Engineering & Mining Journal

NOV 2018

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Page 60 of 83

FILTRATION & THICKENERS NOVEMBER 2018 • E&MJ 59 Keirn recommended "working back- ward" to arrive at the best possible solu- tion. Consider first the desired outcome in the placement area, he said. "You want to define the method of placement and consider mechanical methods, what kinds of pumps you are going to be using, and whether it is going to be gravity flow to that final method or final placement area," Keirn said. "And then you want to consider settling options. Finally, you want to define the equipment and the lo- cation of that equipment in your plant." Different thickeners will have different settling rates, which is largely determined by the type and amount of flocculants applied. The flocculants must overcome the rise rate of the exiting clean water, Keirn said. "Higher floc dosage generally results in faster settling rates; however, it can result in decreased underflow densi- ty," Keirn said. Underflow density refers to the packing of the solids as mud at the bottom. The density of the mud is a factor in determining how it can be further pro- cessed and handled downstream. "A settling rate double the rise rate is generally recommended," Keirn said. How the flocculant ages plays into how it needs to be dispersed. "In the polymer (flocculant) dosing systems, you need to take into consideration the concentration of the polymer, the dispersion of it in the slur- ry coming in, and the aging of the polymer," Keirn said. "It is important to get the proper aging of the polymer in your mixing." How much polymer is absorbed is de- termined by the amount of solids entering the thickener. The more tonnage per hour of solids being piped in, the more poly- mers absorbed. Too little polymers per ton per hour of solids results in low mud den- sity and poor overflow clarity. Too much flocculant is wasteful and inefficient. A balance should be sought and ad- justments made by either adding poly- mers or diluting the incoming slurry with water, Keirn said. "You can add dilution water to the feed tank of the thickener, or you can add dilution water to the feed well or the feed pipe coming in," he said. "A couple of ways to do that would be active or forced, which would be a pump system; or a passive, which, for example, would be a venturi principle." An imbalance that results in leftover polymers can cause problems and rep- resents wasted money. "The dosage is typically 30 to 50 grams of polymer per ton of solids," Keirn said. "In general, cost is $0.25 per ton." The main components of many thick- ener systems are the bridge or trusses, the drives, which can be electric or hy- draulic, and the rake system, which, pow- ered by a drive, moves the mud towards the underflow exit. The size and shape of the thickener and its usage of those components de- termines its unique capabilities, which should be matched to the site needs. For example, typically "underflow density de- pends on the particle size of the thickener feed," Keirn said. Therefore, a plant with a feed comprised of larger particles might do well to consider a type of thickener with the capability to handle denser mud. Each of the six types of thickeners offered by McLanahan present both benefits and drawbacks. For example, a thickener with a small footprint may have reduced overflow clarity. "Or we sacrifice small area for overflow clarity," he said. "We sacrifice polymer costs for small area, and we sacrifice underflow density for a small footprint." Traditional thickeners often have a large diameter, long residence time and offer low underflow density. Sloped plate clarifier thickeners achieve a high settling area, have a small footprint, offer low underflow density, but are prone to plugging. Drag chain thickeners are semi-porta- ble, large, and can produce a discharge that can be stacked. High-rate thickeners can be elevat- ed and employ radial rakes. "It offers a relatively dense overflow," Keirn said. "The on-ground tanks have long suction pipes that can plug and limit underflow concentration." Ultra-high-rate thickeners are typically rake-less with few moving parts or wear items. "It has a higher flocculent con- sumption than high-rate or paste thick- eners," Keirn said. "It offers a decreased underflow density do to the lack of rake and the high polymer dosing." Paste thickeners are so named due to their higher underflow density. "The steep floors help mud report to the un- derflow," Keirn said. "The higher densi- ty underflow results in approximately 10 times the typical required rake torque of a high-rate thickener." Paste thickeners offer the highest achievable mud density of all styles, which means they get the maximum wa- ter recovery. "The mud is often too vis- cous to pump," Keirn said. "The high mud density requires a high-hp rake or picket drive." Paste thickeners are less prone to suction line plugging, he said. Paste thickeners present a "complex- ity vs. performance tradeoff," Keirn said. That translates to "an increase in costs, but you get a higher density underflow and a higher viscosity in the underflow." The controls on thickeners made by McLanahan manage bed level, bed pres- sure, torque and rake height. "Optional is the PLC, clarity meters, beacon, and rake position controls and different options for monitoring underflow and mud level with- in the thickener," Keirn said. The final branch on the decision tree is the level of commitment to monitoring and maintenance. Some require more work than others, but for each "you do need to monitor your thickener perfor- mance," Keirn said. "Some good indica- tors of thickener performance would be the sludge level within the thickener, the floc size, the size of the floc within your thickener, torque reading on your rake and then monitoring the underflow density and making sure you are hitting your target un- derflow density that was designed." A typical McLanahan thickener fea- tures sensors that enable the monitoring of "rake torque, the underflow valve, flow to make sure we've got continued flow and that continually feeds back to the PLC to make sure the pump is pumping, bed level in the thickener and then density to make sure that we are optimizing the water re- covery of the thickener to make sure the underflow of the thickener is maximized," Keirn said. "It is still a piece of mechan- ical equipment, so you do need to do in- spections and checks on the equipment to make sure everything is operating proper- ly," he said. "There are daily inspections, daily checks on it, and also annual inspec- tions or 1,000-hour inspections." Lower Cake Moisture in a Smaller Footprint Peterson Filters Corp. reported the feed- back from a Wollastonite miner in Mex- ico that deployed two prototype Pocket vacuum disc filters continues to be "very positive," even after more than a decade of almost continuous use. The machines, designed in 2006 spe- cifically for the project and built in 2007,

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