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Improved process for treating AMD waste recovers more rare earth elements

By Scott Jenkins |

By modifying a traditional treatment method for acid mine drainage (AMD), researchers at Penn State University (PSU; State College, Pa.; www.psu.edu) were able to both reduce the amount of chemicals required for treatment and increase the amount of rare earth elements (REEs) recovered from the waste.

AMD is an acidic aqueous runoff that occurs when iron sulfide exposed to air and water by coal mining activity oxidizes and generates sulfuric acid. The sulfuric acid solution dissolves surrounding rocks, drawing a variety of toxic metals into the water. AMD is a major environmental hazard for plants, animals and humans, and is treated traditionally by collecting the AMD in retention ponds and neutralizing the acid with hydroxides. This causes toxic metals to precipitate as a sludge.

The PSU team developed a novel, staged-precipitation processes for AMD that incorporates CO2 mineralization. Adding gaseous CO2 to the AMD produces chemical reactions that result in the formation of insoluble metal carbonates, the scientists say. The rare earth elements bond with the extra carbonate anions and precipitate out of the water at lower pH values. The PSU study, published in Chem. Eng. J., represents the first time CO2 mineralization has been used to recover REEs from AMD.

The staged precipitation process with CO2 mineralization raised the level of REEs recovered by 15 percentage points compared to traditional methods, while also suppressing the formation of iron precipitates, which can complicate downstream REE purification.

The PSU researchers say the process can also be applied for the recovery of critical elements from other low-concentration pregnant leach solutions.

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