On the Use of Amino Acids to Leach Precious Metals from Primary and Secondary Resources

During hydrometallurgical processing of precious metals, the leaching process is the critical stage mainly contributing to environmental pollution problems. The most important process variables affecting leaching kinetics are temperature, lixiviant concentration, solid-to-liquid ratio, presence of catalysts and synergists, and the type of oxidant. There are significant efforts to replace harmful lixiviants with environmentally friendly solvents in the leaching of precious metals during hydrometallurgical processing of precious metals. Of the different lixiviants researched, amino acids demonstrate much potential to compete with traditional lixiviants like cyanide and aqua regia. We have reviewed the chemistry and performance of amino acids in leaching of precious metals from primary and secondary resources and compared them with traditional approaches. The use of different approaches to enhance the leaching kinetics including starved cyanide, addition of strong oxidant, addition of synergists and catalysts, concentration, and temperature were reviewed. Commonly used amino acids were reported and the future for commercialization of amino acids for precious metal leaching was discussed. The use of amino acids in absence of cyanide requires the use of strong oxidants and further research is still necessary on green amino acid–oxidant combinations capable of leaching secondary resources and producing high leaching efficiencies at a low cost.