A method for preparing protective slag using coal gangue

Abstract

The present invention belongs to the technical field of protective slag, and in particular relates to a method for preparing protective slag using coal gangue, comprising: step 1, sorting, crushing and homogenizing the coal gangue to separate Fe2O3, sulfide and radioactive minerals; step 2, calcining the pretreated coal gangue at 800-1000°C in a gradient manner to remove sulfur elements and part of organic carbon to generate a desulfurized coal gangue base material; step 3, adding a CaO source, an MgO source and a flux to the desulfurized coal gangue base material to adjust the mixture. The CaO / SiO2 mass ratio is 0.9-1.2, and the Al2O3 content is ≤14wt%; step 4, controlling the total carbon content to 3-8wt% by adding graphite powder or performing secondary calcination on the desulfurized coal gangue base material; step 5, melting the mixture at 1350-1450°C and quenching it with water into glassy particles. The finished continuous casting mold slag prepared by the present invention has the beneficial effects of synergistic removal of impurities and improved environmental protection, precise control of carbon content, saving resource costs and stable product performance.

Description

Technical Field

[0001] The invention belongs to the technical field of mold slag, and in particular relates to a method for preparing mold slag by utilizing coal gangue. Background Art

[0002] Continuous casting mold slag is a key material for regulating heat transfer, lubricating the crystallizer and absorbing inclusions in the continuous casting process of steel. Its performance directly affects the surface quality of the ingot and the continuous casting efficiency. Traditional mold slag uses natural minerals such as wollastonite and fluorite as the main raw materials, but faces two major pain points: first, high-quality wollastonite resources are becoming increasingly depleted, and the raw material cost accounts for as much as 35%-40%; second, fluorite releases fluoride at high temperatures, causing equipment corrosion and environmental pollution. As a solid waste associated with coal mining, the chemical composition of coal gangue is highly consistent with the requirements of the base material of mold slag, but the existing coal gangue resource technology has significant defects:

[0003] The efficiency of synergistic removal of impurities is low: Fe2O3 in coal gangue (5% causes deterioration of the transmittance of the slag layer), sulfides (FeS2 decomposes at high temperature and causes sulfur increase in molten steel), heavy metals (As and Pb migrate to the ingot) and radioactive substances (U, Th series) are difficult to remove simultaneously.

[0004] Inaccurate carbon content control: The carbon content in the mold slag (3-8wt%) needs to be precisely controlled to balance insulation and melting rate, but the residual carbon after calcining coal gangue fluctuates greatly (2-10%).

[0005] Insufficient melt homogenization: The one-step melting process is prone to produce unreacted Al2O3 inclusions and pores (porosity > 8%), resulting in viscosity fluctuations (±0.2 Pa·s) and uneven spreading (slag layer thickness deviation >0.5 mm).

[0006] Based on the above background, a new mold slag preparation method is urgently needed to break through the following technical bottlenecks:

[0007] How to achieve efficient and simultaneous removal of Fe, S, heavy metals and radioactive substances from coal gangue through multi-stage coordinated treatment, so that Fe2O3≤5%, S≤0.3%, and radioactivity <1Bq / g;

[0008] How to build a dynamic carbon compensation mechanism to precisely control the total carbon content within the range of 3-8wt% (with an error of ±0.5%) to meet the continuous casting requirements of different steel grades?

[0009] The melting process is optimized to eliminate porosity and composition segregation, stabilize the mold slag viscosity at 0.1-0.5 Pa·s (1300℃), and keep the porosity ≤5%, thereby improving the sphericity of the glass particles. Summary of the Invention

[0010] In response to the above problems, the present invention proposes a method for preparing protective slag using coal gangue, which effectively solves the problems in the existing technology of high raw material mineral cost, low impurity synergistic removal efficiency, inaccurate carbon content control and insufficient melt homogenization when using coal gangue to prepare protective slag.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing protective slag using coal gangue comprises the following steps:

[0012] Step 1: sorting, crushing and homogenizing the gangue to separate Fe2O3, sulfides and radioactive minerals;

[0013] Step 2: calcining the pretreated gangue at 800-1000° C. in a gradient manner to remove sulfur and part of organic carbon to generate a desulfurized gangue base material;

[0014] Step 3: Add CaO source, MgO source and flux to the desulfurized coal gangue base material, adjust the CaO / SiO2 mass ratio in the mixture to 0.9-1.2, and adjust the Al2O3 content to ≤14wt%;

[0015] Step 4: controlling the total carbon content to 3-8wt% by adding graphite powder or performing secondary calcination on the desulfurized coal gangue base material;

[0016] Step 5: Melt the mixture in step 4 at 1350-1450° C., quench with water into glassy particles, and obtain finished continuous casting mold slag.

[0017] Furthermore, the step 1 includes:

[0018] Magnetic separation was used to remove mineral phases with Fe2O3 content > 5wt%;

[0019] Separate sulfide and carbonaceous components by flotation to reduce the sulfur content to ≤0.3wt%;

[0020] A gamma ray detector is used to screen coal gangue raw materials with radioactivity less than 1Bq / g.

[0021] Furthermore, the gradient calcination in step 2 includes:

[0022] The first stage: calcination at 800-850℃ for 1-2 hours to oxidize FeS2 into Fe2O3 and release SO2 gas;

[0023] The second stage: calcination at 950-1000℃ for 0.5-1 hour to remove organic carbon and activate the SiO2 / Al2O3 mineral phase.

[0024] Furthermore, in step 3, the CaO source is limestone or dolomite, the MgO source is magnesite, and the flux is fluorite or borax, wherein the amount of fluorite added is 2-5wt% of the total mass of the mixture.

[0025] Furthermore, the carbon content control in step 4 is achieved by:

[0026] When the residual carbon content of coal gangue after calcination is less than 3wt%, graphite powder is added to reach the target carbon content;

[0027] When the residual carbon is greater than 8wt%, decarburization is performed to the target range through secondary calcination, and the secondary calcination decarburization temperature is 600-700℃.

[0028] Furthermore, step 1 also includes acid washing pretreatment of the coal gangue, specifically:

[0029] Soak the gangue particles in a 5-10wt% dilute hydrochloric acid solution for 2-4 hours to remove heavy metal impurities such as As and Pb;

[0030] Heavy metal hydroxides are recovered from the wastewater after pickling through neutralization and precipitation.

[0031] Furthermore, the mixture in step 3 also contains 0.5-2 wt % of sodium phosphate, which is used to solidify the residual heavy metal ions into phosphate glass during the melting process.

[0032] Furthermore, the melting process in step 5 adopts a gradient heating system, including: heating to 1200°C at 5°C / min and keeping warm for 30 minutes, and then heating to 1400-1450°C at 3°C / min and keeping warm for 1 hour to achieve complete homogenization.

[0033] Furthermore, in step 5, the water quenching process uses a high-speed water flow with a flow rate of ≥10 m / s to crush the melt into glass particles with a particle size of 0.1-1 mm.

[0034] Furthermore, the properties of the finished continuous casting mold slag meet the following requirements: viscosity of 0.1-0.5 Pa·s at 1300° C., melting point of 1100-1200° C., basicity of 1.0-1.2, and porosity ≤5%.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] Synergistic impurity removal and environmental protection improvement: Through the multi-stage separation process of magnetic separation-flotation-pickling, the Fe2O3 content is ≤5%, the sulfur content is ≤0.3%, and heavy metals (As, Pb) are removed, with the radioactive specific activity less than 1Bq / g; the pickling wastewater is neutralized and precipitated to recover heavy metal hydroxides, reducing secondary pollution.

[0037] Precise control of carbon content: Gradient calcination combined with dynamic carbon replenishment technology (external graphite addition / secondary calcination decarburization) stably controls the total carbon content at 3-8wt%, meeting the precise carbon activity requirements of the continuous casting process.

[0038] Melt homogenization and performance optimization: Gradient temperature melting (1200°C holding + 1400°C homogenization) reduces porosity to ≤5% and narrows the viscosity fluctuation range to ±0.05 Pa·s (1300°C). Combined with a high-speed water quenching process (≥10m / s), it produces glass particles with high sphericity, improving the spreadability and lubricity of the mold slag.

[0039] Resource utilization and cost advantages: Gangue can replace more than 60% of wollastonite raw materials, reducing raw material costs by 40%-50%. At the same time, the disposal of gangue has both economic and environmental benefits.

[0040] Heavy metal stabilization: Sodium phosphate is added to solidify residual heavy metal ions into inert phosphate glass, preventing the migration of harmful elements during service and ensuring the cleanliness of the steel.

[0041] Finished product performance indicators: melting point 1100-1200℃, basicity 1.0-1.2, which fully matches the technical requirements of continuous casting mold slag, can replace traditional high-fluorine formulas, reduce fluorite consumption by more than 30%, and significantly reduce the risk of fluorine pollution. DETAILED DESCRIPTION

[0042] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0043] The gamma-ray detector used in this embodiment is a FD-3013B digital gamma-ray detector. The protective slag performance test method adopts the YB / T185-2017 continuous casting protective slag viscosity test method for experiment. The protective slag porosity test is based on GB / T2997-2015 "Test method for apparent porosity, water absorption, bulk density and true porosity of dense shaped refractory products". The pH value of the protective slag is obtained by calculating the mass ratio of CaO / SiO2 by chemical wet analysis. The melting point and complete melting time of the protective slag are obtained according to the YB / T186-continuous casting protective slag melting temperature test method. Example 1

[0044] Step 1: Take the coal gangue raw material and crush it to a particle size of ≤5mm with a jaw crusher, and use a permanent magnetic drum magnetic separator (magnetic field strength 0.8T) to separate the mineral phase with Fe2O3 content >5wt%; put the magnetically separated material into a flotation tank, add diesel as a collector (dosage 0.5kg / t), and separate sulfide and carbonaceous components to a sulfur content of ≤0.3wt%; use a gamma-ray detector to screen qualified raw materials with a radioactive specific activity of <1Bq / g, and then soak them in a 5wt% dilute hydrochloric acid solution for 3 hours. After filtering, add lime milk to the wastewater to adjust the pH to 8.5, and precipitate and recover hydroxides containing As and Pb.

[0045] Step 2: Place the pretreated coal gangue in a rotary kiln for gradient calcination: in the first stage, heat it to 800℃ at a rate of 10℃ / min, introduce air at a flow rate of 2m³ / h, and calcine for 1.5 hours to completely oxidize FeS2; in the second stage, heat it to 1000℃ and keep it at that temperature for 40 minutes to generate a desulfurized coal gangue base material with a residual carbon content reduced to 2.8wt%.

[0046] Step 3: Add limestone (CaO content 52%), magnesite (MgO content 45%) and fluorite (CaF2 content 95%) to the desulfurized coal gangue base material, control the CaO / SiO2 mass ratio in the mixture to 1.2, the Al2O3 content to 12wt%, the fluorite addition amount to 3.5wt%, and add 1.2wt% sodium phosphate.

[0047] Step 4: After calcination, the residual carbon content of the base material is detected to be 2.8wt%, and 200 mesh flake graphite powder is added to make the total carbon content 5.5wt%.

[0048] Step 5: The mixture prepared in step 4 is charged into an electric arc furnace and heated according to a gradient heating system: the temperature is raised to 1200°C at 5°C / min and kept for 30 minutes, and then raised to 1420°C at 3°C / min and kept for 1 hour; the melt is crushed in a water quenching tower at a water flow rate of 12 m / s to obtain glass particles with a particle size of 0.2-0.8 mm and a high sphericity.

[0049] Performance testing:

[0050] Viscosity at 1300℃ is 0.3Pa·s, melting point is 1150℃, basicity (CaO / SiO2) is 1.15, porosity is 3.8%, and complete melting time is 16 minutes. Example 2

[0051] Step 1: After the gangue is crushed to 3mm by a high-pressure roller mill, a high-gradient magnetic separator (1.2T) is used to remove iron-containing minerals; xanthate (0.3kg / t) is added during flotation to enhance sulfide removal, and the final sulfur content is 0.25wt%; pickling is carried out by soaking in 8wt% hydrochloric acid for 4 hours to neutralize Pb in the wastewater. 2+ Concentration <0.1mg / L.

[0052] Step 2: Gradient calcination: The first stage is 850℃ for 2 hours to desulfurize; the second stage is decarbonization at 950℃ to 7.5wt% residual carbon, and the secondary calcination (650℃ for 1 hour) is used to decarbonize to 4.9wt% to generate desulfurized coal gangue base material.

[0053] Step 3: Add dolomite (CaO30%, MgO20%) to adjust the CaO / SiO2 ratio to 0.9, the Al2O3 content to 14wt%, the borax addition amount to 4wt%, and the sodium phosphate addition amount to 0.8wt%.

[0054] Step 4: The total carbon content of the desulfurized coal gangue base material has reached 4.9wt% through step 2, so step 4 is omitted.

[0055] Step 5: In the melting stage, an induction furnace is used, and the temperature is kept at 1400℃ for 1.5 hours. The melt is crushed in a water quenching tower with a water flow rate of 10m / s, and the particle size of the water-quenched particles is 0.5-1mm.

[0056] Performance testing:

[0057] Viscosity at 1300℃ is 0.4Pa·s, melting point is 1180℃, basicity is 1.05, porosity is 4.5%, and complete melting time is 18 minutes. Example 3

[0058] The difference between step 1 to step 3 and Example 1 is that the amount of sodium phosphate added is 0.5 wt %;

[0059] Step 4: When the residual carbon reaches 8.5wt% after calcination, a secondary calcination at 700℃ is carried out under nitrogen protection to decarbonize to 6.2wt%; the fluorite added in step 3 is 5wt%, and 200 mesh flake graphite powder is added to the mixture to achieve a total carbon content of 7.8wt%.

[0060] Step 5: The mixture in step 4 is heated to 1200°C at 5°C / min and kept warm for 30 minutes, then heated to 1400°C at 3°C / min and kept warm for 1 hour. The melt is water quenched at a flow rate of 15 m / s to obtain ultrafine particles with a particle size of 0.1-0.3 mm.

[0061] Performance testing:

[0062] Viscosity at 1300℃ is 0.2Pa·s, melting point is 1120℃, basicity is 1.2, porosity is 2.9%, and complete melting time is 14 minutes. Example 4

[0063] The difference between step 1 to step 5 and embodiment 1 is that the Al2O3 content in step 3 is 1wt%;

[0064] Performance testing:

[0065] Viscosity at 1300℃ is 0.25Pa·s, melting point is 1100℃, basicity is 1.20, porosity is 3%, and complete melting time is 14 minutes.

[0066] Comparative Example 1:

[0067] Step 1: directly crushing the gangue to a particle size of 10 mm without magnetic separation, flotation or radioactive screening;

[0068] Step 2: calcining at 700°C for 3 hours without using a gradient temperature increase;

[0069] Step 3: adding limestone to adjust the CaO / SiO2 ratio to 0.5, without controlling the Al2O3 content;

[0070] Step 4: No external carbon source is added, and the residual carbon content after calcination is 2 wt%;

[0071] Step 5: Melt at 1250° C. for 1 hour and quench with water at a flow rate of 5 m / s to obtain particles with a particle size of 1-3 mm.

[0072] Performance testing:

[0073] The viscosity at 1300℃ is 0.85Pa·s, the melting point is 1250℃, the basicity is 0.50, the porosity is 9.7%, and the complete melting time is 32 minutes.

[0074] Comparative Example 1 shows that the lack of magnetic flotation results in residual Fe2O3 (>8wt%) and sulfides, and the increase of impurity phases hinders the flow of the melt, resulting in abnormal viscosity; the excessively high Al2O3 content forms a high-melting-point corundum phase, and the liquidus temperature significantly increases, resulting in abnormal melting point; the uncontrolled carbon content (only 2wt%) leads to insufficient sintering driving force, and the pores between the particles cannot be effectively filled.

[0075] Comparative Example 2:

[0076] Step 1: Only Fe2O3 was removed by magnetic separation, and the sulfide content was not treated (sulfur content 1.2 wt%);

[0077] Step 2: calcining directly at 1000°C for 2 hours without desulfurization and decarburization in stages;

[0078] Step 3: Add fluorite (8 wt%) and magnesite to make the CaO / SiO2 ratio reach 2.0;

[0079] Step 4: adding graphite powder to a total carbon content of 12 wt%;

[0080] Step 5: After melting at 1400°C, water quenching is performed at a water flow rate of 5 m / s to obtain particles with a particle size of 1-3 mm, and bubbles and cavities appear inside the particles.

[0081] Performance testing:

[0082] Viscosity at 1300℃ is 0.12 Pa·s, melting point is 1360℃, basicity is 2.00, porosity is 13.5%, and complete melting time is 9 minutes.

[0083] Comparative Example 2 shows that excessive fluorite (8wt%) excessively reduces the viscosity, while the sulfur content of 1.2wt% causes a sudden change in the gas-liquid interfacial tension, CaO / SiO2=2.0 makes it difficult for the silicate network to disintegrate, and melting requires higher energy input. At the same time, the total carbon content is too high, and excessive CO gas is generated during the cooling water quenching process, which reduces the quality of the particles.

[0084] The experimental data of the above embodiments show that by performing magnetic separation, flotation and radioactivity detection on coal gangue, combined with key technologies such as gradient calcination desulfurization and decarburization, and water quenching process control, the performance of the protective slag obtained fully meets the standard requirements of YB / T185-2017 continuous casting protective slag viscosity test method, providing a reliable technical path for the high-value utilization of coal gangue, and is suitable for the production of protective slag under high-speed continuous casting conditions.

[0085] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing mold slag using coal gangue, characterized in that: The following steps are involved: Step 1: sorting, crushing, and homogenizing the gangue to separate Fe2O3, sulfides, and radioactive minerals, including: using magnetic separation to remove mineral phases with an Fe2O3 content greater than 5wt%, separating sulfides and carbonaceous components by flotation to reduce the sulfur content to ≤0.3wt%, and using a gamma-ray detector to screen gangue raw materials with a radioactivity specific activity less than 1Bq / g; Step 2: calcining the pretreated gangue at 800-1000°C in a gradient manner to remove sulfur and some organic carbon to produce a desulfurized gangue base material. The gradient calcination comprises: a first stage: calcining at 800-850°C for 1-2 hours to oxidize FeS2 to Fe2O3 and release SO2 gas; a second stage: calcining at 950-1000°C for 0.5-1 hour to remove organic carbon and activate the SiO2 / Al2O3 mineral phase; Step 3: Add CaO source, MgO source and flux to the desulfurized coal gangue base material, adjust the CaO / SiO2 mass ratio in the mixture to 0.9-1.2, and adjust the Al2O3 content to ≤14wt%; Step 4: controlling the total carbon content to 3-8 wt% by adding graphite powder or performing secondary calcination on the desulfurized gangue base material. The carbon content control is achieved by: when the residual carbon after calcining the gangue is less than 3 wt%, adding graphite powder to the target carbon content; when the residual carbon is greater than 8 wt%, performing secondary calcination to decarbonize the material to the target range; the secondary calcination decarbonization temperature is 600-700°C; Step 5: Melt the mixture in step 4 at 1350-1450° C., quench with water into glassy particles, and obtain finished continuous casting mold slag.

2. The method for preparing mold slag using coal gangue according to claim 1, wherein: In step 3, the CaO source is limestone or dolomite, the MgO source is magnesite, and the flux is fluorite or borax, wherein the amount of fluorite added is 2-5wt% of the total mass of the mixture.

3. The method for preparing mold slag using coal gangue according to claim 1, wherein: Step 1 also includes acid washing pretreatment of the coal gangue, specifically: The coal gangue particles are soaked in 5-10wt% dilute hydrochloric acid solution for 2-4 hours to remove As and Pb heavy metal impurities. The heavy metal hydroxides are recovered from the wastewater after acid washing by neutralization precipitation method.

4. The method for preparing mold slag using coal gangue according to claim 1, wherein: The mixture in step 3 also contains 0.5-2 wt % of sodium phosphate, which is used to solidify the residual heavy metal ions into phosphate glass during the melting process.

5. The method for preparing mold slag using coal gangue according to claim 1, wherein: The melting process in step 5 adopts a gradient heating system, including: heating to 1200°C at 5°C / min and keeping warm for 30 minutes, and then heating to 1400-1450°C at 3°C / min and keeping warm for 1 hour to achieve complete homogenization.

6. The method for preparing mold slag using coal gangue according to claim 1, wherein: In step 5, the water quenching process uses a high-speed water flow with a flow rate of ≥10 m / s to crush the melt into glass particles with a particle size of 0.1-1 mm.

7. The method for preparing mold slag using coal gangue according to claim 1, wherein: The properties of the finished continuous casting mold slag meet the following requirements: viscosity of 0.1-0.5 Pa·s at 1300° C., melting point of 1100-1200° C., basicity of 1.0-1.2, and porosity ≤5%.