The kinetics of the organic elution of gold cyanide from activated granular carbon using an aqueous caustic acetone solution

The kinetics of the elution of adsorbed gold cyanide from granular activated carbon in an aqueous caustic acetone solution in the absence of cyanide were investigated in a batch stirred reactor. The equilibrium and rate experiments were performed with a 1:1 by volume mixture of acetone and 4g/1 NaOH aqueous solution at temperatures ranging between 20 and 60°C. In the rate experiments the effect of different solution to carbon mass ratios, carbon particle size and rate of stirring were investigated. The elution equilibrium was described in terms of Freundlich isotherms. These isotherms were significantly more favourable for elution than those for inorganic solutions used in the Zadra process. The elution rate was described mathematically by a first-order desorption reaction rate equation at the outer surface of the carbon granule. The proposed model accurately desribes the batch elution process in terms of the parameters investigated. The rate of mass transfer across the hydrodynamic boundary layer does not affect the overall rate of elution under the conditions investigated and was therefore not included in the model. High rates of elution were obtained with equilibrium being approached within two hours at 50°C. The elution rate was very sensitive to variations in temperature with an apparent activation energy of 56 kJ/mol. This indicated that the rate of elution was indeed limited by the rate of the desorption reaction.