Oil Sands Operators Tackle Tailings Management Challenges
Alberta’s oil sands miners are testing a variety of methods to process and manage tailings that meet the requirements of a tough new governmental directive
By Russell A. Carter, Managing Editor
Tailings are a byproduct of just about every mining and mineral recovery operation on the planet, and at no time in the past has tailings management, safety and reclamation received more regulatory and public scrutiny than in the current business environment. Against this backdrop, and simply by the nature of their size, the massive oil sands mining and processing operations in northern Alberta, Canada, are leading recipients of this attention. However, a recent regulatory development will increase pressure even more on the oil sands industry to demonstrate improved methods for handling and stabilizing the estimated 720 million m3 of fluid fine tailings currently stored in impoundments as well as future tailings production.
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 about tailings management escalated when approximately 500 ducks died after landing on a large Syncrude tailings pond in April 2008.
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 backgrounder 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 non-segregating 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.
In December 2008, the University of Alberta’s Oil Sands Tailings Research Facility (OSTRF) and the Canadian Oil Sands Network for Research and Development (CONRAD) hosted the First International Oil Sands Tailings Conference (IOSTC) in Edmonton, Alberta, where more than three dozen papers were presented on various aspects of tailings management. The conference’s comprehensive technical program illustrated that the oil sands operators are far from disinterested in finding better methods to manage and reclaim their vast tailings volumes, but success leading to compliance with Directive 074 will most likely not be easy, cheap, quick or achievable without cooperation from numerous stakeholders in the oil sands sector.
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 two-step 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 1.0% 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 freeze-thaw 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 two-month-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.
Another technique being studied by Syncrude is MFT accelerated dewatering, a process used in the Florida phosphate industry in which a 30-m-plus-deep impoundment is constructed in a mined-out pit and filled with MFT. Water removal is enabled and actively managed via a decant structure and rim ditching, resulting in a reduction of MFT volume by up to 50% after 3 to 5 years. The method, said Lahaie, delivers a soft soil material that when suitably capped can be reclaimed, and offers promise for implementation in Syncrude’s mined-out pits. In 2009, a field test will begin in which an 80 x 80-m x 10-m-deep MFT test pond will be constructed, filled with MFT and operated for several years, with the objective of providing planning and geotechnical parameters and to act as a platform on which environmental assessments can be carried out over the complete development cycle—pond filling through to reclamation.
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 consolidated tailings (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 water-capped 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 towards 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).
The newest oil sands project to start production—Canadian Natural Resources’ Horizon mine 70 km north of Fort McMurray—began shipping synthetic crude oil to the sales pipeline in mid-March of this year, and will employ an innovative method for managing its tailings using coarse-sand cyclones, fine tailings thickeners and injection of carbon dioxide as a chemical additive, promoting faster consolidation of solids and settling of fines. The consolidated tailings that Horizon hopes to produce will provide a number of benefits, according to CNRL’s Theo Paradis, including reduced tailings pond size, reuse of reclaimed warm process water, additional bitumen recovery, accelerated pond site reclamation and reduced green house gas emissions.
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 5 hours.
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.”
Behind the Directive
According to a background paper issued by the Alberta Energy Resources Conservation Board, four categories of performance criteria were considered to achieve Directive 074’s goals and objectives. Ultimately, fine tailings consolidation was selected by the board as the most appropriate initial tailings performance criterion. These four categories and the board’s opinions regarding possible performance criteria within each category are summarized below from the backgrounder:
Fine Tailings Consolidation: Fine tailings consolidation is achieved by making consolidated tailings (CT). CT may also be referred to as non-segregating tailings, engineered tailings, and composite tailings. CT technology is being applied at Suncor and Syncrude operations and is planned for all other approved projects. Measurement and reporting of the production and formation of CT is required for Suncor’s operations. Syncrude also monitors CT production. A similar regulatory approach could be applied at all approved schemes, with project-specific modifications if necessary, due to production technology and reclamation plan differences
Reduction of Fine Tailings Accumulation: All [operating] applications present a profile of expected rates of tailings production, management methods, and accumulations over the life of a mine. Applicants expect the accumulation of fluid fine tailings to be near zero or negative after CT production starts. All approved operations also monitor tailings production and tailings accumulation. However, there are complications to this. First, the actual tailings accumulations have greatly exceeded the projected volumes. Second, tailings ponds are very large and complex; their fluid contents are changing, due to ongoing settling and consolidation, tailings input, and water withdrawal. Measurement of ponds requires extensive sampling and a model to estimate the contents (solids, semi-solids, fluid tailings, and water).
A potential criterion for fine tailings accumulation would be expressed on the basis of oil sands processed (i.e., cubic meters of fluid fine tailings per metric ton of oil sands). Another potential criterion is that fine tailings accumulation never exceeds the volume of fine tailings measured at the time the first pond is available for CT production.
Water Management: Water management at oil sands projects is interdependent with tailings management. Ponds hold, clarify, and recycle tailings water. A tailings plan that leads to faster recovery, recycling, and eventual release of water would reduce fluid tailings accumulation. The faster recovery of water would reduce fresh water demand and require treatment to ensure that water quality is acceptable for reuse or release. Tailings performance could be managed indirectly by limiting the water withdrawal rates and water accumulation from the river and by increasing the required recycle rates from tailings ponds.
Water-based tailings performance criteria could be applicable to all mining projects. However, due to differences between projects, the values may vary from project to project. Ongoing monitoring and reporting will provide additional information necessary for the consideration of performance criteria associated with water management. Further work with operators is required to investigate water-based criteria as a means to regulate tailings performance.
Preparation for Reclamation: The ERCB assists in the reclamation process by regulating tailings ponds during the operational phase of a mine. The ERCB regulates tailings deposits up to and including their abandonment, thus preparing for the land reclamation stage.
Some potential policies were considered to link tailings management to reclamation, including:
- limiting land disturbance to a maximum footprint relative to land reclaimed,
- tracking and managing liability on a feature-specific basis,
- managing specific landforms and eventual state of the entire project area,
- adding project-specific conditions of approval (e.g., tailings performance requirements prior to production rate increases and/or new mine development), and
- applying project-specific and/or sector-wide checklists requiring structure-specific abandonment plans, consistent with closure and reclamation plans.