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The effect of temperature and mineralogy on reaction processes during the sulfuric acid baking of Monazite ores

Demol, John (2020) The effect of temperature and mineralogy on reaction processes during the sulfuric acid baking of Monazite ores. PhD thesis, Murdoch University.

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Embargoed until April 2022.


Monazite is one of the two major rare earth minerals currently processed for commercial production of rare earth metals. In industry, sulfuric acid baking is one of the main processes used for cracking the monazite mineral structure, allowing extraction of the rare earth elements. However, the chemistry involved in the process is not well understood. In this study, the effects of bake temperature and non-rare earth mineralogy on the sulfuric acid baking of monazite were studied using a combination of chemical analyses and standard characterisation techniques (XRD, SEM-EDS, FT-IR and TG-DSC).

For acid baking of monazite alone, treatment at 250°C for 2 h resulted in >90% solubilisation of the rare earth elements, thorium and phosphate. For bake temperatures >300°C, the dissolution of thorium, phosphorus and, to a lesser extent, the rare earth elements decreased due to formation of insoluble thorium-rare earth polyphosphates (e.g. Th(PO3)4, Ln(PO3)3). The formation of insoluble polyphosphates was attributed to the dehydration of orthophosphoric acid, produced in the initial reaction of monazite with sulfuric acid, to form polyphosphoric acids (e.g. HPO3) which react with thorium and rare earth sulfates.

Addition of apatite to the monazite acid bake resulted in a sharp decrease in rare earth extraction for bake temperatures >300°C. Apatite acted as an additional source of phosphate, resulting in more extensive formation of insoluble rare earth bearing polyphosphates. In contrast, addition of the iron oxide minerals hematite, goethite and magnetite led to an increase in rare earth dissolution for bake temperatures >300°C. The mechanism was through formation of iron sulfate-polyphosphate type species in preference to thorium-rare earth polyphosphates, which allowed the rare earth elements to remain as soluble sulfates. Similarly, addition of the silicate minerals muscovite and kaolinite maintained rare earth dissolutions of >95% for bake temperatures up to 650°C. For the silicate minerals, these effects were attributed to the formation of partially soluble silicon and aluminium-silicon phosphate type species in the bake, which prevented the rare earth sulfates from forming insoluble phosphates.

Finally, the acid baking of a monazite/apatite ore containing minerals from each of the mineral groups previously examined was studied. The reaction products formed in the more complex system and the dissolution results for the rare earths and impurities were found to be as predicted based on the contributions of the individual mineral components, with the dominant effect being due to apatite, which made up 69% (w/w) of the ore. The effect of time on the bake reactions at 250 and 500°C was examined, and it was shown that the conversion of the rare earths to soluble sulfates was practically complete after 40 min at 250°C. Subsequent transformations at 500°C (after treatment at 250°C), leading to formation of a rare earth containing insoluble calcium phosphate type species, were complete after 10 min at temperature.

The results of this work demonstrate the major impact that bake temperature and feed ore/concentrate mineralogy can have on the deportment of rare earth elements and other elements such as thorium, phosphorus, iron, aluminium and silicon during subsequent leaching. These findings are highly relevant in assisting the understanding and development of commercial acid bake processes for rare earth ores or concentrates, and in improving understanding of how a given mineralogy will impact on the chemistry of the acid bake process.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Chemistry and Physics
Supervisor(s): Senanayake, Gamini and Ho, E.
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