Mecanismo de adsorção de cianocomplexos de cobre em carvão ativado e implicações na adsorção seletiva de ouro

Abstract

The trend in the increase of mining complexes, low-grade ores has led to the exploitation of gold ores with a high Cu/Au ratio. The presence of soluble copper in CIP(carbon-in-pulp)/CIL(carbon-in-leach) circuits results in the competition of its complexes by adsorption sites on activated carbon. The study of copper adsorption on activated carbon can contribute to minimize the detrimental effects of copper on gold cyanidation. Initially, the speciation of the Cu-CN-H2O system was investigated by means of thermodynamic stability diagrams, Raman spectroscopy calculations DFT (density functional theory) calculation. The DFT calculations demonstrated that the linear (Cu(CN)2 -), trigonal plane (Cu (CN)3 2-) and tetrahedral (Cu(CN)4 3-) complexes are the most stable ones amongst the cyanide/hydrated species and thus, these are the expected predominant geometries in cyanide solutions. Raman spectroscopy indicated that the Cu(CN)3 2- is the dominant species in a wide range of pH (5-10.5), copper concentration (0.02 to 0.2mol.L-1) and CN/Cu molar ratio (<7). The predominance of Cu (CN)4 3- is observed only in high pH values (>10.5) and high CN/Cu ratio (7). The Cu(CN)2 - species was not detected. Partial charge values, calculated by the principle of electronegativity equalization, have shown that the strength of the bond CN decreases with the increase of the number of cyanide groups in the complex, which implies in the decrease of the respective Raman vibration frequencies and the complexs stability. In a second approach, the mechanism of copper cyanide complexes adsorption on different samples of activated carbon was investigated. The electrostatic interaction model proposed here considers the effects of the physical chemistry features of the samples, such as the point of zero charge (PZC) and the functional group density, and was consistent with the experimental results. The adsorption of copper was favored in pH <PZC by the attraction between the positively charged surface of the activated carbon and the negatively charged, copper cyanide complexes. The adsorption of the species Cu(CN)3 2- (CN/Cu=3) was favored by the increase in the ionic strength. The presence of calcium ions (Ca2+) promoted additional copper adsorption by the establishment of a local excess of positive charges that are balanced by adsorption of the copper(I) cyanide complexes. Finally, the effects of CIP process parameters (i.e. type of coal, excess of cyanide, presence of Ca2+ and aeration) in the selective adsorption of gold were studied. The activated carbons with lower functional group density (1.2meq.m-2) were more selective to gold adsorption. The excess of cyanide (CN/Cu> 4) favored selectivity, however, high concentrations of cyanide decreased gold adsorption. The presence of Ca2+ increased gold adsorption in 22%, which canceled the positive effect of calcium on the copper adsorption, as observed in single metal solutions. The aeration of the solution increased the adsorption of copper by 4 times and decreased adsorption of gold. This result was related to the greater stability of the Cu (CN)3 2-, as indicated by Raman spectroscopy.