A preliminary investigation of the ferric leaching of a pyrite-arsenopyrite flotation concentrate

INTRODUCTION Bioleaching is now an established technology for the pre-treatmentof refractorygold ores and concentrates. It offers economic, environmental and technical advantages over pressure oxidation and roasting. Furthermore,the spontaneous bioleaching of wastes containing sulphide minerals contributes to acid mine
FeAsS + SF$+ + so + As?’ + 6FG2’
Furthermore, the results obtainedare consistentwiththe hypothesisthatthe leaching of arsenopyrite,the oxidationof As(IU)to As(V), the leaching of pyrite, and theprecipitation of ferric arsenate competefor the available Fe(IIt). 0 1997 Elsevier Science L.td Keywords Gold ores; sulphide ores; leaching: oxidation; extractive metallurgy
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10,No.9.p~.1023-1030. 1997 0 1997ElscviaScieoceLtd RinkdioGreatBritain.Allrigbtsrcaavcd 0892-5875/97$17.00+0.00
ABSTRACT Although considerable work on the leaching of pyrite using FeWI) is reported in the literature,to date very littleworkon the chemical leaching of arsenopyrite,using Fe(IIl) has been reported. Batch,ferric leaching experiments,each 48 hours in length, were carried out in sealed, bafled, agitated vessels. Varying quantitiesof Fe(Ill), ranging from 0 to 0.54 M were added to a slurry, containing 10 g.P-I concentrate. The redox potential was monitored continuously.Samples were taken at regular intervals and the total iron and arsenic concentrationof the supernatantwas determined The results showed that no leaching occurred in the absence of Fe(lII). However, considerable leaching of arsenic occurred in the reactors to which Fe(N) was added Furthermore the resultssuggest thatthe chemical leaching rate may be a function of the redox potential, and not of the absolute Fe(lIl) concentration. The stoichiometry determinedfor theferric leaching of arsenopyriteagreed withthe stoichiometry postulated by previous researchers [I], viz.:
1023
1024
A. W. Breed et al.
drainage, and the contamination of ground water by metals trapped within the matrices of these minerals. Recent research suggests that the bioleaching of sulphide minerals occurs via a two-step mechanism [2], i.e. the mineral is leached chemically by Fe(lII), and the role of the bacteria is to regenerate Fe(III), thereby maintaining a high redox potential within the system. A potential complication in the bioleaching of minerals is the solubilisation of metals to concentrations toxic to the micro-organisms [3-6], hence knowledge of the stoichiometry and kinetics of both the bacterial and chemical reactions may aid the design and modelling of the overall process. Although considerable work on the leaching of pyrite using Fe(III) has been reported in the literature [7-g], to date very little work on the leaching of arsenopyrite, using Fe(IID at concentrations and conditions similar to those used in bioleaching, has been reported. For thii reason some controversy exists in the literature with regard to the stoichiometry of the leaching reaction. Some of the leaching reactions postulated to date are listed below:
Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa. E-mail: AWB@chemeng.uct.ac.za (Received 8 April 1997; accepted 15 May 1997)
FeAsS + llFe3’ + 7H,O + 12Fe2’ + H&O3
+ HSO;
+ 1OH’ UOI
[Ill
(1)
(21 (3) (4)
FeAsS + 13Fe3’ + 8H20 -+ 14Fe2’ + H&vOd + SO:- + 13H’ FeAsS + 7Fe3’ + 4H20 - 8Fe” FeAsS + 5Fe3’ + So+ h3++6 + H&O4
[l]
+ So + 5H’
[II]
Fe2’
The objective of this work was to determine whether or not a sample of pyritelarsenopyrite flotation concentrate from the Fairview Gold Mine in Barberton, South Africa, could be leached by FeOII), at concentrations similar to those observed during normal bioleachmg operations. In view of the disparity with regard to the stoichiometry proposed by other workers, a further objective of the work was to attempt to determine the stoichiometry for the ferric leaching of this concentrate.
MATERIALS
AND METHODS
The experiments were carried out in sealed, baffled, agitated 2.0 Q,glass Quick& culture vessels. A slurry volume of 1.75 4 was used. Varying quantities of Fe(III), ranging from 0 to 0.54 M were added to a slurry, containing 10 g.Q1 (1% (m.v-1)) concentrate. The concentrate sample contained 5.84% As, 21.71% S and 24.01% Fe. 86.35% of the material was finer than 75 micron. The mineral analysis of the concentrate, estimated from the elemental analysis, was 12.70% FeAsS and 37.18% FeS,. The slurry temperatnre was controlled at 40°C by placing the reactors in a water bath. However, the pH was not controlled, and the reactors were not aerated. All the leach tests were run for 48 hours. The leaching rate was monitored by observing the variation in the iron and arsenic concentrations and the redox potential of the slurry. The redox potential of the slurry in the reactors was monitored continuously. It was obtained by direct millivolt measurement using an AS1 OR101431 Pt combination, double junction Ag/AgCl ORP electrode and a Hitech Micro Systems UCT Redox Controller. Samples were taken at regular intervals and the total iron and arsenic concentration of the supematant were determined by Atomic Adsorption Spectroscopy (AA).