Understanding Polymer Flocculants in the Treatment of Oil Sands Tailings
Creators: Dixon, Daniel V.
Publisher: name: University of Alberta Library
Year: 2023

<p>The extraction of bitumen from oil sands is an energy and water-intensive process that produces a tailings byproduct containing sands, clays, water, and residual bitumen. Larger particles eventually settle out of the tailings, but fine particles – stabilized by the residual bitumen – remain suspended, forming a thick slurry called mature fine tailings (MFT). The stability of MFT makes it resistant to conventional treatment methods used for other types of mineral tailings. As a result, large quantities of MFT have accumulated in tailings ponds that must be remediated. Polymer flocculants are widely used to aggregate the fine particles and recover some water from tailings, but commercial anionic polyacrylamide-based flocculants do not perform well when treating MFTs. Research into modifications of polyacrylamides and novel polymers has found promising results for improved dewatering of MFT, but these polymers are often limited by high dose requirements or generate sediments with insufficient solids contents. <br>
Polymer flocculants, like any other polymer, have a statistical distribution of chain lengths. The study of how this and other polymer microstructural details affect flocculation performance has been underexplored in the field of oil sands and other mineral tailings treatment. In this thesis, three studies were done to investigate how polymer properties impact their flocculation performance in clays and MFT.<br>
First, the effect of polymer molecular weight distribution (MWD) on flocculation was investigated. A series of high molecular weight polyacrylamide polymers with a narrow MWD were synthesized using a controlled radical polymerization technique. These polymers were then used alone or in blends to emulate broad MWD flocculants. These polymers flocculated kaolinite and the size of the aggregates formed was monitored. Polyacrylamide flocculants with narrow MWDs made larger clay aggregates than those with broad MWDs at equal doses. In the polymer blends, the fraction of the high molecular weight component was the best indicator of aggregate size formation; the low molecular weight fraction had less influence on final aggregate size. These findings suggest that polymers with narrow MWDs can be used in lower dosages than standard flocculants that have broad MWDs with a similar concentration of high molecular weight polymer chains.<br>
Second, a two-step flocculation process using anionic hydrolyzed polyacrylamide (HPAM) and cationic poly(vinylbenzyl trimethylammonium chloride) (PVBTMAC) was studied for the treatment of MFT. The charge density of the HPAM and the dose of HPAM and PVBTMAC were investigated for their influence on several flocculation performance metrics, including settling rate, capillary suction time (a measure of sediment dewaterability), supernatant turbidity, and zeta potential. The charge density of HPAM had a different optimum value depending on which metric you wish to optimize for – settling rate or dewaterability. PVBTMAC played a role in charge neutralization after the addition of HPAM to further consolidate the flocs formed and produce a clear supernatant. Additionally, the two-step flocculation process could treat undiluted MFT slurries, resulting in high solids content after low pressure filtration.<br>
Third, a population balance model describing the change in aggregate size during MFT flocculation was developed. Experimental data of PVBTMAC adsorption and flocculation of MFT was used for model development. The knowledge of the polymer adsorption behaviour was used in the model to determine how polymer dose impacts aggregate size. The developed model is another step toward a more quantitative understanding of how polymer properties affect the flocculation of MFT.</p>