ENVIRONMENTALLY BENIGN RECOVERY OF CRITICAL MINERALS USING MIXTURE OF CITRIC ACID AND SULFATE SALT

Decarbonization requires widespread deployment of key clean energy technologies over the coming decades to meet the world’s climate goals. The demand for key critical minerals (CMs) in these technologies could grow over 450% by 2050. The U.S. currently relies heavily on import. Existing CM extraction processes are labor-intensive and take a devastating toll on the environment due to the generation of large volumes of waste streams. Assuring a secure, reliable, and sustainable domestic supply of CMs is essential to both national security and net-zero carbon goals by 2050. Its success depends upon enabling environmentally friendly extraction from unconventional and secondary domestic sources.

Citric acid and other organic acids have been used as chelating agents to leach metals from coal and coal ash ¹. Sulfate salt alone, such as magnesium sulfate, has been used as a leaching agent to recover metals from clay-based ion adsorption type rare earth element (REE) ores ². Magnesium sulfate salt alone can barely leach metals from non-ion adsorption type metal sources, such as, coal ash, sandstone ore deposits, mine and slag tailings, and shale. However, the mixture of citric acid and sulfate salt is shown to have a 50-100% extraction efficiency increase in removing REEs from coal ash originated from powder river basin, and an REE-bearing sandstone deposit, a Nevada lithium clay, and slag tailings when compared to using citric acid alone.

The citric-sulfate mixture leaching has several advantages: (1) the critical metal extraction efficiency increases by a factor of two or more; (2) the heat released (heat of dissociation) when sulfate salt dissolves in water increases the temperature of the resulting solution and increases the rate of critical metal extraction and reduces the required contact time between the leaching solution and the critical mineral source; (3) sulfate salt is an inexpensive additive; and (4) both citric acid and sulfate salt are food grade reagents and thus are environmentally friendly.

References

1. Zabiszak et al., 2018, DOI: 10.1016/j.jinorgbio.2018.01.017
2. Xiao et al., 2015, DOI: 10.1016/j.hydromet.2015.02.011

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