The aim of the work is determination of production temperature and salt regimes for an industrial threevessel vacuum evaporation unit for concentrating magnesium chloride solution, included within technology for artificial carnallite preparation. A scheme is provided, and a unit developed and manufactured for this purpose is described. Experimental data are provided that were obtained during magnesium chloride solution concentration within a single-tube evaporation unit with a descending film (unit) under different regimes. A distribution device is selected for provision of stable film flow over the inner surface of a heatexchange tube. The possibility is confirmed by experiment for the first time of using equipment with a descending film for concentrating actual magnesium chloride solution. Data are obtained for the limiting salt content within the concentrated solution providing operation of the unit in a crystallization-free regime.The Russkii Magnii company entered into construction of a plant for preparation of magnesium metal. One of the semi-finished products of magnesium production is synthetic carnallite. For the production process of magnesium chloride solution, obtained during salt-acid leaching of serpentinite, before supply for synthesis of artificial carnallite it is concentrated in a three-vessel vacuum-evaporation unit (VEU) fitted with evaporation equipment with a descending film.Magnesium chloride solution is a complex multicomponent system of the following chemical composition, wt.%: MgCl 2 -20.06; CaCl 2 -0.42; NaCl -1.2; KCl -1.39. The composition of concentrated solution (in wt.%) is: MgCl 2 -34.5; CaCl 2 -0.72; NaCl -2.08; KCl -2.4. During concentration by a factor of 1.72-1.73, salt crystals do not separate from the solution.Experience of industrial operation of a VEU shows that under actual production conditions quite broad deviations of compositions from regulated values are possible for the solutions being processed, which leads to over-evaporation of solutions and separation of crystals on the heat-transfer surfaces of heating pipes and other internal surfaces of the evaporation equipment in the form of solid incrustations. In addition, presence within a solution of calcium and silicon compounds under
Preparation of magnesium from multimillion ton asbestos production dumps, i.e., serpentinite, provides its competitiveness in the world and internal markets due to the low raw material costs, whose price in the net cost of finished metal reaches 30%. Artificial carnallite for magnesium electrolysis is separated by crystallization in multistage vacuum-crystallization units from a solution formed as a result of synthesizing magnesium chloride solution and added potassium chloride. Chromium-magnesium solution forms on leaching serpentinite with hydrochloric acid and separation of impurities, but potassium chloride from the electrolysis bath after dissolution and crystallization is returned again to the technology. Crystallization units for carnallite and potassium chloride are fitted with self-evaporation crystallizers of SverdNIIkhimmash construction, which are used successfully in the production of many inorganic salts.Synthetic carnallite KCl·MgCl 2 ·6H 2 O in magnesium production is separated by crystallization from a hot carnallite suspension, prepared by synthesis [1] of potassium chloride and chromium-magnesium solution. The chromium-magnesium solution forms on leaching serpentinite with hydrochloric acid, and potassium chloride is prepared by crystallization from a solution of exhausted electrolyte. A multi-vessel vacuum-crystallization unit (VCU) is used for crystallizing potassium chloride and carnallite, fitted with double-loop crystallizers [2,3]. The construction of the crystallizers has been developed in Sverdlovskii Research and Design Institute of Chemical Machine Building (SverdNIIkhimmash) and is recommended in the production of various mineral salts, including carnallite, potassium chloride, and cooking salt.The potassium chloride and carnallite VCU should provide productivity, chemical and grain size composition of crystals separated from a multicomponent solution.As a result of self-evaporation of hot solution under vacuum there is cooling and crystallization from a salt solution in accordance with their solubility in the system KCl-NaCl-MgCl 2 -CaCl 2 -H 2 O.The solid phase separated during crystallization of exhausted electrolyte solution contains 75.8 wt.% KCl and 24.2 wt.% NaCl. The crystalline product of the carnallite VCU contains 91.2 wt.% KCl·MgCl 2 ·6H 2 O, 4.5 wt.% KCl, and 4.3 wt.% NaCl.
The article is devoted to choice of structural material for equipping a projected vacuum crystallization unit (VCU) for salt crystallization with intense solution concentration. Results of studies show that the low corrosion resistance of steel 12Kh18N10T, 10Kh17N13M3T, 08Kh22N6T, 08Kh21N6M2T, and alloy 06KhN28MDT specimens in vapor phases and solutions under operating conditions for the projected VCU do not permit their application for manufacturing equipment and pipelines operating in contact with weakly acid saturated potassium chloride solutions and vapor phases of these solutions boiling under vacuum. Under the operating conditions for the projected VCU titanium is the sole metallic material that has in a broad metallurgical range high production and mechanical properties, and corrosion resistance, which makes it possible to provide a standard service life both for individual assemblies and a VCU as a whole. Successful experience of using titanium as the main structural material for evaporation unit equipment in the Slavyansk and Mozyr Salt Extraction Combines, Uralkalii, the Ufalei Nickel Combine, and a number of other producers for over 35 years demonstrates the high reliability of using titanium for manufacturing equipment of a projected VCU, processing corrosive weakly-acid saturated potassium, manganese, and sodium chloride solutions, containing a solid phase in the form of salt crystals. In order to manufacture vessels for a projected VCU, operating at atmospheric pressure, it is possible to use nonmetallic materials or carbon steel with coating. The greatest chemical resistance, according to results of research in saturated chloride solutions with a solid phase as applied to equipment and pipeline operating conditions for a projected VCU, applies to a Dion 9400 coating produced by the companies Reichold and Asplit Dupont.Under conditions of an enhanced crystallization regime for saturated chloride solutions steel is subject to intense uniform corrosion with a corrosion penetration rate of 10-15 mm/yr, which makes its use impossible. Brass and bronze are subject to intense corrosive wear by salt crystals. Bronze and brass corrosion is increased in a weakly-acid medium, and these materials develop a tendency towards dezincing.
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