Adsorption kinetics of dicyanoaurate and dicyanoargentate ions in activated carbon
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INTRODUCTION
IN the last half of this century, activated carbon has become a very common and useful option for the elimination of metallic ions from solutions. One of the most interesting and earliest applications is the treatment of cyanidation liquors containing dicyanoaurate and dicyanoargentate ions or any solutions containing gold or silver cyanides (such as the electroplating baths, for example) with granulated or extruded activated carbon to recover the gold and/or silver selectively. As a consequence of this industrial interest, kinetic studies have been carried out in conditions similar to those in industrial practice. Menne ~] and Nicol e t al. t21 have modeled the adsorption of gold on activated carbon by using empirical rate expressions obtained from batch experiments on cyanided pulps. They found an activation energy of 2.6 kcal/mol. The rate was found to be proportional to the inverse of the particle diameter. Cho e t a l . t3~ studied the kinetics of gold adsorption on activated charcoal. The experimental rate data were explained by using a diffusion-controlled model. Pore diffusion was assumed to be the rate-controlling step, and an activation energy of 3.3 kcal/mol was determined. For the dicyanoargentate ion, Cho and Pitt t4,51 found that calcium and sodium ions increase the amount of silver cyanide adsorbed, while increasing free cyanide ions causes a decrease. Sodium and calcium ions were found to adsorb only in the presence of silver cyanide in solution. These authors also found that large ions have higher adsorbability on charcoal in the order: gold cyanide > silver cyanide > cyanide. Van Deventer t6] provides experimental evidence that the adsorption of the aurocyanide complex on activated carbon is reversible and proposes a model based on the intraparticle diffusion as the controlling step. Fuerstenau e t a l . I71 found the adsorption rate proportional to the temperature, the concentration of charcoal W. YAPU, Professor, is with the Department of Physical Chemistry, Paz University, Bolivia, Spain. M. SEGARRA and M. FERNANDEZ, former Graduate Students, and F. ESPIELL, Professor, are with the Department of Chemical Engineering and Metallurgy, Barcelona University, Martl Franqurs 1, 08028 Barcelona, Spain. Manuscript submitted February 19, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
and the initial gold concentration. The value for the activation energy was 2.0 kcal/mol. They found a firstorder relation between the rate of gold adsorption and the concentration of dicyanoaurate in solution. The kinetic model was tested only for concentrations of 4 parts per million (ppm) or less and with very moderate loadings of the solid phase. They also assumed rate control by film diffusion. A rapid initial uptake phase followed by a slow approach to equilibrium was observed by McDougall e t al. ~s~ and Nicol e t a l . ~21 The latter attributed this behavior to the pore distribution of the carbon (macroporemicropore). Many of the empirical rate models proposed to describe the adsorption of gold f
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