Red mud effect on dicalcium silicate polymorphism and sinter self-destruction prevention

The purpose of the paper is to provide a quantitative estimate of the red mud effect on bicalcium silicate β-phase stability based on the previously identified ability of the red mud to increase sinter strength. The work is aimed at solving the problem of reduction of the amount of stored red mud waste resulting from the production of alumina from bauxites. Red mud accumulators occupy large areas, damage the environment and cause periodical emergencies. A promising field of red mud utilization is associated with its use in ferrous metallurgy as a protector of spontaneous destruction of blast furnace sinter. In this case, the productivity of blast furnaces grows and coke consumption drops in equivalent to the reduction of the amount of fines in the charge (particles of 0-5 mm fraction). Spontaneous destruction of the flux iron ore sinter is explained by the occurrence of internal stresses, which are caused by the 10% increase in the volume of the crystal lattice of bicalcium silicate 2CaO^.SiO₂ (Ca₂SiO₄) and chemical instability of free lime as a sinter component. When cooled up to 675°C, bicalcium silicate undergoes a polymorphic transformation associated with the change in the crystal structure during the transition from β - to γ-modification and its volume increase. CaO grains interact with the moisture contained in the air and transform into Ca(OH)_2. The agglomeration process has been reproduced on example of the charge of given composition and its sintering at the temperature of 1200-1250°C. Self-destruction resistance of the briquetted charge after its heat treatment is determined by the content of the resulting fines (particles of 0-5 mm) after sinter cooling. Red muds as technogenic raw materials for alumina production at one of the domestic plants serve as a material for investigation. Research results have shown that the content of red mud of at least 3-5% wt in the charge with the ratio of CaO/SiO₂ = 2 provides the resistance of the resulting sintering products to self-destruction. All cake samples obtained with the addition of red mud in the amount of 3-5% wt. have the ability to prevent bicalcium silicate polymorphism and, therefore, prevent self-destruction of the agglomerated material. It is found that this effect is enhanced if the content of Al₂O₃ and Na₂O(K₂O) in red mud increases.

red mud, technogenic raw material, dicalcium silicate, flux sinter, polymorphism