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Koivula, Risto Tapio
University of Helsinki
in Cooperation with on an Cooperation-Score of 37%
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article
Solid-State Conversion of Scandium Phosphate into Scandium Oxide with Sodium Compounds
Abstract
<p>The application of scandium (Sc) is hindered by insufficient supply. The majority of the world Sc supply is sourced from industrial byproducts, where Sc needs to be separated from other components. Phosphate precipitation is an effective separation and purification method to harvest dissolved Sc ions from acidic leachate solutions; however the obtained Sc phosphate currently has no direct application. To this end, a solid-state conversion route of Sc phosphate to oxide was investigated by using five different sodium compounds, as sodium forms very stable phosphate compounds. The thermal conversion (up to 1000 degrees C) of Sc phosphate with high melting point sodium compounds (sodium sulfate, carbonate, and chloride) yielded a stable mixed sodium-scandium phosphate phase with a formula of Na3Sc2 (PO4)(3). The thermal conversion with lower melting point sodium compounds (sodium hydroxide and nitrate) resulted in the separation of Sc from phosphate moieties, forming respectively Sc oxides (NaScO2 or Sc2O3) and sodium phosphate. In situ high temperature X-ray diffraction, differential scanning calorimetry (DSC), and thermogravimetry (TGA) were employed to investigate the solid-state conversion process by sodium nitrate. Slower heating rate (120 degrees C/h) and the evolution of oxygen gas (as a result of sodium nitrate decomposition) favored the formation of Sc2O3 phase over NaScO2 phase, and the conversion reaction was completed at 670 degrees C. The conversion process was further explored as a purification step toward Sc-containing mixed phosphate precipitates, where the impurities (aluminum and iron phosphates) were converted into sodium aluminate and ferrite and could then be separated from Sc2O3 by their differences in acid/base solubility.</p>