*3.1. Effect of CaCl<sup>2</sup> Dosage*

Figure 1 shows the effect of CaCl<sup>2</sup> dosage on Co volatilization percentage in the CoO– Fe2O3–CaCl<sup>2</sup> and CoO–SiO2–CaCl<sup>2</sup> systems. The Co volatilization percentage obviously increases as the CaCl<sup>2</sup> dosage in the CoO–SiO2–CaCl<sup>2</sup> system is strengthened at 1173 K, and slightly increases with increasing CaCl<sup>2</sup> dosage at 1273 K and 1373 K. Similar trends are observed in the CoO–Fe2O3–CaCl<sup>2</sup> system. It should be noted that the Co volatilization percentages in the CoO–SiO2–CaCl<sup>2</sup> system are much larger than those in the CoO–Fe2O3– CaCl<sup>2</sup> system.

**Figure 1.** Effect of CaCl<sup>2</sup> dosage on the Co volatilization percentage.

Chlorination could be divided into direct chlorination and indirect chlorination using calcium chloride. It reacts directly with cobalt oxide in the former case, which could be represented as follows:

$$\text{CaCl}\_2 + \text{CoO} = \text{CoCl}\_2 + \text{CaO}\_2 \tag{2}$$

CaCl<sup>2</sup> firstly reacts with oxygen to release Cl<sup>2</sup> in the latter case, followed by cobalt oxide being chlorinated by Cl2. It could be expressed as follows:

$$2\text{CaCl}\_2 + \text{O}\_2 = 2\text{CaO} + 2\text{Cl}\_2 \tag{3}$$

$$\text{2CoO} + \text{2Cl}\_2 = \text{2CoCl}\_2 + \text{O}\_{2'} \tag{4}$$

Figure 1 shows that the Co volatilization percentage of the CoO–Fe2O3–CaCl<sup>2</sup> system is not larger than 12.1%, even with a higher CaCl<sup>2</sup> dosage, and it becomes much larger in the CoO–SiO2–CaCl<sup>2</sup> system. This indicates that SiO<sup>2</sup> is more beneficial to the chlorination– volatilization of cobalt oxide compared with Fe2O<sup>3</sup> in both the cases of direct and indirect chlorination. The direct chlorination of cobalt oxide by calcium chloride, in the presence of SiO2, could be expressed as follows:

$$\text{CaCl}\_2 + \text{CoO} + \text{SiO}\_2 = \text{CoCl}\_2 + \text{CaSiO}\_3 \tag{5}$$

The generation of Cl<sup>2</sup> by the decomposition of CaCl2, in the presence of SiO2, could be expressed as follows:

$$\text{2CaCl}\_2 + \text{O}\_2 + \text{2SiO}\_2 = \text{2CaSiO}\_3 + \text{2Cl}\_2 \tag{6}$$

The standard Gibbs free energies of Equations (2)–(6) are calculated according to the data from [17], and are shown in Figure 2. Equations (7)–(9) in Section 3.4 are also listed in Figure 2. CoCl<sup>2</sup> is considered to be in the gaseous state. The standard Gibbs free energies

of Equations (5) and (6) are much lower than those of Equations (2) and (3), respectively, which means that the chlorination of cobalt oxide is promoted in the presence of SiO2. A larger CaCl<sup>2</sup> dosage is helpful to improve the Co volatilization percentage in the presence of SiO<sup>2</sup> in both the cases of direct and indirect chlorination.

**Figure 2.** The standard Gibbs free energy of reactions: (**a**) solid or liquid CaCl<sup>2</sup> ; (**b**)gaseous CaCl<sup>2</sup> .
