The chemical composition of magnesia-alumina spinel brick has an important influence on its properties. The suitable chemical composition of spinel is 8%~20%, calcium oxide 0.5% ~ 65%, iron oxide 30.2%~8% and silicon dioxide.
Development history:
The research on the synthesis of magnesia-alumina spinel from MgO-containing raw materials and AL2O3 raw materials (calcined alumina or bauxite) began as early as the 1930s. 1939 magnesia-alumina spinel was synthesized from industrial alumina and activated alumina for brick making. In the 1940s, it was found that adding alumina powder to magnesia can improve its thermal shock stability, which is actually magnesia brick combined with magnesia-alumina spinel. At that time, this kind of magnesia brick was mainly used in cement kilns, but later it was replaced by magnesia-chrome refractories except kilns for producing white cement.
The main reasons are: first, the development of magnesia manufacturing technology improves the purity of magnesia, and magnesia with low silicate phase content also has good spalling resistance, so magnesia bricks with good thermal shock stability can be produced without adding alumina; Second, foreign high-quality chromite is rich in resources, and the produced magnesia-chrome refractories have good thermal shock stability and corrosion resistance.
Due to the lack of high-quality chromite resources in China, magnesia bricks containing magnesia-alumina spinel (magnesia-alumina bricks) have been studied since 1950s. Alumina or super-grade bauxite is added to the matrix of magnesia brick, and magnesia-alumina spinel is formed in the matrix during firing. Replacing silicon brick with magnesia-alumina brick improves the life of open hearth furnace top and the operation rate of open hearth furnace.
Although magnesia-alumina spinel refractories have outstanding slag resistance, spalling resistance and better creep resistance compared with magnesia-chrome bricks, spinel was rarely used as raw material to produce alkaline products in Europe and America until the 1970s. At the end of 1970s, Japan first realized that magnesia-chrome bricks reacted with alkali in cement clinker to produce soluble K2CrO4 and (K, Na)Cr(SO4)2, which were harmful to human health (chromium pollution). Secondly, Japan also found that the service life of ordinary magnesia-chrome bricks in the transition zone and the front end of firing zone of modern cement kiln is not ideal. Therefore, Japan began to use spinel refractories in cement industry in 1976.
Another important reason why many countries popularized spinel refractories in cement kilns at the end of 1970s was that the fuel in cement kilns was changed from burning oil to burning coal. The atmosphere in the kiln was unstable, and the valence state of iron in the kiln lining would change, which led to the increase of the porosity of the kiln lining, the weakening of the structure and the easy damage. Magnesium aluminate spinel products with low iron content can avoid this phenomenon.
The production of high-purity sintered spinel started around 1980. It makes the price of spinel refractory lower than that of fused spinel; Due to the purity of raw materials, the properties of spinel refractories have also been improved. It is not only used in cement rotary kiln, but also used in regenerator, flat roof and ladle lining of glass kiln, and has achieved good results. The consumption of magnesia-alumina spinel is increasing.
In the late 1980s, China also conducted a lot of research and trial-production on the synthesis of spinel. Especially in view of the rich high-quality magnesite and bauxite resources in China, bauxite-based spinel has been developed and used in steel lining bricks and castables, and certain results have been achieved. At present, China is also vigorously strengthening the research and production of various high-tech spinel raw materials and products.