By Joseph White
In recent years, traditional fine coal refuse disposal techniques have become increasingly challenging. Increased state and federal regulations on slurry impoundments have required significant modification of disposal techniques within the industry. Secondary options of fine-coal refuse disposal include underground slurry injection, slurry cells, and combined-fill refuse area using belt presses or plate and frame presses.
Midland Trail Energy, an affiliate of Patriot Coal Corp., commissioned its Blue Creek Operation in September 2009 with permits that allowed the placement of fine coal refuse in a slurry cell area. Blue Creek faced challenges building and covering each slurry cell with the appropriate amount of coarse refuse required to achieve structural stability. The preliminary ratio calculations of coarse to fine refuse called for 85% coarse material and 15% fine refuse. This presented a construction challenge that could have delayed underground production. Blue Creek needed to consider the future of the refuse area design.
Combined Coal-Refuse Placement
Midland Trail Energy decided to explore the option of modifying the existing slurry cell permit to allow combined coal-refuse placement. This is also known as a “combined fill” refuse area. This technique combines coarse coal refuse and dewatered fine coal refuse resulting in a homogenous construction material throughout the dump area. The combined fill is permitted to be placed directly above the existing slurry cells, which allows for the current infrastructure to be used for the new refuse area.
Selection of appropriate equipment was the next challenge. The two most viable options were to use belt presses or plate and frame presses. Belt presses historically produce a moisture content of 35%-40%, while the plate and frame presses consistently provide fine coal refuse moisture contents of 18%-20% (Parker, 2011).
The Blue Creek Prep Plant
With this information Midland Trail Energy commissioned two Tons Per Hour/Jingjin plate and frame presses. The plate and frame circuit would need to be able to process the entire fine refuse stream produced by the prep plant.
The Blue Creek prep plant operates in the Campbell Creek area about 10 miles east of Charleston, W.Va. It washes coal from one deep mine, cutting coal from the Stockton seam. With a raw feed capacity of 900 tons per hour (tph), it is a heavy-media cyclone (HMC) plant. The primary components include: one 48-inch Krebs HMC, one bank of six 15-inch Krebs raw-coal classifying cyclones, two banks of eight MDL triple-start, compound spirals, and one bank of four Krebs clean-coal classifying cyclones.
Secondary components include: two Conn-Weld 10- x 16-ft double-deck, raw-coal screens, two Conn-Weld 10- x 16-ft, double-deck, clean-coal screens, two Conn-Weld 8- x 16-ft, double-deck, cyclone-refuse screens, two Conn-Weld 6- x 12-ft fine-refuse dewatering screens, two Conn-Weld 72- x 80-inch (rad.) fine clean-coal sieve bends, two Ludowici VM-1400 clean-coal centrifuges, one Decanter 44- x 132-inch screen-bowl centrifuge, and one 120-ft static thickener.
The Blue Creek prep plant washes the entire raw coal stream. The typical raw feed coal characteristics for Stockton are 58% ash and 0.4% sulfur. The prep plant produces approximately 585 tph of coarse and fine refuse.
The coarse refuse consists of refuse screen over-sized material, and is transferred by refuse belts to a coarse refuse loadout. This coarse refuse stream consists of all plus 1-mm refuse material, and accounts for about 500 tph (55.5% of the plant feed). The remaining material, about 85 tph (or 9.4% of the plant feed), is fine refuse material, minus 1 mm, which reports to the thickener.
Currently the thickener underflow produces 800 gpm with 32% solids (or 1.2 sg) of minus 1-mm fine coal refuse, all of which feeds the plate and frame circuit. The 800-gpm thickener underflow delivers 262 tph of slurry mass and 85 tph of solid refuse. Roughly 90% of the plate and frame feed is minus 60 mm.
With this particle size distribution and quantity of slurry, Blue Creek plant had to select the appropriate plate and frame system for the application.
Plate and Frame Presses
Plate and frame presses (filter presses) work in a “batch” loading style. They are filled with slurry at the start of a filtering cycle. During the filtration cycle, the filter press produces a batch of solid (dewatered) material or “filter cake” and clarified water. The clarified water or “filtrate” is collected during the filtration cycle, and is either reused in the plant or discharged.
After discharging the filter cake, the filter press is re-loaded with slurry for another filtration cycle. The filter press uses pressure placed on the solid-liquid material to displace the filtrate from the solid material. The increased pressure from a plate and frame system can produce moisture levels less than 25%.
The plate and frame system consist of a series of plates with recessed chambers. These plates are forced together with hydraulic pressure typically in the range of 2,500 psi (19 MPa). This compression creates sealed chambers that trap the slurry until pressure builds from the feed pump. As pressure increases, water in the slurry passes through the filter media leaving the solids trapped in the concave cavity. The chamber continues to pack with solids forming the filter cake. Once the chamber fills completely and the water has been forced from the solids,the press opens releasing the solid cake. Clear water is collected in the filtrate pans and returned to the process for re-use.
Membrane vs. Chambers
There are two styles of filter presses: recessed chamber and membrane. In a recessed-chamber filter press, all of the plates are rigid and concave. The slurry is trapped in the chamber created by the plates being pressed together. This design relies entirely on the hydraulic pressure (usually in excess of 200 psi) of the feed pump to achieve dewatering. As water bleeds from the cake, more slurry is packed into the void until dewatering is complete. This design requires less initial capital but may lead to longer cycle times (fewer cycles per hour), greater wear on the feed pump, and higher moisture content of the cake. To reach the desired pressure, positive displacement pumps or dual-stage slurry pumps are frequently used.
With membrane filter presses, every other plate is recessed (chamber plates), the rest of the plates are hollow with flexible elastomer faces (membrane plates). Once the chamber is full and pressurized to about 120 psi, the feed pump stops, feed valve closes and air (or water) inflates the membrane plates to a pressure greater than the pressure in the chamber (usually about 180 psi).
The even pressure across the face of the cake forces water from the cake more rapidly, thus reducing dead-head conditions on the feed pump and reducing wear. It also facilitates faster dewatering. The membrane press consists of chamber plates with concave interiors that create the compartment for the slurry to form a filter cake. The chamber press uses pressure from a two stage feed pump to fill the chambers with the appropriate pressure to dewater the slurry. With this style, the feed pressure to the system is the only dewatering tool. Depending on the design, the feed pressure is determined by the desired moisture content of the filter cake. Unlike the chamber press that uses only chamber plates, the membrane filter press uses alternating chamber and membrane plates. This membrane plate is a flat plate that inflates during a “squeeze” cycle, placing additional pressure on the solid/liquid material. The membrane press uses a single stage slurry pump to fill the plates to the desired pressure. When the correct pressure is reached, the feed will stop and the membrane plates will be inflated with air, which presses the material in the chamber to the final filter cake.
After looking at the data received from the competing manufactures, Midland Trail Energy’s Blue Creek Operation decided to use the Tons Per Hour/Jingjin 2-m x 2-m x 40-mm quick-opening automatic membrane filter press. Tons Per Hour worked in conjunction with Conn-Weld Industries and Taggart Global to construct the plate and frame system. It was designed to process the entire 85-tph stream of dry solids of the thickener underflow. Tests concluded that one membrane press would be able to produce a 25% moisture product at 60 tph; with two presses the circuit could produce 120 tph of filter cake. Accounting for the filter cakes’ 25% moisture, the two presses could process 90 tph of 2.155 density dry solid material. Below are the calculations used to verify these assumptions:
Installing the Filter Press Circuit
Tons Per Hour, Conn-Weld Industries and Taggart Global worked together to construct all of the necessary components around the filter presses. In addition to the two filter presses, the circuit also included one 30,000-gallon surge tank, two 6 x 4 Schurco slurry pumps, two 75-hp air compressors, one 3,000-gallon air tank, one 110-ton refuse bin, one 72- x 285-inch collecting conveyor with structure and drive, and a wash-water tank.
A full cycle of the Blue Creek plate and frame press begins with the thickener underflow material being pumped with the existing underflow slurry pumps to the valve house located at the press building. Once through the valve house the slurry is directed to a 30,000 gallon surge tank. This is the feeding source for both presses. The two Schurco slurry pumps pull from the bottom of the surge tank which pumps the slurry to each press.
At the beginning of each cycle, the presses hydraulically seal and send a signal for feed from each of these slurry pumps. These pumps feed the presses to a desired pressure. Once the desired pressure is reached, a PLC timer starts and the pump will continue pumping for the designed amount of time.
When the timed feed function is complete, the feed pumps are turned off and the press is sealed with automatic valves, this allows the membrane squeeze cycle to start. Filled from the two 75-hp air compressors, the 3,000-gallon tank now releases the stored air into the membrane plates. This squeeze cycle allows for further dewatering of the material without the addition of more slurry. Similar to the feed cycle, the squeeze cycle is set up on a designed pressure (around 180 psi) with a PLC timer to manipulate the density of the final filter cake. As the membrane cycle ends, air is blown into the slurry feed port and the material that is not made into cake is recycled to the refuse surge tank. This segment is referred to as “core-blow.”
Once the core-blow cycle is complete, the hydraulic cylinder of the press will begin to retract and pull the plates open. When the press is completely open, the plate shifting mechanism, or “rabbit,” is sent to open those plates that have not opened by the cylinder. This rabbit is a chain driven device that uses levers that are able to pull plates forward and backward. The rabbit will pull five plates at a time, allowing the filter cake to drop (gravity) from the plate cavity onto the collection conveyor belt beneath it. Upon completion of this plate shifting cycle, the hydraulic ram presses the plates back together and starts the feed process again.
The material on the belt is conveyed into the 100-ton refuse bin.
Press cycle times average 20 minutes or about three cycles per hour. In combination, the filter press circuit can make six cycles per hour. Each press delivers 20 tons per cycle at 25% moisture. With these cycle times, the plate and frame system processes the necessary 120 tph fine tailings to allow the Blue Creek prep plant to operate at its raw feed capacity (900 tph). With the integration of PLC software, cycle times can be manipulated to produce the desired product.
The Blue Creek prep plant has had no issues producing a filter cake with 25% or better moisture since the commencement of this project. Six months of sampling display the moisture levels being right at or below 25%.
Mobile equipment transports the filter cake from the refuse bin to the permitted combined fill area. When the trucks are loaded, the dewatered fine refuse material is hauled to the existing dump area and is incorporated with the coarse.
The permit requires this combined coal refuse material to be placed in 24-inch thick lifts (maximum). Each area is then graded to minimize precipitation from permeating the combined material. Following the completion of each lift, representative samples are taken for compaction records. Five such samples were taken on three days in August 2011; these samples confirmed the average moisture of the combined refuse area is 3.67%. The same samples produced an average compaction density of 116 lb/ft3. With a required compaction density of 113.6 lb/ft3, the samples demonstrate that the compaction completion is at 102%.
The Tons Per Hour/Jingjin membrane filter presses allowed Patriot Coal’s Blue Creek prep plant to process 900 tph of Stockton seam coal without stopping production. The plate and frame system processes the entire 85 tph solid tons of minus 1-mm fine refuse material by dewatering the material to a 25% moisture filter cake. These presses are capable of creating compaction densities sufficient for combined refuse areas. The Tons Per Hour/Jingjin filter presses have proven to be a viable option for fine coal refuse disposal.
White is a preparation plant engineer working for Patriot Coal Services. This article is adapted from a presentation he made at Coal Prep 2012, which took place in May 2012 in Lexington, Ky. To see the original article, visit: www.coalprepshow.com.
The installation of the Blue Creek plate and frame system was supervised by Roger Graley, director of preparation plants for Patriot Coal Services, LLC. His assistance in this installation was greatly appreciated. Tons Per Hour, Inc. and Michael Parker deserve a special thank you for their assistance in the installation as well.