Purification of pigment grade titanium dioxide by processing in molten salts

摘要

An alternative method of purifying the mineral rutile (TiO_2) 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%TiO_2) containing transition metal oxides (Fe_2O_3, SiO_2, ZrO_2, Cr_2O_3, V_2O_5 and Al_2O_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 TiO_2 was enhanced by partial replacement of chloride by fluoride in the melt to allow the complex ion TiF_6~(2-) to dominate the titanium speciation. This had the additional advantage of minimising losses of Ti as volatile TiCI_4.