Chapter IV: Introduction to Leaching

Leaching is the dissolution of solid materials into solution, invariably an aqueous solution in hydrometallurgy. The solids are usually natural minerals, but also include intermediate products of a process. Since we are always after one or a few components, leaching should, ideally, be highly selective for the desired mineral(s)/material(s). In practice we usually achieve partial selectivity, and, we never completely dissolve the whole ore or feed material.

Leaching is virtually ubiquitous in hydrometallurgy and is often the key step upon which the rest of the process hinges. Much has been written about the subject. A key to understanding leaching is that it is an interfacial phenomenon; we are utilizing heterogeneous chemical reactions. We are concerned with the solid-solution interface, but also the gas-solution interface, whenever gaseous reactants (e.g. O_{2 g}) or products are involved. Thermodynamics, kinetics, transport phenomena, inorganic reaction chemistry and other knowledge are involved.

This material provides an introduction to the topic. Underlying aspects of the chemistry required to develop a flowsheet will be presented. This is followed by an overview of leaching processes. Leaching may involve “co-current” and “counter-current” flows of solids and solutions. This will be presented. A brief overview of factors associated with agitation in leaching will be given. In a separate set of notes, kinetic models for leaching will be developed.

The desired mineral(s)/material(s) need to be soluble in the chemical solution to be used, and under practicable conditions:

• Practicable conditions have three aspects. The first is technical feasibility. So, for instance, reactions rates need to be reasonably fast. Otherwise excessively large equipment will be needed to allow suitably long enough retention times. Reactions need to proceed to a high degree of completion (high equilibrium constants), or else little conversion will be achieved. Materials of construction for tanks, pipes, etc., need to be compatible with the chemicals and conditions. There are many such considerations.

• The second aspect of practicable conditions is economic. Ideally we want to use cheap reagents that will work with inexpensive materials, and so on. A technically feasible process that is not cost effective will not succeed. Together, these two factors greatly limit the number of available processes.

• The third aspect has to do with legislative and societal issues. For instance, it is becoming increasingly difficult to build gold plants that use cyanide, due to some recent environmental tragedies and public perceptions.