Recovery of Scandium using Antisolvent Crystallization in the Valorization of Scandium-containing Waste Streams
Time: Fri 2022-02-18 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Subject area: Chemical Engineering
Doctoral student: Doktorand Edward Peters , Kemiteknik, Resource Recovery
Opponent: Professor Jens-Petter Andreassen, Kemiteknik, Norwegian University of Science and Technology
Supervisor: Universitetslektor Kerstin Forsberg, Resursåtervinning; Docent Michael Svärd, Teknisk strömningslära, Resursåtervinning
Scandium is widely dispersed in the earth’s crust and is rarely concentrated in ores, and a viable option to guarantee a secure supply of scandium is to recover the metal from waste streams of other mining and metallurgical facilities. The valorization of such streams to recover metals of value is a prerequisite to alleviate the global shortages of scandium and other rare earth elements. The purpose of this research is to exploit crystallization techniques amongst other unit operations in the valorization of scandium from waste streams such as bauxite residue and titanium dioxide acid waste. The entire process is envisaged as consisting of unit operations such as leaching of the bauxite residue, solvent extraction of the pregnant leach liquors and stripping, crystallization of a scandium salt from the strip liquors, calcination and metallothermic reduction.
Synthetic and real strip liquors with ammonium fluoride matrix were used in this study. The real strip liquors were obtained by leaching and solvent extraction of typical industrial waste streams and stripping the metals from the organic phase to the aqueous phase using NH4F solution. Antisolvent crystallization using alcohol solvents proved to be a more effective method for recovering scandium as ammonium scandium hexafluoride, (NH4)3ScF6, from such strip liquors, since a higher percentage recovery of scandium was obtained in comparison to cooling crystallization. Therefore, the phase equilibria of ammonium scandium fluorides has been investigated in pure NH4F solutions and in 3 mol/L NH4F-alcohol mixtures for methanol, ethanol, 2-propanol and 1,3-propane-diol in the concentration range 0.5 – 9 mol/L. Thesolubility of ammonium metal fluorides of the impurity metals such as Ti, Zr, Al and Fe was also determined in 3 mol/L NH4F-ethanol mixtures. (NH4)2TiF6 was observed to have exceptionally high solubility in these solutions possibly due to the prevalence of the titanyl ion in solution. The other ammonium metal fluorides investigated exhibited comparable or considerably lower solubilities than (NH4)3ScF6. Antisolvent crystallization using strip liquors with varying scandium to impurity ratios revealed that the uptake of impurity metals into the final solid product occurs in proportions that reflect their relative abundances in the strip liquor. However, the uptake of Ti into the solid product is minimal since Ti remains solubilized.
The impact of processing conditions on the crystal size distribution, morphology and purity of (NH4)3ScF6 in a batch antisolvent crystallization process was investigated. These include the control of supersaturation, antisolvent feeding mode, agitation mechanism, external seeding, and two-stage internal seeding. The control of supersaturation by reducing the antisolvent concentration and adding the dilute antisolvent at a sufficiently low addition rate had the greatest effect on increasing the crystal sizes, although it caused significant broadening of the product CSD. The use of an overhead pitched blade impeller also resulted in remarkable increase in crystal sizes compared to a magnetic stirrer, possibly due to reduced crystal attrition and more effective mixing, which reduces the local supersaturation generated, thereby suppressing nucleation. The addition of dilute ethanol (70 and 60% v/v), when added all at once, caused morphological modifications of (NH4)3ScF6 from isodimensional prismatic crystals to elongated crystals, but this was not observed under controlled addition of the dilute ethanol. This clearly shows the significance of operating conditions in manipulating the product quality obtained. The technical feasibility of recovering the antisolvent for reuse has also been demonstrated.