The Alberta oil Sands are mined in Fort McMurray to recover bitumen by flotation. For good processing ores, bitumen recovery is higher than 92% but lower than 70% for poor ores. We have identified that the depression of bitumen recovery results from the loss of bitumen hydrophobicity due to the interactions between calcium and magnesium ions in water and montmorillonite clays in the ores. We have also established that bitumen recovery correlated with the receding water contact angle.

Our contribution to explain depression of bitumen recovery was based on doping tests that consists in poisoning good processing ores samples with additives to make them behave like poor processing ores. Our imaging technique allowed visualizing the results of doping tests on bitumen-air bubbles attachment. We have found that individual additives had marginal effect on bitumen recovery, but bitumen recoveries were decreased when montmorillonite clay at 1 wt % of the oil sands ore and calcium or magnesium ions at 40 ppm in the feed water were present together; that depression of bitumen recovery depended on the valence and concentrations of canons, on the type and concentration of clays than on the ionic strength of the slurries, that bitumen recovery was sensitive to temperature variations only below 50°C and that bitumen recovery was affected by the mechanical energy provided by the agitation.

Depression of bitumen recovery was due to the bridging of divalent calcium ions that interact with carboxylic groups of natural surfactants on the surface of bitumen and with montmonllonite clay particles. They form a slime coating barrier that is confirmed by the use of the Atomic Force Microscopy and by the induction time techniques. The slime coating masks bitumen surface, and prevents its attachment to air bubbles, resulting in poor bitumen floatability and thereby in depressed bitumen recovery.

Our finding suggests that the effect of calcium ion in water would not be observed for low fine contents oil sands ores. Addition of sodium hydroxide at 0.04% of the oil sands ores corrected the depression of bitumen recovery from depressed good processing ore but not for poor ore samples. Addition of sodium bicarbonate at 1000 ppm prevented depression of bitumen recovery by softening feed water but did not restore bitumen recovery once depressed.

More work on more operating variables is still needed to expand our understanding of the mechanisms of depression of bitumen recovery and to find synergy required to prevent clays from interacting with metal ions and therefore to minimize of depression of bitumen from poor processing ore during flotation.