HYPOCHLORITE GOLD LEACHING AN ALTERNATIVE TO CYANIDE TECHNOLOGY

Abstract

Traditional technologies of gold mining are exhausted due to the depletion of stocks of conditioned raw materials at functioning gold mining enterprises, in addition, the preservation or disposal of large-scale waste of these technologies requires significant financial costs for environmental protection measures. Recently, the search for alternative methods of gold leaching, which involve the use of non-toxic factors, but which in their physicochemical properties can compete with traditional levels, and especially with cyanides. The authors of this work investigated the mechanism and kinetics of the process of dissolving metallic gold in chloride-hypochlorite solutions and believe that alkali metal hypochlorites (first of all, sodium hypochlorite NaOCl, which is easily obtained by electrolysis from a solution of food’s salt NaCl, or from sea water) are a very promising replacement for cyanide-containing leachates. Two series of experiments were carried out to study sodium hypochlorite as a gold leaching agent using a traditional gold disk and finely dispersed native gold as dissolution objects. Found fundamental differences in methodological techniques when working with a traditional model object and native gold. The dependences of the dissolution rate on the solution pH, sodium hypochlorite concentration, and temperature are determined. Conditions of the gold surface passivation during its dissolution are discussed. The first-order rate constant of the gold dissolution 1 0,079 - 0,4030 \uf02d ki \uf03d h at temperatures from 277 K to 304 K and others are calculated. The activation energy from the temperature dependence of the rate constants (40,3 kJ/mol) evidences a diffusion-kinetic control of the gold dissolution. Electron microscopy (using electron probe scanning on an energy dispersive spectrometer) of native gold particles revealed foreign inclusions - adsorbed mineral particles of calcite (CaCO3) and, presumably, a surface film consisting of aluminum oxide (Al2O3), which create a significant obstacle to the contact of leaching agents with the surface of the target gold grains. Quantitative data on the composition of surface adsorption films, formed by model gold electrode dissolution products, are obtained using atomic adsorption spectroscopy.