Scandium (Sc), declared a critical raw material in the European Union (EU), could face further supply issues as the EU depends almost entirely on imports from China, Russia, and Ukraine. In this study, a tandem nanofiltration-solvent extraction procedure for Sc recovery from titania (TiO2) acid waste was piloted and then augmented by antisolvent crystallization. The new process, comprising advanced filtration (hydroxide precipitation, micro-, ultra-, and nanofiltration), solvent extraction, and antisolvent crystallization, was assessed in relation to material and energy inputs and benchmarked on ScF3 production. From ∼1 m3 of European acid waste containing traces of Sc (81 mg L–1), ∼13 g of Sc (43% yield, nine stages) was recovered as (NH4)3ScF6 with a purity of approximately 95%, demonstrating the technical feasibility of the approach. The production costs per kilogram of ScF3 were lower than reported market prices, which underscores a competitive process at scale. Although a few technical bottlenecks (e.g., S/L separation and electricity consumption) need to be overcome, combining advanced filtration with solvent extraction and antisolvent crystallization promises a future supply of this critical raw material from European secondary sources. 

Evaporative crystallisation has been used to recover water for industrial reuse but it presents serious problems related to incrustation. In this perspective, alkaline precipitation is studied in the present work, aiming to remove scale-forming salts prior to evaporative crystallisation of a reverse electrodialysis concentrate (EDC). The concentration of inorganic species in the feed and filtrate streams were determined by ICP-OES. The total organic carbon and inorganic carbon were analysed by thermo-catalytic oxidation with high temperature to assess the removal of organic compounds from EDC. The software PHREEQC was used to model the systems and a comparison with experimental results confirmed the credibility of the experiments. The technique proved to be a favourable method for removal of almost 100% of Ca and Mg by adding 0.04 wt. % of caustic soda. Furthermore, this would improve the downstream evaporative crystallisation efficiency due to reduced incrustation potential.

The recovery of scandium from waste streams of other mining and metallurgical processing industries is gaining research interest due to the scarcity of scandium-containing ores. Hydrometallurgical techniques such as leaching, solvent extraction and crystallization amongst others have been successfully applied to recover scandium salts from such waste streams. Scandium can be recovered as (NH4)(3)ScF6 by antisolvent crystallization from NH4F strip liquors obtained after solvent extraction. The coextraction of metal impurities such as Fe, Al, Zr and Ti causes contamination of the final solid product. The extent of coprecipitation of ammonium metal fluorides depends on their initial concentration in the strip liquor and their solubility in the NH4F-antisolvent mixtures. Here, the solubility of ammonium metal fluorides of Sc, Zr, Fe, Al and Ti is reported separately in 3 mol L-1 NH4F-ethanol mixtures at 25 degrees C as well as in a system containing all five solid phases. The solubility of (NH4)(3)ZrF7 is slightly higher than that of (NH4)(3)ScF6, while the solubilities of (NH4)(3)FeF6 and (NH4)(3)AlF6 are significantly lower in comparison to (NH4)(3)ScF6. The solubility of (NH4)(2)TiF6 is 1-2 orders of magnitude higher than those of other ammonium metal fluorides. When a mixture of ammonium metal fluoride salts is dissolved in the same 3 mol L-1 NH4F-ethanol mixture as for the individual salts, the resultant solubility of the ammonium metal fluorides of Sc, Zr and Fe decreases significantly, while the resultant solubility of ammonium aluminum hexafluoride increases. This is likely due to changes in solution speciation with increased NH4F concentration and ionic strength.

Scandium is a metal of value with increasing demand but limited supply. During the valorization of bauxite residue, scandium becomes concentrated in an NH4F strip liquor, from which it can be recovered as (NH4)3ScF6 by antisolvent crystallization. This study investigates the use of different antisolvents and their concentrations on the morphology and crystal size of the (NH4)3ScF6 crystals produced. The antisolvents include alcohols, ketones, and a sulfoxide. These were added all at once, either pure or diluted, to the aqueous solution to attain final concentration of 2, 4, or 8 mol/L total solution in separate experiments. Changes in the functional group and concentration of the antisolvents were observed to induce morphological and crystal size modifications of (NH4)3ScF6. The differences are likely due to the specific interactions of the antisolvent molecules with the atoms at the different faces of the crystals.

The recovery of scandium from waste streams of mining and metallurgical operations presents an opportunity to balance supply and demand of this commodity. This study expands on the research focusing on the recovery of scandium as (NH4)3ScF6 from strip liquors by antisolvent crystallization using ethanol as the antisolvent. The effect of process conditions including reducing the rate of supersaturation generation, agitation mechanism, feeding point location with respect to local supersaturation, and seeding are assessed with emphasis on the final crystal size distributions (CSD) and morphology. Reducing the rate of supersaturation generation by reducing the ethanol concentration and by controlling the rate of antisolvent addition had the greatest effect on increasing the crystal sizes, although broader CSDs were obtained. Morphological modifications, without polymorphic transformations, were also observed when one-pot addition of 60 or 70% v/v ethanol was conducted, but not observed when the dilute antisolvents were fed at a controlled low addition rate.

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.

Skandium återfinns brett spridd i jordskorpan och är sällan koncentrerad i malmer. Ett lönsamt alternativ för att garantera en säker tillgång på skandium är att återvinna metallen från avfallsströmmar från gruv- och metallindustri. Valoriseringen av sådana strömmar för att återvinna värdefulla metaller är en förutsättning för att motverka den globala bristen på skandium och andra sällsynta jordartsmetaller. Syftet med denna forskning är att utnyttja kristallisation tillsammans med andra enhetsoperationer i valoriseringen av skandium från avfallsströmmar från produktion av aluminium från bauxit och produktion av titandioxid. En framtida process är tänkt att bestå av enhetsoperationer såsom syralakning, vätske-vätske-extraktion, kristallisation av ett skandiumsalt, kalcinering och metallotermisk reduktion.

Syntetiska stripplösningar samt riktiga stripplösningar med ammoniumfluoridmatris har använts i denna studie. De riktiga stripplösningarna erhölls genom syralakning och vätske-vätske-extraktion av typiska industriella avfallsströmmar och strippning av metallerna från den organiska fasen till en vattenhaltig NH4F-lösning. Förträngningskristallisation med alkohol visade sig vara en effektiv metod för att separera skandium som ammoniumskandiumhexafluorid, (NH4)3ScF6, från sådana stripplösningar, med en högre procentuell återvinning av skandium jämfört med kylkristallisation. Fasjämvikt av ammoniumskandiumfluorider har undersökts i rena NH4F-lösningar och i 3 mol/L NH4F-alkoholblandningar för metanol, etanol, 2- propanol och 1.3-propandiol i koncentrationsområdet 0.5 – 9 mol/L. Lösligheten minskade i ordningen metanol, etanol, 1,3-propandiol och 2-propanol och detta korrelerar med minskningen av den effektiva dielektricitetskonstanten för NH4Falkoholblandningarna. Lösligheten av ammonium-metallfluorider av Ti, Zr, Al och Fe bestämdes i 3 mol/L NH4F-etanolblandningar. (NH4)2TiF6 observerades ha hög löslighet i dessa lösningar, möjligen på grund av förekomsten av titanyljonen i lösning. De andra undersökta ammoniummetallfluoriderna uppvisade jämförbara eller betydligt lägre lösligheter än (NH4)3ScF6. Förträngningskristallisation från stripplösningar innehållande skandium och andra metaller visade att upptaget av metallföroreningarna i den slutliga fasta produkten sker i proportioner som återspeglar deras relativa mängder i stripplösningarna. Emellertid är upptaget av Ti i den fasta produkten minimalt och Ti förblir i lösning.

Effekten av olika processparametrar på kristallstorleksfördelning, morfologi och renhet av (NH4)3ScF6 i en satsvis förträngningskristallisationsprocess undersöktes. Kontroll av övermättnad, tillsatsmetod för lösningsmedlet, omröringsmekanism, extern groddning och tvåstegs-intern groddning studerades. Kontroll av övermättnad genom att reducera koncentrationen lösningsmedel och genom att tillsätta det utspädda lösningsmedlet med en tillräckligt låg tillsatshastighet gav fler större kristaller, men orsakade dock en signifikant breddning av produktens storleksfördelning. Omrörning medelst en turbin med vinklade blad resulterade också i en anmärkningsvärd ökning av kristallstorlek jämfört med en magnetisk omrörare, möjligen på grund av minskad kristallnötning och effektivare omblandning, vilket minskar den lokala övermättnaden som genereras och därigenom undertrycker kärnbildning. Tillsatsen av utspädd etanol (70 och 60 % v/v), när den tillsattes på en gång, orsakade morfologiska förändringar av (NH4)3ScF6 från isodimensionella prismatiska kristaller till långsträckta kristaller. Denna typ av förändring kunde inte observeras under kontrollerad tillsats av utspädd etanol. Detta visar tydligt betydelsen av driftsförhållanden för att manipulera den erhållna produktkvaliteten. Den tekniska genomförbarheten för att återvinna lösningsmedel har också påvisats.

Scandium can be extracted from waste streams of other industrial processes, particularly the bauxite residue and TiO2 acid waste, by acidic leaching and solvent extraction of the leach solutions. Stripping of the organic phase using NH4F solutions produces strip liquors containing Sc (>2000 mg/L). Scandium can be separated from these liquors by anti-solvent crystallization of (NH4)3ScF6. In this study, the extent to which impurities co-precipitate as separate crystalline phases or are incorporated into the crystal lattice of (NH4)3ScF6 was investigated. The impurity metals Fe, Zr, and U co-precipitated with the Sc phase. Moderate Ti precipitation was only observed from strip liquors containing mainly Fe and Ti impurities. Detection of these phases by powder XRD was difficult due to almost similar peak positions of the ammonium metal hexafluoride salts. However, EDS confirmed that the impurity metals were present in the precipitates in relative abundances that matched non-proportionally those of the initial strip liquors, except for Ti. SEM images showed that (NH4)3ScF6 crystals obtained from strip liquors containing predominantly scandium were bigger (2–3 μm) compared to crystals of the mixed precipitate samples (<2 μm) obtained from strip liquors containing relatively high impurity levels. This could be attributed to surface diffusion impediment of one metal ion by other metal ions at the solid–liquid interface and surface incorporation of foreign metal ions on the growth steps or kinks of one solid phase, thereby reducing the crystal growth rate of that phase. The excess supersaturation is then consumed by crystal nucleation as observed.

This paper investigates the phase equilibria of ammonium scandium fluoride phases in pure water, aqueous NH4F solutions and in mixtures of NH4F and alcohols. The solubility of the solid phases was determined at 25 degrees C. It was observed that (NH4)(3)ScF6 transforms into NH4ScF4 when contacted with pure water for 24 h. Solid phase transformation was also noted in NH4F solutions of concentration <= 0.5 mol/L. At NH4F concentra-tions >= 0.8 mol/L, no phase change has been observed, and the solubility of (NH4)(3)ScF6 decreases with in-creasing NH4F concentration due to the common ion effect. In NH4F-alcohol mixtures, the solubility of (NH4)(3)ScF6 decreases in the order: methanol > ethanol > 1,3-propane-diol > 2-propanol. This aligns with the decrease in the effective dielectric constant of the resulting solvent mixture, which is very similar for NH4(F)-alcohol mixtures formed by ethanol and 1,3-propane-diol.

Scandium can be recovered from waste streams of other metallurgical processes such as red mud from the Bayer process and titanium dioxide acid waste. This is accomplished by an integrated process comprising leaching using acidic media, solvent extraction using organic solvents and stripping of Sc from the organic phase using NH4F solutions. The precipitation of scandium as (NH4)3ScF6 from NH4F strip liquors has been demonstrated at lab-scale with recoveries and purities greater than 98% and 98.3%, respectively. It is desired to assess the possibility to recover the solvents for re-use in the process to improve its economy and environmental friendliness. Investigations have been conducted to recover alcohol by simple distillation from strip liquor-alcohol mixtures containing about 50 v/v% alcohol. The mixtures were obtained after anti-solvent precipitation of (NH4)3ScF6 using an alcohol to strip liquor volumetric ratio of 1:1. The starting strip liquor contained about 2000 mg/L scandium in 3 mol/L NH4F solution. In the recovery of methanol, the first distillate product collected had a purity of about 87 v/v% and this decreased slightly with time over 2 hours. The methanol distillate collected in the first 30 minutes with a purity of ca. 85 v/v% was re-used in anti-solvent crystallization of (NH4)3ScF6 from the initial strip liquor, and the scandium recovery attained was about 98.6 w/w%, almost similar to that of 99.2 w/w% obtained using 100 v/v% methanol. For ethanol recovery, the first distillate product collected was approx. 84 v/v% pure and the distillate collected in the first 30 minutes with a purity of about 75 v/v% achieved a scandium recovery of about 97.2 w/w% in comparison to 98.5 w/w% obtained using 100 v/v% ethanol during anti-solvent crystallization.

Bauxite residues, i.e., red mud, can be processed to recover various valuable end products, while reducing the environmental impact of the waste. Scandium is one of the valuable elements in bauxite residues. It is possible to extract and enrich scandium from red mud by leaching and solvent extraction. Scandium can then be recovered from the pregnant strip liquor by crystallization. Different crystallization techniques can be used to generate the supersaturation required for scandium to crystallize out as a salt. In the present study, the crystallization of an ammonium scandium fluoride phase by cooling and antisolvent crystallization techniques is presented. Cooling crystallization gave a low yield of ammonium scandium hexafluoride, (NH₄)₃ScF₆, below 50% at the lowest temperature of 1 °C investigated. Antisolvent crystallization using ethanol gave almost complete recovery with precipitation efficiency greater than 98% for an ethanol-to-strip liquor volumetric ratio of 0.8. Solubility data of (NH₄)₃ScF₆ under different temperatures and in different ethanol–strip liquor mixtures is herein presented. The product obtained by antisolvent crystallization had very minute crystals (< 2 µm) due to the high supersaturation generated upon adding ethanol to the strip liquor, while it was easier to obtain larger crystals by cooling crystallization. Fe and Ti impurities were detected in the solid product, and an insight into the mechanism of impurity uptake is discussed.