Hydrometallurgy

Hydrometallurgical processes are becoming more and more important. They involve leaching the metal content from the ore, from a concentrate or from process side products in an aqueous solution. After purifying the solution to remove deleterious elements the required metal is isolated and can be extracted.

Bioleaching of low-grade ores is another example of hydrometallurgy. In this case bacterial action liberates metal ions from the ore. Bio-leaching has been successfully applied to sulphide ores, gold bearing pyrites and arsenopyrites.

Teck-Cominco developed a new, low cost process (HydroZinc Process) for the direct extraction of zinc metal from sulphide ores. The new process involves bio-leaching of ore in heaps with naturally occurring bacteria, followed by neutralization, solvent extraction and electrowinning. The process is a direct ore to metal procedure, which does not require grinding and flotation. It is applicable to a variety of zinc ores and, last not least, it is environmentally sound with no sulphur dioxide emissions involved.

The Albion Process was developed by Mount Isa Mines in order to optimise metal recovery from the extremely fine-grained ore of the McArthur River deposit in Australia’s Northern Territory. The process requires ultrafine grinding of an ore or concentrate to particle sizes of 80% less than 20 microns. The resulting slurry then undergoes autothermal oxidation in open agitated tanks (the oxidation of sulphide minerals as an exothermic process is self-sustaining and needs no external energy). After oxidation the slurry is leached with sulfuric acid in leaching tanks and the leached slurry is neutralised to precipitate impurities, ideally to produce a goethite residue. Harmful impurities such as arsenic are fixed in the neutralised solids as ferric arsenate. The neutralised slurry is subsequently thickened, and vacuum filters are used to produce a filter cake. The filter cake is then converted back into a slurry and sent for cyanidation for gold and silver recovery. The filtrate undergoes conventional solvent-extraction and electrowinning to recover metals such as copper, zinc and nickel. Base-metal recoveries for the Albion process are typically in excess of 95%, and those for gold and silver are 90-95%.