Kirsten
Remmen‡
a,
Roman
Schäfer‡
a,
Sebastian
Hedwig
a,
Thomas
Wintgens
*a,
Matthias
Wessling
b and
Markus
Lenz
ac
aFHNW, School of Life Sciences, Institute for Ecopreneurship, Hofackerstrasse 30, 4132 Muttenz, Switzerland. E-mail: thomas.wintgens@fhnw.ch
bChemical Process Engineering, RWTH Aachen University, Forckenbeckstrasse 51, 52074 Aachen, Germany
cSub-Department of Environmental Technology, Wageningen University, 6700 AA, Wageningen, The Netherlands
First published on 31st July 2019
Aluminium scandium (Sc) alloys are stronger, more corrosion resistant and more heat tolerant than classical aluminium alloys and allow for 3D printing. In particular, the aerospace industry benefits from better fuel efficiency due to lighter materials as well as the advantages of additive manufacturing. However, Sc is currently not available in sufficient quantities and has recently been identified as a raw material critical to the economy. Due to the recentness of the demand, technologies for recovery of Sc from secondary sources are in their infancy. In this study, Sc recovery from titanium dioxide pigment production waste by nanofiltration was investigated. Custom-made layer-by-layer (LbL) modified membranes were optimized with regards to their elemental retention (i.e., selectivity towards Sc) as well as their acid resistance. In model solutions, the optimized membrane retained up to 64% ± 4% Sc, removing the major impurity, iron (Fe), efficiently (12% ± 7% retention) while achieving high flux [32 L m−2 h−1] at a low transmembrane pressure of 5 bar. Acid resistance was shown down to a pH of 0.1, which could be even further increased (up to ≤3 M HCl) by adding more bi-layers and changing the coating conditions. In real wastes, the optimized LbL membrane showed higher Sc retention (60% vs. 50%) compared to a commercial acid resistant membrane, while achieving considerably higher fluxes [27 L m−2 h−1versus 1 L m−2 h−1, respectively at 5 bar]. It was possible to operate filtration at low transmembrane pressure with up to 70% permeate recovery and flux that was still high [∼10 L m−2 h−1]. In a nutshell, titanium dioxide pigment wastes contained sufficient amounts to satisfy the growing demand for Sc and can be exploited to their full extent by LbL nanofiltration due to the proven advantages of acid stability, Sc retention and selectivity and high achievable fluxes at low pressures.
Water impactThe paper presents an innovative approach to recover scandium from titanium dioxide pigment production waste by the use of newly produced nanofiltration membranes applying the layer-by-layer method. The novel separation technique offers potential to replace conventional processing routes lowering the environmental impact related to the use of organic solvents, which are a burden to the aquatic environment. |
Some attempts have been made to recover Sc from secondary sources; in particular, recovery from bauxite residues (so-called red mud) has been found promising.1,2,7–10 Apart from red mud, there are further stockpiled waste materials that may help satisfy the growing global Sc demand, in particular residues from titanium dioxide (TiO2) production. These are available on a millions-of-tonnes-per-annum scale. Currently, an estimated 5400000 metric tons of ilmenite and 750000 metric tons of rutile are mined globally.11 From these, TiO2 is extracted/purified through either the traditional “sulfate” or the “chloride” route.12 However, very few attempts have been made for Sc recovery via the sulphate route,13,14 and to the authors' best knowledge, none via the chloride route.
Briefly, during TiO2 pigment production via the chloride route, ore is processed with coke and gaseous chlorine in a fluidized bed reactor at high temperatures of 900–1000 °C.8,15,16 The process produces HCl waste containing high dissolved metal concentrations, as well as unreacted ores/coke overblow. Depending on the original ore and processing conditions, varying Sc concentrations may be found in these waste streams. A small level of Sc (0.5%) in Al alloys in only 0.1% of the global annual Al market would result to an annual scandia (Sc2O3) need of ∼350 tonnes2 (∼230 t Sc). Currently, the global supply of Sc is estimated to be between 10–15 tonnes per year only11 and primary Sc deposits above 100 ppm are rarely reported.2 It has been estimated that about 60% of the 4.5 million tons of pigment production world-wide is generated by the chlorine process.15 The volume of aqueous acid waste of the chloride route as well as Sc concentrations will certainly depend on the operational conditions of the respective plant (e.g., type and grade of ore used, dimensions of scrubber, etc.). Nevertheless such wastes may contain sufficient Sc to meet even future demand.
Sc is commonly concentrated and purified by solvent extraction/precipitation.14 For some elements, in particular the solvent extraction process step may contribute considerably to the overall environmental impact of the processing chain (e.g., up to one third in the case of neodymium oxide17). For Sc recovery, it has been demonstrated that a pre-enrichment is needed for sufficiently concentrating Sc for later selective extraction.14 Here, nanofiltration (NF) can offer two crucial advantages: firstly, since it is based on a different separation principle, it may offer selectivity while, secondly, decreasing the volume to be extracted downstream, decreasing the environmental impact of the solvent extraction steps.18 However, a severe limitation of NF is that in high ionic strength solutions satisfactory fluxes can only be achieved via high operational pressures, increasing operational costs.19–21 Furthermore, only a very limited number of commercial membranes can withstand strongly acidic conditions (regularly found in hydrometallurgy).
LbL modified membranes are assembled by depositing several layers of oppositely charged polyelectrolytes (PE) on an ultrafiltration (UF) membrane. This technology allows tailoring of the membrane characteristics (porosity, selectivity, stability, etc.)22–24 towards a target ion and application. This is achieved through selection of a suitable PE, varying the number of layers, and/or varying the coating method (e.g., the ionic strength, the pH).22,25 LbL membranes often have a higher permeability during acidic filtration than conventional NF membranes. For instance, LbL membranes showed 16 times higher permeability in comparison to a commercial membrane (AS 3012, AMS, TelAviv, Israel) in P recovery from acidic leachates26 with increased P yield.25 In consequence, the filtration units can be smaller, or a lower transmembrane pressure (TMP) can be applied, leading to a considerably lower operational (energy consumption) and capital costs.25,26 A limitation to LbL membranes may be their instability in strong acids. So far, studies focused on stability in H3PO4 (ref. 25 and 26) and acid stability in HCl was only shown by immersing the membrane in 1 M HCl.22,27,28 No studies have been carried out post-filtration regarding element retention or flux behaviour, so that it remains unclear if sufficient acid stability can be achieved over the long term.
Thus, this study is the first to apply LbL membranes in concentrated HCl matrixes for Sc recovery. LbL membranes were optimized with regards to their selectivity towards the target ion (Sc) as well as to their acid resistance by modifying coating parameters (number of layers, ionic strength) in model solutions. The best LbL membrane was applied for filtration of real TiO2 wastes and compared to commercial membranes.
Fig. 1 Sc (A) and Fe (B) retention by (PDADMAC/PSS)3 in model solution as a function of coating condition. |
Additional bi-layers may result in improved membrane properties with regards to element retention as well as stability,22,27,30 yet come at the price of requiring more time and chemicals for membrane preparation. Increasing the number of bi-layers in PDADMAC/PSS systems impacting filtration has been ascribed to the charge overcompensation of PDADMAC, leading to diffusion of the PE from top layers into lower layers.27,36 Here, the addition of more than three bi-layers did not result in improved Sc retention, and the (PDADMAC/PSS)3 system showed satisfactory acid stability (i.e., 45.7 ± 7.2% Sc retention after 60 min; Fig. 2). The addition of more bi-layers only had a minor beneficial effect on Fe retention (minimal retention of 4.2 ± 1.3% with seven bi-layers, Fig. 2), which is too little to justify additional preparatory work due to the low retention using three bilayers (12.3 ± 1.1%). The results are in line with previous studies that have shown already three bi-layers giving high magnesium retention and thus NF membrane properties.27
Fig. 2 Sc and Fe retention after 60 min of model solution filtration for (PDADMAC/PSS)x membranes as a function of the number of bi-layers applied. |
Acidity may influence both chemical (i.e., degradation) and physical (e.g., swelling, structural order, disintegration) integrity of the LbL membranes.23 Regarding the application in hydrometallurgy in general and for TiO2 wastes in particular, high resistivity towards acid is desirable. Considering that a solution with high ionic strength can diminish the electrostatic interaction of the PE, less stable membranes are expected in high acidic environments. Indeed, at low (0.1) pH, the (PDADMAC/PSS)3 system was indicated to be insufficiently stable, underlined by decreased Sc retention before and after filtration (53.5 ± 3.6% to 45.7 ± 7.2%, Fig. 3A and B). Adjusting the pH to 1.5 had a positive effect on Sc retention and membrane stability (63.3 ± 4.4% before to 64.6 ± 10.7% after 60 min of filtration; Fig. 3A and B), while achieving high selectivity towards undesired Fe (14.4 ± 0.9% after filtration). Acid stability down to pH ≥1.5 constitutes an expansion of (PDADMAC/PSS)3 systems that have previously been shown to be stable down to pH 2.5 (PDADMAC/PSS coated Si capillaries for chromatography).28 Improving acid resistivity even further for other hydrometallurgical applications while conserving selectivity for the target ions remains the subject of future work. Certainly, increasing the number of bi-layers and modifying the coating conditions can still considerably improve acid resistivity (e.g., up to 3 M HCl, which is considerably higher than previously reported,28,37 see ESI†). Alternatively, in the future cross-linking of other PE may result in high acid resistivity, however, at the expense of more steps/chemicals necessary and potential loss of selectivity. Here, increasing the pH of the waste had an additional advantage, since some unwanted elements (impurities such as radioactive uranium (U) and thorium (Th)) already precipitate at this value, whereas most Sc (80%) remained in solution (see ESI†). Therefore, acid resistivity towards pH 1.5 solutions was found sufficient for Sc recovery from TiO2 wastes.
Fig. 3 Acid stability in term of Sc and Fe retention as a function pH before (A) and after 60 min (B) of model solution filtration. |
Other impurities such as Fe and U were efficiently removed by LbL filtration, as underlined by the even negative retentions (Fig. 4). Since no Fe or U are concentrated in the retentate, the Sc:Fe ratio was shifted in an even more favourable direction. In previous studies, low U retention was observed for NF membranes and a decrease in pH led to lower retention values as well.40,41 Other studies report a decreased influence of Donnan exclusion for anions such as chloride in a lower pH environment.42 Thus, dielectric and steric exclusion play a major role in retention. Two other studies claim that large hydrated ions with a strong hydration shell are retained best.43,44 This seems to apply to Sc and Th in the here presented experiments. In a study presented by Tansel et al. (2012), Fe2+ was categorized as a large hydrated ion but with a weak hydration shell, leading to lower retention values, which might also be the case for U.44
To benchmark the obtained membrane performance, a filtration experiment was conducted using a commercially available acid resistant Duracid membrane at 0% permeate recovery. The Duracid membrane achieved less Sc retention (50%) and retained more Fe (4%) and U (31%), whereas less Th was retained (69%). The results are in accordance with a previous study that assigned lower retention for monovalent, divalent, and trivalent ions to HCl used to acidify the feed solution using the same membrane.18 Most importantly, it has to be noted that even at 0% permeate recovery, a flux of as little as 1 L m−2 h−1 was measured, which was 3% of the flux that was reached for the LbL membranes at the same conditions (Fig. 4B). Thus, when aiming at an identical amount of permeate volume, a 28-fold membrane area or notable pressure increase would be needed. In conclusion, the LbL membranes used here offer advantages regarding not only selectivity but also lower operational costs and/or energy demand.
Recovery of critical raw materials by nanofiltration in general offers the major advantages of relying on a different separation principle than conventional techniques (i.e., ion exchange, solvent extraction, precipitation) as well as generating concentrates depleted in impurities, beneficial for downstream processing by conventional techniques.18 One challenge regarding the filtration of strong acidic solutions is their corrosivity. Since LbL membranes operate at lower pressure ranges (here, 5 bar) but still reach high fluxes [here, up to 27 L m−2 h−1 in real wastes], the filtration units can be built out of corrosion resistant plastic parts. Though the proof of principle for LbL filtration of TiO2 wastes was made here, a challenge for future research remains the acid resistivity and long-term stability23 of the membranes themselves. Although acid resistivity can be improved to some extent by the addition of more layers or changed coating conditions (see ESI†), some wastes may still remain inaccessible without pre-treatment.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ew00509a |
‡ Both first authors made an equal contribution to this publication. |
This journal is © The Royal Society of Chemistry 2019 |