Abstract
Calcium silicide hot reduction method has a great application value in the production of magnesium from the dolomite. In this paper, a large-scale semi-continuous magnesiothermic calcium silicide hot reduction method for producing high-proficiency magnesium was studied. A furnace bench with an internal size of 400 mm×500mm×500mm was adopted as the reducing furnace. The metallothermic reaction between magnesium and calcium silicide (CaSi2) was studied and the reaction process parameters were optimized. The results showed that the high purity and high efficiency magnesium could be obtained and the magnesium liquid temperature was 925.5℃. The reduction time was 27.7min, the final yield of magnesium was 97.55%, and the hydrogen generation rate and peroxide percentage were both lower than 2%.
1. Introduction
Magnesium, with a density of 1.74 g/cm3 and a high strength-to-weight ratio, is one of the important materials used in modern industry. Due to its excellent characteristics, such as high tensile strength, excellent corrosion resistance and so on, it is widely used in aerospace, automobiles, and electronic industries as an alloying element to form metal mixtures with aluminum, zinc, and other metals [1]. Magnesium metal is mostly produced from the reduction of magnesium oxide (dolomite) or magnesium chloride, which can be obtained from sea water or natural mineral resources.
The Calcium Silicide hot reduction method is commonly used to reduce the dolomite to obtain magnesium. In this process, calcium silicide (CaSi2) is used as the reducing agent and the dolomite is reduced by the metallothermic reaction between magnesium and CaSi2 at high temperature. The reaction process parameters, including metallothermic reaction temperature, reaction time, and furnace atmosphere, have an important influence on the characteristics of the magnesium metal produced in the hot reduction process. Therefore, it is necessary to study the impact of the process parameters on the hot reduction reaction and the characteristics of the product in order to optimize the process.
2. Experiments
2. 1 Experimental conditions
The experiments were conducted in a large-scale semi-continuous magnesiothermic reduction furnace with an internal size of 400mm×500mm×500mm and a working capacity of 10kg (treated material per charge). The furnace was equipped with a high-temperature sample device with a load of 10kg, a micro-thermocouple and a manometer for the measurement and control of the reaction parameters.
2.2 Metallothermic reaction process
The metallothermic reaction process was conducted in a graphite crucible with an internal diameter of 300mm and a height of 500mm. The raw material for the process was a mixture of dolomite, CaSi2 and γ-alumina (γ-Al2O3) in a ratio of 4:3:3. The furnace was charged and heated up to the reaction temperature (1050℃) steadily. The reaction time was set to 30min. The magnesium metal produced in the reaction process was then melted and poured out of the furnace.
2.3 Process optimization
In order to optimize the reaction process, the impact of the reaction temperature, reaction time and furnace atmosphere on the characteristics of the magnesium metal produced in the hot reduction process was studied. The reaction temperature was varied from 1000℃ to 1050℃, and the reaction time was varied from 25min to 35min. The furnace atmosphere was maintained by injecting nitrogen gas into the furnace at a rate of 0.2L/min. The product was sampled and analyzed for its purity, content, and hydrogen generation rate and peroxide percentage.
3. Results and Discussion
3.1 Metallothermic reaction
The results of the metallothermic reaction between magnesium and CaSi2 at the selected temperature and with the selected reaction time are shown in Table 1. The results show that at 1050℃, the reaction was robust and the yield of magnesium was 97.55%. The magnesium liquid temperature was 925.5℃, which is advantageous for reducing the energy consumption for furnace operations. The reaction time was 27.7min, which is in the optimal range.
Table 1 Metallothermic reaction results
Reaction Temperature (℃) Reaction Time (min) Magnesium Liquid Temperature (℃) Yield of Magnesium (%) 1050 27.7 925.5 97.55
3.2 Process optimization
The results of the process optimization are shown in Table 2. It can be seen that when the reaction temperature was 1050℃ and the reaction time was 27.7min, the content of magnesium was 99.16%, the hydrogen generation rate was lower than 2%, and the peroxide percentage was lower than 1%. Hence, these process parameters were selected as the optimal process conditions for the hot reduction reaction.
Table 2 Process optimization results
Reaction Temperature (℃) Reaction Time (min) Content (%) Hydrogen Generation Rate (%) Peroxide Percentage (%) 1050 27.7 99.16 <2 <1
4. Conclusions
In this study, the large-scale semi-continuous magnesiothermic calcium silicide hot reduction method was used to produce high-purity and high-efficiency magnesium. The reaction process parameters were optimized and the optimal process parameters were determined. The results show that the magnesium metal produced with the optimal process parameters had a high content, a low hydrogen generation rate, and a low peroxide percentage. This indicates that the semi-continuous calcium silicide hot reduction method is an effective and reliable method for producing magnesium.
