Effects of mechanical activation on the digestion of ilmenite in dilute H2SO4

The commercial sulfate process for pigment production uses concentrated sulfuric acid (>85 wt% H₂SO₄) as feeding material and discharges 8-10 tons of spend dilute acid (20 wt% H₂SO₄) per ton of product. Re-using spend acid to leach ilmenite can cut the waste emission and save fresh feeding acid. However, the leaching reaction with dilute acid is very slow and the digestion efficiency is fairly low. This paper describes a wet-milling process to enhance the dilute-acid leaching of ilmenite that makes it possible to produce TiO₂ pigment in a more environmentally benign routine. The leaching kinetic study of unmilled ilmenite, dry milled 60 min ilmenite and wet milled 60 min ilmenite was conducted by revision of the shrinking core model (SCM), incorporation of particle size distribution (PSD) into SCM. The results revealed that mechano-chemical activation method significantly increased the leaching efficiency of titanium from 36% to 76% by reducing the particle size and increasing the reaction contact area. On the other hand, the milling process increased the lattice deformation and amorphization of crystalline, which lowered the activation energies in the leaching process. Compared with dry milling operation, wet milling is more effective, the particle size distribution of wet-milled ilmenite was much narrower, smaller, and more uniform. Wet milling of ilmenite makes the leaching reaction with dilute acid (60 wt% H₂SO₄) practicable and the re-use of spend acid becomes possible and economical.