Concentration of Minerals
Concentration of Minerals
(or ore dressing), the processes for the primary treatment of solid mineral raw materials in order to separate products that are suitable for subsequent technologically and economically feasible chemical or metallurgical processing or use. Concentration includes processes that involve separation of minerals without a change in their chemical composition, structure, or state of aggregation (see Figure 1). These processes are becoming increasingly integrated with hydrometallurgy and chemical processing (combined systems).
In the vast majority of cases, direct extraction of the useful components from natural ores and coals is economically unprofitable and frequently technologically unfeasible. Mineral concentration is important because metallurgical, chemical, and other industrial processes are based on the treatment of products (concentrates) that are enriched in useful components. For example, the content of lead in ore is usually less than 1.5 percent, whereas smelting requires a metal content of 30–70 percent. Ores of rare metals have an even greater concentration gap. For example, the molybdenum content of ores does not exceed several tenths of a percent, whereas metallurgical processing requires a 40–50 percent content, as well as lower levels of undesirable impurities (such as arsenic and copper) than are found in nature.
The results of mineral concentration are two main products: the concentrate and tailings. In some cases (for example, in the concentration of asbestos or anthracite), the concentrates differ from the tailings mainly in the size of mineral particles. If an ore contains several useful components, several concentrates are produced from it. For example, the processing of complex metal ores, containing minerals of lead, zinc, copper, and sulfur, yields lead, zinc, copper, and sulfur concentrates. Production of various grades of concentrates is also possible. In a number of cases, complex concentrates are obtained; examples are copper-gold or nickel-cobalt concentrates, in which the components are separated during metallurgical processing.
In most cases, because of the very intimate conglomeration of minerals in concentrates, the concentrates contain small quantities of impurities, and the tailings contain small quantities of useful minerals. Mineral concentration is characterized by two main indicators: content of the useful component in the concentrate and extraction of the useful component (in percent). As of 1974, up to 92–95 percent of the useful components were being extracted. In this case, the concentration of such components increases by factors of 10–100. For example, 50-percent concentrates may be obtained from molybdenum ores containing 0.1 percent molybdenum.
Concentration of minerals is performed by means of a series of consecutive operations, which constitute the enrichment program. The initial operation is crushing and pulverization of the raw material to reduce it to a particle size that is suitable for existing concentration processes and equipment, as well as for the separation of concretions and formation of particles of individual minerals. Crushing and pulverization are performed in several stages; to avoid unnecessary excess pulverization, final products may be separated between stages. The machines used are crushers, which reduce materials to a particle size of 20–30 mm. Fine pulverization is performed in mills. Products of the required particle size are separated using screens for coarse grains and classifiers for fine grains.
The concentration itself is performed by using various physical and physicochemical properties of minerals.
External differences, such as color and luster, are used for ore-picking by means of automatic equipment. Differences in natural and induced radioactivity of minerals are the basis of radiometric concentration. Density differences are the basis of numerous methods of gravity dressing, which take advantage of differences in the velocity of motion of the particles in water or air under the influence of gravity or centrifugal force. Such methods include jigging, dense-medium washing, concentration by tabling, and concentration in sluices. Differences in the surface physicochemical properties of the minerals to be separated are the basis for flotation methods of concentration. If the minerals have differing magnetic susceptibility, they may be separated magnetically. Electrical separation is used for minerals with differing electrical properties (electrical conductivity, dielectric constant, and ability to acquire a charge by friction).
Ores containing minerals that undergo changes at high temperatures—for example, loss of water of crystallization or CO2, changes in magnetic susceptibility or density, or cracking—may be prepared for subsequent concentration by roasting. In a number of cases, roasting is also used for the removal of undesirable impurities. Differences in the size, shape, brittleness, and coefficient of friction make possible separation according to these characteristics. However, such processes are less effective. Gravity and flotation methods are used most frequently.
All the methods of mineral processing mentioned above are used singly or in combinations. When the minerals contain contaminating impurities (mainly clays), washing is included in the flow sheet. Mineral concentrates obtained by means of wet-cleaning are dehydrated. Coarse-grained products are usually dehydrated on screens and by drainage, followed by drying. Fine-grained products are first thickened and then filtered and dried.
The variety of the types of mineralogical and petrologic characteristics of useful minerals almost completely precludes the possibility of using uniform methods and conditions of mineral concentration. In each case, the most feasible method is determined on the basis of laboratory and pilot-plant studies of wash-ability.
The main trends in the development of mineral concentration are (1) improvement of individual concentration processes and use of combined processes, with the goal of producing the greatest possible improvement in the quality of concentrates; (2) an increase in the productivity of individual plants through intensification of processes and an increase in equipment capacity; (3) integrated use of minerals, with extraction of all valuable components and use of tailings (mainly in the production of building materials); and (4) maximum automation of production.
An important task is reduction of pollution of the environment through recycling of water and wider use of dry methods of concentration. The scale of use of minerals is increasing steadily, whereas the quality of mineral raw materials is deteriorating in a like manner. The mineral content and washability of ores are decreasing, and the ash content of coals is increasing. These factors determine the further increase of the role of mineral concentration in industry.
Mineral concentration has been known since ancient times. The first description of many processes for concentration of minerals (the processes were, of course, primitive) was given by Agricola (1556). The development of mineral concentration in Russia was associated with the extraction of gold from ores. In 1488, Ivan III sought to attract craftsmen who knew how to separate gold ore from barren rock. The first concentration plant for gold extraction was built on the Iset’ River in 1748, and in 1763, M. V. Lomonosov provided descriptions of a number of concentration processes in his work The First Foundations of Metallurgy or Ore Processing. His contemporaries I. I. Polzunov, K. D. Frolov, and V. A. Kulibin built several concentration plants. Before 1917 there were 16 medium-size plants in Russia.
Hundreds of plants for concentrating various ores are in operation in the USSR. Among them are several dozen that process more than 25,000 tons of ore each day. In 1971, about 900 million tons of various ores and 300 million tons of coal were concentrated in the USSR.
The development of the theory and practice of mineral concentration in the USSR is closely associated with the establishment and activity of numerous major research, educational, and design institutes. The first scientific research institute for mechanical ore concentration (Mekhanobr) was founded in Leningrad in 1920. Among the Soviet scientists and engineers who have made great contributions to the advancement of mineral concentration are S. E. Andreev, K. F. Beloglazov, O. S. Bogdanov, V. G. Derkach, M. A. Eigeles, V. A. Glembotskii, V. A. Gus’kov, S. M. Iasiukevich, G. I. Iudenich, L. B. Levenson, P. V. Liashchenko, S. I. Mitrofanov, V. A. Mokrousov, V. Ia. Mostovich, M. T. Ortin, I. N. Plaksin, S. I. Pol’kin, K. A. Razumov, P. A. Rebinder, A. V. Troitskii, V. I. Trushlevich, and I. M. Verkhovskii. Significant research abroad has been conducted by the American scientists A. M. Gaudin and A. F. Taggart and the Australian scientist I. Wark.
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V. I. KLASSEN