Extractive metallurgy
The first step in the hydrometallurgical process is leaching, which involves dissolution of the valuable metals into the aqueous solution and or a suitable solvent.
[2] Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials.
The energy required to sustain the high temperature pyrometallurgical processes may derive from the exothermic nature of the chemical reactions taking place.
Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode.
The fused salt electrolysis process is conducted at temperatures sufficient to keep both the electrolyte and the metal being produced in the molten state.
Consequently, mining activities and waste recycling must evolve towards the development of more selective, efficient and environmentally friendly mineral and metal processing routes.
[4] Moreover, despite the environmental concern and the use restriction imposed by some countries, cyanidation is still considered the prime process technology to recover gold from ores.
Bio-hydro-metallurgy make use of living organisms, such as bacteria and fungi, and although this method demands only the input of O2 and CO2 from the atmosphere, it requires low solid-to-liquid ratios and long contact times, which significantly reduces space-time yields.
DESs are generally liquid at temperatures lower than 100 °C, and they exhibit similar physico-chemical properties to traditional ILs, while being much cheaper and environmentally friendlier.
[6] This results in lower plating rates and poorer throwing power and for this reason chloride-based DES systems are still favoured.
For instance, Reline (a 1:2 mixture of choline chloride and urea) has been used to selectively recover Zn and Pb from a mixed metal oxide matrix.
[7] Similarly, Ethaline (a 1: 2 mixture of choline chloride and ethylene glycol) facilitates metal dissolution in electropolishing of steels.
Extraction of these metals from their corresponding hosting minerals would typically require pyrometallurgy (e.g., roasting), hydrometallurgy (cyanidation), or both as processing routes.
Early studies have demonstrated that gold dissolution rate in Ethaline compares very favourably to the cyanidation method, which is further enhanced by the addition of iodine as an oxidising agent.
Metal sulfides (e.g., pyrite FeS2, arsenopyrite FeAsS, chalcopyrite CuFeS2) are normally processed by chemical oxidation either in aqueous media or at high temperatures.
In aqueous media chalcopyrite, for instance, is more difficult to dissolve chemically than covellite and chalcocite due to surface effects (formation of polysulfide species,[14][15]).
However, ionometallurgy has the potential to effectively recover metals in a more selective and sustainable way, as it considers environmentally benign solvents, reduction of greenhouse gas emissions and avoidance of corrosive and harmful reagents.
