Engineering & Mining Journal

JUL 2019

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Page 37 of 67

TAILINGS FILTRATION 36 E&MJ • JULY 2019 greatly increase the size and number of plates, achieving a filter volume three times that of existing filters using the industry-standard 2-m x 2-m plate size. Likewise, large plates made from high- strength fiber-reinforced composite materials enable thinner construction than current standard polypropylene plates, which offer light weight but low strength. The new type of plate, although more mechanically complex than conventional plates, will result in shorter filter pack length, reducing the filter press footprint, and would enable a significant increase in plate quantity – from a typical 80 to 100 plates served by a single-ended feed mechanism to 160 plates fed from both ends. These features, combined with 1) drastically reduced dewatering time and elimination of the need for an air blow; 2) open/close travel times twice as fast as existing models; and 3) improved filter cake consistency due to multiple feed ports will provide filtration cycles 20% to 50% quicker than those of tra- ditional plate and frame filters, said Hille. Perhaps equally important, these innovations are expected to deliver higher filter availability with run times of greater than 90% along with reduced maintenance needs, resulting in low unit operating costs. It's also anticipat- ed that a change from use of draped fil- ter media, with attendant problems of mechanical failure causing by flexing and abrasion at the chamber perimeter, to clamped nylon media that lies flat in the chamber will eliminate that type of wear and increase media life. Hille said testing at Newmont Gold- corp's Marlin and Peñasquito mines had achieved 6,000 cycles of media life to date and shown that the fast-filtered tailings cake could be successfully and efficiently mixed with waste rock on the tailings conveyor to form GeoWaste. The GeoWaste did not liquefy on a convey- or belt for distances up to 10,000 m. In fact, said Hille, testing showed that filtered tailings alone could be trans- ported via conveyor for a reasonable dis- tance without liquefaction and isolated within a GeoWaste stack, if necessary. The key technical objectives of New- mont Goldcorp's mine testing of Geo- Waste were aimed at determination of its non-saturation characteristics — the ability to retain water yet still behave like a self-supporting sand-gravel pile; and its capability to achieve reduced oxygen flux — decreasing the likeli- hood of air reaching sulphide particles in rock and causing oxidation. Accord- ing to Hille, results to date show there has been no measurable seepage from GeoWaste piles at either mine test site. The O 2 content in the GeoWaste pile dropped to 0% at depth from 21% in less than six months while a waste con- trol pile measured greater than15% O 2 . Planning and Proactivity Todd Wisdom, FLSmidth's director of tailings solutions–mining, said the keys to controlling operating expenses asso- ciated with large filter presses are plan- ning and proactive maintenance. Oper- ators should select efficient machines that can provide high availability at a reasonable level of energy consump- tion. Quick ramp-up is another consid- eration: "If you just spent $2 billion on a new mine, are you going to want to take four years to start up the plant?" Wisdom said. "Nobody is going to be happy about that." Proactive maintenance is crucial for high availability. The company recom- mends, as part of a proactive mainte- nance approach, a 12-hour shutdown every two weeks, during which filter inspection, hydraulic maintenance, valve or seal replacements, feed pump maintenance, pipe or hose replace- ments and instrument calibration can be performed. Other proactive program events include plate pack changeouts on a scheduled cycle count, and routine tasks such as walkarounds, greasing or bolt tightening that can be done while the filter is running. Wisdom said FLSmidth designed features into the 5-m x 3-m EcoTails filter such as sensors in each plate that can wirelessly transmit a signal and illuminate an external LED to quick- ly identify a plate that needs service. Semi-automated plate handling systems for batch-refurbishing stacks of plates are available for users with unique site conditions, such as operations at ex- tremely high altitudes where strenuous manual labor is difficult, for example. Planning for variability is also im- portant. "Nobody operates a mine to make tailings, it's just what's left over from the production process," he point- ed out. Depending on what part of a mine the ore is coming from, concen- trator throughput and feed character- istics can change. "Understanding the amount of variability you may see in your mine, and knowing approximate- ly when you're going to see it, allows you to plan ahead to optimize opex spending." Looking beyond the filter assembly itself is also important, according to Wisdom. Can filter pumps and valves be accessed conveniently and safely for regular inspection? If filter pumps or valves break down or wear out, are spares available? If the plant has one overland conveyor for tailings that is fed by a single chute, will a malfunc- tioning chute bring down the entire concentrator? Does the plant setup have sufficient surge-handling capa- city to absorb scheduled or unplanned filter system downtime? "In my opinion, when it comes to large-volume filtration, opex is king," particularly for large operations with decades of mine life ahead of them, he concluded. A cake sample from the test rig. By adding waste rock to fast-filtered tailings, the resultant GeoWaste can be tailored to achieve optimum moisture content.

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