Preparation of synthetic rutile via selective sulfation of ilmenite with (NH4)2SO4 followed by targeted removal of impurities

This paper describes a novel, facile chemical pathway for preparing synthetic rutile from ilmenite. The pathway consists of two primary units, i.e., selectively sulfating ilmenite, which was realized via roasting ilmenite with (NH4)2SO4 followed by selective thermal decomposition of the sulfated ilmenite, and targeted leaching of the impurities. The effects of the process parameters were systematically investigated. The results showed that the optimum sulfation conditions were a mass ratio of (NH4)2SO4 to ilmenite of 14, temperature of 360 °C, and time of 120 min with a sulfation ratio of ~ 95%. The optimum thermal decomposition conditions were 480 °C in N2 atmosphere, and nearly all TiOSO4 were decomposed with co-decomposition of FeSO4 of 23%. For acid leaching, the optimum conditions were 2.5 wt% HCl, 98 °C and 120 min. Under those conditions, 94.2% iron was removed with a TiO2 dissolution loss < 1%. For alkali leaching, 67% SiO2 was removed in 5 wt% NaOH at 102 °C for 1 h. A synthetic rutile with a TiO2 content > 92 wt% and total MgO + CaO < 1.5 wt% was obtained. Based on these results, a schematic flowsheet was proposed. Additionally, it was found that the decomposition of FeSO4 mixed with TiOSO4 under N2 was inhibited due to its oxidation to a higher thermal stability Fe2(SO4)3 by oxygen emitted from the decomposition of TiOSO4. At the same time, TiOSO4 decomposition was promoted due to the immediate in situ consumption of oxygen by FeSO4. The synergetic effect might be responsible for the enhanced selectivity of sulfated ilmenite thermal decomposition.