Extract rare earth from fly ash

The role of ionic liquids

Researchers at Georgia Institute of Technology have developed a simple and easy process to extract rare earths from fly ash using ionic liquids without creating toxic substances.

In a paper published in Environmental Science & Technology, scientists explain that ionic liquids are environmentally friendly and reusable. In particular, betainium bisimide or [Hbet][Tf2N] can selectively dissolve rare-earth oxides rather than other metal oxides.

Scientists say ionic liquids dissolve in water when heated and change into two phases when cooled. Knowing this, they began to test whether useful elements could be effectively and selectively separated from the fly ash, and whether they could be effectively cleaned and developed in a process that was safe and did not cause major pollution.

Extraction process

To do this, the team pretreated the fly ash with an alkaline solution and dried it. Then, the fly ash in water with [Hbet] [Tf2N] is heated to form a single phase. The solution is separated after cooling. Ionic liquids can extract more than 70% of the rare earth elements from fresh fly ash, and even higher recovery rates (97%) from weathered fly ash that has been stored in tailings ponds for many years. The final step in the process is to extract rare earth elements from the ionic liquid with dilute acid.

The researchers found that adding betaine to the leaching process increased the amount of rare earth elements extracted.

Rare earth elements that can be recovered include scandium, yttrium, lanthanum, cerium, neodymium and dysprosium.

Finally, the team also tested the reusability of ionic liquids, using cold water to remove excess acid, and found that extraction efficiencies did not change after three rounds of leached cleaning.

"This low-emission method can produce solutions rich in rare earths with limited impurities and can also be used to recover precious metals from fly ash in tailings ponds," the scientists said in a media release.

This approach is important in coal-producing states like Wyoming. They are looking to reinvest in local industries at a time when demand for fossil fuels is falling.