Amino acids as lixiviants for metals extraction from natural and secondary resources with emphasis on glycine: A literature review

The past decade has seen significant interest in non-conventional lixiviants, particularly the growing interest in organic acids and amino acids. Of the amino acids, glycine has much coverage in the hydrometallurgical and mineral processing literature as a reagent that is non-toxic, non-volatile, and recyclable. The glycine leaching system (i.e., an aqueous system typically consisting of glycine or one of its alkali metal salts, a pH modifier, an oxidant and sometimes, other catalysts or modifiers)has been shown effective in selectively extracting base and precious metals from various resources. Using a glycine leaching system can, therefore, be considered a promising substitute for traditional methods in the hydrometallurgical industry. This paper reviews the application of amino acids and emphasizes the glycine leaching system to extract valuable metals from natural and secondary resources. In the study, variations of the glycine leaching systems with different oxidants, synergists and catalysts are summarised and discussed. Different approaches to downstream metal recovery from glycine-containing leachates are reviewed. Other less commonly reported amino acids are also summarised and discussed. Finally, the perspectives and outlooks of the area are given. It is hoped that the present review can help to understand the relatively new area with state-of-the-art advancements, and to promote the development of glycine leaching system in hydrometallurgy.