## 3 Results
### Vacuum Carbothermal Reduction  
Based on the previous theoretical analysis, the removal of As from the feedstock mainly occurred around 723 K and was mainly caused by the decomposition of arsenate. However, the changes in the fugitive form of As during As removal are not clear. Therefore, the temperature range of the experiment was set at 300-723 K, and the heating rate was set to 2-5 K/min to ensure the accuracy of the experiment. The weight-loss and temperature profiles of the material are shown in Figure 7a, and the XRD pattern of the residue remaining at the end of the experiment is shown in Figure 7b. Figure 7a shows two weight-loss plateaus. We believe that under vacuum conditions, there is no material exchange between the system and the external environment, which means that during the experimental process, there will be no weight gain during the differential weight experiment without the addition of external substances, Therefore, the fluctuating changes in weight in Figure 4a at the beginning of the experiment and the sudden increase in weight during the experiment are caused by experimental errors. Figure 7: Vacuum thermogravimetry experimental results. Because of the high precision of the weighing system, changes in the system pressure cause a change in the scale reading; hence, the mass increase is considered an error caused by the change in the system pressure. The entire curve is divided into two weight-loss stages, the first starting at a temperature of \(\sim\)557 K and the second at \(\sim\)723 K. The XRD results of the residue showed that under the experimental conditions, lead arsenate and bismuth arsenate phases are not present in the residue, but the metallic Bi phase is observed. According to previous thermodynamic simulation results, it is known that the metallic Bi phase will not appear in the reduction reaction of bismuth sulfate under the experimental conditions; hence, the metallic Bi phase was formed by the thermal reduction of bismuth arsenate. The presence of sulfate phases in the residue may be due to incomplete sulfate reduction owing to an insufficient reaction time. It was initially determined that the weight losses during the experiments were mainly caused by the thermal reduction of arsenate and product volatilization. To further investigate the phase changes in each weight-loss section, we refined the experimental parameters and performed experiments with a heating rate of 2-3 K/min. Under these conditions, the first weight-loss temperature shifted from 557 K (Figure 7c) to \(\sim\)500 K, and the second weight loss started at \(\sim\)600 K (Figure 7a). Therefore, experiments were conducted to verify the material phases present at 500 and 600 K.  
According to Figure 8b, it can be seen that at the beginning of the reaction, bismuth sulfate will first react with lead arsenate to form lead sulfate and bismuth arsenate. The disappearance of the bismuth sulfate phase in the residue at 500 K (Figure 8a) also proves that bismuth sulfate and lead arsenate reacted to form lead sulfate and bismuth arsenate. According to the thermodynamic equilibrium simulation results of the mixture, the system should not contain bismuth sulfide, but the XRD results in Figure 8b identified Pb\({}_{2}\)Bi\({}_{6}\)S\({}_{11}\) in the residue. Figure 8c shows the effect of different carbon content and system pressure on the M system. The amount of carbon content directly affects the amount of sulfate/arsenate reduction. The experiments were carried out in the presence of carbon excess and the sulfate was reduced to monomers and metal sulfides at 500 K. Arsenate was then reduced to monomers. The system pressure directly affects the size of the corresponding reduction product stabilization zone. Only when the carbon content of the system is insufficient, arsenic exists in the form of oxides. Comparing Figure 8c,f, the increase in temperature reduces the stable presence phase of sulfate/arsenate and expands the phase of reduction products. There is little effect on the type of reduction products.