Researchers from Penn State found a way to treat acid mine damage (AMD) and turn its environmental pollution into a source of rare earth elements.

Acid mine drainage pollution in a stream in Cambria County, Pennsylvania. Source: Penn StateAcid mine drainage pollution in a stream in Cambria County, Pennsylvania. Source: Penn State

The team’s new two-stage treatment process allowed them to recover high concentrations of rare earth elements, a group of 17 minerals that can create advanced technology, with minimal chemical use.

AMD is a promising domestic source of rare earth elements because it contains high levels of minerals. AMD occurs when pyrite rock is unearthed by mining activity, which oxidizes when it touches water and air, and creates sulfuric acid. Sulfuric acid breaks down surrounding rocks and causes toxic metals to dissolve into water. In the past, it was collected and treated because of environmental concerns. However, using their new method, the team focused on turning more AMD waste into rare earth elements.

Traditionally, to lower AMD contamination, AMD was collected in retention ponds and chemicals were added to neutralize the pH. The chemicals then dissolved metals to precipitate and settle out of the water where it was then collected. With this method, 70% of the rare earth elements were extracted as sludge and the remainder was released with treated water.

The team found that they could extract more rare earth elements and critical minerals by adding carbon dioxide to the AMD. Carbon dioxide brings AMD to a neutral pH 7 in two steps using fewer chemicals. With this method, 90% of aluminum had a pH of 5 and 85% of the rare earth elements recovered had a pH of 7.

Adding carbon dioxide to AMD produces chemical reactions that create carbonites. Rare earth elements bind to the carbonite and precipitate out of the water at lower pH values. This process is called carbon dioxide mineralization.

Carbon dioxide mineralization has been used in the past to remove excess carbon dioxide from the atmosphere. This is the first time this process has been used to recover large concentrations of rare earth elements from AMD. To gather these amounts of elements with traditional methods, researchers would have had to add a large number of chemicals to increase the pH.

Researchers say that their new method could make the rare earth elements market more competitive.

A paper on this research was published in the Chemical Engineering Journal.