Removal of silicon from highly acidic HCl medium to produce purified TiO2

- Equilibrium Si concentrations (suspended & dissolved) are reported for highly acidic conditions.
- Changes in temperature have very little effect on total equilibrium Si at concentrations ≥ 8 M HCl.
- The presence of Na and Fe(III) ions have very little effect on equilibrium Si conc. in HCl soln > 2 M.
- Dissolved Ti(IV) has a strong and surprising effect on Si (< 0.2 μm) concentration in highly acidic solutions
- A desilication strategy using silica-gel in ≅ 8 M HCl solution can achieve < 20 mg/L Si.

A low content of Si is an important characteristic of most Ti metal alloys, and low Si content in Ti sponge from the Kroll process is consistently achievable by the chlorination and distillation step. However, alternative routes are being explored to reduce Ti-metal production costs. One promising approach is a direct TiO2 reduction route called the HAMR (Hydrogen-Assisted Magnesio-thermic Reduction) process, which requires a low-cost, high-purity TiO2-preferably with a coarse particle size distribution (PSD). Although a satisfactory TiO2 can be produced by the chloride process, an alternative route called the ARH (Alkaline-Roasting and Hydrolysis) process has been developed, which produces an excellent feedstock for the HAMR process via hydrolysis in HCl medium. But, the purity of hydrolyzed TiO2 powder produced by the ARH process is affected by the purity of the Ti-bearing acid solution before the hydrolysis step, and Si has been determined to be the most detrimental impurity in the solution, since it will co-precipitate with titanium. Thus, the objective of this work is to clarify the behavior of Si in HCl solution, and to identify a viable approach for the desilication of Ti-bearing acid solution. The effects of acid concentration, temperature, and select ionic species on Si behavior in the acid medium were investigated, and a seeded silica-gel adsorption strategy was tested. The desilication treatment developed in this work has been shown to be capable of achieving Si solution concentrations of < 20 mg/L in acidic solution, corresponding to a Si concentration in the intermediate TiO2 of < 0.024% wt/wt, which is sufficient to meet ASTM standards for Si in the Ti-powder after direct reduction.