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Currently used rare earth silicon iron alloy production process, i.e., in the lean-rich iron ore by blast furnace slag after removal of the iron-rich rare earth, rare earth or rare earth ore fine dross removal lime, silicon arc furnace smelting iron ore suddenly Alloy, which is a process developed in combination with the characteristics of China's rare earth resources. The process is relatively stable and mature, with high production capacity and high output, low cost, and originality compared with foreign countries.
Although the process has been continuously reformed and improved over the years, it is still in the development stage and has great potential. The main problems existing are long process, complicated process, high energy consumption, total energy consumption of 12,000~1400kW·h per ton, making the alloy a high-energy product, and the recovery rate of rare earth is low, industrial production is usually only 65%. about.
Over the years, many research units in China have continuously explored and expanded to simplify the process, improve the recovery rate, reduce costs, and improve technical and economic indicators. These exploratory work is mainly reflected in changing the structure of rare earth raw materials, reducing agent selection and process operation according to different uses.
(1) Selection of concentrate
With the development of China's rare earth alloy industry, rare earth raw materials have undergone changes in ironmaking blast furnace slag (REO 4% to 6%), rare earth rich slag (REO 10% to 17%) and rare earth concentrate (REO > 30%). The raw materials are from coarse to fine, which expands the output, saves energy, reduces environmental pollution, and promotes the development of rare earth intermediate alloys in China, but the high price of rare earth concentrates affects the cost of alloys.
In recent years, China has developed a new production process for the direct smelting of rare earth ferrosilicon alloys using Baiyun Ebo or Shandong Weishan Lake low-grade rare earth concentrate (REO>30%). The concentrate can be directly into the furnace without agglomeration and de-ironing, and the rare earth ferrosilicon alloy with rare earth grade greater than 30% can be smelted, and the rare earth recovery rate is 75%. Smelting contains 21% to 27% of rare earth alloys, and the recovery rate of rare earths is 80%. The electricity consumption per ton is less than 3000kW·h. Compared with the current process, it saves 30% of power consumption. The rare earth intermediate metal can be directly produced by using low-grade rare earth concentrate which is not suitable for hydrometallurgy. The rare earth concentrate and white ash are uniformly distributed in the furnace, and the power is raised. After the furnace is fully melted, the temperature is raised to 1350 °C, the power is cut off to strengthen the reduction, and then the temperature is raised again to strengthen the reduction. When the rare earth alloy content in the rare earth alloy is close to 31.5%, the furnace temperature can reach 1400 °C. The melt is injected into the iron slag separation system, and is allowed to stand for several minutes to inject the alloy into the ingot mold, and the slag is water quenched and recovered for comprehensive utilization.
This process not only has low cost, but also reduces environmental pollution after comprehensive utilization of the final slag.
(2) Improve the reducing power of reducing agent
The reducing agents used in current industrial production are silicon, silicon calcium, aluminum and calcium carbide, etc. From the economic point of view, mainly using ferrosilicon, mainly 75 ferrosilicon. How to improve the utilization of silicon and reduce the oxidation loss is essential. From the change of silicon content in the smelting process, it is found that the amount of alloy silicon content decreased during the reduction process is close to the amount of rare earth in the alloy. That is to say, in the case of normal smelting, the substitution equivalent of rare earth and silicon is about 1, which is quite different from the data calculated in the chemical reaction formula.
This can be illustrated by the following analysis.
In the process of rare earth intermediate alloy smelting, the main functions of silicon are: 1 reduction of rare earth, calcium, magnesium , manganese , titanium and aluminum in the alloy; 2 formation of stable silicide with each element, such as RESi, CaSi, MnSi, TiSi2, MgSi, FeSi, etc.; 3 oxidation, burning loss of silicon. In the metallurgical process, only "free silicon" can participate in the reduction reaction, affecting the yield of rare earth elements in the metallurgical process and the rare earth grade in the alloy. The Fe-Si system is composed of a compound such as FeSi or Fe2Si5 in a solid state, and the activity of silicon remarkably exhibits a negative deviation due to a strong interaction between iron and silicon.
In order to say that it is a practical problem, the following rough calculations can be made.
The composition of a rare earth ferrosilicon alloy is: RE 30%, Ca 3%, Mn 3%, Ti 0.5%, Mg 0.1%, Al 1%, Fe 23%, and Si 40%. To prepare the above-mentioned alloy 1t, it is necessary to add 75 ferrosilicon to 0.8t, and the amount of silicon used to reduce the above elements (except for the iron contained in silicon and ferrosilicon) and the corresponding silicide formed by these elements can be calculated. The amount of silicon, these two are reasonable consumption. According to the amount of ferrosilicon added and the actual silicon content in the alloy, the amount of free silicon remaining and the amount of silicon oxide lost by oxide can be calculated. Calculations show that oxidative burning accounts for about 21% of the total silicon, and the consumption of this part of silicon should be minimized. The compressed air agitation used in the previous large-scale smelting is the main reason for the large amount of silicon burning. The newly developed intensive smelting technology is switched to compressed nitrogen to significantly reduce the oxidation loss of silicon. The rare earth yield is increased by about 10% compared with the original process.
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