Coal Age

MAR 2014

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Recessed chamber and membrane plate filter presses are gaining more popularity in the coalfields. The systems are used to dewater fine refuse that would tradition- ally be pumped to a thickener and even- tually 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 run- ning according to plan. The alternative technology for dewater- ing fines is a belt press. Belt presses are less expensive to purchase, but they have higher operating costs. They use chemi- cals and require a full-time technician. They achieve about 10% less dryness than the filter press. T h e c o a l i n d u s t r y h a s b e e n t e s t i n g recessed plate and membrane plate filter press technology for about four or five years and there has been a bit of a learn- i n g c u r v e . T r a g i c a l l y , a n i n c i d e n t l a s t year at a different installation manufac- tured by a different vendor claimed the life of an engineer and injured another coal worker. The technology, however, continues to improve, and the Bishop i n s t a l l a t i o n i s a t e s t a m e n t t o t h o s e advances. Taking what they learned from B i s h o p , M c L a n a h a n a n d F o r g e a r e 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 m e m b r a n e . B i s h o p i s u s i n g r e c e s s e d chamber plates. "A recessed plate uses fluid pressure for the dewatering pro- c e s s , " J e n s o n s a i d . " N o t h i n g o n t h e p l a t e s m o v e . S l u r r y i s p u m p e d i n a t a h i g h p r e s s u r e . A s m o r e s o l i d s e n t e r , water is forced out." The membrane filter will produce a dri- e r c a k e , b u t t h e r e a r e t r a d e - o f f s . T h e membrane acts as a diaphragm on the s i d e o f e a c h p l a t e . T h e c h a m b e r s a r e f i l l e d w i t h s l u r r y , a n d t h e n t h e d i a p h r a g m s q u e e z e s t h e c a k e s . T h e trade-off, Jenson explained, is that the m e m b r a n e f l e x e s w i t h e v e r y c y c l e . "There is a limited life cycle of maybe 30,000 to 60,000 cycles before the mem- branes will have to be replaced." Forge specified the McLanahan filter p r e s s e s f o r t h i s i n s t a l l a t i o n , a n d t h e d e wat e r i n g f i n e s B Y S T E V E F I S C O R , E D I T O R - I N - C H I E F Advances in filter press technology Improved dewatering techniques offer an alternative to impounding coal fines The Bishop prep plant uses two McLanahan filter presses aligned end-to-end with a single conveyor underneath both machines. 38 www.coalage.com March 2014 CA_pg38-41_V3_CA_pg46-47 3/11/14 2:46 PM Page 38

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