Abstract
The predominant phase in commercial high-grade solidified titania slags is a compound with the pseudobrookite or M3 O5 structure in which a substantial proportion of the titanium is in the trivalent oxidation state. Cooling of the slag in air results in the oxidation of the pseudobrookite to form phases that are associated with the disintegration of the solid slag. The presence of excessive amounts of fine-grained oxidised material renders it unsuitable for subsequent fluidised bed chlorination for the production of TiO2 pigment. Depending on the degree of reduction, the pseudobrookite phase has a composition with variable iron content. In high-grade slag, the stoichiometry of this phase is approximating that of (Fe0.27 Mg0.07 Al0.04 Ti3+ 1.35Ti4+ 1.35)O5. Single crystal structure analysis revealed no significant change in the room temperature structure at temperatures of 250°C and 350°C in air. At temperatures above 700°C, the oxidation of the slag is associated with the formation of mainly anatase, rutile, and oxidised M3O 5. No discernible disintegration takes place during this reaction. Between ∼550°C and 700°C, the reaction is a slow one involving the formation of anatase and oxidised M3O5. Very little disintegration is associated with this reaction. Below ∼550°C, the dominant reaction changes to a fast reaction involving the formation of a single disordered phase (MOx), related to both M3 O5 and anatase, and this results in extensive disintegration. This oxidation takes place in two steps. Initially, large cracks form parallel to the a-crystallographic axis direction in the precursor M3 O5 crystals. Triangular domains of the oxidised phase having a composition MOx form on the crack surfaces. Secondary cracks form along the c-axis directions, dissecting the domains. The cracks originating from adjacent domains join up to segment the crystal into small rectangular fragments as small as 10μm in size.
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