Search

In the current study, a numerical mechanistic kinetic model for direct leaching of zinc from iron containing sphalerite concentrate is presented. The model is based on the well-known shrinking-core model approach. However, in order to consider possible chemical changes that might take place in the liquid phase the model is solved numerically. This also makes it possible to separate and quantify different factors that might limit the leaching process. It is suggested that the most probable leaching mechanism is a combination of the acid attack and electrochemical reaction mechanisms. At first, zinc sulphide reacts with hydrogen ions at the surface of particles liberating zinc ions and forming hydrogen sulphide. Then in the next step, hydrogen sulphide is oxidized by ferric ions or molecular oxygen according to an electrochemical reaction mechanism. The estimated reaction orders for the oxidants are close to 0.5 which supports the electrochemical mechanism. The mechanism of sphalerite leaching also involves simultaneous surface reactions and internal diffusion of oxidants. The obtained results show that the rate of the leaching process is mainly limited by the surface reactions. However, internal diffusion might decrease the leaching rate in some cases up to 50% at higher sphalerite conversions. It is also demonstrated in the study that by using advanced statistical mathematical methods, such as the Markov chain Monte Carlo method, new information can be obtained about the kinetics of sphalerite leaching, the parameters affecting the leaching rate and the uncertainties involved in the model predictions.

A non-cyanide lixiviant for gold dissolution, cupric chloride, was studied. The work used a rotating disk electrode technique to investigate the dissolution reaction mechanism and kinetics of gold dissolution in concentrated chloride (3 M) solution with cupric ion concentration in the range 0.02–1.0 M and temperature 65–95 °C. A reaction mechanism including a disproportionation step of Au(I) is proposed. Increase in the gold dissolution rate was found to be proportional to temperature and cupric ion concentration up to 0.5 M. The dissolution rate was observed to decrease with cupric ion concentration of 1.0 M. A mechanistic kinetic model was developed to study the rate-controlling steps in the dissolution process. Uncertainties in the model parameters of the mechanistic kinetic model were studied with Markov chain Monte Carlo (MCMC) methods. The modeling results showed that the gold dissolution is mainly under mixed control, where both the intrinsic surface reaction and mass transfer have an effect on the overall dissolution rate.

Gold dissolution was investigated in ferric chloride solution, being one alternative cyanide-free leaching media of increasing interest. The effect of process variables ([Fe3+] = 0.02–1.0 M, [Cl−] = 2–5 M, pH = 0–1.0, T = 25–95 °C) on reaction mechanism and kinetics were studied electrochemically using rotating disk electrode with ωcyc = 100–2500 RPM and Tafel method. The highest gold dissolution rate (7.3 · 10−4 mol m−2 s−1) was achieved at 95 °C with [Fe3+] = 0.5 M, [Cl−] = 4 M, pH =1.0 and ωcyc = 2500 RPM. Increase in gold dissolution rate was observed with increase in temperature, ferric ion concentration and chloride concentration, but gold dissolution rate did not have a clear dependency on pH. Redox potential was found to vary between 636 and 741 mV vs. SCE during experiments. According to the calculated equilibrium and measured open circuit potentials, gold was suggested to dissolve as aurous ion Au+ and form AuCl2 −, rather than auric ion Au3+ and form AuCl4 −. Further, it is suggested that AuCl2 − does not oxidize to AuCl4 − under the investigated conditions. Levich plot and the calculated activation energies suggested that gold dissolution was limited by mass and electron transfer. According to a mechanistic kinetic model developed in the current work, intrinsic surface reaction mainly controls gold dissolution, especially at higher rotational speeds (>1000 RPM). Uncertainties in the model parameters of the mechanistic kinetic model were studied with Markov chain Monte Carlo methods.