High alumina bricks can be said to be one of the most widely used refractory bricks in the refractory industry. High-alumina bricks are divided into hundreds of specifications and sizes, and can be used in the iron and steel industry, building materials industry, electric power industry and other kinds of thermal kilns.

Generally speaking, the Al₂O₃ content of more than 48% of the alumino-silicate refractories are collectively referred to as high-alumina refractory materials, stereotypes of products that are commonly used high-alumina bricks are generally divided into three grades: Ⅰ, etc.: Al₂O₃>75%; Ⅱ, etc.: Al₂O₃ 60% ~ 75%; Ⅲ, etc.: Al₂O₃ 48% ~ 60%.

Firing of first-grade high-alumina refractory bricks, usually firing temperature of 1700 to 1800 ℃.

Firing secondary high alumina refractory bricks, usually firing temperature is 1600 to 1700℃.

Tertiary high alumina refractory bricks are usually fired at 1500 to 1600℃.

High-alumina brick and multi-clinker clay brick production process is similar, its main refractory raw materials are: water-containing alumina-oxygen minerals (alumina monohydrate, alumina trihydrate, etc.) as the main composition of high alumina bauxite; silica minerals (including bluestone, erythrite, silica, etc.); synthetic raw materials, such as industrial alumina, synthetic mullite, electrolytic fusion corundum, and so on. The difference is that the proportion of clinker in the ingredients is higher, can be as high as 90% ~ 95%, clinker in the crushing before the need for grading picking and screening iron removal, firing temperature is higher, such as Ⅰ, Ⅱ, such as high alumina bricks with a tunnel kiln firing is generally 1500 ~ 1600 ℃.

For high alumina brick models and specifications in this article we do not do too much elaboration, if you want to know more about the model specifications size can be left a message below, I will send a separate document to you.

About physical and chemical indicators

High alumina brick refractoriness fluctuation range, generally 1770 ~ 2000 ℃, mainly by the Al₂O₃ content of the impact, with the products in the Al₂O₃ content increases and improves. At the same time, the refractoriness is also affected by the content and type of impurities, and is related to the mineral structure of the products.

The load softening onset deformation temperature of high alumina bricks is greater than 1400°C and increases with the increase of Al₂O₃ content.The load softening temperature of the products with Al₂O₃ content less than 71.8% depends on the ratio of the amount of mullite to the amount of glassy phases, and increases with the increase of the amount of mullite. The number and nature of the glass phase has a significant effect on the load softening point. 71.8% ~ 90% Al₂O₃ content of mullite, corundum products, with the increase in Al₂O₃ content, the amount of glass phase is basically unchanged, corundum, although there is an increase in the number of mullite, but mullite but there is a decrease in the load softening temperature; therefore, the softening temperature of the load is not significant. After the content of Al₂O₃ exceeds 90%, with the increase of Al₂O₃ content, the glass phase decreases, and the loading softening temperature increases significantly, from 1630℃ when Al₂O₃ 90% to 1900℃ when Al₂O₃ 100%, the loading softening temperature are increased with the increase of Al₂O₃ content. The thermal conductivity of high alumina bricks decreases with increasing temperature. When the higher the content of Al₂O₃ in the high alumina bricks, the more mullite and corundum crystals it has, the more obvious the tendency of the thermal conductivity decreasing with the increase of temperature. However, above 1000 ℃, its decline decreases. The current national standards, ferrous metallurgy standards, and building materials industry standards for high alumina bricks are as follows: YB∕T 4577-2016 Anti-permeability high alumina bricks GB/T 2988-2012 High-alumina bricks JC/T 1063-2007 Anti-scaling high alumina bricks for cement kilns YB/T 4439-2014 High-alumina anchored bricks for heating furnaces JC/T 350-2013 Phosphate bonded high alumina brick GB/T 3995-2014 High-alumina heat-insulating refractory brick

About the grade of high alumina bricks

Ordinary high alumina bricks are divided into six grades according to the percentage content of Al₂O₃, namely, LZ-80, LZ-75, LZ-70, LZ-65, LZ-55 and LZ-48, and their physical and chemical indexes are shown in Table 1.

National standard GB/T2988-2012 on ordinary high alumina brick regulations low creep high alumina brick type classification and international regulations

On the production of high alumina bricks

Taking grade 1 bauxite clinker and electrofused white corundum powder as raw materials, matched with appropriate amount of expansion agent, after molding and heat treatment at 800℃, high alumina bricks with good thermal shock stability can be produced. To Ⅰ grade bauxite clinker and electrofusion white corundum powder as raw materials, with appropriate amount of expansion agent, through the molding and 800 ℃ heat treatment, can be made of high alumina bricks with good thermal shock stability. In order to reduce the porosity of high alumina bricks, from the selection of raw materials, batching, mixing until the molding, firing process should take corresponding measures. Selection of a high alumina bauxite clinker water absorption rate of less than 5% is appropriate, the second high alumina material water absorption rate of less than 7% is appropriate. The water content of less than 4% of the soft clay and bauxite clinker mixed fine grinding. With this mixture of fine grinding powder batching, can reduce the porosity of brick billet. Batching particle gradation, coarse in the ratio of fine for 4:2:4 is appropriate, the largest particles should not exceed 5mm. mixing of the feeding order is as follows: the first to add coarse particles, and then add the sulfite pulp waste pre-mixed for 3min, and then add the high alumina fine powder for mixing.

Defects caused by firing process

Generally, after firing almost all the problems in high alumina bricks will be exposed and produce all kinds of defects. Among them, the main defects are underfired, overfired, broken, contaminated, black heart, twisted, cracked, etc. 1, underfired underfired is due to the firing temperature is not enough, or insulation time is not enough, or leakage into the cold air caused by the problem. Generally speaking, the mechanical strength of underfired high alumina bricks is low, loose combination, low sound when knocking, easy to break when handling, high temperature performance and poor erosion resistance. 2, overfired overfired is the firing temperature is too high, or the insulation time is too long, or the flame is directly in contact with the surface of the bricks caused by the problem. In general, over-fired high alumina bricks with high mechanical strength, low porosity, dimensional shrinkage, brick deformation, too tightly bonded, serious porcelain, knocking when when the sound, thermal shock resistance is poor. However, when the raw material contains high Fe₂O₃, TiO₂ and other transitional metal oxides impurities, overcooking may also make high alumina brick foaming, density reduction, porosity increase, volume expansion increase phenomenon. 3, breakage breakage is the burned after the emergence of the lack of prongs, corners, drop grain and other defects. The reason may be the matrix quantity is small, the bonding agent strength is insufficient, handling force is too violent, mixing practice is not uniform, particle segregation, molding pressure is not enough or firing temperature is low, etc. 4, pollution pollution refers to the melting hole, iron spots and other defects. The cause of the melt hole may be cinder, debris and other low-melting materials mixed into the brick material, firing high aluminum brick melting caused. Iron spot is the manufacture of ferrous materials mixed into the raw materials, firing of ferrous materials oxidation diffusion caused by dark patches. The reason may be in the crushing, grinding iron substances mixed into the refractory raw materials. 5, black heart black heart is burned refractory products with black center and present defects. The cause of the black heart may be products within the organic matter is oxidized before the surface is produced by the glass phase seal. It may also be products in the reducing atmosphere under the firing, cooling by the oxidizing atmosphere caused by the impact. In general, the black heart of the products of poor erosion resistance. 6, twisted and distorted is a high-alumina brick products at high temperatures produced by pressure deformation, which is marked by the bottom of the high-alumina brick prisms are not in the same plane. If the molding, kiln loading, firing improper, may lead to high alumina brick distortion. 7, crack crack refers to the burned refractory products with a variety of cracks, including surface, internal, visible, hidden cracks. High alumina brick cracks have different causes, such as improper molding may appear layer cracks; drying, firing process contraction, expansion is too large, or contraction, expansion is not uniform will cause cracks; firing brick billet heating, cooling is not uniform, inconsistent firing line changes or withstand excessive thermal stress will also cause cracking. The reasons for cracks are generally the following:

(1) raw material reasons.

Improvement measures: the main mineral phase composition of high aluminum brick is mullite, corundum and glass phase. With the increase of Al₂O₃ content in the product, the number of mullite and corundum phase also increases, the glass phase is reduced accordingly, the refractoriness and high temperature performance of the product is improved. The actual production of high alumina bricks, pay attention to the impurity content of the used high alumina bauxite clinker control, in accordance with the requirements of YB/T5179-2005 (high alumina bauxite clinker), K ₂ O and NaO content of <0.35% ~ 0.6%, try to use the raw materials with low content of impurities and sintering degree of good production.

(2) Reason of mud material

Critical particle size in the mud, the amount of fine powder added, the mixing quality of the mud will affect the high alumina brick semi-finished products to produce cracks. Such as matrix material is generally composed of fine powder, the firing process produces contraction, while the particles are generally produced expansion, between the two produce a large deformation difference and cause internal stress, so that the products produce cracks.

(3) molding reasons. Containing the brick type of reasons, molding mold design is unreasonable, fabric uneven reasons, molding operations and molding pressure reasons

(4) Sintering reasons

High alumina brick sintering is liquid phase sintering, the formation temperature of the liquid phase and the amount of liquid phase, sintering temperature speed, atmosphere conditions, the billet in the sintering process of contraction as well as the secondary mullite compound corundum re-crystallization will lead to inconsistent contraction, so that the surface of the product cracks.

There are generally pre-tropical cracks, firing zone cracks, cooling zone cracks. High-alumina brick crack causes are manifold, but there is one or several factors work together, but from the essence of the analysis, mainly due to the role of stress more than the high alumina brick itself to withstand the stress leads to the occurrence of cracks. The actual production process due to physical and chemical changes in products and micro-cracks sometimes on products to improve thermal shock resistance is beneficial, according to the high alumina brick cracks caused by a variety of factors to be analyzed to make improvements to improve the product's finished product qualification rate.

Application of high alumina bricks

High alumina bricks are mainly divided into ordinary high alumina bricks and modified high alumina bricks. Ordinary high alumina brick refers to the commonly used standard fired brick, and modified high alumina brick mainly refers to: high load soft high alumina brick, micro-expansion high alumina brick, low creep high alumina bricks and phosphate bonded high alumina bricks four. First, ordinary high alumina brick. The main mineral composition of high alumina brick is mullite, corundum and glass phase. Ordinary high alumina brick is mainly used for masonry blast furnace, hot blast furnace, electric furnace roof, blast furnace, reflector furnace, rotary kiln lining. In addition, high alumina bricks are also widely used in flat furnace heat storage type lattice bricks, pouring system with plugs, water spout bricks and so on. Second, high load soft high alumina brick. High load soft high alumina bricks and ordinary high alumina bricks, compared with the difference is the matrix part and combining agent part. (1) matrix part in addition to add three stone concentrate, according to the chemical composition after burning close to the theoretical composition of mullite, the reasonable introduction of high alumina materials (grinding fine high alumina bauxite, industrial alumina or a-Al2O3 micropowder, corundum powder, high alumina corundum powder). (2) Combining agent selection of high-quality ball clay, etc., depending on the variety of different clay composite combining agent, or mullite-based combining agent. Through the use of the above methods, so that the load softening temperature of high alumina bricks to improve 50-70 ℃. Third, micro-expansion of high alumina brick micro-expansion of high alumina brick is high alumina bauxite as the main raw material, add three stone concentrate, according to the production process of high alumina brick made. In order to make high alumina bricks in the use of the process of moderate expansion, the key to choose a good three stone minerals and their particle size, control the firing temperature, so that the selected stone minerals part of the mullite, residual part of the three stone minerals. The residual trilithon minerals are further mullitized (primary or secondary mullitization) in the process of use, accompanied by expansion reaction. The choice of tridymite minerals is preferable to composite materials. The decomposition temperature of the minerals varies. Mullite produced by the expansion of the differences, the use of this feature, high alumina bricks due to different working temperatures and the corresponding expansion effect, squeeze the brick seam, improve the overall densification of the brick, thus improving the brick resistance to slag penetration ability. Fourth, low creep high alumina bricks in our country for a long time in the hot stove using clay bricks, high alumina bricks in the assessment indicators, mainly load softening temperature, did not put forward the butterfly rate of the index requirements. After years of use, the brick deformation, cracking, sinking and other problems, if the hot blast furnace wind temperature increases, such problems are more prominent. With the development of ironmaking blast furnace large volume, high wind temperature technology and longevity, the refractory materials for hot blast furnace put forward higher requirements, requiring materials to withstand long-term thermal stress and high wind temperature and other roles and not easy to damage. Therefore, the hot blast stove with high aluminum bricks with low creep rate. This plays an important role in the longevity of the hot blast stove. Low creep high alumina brick physical and chemical indicators are as follows: in order to solve the high alumina brick creep resistance, the use of adding beneficial minerals, the use of the so-called non-equilibrium reaction to solve, when the creep temperature of 1550 ℃, 1500 ℃ when the addition of quartz, trilithonite minerals; creep temperature of 1450 ℃, 1400 ℃, 1350 ℃, additives for the trilithonite minerals, and accordingly the introduction of corundum, a-Al2O3 etc.; creep temperature of 1300 ° C, 1270 ° C, 1250 ° C, the additives are trilithon minerals. Among them, trilithon minerals and activated alumina are mainly added in the form of matrix, and the complete or nearly complete mullitization of matrix is the key. Because the matrix of mullite, will certainly increase the mullite content of the material, reduce the glass phase content, and mullite excellent mechanical and thermal properties are conducive to the improvement of the material's high temperature performance or enhancement. It has been shown that in the production of high alumina bricks, the addition of 15% -35% of the silica concentrates, can be produced for the creep temperature to 1400-1450 ℃ of the blast furnace hot stove with high load soft, low creep high alumina bricks. For the creep temperature of 1500 ~ -1550 ℃ low creep high alumina bricks, in addition to adding the appropriate amount of silica, but also need to add a certain amount of mullite; or use part of the fused corundum and silica raw materials. Fifth, phosphate bonded high alumina brick phosphate bonded high alumina brick is a dense special or first-class high alumina bauxite clinker as the main raw material, phosphoric acid solution or aluminum phosphate solution for the bonding agent, the semi-dry machine molding, heat treatment at 400-600 ℃ and made of chemical bonded refractory products. It belongs to the no-burn bricks, in order to avoid the product shrinkage in the process of high-temperature use, the ingredients need to be introduced into the heating expansion of raw materials, such as bluestone, silica wire stone, foliated paraffin, silica and so on. Compared with ceramic-bonded fired high alumina, it has better resistance to spalling, but its load softening temperature is lower, and its resistance to erosion is poorer, so it is necessary to add a small amount of electrofused corundum mullite and so on, in order to strengthen the matrix. Phosphate bonded high alumina bricks are suitable for cement kilns, electric furnace tops and steel ladles.