New, environmentally friendly method to extract and separate rare earth elements from each other
From:
Penn State University Eberly College of Science
By Gail McCormick
October 8, 2021 -- A new method improves
the extraction and separation of rare earth elements—a group of 17 elements
critical for technologies such as smart phones and electric car batteries—from
unconventional sources. New research led by scientists at Penn State and the
Lawrence Livermore National Laboratory (LLNL) demonstrates how a protein
isolated from bacteria can provide a more environmentally friendly way to
extract these metals and to separate them from other metals and from each
other. The method could eventually be scaled up to help develop a domestic
supply of rare earth metals from industrial waste and electronics due to be
recycled.
“In order to meet the increasing demand
for rare earth elements for use in emerging clean energy technologies, we need
to address several challenges in the supply chain,” said Joseph Cotruvo Jr.,
assistant professor and Louis Martarano Career Development Professor of
Chemistry at Penn State, a member of Penn State’s Center for Critical Minerals,
and co-corresponding authors of the study. “This includes improving the
efficiency and alleviating the environmental burden of the extraction and
separation processes for these metals. In this study, we demonstrate a
promising new method using a natural protein that could be scaled up to extract
and separate rare earth elements from low-grade sources, including industrial
wastes.”
Because the U.S. currently imports most
of the rare earth elements it needs, a new focus has been placed on
establishing a domestic supply from unconventional sources, including
industrial waste from burning coal and mining other metals as well as
electronic waste from cell phones and many other materials. These sources are
vast but considered “low grade,” because the rare earths are mixed with many
other metals and the amount of rare earths present is too low for traditional
processes to work well. Furthermore, current methods for extraction and
separation rely on harsh chemicals, are labor intensive, sometimes involve
hundreds of steps, produce a high volume of waste, and are high cost.
The new method takes advantage of a
bacterial protein called lanmodulin, previously discovered by the research
team, that is almost a billion times better at binding to rare earth elements
than to other metals. A paper
describing the process appears online October 8 in
the journal ACS Central Science.
The protein is first immobilized onto
tiny beads within a column—a vertical tube commonly used in industrial processes—to
which the liquid source material is added. The protein then binds to the rare
earth elements in the sample, which allows only the rare earths to be retained
in the column and the remaining liquid drained off. Then, by changing the
conditions, for example by changing the acidity or adding additional
ingredients, the metals unbind from the protein and can be drained and
collected. By carefully changing the conditions in sequence, individual rare
earth elements could be separated.
“We first demonstrated that the method
is exceptionally good at separating the rare earth elements from other metals,
which is essential when dealing with low grade sources that are a hodgepodge of
metals to start with,” said Cotruvo. “Even in a very complex solution where less
than 0.1% of the metals are rare earths—an exceedingly low amount—we
successfully extracted and then separated a grouping of the lighter rare earths
from a grouping of the heavier rare earths in one step. This separation is an
essential simplifying step because the rare earths have to be separated into
individual elements to be incorporated into technologies.”
The research team separated yttrium (Y)
from neodymium (Nd)—both abundant in primary rare earth deposits and coal
byproducts—with greater than 99% purity. They also separated neodymium from
dysprosium (Dy)—a crucial pairing that is common in electronic waste—with
greater than 99.9% purity in just one or two cycles, depending on the initial
metal composition.
“The high-purity of the recovered
neodymium and dysprosium is comparable to other separation methods and was
accomplished in as many or fewer steps without using harsh organic solvents,”
said Ziye Dong, a postdoctoral researcher at LLNL and first author of the
study. “Because the protein is able to be used for many cycles, it offers an
attractive eco-friendly alternative to the methods currently used.”
The researchers do not think their
method will necessarily supplant the current liquid-liquid extraction process
that is commonly used for high-volume production of lighter rare earth elements
from high-grade sources. Instead, it will allow for efficient use of low-grade
sources and especially for extraction and separation of the rarer and generally
far more valuable heavy rare earths.
“Other recent methods are capable of
extracting rare earth elements from low-grade sources, but they typically stop
at a ‘total’ product that has all the rare earths lumped together, which has
relatively little value and then needs to be funneled into more conventional schemes
for further purification of individual rare earth elements,” said Dan Park,
staff scientist at LLNL and co-corresponding author of the study. “The value is
really in the production of individual rare earths and especially the heavier
elements.”
“Our process is particularly convenient
because these high-value metals can be purified off the column first,” added
Cotruvo.
The researchers plan to optimize the
method so fewer cycles are required to obtain the highest-purity products and
so it can be scaled up for industrial use.
“If we can engineer derivatives of the
lanmodulin protein with greater selectivity for specific elements, we could
recover and separate all 17 rare earth elements in a relatively small number of
steps, even from the most complex mixtures, and without any organic solvents or
toxic chemicals, which would be a very big deal,” said Cotruvo. “Our work shows
that this goal should be achievable.”
In addition to Cotruvo, Dong, and Park,
the research team includes Joseph Mattocks at Penn State, Gauthier Deblonde and
Yongqin Jiao at Lawrence Livermore National Laboratory, and Dehong Hu at the
Pacific Northwest National Laboratory. This work was supported by the Critical
Materials Institute, an Energy Innovation Hub funded by the U.S. Department of
Energy, and the DOE Office of Science.
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