For a century, the synthesis of magnesia-alumina spinel and the production of spanioli refractories have been known, but the volume expansion of 5-7% during the process of spinel formation from primary acids, magnesia and alumina, does not allow the material to condense during heating and baking. Therefore, a two-stage thermal program is used for the synthesis of high-density spinel, where the first stage is formed to complete the spinel stages and the second stage is formed in order to densify. The use of a two-stage thermal program will be economically appropriate. Magnesia chromite refractories with properties similar to magnesia spinel bricks have a lower price and are more economical. In fact, chromite is used to improve the quality of magnesia bricks. Chromium in the oxidizing atmosphere of rotary cement kilns below 3+, however, materials containing chromium are dangerous because chromium is converted (Ref. 1 and 2). On the one hand, the environment and very good physical and chemical properties of magnesia alumina spinel, on the other hand, caused the desire to replace magnesia chromite bricks with magnesium spinel bricks. Today, the use of magnesium aluminate as an alternative phase in magnesium-based refractories is considered. The main fields of application of magnesium spinel refractories are in the firing and transfer part of the cement rotary kiln and alumina spinel casting in the side walls and bottom of steel pans. 
In terms of raw materials, the synthesis of magnesia spinel products is classified into two main methods. In the first method, preformed spinel is used next to magnesia, while the second method is based on the use of in situ spinel. In other words, in the second method, molten corundum, sinter corundum or calcined alumina is used. It is used along with magnesia, and then spinel is formed during the process and under favorable conditions. The hot bending strength curve at a temperature of 1600 C with an increase in the percentage of sintered spinel and also an increase in sintered corundum is shown in Figure 3. In both curves A and b, by adding sintered spinel and sintered corundum to magnesia, the hot bending strength decreases, but the significant difference is the rate of this decrease. magnesia alumina spinel bricks such as excellent fire resistance, high mechanical strength, very good resistance to thermal shock, high resistance to thermomechanical stresses and high resistance to corrosive conditions (alkaline and acid slag); although the composition The phase in these bricks has a significant effect on the properties. The phase composition in magnesia bricks is shown in Table 2. Meanwhile, the molar ratio of SiO2/CaO plays an important role in the formation of the main phases. Stoichiometric magnesia alumina spinel was synthesized at 1400 C temperature. The optimum temperature for sintering refractory magnesia C spinel was determined to be 1680 o. The addition of spinel significantly improved the load-retarding properties and residual cold flexural strength after thermal shock of the periclase body. The mismatch of thermal expansion between periclase and spinel phases caused the creation of microcracks and tensile stresses around the spinel grains, which led to an increase in the strength of the body. 
However, excessive expansion of microcracks in the body caused the failure of the sample at lower relative stresses and a decrease in thermo-mechanical properties was observed. The main reason for the spread of these refractories has been the environmental issues of chromite magnesite refractories and the prohibition of their use in advanced countries. On the other hand, interesting thermal, mechanical and chemical properties of these bricks have caused their increasing expansion. These refractories are generally divided into two groups: - The first type, the formation of spinel occurs as a result of the reaction of alumina particles with MgO during brick firing. These refractories contain about 3 to 8% alumina, 1 to 2% calcium oxide and 1 to 4% silica. - The second type of spinel (MgAL2 O4) prepared in advance is added to the nitrided magnesite. These bricks usually have 10-15% aluminum oxide and 80-90% magnesium oxide, and the total amount of impurities of silica, iron oxide and calcium oxide is less than 2%. Another division of magnesite-spinel refractories is according to the type of magnesite used. To prepare spinel in type 2 bricks, magnesia and aluminum in a weight ratio of 71.8/28.2% are heated at 1650C in an oxide atmosphere after mixing and molding. The choice of alumina and its granulation is important. The more active the alumina (eg type x compared to x) and the finer it is, the less time the spinel product is made. If other alumina-containing raw materials are used, its impurities and especially the amount of silica should be low. We should note that the melting point of spinel is 2100, alumina is 2000, and MgO is 2800, so a large increase in spinel reduces its fireproof properties. 
alumina bricks price
One of the most widely used products is refractory material, the amount of alumina used in it is very high, and this brick is made of non-metallic mineral materials, which consists of SiO2 and Al2O3, and the amount of Al2O3 is higher than chamotte brick. - Sometimes it is made from alumibi firebricks of kaolin and calcined bauxite and high purity clay so that it has very little impurities. The firing of these bricks is about 120 to 1800 degrees. Application: This type of alumina bricks are used to cover the inner walls of steel melting furnaces, because of the resistance of these bricks against alkaline materials, they are also used to cover the inner walls of cement and glass furnaces. types of: 50% alumina group: temperature 1600 degrees 60% alumina group: 1600 degree temperature 70% alumina group: It is the most widely used alumina brick because of its excellent and effective performance 80% alumina group: It is made from calcined bauxite along with various amounts of alumina under the clay. According to the types of these bricks, the prices of these products are different High aluminum bricks are mainly used in masonry blast furnaces, hot blast furnaces, electric furnace floors, blast furnaces, inverting furnaces, rotary kiln linings, rotary cement rotary belts, preheaters and other heating equipment that require resistance. It is used against thermal shock. The above aluminum bricks are also used as diaphragm rebuilding mesh bricks, spray system fittings and nozzle bricks. 
These bricks contain a high percentage of aluminum (Al2O3). They are made from a mixture of kaolin, bauxite and corundum, which has more than 70% alumina. The firing temperature of these bricks is about 1200 to 1800 degrees Celsius. This type of brick is used to cover the inner walls of steel melting furnaces. Due to the resistance of these bricks against alkaline materials, they are also used as lining of the inner walls of cement and glass furnaces.