Research on the Preparation of 7N-Grade Ultra-High-Purity Arsenic via Transition-State-Controlled Processes.

To meet the demand for ultra-high-purity arsenic (≥7N) from crude arsenic (As ≥ 99.3%, Sb ≤ 0.6%), an integrated process combining chlorination, distillation and hydrogen reduction was developed. After preliminary purification of crude arsenic by vacuum distillation, chlorine was applied to convert arsenic and its impurities into chlorides. Low-boiling chlorides such as SbCl3, S2Cl2 and Se2Cl2 were separated by distillation, and ultra-pure AsCl3 was finally reduced by hydrogen to obtain ultra-high-purity arsenic. Under optimal conditions-10 mL·min-1 Cl2 flow, 20 mm-30 mm arsenic particle size and 80 mm-90 mm packing height-the chlorine utilization reached 92.3%. Distillation at 433 K with 4 h total reflux and a 5:1 volumetric reflux ratio yielded AsCl3 of 99.99999% purity, with S and Se below 0.02 ppm and 0.01 ppm, respectively. Hydrogen reduction at 1123 K, H2/AsCl3 molar ratio 1.8 and 623 K condensation temperature achieved an arsenic recovery of 99.13%, a chlorine residue of 20 ppb and a final arsenic purity of 99.9999%. This study provides a feasible route for large-scale production of high-purity arsenic.