40 FILTRATION AND DRYING

Can alternative tailings disposal become the norm in mining? New technology promises to reduce water, create a smaller environmental footprint and reduce long-term risks for mines

By Amanda Adams, Principal, Mining at Stantec and Theo Gerritsen, principal geotechnical engineer, Stantec’s Global Tailings Group

here is no doubt that the time tailings management is an una- Mining low grades of ore has resources extracted from voidable necessity at many mines. resulted in increased water use per Tmining are critical to our way unit of production. In some cases, of life. Likely, you’re reading this WATER, WATER, WATER the availability of water is one of the on your computer, smartphone, or What are the challenges associated greatest constraints on mine develop- tablet. Mining touches all our lives with conventional tailings? Well, ment. By optimizing water recovery, daily (in addition to metals used in it is all about water. Managing seep- ATD technologies promise to reduce electronics, think about that can of age from the tailings themselves, water use at mines. This is of great soda you are drinking, or the aggre- monitoring pore water pressure advantage in some settings, and in gate used to pave your driveway). within the tailings dams or designing others ATD technologies promise a The industry works hard to make sure facilities to safely store or pass flood smaller environmental footprint and the net impact is positive. However, events. Because such a significant reduced long-term risks. the waste associated with mining, quantity of water is added during both past and present, is an unavoid- mineral processing, ‘conventional’ ALTERNATIVE TAILING able result of conventional mineral tailings dams store the tailings as a DISPOSAL OPTIONS By optimising processing. In the future it may be slurry. Water within the tailing mate- When considering alternative tail- water recovery, possible to extract metals like cop- rial is either evaporated, migrates ings disposal there are three major ATD per and gold without crushing and as seepage or remains entrained types. Each has advantages and technologies grinding the ore down to fine parti- within the tailings impoundment. disadvantages. promise to cle size and adding considerable Removing water prior to disposal • Thickened tailing disposal: reduce water quantities of liquid, resulting in con- through various means is considered Tailing materials are “thickened” use at mines ventional tailings. But, at the present ‘alternative’ disposal. through the use of high-density or

July/August 2021 www. .com 42 FILTRATION AND DRYING

ATD technologies deep-cone thickeners to about promise to 60-70% solids so the material reduce water does not segregate upon deposi- use at mines tion. This can have some benefits with respect to permeability and lower seepage, along with the higher initial water recovery, allowing immediate reuse of a greater fraction of the process water. Advantages: improved water conservation with lower energy input required (compared to other meth- ods) the equipment is relatively com- mon and easier to procure and it is widely used (there is a lot of experi- ence with the technology) and thickening is a continuous process Disadvantages: potential limita- tation is more attractive in, and (as opposed to a batch process). tions for filtering at the scale of this aided by, a dry climate. It takes Disadvantages: the capital cost of project and the large required time and money to remove water the thickeners and, because the tail- CAPEX (both a thickener and filter from the tailings and you don’t ings is in slurry form, a sizeable dam press are required), higher operating want to be in an environment that is still required for containment. costs including the cost of operating will put it right back in. the filter plant and the added cost of • Paste disposal: Produced in conveyance and placement (either in WHAT WILL IT TAKE TO specialized paste thickeners, trucks or with conveyors/stackers) ADOPT ATD? or ultra-high-density thickeners and the unsaturated filtered tailings Mining is necessary for the technol- and transported by positive dis- may tend toward oxidation which ogy we use, vehicles we drive, “ATD placement pumps, the paste is can result in acid generation and medical devices that save lives, even generally used for backfill in drainage in acid generating tailings. that can of soda. However, tailings sounds like underground workings or in are the result of mining and how a win-win other situations where gravity IS ATD RIGHT FOR YOU? we handle tailings today may have opportunity, helps the flow. ATD sounds like a win-win opportu- a significant impact on the environ- Advantages: paste can be used nity, but it is not always easy. Each ment and communities in the very but it is not as backfill, reduced environmental mine is unique. The mine’s physical long term. We need to plan today always impact and disturbing less land, setting, the ore body, and the area’s to safely manage tailings long into absence of segregation, limited climate all greatly impact how the the future. easy” consolidation and associated seep- tailings can be handled. Considering Pair that future planning with age, no need for reclaim pond, as that the social setting is even more water scarcity and potential dam the material is partially saturated important, a tailings facility design safety concerns and the case for so no oxidization, higher in place has to be fit for purpose. While ATD gets even stronger. Although tailings density, therefore, lower recent failures have increased scru- capital costs for ATD alternatives embankment needed. tiny on the selection of conventional are typically higher than conven- Disadvantages: pumping materi- options, it remains important that tional, if future risk management and als of this consistency is not energy mine owners consider the issues at long-term liability reduction can be or cost efficient, so when the dis- each mine site and establish what quantified and considered in the posal site is some distance away would be best there. decision, the choice can be a lot from the mill it may be beneficial to There are many major considera- more attractive. If consideration of pump slurry with lower solids con- tions that limit ATD’s use: total cost (including closure and tent and have the water removal • Topography: Some technologies post-closure) is given greater weight operations close to the disposal site. lend themselves toward flat in the tailing disposal alternative topographies and sometimes are selection, ATD can start to become • Filtered tailings disposal: Tailing not even feasible at sites with ‘the norm’. materials are dewatered to 85% or moderately steep terrain. greater solids by weight ratio • Production rates: Operators seek THE NEXT STEPS using filers aided by a vacuum or reliable equipment and stable This is an exciting time to work confining pressure. The dewatered operations. Removing water using in the mining industry, especially tailing materials are transported filters or paste thickeners from in support of smart and safe tail- by conveyor or trucked to a dis- some tailings can be a challenge at ings management. Technology is posal area. high-production rates. Currently, changing the way we work. Stantec Advantages: reduced environ- conventional tailing disposal brings the whole toolbox with us mental risks, improved water conser- remains the default technology at when we come to a mine. We are vation, improved compliance, space high-production mines. experts in tailings design, alterna- conservation, better handling in cold • Climate: Although ATD methods tives assessments, operations condition and fewer topographic have been implemented in wet cli- support, closure and post-closure limitations. mates, their successful implemen- management.

July/August 2021 www. .com