The Roles of Constituent Minerals and Residual Bitumen in the Filtration of Alberta Oil Sands Tailings
Creators: Wang, Dong
Publisher: name: University of Alberta Library
Year: 2022

<p>The dewatering of Alberta oil sands tailings is one of the most challenging problems faced by the oil sands industry. Despite significant efforts in the past several decades, the industry still lacks viable technologies that can quickly dewater and reclaim the oil sands tailings and eliminate their environmental impact. <br>
Pressure filtration of mine tailings has been gaining popularity in the past ten years or so due to the needs to quickly recycle process water and to eliminate the risks of impounding large volumes of unconsolidated wet tailings. However, despite its success in treating mine tailings, the pressure filtration of Alberta oil sands tailings is laden with low filtration rates, low filter cake solid contents, and high and uneconomical  reagent dosages. It has been unclear why the Alberta oil sands tailings filtered so differently from typical mine tailings. The work reported in this thesis is the first such effort to address this issue to systematically investigate the roles of the constituent minerals and residual bitumen in the filtration of Alberta oil sands tailings.<br><br>
Three different types of minerals, including non-clay minerals (rutile, quartz), non-swelling clays (kaolinite, illite), and swelling clays (illite–smectite, montmorillonite) were selected as model solids for the study. The dewatering of each mineral slurry was studied using filtration and capillary suction time measurements. It was found that the filtration did not depend on mineral particle size but rather, on the type of minerals. The presence of the swelling clays, even in small quantities, significantly affected the filterability of other minerals. It was thus deduced that swelling clays in the oil sands tailings were the main reason for the poor filterability. It was further revealed that the filtration behaviors of the different minerals could be explained through the concept of the “effective volume fraction of the solids”, which accounts for the volume taken by the solid particles and their surrounding electrical double layers. Swelling clays had a high effective solid volume fraction in the aqueous slurry due to their high specific surface areas. Any methods to lower their specific surface area and/or reduce their electrical double layer thickness could help improve filtration.<br><br>
The residual bitumen in the Alberta oil sands tailings can be present in three different forms, i.e., the bulk free bitumen, the bitumen-in-water emulsion, and the bitumen adsorbed on mineral surfaces. The effects of the different forms of the residual bitumen were investigated in this work. It was found that the bulk bitumen and emulsified bitumen had negligible effects on the filtration of oil sands tailings, while the effects of the adsorbed bitumen were more complex. <br>
To investigate the effect of adsorbed bitumen on filtration performance, the model minerals were treated in bitumen-toluene solutions at different bitumen concentrations. Bitumen-coated quartz was compared with silanized quartz to decouple the effects of surface hydrophobicity and adsorbed bitumen layer on filtration.  It was found that the filtration rate of silanized quartz increased linearly with increasing contact angle. Similarly, when the minerals were treated at a low bitumen-in-toluene concentration, the contact angle and filtration rate of all tested minerals were increased. However, at high bitumen-in-toluene concentration, the filtration rates were reduced drastically despite the high contact angle. CHNS elemental analyses and X-ray photoelectron spectroscopy surface composition and depth profile analyses showed that the minerals had higher C, N, S contents and thicker and patchy bitumen layer when they were treated at higher bitumen-in-toluene concentration. It was hypothesized that under the pressure gradient during filtration, the thick and patchy bitumen coating layers could interpenetrate when the mineral particles were pressed together, to form an inter-locked film to close off the pores in the filter cakes. It was concluded that a thin layer of bitumen coating helped improve filtration due to the induced hydrophobicity, but a thick and patchy bitumen coating impeded filtration.<br>
The findings from this work, i.e., that the filtration of the Alberta oil sands tailings seemed to be controlled by the specific surface area and the thickness of the electrical double layer of the clay minerals, and by the thickness of the bitumen coating on the clay minerals, were expected to guide the development of suitable filtration treatment strategies for the Alberta oil sands tailings. </p>