Patents

The characteristic process step is that bauxite at first is partially beneficiated with 5 to 15% cone. HCI at room temperature to remove one of the detrimental impurities, namely lime. In the next step, rice husk ash, which is a waste and abundantly available material, is added to this bauxite so that the final composition ranges between 70 to 80% AI2O3. The rest of the impurities of bauxite - Fe2O3 and TiO2 - enter into the lattice structure of mullite as a solid solution. AI3+ present in the octahedral site of mullite crystal is substituted by Ti4+, causing aluminum ion vacancies, which enhance the mass transport and sintering rate. The firing schedule is particularly controlled between 1050° to 1250°C, so that mullite formation is completed and the low melting phase from Fe2O3 and TiO2l cannot develop. The ultimate high alumina mullite aggregates thus developed are much superior compared to directly calcined bauxite in terms of physical, micro-structural and thermo-mechanical properties. The final product is characterized by 96% densification, compact and homogeneous microstructure with an average grain size of 3 micro-m, and refractoriness under a load value of 1600°C.