Historical Developments in Hydrometallurgy
A brief summary of some of the historical developments in hydrometallurgy is summarized below in Table 10. Long ago it was noticed that when iron metal was brought into contact with a solution containing copper ions (not known to be copper ions at the time) copper metal plated out onto the surface. It appeared as if the iron had been transformed into copper! The reason this works is that Cu+2 is able to oxidize Fe in a favourable and fast reaction:
Cu+2aq + Fe s = Cu s + Fe+2aq
(This is perhaps where the ideas around transmutation of the elements came about. Now, since iron and copper were similar, say in terms of density, and since lead and gold are similar in the same respect, why not transmutation of lead into gold?) Cementation of copper onto scarp iron continued to be a major means of recovering copper from leach solutions up until solvent extraction for copper was implemented in the 1960’s. This methodology, long known to analytical chemists on the lab scale, is now being practiced to recover copper in individual plants on a scale of tens of thousands of tonnes per year. (Solvent extraction involves the transfer of a solute in water into an immiscible organic solution. Purification and concentration of the copper are major advantages.)
Cyanidation of gold ores (dissolution of Au to form Au(CN)2-) and the Bayer process for recovering pure aluminum hydroxide, Al(OH)3, from sodium aluminate solutions were the developments that ushered hydrometallurgy into being as a modern metal production method. The impetus to develop the atomic bomb during World War II resulted in much research into recovery of uranium. New leaching processes were developed. Much development in solvent extraction for metals recovery was also developed in this context. A major discovery was that the lanthanide elements could be separated from one another through ion exchange resins in columns. The lanthanides are chemically very similar and they tend to occur together in nature. Their chemical similarity made traditional methods of separation very difficult. But, slight differences in affinity for ion exchange resins made these materials ideal for separation.
In the 1950’s Frank Forward at the University of British Columbia pioneered research into oxidative pressure leaching of nickel sulfides. The work was patented and later adopted by Sherritt Gordon Mines in Alberta. Hydrogen reduction of many metals was developed in the same decade (Frank Forward again) and is still an important technology for some metals. In the 1960’s it was discovered that certain classes of naturally occurring bacteria readily facilitate oxidation of metal sulfides. These bacteria are instrumental in the Earth’s natural sulfur cycle. As a result low grade ores, particularly of copper, became amenable to metal extraction. Large heaps of rock are piled up, inoculated with bacteria (e.g. thiobacillus ferrooxidans) and irrigated with aqueous solutions. Leachates containing copper and other metal ions are collected and treated. Alternatively, an ore body may not even be mined, but rather is fractured by explosives (if necessary) and leach solutions are pumped through. The leachate solutions return to surface at various points and are collected. The downside to the marvelous biological sulfide leaching process is that where mining has exposed sulfide minerals to air and water, acid mine drainage occurs as a result of the same bacterial action. The Britannia Mines site on Howe Sound near Squamish is just one of many such situations.
Table 10
More recent developments include better understanding of the chemical mechanisms involved in leaching of minerals, implementation of leaching in vessels under pressure and recovery of metals present at low concentrations in solution wastes and tailings. In some cases, with advances in technology, what were once tailings wastes are now resources. Since the 1980’s gold recovery has greatly improved through the use of activated carbon. Gold cyanide complex, i.e. [Au(CN)2]-, in solution can be adsorbed onto activated carbon, stripped off as a concentrated solution and then the gold is electrowon. It had been long known that gold could be adsorbed onto carbon in this way, but it took the development of efficient means of desorbing it and development of appropriate activated carbons before it was commercialized. Some of the significant developments in hydrometallurgy have arisen simply by being able to implement well known chemical processes and methods. This often involves innovation and putting together pieces that already exist, but in disparate fields.
Electrolysis of copper leach solutions to recover copper metal began in Chile. The use of electrolysis to produce aluminum metal was invented in 1886. Today electro-winning is an indispensable method of producing many high purity metals, such as copper, zinc, lead, gold, nickel and cobalt. In addition, electrolysis is used extensively to refine metals. Pioneering work on zinc leaching and electrowinning was carried out in Trail B.C.
