Membrane Technologies For Gold And Silver Processing Improvements

Membrane technology has been available for about 30 years, but has been used very sparingly within the general mining industry. However, recent developments in polymer chemistry, spiral wound element construction, pretreatment equipment and element cleaning techniques have improved the reliability and robustness of membrane based systems. These improvements have expanded the potential to use this "new" technology to enhance critical recoveries of gold and silver in heap leaching operations or in the extraction circuit. Another opportunity for the mining industry will be to use membrane technology in new processes like controlling organic acids or ammonium thiosul­fate to enhance gold and silver extraction during leaching opera­tions. Finally, the most common use for membrane technology is for the control of mining water balance and acid mine drainage issues. Polymeric membranes, specifically thin-film composites, have been developed that will purify, fractionate or concentrate gold or silver. Recent applications have been developed for spi­ral wound modules that operate across the pH spectrum from 0 to 14. Systems using membranes have been constructed that operated on feed streams from 200° C to 3000 centipoise. Most recently, membrane based systems have been pilot tested suc­cessfully to purify solvents, like LIX. Opportunities exist to apply membrane technology to concen­trate gold and silver cyanide solutions with thin-film composite Reverse Osmosis (RO) membranes at elevated pH values above 10. Incorporating membrane technology could allow one to increase production without incurring additional expansion of the gold extraction side of the process. Furthermore, free cyanide is not rejected by the Reverse Osmosis membranes, but other inorganic salts are rejected 98+%, thereby, allowing an essentially ion free, except for the free cyanide, to be recirculat­ed back to the heap leach operation for enhanced extraction.