Bubble attachment and Amyl Xanthate adsorption to copper-activated sphalerite

Abstract

Sphalerite (ZnS) is a primary source of zinc metal. It usually coexists with other sulphide minerals such as pyrite (FeS2) and galena (PbS). Its concentration is accomplished by froth flotation using short-chain thiol collectors such as xanthate. Due to its unique chemical and structural characteristics, the concentration of sphalerite by flotation requires Cu-activation, which is a process of making the mineral susceptible to reactions with the thiol collectors. The high amount of copper sulphate consumption is a big challenge for zinc processing plants, and the parameters that lead to a significant amount of copper sulphate usage for sphalerite activation is not well-understood. Based on the literature review and zinc processing plants reports, process water quality and the presence of pyrite during sphalerite Cu-activation could be two main factors affecting the activation that are the main focus of this thesis. To develop insight into mechanisms of Cu-activation of sphalerite and pyrite, Cryogenic X-ray Photoelectron Spectroscopic (Cryo-XPS) and zeta potential measurements were applied. The Cryo-XPS results showed that Cu-activation of sphalerite was an ion exchange reaction with CuS-type layer formation, while pyrite Cu-activation was governed by redox reactions with CuFeS2-type layer development. Depth profiling after 10 min activation revealed Cu diffusion into the sphalerite lattice up to 10 nm, while the thickness of the activation product on pyrite surface was less than 3 nm. The zeta potential studies confirmed proposed surface products on the minerals by Cryo-XPS. Hydrophobicity and many other surface properties of Cu-activated sphalerite can be changed by dissolved ions available in the process water (e.g. calcium, magnesium and sulphate). The effect of Ca(NO3)2, Mg(NO3)2, MgSO4 and CaSO4 on the surface properties of Cu-activated sphalerite was studied using sessile drop Contact Angle (CA), Cryo-XPS and zeta potential measurements. The CA measurements showed that the hydrophobicity of sphalerite developed after Cu-activation, but the presence of 3×10-2 M Ca(NO3)2 or Mg(NO3)2 or MgSO4 decreased its hydrophobicity. The XPS data showed that the effect of these ions appeared to be the same, namely a decrease in Cu adsorption and polysulphide formation. Zeta potential measurements confirmed the presence of calcium and magnesium ions on the sphalerite surface. Sphalerite is not the only sulphide mineral whose flotation behaviour can be affected by the presence of Cu ions in solution. The possibility of inadvertent Cu-activation of pyrite and its undesirable flotation is one factor accounting for the effective separation of sphalerite from pyrite. The surface hydrophobicity and components of the Cu-activated sphalerite were measured to investigate Cu adsorption kinetics of sphalerite in the presence of pyrite using a novel experimental approach of High-Speed Video Microscopy (HSVM) and Cryo-XPS. HSVM method allowed for in-situ liquid film Drainage rate (DR) and bubble-mineral CA measurements. The results showed that, after the activation, sphalerite became hydrophobic by the formation of polysulphide as confirmed by Cryo-XPS while pyrite remained hydrophilic. With increasing activation time and Cu concentration, both DR and CA on sphalerite rapidly increased, reaching constant values after a long time, while those on pyrite remained unchanged. The change in CA was successfully modelled by considering a second-order rate process for Cu-activation which was proportional to initial copper concentration and available Zn active sites on the sphalerite surface. If both sphalerite and pyrite were simultaneously exposed to Cu solutions (but not in physical contact), the activation rate of sphalerite significantly dropped with increasing the sphalerite:pyrite surface area ratio from 1:3 to 1:6. Cryo-XPS results for the mixed minerals Cu-activation showed a reduction in sphalerite Cu/Zn exchange and polysulphide/Cu ratios with increasing available pyrite surface area in Cu solutions. Kinetic studies of Amyl Xanthate (AX) adsorption on single and mixed sphalerite and pyrite activated by Cu with different ratios were also evaluated by HSVM and Cryo-XPS. AX adsorption rate exhibited higher value on the activated sphalerite than on the activated pyrite. Modeling AX adsorption rate on un-activated and activated minerals by the second order rate equation revealed the dependence of the adsorption rate on the initial AX concentration and the available active sites on the minerals surfaces. For the two mineral system, Cu-activation and AX treatment results showed a high drop in the adsorption rate of AX on the activated sphalerite with increasing the sphalerite to pyrite surface area ratio from 1:1 to 1:6. Cu-xanthate complex formation on the Cu-activated sphalerite and dixanthogen as a dominant surface component on the Cu-activated pyrite after treatment with AX were confirmed by Cryo-XPS. It is envisaged that the outcomes of this work will contribute to a better understanding of the mechanisms and parameters responsible for the high amount of copper sulphate consumption in sphalerite activation, saving millions of dollars for the mining industry by using less copper sulphate.