RU2488639C1 - Method for silicothermic magnesium production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method for silicothermic magnesium production involves annealing of dolomite, crushing of burned dolomite and ferrosilicon, their mutual crushing and briquetting. Then, magnesium is recovered from briquettes in the reducing furnace at the temperature of 1150-1200°C in vacuum with magnesium steam condensation in solid form in cold zone. Magnesium condensate is molten in the melting furnace under flux. Exit gases of the reducing furnace are supplied to the dolomite burning furnace and to the magnesium condensate melting furnace. At that, speed of supply of the above exit gases of the reducing furnace is controlled based on condition of temperature maintenance of exit gases from the burning furnace, which is equal to 100-150°C, and melting temperature of magnesium condensate in the melting furnace is 700-750°C.

EFFECT: reduction of prime cost of magnesium and provision of an environmentally safe process, reduction of the number of fuel incinerators that are used in a flow diagram.

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Description

The invention relates to the field of primary metallurgy of non-ferrous metals, in particular to a method for silicothermic production of magnesium.

A known method of silicothermal production of magnesium (method 1), which consists in roasting dolomite, crushing the calcined dolomite and ferrosilicon, their joint grinding and briquetting, the restoration of magnesium from briquettes in a retort located in the furnace. The reduction of magnesium occurs at a temperature of 1150-1200 ° C in vacuum with the condensation of magnesium vapor in solid form in the cold zone of the retort. After the recovery process, the magnesium condensate is melted in a submerged arc furnace at a temperature of 700-750 ° C / L.M. Pidgeon, W.A. Alexander Thermal production of Magnesium-Pilot-plant Studies on the Retort Ferrosilicon Process. New York Meeting: Trans. Am. Inst. Min. Mater.Eng. 1944, 159, 315-352 /. The recovery process is carried out in stainless steel retorts, in the recovery furnace their number reaches 50. The vacuum in the retorts is 13-15 Pa. This method is called the "Pigeon Method" by the name of the author Pigeon Lloyd - a professor at the Canadian Research Center. Thermal conditions in the kiln, recovery furnace and smelting furnace can be created by burning solid, liquid or gaseous fuels. Currently, China's plants use natural or coke oven gas. In factories of Japan, unlike the Pigeon method, electric energy was used to heat recovery and smelting furnaces, which led to an increase in the cost of magnesium due to the high price of electricity and, as a result, the shutdown of these plants in 2002.

A known method of silicothermal production of magnesium (method 2), which consists in roasting dolomite, crushing the calcined dolomite and ferrosilicon, their joint grinding and briquetting, the restoration of magnesium from briquettes at a temperature of 1150-1200 ° C in vacuum with the condensation of magnesium vapor in solid form in a cold zone smelting magnesium condensate in a submerged arc furnace at a temperature of 700-750 ° C. / C. Bettanini et al. Soc. Ital. Magnesio, Pat. US 4238223, 4238224, 1980 /. This method is known as the "Bolzano method" in Italy and the "BrazMag method" in Brazil. The physicochemical basis of this method is completely identical to the Pigeon method. Vacuum in the recovery zone is less than 400 Pa. The difference from method 1 is that the recovery process is carried out in special reactors, inside which there is a structure similar to a whatnot, where the briquetted charge is placed, and the thermal regime is created using electric heaters. In the reduction reactor, about 7000 kWh of electricity is consumed for the production of 1 ton of magnesium. The plant in Bolzano (Italy) was shut down in 2000, unable to withstand competition from Chinese magnesium suppliers due to the high cost of electricity and, accordingly, the cost of magnesium.

A known method of silicothermal production of magnesium (method 3), which consists in roasting dolomite, crushing calcined dolomite, ferrosilicon and bauxite (or alumina) and loading them into a reduction furnace, reducing magnesium from a molten charge at a temperature of 1650-1950 ° C in a vacuum of 400-670 Pa with condensation of magnesium vapor in liquid form in a cold zone, refining remelting of magnesium condensate in a submerged furnace at a temperature of 700-750 ° C / C. Faure, J. Marchal, Metals, 16 (1964) 721 /. The recovery furnace is an electric arc furnace with an electric power consumption of about 9000 kWh per 1 ton of magnesium, and bauxite or alumina is added to the mixture to obtain a liquid reaction mass. This method is called the “Magneterm method” and was first introduced in 1963 at the Pechiney Electrometallurgie plant (France), as well as North West Alloys (USA), Japan Metals and Chemicals (Japan) and Magnohrom (Yugoslavia). Despite the fact that method 3, compared to method 2, technologically more advanced plants were stopped in 2000-2002 due to the high energy consumption and, accordingly, the higher cost of magnesium.

In the above sources there is no information about the firing temperature of dolomite. However, from literature data it is known that it varies in a rather wide range of 1100-1250 ° C (Strelets Kh.L., Taits A.Yu., Gulyanitsky B.S. Magnesium metallurgy. M., Metallurgizdat, 1960, p. 45) .

Method 1 is taken as a prototype according to the dominant feature determined by thermal conditions in each of the stages of the technological chain - in kilns of firing, recovery and smelting. In addition, in this method for creating thermal conditions, the same energy source is used as in the claimed one.

The prototype has a significant drawback, which consists in a significant consumption of thermal energy, therefore, natural gas in total and in each of the process chain redistributions - in kilns of firing, reduction and smelting. At the same time, all furnaces have their own individual fuel-burning units - nozzles, fuel lines, air compressors, control and regulation equipment and blowers for creating draft and exhaust gas emission into the atmosphere. They create for each specific redistribution their individual thermal regime. In addition, the operation of technological equipment requires about 500 kWh of electricity. The material balance of the whole technological scheme of silicothermal magnesium production by the prototype method showed that 10.9 tons of dolomite are consumed for the production of 1 ton of magnesium, and when it is fired, 5.8 tons of calcined dolomite are obtained. The heat from burning fuel goes to preheating and dehydrating the initial dolomite (therefore its technical name is raw dolomite) to 200 ° C, to firing dolomite (chemical decomposition of calcium and magnesium carbonates into calcium and magnesium oxides, carbon dioxide), which ends at a temperature 950 ° С and a certain “burn-out” for guaranteed complete firing of dolomite so that there are no traces of carbonate ions in the fired dolomite in order to avoid violation of the vacuum in the reaction zone of the retort. Therefore, a possible increase in firing temperature to 1250 ° C is allowed. For the redistribution of firing dolomite, 8700 kWh of heat or 940 m ^{3 of} natural gas are required. The briquetting mixture consists of 5.8 tons of calcined dolomite and 1.1 tons of ferrosilicon-75 (silicon content 75%). To heat the recovery furnace with 50 retorts and to create a thermal regime in the range of 1150-1200 ° C, 15556 kWh or 1600 m ^{3 of} natural gas are required based on the production of 1 ton of magnesium. Magnesium has a melting point of 650 ° C, an overheat of 50-100 ° C should be provided in the smelting furnace, respectively, the temperature in the furnace under the flux is 700-750 ° C. The technology of smelting magnesium condensate involves pre-loading a salt coating flux, for example, VI-2, into a crucible, melting it, pieces of magnesium condensate must be smelted into a molten flux, after a complete set of the magnesium condensate required by mass and its full melting is carried out, mixing and settling is carried out for 1 , 0-1.5 hours of the entire contents of the crucible. After that, the metal is drunk and poured into ingots or another type of marketable product. In this redistribution for melting 1 ton of magnesium, about 4764 kWh of energy or 490 m of natural gas are required. As can be seen from the description of the prototype method, the temperature of 1150-1200 ° C is maintained in the recovery furnace, and accordingly, the exhaust gases at the outlet of the furnace have the same temperature and are then sent to the chimney. Thus, a total of 29459 kWh of energy or 3030 m ^{3 of} natural gas will be required to produce 1 ton of magnesium.

The aim of the proposed method is to reduce the consumption of thermal energy to create the necessary thermal regime, reduce emissions of exhaust gases, improve the performance of the firing process of dolomite. In addition, the number of fuel-burning units that are used in the technological scheme is reduced by three times. A significant novelty of the invention lies in the fact that the exhaust gases from the reduction furnace are sent to the dolomite kiln and to the magnesium condensate melting furnace, and their feed rate is controlled so that the exhaust gases from the kiln have a temperature of 100-150 ° C and the temperature of the magnesium condensate in the smelting furnace was 700-750 ° C. At the same time, air dilution of the exhaust gases is not required, and dolomite overheating will only lead to a certain “burnout” required by the technology of preparing the calcined dolomite for vacuum reduction by ferrosilicon. The kiln is a shaft furnace with a backflow of calcined dolomite and hot gases. Lowering the temperature of the exhaust gases of the kiln below 100 ° C will lead to condensation of water vapor due to the natural humidity of the raw dolomite in the upper part of the shaft furnace, which will negatively affect the quality of the calcined dolomite, and dolomite may be burned and the presence of carbonate ions in the finished intermediate. Exceeding the temperature of the exhaust gases of the kiln more than 150 ° C will cause an increased consumption of hot gases and, accordingly, thermal energy for the redistribution of firing dolomite. The forced supply of hot gases from the reduction furnace to the kiln and, correspondingly, the forced removal of exhaust gases from the kiln will positively affect the kinetics of the decomposition of dolomite - double calcium and magnesium carbonate, which is explained by the Le Chatelier principle, according to which, to shift the reaction to the side decomposition of dolomite requires the removal of carbon dioxide from the decomposition reaction zone. Maintaining the temperature of the molten magnesium condensate in the melting furnace in the range of 700-750 ° C is explained by the fact that lowering the temperature below 700 ° C will increase the melting time and, accordingly, heat and labor costs. An increase in the melt temperature of more than 750 ° C is associated with possible losses of magnesium due to an increase in the partial pressure of magnesium vapor, with an additional consumption of thermal energy and worsening working conditions near the smelting furnace for production personnel. Reducing energy costs reduces the cost of raw magnesium and increases the competitiveness of the resulting commercial products. In addition, reducing the amount of flue gases, which are greenhouse, improves the environmental situation.

Example

Calcined dolomite with the addition of ferrosilicon as a reducing agent was used as a raw material for the magnesium production process. The exhaust gases of the recovery furnace at the exit from it had a temperature of 1150-1200 ° C, some of them (60-65% by volume) were sent to the dolomite kiln. The feed rate of hot gases from the reduction furnace was controlled so that the exhaust gases leaving the calcination furnace had a temperature of 100-150 ° C. The gas temperature at the inlet to the firing furnace was 1100-1150 ° C and no carbonate ions were found in the analysis of the products of firing, therefore, the thermal dissociation of dolomite went completely. According to the results of the experiment, the total consumption of natural gas decreased by 30-32% compared with the prototype.

The exhaust gases of the reduction furnace (28-32% by volume) were sent to the smelting furnace to heat the magnesium condensate melt to a temperature of 690-740 ° C. As a result, there was a decrease in the total consumption of natural gas by 15-17% compared with the prototype. As a result, the total reduction in the consumption of natural gas to create the necessary thermal regime in two stages - during the firing of dolomite and the smelting of magnesium condensate, amounted to 46-48%. The calculation results of the summary heat balance of the prototype and the present invention are presented in table 1.

Table 1 Consolidated heat balance of the prototype and the invention Name of indicators and redistributions Way prototype proposed 1. The consumption of natural gas in the redistribution, m ³ burning 940 0 recovery 1600 1600 melting 490 0 The total consumption of redistribution, m ³ 3030 1600 2. Energy intensity of production, kW · h burning 9139 0 recovery 15556 15556 melting 4764 0 Total consumption for redistribution, kWh 29459 15556 3. The volume of gases released into the atmosphere, m ³ burning 10340 10340 recovery 17600 1870 melting 5390 5390 The total amount of exhaust gas in the redistribution, m ³ 33330 17600

Claims (1)

Method for silicothermic production of magnesium, including roasting of dolomite, crushing of calcined dolomite and ferrosilicon, their joint grinding and briquetting, reduction of magnesium from briquettes in a reduction furnace at a temperature of 1150-1200 ° C in vacuum with condensation of magnesium vapor in solid form in a cold zone, melting of magnesium condensate in a submerged arc melting furnace, characterized in that the exhaust gases of the reduction furnace are sent to a dolomite roasting furnace and to a magnesium condensate smelting furnace, and their feed rate is controlled from the condition dderzhaniya exhaust gases from the kiln temperature equal to 100-150 ° C and the melt temperature of the condensate in the magnesium melting furnace - 700-750 ° C.