Direct desulfurization method for converter tapping

Abstract

A direct desulfurization method for converter tapping comprises the following steps: acquiring the mass of to-be-treated molten steel and the mass fraction of converter end-point sulfur; calculating the mass of sulfur in the molten steel according to the molten steel mass and the sulfur mass fraction; calculating the mass of required sodium carbonate according to the equal molar ratio of the amount of substance; in the converter tapping process, the calculated sodium carbonate mass is added into molten steel in a tapping ladle in one time or in several times; after addition and reaction are completed, the sulfur content of molten steel is detected, direct desulfurization at the tapping end is achieved, and the sulfur content of each furnace is stably controlled to be smaller than or equal to 0.045%, so that the number of times of temporary desulfurization in LF and other refining furnaces is remarkably reduced, materials and energy consumption are reduced, and the machining cost is saved; according to the method, the heat loss and logistics load caused by furnace front tank folding and long-time stirring are avoided, corrosion to a converter lining is relieved, automatic closed-loop adding and controllable management of CO2 emission, dust and sulfur-containing slag can be achieved through casting blank weighing and MES / DCS networking, and the method has the advantages of being small in investment, high in efficiency and good in popularization value.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and in particular to a method for direct desulfurization of steel produced from a converter. Background Technology

[0002] Currently, steel plants commonly employ an external desulfurization process before molten iron is tapped or added to the ladle: a desulfurizing agent is added to the ladle or tundish and stirred or aerated to transfer sulfur from the liquid steel to the slag phase or to form soluble / separable sulfides. The slag is then removed before the steel is fed into the converter or continuous casting. For converters, due to the oxidizing atmosphere, top-blown oxygen, and high temperature, the sulfur removal capacity of post-blowing alone is limited. The significant sulfur reduction rate in converters is usually low (typically far below the ideal value). Therefore, refining equipment such as LF (vacuum refining furnace) is often required for temporary desulfurization after tapping to meet the steel composition requirements.

[0003] The main problems caused by the existing practices include: the need for additional desulfurizing agent and forced stirring before the furnace, and sometimes the need for ladle breaking, which leads to heat loss and temperature drop in molten iron, which is not conducive to reducing iron consumption and affects continuous casting and rolling performance; it increases the investment and energy consumption of intermediate ladle, hoisting, shutdown and equipment, and increases material and energy costs; the high frequency of post-processing such as LF causes furnace fluctuations, high load on refining equipment, and increased operating costs and waste slag (including S disposal); if high temperature and high basicity "post-blowing desulfurization" or strong basic blowing is used in the converter to try to desulfurize, it will aggravate furnace lining erosion, shorten the service life of refractory materials, and affect the stability of furnace conditions. Summary of the Invention

[0004] In order to overcome the above-mentioned shortcomings of the prior art, the purpose of this invention is to provide a method for direct desulfurization of steel tapping from a converter.

[0005] The technical solution adopted by this invention to solve its technical problem is: a method for direct desulfurization of steel tapped from a converter, comprising the following steps:

[0006] S1. Obtain the mass of the molten steel to be processed and the sulfur mass fraction at the converter endpoint;

[0007] S2. Calculate the mass of sulfur in the molten steel based on the mass of the molten steel and the mass fraction of sulfur.

[0008] S3. Based on the reaction formula Na2CO3+S=Na2S+CO2, calculate the required mass of sodium carbonate according to the equimolar ratio of substances.

[0009] S4. During the tapping process of the converter, the calculated mass of sodium carbonate is added to the molten steel in the ladle in one or several batches, and the mixture is stirred after addition.

[0010] S5. After the addition and reaction are completed, the sulfur content of the molten steel is tested.

[0011] As a further improvement of the present invention, it includes a predetermined desulfurization target of ≤0.045% sulfur mass fraction in molten steel.

[0012] As a further improvement of the present invention: the final sulfur mass fraction of the converter obtained in step S1 is obtained by online spectrometer, external furnace test or prediction model based on steel composition and process parameters.

[0013] As a further improvement of the present invention: the sodium carbonate is added in layers / sections inside the ladle or from the converter tapping port toward the ladle, so as to promote the reaction by utilizing the flow of molten steel and the temperature field.

[0014] As a further improvement of the present invention: the calculation of the dosage further considers the actual utilization rate η of the desulfurization reaction, and is corrected by the following formula: m(Na2CO3)=m(Na2CO3) / η, where η is the desulfurization utilization rate determined by experience or experiment.

[0015] As a further improvement of the present invention: the value of η is 0 < η ≤ 1.

[0016] As a further improvement of the present invention: the sodium carbonate in step S4 is added in the form of powder, granules or alkaline solution.

[0017] As a further improvement of the present invention, it also includes: according to the final S content, compound stirring for 4-7 minutes, the desulfurization rate reaches the preset value, and after the desulfurization is completed, the steel tapping operation is performed according to the normal procedure.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] By precisely calculating and adding sodium carbonate according to the final sulfur content during the converter tapping process, the sulfur content of molten steel can be effectively controlled without increasing or only minimally increasing the number of process steps. This significantly reduces the number of furnaces that need to undergo temporary desulfurization in refining furnaces such as the LF. This reduces the refining furnace load, decreases material and energy consumption from refining cycles, and improves the stability of production rhythm.

[0020] Eliminating or reducing pretreatment, ladle folding, and stirring steps can avoid heat loss of molten iron caused by ladle folding and prolonged stirring, reduce the number of times intermediate ladles and cranes are operated and reduce mechanical power consumption, thereby reducing energy consumption and equipment depreciation; at the same time, it reduces the frequency of on-site personnel / equipment operations and improves production line utilization.

[0021] Compared to desulfurization measures such as high-temperature and high-alkalinity post-blowing or harsh chemical conditions inside the converter, this method avoids frequent adjustments to converter post-blowing parameters or high-alkalinity and high-temperature post-blowing by adding the product to the ladle and controlling the amount and method of addition. This reduces the chemical and mechanical erosion of the furnace lining, which is beneficial for furnace stability and extending the service life of refractory materials. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0023] It should be noted that the terms "first," "second," etc., used in the specification and claims of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0024] Currently, desulfurization in steel enterprises is mostly done through hot metal pretreatment, specifically outside the furnace before molten iron is added. Most processes utilize desulfurization devices and agents. After treatment, the molten iron slag is removed before it enters the converter. The hot metal requires the addition of desulfurizing agents and stirring, and in some cases, even ladle breaking, increasing heat loss (a decrease in temperature), which is detrimental to low-iron-consumption production. Pretreatment before molten iron processing increases the number of ladles required and the workload of cranes, thus increasing material input and electricity consumption costs.

[0025] Sulfur is a harmful impurity in steel, and its content directly affects the steel's plasticity, toughness, weldability, and surface quality. Special steel grades have very strict requirements for sulfur control. The chemical form of sulfur in liquid-phase steel, the thermodynamic driving force of desulfurization, and the reaction kinetics are all affected by temperature, composition (such as C, Mn, O), slag basicity ([CaO] / [SiO2], etc.), and furnace stirring / contact conditions. During converter top blowing, due to the strong oxidizing environment, sulfur tends to remain in the metallic phase or is incompletely oxidized / transferred, making efficient desulfurization difficult in the converter section. Effective desulfurization typically requires: a high-basicity slag phase to promote sulfur transfer to the slag, good metal-slag contact (achieved through stirring or air blowing), and appropriate desulfurizing agent metering and timing.

[0026] Traditional desulfurizing agents are mainly Ca-based (lime, steelmaking slag modifiers) and Ca-containing composite agents, which often fix sulfur in the slag by forming CaS. In addition, reducing / desulfurizing agents such as Mg and Al are used in special cases. To achieve precise addition and avoid over- or under-addition, reliable online / semi-online sulfur content measurement, accurate single-furnace molten steel quality measurement (including actual mass / density calculation of continuously cast billets), and automatic addition and closed-loop control interconnected with DCS / MES are essential to transform computational chemometrics into a stable production control method.

[0027] To address the aforementioned issues, during the converter tapping process, the amount of sodium carbonate added is precisely calculated based on the final sulfur content, thereby achieving the goal of sulfur removal during tapping. After the billet is cut to the standard length, it is hotly sent to the rolling mill for weighing. The weight of each billet is divided by the actual end face area and the standard length at the time of cutting to obtain the density of the billet. The drawing speed and steel composition during the production of this billet are obtained through networking. A corresponding relationship is established between the drawing speed, steel composition, the standard weight required for billet rolling, and the accurately calculated end face area. Using the accurately calculated end face area of ​​the billet as the main condition, the standard length that the billet should be cut to is calculated, guiding the cutting device to cut to the calculated standard length, and the data is compared and adjusted with the weighing data from the rolling mill.

[0028] This technology primarily addresses the issue of steel being forced to undergo temporary desulfurization in the LF furnace due to excessive sulfur content during the steel production process. It ensures stable production of the continuous casting machine and reduces the number of furnaces requiring temporary desulfurization in the LF furnace. It effectively reduces material costs and electricity consumption, prevents and reduces scrap generated due to high sulfur content in molten iron, and achieves the goal of stable production rhythm and cost for steelmaking plants. Furthermore, it controls the desulfurization content of each furnace to ≤0.045%.

[0029] The present invention will now be further described with reference to the embodiments: A method for direct desulfurization of steel tapped from a converter includes the following steps:

[0030] S1. Obtain the mass of the molten steel to be processed and the sulfur mass fraction at the converter endpoint;

[0031] S2. Calculate the mass of sulfur in the molten steel based on the mass of the molten steel and the mass fraction of sulfur.

[0032] S3. Based on the reaction formula Na2CO3+S=Na2S+CO2, calculate the required mass of sodium carbonate according to the equimolar ratio of substances.

[0033] S4. During the tapping process of the converter, the calculated mass of sodium carbonate is added to the molten steel in the ladle in one or several batches, and the mixture is stirred after addition.

[0034] S5. After the addition and reaction are completed, the sulfur content of the molten steel is tested.

[0035] Furthermore, the specific principle is as follows:

[0036] Analysis of desulfurization chemical reactions:

[0037] The main reaction involved in desulfurization using sodium carbonate (Na2CO3) is: Na2CO3 + S = Na2S + CO2↑ (this reaction is simplified under the high-temperature environment of steelmaking).

[0038] The molten steel weighs 120t = 120000kg, and the sulfur content in the molten steel is 0.010%. Therefore, the mass fraction of sulfur in the molten steel, m(S), is:

[0039] m(S)=120*1000*0.010%=12kg.

[0040] 2. Calculate the required mass of sodium carbonate:

[0041] Based on the reaction Na₂CO₃ + S = Na₂S + CO₂↑, the molar ratio of sodium carbonate to sulfur can be determined.

[0042] n(Na2CO3):n(S) = 1:1.

[0043] The molar mass of sulfur (S) is M(S) = 32 g / mol. 12 kg = 12000 g. The amount of substance of sulfur is: 12000 / 32 = 375 mol.

[0044] The molar mass of sodium carbonate (Na₂CO₃) is M(Na₂CO₃) = (23 x 2 + 12 + 16 x 3) g / mol = 10⁶ g / mmol

[0045] Because n(Na2CO3) = n(S) = 375 mmol,

[0046] Therefore, the mass of sodium carbonate m(Na2CO3) = n(Na2CO3) x M(Na2CO3) = 375mol x 106g / mol = 39750g = 39.75kg.

[0047] 3. Calculate the volume of carbon dioxide gas produced:

[0048] From the reaction Na₂CO₃ + S = Na₂S + CO₂, we can know the molar ratio of carbon dioxide (CO₂) to sulfur produced.

[0049] The ratio of n(CO2):n(S) is 1:1, therefore n(CO2) = n(S) = 375 mol.

[0050] Under standard conditions (0℃, 101kPa), the molar volume of the gas is 22.4 L / mol. Therefore, the volume of carbon dioxide, V(CO2), is n(CO2)x = 375mol x 22.4 L / mol = 8400 L = 8.4 mm. In summary, 39.75 kg of sodium carbonate needs to be added to produce 8.4 cubic meters of carbon dioxide gas (under standard conditions).

[0051] Specifically, a specific embodiment of the method for direct desulfurization of steel tapping from a converter according to the present invention includes the following steps:

[0052] S1. Obtain the mass of the molten steel to be processed and the sulfur mass fraction at the converter endpoint;

[0053] S2. Calculate the mass of sulfur in the molten steel based on the mass of the molten steel and the mass fraction of sulfur.

[0054] S3. Based on the reaction formula Na2CO3+S=Na2S+CO2, calculate the required mass of sodium carbonate according to the equimolar ratio of substances.

[0055] S4. During the tapping process of the converter, the calculated mass of sodium carbonate is added to the molten steel in the ladle in one or several batches, and the mixture is stirred after addition.

[0056] S5. After the addition and reaction are completed, the sulfur content of the molten steel is tested.

[0057] The sulfur content of the molten iron was 0.0518%, and the final sulfur content after converter smelting was 0.0471%. The converter manager added 12.5 kg of sodium carbonate and 5 bags of lime granules during the tapping process. The sulfur content of sample Y1 entering the station was 0.0436%, and the sulfur content of sample Y2 leaving the station was 0.0429%. The composition meets the requirement of ≤0.045% for steel grades.

[0058] As one embodiment of the present invention, the sulfur content (S) of the molten iron entering the furnace is 0.0518%; the converter manager adds 12.5 kg of sodium carbonate + 5 bags of lime granules during the tapping process, and the sulfur content of the sample taken at station (Y1) is 0.0436%; the sulfur content of the sample taken at station (Y2) is 0.0429%.

[0059] The mass of sulfur in molten steel: m(S)in 62.16 kg at the furnace inlet stage; m(S)term at the converter end point = 56.52 kg; Y1 m(S)Y1 52.32 kg at the station inlet; Y2 m(S)Y2 51.48 kg at the station outlet.

[0060] Desulfurization amount before and after this application:

[0061] Calculate the required mass of Na2CO3 based on stoichiometry (ideal, no loss):

[0062] Reaction: Na₂CO₃ + S = Na₂S + CO₂; Molar mass M(Na₂CO₃) = 10⁶ g / mol, M(S) = 32 g / mol. Theoretically, the amount of Na₂CO₃ needed to remove 5.04 kg of S is 5040 g × (10⁶ / 32) = 16,710 g ≈ 16.71 kg.

[0063] Actual Na₂CO₃ added = 12.5 kg (theoretically, the mass of S removed by the reaction = 12500 / 10⁶ × 3² = 3.77 kg S);

[0064] The actual removal of S = 5.04 kg was higher than the theoretical removal of 3.77 kg from 12.5 kg of Na2CO3 alone. This indicates that the synergistic effect of lime particles (5 bags) with the molten steel / slag phase, as well as process flow and slag-metal distribution, jointly promoted additional desulfurization.

[0065] If we assume that lime contributes approximately (5.04-3.77) = 1.27 kg of S to the removal of S.

[0066] n(Na₂CO₃) = 12500 / 10⁶ ≈ 117.92 mol → If all of it is converted to CO₂, then V(CO₂, STP) = 117.92 × 22.4 L ≈ 2640 L ≈ 2.64 m 3 .

[0067] Specifically, before tapping steel from the converter, the DCS / MES system provides the final analysis value of this furnace (S = 0.0471%) and the estimated quality of the molten steel in this furnace (example 120t); and calculates the required removal amount and recommended Na2CO3 addition amount based on the preset target (S ≤ 0.045%) (considering empirical utilization rate and safety margin).

[0068] Prepare the following reagents: 12.5 kg of powdered / fine granular Na2CO3 (weighed), and 5 bags of lime granules (as per on-site packaging). Prepare localized dust collection / exhaust systems and protective equipment.

[0069] The feed is added in two stages as the converter taphole opens and begins to release water into the ladle:

[0070] Initial addition (main dosage): Within 30–60 seconds after tapping begins, evenly sprinkle approximately 70% Na2CO3 (~8.75 kg) onto the surface of the molten steel in the ladle or along the direction of the steel flow to ensure that the reagent is carried in by the steel flow and comes into direct contact with the metal surface; at the same time, add some lime particles to help form a local alkaline slag layer.

[0071] Second addition (supplementary dosage): Add the remaining 30% Na2CO3 (~3.75kg) and remaining lime particles in the middle and later stages of steel tapping to make up for insufficient initial contact and mixing.

[0072] This segmented addition helps to increase the contact time and utilization efficiency between the reagent and the molten steel, and reduces the risk of local over-reaction or reagent scattering caused by a single addition.

[0073] Preferably add the agent into the ladle (the flow path area from the ladle opening to the center of the ladle), or during the tapping process, directly drop the agent into the flowing steel through a fixed-point dosing box / dosing hopper; if an inert gas stirring (asphyxiation / argon) or a flow propulsion device is available on site, it can be used in conjunction with it under safe conditions to improve mixing.

[0074] After the reagent is added, the self-flowing molten steel and the kinetics during tapping ensure that the reagent comes into full contact with the molten steel. If necessary, the ladle can be lightly stirred briefly (while taking into account temperature and production rhythm) to promote the reaction and the transfer of sulfur to the slag.

[0075] Lime particles help create an alkaline environment in the slag phase and enter the slag phase along with desulfurization products, facilitating subsequent slag removal or discharge with the LF / continuous casting slag system.

[0076] (D) Sampling and Closed-Loop Adjustment

[0077] After the chemical is added to the ladle, rapid chemical testing is carried out at the agreed-upon points according to the process: sampling is taken at Y1 upon entering the station (before the ladle enters the rolling mill / cast billet) and at Y2 upon leaving the station (after leaving the station), and the test data is uploaded to DCS / MES.

[0078] If the Y1 / Y2 test results do not meet the target (S>0.045%), then based on the difference and the empirical utilization rate, quickly calculate whether it is necessary to increase the dosage or change the dosage method in subsequent furnaces (e.g., increase the Na2CO3 dosage, increase the amount of lime, or change to finer powder to increase the reaction area).

[0079] The lime particles added this time help form an alkaline slag layer that facilitates the migration of sulfur to the slag phase, and may also help fix some sulfur through the form of CaS, thus explaining the phenomenon that the actual desulfurization amount (5.04 kg) is greater than the theoretical removal amount of Na2CO3 alone (3.77 kg).

[0080] On-site, a database of "pesticide utilization rate η" (η = actual removal S / stoichiometric removal S) is established from a statistical perspective, and η is incorporated into the automatic dosing correction factor. With accumulated experience, the safety margin can be gradually reduced and costs optimized.

[0081] Based on the estimated total CO2 production from the addition of 12.5 kg of Na2CO3 and the complete reaction, the CO2 produced is approximately 2.64 m³. 3 (STP) The local ventilation capacity of the steel tapping area should be confirmed on site and necessary exhaust / detection devices should be installed to prevent the accumulation of high concentrations of gas in the local area.

[0082] When adding Na2CO3 and lime powder, local dust collection, wet addition, or directional addition via a dosing hopper should be used to prevent dust from spreading and affecting workers and equipment.

[0083] The resulting Na2S sulfur-containing slag must be disposed of in accordance with hazardous and environmental regulations or managed with the existing slag treatment system.

[0084] All dosage, administration time, and test values ​​(Y1 / Y2) are included in the MES record as evidence for closed-loop optimization and patent implementation.

[0085] Heat 25105404 achieved a reduction in sulfur dioxide concentration (S) from 0.0471% at the converter endpoint to 0.0429% at the outlet by adding 12.5 kg of sodium carbonate and 5 bags of lime granules during the converter tapping stage, meeting and exceeding the technical target of S≤0.045%. This embodiment demonstrates that by moving the pre-furnace pretreatment to the converter tapping stage and employing Na2CO3 + lime synergistic desulfurization, precise and economical desulfurization control can be achieved without affecting the converter and continuous casting production rhythm, verifying the practicality and economic benefits of the invention.

[0086] The main functions of this invention are as follows: By accurately calculating and adding sodium carbonate at the final sulfur content of molten steel during the converter tapping process, this invention achieves direct desulfurization at the tapping end and stably controls the sulfur content of each heat to ≤0.045%, thereby significantly reducing the number of heats requiring temporary desulfurization in refining furnaces such as LF, reducing material and energy consumption, and saving processing costs. This method avoids heat loss and material load caused by furnace-front ladle breaking and long-term stirring, reduces erosion of the converter lining, balances compositional stability and continuous casting production rhythm, and can achieve automated closed-loop addition and controllable management of CO2 emissions, dust and sulfur-containing slag through billet weighing and MES / DCS networking. It has the advantages of low investment, high efficiency and good promotion value.

[0087] In summary, after reading this invention document, those skilled in the art can make various other corresponding modifications to the technical solutions and concepts based on this invention without creative mental effort, and all of these modifications fall within the scope of protection of this invention.

Claims

1. A method for direct desulfurization of steel tapped from a converter, characterized in that, Includes the following steps: S1. Obtain the mass of the molten steel to be processed and the sulfur mass fraction at the converter endpoint; S2. Calculate the mass of sulfur in the molten steel based on the mass of the molten steel and the mass fraction of sulfur. S3. Based on the reaction formula Na2CO3+S=Na2S+CO2, calculate the required mass of sodium carbonate according to the equimolar ratio of substances. S4. During the tapping process of the converter, the calculated mass of sodium carbonate is added to the molten steel in the ladle in one or several batches, and the mixture is stirred after addition. S5. After the addition and reaction are completed, the sulfur content of the molten steel is tested.

2. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, The predetermined desulfurization target is ≤0.045% sulfur mass fraction in molten steel.

3. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, The final sulfur mass fraction obtained in step S1 is obtained by online spectrometer, external laboratory analysis, or prediction model based on steel composition and process parameters.

4. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, The sodium carbonate is added either inside the ladle or in layers / sections from the converter tapping port toward the ladle.

5. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, The calculation of the dosage further considers the actual utilization rate η of the desulfurization reaction and is corrected by the following formula: m(Na2CO3)=m(Na2CO3) / η, where η is the desulfurization utilization rate determined by experience or experiment.

6. The method for direct desulfurization of steel tapped from a converter according to claim 5, characterized in that, The value of η is 0 < η ≤ 1.

7. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, In step S4, sodium carbonate is added in the form of powder, granules, or an alkaline solution.

8. The method for direct desulfurization of steel tapped from a converter according to claim 1, characterized in that, It also includes compound stirring for 4-7 minutes according to the final sulfur content, until the desulfurization rate reaches the preset value, and after desulfurization is completed, the steel tapping operation is carried out according to the normal procedure.