Sulphide Minerals: Surface Oxidation and Selectivity in Complex Sulphide Ore Flotation

Sammanfattning: Metal and energy extractive industries play a strategic role in the economic development of Sweden. At the same time these industries present a major threat to the environment due to multidimensional environmental pollution produced in the course of ageing of ore processing tailings and waste rocks. In the context of valuable sulphide mineral recovery from sulphide ore, the complex chemistry of the sulphide surface reactions in a pulp, coupled with surface oxidation and instability of the adsorbed species, makes the adsorption processes and selective flotation of a given sulphide mineral from other sulphides have always been problematic and scientifically a great challenge. Invariably, the problems associated with acid mine drainage and selectivity in flotation are explained to be associated with the oxidation of metal sulphides. Although metal sulphides oxidation and galvanic effects were well known in flotation and leaching of sulphides, recent studies reveal the formation of reactive, oxidizing oxygen species and H2O2 by sulphides due to the catalytic activity of sulphide surfaces. The inherent formation of H2O2 by single and mixture of sulphide minerals during wet and dry grinding systems and in open and closed environments have been investigated. It was found that pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite ((Zn,Fe)S), and galena (PbS) generated H2O2 in pulp liquid during wet grinding in the presence and absence of dissolved oxygen in water and also when the freshly dry ground solids are placed in water immediately after grinding. Pyrite generated more H2O2 than other sulphide minerals and the order of H2O2 production by the minerals found to be pyrite > chalcopyrite > sphalerite > galena. The pH of water influenced the extent of hydrogen peroxide formation where higher amounts of H2O2 are produced at highly acidic pH. The amount of H2O2 formed also increased with increasing sulphide mineral loading and grinding time due to increased surface area and its interaction with water. The sulphide surfaces are highly catalytically active and capable of breaking down the water molecule leading to hydroxyl free radicals. Type of grinding medium on formation of hydrogen peroxide by pyrite and galena revealed that the mild steel produced more H2O2 than stainless steel grinding medium, where Fe2+ and/or Fe3+ ions played a key role in producing higher amounts of H2O2. In addition, the effect of mixed sulphide minerals, i.e., pyrite–chalcopyrite, pyrite–galena, chalcopyrite–galena and sphalerite–pyrite, sphalerite–chalcopyrite and sphalerite–galena on the formation of H2O2 showed increasing H2O2 formation with increasing the content of a nobler mineral or higher rest potential mineral in a mixed composition. The results of H2O2 formation in pulp liquid of sulphide minerals and mixed minerals at different experimental conditions have been explained by Eh–pH diagrams of these minerals and the existence of free metal ions that are equally responsible for H2O2 formation besides surfaces catalytic activity. The results alsocorroborate the amount of H2O2 production with the rest potential of the sulphide minerals; higher is the rest potential more is the formation of H2O2. Most likely H2O2 is answerable for the oxidation of sulphide minerals and dissolution of non-ferrous metal sulphides in the presence of ferrous sulphide besides the galvanic interactions. Studies have also been carried out to build correlation between percentage of pyrite in the concentrate, grinding conditions and concentration of OH•/H2O2 in the pulp and as well of controlling the formation of these species through known chemical means for depressing the generation of the oxidant. Flotation tests using a complex sulphide ore with the same reagent scheme that is being used at Boliden concentrator but with the addition of collector and depressant during grinding stage have been performed to judge the beneficial or detrimental role of H2O2 on the selective flotation of sulphides. The results demonstrate that the selectivity of metal sulphides against pyrite increases with increasing generation of H2O2 in the pulp liquid. This study highlights the necessity of revisiting into the electrochemical and/or galvanic interactions between the grinding medium and sulphide minerals, and interaction mechanisms between pyrite and other sulphide minerals in terms of their flotation behaviour, leaching and environmental degradation in the context of inevitable H2O2 existence in the pulp liquid.