CHARACTERIZATION OF MILL PRODUCTS TO UNDERSTAND SOLID-PHASE ARSENIC SPECIATION AROUND GOLD-ORE ROASTERS

The mobility of arsenic in the environment is largely determined by its solid form and solubility under prevailing hydrogeochemical and biogeochemical conditions. Arsenopyrite, arsenical pyrite and other As-bearing sulfides and sulfosalts are primary reduced-As phases that may be exposed to oxidizing environments. The converse situation may also occur where secondary oxidized-As phases (especially As-bearing Fe oxyhydroxides, Fe arsenates and Ca-Fe arsenates) can later be subjected to reducing conditions. Solid-phase As speciation remains the subject of study in a variety of natural and anthropogenically-influenced systems. However, the form and behaviour of As derived from the pyrometallurgical treatment of As-bearing concentrates has received comparatively less attention from an environmental perspective. The Fe oxides originating from gold-ore roasting should be considered primary anthropogenic phases that are likely to have distinct behaviour in the environment in comparison to primary and secondary natural sources.

Our study of As-speciation in Fe oxides derived from pyrometallurgical treatment at abandoned gold mines in Canada, using synchrotron-based micro-XRD and micro-XANES, has revealed a complex As association. The Fe oxides (maghemite and hematite) are nano- to finely crystalline, composite grains of diverse morphology, containing mixed oxidation state As (As^III and As^V) and total As concentrations up to 15 % (w/w). These grains are porous at the micron to nano-scale, by virtue of the 40% reduction in molecular volume during the sulfide to oxide transformation in roaster-controlled, gas-solid reactions. The highest ratios of As^III/As^V (2 to 3) are present in Fe oxides associated with relic sulfides suggesting that the As oxidation state within individual composite grains is controlled by the O₂ partial pressure during roasting. More typically the As^III/As^V ratios are 0.3 to 0.5 without the presence of sulfide. When hematite is present, it occurs as coarser crystallites with lower arsenic content than coexisting maghemite. Analysis of 50-year-old sub-aerial tailings indicates that the nano-crystalline nature of the Fe oxides and the imprint of the mixed oxidation state by the roaster persists in the environment, even though As^III should be oxidized to As^V under these conditions.