Kaolinite Surface Charges Developed in Cyclohexane  in the Presence of Bitumen
Creators: Zhang, Hanyu
Publisher: name: University of Alberta Library
Year: 2023

<p>Non-aqueous extraction (NAE) process was explored as an alternative to the Clark hot water extraction process (CHWE) to recover bitumen from Alberta oil sands. The upshot of the NAE process is that it eliminates the latter’s environmental issues concerning high fresh water and energy consumptions. Significant efforts have been devoted to improving the performance of NAE over the past several decades. However, the inability to remove fine mineral solids from bitumen to produce a refinery-ready bitumen product remains one of the major challenges that prevent the NAE process from being commercially viable.<br>
A novel approach to address this challenge involves using an external electric field to remove fine solids from NAE bitumen product. However, there is limited research on this concept, and fundamental questions regarding the charging mechanism of fine mineral particles in nonpolar organic solvents with dissolved bitumen remain largely unanswered. In this work, we focused on kaolinite, a representative clay mineral, to investigate its charging mechanism in cyclohexane suspension with the presence of bitumen by electrophoretic deposition experiments and several analytical methods, including phase analysis light scattering (PALS) zeta potential measurement, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical quartz crystal microbalance with dissipation (EQCM-D) measurements.<br>
Electrophoretic deposition and zeta potential measurement showed that kaolinite particles remained uncharged in cyclohexane but acquired charges when coated with bitumen. FTIR spectra of the deposited films on the electrodes showed that carboxyl groups were the primary contributor to the negative charges of kaolinite deposited on the anode. XPS analyses showed that the deposited organic-coated kaolinite films on the cathode had a higher nitrogen content, and the films deposited on the anode showed carbon and sulfur species with higher oxidation states that were associated with oxygen.<br>
Subsequently, we conducted experiments to investigate the charging of kaolinite in cyclohexane with the addition of bitumen and compared it with adding a surfactant Span 80. It turned out pristine kaolinite suspended in cyclohexane remained uncharged, but the addition of bitumen induced negative charges and the addition of Span 80 caused positive charges on kaolinite. The removal of Span 80 eliminated the positive charges, suggesting that Span 80 stabilized negative charges by reverse micelles and made kaolinite positively charged. This is consistent with classical charging mechanism in nonpolar solvents. Notably, removing bitumen did not affect the negative charge on kaolinite, indicating that it is the coated bitumen rather than dissolved bitumen that made kaolinite get charged in cyclohexane. <br>
Analysis of the kaolinite deposited on the cathode after filtering the cyclohexane-bitumen-kaolinite suspension and re-pulping kaolinite showed that it contained a lower carbon content than the kaolinite dispersed in the suspension before electrophoretic deposition. This implied that it was the desorption of a small fraction of adsorbed bitumen on kaolinite surface that caused the kaolinite to carry a net negative charge. Indeed, when the bitumen-coated kaolinite was dispersed in toluene, which is a stronger solvent than cyclohexane, more kaolinite was found to deposit on the anode than from cyclohexane. In contrast, when the bitumen-coated kaolinite was dispersed in n-pentane, which is a weaker solvent than cyclohexane, much less kaolinite was found to deposit on the anode. EQCM-D measurement showed that an applied external electric field could indeed induce bitumen desorption from the bitumen layer previously adsorbed on kaolinite surface.<br>
The findings from this work provided a new perspective on fine particles charging mechanism in nonpolar liquid, revealed the functions of bitumen in kaolinite charging when it is suspended in cyclohexane, and could guide the development of electric field-enhanced fine solids removal technology for nonaqueous extraction of oil sands.</p>