*4.5. E*ff*ect of Carbon Mass Ratio on Arsenic Removal Products of Dust Ash by Roasting*

According to the dust ash in Table 2, the thermodynamic calculation of the effect of carbon powder on arsenic removal products was done using FactSage 7.0. Figure 8 shows the results of the effect of carbon ratio on arsenic removal products of dust ash by roasting using thermodynamic calculations. When the arsenic-bearing dust ash was roasted with a carbon mass ratio increasing from 0 to 1.83%, the percentage of residual solid AlAsO4(s) in dust ash gradually decreased from 100% to 0, and the percentage of gaseous As2O3(g) gradually increased to 100%. When the arsenic-bearing dust ash was roasted with a carbon mass ratio was below 1.63%, the arsenic removal products were the majority of AlAsO4(s) and a small amount of As2O3(g). When the carbon mass ratio was 1.83%, the arsenic removal product was almost volatilized in the form of As2O3(g). Subsequently, with the increase of carbon mass ratio, the percentage of volatile As2O3(g) gradually decreased, while the percentages of As2(g) and As4(g) gradually increased. When the carbon mass ratio increased to 5%, arsenic was almost removed by volatilization of As2(g) and As4(g).

**Figure 8.** Effect of carbon ratio on arsenic removal products of dust ash by roasting.

The arsenic recovery experiment by roasting in the reducing atmosphere was carried out with dust ash containing 2% carbon powder. Figure 9a shows the recovered products condensed on the edge wall of the quartz tube. Figure 9b exhibits the XRD spectrum of the recovered products. The recovered products are almost As2O3(g). Figure 9c is the XRD spectrum of the roasted dust ash, and the peak of arsenic is almost invisible in the XRD spectrum. By comparing dust ash before roasting in Figure 2, it can be seen that almost all arsenic in the dust ash has been volatilized and recovered in the low-temperature section at the end of the quartz tube. Thermodynamic calculation of Figure 8 shows that arsenic volatilized in the form of gaseous As2O3(g) when dust ash was roasted with 2% carbon. The results demonstrate that the thermodynamic calculation results are in good agreement with the experimental results.

**Figure 9.** (**a**) Volatile compounds photograph, (**b**) XRD spectrum of volatile compounds collected during roasting dust ash, and (**c**) the dust ash after roasting.
