Coagulation of bitumen with fine silica in model systems

A prerequisite for recovering bitumen from oil sands is to liberate the bitumen phase from sand (clay) surfaces and to avoid bitumen/sand aggregation, both being related to bitumen/solids interactions. Coagulation of bitumen with fine silica (- 5 mu m) was investigated in this study as a function of pH and calcium ion concentrations. To explore the effect of surfactant present in bitumen on bitumen/silica coagulation, a model system was developed by dissolving surfactant (dodecylamine or/and palmitic acid) in a hydrocarbon oil (hexadecane). It was found that at acidic pHs, silica was rendered hydrophobic by attracting the positively charged amine head groups of surfactants, thereby inducing coagulation. Adding Ca2+ ions decreased coagulation efficiency by setting up barriers for cationic amine interacting with silica. Over alkaline pH range, palmitic acid did not induce coagulation in the absence of Ca2+ ions. Calcium ions triggered bitumen (or oil)/silica coagulation at pH > 10. The results obtained with model oil systems suggest that chemisorbed CaOH+ cations on silica surface interact with anionic surfactant on oil droplets surface, thus bridging the oil droplets and silica. It appears that electrostatic interaction alone cannot account for the observed coagulation of oil with silica, and thermodynamic conditions have to be considered. A good agreement in coagulation behavior between bitumen/silica and oil (with dissolved surfactants)/silica systems was obtained only when both cationic and anionic surfactant were present in oil, suggesting a synergetic effect of various surfactant constituents. The present study is a good example of simulating the coagulation behavior of complex bitumen systems by using a simple model system. (C) 1999 Elsevier Science B.V. All rights reserved.