Hydrometallurgy

Hydrometallurgy extraction is a technique in the field of metallurgy , which obtains metals from their ores. Hydrometallurgy involves the use of aqueous solutions for the recovery of metals from ores, concentrates, and recycled or residual materials . [1] [2] Processing techniques that complement hydrometallurgy are pyrometallurgy , vapor metallurgy, and molten salt electrometallurgy. Hydrometallurgy is generally divided into three general areas:

• leaching
• Solution Concentration and Purification
• metal or metal compound recovery

leaching

Leaching involves the use of an aqueous solution to remove metal from a metal-bearing material that has been exposed to a material containing a valuable metal. [3] The first examples come from 17th-century Germany and Spain where it was applied to the extraction of copper. [4]

Lixiviant solution conditions vary in terms of pH, oxidizing reducing capacity, presence of chelating agent and temperature, to optimize the rate, expansion and dissolution selectivity of the desired metal component in the aqueous phase. Through the use of chelating agents , some metals can be selectively extracted. Such chelating agents are usually amines of Schiff bases.

The five basic leaching reactor configurations are in-situ, heap, vat, tank and autoclave.

In-situ leaching

In-situ leaching is also called “solution mining”. The process initially involves drilling holes in ore deposits. Explosive or hydraulic fracturing is used to create open paths within the deposit for the solution to enter . The leaching solution is pumped into the deposit where it makes contact with the ore. Then the solution is collected and processed. The Beverly uranium deposit is an example of in-situ leaching and also a Trojan mine in Zimbabwe.

Heap leaching

In pile leaching processes, crushed (and sometimes piled) ore is piled into a pile that is lined with an impermeable layer. Leach solution is sprinkled over the pile, and allowed to seep down through the pile. Pile designs typically include collection sumps, which allow “impregnated” leach solutions (i.e. solutions with dissolved valuable metals) to be pumped out for further processing. An example is gold cyanidation , where pulverized ores are precipitated with a solution of sodium cyanide , which, in the presence of air, dissolves gold, leaving behind a non-precious residue.

VAT leaching

Vat leaching involves contact material, which usually undergoes size reduction and classification with a leech solution in larger vats.

Tank leaching

Agitated tanks, also called agitation leaching, consist of contact materials, which typically undergo size reduction and classification, with the leach solution in agitated tanks. The agitation can enhance the reaction kinetics by increasing the mass transfer. Tanks are often configured as reactors in series.

Autoclave leaching

Autoclave reactors are used for reactions at high temperatures, which can increase the rate of the reaction. Similarly, use in autoclaved systems enables gaseous reagents.

Solution Concentration and Purification

After leaching, the leach alcohol should normally undergo a concentration of metal ions that are to be recovered. Additionally, unwanted metal ions sometimes need to be removed. 

• Precipitation is the selective removal of a compound of a target metal or a major impurity by precipitation of one of its compounds. Copper is precipitated in its sulfide form as a means of purifying nickel leachates .
• Cementation is the conversion of a metal ion to a metal by a redox reaction . A typical application involves the addition of scrap iron to a solution of copper ions. The iron dissolves and the copper metal is deposited.
• solvent extraction
• ion exchange
• ran out of gas. Nickel metal is powdered by treating a solution of nickel and ammonia with hydrogen.
• Electrowinning is particularly selective if expensive electrolysis processes are applied for the separation of precious metals. Gold can be electroplated from its solution.

Solvent extraction

In solvent extraction there is a mixture of a diluent in extraction one used to extract a metal from one phase to another. In solvent extraction this mixture is often called “organic” because the main ingredient (diluent) is some kind of oil.

PLS (Impregnated Leach Solution) is mixed with stripped organic in emulsification and allowed to separate. [ citation needed ] The metal will be converted from PLS to the organic they modify. [ clarification needed ] The resulting streams shall be one filled organic and one raffinate . When working with electrowinning, the loaded organic is then mixed in emulsification with a lean electrolyte and allowed to separate. The metal will be exchanged from the organic to the electrolyte. The resulting currents would be a stripped-down organic and a rich electrolyte. The organic stream is recycled through a solvent extraction process. Whereas aqueous currents cycle through the processes of leaching and electrowetting respectively.

Ion exchange

Chelating agents, natural zeolites , activated carbons, resins, and liquid organics impregnated with chelating agents are all used to exchange cations or anions with solutions. Selectivity and recovery are a function of the reagents used and the contaminants present.

Metal recovery

Metal recovery is the final step in the hydrometallurgical process. Metals suitable for sale as raw materials are often produced directly in the metal recovery stage. Sometimes, however, further refining is required if very 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 readily obtained by electrolysis. Cu ²⁺ ions reduce at mild potentials, leaving behind other contaminating metals such as Fe ²⁺ and Zn ^{2+ .}

Electrolysis

Electrowinning and electrorefining respectively involve the recovery and purification of metals using electrodeposition of metals at the cathode , and either metal dissolution or a competitive oxidation reaction at the anode .

Swiftness

Precipitation in hydrometallurgy involves the chemical precipitation of metals and their compounds or contaminants from aqueous solutions. Precipitation will occur when , through reagent addition, evaporation , pH change or temperature manipulation, any species exceeds the solubility limit.