Recovering rare earths from fertilizer waste

Rare-earth elements are widely used in technology applications such as electronic devices and strong magnets. Chinese mines supply more than 80% of the critical minerals, so other countries are keen to develop domestic sources. Richard E. Riman of Rutgers University and colleagues have now shown that rare earths can be extracted from phosphogypsum, a calcium sulfate waste product of fertilizer manufacturing (J. Chem. Thermodynamics 2019, DOI: 10.1016/j.jct.2018.12.034). An estimated 100,000 metric tons of rare earths—equivalent to about three-quarters of the world’s current production—are consigned to this waste stream every year. Riman’s team prepared phosphogypsum samples and spiked them with six different rare earths, then tested four acids for their ability to extract the metals. Sulfuric acid was the most effective, teasing out 77–94% of the rare earths at pH 0.7. The team also saw promising results from a mixture of gluconic acid and other organic compounds produced by the bacterium Gluconobacter oxydans. This bioacid could extract some rare earths at a milder pH of 2.1. “A bioacid might be more cost effective” because treating the less acidic material would be less expensive, says Paul J. Antonick, part of the Rutgers team. The researchers are now running similar tests on other forms of rare-earth-containing waste.