Method for producing chromium-containing ferrometallurgy

Abstract

This invention provides a method for manufacturing chromium-containing molten iron that is inexpensive and generates minimal waste. It involves melting, heating, and oxygen-based decarburization of a raw material containing chromium in a steelmaking refining furnace to produce chromium-containing steel. The method comprises: a first step in which the slag basicity before oxygen-based decarburization is adjusted to a range of 1.5 to 3.0, and the slag basicity after oxygen-based decarburization is adjusted to a range of 2.0 to 3.5, while leaving slag containing chromium oxides generated during oxygen blowing remaining in the furnace; and a second step in which the remaining slag containing chromium oxides is reduced using a newly added carbon or metal source in the same furnace, recovering chromium into the molten iron. The basicity of the slag is obtained by dividing the CaO concentration by the SiO2 concentration based on the mass of the slag.

Description

Technical Field

[0001] This invention relates to a method for producing molten chromium iron using a steelmaking refining furnace. Background Technology

[0002] In the refining process of chromium-containing molten iron, the mainstream method is as follows: after melting the raw materials containing chromium, oxygen is supplied for decarburization blowing. After stopping the oxygen blowing, reducing agents such as Si-containing raw materials or Al-containing raw materials are added. In this way, the chromium component, which is a useful metal, is recovered from the chromium oxide generated at the same time as the oxidation of carbon and returned to the molten iron. Then the molten iron is discharged.

[0003] To reduce the generated chromium oxides, a stoichiometric amount of reducing agent is required, necessitating a considerable quantity of expensive Si or Al alloys. In addition, to adjust the slag composition, CaO is added to address the SiO2 and Al2O3 generated during the reduction of chromium oxides, leading to increased lime costs and a greater amount of slag produced. Furthermore, the slag produced in this process contains a certain concentration of chromium oxides, raising concerns about the potential leaching of hexavalent chromium when used as a byproduct in roadbed materials or aggregates.

[0004] To reduce the amount of Si-containing or Al-containing raw materials used, the following method has been studied: ending the treatment without reducing the slag containing chromium oxides, and then using molten iron or carbon-containing raw materials for reduction. For example, Patent Document 1 discloses a method in which, after stopping oxygen blowing, slag is discharged or removed without reduction, and the slag is reduced in an electric furnace using carbon and silicon, thereby recovering chromium into the molten iron.

[0005] Patent document 2 discloses the following method: after generating slag containing chromium oxides, the molten iron is discharged without reduction, and then loaded into a refining furnace to add carbon materials and blow oxygen, thereby recovering chromium into the molten iron.

[0006] Patent document 3 discloses the following method: after generating slag containing chromium oxides, calcium carbonate is added to solidify the slag without reduction, thereby allowing only molten iron to be discharged, while the slag containing chromium oxides is separately discharged and loaded into an electric furnace for reduction treatment, thereby recovering chromium into the molten iron; etc.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2013-79449

[0010] Patent Document 2: Japanese Patent Application Publication No. 2002-256323

[0011] Patent Document 3: Japanese Patent Application Publication No. 2012-211372 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, the aforementioned prior art has the following problems.

[0014] Regarding the method described in Patent Document 1, it is necessary to re-load the slag containing chromium oxides into another refining container after slag discharge, which raises the issues of increased heat loss, increased electricity costs, or increased heating costs for carbon materials, etc.

[0015] Regarding the method described in Patent Document 2, when slag is discharged while slag containing chromium oxide remains, there is a problem that the slag solidifies on the furnace wall, furnace opening, and near the liquid outlet, leading to operational deterioration.

[0016] Regarding the method described in Patent Document 3, the above-mentioned problem of slag consolidation can be solved by solidifying the slag containing chromium oxides. However, since the decomposition reaction of calcium carbonate is an endothermic reaction, the temperature of the slag decreases. Therefore, there is a problem that the electricity cost or the heating cost of carbon materials, etc., increases in the chromium recovery process.

[0017] The present invention was made in view of the following circumstances, with the aim of providing a method for manufacturing molten chromium iron that is inexpensive and generates little waste without affecting operation.

[0018] Methods for solving problems

[0019] To address the aforementioned issues, the inventors of this application conducted numerous experiments and discovered that, regarding slag containing chromium oxides after oxygen supply, considering the significant changes in solid fraction and viscosity with basicity, by leaving the slag containing chromium oxides in the furnace and reducing it within the same refining furnace using a newly added carbon or metal source, it is possible to produce inexpensive molten chromium iron with minimal waste generation without affecting operation. This invention is based on the above insights, and its main points are as follows.

[0020] The present invention advantageously solves the above-mentioned problems by providing a method for manufacturing chromium-containing molten iron, which is a method for manufacturing chromium-containing steel by melting, heating, and oxygen-based rough decarburization of raw materials containing chromium in a steelmaking refining furnace. The method is characterized by comprising: a first step in which the slag basicity before oxygen-based rough decarburization is adjusted to a range of 1.5 to 3.0, and the slag basicity after oxygen-based rough decarburization is adjusted to a range of 2.0 to 3.5, and the molten iron is discharged while the slag containing chromium oxides generated by oxygen blowing remains in the furnace; and a second step in which the remaining slag containing chromium oxides is reduced using a newly added carbon or metal source in the same furnace, and chromium is recovered into the molten iron, wherein the basicity of the slag is obtained by dividing the CaO concentration by the SiO2 concentration based on the mass of the slag.

[0021] Regarding the method for manufacturing chromium-containing molten iron according to the present invention, the following means are considered to be more preferred solutions:

[0022] (a) During the oxygen blowing process in the aforementioned first step, additional raw materials containing CaO are added for melting;

[0023] (b) The basicity (C / S)pre of the slag before the oxygen blowing-based coarse decarburization in the first process and the basicity (C / S)post of the slag after the oxygen blowing-based coarse decarburization satisfy the following formula (1);

[0024] (c) In the aforementioned first step, after oxygen blowing, one or two of the raw materials selected from Si-containing raw materials and Al-containing raw materials are used to make the concentration of chromium oxide in the slag within the range of 5% by mass to 50% by mass.

[0025] (C / S)post-(C / S)pre≥-0.2···(1)

[0026] The effects of the invention

[0027] According to the present invention, by leaving slag containing chromium oxides in the furnace without affecting operation, and by reducing it by adding a new carbon source or metal source to the same refining furnace, molten chromium iron can be produced in a cheap manner with less slag, which can help reduce the environmental impact. Attached Figure Description

[0028] [ Figure 1 [This is a basic flowchart of a method for manufacturing chromium-containing molten iron according to one embodiment of the present invention.]

[0029] [ Figure 2The graph shows the relationship between the alkalinity of the slag and the liquid phase fraction at 1700℃, calculated using thermodynamic calculation software, based on the chromium oxide concentration in the slag. Detailed Implementation

[0030] The concept that led to the discovery of this invention will be explained.

[0031] Chromium oxide has a very high melting point of 2300℃. The slag containing chromium oxides, produced after oxygen is supplied to the molten chromium metal, reaches a temperature of around 1700℃ and contains a large amount of chromium oxide, resulting in a low liquidus ratio and very high viscosity. Therefore, when molten iron is tapped without reducing the slag containing chromium oxides, this slag remains in large quantities as solidified slag on the furnace walls, furnace opening, or near the tap hole, hindering operation.

[0032] For example, in large-volume furnaces such as converters and AOD furnaces, the narrowing of the furnace opening becomes more pronounced. Consequently, the linear velocity of the exhaust gas exiting the furnace increases, leading to a greater generation of chromium-containing dust. As a result, the amount of Cr discharged from the system increases.

[0033] Therefore, the inventors of this application, focusing on the significant variation in the relationship between the liquid phase fraction and the basicity of slag containing chromium oxides with the concentration of chromium oxide (Cr2O3), discovered a method for retaining slag containing chromium oxides in the furnace without affecting operation. Furthermore, a method for producing molten chromium iron with low slag content by using a newly added carbon or metal source for reduction within the same refining furnace was discovered, which is inexpensive.

[0034] Specifically, it is known that for slag with high chromium oxide concentration, the liquid phase fraction is high under high basicity, while for slag with low chromium oxide concentration, the liquid phase fraction is high under low basicity. For example, after the heating process and during the peak decarburization period, the chromium oxide concentration in the slag is not high. Therefore, if the basicity of the slag is reduced, the liquid phase fraction increases, and the slag adheres less to the furnace wall. On the other hand, at the end of the oxygen delivery process, when the amount of chromium oxide generated is high, increasing the basicity of the slag further increases the liquid phase fraction, and the slag adheres less to the furnace wall. In the typical oxygen delivery process, the Si in the main and auxiliary raw materials is burned, and the basicity of the slag decreases in the latter half of the blowing process. Therefore, it is necessary to design the composition of both the slag before and after oxygen delivery decarburization.

[0035] Here, as a heating process, heating is sometimes achieved by adding carbon materials and supplying oxygen. In this case, the heating process can be considered to continue until the temperature T(K) during the oxygen supply process reaches a temperature higher than that represented by the equilibrium equation (2) shown below. Here, a Cr a represents the activity of Cr in molten metal. C P represents the activity of carbon (C) in molten metal. CO The partial pressure of CO in the atmosphere (atm).

[0036] [Mathematical formula 1]

[0037]

[0038] Figure 1 This is a basic flow chart of a method for manufacturing chromium-containing molten iron according to one embodiment of the present invention. As a first step, a raw material containing chromium is melted using a steelmaking refining furnace (S0). Next, the temperature is raised using electricity or by adding a heat source (S1). Then, with the slag basicity adjusted to a range of 1.5 to 3.0, rough decarburization based on oxygen blowing is performed (S2). Then, with the slag basicity adjusted to a range of 2.0 to 3.5, the molten iron is discharged while leaving slag containing chromium oxides generated by oxygen blowing remaining in the furnace (S3). Here, the slag basicity is obtained by dividing the CaO concentration by the SiO2 concentration based on the mass of the slag. Heating is performed, for example, by electric heating, or by adding carbon materials, silicon-containing substances, and oxygen.

[0039] Next, as a second step, the slag containing chromium oxides remaining in the furnace is reduced by using a newly added carbon or metal source in the same furnace, and the chromium is recovered into the molten iron (S4), thereby effectively producing chromium-containing molten iron.

[0040] In the first process, in order to adjust the basicity of the slag to a suitable range according to the change in the concentration of chromium oxides in the slag during oxygen blowing, a raw material containing CaO can be added during the oxygen blowing process. Furthermore, it is preferable to add a raw material containing CaO such that the basicity (C / S)pre of the slag before oxygen blowing-based coarse decarburization and the basicity (C / S)post of the slag after oxygen blowing-based coarse decarburization satisfy the following formula (1). This allows for the stable production of chromium-containing molten iron.

[0041] (C / S)post-(C / S)pre≥-0.2···(1)

[0042] Regarding the additional melting of CaO-containing raw materials, the raw materials can be added freshly during oxygen blowing, or they can be added from the beginning by melting lumpy CaO-containing raw materials during oxygen blowing. The melting rate of the raw materials varies depending on the properties of the raw materials, the shape of the furnace, and the stirring conditions. Various estimation formulas have been reported, but it can also be estimated empirically based on actual changes in basicity.

[0043] In the first process, by implementing a weak reduction (S5) using one or two of the raw materials containing Si and Al to adjust the concentration of chromium oxide in the slag to a range of 5% by mass to 50% by mass, chromium-containing molten iron can be produced more stably.

[0044] The following is a detailed description of the present invention.

[0045] Prior to making this invention, the inventors of this application conducted thermodynamic studies on the equilibrium phases of slags with different chromium oxide concentrations. Figure 2 The effect of chromium oxide concentration on the relationship between slag basicity and the liquid phase fraction at 1700℃, calculated using the thermodynamic calculation software Factsage, is illustrated using a graph. Here, the MgO and Al2O3 concentrations in the slag are pre-set to 10% by mass and 10% by mass, respectively. These MgO and Al2O3 concentrations represent the typical composition of slag in a refining furnace, and their magnitude does not significantly affect the calculation results.

[0046] In low chromium concentrations, for example Figure 2 When the Cr2O3 concentration in the slag shown is 5% by mass, a high liquid phase fraction can be maintained within a low basicity range of 1.0 to 3.0. Furthermore, the liquid phase fraction decreases as the basicity increases. From the viewpoint of promoting chromium oxide reduction, the basicity of the slag is preferably maintained at 1.5 or higher, and the basicity during initial decarburization based on oxygen supply is preferably within the range of 1.5 to 3.0.

[0047] On the other hand, it was found that in high chromium concentrations, for example... Figure 2 When the chromium oxide concentration in the slag shown is 40% by mass, the liquid phase fraction increases with increasing basicity within the range of slag basicity above 2.0. The liquid phase generated within the range of basicity below 2.0 becomes a highly viscous liquid phase with a basicity of around 1.2, which contributes to adhesion to the furnace walls. The basicity in the liquid phase is obtained by dividing the CaO concentration by the SiO2 concentration based on the mass of the liquid phase. In the low chromium concentration region, the liquid phase fraction is high, so even if it becomes a highly viscous liquid phase, there are no operational problems. However, in the high chromium concentration region of the latter half of the blowing process, when the solid phase fraction of the slag increases, the effect of viscosity on operation becomes significant. When the slag basicity exceeds 3.5, hexavalent chromium will be generated in the slag. From an environmental perspective, the slag basicity needs to be below 3.5. Therefore, the slag basicity at the end of oxygen feeding is preferably set within the range of 2.0 to 3.5.

[0048] As mentioned above, the basicity that results in the highest liquid phase fraction changes during oxygen blowing due to the change in chromium oxide concentration. Therefore, it is preferable to actively add CaO-containing raw materials for melting during oxygen blowing, so that the basicity (C / S)pre of the slag before oxygen blowing-based coarse decarburization and the basicity (C / S)post of the slag after oxygen blowing-based coarse decarburization satisfy the following equation (1). The upper limit of the following equation (1) is 2.0.

[0049] (C / S)post-(C / S)pre≥-0.2···(1)

[0050] If the concentration of chromium oxide is around 60% by mass, the basicity of the liquid phase will not increase significantly even if the basicity of the slag is increased. Therefore, it is preferable to keep the concentration of chromium oxide in the slag below 50% by mass. When the concentration of chromium oxide in the slag is less than 5%, the disadvantage of increased slag volume may become more significant compared to the advantage of being able to recover chromium through newly added carbon or metal sources. It is preferable to ensure that the concentration is above 5%. After the oxygen supply is stopped, appropriate amounts of Si-containing and Al-containing feedstocks are added, thereby adjusting the concentration of chromium oxide to this range. In this case, the basicity of the slag changes after oxygen supply and after weak reduction due to the addition of Si-containing feedstocks, but it is preferable to keep the basicity of the slag after oxygen supply and after weak reduction in the range of 2.0 to 3.5.

[0051] The slag basicity before, after, and after weak reduction of oxygen-based coarse decarburization can be controlled by designing the weight and timing of the auxiliary raw materials charged during the blowing process.

[0052] When slag containing chromium oxides remains in the furnace, chromium can be reduced and recovered by adding a carbon source or a metal source to the slag. Examples of carbon sources include separately prepared molten iron, high-carbon ferrochrome, and carbon materials. Examples of metal sources include ferrosilicon, aluminum pellets, and aluminum slag. The reduction reaction of chromium oxide using carbon is endothermic, therefore it can be heated electrically or by supplying oxygen for heat. In subsequent processes, the slag composition can be adjusted again, allowing the slag containing chromium oxides to remain in the furnace before molten metal is discharged, or a metal reducing agent can be added to perform a single slag discharge before molten metal discharge.

[0053] Example

[0054] (Example 1)

[0055] Molten iron was charged into a 150t converter, and after the addition of ferrochrome, decarburization blowing was performed, followed by heating and decarburization. Then, the molten iron was tapped out. After tapping, the molten iron was charged back into the furnace, the slag containing chromium oxides was reduced, the chromium was recovered, and heating was performed based on oxygen-assisted decarburization. The experimental conditions are shown in Table 1.

[0056] [Table 1]

[0057]

[0058] Next, the condition of the furnace opening was confirmed under the experimental conditions shown in Table 1, and the operational impact was evaluated. The results are shown in Table 2. Here, in the evaluation of operational stability, the rate of reduction of the furnace opening area was evaluated based on photographs of the furnace opening to assess the degree of blockage. A reduction of more than 1% of the furnace opening area after treatment was rated as ×, less than 1% as ○, and no reduction in furnace opening area was observed as ◎. As for the evaluation of the amount of reducing agent used, a × was rated if the amount of metal reducing agent used, i.e., the amount of ferrosilicon and aluminum pellets used, remained the same as before, and a ○ was rated if it decreased compared to before. Furthermore, the proportion of chromium that could be recovered in the second process was evaluated as the chromium recovery rate. Here, the chromium recovery rate was calculated on a mass basis as (chromium concentration of molten metal (%) × output volume (t) / 100 - amount of added chromium (t)) / amount of residual chromium in the furnace (t). A chromium recovery rate less than 0.3 was rated as ×, and a rate greater than 0.3 was rated as ○. As a comprehensive evaluation, conditions in which any one of the operational stability evaluation, reducing agent usage evaluation, and chromium recovery rate is rated as × are rated as ×. Among the conditions that do not include ×, conditions in which the operational stability evaluation is rated as ◎ are rated as ◎, and all other conditions are rated as ○.

[0059] [Table 2]

[0060] No. Operational stability evaluation Evaluation of reducing agent usage Chromium recovery rate Overall evaluation Remark 1 ◎ × × × Comparative example 2 × ○ × × Comparative example 3 × ○ × × Comparative example 4 × ○ × × Comparative example 5 ○ ○ ○ ○ Invention Examples 6 ◎ ○ ○ ◎ Invention Examples 7 ○ ○ ○ ○ Invention Examples 8 ◎ ○ ○ ◎ Invention Examples

[0061] Regarding the processing conditions No. 5 to 8, which are examples of the invention, chromium reduction and recovery can be achieved under all conditions, and the amount of reducing agent used can be reduced. Furthermore, for processing conditions No. 6 and 8, which adjust the concentration of chromium oxide in the slag to 5% by mass or more and 50% by mass or less, the narrowing rate of the furnace opening is more suppressed.

[0062] The examples above illustrate an inexpensive and stable method for smelting chromium-containing molten iron without affecting operation. The chromium-containing molten iron produced by this invention has a high oxygen concentration, making it less prone to nitrogen absorption; therefore, it is also useful as a method for obtaining high-purity molten iron. The molten iron produced using this method has a high oxygen concentration, resulting in a high sulfur concentration at the point of production. However, desulfurization can be achieved without problems through reduction treatment in subsequent processes. Furthermore, there is no need to reduce oxides in the slag, thus significantly reducing the amount of alloying reducing agents such as Si-containing and Al-containing raw materials used. It is also useful to combine the molten iron produced using this method with molten iron obtained from smelting in another refining vessel to smelt molten iron of a specified composition concentration.

[0063] Industrial availability

[0064] The method for producing chromium-containing molten iron of the present invention is inexpensive and can suppress slag formation, thereby reducing environmental impact, and is therefore useful in industry.

Claims

1. A method for producing chromium-containing molten iron, comprising melting, heating, and oxygen-blowing-based decarburization of raw materials containing chromium in a steelmaking refining furnace to produce chromium-containing steel, characterized in that, have: In the first step, the slag basicity before oxygen blowing-based coarse decarburization is adjusted to a range of 1.5 to 3.0, and the slag basicity after oxygen blowing-based coarse decarburization is adjusted to a range of 2.0 to 3.5, while the slag containing chromium oxides generated by oxygen blowing remains in the furnace, is discharged. and The second step involves using a newly added carbon or metal source to the same furnace to reduce the remaining slag containing chromium oxides, thereby recovering the chromium into the molten iron. The alkalinity of the slag is obtained by dividing the CaO concentration by the SiO2 concentration based on the mass of the slag. In the first step, after oxygen blowing, one or two of the raw materials selected from Si-containing raw materials and Al-containing raw materials are used to make the concentration of chromium oxides in the slag range from 5% by mass to 50% by mass.

2. The method for manufacturing chromium-containing molten iron as described in claim 1, characterized in that, During the oxygen blowing process in the first step, additional raw materials containing CaO are added and melted.

3. The method for manufacturing chromium-containing molten iron as described in claim 1 or 2, characterized in that, The basicity (C / S)pre of the slag before oxygen blowing-based coarse decarburization in the first process and the basicity (C / S)post of the slag after oxygen blowing-based coarse decarburization satisfy the following formula (1): (C / S)post-(C / S)pre≥-0.2··· (1).

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