Combined production of synthetic rutile in the sulfate TiO2 process

This paper describes a novel process for preparation of synthetic rutile using an intermediate, sulfated ilmenite from the sulfate TiO2 process as the feedstock. The synthetic rutile can be obtained by selective thermal decomposition of the sulfated ilmenite, followed by targeted leaching for removal of various impurities. The results of the decomposition unit showed that almost all the TiOSO4 in the sulfated ilmenite decomposed to TiO2, while the iron component mainly existed in the form of sulfates in the optimal thermal decomposition conditions, i.e., a roasting temperature of 540 °C and a roasting time of 120 min under air flow or in stagnant air or a roasting temperature of 510 °C and a roasting time of 120 min under nitrogen flow. The thermal decomposition can be divided into three stages. The sulfates of titanium and iron in the sulfated ilmenite were first decomposed to TiO2 and water-insoluble FeOHSO4, respectively, at a temperature less than 500 °C. The FeOHSO4 was further converted into water-soluble Fe2O(SO4)2 at 500-560 °C. Finally, the Fe2O(SO4)2 was decomposed to Fe2(SO4)3 and Fe2O3 at a temperature above 560 °C. The water-soluble metal sulfates, the water-insoluble FeOHSO4/Fe2O3 and the SiO2 in the TiO2-containing slag can be removed through leaching by using water, dilute sulfuric acid and sodium hydroxide, respectively. The results showed that 70-85% of the iron, as well as a majority of the magnesium, calcium, etc. impurities, could be leached by water, and up to 92% of the total iron could be removed after the subsequent acid leaching in a 15 wt% H2SO4 solution, while the silicon removal reached 65% in a 5 wt% NaOH solution. A synthetic rutile with a TiO2 content over 90 wt% and total MgO + CaO less than 1 wt% was obtained under the optimal conditions listed above. The present process can be integrated with the sulfate TiO2 process, producing titania pigment and synthetic rutile simultaneously. The advantages of the TiO2 beneficiation process include the moderate reaction conditions, the capability of recycling the H2SO4 completely and the recovery of a large part of the iron, as well as comprehensive utilization of the waste sulfuric acid discharged from the sulfate TiO2 production process.