According to scientists at the University of Pennsylvania, a new way of treating acid mine effluent (AMD) could help turn environmental pollution into an important internal source of critical rare earths for technologies from smartphones to fighter jets. The results of the new study are published by the Chemical Engineering Journal.
Mine acid effluent has been a serious environmental problem for decades. New research shows that existing processing processes can be modified to not only address environmental concerns but at the same time restore valuable minerals. In addition, it actually reduces the cost of processing mines.
According to the scientists, a group of scientists from the state of Pennsylvania developed a two-step purification process that allowed them to recover higher concentrations of rare earths using fewer chemicals than was previously possible.
Rare earth elements are a group of 17 minerals widely used in advanced technology and recognized in the United States as critical to the country’s economic and national security. Currently, the US imports nearly 100% of these materials, while China produces about 85%.
The acid effluent from AMD mines from the Appalachian coal mines is a promising domestic source of rare earths, scientists say. The fact is that it often contains high concentrations of minerals and is already being collected and processed due to environmental concerns.
Currently, companies only spend money on cleaning the water after cleaning the mines, and in many cases, no one collects all of these minerals. Now scientists are going to turn what was previously considered waste into a valuable resource.
Acidic mine effluent occurs when pyrite rock, an iron sulfide found in mining, interacts with water and air and then oxidizes to form sulfuric acid. The acid then destroys the surrounding rocks, causing toxic metals to dissolve in the water.
Traditional cleaning methods include collecting AMD in sedimentation tanks and adding chemicals to neutralize the pH – a measure of the acidity or alkalinity of a substance. This leads to the precipitation of dissolved metals. Or they can turn from water to solid particles and settle. According to the researchers, up to 70% of the rare earths can be recovered as sludge by this process, and the rest will be released along with the purified water.
Sludge – wastes from the engineering development of a mining product, constituting dust and its smallest parts, obtained in the form of sediment when washing any ore material.
Scientists have found that they can extract higher concentrations of rare earths and other important minerals by adding carbon dioxide to AMD and then bringing it to a neutral pH of 7 to restore the environment. The process will take place in two stages.
Using this method, 90% of the aluminum was recovered at pH 5, and 85% of the rare earths were recovered at pH 7, the scientists said.
According to scientists, the addition of carbon dioxide to acid mine effluents triggers chemical reactions that lead to the formation of hard minerals called carbonates. Rare earths bind to additional carbonites and precipitate out of the water at lower pH values.
This carbon dioxide mineralization process is a new technology that is used to remove excess carbon dioxide from the atmosphere. The study is the first time it has been used to extract high concentrations of rare earth elements from acid mine wastewater, according to scientists.
By simply modifying existing treatment processes, the industry could use fewer chemicals and get more value from acid mine waste streams. This is the main value of this study, the scientists conclude.