*2.2. Aeration Conditions*

The aeration leaching experiments were performed ina1L stirred reactor. Details of the experimental apparatus are illustrated in Figure 2. The inner diameter of the stirred reactor was 80 mm and the agitator was a four-blade propeller. The blade length as 30 mm.

**Figure 2.** Aeration leaching reactor.

The initial reaction mixture comprised 640 mL solution and 320 g reduced ilmenite, which were added to the stirred reactor. The solution contained different concentrations of ammonium chloride and/or hydrochloric acid. The pulp was stirred by a four-blade agitator. Aeration gas was then introduced and passed through the pulp for the entire duration of the experiment. After 4 h, fine iron oxides were separated from the SR by wet screening. Particles of iron oxides and SR were washed and dried for analysis. We used the content of metallic iron (MFe) remaining in the SR to measure the efficiency of aeration leaching: it was found that the lower the residual iron content, the better the effect.

Aeration leaching is a process of oxygen absorption corrosion of metal iron. Three kinds of corrosion systems are generally selected: ammonium chloride, ammonium chloride plus hydrochloric acid, and hydrochloric acid. It is recognized that the anion provided by hydrochloric acid can destroy the passivation film on the surface of metallic iron in the aeration process [23]. The role of NH4 <sup>+</sup> is to combine with ferrous ions in the ore particles to form a complex that cannot be separated, so as to prevent oxidation and hydrolysis in the ore particles. The complex immediately decomposes when encountering water, and so acts as a carrier. The effect of ammonium chloride was examined using concentrations of 2%, 4%, 6%, and 8% (m/v) NH4Cl. The stirring speed was 800 rpm. An ambient temperature was employed. Wet separation of the fine iron oxide from the coarse titanium mineral particles was done by using hydrocyclones and spiral classifiers. We measured the MFe and TiO2 contents in the SR after aeration leaching for 4 h.

The particle size of the samples was analyzed by a laser diffraction particle size analyzer (Bettersize V8.0, Dandong Baite Instrument Co., Ltd., Dandong city, China). The structure and morphology of the reduced ilmenite samples and the product after aeration leaching were characterized by an X-ray diffractometer (BRUKER Inc., Karlsruhe, Germany), applying Cu Kα radiation at 40 kV and 40 mA, with 2θ recording from 10◦ to 80◦ with a step size of 0.02◦ and a counting time of 0.1 s per step. The metallic iron content of the solids was determined by potassium dichromate titration in FeCl3 solution. Other elements were determined by a ZSX PrimusIV X-ray fluorescence spectrum (Japan Neo Confucianism Co., Ltd., Tokyo, Japan).

### **3. Results and Discussion**
