*4.1. Thermodynamic Calculation of Mixed Ore Subjected to Roasting*

In order to explain the reason for the increase in arsenic removal rate with increasing temperature during roasting, the thermodynamic calculations of the roasting mixed ore in the air atmosphere were carried out by FactSage 7.0 (version 7.0, Thermfact/CRCT, Montreal, Canada, GTT-Technologies, Ahern, Germany) thermodynamic software. The effect of partial pressure of oxygen on the residual arsenic rate at different temperatures was calculated as shown in Figure 4. The results show that arsenate is the residual product in air roasting of arsenic-bearing ores at 700–1000 ◦C. Figure 4 shows that excessive partial pressure of oxygen is not beneficial to arsenic removal. Moreover, with the decrease of roasting temperature from 1000 to 700 ◦C, arsenic removal requires lower partial pressure of oxygen, which is the reason why the arsenic removal rate at 700 ◦C is lower than that at 1000 ◦C.

**Figure 4.** Effect of partial pressure of oxygen on residual rate at different temperatures.

The arsenic removal rate by roasting method in the air atmosphere is poor, which is probably attributed to the reaction between As2O3 and oxygen to generate As2O5, and then the As2O5 reacts with other oxides (Fe2O3, Al2O3, CaO) via Equations (2) to (4) to generate arsenate. Thus, the roasting product contains a variety of arsenic residues. The arsenic removal rate at 900–1000 ◦C increases to 79.58–86.77% in air; the formation rate of As2O3 is accelerated at high temperature, and a large amount of gas escapes rapidly.

$$\text{AszO3(g) } + \text{O}\_2(\text{g}) + \text{Fe}\_2\text{O3} = \text{ 2FeAsO4} \tag{2}$$

$$\rm As\_2O\_3(g) \, + O\_2(g) + Al\_2O\_3 = \, 2AlAsO\_4 \tag{3}$$

$$\text{As}\_2\text{O}\_3(\text{g}) \, + \text{O}\_2(\text{g}) + \text{3CaO} \, = \text{ Ca}\_3(\text{AsO}\_4)\_2 \tag{4}$$

#### *4.2. X-Ray Di*ff*raction Analysis of the Roasted Ore and Dust in Di*ff*erent Atmospheres*

The XRD spectra of the roasted ore in air and nitrogen atmospheres are shown in Figure 5a,b, respectively. In the raw material ore, arsenic exists in the form of FeAsS (Figure 5a, bottom). Figure 5a shows the disappearance of peaks of FeAsS due to its decomposition when the ore was roasted at 700–1000 ◦C, and a small amount of peaks of AlAsO4 at 800 ◦C and As2O3 at 1000 ◦C appear for the roasted ore, indicating that FeAsS underwent decomposition via Equation (1) and As2O3(g) underwent reaction via Equation (3). Figure 5b demonstrates that the arsenic removal by roasting in the nitrogen atmosphere is more thorough, and the peaks of arsenates and As2O3(s) are not found in XRD spectra of roasted ore. The reaction of arsenic removal is mainly carried out via Equation (1). The investigation of the mechanism on arsenic removal by roasting method proves that arsenic is mainly removed in the form of gaseous As2O3(g) in the oxidation or nitrogen atmosphere, while the residual arsenic is mostly arsenate.

**Figure 5.** XRD spectra of the roasted ore and dust in: (**a**) Air and (**b**) nitrogen atmosphere.

*4.3. Mechanism Research on Arsenic Removal by Roasting Method and Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy Analysis*

Arsenic removal efficiency in the air atmosphere was found to be poor. Therefore, the arsenic residual form in the roasted sample under an air atmosphere was further studied by SEM coupled with EDS. Figure 6 shows the SEM images of the roasted ore under an air atmosphere at different roasting temperatures. Chemical composition of the raw ore and roasted ore is shown in Table 4. Figure 6a demonstrates that arsenic and sulfur occur simultaneously in the raw material, and arsenic is present in the form of FeAsS. Figure 6b shows that arsenic is found in the samples roasted at 700 ◦C; however, sulfur is not found, which indicates the decomposition of FeAsS, where arsenic is present in FeAsO4 and Ca3(AsO4)2. Figures 5 and 6c show that the arsenic in the roasted ore is present as AlAsO4 and FeAsO4 at 800 ◦C, respectively. The results exhibited in Figure 6d,e are similar to those shown in Figure 6c. The abovementioned SEM and EDS results further confirm that arsenic is removed in the form of As2O3(g) by roasting in the air atmosphere, and the residual arsenic reacts with oxide in the ore to generate arsenates.


**Table 4.** Chemical composition of the raw ore and roasted ore.

Note: "-" Represents "below the detection limit".

**Figure 6.** SEM images of the roasted ore in air atmosphere and at different roasting temperatures: (**a**) Raw material; (**b**) 700 ◦C; (**c**) 800 ◦C; (**d**) 900 ◦C; (**e**) 1000 ◦C.
