PASTE & FILTERED TAILINGS
Demystified dewatering
Paul Moore caught up with FLSmidth ’ s Ken Rahal - Director of Tailings Solutions ; and Fred Schoenbrunn , Global Director for Thickeners , for a discussion on the latest approaches and technologies in paste and filtered tailings
Q How would you summarise the tailings method decision for mining customers ? KR : We spend a lot of our time doing technology trade-offs on unthickened tailings versus paste or filtered tailings . This includes studies based on lab data – we ’ ll look through costs of the different solutions . Paste is typically less expensive on both CAPEX and OPEX than filtered tailings . So if the customer is looking at needing a certain amount of water recovery to close their water balance or reduce their fresh water intake , then we will evaluate it and see if paste can achieve what they need and if so go down that route and support them throughout . FS : Generally , in terms of project evaluation , yes we look at what they are trying to achieve either in terms of water recovery or tailings management . You can have typical high-rate thickener underflow that is very fluid going to a large tailings dam and there are good options for that in terms of dewatering . Paste thickening is step 1 – then you have a variety of filtration options using everything from centrifuges to belt presses – you have a number of mechanical dewatering options that produce higher rheologies than what you get out of a paste thickener . For smaller tonnage throughput mines you have the most options . Once you get to the big mines , however , the tailings facility starts to become very big if you can ’ t stack material on top of itself and get some depth in the tailings deposit . There are a number of technologies that produce a material that is difficult to deal with on a large scale . Small tonnages you can use trucks and dozers – but higher up the volume chain you need to start looking at mobile conveyors and stackers .
Q What role do climate and topography play ?
FS : Topography plays a huge role because it has a big impact on how far you have to transport the material to the TSF and how easy is it to store the material . If you have a big flat area and you need a complete perimeter dam – for a wet impoundment , that ’ s very expensive . It also means a push towards a higher degree of dewatering . If you are in mountainous terrain ; you may have the option to place filtered material easily into ravine or valley type areas , even in mined out open pits . Here a wet impoundment will need to be very tall , depending on the topography shapes , to accommodate the volumes of tailings needed .
Q A few operations like Bulyanhulu have used surface stacked paste tailings disposal but it is not widely used – why ? FS : Getting it to form a nice cone with optimal beach slope and having enough surface area for
Ken Rahal
FLSmidth developed a dry stack tailings solution for Hindustan Zinc ’ s Zawar mines
evaporation is challenging . And it is climate sensitive – it obviously wouldn ’ t work in a high rainfall area like Brazil . The stack has to be almost dry – that ’ s why it worked it at Bulyanhulu in Tanzania . KR : Paste if not produced to a consistent rheology can mean challenges in the deposition area such as channelling and you may increase OPEX as you have to start moving distributor heads . This gets magnified at higher tonnages if you don ’ t have a properly designed disposal system that ’ s going to evenly distribute the paste so that you can achieve the design slopes – because if you don ’ t hit those then the capacity in your storage area will be insufficient .
Q Are there less dewatering equipment options for very large tonnage filtered tailings operations ? FS : As mentioned to stack large volumes of filtered tailings successfully you have to be able to operate equipment on top of the stack so it needs to be structurally stable . That pushes dewatering options to just pressure filters as they are about the only mechanical dewatering method that produces dry enough tailings to support mobile stacking equipment so you can keep raising the stack instead of spreading it out over a large footprint area .
Q What role does particle size play in successful tailings disposal ? KR : When you look at coarser , sandy material – this is often used as hydraulic backfill in underground mines ( versus paste backfill ). You need a certain proportion of fines to create paste – with hydraulic backfill you have to use expensive binders as you don ’ t have the fines . But if you have too many fines with paste it increases the cost of your dewatering whether its thickening or filtration .
Q With the Global Industry Tailings Standard , is the nature of your tailings conversations with mining companies changing ? KR : I would say over the last five years we have seen a lot more interest in conducting dewatering and tailings studies . Many lab-based studies have now been completed and customers are moving towards a PFS or pilot stage to continue refining OPEX and CAPEX numbers and home in on what solution is going to work best for the site they are investigating . We are conducting a pilot study now in South America that is looking at different forms of comingled tailings including mixing paste or filtercake with waste rock . FS : Market activity is increasing – we also recently got an order from a major copper mine
P6 International Mining SUPPLEMENT | JULY 2022