Disintegration in high grade titania slags: low temperature oxidation reactions and associated fracture mechanics of pseudobrookite

Cooling of solidified titania slag in air results in the oxidation of a pseudobrookite phase (M₃O₅ where M = Fe, Ti) to form phases that are associated with the disintegration of the solid slag. Extensive disintegration of the slag at temperatures below 773 K is accompanied by the formation of a disordered MO_x phase, which is a unit cell intergrowth of anatase (TiO₂), and M₃O₅ structural elements. In situ X-ray diffraction measurement and Rietveld refinement of the pseudobrookite unit cell parameters yielded values for spontaneous strain associated with the oxidation of the slag. In the most reactive samples oxidised isothermally at 473 K there is 0·6% contraction along the a-axis, a 0·3% expansion along the b-axis and the c-axis expands by 1%, while still retaining the pseudobrookite structure. Values for the elastic constants and Young's moduli for M₃O₅ were calculated using the GULP crystal optimisation code. These values, together with experimental values for the spontaneous strain tensor and the fracture toughness of M₃O₅, were used to calculate the critical crack size for crack propagation. Crack sizes of 0·9–9 μm are sufficiently large to explain the cracking that preceded the extensive formation of MO_x and the subsequent segmentation of the solid slag into very small particle sizes. Lattice energy calculations of the compounds anatase (TiO₂), FeTi₂O₅, and FeTi₅O₁₁ with GULP showed that the latter is unstable by 173 kJ mole⁻¹ as compared to a mixture of anatase (TiO₂) and FeTi₂O₅ and will eventually revert to anatase (TiO₂) and M₃O₅.