“The effects of water quality on flotation in the lead circuit at Cannington”
The process water at Cannington contains significant levels of calcium, magnesium, sodium, chloride, carbonate and sulphate ions as well as lesser amounts of other metallic species. Previous testing has indicated that the majority of zinc losses to the final lead concentrate are attributable to the composition of the process water. Zinc lost to the lead concentrate is in the order of 12-15% with respect to the plant feed.
The literature revealed that the activation of sphalerite is a pre-requisite for sphalerite flotation. Sphalerite activation is achieved by the sphalerite reacting with a metal ion whose metal sulphide is more stable than zinc sulphide. Common sphalerite activators are copper, lead and silver ions. These cations may be deliberately added as reagents, or accidentally from the oxidation of the ore or grinding media, or from the raw water additions to the circuit.
From the literature and previous testing conducted at Cannington, it was hypothesised that, “The activation and flotation of sphalerite in the galena circuit at Cannington are caused by heavy metal ions that are present in the process water added to the grinding stage”.
The results of a water survey on the March 18, 2002 showed that the raw water is relatively clean compared to the process water. The process water was similar in composition to the zinc circuit water and the tailing thickener overflow indicated that the high concentration of dissolved solids in the process water is due to the accumulation of these species caused by water recycling within the Cannington site.
Laboratory rougher flotation tests investigating the effects of various process water components on flotation performance were conducted. From these diagnostic flotation tests, the effects of process water quality on flotation performance were determined at a lead recovery of 92.5%. This lead recovery coincides with the lead recovery seen in Cannington’s lead rougher circuit during metallurgical surveys conducted in January 2002. The key criterion in examining performance in this case was the zinc recovery to the lead rougher concentrate.
The testing revealed that use of process water during the grinding stage significantly reduced the recovery of sphalerite to the lead rougher concentrate (with respect to flotation feed) when compared to distilled water. Zinc recovery to the lead rougher concentrate attributable to the effects of caused by the process water was approximately 11% at a lead recovery of 92.5%.
The overall reduction in zinc recovery was assumed to be the cumulative sum of the individual effects caused by various species in the process water. The components in the process water that affected zinc recovery were organic carbon, zinc, lead and aluminium. Table 1 details the change caused by thesespecies at a lead recovery of 92.5% at their concentration in the process water. The reference point from which the change was measured is that performance observed with distilled water.
It is recommended that further tests be performed to verify if the cumulative sum of the independent effects caused by the individual components in the process water is equal to the combined effect seen in the process water. The nature of the organics within the process water system and their individual effects on flotation performance should also be determined. After these factors are fully understood, the removal of the process water components hindering lead-zinc selectivity should be investigated.