With a breakthrough in filtration technology, FLSmidth is proving that a full dry stacked tailings solution is technologically feasible for large scale operations – and that the recovery of up to 95% of process water is economically competitive with desalination, even for high tonnages.
"This is no less than a step change in water management. It is the needed piece of the puzzle towards the only sustainable tailings management; dry stacked tailings maximizing the water recirculated in the process,” states Director of Tailings Solutions in FLSmidth, Todd Wisdom.
As the need for dry stacked tailings has increased, so has the need for larger and larger dewatering equipment to accommodate the huge volumes of tailings requiring treatment. Some large mining operations produce well over 100,000 dry tonnes of tailings per day. Dewatering this massive volume of solids to the geotechnical standard needed for dry stacking requires large filtration areas which, using old technology, would require football fields of filters. These in turn would require an unsustainable amount of piping, valves, pumping, conveyance and maintenance. To reduce the number of filters, FLSmidth decided to develop an economically viable system for dewatering high tonnages to the scale of more than 120,000 tonne/day of tailings.
"As the ore grades decline, miners' revenue per tonne decreases accordingly, so we need to decrease the cost of tailings processing to make the operation economically viable and our solution cost competitive with alternate technologies," explains Todd Wisdom.
Up to 95% water reuse
To optimize the capital and operating costs of plants operating at large tonnages, FLSmidth came up with the AFP-IV® Colossal™ filter. The filter enables effective dewatering of large volumes of tailings with less than half the number of filters required with competing large filters. The new filter not only increases the amount of water that is returned to the plant for reuse, it is also able to decrease the amount of water that is lost to the environment through evaporation and seepage. These are two major hurdles that can be expensive and difficult to overcome in traditional wet tailings facilities.
To prove the operating and financial benefits of their large format tailings dewatering facility, FLSmidth partnered with a large mining company in Chile and installed a complete tailings filter system to process approximately 10,000 tonnes per day of whole tailings. A full scale implementation of this technology would allow mines to operate with a water make-up ratio of 0.2 m3/tonne, compared to traditional sand dams with a water ratio of 0.7 m3/tonne.
By enabling mines to reuse between 90-95% of their process water, the filter solution would thus enable mining companies in Chile to reduce their fresh water intake by more than 70%.
Beats desalination and eliminates tailings dams
"With this breakthrough development in filtration technology, we have made the operating and capital costs of dry stacked tailings competitive with traditional technologies for large scale mines. A large copper concentrator with a tailings output of 100,000 tonnes per day, for example, could save 200 million m3 of water over ten years by using our dry stacked tailings technology combining thickener and filter with our RAHCO® mobile stacking and conveying system to dispose of the tailings as dry filter cake," states Todd Wisdom. In Chile that would translate into $USD1 billion for desalination.
“Furthermore, our filter solution eliminates tailings dams. Desalinated water will still end up in a pool and risk causing damage. We need to eliminate the need for any pool at all,” he adds.
Technology for high tonnage, high altitudes
FLSmidth's AFP-IV Colossal filter is a filter press, which uses high pressure and fast filtering to achieve much larger single machine capacity and optimal cake moisture concentration. Where vacuum filters are not effective at higher altitudes, and belt presses and centrifuges are only effective for small tonnages, filter presses are able to use higher driving forces to achieve high dewatering rates and can operate effectively at high altitudes. This allows them to have the smallest installed footprint and low installation costs, alongside greater operational flexibility.
"Operational flexibility is needed as the tailings filtration characteristics change over the life of the mine. For these reasons, we believe that large filter presses like this one will become the equipment of choice for high tonnage tailings dewatering applications," says Todd Wisdom.
The feed to the filter press is pumped from a buffer tank, which has sufficient capacity to allow a constant flow from the tailings thickener during the batch filtration process. The pumping of the feed slurry under pressure into the chambers/cloths, provides the force to build a cake within the chamber. As the cakes form, the pressure to produce a properly compacted cake rises steadily. Most filter presses on tailings applications operate with up to a 15 bar driving force. Using FLSmidth technology, this driving force can easily be reached using just the feed pumps to the filter. After the cake is formed, high pressure air, in the range of 7 to 10 bar, can be blown through the cake to further reduce its moisture content and produce a non-saturated cake, if required.
The cake is formed by the capture of solids on filter cloths which encapsulate the filter chambers. The press is closed by means of hydraulic cylinders and multiple plates, which, on closure, form the chambers that capture the filter cake. The moisture of the discharged filter press cake is in the range of 10 to 25 wt % and can be either saturated or unsaturated depending on the filter design and geotechnical requirements.
The Colossal filter is the largest capacity filter in the industry. It is capable of discharging 20,000 tonnes of filter cake per day and can recover 600 m3 of process water per hour. That is the equivalent of six Olympic sized pools each day.
"To cover the need of a large mine with 150,000 t/d of tailings, ten filters would probably need to be in operation. In arid climates, the amount of water saved more than offsets the costs of this solution, which notably also saves vast amounts of energy. And it enables mining to continue in areas of the world where access to water is extremely scarce and problematic," Todd Wisdom concludes.
Todd Wisdom, Director of Tailings Solutions: Todd.Wisdom@flsmidth.com