USA: Sustainable Mining SNL Develops Environmentally Friendly Method for Mining Rare-Earth Metals
Coal ash is the unwanted but widely present residue of coal-fired power. Rare-earth metals are used for a variety of high-tech equipment from smartphones to submarines. The separation method, which uses carbon dioxide, water and food-grade citric acid, is the subject of a Sandia National Laboratories patent application.
Livermore/USA — A new technique developed at Sandia National Laboratories in the USA, not only recovers rare-earth metals in an environmentally harmless manner but would actually improve environments by reducing the toxicity of coal waste. The Sandia process, which uses citric acid as a carrier for rare-earth metals, so they separate from coal ash, the host material, was implemented by Guangping Xu, lead Sandia researcher on the project. The extraction process is facilitated by using supercritical carbon dioxide solvent. Xu’s Sandia colleague Yongliang Xiong suggested citric acid, a commonly used and environmentally friendly chemical for holding metals in solution.
Xu found that in less than a day, at 158° Fahrenheit (70°C) and 1100 pounds per square inch pressure (about 70 times ordinary atmospheric pressure), the method extracted 42 percent of rare-earth metals present in coal waste samples. Chinese mines, where 95 percent of the world’s resources of rare-earth metals are located, achieve less efficient separation while using environmentally damaging methods.
“Harmless extraction of rare-earth metals from coal ash not only provides a national source of materials essential for computer chips, smart phones and other high-tech products — including fighter jets and submarines — but also makes the coal ash cleaner and less toxic, enabling its direct reuse as concrete filler or agricultural topsoil,” said Guangping Xu.
This technology also could open a new avenue for carbon-dioxide reutilization and sequestration, said Xu’s Sandia colleague Mark Rigali, who with Xu is exploring the use of citric acid and supercritical carbon dioxide to mine metals from oil and gas shales that are often rich in metals. “Using existing oil and gas fracking wells, the citric acid and supercritical carbon dioxide can be used cost-effectively to mine metals while disposing of carbon dioxide below ground,” Rigali said.