Physical and Chemical Properties of Direct Reduced Iron
1.1 Regular shape: In the process of electric furnace smelting, the application of DRI (direct reduced iron) requires DRI to be of uniform and regular shape, which is convenient for material management and continuous charging operation. DRI in the frame can fill the scrap gap, increase the density of the furnace material, reduce the number of charging.
1.2 Particle size: In order to ensure that the direct reduction of iron can quickly pass through the slag layer and contact with the liquid steel, the particle size of DRI also has certain requirements. Generally the particle size of the DRI should be more than 5mm, particle size is too small may lead to DRI by the furnace gas or slag to take away, increase the loss.
1.3 Density: In determining the appropriate particle size of DRI, need to be analyzed from the density of the DRI. Generally, the particle size of DRI produced by the rotary cellar is around 5mm~20mm, the average particle size is 10mm~20mm, and the pile density is 1.8~2.2t/m3. In the process of continuous charging of the electric furnace, it can be effectively applied to high-density loading, and the scope of application is relatively wide. Long columns of direct-reduced iron can only be added to the furnace by means of frame loading. Powdered DRI up to 3 mm must be briquetted before it can be used in the EAF. Usually, the density of DRI obtained by gas direct reduction method is relatively low, only 1.8~2.0t/m3. The higher density of direct reduced iron in the process of electric furnace production can improve the production efficiency of the electric furnace and reduce the reoxidation of DRI. Increasing the operating temperature in the operation process can improve the density of DRI. But it will affect the direct reduction operation level and energy consumption to a certain extent. The density and operating conditions need to be reasonably selected according to the specific requirements of the products smelted in the electric furnace in the production process.
2. Chemical properties
In the process of analyzing the chemical properties of DRI, it is necessary to accurately judge the quality of DRI from two aspects, namely, the full iron content and the amount of chalcopyrite content. The total iron content will have a direct impact on the iron yield. And the chalcopyrite content to a certain extent determines the DRI in the process of application of the dosing amount.
2.1 When determining the content of chalcopyrite, it is necessary to start from different properties of chalcopyrite, including acidic and alkaline chalcopyrite.
Acidic chalcopyrite is mainly composed of SiO2 and Al2O3, and a high content of acidic chalcopyrite leads to an increase in the amount of slag in the steelmaking process of the electric furnace, which has a direct impact on the power loss of the electric furnace. If the acidic chalcopyrite content of the DRI is 4%, and the proportion of DRI is 50%, the amount of slag in the EAF steelmaking process will be more than that in the whole scrap smelting process. This significantly increases the overall energy consumption of the furnace and affects the yield of the smelted product. Therefore, in order to prevent excessive slag in the smelting process, it is necessary to accurately control the maximum dosage according to the acidic chalcopyrite content of the DRI. The higher the acidic chondrite content. The lower the allowable amount of DRI.
If the alkali content of DRI is relatively high, the chalcopyrite content can be increased, but care should be taken to keep the total chalcopyrite content within the reasonable range of the total slag of the electric furnace smelting without exceeding the standard.
2.2 DRI oxide will enter the slag in the steelmaking process, resulting in lower metal recovery.
In order to improve the metal recovery rate, it is necessary to supplement carbon and heat to achieve the purpose of metal recovery through chemical reaction. Therefore, the metallization rate of DRI is directly related to the electric energy consumption of the electric furnace. At the same time, the DRI metallization rate on the electric energy consumption of the electric furnace will, to a certain extent, determine the smelting steel type, scrap composition, carbon and operating conditions and other elements. If the quality of the scrap is relatively poor, the carbon content of the scrap is relatively high, the decarburization reaction in the smelting process can compensate for the addition of DRI resulting in power consumption, thus reducing the total power consumption. However, the use of DRI with a high metallization rate may have an effect on the intense boiling of the melt pool and increase the power consumption.
When smelting with continuous addition of DRI, the power consumption corresponds to the addition rate of DRI, which can shorten the smelting time, thus ensuring that the electric furnace is at the state of maximum input power, and improving the overall output of the electric furnace. Smelting with batch addition of DRI, if the addition is not proper, for example, the DRI is too concentrated or close to the furnace wall, it may lead to the accumulation of DRI or bonding on the furnace wall, resulting in the prolongation of the melting time and the increase of electric energy consumption. Therefore, in the application process of DRI, its metallization rate is closely related to the smelting conditions of the electric furnace and the smelting steel type.
At this stage, the international requirements for the metallization rate of DRI in the process of electric furnace smelting is above 90%, many electric furnace steel mills in China are smelting with pig iron and molten iron, the electric furnace is a more cumbersome task, and the metallization rate of direct reduction iron is relatively low. Therefore, it is necessary to improve the smelting conditions of the electric furnace to produce a lower level of metallization rate of DRI, and at the same time reduce the energy consumption of DRI to ensure the productivity of direct reduction equipment. In addition, the sulfur and phosphorus content within the DRI will have an impact on its application value. Usually the sulfur and phosphorus content in steel must be below 0.03%, and the content of some high-quality steel is even required to be below 0.015%. Therefore, in the application process of DRI, the content of sulfur and phosphorus should be below 0.03%. For some high-quality steel smelting, the sulfur and phosphorus content of DRI should be controlled below 0.02%. Excessive sulfur and phosphorus content in the DRI may lead to additional desulfurization and dephosphorization burden on the electric furnace, which will not only increase the consumption of electric power, but also lead to a decrease in the dosage of direct reduced iron. In addition, the content of non-ferrous elements in the direct reduced iron will also have a direct and important impact on the use value of DRI. Because Cu, Sn, Sb, Pb, Cr, Ni and other elements in the scrap cycle removal efficiency is relatively low, in the process of smelting clean high-quality steel grades, these residual elements over the standard will have a direct impact on the quality of steel grades. And the use of DRI can be a variety of non-ferrous element content can be effectively controlled and diluted, especially some of the requirements of high-grade steel grade impurity element content is relatively low, can only use DRI as raw materials. Because the smelting of different steel grades on the non-ferrous element limitations also have certain differences. Therefore, when controlling the content of non-ferrous elements in DRI, it is necessary to select DRI according to the specific requirements of the smelting steel grade.
