A new purification process, based on a previously developed three-stage AMD treatment process, is designed to recover high-grade aluminum, REE, cobalt and manganese products from materials of sludge by processing water with several cleaner steps and precise control of process parameters. , according to the researchers.
“Obtaining REEs and CMs directly from AMD eliminates the need for sludge dissolution and associated costs of reagents and processing, resulting in more sustainable waste disposal methods with lower costs,” said Mohammad Rezaee, assistant professor of mining engineering at Penn State and co-author of the study. “We’ve shown that we can turn these waste streams, which have had environmental concerns for decades, into valuable resources, so it’s a win for the environment, the commonwealth and the country.”
AMD is usually treated by adding lime or other chemicals to raise the pH to 7.
“Typically, AMD is neutralized by adding various alkaline chemicals,” said Rezaee, who also holds the Centennial Career Development Professorship in Mining Engineering. “As the pH of AMD increases during the treatment process, metals precipitate as metal hydroxides or other complexes.”
In the new system, the pH is still raised to a pH of 7, but in stages.
“Instead of adding sodium hydroxide, calcium hydroxide or lime all at once to raise the pH, we raise it in stages,” says Pisupati. “The advantage of this method is that it allows certain minerals to precipitate at different pH levels. If we add our base at once and bring the pH to 7, all these things precipitate at the same time. Then we have to go back and separate them.”
The researchers raised the pH to the level needed for iron precipitation and then to the pH needed for aluminum to precipitate. After the iron and aluminum are removed, the REEs are recovered by carbonate precipitation, according to the researchers.
“Our challenge is that we cannot get 100% of the iron and aluminum that is available; there is a small residue in the REE concentration,” said Pisupati. “Even if you have only 1% aluminum content in the mixture it dominates, and your quality of rare earths is not pure. This is addressed by the new purification process.”
In the purification process, the precipitates removed are returned though the cycle to remove iron, aluminum and other residues.
“In the purification process, we go through the cycle again, return to a pH of 3 or 3.5 and start all over,” Pisupati said. “We remove some residues slowly, maybe two or three times in the cycle, to increase the purity of the REE. In our previous research we were at about 17% to 18% grade, so this is a significant achievement .”
