(401f) Life Cycle Assessment of Treating Acid Mine Drainage Treatment and Producing Rare Earth Element Using Coal Combustion By-Products

Rare earth elements (REEs) are essential components of many present technologies, such as smartphones, computers, and televisions. As a result, global demand for REEs (e.g., Scandium, Yttrium, Neodymium) has increased substantially over the past fifteen years. Yet the economic benefits of the use of REEs can result in geopolitical challenges, in part because natural deposits that are substantial and cost-effective are limited. In fact, only ~1.1% of REEs were produced within the United States, and 80% of REEs that imported into the United States were from China (United States Geological Survey, 2020). Furthermore, traditional extraction processes for REEs involve intensive mining practices that can have substantial negative environmental impacts. These issues suggest the need to identify alternative sources of REEs and develop environmentally benign methods for their production.

One potential source of REEs is acid mine drainage (AMD) from abandoned coal mines. The typical AMD discharge in the United States contains 568 ng/L to 2580 µg/L REEs (Soyol-Erdene et al., 2018). In addition, AMD impairs an estimated 20,000 km of U.S. waterways each year, with potentially serious ecological and human health effects (Skousen et al., 2000). Typically, AMD is generated when exposed minerals are exposed to oxygen, water, and bacteria, which leads to metal solubilization. As a result, most AMD waters have high concentrations of elements, like arsenic and iron (Science Direct, 2019). Typically, AMD is basic, with pH values typically between 2.5 and 6.0, and could be neutralized by interaction with alkaline materials, such as lime sludge from wastewater treatment plants (WTP sludge) and stabilized flue gas desulfurization (sFGD) material which could also trap the REEs for extraction by other processes.

We conducted techno-economic assessments (TEA) and lifecycle assessments (LCA) of a novel approach that uses sFGD material or WTP sludge to neutralize AMD and extract REEs. The process includes a passive treatment cell and a facility in which the REEs are extracted, both of which are assumed to be located at or near the AMD site. Passive treatment cells are assumed to operate continuously with minimal interruption, which makes them ideal for addressing the slow and continuous discharge of AMD. This passive operation is briefly interrupted to replace the sFGD material or the WTP sludge when the neutralizing capacity has decreased to the point where fresh material is needed: three times a year for WTP sludge or seven times a year for sFGD material. These replacements will take approximately three days to complete. The spent sFGD material or WTP sludge is then moved to the facility—which is sized for the amount of spent material that is used with the particular AMD site—and subjected to a series of extraction, aeration, and sedimentation steps to produce a concentrated 7.5 wt. % REE feedstock that can be sold commercially.

The TEAs and LCAs we conducted are based on results from bench-scale experimental results using AMD from two sites in the U.S. state of Ohio. The system design requires ~3,500 kg of WTP sludge or ~3,300 kg of sFGD material to produce 1 kg REE annually, which can remediate 348,000 L or 166,000 L of AMD, respectively. Present work is addressing economies of scale for increasing the annual production of T-REEs. In the TEA, we assumed that the process would operate for 20 years and that the discount rate was 8%. With these parameters, the unsubsidized levelized cost of the process with sFGD was $277/g of REE produced and with WTP sludge was $80/g of REE produced—inclusive of all capital and operating costs. This cost does not include potential revenue from handling the sFGD or WTP sludge (e.g., $3-5/ ton of sFGD) or the avoided costs of landfilling it (e.g., $10-13/ton of sFGD) , or selling the REEs when applied to AMD discharge at two sites in the U.S. state of Ohio (Burkhart, 2020). WTP sludge is currently disposed by filling an empty quarry, where the material is left to sit, and our process could mitigate the need for this disposal. Further it is difficult to account for the value of reducing the need for treatment at water treatment facilities. Our TEA results are consistent with prior work on bioleaching REEs from waste material primarily depended on the tipping fees that could be charged for handling by-product material for industrial producers. Current and future prices of REEs might influence revenue streams as well (Thompson et al., 2018).

For the LCA, we implemented the 2016 ReCiPe Heirarchist impact assessment method. The functional unit was 1 kg of REEs produced, which is the annual amount we expect to produce from this system design for the AMD sites in this work. Using sFGD, the final endpoint indicator for human health was 0.56 disability-adjusted life-years (DALY), which indicates that the net impact of this process on human health is the loss of a half year of a healthy life. The estimated impact on ecosystems is a loss of 0.0012 species/year, which, as with the effect on human health, is almost negligible. The LCA also estimated a loss of $10,052 (2013 USD) in resources, most of which is due to the use of coal and natural gas to provide electricity in Ohio for the process. With WTP instead of sFGD, the impacts where 0.18 DALY, 3.7x10^-4 species/year, and $3,250 (2013 USD). These initial results suggest that it might be favorable, from resource consumption and human and environmental health perspectives, to use WTP sludge instead of sFGD material. Most of the negative impacts from the process (i.e., >99%) result from the REE extraction process. The inclusion of the AMD treatment cell decreases the net impacts on human health and ecosystems, which can be attributed to the removal of several elements—including phosphorous, iron, chromium, aluminum, and lead—prior to entering waterways. Moreover, there are end-of-life benefits to the process that have not been included in these LCAs. After the REEs have been extracted and the concentrated REE feedstock has been produced, the residual material can be used to backfill abandoned mine lands. Doing so could eventually mitigate the production of AMD and further negate the need for cost-intensive treatment and disposal processes.