The extraction and recovery of metals from their ores by processes in which aqueous solutions play a predominant role. Two distinct processes are involved in hydrometallurgy: putting the metal values in the ore into solution via the operation known as leaching; and recovering the metal values from solution, usually after a suitable solution purification or concentration step, or both. The scope of hydrometallurgy is quite broad and extends beyond the processing of ores to the treatment of metal concentrates, metal scrap and revert materials, and intermediate products in metallurgical processes. Hydrometallurgy enters into the production of practically all nonferrous metals and of metalloids, such as selenium and tellurium.
A generalized metallurgical flow sheet (see illustration) provides an indication of the nature and extent of the role of hydrometallurgy in metal production. It also shows the manner in which hydrometallurgical and pyrometallurgical processes complement each other. See also: Pyrometallurgy
The advantages of hydrometallurgy include applicability to low-grade ores (copper, uranium, gold, silver) and complex sulfide ores, amenability to the treatment of materials of quite different compositions and concentrations, adaptability to separation of highly similar materials (hafnium from zirconium), and flexibility in terms of the scale of operations. Hydrometallurgical operations are amenable to effective control leading to automation and continuous operation.
There are some important disadvantages in hydrometallurgical processes. The processes are generally energy-intensive and can involve the handling of large volumes of dilute solutions that may be corrosive or hazardous. In addition, the processes yield residues and effluents that must be disposed of in an environmentally acceptable manner.
The first commercial hydrometallurgical operation was the application of cyanidation to the treatment of gold ores in 1889, more than 40 years after the discovery that gold can be dissolved in dilute alkaline solutions of sodium cyanide, as shown in reaction (1).
A major unit process in hydrometallurgy is the leaching of ores and concentrates. A particularly noteworthy and time-honored dissolution process is the caustic pressure leaching of bauxite ore to produce pure alumina. This process, which was developed in the nineteenth century, is still in use as the principal method of producing alumina. The ever-increasing demand for aluminum and the rising cost of bauxite stimulated research on the development of processes for leaching nonbauxite alumina materials such as a alunite, anorthosite, clays, and coal shales. See also: Aluminum; Leaching
In the case of sulfide concentrates, leaching in acid or ammoniacal solution under oxygen pressure provides a number of advantages, including accelerated leaching and the recovery of sulfur in a suitable form, such as ammonium sulfate in the ammoniacal leaching of nickel concentrates in the Sherritt-Gordon process or as elemental sulfur in the acid pressure leaching of zinc concentrates.
The economic recovery of metals from low-grade ores became increasingly important as mining costs rose and as higher-grade materials became more difficult to find. In many operations, what was at one time waste is now considered as ore to be treated by never leaching methods such as heap, dump, and in situ leaching. In the United States, copper in excess of 160,000 tons (145,000 metric tons) is extracted annually by such processes. The same technology is applied in the leaching of uranium ores and in the treatment of gold and silver ores.
Biotechnology has come to play a more significant role in mineral leaching and metal separation processes. Of particular importance is the microbiological leaching of low-grade sulfide copper ores and of uranium ores in the presence of pyrite utilizing the action of the bacterium Thiobacillus ferrooxidans. The same organism provides an attractive option in the treatment of refractory gold ores. The bioleaching of minerals such as pyrite and arsenopyrite, in which gold is entrapped, facilitates the subsequent recovery of the gold by cyanide leaching. See also: Bioleaching
The establishment of more stringent regulations in many countries concerning sulfur dioxide emissions from smelters has led not only to the adoption of new and improved smelting processes but also to a great deal of research on hydrometallurgical alternatives to smelting, especially of copper concentrates. In the case of copper concentrates, the principal difficulty has been the realization of a hydrometallurgical process that has sufficiently well-defined economic advantages. From an environmental standpoint, there are definite advantages which are afforded by hydrometallurgy, principally the elimination of sulfur dioxide emissions and the conversion of sulfur to elemental sulfur or a sulfate. The overall economics of copper hydrometallurgy can be improved by the recovery of copper in final form by a modified electrowinning process or by an alternative process such as the hydrogen reduction of cuprous chloride.
Environmental and other considerations led to the cessation of the production of zinc by the carbothermic reduction of zinc calcine at 2200–2400°F (1200–1300°C) in horizontal retorts by reaction (2),
where (s) and (g) denote solid and gas. This process has been replaced by the sulfuric acid leaching of zinc calcine, purification of the resulting leach solution, and the recovery of zinc by electrowinning. An important alternative to the production of zinc calcine by fluid-bed roasting is the acid pressure leaching of zinc concentrates, in which the sulfide is oxidized to yield elemental sulfur.
The recovery of metals from leach solutions is often preceded by the purification or concentration of the metal to be isolated. Two processes that can be used to achieve this objective are ion exchange and solvent extraction.
Ion exchange involves the selective adsorption of metal ions onto a synthetic organic ion-exchange resin (commonly a styrene-divinylbenzene copolymer containing suitable functional groups for ion exchange). The process is stoichiometric and reversible, so that the adsorbed ions are subsequently desorbed or eluted from the resin by a solution known as the eluant. The resulting solution or eluate is treated for metal recovery or for production of a metal compound, such as ammonium diuranate, (NH4)2U2O7, in uranium extraction. Ion exchange, which is widely used in water purification, is employed extensively in uranium recovery. Here continuous ion-exchange processes are in commercial use, with resultant increases in efficiency and productivity. Chelating resins should find increasing use in the separation of metals such as copper, nickel, cobalt, and zinc. See also: Chelation; Ion exchange
As applied in hydrometallurgy, solvent extraction relates to the selective transfer of metal species from an aqueous solution to an immiscible organic solvent with which it is in contact. The solvent itself can be the active reagent in the process, as in the extraction of uranium by tributylphosphate. On the other hand, the solvent can contain an organic extractant which forms a complex or chelation compound with the metal ion from the aqueous phase. The extraction process is reversible, so that the extracted metal can be stripped from the organic solution and returned to the aqueous phase.
With the development of suitable chelating compounds, solvent extraction became established as an excellent technique for the upgrading of copper leach solutions. The application of solvent extraction is especially suited to the treatment of the dilute solutions resulting from the heap and dump leaching of low-grade copper ores. The selectivity of the organic reagents, notably the hydroxyoximes, is such as to permit an excellent separation from iron. The stripping of the organic phase by spent electrowinning electrolyte leads to a strong solution, from which copper can be electrowon without difficulty.
Solvent extraction is used commercially in the separation of many metals, including copper, nickel, cobalt, zinc, uranium, thorium, zirconium, hafnium, molybdenum, tungsten, niobium, tantalum, and beryllium. See also: Solvent extraction
Recovery from aqueous solution
Metal recovery from aqueous solution can be effected by a number of reduction processes, the most prominent being electrowinning and gaseous reduction using hydrogen. The latter process yields metal powder, whereas the former results in cathodes which are melted and cast to produce various marketable shapes.
Hydrometallurgy occupies an important role in the production of aluminum, copper, nickel, cobalt, zinc, gold, silver, platinum metals, selenium, tellurium, tungsten, molybdenum, uranium, zirconium, and other metals. Considering the versatility of hydrometallurgy and the need to process more complex ores, as well as lower-grade ores and secondary materials, and to produce high-purity advanced materials, hydrometallurgical processes are expected to play an even greater part in the production of metals and materials in the future. See also: Electrometallurgy; Metallurgy