bastnaesite concentrate, rare earths
Bastnaesite-(Ce) crystal (credit: Stephan Wolfsried)
further information, ANZAPLAN Services

Rare Earth Metals Extraction (Hydrometallurgy)

After concentration REE bearing minerals, rare earth elements have to be extracted from the concentrate.

Several procedures for decomposition of REE bearing minerals are available. The major part includes thermal treatment of the ore in the presence of acidic or caustic reagents. Depending on the composition of the ore concentrate an appropriate method is identified.

Acid baking with sulfuric acid is a very common process. The powdered ore is mixed with concentrated sulfuric acid and baked at temperatures between 200 and 400 °C for several hours. The resulting cake is leached with water to dissolve REE as sulfates. Optimal reaction conditions and reagent use have to be matched specifically with each tested ore. There are different factors influencing the reaction, e.g. the presence of iron oxide leading to an increased consumption of acid. At roasting temperatures above 300 °C the recovery of REE decreases in most cases, while Th leaching is also reduced. Since thorium is generally an undesired leaching product, roasting temperature will be a trade-off between REE recovery and Th leaching.

Acid baking is a standard process since it is applicable for many of the common rare earth minerals such as monazite, bastnaesite, xenotime, apatite or aeschynite.

Decomposition in HCl is commonly applied for carbonate minerals like bastnaesite, parisite, synchisite or similar minerals but can be also used to decompose allanite, cerite or gadolinite. The ore is stirred in concentrated HCl at temperatures > 90 °C. If the ore contains fluorine (e.g. bastnaesite), a part of the REE forms insoluble REE-fluorides remaining in the solid residue. To recover those REE the solid residue has to undergo an additional decomposition with sodium hydroxide, to convert the fluorides into hydroxides and soluble sodium fluoride. Fluorides are washed away and REE hydroxides are dissolved by excess HCl in the leaching liquor from the HCl decomposition step.

In the presence of calcite or similar carbonate phases in the ore, a leach with diluted HCl at room temperature is appropriate to purify the ore prior to decomposition as these reaction conditions will dissolve unwanted carbonate phases without attacking bastnaesite.

A special ore is eudialyte. It is easily soluble in any mineral acid thus decomposition is rather simple. Nevertheless, it is often accompanied by zeolites, which tend to form silica gels upon dissolution in acid.  Depending on the zeolite content different precautions have to be taken during decomposition to minimize the formation of gels.

Alternatively caustic decomposition can be applied to specific ores. The most common process is decomposition with sodium hydroxide being applicable for monazite and bastnaesite. The ore is mixed with 50-60 wt.-% NaOH and is decomposed at T > 140 °C. Rare earths are transformed to hydroxides, while phosphates (from monazite) or carbonates and fluorides (from bastnaesite) are transformed into soluble sodium salts that can be washed off. The resulting solids are leached in diluted HCl.

The choice of decomposition method depends on various factors. Often caustic decomposition results in more pure products, but if the ore contains several different minerals a process capable of decomposing all of them has to be defined. Since in most cases non-neglibile amounts of non REE-containing side components are present in the mineral concentrates, the effectivity of a process will usually be affected by the latter. Another point to be taken into consideration is regional availability of reagents.

Once REE are solubilized, they have to be separated from co-leached elements. After removal of impurities e.g. by pH dependent precipitation, REE are typically precipitated as oxalates or carbonates, from which REE-oxides can be obtained by calcination. If necessary, residual thorium can be removed from the oxides by dissolving them in nitric acid and extraction of the resulting solution with organophosphates. 

Read more:
> Download our white papers: "REE-Extraction from Phosphate minerals" and
   "Extraction of rare earths from bastnaesite"