In oil sands open-pit mining processes, hot water is used to separate very heavy oil (bitumen) from the sticky sands. This water is then sent to a tailings pond, where the mixture of water, clay, sand and residual bitumen begin to settle out, allowing water to be recycled and used again in the mining operation.
The Filter Press is a technology that COSIA companies are assessing to speed water reclamation by mechanically pressing water out of Mature Fine Tailings (MFT). MFT is a settled out layer of water and miniscule clay particles, called fines, which can remain suspended for decades if not treated. Filtration technology had long been dismissed as being impractical due to the eventual clogging of filter materials by fines and residual bitumen. However, this new Filter Press Project includes a two stage chemical process, which keeps the fluid fine tailings from sticking to the filter, allowing the press to squeeze water out of the MFT. Once the water is pressed out, the resulting clay fines cake is dense enough, and strong enough, to be shipped directly into a reclamation area to become the basis for efficient land reclamation.
The Filter Press Project combines chemically treating MFT and then mechanically pressing the treated MFT between large metal plates lined with filter materials. The filter and mechanical press is a well-known technology, but it is the chemical treatment process and successful application of mechanical filtration to MFT that makes this project unique and innovative.
The chemical treatments applied to MFT in this project are both flocculation processes. Flocculation, in the field of chemistry, is a process where suspended particles in a mixture come out of suspension and bind together to form larger ‘flocs’, or flakes. There are two types of flocculation used in the Filter Press Project: an anionic and cationic process. The anionic flocculation causes fines to group together to make larger chunks using negatively charged ions. Cationic flocculation does the same, but using positive charged ions. Using these two processes, in the right combination, keeps the filter cloths free of binding fines while allowing released water to pass through.
The pressed out water is captured and returned to tailings ponds to be recycled into mining operations. The clay cakes left behind are of varying consistencies, depending on the length of time they were pressed and the nature of the chemical treatments applied to the MFT. A three-month study was undertaken in 2012 to assess the combinations of pressing time and variations of chemical treatments to achieve the desired outcome.
Various press parameters such as cake thicknesses, time of pressing and chemical inline mixing were tested, with varying results. Batch mixed flocculants added to MFT showed the solids content of the cakes increased with press cycle time. However, a trademarked mixture of flocculants added ‘in-line’ to the MFT showed solids content of the cake was significantly higher at the same cycle time as the batch, and also found the solids content reached a plateau at approximately 30 minutes.
Filter pressed cakes at various cycle times
The environmental benefits are that more water is released from tailings to be used as processed recycled water, reducing the need to make up that volume of water from other sources. The residual clay fine cakes meet the Alberta government’s regulatory requirement for density and strength of tailings materials that can be used in reclamation. Disposing of these fines in a disposal area that is being prepared for reclamation, and releasing the water back into the recycling system, reduces the size of tailings ponds over the long term, the need for additional water, and increases the speed by which companies can reclaim disturbed lands on their mine sites.
Albian Sands and Teck Resources joined efforts with Ledcor Nalco Services (LNS) to test and validate the effectiveness of using these flocculation chemicals and pressure filtering via a large pilot plant press. The pilot press was built by LNS in 2012, for the purpose of proving the technology. The value of COSIA was linking a number of companies together to work with one supplier. If the decision is made to move forward with a commercial-scale demonstration it would be through a COSIA Joint Industry Project designed to evaluate and trial the technology.
In June 2017, Canadian Natural acquired 70 per cent ownership of the Athabasca Oil Sands Project and became operator of Albian Sands and all of its technology.