The raw materials for producing magnesia-chromium bricks are natural raw materials, synthetic raw materials, industrial chromium oxide and industrial alumina, etc. Natural raw materials for regenerative brick: such as sintered magnesia of various grades, ordinary chromium ore and chromium concentrate with low impurity content. Artificial synthetic raw materials: co-sintered magnesia-chromium material prepared by fine grinding, mixing, compacting of magnesia and chromium concentrate and calcining at high temperature; fused magnesia-chromium material produced by electrofusion of magnesite and chromium ore; and fused magnesia-chromium sand. Synthetic materials should generally be raw materials with low impurity content.
The magnesia-chrome bricks with various names can be made from the above raw materials by different combinations and formulas. Moreover, the manufacturers are unwilling to explain the formula more. Therefore, the varieties and names of magnesia-chrome bricks are confusing and easy to confuse. According to the existing names of magnesia-chrome bricks at home and abroad, the following standardization can be roughly carried out.
(1) Ordinary magnesia-chrome bricks (i.e. silicate-bonded magnesia-chrome bricks), which are made of chrome ore and magnesia with more impurities (SiO2 and CaO). The firing temperature is not higher than 1550 C. The microstructural characteristics of bricks are that the refractory grains are bound by silicates.
The fine powder of this refractory brick is made by grinding, mixing, compacting and calcining magnesia-chromium ore with magnesia, which is still combined with silicate. But the performance is better than that of ordinary magnesia brick.
(2) Direct bonded magnesia brick is made from chromium concentrate with low impurity content and pure magnesia. The firing temperature is above 1700 C. The structural characteristics of this kind of refractory brick are that the grain of refractory is in direct contact with each other. Therefore, its high temperature performance, corrosion resistance and erosion resistance are better than ordinary magnesia-chrome bricks.
(3) Re-bonded magnesia-chromium bricks are commonly referred to as re-bonded magnesia-chromium bricks abroad, which are made entirely of co-sintered magnesia-chromium materials or fused magnesia-chromium materials (or some fused magnesia). In China, the magnesia-chromium bricks made of all fused magnesia-chromium materials are called recrystallized magnesia-chromium bricks. In order to be consistent with the international situation, it is advisable to use co-sintered magnesia-chrome bricks and fused materials to combine magnesia-chrome bricks or fused grains to combine magnesia-chrome bricks.