**1. Introduction**

Titanium dioxide (TiO2) is the most widely used titanium product, being employed as pigment, as filler in the paper, plastic, and rubber industries, and as flux in glass manufacture. Synthetic rutile (SR) is one of the major sources of TiO2 [1–3]. Industrial processes usually involve the initial preparation of titanium dioxide, followed by titanium metal production [4,5]. Several commercial or proposed processes are available to produce SR or high-grade titanium slag from ilmenite which is mainly composed of FeTiO3. These involve a combination of thermal oxidation and reduction by roasting, leaching, and physical separation steps. Iron is converted to soluble ferrous or elemental forms by reduction at a high temperature, followed by acid leaching to obtain a SR product.

Ilmenite generally contains impurities such as iron, which leads to its low grade and cannot be directly used. Synthetic rutile is a kind of titanium rich raw material with the same composition and structural properties as natural rutile by separating most iron components from ilmenite. An industrial process for upgrading ilmenite to SR is typically represented by the Becher process [6–8]. Ilmenite contains 40–65% titanium as TiO2, with the rest being iron oxide. The Becher process removes the iron oxide, leaving a residue of SR that contains more than 90% TiO2. The Becher process comprises

four major steps: oxidation, reduction, aeration, and acid leaching [9,10]. Oxidation involves heating the ilmenite in a rotary kiln with air to convert the contained iron to iron oxide:

$$\text{4FeTiO}\_{3(s)} + \text{O}\_{2(g)} \rightarrow 2\text{Fe}\_2\text{O}\_3 \cdot \text{TiO}\_{2(s)} + \text{TiO}\_{2(s)}\tag{1}$$

This allows for the use of a wide range of ilmenite materials with various Fe(II) and Fe(III) contents for the subsequent step. Reduction is performed in a rotary kiln with a mixture of pseudobrookite (Fe2O3·TiO2) and coal at about 1200 ◦C to reduce iron oxide to metallic iron:

$$\text{Fe}\_2\text{O}\_3 \cdot \text{TiO}\_{2(s)} \cdot + \text{3CO} \rightarrow 2\text{Fe}\_{(s)} + 2\text{TiO}\_{2(s)} \; + \text{3CO}\_{2(g)} \tag{2}$$

Metallic iron is then oxidized and precipitated from the solution as a slime in an aeration or 'rusting' step in large tanks using 1% ammonium chloride solution at 80 ◦C:

$$\text{4Fe}\_{(s)} + \text{3O}\_{2(g)} \to \text{2Fe}\_2\text{O}\_3\tag{3}$$

The finer iron oxide is then separated from the larger SR particles. When most of the iron oxide is removed, the residual portion is leached using 0.5 M sulfuric acid and then separated from the SR. In the aeration leaching step, the removal of metallic iron from the reduced ilmenite (RI) grains is essentially a redox reaction, which can be represented by the following half-cell reactions:

$$2\text{Fe} \rightarrow 2\text{Fe}^{2+} + 4\text{e}^- \text{ (anodic reaction)}\tag{4}$$

$$\text{H}\_2 + 4\text{H}^+ + 4\text{e}^- \rightarrow 2\text{H}\_2\text{O} \text{ (cathodic reaction)}\tag{5}$$

The oxidation of ferrous ions is then given by:

$$2\text{Fe}^{2+} + 4\text{OH}^- + 1/2\text{O}\_2 \rightarrow \text{Fe}\_2\text{O}\_3 \cdot \text{H}\_2\text{O} + \text{H}\_2\text{O} \tag{6}$$

In current industrial practice, the aeration step of the Becher process can take as long as 22 h to complete [11]. Some reports show that the rusting process can be accelerated by improving aeration [12] or by adding a component such as acetic, tartaric, or citric acid [13,14]; a ligand, such as ethylenediammonium dichloride; various phenolic and aldehyde compounds, such as pyrogallol, saccharin, starch, and formaldehyde; sugars, such as glucose and sucrose; and water-soluble redox catalysts, namely, methyl viologen dichloride and diquat dibromide [11,15–18]. These additives differ in effectiveness and cost. Most prior research was carried out at relatively high temperature (70 ◦C). Other related hydrometallurgical processes include, for example, ultrasonic-assisted acid leaching for iron removal from quartz sand [19–21] and the goethite process for iron removal from hydrochloric acid leaching solution of reduced laterite [22].

In the present work, we report a study of aeration leaching of reduced ilmenite at room temperature. Aeration leaching experiments using the hydrochloric acid system with oxygen injection at room temperature are rarely studied. The effects of hydrochloric acid and ammonia chloride in improving the aeration efficiency were evaluated. The effects of leaching parameters, including stirring speed and NH4Cl and hydrochloric acid concentrations, were investigated. Through the above research, the method of strengthening the aeration process at room temperature is explored to provide a new way to obtain high-grade SR.

#### **2. Materials and Methods**

#### *2.1. Materials*

A Chinese source of reduced ilmenite, produced by carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1100 ◦C, was used. The chemical composition and particle size is reported in Table 1 and Figure 1, respectively. MFe stands for metal iron and TFe stands for all iron in Table 1. The composition of reduced ilmenite and SR obtained by XRF analysis and MFe was determined by chemical titration. Figure 1 shows that almost 80% of the particles were distributed between 90 and 400 μm, with a mode value of about 200 μm and a uniform distribution.


**Figure 1.** Particle size distribution of reduced ilmenite.
