Dry ramming material for intermediate frequency furnace and preparation method of dry ramming material

Abstract

The invention relates to the field of refractory materials, in particular to a dry ramming mass for an intermediate frequency furnace and a preparation method thereof.The dry ramming mass is prepared from, by weight, 45-55 parts of fused corundum, 8-12 parts of fused magnesia, 15-20 parts of tabular corundum, 3-5 parts of fused spinel, 3-4 parts of activated aluminum oxide, 1-2 parts of silicon carbide, 1-2 parts of silica sol and 0.5-1 part of boric acid. 3.5-4.5 parts of a binding agent and 3-5 parts of an anti-penetrating agent; wherein the anti-penetrating agent is chromium oxide coated nickel aluminate microspheres. The invention solves the two major problems of easy penetration and thermal shock peeling of the traditional furnace lining, is especially suitable for harsh smelting environments of high manganese steel, stainless steel and the like, and has long service life and convenience in construction.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of refractory materials, in particular to a dry ramming material for medium frequency furnace and a preparation method thereof. BACKGROUND

[0002] The medium frequency induction furnace has become the mainstream equipment for casting, special alloy and stainless steel smelting due to its high smelting efficiency and precise temperature control. The furnace lining is one of the most important components of the induction furnace, which plays a role in containing the metal liquid and protecting the furnace body. With the requirement of high-quality steelmaking process, the requirement for the furnace lining material is also becoming more and more demanding. During the smelting process, the furnace lining itself is in a very harsh environment, facing a series of problems such as the impact of the metal liquid, the penetration of metal ions, the thermal stress caused by the rapid change of temperature, etc., which seriously shorten the service life of the furnace lining. The quality of the furnace lining directly determines the production efficiency and service life. With the rapid development of short process steelmaking, the demand for high-quality dry ramming material for medium frequency induction furnace is also increasing. Especially when smelting low alloy steel in the induction furnace, the MnO content in the slag is high, which seriously penetrates the furnace lining material, which has become a technical problem to be solved.

[0003] At present, corundum and magnesia ramming material is commonly used as the furnace lining of the medium frequency induction furnace. This traditional dry ramming material faces abnormal expansion caused by in-situ spinelization during use, which destroys the structure of the material, making the dry ramming material for medium frequency furnace more easily penetrated by slag during use, thereby reducing the service life of the furnace lining. And this traditional dry ramming material has the disadvantages of high porosity, loose structure and low strength. Therefore, how to improve the physical properties and anti-penetration properties of the dry ramming material for medium frequency furnace is a problem to be solved. SUMMARY

[0004] In view of the problems in the prior art, the purpose of the present application is to provide a dry ramming material for medium frequency furnace and a preparation method thereof.

[0005] The purpose of the present application is achieved by the following technical solutions: In a first aspect, the present application provides a dry ramming material for medium frequency furnace, which comprises the following components in terms of weight fraction: 45-55 parts of fused corundum, 8-12 parts of fused magnesia, 15-20 parts of tabular corundum, 3-5 parts of fused spinel, 3-4 parts of active alumina, 1-2 parts of silicon carbide, 1-2 parts of silica sol, 0.5-1 part of boric acid, 3.5-4.5 parts of binder, and 3-5 parts of anti-penetration agent.

[0006] Preferably, the fused corundum comprises fused corundum fine powder with a particle size of 1-3 mm and fused corundum coarse powder with a particle size of 4-6 mm, wherein the mass ratio of the fused corundum fine powder to the fused corundum coarse powder is 0.6-1:1.

[0007] Preferably, the grain size of the electrically fused magnesia is 1-3mm.

[0008] Preferably, the grain size of the tabular corundum is not more than 0.088mm.

[0009] Preferably, the grain size of the electrically fused spinel is not more than 0.074mm.

[0010] Preferably, the active alumina is active alumina (α-Al2O3).

[0011] Preferably, the grain size of the silicon carbide is not more than 0.044mm.

[0012] Preferably, the binding agent is a composite phosphate, including aluminum dihydrogen phosphate and sodium hexametaphosphate in a mass ratio of 6-7:3-4.

[0013] Preferably, the anti-permeation agent is chromium oxide coated nickel aluminate microspheres, and the preparation method includes: S1, weigh the nickel aluminate powder, first perform high-temperature calcination treatment to remove surface adsorbed water and impurities; then place it in an acid solution for activation treatment to obtain active nickel aluminate powder; S2, weigh ammonium chromate and chromium acetylacetonate into anhydrous ethanol, fully dissolve to obtain solution A; weigh polyvinylpyrrolidone and silica sol into anhydrous ethanol, fully stir to obtain solution B; gradually add solution B to solution A, stir at a speed of 200-500rpm for 2-3h to form a light green transparent sol, i.e. sol coating liquid; S3, add the active nickel aluminate powder to the sol coating liquid, ultrasonically disperse to form a stable suspension, then dry by a spray dryer to obtain composite microspheres; S4, place the sprayed composite microspheres in a muffle furnace in an inert atmosphere, heat to 800-850℃ and keep for 2-3h to obtain chromium oxide coated nickel aluminate microspheres.

[0014] Preferably, in S1, the high-temperature calcination treatment is calcination in a muffle furnace at 550℃ for 2h; the activation treatment is soaking in 5% dilute hydrochloric acid for 30min.

[0015] Preferably, in S1, the mass-volume ratio of the nickel aluminate powder and dilute hydrochloric acid is 1g:(10-20)mL.

[0016] Preferably, in solution A of S2, the mass-volume ratio of ammonium chromate, chromium acetylacetonate and anhydrous ethanol is (20-30)g:(5-8)g:100mL.

[0017] Preferably, in solution B of S2, the mass ratio of SiO2 in the silica sol is 30%-40%, and the mass ratio of the silica sol, polyvinylpyrrolidone and anhydrous ethanol is (3-5) g:(0.5-1) g:50 mL.

[0018] Preferably, in S2, the volume ratio of solution A to solution B is 1:0.8-1.2.

[0019] Preferably, in S3, the mass-volume ratio of active nickel aluminate powder to sol coating liquid is 100 g:(150-200) mL.

[0020] Preferably, in S3, the power of ultrasonic dispersion is 300 W, and the time is 30 min; the inlet temperature of the spray dryer is 200 DEG C, the outlet temperature is 90 DEG C, the atomization pressure is 0.3 MPa, and the feeding rate is 10 mL / min.

[0021] In a second aspect, the application provides a preparation method of dry ramming mixture for intermediate frequency furnaces, comprising the following steps: Step 1, fine powder premixing: mixing tabular corundum, active alumina, fused spinel, silicon carbide and boric acid in a blender for 10-20 min; Step 2, adding aggregate: adding fused corundum powder and fused magnesia respectively, and continuing to mix and stir for 5-10 min; Step 3, adding liquid phase: adding silica sol and composite phosphate binder, and mixing and grinding until loose and free of lumps, and then drying until the water content is less than 1%, to obtain the dry ramming mixture for intermediate frequency furnaces.

[0022] The application has the following beneficial effects: 1. The application prepares a dry ramming mixture for intermediate frequency furnaces, which uses multi-scale particle grading of fused corundum, fused magnesia, tabular corundum and fused spinel, uses a combined system of phosphate and sol, and uses a special anti-permeation agent of chromium oxide coated nickel aluminate microspheres, thereby solving the two major problems of easy penetration and thermal shock spalling of traditional furnace linings, and being especially suitable for harsh smelting environments such as high manganese steel and stainless steel, and having long service life and convenient construction.

[0023] 2. The application uses chromium oxide coated nickel aluminate microspheres as an anti-permeation agent, which has a core-shell structure after coating a chromium oxide shell on the surface of the nickel aluminate powder core, and the shell greatly reduces the risk of nickel reduction.

[0024] 3、The prepared core-shell anti-infiltration agent has multiple protection mechanisms, first, the Cr2O3 shell layer with ultra-high melting point can directly block the infiltration of slag or molten metal, second, the interface NiCr2O4 spinel can improve the high-temperature bonding strength, and under high-temperature conditions, Cr2O3 will diffuse to the matrix, thereby repairing micro-cracks, in addition, when used alone, nickel aluminate will easily generate metallic nickel (Ni) in a reducing atmosphere. 2+ →Ni), resulting in furnace lining perforation.

[0025] 4、The prepared core-shell anti-infiltration agent can retain the advantages of the nickel aluminate spinel framework while using the Cr2O3 shell layer to isolate slag and reducing atmosphere, thereby solving the safety hazards of traditional aluminate infiltration agents, achieving the synergistic improvement of anti-infiltration and high-temperature stability, and significantly improving the service safety and life of the ramming mixture, and the service life can be extended by more than 2 times. DETAILED DESCRIPTION

[0026] The technical solutions of the present application are described below through specific examples. It should be understood that the one or more method steps mentioned in the present application do not exclude other method steps before and after the combination steps or other method steps inserted between the explicitly mentioned steps; it should also be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. Moreover, unless otherwise stated, the numbering of each method step is only a convenient tool to identify each method step, and is not a limitation on the arrangement order or the scope of the present application that can be implemented, and changes or adjustments of the relative relationship without substantial changes in technical content are also considered as the scope of the present application that can be implemented.

[0027] In order to better understand the above technical solutions, the exemplary embodiments of the present application are described in more detail below. Although exemplary embodiments of the present application are shown, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present application and to fully convey the scope of the present application to those skilled in the art.

[0028] The raw materials used in the present application include: MgO≥97%, SiO2≤1.2%, CaO≤1.5% for fused magnesia, Al2O3≥98%, Fe2O3+TiO2≤0.5% for fused corundum, Al2O3≥99%, Na2O≤0.4%, SiO2≤0.15% for tabular corundum, MgAl2O4 (MgO content 28-30%) for fused spinel, and the total amount of impurities ≤2%, and the mass content of SiO2 in the composition of silica sol is 35%±1%.

[0029] The present application is further described below in conjunction with the following examples. Embodiment 1

[0030] A dry ramming mixture for intermediate frequency furnace comprises the following components in parts by weight: 50 parts of fused corundum, 10 parts of fused magnesite, 18 parts of tabular corundum, 4 parts of fused spinel, 3.5 parts of active alumina, 1.6 parts of silicon carbide, 1.8 parts of silica sol, 0.7 parts of boric acid, 4 parts of binder, and 4 parts of anti-permeation agent.

[0031] The fused corundum comprises fused corundum fine powder with a particle size of 1-3 mm and fused corundum coarse powder with a particle size of 4-6 mm, and the mass ratio of the fused corundum fine powder to the fused corundum coarse powder is 0.8:1.

[0032] The fused magnesite has a particle size of 1-3 mm; the tabular corundum has a particle size of no more than 0.088 mm; the fused spinel has a particle size of no more than 0.074 mm; the active alumina is α-Al2O3; and the silicon carbide has a particle size of no more than 0.044 mm.

[0033] The binder is a composite phosphate salt comprising aluminum dihydrogen phosphate and sodium hexametaphosphate at a mass ratio of 7:3.

[0034] The anti-permeation agent is chromium oxide-coated nickel aluminate microspheres, and the preparation method comprises the following steps: S1. 1 g of nickel aluminate (NiAl2O4) powder is weighed and first placed in a muffle furnace at 550°C for calcination for 2 h to remove surface adsorbed water and impurities; then the nickel aluminate is placed in 15 mL of 5% dilute hydrochloric acid for 30 min, centrifuged to remove the liquid, washed with water until neutral, and dried at 110°C to obtain active nickel aluminate powder; S2. 25 g of ammonium chromate and 6.5 g of chromium acetylacetonate are placed in 100 mL of anhydrous ethanol, and after being fully dissolved, a solution A is obtained; 0.8 g of polyvinylpyrrolidone (PVP) and 4 g of 35% silica sol are added to 50 mL of anhydrous ethanol, and after being fully stirred, a solution B is obtained; solution B is gradually added to solution A, and the volume ratio of solution A to solution B is 1:1, and the mixture is stirred at a speed of 400 rpm for 3 h to form a light green transparent sol, i.e., a sol coating solution; S3. 100 g of the active nickel aluminate powder is added to 180 mL of the sol coating solution, and ultrasonic dispersion is performed (power 300 W, 30 min) to form a stable suspension liquid, which is then passed through a spray dryer (inlet temperature 200°C, outlet temperature 90°C) at an atomization pressure of 0.3 MPa and a feeding rate of 10 mL / min to make the precursor uniformly deposited on the surface of the core, and the suspension liquid is atomized into composite microspheres; S4, the sprayed composite microspheres were placed in a muffle furnace (Ar) in an inert atmosphere, and heated to 800℃ at a rate of 5℃ / min and maintained for 3h, so that the chromium precursor was decomposed into Cr2O3 and densified, to obtain chromium oxide coated nickel aluminate microspheres.

[0035] The preparation method of the dry ramming mixture for the medium-frequency furnace comprises the following steps: Step 1, fine powder premixing: mix tabular corundum, active alumina, fused spinel, silicon carbide and boric acid in a blender for 15 minutes; Step 2, adding aggregate: add fused corundum powder and fused magnesia respectively, and continue to mix and stir for 8 minutes; Step 3, adding liquid phase: add silica sol and composite phosphate binder, and mix and grind until loose and free of lumps, and then dry to a water content of less than 1%, to obtain the dry ramming mixture for the medium-frequency furnace. Example 2

[0036] A dry ramming mixture for a medium-frequency furnace comprises the following components in terms of weight fractions: 45 parts of fused corundum, 8 parts of fused magnesia, 15 parts of tabular corundum, 3 parts of fused spinel, 3 parts of active alumina, 1 part of silicon carbide, 1 part of silica sol, 0.5 parts of boric acid, 3.5 parts of binder, and 3 parts of anti-permeation agent.

[0037] The fused corundum comprises fused corundum fine powder with a particle size of 1-3mm and fused corundum coarse powder with a particle size of 4-6mm, and the mass ratio of the fused corundum fine powder to the fused corundum coarse powder is 0.6:1.

[0038] The particle size of the fused magnesia is 1-3mm; the particle size of the tabular corundum is not greater than 0.088mm; the particle size of the fused spinel is not greater than 0.074mm; the active alumina is α-Al2O3; and the particle size of the silicon carbide is not greater than 0.044mm.

[0039] The binder is a composite phosphate, comprising aluminum dihydrogen phosphate and sodium hexametaphosphate in a mass ratio of 6:4.

[0040] The anti-permeation agent is prepared by the same method as in Example 1.

[0041] The preparation method of the dry ramming mixture for the medium-frequency furnace comprises the following steps: Step 1, fine powder premixing: mix tabular corundum, active alumina, fused spinel, silicon carbide and boric acid in a blender for 10 minutes; Step 2, adding aggregate: add fused corundum powder and fused magnesia respectively, and continue to mix and stir for 5 minutes; Step 3, liquid phase addition: add silica sol and composite phosphate binder thereto, and mix and mill until loose and free of lumps, and dry to a moisture content of <1%, to obtain the dry ramming mixture for medium frequency furnace. Example 3

[0042] A dry ramming mixture for medium frequency furnace, comprising the following components in parts by weight: 55 parts of fused corundum, 12 parts of fused magnesia, 20 parts of tabular corundum, 5 parts of fused spinel, 4 parts of active alumina, 2 parts of silicon carbide, 2 parts of silica sol, 1 part of boric acid, 4.5 parts of binder, and 5 parts of anti-permeation agent.

[0043] The fused corundum includes fused corundum fine powder with a particle size of 1-3 mm and fused corundum coarse powder with a particle size of 4-6 mm, and the mass ratio of the fused corundum fine powder to the fused corundum coarse powder is 1:1.

[0044] The particle size of the fused magnesia is 1-3 mm; the particle size of the tabular corundum is not greater than 0.088 mm; the particle size of the fused spinel is not greater than 0.074 mm; the active alumina is α-Al2O3; and the particle size of the silicon carbide is not greater than 0.044 mm.

[0045] The binder is a composite phosphate, including aluminum dihydrogen phosphate and sodium hexametaphosphate in a mass ratio of 7:3.

[0046] The preparation method of the anti-permeation agent is the same as that of Example 1.

[0047] The preparation method of the dry ramming mixture for medium frequency furnace includes the following steps: Step 1, fine powder premixing: mix the tabular corundum, active alumina, fused spinel, silicon carbide, and boric acid in a blender for 20 min; Step 2, addition of aggregate: add the fused corundum powder and the fused magnesia, and continue to mix and stir for 10 min; Step 3, liquid phase addition: add silica sol and composite phosphate binder thereto, and mix and mill until loose and free of lumps, and dry to a moisture content of <1%, to obtain the dry ramming mixture for medium frequency furnace. Example 4

[0048] A dry ramming mixture for medium frequency furnace, which is different from Example 1 only in that the preparation method of the anti-permeation agent is different.

[0049] The anti-permeation agent is chromium oxide-coated nickel aluminate microspheres, and the preparation method includes: S1, 1g of nickel aluminate (NiAl2O4) powder is weighed and first placed in a muffle furnace at 550°C for calcination for 2h to remove surface adsorbed water and impurities; then placed in 20mL of 5% dilute hydrochloric acid for 30min, centrifuged to remove the liquid, washed with water to neutral, and dried at 110°C to obtain active nickel aluminate powder; S2, 30g of ammonium chromate and 8g of chromium acetylacetone are placed in 100mL of anhydrous ethanol, fully dissolved to obtain solution A; 1g of polyvinylpyrrolidone PVP and 5g of 40% silica sol are added to 50mL of anhydrous ethanol, fully stirred to obtain solution B; solution B is gradually added to solution A, the volume ratio of solution A to solution B is 1:1.2, stirred at a speed of 500rpm for 3h to form a light green transparent sol, which is a sol coating liquid; S3, 100g of active nickel aluminate powder is added to 150mL of sol coating liquid, ultrasonic dispersion (power 300W, 30min), to form a stable suspension, then through a spray dryer (inlet temperature 200°C, outlet temperature 90°C), atomization pressure is 0.3MPa, feeding rate is 10mL / min, to make the precursor uniformly deposited on the surface of the core, and the suspension is atomized into composite microspheres; S4, the sprayed composite microspheres are placed in a muffle furnace (Ar) in an inert atmosphere, heated to 800°C at a rate of 5°C / min and kept for 3h to make the chromium precursor decompose into Cr2O3 and densify, to obtain chromium oxide coated nickel aluminate microspheres. Example 5

[0050] A dry ramming material for a medium frequency furnace, which is different from example 1 only in that the preparation method of the anti-permeation agent is different.

[0051] The anti-permeation agent is chromium oxide coated nickel aluminate microspheres, and the preparation method comprises: S1, 1g of nickel aluminate (NiAl2O4) powder is weighed and first placed in a muffle furnace at 550°C for calcination for 2h to remove surface adsorbed water and impurities; then placed in 20mL of 5% dilute hydrochloric acid for 30min, centrifuged to remove the liquid, washed with water to neutral, and dried at 110°C to obtain active nickel aluminate powder; S2, 30g of ammonium chromate and 8g of chromium acetylacetone are placed in 100mL of anhydrous ethanol, fully dissolved to obtain solution A; 1g of polyvinylpyrrolidone PVP and 5g of 40% silica sol are added to 50mL of anhydrous ethanol, fully stirred to obtain solution B; solution B is gradually added to solution A, the volume ratio of solution A to solution B is 1:1.2, stirred at a speed of 500rpm for 3h to form a light green transparent sol, which is a sol coating liquid; S3, 100 g of active nickel aluminate powder was added into 200 mL of sol coating solution, and ultrasonic dispersion was performed (power 300 W, 30 min) to form a stable suspension, and then a spray dryer (inlet temperature 200 ℃, outlet temperature 90 ℃) was used to atomize the suspension into composite microspheres at an atomization pressure of 0.3 MPa and a feeding rate of 10 mL / min, so that the precursor was uniformly deposited on the surface of the core; S4, the sprayed composite microspheres were placed in an inert atmosphere muffle furnace (Ar) and heated to 800 ℃ at a rate of 5 ℃ / min and kept for 3 h, so that the chromium precursor was decomposed into Cr2O3 and densified, and nickel aluminate microspheres coated with chromium oxide were obtained.

[0052] Comparative Example 1 A dry ramming material for a medium frequency furnace, which is different from Example 1 only in that no anti-permeation agent is added.

[0053] Comparative Example 2 A dry ramming material for a medium frequency furnace, which is different from Example 1 only in that the anti-permeation agent is replaced by nickel aluminate powder.

[0054] Comparative Example 3 A dry ramming material for a medium frequency furnace, which is different from Example 1 only in that the anti-permeation agent is replaced by a mixture of nickel aluminate powder and Cr2O3.

[0055] In order to illustrate the content of the present application, the performance of the ramming materials prepared in the optimal embodiment (Example 1) and Comparative Examples 1-3 was detected, and the detection items included: (1) slag penetration resistance: reference YB / T 4131-2005 (static crucible method), slag composition: CaO 40%, SiO2 35%, Al2O3 10%, FeO 15%; treatment method: sintering at 1600 ℃ for 4 h; (2) thermal shock stability: reference GB / T 30873-2014 (1100 ℃→water cooling, 20 cycles); (3) thermal shock stability: reference GB / T 5072-2008 (1600 ℃×3 h after sintering, compressive strength); (4) linear change rate: reference GB / T 5988-2007 (1600 ℃×3 h after sintering, size change).

[0056] The detection results are shown in Table 1:

[0057] From the above detection results, it can be seen that the dry ramming mixture prepared in Example 1 of the present application has more excellent performance compared with the traditional dry ramming mixture. The dry ramming mixture prepared in Example 1 of the present application solves the problems of uneven dispersion and nickel reduction of the traditional material through the way of interfacial chemical bonding and physical barrier, and significantly surpasses the traditional ramming mixture in terms of permeability resistance, thermal shock stability and service life.

[0058] In the description of the present specification, the description referring to the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and integrate different embodiments or examples described in the present specification.

[0059] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A dry ramming mix for use in intermediate frequency furnaces, characterized in that, According to the weight parts, the following ingredients are included: 45-55 parts of fused corundum, 8-12 parts of fused magnesite, 15-20 parts of tabular corundum, 3-5 parts of fused spinel, 3-4 parts of active alumina, 1-2 parts of silicon carbide, 1-2 parts of silica sol, 0.5-1 part of boric acid, 3.5-4.5 parts of binder and 3-5 parts of anti-permeation agent; The anti-permeation agent is chromium oxide coated nickel aluminate microspheres.

2. A dry ramming mix for use in an intermediate frequency furnace according to claim 1, characterized in that, The fused corundum includes fused corundum fine powder with a particle size of 1-3 mm and fused corundum coarse powder with a particle size of 4-6 mm, wherein the mass ratio of the fused corundum fine powder to the fused corundum coarse powder is 0.6-1:

1.

3. A dry ramming mix for use in an intermediate frequency furnace according to claim 1, wherein The particle size of the fused magnesite is 1-3 mm; the particle size of the tabular corundum is not greater than 0.088 mm; the particle size of the fused spinel is not greater than 0.074 mm; and the particle size of the silicon carbide is not greater than 0.044 mm.

4. A dry ramming mix for use in an intermediate frequency furnace according to claim 1, wherein The active alumina is α-Al2O3.

5. A dry ramming mix for use in an intermediate frequency furnace according to claim 1, wherein The binder is a composite phosphate salt including aluminum dihydrogen phosphate and sodium hexametaphosphate with a mass ratio of 6-7:3-4.

6. A dry ramming mix for use in an intermediate frequency furnace according to claim 1, wherein The preparation method of the anti-permeation agent includes: S1, weigh the nickel aluminate powder, first perform high-temperature calcination treatment to remove surface adsorbed water and impurities, then activate the nickel aluminate powder in acid solution to obtain active nickel aluminate powder; S2, weigh ammonium chromate and chromium acetylacetonate into anhydrous ethanol, fully dissolve to obtain solution A; weigh polyvinylpyrrolidone and silica sol into anhydrous ethanol, fully stir to obtain solution B; gradually add solution B into solution A, stir at a speed of 200-500 rpm for 2-3 h to form a light green transparent sol, i.e. sol coating liquid; S3, add the active nickel aluminate powder into the sol coating liquid, ultrasonic dispersion to form a stable suspension, then dry by a spray dryer to obtain composite microspheres; S4, place the sprayed composite microspheres in a muffle furnace in an inert atmosphere, heat to 800-850℃ and keep for 2-3 h to obtain chromium oxide coated nickel aluminate microspheres.

7. A dry ramming mix for use in an intermediate frequency furnace according to claim 6, wherein In S1, the high-temperature calcination treatment is calcination in a muffle furnace at 550℃ for 2 h; the activation treatment is soaking in 5% dilute hydrochloric acid for 30 min; the mass-volume ratio of the nickel aluminate powder to the dilute hydrochloric acid is 1 g:(10-20) mL.

8. A dry ramming mix for use in an intermediate frequency furnace according to claim 6, wherein In solution A of S2, the mass-volume ratio of ammonium chromate, chromium acetylacetonate and anhydrous ethanol is (20-30) g:(5-8) g:100 mL; in solution B of S2, the mass fraction of SiO2 in the silica sol is 30%-40%, and the mass ratio of the silica sol, polyvinylpyrrolidone and anhydrous ethanol is (3-5) g:(0.5-1) g:50 mL; the volume ratio of solution A to solution B is 1:0.8-1.

2.

9. A dry ramming mix for use in an intermediate frequency furnace according to claim 6, wherein In S3, the mass-volume ratio of the active nickel aluminate powder to the sol coating liquid is 100 g:(150-200) mL.

10. A method of producing a dry ramming mix for intermediate frequency furnaces according to claim 1, characterized in that The method includes the following steps: Step 1, mix the tabular corundum, active alumina, fused spinel, silicon carbide and boric acid in a blender for 10-20 min; Step 2, add the fused corundum powder and the fused magnesite respectively, and continue to mix and stir for 5-10 min; Step 3, add the silica sol and composite phosphate binder into it and mix and grind until loose and no lump, dry to moisture content <1%, to obtain the dry ramming material for medium frequency furnace.

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