Geosynthetics October/November 2019 - 8

UPDATE

Recovery of rare earth elements from acid
mine drainage using geotextile tubes
By Tom Stephens

A

FIGURE 1 Geotextile tube
dewatering cell

Tom Stephens is director of
TenCate Geosynthetics Americas
in Bedford, Va.
All photographs courtesy of the author.

8

Geosynthetics | October November 2019

cid 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



Geosynthetics October/November 2019

Table of Contents for the Digital Edition of Geosynthetics October/November 2019

Geosynthetics October/November 2019 - Cover1
Geosynthetics October/November 2019 - Cover2
Geosynthetics October/November 2019 - 1
Geosynthetics October/November 2019 - 2
Geosynthetics October/November 2019 - 3
Geosynthetics October/November 2019 - 4
Geosynthetics October/November 2019 - 5
Geosynthetics October/November 2019 - 6
Geosynthetics October/November 2019 - 7
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Geosynthetics October/November 2019 - Cover3
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Geosynthetics October/November 2019 - Blank1
Geosynthetics October/November 2019 - GeoConf20_1
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