Controlled Hydrolysis of TiO2 from HCl Digestion Liquors of Ilmenite

Traditionally, industrial scale production of the TiO2 pigment is achieved by hydrolysis from H2SO4 solution or by hydrolysis of TiCl4. However, the H2SO4 route produces FeSO4 waste, which is problematic, and the TiCl4 route requires a high grade rutile feedstock or chemically upgraded ilmenite (FeTiO3). Here, we investigate a direct route from ilmenite to TiO2 using aqueous HCl. New Zealand ilmenite digested in 35 wt % HCl to achieve a solution containing typically 1.18 mol kg–1 Fe(aq)2+ and 1.14 mol kg–1 Ti(aq)4+ was hydrolyzed under reflux, after seed preparation in water, or with phosphoric or citric acid. The structure of the seed was determined by Raman spectroscopy and X-ray powder diffraction using pair distribution function analysis, the latter enabling the identification of short-range order in poorly crystalline materials. TiO2 hydrate was precipitated from HCl in either the anatase or the rutile structure. Unlike from H2SO4, the natural structure formed without the use of structure determining agents is rutile. However, seed preparation using 0.4 mol H3PO4 per mole of Ti (resulting in 0.35 wt% H3PO4 in the hydrate) results in anatase hydrate formation. Sodium citrate or citric acid addition also seed anatase hydrate. The mechanism for polymorph control may be kinetic rather than a structural template or surface adsorption. This process has the potential to be used for the commercial scale production of the TiO2 pigment. Anatase hydrate has the advantage that traces of iron may be more readily removed by washing than from rutile precipitate, making the HCl process from ilmenite feasible for pigment grade material.