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Refractories made from alumina
- Nov 16, 2018 -

Refractories made from alumina

corundum refractory material refers to a refractory material mainly composed of Al2O3 and a main crystal phase of α-AL2O3 (corundum). Because of its high temperature resistance, high hardness, high strength, oxidation resistance, corrosion resistance, electrical insulation, and good air tightness, it has a wide range of applications.

Natural corundum is distributed in nature and is not concentrated. It is mainly used as abrasive materials for machinery industry and precious crafts. So far, there have been no reports on the use of natural corundum to produce refractory materials. The corundum raw materials used in the refractory industry are extracted from ores containing AL2O3 minerals, mainly bauxite, and then alumina.

1. Sintered corundum refractory

sintered corundum refractory material, due to the different raw materials used, can be divided into:

(1) corundum products made of sintered corundum clinker generally called recombined sintered corundum products;

(2) burnt corundum powder Corundum products made of granules and fine powder, also known as sintered alumina products.

In addition to the high purity of corundum raw materials, sintered corundum refractories have no difference from the traditional refractory production process.

1.1sintered alumina products

Raw materials used are mainly industrial alumina. Industrial alumina is difficult to sinter. In order to improve the sinterability and lower the sintering temperature, it is necessary to finely pulverize and pre-fire the industrial alumina high-temperature amount of the additive for promoting sintering. Industrial alumina should be at -1600 before use. C calcination converts γ-L2O3 into stable α-AL2O3 and reduces the firing shrinkage of the product. The disadvantage is that the material becomes hard after burning and is not easy to finely grind. In order to remove the iron brought in during the pulverization process, it may be acid-washed with hydrochloric acid, and after repeated washing with fluids, white α-AL2O3 is separated, and then dried and crushed to obtain a billet.

The molding method is determined according to the use, the property requirements, and the shape and size of the product. The specific molding methods include: applying fluids suspension (mud) to mud casting in a plaster mold (crucible, pipe, and other hollow products); products with complicated shapes and small dimensions are formed by hot pressing or extrusion; large products It can be formed by machine pressure or air hammer. The products are formed at 1600-1800 °C, and the firing equipment has a small-section high-temperature tunnel kiln and a downdraft kiln. For tubular products, they should be hoisted and fired in a live top downdraft kiln to prevent deformation during firing. After the firing, the cutting process is performed.

1.2 Combined with sintered corundum products,

sintered alumina (sintered corundum) is used as pellets, and corundum fine powder is formulated into a slurry, which is formed, dried and fired, also known as corundum clinker refractory.

In high-purity sintered corundum products, pure corundum clinker of the same composition should be used for the granular material and fine powder (fine powder of fine-grained alumina of n-AL2O3 type can also be used). In the production of ordinary sintered corundum products, in order to improve its thermal shock resistance or slag resistance, it is generally allowed to add less than 10% of the second component, the second component being mainly mullite. In order to improve the molding properties, an appropriate amount of organic binder is often added. The mud material should be thoroughly mixed, and if necessary, the material moisture will fluctuate from 5% to 8%. Ordinary products are formed by semi-dry pressing. Products with complex shapes and large dimensions are hammered or vibrated. Special products are specially designed molds that are hydrostatically molded. Corundum clinker products must be fired at high temperatures. The firing temperature is between 1650 and ~. The question of C sometimes reaches 1850. C. Appropriately increasing the firing temperature can increase the strength and density of the article accordingly.

2, fused corundum refractory

fused alumina corundum refractory material can be divided into fused cast corundum brick and recombined (semi-recombined) fused corundum brick according to the manufacturing process. The cast-cast corundum brick is melted in an electric arc furnace by industrial alumina and a small amount of soda ash and quartz powder, and then subjected to a process such as casting and annealing, and finally mechanically processed into a desired shape and size. Combined with (semi-recombination) fused corundum bricks using fused corundum granules and fine powder, adding a binder, after thorough mixing, forming with a brick press, the bricks are dried and then fired at a high temperature.

2.1 Casting of corundum products The products

of alumina raw materials are melted at a temperature higher than the melting temperature, poured into a prefabricated refractory model, and formed by cooling and solidification to grow the crystals, which is called a cast-cast corundum refractory. It is generally melted by electrofusion.

The fused cast corundum refractory material which is electrofused with industrial alumina as the main raw material is melted, refined and then cast into the model through the batch, and the casting is solidified, annealed, cut, ground and pre-assembled to become a qualified product. There are three main types of chemical minerals:

(1) α-AL2O3 bricks. Also known as fused α-AL2O3 brick, it is a fused cast refractory material with α-A12O3 crystal phase. α-AL2O3, the content of about 95%, the β-AL2O3 content of 5% to 10%, less glass phase. The brick structure is compact, the refractoriness and the high-temperature structural strength are high, and the high-temperature chemical stability is good, but under the action of the alkali metal oxide, it may be converted into β-AL2O3 and the volume is increased, which is suitable for the cooling of the masonry glass kiln. Pool parts and superstructure, as well as non-ferrous metal smelting furnaces and high-temperature tunnel kiln.

(2) Casting β-AL2O3 bricks. Also known as fused β-AL2O3 brick, it is a fused cast refractory material with β-AL2O3 as the main crystal phase. The content of β-AL2O3 is about 99.5%, and the larger plate crystals are intermeshing with each other, and the glass phase is less, about 0.5% or less, and is inert to alkali vapor. In an alkali-free atmosphere, the base is easily separated and converted into α-AL2O3, accompanied by volume shrinkage, which may cause chipping of the brick. The cast-cast β-AL2O3 brick has good thermal shock resistance, and the air circulation from 700 °C to normal temperature can reach about 40 times, which is generally suitable for the upper structure of the masonry glass melting furnace.

(3) Casting α, β-AL2O3 bricks. Also known as fused alpha, beta-AL2O3 bricks, are melt cast products containing alpha-AL2O3 and beta-AL2O3. The content of α-AL2O3 is about 40% to 55%, and the content of β-AL2O3 is about 45% to 60%, and the glass phase is extremely small. Compared with the β-AL2O3 type product, it has strong anti-glass liquid erosion ability and good anti-alkali vapor at high temperature. It can be used in the upper structure of the glass kiln, and can also be used in the low-temperature part directly contacting the glass liquid. The physical and chemical indicators of the fused corundum brick produced by Henan Refractory Material Factory are as follows.

2.2 Combined with the fused corundum product

and then combined with the fused corundum product, the fused corundum clinker is used as the granule, the fused corundum powder or the sintered corundum powder is used as the matrix,

and the sintered product is combined with the sintered product.

As the raw material of fused brown corundum or white corundum, the pellets are magnetically removed by iron, and the finely ground powder is washed with iron to remove iron. The particle size ratio should be a multi-stage ratio, reduce the intermediate particles, increase the amount of fine powder (including a certain amount of ultra-fine powder), and help to improve the density and sintering performance of the product. Add a certain amount of binder, mix well,  fluids content is about 3% to 4%. Formed by high pressure, a dense brick is obtained. The fused corundum product has high purity and is difficult to sinter, and needs to be fired at above 1800 °C. In the case of small batch production, the high-temperature intermittent kiln is suitable, and small high-temperature tunnel kiln can be used in batch production. The chemical composition and properties of the fused white corundum brick are combined with the following figure.

3. Application

of corundum refractory materials Due to the high-temperature performance of corundum refractory materials, it is suitable for various use conditions. Not only can the blast furnace belly be built, but also the hot blast stove for blast furnaces and other kilns. It can be used for furnace wall, furnace top and checker brick of 1500~1700°C, gas preheater, equipment lining and brick lattice under direct action of carbon, methane, natural gas and coke oven gas, vacuum treatment outside the furnace of steel and Continuous casting equipment, vertical furnace for high temperature treatment of anthracite and other carbonaceous materials, high temperature carbonization furnace, kiln with high temperature load, tunnel kiln with operating temperature of 1800 °C and other kiln (low alkaline oxidation) Aluminum-based corundum products can be used at temperatures up to 1920~1950°C as catalysts and heat carrier nozzle balls, boiling furnace tuyere belts (such as calcined mercury mines), heating furnace bottoms, replacing metal parts and machines with fast wear. Lining, production of glass balls and glass fiber high-temperature glass melting furnace tops and linings.