*3.1. E*ff*ect of Solution Composition*

The results are shown in Figure 3. With increasing NH4Cl concentration from 2% to 8%, the metallic iron content in the SR increased from 1.85% to 6.75%, the total iron content decreased from 19.63% to about 16%, and the TiO2 content increased from 64.97% to about 70%. It can be deduced that an increase in NH4Cl concentration is not conducive to the aeration leaching process. When the concentration of 2% ammonium chloride was the same as that in Reference [21] and the reaction time was reduced by one hour, the removal rate of MFe in this paper was as high as 98.15%, while the removal rate of iron in Reference [21] was less than 50%. The TFe content was close to 20% and this is not sufficient for the aeration leaching products.

**Figure 3.** Effect of ammonium chloride concentration on composition of synthetic rutile.

Aeration leaching tests using 2% NH4Cl with 0% to 3% hydrochloric acid were then carried out. The results are shown in Figure 4. For the same reaction time and other conditions, the contents of TFe and MFe in the SR monotonically decreased with an increase in hydrochloric acid concentration from 1% to 3%, while the TiO2 content increased. The addition of hydrochloric acid helped to improve the aeration leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. Similar to Figure 3, the reaction rate of MFe was high but there was over 10% content of TFe in the aeration leaching products.

**Figure 4.** Effect of 2% NH4Cl with hydrochloric acid on composition of synthetic rutile.

The aeration leaching was better with the addition of hydrochloric acid than with the NH4Cl alone, so pure hydrochloric acid was considered for comparative analysis. The hydrochloric acid concentration was selected as 1.5% (m/v). The resulting MFe and TiO2 contents in the SR are shown in Figure 5.

In the hydrochloric acid system, the TFe content in the SR was about 4%, compared with over 10%, and even up to 18%, in the NH4Cl system. This proved that hydrochloric acid is better for aeration leaching than ammonium chloride. However, it is more difficult to store and transport hydrochloric acid, and the accumulation of chloride ion is not conducive to recycling of the corrosion solution. Therefore, comprehensive consideration is needed to select the best aeration leaching solution.

**Figure 5.** Effect of hydrochloric acid on composition of synthetic rutile.

#### *3.2. E*ff*ect of Stirring Speed*

Stirring is one of the most important factors in mixing processes in the chemical industry and metallurgy. The purpose is to mix evenly, accelerate the dissolution, or accelerate the reaction process. Generally, too slow a stirring speed will lead to uneven mixing and too fast a stirring speed can damage the product. High-speed mixing consumes more electric energy, which results in an increase in production cost. Selection of an appropriate mixing speed is, therefore, essential [23]. The effect of stirring speed on the removal of metallic iron from the reduced ilmenite was investigated by varying the impeller speed in the range of 400 to 1000 rpm. The concentration of hydrochloric acid was 1.5% m/v, the reaction time was 4 h, and the aeration gas was present in excess.

As shown in Figure 6, the metallic iron remaining in the SR decreased from 6.95% at 400 rpm to 0.49% at 800 rpm, corresponding to a reduction in metallic iron content of 6.46% points. The metallic iron content increased to 2.85% at 1000 rpm and the iron content increased by 2.36% compared with the value of 2.85% at 800 rpm.

**Figure 6.** Effect of stirring speed on metallic iron content of synthetic rutile.

The presence of agitation can break up bubbles, increase the specific surface area of bubbles, and accelerate mass transfer from the gas phase to the liquid phase. Agitation can also promote uniform suspension of reduced ilmenite particles, increase the liquid–solid contact area, accelerate the internal diffusion process, and prevent corroded iron ions from reducing ilmenite particles in an in situ reaction. The vortex will be formed at high speed, which will lead to uneven mixing of gas, liquid, and solid.

#### *3.3. Phase Analysis*

Figure 7 presents XRD spectra of reduced ilmenite before and after iron removal by aeration under the conditions: room temperature, t = 4 h, excess oxygen, 1.5% hydrochloric acid. The major phases in the reduced ilmenite before iron removal were Fe, TiO2, and FeTi2O5. The diffraction peaks of FeTi2O5 and TiO2 were strong. Peaks for the metallic iron phase were not to be found in the sample after aeration leaching, but diffraction peaks of FeO(OH) were detected. The diffraction peaks of FeO(OH) and TiO2 had the same intensity, which indicated that metallic iron transformed into FeO(OH). There were just two main phases in the sample after the aeration leaching process. These results indicate that the transformation of reduced ilmenite into rutile was achieved under these experimental conditions.

**Figure 7.** X-ray diffraction patterns of reduced ilmenite before and after iron removal.

Philips ssx-550 scanning electron microscope (SEM) images of the sample before and after aeration leaching are shown in Figure 8. There are obvious differences between the raw material and the product of the aeration leaching process: the sample before aeration leaching was compact and we could not find holes in the surface; after aeration leaching, the interior was full of holes, giving rise to a network structure, which maintained the sample integrity. The holes are attributed to the transformation of metallic iron into iron oxide by the aeration leaching reaction and its removal from the interior of sample.

(**b**)

**Figure 8.** *Cont.*

**Figure 8.** Scanning electron micrographs of sample (**a**) before and (**b**) after aeration leaching process (**c**) after aeration leaching process (enlarged).
