Recovery of elemental arsenic from calcium arsenate slag by carbothermic reduction: Ca3(AsO4)2 → Ca3(AsO3)2 → As4(g) pathway

Abstract

With the increasing application of As(0) in semiconductor, equipment manufacturing fields, recovery of As(0) from hazardous wastes-bearing arsenic has attracted great attention. As alkali leaching method is widely used for disposal of wastes-bearing arsenic, secondary arsenic pollution has become an inevitable environmental problem due to the generated calcium arsenate slag. To solve secondary arsenic contamination, a reduction roasting process was proposed to recover As(0) from calcium arsenate slag. Thermodynamic calculation suggested that reduction pathway Ca3(AsO4)2 → As4(g) is the most feasible, followed by As(s) and As(g). Increasing temperature is conducive to reduction of Ca3(AsO4)2(s) to As4(g). To achieve efficient reduction of Ca3(AsO4)2 and avoid generation of As4O6(g), the temperature should be 650 °C ∼ 952.88 °C and molar ratio of C/Ca3(AsO4)2 should be greater than 3.0. The optimum operating parameters were determined to be C dosage of 14 %, temperature of 900 °C, and reduction time of 2 h, arsenic recovery efficiency and purity of As(0) reached 99.22 % and 99.41 %, respectively. Besides, mechanism of reduction process was elucidated and the findings revealed that reduction process is carried out in steps by Ca3(AsO4)2 → Ca3(AsO3)2 → As4(g) pathway. Moreover, this method has been successfully applied for semi-industrial test, providing an alternative solution for treatment of arsenic-bearing wastes.