Selective separation of pyrite and chalcopyrite by biomodulation

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Abstract

Selective separation of pyrite from other associated ferrous sulphides at acidic and neutral pH has been a challenging problem. This paper discusses the utility of Acidithiobacillus ferrooxidans for the selective flotation of chalcopyrite from pyrite. Consequent to interaction with bacterial cells, pyrite remained depressed even in the presence of potassium isopropyl xanthate collector while chalcopyrite exhibited significant flotability. However, when the minerals were conditioned together, the selectivity achieved was poor due to the activation of pyrite surface by the copper ions in solution. The selectivity was improved when the sequence of conditioning with bacterial cells and collector was reversed, since the bacterial cells were able to depress collector interacted pyrite effectively, while having negligible effect on chalcopyrite. The observed behaviour is analysed and discussed in detail. The separation obtained was significant both at acidic and alkaline pH. This selectivity achieved was retained when the minerals were interacted with both bacterial cells and collector simultaneously.

Introduction

Pyrite is often associated with other valuable metal sulphides such as chalcopyrite, arsenopyrite, galena, sphalerite etc. Economical extraction of these valuable metals demands selective depression of pyrite from the associated metallic sulphides during froth flotation. Most of the inorganic reagents used for pyrite depression are toxic and thus, an alterative environmentally benign process becomes imperative.

During the past decade, Acidithiobacillus ferrooxidans, a chemolithoautotroph that thrives on both iron and sulphur has been successfully utilised for the bioleaching of sulphide minerals and biooxidation of refractory gold ores [1], [2]. In recent years, the utility of this bacterium for desulphurization of pyretic coal by selective depression of pyrite has also been investigated [3]. The attachment of this bacterium to mineral surfaces has been an area of much research [4], [5], [6], [7]. The interactions of the bacterial cells with minerals were shown to alter the surface chemistry of both the cells and minerals [8], [9]. More recent work to investigate the mechanism of bacterial adhesion suggests that the specific attachment of A. ferrooxidans to pyrite was due to the specific protein apo-rusticyanin acting as a receptor for initial attachment [10] and the extracellular polysaccharides play an important role in attachment to mineral surfaces [11], [12].

Very little work has been reported on the utility of this bacterium in the selective flotation of sulphide minerals. Yelloji Rao et al. [13] and Santhiya et al. [14] studied the separation of galena from sphalerite using Thiobacillus ferrooxidans and Thiobacillus thiooxidans, respectively. Nagaoka et al. [15] investigated the ability of T. ferrooxidans to selectively depress pyrite from other non-ferrous sulphides due to selective adhesion. Patra and Natarajan [16] investigated selective depression of pyrite from associated oxides using Bacillus polymyxa. However, all these studies were aimed at separating pyrite from either oxides or other non-ferrous sulphides, but no significant research has been reported on the separation of pyrite from associated ferrous sulphides. Recently, Sharma et al. [17] studied the effect of bacterial conditioning on the behaviour of pyrite and chalcopyrite, but their study is restricted to individual minerals and the problems associated with selectivity when the minerals are present together have not been analysed. Hence, a systematic study of the mixed system is required to determine the conditions for good selectivity.

In the present study selective separation of pyrite from a mixture of pyrite and chalcopyrite by biomodulation and collector interaction is investigated in detail.

Section snippets

Minerals

Pure handpicked mineral samples of pyrite were obtained from Alminrock Indser Fabricks, Bangalore and chalcopyrite from Gregory, Bottley and Lloyd, UK. The mineral samples were dry-ground with a porcelain ball mill and dry-sieved to obtain different size fractions. The −106 + 75 μm fraction was used for flotation studies. The −37 μm fraction was further ground in a Retsch mortar grinder. The particle size analysis of this sample was done using a Malvern Mastersizer 3000-model and the mean size

Growth of bacteria

The growth of A. ferrooxidans on ferrous iron under optimum conditions of temperature (30 °C) and pH (2) was characterized by a short lag phase (5–6 h) followed by a steep log phase. The maximum growth was attained in 48–50 h. The cell number at the end of log phase was 2.25 × 108 cells/ml.

Adsorption studies

Adhesion studies were carried out to determine the kinetics and extent of adhesion of A. ferrooxidans cells on the mineral surfaces. The adhesion kinetics of bacterial cells onto pyrite and chalcopyrite as a

Conclusions

A. ferrooxidans was successfully utilised for the selective removal of pyrite from a mixture of pyrite and chalcopyrite. When the individual minerals were conditioned with bacterial cells prior to flotation using PIPX as collector, pyrite was depressed while chalcopyrite exhibited significant flotability. When the minerals were conditioned together with bacterial cells prior to collector interaction, the selectivity achieved was poor and it was shown to be due to copper dissolution from

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