Magnesia chrome brick is a type of basic refractory that contains 55-80% MgO and 8-20% Cr2O3. It is composed of periclase, spinel and a small amount of silicate compound phase. 

Magnesia chrome brick has seen a rapid development due to the improvement of purity of raw materials and firing temperature since 1960s. Currently, it can be divided into common brick, directly bonded brick, co-fired brick, rebonded brick and cast brick. 

 

It is a traditional refractory product. It is made of chrome ore as coarse particles and magnesia sand as fine powder or graded particles of the two materials. The firing temperature is generally 1550-1600℃. In this brick, chrome ore particles and periclase are seldom directly bonded but cemented by silicates (CMS) or isolated by gaps. The desolventizing phase in the periclase is small. It is seldom directly bonded in the matrix. This brick has poor mechanical properties and poor resistance to slags. 

 

It is developed on the basis of the common magnesia chrome brick. It is made of purer raw materials and higher firing temperature. ‘Directly bonded’ refers to that there are more direct contact between the chrome ore and periclase. Since there are only a little SiO2 in the raw materials (less than 1%-2%), there are only a little silicates produced. The silicates are squeezed into the corners of the solid particles and thus improve the directly bonding of the solid phases. This brick, due to its high degree of directly bonding, has high strength, slag resistance, corrosion resistance, erosion resistance, good thermal shock and volume stability under 1800℃. 

 

This brick is made by firing the mixture of magnesia sand and chrome ore fined powder at high temperature to acquire co-fired materials by directly bonding the secondary spinel, magnesia sand and chrome ore. The co-fired brick has better homogeneity and better microstructure. Since the desolventizing phase in the periclase and secondary spinel are more, it has better properties than directly bonded magnesia chrome bricks, such as high strength, resistance to rapid temperature changes and good slag resistance. 

 

It can be divided into two types, fully co-fired brick and partially co-fired brick. The particles and the fined powder of fully co-fired bricks are both co-fired materials. Both fired and chemically bonded bricks have similar microstructure. For partially co-fired bricks, just a part of raw materials are co-fired materials, so the microstructure of the fired brick and the chemically bonded brick are different. 

 

It is made of fused magnesia chrome. First, melt the mixture of magnesia and chrome materials in an electric arc furnace. Then the melt is crystallized to form a material with a uniform microstructure composed of magnesium chrome spinel and periclase. Finally the fused magnesia chrome is crushed into particles, mixed, molded and fired into rebonded brick or made into chemically bonded brick. 

 

The microstructure of the rebonded brick is characterized by highly directly bonding and a large amount of spinel desolventizing phase. The matrix with a large amount of desolventizing phase changed the physical and chemical properties of periclase, such as the lower thermal expansion coefficient, the better thermal shock resistance and better resistance to acid and alkali slags. This brick has similar properties as cast brick, but better resistance to rapid temperature changes and more uniform microstructure. It has fine-grained matrix with uniformly distributed pores and micro cracks, so it has better resistance to rapid temperature changes than cast bricks. Its high temperature properties are between cast brick and directly bonded brick. 

 

It is made of the magnesia sand and the chrome ore by completely melting them in an electric arc furnace, then casting the melt into a mold and cooled into a solid. During the solidification process, stable periclase and spinel are generated and form a fine crystalline structure, so it has better high temperature strength and slag resistance. Like fused magnesia chrome, cast brick has highly directly bonded and a large amount of spinel desolventizing phase. It has dense structure which is difficult to penetrate by the molten slags, so it has better slag resistance than the directly bonded brick but poor thermal shock resistance than the above two bricks.