Geochemical investigation of fluid petroleum coke deposits at an oil sands mine in northern Alberta, Canada

Abstract

The geochemical characteristics of three petroleum coke deposits at an oil sands mine near Fort McMurray, Alberta were investigated. This coke was generated in large volumes via fluidized-bed coking during the upgrading of oil sands bitumen and slurried with process-affected water to deposits onsite. This research examined variations in trace element speciation and release under varied field conditions. Continuous core sampling and multi-level well groundwater sampling were performed to 8 m depth throughout the deposits to acquire samples from differing storage and redox conditions. Bulk elemental analyses confirmed that this coke was composed mainly of C and S, with a secondary fraction of clays and oxides (Si, Al, Fe, Ti, Ca, K, Mg) and a suite of trace metals dominated by V (1280 ± 120 mg/kg) and Ni (230 ± 80 mg/kg). Synchrotron powder X-ray diffraction revealed the presence of Si and Ti oxides, organically-complexed V, hydrated Ni sulfate, and provided information about the aromatic carbon matrix. Electron microprobe analyses, scanning electron microscopy, and X-ray fluorescence mapping revealed concentric rings within coke grains, with elevated concentration of inorganics, V, and Ni in the ring margins. Vanadium and Ni K-edge X-ray absorption spectroscopy (XAS) showed that these elements were hosted in porphyrins and similar organic complexes throughout coke grains. Micro-focused XAS showed that V and Ni speciation is different in the concentric ring margins. Vanadium coordination is consistent with substitution into the distorted octahedral site of phyllosilicates, and Ni is either hosted in distorted asphaltenic metal sites or in Ni oxide. Sulfur and Fe XANES confirmed that thiophenic coordination and pyritic-ilmenitic coordination are predominant, respectively. Groundwater depth profiles of pH, Eh, electrical conductivity, major ions, and stable isotopes of water showed evidence of a mixing zone between infiltrating, oxic, sub-neutral pH precipitation and underlying, anoxic, alkaline pH process water. Vanadium and Ni are mobilized only at the top of the saturated zone due to ingress of oxygen and precipitation. Vanadium was found to undergo oxidative mobilization in the form of oxyanionic species whereas Ni mobilization occurred as pH decreased.