The chemistry of the carbon-in-pulp process

Abstract

Several conflicting theories of the adsorption of aurocyanide onto activated carbon presently exist. To resolve the mechanism, adsorption and elution of aurocyanide are examined by several techniques, including Mossoauer spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractometry, Fourier Transform Infrared spectrophotometry, ultraviolet-visible spectrophotometry and scanning electron microscopy. The evidence gathered indicates that, under normal plant conditions, aurocyanide is extracted onto activated carbon in the form of an ion pair M n+ [Au(CN) 2 ] n, and eluted by hydroxide or cyanide. The hydroxide or cyanide ions react with the carbon surface, rendering it relatively hydrophilic with a decreased affinity for neutral species. Additional adsorption mechanisms are shown to operate under other conditions of ionic strength, pH, and temperature. The poor agreement in the literature regarding the mechanism of adsorption of aurocyanide onto activated carbon is shown to be due to the fact that different mechanisms operate under different experimental conditions. The AuCN produced on the carbon surface by acid treatment is shown to react with hydroxide ion via the reduction of AuCN to metallic gold with formation of Au (CN) 2 , and the oxidation of cyanide to cyanate. Other species, such as An(CN)5 and Ag(CN)g adsorb onto activated carbon by a similar mechanism to that postulated for Au(CN)2 . Ion association of MAu(CN) 2 salts in aqueous solution is demonstrated by means of potentiometric titration and conductivity measurements, and various associated species of KAu(CN), salts are shown to occur in organic solvents by means of infrared spectrophoteaietric and distribution measurements. A kinetic model was developed for elution of aurocyanide from activated carbon and was found to predict gold elution performance successfully using the Zadra procedure. The influence of the surface chemistry and structure of activated carbon on adsorption of aurocyanide was investigated by characterization of activated carbons that were synthesized or oxidized under various conditions. Synthetic polymeric adsorbents with characteristics similar to activated carbons were also studied. The evidence suggests that a large micropore volume is important in providing suitable active sites for adsorption. Another important factor is the presence of basic functional groups within the micropore, which act as solvating agents for the ion pair. The aim is to provide a self-consistent adsorption mechanism that accounts for all observations presented in the literature. Interpretation of results in terms of preconceived ideas, and neglect of observations of other authors has greatly contributed to current disagreement in the literature.