The non-flooded bed leaching process, by spraying acid solutions, is nowadays the most widely used in copper recovery operations from oxidized species. The technical-operational feasibility of the process lies largely in the optimization of the acid consumption rate as a function of the bed mineralogy and the expected degree of extraction for the metal of interest.
Although the increase in acid concentration of the leaching solution is directly proportional to the cupric ion recovery kinetics, high acid consumption could negatively affect the economic viability of the hydrometallurgical project.
Sulfuric acid (H2SO4) is highly selective with respect to the gangue, except if this is mostly made up of minerals of a calcareous nature, such as calcium carbonates (CaCO3), which together with iron oxides and hydroxides generate reactions that compete with the release of copper and increase the consumption of reagents.
Although certain secondary reactions are beneficial to the operation, such as the formation of ferric sulfate (Fe2(SO4)3) and ferric chloride (FeCl3), which cooperate in the leaching of some copper sulfide species such as chalcocite (Cu2S). The problem lies mainly in the fact that carbonates have higher reaction kinetics than copper, so they react before the solution can reach the species of interest.
The equation representing the acid consumption of carbonates is as follows:
CaCO3(s) + H2SO4(l) 🡪 CaSO4(s) + CO2(g) + H2O
As can be observed, a collateral effect of the reaction is the formation of solid calcium sulfate, which could deposit on the dissolving species, thus preventing contact between the leaching solution and the soluble species. This effect limits the dissolution kinetics and may even stop the reaction in some sectors due to the formation of blind pockets and an increase in the water table of the bed.
To quantify the acid consumption of the ore to be treated, it is common to perform ISO-pH tests which, by means of the agitated leaching method for representative samples with a particle size of 100% less than 10#, provide the maximum copper recovery and the maximum acid consumption. For a hydrometallurgical project to be viable, the consumption rate must be less than 40 or 50kg of acid per dry metric ton of ore, a rate that depends on the value of acid in the market at the time the project is managed. This value is often achieved by blending ores.
Depending on the mineralogy of the material to be treated, an alternative to oxide processing when high amounts of acid consumers are present, is to approach alkaline leaching with ammonium hydroxide (NH4OH) which is selective to some oxidized species, keeps the cupric ion in solution preventing it from precipitating and favors copper recovery as a copper tetra-amine complex (Cu(NH3)4+2).
