Arsenic Removal and Recovery of Germanium and Tungsten in Toxic Coal Fly Ash from Lignite by Vacuum Distillation with a Sulfurizing Reagent

Every year, billions of tons of lignite are burnt to generate electricity, meanwhile generating large amounts of coal fly ash (CFA) that is regarded as an industrial waste. During lignite combustion, arsenic and scarce metals are simultaneously volatilized in the form of oxide into CFA. This study proposed an effective vacuum distillation method to remove As and recover Ge and W from CFA. The feasibility of separating As and recycling Ge and W from CFA was verified by the theoretical analysis. The experimental result indicated that the removal ratio of As was 96 ± 1% and the contents of Ge and W reached 0.75 ± 0.023 and 0.24 ± 0.016 wt % in the residue, which were enriched 17.2 and 1.2 times, respectively, at a temperature of 550 °C, with 50 wt % sulfurizing agent added under pressure of 1 Pa and 240 min of heating. For the condensed product, chemical species As₂S₃ and As₄S₄ were detected by X-ray photoelectron spectroscopy analysis. For Ge and W in the residue, GeO_x (x < 2), GeS, WO_x (x < 3), and WS₂ were the main chemical species. The potential mechanism involved in the release of arsenic from CFA, vacuum sulfurization, evaporation, and condensation was proposed. The kinetic analysis indicated that the apparent activation energy (E_α) was 31.24 kJ mol^-1. Those results encourage further exploration of vacuum separation technology to environmentally friendly recycle CFA.