Researchers have studied a method for extracting rare earth elements from mining waste that provides the world with a reliable supply of valuable materials.
Research led by the Idaho National Laboratory and Rutgers University is supported by the Key Materials Institute, which is published online in the Journal of Chemical Thermodynamics .
Rare Earth Elements (REE) – A class of metallic elements, including lanthanum and cerium – are required to manufacture many high-tech equipment, including cell phones, computers and wind turbines.
REE is difficult to obtain, and the United States does not currently produce domestic supplies. This scarcity makes manufacturers vulnerable to supply disruptions.
However, a large amount of rare earth is present in phosphogypsum (PG), which is a waste of phosphoric acid produced from phosphate rock. In 2017, approximately 28 million tons of phosphate rock were mined in the United States. Phosphoric acid is used in the production of fertilizers and other products.
Researchers estimate that more than 1 billion tons of PG waste are piled up across the country, especially in Idaho and Florida. Around the world, about 100,000 tons of rare earth elements will eventually produce PG waste. This is almost as much as nearly 126,000 tons of rare earth oxides produced annually worldwide.
To test whether rare earth elements can be obtained from PG, the researchers used six rare earth elements – lanthanum, cerium, lanthanum, cerium, lanthanum and cerium to dope to synthesize phosphogypsum. Then they studied various solutions that could be used to extract elements.
In particular, a solution, a mixture of chemicals produced by the bacteria Gluconobacter oxydans, is particularly attractive for the recovery of rare earth elements. Gluconobacter is a common bacterium that is easily found in the environment, including decaying fruits.
Gluconobacter bacterium produces an organic acid, such as gluconic acid, which dissolves rare earth elements from surrounding materials and absorbs them into solution during a process known as "bioleaching." The REE can then be precipitated from the solution and purified for industrial use.
INL researchers previously used Glucosamine to recover rare earth elements from waste fluid catalytic cracking catalysts (a material used for petroleum refining) and other sources of rare earth elements.
In a recent study, a mixture of chemicals produced by Gluconobacter was superior to other acids, such as phosphoric acid and gluconic acid. Sulfuric acid works best in the chemicals studied.
INL researcher Yoshiko Fujita said that the huge difference between the research and research team's past studies using Glucosamine to recover rare earth elements from waste products or industrial waste is that PG is sufficient to alleviate the world's rare earth supply shortage. David Reed, a scientist in the INL Biological and Chemical Processing Division.
Early studies have shown that bioleaching with Glucospora can be economically feasible and has less environmental impact, especially when compared to sulfuric acid. Conventional methods for extracting rare earth elements from ore produce millions of tons of toxic and acidic contaminants.
"By bioleaching, the organic acids we use are less harmful to the environment," Fujita said.
There are two major challenges in using PG as a source of rare earth elements. First, PGs are usually lightly radioactive. “Uranium and thorium are usually present in these phosphate deposits,” Fujita said. “But some of these elements have very few deposits.”
Second, PGs are classified as waste, which may cause regulators to restrict access to the PG heap.
However, there is a high interest in using this potential source of large rare earth elements. Mining companies have begun to ask about this process. Next, the researchers hope to test bio-acids in industrial PG and other wastes produced during the phosphoric acid production process, and also contain rare earth elements.
"I think there is such a huge reserve there," Reid said. “At some point, the impetus will drive, and we will have to see PG as a viable resource. If REE material flows, these PG sources are important.”
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