A method for recycling solid waste from molten iron pretreatment

Abstract

The present invention belongs to the technical field of steelmaking, and specifically relates to a method for recycling solid waste from molten iron pretreatment, specifically a method for applying KR desulfurization slag produced by KR molten iron pretreatment to the smelting of sulfur-containing free-cutting steel. The present invention adds KR desulfurization slag during LF refining after deoxidation and alloying in normal converter smelting, fully utilizing the combined use of KR desulfurization slag, converter deoxidation products, and converter final slag, successfully recovering sulfur from KR desulfurization slag, realizing waste slag recycling, reducing the consumption of ferrosulfur in steelmaking, reducing smelting costs, reducing the pollution and harm of waste slag to the environment, and at the same time increasing the spindle rate of manganese sulfide. The method provided by the present invention does not require changing the smelting process flow of sulfur-containing free-cutting steel, is more convenient for handling desulfurization slag, and is easy to promote throughout the industry.

Description

Technical Field

[0001] The invention belongs to the technical field of steelmaking, in particular to the field of solid waste recycling steelmaking, and specifically relates to a method for recycling solid waste from molten iron pretreatment. Background Art

[0002] Steelmaking generates a large amount of solid waste, including hot metal pretreatment slag, converter slag, LF refining slag, and dust removal ash. KR desulfurization slag is a representative example. KR desulfurization is a process for pre-treating hot metal outside the converter prior to converter steelmaking. It is characterized by its simplicity, low cost, and high desulfurization efficiency, making it widely adopted by domestic steel companies. Desulfurizers used in KR desulfurization include lime, calcium carbide, soda, and magnesium. Lime-based desulfurizers are readily available and low-cost, making them the most common. KR desulfurization slag is a byproduct of the desulfurization process, producing approximately 7-9 kg per ton of steel. Given that my country's annual crude steel production is 1 billion tons, and approximately one-third of this crude steel is pre-treated using the KR desulfurization process, the annual production of KR desulfurization slag exceeds 2 million tons.

[0003] At present, the main treatment method for KR desulfurization slag in China is to recover the iron in the slag through magnetic separation. The desulfurization slag is then converted into solid waste and piled up for disposal. With the continuous increase in steel production, the stockpile of KR desulfurization slag continues to increase, and the pollution and harm of sulfur-containing substances to the environment are relatively large, which has become a serious environmental problem. How to recover the sulfur in KR desulfurization slag, realize the resource recycling and utilization of waste slag, reduce smelting costs, and at the same time reduce the pollution and harm of waste slag to the environment, promote the green development of steel enterprises, has become a problem that steel enterprises urgently need to solve. CN202110543363.X A method for smelting sulfur-containing steel grades using desulfurization slag. The sulfur in the desulfurization slag can be replaced by [S] through oxidation. By optimizing the smelting process, the [S] element can be stably introduced into the molten steel. Based on this, the desulfurization slag can be recycled. It adds desulfurization slag to the ladle before the converter is tapped, requiring that the deoxidation and alloying of the converter smelting be delayed, which has a certain impact on the normal process flow of the converter smelting, and special deoxidizers such as SiC must be used to ensure the sulfur recovery efficiency. CN202110201652.1 A process method for recycling slag to produce high-sulfur steel, which adds KR desulfurization slag at the beginning of converter smelting and tapping. This operation will have an impact on normal smelting, such as the delay of converter smelting deoxidation and alloying, and the use of cord steel refining recovery slag, a special steelmaking product. It is still necessary to use special raw and auxiliary materials such as "special deoxidizers" and "cord steel refining recovery slag", which is not conducive to promotion throughout the industry. In addition, the effect of recycling KR desulfurization slag on the spindle rate of manganese sulfide has not been studied.

[0004] Therefore, the technical problem to be solved by the present invention is how to improve the sulfur recovery efficiency in the desulfurization slag and the spindle rate of manganese sulfide without adding special slag and without causing any impact on the smelting process. Summary of the Invention

[0005] To solve the above problems, the present invention adds desulfurization slag during LF refining after deoxidation and alloying in normal converter smelting, and utilizes converter final slag to effectively increase the SiO2 content in LF refining slag, so as to improve the sulfur recovery efficiency in desulfurization slag, without the need to add special deoxidizers. By making full use of the combined use of KR desulfurization slag, converter deoxidation products, and converter final slag, sulfur in KR desulfurization slag is successfully recovered, waste slag recycling is realized, the consumption of ferrosulfur in steelmaking is reduced, the smelting cost is reduced, the pollution and harm of waste slag to the environment are reduced, and at the same time the spindle rate of manganese sulfide is improved, which is conducive to promotion within the industry.

[0006] A method for recycling solid waste from molten iron pretreatment, wherein the tailings remaining after KR desulfurization slag is subjected to heat stuffing, crushing, magnetic separation, and drying are recycled and used in the smelting of sulfur-containing free-cutting steel to increase the sulfur content of the molten steel.

[0007] The smelting method of the sulfur-containing free-cutting steel of the present invention is described in detail below.

[0008] Step 1: Converter Smelting: The converter charge consists of 90% molten iron and 10% scrap steel, with a charge capacity of 120 tons. The scrap steel is standard commercial steel, with no special requirements. The converter uses lime and light-burned dolomite to produce a low-basicity slag. The slag-forming amount of lime is 18-20 kg per ton of steel, and the amount of light-burned dolomite is 10-15 kg per ton of steel. The slag basicity is controlled between 1.2 and 1.6. The final converter temperature is controlled between 1600 and 1630°C, and the final carbon content is controlled between 0.05 and 0.08%.

[0009] Step 2: Tapping from the converter: Deoxidation and alloying are carried out during the tapping process. The alloys added for deoxidation and alloying are: silicon manganese (4.6-5.0 kg / ton of steel), low-carbon ferromanganese (17.8-18.0 kg / ton of steel), and ferrophosphorus (1.50-1.52 kg / ton of steel). No ferrosulfur is required. Slag is manually added during the tapping process, with a slag amount of 600 kg / furnace. The silicon content at the tapping end is controlled to be ≤0.03%.

[0010] Step 3: LF refining: After the molten steel reaches LF refining, it is heated slightly (heating rate ≤ 0.05℃ / s, temperature rise ≤ 30℃), and slag is not added. The treated KR desulfurization slag is first added as slag and to add sulfur to the molten steel. The amount of KR desulfurization slag used is 100kg / ton of steel. Then the active oxygen of the molten steel is sampled and analyzed. The active oxygen content of free-cutting steel is controlled at 30-40ppm. After the oxygen determination is completed, calcium carbide is used to deoxidize the slag surface according to the oxygen content, and the active oxygen content is adjusted to the required range. Sulfur iron (5.0kg / ton of steel) is added before the end of LF smelting, and then it is hoisted into the continuous casting process for normal casting.

[0011] The components of the treated KR desulfurization slag in the present invention are calculated by weight percentage as follows: CaO: 52.5-61.8%, SiO2: 9.1-12.5%, CaS: 4.4-5.6%, CaF2: 1.0-3.8%, Al2O3: ≤3.5%, MgO: ≤3.0%, moisture ≤0.5%, and the rest are impurities.

[0012] In the present invention, the sulfur in the KR desulfurization slag is mainly present in the slag in the form of CaS. In order for it to enter the molten steel, it needs to be reduced through a chemical reaction. According to the chemical reaction mechanism, it is necessary to promote the chemical reaction in the opposite direction of KR desulfurization to generate CaS. It is necessary to increase the SiO2 content in the slag, reduce the Si content in the molten steel, and appropriately reduce the molten steel temperature to improve the efficiency of the reverse reaction. The present invention uses a large amount of silicon manganese to deoxidize the molten steel and utilizes the final slag of the converter to effectively increase the SiO2 content in the LF refining slag. At the same time, the silicon content at the end of the converter steel tapping is controlled to ensure that the reverse reaction is fully carried out.

[0013] Beneficial effects

[0014] The present invention provides a method for recycling solid waste from molten iron pretreatment and its application in the smelting of sulfur-containing free-cutting steel. Desulfurization slag is added during LF refining, and the final slag of the converter is used to improve the recovery rate of S in the desulfurization slag. By fully utilizing the combined use of KR desulfurization slag, converter deoxidation products, and converter final slag, sulfur in the KR desulfurization slag is successfully recovered, thereby achieving waste slag recycling, reducing the consumption of ferrosulfur in steelmaking, having no impact on the smelting process, reducing smelting costs, reducing the pollution and harm of waste slag to the environment, and simultaneously improving the spindle rate of manganese sulfide. DETAILED DESCRIPTION

[0015] Taking our company's sulfur-containing free-cutting steel Y1215 as an example, the smelting process of Y1215 is: converter smelting - LF refining - continuous casting. The specific chemical composition by weight percentage is C: ≤0.09%, Si: ≤0.08%, Mn: 1.10~1.40%, P: 0.04~0.09%, S: 0.33~0.42%, Cr, Ni, Cu ≤0.20%, and the rest is iron and unavoidable impurities.

[0016] Example 1

[0017] 1. Converter Smelting: The converter charge consists of 90% molten iron and 10% scrap steel, with a charge capacity of 120 tons. The scrap steel is standard commercial steel, with no special requirements. The converter uses lime and light-burned dolomite to produce low-basicity slag. The lime addition rate is 18 kg per ton of steel, and the light-burned dolomite addition rate is 14 kg per ton of steel. The slag basicity is controlled at 1.3. The converter smelting endpoint temperature is controlled at 1612°C, and the endpoint carbon content is controlled at 0.06%.

[0018] 2. Converter tapping: Deoxidation and alloying are carried out during the tapping process. The alloys added for deoxidation and alloying are: silicon manganese (4.8kg / ton steel), low carbon ferromanganese (17.9kg / ton steel), and ferrophosphorus (1.5kg / ton steel). There is no need to add ferrosulfur. Slag is manually discharged during the tapping process, with a slag discharge volume of 600kg / furnace. The silicon content at the tapping end point is controlled at 0.02%.

[0019] 3. LF Refining: After the molten steel reaches the LF refining stage, it is heated slightly (heating rate ≤ 0.05°C / s, temperature rise ≤ 30°C). No slag is added. Instead, treated KR desulfurization slag is added to act as slag and to add sulfur to the molten steel. The KR desulfurization slag usage is 100kg / ton of steel. The composition of KR desulfurization slag, by weight, is as follows: CaO: 52.5-61.8%, SiO2: 9.1-12.5%, CaS: 4.4-5.6%, CaF2: 1.0-3.8%, Al2O3: ≤3.5%, MgO: ≤3.0%, moisture ≤0.5%, and the remainder is impurities. The molten steel is then sampled and analyzed for active oxygen. After oxygen determination is complete, calcium carbide is used to deoxidize the slag surface, adjusting the active oxygen content to 35ppm. Ferrosulfur (5.0kg / ton of steel) is added before the LF refining phase ends. The steel is then hoisted into the continuous casting process for normal casting.

[0020] Example 2

[0021] 1. Converter Smelting: The converter charge consists of 90% molten iron and 10% scrap steel, with a charge capacity of 120 tons. The scrap steel is standard commercial steel, with no special requirements. The converter uses lime and light-burned dolomite to produce low-basicity slag. The lime addition rate is 20 kg per ton of steel, and the light-burned dolomite is 15 kg per ton of steel. The slag basicity is controlled at 1.5. The converter smelting endpoint temperature is controlled at 1622°C, and the endpoint carbon content is controlled at 0.07%.

[0022] 2. Converter tapping: Deoxidation and alloying are carried out during the tapping process. The alloys added for deoxidation and alloying are: silicon manganese (5.0kg / ton steel), low carbon ferromanganese (17.8kg / ton steel), and ferrophosphorus (1.51kg / ton steel). There is no need to add ferrosulfur. Slag is manually discharged during the tapping process, with a slag discharge volume of 600kg / furnace. The silicon content at the tapping end is controlled at 0.01%.

[0023] 3. LF Refining: After the molten steel reaches the LF refining stage, it is heated slightly (heating rate ≤ 0.05°C / s, temperature rise ≤ 30°C). No slag is added. Instead, treated KR desulfurization slag is added to act as slag and to add sulfur to the molten steel. The KR desulfurization slag usage rate is 100 kg per ton of steel. The composition of KR desulfurization slag, by weight, is as follows: CaO: 52.5-61.8%, SiO2: 9.1-12.5%, CaS: 4.4-5.6%, CaF2: 1.0-3.8%, Al2O3: ≤3.5%, MgO: ≤3.0%, moisture ≤0.5%, and the remainder is impurities. The molten steel is then sampled and analyzed for active oxygen. After oxygen determination is complete, calcium carbide is used to deoxidize the slag surface, adjusting the active oxygen content to 37 ppm. Ferrosulfur (5.0 kg per ton of steel) is added before the LF refining phase ends. The slag is then hoisted into the continuous casting process for normal casting.

[0024] Comparative Example 1

[0025] Using conventional smelting process:

[0026] 1. Converter Smelting: The converter charge consists of 90% molten iron and 10% scrap steel, with a charge capacity of 120 tons. The scrap steel is standard commercial steel, with no special requirements. The converter uses lime and light-burned dolomite to produce low-basicity slag, with the lime addition rate at 35 kg / ton of steel. The slag basicity is controlled at 2.0. The converter smelting endpoint temperature is controlled at 1640°C, the endpoint carbon content is controlled at 0.04%, and the endpoint silicon content is controlled at 0.05%.

[0027] 2. Steel tapping from the converter: Deoxidation and alloying are carried out during the steel tapping process. The alloys added for deoxidation and alloying are: low-aluminum ferroaluminum (1.75kg / ton of steel), low-carbon ferromanganese (21.7kg / ton of steel), ferrophosphorus (1.5kg / ton of steel), and ferrosulfur (10.2kg / ton of steel).

[0028] 3. LF refining: After the molten steel reaches LF refining, it is rapidly heated (heating rate ≥ 0.10℃ / s, temperature rise range ≥ 50℃), a small amount of slag-reducing agent is added to supplement the slag material, and then samples are taken to analyze the active oxygen content of the molten steel. After the oxygen content is determined, calcium carbide is used to deoxidize the slag surface and adjust the active oxygen content to 36ppm. After that, it is hoisted into the continuous casting process for normal casting.

[0029] Comparative Example 2

[0030] 1. Converter Smelting: The converter charge consists of 90% molten iron and 10% scrap steel, with a charge capacity of 120 tons. The scrap steel is standard commercial steel, with no special requirements. The converter uses lime and light-burned dolomite to produce low-basicity slag, with the lime addition rate at 35 kg / ton of steel. The slag basicity is controlled at 2.0. The converter smelting endpoint temperature is controlled at 1640°C, the carbon content at 0.04%, and the silicon content at 0.04%.

[0031] 2. Converter tapping: Deoxidation and alloying are carried out during the tapping process. The alloys added for deoxidation and alloying are: silicon manganese (4.9kg / ton steel), low carbon ferromanganese (17.7kg / ton steel), ferrophosphorus (1.51kg / ton steel), and ferrosulfur (10.1kg / ton steel). Slag is manually discharged during the tapping process, with a slag discharge capacity of 600kg / furnace.

[0032] 3. LF refining: After the molten steel reaches LF refining, it is rapidly heated (heating rate ≥ 0.10℃ / s, temperature rise range ≥ 50℃), a small amount of slag-reducing agent is added to supplement the slag material, and then samples are taken to analyze the active oxygen content of the molten steel. After the oxygen content is determined, calcium carbide is used to deoxidize the slag surface and adjust the active oxygen content to 39ppm. After that, it is hoisted into the continuous casting process for normal casting.

[0033] Comparative Example 3

[0034] The "manual slag removal during tapping, with a slag quantity of 600 kg / furnace" operation in step 2 of Example 1 was omitted. Other operations were the same as in Example 1. Due to this, to ensure that the sulfur content met the requirements, the amount of ferrosulfur added before the end of LF smelting was increased to 8.5 kg / ton of steel.

[0035] The ferrosulphur consumption and manganese sulfide spindle rate of sulfur-containing free-cutting steel Y1215 produced using the embodiments of the present invention and the comparative examples are shown in Table 1:

[0036] Table 1

[0037] Plan No. Ferrosulfur consumption (kg / ton steel) Manganese sulfide spindle rate (%) Example 1 5.0 38 Example 2 5.0 36 Comparative Example 1 10.2 16 Comparative Example 2 10.1 19 Comparative Example 3 8.5 21

[0038] Note: 1. The higher the manganese sulfide spindle rate in the steel, the better the turning performance of the steel.

[0039] 2. Manganese sulfide is examined using an optical microscope, and those with a length / width ≤5 are considered spindle-shaped manganese sulfide.

Claims

1. A method for recycling solid waste from molten iron pretreatment, characterized by: The specific steps are as follows: Step 1: Converter smelting: The converter charge is molten iron + scrap steel. The converter uses lime and light-burned dolomite to make low-basicity slag. The slag basicity is controlled at 1.2-1.

6. The converter smelting end temperature is controlled at 1600-1630℃, and the end carbon content is controlled at 0.05-0.08%. Step 2: Tapping from the converter: Deoxidation and alloying are performed during the tapping process. The alloys added for deoxidation and alloying are: silicon manganese, low carbon ferromanganese, and ferrophosphorus. No ferrosulfur is required. Slag is manually removed during the tapping process, and the silicon content at the tapping end is controlled to ≤0.03%. The amount of silicon manganese added is 4.6-5.0 kg / ton of steel. Step 3: LF refining: After the molten steel reaches LF refining, it is heated slightly without slag replenishment. KR desulfurization slag is first added as slag and to add sulfur to the molten steel. Then samples are taken to analyze the active oxygen content of the molten steel. After the oxygen is determined, calcium carbide is used to deoxidize the slag surface according to the oxygen content, and the active oxygen content of free-cutting steel is controlled at 30~40ppm. Sulfur iron is added before the end of LF smelting, and then it is hoisted into the continuous casting process for normal casting; the composition of KR desulfurization slag is calculated by weight percentage: CaO: 52.5~61.8%, SiO2: 9.1~12.5%, CaS: 4.4~5.6%, CaF2: 1.0~3.8%, Al2O3: ≤3.5%, MgO: ≤3.0%, moisture ≤0.5%, and the rest are impurities; the sulfur in the KR desulfurization slag is mainly present in the slag in the form of CaS.

2. The method for recycling solid waste from molten iron pretreatment according to claim 1, characterized in that: In step 1, the converter charge is 90% molten iron + 10% scrap steel, the amount of lime added is 18-20 kg / ton of steel, and the amount of light-burned dolomite added is 10-15 kg / ton of steel.

3. The method for recycling solid waste from molten iron pretreatment according to claim 1, characterized in that: The amount of low-carbon ferromanganese added in step 2 is 17.8-18.0 kg / ton of steel, and the amount of ferrophosphorus added is 1.50-1.52 kg / ton of steel.

4. The method for recycling solid waste from molten iron pretreatment according to claim 1, characterized in that: Step 3: Slightly increasing the temperature means that the heating rate is ≤ 0.05°C / s and the heating range is ≤ 30°C.

5. The method for recycling solid waste from molten iron pretreatment according to claim 1, characterized in that: The usage of KR desulfurization slag is 100kg / ton of steel; ferrosulfur is added before the end of LF smelting, and the usage of ferrosulfur is 5.0kg / ton of steel.