Composition Cu Cd Ni Sb As Content/(mg/L) 82.43 0.45 0.02 0.11 2.84 atomic absorption spectrophotometer (Japan, Shimadzu

100 3.0

2.5 80 2.0 60 1.5 Si 40 Fe Recovery/% Recovery/% 1.0

20 0.5

0 0.0 024 6 8 10 12 14 16 024 6 8 10 12 14 16 Leaching time/d Leaching time/d

Fig. 4. Column leaching of zinc oxide ores (a) % recovery of zinc; (b) % recovery of iron and silica. W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700 699

3.2. Extraction and stripping recycled to leaching without the addition of acid, which reduces the acid consumption. The results for the extraction of various metals with 10% The zinc concentration of the leach liquor in the first D2EHPA are shown in Fig. 5. leaching cycle was 32.57 g/L. From Fig. 1, some of the At the equilibrium pH values about 2.0, the %zinc and leach liquor was treated by solvent extraction, and the raff- calcium extraction were 90% and 70%, respectively, while inate is recycled back to leaching section. After the zinc both of the %copper and cadmium extraction were less extraction operation, a amount of zinc (about 8.27 g/l) is than 5%, and the %cobalt and nickel extraction were neg- not extracted and remains in the raffinate when an O/A ligible. Therefore, it may be seen that zinc is easily sepa- ratio close to 1.5 is chosen. So the zinc concentration of rated from copper, cadmium, cobalt and nickel at an leach liquor was decreased to 12.36 g/L in the 16th cycle. equilibrium pH value of 2.0. However, zinc cannot be eas- The co-extraction of impurity metals with D2EHPA is ily separated from calcium. At an equilibrium pH value of inevitable particularly at high concentrations. Therefore low than 1.5, Ca concentration will be around its satura- to obtain a pregnant solution with low contamination tion value under the conditions in the liquor owing to pre- the loaded organic phase must be scrubbed to remove cipitation of gypsum on the bed. Low calcium was impurities. extracted than zinc. Fortunately, calcium at less concentra- The scrubbing solution was zinc sulfate solution at a pH tion has no effect on zinc electrowinning. value of 1.5. To maintain the water balance, a high phase The solvent extraction process is shown in Fig. 3. One ratio (O/A) was necessary. It was found that the co- extraction process was carried out in each leaching circuit, extracted calcium and the entrainment of silica in loaded and about 30 L leach liquor was used in each extraction organic can be removed by scrubbing, and a phase ratio process. The data for a closed loop leaching–extraction cir- (O/A) of 10:1 was acceptable. Iron cannot be removed by cuit are shown in Table 5. The pH value of the extraction scrubbing. solution was less than 2.0, after zinc extraction the aqueous Hence, zinc was separated from iron by selective strip- pH value decreases to about 1.0, so the raffinate can be ping. The stripping is the reverse process of extraction, in which zinc transfers from organic phase to aqueous phase, the equation is as follow: 100 þ þ ZnR2 2HRorg þ 2Haq () 2H2R2ðorgÞ þ Znaq ð4Þ 80 Hence the aqueous phase in stripping must be of high 60 Zn acidity to keep the equilibrium move rightwards. The zinc Ca loaded organic phase contained iron(III), and minim of 40 Cu Cd calcium and silica. The percentage stripping increased with Extracted / % 20 Co Ni the increase of acidity. It was found the iron in the loaded 0 organic phase was hardly stripped by 1.53 mol/L sulphuric 1235 4 6 acid. When the zinc loaded organic was stripped by sulfuric Equilibrium pH acid, zinc transfers from organic phase to aqueous phase Fig. 5. D2EHPA pH-extraction isotherms for six elements at 25. while iron(III) remains in the organic phase.

Table 5 Closed loop of leaching–extraction circuit Cycle pH q(Zn)/g/L Recovery of zinc (%) Extraction of zinc (%) Leach liquor Raﬃnate Leach liquor Raﬃnate 1 1.84 0.98 32.57 22.15 30.51 32.00 2 1.80 0.97 28.62 19.99 36.51 30.14 3 1.70 0.98 24.87 17.40 41.14 30.00 4 1.75 0.96 22.49 15.36 45.91 31.70 5 1.60 1.00 22.20 15.65 51.94 29.50 6 1.82 0.98 24.80 17.36 58.98 30.00 7 1.63 0.99 22.79 16.34 63.99 28.00 8 1.68 0.98 21.23 15.24 68.56 28.20 9 1.55 1.05 18.18 13.36 71.32 26.50 10 1.63 1.03 17.87 12.69 75.55 29.00 11 1.58 0.97 16.02 11.05 78.67 31.00 12 1.49 0.98 14.94 10.76 82.31 28.00 13 1.56 0.97 13.64 9.71 85.01 28.80 14 1.70 0.96 13.95 9.54 88.98 31.60 15 1.55 1.00 12.63 8.80 91.88 30.30 16 1.60 0.98 12.36 8.27 95.21 33.10 700 W.-q. Qin et al. / Minerals Engineering 20 (2007) 694–700

Table 6 Amer, S., Figueiredo, Luis, A., 1995. The recovery of zinc from the leach Chemical components of pregnant solution q/(mg/L) liquors of the CENIM-LNETI process by solvent extraction with di(2-ethylhexyl)phosphoric acid. Hydrometallurgy (37), 323–337. Zn Fe Cu Mn Co Cd CaO MgO Bart, H.H., Marr, R., Scheks, J., Koncar, M., 1992. Modelling of solvent 68000.00 0.30 0.03 25.00 0.03 0.11 0.48 0.13 extraction equilibria of Zn(II) from sulfate solutions with bis(-2- Ge F Cl Ni Sb As SiO H SO 2 2 4 ethylhexyl)phosphoric acid. Hydrometallurgy (31), 13–28. 0.02 <5.00 <10.00 0.09 0.02 0.04 0.18 150.00 Bodas, M.G., 1996. Hydrometallurgical treatment of zinc silicate ore from Thailand [J]. Hydrometallurgy 35 (1), 37–49. Chen, Shi-ming, Qu, Kai-liu, 1998. On the treatment of oxidized zinc ore Though the iron concentration in the leach liquor was in lanping [J]. Yunnan Metallurgy 27 (5), 31–35. low, it accumulated in the recycle organic, which decreased Choi, W.K., Torma, A.E., Ohline, R.W., 1993. Electrochemical aspects of the loading capacity of the extractant. Hence the recycle zinc sulphide leaching by Thiobacillus ferrooxidans. Hydrometallurgy organic must be treated periodically to remove the iron(III). 33 (1), 137–152. The conventional method is that the Fe3+ is directly Doepker, R.D., O’Connor, W.K., 1990. Column leach study II. Heavy metal dissolution characteristics from selected lead–zinc mine stripped by strong hydrochloric acid. It was found that a 3+ tailings. In: Proceedings of the Western Regional Symposium on strong acid solution (6 mol/L) was required to strip Fe Mining and Mineral Processing Wastes, May 30, Berkeley, CA, USA, from D2EHPA (Gu et al., 2000). pp. 69–80. The long run test indicated that the performance of Duan, Xiu-mei, Lou, Lin, 2000. Review on present situation of the organic phase is stable in the recycle process and neither flotation of oxidized zinc ore [J]. Mining and Metallurgy 9 (4), 47–51. Forrest, C., Hughes, M.A., 1978. The separation of Zn from Cu by emulsion nor third phase formation was observed in the DZEHPA – an equilibrium study. Hydrometallurgy (3), 327–342. solvent extraction process. Gu, H., Chang, C.-M., et al., 2000. Preliminary design of a solvent The chemical analysis of the pregnant zinc sulfate pro- extraction processing for the galvanic stripping of iron from D2EHPA duced from solvent extraction is shown in Table 6. Zinc [J]. Mineral and Metallurgical Processing 17 (1), 16–22. electrowinning is dependent on high hydrogen over poten- Hsu, C.H., Harrison, R.G., 1995. Bacterial leaching of zinc and copper from mining wastes. Hydrometallurgy 37 (2), 169–179. tial on the zinc metal; a small quantity of impurities can Kongolo, K., Mwema, D.M., Kyony, P.M., Mfumu, K., 2000. Manganese result in the decrease of this hydrogen overpotential, which and zinc removal from cobalt sulphate solutions by means of decreases the current efficiency and the quality of cathode solvent extraction. In: Proceedings of the 21st IMPC, July 23–27, zinc. The impurities in the electrolyte were at levels as fol- Rome, Italy. lows (mg/L): Cu < 0.10, Ni < 0.30, Co < 0.30, Fe < 10.00, Kongolo, K., Mwema, M.D., Banza, A.N., Gock, E., 2003. Cobalt and zinc recovery from copper sulphate solution by solvent extraction. Cd < 0.30, Sb < 0.03, As < 1.00, Ge < 0.03, Sn < 0.10, the Minerals Engineering 16 (12), 1371–1374. electrolyte is suitable to electrowinning (Peng, 1992; Yang Mansur, M.B., Slater, M.J., Biscaia Jr., E.C., 2002. Equilibrium analysis and Liu, 1988). It can be seen that the impurities in the of the reactive liquid–liquid test system ZnSO4/D2EHPA/n-heptane. pregnant solution are less than the above standards. Hydrometallurgy (63), 117–126. Mellah, A., Benachour, D., 2006. 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