*3.3. Effect of Al2O3, MgO and CaO*

Figure 4 shows the effects of Al2O3, MgO, and CaO on the Co volatilization percentage in the CoO–SiO2–(Al2O3/MgO/CaO)–Fe2O3–CaCl<sup>2</sup> systems. The Co volatilization percentage remains almost constant with increasing CaO concentration, with a molar ratio of CaCl<sup>2</sup> to CoO of 8.3 and a molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> of eight. This could be attributed to the sufficient amount of SiO<sup>2</sup> existing in the system. The molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> is still larger than two, taking into consideration that some SiO<sup>2</sup> could react with CaO. However, it decreases with increasing Al2O<sup>3</sup> and MgO concentrations under the same conditions. The reasons for this need further study. Table 2 shows the comparisons of the effects of Al2O3, MgO, CaO, and SiO<sup>2</sup> on the volatilization percentages of CoO, ZnO [19], Zn•Fe2O<sup>3</sup> [19], Cu2O [21], and CuO [21]. It should be noted that the molar ratios of CaCl<sup>2</sup> to ZnO, CaCl<sup>2</sup> to Zn•Fe2O3, CaCl<sup>2</sup> to Cu2O, and CaCl<sup>2</sup> to CuO are much lower than the molar ratio of CaCl<sup>2</sup> to CoO in this study.

**Figure 4.** Effect of (**a**) Al2O<sup>3</sup> , (**b**) MgO and (**c**) CaO on Co volatilization percentage.


**Table 2.** The effects of Al2O<sup>3</sup> , MgO, CaO and SiO<sup>2</sup> on the volatilization percentage.

#### *3.4. Phases of Calcines*

Figure 5 shows the XRD results of the calcines from the CoO–Fe2O3–CaCl<sup>2</sup> system after roasting with a molar ratio of CaCl<sup>2</sup> to CoO of 16.6. Fe2O3, CaFe2O4, Ca2Fe2O5, and CaCO<sup>3</sup> are identified in the calcines at 973 K, which is a temperature lower than the melting point of CaCl<sup>2</sup> (1055 K). The diffraction peaks of CaCO<sup>3</sup> disappear at temperatures higher than 1173 K, which could be attributed to the decomposition of CaCO3. The generation of Cl2, by the decomposition of CaCl2, in the CoO–Fe2O3–CaCl<sup>2</sup> system could be expressed as follows:

$$2\text{CaCl}\_2 + \text{O}\_2 + \text{Fe}\_2\text{O}\_3 = \text{Ca}\_2\text{Fe}\_2\text{O}\_5 + 2\text{Cl}\_2\tag{7}$$

$$2\text{CaCl}\_2 + \text{O}\_2 + 2\text{Fe}\_2\text{O}\_3 = 2\text{CaFe}\_2\text{O}\_4 + 2\text{Cl}\_2\tag{8}$$

$$\text{2CaCl}\_2 + \text{O}\_2 + \text{2CO}\_2 = \text{2CaCO}\_3 + \text{2Cl}\_2 \tag{9}$$

where Equation (9) occurs at temperatures no more than 1173 K. The equilibrated chlorine partial pressure of Equation (9) is calculated according to Equation (10), and is shown in Figure 6, where the O<sup>2</sup> and CO<sup>2</sup> partial pressures are 0.209 and 3.1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , respectively. The chlorine partial pressure is approximately 10−<sup>9</sup> when the activity of solid or liquid CaCl<sup>2</sup> is set to one. This means that Equation (9) could theoretically take place when the chlorine partial pressure is less than 10−<sup>9</sup> . The equilibrated chlorine partial pressure becomes much larger when CaCl<sup>2</sup> changes from the solid or liquid state to the gaseous state.

$$\frac{\Delta\mathcal{G}\_{\text{q}}^{0}}{\frac{-RT}{-RT}} = \ln \frac{\left(p\_{\text{C}\_{2}}/P^{0}\right)^{2}}{a\_{\text{CaC}\_{2}}\left(p\_{\text{CO}\_{2}}/P^{0}\right)^{2}\left(p\_{\text{O}\_{2}}/P^{0}\right)^{0.5}} \\ or \\ = \ln \frac{\left(p\_{\text{C}\_{2}}/P^{0}\right)^{2}}{\left(p\_{\text{CaC}\_{2}}/P^{0}\right)^{2}\left(p\_{\text{CO}\_{2}}/P^{0}\right)^{2}\left(p\_{\text{O}\_{2}}/P^{0}\right)^{0.5}} \\ \tag{10}$$

Figure 7 shows the XRD results of the calcines from the CoO–SiO2–CaCl<sup>2</sup> system after roasting with a molar ratio of CaCl<sup>2</sup> to CoO of 16.6. There is newly generated CaSiO3, and unreacted SiO<sup>2</sup> and CaCl2, at 973 K. Newly generated Ca2SiO3Cl<sup>2</sup> exists alongside CaSiO<sup>3</sup> at 1073 K and 1173 K. The formation reaction of Ca2SiO3Cl<sup>2</sup> could be expressed as follows:

$$4\text{CaCl}\_2 + \text{O}\_2 + 2\text{SiO}\_2 = 2\text{Ca}\_2\text{SiO}\_3\text{Cl}\_2 + 2\text{Cl}\_2\tag{11}$$

The diffraction peaks of Ca2SiO3Cl<sup>2</sup> disappear at temperatures in excess of 1273 K, and only newly generated CaSiO<sup>3</sup> and unreacted SiO<sup>2</sup> remain in the calcines. This could be attributed to the instability of Ca2SiO3Cl<sup>2</sup> at higher temperatures. The reaction could be expressed as follows:

$$2\text{Ca}\_2\text{SiO}\_3\text{Cl}\_2 + \text{O}\_2 + 2\text{SiO}\_2 = 4\text{CaSiO}\_3 + 2\text{Cl}\_2 \tag{12}$$

**Figure 5.** XRD results of calcines from the CoO–Fe2O3–CaCl<sup>2</sup> system.

**Figure 6.** The equilibrated chlorine partial pressure of the following reaction: 2CaCl<sup>2</sup> + O<sup>2</sup> + 2CO<sup>2</sup> = 2CaCO<sup>3</sup> + 2Cl<sup>2</sup> .

Ca2SiO3Cl<sup>2</sup> was formed when the molar ratios of SiO<sup>2</sup> to CaCl<sup>2</sup> were two and one at 1023 K and 1073 K, respectively [22]. In this study, Ca2SiO3Cl<sup>2</sup> is formed when the molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> is approximately 7.4 at 1073 K and 1173 K. Considering the results presented by Zhang [22], along with those from this study, it is suggested that the intermediate product Ca2SiO3Cl<sup>2</sup> is formed when the molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> is larger than one at temperatures between 1023 K and 1173 K.

CaSiO3, CaFe2O4, Ca2Fe2O5, and Ca3Fe2(SiO4)<sup>3</sup> are generated in the CoO–SiO2– Fe2O3–CaCl<sup>2</sup> system at 1273 K, with a molar ratio of CaCl<sup>2</sup> to CoO of 8.3 and a molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> of eight [16]. CaAl2Si2O<sup>8</sup> and CaMg(SiO3)<sup>2</sup> are generated after the addition of Al2O<sup>3</sup> and MgO, respectively, as shown in Figure 8.

**Figure 7.** XRD results of calcines from the CoO–SiO2–CaCl<sup>2</sup> system (**a**) at 973 K; (**b**) at 1073 K and 1173 K; (**c**) between 1273 K and 1473 K.

**Figure 8.** XRD results of calcines from the CoO–SiO2–Fe2O3–(Al2O3/MgO)–CaCl<sup>2</sup> system at 1273 K.
