Extraction of bitumen from oil sands widely employs a water-based extraction process—the ore is mixed with hot water and agitated, and 75% to 90% or more of the bitumen in the ore is floated off and recovered. Tailings from the process are composed of water, sand, silt, clay and residual bitumen.

The core of the problem is treatment and final disposition of what is called mature fine tailings (MFT). The coarse solids in oil sands tailings settle rapidly and up to 85% of the water contained in the tailings stream is recycled, but fine solids remain in suspension. These “fluid fine tailings” eventually thicken to about 30% mass solids over the course of a few years but only very slowly thereafter. This semi-consolidated material—mature fine tailings—is impounded in the early years of a project in external ponds or in disused pits as mining advances. MFT is a concern because it may have to be impounded indefinitely and because there is currently no way to reclaim it without further processing. About 1.5 barrels of MFT are produced for every barrel of bitumen.

According to the Energy Resources Conservation Board (ERCB), which regulates more than three dozen commercial oil sands plants in Alberta, no oil sands tailings pond in the province has yet been reclaimed to the ECRB’s desired standards. The ERCB and various joint review panels have expressed increasing concern for the overall tailings performance of the oil sands industry, and the level of concern escalated when approximately 500 ducks died after landing on a large tailings pond in April 2008.

Directive Establishes Tailings Criteria
The ERCB subsequently issued a draft directive on “Tailings Performance Criteria and Requirements for Oil Sands Mining Schemes” in June 2008. After an industry and public comment period, a final document was released on February 3, 2009, as Directive 074: Tailings Performance Criteria and Requirements for Oil Sands Mining Schemes. According to the ERCB, the directive is the first component of a larger initiative for the ERCB to regulate tailings management, and sets out industry-wide performance criteria for the reduction of fluid fine tailings and the formation of trafficable deposits for all oil sands operators.

Operators will be required to satisfy three categories of compliance to achieve the purposes of this directive: 1) management of fluid fine tailings—i.e., reduction of fluid fine tailings by production of consolidated tailings or equivalent means; 2) management of tailings ponds over their full life cycle; and 3) satisfactory measurement methods, record keeping and reporting. Failure to comply with these new requirements could lead to enforcement actions from the ECRB.

In a background document issued by the ERCB in conjunction with Directive 074, the agency explained that the most common approach for consolidating fluid fine tailings at existing operations is consolidated tailings (CT) technology, also referred to as non-segregating tailings (NS). CT involves mixing fluid fine tailings, coarse sand and chemical additives to form a nonsegregating mixture, which may be reclaimed to a solid deposit. The remaining fluid fine tailings are placed in end-pit lakes (EPLs) and reclaimed by water-capping. The agency maintains that, among other problems, water capping of fluid fine tailings in EPLs has yet to be proven on a commercial scale, and operators face a number of challenges in producing CT on specification and transforming CT ponds into solid, trafficable deposits.

During its 2008 test program for producing consolidated tailings by centrifuge dewatering, Syncrude began
depositing conveyor-borne centrifuged tailings in an impoundment and was pleased to observe the immediate
release of additional water (foreground) upon deposition. (Photo courtesy of Sycrude Canada Ltd.)

In a paper presented at the IOSTC, Syncrude’s Rick Lahaie said the company has been working closely with internal experts, fellow oil sands operators and others to advance several new tailings concepts through laboratory and field testing. The company is specifically looking at two dewatering techniques—MFT centrifuge and MFT accelerated dewatering—for possible implementation at its Mildred Lake operation, although they could be applied at other Syncrude leases.

Syncrude sees MFT centrifuge as a twostep process. The first step involves MFT dewatering using horizontal solid bowl scroll centrifuge technology with flocculent addition, forming two streams: centrate— i.e., relatively solids-free water having 0.5% to 1% wt solids, returned to the tailings water system for recycle; and cake, a 60% wt solids soft soil material capturing greater than 95% of the solids. Cake is roughly half the volume of the original tailings.

The second step involves subsequent dewatering of the cake by natural processes; consolidation, desiccation and freezethaw via 1- to 2-m-thick annual lifts, delivering a trafficable surface that can be reclaimed.

Syncrude began bench trials of this technique in 2005, progressing to a twomonth- long test in 2008 focusing on MFT flocculation and the centrifuge process, as well as on preliminary cake-transportation assessment involving conveyors, positive displacement pumps and a pipeline. Two “pods” of oilfield-scale centrifuges were operated in parallel, using two Alfa Laval Lynx 40 (nominal 400-mm diam) machines running in parallel to provide centrifuge cake for transportation assessment and bulk materials for geotechnical and environmental studies. Another MiSWACO 518 centrifuge (nominal 355- mm diam) was used to gain MFT flocculation and MFT centrifuge process understanding, flocculent optimization studies and centrifuge operational parameter assessments.

These tests showed promise, Lahaie noted. The technology is robust, with solids throughput on target and high solids capture in cake. Centrifuge cake transportation and deposition results also were encouraging and equipment scoping studies are planned for 2009, with centrifuge scale-up progressing from the nominal 400-mm to 1,000-mm diameter machines.

Alberta oil sands operators are looking for ways to produce quicker-settling, drier tailings (left) and reduce the
amount of harder-to-handle fine fluid tailings (right) that must be impounded and reprocessed to achieve eventual
reclamation. (Photos courtesy of Shell Canada.)

To consolidate its tailings, Suncor adds gypsum, a byproduct of its flue gas system for scrubbing sulphur in the process train. Suncor Energy’s Mark Shaw said Suncor actually began large-scale production trials of CT as early as 1996, and in the course of ongoing trials has studied additional technologies that include standardized pond assessment methods, enhanced pond modeling techniques, new field deposition systems, process control enhancements, water chemistry control methods and CT consolidation improvement. Suncor’s studies showed tailings technology advances could manage MFT inventory and free water for re-use, with the “final closure” landform requiring a watercapped EPL lake containing MFT.

Suncor’s Industrial-scale CT studies indicate that this approach results in a mixture of deposit types, including large deposits that exhibit sand-like behavior, consolidate quickly and provide water for re-use; along with volumes of CT that consolidate more slowly and require additional closure technologies to speed reclamation (coke capping, dewatering, etc.).

Shaw said Suncor’s Pond No. 5—the industry’s first CT pond—will be filled with CT in 2009 and focus will move toward final closure. A CT operational cycle would include the following steps:
a) Construction of a large-scale fluid retaining structure (pond),
b) Placement of a small volume of MFT into the structure to reduce segregation of future CT deposits,
c) Installation and operation of MFT and water removal systems,
d) Deposition of a mixture of CT and sand until all fluid deposits are removed and pond structure is filled, then eventual final reclamation,
e) Return to (a).

DCNRL’s Horizon operation, the newest oil sands mine, will use horizontal and vertical cyclones, high rate
thickeners and CO2 injection to process its tailings. Shown here is an illustration of the Horizon cyclone plant.
(Photo courtesy of Canadian Natural Resources)

The Horizon tailings consolidation scheme will employ 55-in.-diam horizontal cyclones fitted with a 13-in.-diam apex to handle rocks. These cyclones will dewater underflow from the extraction plant’s primary froth separation cell (PSC). A bank of 26-in.-diam vertical cyclones with 7-in.- diam vortexes follows; together, this combination is expected to achieve a targeted underflow density of 72%.

The tailings treatment setup also includes several 70-m-diam high-rate thickeners designed to dewater fine flotation tailings and to clarify and recover the warm process water for reuse in the extraction processes. The thickeners will produce underflow with a density of >50 wt% solids, along with overflow warm water containing less than 0.5 wt% solids. Retention time will be about five hours.

How Much?
In the weeks following the release of the tailings directive, the province’s oil sands operators declined to offer specific comments on how they were going to achieve compliance with its requirements, or how much it would cost, until they had more time to study the situation. As most operators are already either producing or preparing to produce consolidated tailings, it seems unlikely that compliance would require major physical changes in their basic plant setups—but the overall tailings performance, reporting and planning requirements imposed by the directive will definitely involve additional costs. One of the few indicators of the rough magnitude of these costs was provided by the Canadian Oil Sands Trust (the largest owner of Syncrude, with 36.74% of the project) in its third-quarter 2008 financial and operating results report, in which it stated that over the next few years Syncrude expected to incur additional charges in its cost per barrel of $5 to $10 per barrel annually for large environmental and infrastructure projects, including tailings system initiatives “…to improve and supplement the effectiveness of systems used to separate water from sand and clay.”