Aluminum in bauxite exists in the form of alumina, and the metallic aluminum obtained by direct reduction cannot be separated from other impurities, so it cannot be directly reduced by reducing agent. In this paper, the principle of direct aluminum smelting from bauxite by carbothermal reduction and decomposition of low-valent chloride in vacuum is studied, which provides reference for related engineering practice.

Keywords: carbothermal reduction aluminum smelting vacuum technology

introduce

Aluminum is a light metal, which is abundant in nature. Its density is 1/3 of steel, its strength/weight ratio is large, and its electrical conductivity and thermal conductivity are good. There is an oxide film on the surface of aluminum to protect it from rust, and the recovery rate of aluminum is high. Therefore, aluminum is widely used in automobile manufacturing, construction and aerospace industry, and it is an important economy and strategic metals. Therefore, some countries lacking bauxite resources are studying the methods of producing alumina from these raw materials, among which the acid method, high pressure hydrochemistry method and limestone sintering method are the most studied. However, among various comprehensive treatment methods of alunite, only reduction roasting method has been used in industrial production, and sintering method has been widely used in industrial production. Based on the analysis of these common methods, this paper studies the principle of aluminum smelting by vacuum carbothermic reduction chlorination.

1 Common aluminum smelting methods

1. 1 acid method

Non-bauxite raw materials are characterized by high silica content, so it is reasonable to treat these raw materials by acid method in principle. According to the different kinds of inorganic acids used, there are various schemes for producing alumina by acid method, but they have the same characteristics and similar production processes, including ore pretreatment (roasting and grinding), acid leaching (transforming alumina into soluble inorganic acid aluminum salt), iron removal from leaching solution, aluminum salt decomposition, aluminum hydroxide calcination and acid recovery. Many countries have studied the acid methods of high silicon and high aluminum raw materials including hydrochloric acid, sulfuric acid and nitric acid. Some people have also estimated the raw materials and energy consumption, the cost and investment per unit product when treating different raw materials with various acid methods in some countries, and made optimistic estimates on the future industrial application of acid methods. However, the acid method is still in the experimental stage and needs to be improved and perfected.

1.2 high pressure hydrochemical method

This method was invented by scholars in the former Soviet Union in 1950s. Wet treatment of high silicon and aluminum-containing raw materials to produce alumina is a new method, that is, it is treated with lime or high concentration alkali liquor, and then evaporated and crystallized to obtain the product. The physical and chemical principles and techniques of this method have been extensively studied for a long time, and great progress has been made.

Compared with sintering method, high-pressure hydrochemical method has many advantages: it avoids the sintering process with large investment and high energy consumption, and can obtain higher dissolution rate of Al2O3 and Na2O under suitable conditions, without consuming high-quality coal, which is beneficial to environmental protection. According to the semi-industrial test results, the technical and economic comparison shows that the energy consumption, investment and product cost are obviously lower than those of sintering method. However, there are still many shortcomings in this method, which need to be studied and improved.

1.3 reduction roasting method

This method is the only way to comprehensively treat alunite in industry at present. Alum is dehydrated, reduced and roasted to remove SO3, and then treated by Bayer process. The products are alumina, potassium sulfate and sulfuric acid.

Because most bauxite mines in China belong to diaspore type with high aluminum and high silicon, and the ratio of aluminum to silicon is moderately low, it is impossible to produce alumina economically by conventional Bayer process abroad, and most of them can only produce alumina by alkaline lime sintering method and mixed method. Compared with foreign conventional Bayer process to treat high-grade bauxite, these two production methods have the following disadvantages: high comprehensive energy consumption per unit product, long process flow and large construction investment. For the construction projects of large and medium-sized alumina plants, the unit product construction of mixed method is much higher than that of conventional Bayer method. With the gradual decline of bauxite grade and the rising of energy price in China, the technical and economic indicators of existing mixed-process plants will gradually deteriorate, and the production cost will also increase accordingly. In recent years, high-grade private mining has been widely used in alumina plants in China, resulting in waste of resources and abandonment of poverty. With the continuous development of production, high-grade ore will soon be used up, which is expected to last only a few years. If low-grade ore is used, the production capacity of aluminum plant will be greatly reduced, the cost will increase and the economic benefit will deteriorate. The above shortcomings make China alumina industry lack competitiveness in the world, which further challenges its survival and development. With the increase of energy price and the decline of bauxite grade year by year, this challenge will become increasingly severe. Principle of aluminum smelting by vacuum carbothermic reduction chlorination

According to the invention, aluminum trichloride gas is introduced into the reaction system of bauxite and coke to react with aluminum trichloride to obtain aluminum monochloride (AlCl), and the obtained aluminum monochloride is decomposed at a lower temperature in the reaction system to obtain metal aluminum and aluminum trichloride, and the aluminum trichloride can be recycled after collection. That is to say, in the whole reaction system, aluminum trichloride is only used as the medium for direct carbon reduction of alumina in bauxite, but because of its existence, the reduction reaction temperature is greatly reduced, and relatively pure metallic aluminum can be obtained, thus making it possible to produce metallic aluminum by direct carbon reduction of bauxite. In this principle, the reaction of forming aluminum chloride with low valence at high temperature is a reaction with increasing gas volume, so it can be carried out in vacuum, which makes this step easy. At the same time, the research of aluminum smelting under vacuum is mainly due to the following reasons:

(1) The first step of the aluminum smelting mechanism of this method is to increase the gas volume (Al2O3+AlC 13 gas +3c...3LCL gas +3CO gas), from 1 gas volume to 6 gas volume. Therefore, reducing the pressure of the reaction system (under vacuum) will greatly promote the reaction, and the low-valent aluminum compound will be carbothermically decomposed to reduce alumina. The first step is that alumina reacts with carbon and high-valent aluminum compounds at high temperature to generate low-valent aluminum compounds, and the second step is that the obtained low-valent aluminum compounds are decomposed at low temperature to obtain metallic aluminum.

(2) Because it is carried out in vacuum, the emission direction of tail gas generated in the whole process is easy to control, so the pollution control is also easy, and finally it can become a low-pollution process;

(3) Because the reaction system is carried out under the condition of continuous vacuum pumping, the gas velocity in the system is very high, and the liquid metallic aluminum obtained by decomposing aluminum monochloride can be quickly separated from the gases (carbon monoxide and aluminum trichloride) existing in the system, so as to avoid the reaction between metallic aluminum and these gases, resulting in the loss of metallic aluminum and the decrease of its purity.

(4) By calculating the theoretical power consumption required for aluminum smelting by carbothermic reduction chlorination, it can be known that carbothermic reduction chlorination has more advantages in energy consumption than other aluminum smelting methods at present.

(5) Other components in bauxite, such as iron oxide, zinc oxide, silicon oxide, etc. Because it will not react to generate gas, it can only remain in the slag, but the reaction temperature can be lower than the temperature under vacuum and normal pressure, and the airflow direction of AlCl gas is certain, so the condensation decomposition point of AlCl is easy to control, thus realizing the possibility of obtaining metallic aluminum from bauxite in one step, and the obtained metallic aluminum has high purity because other components remain in the slag.

label

With the gradual decline of bauxite grade and rising energy price in China, the manufacturing cost will increase accordingly, and high-grade ore will be used up soon. If low-grade ore is used, the production capacity of aluminum plant will be greatly reduced, the cost will increase and the economic benefit will deteriorate. The aluminum smelting method described in this paper can change the traditional production process of metallic aluminum, which is a major reform, so the research on this method has great development value and application prospect.

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