Novel Functionalized Polyolefin Flocculant for Treatment of Oil Sands Tailings
Creators: Rostami Najafabadi, Zahra
Publisher: name: University of Alberta Library
Year: 2023

<p>Bitumen is most commonly extracted from oil sands using the Clark hot water extraction process that produce slurry residues, called tailings. Sand particles in tailings quickly settle and produce mature fine tailings (MFT), which are stored in large tailing ponds. Currently, tailing ponds cover about 257 km2 of Alberta’s oil sands region, and only a small fraction of this large area has been reclaimed. Paste technology, which involves the use of polymeric flocculants, is commonly used to treat oil sands tailings. However, existing commercial flocculants—polyacrylamide (PAM) or partially hydrolyzed anionic polyacrylamide (HPAM)—still suffer from limitations such as shear sensitivity, poor dispersion in tailings, inability to flocculate fine clay particles effectively, and retention of large amounts of water in the sediments.Many investigations such as, modification of acrylamide polymers by addition of hydrophobic groups, and synthesizing partially hydrophobic flocculants with novel chemistries have been done to improve MFT dewatering. Most of these studies highlighted how hydrophobic modifications could enhance MFT dewatering, but the relation between flocculant microstructure and its flocculation and dewatering performance is still missing. This thesis addresses some of these challenges by synthesizing a family of novel partially hydrophobic flocculants composed of hydrolyzed poly(methyl acrylate) side chains of different molecular weights and densities grafted via reverse ATRP onto two ethylene-propylene-diene (EPDM) backbones of different molecular weights. We investigated how graft molecular weight and density, as well as backbone length, affected the flocculation and dewatering of oil sands mature fine tailings (MFT) through a statistical approach. This research was broken down into three main investigations:1-   Synthesis of ethylene/propylene/diene terpolymers grafted with hydrolyzed poly(methyl acrylate) (EPDM-g-HPMA).EPDM terpolymers were grafted with PMA by reverse ATRP. Two EPDM samples with the same 5-ethylidene-2-norbornene, ethylene, and propylene fractions, but different weight average molecular weights, were used as backbones. The PMA graft molecular weights and densities were controlled by varying the concentration and conversion of methyl acrylate, as well as the concentration of initiator. The grafted copolymers were characterized by Fourier-transform infrared spectroscopy, proton nuclear magnetic resonance, and gel permeation chromatography to measure the graft densities and molecular weights of the grafted copolymer. Copolymers with graft molecular weights varying from 12 to 179 kDa and grafting densities from 11 to 53 % were made following this procedure. 2-  Assessing the characteristics of the novel partially hydrophobic flocculant (EPDM-g-HPMA) on the flocculation and dewatering of mature fine tailings.In this study, the impact of the side chains molecular weight and density of EPDM-g-HPMA on the flocculation and dewatering of oil sands mature fine tailings were assessed using a central composite experimental design by measurements of initial settling rate, capillary suction time, supernatant turbidity, and sediment solids content. Flocculants with high molecular weight grafts and low grafting densities reduced initial settling rates, decreased supernatant turbidities, and increased the sediments solids content. The capillary suction time, however, depended mostly on the charge density and on the hydrophobicity of the grafted copolymer. The effect of the EPDM backbone molecular weight was also investigated. Shorter backbones resulted in flocculants that dispersed more easily in water and consequently, settled the sediments faster, decreased the supernatant turbidity and increased the solids content in the sediments. 3- Evaluation of floc structure and its effect on dewatering and rheology of the flocculated MFT. In this study, a combination of confocal microscopy and rheology was used to investigate the effect of EPDM-g-HPMA graft density on its dispersion in water, sediment solids content and long-term dewatering of oil sands tailings. Microscopy observations and rheology measurements showed that increasing graft density from 30 % to 50 % makes the flocculant easier to disperse in the medium and consequently, leads the flocculant with higher graft density to produce sediments with higher solids contents, increase the rate of initial dewatering, and also enhance the viscoelastic response of the flocculated sediments right after sedimentation. However, the long-term rheological properties of the flocculated sediments were similar for all flocculants. Tri-dimensional microscopic details of the spatial distribution of water within the flocculated sludge showed that increasing the graft density in EPDM-g-HPMA causes trapping more water within the individual flocs and consequently, decreases the post-flocculation dewatering rate. </p>