Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1990-01-09
1992-02-25
Andrews, Melvyn J.
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 1033, 75597, C22B 400
Patent
active
050909967
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to magnesium production.
Magnesium is produced industrially by both electrolytic and pyrometallurgical techniques with the former accounting for the bulk of magnesium production. So far as the pyrometallurgical techniques are concerned these may be subdivided into carbothermic and metallothermic reduction techniques. The metallothermic technique, with which the present invention is concerned, involves the reduction of MgO by a metal (which term is used herein to include silicon). For economic reasons, the reducing metal is usually silicon (provided in the form of ferrosilicon) although it is possible to use aluminium, calcium or their alloys as reducing metal.
The Magnetherm process involving the silicothermic reduction of MgO accounts for about 20% of current world magnesium production, the other 80% being produced by electrolytic techniques. More specifically, the Magnetherm process involves the silicothermic reduction of MgO in the form of calcined dolomite (dolomite MgCO.sub.3 CaCO.sub.3) from a molten slag bath according to the overall equation. O.sub.3 +2Mg+xFe
The process does however suffer from a number of disadvantages, as set out in the following description.
The reaction is promoted by the low silica activity in the resultant slag and by operation under a vacuum of 0.05 atm. The slag composition is held at or close to 55% CaO, 25% SiO.sub.2, 14% Al.sub.2 O.sub.3 and 6% MgO (all % by weight) and reaction takes place at 1550.degree. C.
Careful control of slag composition is essential. At the operating temperature of 1550.degree. C. the Magnetherm slag system is not fully molten and contains 40% solids as dicalcium silicate (2CaO.SiO.sub.2), (Christini, R.A. "Equilibria Among Metal, Slag, and Gas Phases in the Magnetherm Process" Light Metals, New York, 1980, pp 981-995.) Successful operation of the process relies on the fact that the remaining fully liquid component has a composition situated on the boundary of the dicalcium silicate and periclase (MgO) phase fields of the quaternary CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -MgO system. Hence the liquid component is saturated with respect to MgO 9.e. it has a thermodynamic activity of MgO which is or is close to unity.
A primary objective of the process is therefore the maintenance of a near constant slag composition. The use of dolomite (containing Ca0) enables the CaO:SiO.sub.2 ratio of the slag to be kept close to 2 as SiO.sub.2 is generated from the reduction reaction. Regular additions of Al.sub.2 O.sub.3 are also required to keep the composition of the liquid slag component on the periclase phase boundary. Published data (Faure, C and Marchal, J "Magnesium By the Magnetherm Process" Journal of Metals, Sept. 1964, pp 721-723), suggest that Ferrosilicon and bauxite are added in roughly equal amounts by weight.
At present the process is conducted in an ac arc furnace with an upper (water cooled) copper electrode. The second electrode is formed by the carbon hearth of the furnace. Heat is generated within the molten slag and has to be transferred to the slag surface (at which the reduction occurs, by convection. At the surface the energy is consumed by the endothermic reduction reaction and in heating the raw materials (including slag additives) to the reaction temperature.
Initially the ferrosilicon droplets will be supported at the slag surface by the combined forces exerted by gas (Mg) evolution, convection within the slag bath and interfacial tension. However as Si is consumed the density difference between slag and FeSi will begin to predominate and as the metal sinks through the slag the continued reaction between FeSi and dissolved MgO becomes thermodynamically less favourable due to the increased pressure exerted by the slag.
The overall reaction can be represented by
The free energy change for this reaction must be negative for reaction to proceed in the desired direction, and is given by ##EQU1## for the fixed and controlled composition of the Magnetherm slag system
The process is operated at 0.05 atms. hence it m
REFERENCES:
patent: 2380449 (1945-07-01), Kirk
patent: 3681053 (1972-08-01), Avery
patent: 4033758 (1977-07-01), Johnston et al.
patent: 4572736 (1986-02-01), Warren et al.
patent: 4699653 (1987-10-01), Barcza et al.
Andrews Melvyn J.
University of Manchester Institute of Science and Technology
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