WorleyParsons Canada Services Ltd. (WorleyParsons) was retained by Brazil Potash Corporation (BPC) to complete a bankable feasibility study of their Autazes Potash Project, which is located near the municipality of Autazes, Amazonas State, Brazil. The project consists of a conventional underground mine, processing plant, tailings management area (TMA), product load-out and transportation system, as well as supporting ancillary facilities and systems.
As with all potash mining projects, the question always arises, how can we best manage the tailings to achieve environmental and fiscal responsibility? Typically, potash waste tailings are piled on the ground and cover a vast surface area. As an example, in Saskatchewan the piles cover many hectares and reach heights of up to 50 metres using a spigot technique of disposal. In Germany, Russia, and Belarus, tailings are disposed of by using a stacking technique with piles reaching over 150 metres in height. With the Autazes Potash Project, WorleyParsons and BPC seized the opportunity to reduce both the environmental footprint and the cost of the project as compared to conventional tailings piles.
The unique situation in the Amazonas State is the amount of annual rainfall, which can be up to seven times greater than other locations. This amount of precipitation creates a significant amount of brine when it comes into contact with the tailings pile (for every tonne of KCl that is refined, approximately two t of NaCl is produced along with one m3 to two m3 of highly concentrated brine). Without proper management, this brine has the potential to contaminate both surface water and deep aquifers.
To mitigate this challenge, WorleyParsons designed the TMA to meet the project’s environmental and financial objectives:
¥ Long-term storage and secure containment of mining and processing plant solid waste material;
¥ Collect, contain, and safely dispose of the brine generated during operations;
¥ Meet or exceed the regulatory requirements for tailings storage and brine disposal;
¥ Low CAPEX and OPEX; and
¥ Allow for environmentally responsible decommissioning and reclamation.
These objectives were achieved by designing a conveyors and staking equipment system to collect the tailings in a set of two large prism-shaped piles, built as two separate units. Each unit is comprised of one pile and one brine collection pond located alongside each pile. The piles each measure 1,200-metres-long by 1,200-metres-wide by 21-metres-high (at the highest point), with a 1:5 slope. The maximum pile storage volume is 25M m3 and the brine ponds are designed to store two weeks of runoff at the peak of the rainy season.
This system capitalizes on the very high rate of precipitation in the area, which creates an opportunity for accelerated decommissioning of the salt tailings. The brine resulting from the natural dissolution is collected in a series of storage ponds where separation of insoluble particles takes place. The clear brine from the ponds is disposed of in a 300-metre-deep brackish aquifer using conventional deep-injection wells. The remaining insoluble residues are sealed from the top, covered by topsoil and re-vegetated as part of mine decommissioning.
This TMA design has the capacity to dispose of 97M m3 of tailings over the 32 years of Life of Mine (LOM) and by using the natural dissolution of the tailings piles, not only will the process of mine decommissioning and site remediation be significantly accelerated, but the environmental footprint is reduced, thereby resulting in an overall cost savings.
By introducing this innovative tailings management technique to the potash industry, WorleyParsons set a new standard for cost efficient and environmentally responsible management of tailings.by