Chromatography techniques yield rare earth elements from coal ash

New chromatography separation techniques could extract rare earth elements from waste coal ash and help introduce the US to a market worth $4bn.

rare earth elements
Linda Wang, Purdue University’s Maxine Spencer Nichols Professor of Chemical Engineering

Rare earth elements (REE) consist largely of Lanthanides (Lns) – a series of 15 metallic elements – and the value of products requiring them is estimated at over $4 trillion per year.

“REEs have many important applications in things such as permanent magnets in power generation and electric cars, batteries, petroleum refining catalysts, phosphors in colour televisions, and many electronics including cellphones,” said Linda Wang, inventor of the technology at Purdue University, Indiana. “The demand for REEs is predicted to grow dramatically over the next several decades.”

Separating rare earth elements is, however, extremely difficult because the elements have the same ionic charge and are similar in size, said Wang. They also occur in ores in concentrations of a few thousand parts per million.

“Extensive processes of mining, grinding, extraction, and purification are needed to transform the ores to very high purity [approximately 99.9 percent minimum] rare earth metals required for commercial applications,” she said. “Typically, old technologies from the 1950s are used for separation and purification. They usually require 1,800 different extraction stages in series and in parallel for purification.”

By contrast, Wang’s new techniques could separate REE first from other impurities and then from each other by using only a few chromatography units.

The processes are said to involve ligand-assisted elution or displacement chromatography methods using low-cost, inorganic sorbent titania or polymeric sorbents.

“These new processes can effectively separate REEs with purities and yields greater than 95 per cent,” Wang said. “Using titania sorbents is what makes this innovation unique. They are robust and inexpensive, making the processes efficient and affordable. We are the first group in the world who developed this technology. Additionally, the by-products of our process include silica gel, aluminum oxide, and other metal oxides of commercial value, making the overall process profitable and economical.”

Wang said that the US has accumulated 1.5 billion tons of coal ash, which is rich in rare earth elements and could be used to produce REEs for decades.

“We have demonstrated the feasibility of this technology at laboratory scale and believe it is going to be a much simpler, more efficient, and lower capital cost option for REE production,” Wang said. “We have had success in many challenging chromatographic separations, including the purification of medical isotopes, sugars, amino acids, chiral drugs, insulin, polymers, and many others. Thus, we are confident that we can produce high purity REEs from coal ash.”

The Purdue Research Foundation’s Office of Technology Commercialisation has patented the technology and it is available for license.