Tracked conveyor bridge, versatile mobile stacking technology, mobile conveyors and advance and retreat stacking. Credit: FLSmidth
Economic growth, urbanization and growing populations are fuelling demand for new urban infrastructure, technology and modern conveniences. These factors, especially in developing economies, are lifting more and more people out of poverty and into a swelling middle class. This demographic change means hundreds of millions more people with disposable income, which in turn is fuelling consumption and placing more and more demand on the mining industry to deliver material to the market.
With a concurrent move towards energy efficiency, green technology and a low-carbon future, we are seeing stronger demand for electric cars, wind and solar energy and energy storage. Meeting these demands requires minerals, which places the mining industry at the centre of a complex conundrum.
Declining ore grades, increasing environmental footprint
The difficulty in delivering the minerals demanded by society while decreasing the environmental footprint is being compounded by declining ore grades. This has direct implications for the amounts of water and energy needed for production, increasing the environmental footprint of the mine. It is a challenge, but it also creates opportunities for us to be part of the solution.
Processing more ore to recover needed metals directly increases the amount of tailings produced from a mine. Tailings management facility (TMF) failures are perhaps the most significant environmental liability for a mining project. In fact, tailings management was listed as the only direct flowsheet risk in KPMG’s 2020 Global Mining Survey Report of top 10 risks facing the mining industry.
High-profile dam failures in Brazil in recent years have highlighted the consequence of TMF failures, including human, economic, and environmental impact and cost, reparations and liabilities running into the billions of dollars, reputational damage and market capitalization devaluation. On average, over the last 30 years, TMFs have experienced around 20 dam failures per decade and a third of them caused serious safety and environmental liabilities, including deaths.
Add to this the fact that access to water is becoming increasingly difficult and mining operations have seen water costs steadily rise. In short, water availability and water contamination are challenges miners have faced for some time, and this trend will only grow in the years ahead, creating huge financial and operational risks.
Change is in the water
Techniques for dewatering large tonnages of tailings, with minimal operational costs and making use of economies of scale, through larger equipment are continuously being sought. A safe and cost-effective solution for tailings management facilities capable of accommodating 150,000 t/d is needed.
Dewatering tailings using pressure filters has been around for a while, but traditional filtered tailings costs are currently too high for high-throughput, low value per tonne ore deposits. Typical filtered tailings using smaller pressure filters and air drying the filter cake to an optimum cake moisture content, and then using trucks for tailings transport and placement has an in-place cost of over US$4 per tonne. For large-scale, low-grade open pit mines, this cost is too high.
To reduce costs associated with dewatering tailings using pressure filters technology, three main problems need to be solved. The first is low filter press volume per filter. Filter presses are a batch process and low volumes per batch mean low production rates per filter. Second relates to maintenance and availability, and lastly, you have slow cycle times, which reduces batches per hour.
Low filter press volume per filter
The solution to low filter press volumes is to increase the plate size and increase the number of filter chambers in the filter press. Larger filter volumes have many benefits, such as a need for fewer filters, fewer support components, reduced number of filter cloths to change and handle. Resolving this was the main driver, for instance, in FLSmidth’s design of a filter that includes a filter plate with dimensions of 5 metres high by 3 metres wide and 160 chambers, that meant a filter volume of over 90 cubic metres and over 4,000 sq. metres of filtration area.
Intensive maintenance and low availability
Most maintenance on a filter press is associated with the filter cloth and filter plates. Filter cloth has a typical lifetime of around 3,000 cycles on most mineral flotation tailings. This leads to changing the filter cloth on the filter approximately twice per month, depending on how often the filter is operated.
High availability, however, can be achieved by performing the cloth and plate maintenance outside of the filter press. Current filter presses can achieve over 90% availability utilizing this method. It is envisioned for the 5 by 3-metre filter that plate cassettes of 10 will be removed and maintained outside of the filter. As soon as the dirty filter plates are put into the maintenance rack, new clean plates are lifted into the filter allowing the filter to operate while the dirty plates are maintained. As the cloth and plate maintenance is performed outside the filter, while the filter is operating, high availability of the filter is maintained. The total downtime to remove a set of plates and reinstall a set of plates has been measured in an operating filter to be 10 minutes.
Slow cycle times
For most flotation tailings, total cycle times of less than eight minutes can be achieved if both the process and mechanical times are minimized. Mechanical times can be minimized by opening the complete plate stack all at once, maximizing the speed of the opening and closing, performing valve openings and closings in a parallel method when possible and optimizing cloth washing and cloth shaking times when possible.
Process times can be minimized by four actions: Fill the filter at high pressures, eliminating the use of membranes for squeezing the cake, reduce or eliminate cake air blow and optimize feed pumping control to take full advantage of pump flow and pressure capacities.
The fast cycle times and a rapid filtering process is referred to as “Fast Filtering” and allows more batches per hour and higher unit filter capacities. However, if the cake airblow time is reduced, it may also reduce the volume of water removed from the filter cake produced. The resulting higher cake moisture content, typically between 18 – 20 wt% moisture, introduces potential geotechnical issues within the TMF.
To give the TMF the required strength needed when the higher cake moisture tailings are deposited, FLSmidth came up with the EcoTails process, which co-mingles waste rock with the tailings. Mixing the low moisture waste rock with the higher moisture tailings gives a product with an average moisture that is acceptable for stacking.
The coarser rock particles also add shear strength and provide a higher density to the pile the tailings are fast filtered with, with the blending taking place during conveying transportation. To get the waste rock on a conveyor, in-pit crushing and conveying (IPCC) can be used which will further improve mine economics and reduce truck emissions for additional sustainability advantages. EcoTails mix ratios are designed for individual mine site requirements, producing a material that is geotechnically and geochemically stable.
Calmer waters ahead?
With the traditional technical challenges faced by large-scale filtration and dewatering gradually melting away through innovation and ambition, the mining industry need not see more sustainable water usage as an insurmountable challenge. Cost analyses, considered over the full lifetime of a mine, are also likely to show that new solutions like EcoTails are actually cost competitive compared to wet tailings dams under many conditions, leaving the main challenge as the industry’s willingness to adopt these methods.
TODD WISDOM is director of Tailings with FLSmidth.