[China Aluminum Network] Aluminothermic method (thermiteprocess)
A method for producing a ferroalloy using aluminum as a reducing agent. By reducing the heat of chemical reactions released by certain metal oxides with aluminum, the oxide reduction reaction can be completed and the separated alloy and slag can be obtained without externally replenishing heat. The aluminothermic method and the silicon thermal method using ferrosilicon as a reducing agent belong to the same method that utilizes an autothermal reaction to produce a ferroalloy. It is called a metal thermal method and is also called an off-furnace method. They use aluminum particles, ferrosilicon powders or aluminum-magnesium alloy powders as reducing agents. The aluminothermic method is mainly used to produce ferroalloys, intermediate alloys, chromium and manganese, which contain high-melting-point metals and hard-to-reduce elements. The product is characterized by a very low carbon content (typically <0.05%). Aluminum heat production equipment is simple, covers an area of ​​small, production scale can be determined according to the task, product variety, short production cycle and other characteristics.
A Brief History In 1859, the Russian scientist H.H. BeKeTOBy mentioned in "Some Reducing Phenomenon" that "reduction of yttrium oxide with aluminum yields a tantalum alloy of 24% Ba and 33% Ba." This is an earlier report of the aluminothermic test, but it was not used industrially at that time. In 1893, H. Goldschmidt discovered that the mixture of metal oxide powder and powdered reducing metal (essentially aluminum) could be automatically initiated after ignition reaction until the charge reaction was completed. In 1898, Goldschmitt made a report on the metal thermal reduction method at the German Electrochemical Society. It was only known that the aluminum thermal method has achieved good results in industrial production, and can produce carbon without economical and large quantities. Ferroalloy and pure metal. This year should be the starting point for the use of the aluminothermic method for industrial production. The iron alloys produced by the aluminum heat method in industry mainly include: titanium iron, molybdenum iron, niobium iron, boron iron, vanadium iron, tungsten iron, metal chromium, manganese metal, and nickel-based, titanium-based, aluminum-based and other intermediate alloys.
China's industrial production of ferroalloys using the aluminothermic method started at the end of 1957 with the production of ferromolybdenum by Jilin Ferroalloy Plant.
All oxides are more easily decomposed with increasing temperature and are therefore more easily reduced. The oxygen potential difference of various oxides becomes smaller at high temperatures. From Figure 1, the reduction situation can be estimated. At ΔF. In the -T diagram, lower-position elements can restore higher-position oxides. Two ΔF. The greater the distance between the -T lines, the more heat is generated by the reduction reaction. The precondition for the thermal reduction of aluminum (or silicon) is ΔF. ≤ 0, that is, the greater the negative value of the free energy of the reaction, the easier the aluminothermic reduction reaction proceeds. From ΔF. -T chart analysis of aluminum (or silicon) thermal reduction reactions does not take into account the kinetic process, so this judgment is qualitative. All metal thermal reduction reactions are at a lower temperature, ΔF. Compare ΔF at high temperatures. The negative value is large, so the reaction temperature is controlled as low as possible under the conditions in which the reaction can proceed, so that the reduction reaction is favorably to the right.
Some of the aluminothermic reduction reactions can displace metals entirely from the relevant oxides, such as iron, tungsten, molybdenum, etc.; while others can only proceed to the equilibrium of the oxides in the molten metal and slag, and some of the oxides remain in the Slag. Some oxides are reduced to low-valent oxides during the aluminothermic reduction process. For example, TiO2 is reduced to TiO, converted from acidic oxides to basic oxides, and combines with AI2O3 produced by the reduction process to form an aluminate and remains in the slag. In the increase of titanium loss. In order to reduce the loss of low-valent oxides in the slag: (1) increase the amount of reducing metal to be added, avoid the generation of low-valent oxides under excess reducing agent conditions; (2) add basic oxides such as CaO, MgO , BaO can reduce the content of TiO, MnO, etc. in slag, and increase the recovery rate of metal elements. Alkaline oxides also lower the melting point of the slag and improve the flowability of the slag. The liquidus changes of the binary slag system consisting of oxides and AI2O3 are shown in Figure 2. The amount of basic oxide added should be as small as possible to avoid increasing the amount of slag that affects the reaction process.
Because of the fast response, it is difficult to reach equilibrium conditions. A part of the reduced metal is not used for reduction and remains in the alloy, forming intermediate compounds such as TiAl, TiAl3, etc., so that the aluminum content of the alloy is high, and it is difficult to obtain a high-grade alloy. In order to promote a near-equilibrium of the reaction, a third element is sometimes added. For example, iron is added to absorb the metal produced by the reaction and the reaction proceeds to the right. This method is feasible in the production of ferroalloys and can also reduce the alloy melting point and reaction temperature. To obtain products with low aluminum content, the amount of aluminum added can be slightly lower than the calculated amount. Figure 1 can provide a reference for selecting the type of reducing agent and the type of oxide. Ferrous alloy smelting commonly used reducing agent is mainly aluminum and ferrosilicon, and occasionally also use a small amount of magnesium (added with magnesium alloy).
The result of the aluminothermic reaction must be such that both the metal and the slag have good fluidity, i.e., are heated above their melting point, so that the produced alloy is clearly separated from the slag; and a higher metal yield can be obtained before it can be considered It is an automatic reaction that is used by industrial production. This problem needs to analyze the thermal balance of the aluminothermic smelting process. The heat balance of the aluminothermic smelting is shown in the table.
In the aluminothermic reduction process, the reduction of reactants, the production of products, the generation of reaction heat, and the heating of the reactants (alloys and slag) are performed at the same time and in the same system. Therefore, heat concentration, reaction speed, short time, high thermal efficiency. The surface of the reaction melt is always covered by the charged charge, so when the reaction proceeds, the heat loss from the heat conduction and heat radiation of the reactor has little effect on the reduction process. Since the reaction time is short, the amount of evaporation loss of the charge and the reactants is small, so the amount of evaporation heat is also small.
The main heat source of the aluminothermic method is the heat of reaction ΔH produced by the thermochemical reaction. 298 (reaction), which can be calculated by the calculation method. In 1914, the Russian chemist Gemchuzhnyi proposed that "if the heat content of the metal and slag obtained, and the heat loss accompanying the reaction process is similar to the various alloys," Normally, the heat generated in the reaction per gram of charge must not be less than 550 cal. The heat produced by the unit charge is used to determine whether the aluminothermic reduction process can be automated. The so-called unit calorific value Q is the heat of aluminum reduction reaction △H. 298 (reaction) divided by the total weight of the charge (oxide, aluminum) W, that is Q = △ H. 298 (reaction)/W, cal/g (1cal=4.19J, the same below). The Semchiqiri rule can be used as a reference in production, or when a preliminary estimate is made at the time of new species development. The reason for this is that the regulation of the degree of oxide reduction is different, the melting point of the alloy and the slag is different, the size of the smelting scale is different, and the phase structure of the ore is different. Therefore, after the charging ratio of the charge material is calculated, the small-scale smelting equipment must be used first. Trials, and then make appropriate adjustments before they can be used for production. In the normal production plant, when the ore is changed, trials are required to correct the ingredient list. The total amount of charge on the production should include the quality of aluminum, ferrosilicon and other reducing agents, oxides (or ore) and impurities (or gangue), flux, etc. The heat of reaction is calculated based on the data generated in the manual (△H.298). Due to different ages and versions, there are different degrees of difference, and the calculated heat of reaction is also different. Actual workers should select a batch of data, use it on a fixed basis, and obtain a correction factor based on practice.
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