Why Scandium Solvent Extraction Is Better Than Traditional Scandium Processing Techniques
Nigel Ricketts, Manager of Studies, Queensland, Engenium Pty Ltd
Scandium is a metal with emerging uses in aluminium alloying, lighting and ceramic fuel cell applications. In recent years, an increase in demand for scandium has been hampered by a lack of supply options. This has driven the development of several projects where scandium production is a major focus, in particular laterite sources in Australia, Turkey and the Philippines. Solvent extraction is commonly used for recovery of scandium from the laterite leach solutions.
Scandium has two dominant uses. The first is an additive to the zirconia ceramic in some types of ceramic oxide fuel cells. The addition of scandium oxide reduces the operating temperature of this style of fuel cell. A second major use is as a strengthening agent in aluminium alloys. A third emerging application is as an additive of scandium iodide to halogen lighting to make the light produced whiter.
Scandium initially found applications in aircraft aluminium alloys in the Soviet aviation industry. In the 1990s, Ashurst Technology, together with a Ukrainian partner, produced scandium from a historic uranium mine in Ukraine. Whilst the venture was short lived, it has provided the impetus for companies to look for scandium in other locations.
Most of the current production of scandium now comes from China from by-product recovery in the processing of zirconium and titanium chemicals, although some has been produced from rare earth deposits. The bauxite leach residue from the aluminium industry known as “red mud” can also contain elevated levels of scandium, with some red muds from Jamaica and Greece containing up to 150 ppm scandium.
It has been known for many years that Ni-Co laterite ore bodies contain varying levels of scandium. Sumitomo Mining and Metals have recently installed a scandium recovery circuit on their Taganito operation in the Philippines, after piloting the process at Coral Bay. Meta Nikel Kobalt in Turkey are also planning to produce scandium products at their Gördes facility in Turkey.
A number of Australian nickel laterites contain scandium at levels well in excess of the levels seen in laterites from other parts of the world. The Sunrise, Flemington, Owendale, Young, Tiger’s Creek and Honeybugle deposits in NSW are each more than capable of supplying the world annual production of scandium for many years, with scandium levels in excess of 400 ppm and at times in excess of 700 ppm.
The New South Wales laterites are weathered pyroxenite-dunite intrusions. The scandium appears to be concentrated at the periphery of the intrusions whilst the cobalt and nickel appears to be more concentrated towards the centre. The feed material to a potential plant will depend on the relative value in the nickel, cobalt and scandium.
Traditional Scandium Processing Routes
The traditional method for scandium recovery from a nickel laterite involves precipitation of scandium with an iron-bearing residue from high pressure acid leach solutions at about pH 4. This is often conducted in two stages. The first stage at a slightly lower pH will be higher in iron and lower in scandium than the second stage precipitate.
The co-precipitation of iron and scandium hydroxides is unavoidable for two reasons. The pH range at which scandium hydroxide precipitates is only slightly higher than the pH range for iron hydroxide to precipitate. If a higher precipitation pH is used, then this overlaps with the nickel and cobalt hydroxide precipitation range, resulting in nickel and cobalt losses. Scandium and iron can also substitute for each other in a range of mineral products. So iron and scandium co-precipitation is always an issue. It should also be noted that some other elements are also co-precipitated by this process. Notably aluminium, thorium, uranium and rare earths (if they are present).
The second stage iron precipitate can then be re-leached in sulfuric acid, maybe even at a second site. The impurities can be further removed from the leach solution by ion exchange or solvent extraction. Scandium is often then precipitated from solution using oxalic acid to produce a scandium oxalate which is calcined to scandium oxide. The oxalate precipitation process works in sulfate, chloride and nitrate solutions across a broad pH range and is in itself an effective purification process.
There two main issues with the traditional process route:
- Precipitation chemicals are expensive to neutralize the acid used in leaching
- The process does not remove all of the impurities
Scandium Recovery Utilising Solvent Extraction
Scandium can be recovered by a range of common solvent extraction reagents. The same applies to a range of ion exchange resins. Often these reagents extract scandium quite selectively at low pH, eliminating the need for neutralisation of the leach solution prior to scandium recovery. However, scandium recovery is often so strong, that release of scandium from the recovery chemical is very difficult. This appears to have been the main drawback as to why scandium solvent extraction has failed to gain acceptance as a recovery method.
HPAL leach solutions containing scandium, even from the high scandium laterites from Australia, will be in solution in the range of 100-200 mg/L. This is low by solvent extraction standards for other metals like copper. The HPAL leach solutions will also contain aluminium, iron, manganese and chrome at levels an order of magnitude higher than the scandium tenor, along with g/L levels of nickel and cobalt.
Scandium can be recovered by a range of extractants, but two stand out for more serious discussion. The final scandium product also needs to be examined carefully as scandium oxide may not be the ideal product for subsequent use in the aluminium industry.
Scandium Recovery Using Primene JM-T
Scandium recovery by the solvent extraction organic primary amine known as Primene JM-T has been in the public domain since the 1960s. This chemical is a mixture of primary aliphatic amines and is produced by Rohm and Haas. It is used in the same manner as most metal industry solvent extractants, in mixer settlers diluted with kerosene type solvents.
The US Bureau of Mines used Primene JM-T for extraction of scandium from uranium waste solution in the 1960s. They used a mixture of 2.5% Primene JM-T in kerosene for extraction and a 2 molar sodium chloride solution, acidified to pH 1 for stripping (no mention of which acid was used but cost information would suggest sulfuric acid). A pilot unit was also constructed that was truck mounted and this is shown in Figure 1. The feed was only 0.3 mg/L, but this was able to be upgraded to 130 mg/L in the strip solution. Scandium was recovered by neutralising the solution to pH 7 and the precipitate contained 1.9% scandium.
Further work by the USBM published in 1964 detailed a further evolution of the process for extraction of scandium from uranium plant iron sludge and wolframite concentrates. Important technical information was supplied including the McCabe-Thiele diagrams for extraction and stripping. Figure 2 shows the extraction diagram for recovery of scandium from iron sludge solution of 0.3 g/L using 10 volume percent Primene JM-T in kerosene.
Figure 1 Pilot scandium extraction unit used in 1962 at the USBM in 1962
Figure 2 Equilibrium extraction isotherm for recovery of scandium from iron sludge solutions.
Further interest in the Primene JM-T route for scandium recovery does not seem to have been significant until Scandium International Mining Corp became interested in this process route. In comparison to the earlier USBM work, the patent by Scandium International details experimentation on various strip solutions, including mixtures of chloride salts and acids. It is important to understand that at low pH values, in strong sulfate solutions, the scandium will be present as a scandium sulfate complex ion. When stripped from the organic phase with chloride-based solutions, the scandium strips as a complex chloride ion. It is therefore important to keep the chloride ion activity high in the strip solution. The presence of acid in the strip solutions will also help the phase disengagement of the organic and aqueous phase.
The downstream markets for the final product will influence the purity of the product produced after solvent extraction. Scandium oxalate precipitation as a penultimate step in making scandium oxide works well across a range of pH levels and is a significant purification process. The presence of iron, sodium, potassium, calcium and other elements in the precipitating solution will have a negative impact on the oxalate product purity.
Many processes for scandium oxalate precipitation use an ammonium hydroxide addition to the solvent extraction strip solution to neutralise the pH back to about 2 before adding scandium oxalate. The low acid strip solution used by Scandium International means that the scandium oxalate precipitation can be conducted without such a pre-neutralisation step, saving significant operating cost as well as eliminating the need for ammonia from the process site.
There are two detracting aspects to Primene JM-T use:
- There are anecdotal reports of degradation of the relatively expensive Primene JM-T over time when using a high acid strip solution
- If there are significant amounts of uranium, thorium and rare earths in the ore body being leached, these can be concentrated up along with the scandium, requiring a secondary purification step.
Scandium Recovery Using D2EHPA
The use of D2EHPA (di-2-ethylhexyl phosphoric acid) for recovery of scandium from solution has been known for many years. This extractant is not particularly selective for scandium (depending on the pH) and the scandium can be problematic to strip from the organic phase.
Canning detailed some work on using D2EHPA solutions for recovery of scandium and rare earths from a “barren liquor” from a tailings deposit in Port Pirie in South Australia. It was found that D2EHPA at pH 1.7 using a fully reduced solution to eliminate ferric ion was successful at extracting 90% of the scandium in solution. It also extracted significant amounts of thorium, yttrium and ytterbium. A scrub with 9N sulfuric acid strip all of the extracted metals except scandium. Scandium was able to be stripped with either 10N HF or sodium hydroxide solution (concentration not specified). The latter was preferred and resulted in a scandium hydroxide precipitate in the aqueous phase which could be centrifuged and recovered.
The use of sodium hydroxide to strip a D2EHPA/TBP mixture compound of scandium was also detailed in a patent by Bloom Energy, despite the prior art by Canning. A paper by Hartley and Liao adds further detail on the extraction of scandium from waste titanium production solutions with a 20% D2EHPA/5% TBP mixture in Exxsol 80 diluent. Titanium, zirconium and iron were scrubbed from the organic phase with 500 g/L sulfuric acid with an addition of 2% v/v hydrogen peroxide. The scandium was then stripped with 100 g/L sodium hydroxide. This produced a strip solution with suspended solids of scandium hydroxide that contains 20-30% scandium as a concentrate. The scandium concentrate was redissolved in sulfuric acid to produce a 10 g/L scandium sulfate solution at pH 0.5. Removal of zirconium is conducted with Alamine 336. Second stage extraction was conducted with a Cyanex 272/TBP mixture combined with adding magnesium sulfate to the aqueous phase. The loaded organic is scrubbed of titanium with a 480 g/L sulfuric acid solution with added hydrogen peroxide. The scandium is then stripped from the organic using an 80-100 g/L oxalic acid solution, producing scandium oxalate precipitate in the strip solution. This is then filtered and calcined to produce scandium oxide.
Work in the 1980s by GTE looked at using D2EHPA to recover scandium from tungsten bearing materials used a 10% weight solution of ammonium carbonate to strip scandium from a 2% D2EHPA organic solution. This strip solution was then evaporated to dryness to recover the scandium as a 1-10% scandium concentrate.
A recent by Turkish researchers details process development for recovery of scandium from Ni-Co HPAL leach solutions. As is normal practice, residual iron is removed from the PLS by addition of lime or other neutralising agents. This is normally conducted in two stages, the first at a pH of 2.7 with limestone and then at a higher pH of around 4.7 with a more reactive base like MgO or soda ash to give finer pH control. It is this second iron precipitate where the scandium can be found. The iron precipitate in his paper was able to be concentrated up to 900-1000 ppm Sc from a feed laterite of 106 ppm. The scandium was then in a readily leachable hydroxide state.
The precipitate is then re-leached in 100 g/L sulfuric acid at 60 °C for 60 minutes and achieved 98% leach efficiency. This leach solution was then contacted with 10% D2EHPA. Iron and scandium were both extracted. The researchers used a 3M ammonium fluoride strip solution to strip 80% of the scandium from the D2EHPA in a single strip stage. By adding sodium hydroxide to the strip solution, a sodium-scandium-fluoride precipitate is produced at pH 9. Once calcined, this product can be used in the production of Al-Sc master alloys.
Work by Turkish researchers from Meta Nikel Kobalt Madencilik is detailed in a patent application where the stripping of scandium from D2EHPA is achieved by pre-conditioning the organic phase with ammonium thiocyanate (NH4SCN). The inventors claim that this allows the D2EHPA loaded organic to be stripped with either nitric acid, sodium hydroxide or ammonium fluoride solutions. A scrubbing solution is used prior to remove impurities prior to stripping the scandium.
Sumitomo Metal Mining started a scandium recovery operation from their Taganito Ni-Co operations in early 2018. It is believed that only an intermediate product is produced on site and is shipped to the Niihama refinery. Scandium is recovered using ion exchange utilising a chelating resin having iminodiacetic acid as a functional group. The eluate from the ion exchange process in neutralised to precipitate scandium hydroxide, then it redissolved in sulfuric acid and extracted from solution using D2EHPA (although an example using PC-88a is also present in the patent). The scandium is stripped from the D2EHPA with 6M sodium hydroxide solution, producing a scandium hydroxide precipitate. After filtration, this precipitate is washed, redissolved again and then precipitated as scandium oxalate using oxalic acid. The oxalate is subsequently calcined at 900 °C for two hours to produce scandium oxide.
There is significant scandium in a number of mineral deposits identified around the world and there is an emerging scandium industry developing. Operating flowsheets using both Primene JM-T and D2EHPA as solvent extractions reagents are able to be developed. Whilst extraction of scandium from highly acidic solutions has been well demonstrated, further work is required on the stripping of scandium from these reagents and on scrubbing solutions to remove co-extracted impurities. When developing these stripping solutions, it should be remembered what market the scandium product is being used for. The use of scandium in aluminium alloys provides potential scope for different stripping solutions than if the product was to be used for solid oxide fuel cell electrolyte.