Reducing Environmental footprint through tailings treatment
Up in the northern region, Alberta, the bitumen (a very viscous form of petroleum) is present in the deposits known as oil-sands. Such deposits believed to form the same way as fossil fuels are composed of bitumen, water, and solids. The solids component made up of coarse sand, fines such as clays, and other minerals could be as high as 85%.
Ther are two ways to extract bitumen from oil-sands, namely, surface mining and in-situ technologies. In surface mining, the first step is to remove forests and few surface layers of soil to reveal the oil-sands deposit. The ground removed in the preparation step is collected and saved for restoring the mined area later. The clumps of oil-sands from shoveling are further reduced in size using crushers. The belts carry the crushed lumps to a slurry preparation plant (SPP) to prepare a thick slurry using water.
From SPP, the slurry lines send this thick liquid to a separation vessel to split the oil-sands into three layers using hot water. Bitumen-rich froth, being lighter, floats on the surface, also containing some fine solids and water. The heavy coarse solids sink to the bottom of the vessel is termed as coarse tails, a waste product. The middle layer is processed further in several steps to extract the remaining bitumen. Each of the subsequent steps produces a waste stream, called fluid tails. Such waste streams combined, consisting of solids, water, and unrecovered bitumen is called regular tails.
There is another type of tails produced during froth treatment. What is done here is the dilution of bitumen froth with a diluent. The most commonly used diluent is naphtha. This addition makes it easier to process froth to obtain a bitumen-naphtha stream. However, not all the naphtha goes with the bitumen product. The leftover stream is treated in a recovery unit to pull out naphtha. Still, some traces of naphtha ends up in the waste product, known as froth treatment tails (FTT).
Tailings ponds
Tailings ponds are massive settling basins, to store and settle tailings from bitumen extraction (explained previously). The coarse solids in the tailings, being heavier, sinks to the pond bottom. As the solids settle, a part of fines entraps along to form a bed at the bottom. The clarified water, in turn, comes to the top, fit for reuse. However, the remaining fines suspended in the tailings pond water do not settle (cross-sectional view in the figure below). This suspension starts forming a sludge-like material which is commonly called thin fluid tailings (TFT) or fine fluid tailings (FFT). TFT unfit for process use can take decades to settle on their own.
In the Alberta region, bitumen extraction and its processing over the decades have produced over a trillion liters of tails. Because there has been no regulation to address the tailings in the past, ponds have been left untreated. With time, TFT starts degrading into another form of tails underneath referred to as mature fine tailings (MFT). Consistency of a gel-like material, MFT holds a lot of water with up to 35% solids.
Why do tailings ponds need consideration?
Bitumen ending up in tails, being lighter, floats on the surface of tailings ponds. It is a concern for resident and migratory birds trying to use these ponds. Some proportion of the birds landing on these tailings ponds die because of oiling.
MFT displays a high water retention capacity. In other words, there is a lot of water entrapped in the MFT, if extracted, increases the amount of water available for reuse in the bitumen extraction process.
An article from Government of Canada lists various compounds that are present in the tailings ponds harmful to the environment. Such compounds, termed as PAHs, are pollutants. While evaporation takes place, PAHs tend to escape from tailings ponds to the atmosphere. Some of these compounds are volatile, meaning more easier entry into the air. Tailings ponds are also known to emit other compounds (VOCs, RSCs), contributing to the overall smog and odour burden in the region.
The Alberta Government has put in place a tailings management program. What this means is the restoration of disturbed landscape to a new land that can support vegetation and wildlife. It is known as the reclamation process. All the fluid tailings, per program, be ready to reclaim within ten years of the end of mine’s life.
There are various microbes present in tailings ponds. Such microbes called bacteria feeds on hydrocarbons and minerals present in the tailings to produce concerning gases. For instance, a form of bacteria, called methanogens, feeds on the naphtha present in the pond from FTT. This process releases methane, a greenhouse gas.
Methods to treat MFT
As we know, MFT holds a lot of water, unavailable for bitumen extraction. One option to facilitate water release from the MFT is through chemicals (like polymers) addition. It is more economical and feasible to add chemicals in the line through which MFT is running. Therefore, the usual practice is to pump out the MFT from the tailings ponds to a disposal area. The polymer, when added to the flowing MFT stream, mixes well and starts binding to the clay particles to form aggregates. As a result, water held in the clay particles starts releasing.
In one technique using the above principle, the line delivers the treated MFT in thin layers to a disposal area. As MFT deposits into a sloped containment area, the released water from the collection area is recycled back. The resulting dried solids can be reclaimed in place or moved. This process is termed as atmospheric drying the MFT.
In another technique, two chemicals are added to the flowing MFT stream a certain distance apart to provide mixing time. The first chemical, coagulant, added to the MFT stream, promote effectivity of the polymer added down the line. As the MFT discharges into the basin, agglomerated solids start settling down. The entrapped water, therefore, releases from the MFT matrix. The water comes to the surface, suitable for reuse. The process treatment of MFT through this method is called accelerated dewatering (ADW).
There are other techniques for MFT treatment. Regardless, the objective behind all the works is to accelerate water release from MFT.
How treating MFT helps the environment?
We know MFT when treated releases reusable water. As a result, there is less requirement to pull fresh water for bitumen extraction from the environment (lakes and rivers).
The other added advantage of treating MFT through proven technologies (like ADW) is the reduction in existing MFT inventory. The need to build more tailings ponds, therefore, reduces.
MFT treatment reduces the volume of tailings ponds at faster rates. The time it will take to reclaim land drops to a fraction of the time it would take with no MFT treatment.
