Engineering & Mining Journal

JUL 2019

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Page 36 of 67

TAILINGS FILTRATION JULY 2019 • E&MJ 35 media has to be changed every month or month-and-a-half," said Flanagan. "We looked at every type of filter me- dia under the sun, and actually came up with a totally different type of filter plate that allows filter media to last three months. It offers significantly less capex and opex." James Chapponel, R&D project manager, explained the various steps needed to work through design and de- velopment of a pressure filter prototype. The proof of concept stage began with assessing the manufacturability of large filter plates. What type of composite material would be best, and what man- ufacturing method? Process validation required testing of filtration rates, cake density/moisture/particle size distribu- tion (PSD), etc. Next on the list were plate assembly and maintenance considerations: How do you handle such large plates and in- stall the media? What tools/jigs would be needed for assembly/disassembly? How will repairs be handled, and what issues need to be addressed in the area of process safety? The proof of concept effort resulted in fabrication of the single-plate, Super- cell rig on display at the Tucson event. It weighs 40 tons, has a plate size of 5 m x 3 m, chamber volume of 0.6 m 3 and filtration area of 25.12 m 3 . Cham- bers are filled through five feed eyes on the plate. Although laboratory scale testing envisaged a pressure rating of 15 bar, which would provide a fill time of 3 minutes, the prototype is rated at 5 bar with a fill time of 27 minutes. In other test results, the 5-m x 3-m test filter correlated quite closely with lab- oratory-filter performance: slurry solids (wt%), 60% (lab) vs. 60% (test); cake moisture (wt%), 18% vs. 15.7%; wet cake density (kg/m 3 ), 2,160 vs. 2,204; and dry cake density (kg/m 3 ), 1,833 vs. 1,857. Chaponnel said strength testing of the composite plates will be conduct- ed after process testing is completed. Destructive testing of the plates will in- clude failure strength of plate support and differential pressure testing. The ultimate dimensions of a com- mercial-scale 5-m x 3-m filter press are imposing. A 160-plate filter would measure 47 m long by 9 m wide and 8.5 m high, with total mass of 1,100 metric tons. The filter's "follower" rail drive alone weighs in at more than 150 mt — about the same as a GE 4500 railroad locomotive. Average output per cycle could reach as high as 207 mt, compared with a 120- or 150-plate machine with 2-m x 2-m plate size's 42 mt per cycle. At 90% online availability and 185 cycles per day for both the smaller and larger machine, daily output would be 7,789 mt vs. 38,295 mt, respectively, accord- ing to FLSmidth's calculations. Sizing Up the Solution Simon Hille, group executive–glob- al projects technical engineering at Newmont Goldcorp, told seminar at- tendees that the design of a produc- tion-scale filter suitable for meet- ing the requirements of the EcoTails concept had to be capable of over- coming three major weaknesses in traditional filtering techniques: • Low filter cycle volumes, based on plate size, thickness and quantity limitations. • Long cycle times caused by a slow multistage dewatering process that uses low feed pressure and results in low filtration rates. Additional limita- tions include slow filter opening and closing time and uneven dewatering in the filter chamber. • Intensive maintenance and low availa- bility, stemming from the need for fre- quent changeout of draped filter cloths and time-consuming plate maintenance. The EcoTails filter envisaged by FLSmidth's designers, said Hille, will During the demonstration, the filter plate was sandwiched into its test fixture, tilted upward into vertical position, filled with slurry and clamped by hydraulic rams. Designing for Deposition The reported results of Newmont Goldcorp's mine-site testing of fast-filtered tailings technology fit well with the operational model of commingling waste and filtered tailings as envisaged by the EcoTails consortium partners: Using a minimal amount of crushed waste, blended with dewatered tailings provided by a continuous high-volume filtration system and transported by conveyor to a stack site, thus reducing risks of stack in- stability and acid rock drainage problems. Then, as the mine progresses through the various stages of its life, take advan- tage of the opportunity to rehabilitate tailings storage areas progressively rather than waiting until final closure. It's a concept that G. Ward Wilson, professor of geo- technical and geo-environmental engineering at the Uni- versity of Alberta, thinks can solve many of the challenges that producers will face trying to implement Best Available Tailings Technology (BATT). Ward refers to GeoWaste and other types of co-mingled tailings products as "designer waste" — not because they're fashionable, but because they offer an opportunity for mine owners to tailor the makeup, deposition and rehabilitation of dry-stack tailings piles ac- cording to specific needs, resources and operational plans. Problems of instability and acid rock drainage associat- ed with waste and tailings piles "are determined at the time of deposition," said Wilson. Segregation of materials is the basic cause of most problems, and producers can avoid com- plications by controlling the physical characteristics of waste before deposition. "If you deposit it hydraulically, it can be- come hydraulic again," he told the seminar audience.

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