By R.L. “Rick” Shaw and Darrell A. Smith
Located in northwest Marion County, near Fairview, W.Va., CONSOL Energy’s Loveridge mine and preparation plant have been in operation since 1957. In its first 20 years of operation the mine used rail haulage and continuous miner production. The mine was converted to a longwall operation in the late 1970s and conveyors began to replace the track haulage. In 1983, Loveridge was ranked as one of the top underground coal mines in the world, producing 3.3 million tons of clean coal from two longwalls.
Today, the mine employs one longwall and four continuous miners. Conveyor haulage is used exclusively after major belt upgrades in the 1980s and 1990s. The 3,300-ft production slope exits the ground near the prep plant while the production faces are several miles distant underground. A small bunker near the production area stores coal and meters a smooth stream to the conveying system. The RoM crusher is the original McLanahan Rockmaster at the slope bottom. Raw coal is stored on the surface in the original 15,000-ton raw coal bin or a newer 40,000 ton stacker.
The mine operates in the Pittsburgh seam and is the sole source of coal for the prep plant. All production is fully washed. Typical annual production for the last five years is 6.5 million tons of clean coal. Yield averages 80% to 82%.
The plant product is a 2-inch x 0 steam coal with thermal drying for the fines. The product is stored in two 10,000-ton clean coal silos (no ground storage for clean coal). The loadout is a conventional unit train loadout operating at 3,500 tons per hour (tph), equipped with a certified scale and automatic sampler. Typically the mine serves a dozen or more utility customers each month.
Plant Upgrade
Built by Fairmont Machinery Co., Loveridge’s original 1,200-tph raw feed design included two Baum jigs for the 6- x 3/16-inch material and Diester tables for the 3/16-inch x 100 mesh. The minus 3/16-inch was dewatered in eight low speed Bird solid bowl centrifuges and thermally dried in three Raymond flash dryers.
In the early to mid-1980s, Loveridge underwent several major modifications. This work was done around the existing circuitry as the plant operated, resulting in widely dispersed equipment, convoluted flow paths, and poor space utilization. By 1986, the rebuilt plant was designed for a 1,400-tph raw feed capacity. Two heavy media vessels replaced the coarse coal jigs, a fine coal jig was installed to eliminate the Diester tables, and two Wemco froth cell banks and two Peterson vacuum disc filters were added. At the same time the Raymond flash dryers were replaced with a single large ENI fluid bed dryer. Krebs 26-inch classifying cyclones were used to make a 60-mesh separation between the jig feed and flotation. Large Warman high pressure slurry pumps replaced older Hazleton pumps. The eight Bird solid bowls were the only remaining process equipment from the 1957 flowsheet.
With the market constricting in the early 1990s, Loveridge was idled multiple times beginning in 1993. This erratic operation continued through 2003 until the longwall went back into production in March 2004. While money was invested to maintain and rehabilitate the aging structure during this period, no funds were invested in the washing circuits.
In 2005, considerable coal losses began to appear in the thickener. This was due in part to changes in size consist, higher feed rate (1,600 tph), and new flotation reagents. Previous work at Loveridge and other plants had pointed out the potential weaknesses in the three circuit design for longwall mines in the Pittsburgh seam: too many fines reported to the jig and the flotation feed contained too much plus 100-mesh material. A decision was made to add a hydro-spiral circuit between the jig and froth cells to enhance fines recovery in several areas. Process efficiency issues included:
Froth feed too coarse—Feed to the froth cells contained 12% plus100-mesh material. This is excessive for these two 2,500-cu-ft banks and resulted in 25% plus 100-mesh low ash coal in the tailings.
Filter feed too fine—The resultant froth was approximately 88% minus 100-mesh, too fine to build a porous filter cake and resulted in a thin cake and low filter capacity.
Jig refuse underflow lost coal to the thickener—Hydraulic transport of the jig refuse resulted in fines laden jig underwater being pumped to the jig refuse screen and lost to the undersize fraction which reported to the thickener. Tests showed that 19% of the jig refuse solids were minus 100-mesh at 33% ash.
Circuit Modifications
A conventional hydro-spiral circuit was selected to wash the 14- x 100-mesh feed, thus unloading the froth cells and the fines from the jig. A secondary benefit of this circuit is the sharper separation of the 15-inch Krebs raw coal classifying cyclones cutting at 100 mesh vs. the flatter curve of the 26-inch cyclones separating at 60 mesh. The 26-inch cyclones had been previously scheduled for replacement and most of the associated hardware (piping, froth cell protection sieves, etc.) in that area was also at the end of its useful life.
Reverse incline screens are typically used to dewater the classifying cyclone undersize fraction. The high-ash ultrafine material reporting with the water to the cyclone underflow end up in product however. At Loveridge, Derrick Stack Sizers were instead used to lower the product ash by removing most of these ultrafines back to the froth feed. The Derrick screen oversize fraction provides the coarse material to improve the filter performance.
The undersize fraction from the jig refuse screen was diverted back to the Bird effluent sump to capture the misplaced minus 100-mesh coal.
Layout & Structural Issues
As previously noted, much of the interior space of the Loveridge plant is underused. With the heavy-media vessels on the west end, the fine coal jig on the north, and the filters on the east, the center of this former Diester table floor is largely vacant. Unfortunately, while this condition exists on most floors, these vacant areas did not coincide vertically and could not be used without inter-stage pumping or transfer conveyors, etc.
In addition, the constraints of the circuit required that the cyclone banks be on opposite ends of the plant. The three resultant streams of the hydro-spiral circuit—refuse, middlings, and product—report to the refuse belt and two sumps in the northwest area of the plant. The product from the classifying cyclones and Derrick Stack Sizer report to the vacuum filters which are located on the east side of the seventh floor. This meant most of the construction occurred in two areas above the seventh floor. The hydro-spiral circuit occupies one bay on four floors at or above the control room (seventh) floor, and resulted in a 130-ft tower in the northwest corner. The classifying cyclones are located in a four floor addition above the froth cells on the east end of the plant, increasing the height of the plant by 23 ft to more than 150 ft.
After 50 years, and several major circuit modifications and structural rehabilitations, much of the area where equipment was added was previously lightly loaded and had not been refurbished in some time. Consequentially, steel rehabilitation had to begin at the bottom floor in the tower areas and proceed upwards before significant weight could be added above. This work was completed in a timely fashion and under budget. However, one deteriorated member in an adjacent area was discovered and led to considerable overtime and unanticipated expense. Overall, structural concerns occupied nearly half of the pre-startup field engineering time.
Circuit Description
The 3/8-inch x 0 jig feed is classified on three 6-ft wide sieve bends with a 60-inch radius and 1-mm opening. The undersize fraction reports to the hydro feed sump on the ground floor. A new 700-hp Warman 14 x 12 pump, with refurbished Toshiba motor and Falk speed reducer, lifts it 120-ft to a bank of 18 Krebs 15-inch hydrocyclones. (This pump replaced the existing 500-hp 10-mesh pump which was scheduled for replacement.) The oversize fraction from the hydrocyclones flow to the classifying cyclone sump and the undersize reports to the spiral feed sump.
The 600-gallon spiral feed sump is equipped with a fast acting level control loop and discharge orifices to maintain constant head and flow rate to the two spiral protection sieves. The sieves are 3-ft wide with a 40-inch radius and 3-mm openings.
Two banks of 10 Krebs double start spirals make the three way separation. The spiral product reports with the hydrocyclone oversize to the classifying sump; the middlings normally go back to the hydrocyclone sump for recycle (although they can be sent to refuse), and the reject goes to a reconditioned Tabor reverse incline screen. The spiral refuse screen undersize fraction reports to the thickener and the oversize goes onto the refuse belt.
The 14-mesh x 0 hydro-spiral product reports to the classifying sump on the ground floor. A new 700-hp Warman 14 x 12 pump, with refurbished Toshiba motor and Falk speed reducer, lifts it 140 ft to a bank of 15 Krebs 15-inch classifying cyclones. The cyclone oversize (minus 100 mesh) report to the two existing 2,500-cu-ft Wemco froth cel1s. The plus 100-mesh undersize drops to two sieve bends that measure 6-ft wide with a 60-inch radius and 1-mm opening to control the load on the Derrick Stack Sizer; the oversize shifts back to the Bird solid bowls.
The sieve bend undersize reports to two flow dividers for the two Derrick 3-deck Stack Sizers. The minus 80-mesh Derrick undersize goes to froth feed and the oversize fraction reports to the froth launders enroute to the two 12.5-ft, 10-disc Peterson vacuum filters.
The existing 5-hp DC drives on the disc filters were replaced with 10-hp drives to accommodate the heavier cake. An automated flocculant addition system was added to each filter to enhance recovery and capacity.
The jig refuse screen undersize fraction now reports back to the Bird effluent sump and is used as pushwater for the raw coal flumes. The only two streams from the plant now reporting to the thickener are the froth tailings and the spiral refuse screen undersize fraction.
Electrical & Control Enhancements
The Loveridge Plant was marginal on transformer capacity prior to this project. Power fluctuations and intermittent problems occurred during periods of heavy loading, particularly in the summer. While the plant upgrade required less than 1,000 hp of incremental power, a new 2,500-KVA high voltage transformer and switchgear was added in the adjacent substation to improve power distribution and provide some future capacity.
The plant’s existing Allen-Bradley ControlLogix platform and RSView software were used to incorporate the control changes. New level controls, start-up diverters, and automated flocculent and reagent controls were added. Several trend screens were added to aid in diagnostics and performance tracking.
Results
The hydro-spiral circuit performed as expected on the 14- x 100-mesh feed with a reject of 70.40 % ash, 10.97% sulfur, and efficiency of 99.58%. The Derrick Stack Sizers performed the intended task by reducing product ash on that stream by 2% and sulfur by 0.25%. The underflow reports to the froth feed to prevent coal loss.
The froth cells, operating on a low 15% ash feed, exhibited an 86% yield and 93% coal recovery. Product was 6.75% ash and 3.05% sulfur. The froth feed is 7% plus 100 mesh, as intended. Feed to the filters met the 40% plus 100-mesh target, averaging 42%.
Thickener underflow improved dramatically with the circuit upgrade. Solids content in the underflow dropped from a first quarter 2008 average of 34.88% to a fourth quarter average of 18.99% while underflow ash increased from 26.60% to 51.47%, surpassing the project goals.
Using an improvement of 2% of plant feed, at Loveridge’s annual production rate and estimated realization, this $4.93 million project will pay for itself in less than one year.
Increased production of filter cake has added greater load to the thermal dryer. Modifications to the dryer are in process to allow it to operate at higher evaporative loads within its design range. Additional mechanical drying will be investigated.
Acknowledgements
The authors would like to acknowledge the efforts and assistance of Randy Tennant, Loveridge plant maintenance foreman; Barry Martino, plant electrical foreman; and Wade Burns, Northern West Virginia operations electrical engineer for their valued assistance in construction and implementation; to Larry Peterson, consultant; Gary Meenan, Robert Egeland, and Tim Walter of CONSOL’s Process Group for their technical expertise, and to Roger Nesselrote of Industrial Resources for his experience and expertise. We would also like to thank the vendors and contractors who made significant positive contributions to this project: Industrial Resources, West Virginia Electric, FLSmidth Krebs, Morris-Coker, and Soles Electric among others.
Shaw is the manager, process/quality control, CONSOL Energy, and Smith is the plant superintendent for CONSOL Energy’s Loveridge preparation plant. This article was adapted from a paper they presented at Coal Prep 2009, which was held during April 2009, in Lexington, Ky.
