Leaching the Unleachable Mineral: Rare Earth Dissolution from Monazite Ore in Condensed Phosphoric Acid

Harry Watts,Tonya Fisher

doi:10.3390/min11090931

Abstract

Monazite is a poorly soluble mineral of rare earth phosphate. It is an ore of the rare earths which is difficult to break down; in industry either concentrated sulphuric acid or caustic soda is used to attack finely ground monazite at between 140 °C and 400 °C. In these processes, the rare earths are converted into different solid compounds, undergoing an incomplete conversion. Here we show a new process for a direct and much faster breakdown of monazite by simple dissolution under milder conditions. Condensed phosphoric acid was used to dissolve rare earths (up to 96 g/L) from unground monazite sand from four sources. Greater than 99% of light rare earths dissolved within 30 min at 260 °C. The cooled solution can be diluted to an extent with water to reduce viscosity for analysis or further processing. This method of dissolution avoids the use of strong acids/bases and reduces the risk of dusk exposure from fine grinding of particles.

Highlights

Rare earth elements are generally taken as the series of increasing atomic numbers from lanthanum to lutetium, as well as yttrium and scandium
Monazite sands from the Perth monazite feedstock were analysed via XRD and solid assay
The results indicate the very high and rapid dissolution of light rare earths in the leach solution, with about 95% dissolution within 15 min and greater than 99.8% dissolution within 30 min

Summary

Introduction

Rare earth elements ( known as rare earth metals, lanthanides, lanthanoids, or lanthanons) are generally taken as the series of increasing atomic numbers from lanthanum to lutetium, as well as yttrium and scandium. Rare earth metals can be obtained from ores of phosphate minerals such as monazite. Monazite is a solid solution which is produced globally as a by-product of heavy mineral sand operations. It comprises mainly the lighter rare earths in terms of atomic mass, from lanthanum to gadolinium, with some yttrium as well [1]. There is a strong demand for the lightweight rare earth magnets used in the motors of electric vehicles.

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