**4. Conclusions**

Thermodynamic calculations and high-temperature experiments confirmed that WO<sup>3</sup> can be effectively recovered from wolframite by the pyrometallurgical process. The recovery of tungsten from wolframite is much higher than that in the conventional process. SiO<sup>2</sup> can form liquid slag with the oxides of iron, manganese and sodium to enhance the decomposition of wolframite. However, excess SiO<sup>2</sup> consumes Na2O and reduces the decomposition rate. High Na2O addition and low temperature are beneficial for maximizing the recovery of tungsten. It is easy to decompose wolframite under reducing conditions where iron is present as Fe2+ .

**Author Contributions:** Conceptualization, B.Z.; methodology, B.Z. and L.X.; Experiments, L.X.; writing—original draft preparation, L.X.; writing—review and editing, B.Z.; supervision, B.Z.; project administration, B.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

**Peiwei Han 1,2,\* , Zhengchen Li 1,2, Xiang Liu 1,2 , Jingmin Yan 1,2 and Shufeng Ye 1,2,\***


**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–CaCl<sup>2</sup> 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–CaCl<sup>2</sup> system, the Co volatilization percentage is initially positively related to the molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> , and remains almost constant when the molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> rises from zero to eight. The critical molar ratios of SiO<sup>2</sup> to CaCl<sup>2</sup> are 1 and 2 when the molar ratios of CaCl<sup>2</sup> 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 Al2O<sup>3</sup> and MgO are added. Ca2SiO3Cl<sup>2</sup> 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–CaCl<sup>2</sup> system. CaSiO<sup>3</sup> always exists in the calcines at temperatures in excess of 973 K.

**Keywords:** chlorination–volatilization; cobalt oxide; calcium chloride; phases of calcines

## **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 [1–6].

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 [7–14]. The chloridizing agents used for chloride roasting include gaseous Cl<sup>2</sup> 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 CaCl<sup>2</sup> dosage on the Co volatilization percentages during the chlorination–volatilization of cobalt oxide, and on the phases of calcines after roasting.

**Citation:** Han, P.; Li, Z.; Liu, X.; Yan, J.; Ye, S. The Role of Silica in the Chlorination–Volatilization of Cobalt Oxide by Using Calcium Chloride. *Metals* **2021**, *11*, 2036. https:// doi.org/10.3390/met11122036

Academic Editors: Baojun Zhao, Jianliang Zhang, Xiaodong Ma and Felix A. Lopez

Received: 10 November 2021 Accepted: 10 December 2021 Published: 15 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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

Cobalt-containing slags were prepared using reagent-grade CoO, SiO2, Fe2O3, Al2O3, MgO and CaO powders. Reagent-grade anhydrous CaCl<sup>2</sup> 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 CaCl<sup>2</sup> dosage on the Co volatilization percentage were investigated from No.1 to No.5. The effects of the molar ratio of SiO<sup>2</sup> to CaCl<sup>2</sup> 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 CaCl<sup>2</sup> dosage.

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).

$$
\eta = \frac{c\_i m\_i - c\_f m\_f}{c\_i m\_i} \times 100\% \tag{1}
$$

where *c<sup>i</sup>* and *c<sup>f</sup>* are Co concentrations of samples before and after roasting, respectively. *m<sup>i</sup>* and *m<sup>f</sup>* 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**
