Magnesia chrome brick is composed of alkaline refractory products containing Mg055%~80%, C,038%~20%, composed of magnesia, spinel and a small amount of silicate phase. The composite spinel consists of MgAl2O4, MgFe2O4, MgCr2Oa and FeAl,O, etc. Magnesium-chrome bricks have developed rapidly since the 1960s due to the improvement of raw material purity and firing temperature. At present, according to different production methods, magnesium-chrome bricks can be divided into ordinary bricks, direct-bonded bricks, co-sintered bricks, rebonded bricks and fusible cast bricks.
(1) Ordinary magnesium-chrome brick This is a traditional product, with chromium ore as coarse particle, magnesia as fine powder. Or the two materials are composed of graded particles, and the firing temperature is generally 1550~1600℃. The microstructure of the brick shows that there is little direct bonding between chromium ore particles and magnesite, but mostly silicate (CMS) cementation or fracture isolation. The brick has poor mechanical properties and slag corrosion resistance because there is little desolubilization phase and little direct bonding in the matrix.
(2) Directly bonded magnesia chrome brick is developed on the basis of ordinary magnesia chrome brick, its production characteristics have two main points, one is the use of pure raw materials, the other is the use of higher firing temperature. The so-called direct combination refers to the more direct contact between the chromium ore particles and the cubic magnesite in the brick, because there is less 502 in the raw material (controlled below 1%~25%), less silicate production, through high temperature sintering means to squeeze the silicate to the corner of the solid particles. Thus improving the direct bonding of the solid phase. Due to the high degree of direct binding, the brick has high high temperature strength, slag resistance, erosion resistance, erosion resistance, corrosion resistance and excellent thermal shock stability and volume stability at 1800℃.
(3) The production process of jointly sintered magnesia chrome brick is characterized by the high temperature furnace firing of the mixture of magnesia and chrome ore fine powder in a certain proportion to achieve the generation of secondary spinel and magnesia. Direct bonding of chromium ore is the solid reaction for the purpose of preparing common sintered material, which is used to manufacture sintered or chemically combined products. The direct bonding and microstructure uniformity of the co-sintered magnesia-chrome brick is better than that of the direct-bonded brick, and the amount of the cubic magnesite desolubilizing phase and intergranular secondary spinel is more. The co-sintered magnesia-chrome brick has a series of better properties than the direct-bonded brick, especially known for its high temperature strength, temperature resistance and slag resistance. Co-sintered bricks can also be divided into two varieties. One is fully co-sintered bricks, in which particles and fine powders are all co-sintered materials, and their microstructure is basically similar whether fired or chemically combined: Part of the ingredients, such as coarse particles with common sintered material, and fine powder part can be fine chromium ore and magnesia paper powder mixed into the brick according to a certain proportion, so that the fired and chemically combined products will be different in the microstructure.
(4) Rebonded magnesia chrome brick is fused by electromelting method to melt the magnesia chrome powder mixture, through the melt crystallization, the formation of a fairly uniform microstructure, magnesia chrome spinel and magnesia magnesia mixed crystal as the main phase of the raw material, the electrofused magnesia chrome material into a certain particle size, mixed forming, after firing to prepare the rebonded brick, or directly used as chemical junction brick. The microstructure of rebonded brick is characterized by a high degree of direct binding and containing a large number of dedissolved spinel phase: the base crystal containing a large number of dedissolved phase, which essentially changes the physical and chemical properties of magnesite, such as reducing the coefficient of thermal expansion, improving the thermal shock resistance, and improving the resistance to acid and alkaline slag erosion. The rebonded brick has similar properties to the fused cast brick, but has better temperature resistance and more uniform microstructure than fused cast brick. The recombined magnesium-chromium brick is a fine grained matrix with evenly distributed pores and micro-cracks, and is more sensitive to rapid temperature changes than casting. The high temperature performance of the product is between the fused cast brick and the directly bonded brick.
(5) The mixture of magnesia and chromium and ore is completely fused in the arc furnace, and then the melt is injected into the refractory casting mold for casting. In the solidification process, the stable crystal phases of magnesite and spinel are formed, and the fine crystalline structure is formed at the same time, so the fused cast magnesium-chromium brick has excellent high temperature strength and slag corrosion resistance. Fusible cast magnesium-chromium brick and electromolten magnesium-chromium material, with a high degree of direct binding and containing a large number of spinel dedissolved phase, this product causes high density, slag is not easy to penetrate, so slag resistance is better than the direct binding brick, but the thermal shock resistance is worse than the above two products.
(6) The pre-reacted magnesium-chrome brick is made of lightly fired magnesium-chrome powder and chromite, which are jointly finely ground into fine powder with particle size less than 0.088mm, pressed into blank or ball, forged and fired into pre-reacted sintered material at 1750-1900℃, and then produced by conventional brick making process, crushed, mixed, formed under high pressure and fired at 16001780℃. The main mineral composition of the prereaction magnesia-chrome brick is magnesite, spinel and a little silicate, and the direct intercrystalline binding degree is high. The product has dense structure, uniform composition, low porosity, high temperature strength, good slag resistance, good thermal shock resistance, and can be used in VOD furnace, F, furnace and other furnace refining equipment slag line parts.
