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The Importance of Magnesium and Its Alloys in Modern Technology and Methods of Shaping Their Structure and Properties
Published in Leszek A. Dobrzański, George E. Totten, Menachem Bamberger, Magnesium and Its Alloys, 2020
Leszek A. Dobrzański, George E. Totten, Menachem Bamberger
Many large plants producing magnesium in the world currently use electrolytic processes requiring a charge in the form of magnesium chloride and apply electric current to the dissociation of the metal to magnesium and chlorine gas, which can also be traded [33,34]. In the United States, natural brine water and sea water account for about two-thirds of the total magnesium industry compound production values. Magnesium is mainly produced in the Dow process, by electrolysis of fused magnesium chloride from brine and sea water. The Dow Chemical Company, as one of the predecessors of DowDuPont Inc., was set up by Herbert Henry Dow (1866–1930), who used electric current in 1891 to separate bromides from brine, with the method known as the Dow process. For magnesium, electrolysis processes comprise two stages: preparing a raw material containing magnesium chloride and dissociation of this compound to the magnesium and the chlorine gas in electrolytic cells. This process used calcined dolomite and 75% FeSi ground, blended and briquetted into pellets that were charged into closed end, externally heated retorts. The salt solution containing ions of Mg2+ is first treated with lime (calcium oxide) to precipitate magnesium hydroxide. Magnesium hydroxide is then converted to a partial hydrate of magnesium chloride by subjecting hydroxide to the activity of hydrochloric acid, and heating the product. The salt is then subjected to electrolysis in a molten state. Mg2+ ion in a cathode is reduced by two electrons to magnesium. In industrial processes, charges consist of various molten salts containing anhydrous or partially dehydrated magnesium chloride or anhydrous carnallite. In order to avoid impurities present in carnallite ores, dehydrated artificial carnallite is produced by controlled crystallisation from heated solutions containing magnesium and potassium. Partially dehydrated magnesium chloride can be obtained in the Dow process, in which seawater is mixed in a flocculator with lightly burned reactive dolomite. The insoluble magnesium hydroxide is precipitated on the bottom of the settling tank, from where it is pumped as a suspension, filtered, and converted to magnesium chloride, by reaction with hydrochloric acid, and then it is subject to the multi-stage drying to 25% of water. The final dehydration takes place during melting. Anhydrous magnesium chloride is produced by two main methods, by dehydration of magnesium chloride brines or chlorination of magnesium oxide.
Application of X-ray diffractometry and scanning electron microscopy to study the transformation of carnallite and thenardite to schoenite at 25 °C
Published in Chemical Engineering Communications, 2020
Qingyu Hai, Huaide Cheng, Haizhou Ma, Jun Li, Xiwei Qin
Based on the phase diagram for the quinary system, Na+, K+, Mg2+//Cl–, and SO42−-H2O at 25 °C (Jin et al., 1980), a direct path should exist to produce schoenite from carnallite and mirabilite/thenardite, which has rarely been reported. Mirabilite, which is known as a common evaporite from sodium-sulfate-bearing brines, exists widely around saline springs and along saline playa lakes (Hill, 1979). Mirabilite is unstable and dehydrates rapidly in dry air. The prismatic crystals convert to a white powder, thenardite. Carnallite is an evaporate mineral that usually forms in saline marine deposits or sedimentary basins. It is distributed widely as ore or is recovered from brine by saline manufacturers. Carnallite is a disproportionate salt, and water solutions yield KCl crystals and MgCl2 solution (Weedon et al., 2007a, 2007b; Wang et al., 2014b; Cheng et al., 2015). As can be seen that carnallite and mirabilite/thenardite are easily obtained from saline resources, so, a new and feasible path may exist to produce schoenite via carnallite and mirabilite/thenardite. Knowledge of the basic principle of the reaction of mirabilite/thenardite and carnallite may help to develop more effective strategies for producing schoenite through double-decomposition reactions.