Enhancing densification by pH-switchable polymers for tailings management

摘要

Paste and thickened tailings are widely applied in the mining sector to reduce the volume of waste slurry and enable rapid recovery of water for recycling. Polymers are often added at an appropriate dosage to flocculate fines before being discharged to tailings ponds. The performance of polymers depends greatly on their charge density and molecular weight. While commercial polyacrylamide polymers address the condition of rapid settling, they are quite limited for achieving the desired sediment properties, such as high sediment shear yield stress and low retained water content. To achieve both high sedimentation rate and high yield stress of the sediment, our research over recent years has focused on the synthesis of switchable polymers that exhibit two different states of conformation, depending on solution temperature and/or pH. It is highly desirable to flocculate fines to achieve rapid settling when the polymer is in an extended state, followed by coiling (increasing hydrophobic interaction or reducing hydrogen bonding) to produce dense floes that release trapped water and increase the strength of the consolidated sediment.With the great relevance of oil sands tailings to Alberta, Canada, we focus on the synthesis of a pH-responsive or pH-switchable polymer, poly(acrylamide-co-DMAEMA) known as PAM-co-DMAEMA. With proper molecular weight and PAM to DMAEMA ratio, the PAM-co-DMAEMA can transition from an extended conformation at pH > 7.5 to a coiled state at pH = 7.5 to 6.8. With the warm water bitumen extraction process operating in a pH range from 8.5 to 9.0, and CO_2 purging as a viable option to lower tailings water pH to 7.0, we are able to fine-tune the properties of the synthesised PAM-co-DMAEMA, so that the desired switching functionality can be achieved for treating oil sands tailings. Settling rate, clarity of supernatant, and sediment bed height of model and actual oil sands tailings were all shown to be a function of the polymer conformation, with the best performance achieved with the pH switch from pH 8.2 to 7.0.