Circular 86—Study of precipitation of copper on iron from acid solutions

By D. W. Mitchell, 1966, 5 pp., 1 table, 2 figs.

Copper is leached from low-grade mine dumps and caved cap rock in increasing amounts in the southwest U.S. mining industry. The red metal is recovered as cement copper by precipitation on iron, usually tin-can scrap. This activity is very important to the copper mining industry because cement copper is the cheapest copper it produces, assuming, of course, that the cost of mining the waste to be leached can be charged legitimately to the usual mining and milling operations. Production of cement copper has increased steadily, particularly during recent years, until it now accounts for several percent of the total Southwest copper output. This study is part of an attempt to elucidate the principles of the cementation process in some detail and to search for the optimum chemical and hydrodynamic conditions for precipitation of copper by cementation on iron. It started with the hypothesis that, in the acid leach solutions encountered in the industry, the iron used for precipitation is polarized with a film of hydrogen in some unspecified activated state and that subsequent reaction of cupric ion with this activated hydrogen accounts for at least part of the copper precipitated at the iron surface, the fraction depending upon concentrations of reacting ions in the leach solutions and the hydrodynamic conditions prevailing.

It is reasoned that depletion of copper from the leach solution at the metal-solution interface offers the opportunity for reduction of hydrogen ion by the metallic iron. Some of this hydrogen is more or less slowly liberated as hydrogen gas and escapes to the atmosphere; some of it is oxidized by cupric ion while it is still in the activated state. The rate of diffusion of hydrogen ion to the interface is large compared to the diffusion rates of other ions in leach solutions because of its relatively high concentration and its large diffusivity. Consequently, there will always be hydrogen on the iron surface, some of it in the process of transforming to hydrogen molecules. The amount of hydrogen escaping from the system as molecules represents non-productive consumption of iron. Possibly this part of the total iron consumption in a cementation operation might be minimized by achieving hydrodynamic conditions that maintain the maximum flux of copper ions through the diffusion layer at the metal-solution interface. Hydrogen in the activated state, but not yet combined into molecules, might then be largely oxidized by cupric ions. The work described in this paper was performed to learn if there are obvious objections to this hypothesis. Other work is being done to find the combinations of variables that provide optimum cementation conditions.