Method for preparing iron and chromium by reducing chromium-containing raw materials through electromagnetic induction heating

Abstract

The invention belongs to the field of iron and steel smelting, and particularly relates to a method for preparing iron and chromium by reducing chromium-containing raw materials through electromagnetic induction heating. The method comprises the following steps: (1) carrying out first pelletizing on raw materials comprising chromium-containing metallurgical solid waste, first chromite, a first carbon source, metal iron particles and a first additive to obtain primary pellets containing the metal iron particles; (2) the primary pellets are subjected to first reduction, and a slag-iron mixture is obtained; (3) crushing and grinding the slag-iron mixture, and carrying out electromagnetic separation to obtain chromium-rich slag and iron particles; (4) the chromium-rich slag, second chromite, a second carbon source, ferrochrome alloy particles, a second additive and a slag former are subjected to second pelletizing, and secondary pellets containing ferrochrome alloy particles are obtained; (5) secondary reduction is conducted on the secondary pellets, and ferrochromium alloy melt and refining slag are obtained; and (6) casting the ferrochrome alloy melt into an alloy ingot, cooling, crushing, and screening to obtain a ferrochrome alloy product. The method can improve the recovery rate of Cr.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of steel smelting, and particularly relates to a method for preparing iron-chromium by using electromagnetic induction heating reduction of chromium-containing raw materials. BACKGROUND

[0002] In the process of stainless steel smelting, a large amount of chromium-containing metallurgical dust is generated in process links such as electric arc furnace (EAF), argon-oxygen decarburization furnace (AOD) and the like, and a large amount of pickling dust and mud and the like containing chromium metallurgical solid waste is also generated in pickling, neutralization and the like. The yield of such solid waste accounts for about 3% to 5% of the total yield of stainless steel, showing a significant scale trend.

[0003] The chromium-containing metallurgical solid waste is rich in iron oxides (FeOx) and chromium oxides (Cr2O3, CrO3) with high content, and is often accompanied by a certain amount of impurity oxides such as nickel oxide (NiO), calcium oxide (CaO), magnesium oxide (MgO), silicon dioxide (SiO2), zinc oxide (ZnO), aluminum oxide (Al2O3) and the like, and contains trace amounts of toxic heavy metal elements such as lead (Pb), cadmium (Cd) and the like. During the natural piling process of these solid wastes, heavy metal elements (such as Zn 2+ , Cr 3+ , Pb 2 +, Cd 2 +) are easily leached with rainwater and seep into the soil and groundwater, and the concentration often exceeds the limit value of environmental protection regulations, seriously threatening the ecological environment and human health.

[0004] Due to the above reasons, the chromium-containing metallurgical solid waste is identified as hazardous waste, and it is legally prohibited to be directly landfilled or arbitrarily discarded. At present, although such solid waste is rich in valuable metal resources such as Cr, Fe, Ni, Zn and the like and has high recycling potential, due to the limitation of existing treatment technology, the resource utilization rate is always at a low level, and most of them are still treated by landfill or mixed into building materials and the like in a low-value manner, which not only causes resource waste, but also hides long-term environmental risks.

[0005] From the perspective of metallurgical thermodynamics, the thermal stability of Cr2O3 is significantly higher than that of Fe2O3, and it is difficult to effectively reduce it to metallic chromium under conventional reduction conditions. Therefore, deep reduction of Cr2O3 usually needs to be carried out under high temperature, strong reducing atmosphere or vacuum conditions, which puts higher requirements on the structure of equipment, energy consumption control and process regulation. The current widely used heat treatment equipment such as rotary kiln and rotary hearth furnace has problems such as low thermal efficiency and difficulty in accurately controlling the reaction atmosphere, which is difficult to meet the needs of efficient reduction of Cr2O3, resulting in low utilization rate of chromium resources.

[0006] As a high-efficiency, controllable and clean heating method, electromagnetic induction heating has shown its superiority in high-end metal smelting and special metallurgy. It has a high heating efficiency, a fast heating speed, a precise temperature control, can realize local heating and protective atmosphere operation, and has a good industrial application prospect. However, due to the poor electrical conductivity of the powdery metallurgical solid waste, the traditional electromagnetic induction heating cannot directly act on the body of the powdery metallurgical solid waste, which limits its direct application in the field of metallurgical solid waste treatment.

[0007] Therefore, it is urgent to develop a green and efficient reduction technology path taking electromagnetic induction heating as the core, supporting the construction of an electrically conductive path and a reaction system, to realize the efficient recovery of valuable metal elements such as iron and chromium in chromium-containing metallurgical solid waste, especially to effectively break through the technical bottleneck of difficult reduction of Cr2O3, and finally prepare chromium-iron alloy products with high added value. This technology not only can improve the utilization rate of solid waste resources and reduce the environmental protection cost of enterprises, but also can significantly reduce the environmental impact of hazardous waste, and has important industrial value and environmental protection significance. SUMMARY

[0008] The purpose of the present application is to provide a method for preparing iron-chromium by reducing chromium-containing raw materials using electromagnetic induction heating. The method in the present application can effectively improve the production efficiency of chromium-iron alloy and improve the economic benefit of the process.

[0009] In order to achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0010] The present application provides a method for preparing chromium-iron by reducing chromium-containing raw materials using electromagnetic induction heating, which comprises:

[0011] (1) preparing primary pellets: first pelletizing raw materials including chromium-containing metallurgical solid waste, first chromium-iron ore, first carbon source, metallic iron particles and first additive to obtain primary pellets containing metallic iron particles;

[0012] (2) first reduction: first reducing the primary pellets obtained in step (1) to obtain slag-iron mixture;

[0013] (3) slag-iron separation: crushing, grinding and electromagnetically separating the slag-iron mixture obtained in step (2) to obtain chromium-rich slag and iron particles;

[0014] (4) preparing secondary pellets: second pelletizing the chromium-rich slag obtained in step (3) with second chromium-iron ore, second carbon source, chromium-iron alloy particles, second additive and slagging agent to obtain secondary pellets containing chromium-iron alloy particles;

[0015] (5) second reduction: second reducing the secondary pellets obtained in step (4) to obtain chromium-iron alloy melt and refining slag;

[0016] (6) Casting and post-processing: the chromium-iron alloy melt obtained in step (5) is cast into alloy ingots, which are cooled and then broken and sieved to obtain chromium-iron alloy products.

[0017] The particle size of the chromium-containing metallurgical solid waste, the first chromium-iron ore, the second chromium-iron ore, the first carbon source, the second carbon source, the first additive, the second additive, and the slagging agent in the present application is not particularly limited, and the particle size range commonly used in conventional pelletizing processes can be used to ensure good pelletizing performance. The present application does not have special restrictions, so it is not necessary to repeat it.

[0018] Preparation of primary pellets

[0019] In step (1), based on the total mass of the chromium-containing metallurgical solid waste, the first chromium-iron ore, the first carbon source, the metallic iron particles, and the first additive, the mass content of the chromium-containing metallurgical solid waste is 50% to 65%, preferably 55% to 60%.

[0020] By controlling the proportion of chromium-containing metallurgical solid waste in the above range, not only can the inexpensive chromium-containing metallurgical solid waste be fully utilized to reduce production costs, but also it is beneficial to control the composition of the chromium-iron product within the target range.

[0021] The chromium-containing metallurgical solid waste refers to solid waste generated during the smelting process of chromium-iron alloy (mainly high-carbon chromium-iron, also including silicon-chromium alloy, medium and low-carbon chromium-iron, etc.), which is mainly composed of chromium-containing compounds, and may also contain oxides of magnesium, calcium, aluminum, iron, silicon, etc. Preferably, the mass content of FeOx (such as Fe2O3, FeO) in the chromium-containing metallurgical solid waste is greater than 30%, more preferably greater than 40%; preferably, the mass content of Cr2O3 in the chromium-containing metallurgical solid waste is greater than 10%, more preferably greater than 20%, and most preferably greater than 30%. The remaining components in the chromium-containing metallurgical solid waste are common oxides such as SiO2, MgO, CaO, Al2O3, and ZnO. By controlling the chemical composition of the chromium-containing metallurgical solid waste within the above range, the present application can play the role of preferred raw materials, thereby reducing production costs.

[0022] As an example, the chromium-containing metallurgical solid waste in the present application has a mass percentage of 29.17% Fe2O3, 20.43% FeO, 18.89% Cr2O3, 2.92% NiO, 2.85% MnO, 0.38% ZnO, 16.43% CaO, 3.14% MgO, 1.24% Al2O3, 4.54% SiO2, and not more than 0.01% S and P.

[0023] Preferably, based on the total mass of the chromium metallurgical solid waste, the first chromium-iron ore, the first carbon source, the metallic iron particles, and the first additive, the mass content of the first chromium-iron ore is 10% to 35%, preferably 15% to 25%.

[0024] The application controls the proportion of the chromite in the first pellet in the above range, can supplement the chromium content in the chromium-containing metallurgical solid waste, ensures sufficient chromium resources in the reduction process, and is beneficial to control the composition of the chromium-iron product in the target range.

[0025] In the application, the first chromite preferably adopts metallurgical-grade chromite in terms of mass percentage, ensuring that the mass ratio of Cr / Fe is > 2.

[0026] As an example, the main chemical components of the first chromite in the application are 52.3% of Cr2O3, 14.8% of FeO, 16.8% of MgO, 7.2% of Al2O3, 3.1% of SiO2, 0.5% of CaO, 0.3% of MnO, and not more than 5% of S and P in terms of mass percentage.

[0027] Preferably, the ratio of the molar amount of C element in the first carbon source to the total molar amount of O element in the reducible metal oxides in the raw material is preferably 1-1.2, and more preferably 1.05-1.1, wherein the reducible metal oxides include Fe2O3, FeO, NiO and MnO.

[0028] In the application, the amount of carbon added is controlled in the above range, which can ensure the sufficient reduction of iron oxides and avoid the premature reduction of chromium during low-temperature reduction, affecting the enrichment of Cr2O3.

[0029] Preferably, the first carbon source includes at least one of coke, graphite and anthracite.

[0030] Preferably, the mass content of the metallic iron particles is 3%-15%, and more preferably 6%-12%, based on the total mass of the chromium-containing metallurgical solid waste, the first chromite, the first carbon source, the metallic iron particles and the first additive.

[0031] The application controls the proportion of the metallic iron particles in the iron ore pellet in the above range, which can ensure that the pellet can be efficiently heated in the electromagnetic induction furnace, promote the reduction reaction, and at the same time control the amount of metallic particles to reduce production cost.

[0032] The application does not have special limitations on the specific morphology of the metallic iron particles, and therefore is not described in detail. The metallic iron particles in the application are preferably direct reduced iron particles.

[0033] Preferably, the particle size of the metallic iron particles is 3-10 mm, and more preferably 4-8 mm.

[0034] The particle size of the metal iron particles is controlled in the range, which is beneficial to realize effective energy coupling and rapid heating in the electromagnetic induction heating process. The larger particle size helps to form a closed path for the induced current in the particle, thereby improving the induction heating efficiency of the unit particle. However, too small particles are prone to insufficient heating or uneven temperature rise due to small volume and short conductive path.

[0035] Preferably, the mass content of the first additive is 1% to 5%, more preferably 2% to 4%, based on the total mass of the chromium-containing metallurgical solid waste, the first chromite, the first carbon source, the metal iron particles and the first additive.

[0036] The present application can better form a ball in the balling process by controlling the proportion of the first additive in the above range.

[0037] Preferably, the first additive includes one or more of bentonite, water glass, dolomite, kaolin, starch and polymer auxiliary materials, preferably bentonite and / or dolomite.

[0038] More preferably, the first additive further includes boron trioxide, and preferably the mass ratio of boron trioxide to bentonite and / or dolomite is 1:(3-6).

[0039] In the present application, it is found that when a certain amount of boron trioxide is contained in the first additive, the Cr recovery rate can be further improved while reducing the C content, which is presumably because boron trioxide can inhibit the formation of Cr-Fe.

[0040] Preferably, the diameter of the primary pellets is 10 to 30 mm, preferably 15 to 25 mm.

[0041] In the present application, the diameter of the pellets is controlled in the range, which is beneficial to improve the reduction reaction rate, and at the same time helps to regulate the mass ratio of the metal iron particles in the pellets, ensuring the stability of the pellet structure and the uniformity of the reaction.

[0042] In the present application, in view of the difficulty of metal iron particles to be fully combined with other balling raw materials, the first balling in step (1) can be carried out in the presence of a viscous liquid, preferably the viscous liquid comprises at least one of mineral oil, water glass solution and cooked starch solution, and of course other industrial liquids with certain bonding properties can also be selected, preferably the viscous liquid is mineral oil, for example the mineral oil can be 15# white oil; the present application can mix the components to obtain the mixed raw materials, in order to mix more uniformly, some components can be fully added and then the remaining components are mixed, for example the chromium-containing metallurgical solid waste, the first chromium iron ore, the first carbon source and the first additive are fully ground and mixed uniformly to obtain a solid mixture; the surface of the metal iron particles is uniformly coated with a layer of viscous liquid, so that the metal iron particles are wetted by the viscous liquid, and then the solid mixture is added for the first balling.

[0043] In the present application, the specific conditions of the first balling in step (1) are not limited, and the diameter of the primary pellets is 10-30mm, preferably 15-25mm.

[0044] In the present application, the balling equipment for the first balling can use conventional equipment such as balling disc and balling drum maturely applied in the art, and the type of equipment is not specially limited, and the specific selection can be determined according to the production conditions and material properties.

[0045] Preferably, the method of the present application further comprises a drying step when preparing the primary pellets, and the drying conditions are not special, as long as the water in the pellets can be removed, for example drying at 80-120℃ for 8-24h.

[0046] By using the drying process instead of the traditional roasting, not only the water content of the pellets is effectively reduced, and the uniform and stable strength is given, but also the energy consumption and processing time are significantly reduced, and the economic efficiency and efficiency of the overall process are improved.

[0047] Preferably, the step of preparing the primary pellets comprises: fully grinding and mixing the chromium-containing metallurgical solid waste, the first chromium iron ore, the first carbon source and the first additive to obtain a solid mixture; uniformly coating the surface of the metal iron particles with a layer of viscous liquid, so that the metal iron particles are wetted by the viscous liquid, and then adding the solid mixture to the balling disc for balling to obtain pellets with a diameter of 10-30mm (preferably 15-25mm), and drying the pellets at 80-120℃ for 8-24h to obtain the primary pellets.

[0048] First reduction

[0049] The equipment for the first reduction in the present application is not specially limited, for example, it can be carried out in an electromagnetic induction heating furnace.

[0050] Preferably, in step (2), the conditions of the first reduction include: temperature of 850-1050℃, preferably 900-1000℃; time of 10-60min, preferably 20-40min.

[0051] The present application limits the temperature and time of the low-temperature reduction in the above range, which can ensure sufficient reduction of iron oxides and effectively inhibit the reduction reaction of chromium oxides, avoiding the premature reduction of chromium elements and affecting the subsequent separation and product quality.

[0052] Preferably, the step of the first reduction includes: loading the primary pellets obtained in step (1) into an electromagnetic induction heating furnace, gradually heating to 850-1050℃ (preferably 900-1000℃), and holding for 10-60min (preferably 20-40min) for the first reduction. After the first reduction is completed, the slag-iron mixture is obtained by natural cooling and discharging.

[0053] Slag-iron separation

[0054] In step (3) of the present application, the slag-iron mixture obtained in step (2) is crushed, ground, and subjected to electromagnetic separation to obtain chromium-rich slag and iron particles. The crushing and grinding can be performed using conventional equipment in the art, and the type of equipment is not limited. The type of electromagnetic separation equipment is also not limited, and suitable equipment can be selected according to actual needs, for example, a magnetic separation column can be used for magnetic separation to achieve electromagnetic separation. The electromagnetic separation (i.e., magnetic separation) can be performed multiple times (e.g., 2-6 times) to improve the purity and recovery rate of the separation and achieve efficient and reliable solid separation. Further, the ground material in step (3) can be sieved as needed, and the sieving method can be a conventional method in the art, which is not limited in the present application, and therefore will not be described in detail.

[0055] Preferably, the step of slag-iron separation in step (3) includes: crushing and grinding the slag-iron mixture obtained in step (2), and performing 2-6 times of magnetic separation using a magnetic separation column to obtain chromium-rich slag and iron particles.

[0056] Preparation of secondary pellets

[0057] Preferably, in step (4), based on the total mass of the chromium-rich slag, the second chromium ore, the second carbon source, the chromium-iron alloy particles, the second additive, and the slag-making agent, the mass content of the chromium-rich slag is 50-60%, preferably 53-58%.

[0058] Preferably, in step (4), based on the total mass of the chromium-rich slag, the second chromium ore, the second carbon source, the chromium-iron alloy particles, the second additive, and the slag-making agent, the mass content of the second chromium ore is 5-20%, preferably 8-15%.

[0059] Preferably, in step (4), the mass content of the chromium-iron alloy particles is 3-15%, preferably 6-12%, based on the total mass of the chromium-rich slag, the second chromite, the second carbon source, the chromium-iron alloy particles, the second additive, and the slagging agent.

[0060] Preferably, in step (4), the mass content of the second additive is 1-5%, preferably 2-4%, based on the total mass of the chromium-rich slag, the second chromite, the second carbon source, the chromium-iron alloy particles, the second additive, and the slagging agent.

[0061] Preferably, in step (4), the mass content of the slagging agent is 5-10%, preferably 6-8%, based on the total mass of the chromium-rich slag, the second chromite, the second carbon source, the chromium-iron alloy particles, the second additive, and the slagging agent.

[0062] Preferably, the molar ratio of the C element in the second carbon source to the O element in Cr2O3, Fe2O3, FeO, MnO, and SiO2 in the second chromite and the chromium-rich slag is 1-1.2, preferably 1.05-1.1.

[0063] In the present application, the raw material composition of the secondary pellets is controlled within the above range, which is conducive to efficient heating and reduction of the pellets under electromagnetic induction conditions, and helps to ensure the stable composition and reliable quality of the final chromium-iron product.

[0064] The main component of the chromium-rich slag is Cr2O3.

[0065] In the present application, the morphology of the chromium-iron alloy particles is not particularly limited, and preferably, the particle size of the chromium-iron alloy particles is 3-10 mm, preferably 4-8 mm.

[0066] The chromium-iron alloy particles in the present application can be any chromium-iron alloy particles in the art, for example, FeCr69C0.03 chromium-iron alloy particles.

[0067] In the present application, the particle size of the chromium-iron alloy particles is controlled within the range, which is conducive to effective energy coupling and rapid heating during electromagnetic induction heating. In the present application, the type of carbon source in the secondary pellets is the same as that in the primary pellets. In the present application, the type of additive in the secondary pellets is the same as that in the primary pellets.

[0068] The second chromite in step (4) of the present application can be the same as the specific type of the first chromite.

[0069] As an example, the main chemical composition of the second chromite in the application is: 52.3% of Cr2O3, 14.8% of FeO, 16.8% of MgO, 7.2% of Al2O3, 3.1% of SiO2, 0.5% of CaO, 0.3% of MnO, and S and P are not higher than 5% in terms of mass percentage.

[0070] The second carbon source in the application can be the same as the first carbon source in the application, that is, the second carbon source includes at least one of coke, graphite and anthracite. That is, the first carbon source and the second carbon source each independently include at least one of coke, graphite and anthracite.

[0071] Preferably, the second carbon source in the application is used after pretreatment, and the pretreatment method includes: immersing 1-3wt% of a magnesium sulfate aqueous solution in the second carbon source, and then filtering, and sintering the solid phase at 600-800℃ with CO2 for 30-50min to obtain the pretreated carbon source.

[0072] In the application, the pretreated first carbon source in the application can effectively increase the recovery rate of Cr and reduce the carbon content.

[0073] The second additive in the application can be the same as the first additive in the application, that is, the first additive and the second additive each independently include one or more of bentonite, water glass, dolomite, kaolin, starch and polymer additive materials, preferably bentonite and / or dolomite.

[0074] Preferably, the first additive and the second additive further include boron trioxide, and preferably the mass ratio of boron trioxide to bentonite and / or dolomite is 1:(3-6).

[0075] Preferably, the slagging agent includes quartz sand and / or fluorite, preferably includes quartz sand and fluorite, and more preferably, the mass ratio of the quartz sand to the fluorite is (30-90):(10-70), preferably (40-60):(40-60).

[0076] Controlling the slagging agent composition in the application within the range is helpful to reduce the melting point of the molten slag, improve the fluidity of the molten slag, thereby promoting the sufficient melting of the molten slag and the separation of the molten slag and the gold in the subsequent melting separation process, and improving the yield of the ferrochrome alloy.

[0077] Preferably, the diameter of the secondary pellet is 10-30mm, preferably 15-25mm.

[0078] Controlling the pellet diameter in the application within the range is beneficial to improve the reduction reaction rate, and is also helpful to regulate the mass ratio of the metallic iron particles in the pellet, and ensure the stability of the pellet structure and the reaction uniformity.

[0079] In the present application, in view of the difficulty of the ferrochrome alloy particles to be fully combined with other balling raw materials, the second balling in step (4) is preferably performed in the presence of a viscous liquid, preferably the viscous liquid comprises at least one of a mineral oil, a water glass solution and a cooked starch solution, and of course other industrial liquids with certain binding properties can also be selected, preferably the viscous liquid is a mineral oil, for example the mineral oil can be 15# white oil; the present application can mix the components to obtain the mixed raw materials, in order to mix more uniformly, some components can be fully mixed and then the remaining components are added for mixing. For example, the chromium-rich residue is fully mixed with the second chromite, the second carbon source, the second additive and the slagging agent to obtain a solid mixture; the surface of the ferrochrome alloy particles is coated with a layer of viscous liquid, so that the ferrochrome alloy particles are wetted by the viscous liquid, and then the solid mixture is added for the second balling.

[0080] In the present application, the specific conditions of the second balling in step (4) are not limited, and the diameter of the secondary pellets is 10-30mm, preferably 15-25mm.

[0081] In the present application, the balling equipment for the second balling can use conventional equipment such as balling disc and balling drum maturely applied in the art, and the type of equipment is not specially limited, and the specific selection can be determined according to the production conditions and material properties.

[0082] Preferably, the method of the present application further comprises a drying step when preparing the secondary pellets, and the drying conditions are not special, as long as the water in the pellets can be removed, for example, drying at 80-120℃ for 8-24h.

[0083] By using the drying process instead of the traditional roasting, not only the water content of the pellets is effectively reduced, and the uniform and stable strength is given, but also the energy consumption and processing time are significantly reduced, and the economic efficiency and efficiency of the overall process are improved.

[0084] Preferably, the step of preparing the secondary pellets comprises: fully mixing the chromium-rich residue obtained in step (4) with the second chromite, the second carbon source, the second additive and the slagging agent to obtain a solid mixture; coating the surface of the ferrochrome alloy particles with a layer of viscous liquid, so that the ferrochrome alloy particles are wetted by the viscous liquid, and then adding the solid mixture into the balling disc for balling, to obtain pellets with a diameter of 10-30mm (preferably 15-25mm), and drying the pellets at 80-120℃ for 8-24h to obtain the secondary pellets containing ferrochrome alloy particles.

[0085] Second reduction

[0086] In the present application, the equipment for the second reduction is not specially limited, for example, it can be performed in an electromagnetic induction heating furnace.

[0087] Preferably, the conditions of the second reduction include: a first stage reduction and a second stage reduction, the temperature of the first stage reduction is 1200-1450℃, preferably 1300-1400℃; the time of the first stage reduction is 15-60min, preferably 30-40min; the second stage reduction is carried out under a reducing agent; the temperature of the second stage reduction is 1500-1600℃, preferably 1530-1580℃; the time of the second stage reduction is 10-30min, more preferably 15-25min.

[0088] The present application limits the temperature and time of the second reduction in the above range, which can effectively promote and ensure the reduction of chromium oxide, fully reduce, promote the formation of stable chromium-iron alloy, and achieve complete separation of slag and gold, while optimizing the time and energy consumption of the reaction process, enhancing the overall economy and operability of the process.

[0089] Further, the present application continues to carry out the second stage reduction in the presence of a reducing agent, which can perform melting separation treatment on the slag-chromium mixture to achieve complete reduction of Cr2O3, and obtain chromium-iron alloy melt and refining slag.

[0090] Preferably, the first stage reduction is carried out in an inert atmosphere for 5-30min (for example, 3-10L / min of argon or nitrogen is introduced at 1200-1450℃ (preferably 1300-1400℃) for 5-30min), and then switched to vacuum conditions until the end of the first stage reduction reaction, preferably the vacuum degree is <1000Pa, preferably <100Pa, and the vacuum reduction time is preferably 10-30min, more preferably 15-25min.

[0091] Preferably, the reducing agent includes a ferrosilicon reducing agent, which can be a conventional ferrosilicon reducing agent in the art, for example, it can be a commercially available 75 ferrosilicon powder.

[0092] Preferably, the mass of the reducing agent is 5-20% of the mass of the second chromium-iron ore, preferably 8-15%.

[0093] The present application limits the reducing agent in the above range, which can ensure the full reduction of chromium oxide, promote the formation of stable chromium-iron alloy, and achieve complete separation of slag and gold, thereby significantly improving the recovery rate of chromium and product quality.

[0094] Preferably, the specific step of the second reduction comprises: placing the secondary pellets obtained in step (4) into an electromagnetic induction heating furnace and gradually heating to 1200-1450℃ (preferably 1300-1400℃), first passing in 3-10 L / min of argon or nitrogen and maintaining for 5-30 min, then vacuumizing to less than <1000 Pa (preferably <100 Pa) and maintaining for 10-30 min (preferably 15-25 min) for vacuum reduction; then heating to 1500-1600℃ (preferably 1530-1580℃), adding a reducing agent, and continuing to smelt for 10-30 min (preferably 15-25 min) to obtain a ferrochrome alloy melt and a refining slag.

[0095] Casting and post-processing

[0096] In the present application, the ferrochrome alloy melt can be obtained by static stratification and slagging.

[0097] The ferrochrome alloy product prepared by the preparation method of the present application is a medium-carbon ferrochrome alloy product, wherein the mass content of carbon in the medium-carbon ferrochrome alloy product is less than 2.5%, preferably 0.5-4%, and more preferably 1.0-2.5% by mass percentage.

[0098] More preferably, the method for preparing ferrochrome by reducing chromium-containing raw materials using electromagnetic induction heating comprises the following steps:

[0099] (1) preparing primary pellets: grinding and mixing the chromium-containing metallurgical solid waste, chromite, first carbon source and first additive uniformly to obtain a solid mixture; uniformly coating a layer of mineral oil on the surface of the metal iron particles so that the metal iron particles are wetted by the viscous liquid, and then adding the solid mixture to the balling disc to form pellets with a diameter of 10-30 mm, and drying the pellets at 105℃ for 12 h to obtain primary pellets;

[0100] (2) first reduction: loading the primary pellets obtained in step (1) into an electromagnetic induction heating furnace, gradually heating to 850-1050℃, and maintaining for 10-60 min for first reduction, after the first reduction is completed, naturally cooling and discharging to obtain a slag-iron mixture;

[0101] (3) slag-iron separation: crushing and grinding the slag-iron mixture obtained in step (2), and using a magnetic separation column for 2-6 times of magnetic separation to obtain chromium-rich slag and iron particles;

[0102] (4) preparing secondary pellets: the chromium-rich residue obtained in step (4) is mixed with chromite, a second carbon source, a second additive and a slagging agent to obtain a solid mixture; the surface of the ferrochrome alloy particles is coated with a layer of viscous liquid, so that the ferrochrome alloy particles are wetted by the viscous liquid, and then the solid mixture is added to the balling disc to form pellets with a diameter of 10-30 mm, and the pellets are dried at 105℃ for 12h to obtain secondary pellets containing ferrochrome alloy particles;

[0103] (5) second reduction: the secondary pellets obtained in step (4) are placed in an electromagnetic induction heating furnace and gradually heated to 1200-1450℃, argon or nitrogen gas is first introduced at a flow rate of 3-10L / min for 5-30min, then vacuum is applied to less than 1000Pa and maintained for 10-30min for vacuum reduction; then the temperature is raised to 1500-1600℃ and silicon-iron reducing agent is added, and the smelting is continued for 10-30min to obtain a ferrochrome alloy melt and refining slag.

[0104] (6) casting and post-processing: after smelting, the layers are separated and the slag is removed to obtain a ferrochrome alloy melt, which is poured into a cast iron mold to cool, and then broken and sieved to obtain ferrochrome alloy particles with uniform particle size.

[0105] Compared with the prior art, the present application has at least the following beneficial effects:

[0106] 1. In the present application, the reduction process is divided into two stages: iron oxide preferential reduction (850-1050℃) and chromium oxide deep reduction (1200-1600℃), which solves the problem of high thermal stability of chromium oxide and the difficulty of efficient reduction in traditional process by matching the temperature gradient with the reducing agent.

[0107] 2. Compared with the traditional rotary kiln or rotary hearth furnace, the present application uses metal iron particles / ferrochrome alloy particles as the induction heating medium, and coats them with a viscous liquid (such as mineral oil) to enhance the combination with the solid mixture, which solves the technical obstacle of non-conductivity of the powdered solid waste, realizes efficient and directional heating, and at the same time cooperates with electromagnetic induction to make the heating speed fast and the thermal efficiency high, which can realize precise temperature control and avoid the effects of local overburning or insufficient reduction. DETAILED DESCRIPTION

[0108] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0109] In the following examples and comparative examples:

[0110] The mass content of C in the charcoal is 85%; the mass content of C in the graphite is 87%; the mass content of C in the anthracite is 82%; the average particle size of the quartz sand is 1 mm; the average particle size of the fluorite is 180 microns; and the average particle size of the dolomite is 150 microns.

[0111] The main chemical components of the chromium-containing metallurgical solid waste are as follows in terms of mass percentage: 29.17% Fe2O3, 20.43% FeO, 18.89% Cr2O3, 2.92% NiO, 2.85% MnO, 0.38% ZnO, 16.43% CaO, 3.14% MgO, 1.24% Al2O3, 4.54% SiO2, and S, P < 0.01%.

[0112] The main chemical components of the chromium-containing metallurgical solid waste are as follows in terms of mass percentage: 29.17% Fe2O3, 20.43% FeO, 18.89% Cr2O3, 2.92% NiO, 2.85% MnO, 0.38% ZnO, 16.43% CaO, 3.14% MgO, 1.24% Al2O3, 4.54% SiO2, and S, P < 0.01%.

[0113] The electromagnetic induction furnace has a capacity of 500 g and a working frequency of 3500 Hz. The loading amount of the primary pellets is about 90% of the furnace capacity, and the loading amount of the secondary pellets is determined according to the content of the chromium-rich slag after the slag-iron separation, and the total amount is less than 90% of the furnace capacity.

[0114] During the temperature rising process of the first reduction and the second reduction procedures, the power of the electromagnetic induction furnace is 4.5 kW and 6 kW, respectively, and the temperature rising power of the smelting process is 6.5 kW. During the constant temperature reduction process and the melting separation stage, the furnace temperature is automatically adjusted by a PID program control system to ensure the stability of the temperature and the controllability of the reduction reaction.

[0115] Example 1

[0116] Preparation of chromium-iron alloy:

[0117] (1) Preparation of primary pellets: 225 g of chromium-containing metallurgical solid waste, 132.34 g of chromite, 39.48 g of coke, and 10.5 g of bentonite were thoroughly ground and mixed to obtain a solid mixture; 42.7 g of direct reduced iron particles (with an average particle size of 6.5 mm) were uniformly coated with a layer of mineral oil to make the direct reduced iron particles wetted with the mineral oil, and then the solid mixture was added to the direct reduced iron particles and balling was performed in a balling disc to obtain pellets with a diameter of 18 mm. The pellets were dried at 105°C for 12 h to obtain the primary pellets;

[0118] (2) First reduction: the primary pellets obtained in step (1) were loaded into the electromagnetic induction heating furnace, and gradually heated to 980°C, and kept for 30 min for the first reduction. After the first reduction was completed, the slag-iron mixture was naturally cooled and discharged.

[0119] (3) Slag-iron separation: the slag-iron mixture obtained in step (2) is crushed, ground and sieved, and 3 times of magnetic separation is performed using a magnetic separation column to obtain chromium-rich slag and iron particles;

[0120] (4) Preparation of secondary pellets: 200 g of the chromium-rich slag obtained in step (4) is mixed with 37 g of chromite, 63 g of coke, 16 g of bentonite and 24 g of slagging agent (12 g of quartz sand and 12 g of fluorite) to obtain a solid mixture; 60 g of chromium-iron alloy particles are coated with a layer of mineral oil to wet the chromium-iron alloy particles, and then added to the solid mixture to form pellets in a pelletizing disc, and pellets with a diameter of 27 mm are prepared; the pellets are dried at 105°C for 12 h to obtain secondary pellets containing chromium-iron alloy particles;

[0121] (5) Second reduction: the secondary pellets obtained in step (4) are placed in an electromagnetic induction heating furnace and gradually heated to 1300°C, 5.5 L / min of argon is introduced for 20 min, then vacuum is applied to 950 Pa and maintained for 15 min for vacuum reduction; then the temperature is raised to 1550°C, 5 g of ferrosilicon reducing agent is added, and the smelting is continued for 15 min to obtain chromium-iron alloy melt and refining slag.

[0122] (6) Casting and post-processing: after smelting, the chromium-iron alloy melt is obtained by layering and slagging, and the chromium-iron alloy melt is poured into a cast iron mold to cool; after cooling, crushing and sieving, chromium-iron alloy particles with uniform particle size are obtained.

[0123] Example 2

[0124] Preparation of chromium-iron alloy:

[0125] (1) Preparation of primary pellets: 248.4 g of chromium-containing metallurgical solid waste, 121.75 g of chromite, 35 g of graphite and 22.5 g of dolomite are thoroughly ground and mixed to obtain a solid mixture; 22.5 g of direct reduced iron particles (average particle size 3.5 mm) are uniformly coated with a layer of mineral oil to wet the direct reduced iron particles, and then added to the solid mixture to form pellets in a pelletizing disc, and pellets with a diameter of 12 mm are prepared; the pellets are dried at 105°C for 12 h to obtain primary pellets;

[0126] (2) First reduction: the primary pellets obtained in step (1) are loaded into an electromagnetic induction heating furnace, and gradually heated to 850°C, and maintained for 60 min for first reduction; after the first reduction is completed, the material is naturally cooled to obtain a slag-iron mixture;

[0127] (3) Slag-iron separation: the slag-iron mixture obtained in step (2) is crushed, ground and sieved, and 3 times of magnetic separation is performed using a magnetic separation column to obtain chromium-rich slag and iron particles;

[0128] (4) Preparation of secondary pellets: 208 g of chromium-rich slag obtained in step (4) is mixed with 53.8 g of chromite, 66.4 g of graphite, 7.2 g of dolomite and 37.6 g of slagging agent (20 g of quartz sand and 17.6 g of fluorite) to obtain a solid mixture; 27 g of ferrochrome alloy particles are coated with a layer of mineral oil so that the ferrochrome alloy particles are wetted with the mineral oil, and then added to the solid mixture to form pellets in a pelletizing disc, and pellets with a diameter of 188 mm are prepared, which are dried at 105°C for 12 h to obtain secondary pellets containing ferrochrome alloy particles;

[0129] (5) Second reduction: the secondary pellets obtained in step (4) are placed in an electromagnetic induction heating furnace and gradually heated to 1200°C, 5.5 L / min of argon or nitrogen is first introduced and maintained for 20 min, then vacuum reduction is carried out by vacuumizing to 200 Pa and maintaining for 10 min; then the temperature is raised to 1600°C and 4 g of ferrosilicon reducing agent is added, and the smelting is continued for 15 min to obtain a ferrochrome alloy melt and refining slag.

[0130] (6) Casting and post-processing: after smelting, the slag is removed by layering and the ferrochrome alloy melt is obtained, which is poured into a cast iron mold and cooled, then broken and sieved to obtain ferrochrome alloy particles with uniform particle size.

[0131] Example 3

[0132] Preparation of ferrochrome alloy:

[0133] (1) Preparation of primary pellets: 276 g of chromium-containing metallurgical solid waste, 58 g of chromite, 43.8 g of anthracite and 9 g of dolomite are thoroughly ground and mixed to obtain a solid mixture; 63 g of direct reduced iron particles are uniformly coated with a layer of mineral oil so that the direct reduced iron particles are wetted with the mineral oil, and then added to the solid mixture to form pellets in a pelletizing disc, and pellets with a diameter of 20 mm are prepared, which are dried at 105°C for 12 h to obtain primary pellets;

[0134] (2) First reduction: the primary pellets obtained in step (1) are loaded into an electromagnetic induction heating furnace, and gradually heated to 1050°C, and maintained for 10 min for first reduction; after the first reduction is completed, the material is naturally cooled and discharged to obtain a slag-iron mixture;

[0135] (3) Slag-iron separation: the slag-iron mixture obtained in step (2) is crushed and ground, and 3 times of magnetic separation is carried out using a magnetic separation column to obtain chromium-rich slag and iron particles;

[0136] (4) Preparation of secondary pellets: 240 g of the chromium-rich slag obtained in step (4) is mixed with 23.2 g of chromite, 72.8 g of graphite, 8 g of dolomite and 40 g of slagging agent (20 g of quartz sand and 20 g of fluorite) to obtain a solid mixture; 16 g of chromium-iron alloy particles are coated with a layer of mineral oil so that the chromium-iron alloy particles are wetted with the mineral oil, and then added to the solid mixture to form pellets in a pelletizing disc, and pellets with a diameter of 11 mm are obtained; the pellets are dried at 105 ℃ for 12 h to obtain secondary pellets containing chromium-iron alloy particles;

[0137] (5) Second reduction: the secondary pellets obtained in step (4) are placed in an electromagnetic induction heating furnace and gradually heated to 1450 ℃, 5.5 L / min of argon is introduced for 10 min, then vacuum reduction is performed by vacuumizing to 500 Pa and maintaining for 20 min; then the temperature is raised to 1500 ℃, 4.1 g of ferrosilicon reducing agent is added, and the smelting is continued for 15 min to obtain chromium-iron alloy melt and refining slag.

[0138] (6) Casting and post-processing: after smelting, the chromium-iron alloy melt is obtained by layering and slagging, and then the chromium-iron alloy melt is poured into a cast iron mold to cool; after cooling, the chromium-iron alloy particles with uniform particle size are obtained by crushing and sieving.

[0139] Example 4

[0140] According to the method of Example 2, the difference is that:

[0141] 22.5 g of dolomite in step (1) is replaced by 18.8 g of dolomite and 3.7 g of boron trioxide; 7.2 g of dolomite in step (4) is replaced by 5.5 g of dolomite and 1.7 g of boron trioxide.

[0142] The rest is the same as Example 2, and finally a chromium-iron alloy is prepared.

[0143] Example 5

[0144] According to the method of Example 1, the difference is that:

[0145] The coke in step (4) is pretreated and then used, the pretreatment method is: 2wt% magnesium sulfate aqueous solution is impregnated in the coke, and then filtered, and the solid phase is sintered at 720 ℃ by introducing CO2 for 40 min to obtain the pretreated carbon source.

[0146] The rest is the same as Example 1, and finally a chromium-iron alloy is prepared.

[0147] Example 6

[0148] According to the method of Example 1, the difference is that:

[0149] The coke in step (4) was pretreated by sintering at 720℃ for 40 min in CO2 to obtain the pretreated carbon source.

[0150] The rest was the same as example 1, and finally the ferrochrome alloy was prepared.

[0151] Comparative example 1

[0152] The preparation of ferrochrome alloy was carried out according to the method of example 1, except that the average particle size of direct reduced iron particles was 0.1 mm, and the results showed that the temperature could not be raised, resulting in reduction failure.

[0153] The rest was the same as example 1, and finally the ferrochrome alloy was prepared.

[0154] Comparative example 2

[0155] The preparation of ferrochrome alloy was carried out according to the method of example 1, except that:

[0156] The chromium-containing metallurgical solid waste 171 g, chromite 94.65 g, coke 18.5 g and bentonite 7.5 g were thoroughly ground and mixed to obtain a solid mixture; the direct reduced iron particles (average particle size 6.5 mm) 158.7 g were uniformly coated with a layer of mineral oil, so that the direct reduced iron particles were wetted with mineral oil, and then added to the solid mixture to form a ball in the balling disc, to obtain a ball with a diameter of 18 mm, and the ball was dried at 105℃ for 12h to obtain a primary ball;

[0157] The rest was the same as example 1, and finally the ferrochrome alloy was prepared.

[0158] Comparative example 3

[0159] The preparation of ferrochrome alloy was carried out according to the method of example 1, except that:

[0160] No slagging agent was added in step (4); in step (5), the temperature was raised to 1550℃, 5g of ferrosilicon reducing agent and 24g (12g of quartz sand and 12g of fluorite) were added, and the smelting was continued for 15 min to obtain a ferrochrome alloy melt and refining slag;

[0161] The rest was the same as example 1, and finally the ferrochrome alloy was prepared.

[0162] Comparative example 4

[0163] The preparation of ferrochrome alloy was carried out according to the method of example 1, except that:

[0164] The secondary pellets obtained in step (4) were placed in an electromagnetic induction heating furnace and gradually heated to 1300°C, 5.5 L / min of argon was introduced and maintained for 15 min; then the temperature was raised to 1550°C, 5 g of ferrosilicon reducing agent was added, and smelting was continued for 15 min to obtain chromium-iron alloy melt and refining slag.

[0165] The rest was the same as in Example 1, and finally chromium-iron alloy was prepared.

[0166] Performance test

[0167] From each of Examples 1-6 and Comparative Examples 1-4, 10 g of chromium-iron alloy sample was taken, and the contents of Cr, Fe and C were determined using a chemical means instrument.

[0168] Recovery rate = mass of recovered Cr / mass of Cr in raw material x 100%;

[0169] V in Table 1 is the rate of gradual temperature increase to the specified temperature at the beginning of the first reduction. L V in Table 1 is the rate of gradual temperature increase to the specified temperature at the beginning of the first reduction. H V in Table 1 is the rate of gradual temperature increase to the specified temperature at the beginning of the first reduction.

[0170] The test results are shown in Table 1.

[0171] Performance test results in Examples 1-6 and Comparative Examples 1-4

[0172]

[0173] From the above performance test results, it can be seen that by controlling and optimizing the process parameters and raw material ratio in Examples 1-6, efficient preparation of medium-carbon chromium-iron alloy is achieved. Specifically, the Cr content in the chromium-iron alloy particle product is >50%, and the C content is strictly controlled within the standard range of medium-carbon chromium-iron alloy (0.5%-4%), indicating the reliability and stability of the process.

[0174] Further, in Example 4, an appropriate amount of boron trioxide was added as an additive, which can reduce the carbon content in the chromium-iron alloy and further improve the recovery rate of Cr, as boron trioxide can inhibit the formation of Fe-Cr-C. In Examples 5-6, the carbon source in the second reduction was pretreated, which can reduce the carbon content in the chromium-iron alloy and further improve the recovery rate of Cr, and the magnesium doped in the carbon source after pretreatment can further improve the reduction activity of the carbon source after pretreatment and inhibit carbon residue.

[0175] In Comparative Example 1, the particle size of the metal iron particles is only 0.1 mm, and due to the small size of the metal iron particles, effective eddy current cannot be formed during electromagnetic induction heating, and finally the reduction fails because the temperature cannot reach 850℃. This shows that the metal particles as the "activation center" of electromagnetic induction heating, their existence is the necessary condition to realize efficient heating. In Comparative Example 2, the addition amount of metal iron particles in the primary pellets is too much, and the chromium iron alloy particles in the secondary pellets are also out of the appropriate range. Although this excessive addition speeds up the heating rate, it does not help to improve the product quality, but on the contrary, it leads to a decrease in the production of chromium iron alloy, indicating that the addition amount of metal particles needs to be controlled within a reasonable range. In Comparative Example 3, the slag-gold separation effect is poor, part of the chromium resources remains in the slag and cannot be fully recovered, at the same time, the energy consumption of the smelting process increases significantly, the reaction time is greatly prolonged, leading to low overall process efficiency, and finally the product quality is seriously affected. In Comparative Example 4, vacuum reduction is not used, resulting in a decrease in the production of chromium iron alloy, indicating that vacuum reduction at a specific stage can better improve the production of chromium iron alloy.

[0176] The above experimental results further prove the importance of the technical solutions defined in the present application to its technical effects.

[0177] The above is the preferred embodiment of the present application, it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, can also make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for producing ferrochrome by reducing a chromium-containing raw material using electromagnetic induction heating, characterized in that, The method comprises: (1) preparing primary pellets: preparing first pellets containing metallic iron particles by first pelletizing raw materials including chromium-containing metallurgical solid waste, first chromite, first carbon source, metallic iron particles and first additive; (2) first reduction: performing first reduction on the primary pellets obtained in step (1) to obtain a slag-iron mixture; (3) slag-iron separation: crushing, grinding and performing electromagnetic separation on the slag-iron mixture obtained in step (2) to obtain chromium-rich slag and iron particles; (4) preparing secondary pellets: preparing secondary pellets containing chromium-iron alloy particles by second pelletizing the chromium-rich slag obtained in step (3) with second chromite, second carbon source, chromium-iron alloy particles, second additive and slagging agent; (5) second reduction: performing second reduction on the secondary pellets obtained in step (4) to obtain chromium-iron alloy melt and refining slag; (6) casting and post-processing: casting the chromium-iron alloy melt obtained in step (5) into alloy ingots, and after cooling, crushing and screening to obtain chromium-iron alloy products.

2. The method of producing chromium iron by reducing a chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized by, Based on the total mass of the chromium-containing metallurgical solid waste, the first chromite, the first carbon source, the metallic iron particles and the first additive: the mass content of the chromium-containing metallurgical solid waste is 50% to 65%; the mass content of the first chromite is 10% to 35%; the mass content of the metallic iron particles is 3% to 15%; and the mass content of the first additive is 1% to 5%.

3. The method of claim 1, wherein the method is characterized by: Based on the total mass of the chromium-rich slag, the second chromite, the second carbon source, the chromium-iron alloy particles, the second additive and the slagging agent: the mass content of the second chromite is 5% to 20%; the mass content of the chromium-iron alloy particles is 3% to 15%; the mass content of the second additive is 1% to 5%; and the mass content of the slagging agent is 5% to 10%.

4. The method of claim 1, wherein the method is characterized by: The ratio of the molar amount of C element in the first carbon source to the total molar amount of O element in the reducible metal oxides in the raw materials is 1 to 1.2; and the ratio of the molar amount of C element in the second carbon source to the molar ratio of O element in Cr2O3, Fe2O3, FeO, MnO and SiO2 in the second chromite and the chromium-rich slag is 1 to 1.

2.

5. The method of producing iron-chromium by reducing chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized in that, The particle size of the metallic iron particles ranges from 3 to 10 mm; the diameter of the primary pellets ranges from 10 to 30 mm; the particle size of the chromium-iron alloy particles ranges from 3 to 10 mm; and the diameter of the secondary pellets ranges from 10 to 30 mm.

6. The method of producing iron-chromium by reducing chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized in that, The metallic iron particles are direct reduced iron particles; the first additive and the second additive each independently include one or more of bentonite, water glass, dolomite, kaolin, starch and polymer auxiliary materials; and the first carbon source and the second carbon source each independently include at least one of coke, graphite and anthracite.

7. The method of producing iron-chromium by reducing chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized in that, The first pelletizing in step (1) is performed in the presence of a viscous liquid, which includes at least one of mineral oil, water glass solution and cooked starch solution; and in step (2), the conditions of the first reduction include a temperature of 850 to 1050°C and a time of 10 to 60 minutes.

8. The method of producing iron-chromium by reducing chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized in that, The second pelletizing in step (4) is performed in the presence of a viscous liquid, which includes at least one of mineral oil, water glass solution and cooked starch solution.

9. The method of producing iron-chromium by reducing chromium-containing raw material using electromagnetic induction heating according to claim 1, characterized in that, The conditions of the second reduction include: a first stage reduction and a second stage reduction, the temperature of the first stage reduction is 1200-1450 DEG C; the time of the first stage reduction is 15-60 min; the second stage reduction is carried out under a reducing agent; the temperature of the second stage reduction is 1500-1600 DEG C; the time of the second stage reduction is 10-30 min.

10. The method of claim 9, wherein the method is characterized by: The first stage reduction is carried out under an inert atmosphere for 5-30 min, and then is switched to be carried out under vacuum until the end of the first stage reduction; the reducing agent includes a ferrosilicon reducing agent; the mass of the reducing agent is 5-20% of the mass of the second chromite.

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