Article pubs.acs.org/EF
Rare Earth Elements in Alberta Oil Sand Process Streams Elliot Roth,†,‡ Tracy Bank,†,‡ Bret Howard,† and Evan Granite*,† †
National Energy Technology Laboratory, United States Department of Energy, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ‡ AECOM, Pittsburgh, Pennsylvania 15236, United States S Supporting Information *
ABSTRACT: The concentrations of rare earth elements in Alberta, Canada oil sands and six oil sand waste streams were determined using inductively coupled plasma mass spectrometry (ICP−MS). The results indicate that the rare earth elements (REEs) are largely concentrated in the tailings solvent recovery unit (TSRU) sample compared to the oil sand itself. The concentration of lanthanide elements is ∼1100 mg/kg (1100 ppm or 0.11 wt %), which represents a >20× increase in the concentration compared to the oil sand itself and a >7× increase compared to the North American Shale Composite (NASC). The process water, which is used to extract the oil from oil sands, and the water fraction associated with the different waste streams had very low concentrations of REEs that were near or below the detection limits of the instrument, with the highest total concentration of REEs in the water fraction being less than 10 μg/L (ppb). Size and density separations were completed, and the REEs and other potentially interesting and valuable metals, such as Ti and Zr, were concentrated in different fractions. These results give insights into the possibility of recovering REEs from waste streams generated from oil sand processing.
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water, dissolved inorganic salts, and organics.1 Kasperski and Mikula1 described the mineral content of the oil sands and the main tailing streams, shown in Table SI-1 of the Supporting Information. In the same review, monazite, a REE phosphate, was identified in the heavy minerals in the Athabasca oil sands using scanning electron microscopy and energy-dispersive Xray spectroscopy (SEM/EDS).10 In another recent review of REE resources, monazite is reported in Athabasca oil sands; however, no REE concentrations are reported.10,11 Other characterization and mineral studies on Canadian oil sands have also been conducted by Kaminsky et al., and it is recommended that the reader consult these studies for more detailed characterizations of mineral content.10,12,13 Although Canadian oil sands and wastes have been extensively studied for organic and mineral content, there has been little detailed analysis of individual REEs using inductively coupled plasma mass spectrometry (ICP−MS). This study adds detailed information on the elemental concentrations of typical waste streams associated with Alberta oil sands and gives direction on which streams would most likely be economic sources of REEs and other metals for recovery and extraction. While it is acknowledged that a large heterogeneity in the concentrations of REEs and other metals could be expected in oil sand tailings, this study is meant to give insight into the possibility of oil sand waste streams as a source of REEs.
INTRODUCTION Oil sands, tar sands, or bituminous sands are large sources of oil in Canada and yield over 1.5 × 106 barrels of oil a day, making Canada one of the world’s top oil producers.1 In addition to being a significant source of unconventional oil, the water-based extraction process produces extremely large volumes of solid and liquid waste.1−3 Approximately 10 wt % of the oil sand is bitumen, and approximately 90% is residual sand, silt, clay, and water that is mostly sent to waste ponds, which may be potential sources of valuable elements.1 For example, titanium and zirconium have been targeted as valuable and extractable metals in these wastes.1 In addition, these byproducts are potential sources of rare earth elements (REEs).4 It is estimated that there are 5.6 trillion barrels of bitumen and heavy oil resources around the world, which could provide enormous amounts of byproduct streams for extraction of valuable or critical elements.5 The National Energy Technology Laboratory (NETL) recently initiated research for the determination and recovery of rare earths from abundant domestic coal byproducts, such as mining wastes, ashes, gasification byproducts, and coal preparation wastes.6−8 Many other research organizations have also initiated efforts for the determination and recovery of rare earths from unconventional sources, such as coal byproducts and phosphate mine wastes. It has been reported that up to 0.3−0.4 wt % of REEs could be concentrated in the combined centrifuge tailings;4,9 however, no details on the rare earth distributions within different oil sand process streams have been reported. The purpose of this study is to quantify the amount of REEs in different process streams that are derived from the production of Canadian oil sands and determine which metals can be concentrated using size and density separations. The three main tailing streams from mined oil sands are coarse tailings, fluid fine tailings, and froth treatment tailings. All of these streams contain different amounts of minerals, © XXXX American Chemical Society
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MATERIALS
Canadian oil sand and oil sand waste samples were received from CanmetENERGY in approximately 100−500 mL quantities. Except for the solid oil sand, these samples all contained some amount of water Received: November 30, 2016 Revised: March 15, 2017
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DOI: 10.1021/acs.energyfuels.6b03184 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels and solids. The sample names, approximate amount of solids, and additional information are provided in Table 1.
using He as the collision gas. For quality control, standard reference materials of similar composition were processed with the study samples. Precision was calculated on the basis of duplicate measurements of samples. The procedure for digesting and analysis of the samples for rare earths has been described in a United States Department of Energy (U.S. DOE) Topical Report.14 The oil sand tailings solvent recovery unit (TSRU) tailings were size-separated using stainless-steel sieves from 100 to 500 mesh. The sample was first dried in a drying oven at 110 °C, gently crushed in an agate stone mortar and pestle, and then sieved in 3 in. stainless-steel sieves. The samples were then fused and analyzed by ICP−MS. Density separations were conducted using sodium polytungstate (Geoliquids, Inc.) as-received, with a reported density of 2.89 kg/L. SEM and X-ray diffraction (XRD) analyses were conducted on the oil sand TSRU samples to study mineralogy. Powder XRD was used to determine the crystalline phase composition of the TSRU solids and dense fraction using a PANalytical X’Pert PRO multipurpose diffractometer equipped with a Cu anode operated at 45 kV and 40 mA and a divergent beam monochromator. The sample was ground in an agate mortar and pestle before analysis. The dense fraction sample was mounted on a zero background quartz slide for analysis. It was analyzed as separated; therefore, preferred orientation of phases was expected. Phase identification was verified by comparison of the collected data to the International Centre for Diffraction Data (ICDD) inorganic compound database. A FEI Company Quanta 600 field emission scanning electron microscope and an Oxford Inca Energy 350 X-act energy-dispersive Xray analyzer were used to investigate the mineralogy of the dense fraction sample. The sample was mounted on conductive tape on aluminum planchets. An accelerating voltage of 20 kV and a working distance of 10 mm were used to examine the mineral particles in lowvacuum backscattered electron mode.
Table 1. Samples Received from CanmetENERGY sample number 1 2 3 4 5 6 7
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sample ID oil sand oil sand process water oil sand TSRU tailings oil sand mature fine tailings oil sand fluid fine tailings oil sand extraction middlings coarse tailings
approximate percentage of solids 0.05 unknown
additional information from field operations from field operations, considered close to froth treatment tails
40 8.6 unknown >60
derived from in-house batch extraction of sample 1 derived from in-house batch extraction of sample 1
EXPERIMENTAL SECTION
The liquid samples were separated from the solid samples by centrifugation at 3000 revolutions per minute (rpm) for 15 min, with a reported relative centrifugal force of 1590. The liquid supernatants were decanted, and half was filtered through a 0.45 μm filter for measurement of metals and REEs. The filtered samples were diluted 10-fold in 2% HNO3 before analysis using Nexion 300d ICP−MS (PerkinElmer). The unfiltered supernatant was used for measurement of pH and conductivity. Solid samples were dried at 60 °C under house nitrogen for 48 h. To digest the material, approximately 50 mg of dried sample was mixed with 400 mg of lithium metaborate in a platinum crucible. Samples were fused at 1100 °C for 5 min and then digested in 100 mL of 5% nitric acid on low heat with constant stirring. Samples were diluted 20-fold in 2% nitric acid before analysis by Nexion ICP−MS. ICP−MS was operated in kinetic energy discrimination (KED) mode
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RESULTS AND DISCUSSION The liquid fraction of samples contained very low concentrations of REEs; measured concentrations were
