Metals precipitation and recovery of cyanide from cyanide leaching solutions

Cyanide leaching, the most common method employed for the extraction of gold/silver from ores, is not a selective method. Copper, zinc etc. minerals present in the ore are also dissolved under cyanide leaching conditions leading to high consumption of cyanide. Ready dissolution of metals and high consumption of cyanide can lead to technical problems and high treatment costs during leaching and solution purification/recovery stages. Recovery of cyanide is of great importance from ores with high cyanide consumption. In view of the shortcomings of the current industrial cyanide recovery process, the development of a new and alternative process with its potential for practical application is considered in this study. The proposed process is based on the precipitation of metals that are complexed with cyanide as metal-TMT compound using an organic trimercapto-s-triazine (TMT) with low toxicity. The experimental studies have shown that silver and zinc can be rapidly and extensively precipitated by using TMT from synthetic and real pregnant leach solutions i.e. >91% Ag&Zn recoveries over 5-15 min. However, the precipitation of copper by TMT was determined to be very slow (55% Cu in 4 days). Increasing the TMT dosage was found to enhance the precipitation of metals while high cyanide concentrations adversely affected precipitation of Zn and Cu. The precipitation tests have also demonstrated that high silver and zinc recoveries (e.g. 99%) with high selectivity over gold can be readily achieved from the pregnant leach solutions obtained from cyanide leaching of different Au/Ag ores. In these tests the precipitation of copper was limited to ≤13.3%. These findings indicated that the proposed precipitation method can be potentially utilised for the treatment of cyanide leaching solutions fort the selective recovery of silver and zinc, in particular, over gold.