The Influence of Lime Quality on Steelmaking

　　1.The effect of effective CaO and SiO2 content

　　Effective CaO refers to the residual amount obtained by subtracting the CaO content in lime from the amount of CaO consumed by SiO2 in lime under specific slag alkalinity conditions. The amount of lime required for a certain composition of molten iron is determined by the final slag alkalinity (CaO/SiO2), and the SiO2 content determines the slag alkalinity.

　　The key factor is that the higher the SiO2 content, the greater the amount of lime required. If the impurity SiO2 contained in lime is calculated based on the slag alkalinity of 3.2, for every 1mol of SiO2 contained in lime, 3.2mol of active CaO is required to neutralize it, greatly reducing the effective CaO content in lime, thereby increasing the amount of lime used for steelmaking and slag. According to reports, when the effective CaO mass fraction decreases by 1%, calculated based on the alkalinity of steelmaking slag of 3.2, the consumption of steelmaking lime will increase by 0.432kg/t. Guanggang Group produces over four million tons of steel annually. If the effective CaO content of steelmaking lime is reduced by 1%, the annual consumption of steelmaking lime will increase by over 17000 tons, equivalent to approximately RMB 750000.

　　2. Impact of sulfur content

　　An increase in sulfur content in lime will reduce the effective CaO content, increase per ton steel consumption, accelerate furnace lining damage, and more importantly, reduce the desulfurization ability of molten steel, affecting its quality. Reducing the effective CaO content, increasing consumption, and increasing sulfur content will reduce the effective CaO content in lime and increase the consumption of per ton of steel lime. Based on an increase of 0.03% in sulfur content, the annual consumption of 250000 tons of lime by Guangzhou Iron and Steel will increase the effective CaO consumption by over 130 tons per year; Calculated based on a purity of 90%, the annual consumption of lime will increase by approximately 145t, equivalent to approximately RMB 65000. The increase in sulfur content in lime causes an increase in sulfur content in molten steel, exacerbates the reaction between slag and furnace lining, and accelerates the damage of furnace lining. Reduce the desulfurization ability of molten steel and deteriorate its quality. One of the purposes of using lime as a slag making agent is to remove sulfur from molten steel. If the sulfur content of lime itself is high, it is obviously unfavorable for the removal of sulfur in molten steel. According to reports, when 0.01% sulfur is added to lime, it will increase the sulfur content of molten steel by 0.001%. As a result, it will increase lime consumption, extend blowing time, reduce desulfurization and dephosphorization rates, and deteriorate the quality of molten steel.

　　Therefore, reducing the sulfur content in lime is an important measure to reduce lime consumption, improve furnace lining life, improve steel quality, and ensure smooth smelting.

　　3 Burn reduction

　　The ignition reduction of lime is generally 2.5% to 3.0%, which is equivalent to about 2% residual CO2 in lime. The amount of residual CO2 in lime also reflects the overburning of lime during calcination, which affects the effective CaO content in lime. According to reports, when the reduction in ignition is reduced by 0.20%, the effective CaO content can be increased by 0.17%, which will reduce the consumption of lime per ton of steel.

　　4 activity

　　The activity of lime refers to its ability to react with other substances in the slag, expressed by the melting rate of lime in the slag. Because it is difficult to directly measure the melting rate (thermal activity) of lime in the slag, it is usually expressed by the reaction rate of lime and water, that is, the Water activity of lime. Lime has high activity, active chemical properties and strong reaction ability, which is conducive to the smelting process. According to statistics, compared with ordinary lime (activity less than 300), using active lime (primary ash, activity greater than 320) can shorten the oxygen blowing time of converter by 10%, increase the yield of molten steel by 10%, reduce lime consumption by 20%, and reduce fluorite consumption by 25%. At the same time, highly active lime is also conducive to improving desulfurization and dephosphorization capabilities, and improving the life of furnace lining.

　　5 Impurities

　　The impurities in lime generally refer to SiO2, Al2O3, Fe2O3, Na2O, and K2O in limestone. These impurities start to react with lime at lower temperatures (900 ℃), promoting the fusion between CaO particles, leading to coarsening of particle crystallization and reduced activity. Iron compounds and aluminum compounds are strong fluxes that can promote the formation of fusible calcium silicate, calcium aluminate, and calcium ferrite. These molten compounds will block the fine pores on the surface of lime, reducing its reactivity, and also blocking the discharge of CO2 gas, forming quick-burned lime in certain parts of the center. More importantly, these molten compounds react with lime and bond together to form slag blocks, causing maladjustment of the lime kiln condition, seriously reducing the activity of lime, and increasing the amount of lime used in steelmaking.