Method for preparing casting powder from coal gangue

Abstract

The invention belongs to the technical field of casting powder, and particularly relates to a method for preparing casting powder from coal gangue, which comprises the following steps: step 1, sorting, crushing and homogenizing the coal gangue, and separating Fe2O3, sulfide and radioactive minerals; step 2, carrying out gradient calcination on the pretreated coal gangue at 800-1000 DEG C, and removing sulfur element and part of organic carbon to generate a desulfurized coal gangue base material; step 3, adding a CaO source, a MgO source and a fluxing agent into the desulfurized coal gangue base material, and adjusting the mass ratio of CaO to SiO2 in the mixture to 0.9-1.2 and the content of Al2O3 to be less than or equal to 14wt%; step 4, adding graphite powder or carrying out secondary calcination on the desulfurized coal gangue base material to control the total carbon content to be 3-8wt%; and step 5, melting the mixture at 1350-1450 DEG C, and performing water quenching to obtain glassy particles. The finished continuous casting mold flux prepared by the method has the beneficial effects that impurities are synergistically removed, the environmental protection property is improved, the carbon content is accurately regulated and controlled, the resource cost is saved, and the performance of a finished product is stable.

Description

Technical Field

[0001] The present invention belongs to the technical field of mold fluxes, and particularly relates to a method for preparing mold fluxes using coal gangue. Background Art

[0002] Continuous casting mold flux is a key material for regulating heat transfer, lubricating the mold, and adsorbing inclusions in the steel continuous casting process. Its performance directly affects the surface quality of the cast slab and the continuous casting efficiency. Traditional mold fluxes mainly use natural minerals such as wollastonite and fluorite as raw materials, but they face two major pain points: one is that high-quality wollastonite resources are becoming increasingly exhausted, and the raw material cost accounts for as high as 35%-40%; the other is that fluorite releases fluorides at high temperatures, causing equipment corrosion and environmental pollution. Coal gangue, as a solid waste associated with coal mining, has a chemical composition that highly matches the requirements of mold flux base materials. However, existing coal gangue resource utilization technologies have significant defects: Low efficiency of synergistic impurity removal: Fe2O3 (when it is 5%, it causes deterioration of the light transmittance of the slag layer), sulfides (the high-temperature decomposition of FeS2 causes sulfur increase in the molten steel), heavy metals (As, Pb migrate to the cast slab), and radioactive substances (U, Th series) in coal gangue are difficult to remove synchronously.

[0003] Inaccurate control of carbon content: The carbon in the mold flux (3-8wt%) needs to be precisely controlled to balance heat insulation and melting rate, but the residual carbon in coal gangue fluctuates greatly (2-10%) after calcination.

[0004] Insufficient melt homogenization: The one-step melting process is prone to generating unreacted Al2O3 inclusions and pores (porosity > 8%), resulting in viscosity fluctuations (±0.2Pa·s) and uneven spreading (slag layer thickness deviation > 0.5mm).

[0005] Based on the above background, there is an urgent need for a new method for preparing mold fluxes to break through the following technical bottlenecks: How to achieve efficient synchronous removal of Fe, S, heavy metals, and radioactive substances in coal gangue through multi-stage collaborative treatment, so that Fe2O3 ≤ 5%, S ≤ 0.3%, and specific radioactivity < 1Bq / g; How to construct a dynamic carbon compensation mechanism to precisely control the total carbon content within the range of 3-8wt% (error ± 0.5%) to adapt to the continuous casting requirements of different steel grades; Optimize the melting process to eliminate pores and composition segregation, so that the viscosity of the mold flux is stable at 0.1-0.5Pa·s (1300°C), the porosity ≤ 5%, and improve the sphericity of glass particles. Summary of the Invention

[0006] In view of the above problems, the present invention proposes a method for preparing mold fluxes using coal gangue, which well solves the problems of high raw material mineral cost in the prior art, low efficiency of synergistic impurity removal, inaccurate control of carbon content, and insufficient melt homogenization when using coal gangue to prepare mold fluxes.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing a mold powder using coal gangue, comprising the following steps: Step 1: Sort, crush and homogenize the coal gangue to separate Fe2O3, sulfides and radioactive minerals; Step 2: Gradiently calcine the pretreated coal gangue at 800 - 1000 °C to remove sulfur elements and part of the organic carbon, generating a desulfurized coal gangue base material; Step 3: Add a CaO source, an MgO source and a flux to the desulfurized coal gangue base material, and adjust the mass ratio of CaO / SiO2 in the mixture to 0.9 - 1.2, with the Al2O3 content ≤ 14 wt%; Step 4: Control the total carbon content to 3 - 8 wt% by externally adding graphite powder or performing secondary calcination on the desulfurized coal gangue base material; Step 5: Melt the mixture in Step 4 at 1350 - 1450 °C and water-quench it into glassy particles to obtain the finished continuous casting mold powder.

[0008] Further, Step 1 includes: Adopt magnetic separation to remove the mineral phase with Fe2O3 content > 5 wt%; Separate sulfides and carbonaceous components by flotation method to make the sulfur content ≤ 0.3 wt%; Use a γ-ray detector to screen coal gangue raw materials with a specific activity of radioactivity < 1 Bq / g.

[0009] Further, the gradient calcination in Step 2 includes: The first stage: Calcinate at 800 - 850 °C for 1 - 2 hours to oxidize FeS2 to Fe2O3 and release SO2 gas; The second stage: Calcinate at 950 - 1000 °C for 0.5 - 1 hour to remove organic carbon and activate the SiO2 / Al2O3 mineral phase.

[0010] Further, in Step 3, the CaO source is limestone or dolomite, the MgO source is magnesite, and the flux is fluorite or borax, where the addition amount of fluorite is 2 - 5 wt% of the total mass of the mixture.

[0011] Further, the control of the carbon content in Step 4 is achieved by the following methods: When the residual carbon after calcining the coal gangue < 3 wt%, externally add graphite powder to the target carbon content; When the residual carbon > 8 wt%, decarbonize to the target range by secondary calcination, and the decarbonization temperature of the secondary calcination is 600 - 700 °C.

[0012] Further, Step 1 also includes performing pickling pretreatment on the coal gangue, specifically: Soak the coal gangue particles in a 5-10 wt% dilute hydrochloric acid solution for 2-4 hours to remove heavy metal impurities such as As and Pb. After pickling, the wastewater is treated by the neutralization precipitation method to recover heavy metal hydroxides.

[0013] Furthermore, the mixture in step 3 also contains 0.5-2 wt% sodium phosphate, which is used to solidify residual heavy metal ions into phosphate glass during the melting process.

[0014] Furthermore, in step 5, the melting process adopts a gradient heating system, including: heating to 1200 °C at a rate of 5 °C / min and holding for 30 minutes, then continuing to heat to 1400-1450 °C at a rate of 3 °C / min and holding for 1 hour to achieve complete homogenization.

[0015] Furthermore, in step 5, the water quenching process uses a high-speed water flow with a velocity ≥10 m / s to break the melt into glass particles with a particle size of 0.1-1 mm.

[0016] Furthermore, the properties of the finished continuous casting powder meet the following requirements: viscosity at 1300 °C is 0.1-0.5 Pa·s, melting point is 1100-1200 °C, basicity is 1.0-1.2, and porosity ≤5%.

[0017] Compared with the prior art, the present invention has the following beneficial effects: Synergistic removal of impurities and improvement of environmental protection: Through the multi-stage separation process of magnetic separation - flotation - pickling, the content of Fe2O3 ≤5%, sulfur content ≤0.3%, heavy metals (As, Pb) are removed, and the specific activity of radioactivity <1 Bq / g; the pickling wastewater is treated by neutralization precipitation to recover heavy metal hydroxides, reducing secondary pollution.

[0018] Precise control of carbon content: Gradient calcination combined with dynamic carbon addition technology (externally doped with graphite / secondary calcination for decarbonization) stably controls the total carbon content at 3-8 wt%, meeting the precise requirements of the continuous casting process for carbon activity.

[0019] Melt homogenization and performance optimization: Gradient heating melting (holding at 1200 °C + homogenization at 1400 °C) makes the porosity ≤5%, and the viscosity fluctuation range is narrowed to ±0.05 Pa·s (at 1300 °C); combined with the high-speed water quenching process (≥10 m / s), spherical glass particles with a high sphericity are generated, improving the spreading and lubricity of the powder.

[0020] Resource utilization and cost advantage: Using coal gangue to replace more than 60% of the wollastonite raw material reduces the raw material cost by 40%-50%, and at the same time consumes coal gangue, having both economic and environmental benefits.

[0021] Heavy metal stabilization: Sodium phosphate is added to solidify residual heavy metal ions into inert phosphate glass bodies, avoiding the migration of harmful elements during service and ensuring the cleanliness of steel.

[0022] Finished product performance indicators: Melting point 1100 - 1200 °C, basicity 1.0 - 1.2, fully meeting the technical requirements of continuous casting mold powder, can replace traditional high-fluorine formulations, reduce the fluorite dosage by more than 30%, and significantly reduce the risk of fluorine pollution. Specific implementation manners

[0023] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] In this embodiment, the γ-ray detector model used is FD-3013B digital γ-radiometer. The performance test method of the mold powder is carried out by the viscosity test method of continuous casting mold powder YB / T185-2017. The porosity test of the mold powder is based on GB / T2997-2015 "Test methods for apparent porosity, water absorption, bulk density and true porosity of dense shaped refractory products". The acidity and alkalinity of the mold powder are obtained by calculating the mass ratio of CaO / SiO2 through chemical wet analysis method. The melting point and complete melting time of the mold powder are obtained according to YB / T186 - test method for melting temperature of continuous casting mold powder. Example 1

[0025] Step 1: Take coal gangue raw materials and crush them to a particle size ≤ 5 mm by a jaw crusher. Use a permanent magnetic drum separator (magnetic field intensity 0.8 T) to separate the mineral phase with Fe2O3 content > 5 wt%. Put the magnetically separated materials into a flotation cell, add diesel as a collector (dosage 0.5 kg / t), and separate sulfides and carbonaceous components until the sulfur content ≤ 0.3 wt%. Use a γ-ray detector to screen qualified raw materials with a specific activity of radioactivity < 1 Bq / g, and then soak them in a 5 wt% dilute hydrochloric acid solution for 3 hours. Add lime milk to the filtered wastewater to adjust the pH to 8.5, and precipitate and recover hydroxides containing As and Pb.

[0026] Step 2: Place the pretreated coal gangue in a rotary kiln for gradient calcination: In the first stage, heat it to 800 °C at a rate of 10 °C / min, and introduce an air flow of 2 m³ / h and calcine for 1.5 hours to completely oxidize FeS2. In the second stage, heat it to 1000 °C and keep it warm for 40 minutes to produce a desulfurized coal gangue base material with the residual carbon content reduced to 2.8 wt%.

[0027] 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 mass ratio of CaO / SiO2 in the mixture to be 1.2, the Al2O3 content to be 12 wt%, the fluorite addition amount to be 3.5 wt%, and another 1.2 wt% sodium phosphate is incorporated.

[0028] Step 4: Detect that the residual carbon content of the base material after calcination is 2.8 wt%, and externally add 200-mesh flake graphite powder until the total carbon content reaches 5.5 wt%.

[0029] Step 5: Load the mixture in Step 4 into an electric arc furnace, and follow the gradient heating system: heat up to 1200 °C at a rate of 5 °C / min and hold for 30 minutes, then heat up to 1420 °C at a rate of 3 °C / min and hold for 1 hour; the melt is broken by 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 relatively high sphericity.

[0030] Performance test: The viscosity at 1300 °C is 0.3 Pa·s, the melting point is 1150 °C, the basicity (CaO / SiO2) is 1.15, the porosity is 3.8%, and the complete melting time is 16 minutes. Example 2

[0031] Step 1: After the coal gangue is crushed to 3 mm by a high-pressure roller mill, use a high-gradient magnetic separator (1.2 T) to remove iron-containing minerals; during flotation, add xanthate (0.3 kg / t) to strengthen the removal of sulfides, and the final sulfur content is 0.25 wt%; for pickling, soak in 8 wt% hydrochloric acid for 4 hours, and the Pb concentration in the neutralized wastewater is < 0.1 mg / L. 2+ The concentration < 0.1 mg / L.

[0032] Step 2: In the first stage of gradient calcination, desulfurization is carried out at 850 °C for 2 hours; in the second stage, decarbonization is carried out at 950 °C until the residual carbon is 7.5 wt%, and after secondary calcination (650 °C for 1 hour), the decarbonization is carried out to 4.9 wt% to produce the desulfurized coal gangue base material.

[0033] Step 3: Add dolomite (CaO 30%, MgO 20%) to adjust the CaO / SiO2 ratio to 0.9, the Al2O3 content to 14 wt%, the borax addition amount to 4 wt%, and the sodium phosphate addition amount to 0.8 wt%.

[0034] Step 4: Through Step 2, the total carbon content of the desulfurized coal gangue base material has reached 4.9 wt%, so Step 4 is omitted.

[0035] Step 5: In the melting stage, use an induction furnace, hold at 1400 °C for 1.5 hours, and the melt is broken by a water quenching tower at a water flow rate of 10 m / s, and the water quenched particle size is 0.5 - 1 mm.

[0036] Performance test: Viscosity at 1300°C is 0.4 Pa·s, melting point is 1180°C, basicity is 1.05, porosity is 4.5%, and the complete melting time is 18 minutes. Example 3

[0037] The differences between Steps 1 - 3 and Example 1 are that the incorporation amount of sodium phosphate is 0.5 wt%; Step 4: When the residual carbon after calcination reaches 8.5 wt%, perform secondary calcination under nitrogen protection at 700°C to decarburize to 6.2 wt%; the addition amount of fluorite in Step 3 is 5 wt%, and add 200 - mesh flake graphite powder externally to the mixture with a total carbon content of 7.8 wt%.

[0038] Step 5: Heat the mixture in Step 4 to 1200°C at a rate of 5°C / min and hold for 30 minutes, then continue to heat to 1400°C at a rate of 3°C / min and hold for 1 hour, and water - quench the melt at a flow rate of 15 m / s to obtain ultrafine particles with a particle size of 0.1 - 0.3 mm.

[0039] Performance test: Viscosity at 1300°C is 0.2 Pa·s, melting point is 1120°C, basicity is 1.2, porosity is 2.9%, and the complete melting time is 14 minutes. Example 4

[0040] The differences between Steps 1 - 5 and Example 1 are that the content of Al2O3 in Step 3 is 1 wt%; Performance test: Viscosity at 1300°C is 0.25 Pa·s, melting point is 1100°C, basicity is 1.20, porosity is 3%, and the complete melting time is 14 minutes.

[0041] Comparative Example 1: Step 1: Directly crush the coal gangue to a particle size of 10 mm without magnetic separation, flotation, and radioactive screening; Step 2: Calcinate at 700°C for 3 hours without using gradient heating; Step 3: Add limestone to adjust the CaO / SiO2 ratio to 0.5 without controlling the Al2O3 content; Step 4: Do not externally add a carbon source, and the residual carbon content after calcination is 2 wt%; Step 5: Melt at 1250°C for 1 hour, and the water - quench water flow rate is 5 m / s, and particles with a particle size of 1 - 3 mm are prepared.

[0042] Performance test: Viscosity at 1300°C is 0.85 Pa·s, melting point is 1250°C, basicity is 0.50, porosity is 9.7%, and the complete melting time is 32 minutes.

[0043] From Comparative Example 1, it can be seen that non-magnetic separation flotation results in residual Fe2O3 (>8 wt%), sulfides, and an increase in impurity phases, which hinder the melt flow and cause abnormal viscosity; too high Al2O3 content forms a high-melting-point corundum phase, and the liquidus temperature increases significantly, resulting in abnormal melting point; failure to control the carbon content (only 2 wt%) leads to insufficient sintering driving force, and the pores between particles cannot be effectively filled.

[0044] Comparative Example 2: Step 1: Only magnetic separation is used to remove Fe2O3, and the sulfide content is not treated (sulfur content 1.2 wt%); Step 2: Directly calcine at 1000 °C for 2 hours without staged desulfurization and decarbonization; Step 3: Add fluorite (8 wt%) and magnesite to make the CaO / SiO2 ratio reach 2.0; Step 4: Externally add graphite powder to make the total carbon content 12 wt%; Step 5: After melting at 1400 °C, the water quenching water flow rate is 5 m / s, and particles with a particle size of 1-3 mm are obtained, and bubbles and voids appear inside the particles.

[0045] Performance test: Viscosity at 1300 °C is 0.12 Pa·s, melting point is 1360 °C, basicity is 2.00, porosity is 13.5%, and the complete melting time is 9 minutes.

[0046] From Comparative Example 2, it can be seen that excessive fluorite (8 wt%) excessively reduces the viscosity. At the same time, a sulfur content of 1.2 wt% causes a sudden change in the gas-liquid interfacial tension. CaO / SiO2 = 2.0 makes it difficult to disintegrate the silicate network, and higher energy input is required for melting. At the same time, too high total carbon content generates excessive CO gas during the water quenching process, resulting in a decrease in particle quality.

[0047] From the experimental data of the above examples, it can be obtained that by performing magnetic separation, flotation, and radioactive detection on coal gangue, combined with key technologies such as gradient calcination for desulfurization and decarbonization, and water quenching process control, the performance of the obtained mold powder fully meets the requirements of the YB / T185-2017 standard for the viscosity test method of continuous casting mold powder, providing a reliable technical path for the high-value utilization of coal gangue and being applicable to the production of mold powder under high-speed continuous casting conditions.

[0048] Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a mold powder using coal gangue, characterized in that: It includes the following steps: Step 1: Sort, crush and homogenize the coal gangue, and separate Fe2O3, sulfides and radioactive minerals; Step 2: Gradiently calcine the pretreated coal gangue at 800 - 1000 °C to remove sulfur elements and part of the organic carbon, and generate a desulfurized coal gangue base material; Step 3: Add a CaO source, an MgO source and a flux to the desulfurized coal gangue base material, and adjust the mass ratio of CaO / SiO2 in the mixture to 0.9 - 1.2, with the Al2O3 content ≤ 14 wt%; Step 4: Control the total carbon content at 3 - 8 wt% by externally adding graphite powder or performing secondary calcination on the desulfurized coal gangue base material; Step 5: Melt the mixture in Step 4 at 1350 - 1450 °C, and water-quench it into glassy particles to obtain the finished continuous casting mold powder.

2. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: The said Step 1 includes: Use magnetic separation to remove the mineral phase with Fe2O3 content > 5 wt%; Separate sulfides and carbonaceous components by flotation to make the sulfur content ≤ 0.3 wt%; Use a γ-ray detector to screen the coal gangue raw materials with a specific radioactivity < 1 Bq / g.

3. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: The said gradient calcination in Step 2 includes: The first stage: Calcinate at 800 - 850 °C for 1 - 2 hours to oxidize FeS2 to Fe2O3 and release SO2 gas; The second stage: Calcinate at 950 - 1000 °C for 0.5 - 1 hour to remove organic carbon and activate the SiO2 / Al2O3 mineral phase.

4. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: In Step 3, the CaO source is limestone or dolomite, the MgO source is magnesite, and the flux is fluorite or borax, with the fluorite addition amount being 2 - 5 wt% of the total mass of the mixture.

5. The method for preparing a mold powder using coal gangue according to claim 1, wherein: The carbon content control in Step 4 is achieved through the following methods: When the residual carbon after coal gangue calcination < 3 wt%, externally add graphite powder to the target carbon content; When the residual carbon > 8 wt%, decarbonize to the target range through secondary calcination, and the decarbonization temperature for secondary calcination is 600 - 700 °C.

6. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: Step 1 also includes performing pickling pretreatment on the coal gangue, specifically: Soak the coal gangue particles with a 5 - 10 wt% dilute hydrochloric acid solution for 2 - 4 hours to remove heavy metal impurities such as As and Pb, and recover heavy metal hydroxides from the pickling wastewater through the neutralization precipitation method.

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

8. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: In Step 5, the melting process adopts a gradient heating system, including: heating to 1200 °C at a rate of 5 °C / min and holding for 30 minutes, then continuing to heat to 1400 - 1450 °C at a rate of 3 °C / min and holding for 1 hour to achieve complete homogenization.

9. The method for preparing a mold powder using coal gangue according to claim 1, characterized in that: In Step 5, the water-quenching process uses a high-speed water flow with a flow rate ≥ 10 m / s to break the melt into glass particles with a particle size of 0.1 - 1 mm.

10. The method for preparing mold powder using coal gangue according to claim 1, characterized in that: The performance of the said finished continuous casting mold powder meets the requirements: the viscosity at 1300 °C is 0.1 - 0.5 Pa·s, the melting point is 1100 - 1200 °C, the basicity is 1.0 - 1.2, and the porosity ≤ 5%.