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Related to Hydrometallurgy: electrometallurgy
the extraction of metals from ores, concentrates, and various waste products by means of aqueous solutions of chemical reagents with the subsequent separation out of the metals from these solutions.
In 1763, M. V. Lomonosov indicated the possibility of using hydrometallurgical processes for extracting metals from ores. An important contribution to the development of hydrometallurgy was made by the Russian scientist P. R. Bagration, who formulated the theory of the cyanidation of gold (1843). At the beginning of the 20th century the hydrometallurgy of copper attained industrial importance. Later, hydrometallurgical methods were developed to extract many other metals.
Hydrometallurgy includes a number of basic technological operations that are carried out in a fixed sequence; these include the mechanical processing of the ore, alteration of the chemical composition of the ore, leaching, separation of the metal-containing solution, preparation of the solutions, precipitation of the metals, and reprocessing (secondary treatment).
The mechanical processing of the ore consists of crushing and grinding it in order to completely or partially expose the grains of the minerals containing the metal to be extracted. Alteration of the chemical composition of the ore or concentrate in order to prepare it for leaching includes chlorination, oxidizing, and sulfation or for reduction roasting and sintering. The objective is to break down the chemical compounds of the metal to be extracted and to reduce them to a soluble form. Leaching is the transfer to an aqueous solution of the metal to be extracted. This operation is sometimes carried out along with the process of wet grinding (in mills and classifiers) or in a special apparatus (leaching vessels and autoclaves). Separation of the metal-containing solution from the ground material is achieved by water removal and washing in thickeners and by filters. The solutions for separating out the compounds or metals are prepared by isolating the suspended particles (purification) or by chemically precipitating the accompanying metals and admixtures. The metals or their compounds are precipitated from the solutions by electrolysis (copper, zinc, and other metals), by cementation, which is reduction by more electronegative metals (such as copper, silver, or gold), by sorption, using ion-exchange resins or carbon, and by liquid extraction of the metal compounds using organic solvents, with the subsequent reextraction into the aqueous solution and the precipitation from it of the pure metal or its chemical compound. Reprocessing of the precipitate, which is done to purify the isolated compound or crude metal further or else to obtain a commercially suitable metal directly, may be carried out in the following ways: recrystallization, volatilization, calcination, smelting, and electrolysis of the aqueous or molten media.
In the chemical interaction of the metal with the solvent the neutral atom of the metal passes into an ionic state, forming a dissolved compound. Such a process of dissolution proceeds easily during the leaching of ores or concentrates in which the metal is present in an oxidized (ionic) form. Examples are oxidized copper and uranium ores, roasted zinc concentrates, and the products of chlorinating roasting. In certain cases, extracting metal by a solvent involves a preliminary oxidation or treatment by another oxidizer (for example, in the soda leaching of ores containing 4-valence uranium, for reducing the latter to 6-valence). In the dissolution of metals (whether original or reduced), it is necessary to oxidize them in order to convert them to their ionic states. The oxidation of a metal together with the simultaneous ionization of the oxidizing agent (for example, molecular oxygen dissolved in water) is possible in the case of the more noble metals only with a loss of energy, which for example may be obtained during the formation of a complex ion (the cyanide processing of gold and silver, the ammoniacal leaching of metallic copper and nickel).
The dissolution of minerals with various types of chemical bonding in a crystal lattice (covalent, metallic, ionic) is characteristic of the leaching of sulfides, arsenides, selenides, and tellurides. The dissolution of these minerals, if a preparatory oxidizing roast is not carried out, in most cases also requires oxidizing in the pulp, for example, in the ammoniacal leaching of copper-nickel sulfide ores in an autoclave under the pressure of oxygen or air. A transfer of the solvent and the elimination of the products of the reaction occurs in the volume of the solution by convection (turbulent diffusion), whereas in the layer contiguous with the mineral it is by molecular (heat) diffusion. Usually the reaction that occurs during hydrometallurgical extraction takes place in the diffusion area; a determining factor is the diffusion velocity of a substance. This limits the course of the reaction. An increase in the speed of the mineral’s dissolution occurs when its relative surface is enlarged (that is, the degree of grinding), as well as when the mixing process is speeded up and when the temperature is increased.
The form of the surface and the particle size of the mineral being dissolved determine the functional dependence of the quantity of the dissolved metal on the time of its contact with the solution; therefore, they affect the degree of grinding and the capacity of the apparatus used for leaching.
The following are the solvents most employed for leaching of compounds: sulfuric acid (vanadium, copper, zinc), soda (vanadium in carbonate ores, molybdenum, and tungsten), sodium hydroxide (aluminum oxide and tungsten), ammonia (copper and nickel), cyanide salts (gold and silver), sodium sulfide (antimony and mercury), chlorine solutions and chlorides (the noble metals, lead, and the rare metals), and thiosulfates (gold and silver).
Various compounds are used for liquid extraction (for example, a solution of tributyl phosphate and di-2-ethyl hexyl phosphate in kerosine). After this extraction process the purified metal compound is extracted from the organic solvent by means of an aqueous solution, frequently with an addition of an acid or some other reagent. Metals are precipitated from the solution by a carburizing, or carbon, method or by hydrogen under pressure. Also used for this purpose are anionic exchangers (anion-exchange resins) or cationic exchangers. After sorption the metal compound is removed by a solvent from the ionic exchanger, and the latter is subjected to regeneration.
In large-scale hydrometallurgical production (for example, in leaching copper from oxidized ores in lumps) the process is sometimes carried out by means of washing piles of ore with weak solutions of sulfuric acid. The copper-containing solutions are drained into collecting tanks and then into cementation units. For crushed and granular-sized graded ores (for example, gold ores), a method is used in which the solution in vessels is percolated through a layer of well-filtered charge. In order to intensify this process, a preliminary saturation with air is sometimes carried out, and a vacuum is created under the filtering bottom layer. For leaching finely ground material, vessels are used for mixing the pulp (by mechanical or pneumatic means or a combination of the two); for continuous leaching they are usually combined sequentially.
Sometimes combined schemes of leaching are possible: for granular sized material by percolation; and for separated fine material (slime) by slurrying. In specific instances other types of leaching apparatus may be set up, for example, in autoclaves operating on a continuous basis or in periodic cycles. Leaching of acidic solutions is carried out in rubber-lined steel, ceramic, or other acid-resistant apparatus; for alkaline solutions steel and sometimes wooden apparatus is suitable. The methods of liquid extraction either supplement leaching or are used for the direct extraction of metal compounds from ores. The extraction is accomplished according to the coun-tercurrent principle in extraction columns (the extract and the waste solution are continuously being drawn off in different directions).
Dewatering and washing are carried out in thickeners (of the paddle type with central and peripheral drive mechanisms, as well as multistage types) and in filters (vacuum filters and filter presses, operating continuously or in periodic cycles). Precipitation from the solutions is accomplished in apparatus with designs that depend on the precipitating agent used. Reactors and filters are used for chemical (soluble) precipitating agents. Powder-form precipitating agents (zinc, aluminum dust) are introduced into the mixing tanks with the solution; the precipitation may then continue within a reciprocating pump, along a main pipeline and through a layer of the precipitating agent in a filter.
A metal or its compounds can be precipitated in the pulp itself (for example, by immersing mesh baskets with ionic exchangers into the pulp). Powder-type precipitating agents, after contact with the solution, can be separated out by flotation. Precipitation by lump-form precipitating agents (iron for copper, zinc shavings or carbon for gold) is carried out in troughs or containers equipped with partitions for the zig-zag motion of the solution above and below through the layer of the precipitating agent. It is possible to separate out alloys (for example, those of iron) by means of hydrolysis from a purified solution with the subsequent obtaining of the basic metal (for example, zinc) by precipitating on a cathode, using electrolysis with nonsoluble anodes.
REFERENCESOsnovy metallurgii, vols. 1-5. Moscow, 1961-68.
Avtoklavnye protsessy v tsvetnoi metallurgii. Moscow, 1969.
Burkin, A. R. The Chemistry of Hydrometallurgical Processes. London, 1966.
Habashi, F. Principles of Extractive Metallurgy, vols. 1-2. NewYork-London-Paris, 1969-70.
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. Hydrometallurgical and pyrometallurgical processes complement each other. See Leaching, Pyrometallurgy
Hydrometallurgy occupies an important role in the production of aluminum, copper, nickel, cobalt, zinc, gold, silver, platinum, selenium, tellurium, tungsten, molybdenum, uranium, zirconium, and other metals. See Metallurgy