A method for preparing low thermal conductivity magnesium aluminate spinel bricks for lime kilns

Abstract

This invention provides a method for preparing low thermal conductivity magnesia-alumina spinel bricks for lime kilns, belonging to the field of finished lime kiln refractory bricks. The method uses magnesia and corundum powder in a mass ratio of 90:10 as the main materials, adding 1% Y₂O₃ sintering aid and 4% Mg(OH)₂-Al(OH)₃ sol binder. The mixture is then pressed, fired at high temperature, and cooled in the kiln. During firing, the sol decomposes to generate nano-MgO and Al₂O₃ particles, filling the grain boundary pores and forming a dense, low thermal conductivity magnesia-alumina spinel. Y₂O₃ effectively reduces the firing temperature. This brick is an alkaline, chromium-free material. When used in the combustion zone of a lime kiln, it exhibits excellent kiln lining adhesion and thermal shock stability, reducing kiln temperature by 30–50°C, achieving energy saving, consumption reduction, and extending kiln lining life.

Description

Technical Field

[0001] This invention relates to a method for preparing low thermal conductivity magnesium aluminum spinel bricks for lime kilns, belonging to the field of finished refractory bricks for lime kilns. Background Technology

[0002] Lime is an important raw material for industrial, chemical, and food processing. The steel, calcium carbide, alumina, and refractory materials industries are major consumers of lime. To completely control the pollution from simple lime kilns, it is essential to promote modern lime kilns that are environmentally friendly, energy-efficient, and highly automated to meet the demand. This has led to a comprehensive upgrade of refractory bricks used in lime kilns and their application technologies, and the rapid development of magnesia-based refractory materials.

[0003] Currently, magnesia-based refractory bricks used in lime kilns mainly include low-chromium bricks, dolomite bricks, magnesia-zirconium bricks, spinel-modified bricks, and various special magnesia bricks. To meet the development needs of low thermal conductivity refractory bricks for lime kilns, spinel-based refractory materials have been widely used. Currently, methods for synthesizing magnesia-alumina spinel include electrofusion synthesis, solid-state sintering, and molten salt synthesis.

[0004] The raw materials used in refractory bricks for lime kilns mainly include high-purity magnesia with a magnesium oxide content of 97%, iron-aluminum spinel, magnesia-iron sand, and magnesia-aluminum spinel sand. These raw materials undergo secondary fusion before being sintered in a tunnel kiln to produce bricks for lime kilns. While there are published patents for the preparation of magnesia-aluminum spinel, achieving complete fusion after the raw materials are fused again is difficult, resulting in uneven crystal distribution in the produced refractory bricks. This reduces the thermal shock resistance of the refractory bricks and the service life of the lime kiln. The advantage of this invention lies in the dense arrangement of MgAl2O4 grains in the sintering process, with fewer pores between grain boundaries, resulting in a dense sample structure. During sintering, Mg(OH)2-Al(OH)3 sol decomposes into nano-sized MgO and Al2O3 particles that are free within the grain boundaries, filling the pores and forming low-thermal-conductivity magnesia-aluminum spinel. Simultaneously, the addition of Y2O3 lowers the sintering temperature. Compared to using ordinary magnesia-aluminum spinel bricks, the cylinder temperature is reduced by 30-50°C, achieving energy saving and consumption reduction. Summary of the Invention

[0005] To address the above technical problems, the present invention provides a method for preparing low thermal conductivity magnesium-aluminum spinel bricks for lime kilns. The obtained magnesium-aluminum spinel bricks exhibit good fusion of magnesium and aluminum phases, uniform crystal distribution, high density, and long service life for the produced lime kiln refractory bricks.

[0006] A method for preparing low thermal conductivity magnesium aluminum spinel bricks for lime kilns includes four parts: preparation of lightly calcined magnesia, synthesis of Mg(OH)2-Al(OH)3 sol, material mixing and pressing, and high-temperature firing.

[0007] In the above technical solution, the calcination temperature range of the crushed magnesite raw material is 1000~1080℃, and the particle size range of the lightly calcined magnesia is 5-3, 3-1, 0.074.

[0008] In the above technical solution, the synthesis step of Mg(OH)2-Al(OH)3 sol involves using magnesium chloride, aluminum sulfate, and ammonia as raw materials, with a concentration of 0.5 mol / L. The reaction equation is as follows: 1) MgCl2+2NH3H2O=Mg(OH)2+2NH4Cl 2) Al2(SO4)3+6NH3H2O=2Al(OH)3+3(NH4)2SO 4。

[0009] In the above technical solution, in the material mixing and pressing step, magnesia and corundum powder are used as raw materials, two-phase sol is used as binder, Y2O3 is used as sintering aid, the ball milling wet mixing time is controlled within 7 hours, the molding pressure range is 300MPa, and it is pressed into a long strip sample of 300×300×60mm.

[0010] In the above technical solution, during the high-temperature firing step, the calcination temperature range of the shaped green body is 1800℃, and the firing time is controlled within 6 hours.

[0011] In the above technical solution, w(SiO2) < 1%, w(Al2O3) = 10%, w(MgO) > 90 wt.%, and the bulk density is 2.98~3.08 g / cm³. 3 The amount of binder added is 3 wt.%.

[0012] Compared with existing methods for preparing magnesium-aluminum spinel, the present invention has significant advantages in the following aspects: Magnesium-aluminum spinel bricks are sintered in an ultra-high temperature kiln using lightly calcined magnesia and corundum powder as raw materials, Mg(OH)2-Al(OH)3 sol as a binder, and Y2O3 as a sintering aid. Through a wet-mixing process, brick forming by a brick press, and ultra-high temperature kiln calcination, low thermal conductivity magnesium-aluminum spinel bricks with good fusion of the magnesium and aluminum phases are obtained in one step. The sol particles formed after high-temperature calcination promote the sintering of the brick body and the homogenization of its internal structure. The high thermal shock and low thermal conductivity magnesium-aluminum spinel bricks prepared in this embodiment utilize Mg(OH)2-Al(OH)3 sol to fill the pores of the spinel bricks, improving the density of the sintered magnesia bricks. Nanoscale MgO and Al2O3 grains are distributed at the grain boundaries of the magnesia, enhancing the thermal shock resistance of the magnesium-aluminum spinel bricks. Adding alumina spinel to magnesia refractories improves thermal shock stability, reduces thermal expansion coefficient, and increases density. This lowers the thermal expansion coefficient of magnesia refractories. Furthermore, due to the significant difference in thermal expansion coefficients between the two materials, microcracks will form due to thermal mismatch after high-temperature treatment, further enhancing thermal shock stability. Simultaneously, sintering shrinkage creates internal cracks, further reducing the linear expansion coefficient. When applied to the combustion zone of lime kilns, this product exhibits excellent kiln lining adhesion and thermal shock stability, extending service life, reducing refractory material consumption, decreasing maintenance costs, and increasing lime production. Compared to ordinary magnesia-alumina spinel bricks, this refractory brick reduces the kiln temperature by 30-50°C, achieving energy conservation and emission reduction. It not only meets the performance requirements for low thermal conductivity refractory bricks for kilns but, more importantly, achieves green and pollution-free production. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the preparation process of low thermal conductivity magnesium-aluminate spinel bricks for the combustion zone of lime kilns. Detailed Implementation

[0014] In embodiments of the present invention, lightly calcined magnesia and corundum powder are used as raw materials, Mg(OH)2-Al(OH)3 sol is used as a binder, and Y2O3 is used as a sintering aid. The particle size range of the lightly calcined magnesia is 5-3, 3-1, 0.074 mm, and the particle size of the corundum powder is less than 0.061 mm.

[0015] A method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns includes sieving, mixing, molding, drying, firing, and cooling. The specific preparation method is as follows: 1) Fine grinding and sieving of raw materials: Lightly calcined magnesia is sieved according to different particle sizes; corundum is crushed and the 0.061mm sieve material is taken. 2) Raw material wet mixing: The sieved lightly calcined magnesia powder and corundum powder are added to a mixer according to the mass ratio. Simultaneously, 3% of the total mass of Mg(OH)2-Al(OH)3 sol and 1% of Y2O3 are added to the mixer in 2-4 equal portions for wet mixing. After mixing for 7 hours, the mixture is dried. Mechanism: Magnesia-alumina spinel sand raw material is added to a mixture of ammonia water containing MgCl2 and Al2(SO4)3. The stirring intensity is 300 rpm until the reaction is complete and no more precipitate is formed. The precipitated reactants coat the surface of the magnesium-alumina spinel sand. The mixture is filtered, and the obtained filter residue is heated to 99℃ in a rotary evaporator at 5℃ / time until all liquid is vaporized, yielding coated magnesium-alumina spinel sand. 3) Mixed material compression molding: After the materials are mixed evenly, they are fed into a brick press to form strip samples of 300×300mm and 60mm in diameter; 4) Drying: After the material is formed, it is dried and dehydrated, and naturally dried for 3 days; 5) High-temperature firing: After the material is dried, it is placed in an ultra-high temperature kiln and fired at 1800℃ for 6 hours; 6) Cooling of sintered balls: Allow the sintered product to cool naturally for 6-7 days. Example 1

[0016] 1) Calcination of light-burned magnesia: Magnesite is used as raw material. After being crushed, it is roasted in a gas-fired light-burning kiln. After calcination at 1000~1080℃, it is finely ground and then screened to obtain light-burned magnesia with different particle sizes. 2) Raw materials: lightly calcined magnesia (90 wt.%), corundum powder (10 wt.%); 3. Sieving: The obtained lightly calcined magnesia is sieved according to the particle size range of 5-3, 3-1, and 0.074; the corundum is crushed and the 0.061mm sieve material is taken. 4) Mixing: Take the crushed and sieved lightly calcined magnesia and corundum powder according to the mass ratio, put the materials into a ball mill for wet mixing for 7 hours, and at the same time add 3% of the total mass of Mg(OH)2-Al(OH)3 sol and 1% of Y2O3 in two equal portions. After mixing evenly, dry. 5) Molding: After the materials are mixed evenly, they are fed into the brick press and pressed into strip-shaped samples of 300×300×60mm under a molding pressure of 300Mpa. 6) Drying: After the material is pelletized, it is dried and dehydrated naturally for 3 days; 7) High-temperature firing: After the material is shaped and dried, it is placed in an ultra-high temperature kiln and fired at 1800℃ for 6 hours to obtain low thermal conductivity magnesium aluminum spinel bricks. 8) Cooling: Allow the magnesium aluminum spinel bricks to cool naturally for 6-7 days. Example 2

[0017] 1) Calcination of lightly calcined magnesia: Magnesite is used as raw material. After being crushed, it is roasted in a gas-fired light-calcining kiln. After calcination at 1000~1080℃, it is crushed and then screened. 2) Raw materials: lightly calcined magnesia powder (90 wt.%) and corundum powder (10 wt.%); 3) Crushing and sieving: Crush the lightly calcined magnesia and take the material passing through the 5-3, 3-1, and 0.074 mm sieves; crush the corundum and take the material passing through the 0.061 mm sieve. 4) Mixing: Take the crushed and sieved light calcined magnesium powder and corundum powder according to the mass ratio, put them into the mixer for wet mixing for 7 hours, and at the same time add 3% of the total mass of Mg(OH)2-Al(OH)3 sol and 1% of the total mass of Y2O3 in 4 equal portions. After mixing evenly, dry. 5) Press molding: After the materials are mixed evenly, they are fed into a brick press and pressed into strip-shaped samples of 300×300×60mm under a molding pressure of 300MPa. 6) Drying: After the material is formed, it is dried and dehydrated, and naturally dried for 3 days; 7) High-temperature firing: After the material is dried into pellets, it is placed in an ultra-high temperature kiln and calcined at 1900℃ for 6 hours to obtain low thermal conductivity magnesium aluminum spinel bricks. 8) Cooling: Allow the magnesium aluminum spinel bricks to cool naturally for 6-7 days. Example 3

[0018] 1) Calcination of lightly calcined magnesia: Magnesite is used as raw material. After being crushed, it is roasted in a gas-fired light-calcining kiln. After calcination at 1000~1080℃, it is crushed and then screened. 2) Raw materials: lightly calcined magnesia powder (90 wt.%) and corundum powder (10 wt.%); 3) Crushing and sieving: Crush the lightly calcined magnesia and take the material passing through the 5-3, 3-1, and 0.074 mm sieves; crush the corundum and take the material passing through the 0.061 mm sieve. 4) Mixing: Take the crushed and sieved light calcined magnesium powder and corundum powder according to the mass ratio, put them into the mixer for wet mixing for 7 hours, and at the same time add 3% of the total mass of Mg(OH)2-Al(OH)3 sol and 1% of the total mass of Y2O3 in 4 equal portions. After mixing evenly, dry. 5) Press molding: After the materials are mixed evenly, they are fed into a brick press and pressed into strip-shaped samples of 300×300×60mm under a molding pressure of 300MPa. 6) Drying: After the material is formed, it is dried and dehydrated, and naturally dried for 3 days; 7) High-temperature firing: After the material is dried into pellets, it is placed in an ultra-high temperature kiln and calcined at 2000℃ for 6 hours to obtain low thermal conductivity magnesium aluminum spinel bricks; 8) Cooling: Allow the magnesium aluminum spinel bricks to cool naturally for 6-7 days.

Claims

1. A method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns, characterized in that, It includes four parts: preparation of lightly calcined magnesia, synthesis of Mg(OH)2-Al(OH)3 sol, material mixing and pressing, and high-temperature firing.

2. The method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns according to claim 1, characterized in that, In the preparation step of the lightly calcined magnesite, the calcination temperature of the crushed magnesite raw material is in the range of 1000~1080℃, and the particle size range of the obtained lightly calcined magnesite is 5-3, 3-1, 0.

074.

3. The method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns according to claim 1, characterized in that, In the synthesis step of the Mg(OH)₂-Al(OH)₃ sol, the Mg(OH)₂-Al(OH)₃ sol is made from magnesium chloride, aluminum sulfate, and ammonia water, with a concentration of 0.5 mol / L. The reaction equation is as follows: 1) MgCl2+2NH3H2O=Mg(OH)2+2NH4Cl 2) Al2(SO4)3+6NH3H2O=2Al(OH)3+3(NH4)2SO4.

4. The method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns according to claim 1, characterized in that, In the material mixing and pressing step, magnesia and corundum powder are used as raw materials, two-phase sol is used as binder, Y2O3 is used as sintering aid, the ball milling wet mixing time is controlled within 7 hours, the molding pressure range is 300MPa, and the sample is pressed into a long strip shape of 300×300×60mm.

5. The method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns according to claim 1, characterized in that, In the high-temperature firing step, the calcination temperature range of the shaped green body is 1800℃, and the firing time is controlled within 6 hours.

6. The method for preparing low thermal conductivity magnesium-aluminate spinel bricks for lime kilns according to claim 1, characterized in that, w(SiO2) < 1%, w(Al2O3) = 10%, w(MgO) > 90 wt.%, bulk density 2.98~3.08 g / cm³ 3 The amount of binder added is 3 wt.%.

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