Synergistic Role of Sodium Hydroxide and Sodium Citrate on Bitumen Slime Coating in Process Water

Abstract

Sodium hydroxide (NaOH) is generally used as the primary processing aid in the water-based oil sands extraction for the recovery of bitumen. It was found by Syncrude Canada Ltd. that the combined use of sodium hydroxide and sodium citrate could effectively improve the bitumen recovery as compared to using sodium hydroxide alone. Thus, sodium citrate is introduced as a secondary process aid (SPA) in the water-based extraction. Slime coating in the bitumen extraction is the attachment of undesired clay particles onto bitumen, thereby reducing bitumen recovery and froth quality. It is believed that the addition of sodium citrate prevents the hetero-coagulation of bitumen droplets and clay particles, enhancing the bitumen recovery. Moreover, the combined use of sodium hydroxide and sodium citrate is believed to further prevent bitumen-clay coagulation by their synergistic effect. In this research, the synergistic role of sodium hydroxide and sodium citrate on preventing slime coating of bitumen with various clay particles were investigated using process water. Adding sodium citrate was able to make the zeta potential of kaolinite, illite, montmorillonite, and bitumen more negative in the process water. The zeta potential of the mixture of bitumen with non-swelling clays (kaolinite and illite) showed two separated distributions in the process water, suggesting that no slime coating had happened. However, single distribution of zeta potential of bitumen-montmorillonite mixture was observed in process water. The addition of sodium citrate before mixing bitumen and montmorillonite could effectively prevent slime coating. Results obtained from zeta potential distribution measurements were verified by Quartz Crystal Microbalance with Dissipation (QCM-D), and consistent results were observed. Sodium citrate alone could not change the zeta potential of real fines in the process water, but it could work synergistically with sodium hydroxide to change the zeta potential of real fines. For the mixture of bitumen and fines, experimental results showed that the consequent effect of more negatively charged colloidal particles (bitumen and fines) due to the combined use of sodium hydroxide and sodium citrate prevented the hetero-coagulation of bitumen and real fines.