With the depletion of high quality ores and an increased demand of metals by society, low quality ores have been processed by the industry. This includes processing of clay ores and grinding the ores to smaller sizes. In an attempt to address the issue relating to processing clay ores, Newcrest’s Telfer operation has examined clay dispersants in the flotation of high clay bearing copper and gold ores in the laboratory and full-scale plant. It was found that a lignosulfonate clay dispersant improved copper and gold flotation to a greater extent in the laboratory than that observed during full scale plant trials. In this research, this discrepancy was studied, with a particular focus on the difference in grinding chemistry between the laboratory and plant conditions which may have led to the different impact of clay dispersants on flotation performance in the two systems.
A review of literature indicates that due to the unique structure of clay minerals (the colloidal size and the charges clay minerals carry at the face and edge), slime coating, an increase in pulp viscosity and high entrainment in flotation are unavoidable, resulting in reduced flotation efficiency. Clay dispersants have been used to improve flotation performance in coal, potash and pentlandite systems through the mitigation of the coating of clay minerals on the surface of valuable minerals. However, the use of lignosulfonates as clay dispersants has not been extensively studied. Researchers have shown that galvanic interactions between sulphide minerals and grinding media greatly affect separation efficiency and the resulting flotation recovery and grade. The use of mild steel grinding media adversely affects flotation performance compared to other more inert grinding media mainly due to the oxidation of iron and the precipitation of iron hydroxides on sulphide mineral surfaces especially in fine size fractions. To mitigate iron contamination from grinding media, the control of oxidation and the addition of a chelating agent during grinding and flotation have been investigated.
The aim of this study was to examine the effect of grinding chemistry on copper and gold flotation in the presence of clay minerals and clay dispersants. A systematic approach involving ore characterisation, viscosity measurement, surface chemistry, flotation testing and size-by-size analysis were employed to understand the underlying mechanism in clay affected flotation. Two types of ore samples with different quantities of clay minerals from Telfer Mine were explored in the present work with the majority of work focused on the ore with the higher clay content.
It was found that slime coatings of the clay minerals on valuable mineral surfaces was the main factor affecting copper and gold flotation performance as opposed to the change in slurry rheology imparted by the clay minerals. Stainless steel grinding media, which produces much less iron species contamination on mineral surfaces than mild steel is favourable for copper and gold recovery. Mild steel media produces a reducing environment and the formation of hydroxide irons, confirmed by surface chemistry analysis, may mask the efficiency of the studied clay dispersant to mitigate the impact of the clay minerals on the process. This is consistent with the previous observation during the plant trial. Furthermore, a size-by-size analysis revealed that fine and ultrafine size fractions were impacted the most with significant lower copper and gold recovery compared to other size fractions.
In attempts to mitigate the negative effect of iron contamination from grinding media to improve the effectiveness of the lignosulfonate clay dispersant, two hypotheses were tested: a coarser primary grind prior to flotation and the removal of hydroxide species from mineral surfaces prior to flotation.
The quantity of fine valuable particles plays an important role in the overall flotation recovery due to their typically poor flotation response. Increasing the grind size decreases the galvanic interaction and formation of iron hydroxides. However, it was found that by increasing the grind size, the proportion of coarser particles became relatively greater while the amount of fine and ultrafine particles remained similar. In addition, clay minerals are extremely easy to fragment due to their natural colloid sizes, so it is unlikely that the distribution of clay minerals were affected by coarser grind. As a result, a coarser grind was not found to be an effective strategy to improve flotation performance in this case.
The second approach investigated, to mitigate the negative effect of iron contamination from grinding media, was to eliminate the iron oxide species chemically. By adjusting the pH and Eh of the slurry, the precipitated iron species can be dissolved back into solution as ferrous iron. Following this, the addition of a chelating agent can prevent these species reforming on the mineral surfaces thereby mitigating their impact on flotation. Three different well know iron chelating agents in conjunction with lignosulfonate dispersant were tested at different dosages with EDTA being the most effective at 250 g/t and it was shown that gold and copper recoveries were increased from 50% to 87% and 36% to 55%, respectively.