Rare earths miner Rainbow Rare Earths has revealed that the results of mineralogy and hydrometallurgical testwork recently carried out on phosphogypsum material from the Mosaic Company’s Uberaba site, in Brazil, indicates similar characteristics to Rainbow’s Phalaborwa project, in South Africa.

The Uberaba site is the subject of a memorandum of understanding between Rainbow and Mosaic.

The phosphogypsum stacks at both Uberaba and Phalaborwa are based on a hard-rock carbonatite phosphate deposit. As such, Rainbow has concluded that Uberaba material is amenable to direct acid leaching, which the testwork demonstrated can recover between 31% and 65% of the total rare earth oxides (TREO).

“These initial mineralogy results are very positive because they demonstrate that the Uberaba material shows good liberation results that bode well for a potentially economic operation to recover the rare earth elements,” Rainbow CEO George Bennett said on April 16.

Rainbow announced in September last year that it had assayed gypsum samples from different areas of the Uberaba stack, which were sent to SGS Laboratories in Lakefield, Canada, for testing.

The assays that were found to have the highest grade were those taken from the most recently deposited phosphogypsum material, which is the by-product of ongoing phosphoric acid production by Mosaic at the Uberaba site.

This material demonstrated a grade of between 4 520 ppm and 7 912 ppm TREO, with neodymium and praseodymium (NdPr) being 24.7% of the rare earths basket.

The TREO grade is therefore about 80% higher, based on current phosphogypsum material from the phosphoric acid plant at Uberaba, and the NdPr grade about 50% higher than that at Phalaborwa.

Mineralogical evaluation of the leach residue carried out at SGS Laboratories has revealed that 50% to 71% of the rare earth oxides are contained in monazite.

Owing to the mineralogy, a complementary route is being studied through hydrometallurgical and monazite concentration testwork at Mosaic’s laboratory in Brazil, which will allow for increasing overall TREO recovery.

The phosphoric acid plant at Uberaba receives its phosphate rock feed from long-life phosphate mines which offers the opportunity to recover rare earths from the current arisings of phosphogypsum from the phosphoric acid plant.

Rainbow said the stack still represent a significant rare earths resource that can be addressed at a later date, should the current arisings become the focus of the initial study.

“We continue to expect that a significant portion of the process flowsheet and intellectual property developed for Phalaborwa can be applied to rare earth extraction at Uberaba.

“However, given the scale of the Uberaba project, we could expect a rare earths processing operation there to be significantly larger than that at Phalaborwa,” Bennett said.