Refractory Bricks

Characteristics and Production Technology of Alumina Hollow Ball Insulation Bricks

Oct 17,2022
The composition and structure of alumina hollow ball bricks The Al2O content is not less than 98%, the SiO2 content is not more than 0.5%, the Fe2O3 content is not more than 0.2%, the bulk density is 1.3~1.48g/cm3, and the displayed porosity is 60%~70% , The compressive strength is not less than 9.8MPa, the softening temperature under load (0.2MPa) is not less than 1700℃, and the thermal conductivity is 0.7~0.8W/(m.K).
There are generally three types of cross-sectional structures of hollow spheres in alumina hollow sphere bricks: thin-shell hollow spheres: (a). Porous honeycomb spheres (b). Thick-walled hollow spheres (c). The structure type of the hollow sphere mainly depends on the chemical composition, which has a great influence on the physical properties of the hollow sphere.
18000 mainly uses alumina hollow ball bricks below ℃ for long-term use. It also has good chemical stability and corrosion resistance at high temperatures.
Alumina hollow sphere is a kind of light aggregate, which can ensure high temperature resistance and reduce density; corundum powder is an inert filler; sillimanite is an expansion agent, which can reduce combustion shrinkage and improve heat resistance; alumina powder. Oxidation Silica micropowder and clay powder are binders that reduce the sintering temperature, form a mullite bond phase, and improve the high temperature resistance and heat resistance of refractory materials.
The bricks are prepared according to the set batching plan, and after mixing, the bricks are made, formed, and naturally dried for 24h, then dried at 110°C for 24h and 1600°CX4h. The insulating bricks are fired. When the content of sillimanite is 12%, the performance of making thermal insulation bricks is: bulk density 1.48g/cm3. compressive strength 26MPa. 1710°C.1600°CX2h reburning line change +0.3%. 1100°C water cooling) 17 times: the thermal insulation material has the characteristics of light weight, high strength, high temperature resistance and thermal shock resistance.
Detailed production steps of alumina hollow ball brick
The preparation methods of alumina hollow spheres include rolling ball burning loss method and fusion blowing method. The rolling ball method is to make a core ball with organic resin or low melt, put it into a rotating disc, and then sprinkle wet alumina powder containing a certain amount of moisture in the disc, adhere to the core ball, and form a shell, just like in the The pills are covered with a layer of sugar coating. The ball is then burned at high temperature, the core ball is burned, and the outer shell is sintered to form a hollow ball. The disadvantage of this method is that the efficiency is low, the spherical surface is not smooth, and the wall thickness and size are not easy to control, so it has not been widely used in industrial production.
The process flow for preparing alumina hollow spheres is shown in Figure 1. Using industrial Al2O3 as raw material, it is directly melted in the electric arc furnace. The electric arc furnace can be single-phase or three-phase, preferably three-phase. The furnace body is equipped with a tilting device, and the discharge port is equipped with a gas nozzle at a certain angle. Graphite is used for furnace lining material. Assume that the diameter of the furnace is 550mm, the height of the furnace is 300mm, the diameter of the graphite electrode is 150mm, the radius of the electrode circle is 150mm, the smelting voltage is 100V, and the current is 1500~1700A. During the melting process, the alumina fines are gradually changed. Interstitial additions are made until the melt reaches a sufficient amount. The temperature of the melt is about 2200~2300℃. At the same time, the furnace body can be tilted to make the melt flow out from the discharge chute, and the air pressure valve is opened at the same time. The exhaust volume is 10m3/min. The pressure is 0.6~0.8MPa due to the melt viscosity. And the action of surface tension, the compressed air solidifies rapidly during flight, forming a shell-shaped sphere, which is collected into the collection chamber. After sorting, sieving or washing, and washing frit fragments, large and small balls with a particle size larger than 5mm or less than 0.2mm are removed, and qualified hollow balls are classified according to particle size from 5mm to 0.2mm.
The natural bulk density of alumina hollow spheres is 0.42~0.97g/Cm3, and the integrity rate is 70%~90%. As the diameter of the sphere decreases, the integrity rate of the sphere increases, and the natural accumulation density of the sphere also increases. The alumina content of the hollow spheres is greater than 99.5%. This hollow sphere can be directly used as a heat dissipation and thermal insulation material. When using, it can be matched with large and small balls, mixed with an appropriate amount of lightly burned α-Al2O3 fine powder, and the heat preservation effect is better. The high service temperature of alumina hollow spheres can reach 1800 ℃.
If the hollow ball is used as the aggregate, the alumina powder is used as the base material, and the carboxymethyl cellulose solution of aluminum sulfate and the binder can be used to make the hollow ball brick. Appropriately match the size of the hollow ball. Alumina fine powder is a kind of α-Al2O3 obtained by calcining, pulverizing and purifying industrial γ-AI2O3 at 1450℃. The content of Al2O3 is more than 99%, and the particle size is less than 0.147mm, of which less than 5μm accounts for about 85%. Aluminum sulfate solution (20% by mass) and carboxymethyl cellulose solution (3% by mass) were added to the binder. The mixed blank is poured into a wooden mold, placed on a vibrating table, and gently vibrated to form to obtain a complete sphere. The evenly distributed blank. After the green body is dried, it can be fired at 1500~1800℃ at different temperatures according to different usage requirements. The porosity of the product is 50%~70%, the compressive strength is 4~6MPa, and the thermal conductivity is 3.3~3.8kJ/m·h·℃. 1100 ℃ ~ air-cooled thermal shock cycle for more than 15 times, is a refractory insulation material, with good thermal insulation performance. With the change of the added amount of alumina fine powder and the sintering temperature, the performance of the product has changed greatly, and the change of the ratio of large and small balls has little effect on the performance of the product.
What are the advantages of alumina hollow ball bricks?
Alumina hollow ball bricks are made of alumina hollow balls and alumina powder as the main raw materials, combined with other binders, and fired at 1750 ℃. An energy-saving thermal insulation material belonging to ultra-high temperature materials.
In order to reduce the weight of the furnace body, the structure is transformed to save materials. The energy saving effect is remarkable.
1. High temperature use:
Good thermal stability, up to 1750 degrees or more. The change rate of the reburning line is small, and the use time is long.
2. Optimize the structure and reduce the weight of the furnace body:
At present, high temperature resistant materials are heavy bricks with a bulk density of 2.6-3.0g/cm, while alumina hollow ball bricks have only 1.1~1.5g/cm, and the same volume of one cubic meter can be Reduced weight by 1.1-1.9 tons.
3. Save materials:
In order to achieve the same service temperature, such as the use of heavy bricks, the price is equivalent to the price of alumina hollow ball bricks, but it also requires considerable insulating refractory materials. If you use alumina hollow ball bricks, you can save 1.1-1 per cubic meter. Using 9 tons of heavy bricks can save 80% of refractory insulation materials.
4. Energy saving:
Alumina hollow spheres have obvious thermal insulation properties and low thermal conductivity, which can play a good role in thermal insulation, reduce heat dissipation and improve thermal efficiency, thereby achieving the purpose of energy saving. The energy saving effect can reach more than 30%.
Alumina hollow spheres and their products. Lightweight refractory materials have excellent energy-saving properties and are very stable in various atmospheres. Especially 1800 ℃ is used in high temperature kiln. Hollow spheres can be used for high temperature, ultra-high temperature insulation filler, high temperature refractory concrete lightweight aggregate, high temperature castable, etc. Hollow ball bricks can be used for high temperature energy saving (>30%) down flame kiln, shuttle kiln, molybdenum wire furnace, tungsten rod furnace, induction furnace, nitriding furnace, etc. Reduce the weight of the furnace body, transform the structure, save materials, and save energy.
α-alumina powder is made of high-quality industrial alumina as raw material, produced by new high-temperature inverted flame kiln and scientific calcination process, artificial selection, clear classification, less impurities, high conversion rate, stable shrinkage rate, and good consistency.
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