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leaching,method of extraction in which a solvent is passed through a mixture to remove some desired substance from it. A simple example is the passage of boiling water through ground coffee to dissolve and carry out the chemicals necessary for producing the beverage. Another example is the removal of sugar from sugar beets using water as the solvent. Leaching is also used to remove metals from their ores. In one procedure certain crushed ores of copper are placed into a series of tanks. As a solvent, such as sulfuric acid, is pumped into the first tank, it dissolves the copper from the ore. Eventually overflowing the first tank, the solution passes into the second, where more copper is dissolved. When this tank overflows, the process is repeated in the third tank and so on. The copper is ultimately removed from the solution by chemical or other treatment.
(lixiviation), the transformation into solution (usually aqueous) of one or several components of a solid by means of an aqueous or organic solvent and often with the participation of gases (oxidation or reduction agents). Examples of leaching are the alkaline extraction of lignin from wood, the solution of sugar from sugar beets and sugarcane in hot water, and the extraction of metals from ores and concentrates. Leaching includes at least two processes: the chemical process, which is the passage of one of the substances into the dissolved state, and the physicochemical process, which is the dissolution in water.
If necessary, solid substances are treated mechanically (crushing or pulverization) and chemically (removal—oxidation or reduction in a pulp, annealing, caking, sulfatization, and so on) before leaching. The purpose of removal is to transform slightly soluble compounds into highly soluble compounds (sulfides into sulfates, higher oxides into lower oxides). Removal is combined with leaching—for example, during oxidation leaching of sulfide ores and concentrates in an autoclave. Typical industrial solvents are water, aqueous solutions of acids (especially sulfuric and hydrochloric acid), bases (ammonia, caustic soda), salts (carbonates of sodium and aluminum), and cyanides.
Leaching is accomplished by agitating fine solid material with a liquid solvent in contact with a gaseous reagent such as air (for the leaching of gold and uranium ores and sulfide concentrates), percolating the liquid reagent through a motionless layer of the solid (the leaching of copper from oxidized ores and of aluminates from caked bauxites).
Periodic or continuous direct or reverse-current leaching usually takes place in vats with mechanical, pneumatic, or pneumatic-mechanical agitation at atmospheric pressure; in vats without agitation (percolators or diffusors); in tubular reactors; or in autoclaves at high pressures and temperatures.
The type of leaching depends upon the chemical properties and concentration of the solvent, the structure and physicochemical properties of the solid, and the solubility of the compounds of the leached substance under the given conditions. The speed of leaching depends on the specific solid-liquid boundary (that is, on the size of the solid particles), the difference in concentration of the solvent and chemical reagents on the surface of the solid and in the volume, the viscosity of the solvent, the value of the diffusion coefficient, the intensity of agitation (the decrease of the diffusion layer and the increase in speed of solution of gaseous reagents), the temperature (increase of the constants of reaction speed and diffusion), the partial pressure of the gaseous reagent (oxygen, sulfur dioxide, and others) above the solution, and the concentration of the dissolved oxidizer—for example, iron sulfate. Most often leaching as a heterogeneous process takes place in the range of diffusion, although mixed diffusion-kinetic or kinetic conditions are possible.
Intensification of leaching is achieved by simultaneous sorption of the leached component on resins (so-called diffusion leaching), the introduction of bacteria (bacterial lixiviation), and the application of high temperatures (up to 300° C) and pressures (up to 5 meganewtons per sq m [MN/m2], or 50 kilograms-force per sq cm [kgf/cm2]), which is called autoclave leaching. Sometimes leaching is accomplished under conditions of a “boiling layer,” using vibration agitation or ultrasonic cavitation.
Leaching is done from piles of poor ore (compact leaching) or directly from the ore (if it is porous or cracked). In order to create a sufficient number of cracks in the ore, it is loosened by explosions using ordinary explosives or atomic charges (underground leaching). In these cases the solutions are supplied to the ore from above; the enriched solutions (which have soaked through the ore) are collected in excavations below and are placed on a device for extracting the metal; and the spent solution, after recovery of the solvent, is returned for repeated use.
The efficiency of leaching is determined by the completeness of extraction of the valuable components; the concentration of extracted components and harmful additives in the final solution; expenditure of materials, electric power, and steam; labor costs; and the speed of the process.
REFERENCESObshchaia khimicheskaia tekhnologiia, vol. 1. Moscow-Leningrad, 1952.
Kasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 7th ed. Moscow, 1960.
Osnovy metallurgti, vol. 1. Moscow, 1961.
N. V. GUDIMA and N. N. RAKOVA
The removal of a soluble fraction, in the form of a solution, from an insoluble, permeable solid with which it is associated. The separation usually involves selective dissolving, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid (as the metal leached from ore) or liquid (as the oil leached from soybeans).
Leaching is closely related to solvent extraction, in which a soluble substance is dissolved from one liquid by a second liquid immiscible with the first. Both leaching and solvent extraction are often called extraction. Because of its variety of applications and its importance to several ancient industries, leaching is known by a number of other names: solid-liquid extraction, lixiviation, percolation, infusion, washing, and decantation-settling. The liquid used to leach away the soluble material (the solute) is termed the solvent. The resulting solution is called the extract or sometimes the miscella.
Leaching processes fall into two principal classes: those in which the leaching is accomplished by percolation (seeping of solvent through a bed of solids), and those in which particulate solids are dispersed into the extracting liquid and subsequently separated from it. In either case, the operation may be a batch process or continuous. See Filtration, Solvent extraction