Article pubs.acs.org/EF
Migration of Fine Solids into Product Bitumen from Solvent Extraction of Alberta Oilsands Hossein Nikakhtari, Sebastian Wolf, Phillip Choi, Qi Liu, and Murray R. Gray* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2 V4, Canada ABSTRACT: The extraction of bitumen from the oilsands using solvents has the potential to avoid the environmental problems with wet tailings associated with water-based extraction technology. One of the key performance parameters for this technology is the concentration of fine solids in the recovered bitumen. In this study, the concentration and properties of the fine solids in cyclohexane-extracted bitumen were examined as a function of the water content of the ore. Total water contents from 3.4−13.4 wt % gave the lowest concentrations of fine solids in the bitumen, ca. 0.2 wt % of the initial mass of a low-fines content ore. Higher fine solids contents in the bitumen were observed with water concentrations outside this range and with higher concentrations of fine solids in the initial oilsands ore. The fine solids exhibited a distribution of surface properties, and the most hydrophobic, carbon rich solids were carried into the bitumen even under the most favorable conditions. When the bitumen was more contaminated with fine solids, due to higher fines in the ore or lack of water to bind the fines to the sand, the carbon content of the fine solids decreased. The majority of the carbon in the fine solids was organic material due to bitumen and surface adsorbed components. Despite carbon contents over 20 wt %, XPS analysis showed significant surface silicon and aluminum, and contact angles were in the range of 80−88°, consistent with partial organic coating of the mineral particles. Characterization of the fine solids by SEM, particle size, and mineralogy suggested that the migration of the fine solids into the bitumen, versus remaining with the coarse sand, was significantly affected by the organic surface deposits on the particles.
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INTRODUCTION The solvent extraction of bitumen from the Alberta oilsands has gained renewed interest due to the difficulty in dealing with the fine solids fraction in the tailings from the water-based extraction processes that are the current best-practice. A solvent-based process would have potential benefits in producing dry solids after extraction, which could be placed immediately into the mined-out area. In order for such a process to be feasible, it must operate on a very large scale, provide very high recovery of the solvent from the sands and fine solids, and give a bitumen product relatively free from contaminating fine solids. A number of studies have examined solvent extraction,1−5 but recent attention has been focused on light hydrocarbon solvents such as cycloalkanes that are good solvents for bitumen but have low toxicity and low boiling point (high vapor-pressure) which makes it easy to enable recovery by vaporization.6,7 Nikakhtari et al.7 found that cyclohexane combined good solvent properties with low solids content in the product bitumen and rapid evaporation of the solvent from the extracted sands and fine solids. The contamination of the product bitumen with fine solids depends on several factors, including the type of solvent,7 the amount of water present in the mixture of ore and solvent,8 and the mechanical shearing forces on the solvent-oilsand mixture during the extraction process, which depend on the specifics of the operation of the separation equipment.8 The fine solids in the oilsands ore include particles with a range of surface properties, from hydrophobic to hydrophilic,9−11 and the former particles tend to appear in the bitumen product in both solvent extraction12 and in the conventional water-based process.9 While the organic material on the fine solids has been analyzed,11,13 the surface properties of the organic coated fine © 2014 American Chemical Society
solids have received much less attention, and no work has appeared on the properties of solids in the product bitumen from solvent extraction of Alberta oilsands using light solvents such as cycloalkanes. The objective of this paper is to determine the migration of fine solids into product bitumen as a function of the ore grade and moisture content of the ore and to define the surface and mineralogical properties of those fine solids.
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MATERIALS AND METHODS
Materials. Three oilsands ore samples (high, medium, and low fines content oilsands) were provided by Syncrude Canada Ltd. Cyclohexane (Certified ACS, Fisher Scientific, Canada) was used as a solvent for nonaqueous bitumen extraction experiments. Experimental Procedures. Extraction. Figure 1 shows the laboratory protocol that was used to extract bitumen from oilsands using cyclohexane. The steps of the protocol were explained in detail in previous work.7,14 In summary, 250 mL solvent was used to extract bitumen from 150 g of oilsands in two mixing steps. The first step used 100 g of solvent in a 500 mL Teflon bottle mounted on a rotary mixer (60 rpm, Rotator drive STR4, Stuart Scientific) for 10 min. The solids were allowed to settle for 30 min, and then the sediment was extracted with 50 g of solvent by the same method. Both steps were carried out at room temperature (24 ± 0.5 °C) and ambient pressure. The extracted ore, or dry gangue, was recovered as the +45 μm fraction after two stages of washing by cyclohexane on a 45-μm aperture sieve (ASTM E-11 Standard test sieve no 325, Fisher Scientific Co., USA on a Forney LA-0439 vibrator). The undersize product from the sieve (i.e., bitumen-solvent solution, or product bitumen) was centrifuged at 4000 relative centrifugal force (RCF) for 1 h to recover the fine solids. Selected samples of the centrifuge solids were extracted with toluene to remove the residual bitumen. This was achieved by placing Received: January 4, 2014 Revised: April 21, 2014 Published: April 22, 2014 2925
dx.doi.org/10.1021/ef500021y | Energy Fuels 2014, 28, 2925−2932
Energy & Fuels
Article
Figure 1. Flow diagram for nonaqueous extraction process used in the laboratory. the centrifuge solids in a 0.22-μm pore-sized Millipore filter and repeatedly rinsing with toluene until the filtrate was colorless. The amount of toluene used depended on the type of oilsands samples; for high-fines ore 400 mL of toluene was used to wash 2 g of centrifuge solids, for medium-fines ore 100 mL toluene was used to wash 0.35 g of fine centrifuge solids, and for low-fines ore 125 mL of toluene was used to wash 0.4 g of centrifuge solids. The extraction test of the low-fines oilsands with added water was also carried out. To study the effect of connate water on the nonaqueous extraction process, the oilsands samples were vacuumdried at 75 °C overnight prior to some extraction tests. The carbon content of these dried oilsands samples was the same as the initial material. Analysis. The Dean−Stark procedure was used to determine the bitumen, water, and mineral solids contents of the samples15 and to isolate the mineral solids for further analysis. In this method, a Soxhlet extraction with toluene was done for 3−4 h followed by measurement of residual toluene in the extracted bitumen by a precalibrated FTIR (Nicolet 6700, Thermo Scientific). The fine solids were separated from the Soxhlet extracted oilsands by wet sieving on a 45-μm aperture sieve. For this purpose, the Soxhlet extracted solids were washed with water on the sieve until the draining liquid was clear. The collected liquid from rinsing the sieve was centrifuged (Avanti J-30I centrifuge, Beckman Coulter, Mississauga, ON, with a JA-10 rotor and 250 mL Teflon containers) at 5800 RCF for 1 h, then the sediment was removed, and the supernatant was centrifuged again at 13800 RCF for 1 h. A particle size analyzer (Mastersizer 2000, Malvern Instruments, Westborough, MA) was used to determine the volume fraction of fines (i.e., particles that are
