Elsevier

Hydrometallurgy

Volume 109, Issues 3–4, October 2011, Pages 187-193
Hydrometallurgy

Beneficiation of a Nigerian sphalerite mineral: Solvent extraction of zinc by Cyanex®272 in hydrochloric acid

https://doi.org/10.1016/j.hydromet.2011.06.004Get rights and content

Abstract

The extraction of Zn(II) from aqueous chloride sphalerite leached liquor using bis(2,4,4-trimethylpentyl)phosphinic acid or Cyanex®272 in kerosene has been studied. The results of fundamental studies on solvent extraction of synthetic solutions of Zn(II) showed that extraction of metal ions increased with increasing pH, extractant concentration and temperature. The stoichiometry of the extracted metal species by Cyanex®272 with Zn(II) was 1:1. The apparent standard molar enthalpy (∆H°), molar entropy(∆S°) and Gibb's free energy(∆G°) of 26.81 ± 0.11 kJ/mol, 107.63 ± 0.05 J mol–1 K–1 mol–1 and − 5.48 ± 0.13 kJ/mol were calculated for the process respectively. These values showed that Zn(II) extraction by Cyanex®272 is thermodynamically favourable. The number of the theoretical stages for this process evaluated by the McCabe-Thiele diagram was six. An extraction efficiency of 95% Zn(II) was obtained with 0.047 mol/L Cyanex®272 in kerosene from an initial sphalerite leach liquor containing mainly 603.4 mg/L Zn, 121.4 mg/L Fe and 16.3 mg/L Pb. The Pb(II), Ag(I), Cu, Sn, and Al (less than 5 mg/L) were firstly separated by cementation with Zn granules and this was followed by Iron removal by precipitating with 4 mol/L ammoniacal solution to a pH of 3.5 at 25 °C ± 2 °C. A 0.1 mol/L HCl was found to be adequate for the stripping of about 95% of Zn from the organic phase. The stripped Zn(II) solution was recovered as zinc oxide (ZnO) via precipitation with sodium hydroxide followed by calcination at 600 °C during 120 min. A practicable hydrometallurgical scheme summarising the operational procedures used for the extraction of Zn(II) and Pb(II) from the sphalerite ore was presented.

Research highlights

► Extraction of Zn(II) from sphalerite leach liquor in HCl by Cyanex 272. ► Fundamental studies adopted to Zn(II) recovery from sphalerite leachate. ► Combination of precipitation, cementation, roasting, calcination adopted for Zn(II) and ZnO recoveries. ► A practicable hydrometallurgical flow diagram proposed.

Introduction

Zinc is mostly extracted from sphalerite (ZnS) ore. Deposits of zinc and lead ores including sphalerite which are usually found as mixture have long been known in Nigeria, but they have only been mined in the past on a very small scale. An estimated 10 million tonnes of zinc/lead veins are spread over eight states of Nigeria. Proven reserves in three prospects in the East-central areas are 5 million tonnes. (Min. and Ind., Nigeria, 2010). In the commercial flow sheet for the production of zinc metal, the sphalerite concentrate is roasted, leached in sulphuric acid and electrolyzed (Alguacil and Martinez, 2001). Zinc is used in metallic coating to improve corrosion resistance of various types of steel. The pickling of steel goods is usually carried out using 10–20% HCl (Regel et al., 2001).

Zinc is primarily produced from sulphidic ores including sphalerite, ZnS. Other sources of zinc include oxide-carbonate ores and different secondary sources such as zinc ash, zinc dross, flue dusts of electric arc furnace, leach residues, etc. Pyrometallurgical and hydrometallurgical routes or combination of the two can be employed for treating secondary materials (Turan et al., 2004). The hydrometallurgical process has been established to be more eco-friendly for treating such materials having low zinc content (Leclerc et al., 2003).

In recent years, the recovery of metals from aqueous chloride solutions has attracted much attention. This is due to the high efficiency of the chloride leaching processes, which are now recognised as a logical choice for treating complex sulphide ore concentrates which cannot be easily or economically treated by other means. Another important aspect of such leaching process is that sulphur is liberated in the elemental form rather than as sulphur dioxide (Cote and Jakubiak, 1996).

In hydrometallurgical processes, valuable metals are generally recovered by a combination of two or more of leaching, precipitation, solvent extraction and electrowinning techniques. During leaching, dissolution of metals with a suitable acid or mixture of acids is possible. The processing of leach liquor solutions containing different concentrations of acid/acids is very complex and separation of the metal ions using various techniques such as precipitation, adsorption, solvent extraction, ion-exchange, etc. can be cumbersome (Sarangi et al., 2007). Of these purification technologies, solvent extraction shows a prominent role as a separation (and concentration) operation within these processes (Alguacil and Martinez, 2001) .

In this regard, the use of organophosphorus extractants in solvent extraction of metals has been steadily increasing because of their excellent selective nature in forming complexes under specific conditions. In particular, the introduction of diakyl phosphoric and phosphoric acids including Cyanex 272 has brought about a vast change in the separation technology (Daoud et al., 2006, Kunungo and Mohapatra, 1995). Thus, Cyanex 272 has been regarded as better reagent for the zinc extraction and very selective for the removal of other metal ions including cobalt to avoid possibility of cross contamination of reagents and has been adopted at the Murrin Murrin plant in Western Australia (Flett, 2005).

In the particular case of chloride-based zinc hydrometallurgy, a number of investigations using different extractants have been reported (Alguacil et al., 1992, Benito et al., 1996, Jia et al., 2002, Li et al., 2003, Mellah and Benachour, 2007). In most of these studies, synthetic solution containing Zn(II) was used and possible efforts in separating Zn(II) from other metal ions have been undertaken with respect to the extraction mechanism and the extracted complexes being formed. The results of some of these investigations are summarised in Table 1.

As evident from Table 1, there is a very limited work on the extraction of Zn from an aqueous leach liquor emanating from sphalerite mineral, (Baba et al., 2004).

The first part of the studies on the beneficiation of the sphalerite mineral had earlier been published (Baba and Adekola, 2010). The present investigation constitutes the second part and it is focused on the extraction and separation of zinc(II) from associated impurities such as lead, iron, Cu, Ag etc., contained in Nigerian sphalerite mineral. This is a first in-depth study on the hydrometallurgical recovery of zinc and lead from a Nigerian sphalerite origin using Cyanex 272.

Section snippets

Experimental

The experimental approach adopted for this study comprises a preliminary work aimed at establishing conditions for the optimal extraction of Zn from synthetic Zn(II) solutions by Cyanex 272 with subsequent application to the recovery of zinc from sphalerite leachate. The leachate obtained from the leaching of 10 g/L sphalerite per litre of 4 mol/L HCl solution at 80 °C for 120 min (Baba and Adekola, 2010) was used for the systematic study of the extraction of Zn(II) and Pb(II). The leachate has the

Influence of initial metal ion concentration

Solutions containing different concentrations of Zn(II) within the range of 0–300 mg/L dissolved in HCl solution whose pH was 3 were prepared (Baba, 2008, Baba et al., 2009). The extraction was studied by 4.17 × 10 2 mol/L Cyanex 272 in kerosene at 25 °C ± 2 °C for 25 min (Daoud et al., 2006). The results obtained are represented in Fig. 1.

It is clear from Fig. 1 that the increase in the initial concentration of Zn(II) is accompanied by an increase in the extraction process before tending to a constant

Conclusions

The results on fundamental studies on solvent extraction of synthetic solutions of Zn(II) showed that extraction of metal ions increased with increasing pH, extractant concentration, and temperature. The stoichiometry of the extracted metal species by Cyanex 272 with Zn(II) was found to be ZnA2. The apparent standard molar enthalpy (∆H°), molar entropy(∆S°) and Gibb's free energy(∆G°) of 26.81 kJ/mol, 107.63 J mol–1 K–1 and − 5.48 kJ/mol were calculated for the process, respectively. These values

Acknowledgements

The authors wish to thank Dr. Oliver Rouher and Mrs Christine Salomon of Cytec Industries, Rungis Cedex, France, for their benevolence by supplying Cyanex 272. A. A. Baba also thanks the University of Ilorin, Ilorin-Nigeria, for the 2005/2006 Staff Development Award for Ph.D research in Chemistry; and the Academy of Sciences for the Developing World (TWAS), Trieste, Italy for the 2010 CSIR-TWAS Fellowship for Postdoctoral Research at Institute of Minerals and Materials Technology,

References (36)

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