Effect of the physical properites of activated carbon in the gold adsorption from cyanide media

Abstract

The effect of the physical properties of an activated carbon such as pore size distribution, specific surface, pore average diameter, in the gold adsorption from cyanide solution with the gold to the Au (CN)^–₂ form, was studied. To meet the proposed objectives two carbons were studied: carbon A with specific surface of 985 m² / g, 57 % of micropores and 1.85 nm as average diameter of pores and carbon B with specific surface of 786 m² / g, 27 % and pores of 2.35 nm as average diameter of pores; both granular carbons made from coconut shell. Batch adsorption tests were performed in a reactor of 500 ml of capacity with mechanical stirring at constant temperature. The effect of cations present in the aqueous solutions such as Ca²⁺, Na⁺, K⁺ and Li+, the effect of pore size distribution, the effect of average pore diameter and surface area were evaluated in function of the rate and amount of gold adsorbed on the activated carbons denominated as A and B. The results to indicate that the physical properties of an activated carbon are an important factor in the gold adsorption process in terms of rate and amount of adsorbed gold. The carbon B with 786 m² / g of specific surface area reached a higher load per unit area (0.02 mg Au/m²) in relation to the carbon B of 985 m² / g which had a load of 0.01 mg Au / m², after 6 h of contact carbon-solution. The rate adsorption of gold in both carbons is controlled by mass transfer in the liquid film surrounding the carbon particles to short times or small loads of gold in the particles, far from equilibrium. Applying a first order kinetic model, it was obtained that the ratio of the kinetic constants for carbons A and B, ie (k_B / k_A), fluctuates in a value of 3 for the different cations in study. In general it is possible to say that the rate adsorption and the amount of adsorbed gold increased with the increase in macropores and with the increasing pore average diameter. The presence of cations favors the gold adsorption in cyanide media, especially those of higher valence.