Thesis Topic: Reduction of Activated Sphalerite in the Lead Flotation Circuit at Cannington.
In the 2003/04 financial year the BHP Billiton Cannington site lost up to 18% of the sphalerite in the plant feed to the lead concentrate. Monthly liberation analysis demonstrated that over 80% of the sphalerite lost to the lead concentrate is liberated. Metal ion activation has been determined through surface analysis to strongly affect the flotation rate of sphalerite. Rejecting the sphalerite from the lead circuit would improve the lead concentrate grade along with increasing the sphalerite reporting to the zinc circuit. To reduce the amount of sphalerite misreporting to the lead concentrate, this thesis aims to identify the extent of metal ion activation, along with possible methods to reduce the recovery.
Review of the literature and previous test work undertaken using Cannington ore has resulted in the following hypothesis:
The activation and recovery of sphalerite in the lead flotation circuit at Cannington is partially caused by heavy metal ions present in the process water. Sphalerite depressants, pH modification and water quality control are possible methods to decrease the extent of metal ion precipitation onto the sphalerite particle surfaces.
Diagnostic flotation tests were conducted to investigate the effects of various metal ion concentrations on flotation performance. The results were then compared at a lead recovery of 90% to determine the effects on lead-zinc selectivity. Zinc depressants were also tested at a variety of pH levels to determine the effects on lead-zinc selectivity. Process water chemistry was modified by changing the pH to determine the level of metal ion precipitation, which may be a possibility for process water quality control.
From the test work it was determined that the performance of the lead flotation rougher was dependant on the concentration of metal ions present in the process water. Table 1 details the most significant effects on zinc recovery caused by these species at a lead recovery of 90% and at concentrations similar to those determined for Cannington process water (Pb <5mg/L and Cu <1mg/L). The reference point from which this change has been measured is the performance observed with distilled water.
Increases in pH from 8-10 using the standard rougher test had little effect on the lead recovery. The lead-zinc selectivity showed a small increase with pH. No effect on silver recovery was evident.
Zinc sulphate (ZnSO4) and sodium metabisulphite (MBS) were tested at three concentrations at pH 8-10. As the concentration of reagent increased the lead-zinc selectivity improved. Significant improvements were then seen as the pH was raised. Zinc sulphate caused a 1-2% decrease in silver recovery as the concentration increased while a 2-3% decrease was seen for MBS.
Modification of the process water using pH has the potential to reduce the lead ions in solution by up to 60% by precipitating the Pb2+ ions out of solution at a pH of 10. Other means to reduce sphalerite recovery may be to use heavy metal ion absorbents. The advantage of using solid heavy metal ion absorbents lies in that it is relatively easy to separate them from recycling water after being used.
It is recommended that the effect of more than one metal ion in solution be tested at the same time in both the grinding and flotation stages. Further flotation test work should be conducted to determine the effects of zinc sulphate and pH modification using process water, rather than tap water.
Investigate the effects of using pH modified process water in flotation along with conducting trials using heavy metal ion absorbents to remove metal ions from the process water.
A risk assessment was conducted to identify all potential risks to the successful completion of the thesis, assess the risks and identify actions required to either mitigate or eliminate unacceptable risks. Microsoft Project was used to plan and manage the progress of the project, and ensure the thesis was completed as scheduled.