Gold Recovery by Solvent Extraction
As the world’s need for gold increases and the grade of gold ores decreases, finding effective and efficient methods for processing gold ores available and recycling previously used gold products, it is important and greater importance to look for new options. There is no doubt that hydrometallurgy provides many of the new processes to recover gold and with equal certainty, the solvent extraction process play an important role in many of these processes. The solvent extraction is well established technology of separation in metallurgy extractive and was first applied to the recovery of uranium for nuclear purposes in the 1940’s and is currently employed in the recovery of uranium. Copper, zinc, cobalt, nickel, vanadium, tungsten and molybdenum as well as in separation involving the platinum group metals.
Basically, the solvent extraction process involves the transfer of a solute from and aqueous to an organic phase, and the subsequent extraction of the solute back into another aqueous phase under a different set of chemical or physical conditions. In this way, for compounds present in an aqueous solution to be able to pass into an organic phase, it must react with an organic component. Metallic ions can be present in aqueous solutions in a variety of forms, either free or as complex compounds and, usually, only a particular ion, or at most a limited number of these ions, will react will the solvent and be extracted. The distribution of the valuable metal between the two phases is influenced by the composition of the aqueous phase. The solvent may be a pure organic liquid or a solution of an organic reagent in inert diluents. The organic reagent may undergo association, salvation or other reactions in the organic phase and these will influence the extraction equilibrium.
The grade or value of the aqueous solution being treated is a key factor at the moment of selecting solvents for gold recovery. Essentially, solvent extraction is preferred for the treatment of small volumes of solutions containing high gold contents. This consideration is important because solvent is lost in solvent extraction and this aspect is important with respect to operating costs when large volumes of solution are processed, whereas resins are effectively insoluble in water and do not suffer from this drawback. Other consideration respect to ion exchange resins is the fact that they are able to treat pregnant solutions containing high concentrations of suspended solids, whereas a solvent extraction process needs a clarification step, otherwise some problem will arise in next stages of the process.