Recessed chamber and membrane plate filter presses are gaining more popularity in the coalfields. The systems are used to dewater fine refuse that would traditionally be pumped to a thickener and eventually to an impoundment or settling pond. Instead, the dry cake produced by the filter press can be backfilled into the pit. This is important as coal operators are finding it more difficult to permit new impoundments.
A prime example of this new approach to refuse dewatering are the filter presses installed at the Bishop prep plant, which sits on the border of Tazewell County, Va., and McDowell County, W.Va., (See Coal Age, January 2012, p. 32). With the help of Taggart Global (now Forge), Southern Coal constructed the new $60 million state-of-the-art prep plant in 2012. The mine plan called for reject from the plant to be backfilled into the pit, and the centerpiece for this strategy was recessed-plate filter presses provided by McLanahan that dewater the reject. The prep plant began operating in January 2013, and so far everything has been running according to plan.
The alternative technology for dewatering fines is a belt press. Belt presses are less expensive to purchase, but they have higher operating costs. They use chemicals and require a full-time technician. They achieve about 10% less dryness than the filter press.
The coal industry has been testing recessed plate and membrane plate filter press technology for about four or five years and there has been a bit of a learning curve. Tragically, an incident last year at a different installation manufactured by a different vendor claimed the life of an engineer and injured another coal worker. The technology, however, continues to improve, and the Bishop installation is a testament to those advances. Taking what they learned from Bishop, McLanahan and Forge are already looking at how to improve future installations.
Dewatering With a Filter Press
The Bishop prep plant uses two McLanahan filter presses aligned end-to-end with a single conveyor underneath both machines. The filter cake drops onto the conveyor belt and it’s transported to a hopper, where it is batch loaded into trucks. The system produces roughly 20 tons of filter cake per cycle. “If they run both presses they can produce about 100 to 120 tons per hour [tph],” said Cory Jenson, general manager of McLanahan’s environmental division, which makes the filter presses. “Currently, they only need to produce about 60 tph.”
The system is an overhead beam filter press, which has a high production capacity and allows better access for maintenance. “Rather than pulling the plates open in groups of 10 and 20 where they may only be open a couple of inches, we open them with a chain carousel between the overhead I-beams,” Jenson said. “They open one-by-one, 30 in. apart, at high rate of speed. There is a lot of movement and a lot of space for the cake to drop.”
The filter press can be operated with two types of plates: recessed chamber or membrane. Bishop is using recessed chamber plates. “A recessed plate uses fluid pressure for the dewatering process,” Jenson said. “Nothing on the plates move. Slurry is pumped in at a high pressure. As more solids enter, water is forced out.”
The membrane filter will produce a drier cake, but there are trade-offs. The membrane acts as a diaphragm on the side of each plate. The chambers are filled with slurry, and then the diaphragm squeezes the cakes. The trade-off, Jenson explained, is that the membrane flexes with every cycle. “There is a limited life cycle of maybe 30,000 to 60,000 cycles before the membranes will have to be replaced.”
Forge specified the McLanahan filter presses for this installation, and the presses met the capacity that was expected. Forge commissioned and operated the plant for the first six months before turning it over to Southern Coal. The startup went well, Jenson explained. “The first cycle surprised everyone with the dryness of the cake,” Jenson said. “We had some problems with the pumps. That has been resolved and they have been producing cake at the dryness and volume that they specified.”
A lot of thought was put into this installation considering the characteristics of the coal and the refuse and the desired quality of the cake, explained Pat Borreggine, vice president procurement, Forge Group North America. “There was testing that figured into the selection of the cloth,” Borreggine said. “We bought the better cloth, which is critical as far as longevity and operations of the plate frames. We had anticipated, or were given the expected moistures for the cake, and we exceeded that by an additional 5%. That allowed us to reduce the cycle times and pressures.” Lessening the cycle times decreases the wear and tear on the machine.
The Development Curve
Believing that the technology could benefit coal operators, McLanahan has been working with these machines in the field for a while. Their first unit at Sunrise Coal was a recess chamber machine purchased from an Italian company. “After that installation, McLanahan worked with a second Italian company before deciding to pursue our own design that we could support ourselves with local parts and service.”
McLanahan now has five of these machines operating with more starting this year in other applications (aggregates and industrial minerals). The units at Bishop are the first two operating in coal. “Six additional McLanahan presses are due to be coming online in coal applications in the next six months,” Jenson said. “Anywhere that chemical or handling costs, permitting or compaction is an issue, we are seeing lots of interest.”
Mine operators have been fairly vocal about the headaches associated with permitting and permitting an impoundment or extending the permit for an impoundment. The filter press offers a closed-loop solution. The cake simply needs to be dry enough to meet compaction requirements.
After working with these systems, McLanahan discovered how it could improve the system using standardized local parts.
Probably the biggest selling point about the McLanhan filter presses is that they are easy to install, integrate and operate, and they provide an efficient, cost-effective operation. The company’s Smart Hydraulic system controls operating pressures and positions for fast, precise opening and closing. With pressures of 225 psi to 870 psi (16 to 60 b), the press can dewater even the most difficult sludge, including those with a high clay content. The PLC-controlled press has many available cake thicknesses or membrane plates for the driest possible cake.
Ultimately, it depends on the specification for the dryness of the filter cake. “Membrane has a higher up-front cost with higher operating costs,” Jenson said. “But, if the cake needs to be drier that’s the best technology commercially available. If a recessed chamber dries to 75% solids, the membrane might dewater to 78%-79% solids; it’s typically only three or four points better, but if that’s what a mine needs to meet specifications as far as compaction, then that’s the determining factor,” he said.
Cloth selection is also important. “Different cloths have different surface tensions depending on the material it contacts,” Jenson said. “One cloth may have less friction or be less electrically attractive. Different weaves have different properties. There are a number of ways to improve cake release, but we never had to use paddles at Bishop.”
McLanahan has made some improvements based on the performance at Bishop. “The main changes were switching from four hydraulic cylinders to one main hydraulic cylinder,” Jenson said. “It reduced the horsepower and reduced the number of valves. For automation, we no longer have to synchronize the cylinders. We kept four tensioned shafts. We kept the advantages of the old design, simplified it and used less parts and horsepower.”
The latest design has reduced the horsepower by one-third. “All of the torsional valves balancing the flow between the four cylinders require a lot of head and horsepower,” Jenson said. “By going with a single cylinder we cut out all of those valves and we no longer have to worry about synchronizing it and maintaining alignment. We were able to simplify everything, use less horsepower. Of course, we have a couple of new pump vendors as well.”
Operating the System at Bishop
At the time Forge was commissioning the filter press at Bishop, a freak accident occurred with a filter press at Patriot Coal’s Blue Creek prep plant. During February 2013, an engineer was killed at the facility when he was struck by one of the hydraulic cylinders on a plate-type filter press from another vendor. A hydraulic cylinder catastrophically failed while the press was in operation, according to the Mine Safety and Health Administration (MSHA). The victim was positioned near the hydraulic cylinders, troubleshooting the operation of the filter press, when the accident occurred. A second employee received non-life threatening injuries.
“Obviously, this raised some serious concerns and we looked at what happened,” said Borreggine. “Blue Creek was also operating two plate-and-frame presses. For whatever reason, they chose to put the operator controls between the two units. The operators panels at Bishop are on the outside of the units, and they are perpendicular to the hydraulic cylinders that operate the presses.”
Borreggine said the key to keeping the machines running is keeping the cake off of the cloth or the panels, and they did not encounter any issues with the cake sticking. A sticking cake can cause issues with cloth tearing or possibly cracking the plates. “The McLanahan machine performed as it was supposed to,” Borreggine said. “It performed very well.”
The Blue Creek prep plant purchased the plate-and-frame press and Taggart installed it. “With this installation, we were involved with the purchasing and we were able to select the plate-and-frame press we wanted to install,” Borreggine said. “We have a great relationship with McLanahan and felt that they, from a technical and service standpoint, had the best plate-and-frame press.”
During startup, there was not much of a learning curve, Borreggine explained. Maintenance, he added, is also fairly simple. “One thing we did not have to deal with at Bishop was clay,” Borreggine said. “If we would have high-clay application, you would need a blow-back system or an automated spray wash system. We did not have those because this is what we call a ‘clean’ refuse dewatering system because it did not have a lot of clays.”
From the time Taggart (Forge) commissioned and operated the system, it has been nothing but standard maintenance — no issues, Borreggine explained. The Southern Coal team took over and it has been smooth sailing so far.
In the future, Borreggine thinks more prep plants will adopt this technique. “They would have to do some due diligence,” Borreggine said. “What are they doing as far as tonnage and coal quality. It is good to minimize chemical consumption. As they become more popular, I think McLanahan will make these units better, more user-friendly, and more cost –effective.”
Forge is looking at installing another facility considering plate-and-frame presses in U.S. coal. They currently operate belt presses. Because conditions are changing and they need more capacity, they are going to add a plate-and-frame press.