Hot water extraction technique has been applied to extract bitumen from oil sands for decades. With the challenge of low oil prices, achieving higher recovery and efficient production becomes more urgent. In recent years, the overall recovery and froth quality have been improved significantly when sodium citrate is used as a secondary process aid during oil sands extraction. The beneficial effects of sodium citrate on preventing slime coating, accelerating the liberation process, and enhancing bubble-bitumen attachment have been demonstrated in industry applications.
Bitumen droplet size is a key factor affecting the flotation recovery. A bigger bitumen droplet tends to attach to an air bubble easily and can be effectively floated. This research focuses on the fundamental effects of sodium citrate on bitumen droplet size and coalescence. In order to study the effects of sodium citrate on bitumen droplet size, experiments were performed using a high-speed camera, Focused beam reflectance measurement (FBRM) and Smart online particle analysis technology (SOPAT) with oil sands ore or bitumen samples under various water chemistries.
The beneficial effects of sodium citrate on bitumen droplet size and bitumen droplet coalescence have been observed when added with caustic into the oil sands system with process water. At an optimum dosage, bitumen droplet coalescence is increased when sodium citrate is added as a secondary processing aid in the bitumen emulsion system with either process water or synthetic process water. However, sodium citrate did not show any beneficial effect at high concentrations in either process water or synthetic process water. The use of sodium citrate alone did not show any beneficial effect on bitumen coalescence at the concentrations studied in deionized water at pH 8.5. The fundamental mechanism affecting bitumen droplet interaction consists of the interplay of three major factors: slime coating, surface properties, and surface forces. The use of sodium citrate as secondary processing aid can prevent slime coating, which keeps the surface clean to enhance the droplet coalescence. Also, the electrical double layer (EDL) repulsion between bitumen droplet could be increased at a high concentration of sodium citrate, which prevents the droplet coalescence. In our recent studies, the use of sodium citrate might soften the interfacial film at bitumen-water interface, which may promote the bitumen droplet coalescence. Depending on the concentration of sodium citrate, the overall effect of sodium citrate on bitumen droplets coalescence depends on the balance of these three factors. The fundamental knowledge obtained in this research indicates that larger bitumen droplets can be induced at an optimum sodium citrate concentration, which in turn results in a higher flotation recovery.