Using Supercritical CO2 to Extract Rare Earth Elements from Coal Ash?

Highlights

• Researchers developed a promising supercritical CO2 extraction method to recover rare earth elements from coal ash, achieving up to 80% extraction efficiency.
• Coal ash contains 200-1,500 parts per million of rare earth elements, presenting a potential alternative to traditional mining methods.
• Experts predict scalable coal ash recycling technologies could be achievable within 10-15 years, offering a more sustainable approach to obtaining critical technological materials.

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Uthej Veerla’s (opens in a new tab) 2024 Master’s Thesis (opens in a new tab) explores a cleaner and more efficient method for extracting rare earth elements (REEs) from coal ash using supercritical carbon dioxide (SCF CO2). The hypothesis is that SCF CO2, combined with chelating agents such as tributyl phosphate (TBP) and nitric acid (HNO3), could provide a sustainable alternative to conventional extraction methods, minimizing waste and environmental harm.

The study focused on three types of coal ash—anthracite, bituminous, and sub-bituminous—optimizing variables like temperature, pressure, and solvent ratios for maximum extraction efficiency.

The research found that anthracite ash achieved the highest extraction efficiency (80%) under optimal conditions, followed by sub-bituminous (58%) and bituminous (49%). This process effectively leveraged CO2’s high diffusivity and low viscosity to recover REEs, which are vital for technology applications like magnets, batteries, and electronics.

However, the study faced limitations such as the complexity of scaling the high-pressure system for industrial use and variations in ash composition that may affect efficiency. The assumption that these conditions are universally applicable to all coal ash types requires further testing. Despite these challenges, this researchhighlights the potential for coal ash as a valuable secondary source ofREEs and emphasizes the importance of innovative, eco-friendly extraction technologies.

Is Coal Ash a New Frontier for Rare Earth Element Recycling?

Coal ash, a byproduct of burning coal in power plants, is gaining attention as a potential source for REEs,  which are vital for technologies like smartphones, wind turbines, and electric vehicles. Scientists have discovered that coal ash contains small amounts of REEs, ranging from 200 to 1,500 parts per million. This has sparked interest because recycling REEs from coal ash could help reduce dependence on traditional mining, cut waste, and support a more sustainable supply chain.

Recent advancements in extraction techniques have brought this idea closer to reality. Researchers have developed methods like acid leaching, bio-leaching with microbes, and supercritical carbon dioxide (CO2) extraction to separate REEs from coal ash. These techniques aim to be more environmentally friendly than conventional mining, using fewer harsh chemicals and producing less waste. Some experiments have shown promising results, with up to 80% of REEs successfully extracted under optimal lab conditions.

Despite this progress, significant challenges remain. The concentration of REEs in coal ash is lower than in natural ores, making extraction less economical at scale. Additionally, coal ash contains many other elements that complicate the separation process. Scaling up these technologies from lab tests to industrial use will require overcoming technical, economic, and logistical hurdles.

Experts predict that with continued research and investment, scalable coal ash recycling for REEs could be achieved within the next 10 to 15 years. While not an immediate solution, coal ash represents a promising step toward a more sustainable and circular rare earth economy. The journey to large-scale implementation may be challenging, but the potential benefits for technology and the environment may be worth the effort.