| Reaction kinetics for gold dissolution in acid thiourea (Tu) solutions were studied using ferric sulfate and formamidine disulfide (FDS) as oxidants. Stability of thiourea and factors causing thiourea consumption in actual leaching systems were examined.; For the thiourea-ferric sulfate system as presented in Chapter 4, it was found that when ferric ion (Fe(III)) concentration is below 0.06 g/l with an initial Tu concentration of 4.0 g/l, pH 1.5, 25°C, and gold disk rotational speed of 800 rpm, the rate of gold dissolution is limited by the mass transfer of Fe(III) to gold surface. When the Fe(III) concentration is above 0.2 g/l, a surface reaction appears to be the rate-limiting step. Under the surface reaction control, the apparent activation energy was determined to be 13.9 kcal/mol (or 58.1 kJ/mol). The empirical equation for the kinetics at pH 1.5 has the following form: dCAuI dt=kTu 0.25; The results using FDS as oxidant presented in Chapter 3 suggest that the rate of gold dissolution is limited by a surface reaction under the conditions studied. The apparent activation energy for the dissolution reaction was determined to be 10.0 kcal/mol (or 41.6 kJ/mol). The empirical equation for the kinetics has the following form: dCAuI dt=kTu FDS0.5 Both of the kinetic equations are substantiated from electrochemical theory using the Butler-Volmer equation.; Chapter 5 presents the results for thiourea stability and factors causing thiourea consumption, including adsorption by mineral particles, thermal degradation, oxidation by Fe(III)/air, and complexation with base metal ions. It was found that the rate of Tu oxidation by Fe(III) is relatively slow in simple solutions and limited by a homogeneous chemical reaction. The apparent activation energy was determined to be 16.0 kcal/mol or 66.7 kJ/mol. The redox rate is first order with respect to thiourea and ferric ion concentrations, respectively. However, the redox reaction is significantly catalyzed by some sulfide minerals (pyrite and chalcopyrite) and cupric ion (Cu(II)). Of all these factors, Tu oxidation by Fe(III) appears to be the most significant. Cu(II) has a deleterious influence on gold leaching kinetics and thiourea stability. |
