Separation of Scandium from HCl-Ethanol Leachate of Red Mud by a Supported Ionic Liquid.

[NNadir]

The paper I'll discuss in this post is this one: Separation of Scandium from Hydrochloric Acid–Ethanol Leachate of Bauxite Residue by a Supported Ionic Liquid Phase (Dženita AvdibegovićDženita Avdibegović
and Koen Binnemans Ind. Eng. Chem. Res. 2020, 59, 34, 15332–15342)

Red Mud, also known as Bauxite residue is a troublesome side product of the aluminum industry which is primarily addressed by impoundment reservoirs, landfills, etc.

A recent news item in the journal Science discussed this problem and included this picture:



Red mud is piling up. Can scientists figure out what to do with it? (Service, Science Aug. 20, 2020).

Scandium is the first "d element" in the periodic table, but is often treated as if it were a lanthanide element, because of the closeness of its chemistry with the lanthanides; however there are no real concentrated ores of the element and thus, even though it is not particularly rare, it is very, very, very expensive, with prices in the thousands of dollars per kg. The element is strong, and light, much like its neighbor in the periodic table, titanium, for which plentiful ores (the wonder substance, TiO2) exist.

It is known that when scandium is alloyed with aluminum, it can greatly improve the strength of the metal.

The paper under discussion addresses the recovery of scandium from red mud. I am familiar with one of the authors, Dr. Binnemans as a result of a very nice review article he authored many years ago, owing to his expertise in the chemistry of ionic liquids with respect to the f elements, the lanthanides and actinides: Lanthanides and Actinides in Ionic Liquids (Binnemans, Chem. Rev. 2007, 107, 6, 2592–2614)

From the introduction to the more recent paper cited at the outset of this post:

Scandium is a scarce and expensive rare-earth element.(1) As a consequence, its commercial applications are still limited. Its major uses are in solid oxide fuel cells and as an alloying metal for aluminum. The addition of no more than 0.35–0.4% of scandium to aluminum alloys results in a material with superior mechanical strength.(2,3) Scandium is rarely found in nature in concentrated ore deposits but is obtained as a byproduct in the extraction processes of other metals such as the rare earths and uranium.(4)

Bauxite residue (BR) or red mud is an alkaline byproduct generated in the Bayer process for production of alumina from bauxite ore. Its global annual average production is estimated at 150 million tonnes.(5) It is commonly disposed by lagooning or “dry stacking” methods. In the lagooning method, BR slurry is pumped into storage ponds. BR disposed in such a way can create safety and environmental issues, such as contamination of surface and ground waters by leaching of alkaline liquor and other contaminants.(5) Dry stacking is used as the preferred method for BR disposal in order to reduce the potential for leakage of alkaline liquor and increase the recoveries of soda and alumina.(5) Both methods for disposal of BR require a substantial area of land, which could be used, for instance, for forests or agriculture. BR has attracted a lot of research attention in the past years as a resource for metals or as a building material.(6?12) BR can also be a valuable resource of scandium, but the scandium concentration is dependent on the type and origin of the bauxite ore.(13) For instance, Greek BR contains around 120 g tonne–1 of scandium, which is much higher than the average abundance of scandium in the Earth’s crust (22 g tonne–1) and high enough to consider this BR as a resource for scandium recovery. The main metals in BR are iron, aluminum, calcium, sodium, silicon, and titanium, and these elements are present in much higher concentrations than scandium.(14) Greek BR also contains other rare-earth elements (e.g., yttrium, lanthanum, neodymium) besides scandium, but their economic value in BR is much lower than that of scandium.

Typically, scandium is recovered from BR by hydrometallurgical methods or by a combination of pyrometallurgical and hydrometallurgical methods.(15) BR or its slag after a pyrometallurgical treatment is leached with mineral acids followed by recovery of the dissolved elements in the leachates by precipitation methods, solvent extraction, or ion exchange...(15?20)

...In the present study, the enhancement of the selectivity of sorbents for scandium is investigated by tuning the composition of the solvent in which scandium is dissolved. The selectivity for scandium over iron is investigated in batch mode from aqueous solutions and solutions with green, organic solvents (ethanol, 2-propanol, ethylene glycol, and polyethylene glycol 200). The investigated sorbents are a supported ionic liquid phase (SILP) betainium sulfonyl(trifluoromethanesulfonylimide) poly(styrene-co-divinylbenzene) [Hbet–STFSI–PS–DVB], bare silica (SiO2) and silica modified with ethylene diaminotetraacetic acid (SiO2–TMS–EDTA) (Figure 1). The SILP has been previously used to recover scandium from BR leachate with nitric acid.(17) Scandium was selectively eluted from the SILP column with dilute phosphoric acid, but the uptake of other major components of the BR leachate was also significant, which diminished the amount of leachate that could be processed. Therefore, an improvement in selectivity of the SILP by a solvometallurgical method is further investigated.

Figure 1:



The caption:

Figure 1. Sorbents tested for scandium recovery from BR leachates: (a) SILP Hbet–STFSI–PS–DVB, (b) silica, and (c) SiO2–TMS–EDTA.

An issue with red mud is that it generally contains quite a bit of iron, and therefore one must achieve selectivity in such a way that the scandium is highly concentrated and the iron rejected. (Since we are living future generations with the best ores depleted, it is possible that red mud will be another form of garbage than we leave them to sift through for materials.) Red mud also contains considerable sodium, which is why ethanol in HCl appears to be a fairly good solvent system for achieving separations:

The intrinsic selectivity of a sorbent for a given metal ion is influenced by several factors: (a) the mechanism of sorption, which is typically governed by the functional groups of sorbents, the coordination sphere, and the charge of metal ions, (b) the kinetics of the sorption, and (c) the sorption medium, including the presence of metal complexing agents. Here, the selectivity of the three sorbents (SILP, SiO2, and the SiO2–TMS–EDTA) was explored by variation of the sorption medium. The selective uptake of scandium was investigated from water, ethanol, isopropanol, ethylene glycol, and PEG-200 solutions containing scandium and iron in equimolar concentrations. In previous ion-exchange studies, it has been pointed out that scandium(III) and iron(III) separation from BR leachates is challenging because of the similar charge density of these ions and their similar hydration enthalpies.(18) As it is also one of the major elements in the leachate of BR, iron was chosen for the sorption studies as a competitive ion to scandium.
Both scandium and iron were nearly quantitatively sorbed by the SILP from their binary aqueous feed (Figure 2a). However, about 88% of scandium was recovered from the ethanolic feed by the SILP with a negligible amount of cosorbed iron. Moreover, the sorption of scandium was still higher (98%) than the sorption of iron (55%) even from the feed comprising ethanol and water in 1:1 volume ratio. The recovery of scandium and iron by the SILP takes place by exchange of their positively charged species in the feed for protons of the carboxyl-group of the SILP.(29) In aqueous acidic solutions of ScCl3 of concentration below 0.255 mol L–1, scandium is predominantly present as hexaaqua complex [Sc(H2O)6]3+. Neutral or anionic species like ScCl3, [ScCl4]?, or [ScCl6]3– are not formed, even in the presence of an excess of chloride ions.(30) Therefore, in the tested 1 mmol L–1 aqueous feed, scandium(III) is present as [Sc(H2O)6]3+ which is exchanged with the protons of the SILP.

SILP = (Supported Ionic Liquid Phase.)

Some more pictures from the text:



The caption:

Figure 2. Sorption (%) of scandium(III) and iron(III) from 2.5 mL of their 1 mmol L–1 aqueous, organic, and aqueous–organic mixtures of ethanol (EtOH), isopropanol (i-Pr), ethylene glycol (EG), and PEG-200 by 25 mg of the sorbents: (a) SILP, (b) SiO2, and (c) SiO2–TMS–EDTA. The 1:1 ethanol, 1:1 isopropanol, and 1:1 PEG-200 are feeds comprising water and the solvent in 1:1 ratio.

The caption:

Figure 3. Concentrations (mg L–1) of elements in the leachates of the Greek BR: (a) minor elements (scandium and yttrium) and (b) major elements (calcium, aluminum, sodium, silicon, titanium, and iron). The BR was leached with 0.7 mol L–1 HCl in water (aqueous leachate) or 0.7 mol L–1 HCl in ethanol (ethanolic leachate) at room temperature and with a liquid-to-solid ratio (L/S) of 10.

The caption:

Figure 4. X-ray diffractograms of the Greek BR (pristine BR) and the solid residues after leaching with 0.7 mol L–1HCl in water (BR after aqueous leaching) or 0.7 mol L–1HCl in ethanol (BR after ethanolic leaching). The dotted red lines emphasize the particular reflections of NaCl in the diffractograms.

The caption:

Figure 5. Breakthrough curves of BR leachates with (a) 0.7 mol L^(–1) HCl in water (aqueous leachate) or (b) 0.7 mol ^(–1) HCl in ethanol (ethanolic leachate) by 2 g of the SILP. The flow rate was 0.1 mL min^(–1).

The caption:

Figure 6. Recovery of elements by 2 g of the SILP from 1 mL of BR leachate with 0.7 mol L–1 HCl in water (aqueous leachate) followed by elution with 9 mL of 0.1 mol L–1 HCl in water, and from 1 mL of BR leachate with 0.7 mol L–1 HCl in ethanol (ethanolic leachate), followed by elution with 9 mL of ethanol.

The caption:

Figure 7. Chromatography separation of scandium (Sc) from (a) aqueous or (b) ethanolic BR leachates. Mobile phases: (A) 1 mL of leachate of BR followed by 9 mL of 0.1 mol L–1 HCl for aqueous leachate (a), or 9 mL of ethanol for ethanolic leachate (b); (B) 0.1 mol L–1 HCl; (C) 0.1 mol L–1 H3PO4; (D) 2 mol L–1 HCl. Flow rate of leachates was 0.1 mL min–1 and of eluents was 0.5 mL min–1. Dashed lines mark the volume of each mobile phase. Dotted lines mark the elution of iron (Fe).

Some more commentary from the text on metal separations:

Generally, iron separation from common minerals of the major base metals (e.g., Cu, Zn, Ni, and Co) is a major challenge in hydrometallurgy.(58) The present study in which iron(III) is separated from the BR leachate using ethanol and the SILP demonstrates the potential of solvometallurgical methods for tuning flowsheets for metal recovery. On the basis of environmental impact and toxicity, ethanol is generally considered as a green solvent.(59) It can be produced from biomass and is usually available in large quantities at a low price.(59,60) Therefore, apart from its performance ethanol is a sustainable solvent, which is the requirement for solvents used in solvometallugry...

Corn is physically green, of course, but I question whether corn ethanol is really "green," in the way many people take it. I do believe that it may be possible, with sufficient energy, to generate from carbon sources, including carbon dioxide, ethylene, from which ethanol can be conveniently made. Ethanol from corn may not be sustainable owing to the depletion of phosphate ores; we'll see.

The authors in any case continue:

...After scandium was separated by elution, the column was regenerated with 2 mol L–1 HCl (Figure 7). The column effluent after elution of the remaining components of the aqueous leachate of BR was mainly composed of a mixture of the major elements, namely iron, aluminum, and calcium (Figure 7a). Silicon and the majority of titanium were separated from other elements in the first fractions. The mixture of silicon and titanium can be used, for instance, in the synthesis of titanium silicate materials for catalysis and adsorptive separations.(61) By elution of the remaining components of the ethanolic leachate of BR (Figure 7b), titanium was collected in fractions together with aluminum and calcium. Their mixture can be considered as a potential precursor of a CaO–Al2O3–TiO2 slag for steel refining.(62) Moreover, their fractions were free from iron, as iron was eluted with ethanol in the initial fraction, along with silicon. The iron-silicate fraction could be considered as a resource for abrasives for blast cleaning. Another potential application is in the production of FeCl3, which is used for wastewater treatment and in the production of printed circuit boards.(63) Yttrium was eluted with 2 mol L–1 HCl along with the major components of both aqueous and ethanolic leachates. The separation of yttrium has not been performed since the focus of the present study falls on opportunities in solvometallurgy for scandium recovery. However, it has been shown by the previous studies that yttrium can be well separated by gradient elution of the SILP with phosphoric acid.(17)

The UV spectra may be out of context outside of the full text, but it bears on iron separations.



The caption:

Figure 8. UV–vis absorption spectra of BR leachates with 0.7 mol L–1 water (aqueous leachate, dashed blue line) or with 0.7 mol L–1 ethanol (ethanolic leachate, full green line).

The peak at 221 corresponds to the aquo/chloro complex of iron (III).

Excerpts from the conclusion:

Screening of the three sorbents (SiO2, SiO2–TMS–EDTA, and the SILP) for recovery of scandium from water, ethanol, isopropanol, ethylene glycol, and PEG-200 solutions revealed the potential of the SILP for scandium separation from the ethanolic leachate of BR. The BR was leached by 0.7 mol L–1 HCl in ethanol or in water. The leaching efficiencies of scandium and a vast majority of other elements were similar to both lixiviants. However, the sodium concentration in the ethanolic leachate was significantly lower compared to that in the aqueous leachate due to the limited solubility of sodium chloride in ethanol. Moreover, silica gel formation was suppressed by leaching with 0.7 mol L–1 HCl in ethanol, unlike when the leaching was performed with 0.7 mol L–1 HCl in water. In the breakthrough curve studies with the aqueous BR leachate, the uptake preference of the elements by the SILP was Si ? Ti < Na < Ca ? Fe ? Al < Sc < Y. The sequence was in part reversed when the uptake of the elements was performed from the ethanolic leachate, that is, Si < Fe ? Ti < Sc < Al < Y < Ca < Na. The reversal in trend was partly rationalized based on the change in solvation of the metal ions in the ethanolic leachate. Iron(III) was easily separated from the majority of other components of the BR by elution with ethanol in column chromatography with the SILP...

... About 84% of scandium was separated from other components of both leachates of the BR by elution with 0.1 mol L–1 H3PO4. Still, a high sample throughput and concentration of scandium from the ethanolic leachate by the SILP was not achieved. Apart of iron and silicon, other major components of the ethanolic BR leachate were recovered by the SILP along with scandium. Nevertheless, the study gives new insights on how a simple change in solvent in which metals are dissolved greatly affects the entire process for metal recovery...

What we are leaving for our children, our grandchildren, and indeed our grandchildren's great-great grandchildren is a huge pile of waste while we squander materials on fantasies like so called "renewable energy."

Be that as it may, we are leaving clues about how something might be done in even harsher times which are surely coming out of our indifference. This little paper is intriguing, I think.

Have a nice day tomorrow.

[KY_EnviroGuy]

1. Interesting post, NNadir.

Never paid any attention to scandium before, probably because it's so rare. I worked with a lot of exotic alloys in my pollution control work (before retirement) and don't recall seeing that element used.

Your post and the photo reminded me of the red mud disaster in Hungary in October of 2010 where a small town was engulfed after a dam failure at an aluminum facility (and mentioned in your cited article):

There was a dam – six metre-high, a couple of miles away, that held back a reservoir of deadly "red mud", a caustic byproduct of aluminium extraction. But it had burst, and a million cubic metres of the slime was rushing toward Devecser, with waves of up to two metres. Within minutes the town was overcome: cars washed down streets and residents lay stricken on the roofs of their ruined homes. The "red mud disaster" claimed 10 lives, 150 were seriously injured.

From: https://www.theguardian.com/environment/2014/jan/08/devecser-hungary-eco-town

From Science's article:

Globally, some 3 billion tons of red mud are now stored in massive waste ponds or dried mounds, making it one of the most abundant industrial wastes on the planet. Aluminum plants generate an additional 150 million tons each year.

I've seen many horror stories about mine ore waste piles in Brazil and our stores of coal-fired power plant ash ponds, many of which I've seen in person. Agree that future generations may look back at our industrial history with disdain, especially as fresh water supplies become scarce.

Keep up your good scientific work and have a great Thursday.......

[NNadir]

2. I'd actually forgotten about that Hungarian event. Thanks for the reminder.

There are so many of these impounding dams around the world, and around this country and one should expect that they will be failing for thousands of years.

I can only imagine the contempt that a person living 500 years from now - should humanity continue to exist - would have for a collapse like, say the Martin County event, having derived no benefit from the use of the coal burned centuries before.

This is why I so often say at the end of my posts that "History will not forgive us, nor should it."

I have had occasion to view huge heaps of coal residues outside of abandoned steel plants; there's one I pass fairly regularly, about once a month.

I often wish I could take a sample of water in a puddle near one of those heaps and run an ICP/MS on it. I expect it would be disturbing.

I'm sure in Kentucky, working in the environmental field, you might have some interesting tales to tell.

It does seem to me, that future generations will need to sift through these slag heaps regarding them as low grade ores. One hears a lot about "urban mining" proposals, by which landfills will become ores.

There's this post about coal ash and uranium that's been on the internet for some time: Coal Combustion: Nuclear Resource or Danger. It points to coal ash as a possible source of uranium.

Effectively however, we have consigned all future generations to pick through our garbage. This strikes me as detestable. An honorable thing would have been for our generation to work to close material cycles throughout our lifetimes. We did make some noise about it, I guess, but basically we rapidly abandoned any pretense of caring relatively quickly. By 1980's, at least here in the US, we devolved into a generation of vicious consumers totally indifferent to the future.

Scandium is a very cool element, I think, if one looks into it, not that there is generally much impetus to do so. I recall reading somewhere that the first kg scale isolation efforts were conducted by the US air force. It is apparently now isolated on the ton scale, generally as the oxide, but it remains too expensive for broad use.

It's never going to be cheap. The reported concentrations in red mud in this paper is on the order of 120 grams/ton. It may become viable as a side product if industrial processes become available for the difficult separation of iron (for use) from the other elements in red mud. In this case, red mud might well become a useful ore for iron. This may be an application for the FFC Cambridge approach to metal isolation, I don't know.

Anyway, scandium; I can't say I've ever had a conversation longer than 5 minutes about this element; in fact, I may have not have ever had one previously even that long. This conversation may be the longest of my life.

When I was a kid, I used to look at the lanthanoid elements with a kind of "so what?" attitude. I was being foolish, but the point of youth I think, now that I am old, is the freedom to be foolish. Of course, some people remain fools long after their youth, and that is a problem, as we're seeing on a national scale, a big problem.

It will be a pleasure to have a real adult as President in 2021, one who perhaps can set us on a path of doing better than we have done.

Thanks for the reminder of the Hungarian event with red mud.