Canadian oil sands are a complex mixture of sand, clays, water, and bitumen. For surface mining oil sands production, a hot water extraction process (HWEP) has been widely applied. In this process, hot water and processing aids are mixed with oil sands to liberate bitumen from the minerals by shearing the oil sand particles. A bitumen-rich froth is formed, and an organic solvent is then used to dilute the froth, decrease the viscosity of bitumen, and facilitate the removal of minerals and water droplets.

This process has a number of environmental drawbacks, including the enormous consumption of water—resulting in 4 m³ of fluid tailings for treating 1 m³ of oil sands. As a result, there is an ever-growing inventory of process-affected waters and tailing pond materials. After years of settling, a stable suspension in water (mature fine tailings) is formed by the fine particles, high concentration of naphthenic acids, sodium sulfate, and unrecovered bitumen. This is banned to discharge because it is toxic to a variety of organisms once released into the environment.

Organic solvent extraction technology was developed as an alternative. However, Li et al. note, it is difficult to extend this technology to the oil sands industry because these organic solvents are volatile, flammable, toxic, and difficult to recycle from the spent sands.

Ionic liquids have also been explored as alternatives—a recent study found that an IL improved the bitumen recovery to 90% in solvent extraction using toluene at room temperature. The issues associated with tailing water were addressed as only a slight amount of water was used to remove IL from the residual sands and clays. However, Li et al. suggest, the extensive application of this method may be limited by the facts that (i) the high viscosity of the IL would make it strongly adhere to the inner wall of pipelines and block the equipment, which consequently increases the difficulty of transportation and gives rise to the loss of ILs, and (ii) toluene tends to cause significant environmental impacts due to its toxicity.

The specific objectives of this study were to (i) find out the feasibility of adding additives to reduce the viscosity of IL, (ii) apply ILs with low viscosity to enhance solvent extraction using organic solvents with less toxicity than aromatic solvents, (iii) evaluate the impacts of various factors on the bitumen recovery, and (iv) determine the optimal solvent extraction conditions.

—Li et al.

The team used the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate ([Emim][BF₄] to enhance a composite solvent (acetone and n-heptane). They found that this combination resulted in a high recovery efficiency of up to 95%, compared to 86% with the use of acetone and n-heptane alone. There was less water consumption, no issues of tailing water, negligible clay fines entrained in the extracted bitumen, no IL residue in the treated sands, and high performance of reusable IL (bitumen recovery >92% after reusing 10 times).

The IL ([Emim][BF4 ]) used in this study is much less viscous than many other ILs, which rendered the operation easier and required less energy consumption for pipeline transportation. Additionally, the acetone played an important role in the solvent extraction as it further decreased the viscosity of IL and improved the solvent ability to solubilize bitumen. However, the ratio of acetone applied should be restrictedly controlled because excess acetone would diminish the enhancement of IL and result in the incorporation of IL with bitumen.

The optimal ratio of acetone to n-heptane in the composite solvent (2:6, v/v), mixing time (10 min), stirring speed (450 rpm), temperature (25 °C), and organic solvents to oil sands ratio of 4:1, v/w were determined for implementing this technology. The overall results encouraged the extension of the bitumen recovery technology using [Emim][BF₄ ] and composite solvent (heptanes and acetone) from bench scale to the oil sand industry.

—Li et al.

Resources

• Xingang Li, Wenjun Sun, Guozhong Wu, Lin He, Hong Li, and Hong Sui (2011) Ionic Liquid Enhanced Solvent Extraction for Bitumen Recovery from Oil Sands. Energy & Fuels DOI: 10.1021/ef2010942