Study On Direct Bonded Magnesia-Alumina Brick

Magnesia-chromite brick is a widely used basic refractory. It was discovered suddenlythat the Cr⁶⁺ ion can cause cancer in 1980s. So all the countries were looking for thesubstitute for the refractories containing chrome, but the effect is faint until now. Theresearch and development work is only on underway stage in our country.Using direct bonded magnesia-alumina brick to substitute for magnesia-chromitebrick was studied in this paper.In the experiment the dense spinel (MA50) was synthesizedby sintering using purer industrial raw materials including active MgO and a-Al₂O₃ powder.The results show that the reaction temperature of synthesis spinel is about 1000℃, and bymeans of controlling the ratio of MgO/Al₂O₃ (weight) at 1, adjusting the moulding pressureabout 100MPa, sintering at 1700℃and holding time 3h in stove, the MgO rich spinel canbe synthesized by 1 step sintering and its BD can come to upwards of 3.30 g·cm^-3.The experiment for brick samples adopted the aggregates and powders composition of6 to 4. The critical granularity is 3mm and the composition includes 3 classes, as follows3～2mm 30%, 2～0.9mm 15%, 0.9～0.088mm 15%. The matrix powders introduce thespinel (MA50) synthesized by former experiment and the optimal dosage is 10 %. Theother parts are mixed powders of active MgO and a-Al2O3 with the ratio of MgO/Al₂O₃equal to 1. The brick samples have excellent properties. The XRD and SEM show that themain phases are magnesia and spinel and the grains come into direct bond.Additives can obviously improve the properties of brick samples. The experimentintroduced three kinds of additives including metal Al powder, TiO₂ powder and ZrSiO₄powder and adopted orthogonal method to analyse the results. It shows that the influencingorder of additives on BD and AP: ZrSiO₄>TiO₂>Al powder; the influencing order ofadditives on strength: TiO₂>ZrSiO₄>Al powder. The SEM picture shows the grains inmatrix are tight and form direct bonded structure. Some spinel phase crystallized fromsolid solution or silicate liquid and especially in interspace the spinel grows well and joinstogether to form direct bond, which improves the strength and thermal shock resistance.