Jenny G. Wiese

Inorganic electrolytes present in the process water used during froth flotation may have both beneficial and detrimental effects. These effects are said to be ion specific, as some ions may result in enhanced froth stability, increased mineral recoveries and decreased concentrate grades, while others may bring the opposite effects. Onsite process water quality variations have intensified the need to understand the relationship between inorganic electrolytes and flotation reagents on flotation performance. The use of mixtures of thiol collectors in sulfide flotation is a common practice across the globe; however, very few investigations have considered these in process waters of varying compositions. This study considers the effect of common cations, Na⁺ and Ca²⁺, in process water on the behavior of mixtures of thiol collectors. Single-salt solutions of NaCl and CaCl₂ at an ionic strength of 0.0213 mol·dm⁻³ were used to investigate the behavior of mixtures of two thiol collectors. These were carefully selected to understand how mixtures of thiol collectors behave in the presence of a monovalent cation versus a polyvalent cation. Bench-scale froth flotation tests were conducted using a Cu-Ni-PGM ore from the Merensky Reef. The results have shown that the divalent cation, Ca²⁺, resulted in higher %Cu and %Ni recoveries at all collector mixtures compared to the monovalent cation, Na⁺. The concentrate grades were, however, slightly compromised, as slightly more gangue reported to the concentrate in the presence of Ca²⁺. This behavior is attributed to the effect of polyvalent cations on bubble coalescence and froth stability.

The use of mixtures of thiol collectors is reportedly beneficial in sulfide flotation. This is becoming standard practice for many concentrators, but process water recirculation and re-use in flotation circuits may compromise the behaviour of such mixtures owing to changes in physicochemical interactions occurring in the pulp phase as a result of water quality variations. It is expected that changes in the pulp chemistry would in turn affect both the pulp and froth phase phenomena, thereby affecting flotation performance. Thus, this study considers mixtures of thiol collectors, sodium iso-butyl xanthate (SIBX), and sodium di-ethyl dithiophosphate (SEDTP) in degrading water quality. Bench-scale flotation tests were conducted on various molar ratios of the selected thiol collectors under different ionic strengths (0.0242 mol·dm⁻³ and 0.1212 mol·dm⁻³) of synthetic plant water. Increasing the ionic strength of synthetic plant water and SEDTP molar ratio resulted in an increase in water, solids, Cu, and Ni recoveries. Cu-Ni grades decreased in increasing SEDTP molar ratios. The highest Cu-Ni grades were obtained in degrading water quality. The increase in water and solids recoveries in increasing SEDTP and ionic strength of plant water is attributed to an increase in froth stability. It can be concluded that the increase in the ionic strength of plant water increased water recoveries and therefore froth stability in parallel with SEDTP’s froth stabilizing effect, thus suggesting an additive interaction on the froth stabilisation effect seen.

Previous studies speculate that hydroxo species present in flotation pulps at pH > 9, particularly those of polyvalent cations, selectively adsorb onto gangue minerals. Such species supposedly enhance the depressive action of carboxymethyl cellulose (CMC) onto gangue via an acid-base interaction between the positively charged mineral surface and the negatively charged CMC molecule. Thus, the hydrophilicity of gangue minerals is enhanced, preventing the dilution of the concentrate. However, as there is little evidence to support these claims for complex process waters of increasing ionic strength, it is important to investigate. Adsorption data and mineral surface charge analyses provide a fundamental understanding of how electrolytes and their ionic strengths affect gangue mineral-depressant adsorption. It is strongly anticipated that decoupling these effects will allow process operators to tailor their process water quality needs towards best flotation operating regimes and, in the long run, effect closed water circuits. Thus, using talc as a proxy for naturally floatable gangue common in sulfidic Cu–Ni–PGM ores, this work investigates the influence of the ionic strength of process water on the adsorption of CMC onto talc for a perspective on how saline water in sulfidic ores would affect the behavior and therefore management of floatable gangue. In the presence of CMC, the microflotation results showed that the rate of talc recovery decreased with increasing ionic strength of process water. Increases in ionic strength resulted in an increase in the adsorption of CMC onto talc. Talc particles proved to have been more coagulated at higher ionic strength since the settling time decreased with increasing ionic strength. Furthermore, the zeta potential of talc particles became less negative at higher ionic strengths of process water. It is thus proposed that increases in the ionic strength of process water increased the zeta potential of talc particles, enhancing the adsorption of CMC onto talc. This in turn created a more coagulated nature on talc particles, increasing their hydrophilicity and thereby retarding floatability.