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INVESTIGATION OF SOLVENT EXTRACTION OF Zn(II) FROM NIGERIAN SPHALERITE BY

CYANEX®272

Dr. Alafara A. BABA (+)

, Lecturer I and Assoc. Prof. Folahan A. ADEKOLA(+)

Department of Chemistry, University of Ilorin,

P.M.B. 1515, Ilorin 240003, Nigeria.

(+) Corresponding authors:

(E-mail: alafara@unilorin.edu.ng (A.A. Baba); Tel: +2348035010302,

fadekola@unilorin.edu.ng (F. A. Adekola); Tel: +2348067332320.

Abstract

The extraction and purification of Zn(II) with bis(2,4,4-trimethylpentyl)phosphinic acid, commonly known

as Cyanex®272 in kerosene from aqueous chloride sphalerite leached liquor was investigated. 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 stochiometry 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 -26.81kJ/mol, 21JK-1 mol-1 and -5.48kJ/mol were calculated for the process respectively.

These values showed that Zn(II) extraction with Cyanex®272 is entropy controlled. The number of the theoretical

stages for this process evaluated by the McCabe-Thiele diagram was six. An extraction efficiency of 95% Zn(II) by

0.047M Cyanex®272 in kerosene was obtained from an initial sphalerite leach liquor containing 603.4mg/L Zn,

121.4mg/L Fe and 16.3mg/L Pb as a major constituents. Pb(II), Ag(I), Cu, Sn, and Al (less than 5mg/L) present in

the liquor were firstly separated by cementation with Zn granules, followed by Iron removal by precipitating with

4M ammoniacal solution to a pH of 3.5 at 25oC±2oC. A 0.1M HCl was found to be adequate for the stripping of at

least 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 600oC during 120 minutes. A practicable

hydrometallurgical scheme summarizing the operational procedure for the extraction of Zn(II) and Pb(II) from the

Sphalerite ore was presented.

Key words: Beneficiation, sphalerite, Zinc, Solvent extraction, hydrochloric acid, Cyanex®272.

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Introduction:

The mineral sphalerite, ZnS, is the most important zinc mineral and is generally treated

pyrometallurgically. However, zinc smelting faces difficulties with respect to the environmental restrictions.

(Adebayo, et al, 2006). Therefore, the hydrometallurgical solvent extraction processes have become major

purification operations in practice with special emphasis on Zn(II) separation from its coexistent species such as

Pb(II), Co(II), Ni(II) and Cu(II), etc. Presence of these impurities in dissolved low grade Zinc ores and need of

concentrated Zinc electrolytes for ultra high grade Zinc production made solvent extraction a very attractive

pretreatment sequence for electrowinning process (Sayar, et al, 2007).

As the world wide high grade ore reserves are falling at appalling rate as most are worked out because of

high metal demand (Kesari, et al, 2009), hydrometallurgical route has been an alternative to pyrometallurgical

processes for sulphidic ores and concentrates, particularly for small scale production and for remote metal resources

not acceptable by pyrometallurgy. (Rotuska and Chmielewski, 2008).

Zinc has a wide variety of application including its use in metallic coating to improve corrosion resistance

of various types of steel. The pickling of steel goods is usually carried out using 20% HCl and the process is stopped

when HCl concentration reaches 10% (Regel, et al, 2001). The use of organophosphorus extractants including

Cyanex®302, Cyanex®921, Cyanex®923 and TBP have been widely reported for the separation and recovery of

Zn(II) from HCl medium (Daoud, et al, 2008; Tait, 1992).

Therefore, the present study centered on the solvent extraction and separation of Zinc(II) from associated

impurities such as lead, iron, copper, silver etc. contained in Nigerian sphalerite mineral. This is a first in-depth

study on the hydrometallurgical recovery of Zinc(II) and lead(II) from a Nigerian sphalerite origin by Cyanex®272.

It is worthy to note that the first part of the studies on the beneficiation of the sphalerite mineral had earlier been

published (Baba and Adekola, 2010).

Experimental

The experimental approach adopted for this study comprises a preliminary work aimed at establishing

conditions for the optimal extraction of Zinc from synthetic Zn(II) solutions by Cyanex®272 with subsequent

application to the recovery and beneficiation of Zinc(II) from sphalerite leachate (Baba, 2008; Daoud, et al, 2006;

Sayar, et al, 2007).

In all extraction experiments, the ratio of Zn(II) in the extract to its concentration in the aqueous phase,

otherwise known as distribution ratio, Dc is given by:

Dc = [Zn(II)]org/[Zn(II)]aq (1)

Dc is the index of metal extractability and it is related to percentage of the metal extracted (Sayar, et al, 2007).

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Result, discussion and conclusion:

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 the system temperature. Also,

the values of the thermodynamic parameters such as the apparent molar enthalpy, molar entropy and Gibb’s free

energy change showed that Zn(II) extraction by Cyanex®272 is entropy controlled. These results were used in the

optimization of the systematic study of the Zinc(II) and Pb(II) extraction and its beneficiation from sphalerite ore.

Consequently, a hydrometallurgical flow diagram summarizing all analytical procedures for the recovery of

Zinc(II) and production of pure Zinc oxide from Nigerian sphalerite origin by Cyanex®272 is presented in Fig. 1.

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Leaching of

Sphalerite in

HCl medium

Sphalerite sample

collection and preparation

(crushing, pulverizing)

Sphalerite leachate in

4M HCl at 360rpm at

80; pH adjusted to 3.0

Zn, other trace

elements

Zn - Cyanex

Cyanex 272

Zn(II) in 0.1M

HCl

Zn: 603.45mg/L

Pb: 16.33mg/L

Fe: 121.40mg/L

* Other elements: Ag,

Mn, Sn, Cu are present

in traces

Zn: 648.65mg/L

Pb: 1.26mg/L

Fe: 104.55mg/L

Ammoniacal solution,

pH 3.5Fe

98.85% efficiency

Zn: 646.22mg/L

Fe: <2mg/L

Zn-Cyanex: 613.98mg/L

Zn-aqueous: 32.24mg/L

Extraction efficiency = 95.01%

0.032M Cyanex 272

pH 3.0

Temperature:

250C±2

0C

Stripping with 0.1M HCl

Zn-aqueous:

586.23mg/L

Zn-recovery = 95.48%

Pb

92.28% efficiency

Cementation

Zn, Fe, other

trace elements

re-u

se

n£3

ZnO

NaOH/Calcination

Figure 1: Hydrometallurgical flow diagram for the recovery of Zn(II) extraction and production of pure ZnO

from Abakaliki (Nigeria) Sphalerite ore.

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Acknowledgements:

The authors wish to thank Dr. Oliber 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-

Nigeria, for the 2005/2006 Staff Development Award for Ph.D. research in Chemistry.

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