AIChE Central Florida September Dinner Meeting 

Speaker:  Patrick Zhang, 

Florida Industrial and Phosphate Research Institute

Phosphate minerals have been identified as significant unconventional rare earths resources. World’s identified phosphate resources total about 300 billion tons, representing 90 million tons of rare earth elements (REE), assuming an average REE content of 300 ppm Under a project funded by the Critical Materials Institute (CMI), the Florida Industrial and Phosphate Research Institute (FIPR Institute) conducted detailed investigation on beneficiation and leaching techniques for recovering rare earth elements (REE) from five phosphate mining and processing streams including amine flotation tails, waste clay, phosphate rock, phosphogypsum (PG), and phosphoric acid sludge.

Based on chemical analyses and minerals characterization, the following were learned: 1) REE in the flotation tails exist primarily in monazite with some in xenotime and heavy minerals such as zircon; 2) In the phosphoric acid manufacturing process over 70% of the REE in phosphate rock is dissolved, but a majority (about 70%) of which is either re-precipitated with PG or get absorbed onto PG; 3) the REE in phosphoric acid is mostly precipitated as the acid is concentrated from about 30% P2O5 to 54%; 4) REE in the waste clay occur in two  major forms, xenotime and calcium substitution in phosphate crystals. These understandings provided the basis for developing beneficiation methods for concentrating REE-containing materials from each stream and leaching schemes for dissolving REE into solutions for REE extraction. The amine flotation tails was first subject to gravity separation to concentrate roughly 40% of the REE in about 12% of the total mass with REE content increased from 200 ppm to 657 ppm. Flotation of the gravity separation concentrate resulted in a concentrate containing nearly 1900 ppm REE at 82% P2O5 recovery and 73% REE recovery. Leaching of either the gravity separation concentrate or the flotation concentrate using relatively concentrated sulfuric acid could recover about 60% of the REE. The flowsheet for waste clay involves sizing using hydrocyclone to split REE in the fine (79%) and coarse (21%) fractions, with the coarse fraction further treated by flotation achieving about 82% REE recovery. High temperature leaching of the flotation concentrate resulted in 90% REE recovery. Both phosphogypsum and acid sludge could be leached using dilute sulfuric acid to obtain over 80% REE recovery.