A mechanistic study of an industrial cementation process of copper

Abstract

The kinetics of cementation of cu+ with iron in a strong FeC 12 medium were observed to follow first-order kinetics. Various physical factors have been noticed to influence the rate. The accumulated copper has a catalytic influence on the cementation reaction. The increase in rate is proportional to the mass of the deposited Cu. The cementation rate increases with stirring speed to a limited value which is independant of further increase in stirring speed. The rate is proportional to the surface area of the iron. Impurities have been observed to slow down the reaction rate as they tend to make the Cu deposit so adherant that the reaction surface is blanked off. The activation parameters were obtained in a temperature study. The low activation enthalpy seems to point to a reaction which is controlled by diffusion through a boundary fiIm. It was further observed that certain chemical factors modify the cementation rate. By increasing the mass of the iron powder the reaction rate is increased. This is ascribed to an increase in the reagent surface area. cu+ concentration increase caused the rate to increase. According to Fick's first law diffusion is proportional to concentration and the higher reaction rate is ascribed to increased diffusion across a boundary layer between the bulk of the solution and the reaction surface. Cu2 + and FeC 12 addition was found to retard the rate of cementation. This can be ascribed to the two electron transfer for Cu2 + addition and an increase in viscosity when FeC 12 is added. The reaction was found to be sensitive to HC I addition and it is concluded that just enough should be added to prevent the hydrolysis of iron. A back reaction, where iron is cemented on copper, is shown to be theoretically and practically unlikely. A counter reaction whereby copper goes back into solution has been proved to exist. It is probably due to direct oxidation of the cemented copper by air oxygen or by air oxidation of Fe2 + to Fe3 + and subsequent oxidation of the cemented copper by Fe3 +. Factors which may influence the rate of this reaction were studied. The factors influencing the consumption of iron were studied and it was concluded that those factors which increase the rate of the cementation reaction, decrease the excessive consumption of iron and those decreasing it, increase the excessive consumption and vice-versa for the counter reaction. By bubbling N2 through the solution it was found that the counter reaction could be almost completely stopped. This was reflected in a lower consumption of iron.