Filtered tailings - Best practices that cut downtime and keep costs low

Tailings storage management has long been treated as a necessary cost of doing business for mine sites. But faced with increasingly complex environmental issues – such as water scarcity – tighter regulation and licensing challenges, as well as a renewed focus on tailings management following recent high-profile tailings dam disasters, mining’s mindset is changing. In short, tailings management carries risks – and these risks, whether water sourcing costs, fines should things go wrong, or licensing issues, can be costly.

It is no surprise that solutions to these challenges are becoming increasingly popular. For instance, pressure‐filtered tailings, remove many of these key risks by increasing the amount of water that can be recycling and reused in mineral processing.

But are these solutions themselves cost competitive?

For a tailings filtration project to be financially competitive compared with other tailings options, high availability of filtration equipment and low operational costs are essential. Automatic pressure filters are easily ruined, for instance, if proper maintenance is not performed. The good news is through high-quality equipment design, building layouts planned around accessibility, maintenance procedures and automation, operating costs can be kept low, while availability greater than 90% is possible.  

Sizing up requirements - getting equipment design right

Pressure filter equipment design – especially when it comes to sizing and a design that allows for easy cloth and plate maintenance – needs to be a top priority when maintaining high availability for pressure filter equipment.

Undersized dewatering equipment will operate at excessive rates and without the proper downtime required for preventative maintenance. Bench scale filtration equipment is usually employed with a high degree of success when it comes to getting size right as it gives reliable results that have been proven to scale up with a high level of accuracy to full sized filter production. Additionally, it is necessary to comprehensively test a complete range of tailings that are representative of the complete mine lifecycle, as opposed to a blend of “typical tailings” material.

Filtration equipment is frequently sized according to a blended sample – often with no other samples tested. If the complete operational envelope is not tested it means taking on a significant risk of undersizing and having no time available for required maintenance. The temptation to take this shortcut is based on an effort to reduce project developmental costs. Additionally, if best filtering material is used, it can lead to the filters being undersized (sometimes by as much as half). Equally, if the worst material is used as the basis of the design, capital costs and operating expenses could be too high for the project and it may never seem viable.

Cutting your cloth lifespan cuts unplanned failure

With 80% of pressure filter maintenance associated with filter clothes and filter plates, it is crucial that equipment design allows for easy plate removal. High availability is achieved by ensuring the vast majority (99%) of cloth changes are planned as opposed to unplanned. An unplanned cloth failure spreads throughout the plate-pack causing a whole filter to have cloth failures.

If properly selected, a filter cloth will achieve between 1,000 and 4,000 cycles of operation before reaching the potential failure line, depending on what type of tailings is being processed (the worst type of tailings for cloth life typically contain a significant portion of clay and clay sized particles).

When the potential failure cycle count for the cloth is reached, it is optimal to remove the filter plate, or set of plates, from the filter to perform the required cloth maintenance. This requires the proper filter design allowing quick and easy removal of filter plates. It is possible to remove a set of up to 20 plates at a time to reduce lifting and removal times.

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 take 10 minutes.

Access all (maintenance) areas

Good maintenance procedures ensure the pressure filter system is operating at peak efficiency. This obviously means high-level daily walkarounds and inspections, change outs and plate and cloth maintenance outside the filter. It also means having the right number of skilled personnel; good practice being one operator for every two filters.

To carry this out maintenance most efficiently, sufficient area is required for plate and cloth maintenance and so the layout out of all filtration equipment must be considered from the project’s instigation.

For plates and clothes, the area should consist of a dirty maintenance rack and a clean ready rack. The maintenance rack should contain enough space to hold approximately 10% of the total number of filter plates installed. This area needs to be contained and drained so that the plates can be washed as needed.

A 5-meter length of rack needs to be available such that plates can be spread apart and maintained before being put in to the clean rack. The clean rack should also contain enough space to hold approximately 10% of the total number of filter plates installed. The rack should have a lower access for maintenance on the lower part of the plate, and an upper walkway to access the top of the plate.

But space planning is not restricted to cloth and plate maintenance. Slurry valves for instance are valves are bulky and heavy and must be freely accessible, with sufficient space for removal and installation. Additionally, every pinch valve should have a flexible connection (bellows) as an interface with the rigid piping system or a removable angled spool where a bellows is not practical or possible.

Some other areas to take into consideration are the two heads on a pressure filter; one at each end of the plate stack. The follower, which is the moving head which on a pressure filter, should be accessed with a walkway (or walkways) and hand‐railings to enable routine unrestricted access for installation, set up and maintenance. Also requiring walkway access are the drip tray components, particularly the hydraulic cylinders and instrumentation components, located under the floor of the filter, which present an added challenge.

Making the most of automation

The most beneficial automation for maintaining high availability on a pressure filter is focussed on the filter cloth. Knowing when a cloth has failed, tracking how long that cloth was in operation and where it failed is key information – and automation can provide it.

Full filter turbidity sensors are commonly used to detect cloth failures in a filter press. Their limitation is not indicating which of the potential hundreds of cloths in a filter has failed. Single-plate turbidity sensors developed by FLSmidth can be used to precisely indicate where the failure has occurred. Turbidity sensing modules can be installed into a filter plate to detect damages in the filter cloth as well as failures. There are also solutions on the market that allow a maintenance person to log where the cloth failed and why it failed using a mobile device. This allows for targeted cloth improvements based on failure modes, predictive maintenance and inventory control, and improved availability and production.

Minimised downtime means higher productivity

Successful implementation of these best practices for tailings filtration facilities have achieved 90% availability. The result is fewer filters need to be installed, reducing capital costs of the installation, while operational costs have been shown to be less than US 0.80/t of tailings dewatered. At that level, filtered tailings are a serious cost-competitive solution for many mines.