Sulfate process

"The sulphate process was the first commercialized technology to obtain the titanium dioxide pigment. In this process, ilmenite (40%-60% TiO₂) or titanium slag (72%-85% TiO₂) is digested with concentrated sulphuric acid (98%). A highly-exothermic reaction is initiated by the addition of measured quantities of steam, water and diluted sulphuric acid. If the feedstock used is ilmenite based, a reduction step is required in which iron is added to convert any ferric ions (Fe³⁺) to the ferrous (Fe²⁺) form to aid separation later in the process. Therefore to ensure that all the iron is in dissolution, the liquor is passed through scrap metal (iron reduction step). Then, it passes to a clarification tank where the undissolved ore and solids are allowed to settle. The titanium liquor is then concentrated and hydrolyzed to titanium dioxide hydrated.  The titanium dioxide hydrated precipitates from the ferrous sulphate and sulphuric acid is separated by filtration. The iron sulphate is separated from the titanium dioxide production process liquors by concentration and cooling. After filtration, the hydrated titanium dioxide slurry is sent to a calciner, where the titanium dioxide crystals grow to their final crystalline size and residual water and H₂SO₄ are removed. The dried titanium dioxide is sent to a finishing phase, which involves any required milling and or chemical treatment, such as surface coating with silica or alumina. Further processing (finishing), is then analogous to the chloride process involving chemical surface treatments (coating), milling and drying operations. About one ton of raw material (ilmenite or ilmenite + slag) is required to produce 0.5 tons of titanium dioxide pigment. Finally, we can say that he sulphate process uses a simpler technology and lower grade and cheaper raw materials to produce a form of pigment called anatase (tetragonal, near octahedral), which is preferred over the pigment from the chloride process for use in papers, ceramics and inks. However, the traditional sulphate process produce lower quality products for most applications and large quantities of waste iron sulphate, due to the high concentration of iron presents in the ilmenite used. The exhaustion of some large deposits exploited during decades have led to some new deposits being explored for its potential working but the trend is to feed the TiO₂ sulphate sites containing ilmenite with lower TiO₂ than in the past." [1]

The general equation for the digestion reaction (dissolution of the raw material):

FeTiO₃ + 2 H₂SO₄ → TiOSO₄ + FeSO₄ + H₂O

The general equation for the TiO₂ precipitation reaction:

TiOSO₄ + H₂O → TiO₂n∙H₂O + H₂SO₄

The general equation for the TiO₂ calcination and conditioning:

TiO₂n∙H₂O → TiO₂+ nH₂O