Arsenic (As) is a common element in many sulfide ores and concentrates and is consequently a significant waste product produced during the extraction of some metals (e.g. Au and Cu). Due to the toxicity of arsenic, its removal from process and waste streams and its stabilization prior to disposal are necessary. For nearly complete removal of waste arsenic species from metallurgical process streams, it is required that arsenic exist in the pentavalent state (As5+).
A typical arsenic removal process from As-containing metallurgical streams involves oxidation of arsenic to the pentavalent state and reaction with ferric iron to precipitate crystalline or amorphous ferric arsenate. A common practice for removal of arsenic from metallurgical process streams comprises oxidizing the arsenic species to the pentavalent state in an oxygenated autoclave at above 90° C. and at a pH below 4, thereby converting the pentavalent arsenic species to stable ferric arsenate compounds. The capital expenditure (“CAPEX”) and operating expenditure (“OPEX”) associated with autoclave processes are relatively high.
Other methods include the stepwise scorodite precipitation and the bio-scorodite precipitation processes. Both of these processes occur at temperatures below 95° C. and at atmospheric pressure. Addition of scorodite seed material can improve the kinetics of precipitation reactions; however, these processes are feasible only when arsenic is in pentavalent state.
Oxidation of arsenic with oxygen under atmospheric conditions is a very slow reaction and the presence of a strong oxidant, such as hydrogen peroxide, ozone or mixture of SO2/O2 gas, is required. The cost associated with these oxidants renders these processes economically unattractive.
There is a need for an alternative atmospheric arsenic oxidation process to produce pentavalent arsenic.