Fluid Tailings Thermal Drying: More Promising Than You Think
In December, Jonathan Matthews from Magpie Consulting Inc. will be presenting his paper at the International Oil Sands Tailing Conference. This paper is a culmination of decades of work and advocacy toward improving the efficiency and sustainability of the oil sands operation, namely in the challenges associated with the fluid fine tailings.
The following selections are taken from the upcoming paper, discussing the numerous benefits of Fluid Tailings Thermal Drying™.
Oil produced from Athabasca Oil Sands mining operations will be required as a responsible source of energy for decades to come. However, the environmental footprints associated with fluid fine tailings, land disturbance, and fugitive GHG emissions need mitigation to a greater extent than has been accomplished to date.
Fluid Tailings Thermal Drying (FTTD™) has exciting potential to convert fluid fine tailings (FFT) into immediately trafficable deposits, enhance water reclamation and conservation efforts, reduce net GHG emissions, and accelerate the pace of oil sands mine reclamation by decades.
FTTD™ can be implemented to deliver continuous throughput and will be most energy and cost-efficient when coupled with incumbent fluid fine tailings treatment technologies such as centrifugation, thickening, accelerated dewatering, or thin lift fines drying.
Contrary to popular perception, FTTD™ is feasible and can reduce bitumen production costs while enhancing environmental performance.
Thermal Drying of Fluid Fine Tailings: An Introduction
The primary confounding challenge with oil sands fluid fine tailings management relates to the stability of the clay suspension created when clay minerals in the oil sands ore become dispersed during the bitumen digestion and flotation processes.
An elegant solution to overcoming the challenge of clay fines suspension stability is to drive the water from the solids via evaporation in a controlled and efficient manner. FTTD™ delivers on this opportunity with precision.
FTTD™ is a straightforward concept, and the operating principle is illustrated in Figure 1. This illustration envisions converting FFT at 50% solids into a final FTTD™ product at 80% solids.
Figure 1: FTTD™ Process
As will be discussed, the 80% final solids target may be more aggressive than needed to achieve all the benefits of the drying process. The key point is that the FTTD™ drying circuit can accommodate a wide range of FFT compositions and the final solids concentration can be optimized to deliver optimal performance according to mine-specific needs and opportunities.
The heart of the drying process is a dryer that enables rapid drying of high-moisture wet cakes, pastes, or slurries without adding a carrier fluid, such as water. High-density fluid fine tailings mixes are fed to the inlet air stream by a mechanical screw auger where high-speed dispersion plates rotate and spread material into a thin layer along the periphery of the dryer. A heated air stream passes through and conveys the wet material, flashing off moisture. The water is separated from the fine mineral solids and non-volatile hydrocarbons. The solids stream leaving the drying vessel is dry to the touch. Average slurry residence times that would be expected in a dryer of suitable scale for oil sands tailings applications would be only a few minutes, enabling high throughput with modest equipment footprint.
Effective management of incremental energy footprint of the fines-drying process is critical to the viability of the process. The gross environmental footprint is primarily a function of energy consumed in the drying process. For the case examples discussed in this paper we assume that the heat source is from combustion of methane with air and electricity is needed to run pumps and blowers. The combination of combustion and electrical demand defines the gross energy footprint of the process.
The flue gas is ‘scrubbed’ in a cyclone to remove any fine particles and the water vapor in the flue gas can be recovered through condensation for reuse in plant operations.
Alternative configurations can be considered to optimize heat and water recovery to support optimal process efficiency and water conservation. For example, the exhaust gases could be directed to a heat exchanger where 90% of the thermal energy used to drive the water-solids separation process can be captured and used to heat process water needed for essential process needs such as bitumen extraction, froth treatment, or utilities service. The high value of the recovered heat and water reclaimed from the fine tailings suggests that integration with bitumen extraction and site utilities operations will realize optimal economic and environmental benefits. After drying, the solids fraction has the consistency of dry fine sand. As produced, the solid fraction will be drier than necessary to facilitate rapid low-cost compaction, capping, and reclamation so unprocessed fluid tailings are be added to the dried product to produce a mixture with an engineered water content to deliver site-specific optimal material handling, compaction and stabilization, and storage efficiency needs.
Exhaust gas temperature and composition will vary according to the type of heater used (i.e., electric or gas fired) and to some extent according to the characteristics of the tailings stream that is being processed. In either respect, most of the heat used to affect separation can be recovered and reused in utilities plants or as hot water to support extraction operations such that the recovered heat offsets natural gas that would otherwise be consumed in support of bitumen extraction activities
The FTTD™ product can be trucked to the point of final disposition and blended with FFT just before placement or blended at the thermal drying site location and the blended product transported to final disposition by truck or conveyor. In either scenario, the final product will be trafficable with heavy mine equipment and highly amenable for incorporation into mine deposits that can be reclaimed to terrestrial landscapes.
For over 100 years, Bepex has resolved industrial challenges in solids processing. We continue this pursuit today, focused on improving the sustainability of drying operations, often the most energy-intensive operation in any plant. Over the last decade, this work has led us to develop our Fluid Tailings Thermal Drying™ process.