Understanding the Chemistry of REE Leaching from Phosphoric Acid Sludge

P. Zhang, H. Liang, A. Medley, T. Levitskaia, Y. Katsenovich
FIPR Institute, Florida Poly,
United States

Keywords: REE in Phosphate, phosphoric acid sludge, leaching


In the wet phosphoric acid manufacturing process, phosphate rock reacts with sulfuric acid to produce phosphoric acid and calcium sulfate dihydrate (phosphogypsum) slurry. This reaction slurry is filtered to generate the phosphoric acid product and phosphogypsum byproduct. The filter acid containing about 28-30% P2O5 is usually further concentrated by a two-stage evaporation process, first to 40% P2O5 and then to 54% P2O5. As the P2O5 content increases in the acid, some solid precipitates fall out forming a slurry called phosphoric acid sludge. Much of the REE originally dissolved in the acid ends up in the sludge making it an attractive REE feedstock. ICP-MS analysis of numerous samples shows that the sludge contains over 2000 ppm REE with Y and heavy REE accounting for about 50%. Depending on how a phosphoric acid manufacturing plant operates, about 5-20% of the phosphate input to the plant ends up in the sludge, which is mainly used to make a low-grade fertilizer. Therefore, phosphate loss in the sludge is also significant. The presence of P, S, Fe and Al in the sludge poses a challenge to both leaching and extraction of REE. Sample aging also showed a dramatic impact on REE leaching recovery using any acid. This presentation attempts to shine some light on the chemistry of REE leaching from the sludge. Some processing strategies are also discussed for economic recovery of both REE and the phosphate value from the sludge.