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