Effect of citric acid on inhibiting hexadecane–quartz coagulation in aqueous solutions containing Ca2+, Mg2+ and Fe3+ ions

Coagulation between oil droplets (hexadecane containing 10− 3 mol/L of sodium oleate) and quartz particles in aqueous solutions containing multivalent metal cations (Ca2+, Mg2+ or Fe3+) was monitored, in the absence and presence of citric acid. It was found that citric acid effectively inhibited oil–quartz coagulation caused by the multivalent metal ions. By using speciation diagrams and zeta potential and metal ion adsorption measurements, it was found that the metal hydroxyl species and metal hydroxide precipitates, which can adsorb on quartz and possibly on the oil droplets, were responsible for the strong oil–quartz coagulation. The addition of citric acid lowered the concentrations of the metal hydroxyl species by forming metal–citrate complexes, which led to a reduction in the oil–quartz coagulation. The citrate anions could also adsorb on the metal hydroxide precipitate giving it a high negative surface charge, thus eliminating the effect of the metal hydroxide precipitate as a promoter for quartz–oil coagulation. This study shows that the attachment of a hydrocarbon liquid to an inorganic mineral surface is not solely governed by the hydrophobic/hydrophilic properties of the solids. Electrostatic interactions also contribute to the attachment process. Since the studied metal ions are inevitable in the bitumen extraction pulp from oil sands, the results of this study have interesting implications with regard to the effect of the metal ions, as well as citric acid, on bitumen liberation from the sand particles.