Cyanide use, recovery, and reuse at the DRDGOLD ERGO Brakpan Gold Tailings Mine

Abstract

Cyanide-containing effluent streams for gold ore leaching represents a potential avenue for cyanide recovery and reuse in gold mining, potentially improving operational profitability and environmental sustainability. The reuse or recycling of cyanide effluent was examined within the context of the DRDGOLD ERGO Brakpan gold tailings reprocessing plant operating in East Gauteng, South Africa. An operational assessment of the plant identified two cyanide-containing effluent streams suitable for testing. In addition, it also identified an available gold bearing coarse material unsuitable for leaching in the existing Carbon-in- Leach (CIL) circuit. The cyanide effluent streams of interest are from the CIL effluent and from the “Policeman Column” effluent. The CIL effluent contains 50 mg/L of free cyanide in solution at an average production of 100 000 tonnes of slurry (50% solids) per day. The policeman column effluent is a clear stream of approximately 400 mg/L free cyanide at an average production of 250 m3 per day. A literature review of current cyanide recovery methods indicated that HCN gas production was an important intermediatory process step in the recovery of cyanide. HCN gas production is prohibited at the ERGO plant. The policeman column effluent provides a working process stream without need for adjustment or alteration due to its high free cyanide concentration and clarity. The reuse of this stream as process water was therefore investigated in the leaching of gold from the available ERGO plant Coarse material for revenue generation. A representative sample was taken from the Coarse material heap for testing. XRD analysis indicated that the sample is primarily composed of silica and the Coarse material contains particle sizes predominantly +5.6 mm in diameter determined by sieve analysis. Initial leaching tests confirmed the presence of preg-robbing carbonaceous material within the Coarse material, accounting for 8.1 % of the ore as determined by Dense Media Separation (DMS) analysis During initial leaching experiments with no agitation, it was observed that the rate of gold extraction from the coarse material exceeded the rate of gold adsorption by carbonaceous material present in the coarse material within the first 30 min of the leaching process. Adding aeration doubled the extraction. However, there was severe preg-robbing leading to a final gold extraction of 1.3 % and 2.5 % after 24 h, compared to peak gold extraction values of 2.5 % and 5.6 % at 30 min for unaerated and aerated leaching respectively. In the case of agitated leaching with both coarse and pulverized Coarse material the severity of preg-robbing increased resulting in negligible gold extraction into the solution. With the knowledge of the carbonaceous component of the Coarse material literature on the methods for preg-robbing mitigation were reviewed. The existing method of using competing activated carbon in CIL-like conditions was chosen for experimentation. The competing activated carbon allowed for gold extraction of 17 % to 19 % for activated carbon samples of different activity. The column percolation leach with reused cyanide effluent experiment was tested with results showing gold extraction of 26.7% over 4 days. The absence of milling and higher extraction favored the column percolation leach process for a preliminary design and economic analysis. The proposed column percolation leach process would operate for 18 years and produce a yearly revenue of R42.1 Million, and yearly cost of operation of R13.8 Million, with an upfront capital cost of R85.1 Million. The payback period is 6.3 years with a Return on Investment (ROI) of 15.7%. The Net Present Value (NPV) at the end of the process is also positive at R1.9 Million. The effect of reused cyanide effluent is seen in the reduction in cost of R2.1 million per year. Without the reused cyanide effluent, the payback period and ROI are negatively affected at 7.1 years and 14.3 % respectively. However, a negative value of -R4,726,805 for the NPV signifies that the investment has been negatively affected, making it unappealing for investment without reusing cyanide effluent. The application of the column percolation leach with reused cyanide effluent process at the ERGO Brakpan plant can allow for an increase in profit of 3.3 % based on ERGO Brakpan plant financial results for the financial year 1 July 2021 – 30 June 2022.