Effect of concurrent ball milling on cementation reactions: The case of Cu-Al system

Abstract

Despite a strong thermodynamic tendency, cementation by aluminum of relatively noble metals from sulfate solutions involves a rather slow kinetics due to the presence of the passive oxide film on the aluminum surface. We know from the literature that small amounts of chloride ion can reduce the dimension of the problem. In this paper, we investigate the additional effect of concurrent ball milling on the enhancement of the cementation kinetics of Cu-Al system as a case study. The cementation tests were carried out in a polyamide jar placed in a planetary ball mill. In the first part of the study, the effect of rotation speed of the milling jar, number of alumina milling balls, particle size of the aluminum powder, and the initial hydrogen ion concentration on the copper recovery was studied and optimized using a central composite design of experiments. In the second part, we investigated the kinetics and mechanism of the cementation process under the optimum conditions obtained from the first part. The results show that the concurrent ball milling can shorten or even remove the initial slow rate period, known as the induction period. It also alters the rate-controlling mechanism from chemical to boundary layer diffusion revealed by a decrease in the activation energy from 79 to 18 kJ mol−1. The morphology of copper deposits obtained under ball milling condition was found to be botryoidal in contrast to the typical dendritic structure. The findings overall support the beneficial role of the simultaneous ball milling in lowering the kinetic barrier of cementation reactions on the aluminum surface.