Method for improving purity of reduced iron

Abstract

The invention discloses a method for improving the purity of reduced iron, and belongs to the field of metallurgical chemical industry. The method comprises the following steps: carrying out gas-based roasting reduction on an iron ore raw material in a roasting furnace to obtain iron ore containing zero-valent iron; iron ore containing zero-valent iron is placed in an electromagnetic induction furnace to be heated and melted, in the melting process and in the melting state, metallic iron and non-metallic impurities exist in a split-phase mode, slag is removed after cooling, cast ingots with TFe larger than or equal to 96% are obtained, and the purity of reduced iron is greatly improved. The method is suitable for iron ore raw materials with 50-65% of TFe, the iron ore raw materials comprise pellets, sintered ore, natural iron ore lump ore and iron ore concentrate powder, any auxiliaries such as a deoxidizing agent and a slag former are not used, harmful inclusion elements can be removed without slag steel interface metallurgical reaction, and the method is simple and belongs to zero-carbon metallurgy.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical chemistry, and particularly relates to a method for improving the purity of reduced iron. Background Art

[0002] In order to achieve the transformation of the low-carbon economy, the steel industry will face a transformation in the production mode. Transitioning from carbon metallurgy to hydrogen metallurgy is an important path. Hydrogen metallurgy generally refers to a non-blast furnace smelting process that uses a reducing gas with a hydrogen content greater than 55% and an H 2 / CO higher than 1.5 to reduce high-grade iron ore lump ore or pellet ore or sinter ore to produce reduced iron, that is, the direct reduction process for producing sponge iron (DRI). The long process of steel smelting includes main processes such as mining → ore dressing → sintering / pelletizing → blast furnace ironmaking → converter steelmaking → continuous casting billet → hot rolling / cold rolling, etc. The process of replacing carbon with hydrogen currently focuses on the change in the blast furnace ironmaking process, which not only involves the adjustment of the energy structure, but also involves the adjustment of the purity of the raw materials for the process of replacing carbon with hydrogen and the adjustment of the subsequent steelmaking process for the process of replacing carbon with hydrogen.

[0003] Currently, the process of replacing carbon with hydrogen only completes the process of reducing iron ore to metallic iron and cannot remove the impurities in the ore itself and the impurities introduced when forming sinter ore or pellet ore. Therefore, high requirements are placed on the purity of the raw materials, and the iron grade TFe, that is, the mass percentage of iron ≥ 67.0%. The performance of the electric arc furnace depends on high-grade reduced iron to maintain a low slag output and high melting efficiency. However, not all iron ores can be selected to the DR grade. For the sponge iron obtained by treating blast furnace grade pellet ore or sinter ore using the DRI process, the existing technology melts it into molten iron using a submerged arc furnace or a resistance furnace, and further removes the impurities in an electric arc furnace (EAF) or a converter (BOF). Therefore, the advantage of using sponge iron for smelting is insufficient compared with traditional EAF melting. Summary of the Invention

[0004] The object of the present invention is to enable the sponge iron obtained after being melted in an electromagnetic induction furnace after being treated by the DRI process to be efficiently separated from impurities, improve the purity, and meet the requirements of subsequent smelting. The present invention effectively removes slag by using the eddy current motion, skin effect motion, and self-stirring motion of metallic iron during the electromagnetic induction melting process. By making good slag, while smelting good iron, it not only matches the subsequent process of DRI, but also relaxes the requirements for the purity of the raw materials.

[0005] The specific content of the invention is as follows: The iron ore raw materials are subjected to hydrogen-based reduction in a roasting furnace to obtain iron ore with zero-valent iron, i.e., reduced iron ore. The reduced iron ore is placed in a crucible of an electromagnetic induction furnace, and a water-cooled electromagnetic induction coil is wound around the outside of the crucible. Medium-frequency alternating current is introduced into the water-cooled coil. Under the action of the induced magnetic field, the zero-valent iron in the reduced iron ore acts as a metal conductor, and its internal electrons move in a circular motion perpendicular to the magnetic field direction. By virtue of the resistance heat generation in the conductor body, the temperature rises rapidly, and the metal melts rapidly, separating from some non-conductive or poorly conductive and poorly fluid non-metallic impurities. After the metal forms a melt, the iron melt sinks due to its high density. At the same time, the viscosity decreases, and a large number of carriers are generated inside to form eddy currents, and the skin effect also occurs, causing the iron melt to move towards the periphery and become denser at the edge. When the iron melt moves, the non-metallic impurities originally embedded in the iron powder particles experience a process of merging and growing and are expelled from the iron melt. The direction of the eddy current movement changes twice within one cycle, forming an internal stirring from the iron melt itself, deeply removing non-metallic impurities, and at the same time making the composition of the molten iron / molten steel uniform and the temperature distribution uniform, maintaining the molten state for 5 - 50 minutes, and then cooling and removing slag to obtain a purified ingot.

[0006] Further, for the iron ore raw materials, the iron grade TFe, i.e., the mass percentage of iron, is 50 - 65%, or it is pellet ore, or sintered ore, or natural iron ore lump ore, or iron concentrate powder. For the hydrogen-based reduction carried out in the roasting furnace, the gas used is either NH 3 , or H 2 , or NH 3 +H 2 , or the above reduction gas + auxiliary gas N 2 . In the reduction gas mixture, the volume percentage of the reducing gas in the mixture is more than 20%, the reaction temperature is 800 - 1100 °C, and the reduction is carried out to zero-valent iron. The heating rate of the electromagnetic induction furnace is higher than 150 °C / min, the maximum temperature is 1700 °C, and the TFe of the purified ingot is ≥ 96%.

[0007] Further, in the electromagnetic induction furnace, the temperature of the melt is controlled to be 1450 - 1650 °C, and the time for maintaining the entire iron in a molten state does not exceed 30 minutes. Then, the heating of the furnace charge is stopped, the crucible is taken out from the induction coil, and the furnace charge cools with the crucible.

[0008] Further, the crucible is a three-layer concentric cylindrical crucible. The inner and outer crucibles are either magnesia crucibles, or calcia crucibles, or corundum crucibles, the middle layer is a graphite crucible, the outer crucible supports the heating body, the middle crucible assists in heating, and the inner crucible loads the furnace charge, i.e., the reduced iron ore.

[0009] Compared with the prior art, the advantages of the present invention are as follows: 1. Electromagnetic induction enables three highly efficient separation methods for metallic iron and non-metallic impurities: (1) Melting separation: The porosity of DRI sponge iron plays an important role in melting separation. The abundant pores in the sponge iron endow the raw material with a large gas-solid interface. When the present invention uses an electromagnetic induction furnace to rapidly melt the sponge iron, the zero-valent iron in the reduced iron, as a metal conductor, is rapidly heated and generates heat, causing the temperature to rise rapidly. The metal quickly melts, and the large gas-solid interface is rapidly transformed into a large slag-liquid interface, which is conducive to the floating of non-metallic impurities with poor fluidity on the interface, thereby separating part of the impurities; (2) Molten separation: The eddy current movement and skin effect movement of the metal in the molten state form a self-centrifugal movement, which can remove non-metallic impurities embedded inside the melt; (3) Self-stirring separation: The self-internal stirring of the melt during the rotational eddy current movement can deeply remove non-metallic impurities. Thus, the purpose of highly efficient slag removal and improving the purity of reduced iron is achieved, and the gas-based reduced iron ore smelting technology is improved.

[0010] 2. For iron ore raw materials with TFe of 50-65%, including pellet ore, sintered ore, natural lump ore, and iron concentrate powder, high-quality iron-based ingots can be obtained through the method of "reduction + smelting".

[0011] 3. The electromagnetic induction smelting process is simple, can efficiently produce slag, does not use any additives, such as deoxidizers and slag formers, and can remove harmful inclusion elements without relying on the slag-steel interface metallurgical reaction. Description of the Drawings

[0012] Figure 1 It is the SEM photograph of the reduced pellet ore powder in Example 1 and the EDS spectrum of the corresponding area.

[0013] Figure 2 It is the photograph of the slag and ingot in Example 1.

[0014] Figure 3 It is the SEM photograph of the cross-section of the ingot in Example 1 and the EDS spectrum of the corresponding area. Detailed Description of the Invention

[0015] The present invention will be described in detail below with examples. Example 1

[0016] The iron ore raw material is pellet ore, with a particle size of 10-12 mm and an iron grade of 64.20%. 20 g of this raw material is subjected to reduction roasting in a roasting shaft furnace. The reducing gas used is NH 3 , which forms a mixed gas with the auxiliary gas N 2 . Among them, the volume percentage of NH 3 is 40%. When the furnace cavity temperature rises to 900 ± 10 °C, the mixed gas is introduced, and the reaction lasts for 3-3.5 hours to reduce the iron in the iron ore raw material to zero-valent iron; after cooling, it is crushed to less than 2 mm. Precautions: Before reduction, the air in the furnace is replaced with N2 After replacement, a reducing gas is introduced. After the reaction, N 2 is used as a cooling and protective gas. The SEM photographs of the reduced pellet powder and the EDS spectra of the corresponding regions are shown in Figure 1 , according to the analysis results of EDS, the iron grade of the reduced pellet is 88.82 - 91.84%, and the remaining components are mainly C, O, Si and Mn.

[0017] The crucible used in the electromagnetic induction heating furnace is a three-layer concentric cylindrical crucible. The inner and outer crucibles are corundum crucibles, the middle layer is a graphite crucible, the outer crucible supports the heating element, the middle crucible assists in heating, and the inner crucible loads the furnace charge. The reduced pellet powder is loaded into the inner crucible, and a water-cooled induction coil is wound around the outer crucible. Intermediate-frequency alternating current is introduced into the water-cooled induction coil and operates at a power of 4KW with a heating rate of 200°C / min. Under the action of the induction magnetic field, the zero-valent iron in the reduced iron ore acts as a metal conductor, and its internal electrons move in a circular motion perpendicular to the magnetic field direction. With the help of the resistance heat generation in the conductor body, the temperature rises rapidly, and the metal melts rapidly, separating from some non-metallic impurities that are non-conductive or have poor conductivity and poor fluidity. This separation process is melting separation; after about 6 - 8 minutes, when the metal forms a melt, the iron melt has a large density and sinks. At the same time, the viscosity decreases, and a large number of carriers are generated inside to form eddy currents, and the skin effect also makes the iron melt move towards the periphery, forming a spontaneous centrifugal movement, which can remove the non-metallic impurities embedded in the melt and densify at the edge. When the iron melt moves, the non-metallic impurities originally embedded in the iron powder particles experience the process of merging and growing and are squeezed out of the iron melt. This separation process is melting separation; the direction of the eddy current movement changes twice within one cycle, forming an internal stirring from the iron melt itself, deeply removing non-metallic impurities. This separation process is stirring separation. At the same time, the internal stirring makes the composition of the molten iron / steel water uniform and the temperature distribution uniform. The highest temperature of the melt is 1600°C, and it remains in a molten state for 10 minutes, and then it is cooled and slag is removed to obtain an ingot with TFe = 97.06%. The photographs of the slag and the ingot are shown in Figure 2 . Figure 2 It shows that the edge of the ingot is thick, dense and smooth, reflecting the effects of the circular motion and skin effect of the iron melt; the slag is also dense, and the contact surface with the ingot is smooth. Combining with the Figure 1 above SEM photographs of the reduced pellet powder and the EDS spectra of the corresponding regions, it is proved that the non-metallic impurities originally embedded in the iron powder particles experience the process of merging and growing and are squeezed out of the iron melt. The SEM photographs of the cross-section of the ingot and the EDS spectra of the corresponding regions are shown in Figure 3 , according to the analysis results of EDS: the iron grade of the ingot is about 96.53%, and the remaining components are mainly carbon. Therefore, this method can significantly improve the purity of the ingot. Example 2

[0018] Based on Example 1 of the present invention, the iron ore raw material used is natural iron concentrate lump ore with a particle size of 10 - 15 mm and an iron grade of 56.26%. 20 g of this raw material is subjected to hydrogen-based reduction roasting in a roasting shaft furnace. The reducing gas used is NH 3 , which forms a mixed gas with the auxiliary gas N 2 . Among them, the volume percentage of NH 3 is 30%. When the furnace chamber temperature rises to 900 ± 10 °C, the mixed gas is introduced and the reaction lasts for 3 hours to reduce the iron in the iron ore raw material to zero-valent iron. After cooling, it is crushed to less than 1 mm. Precautions: Before reduction, the air in the furnace is replaced with N 2 and then the reducing gas is introduced. After the reaction, N 2 is used as the cooling protective gas.

[0019] The reduced iron concentrate lump ore powder is loaded into a crucible (the same as in Example 1), placed in an electromagnetic induction heating furnace, and rapidly melted with 4 KW. The heating rate is 200 °C / min. All the iron powder is melted, the highest temperature is 1600 °C, and the total heating time is 15 min. Then, it is cooled and slag is removed to obtain an ingot with TFe = 96.50%. Example 3

[0020] Based on Example 1 of the present invention, the iron ore raw material used is sintered ore with a particle size of 10 - 15 mm and an iron grade of 56.48%. 20 g of this raw material is subjected to hydrogen-based roasting reduction in a roasting shaft furnace. The reducing gas used is NH 3 , which forms a mixed gas with the auxiliary gas N 2 . Among them, the volume percentage of NH 3 is 60%. When the furnace chamber temperature rises to 900 ± 10 °C, the mixed gas is introduced, the reduction temperature is 900 - 950 °C, and the reaction time is 2.5 - 3 hours to reduce the iron in the iron ore raw material to zero-valent iron. After cooling, it is crushed to less than 2 mm. Precautions: Before reduction, the air in the furnace is replaced with N 2 and then the reducing gas is introduced. After the reaction, N 2 is used as the cooling protective gas.

[0021] The reduced sintered ore powder is loaded into a crucible (the same as in Example 1), placed in an electromagnetic induction heating furnace, and rapidly melted with a power of 4.5 KW. The heating rate is 230 °C / min. All the iron powder is melted, the highest temperature is 1600 °C, and it is kept in a molten state for 6 - 10 min. Then, it is cooled and slag is removed to obtain an ingot with TFe = 97.11%. Example 4

[0022] Based on Example 1 of the present invention, the iron concentrate powder used, the main component is Fe 2 O 3, the iron grade is 52.63% and the proportion of -325 mesh is 84.0%. 30 g of this raw material is subjected to hydrogen-based roasting reduction in a roasting furnace, and the reducing gas used is H 2 . When the furnace cavity temperature rises to 900 ± 10 °C, H 2 is introduced and the reaction lasts for 1 hour to reduce the iron component in the iron ore raw material to zero-valent iron; after cooling, reduced iron ore powder is obtained. Precautions: Before reduction, the air in the furnace is replaced with N 2 and then hydrogen is introduced. After the reaction, N 2 is used as the cooling protective gas.

[0023] The reduced concentrate powder is loaded into a crucible (the same as in Example 1), placed in an electromagnetic induction heating furnace, and rapidly melted with a power of 4.5 KW at a heating rate of 230 °C / min. All the iron powder is melted, the highest temperature is 1600 °C, and the molten state is maintained for 15 - 20 min, and then it is cooled and slag is removed to obtain an ingot with TFe = 96.27%.

[0024] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them.

Claims

1. A method for improving the purity of reduced iron, characterized in that: The iron ore raw material is subjected to hydrogen-based reduction in a roasting furnace to obtain iron ore with zero-valent iron, i.e., reduced iron ore; The reduced iron ore is placed in the crucible of an electromagnetic induction furnace. A water-cooled electromagnetic induction coil surrounds the outside of the crucible. Medium-frequency alternating current is introduced into this water-cooled coil. Under the action of the induced magnetic field, the zero-valent iron in the reduced iron ore acts as a metal conductor, and its internal electrons move in a circular motion perpendicular to the magnetic field direction. By virtue of the resistance heat generation in the conductor body, the temperature rises rapidly, the metal melts rapidly, and is separated from some non-metallic impurities that are non-conductive, or have poor conductivity and poor fluidity; after the metal forms a melt, the iron melt has a large density and sinks. At the same time, the viscosity decreases, and a large number of carriers are generated inside to form eddy currents, and the skin effect also makes the iron melt move towards the periphery and densify at the edge. When the iron melt moves, the non-metallic impurities originally embedded in the iron powder particles go through the processes of merging and growing, and are squeezed out of the iron melt; the direction of the eddy current motion changes twice within one cycle, forming an internal agitation from the iron melt itself, deeply removing non-metallic impurities, and at the same time making the composition of the molten iron / steel water uniform and the temperature distribution uniform, maintaining the molten state for 5 - 50 minutes, and then cooling and removing slag to obtain a purified ingot.

2. The method for improving the purity of reduced iron according to claim 1, characterized in that: The iron ore raw material has an iron grade of TFe, that is, an iron content of 50-65% by mass, or is a pellet, a sintered ore, a natural iron ore lump, or an iron ore concentrate powder; the gas-based reduction in the roasting furnace is either NH 3 , or H 2 , or NH 3 +H 2 , or the above reducing gas + auxiliary gas N 2 In the reducing mixed gas, the reducing gas accounts for more than 20% of the volume percentage of the mixed gas, the reaction temperature is 800-1100°C, and the reduction is to zero-valent iron; the heating rate of the electromagnetic induction furnace is higher than 150°C / min, the maximum temperature is 1700°C, and the purified ingot TFe is ≥96%.

3. The method for improving the purity of reduced iron according to claim 1, characterized in that: In the electromagnetic induction furnace described above, the temperature of the melt is controlled at 1450 - 1650 °C, and the time for maintaining the entire iron in the molten state does not exceed 30 minutes. Then, the heating of the furnace charge is stopped, the crucible is taken out from the induction coil, and the furnace charge cools with the crucible.

4. The method for improving the purity of reduced iron according to claim 1, characterized in that: The crucible is a three-layer concentric cylindrical crucible. The inner and outer crucibles are either magnesia crucibles, or calcium oxide crucibles, or corundum crucibles, the middle layer is a graphite crucible, the outer crucible supports the heating body, the middle crucible assists in heating, and the inner crucible loads the furnace charge, i.e., the reduced iron ore.

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