Rare earth metals are used in many modern-day and cleantech devices including hybrid cars, cell phones, laptops, flat screen TVs and numerous defense technologies. Rare earths can be found all over the world, but they’re expensive and often in short supply. China, which mines and produces 97 percent of rare earths, has placed limits on shipments of the elements.
Previous attempts to recycle valuable REEs from wastewater have proven too expensive. The elements are typically highly diluted in the wastewater, making it difficult and costly to retrieve them.
The team of scientists produced inexpensive nano-Mg(OH)2 particles whose shapes resemble flowers under a high-powered microscope. Scientists found that the particles captured more than 99 percent of the REEs that were diluted in wastewater. Further analysis revealed the REEs could be collected on the surface of Mg(OH)2 as metal hydroxide nanoparticles.
The scientists also developed a method to further separate immobilized REEs and the residual magnesium hydroxide by varying the solution pH, according to the study published in ACS Applied Materials & Interfaces. In a pilot-scale experiment, the REEs from practical wastewater were immobilized effectively at a high flow rate. The team of scientists say this work can provide a good example for the recycling of valuable REEs in practical industrial applications.
One of the only rare earth mining operations in the country, the Project Phoenix facility in Mountain Pass, Calif., is designed to have near-zero wastewater discharge. The $895 million project, which began sequential startup in February 2012, mines for the minerals bastnasite and monazite, and will use acids and bases – mostly hydrochloric acid and sodium hydroxide – to separate out 17 rare earth minerals. This process results in a salt-water byproduct, which traditionally companies will pump into evaporation ponds.
Mine owner Molycorp is using a chlor-alkali process to split the water back up into hydrochloric acid and sodium hydroxide, to start the process anew. This not only saves wastewater storage and associated permitting costs, but eliminates the need to buy 15 to 20 truckloads of acids and bases each day.