3.1. Measurement of Permittivity
The effect of the temperature and apparent densities on the dielectric properties of CuCl residue at a frequency of 2.45 GHz is shown in
Figure 5 and
Figure 6.
As shown in
Figure 5a,b, when the temperature increased from 20 to 550 °C at an apparent density of 1.70 g/cm
3, ε’ increased remarkably from 2.471 to 2.916, and ε’’ also rapidly increased from 0.0270 to 0.1180. This could be because the energy of molecules increases with temperature increases, so the response to the external electric field is faster, and the dielectric constant of CuCl residue concentrate increases immediately [
13]. The trend of the dielectric constant of CuCl residue with temperature was observed in other ores and minerals [
17,
18]. ε’’ increased significantly above 400 °C because the number of active ions and electrons in the CuCl residue sharply increased since the melting point of CuCl is about 427 °C [
21]. There are many ions and electrons at the interfaces of various compounds, and the majority of these ions and electrons will relax in the presence of a microwave field. The loss factor of a material is directly proportional to its conductivity, making the loss factor increase with increasing temperature [
13]. In addition, the dielectric properties of the CuCl residue with an apparent density of 1.9 g/cm
3 were mostly greater than 1.7 g/cm
3 at the same temperature. This is because the higher the apparent density, the better the ability to reduce the internal clearance of the residue, increase the number of raw materials, multiply the number of polar molecules per unit volume space, and reduce the air content in the material. Additionally, because air is a low-loss medium, it reduces the material’s overall dielectric properties. Similarly, the higher the water content of a material, the higher the measured dielectric constant and dielectric loss factor. Overall, the microwave absorption characteristics of the CuCl residue improved as temperature increased.
The loss tangent of the CuCl residue at different temperatures is shown in
Figure 6, which shows the same trend as the loss factor in
Figure 5b. The values of loss tangent changed slightly between 20–400 °C. When the temperature exceeded 400 °C, the loss tangent increased rapidly from 0.0131 to 0.0440 at an apparent density of 1.70 g/cm
3. By comparing
Figure 5, it was discovered that as the temperature exceeded 400 °C, the apparent density’s influence on the loss tangent gradually decreased, and the temperature became a significant factor affecting the material’s microwave absorption. Because a high apparent density can interfere with oxygen adsorption and chlorine diffusion during the oxidative dechlorination process, the apparent density naturally stored at 1.70 g/cm
3 was chosen for the dechlorination process.
3.4. Microwave Roasting
The effect of roasting temperature and holding time on the chlorine removal rate of CuCl residue was investigated under the conditions of an oxygen flow of 150 mL/min, microwave power of 1200 W, and particle size of 120 μm. The experimental results are shown in
Figure 9.
Figure 9 reveals that the degree of dechlorination of CuCl residue increased with the temperature for both microwave roasting and conventional roasting processes; however, the microwave heating method achieved the same effect as conventional heating in a shorter time, as shown in
Figure 10a,b. For instance, when the temperature reached 500 °C, the degree of dechlorination exceeded 94% within 60 min of the microwave roasting processes. Under the same conditions, the degree of dechlorination after roasting in conventional heating was only ~80% and required an extra 30 min to obtain the same result. Furthermore, when the roasting temperature exceeded 450 °C, the effect of temperature on the chlorine removal rate was significantly weaker under microwave roasting than it was under conventional heating. The main reason is that microwave heating has both bulk and selective heating characteristics, and the CuCl residue is a strong microwave absorber, causing it to be heated faster than other components in the residue [
9]. The optimal experimental conditions were determined by the process characteristics, which were 450 °C and 90 min of holding time.
To study the control steps of the dechlorination process, all collected data from microwave heating and conventional heating processes were fitted with Equations (1) and (2). The fitting results of microwave heating and conventional heating by various dynamics models are shown in
Figure 10. The rate constant
k for each of the temperatures is also listed in
Table 3.
The results in
Figure 10a,b demonstrate that the
R2 values of >0.99 for the linear fit of both microwave heating and conventional heating process are higher than other models in
Table 3. Thus, the chemical reaction on the surface control model is an appropriate model, which was utilized to estimate the activation energy and pre-exponential factor for the reactions. The results are shown in
Figure 11 and
Table 4.
The apparent activation energy of the dechlorination of the CuCl residue was calculated to be 42.36 kJ/mol and 52.39 kJ/mol for microwave roasting and conventional roasting, respectively. The apparent activation energy for the dechlorination reaction by microwave roasting was 19.61% lower, indicating that microwave roasting improved the reactions between O2 and CuCl to form Cl2.
3.6. SEM-EDS
As shown in
Figure 13, all products after microwave roasting were composed of bright, white particles, accompanied by a small number of regular rectangular crystals. When compared to the SEM image of the raw materials (
Figure 2), more burrs and convex parts formed on the particle surface, mainly due to the evaporation of chlorine when the chloride in the material was oxidized. Based on the EDS analysis, the regular rectangular crystal (Spot 1 and Spot 2) was mainly composed of Cu and O. The bright particles with burrs and protrusions (Spot 3) were also mainly composed of Cu and O, and a small amount of S and Ca, which were determined by XRD analysis to be mainly CuO.
According to
Figure 14, after microwave roasting, the product was primarily composed of Cu, Zn, and S elements, with a very small amount of Cl, which is consistent with the results shown in
Figure 12;
Figure 13; thus, the product was primarily composed of CuO, with a minor amount of zinc and silicon.
As can be seen from
Figure 15, Cu accounted for most of the atoms in the granules during roasting, and the Cl content in the granule gradually decreased upon extending the holding time, while the O content gradually increased. The distribution of chlorine and oxygen in the particles was observed after 20 min of microwave roasting. The chlorine content was lower at particle boundaries and higher in the center of the particles, whereas oxygen distribution was reversed. CuO was formed in the material after 60 min of microwave roasting, along with a trace of Zn and S. The chlorine evaporated, leaving only a trace in the particle’s center, which was consistent with XRD and SEM findings.
3.8. Analysis of Microwave-Enhanced Dechlorination Process
According to the previous analysis, the main phase involved in the reaction during dechlorination included CuCl, Cu
2O, CuO, ZnO, ZnS. The microwave roasting process is significantly influenced by the heating behavior of each phase of the residue in the microwave field. In the microwave field, the pure phase, according to Haque [
9], can be classified into four types: hyperactive, active, difficult to heat, and inactive.
Table 6 shows the phase classification of the materials.
Among them, Cu2O was easily oxidized to CuO and was not measured separately. The chlorine in the CuCl residue is predominantly in the form of CuCl, which is an excellent microwave absorber. Because the other components of the material have relatively weak microwave absorption, the material can be selectively heated internally, allowing CuCl to combine with oxygen via the chloride oxidation reaction, releasing chlorine that can be recycled.
Because the loss of electromagnetic energy in minerals is relatively small at low temperatures, the penetration depth of microwaves is higher. As a result, flash heating occurred inside the material’s interior because the electric field was concentrated in areas with highly absorbing CuCl, as shown in
Figure 16a. At the same time, rapid heat transfer occurs between different phases in the material. With an increase in the overall temperature of the material, the microwave absorption of the material gradually increases, as shown in
Figure 5, which depicts the dielectric constant and loss factor.
As the temperature of the electric field concentration area and the strong absorbing material concentration area reached the temperature required for the oxidation and dechlorination reactions, the CuCl in the material underwent a reaction with the surrounding oxygen. CuCl exists as a floating liquid during the dechlorination process due to its melting point of approximately 427 °C, which significantly increases the number of polar molecules in the material. The microwave field changes at a frequency of 2.45 GHz, causing the reactant molecules to always lag behind the field’s effect, causing them to constantly move or rotate to achieve a state of dynamic equilibrium, greatly increasing the effective collision frequency. Macroscopically, it reduces the apparent activation energy of the reaction and strongly promotes the oxidation of CuCl into CuO. This mechanism is consistent with the observed trend of sharp increases in the loss factor and loss tangent values of CuCl residue at temperatures above 400 °C.
In addition, microwave preferentially heats CuCl in the material, and cracks caused by uneven temperature distribution provide a new channel for gas diffusion, as shown in
Figure 16b, which promotes and intensifies the reaction between CuCl molecules and oxygen.
Additionally, because the physical properties of CuCl and Cu2O in the residue are different from those in the product, such as hardness and density, the shrinkage surface or reaction layer forms in the residue. Simultaneously, the dechlorination process produces a large number of gaseous by-products, resulting in a large number of pores of various sizes. These particles are not uniform, have a loose structure, and have a large number of pores, which allow microwave penetration and multiple reflections in residue and improve microwave heating efficiency.
Furthermore, as illustrated in
Figure 16c, these porous microstructures can act as mass transfer channels, promoting the reaction of CuCl molecules with oxygen within the particles.