By Tom Stephens
Acid mine drainage (AMD) has been occurring in the Appalachian region since coal mining began in the early 18th century. Over time, many mines reached the end of their useful life and production ceased or companies went out of business, resulting in AMD discharging from these abandoned mines into native streams and waterways. AMD occurs naturally within some environments as part of the rock-weathering process but is exacerbated by large-scale earth disturbances characteristic of coal mining. Ground flow of water through a chemical process of oxidation absorbs the sulfur and becomes acidic.
The 1972 U.S. Clean Water Act required collection and treatment of AMD clay or geomembrane-lined lagoons. However, this created numerous lagoons filled with high-liquid-content “red mud.” These lagoons tended to overflow or have berm failures during high rainfall events.
The first recorded large-scale use of geotextile tubes to contain and dewater AMD was in 2006 for a section of construction of Interstate 99 for the Pennsylvania Department of Transportation. This project created a proof of concept that could be adapted to the mining industry for AMD treatment, containment and dewatering.
West Virginia has the largest number of abandoned coal mines in the U.S. and has taken the Clean Water Act regulation process one step further by establishing a focused AMD Task Force within its Department of Environmental Protection (DEP) to acquire these abandoned mines and manage the AMD. After reviewing the success of the I-99 AMD project, the DEP designed its first large-scale project for the Omega mining complex south of Morgantown. The site consisted of three abandoned mines generating 35 to 71 cubic feet per minute (1.0 to 2.0 m3/min) of ADM, depending on the season, with a pH as low as 2.8. The flow was collected at the three different sources and pumped to a central point.
From the central collection point, the AMD with 0.2% solids flowed into an equalization tank where hydrated lime was injected at a rate to raise the pH to 6.0 and precipitate the dissolved solids. Also, a small amount of anionic polymer was injected to agglomerate the precipitated solids. From the equalization tank, the AMD flowed to the clarifier where a slurry formed that settled to the conical bottom. As the settled slurry level raised in the clarifier to a certain level, pumps in the control house automatically turned on and pumped the now 2.0% solids slurry to the geotextile tube dewatering cell (Figure 1).
The dewatering cell has capacity for sixteen 45-feet (13.7-m) circumference × 243-feet (74-m) long geotextile tubes per layer. These geotextile tubes contain the slurry as the solids separate and the clear effluent weeps through the geotextile pores. The AMD retained and dewatered within the geotextile tubes increased to 45% solids by weight within seven days and eventually reached 65% solids within 30 days (Figure 2). Currently, the Omega site has been in operation for three years, and the geotextile tubes are on level 3. The current dewatering cell site has a capacity to receive and dewater the current rate of AMD flow for 20 years of 24/7 operation. If the story ended here, by any method of measurements, the Omega AMD project would be a success. In fact, the DEP is adding three of these automated geotextile tube AMD management sites. One of the new sites has flow rates up to ten times the Omega project.
In 2017, the U.S. Department of Defense (DOD) and U.S. Department of Energy (DOE) initiated a program to develop domestic sources of rare earth elements (REE). The REE is a family of 17 elements that are critical and strategic elements for the manufacturer of communication, energy, defense, and aerospace products and systems. Since the early 1990s, more than 90% of all REE have been mined and refined in China, and the global demand is growing. Since 2013, domestic REE U.S. sources have been a congressional concern, according to the Congressional Research Service. DOE Secretary Rick Perry stated in 2018 at the National Energy Technology Laboratory (NETL) Conference that without a domestic source of REE, the U.S. economy would be at risk.
In 2017, the University of West Virginia Water Research Institute began a study to identify and quantify potential sources of REE in existing Appalachian AMD point sources. Some 152 sources of raw AMD water were sampled in the Appalachian Basin.
The average concentration of REE in the AMD point sources samples was 410.6 grams per ton (g/t) of dry solids (Table 1). According to Ziemkiewicz, Xingbo and Noble’s (2018a) study at the University of West Virginia, the dewatered solids in the geotextile tubes at the Omega mine located in the Central Appalachian (CAPP) region totaled 397 g/t. Therefore, the geotextile tube technology was retaining 96.7% of available REE from the AMD flow. In the same study, it was calculated that each geotextile tube in the Omega dewatering cell contains 146 dry metric tons of AMD or 128 pounds (58 kg) of recoverable REE ore. The NETL April 2018 Summary Report set the REE Basket Price of Appalachian Basin recoverable REE ore at $225 per kilogram. Therefore, each geotextile tube contains approximately $13,050 of recoverable REE ore.
When geotextile tube technology was analyzed as a method for recovering rare earth elements from acid mine drainage, it was proven to be extremely efficient and cost-effective in capturing more than 90% of available REE. Also, the recovery of REE from the AMD will have a tremendous economic and environmental benefit to the Appalachian region.
Tom Stephens is director of TenCate Geosynthetics Americas in Bedford, Va.
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