What are REEs?
The rare earth elements (REEs) group consists of seventeen chemical elements, including fifteen lanthanides and yttrium and scandium (occur in the same ore deposits and have similar chemical properties). All these elements exist in the form of trace quantities in natural materials, which are unevenly distributed around the world (estimated average concentration in the Earth’s crust ranging from around 130 to 240 µg/g). They could be categorized in many ways, among others as critical (Nd, Y, Eu, Dy, Tb, and Er), uncritical (Pr, Sm, La, and Gd), and excessive (Tm, Ce, Ho, Lu, and Yb) or as light rare earth elements (LREEs) and heavy rare earth elements (HREEs). LREEs are in the form of bastnasite ((Ce, La)CO3(F,OH)) and monazite ((Ce,La,Nd,Th)PO4), while HREEs are usually present in the form of apatite, cheralite, eudialyte, loparite, phosphorites, rare-earth-bearing (ion adsorption) clays, secondary monazite, and xenotime.
Why are REEs so important?
Currently, REE become more and more important because of their unique chemical, physical, catalytic, luminescent, and magnetic features and of course many industrial applications (electronics, clean energy technologies, defense, optics, automotive, agriculture, medicine, energy, etc.). For instance, neodymium is applied in super magnets for disk drives, cerium in autocatalysts and all in flat-panel TVs production and smart batteries for electric vehicles. Due to their application all modern gadgets can be more efficient, smaller, faster, and lighter. Therefore, the demand for REE will continue to increase in the near future.
Where are REEs coming from?
In the twentieth century, REE were mined in California and Australia. Currently, the main REEs producers are China (63%), Russia (14%), Australia (10%), USA, Myanmar, Brazil, and India. Also Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan, and Turkmenistan show some REE mineral occurrence. However, the largest REE exporter in the world is China. Its monopoly forces other countries to look for other alternative sources and developing REE recovery techniques. Except that the conventional REE mining techniques, which include ore deposits, are energy-intensive and generate significant volumes of toxic wastes and acidic wastewater. Therefore, new sources of REE should be sought. It turned out that coal fly ashes could be one of their sources.
Source: Rybak, A.; Rybak, A. Characteristics of Some Selected Methods of Rare Earth Elements Recovery from Coal Fly Ashes. Metals 2021, 11, 142. https://doi.org/10.3390/met11010142
images: nettetal10,jdn2001cn0, Buffik z Pixabay
