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Leaching Edit Leaching involves the use of aqueous solutions to extract metal from metal bearing materials which is brought into contact with a material containing a valuable metal. Through the use of chelating agents , one can selectively extract certain metals.
Such chelating agents are typically amines of schiff bases. In-situ leaching Edit In-situ leaching is also called "solution mining. Explosives or hydraulic fracturing are used to create open pathways within the deposit for solution to penetrate into.
Leaching solution is pumped into the deposit where it makes contact with the ore. The solution is then collected and processed. The Beverley uranium deposit is an example of in-situ leaching and also Trojan Mine in Zimbabwe.
Leach solution is sprayed over the top of the heap, and allowed to percolate downward through the heap. The heap design usually incorporates collection sumps, which allow the "pregnant" leach solution i. An example is gold cyanidation , where pulverized ores are extracted with a solution of sodium cyanide , which, in the presence of air, dissolves the gold, leaving behind the nonprecious residue. Vat leaching Edit Vat leaching involves contacting material, which has usually undergone size reduction and classification, with leach solution in large vats.
Tank leaching Edit Stirred tank , also called agitation leaching, involves contacting material, which has usually undergone size reduction and classification, with leach solution in agitated tanks. The agitation can enhance reaction kinetics by enhancing mass transfer.
Tanks are often configured as reactors in series. Autoclave leaching Edit Autoclave reactors are used for reactions at higher temperatures, which can enhance the rate of the reaction. Similarly, autoclaved enable the use gaseous reagents in the system. Solution concentration and purification Edit After leaching, the leach liquor must normally undergo concentration of the metal ions that are to be recovered. Additionally, undesirable metal ions sometimes require removal.
Copper is precipitated as its sulfide as a means to purify nickel leachates. Cementation is the conversion of the metal ion to the metal by a redox reaction. A typical application involves addition of scrap iron to a solution of copper ions. Iron dissolves and copper metal is deposited. Ion Exchange Gas reduction. Treating a solution of nickel and ammonia with hydrogen affords nickel metal as its powder. Electrowinning is a particularly selective if expensive electrolysis process applied to the isolation of precious metals.
Gold can be electroplated from its solutions. Solvent extraction Edit In the solvent extraction is a mixture of an extractant in a diluent is used to extract a metal from one phase to another.
In solvent extraction this mixture is often referred to as the "organic" because the main constituent diluent is some type of oil. The PLS pregnant leach solution is mixed to emulsification with the stripped organic and allowed to separate. When dealing with electrowinning, the loaded organic is then mixed to emulsification with a lean electrolyte and allowed to separate. The metal will be exchanged from the organic to the electrolyte.
The resulting streams will be a stripped organic and a rich electrolyte. The organic stream is recycled through the solvent extraction process while the aqueous streams cycle through leaching and electrowinning[ clarification needed ] processes respectively. Metal recovery Edit Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step.
Sometimes, however, further refining is required if ultra-high purity metals are to be produced. The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation. For example, a major target of hydrometallurgy is copper, which is conveniently obtained by electrolysis. Electrolysis Edit Electrowinning and electrorefining respectively involve the recovery and purification of metals using electrodeposition of metals at the cathode, and either metal dissolution or a competing oxidation reaction at the anode.
Precipitation Edit Precipitation in hydrometallurgy involves the chemical precipitation of either metals and their compounds or of the contaminants from aqueous solutions.
Precipitation will proceed when, through reagent addition, evaporation , pH change or temperature manipulation, any given species exceeds its limit of solubility. Hydrometallurgical recovery process of rare earth elements from waste: main application of acid leaching with devised diagram.
Hydrometallurgy in Extraction Processes, Volume I
Hydrometallurgy in extraction processes
Hydrometallurgy in Extraction Processes Gupta