The Role of Silica in the Chlorination–Volatilization of Cobalt Oxide by Using Calcium Chloride

Abstract

The role of silica in the chlorination–volatilization of cobalt oxide, using calcium chloride, is investigated in this paper. It is found that the Co volatilization percentage of the CoO–Fe₂O₃–CaCl₂ system is not larger than 12.1%. Silica plays an important role in the chlorination–volatilization of cobalt oxide by using calcium chloride. In the CoO–SiO₂–Fe₂O₃–CaCl₂ system, the Co volatilization percentage is initially positively related to the molar ratio of SiO₂ to CaCl₂, and remains almost constant when the molar ratio of SiO₂ to CaCl₂ rises from zero to eight. The critical molar ratios of SiO₂ to CaCl₂ are 1 and 2 when the molar ratios of CaCl₂ to CoO are 8.3 and 16.6, respectively. The Co volatilization percentage remains almost constant with the increase in CaO concentration, and decreases when Al₂O₃ and MgO are added. Ca₂SiO₃Cl₂ is determined after roasting at 1073 K and 1173 K, and disappears at temperatures in excess of 1273 K in the calcines from the CoO–SiO₂–CaCl₂ system. CaSiO₃ always exists in the calcines at temperatures in excess of 973 K.

1. Introduction

When roasted at a high temperature, nonferrous metal oxides are converted into corresponding chlorides and volatilize in the form of gaseous chlorides in the presence of chlorinating agents. The chlorination–volatilization method has been used for the recovery of valuable metals from slags or refractory ores, such as Au, Ag, Cu, Pb, and Zn [1,2,3,4,5,6].

In regards to cobalt recovery, many works have concentrated on chloride roasting as the pretreatment to convert cobalt compounds into soluble chloride, followed by a subsequent hydrometallurgical step [7,8,9,10,11,12,13,14]. The chloridizing agents used for chloride roasting include gaseous Cl₂ and HCl, and solid MgCl₂•6H₂O, NaCl, AlCl₃•6H₂O, and NH₄Cl. However, there are few investigations about the chlorination–volatilization of cobalt at present. It was reported that to obtain approximately 50% volatilization percentage, a chlorine consumption of approximately 2.5 times the stoichiometric amount was needed, in the case of producing iron ore pellets from pyrite cinders containing nonferrous metals by using chlorine [15].

Calcium chloride is a popular chloridizing agent because of its high stability and lack of toxicity in practice. The chlorination–volatilization of cobalt, using calcium chloride, was investigated in our previous paper [16]. The effects of different variables on the cobalt volatilization percentage were investigated, including flow rate, oxygen partial pressure and water vapor content of the carrier gas, roasting time, and temperature. The aims of this paper are to investigate the effects of silica, other gangues (Al₂O₃, CaO, and MgO) and CaCl₂ dosage on the Co volatilization percentages during the chlorination–volatilization of cobalt oxide, and on the phases of calcines after roasting.

2. Materials and Methods

Cobalt-containing slags were prepared using reagent-grade CoO, SiO₂, Fe₂O₃, Al₂O₃, MgO and CaO powders. Reagent-grade anhydrous CaCl₂ was adopted as the chlorinating agent. All the reagents were precisely weighed according to the compositions shown in Table 1 and pressed into briquettes after sufficient mixing. The effects of CaCl₂ dosage on the Co volatilization percentage were investigated from No.1 to No.5. The effects of the molar ratio of SiO₂ to CaCl₂ on the Co volatilization percentage were also investigated from No.4 to No.14. Finally, the effects of Al₂O₃, MgO and CaO additions on the Co volatilization percentage were investigated from No.14 to No.20.

Table 1. Sample compositions and CaCl₂ dosage.

No.	Slag Composition (Mass%)						CaCl2 Dosage (Mass%)	Molar Ratio of CaCl2 to CoO	Molar Ratio of SiO2 to CaCl2
	CoO	SiO2	Al2O3	MgO	CaO	Fe2O3
1	1	0	0	0	0	99	12.3	8.3	0
2	1	0	0	0	0	99	18.5	12.5	0
3	1	0	0	0	0	99	24.6	16.6	0
4	1	99	0	0	0	0	12.3	8.3	14.9
5	1	99	0	0	0	0	24.6	16.6	7.7
6	1	6.66	0	0	0	92.34	24.6	16.6	0.5
7	1	13.33	0	0	0	85.67	24.6	16.6	1
8	1	26.66	0	0	0	72.34	24.6	16.6	2
9	1	53.31	0	0	0	45.69	24.6	16.6	4
10	1	3.33	0	0	0	95.67	12.3	8.3	0.5
11	1	6.66	0	0	0	92.34	12.3	8.3	1
12	1	13.33	0	0	0	85.67	12.3	8.3	2
13	1	26.66	0	0	0	72.34	12.3	8.3	4
14	1	53.31	0	0	0	45.69	12.3	8.3	8
15	1	53.31	10	0	0	35.69	12.3	8.	8
16	1	53.31	20	0	0	25.69	12.3	8.3	8
17	1	53.31	0	10	0	35.69	12.3	8.3	8
18	1	53.31	0	20	0	25.69	12.3	8.3	8
19	1	53.31	0	0	10	35.69	12.3	8.3	8
20	1	53.31	0	0	20	25.69	12.3	8.3	8

The chlorination–volatilization experiments were carried out in a muffle furnace. Furnace temperature was set to increase at 25 K/min. An alumina boat (length: 60 mm, width: 30 mm) containing approximately 10 g of briquettes was located at the center of the furnace at approximately 873 K. The furnace was turned off after the sample was held at the desired temperatures for 1 h, which was sufficient to reach the maximum cobalt volatilization percentage according to previous work [16]. There was an air inlet under the thermocouple and an air outlet at the top of the furnace hearth to connect with the atmosphere. Hot air in the furnace hearth escaped through the outlet and was exhausted by a negative pressure fan above the furnace into the air after alkaline solution treatment. Samples were cooled inside the furnace to approximately 873 K and then naturally cooled to room temperature.

Samples after roasting were prepared carefully for the chemical and phase analyses. The phases of calcines were detected by X-ray diffraction (XRD, X’PertPro, PANalytical, Almelo, The Netherlands) and identified by comparisons between diffraction peaks of XRD data and no less than three main characteristic peaks of substances. Cobalt concentration was measured by inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, PerkinElmer, Waltham, Massachusetts, MA, USA). The Co volatilization percentage was calculated according to Equation (1).

$$\eta =\frac{c_i{m}_i-c_f{m}_f}{c_i{m}_i}\times 100\%,$$

(1)

where c_i and c_f are Co concentrations of samples before and after roasting, respectively. m_i and m_f are the masses of the samples before and after roasting, respectively. The analysis results of cobalt concentrations are within ±0.015 mass%. Therefore, the errors of volatilization percentages are calculated to be within ±2.5% according to Equation (1).

3. Results and Discussions

3.1. Effect of CaCl₂ Dosage

Figure 1 shows the effect of CaCl₂ dosage on Co volatilization percentage in the CoO–Fe₂O₃–CaCl₂ and CoO–SiO₂–CaCl₂ systems. The Co volatilization percentage obviously increases as the CaCl₂ dosage in the CoO–SiO₂–CaCl₂ system is strengthened at 1173 K, and slightly increases with increasing CaCl₂ dosage at 1273 K and 1373 K. Similar trends are observed in the CoO–Fe₂O₃–CaCl₂ system. It should be noted that the Co volatilization percentages in the CoO–SiO₂–CaCl₂ system are much larger than those in the CoO–Fe₂O₃–CaCl₂ system.

Figure 1. Effect of CaCl₂ 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:

CaCl₂ + CoO = CoCl₂ + CaO,

(2)

CaCl₂ firstly reacts with oxygen to release Cl₂ in the latter case, followed by cobalt oxide being chlorinated by Cl₂. It could be expressed as follows:

2CaCl₂ + O₂ = 2CaO + 2Cl₂,

(3)

2CoO + 2Cl₂ = 2CoCl₂ + O₂,

(4)

Figure 1 shows that the Co volatilization percentage of the CoO–Fe₂O₃–CaCl₂ system is not larger than 12.1%, even with a higher CaCl₂ dosage, and it becomes much larger in the CoO–SiO₂–CaCl₂ system. This indicates that SiO₂ is more beneficial to the chlorination–volatilization of cobalt oxide compared with Fe₂O₃ 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:

CaCl₂ + CoO + SiO₂ = CoCl₂ + CaSiO₃,

(5)

The generation of Cl₂ by the decomposition of CaCl₂, in the presence of SiO₂, could be expressed as follows:

2CaCl₂ + O₂ + 2SiO₂ = 2CaSiO₃ + 2Cl₂,

(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. CoCl₂ 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 CaCl₂ dosage is helpful to improve the Co volatilization percentage in the presence of SiO₂ in both the cases of direct and indirect chlorination.

Figure 2. The standard Gibbs free energy of reactions: (a) solid or liquid CaCl₂; (b)gaseous CaCl₂.

3.2. Effect of the Molar Ratio of SiO₂ to CaCl₂

Figure 3 shows the effect of the molar ratio of SiO₂ to CaCl₂ on the Co volatilization percentage in the CoO–SiO₂–Fe₂O₃–CaCl₂ system. When the molar ratio of CaCl₂ to CoO is 8.3, the Co volatilization percentage increases significantly, as the molar ratio of SiO₂ to CaCl₂ rises from 0 to 1, and remains almost constant when the molar ratio of SiO₂ to CaCl₂ increases from 1 to 8 at all the desired temperatures. Similar trends are observed when the molar ratio of CaCl₂ to CoO is 16.6 at 1273 K and 1373 K. The Co volatilization percentage increases significantly and remains almost constant when the critical molar ratio of SiO₂ to CaCl₂ is two. However, it increases significantly as the molar ratio of SiO₂ to CaCl₂ is amplified at 1173 K.

Figure 3. Effect of the molar ratio of SiO₂ to CaCl₂ on the Co volatilization percentage.

Equation (5) is initially promoted with an increase in the molar ratio of SiO₂ to CaCl₂ from zero. The enhancement becomes weaker when the molar ratio of SiO₂ to CaCl₂ is large enough. As shown in Equation (6), SiO₂ plays an important role in the generation of Cl₂, by the decomposition of CaCl₂. This result is consistent with the works of Liu et al. [18], Zhu et al. [19], and Ding [20]. The Co volatilization percentage is very low when the molar ratio of SiO₂ to CaCl₂ is zero. Most of the CaCl₂ is lost in the form of volatilization in this case. This also means that the role of Fe₂O₃ in the generation of Cl₂, by the decomposition of CaCl₂, is much weaker than that of SiO₂. More Cl₂ is released when the molar ratio of SiO₂ to CaCl₂ is increased, which leads to an increase in the Co volatilization percentage. There is sufficient Cl₂ at a high molar ratio of SiO₂, resulting in a constant Co volatilization percentage.

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–CaCl₂ systems. The Co volatilization percentage remains almost constant with increasing CaO concentration, with a molar ratio of CaCl₂ to CoO of 8.3 and a molar ratio of SiO₂ to CaCl₂ of eight. This could be attributed to the sufficient amount of SiO₂ existing in the system. The molar ratio of SiO₂ to CaCl₂ is still larger than two, taking into consideration that some SiO₂ could react with CaO. However, it decreases with increasing Al₂O₃ 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 SiO₂ on the volatilization percentages of CoO, ZnO [19], Zn•Fe₂O₃ [19], Cu₂O [21], and CuO [21]. It should be noted that the molar ratios of CaCl₂ to ZnO, CaCl₂ to Zn•Fe₂O₃, CaCl₂ to Cu₂O, and CaCl₂ to CuO are much lower than the molar ratio of CaCl₂ to CoO in this study.

Figure 4. Effect of (a) Al₂O₃, (b) MgO and (c) CaO on Co volatilization percentage.

Table 2. The effects of Al₂O₃, MgO, CaO and SiO₂ on the volatilization percentage.

Phases	System	Al2O3	MgO	CaO	SiO2	Reference
CoO	CoO–SiO2–Fe2O3–Al2O3–CaCl2	Negatively				This study
	CoO–SiO2–Fe2O3–MgO–CaCl2		Negatively
	CoO–SiO2–Fe2O3–CaO–CaCl2			None
	CoO–SiO2–Fe2O3–CaCl2				Positively
ZnO	ZnO–Fe2O3–Al2O3–CaCl2	Negatively				[19]
	ZnO–Fe2O3–MgO–CaCl2		Negatively
	ZnO–Fe2O3–CaO–CaCl2			Positively
	ZnO–Fe2O3–SiO2–CaCl2				Positively
Zn•Fe2O3	Zn•Fe2O3–Fe2O3–Al2O3–CaCl2	Negatively				[19]
	Zn•Fe2O3–Fe2O3–MgO–CaCl2		Negatively
	Zn•Fe2O3–Fe2O3–CaO–CaCl2			Negatively
	Zn•Fe2O3–Fe2O3–SiO2–CaCl2				Negatively
Cu2O	Cu2O–Fe2O3–Al2O3–CaCl2	Negatively				[21]
	Cu2O–Fe2O3–MgO–CaCl2		Negatively
	Cu2O–Fe2O3–CaO–CaCl2			Negatively
	Cu2O–Fe2O3–SiO2–CaCl2				Negatively
CuO	CuO–Fe2O3–Al2O3–CaCl2	None				[21]
	CuO–Fe2O3–MgO–CaCl2		Negatively
	CuO–Fe2O3–CaO–CaCl2			Positively
	CuO–Fe2O3–SiO2–CaCl2				Positively

3.4. Phases of Calcines

Figure 5 shows the XRD results of the calcines from the CoO–Fe₂O₃–CaCl₂ system after roasting with a molar ratio of CaCl₂ to CoO of 16.6. Fe₂O₃, CaFe₂O₄, Ca₂Fe₂O₅, and CaCO₃ are identified in the calcines at 973 K, which is a temperature lower than the melting point of CaCl₂ (1055 K). The diffraction peaks of CaCO₃ 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–CaCl₂ system could be expressed as follows:

2CaCl₂ + O₂ + Fe₂O₃ = Ca₂Fe₂O₅ + 2Cl₂,

(7)

2CaCl₂ + O₂ + 2Fe₂O₃ = 2CaFe₂O₄ + 2Cl₂,

(8)

2CaCl₂ + O₂ + 2CO₂ = 2CaCO₃ + 2Cl₂,

(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 O₂ and CO₂ partial pressures are 0.209 and 3.1 × 10⁻⁴, respectively. The chlorine partial pressure is approximately 10⁻⁹ when the activity of solid or liquid CaCl₂ is set to one. This means that Equation (9) could theoretically take place when the chlorine partial pressure is less than 10⁻⁹. The equilibrated chlorine partial pressure becomes much larger when CaCl₂ changes from the solid or liquid state to the gaseous state.

$$\frac{\Delta G_9^0}{-RT}=\ln \frac{{(P_{{\mathrm{Cl}}_2}/P^0)}^2}{a_{{\mathrm{CaCl}}_2}{(P_{{\mathrm{CO}}_2}/P^0)}^2{(P_{{\mathrm{O}}_2}/P^0)}^{0.5}}or=\ln \frac{{(P_{{\mathrm{Cl}}_2}/P^0)}^2}{{(P_{{\mathrm{CaCl}}_2}/P^0)}^2{(P_{{\mathrm{CO}}_2}/P^0)}^2{(P_{{\mathrm{O}}_2}/P^0)}^{0.5}}$$

(10)

Figure 5. XRD results of calcines from the CoO–Fe₂O₃–CaCl₂ system.

Figure 6. The equilibrated chlorine partial pressure of the following reaction: 2CaCl₂ + O₂ + 2CO₂ = 2CaCO₃ + 2Cl₂.

Figure 7 shows the XRD results of the calcines from the CoO–SiO₂–CaCl₂ system after roasting with a molar ratio of CaCl₂ to CoO of 16.6. There is newly generated CaSiO₃, and unreacted SiO₂ and CaCl₂, at 973 K. Newly generated Ca₂SiO₃Cl₂ exists alongside CaSiO₃ at 1073 K and 1173 K. The formation reaction of Ca₂SiO₃Cl₂ could be expressed as follows:

4CaCl₂ + O₂ + 2SiO₂ = 2Ca₂SiO₃Cl₂ + 2Cl₂,

(11)

Figure 7. XRD results of calcines from the CoO–SiO₂–CaCl₂ system (a) at 973 K; (b) at 1073 K and 1173 K; (c) between 1273 K and 1473 K.

The diffraction peaks of Ca₂SiO₃Cl₂ disappear at temperatures in excess of 1273 K, and only newly generated CaSiO₃ and unreacted SiO₂ remain in the calcines. This could be attributed to the instability of Ca₂SiO₃Cl₂ at higher temperatures. The reaction could be expressed as follows:

2Ca₂SiO₃Cl₂ + O₂ + 2SiO₂ = 4CaSiO₃ + 2Cl₂,

(12)

Ca₂SiO₃Cl₂ was formed when the molar ratios of SiO₂ to CaCl₂ were two and one at 1023 K and 1073 K, respectively [22]. In this study, Ca₂SiO₃Cl₂ is formed when the molar ratio of SiO₂ to CaCl₂ 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 Ca₂SiO₃Cl₂ is formed when the molar ratio of SiO₂ to CaCl₂ is larger than one at temperatures between 1023 K and 1173 K.

CaSiO₃, CaFe₂O₄, Ca₂Fe₂O₅, and Ca₃Fe₂(SiO₄)₃ are generated in the CoO–SiO₂–Fe₂O₃–CaCl₂ system at 1273 K, with a molar ratio of CaCl₂ to CoO of 8.3 and a molar ratio of SiO₂ to CaCl₂ of eight [16]. CaAl₂Si₂O₈ and CaMg(SiO₃)₂ are generated after the addition of Al₂O₃ and MgO, respectively, as shown in Figure 8.

Figure 8. XRD results of calcines from the CoO–SiO₂–Fe₂O₃–(Al₂O₃/MgO)–CaCl₂ system at 1273 K.

4. Conclusions

The following conclusions could be drawn from this study:

• The Co volatilization percentage of the CoO–Fe₂O₃–CaCl₂ system is not larger than 12.1%.
• SiO₂ plays an important role in the chlorination–volatilization of cobalt oxide, using calcium chloride. In the CoO–SiO₂–Fe₂O₃–CaCl₂ system, the Co volatilization percentage is initially positively related to the molar ratio of SiO₂ to CaCl₂, and remains almost constant when the molar ratio of SiO₂ to CaCl₂ rises from zero to eight. The critical molar ratios of SiO₂ to CaCl₂ are 1 and 2 when the molar ratios of CaCl₂ to CoO are 8.3 and 16.6, respectively.
• The Co volatilization percentage remains almost constant with increasing CaO concentration, and decreases when Al₂O₃ and MgO are added, with a molar ratio of CaCl₂ to CoO of 8.3 and a molar ratio of SiO₂ to CaCl₂ of eight.
• Fe₂O₃, CaFe₂O₄, Ca₂Fe₂O₅, and CaCO₃ are identified in the calcines from the CoO–Fe₂O₃–CaCl₂ system at 973 K, and CaCO₃ disappears at temperatures higher than 1173 K.
• Ca₂SiO₃Cl₂ exists in the calcines from the CoO–SiO₂–CaCl₂ system at 1073 K and 1173 K, and disappears at temperatures in excess of 1273 K. CaSiO₃ always exists in the calcines at temperatures in excess of 973 K.