Mine Waste Project Aims to Reduce Potash Mines' Environmental Footprint
PotashCorp has developed an innovative way to deal with one portion of its potash mine waste stream.
All but two of the potash mines in Saskatchewan produce a waste stream of water-insoluble material and primarily water-soluble sodium chloride salts collectively known as "fine tailings." These tailings are discarded from the mill in the form of salt brine that is 70 percent solid and 30 percent liquid. It naturally flows and cannot be successfully stacked as is done with mines' coarse tailings.
Fine tailings have been stored separately in shallow engineered ponds, or cells, that take up a great deal of land and remain soft indefinitely, providing little opportunity for capping with soil and then vegetation.
In 1999, PotashCorp entered into an agreement with Saskatchewan's Ministry of the Environment that allowed it to build a new fine tailings cell at its Lanigan operation, provided it did extensive research to find a more environmentally acceptable solution to fine tailings disposal by 2010.
In the eight years since, the company has invested millions of dollars in research, focusing first on two alternatives.
The first method involved ''washing" the fine tailings to remove all water-soluble materials (salts), leaving relatively clean solids that could be de-watered and stacked, or possibly distributed on farmland. It required exceedingly large amounts of fresh water and was determined not to be environmentally sound.
"So much water would be needed to wash these fine tailings that you might even find yourself drawing down underground aquifers and affecting local wells," said PotashCorp Environmental Director Mark Getzlaf.
The second method, pulling the liquid out of the fine tailings by centrifuge, was also seen as environmentally and economically unfeasible because, while conserving water and land, it was energy- and chemical-intensive.
All was not lost, however. Through this exhaustive research on removing brine from fine tailings, it was discovered that the liquid brine could be removed in situ, after fine tailings were in modified engineered tailings ponds that would allow free brine to drain from the fine tailings into brine storage ponds – essentially leaving the tailings high and dry.
The most promising approach to future fine tailings management appears to be a method in which the tailings are pumped to a containment cell and the brine is allowed to flow freely through a permeable barrier – a porous berm constructed at the downstream end of the cell. This brine is recovered and the remaining fine tailings consolidate into a solid form similar to what can be achieved by centrifuge.
"This approach still needs more testing and research, but the initial results are very positive," Getzlaf said. "Not only do we have a potential solution for new fine tailings facilities, but existing facilities can be retrofitted to this new technology.
"The fine tailings that are left over will be solid enough to stack vertically, which conserves land, and will be more easily capped and planted when the facility is ready to be decommissioned."
The ultimate fate of the fine tailings cells would be capping and revegetation. Most of the exposed surface area would be earth, which could be seeded to grass.
PotashCorp is working with another potash company in the province to test this method, which may one day be the standard way of dealing with fine tailings in Saskatchewan.