Exploiting the malleability of gold for placer concentrate extraction and recovery

A novel, chemical-free method for extracting gold from gravity middlings is described. This method exploits the differences in relative malleability, rather than density, of gold and waste products by submitting concentrates to grinding in a rod mill. The grind products are separated through a sieve, where flattened gold particles are captured on the oversize while brittle waste reports to the fines. This paper presents the results of a field tour of this method throughout the Yukon placer gold fields, where gold was extracted from difficult to process stockpiled middlings. Recoveries were often well over 90%, with nearly pure raw gold effectively separated after only minutes of grinding in a batch mill. Gold remaining in the fine and evenly classified loss material is now more amenable to gravity processing than similarly sized unground concentrate. Further lab testing indicated that gold recoveries were most sensitive to the mill charge amount and total grind time. While these results are based on placer properties, the method has potential in any mill involving a gravity recovery circuit.