Precipitation of metals by metals (cementation) - kinetics, equilibria

The generally accepted equation for describing the kinetics of the cementation process has been derived under several simplilying assumptions. Consequently, it does not explain some experimental facts and sometimes even leads to conclusions contradicted by the experiments. The main assumption is that the kinetics of the cementation is determined by the rate of transport of the metal A ions to be precipitated to the surface of the precipitating metal B. It does not explain why the rate of cementation of A, by a different metal B, differ from each other. It also does not explain why maxima appears on the potential epsilon(A/B)=f(t) curves. In the case of cementation from complex salt solutions, the assumption that the cementation runs to the point of zero concentration of A is oversimplified. Doubtful also is the assumption that the reaction surface, identified usually with that of the precipitating metal B, does not change with the progress of reaction. Problems of the direct determination of the cementation rate cause that, as a rule, the basic differential equation is not used but instead its secondary integral form is confronted with the experimental results. Therefore, an attempt has been made to perform cementation in a model cell. This enables determination of the rate of the process and also the potential epsilon(A/B) as a function of time. Results of these experiments have proved that the rate of the process does not diminish continuously, as follows from the differential equations. It rises initially and then decreases. The same concerns the epsilon(A/B)=f(t) dependence. Hypothetical runs of the polarization curves have been presented which explain these effects. They also help to explain why the cementation rates of the same metal A by various metal B may differ from each other. Reduction of AgCl has been also performed in the cementation model cell. Results of these experiments have helped to formulate some conclusions on the mechanism of that process. A method of Ag recovery from AgCl, presenting several advantages over that at present applied in industry, has been patented. This text presents a synthesis of results of several years of research. (C) 2002 Elsevier Science B.V. All rights reserved.