| ArticleName |
Redox characteristics of gold in HCl – Cl2, HCl – MnO2, and H2SO4 – NaCl – MnO2
solutions: A review
|
| Abstract |
Gold dissolution in acidic chloride solutions occurs both under natural conditions and in industrial hydrochlorination (chlorination) processes. The mechanism of gold dissolution and redox interactions using aqueous solutions of HCl – Cl2, HCl – MnO2, and H2SO4 – NaCl – MnO2 are discussed. It is shown that the sources of information are Eh-pH diagrams, which take into account, among other things, the nanostate ofsuch oxid izing minerals as pyrolusite, bixbyite, hausmannite, and birnessite, with characteristic oxidation potential values sufficient for the dissolution of elemental gold. The oxidation of Au(0) occurs at oxidation potential values above 1.2 V (pH < 2 and temperature >50 °C); in addition to Cl2, HClO (ClO–), ClO3– or O3 can serve as oxidizing agents. The process of gold dissolution in aqueous chloride solutions is staged. In the first stage, simple gold chloride is formed as an intermediate product, and in the second stage, gold(I) chloride is formed, which is an intermediate compound that is either oxidized to Au(III) or, depending on the potential, is retained in solution. At potential values exceeding 1.4 V, the gold surface is passivated due to the formation of an oxide layer. In a natural environment, elemental gold dissolves in oxidized, cold, acidic aqueous surface solutions in the presence of Mn(IV) oxides and chloride ions. The electrochemical characteristics of a number of solid-phase compounds inan aqueous environment are determined by the varying oxidation states of manganese (II, III, and IV) and are manifested in their stability in the alkaline zone. The zones of existence of metastable manganese compounds (β-MnOOH, γ-MnOOH, and R-MnO2) are shown. These compounds play an important role as intermediates in hydrothermal solutions, metamorphosed sediments, and soil layers. This review is intended for scientific and technical departments of gold mining enterprises, employees of research institutes, and undergraduate and graduate students of higher education institutions. |
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