Purification of pigment grade titanium dioxide by processing in molten salts

An alternative method of purifying the mineral rutile (TiO₂) is reported. This offers potential savings in process energy and reduced waste when compared with current technology. The work reported here focused on refining rutile sand (95%TiO₂) containing transition metal oxides (Fe ₂O₃, SiO₂, ZrO₂, Cr ₂O₃, V₂O₅ and Al₂O ₃), which impart a strong colour to the mineral concentrate. The objective was to remove the colour giving oxides to produce rutile of over 99% purity (which is white in colour). Complete dissolution in molten salt (alkali chloride-fluoride) at 750°C allowed electroseparation of the transition metals between a graphite anode and a stainless steel cathode. The voltage maintained across the cell ensured removal of transition metal ions from the solution, with minimal loss of titanium. In this process, dissolution of TiO₂ was enhanced by partial replacement of chloride by fluoride in the melt to allow the complex ion TiF^2- ₆ to dominate the titanium speciation. This had the additional advantage of minimising losses of Ti as volatile TiCl₄.