Gold Leaching with Organic Nitriles

As part of a U.S. Bureau of Mines program to evaluate alternatives to sodium cyanide in gold leaching, three organic nitrile compounds were tested for their effectiveness as gold lixiviants. The organic compounds examined were malononitrile, acetonitrile, and cyanoacetamide, tested on an oxide ore, two carbonaceous ores, and a sulfide ore. Acetonitrile and cyanoacetamide were shown to be ineffective gold leachants. Conversely, malononitrile demonstrated significant effectiveness, performing similarly to sodium cyanide with extractions of around 80% on oxide ore and extracting 24% of the gold from carbonaceous ores, where sodium cyanide was completely ineffective. Additionally, when using a resin-in-leach technique, the extraction with sodium cyanide increased to 60%, while malononitrile achieved 80%. For sulfide ores, both sodium cyanide and malononitrile reached over 80% extraction, contingent on maintaining the pH above 10.5 for malononitrile. Importantly, malononitrile’s toxicity is significantly lower than that of sodium cyanide, presenting a potential advantage. However, its high cost and uncertain utility could hinder its widespread acceptance.