Ïåðâîèñòî÷íèê ìàòåðèàëà: http://www.cdc.gov/NIOSH/mining/pubs/pubreference/outputid132.htm
DISCHARGE WATER HANDLING AND TREATMENT: PROBLEMS AND SOLUTIONS AT A LARGE PITTSBURGH SEAM COAL MINE
By J. D. Byars RAG Emerald Resrc(s) LP Waynesburg, PA, USA, T. P. Mucho NIOSH Pittsburgh, PA, USA, R. L. Zick US Filter Opg Svc(s) Moon Township, PA, USA
ABSTRACT
Recently, a large Pittsburgh seam longwall mine was nearing depletion of a major portion of its current reserves and had begun to develop in a different direction from its original portal area to access additional reserves. This meant that a large area of the mine would be abandoned and sealed. However, continued use of the original mine portal area required that the water accumulating in the abandoned mine would eventually need to be pumped to the surface. This would be in addition to the discharge water associated with the new portion of the mine. Several pumping and sump options were investigated to handle the quantity and quality of the anticipated discharge water. This paper describes some of these options, their advantages and disadvantages, and the final engineering decisions. Some problems and unanticipated outcomes, as well as the eventual solutions are discussed.
INTRODUCTION
The act of creating openings and subsiding strata during coal mining almost always produces inflows of unwanted ground water. These water inflows into the underground mine sometimes present considerable problems to mine operators. Depending on amounts and location of inflows, the water needs to be removed from active mining areas. Environmental concerns, law, and regulation require coal mines only to discharge water of an acceptable water quality. However, contact of the ground water with contaminates in and around the coal seam, especially minerals such as pyrite, often requires costly water treatment facilities to meet current criteria. As a result, many operators choose to store as much water in abandoned portions of the mine and/or use this discharge water in the coal preparation process, which can minimize or eliminate treatment costs. Due to abandoning a major portion of a large Pittsburgh seam longwall mine, one operator was challenged to devise a new handling system to deal with the accumulating water in the abandoned portion and the eventual inflows from the projected future mining areas.
Prior Operating Plan
As the current mine dewater system became unable to handle the total mine discharge, new options were considered for the removal of mine water. The new pumping system design had to encompass many design considerations. Some of the considerations are listed below:
- safety and operational risks;
- water system integration;
- future mine plan design;
- minimize operating and capital investment;
- maximize efficiency and minimize operational delays;
The existing pumping system, consisting of a small, above-mine floor storage sump, and the #1 sump pumps located at the main portal area, was delivering about 300 gpm of water to the surface. All of the water pumped to the surface was collected in a series of ponds and then used by the coal preparation plant as make-up water. Water from this sump was the only water pumped from the mine with the nearby sealed area accumulating the remainder of the mine make of ground water. This original system had a number of problems. The #1 sump was small and the system was under-designed to handle the amount of water and solids that was being developed. The system was down frequently due to maintenance problems requiring the water to be directed to the abandoned workings behind the western seals. This original sump was limited in size and would fill up with solids periodically. The high head pumps would only last several months, due primarily to the abrasive material in the water. The discharge line to the surface would burst on an average of six times a year because of the high head and the age of the line. Also, the #1 sump was very inefficient and required a high amount of maintenance to remove accumulated solids. After a cursory evaluation, other system options were investigated.
Determining Requirements of Pumping System
Prior to the design of a pumping system, a general flow diagram was developed. The data for this diagram was gathered in several ways. The ground water inflow behind the seals was determined by a meeting with several foremen that managed the water when the mine was active in that area. The foremen indicated that the water flow, after the initial mining, was fairly consistent at approximately 300 gpm. The water inflow, pumped via pipeline from the southern end of the mine, was approximately 350 gpm. The water from the active eastern reserves was determined to be about 150 gpm. The total maximum projected flow was calculated to be 800 gpm. An 1100 gpm flow rate was selected for the design flow. The 1100 gpm flow rate allows for approximately a 33% increase on the total flow. Using a piezometer, water levels were taken from the abandoned shaft to record the water elevations in the sealed area. An attempt to calculate the water inflow rate into the sealed area using the water level increase was difficult. The natural sump, located behind the western seals, had several sources of water inflow. This area was the natural low spot of the mine and consisted of previously retreated room and pillar, longwall panels and associated standing entries. Inflows of natural ground water and discharge from active mining areas pumped via a pipeline system into the area, accumulated there. Theoretically, the groundwater inflow rate could be calculated by documenting the rise in water level in the sump and subtracting out the known mine discharge inflow. However, the required data is very difficult to obtain. This is due to difficulty determining the volume of the sump given the caving and remaining pillars. A reasonable approximation can be made of the volume of the standing entries. However, determining the volume in the caved areas is more speculative. This is due to problems obtaining the height within the caved area, which can be filled with water, and estimating the void space within the broken rock in the gob. The gob void space calculation was attempted using the following analysis: The gob was defined in area to originally have a 7-ft mining height. It was assumed that the main contributor for sump capacity in the gob area would be the “caved zone” area as depicted by Peng and Chaing and Singh and Kendorski1,2. The contribution of the “fractured zone” and other subsided areas of the gob were considered to be negligible. The height of the caving zone can be estimated by formulas and bulking factors given by Peng1 and Chen3. In this case, a caving height of 5 times the mining height and a bulking factor of 1.25 were used. This results in a maximum water fill height of 35 feet from the original bottom with 25% of this volume being available for water storage. This calculation could be assumed across the width of the longwall panel and added to the volume of the standing entries. Therefore, at various water pool elevations, and given the original coal bed bottom elevations, the total water storage volume available could be estimated at incremental, measured, sump pool heights. The main point of determining the volume of the inflow into the sealed area was to develop a good engineering estimate of the total flow rate that would need to be pumped. While a reasonable estimate was made, the ground water inflow rate of the sealed gob was very difficult to determine precisely by this volume method.