Application of common flux for sintering

An alkaline flux is added to the sinter, the purpose of which is to: 1) obtain a certain alkalinity sinter, so that no smelting agent is added during the blasting of the blast furnace, so as to improve the smelting strength of the blast furnace and reduce the coke ratio. 2) During the sintering process The alkaline substances CaO and MgO in the flux can form a low melting point with the acidic gangues SiO ₂ and Al ₂ O ₃ in the ore powder, so that a sufficient liquid phase can be obtained to improve the sinter in the case of low fuel consumption. strength and metallurgical properties (reduced).
The requirements for flux are as follows:
1) The basic oxide content is high. Pure limestone ( calcite ) has a theoretical CaO content of 56% and a CO ₂ content of 44%. However, limestone in nature contains impurities such as magnesium , iron and manganese , and a certain amount of acidic oxide. Therefore, the limestone CaO content in the industry is lower than the theoretical one. Generally in 49 ~ 52%.
2) The lower the content of the acidic oxide (SiO ₂ + Al ₂ O ₃ ), the better. Increased acid oxide content will reduce CaO in limestone and reduce effective CaO, generally requiring SiO ₂ <3% in limestone.
3) Less harmful impurities such as sulfur and phosphorus . Generally, it contains <0.08% and phosphorus is <0.03%.
4) The particle size should not be too large, and the general particle size should be below 40 mm. The flux size for sintering is <3 mm, which increases the cost of crushing and complicates the process.
Common fluxes for sintering are limestone, self-cloud, quicklime and hydrated lime. Their characteristics are as follows:
The main chemical composition of limestone is CaCO ₃ and the main mineral is calcite. Most are milky white, with a hardness of 3 and a specific gravity of 2.6 to 2.8. The most common crystal is a rhombohedron, dense block. Because it is widely distributed on the earth's crust, it is cheap and easy to obtain, it is widely used.
The Longquan Limestone used by Shougang is a dolomitic limestone containing calcium oxide CaO40% and MgO10%.
Dolomite: The main chemical composition is CaMg(CO ₃ ) ₂ , CaO30.4%, MgO21.7%, and its morphology is the same as calcite. The difference is that the crystal face is bent into a saddle shape, grayish white or other light color, hardness of 3.5 to 4, specific gravity of 1.8 to 2.9. Dolomite is one of alkaline fluxes. However, adding dolomite to the sinter is mainly to increase the content of MgO in the sinter, thereby improving the quality (strength) of the sinter and improving the physical properties (fluidity) of the blast furnace slag.
In order to make full use of local resources, some factories in Jiangsu and Shandong have completely used dolomite as a sintering flux. Nanjing, Suzhou, Xuzhou and other steel plants use the waste of local dolomite crushing process instead of limestone to produce high alkalinity high MgO sinter , obtained good economic benefits and smelting effects.
Quicklime: It is a product of calcined limestone, which has a high CaO content, generally 64 to 68%. Due to the exothermic reaction of quicklime and water, it can preheat the sintering material, and it also helps to make it after the water turns into slaked lime. ball. The use of quicklime is a measure to increase production in sintering production. However, due to the large dust and poor working conditions during roasting and transportation, it has not been adopted by some factories. As early as 1973, Maanshan Iron and Steel Co., Ltd. used rotary kiln roasting and pneumatic transport of quicklime into the warehouse, which greatly improved working conditions. Technical characteristics of existing equipment: rotary kiln Ф2.5 × 58 m, 65,000 tons annual active lime, a mixed gas of coal (1600 to 1800 most heat Kcal / m ^3, the particle size of limestone kiln 3 ~ 10mm or 7~20mm, 0~10mm after roasting, sent to the sintering batching room with double-storage pump, 10~20mm, sent to the steel mill by tanker, the activity of active lime can reach 370ml or more The device can also recover waste heat and produce 8-10 tons of steam per hour, which can be used for preheating the sintering mixture. Due to the light weight of the quicklime pile, it can not be used excessively in the sintering ingredients. The general usage is not more than 5%.
Slaked lime: Its main chemical composition is Ca(OH) ₂ , generally about 65% CaO. Because of its strong adhesion, it is advantageous for the spheroidizing of the sinter. Due to its light weight, it can not be used too much in the sintering ingredients, generally 3 to 5% in the ingredients. The current digestion equipment needs further improvement. The water content of slaked lime does not exceed 15%.

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