Using Surface Geopolymerisation Reactions to Strengthen Athabasca Oil Sands Mature Fine Tailings

Abstract

Bitumen extraction from oil sands operation in Fort McMurray, Canada produces an immense quantity of slurry waste that is discharged to tailings ponds. These ponds now stretch over a region which is roughly 1.5 times the area occupied by Vancouver city mainly due to slow consolidation process of the mature fine tailings (MFT). The MFT comprises of a mixture of water and fine solids that is characteristically known for very low shear strength. While the current research on tailings consolidation has been focused on removing water from the settled tailings sludge, there is presently no known technology to effectively consolidate tailings and strengthen it for land reclamation. This dissertation describes a novel method to address the oil sands tailings reclamation problems using surface geopolymerisation reactions. Geopolymerisation involves dissolution of aluminosilicates and re-solidification reactions that result in the formation of three dimensional inorganic polymers, which have significant strengths. Since the fine solids in oil sands mature fine tailings (MFT) mainly consist of aluminosilicate minerals (clays), it is hypothesized that by adding appropriate reagents, the surfaces of the clay minerals can be activated to go through geopolymerisation reactions. The resulting geopolymers formed on the surfaces of the clay minerals bind the clay particles together and strengthen the tailings even without further dewatering and consolidation. Characterizations conducted on kaolinite samples, which was chosen as model solids for MFT, showed that under the test conditions the geopolymerisation reactions only occurred on the surface of the kaolinite mineral. The “surface geopolymers” held the kaolinite particles together leading to an increase in shear strength. The results also indicate that shear strength of MFT is considerably increased on addition of NaOH and Na2SiO3, or alkali activators, at the optimum dosage. When a 51 wt% solids centrifuged MFT was treated with 1 mole/L sodium hydroxide and 0.5 moles/L sodium silicate, it was found that the shear strength increased from 115 Pa to 4,880 Pa after 90 days. At the same activator concentrations, the original un-centrifuged MFT (37.9 wt% solids) saw an increase of its shear strength from 19 Pa to 1,245 Pa. Similarly, the shear strength of a 48 wt% solids MFT sample flocculated by adding 1000 g/t of A3335 polymer flocculant increased from 395 Pa to 3,950 Pa 90 days after the addition of the same concentration of alkali activators. Addition of A3335 polymer flocculant to the MFT is observed to hamper the surface geopolymerisation process leading to a lower than expected increase in shear strength. However, low capillary suction time is seen when coupling flocculation with surface geopolymerisation, indicating that the treated MFT is amenable for water release.