Penn State Develops AMD Process to Extract Rare Earth from Wastewater

Pennsylvania State University (Penn State) scientists have developed a two-stage acid mine drainage (AMD) treatment process, which can recover higher concentrations of rare earth elements from acidic wastewater with fewer chemicals than before. The new way to treat AMD could help transform the environmental pollution problem into an important domestic source of the critical rare metal elements needed to produce technology ranging from smartphones to fighter jets.

Scientists proposed in a paper published in the Chemical Engineering Journal that the acid wastewater discharged from coal mines in the Appalachian area can be used as an important source of rare earths in the United States. This kind of wastewater containing high concentrations of minerals relating to environmental pollution has already been recycled.

Rocks containing iron sulfide, namely pyrite (858, -2.00, -0.23%) react with water and are exposed to air to oxidize to produce sulfuric acid. The acidic liquid erodes the rock, and the toxic metal dissolves in the water.

According to researchers at Penn State, traditional treatment methods involve collecting the AMD in retention ponds and adding chemicals to neutralize the pH -an indicator of how acidic or basic a substance is. This causes the dissolved metals to precipitate, or form into solids, and settle out of the water. Up to 70% of rare earth elements can be extracted as a sludge using this process, and the rest are released along with the treated water.

The first author of the paper, Behzad Vaziri Hassas, and his colleagues found that adding carbon dioxide to acid mine drainage can extract high concentrations of rare earths and other key minerals, and neutralize the pH value to reach the goal of environmental restoration.

The principle is that CO₂ produces a chemical reaction to form carbonate solid minerals. When the pH value decreases, the rare elements combine with saturated carbonates and precipitate out of the water. The researchers introduced in the article that using this method, at a pH of 5, 90% of aluminum in acid mine drainage can be recovered; at a pH of 7, 85% of rare earth elements can be recovered.

At present, the use of traditional treatment processes to recover the same amount of rare earth elements from acid mine drainage requires the addition of more chemicals to increase the pH value above 7. Therefore, they believe that by reducing recycling costs, the new treatment process will make the domestic rare earth market more competitive in the United States.

Mohammad Rezaee, one of the authors of the paper from Penn State, said in a news briefing that through simple improvements to the existing wastewater treatment process, a smaller amount of chemicals can be used to obtain higher-value substances from acid mine drainage.

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