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Licensed Unlicensed Requires Authentication Published by De Gruyter 2021

Chapter 16 Rare-earth elements in phosphogypsum and mineral processing residues from phosphate-rich weathered alkaline ultramafic rocks, Brazil

From the book Industrial Waste

  • Tim Rödel , Stefan Kiefer and Gregor Borg


Abstracts: Phosphogypsum is regarded today mainly as a by-product or waste generated during industrial processes such as the production of phosphoric acid for fertilizer. During the process of sulphuric acid leaching of apatite-rich concentrates from weathered alkaline ultramafic rocks large volumes of gypsum and accessory minerals are produced. Recently nine phosphogypsum samples from Catalão, Brazil have been investigated as a potential secondary source for rare-earth elements (REEs). Identifying the minerals hosting REE, the mineral composition and modal abundance are key in evaluating the economic potential of phosphogypsum as a source for critical metals. A combination of detailed petrographic investigations, SEM-based mineral distribution analyses, EPMA and geochemical analyses using ICP-MS/AES was successfully applied to identify REE carrier minerals and the fraction of associated REE. The analysed phosphogypsum samples are mainly composed of euhdral gypsum crystals constituting around 93% of the total mass. Accessory minerals identified include quartz, octahedral REE-bearing, (Ca-Al-) fluorides, monazite, celestine, Fe-oxides, ilmenite, barite, pyrochlore, and baddeleyite. Apart from gypsum most minerals also occur in the weathered phosphate-rich rocks and are therefore carried over throughout the sulphuric acid leaching process of apatite concentrates. Monazite has been identified as the most important carrier for REE in phosphogypsum. The mineral mainly consists of Ce, La, Nd and P. The total rare-earth content in the mineral amounts to a mean 57 %. Furthermore the Th concentration, a major contaminant, are comparably low. The mean abundance of monazite in phosphogypsum amounts to 0.6wt%. Overall monazite hosts 50% to 60% of the total REE in phosphogypsum, while the remaining 50% are locked in gypsum and Ca-Al-fluorides. Therefore only a fraction of the geochemically available total rare-earth oxide content of 0.6%in phosphogypsum is likely to be recoverable. A good complete particle liberation of 60% to 80% and a grain size range of 15mμm to 50μm is promising for a further beneficiation of monazite by means of physical separation prior to winning the REE. Based on the data at hand phosphogypsum from the Catalão region in Brazil could potentially provide a significant supply of REE as a secondary resource.

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