Recovery of critical materials from mine tailings: A comparative study of the solvent extraction of rare earths using acidic, solvating and mixed extractant systems

Journal of Cleaner Production 218 (2019) 425e437

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Recovery of critical materials from mine tailings: A comparative study of the solvent extraction of rare earths using acidic, solvating and mixed extractant systems Cristian Tunsu a, *, Yannick Menard b, Dag Øistein Eriksen c, Christian Ekberg a, Martina Petranikova a €teborg, Chalmers University of Technology, Department of Chemistry and Chemical Engineering, Nuclear Chemistry and Industrial Materials Recycling, Go Sweden French Geological Survey, Water, Environment and Ecotechnologies Division, Orl eans, France c Primus.inter.pares AS, Oslo, Norway a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 August 2018 Received in revised form 25 January 2019 Accepted 28 January 2019 Available online 29 January 2019

Our society heavily depends on the availability of raw materials. Technology metals such as rare earth elements (REEs) are vital in many applications. Because their virgin mining and production is constrained by a multitude of factors, future exploitation of secondary sources is strongly considered. Tailings from past and present mining activities are important sources of REEs and other critical raw materials, e.g., tungsten and phosphate. The possibility of processing such tailings was thoroughly investigated in the ENVIREE European Project (2015e2018). In this paper, we assess the use of solvent extraction to recover REEs from tailings originating from New Kankberg (Sweden) and Covas (Portugal). Extraction of REEs from common mineral acid solutions was carried out using solvating (Cyanex 923 and TODGA) and acidic extractants (DEHPA and Cyanex 572). Extraction was studied in the presence of high amounts of phosphate, iron and copper in solution. This was to identify bottlenecks in the separation process and ways to mitigate them. While copper and phosphate didn't pose significant issues, iron was co-extracted with the REEs in several systems, e.g., DEHPA e sulfuric acid. Co-extraction was reduced by using a blended DEHPA e Cyanex 923 organic phase. At the same time, the extraction efficiency of REEs improved. Control of the contact time between the aqueous and organic phase, and selective stripping were also used to effectively mitigate the extraction of iron. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Tailings Urban mining Critical metals Rare earth elements Solvent extraction

1. Introduction Excessive exploitation of Earth's natural deposits to extract raw materials is a major concern. The speed at which deposits are depleted has accelerated with the start of the industrial era. Presently, there is a high demand for specific raw materials, notably speciality metals for sustainable applications and hi-tech (European Commission, 2011; US Department of Energy, 2011). Rare earth elements (REEs) are needed in, among others, rechargeable batteries, electric transportation, wind turbines and low-energy lighting. This makes them highly desirable for the transition towards a low-carbon economy. It was estimated that, to

* Corresponding author. E-mail address: [email protected] (C. Tunsu). https://doi.org/10.1016/j.jclepro.2019.01.312 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

meet the Paris Agreement requirements, production of rechargeable batteries will significantly increase the already high demand for lithium, cobalt, nickel and REEs (Hodgkinson and Smith, 2018). The preponderant production in China, tight dependency of industrialized countries on imports, geo-political factors, high specificity and wide variety of applications led to the critical status of REEs (US Department of Energy, 2011; European Commission, 2017). There is a very tight connection between REEs prices, supply and demand, and this was most evident in the beginning of the current decade. The so-called rare earth crisis, fuelled by geo-political factors and by the export quotas imposed by China, greatly affected the price of all REEs. Since the long-term growth of numerous industries depends on the ability to secure stable and diverse sources of speciality metals (Hatch, 2012), this placed focus on finding alternative sources, primary and secondary sources being both considered. Previous mines such as Mountain Pass re-opened

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C. Tunsu et al. / Journal of Cleaner Production 218 (2019) 425e437

briefly, and a significant amount of research was aimed at recovery of REEs from end-of-life electronics and other secondary streams (urban mining) (Binnemans et al., 2013a, 2013b; Ferron and Henry, 2015; Tunsu et al., 2015; Xie et al., 2014). In the aftermath of the crisis, the prices of all REEs fell significantly. These events highlighted the cyclic nature of supply and demand for speciality metals with extremely localized and controlled production. Moreover, it articulated that society cannot solely rely on virgin mining and, in the context of a circular economy (Kirchherr et al., 2017), effective recycling and processing of secondary sources is a must. Unless recycling rates increase dramatically and population growth is curbed, many resources can become scarce this century ttir et al., 2012; Sverdrup and Ragnarsdo ttir, 2014a). As (Ragnarsdo the World's population increases, there is need to improve the efficiencies and yields in the recycling and production chains to reduce net consumption per capita and conserve resources ttir, 2014b). (Sverdrup and Ragnarsdo Virgin production of REEs is associated with numerous environmental drawbacks. Local ecosystems are negatively affected by the large amounts of secondary wastes generated during mining of REEs, including waste gas containing dust concentrate, hydrofluoric acid, sulphur dioxide and sulphuric acid, acidic wastewater and radioactive residues (Hurst, 2010). This made the argument for urban mining stronger, pointing out notable advantages: the presence of desirable metals in a smaller volume, in more concentrated form, less mobilization of harmful compounds, reduction of landfill areas and land reclamation for existing landfills. In addition, the contribution of secondary sources to the available supply of REEs will continue to increase in the future (Guyonnet et al., 2015) and can be a responsible sourcing strategy for critical metals (Wall et al., 2017). End-of-life electronics, metallurgical slags, phosphogypsum, and mine tailings have been acknowledged as secondary sources of REEs and other valuable compounds (Binnemans et al., 2013a, 2013b; Ferron and Henry, 2015; Tunsu et al., 2015; Mueller et al., 2015; Innocenzi et al., 2014; Jha et al., 2016; Tan et al., 2015). Investigations performed in the Baltic Region have shown that fine-grained fractions of excavated waste, including clay and colloidal matter, contain significant amounts of potentially valuable metals, including REEs (Burlakovs et al., 2018). Despite efforts, the current recycling rates of REEs remain extremely low, below 1% (European Commission, 2017; UN Environment Programme, 2011), partly due to the significant aforementioned price decreases and lack of governmental policies. In recent years, there has been extensive research on the recovery of REEs from permanent magnets, nickel-metal hydride batteries and fluorescent lamps. In contrast, detailed studies of the recovery of REEs from mine tailings are lacking. While the REEs content in tailings can be lower than in electronics, their processing may be worthwhile if environmental benefits are also considered or imposed, e.g., remediation of mining sites and land reclamation. Governmental regulations can play a pivotal role in closing material loops (Machacek et al., 2017). In addition, exploitation of tailings does not depend on an effective collection-transportation-sorting scheme, which is required for end-of-life products containing REEs; the recyclable material is simply available on site. The present study focuses on aspects related to the hydrometallurgical recovery of REEs found in tailings from past and present mining operations in Europe, an important topic scarcely covered in literature. This was investigated in the ENVIREE Project (2015e2018), which screened various locations across Europe and South Africa to find secondary sources of REEs and assess the potential to recover contained strategic metals. Here, we look at tailings from two sites, New Kankberg (Sweden) and Covas (Portugal). These were selected on the basis of material availability, amount, mineral composition, REE content and former processing

techniques that produced the tailings. Aspects about the mineralogy and beneficiation of samples are discussed but emphasis is placed on the solvent extraction of REEs in the presence of high amounts of interfering anions and cations, phosphate, iron and copper, which are encountered in such streams. The study thoroughly compares extraction of trace (mg/L) and g/L amounts of REEs from common mineral acid solutions (nitric, hydrochloric and sulfuric) with various solvating (Cyanex 923 and tetraoctyl digylcol amide) and acidic extractants (bis-(2-ethylhexyl)phosphoric acid and Cyanex 572). We report on the use of mixed organic phases containing two or more commercial extractants (bis-(2-ethylhexyl) phosphoric acid Cyanex 923 and/or methyltrioctylammonium chloride) as method to minimize co-extraction of iron from complex mixtures and improve extraction of trace REEs.

2. Background Past processing methods for REE ores were not very efficient (Binnemans et al., 2013b). In addition, targeted mining of non-REEs ignored REEs, even if present in the ore. This has led to dumping and accumulation of significant amounts of REE-containing tailings close to mines and processing plants. In the ENVIREE Project, a high number of such sites were initially identified in the Czech Republic, South Africa, Sweden, Poland and Portugal. Tailings from New Krankberg (Sweden) and Covas (Portugal) contain REEs but also other materials presently considered critical in the European Union, e.g., tungsten and phosphate. The New Kankberg tailings originate from gold and tellurium production. These are deposited together with tailings from other sulphide ores in a pond which is around 5 km wide and 10 km long. Roughly 700 million tons tailings has so far been stored and around 1300 million tons more is planned to be stored until 2045. The main mineralogy includes quartz, muscovite, kaolinite and K-feldspar. This material has the highest concentration of REEs among the other European tailings investigated in this project. Mineralogic data revealed the presence of phosphates as monazite and apatite, and minor amounts of xenotime and berlinite. Most of the REEs is present in monazite, thus light REEs predominate (lanthanum, cerium and neodymium). QEMSCAN analysis on representative samples showed that monazite is for 40% associated with other minerals and has a free surface that totals 55%. Phosphate and REEs are present in all size class of the tailings; a slight increase in content was observed in the classes below 40 mm. The Covas tailings originate from past underground mining of tungsten (scheelite and minor wolframite), which occurred for 30 years after the second half of the twentieth century. The mineralogy also includes muscovite, quartz, kaolinite, chlinochlore, K-feldspar and some heavy magnetic minerals (hematite, chalcopyrite, ferberite and arsenopyrite). The deposit consists of 1e3 m thick lenticular skarn levels hosted by schists. The skarn levels are constituted essentially by zones of massif sulphides with associated wolframite, scheelite and ferberite pseudomorphs after scheelite. Additional information

Fig. 1. Bench-scale beneficiation of New Kankberg tailings (a) and Covas tailings (b).

C. Tunsu et al. / Journal of Cleaner Production 218 (2019) 425e437

about the REEs content and bench-scale beneficiation of the tailings are given in Fig. 1 and are further discussed in section 4.1. Due to this mineralogy, hydrometallurgy is required to reclaim the REEs. Pyrometallurgical processing in electric arc furnaces or non-ferrous smelters will divert the REEs to the slag phase, hindering their recovery (Frohlich et al., 2017). Typical hydrometallurgical steps include leaching, followed by separation of metal ions in the resulting leachate using precipitation, solvent extraction or a combination of these. For Covas tailings, selective leaching of tungsten and REEs is possible. Tungsten in scheelite and wolframite is not easily dissolved with acidic solutions. Leaching of tungsten can be done with base solutions, in which the REEs will dissolve. To leach the REEs, acidic conditions are needed. Alkaline cracking and acid baking can be used to modify the mineralogy and facilitate easier dissolution of REEs in weak acidic solutions; such processes have recently been reviewed (Sadri et al., 2017). For solutions containing two or more REEs, precipitation will give a mixed product due to the similar chemical behaviour of these elements. This is not desirable if individual separation is wanted, or if concentrated impurities in solution can precipitate alongside REEs (low product purity). Solvent extraction allows for the recovery of individual REEs with high purities from chemically-complex feeds. Separation involves the transfer of metal ions from the aqueous leachate to an immiscible organic phase, which contains one or more selective extractants (Nash, 1999). Individual separation is possible due to the small differences in ionic radii (the lanthanide contraction), which affects the strength of cation-anion, ion-dipole and ion-induced dipole interactions of REE ions. Heavier REEs create stronger complexes than lighter ones, facilitating their separation. The metals extracted are recovered from the organic phase using a similar process, called stripping or back-extraction. Amines, e.g., Aliquat 336 and Alamine 336, are the most promising extractants for separation of tungsten from alkaline media (Nguyen and Lee, 2016). For REEs, a variety of solvent extraction reagents is available. Extensive research on the separation of REEs has been performed in the nuclear field, starting with the second half of the last decade. This aimed at separating f-block elements from other constituents in spent nuclear fuel (group actinide-lanthanide extraction), and at separating f-block REEs from actinides (Nash, 1993, 1999; Nash and Jensen, 2001; Nilsson and Nash, 2007; Lumetta et al., 2014). Research was also carried out for non-nuclear applications and, to date, various compounds have been suggested for individual or group separation of REEs. Examples are: bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) (Swain and Otu, 2011), Cyanex 925 (Li et al., 2007), Cyanex 923 (Gupta et al., 2003), Cyanex 572 (Wang et al., 2015), bis-(2ethylhexyl)phosphoric acid (DEHPA) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP) (Morais and Ciminelli, 2004; Xu et al., 1992), tetraoctyl digylcol amide (TODGA) (Apichaibukol et al., 2004; Narbutt et al., 2015) and multidentate diamides (Narita et al., 2004a, 2004b), polyamides bearing tetrabutylmalonamide functional groups (Tyumentsev et al., 2016) and various ionic liquid systems (Rout and Binnemans, 2014; Rout et al., 2014; Sun and Waters, 2014; Yang et al., 2012). In recent years, several reviews on the separation of REEs have been published (Binnemans et al., 2013a, 2013b; Ferron and Henry, 2015; Tunsu et al., 2015; Xie et al., 2014; Innocenzi et al., 2014; Jha et al., 2016; Tan et al., 2015; Kolarik, 2012; Kubota et al., 2012; Yoon et al., 2016). These discuss the mechanisms of extraction and potential applications for urban mining. For REEs, three type of extractants are typically mentioned: solvating (Cyanex 923, TODGA, tributyl phosphate), acidic (DEHPA, HEHEHP, Cyanex 272, Cyanex 572) and ion-pair (Aliquat 336, Trihexyl(tetradecyl)phosphonium chloride and other ionic liquids). Solvating extractants extract metal ions according to Equation (1). The metal ions in the aqueous phase

427

(Mzþ) form complexes with the counter-ions present (X). The remaining coordination sites are occupied by water. Due to their water content, these complexes have very low distribution coefficients in, e.g., a hydrocarbon. In the presence of a solvating extractant (B), the water molecules are replaced by extractant molecules. More lipophilic complexes are formed, which are extracted into the organic phase. Acidic extractants extract REE ions according to Equation (2). Since protons are involved in the equilibrium reaction, distribution is pH-dependent. Extraction of REEs is favoured by increases in pH and the opposite process (stripping) is favoured at high acidity. Most commonly, ion-pair extraction involves an anion exchange mechanism between anionic metal complexes and one or more large organic cations (Equation (3)). Mzþ

(aq)

REE3þ

þ z X

(aq)

(aq)

þbB

þ 3 (HA)2

(org)

(org)

4 MXzBb

4 REE(HA2)3

(1)

(org)

(org)

þ 3 Hþ

(aq)

(2)

where (HA)2 is the dimer form of the extractant. MLx n

(aq)

þ x QL

(org)

4 QxMLn

(org)

þ x L

(aq)

(3)

Consensus is that acidic extractants such as DEHPA and HEHEHP are the industrial standard for individual REEs separation. They allow for separation factors of approximately 2.5 between adjacent REEs, highest among traditional extractants. Separation of REEs with high purity is, however, not an easy task. Tenths of countercurrent separation stages are sometimes needed to isolate adjacent members of the group. Chemically-complex streams pose additional challenges. Their efficient processing can be reagentintensive and can generate significant amounts of secondary wastes. This is especially the case for selective extraction of trace REEs from solutions with high contents of interfering species. Iron, a common constituent in ores and tailings, is problematic due to its high extractability by many of the aforementioned conventional reagents, notably DEHPA, Cyanex 923 and Cyanex 272. Tailoring more selective extraction processes is, therefore, very important in the extractive metallurgy of REEs, especially for low-grade feeds. In this paper, we look at the extraction of iron, copper, phosphate and REEs from common mineral acids, using acidic and solvating reagents. We propose an extraction process with mixed conventional acidic and solvating reagents (DEHPA and Cyanex 923), coupled with selective stripping. This minimizes co-extraction of iron from solutions with low amounts of REEs and significantly higher iron and copper content, while enhancing distribution of REEs. 3. Experimental 3.1. Beneficiation of tailings Two barrels of 250 L containing tailings from New Kankberg were emptied in a stirred tank and homogenized. Bench-scale beneficiation was carried out using phosphate flotation and magnetic separation. Flotation was done in a 2.5 L Denver flotation cell in batches of approx. 1 kg. The stirring speed was 1500 rpm and the solid concentration was 40%. Pulp conditioning included depressant (water glass), collector (Resinoline BD2) and frother (polypropylene glycol). Operating parameters are given in Fig. 1. Following flotation, the concentrate (a mix of phosphates, apatite and monazite) was further enriched using magnetic separation (apatite is non-magnetic while monazite is paramagnetic). This was done in a BoxMag High Intensity Magnetic Separation batch cell using tenths of grams of sample. Two stages, the first at magnetic intensity of 4000 G followed by a scavenging step at 15000 G, produced a concentrate enriched in REEs and phosphate.

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One ton of Covas tailings with particle size distribution between 0 and 30 mm was dried and screened (2 mm) to remove coarser particles. The coarser particles were crushed to below 2 mm and mixed with the sample after screening, followed by grinding to below 100 mm. Multi gravimetric concentration followed by magnetic separation was used for beneficiation. A wet shaking table with a deck surface area of 0.8 m2 was used. The feed was diluted with water (25e30% w/w solid). The shaking pulse rate was 500 pulses/min and the stroke length 10 mm. Wet magnetic separation followed. Rare earths were recovered at intensities of magnetic field in the range 900e14 000 G.

1 min for Cyanex 923, 45 min for TODGA, and 30 min for DEHPA and Cyanex 572. After mixing, the vials were centrifuged to assure complete phase separation. Extraction with DEHPA and Cyanex 572 was also carried out at higher pH. For this, the pH of the aqueous feeds was increased with sodium hydroxide. Stripping of Cyanex 923 organic phases after extraction was done with 1.6 mol/L hydrochloric acid and 4 mol/L nitric acid solutions, respectively. Mass balance was used to calculate the distribution ratios (D) and percentage extracted (%E), according to Equations (4) and (5), respectively.

D¼ 3.2. Hydrometallurgical separation of REEs 3.2.1. Feeds, screening of extractants and typical bench-scale extraction procedure Screening was first carried out to find suitable extractants to separate REEs from model solutions also containing major impurities found in acidic tailing leachates. Six aqueous feeds were investigated, as follows (Table 1): i) nitric, hydrochloric and sulfuric acid solutions meant to resemble Covas leachates (over 10 g/L iron and copper as impurities, and about a hundred ppm REEs), and ii) nitric, hydrochloric and sulfuric acid solutions meant to resemble New Kankberg leachates (gram amounts of REEs, and 10 g/L phosphate). The feeds were prepared by dissolving appropriate metal compounds into each acid: iron, copper and REE nitrates in nitric acid; iron, copper and REE chlorides in hydrochloric acid; and iron sulfate, copper sulfate and REE oxides and sulfates in sulfuric acid. The amounts of REEs in solution were selected assuming a solid-toliquid ratio of approx. 1:5 w/v during leaching of beneficiated fractions and approx. 80% leaching efficiency. Sodium phosphate was added in the New Kankberg feeds. The copper in the compounds used was in oxidation state II. The REEs and iron were in oxidation state III. The stock acid solutions used were of analytical grade purity or higher. Pure water (MilliQ, Millipore, >18 MU/cm) was used as diluent. Cyanex 923 (93%, Cytec), TODGA (prepared in-house), DEHPA (97%, Sigma-Aldrich) and Cyanex 572 (Cytec) were the extractants screened. They were dissolved in kerosene with low aromatic content (Solvent 70, Statoil) to obtain the following concentrations: 0.59 mol/L and 1.18 mol/L Cyanex 923, 0.1 mol/L TODGA, 1 mol/L DEHPA and 1 mol/L Cyanex 572. All commercial organic compounds were used as supplied, without further purification. Bench-scale extraction was carried out in glass vials (3.5 mL, 46  13  0.8 mm) secured with lids. Phase mixing was achieved using a thermostatic shaking machine set to 1500 vibrations per minute (vpm), at 21 ± 1  C. Unless stated otherwise, the organic:aqueous (O:A) phase ratios were 1:1. The mixing times were:

%E ¼

Ci  Cf Cf

(4)

Ci  Cf 100 Ci

(5)

where Ci and Cf are the metal concentrations in the aqueous phase before and after extraction, respectively. Elemental quantification was carried out with Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) (iCAP 6500, Thermo Fischer) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (iCAP Q, Thermo Fischer). 3.2.2. Extraction with acidic extractants from covas sulphate model solution The influence of the aqueous pH and phase contact time was further investigated for the Covas sulfate e 1 mol/L DEHPA system. The model solution was partly neutralized with solid sodium hydroxide to produce feeds with similar metal content but different pH, in the range 0.2e4. Extraction tests were done for 30 min using the aforementioned procedure. A similar test was also carried out with 1 mol/L Cyanex 572. The effect of time on the extraction with 1 M DEHPA was investigated using model solution with initial pH 1.65. Extraction was carried out in a similar manner using several different phase contact times. To investigate stripping, aqueous solution with initial pH 1.65 was manually shaken for 1 min with 1 mol/L DEHPA (O:A 1:1). After 7 min resting, the organic phase was isolated. Organic aliquots were stripped with 0.5 mol/L citric acid, and with hydrochloric and nitric acid solutions of various concentrations (1 mol/L, 2 mol/L, 4 mol/L and 6 mol/L, respectively). Stripping was done using O:A ratios 1:1, for 30 min, at 1500 vpm and 21  C. 3.2.2.1. Minimizing co-extraction of iron by blending DEHPA with other extractants. Extraction of metal ions from Covas sulfate solution was studied using DEHPA and several organic phases consisting of DEHPA blended with Cyanex 923 and/or methyltrioctylammonium chloride (Aliquat 336). Their

Table 1 Aqueous feeds investigated. Ionic specie/others

Sample type, acid media and metal content (mg/L) Covas

3þ

Ce Nd3þ La3þ Dy3þ Fe3þ Cu2þ (PO4)3Acid (mol/L) Measured pH

New Kankberg

HNO3 media

HCl media

H2SO4 media

HNO3 media

HCl media

H2SO4 media

41 23 21 6 9980 1010

39 21 23 5 9980 9975

41 22 17 6 9000 1000

810 480 460 100

730 400 430 90

750 470 460 105

1 0

0.4 0.4

1 0

9550 1 0.2

9080 0.92 0.1

9670 1 0.1

C. Tunsu et al. / Journal of Cleaner Production 218 (2019) 425e437 Table 2 The DEHPA e Cyanex 923 organic phases tested with the Covas sulfuric acid model feed. Extractant and concentration (mol/L) DEHPA

Cyanex 923

0.5 0.5 0.5 0.5 0.5 1 1 1 1 1

0 0.117 0.235 0.47 0.705 0 0.117 0.235 0.47 0.705

Volumetric composition (% vol.)

15 15 15 15 15 30 30 30 30 30

%vol. %vol. %vol. %vol. %vol. %vol. %vol. %vol. %vol. %vol.

DEHPA DEHPA, DEHPA, DEHPA, DEHPA, DEHPA DEHPA, DEHPA, DEHPA, DEHPA,

5 %vol. Cyanex 923 10 %vol. Cyanex 923 20 %vol. Cyanex 923 30 %vol. Cyanex 923 5 %vol. Cyanex 923 10 %vol. Cyanex 923 20 %vol. Cyanex 923 30 %vol. Cyanex 923

composition is given in Fig. 10. Extraction was carried out from solution partly neutralized with solid sodium hydroxide to pH 2. The extraction conditions were: O:A 1:1, 2.5 min contact time, 1500 vpm and 21  C. The DEHPA e Cyanex 923 organic system was further investigated with the phases in Table 2. Extraction was carried out from the aforementioned aqueous solution using O:A ratios 1:1, 1 min contact time, 1500 vpm and 21  C. A mixed 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 solution in kerosene was further tested on an aqueous feed with lower pH (initial pH 1.2). This was done using similar experimental conditions and at different O:A ratios, in the range 1:4e4:1. 3.2.2.2. Extraction in mixer-settlers. A solution containing 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 in kerosene was used as organic phase. 2 %vol. 1-nonanol was preventively added to this as third phase inhibitor. A new batch of Covas sulfate model solution was prepared, in which the concentration of dysprosium was double compared to that in Table 1. The acidity was lower, as the compounds were dissolved in approx. 0.1 mol/L sulfuric acid solution. Part of this feed was used as prepared (initial pH 1.19). The other part was pre-neutralized to pH 1.95 using solid sodium hydroxide. This was done to assess extraction at slightly higher pH. The two feeds were separately pumped in a mixer-settler countercurrent cascade with 5 extraction stages. The O:A ratio was 1:3 and the mixing speed in each chamber was 1000 rpm. The flow rates for the organic and aqueous solutions were 2 mL/min and 6 mL/min, respectively. The aqueous phases in each settling chamber were sampled after the system reached steady state. Their pH was measured and their metal content was determined using ICP-MS. Mass balance was used to estimate the percentages extracted (Equation (2)). 4. Results and discussions 4.1. Beneficiation of tailings Bench-scale beneficiation of New Kankberg tailings was carried out using phosphate flotation and magnetic separation. A diagram of the process, together with the recovery yields and concentrations of REEs are presented in Fig. 1a. Flotation consisted of one rougher step, two scavenging steps and one to two washing steps. Phosphate recovery reached 70%, with a concentration factor of 10. Rare earths recovery reached 50%, with a concentration factor of 9. The concentrate containing a mix of phosphates (apatite and monazite) was further enriched using two-stage magnetic separation. This exploited the differences in magnetism of apatite and monazite: monazite is paramagnetic while apatite is non-magnetic. The first stage was carried out using

429

a magnetic intensity of 4000 G. A scavenging step at 15000 G followed. This led to a concentrate with 17.5% of the initial phosphate content (monazite mainly) and 37.5% of the initial REEs. The phosphate content increased from 0.17% to 2.5%. The REE content increased from 170 ppm to 5000 ppm for cerium, 90 ppme2800 ppm for lanthanum and 70e2300 ppm for neodymium. In grinded Covas tailings, the REEs and tungsten were mainly found in fine particles, 63 mm and smaller. Flotation was not efficient to recover the REEs and phosphate, regardless of operating conditions. This is mainly due to the presence of calcite (14%), which hindered the process. The presence of scheelite and wolframite, together with other heavy magnetic minerals (hematite, chalcopyrite, ferberite and arsenopyrite), suggest that a combination of gravity and magnetic separation can be used for beneficiation. It was noted down that REEs followed hematite and other iron oxides (ferberite) or sulfides (chalcopyrite and arsenopyrite). Because these minerals and scheelite are heavy, they were easily recoverable using gravimetric techniques. The recovery yields obtained with gravity separation were high. Rare earths recovery exceeded 70%. For tungsten, recovery was 50%. The concentration factor for REEs was 5.5 and for tungsten 2.5 (Fig. 1b). The technique could be implemented on site, with very low CAPEX and OPEX; the only requirements are water and power supply. Magnetic separation allowed further concentration of REEs (8 times more compared to the initial material) and tungsten (12.6 times more). The final recovery of REEs was 55%. For tungsten, this was 35%. 4.2. Separation of REEs from New Kankberg and Covas model leachates The above results show that iron and tungsten are major constituents in Covas concentrates; for New Kankberg, phosphate is concentrated during beneficiation. Tungsten was not regarded as problematic since effective leaching of REEs requires acidic conditions. Acid dissolution of tungsten from scheelite and wolframite is expected to be low. Tungsten can be selectively dissolved using basic solutions, then further recovered using solvent extraction with amines (Nguyen and Lee, 2016). On the other hand, iron and copper will be effectively leached from the iron oxides and sulphides by mineral acids. The focus was to assess how extraction and separation of REEs is influenced by the nature of the aqueous media (nitric, hydrochloric or sulfuric acid) and by impurities. We chose simple model solutions to have precise control over the composition of the aqueous solutions, and the amount and nature of impurities, e.g., oxidation state and counter-ions present. This facilitated an easier and better understanding of the extraction chemistry, and allowed for good comparison between the different extraction systems tested. For Covas, the extraction behaviour of trace REEs was studied in the presence of significantly higher amounts of iron and copper. For New Kankberg, phosphate will be a major anionic impurity in solution and may influence distribution of REEs. The model solutions in Table 1 were investigated. A low aqueous pH was chosen to assure the stability of the elemental ions in solution. All Covas solutions were stable after storage for 3 months at ambient conditions. The hydrochloric and nitric acid New Kankberg solutions were also stable. Precipitation occurred in the New Kankberg sulfuric acid solution after a few weeks. Addition of concentrated sulfuric acid did not dissolve the pale pink precipitate formed. Analysis of the solution after one month showed significant decreases of the cerium, neodymium and lanthanum concentrations. The values were 20e25% of the initial concentrations in Table 1. The concentrations of dysprosium and phosphorous were similar to

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their original values. Precipitation occurred due to formation of low soluble double sulfate salts between light REEs and sodium, present in solution from the addition of sodium phosphate (Equation (6)). 2 REE3þ (aq) þ 2 Naþ (aq) þ 4 SO2 4 SO4)3$Na2SO4$nH2O (s)

(aq)

þ n H2O

(aq)/

REE2((6)

It was possible to increase the pH of the Covas solutions above 2 without precipitation and the resulting solutions were stable in time. For New Kankberg, precipitation become an issue above pH 0.5e1. This was most preeminent in sulfuric acid media due to formation of double sulfate salts. 4.2.1. Screening of solvating extractants Cyanex 923, a commercially available mixture of four trialkylphosphine oxides (Dziwinski and Szymanowski, 1998), and TODGA were screened. Cyanex 923 extracts iron but this is significantly slower compared to extraction of REEs. A past study (Tunsu et al., 2014), using a different aqueous system, showed that extraction of REEs from nitrated media reached equilibrium in less than 1 min. Iron was extracted over much longer time, with lower distribution ratios in the beginning. A short phase contact time can achieve partial separation of REEs from iron. In addition, selective stripping is possible. Based on these observations, extraction with Cyanex 923 was done here for 1 min. Two Cyanex 923 solutions were tested: 0.59 mol/L (approx. 25 %vol.) and 1.18 mol/L (50 %vol.). The distribution ratios of elements are given in Fig. 2. Satisfactory extraction of REEs was only achieved from nitric acid media. Iron was not significantly extracted due to the short phase contact time. Phosphate ions showed noticeable extraction from hydrochloric and sulfuric acid but less from nitric acid. This may be due to the ability of Cyanex 923 to extract protons (Alguacil and pez, 1996; Ansari et al., 2004), according to Equation (7). Lo Competing effects between REEs and phosphate are likely responsible for the lower extraction of the latter from nitric acid. It is possible to extract phosphate with Cyanex 923, of importance as this is currently a critical raw material (European Commission, 2017). x Hþ

(aq)

þ x X

(aq)

þyB

(org)

4 yB∙xHX

(org)

(7)

where X is an anion in solution (phosphate or other) and B the extractant. As expected, the distribution ratios of REEs increased with increasing atomic number, a consequence of the lanthanide contraction (Peppard et al., 1969). Extraction selectivity for individual REEs was, however, not achieved. A large number of separation/stripping stages will be needed to separate adjacent REEs from each other. Increasing the extractant concentration led to increased extraction, also expected, as the equilibrium in Equation (1) is pushed towards formation of extractable complexes. Stripping of Cyanex 923 after extraction from nitric acid media was effectively carried out with acidic solutions (Fig. 3). Hydrochloric acid can be used to selectively strip REEs from co-extracted iron, as detailed in (Tunsu et al., 2014). Similar to Cyanex 923, TODGA showed very good extraction of REEs from nitric acid. Heavy REEs (dysprosium) were also extracted from hydrochloric acid, notably from Covas model solution (Fig. 4). Extraction from sulfuric acid was not satisfactory. A clear advantage of TODGA is the extraction selectivity over iron, copper and phosphorus, regardless of acid media. To conclude, extraction of REEs with solvating extractants was most effective from nitric acid. The REEs were well extracted even in the presence of over 9 g/L phosphate. Generally, extraction of REEs from the acids tested decreased in the order: nitric acid, hydrochloric acid, sulfuric acid. The last gave unsatisfactory extraction regardless

Fig. 2. Distribution ratios of elements with 0.59 mol/L and 1.18 mol/L Cyanex 923 in kerosene. Extraction conditions: O:A 1:1, 1 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a single sample replicate. Distribution ratios calculated to be below 0.01 or above 100 are reported as 0.01 and 100, respectively. Distribution ratios calculated to be between 0.01 and 0.05 are reported as 0.05.

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Fig. 3. Distribution ratios for stripping of Cyanex 923 previously contacted with New Kankberg and Covas nitric acid feeds (see Fig. 2). Stripping conditions: 1.6 mol/L hydrochloric acid or 4 mol/L nitric acid, O:A 1:1, 30 min phase contact time, 1500 vpm and 21  C. Calculations using mass balance for a single sample replicate. Distribution ratios calculated to be above 100 are reported as 100.

of feed. Extraction of acid is possible (Equation (7)). Cyanex 923 e nitric acid complexes of the type 2:1, 1:1 and 1:2 can form, depending on the initial extractant and acid concentrations (Ansari et al., 2004). Extraction of acid leads to an increase in aqueous pH, which can affect the stability of the system, e.g., the aforementioned precipitation for New Kankberg sulfuric acid solution. This, however, did not occur under the conditions tested here. 4.2.2. Screening of acidic extractants Bis-2-ethylhexyl phosphoric acid (DEHPA) and Cyanex 572 were screened. Two tests were carried out: one with the original aqueous feeds and one at slightly higher pH, where these solutions were partly neutralized with sodium hydroxide. The distribution ratios with DEHPA are presented in Fig. 5. Dysprosium was effectively extracted and could be separated from lighter REEs without any pH increase. This is advantageous, as heavier REEs are much desired and are more expensive than lighter ones. In all cases, iron was extracted with the REEs. Copper and phosphate extraction was not significant, regardless of feed or pH tested. Selective stripping of REEs from iron was possible with nitric acid or diluted hydrochloric acid, and this will be discussed later. The extraction of REEs from the acids tested decreased in the order: nitric acid, hydrochloric acid, sulfuric acid. In contrast with the solvating extractants tested, DEHPA extracted REEs from sulfuric acid. Due to economic considerations (sulfuric acid is plentiful and cheap, and DEHPA is commercially available), this system was further investigated. This will be discussed in the next section. The distribution ratios with Cyanex 572 are presented in Fig. 6. Compared to DEHPA, extraction of REEs was not as effective. Iron was extracted in all cases. A 1.6 mol/L hydrochloric solution didn't manage to selectively strip REEs from iron. Nitric acid was, however, able to selectively recover the REEs. It was concluded that acidic extractants are suitable for extraction of REEs from sulfuric acid, where solvating reagents failed. DEHPA was preferred due to better distribution of REEs at low pH (stability of the New Kankberg sulfate solution at high pH was a concern). Selective stripping of REEs with hydrochloric acid was possible. In contrast with solvating reagents, extraction of phosphate was not evident; this allows for higher product purity. Coextraction of iron is a main concern, as this will also affect the loading capacity of the organic phase. The Covas sulfuric acid e DEHPA system was further investigated to get a better picture of the extraction process and to find effective ways to mitigate coextraction of iron. 4.2.3. Extraction of REEs from covas sulfuric acid model solution with acidic extractants 4.2.3.1. pH dependency. The Covas sulfuric acid solution was partly neutralized with solid sodium hydroxide. This was preferred over

431

sodium hydroxide solution to prevent significant volume changes, which would affect the metal concentration. This allowed preparation of aqueous feeds with similar metal content and different pH. The effect of equilibrium pH on the extraction with 1 mol/L DEHPA is presented in Fig. 7. Iron was well extracted and its distribution increased with increasing pH. Copper was not significantly extracted across the pH range investigated. Dysprosium could be selectively recovered from the other REEs at pH 0, which did not require any addition of sodium hydroxide. At this pH, iron was extracted. Further recovery of the other REEs required an equilibrium pH above 1. Neodymium and cerium were extracted first, followed by lanthanum at higher pH. The results showed good selectivity between heavy and light REEs but not much between cerium and neodymium. Extraction with 1 mol/L Cyanex 572 was investigated in a similar manner. During extraction, all aqueous feeds with an initial pH above 1.3 led to formation of third phases and had to be discarded. No third phase modifiers were used in this study. Extraction of REEs from the aqueous phases with initial pH lower than 1.3 was very poor. Only minor amounts of dysprosium were extracted as the pH increased. Iron was the only metal ion which was effectively extracted. Between 25 and 65% of the iron was extracted across the pH range in which third phase didn't occur. Extraction with Cyanex 572 was not investigated further.

4.2.3.2. The extraction behaviour of metal ions in time in the DEHPA system. To study the extraction behaviour of metal ions in time, the Covas sulfate feed was partly neutralized with solid sodium hydroxide to pH 1.65. Aliquots of this solution were contacted with 1 mol/L DEHPA for 1e60 min. The results are presented in Fig. 8. Extraction of iron occurred slowly and did do reach equilibrium during the investigated time. About 90% of the iron was extracted in 1 h. The pH in the aqueous phase changes during the slow uptake of iron due to the liberation of protons by the extractant (similar to Equation (2)) and this affects the distribution of REEs in time (Fig. 7). Initially, due to higher pH, the light REEs were also extracted. As iron continued extracting (continuously lowering the aqueous pH), these REEs were back-extracted in the aqueous phase. Dysprosium was the only REEs not significantly affected. This is explained by its effective extraction, even at low pH. Nonetheless, minor amounts of dysprosium were back-extracted as extraction of iron tended towards equilibrium. Iron co-extraction can be reduced by maintaining a short phase contact time. Similar behaviour was also reported for centrifugal extraction of REEs from wet-process phosphoric acid (Wang et al., 2011). The non-equilibrium extraction is, in this case, a simple option to increase the REEs/iron separation factors. This also mitigates the pH decrease, allowing better extraction of lighter REEs.

4.2.3.3. Stripping of metal ions extracted with DEHPA. Stripping of 1 mol/L DEHPA previously contacted with Covas sulfuric acid solution (pH after extraction 1.15) was done with 0.5 mol/ L citric acid, and with 1e6 mol/L hydrochloric and nitric acid (Fig. 9). Light REEs were easiest to strip, as they form weaker complexes due to their larger ionic radius. Citric acid performed very poorly. Over 75% of the cerium and 85% of the neodymium were recovered in one step with the 1 mol/L acidic solutions. Satisfactory recovery of dysprosium required increased acidity. There weren't significant differences between the performance of the two mineral acids. Iron was not striped in significant amounts by nitric acid. The same was observed for diluted hydrochloric acid. Minor stripping of copper was observed. The amount of copper initially extracted by DEHPA was, however, low, about 1.5%.

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Fig. 4. Distribution ratios of elements with 0.1 mol/L TODGA. Extraction conditions: O:A 1:1, 45 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a single sample replicate. Distribution ratios calculated to be below 0.01 or above 100 are reported as 0.01 and 100, respectively. Distribution ratios calculated to be between 0.01 and 0.05 are reported as 0.05.

4.2.3.4. Minimizing co-extraction of iron by blending DEHPA with other extractants. Prior investigations showed that co-extraction of iron with DEHPA can be minimized by maintaining a short contact time between the aqueous and the organic phases. Nonetheless, some iron will be extracted. For high amounts of iron in solution, as is the case for the tailings investigated here, a small extractable fraction can still be problematic for separating mg/L amounts of REEs with high purity. Although selective stripping of REEs is possible, minimizing iron co-extraction simplifies the separation process, allows for higher purity of REEs and produces less secondary wastes. The premise tested here was that extraction of iron could be reduced by mixing DEHPA with Cyanex 923 and/or Aliquat 336. The distribution ratios for several such systems are given in Fig. 10. Here, the mixing time was set to 2.5 min to extract some of the iron and be able to compare the efficiency of different organic blends. Halving the concentration of DEHPA led to an expected decrease in the extraction efficiency of all metal ions. Addition of Aliquat 336 significantly supressed extraction. Dysprosium was still extracted but its distribution ratios were much lower compared to DEHPA alone. By using organic phases consisting of DEHPA, Cyanex 923 and kerosene only, iron co-extraction was reduced. At the same time, the distribution of REEs increased. This positive outcome led to further investigations of the DEHPA e Cyanex 923 system. Several compositions were tested (Table 2) to see how much Cyanex 923 is needed to supress co-extraction of iron without significantly affecting the distribution of REEs. The mixing time was reduced to 1 min to further minimize co-extraction of iron. The results are presented in Fig. 11. The distribution ratios of iron decreased with increasing the concentration of Cyanex 923 in the organic phase. Increasing the concentration of Cyanex 923 past 0.235 mol/L (10 %vol.) also led to decreased distribution of all REEs. It was previously noted that Cyanex 923 alone did not extract REEs from sulfuric acid media (Fig. 2). Extraction was further studied with 1 mol/L DEHPA þ0.235 mol/ L Cyanex 923 at O:A ratios in the range 4:1e1:4 (Fig. 12). Based on the observations, it was decided to select an O:A ratio of 1:3 for a mixer-settler separation trial. Co-extraction of iron was minimal at this phase ratio, while extraction of REEs was satisfactory. Using less organic phase is beneficial, as it leads less consumption and losses of chemicals, and less secondary wastes. This is due to higher metal load in the organic phase compared to extraction at higher O:A ratio, and lower volumes of stripping solution needed. 4.2.3.5. Extraction in mixer-settlers. Extraction of REEs from Covas

sulfuric acid model solutions was studied in a counter-current mixer-settler system comprising five extraction stages. The organic phase consisted of a mix of 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 in kerosene. Two aqueous feeds were tested, one with an initial pH of 1.19 and one with a pH of 1.95. The extraction behaviour of iron and REEs is shown in Fig. 13. Extraction of dysprosium from both feeds was very effective. Three extraction stages were sufficient to recover most of the dysprosium. At lower pH (Fig. 13a), extraction of the other REEs was not significant in the first three stages but started to become noticeable afterwards. Co-extraction of iron was around 8% in the first three stages and reached 18% in the last stage. Upon increasing the pH of the starting solution to 1.95, extraction of all REEs improved (Fig. 13b). Less iron was co-extracted here, likely due to competition with the REEs. The extraction efficiency increased in the order: lanthanum, cerium, neodymium, dysprosium. The last one was effectively recovered in the first two stages. At the same time, co-extraction of the other REEs was more preeminent compared to the first feed. A complete separation of cerium and neodymium will require a large number of extraction and stripping stages. Their extraction occurred concomitantly and the differences in extraction percentages at a given pH were not significant to allow good selectivity. Lanthanum, on the other hand, was less extracted and its complete separation will require a lower number of extraction/stripping stages. 4.2.4. Potential processing of New Kankberg and Covas leachates The most desirable REE in the New Kankberg and Covas feeds is dysprosium but this is present in lowest amounts. Nevertheless, although lanthanum and cerium have lower market value, they are now regarded as the most critical REEs for the European Union (European Commission, 2017). These are used together with neodymium as mischmetal alloy in nickel metal hydride (NiMH) rechargeable batteries. These have important applications in the automotive industry, specifically hybrid vehicles and buses. The pH of the aqueous feed needs to be above 1 to get satisfactory extraction of lighter REEs with DEHPA. This was not possible for the New Kankberg sulfuric acid model solution due to precipitation. For this reason, separation was studied in mixer-settlers with Covas solution. For the New Kankberg sulfuric acid model solution, which contained significantly higher amounts of REEs, no copper and no iron, precipitation at higher pH is an option. This did not significantly affect the solubility of dysprosium and phosphate. The light REEs can be recovered as a group and used for mischmetal production after subsequent treatment. Cerium, lanthanum and neodymium in the New Kankberg sulfate solution were easily

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Fig. 5. Distribution ratios of elements with 1 mol/L DEHPA. Extraction conditions: O:A 1:1, 30 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a single sample replicate. Distribution ratios calculated to be below 0.01 or above 100 are reported as 0.01 and 100, respectively. Distribution ratios calculated to be between 0.01 and 0.05 are reported as 0.05. Data for the New Kankberg sulfuric acid solution at higher pH was omitted due to precipitation.

433

Fig. 6. Distribution ratios of elements upon extraction with 1 mol/L Cyanex 572. Extraction conditions: O:A 1:1, 30 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a single sample replicate. Distribution ratios calculated to be below 0.01 or above 100 are reported as 0.01 and 100, respectively. Distribution ratios calculated to be between 0.01 and 0.05 are reported as 0.05. Data for the New Kankberg nitric and sulfuric acid solutions at higher pH was omitted due to precipitation.

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Fig. 7. Extraction as a function of pH in the Covas sulfuric acid e 1 mol/L DEHPA system. Extraction conditions: O:A 1:1, 30 min phase contact time, 1500 vpm, 21  C. Calculations using mass balance for a single sample replicate.

Fig. 9. Striping of metal ions after extraction of metal ions from Covas sulfuric acid model solution (initial pH 1.65) with 1 mol/L DEHPA. Striping conditions: O:A 1:1, 30 min phase contact time, 1500 vpm, 21  C. Copper data for the 6 mol/L systems not available. Calculations using mass balance for a triplicate test.

Fig. 8. Extraction of metal ions as a function of time for the Covas sulfuric acid e 1 mol/L DEHPA system. Extraction conditions: initial aqueous pH 1.65, O:A 1:1, 1500 vpm, 21  C. Calculations using mass balance for a triplicate test.

recovered as double sulfate salts at pH 1 upon adding sodium hydroxide. For Covas, dysprosium can be extracted at low pH with mixtures of DEHPA and Cyanex 923; selective stripping from any coextracted iron is easy. The organic and stripping solutions can be run in a loop so that dysprosium is concentrated in a small volume. This REE can be subsequently precipitated as oxalate (Equation (8)) and afterwards converted to oxide via thermal treatment (Equation (9)). 2 REE3þ(aq) þ 3 C2O2 4 (aq) / REE2(C2O4)3 2 REE2(C2O4)3

(s)

þ 3 O2

(g)

/ 2 REE2O3

(8)

(s)

(s)

þ 12 CO2

(g)

(9)

The Covas model solution after solvent extraction of dysprosium contains copper, most of the iron, and the other three REEs. Copper

Fig. 10. Distribution ratios of elements with various blended organic phases. Extraction conditions: initial aqueous pH 2, O:A 1:1, 2.5 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a triplicate test. Distribution ratios calculated to be below 0.01 or above 100 are reported as 0.01 and 100, respectively. Distribution ratios calculated to be between 0.01 and 0.05 are reported as 0.05.

can be selectively extracted with copper-specific reagents, e.g., commercial LIX or Acorga extractants (Szymanowski, 1993). The

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Fig. 11. Extraction of metal ions from Covas sulfuric acid model solution with mixed DEHPA e Cyanex 923. Extraction conditions: initial aqueous pH 2, O:A 1:1, 1 min phase contact time, 1500 vpm, 21  C. Calculations using mass balance for a triplicate test.

435

Fig. 14. Potential options to process Covas-type sulfuric acid feed.

mentioned above, is presented in Fig. 14. A combination of lower O:A ratio for extraction and higher O:A ratio for stripping can be used to overcome for the lower concentration of REEs in the feed. This can be coupled with re-circulation of the feeds to further enrich the REE content in solution and minimize the amounts of iron. We tested stripping of metals from a mixed 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 organic solution. Stripping was done with 2 mol/L nitric acid solution at O:A ratio 10:1. In one stage, 40% of the extracted lanthanum, 20% of the cerium, 15% of the neodymium and 1% of the dysprosium were back-extracted. The stripping efficiency of iron was low, 0.1% of the extracted amount. Stripping of the organic after the mixer-settler experiment (the aqueous feed with initial pH 1.95) with 2 mol/L nitric acid solution at O:A 10:1 gave a solution containing 200 mg/L REEs and 3 mg/L iron. Fig. 12. Extraction percentages of metal ions with 1 mol/L DEHPA þ0.235 mol/L Cyanex 923 at different O:A ratios. Aqueous phase: Covas sulfuric acid solution partly neutralized to pH 1.2. Extraction conditions: 1 min phase contact time at 1500 vpm, 21  C. Calculations using mass balance for a single test.

Fig. 13. Extraction of metal ions from Covas sulfuric acid model solutions with a mix of 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 in kerosene. Extraction conditions: O:A 1:3, 1000 rpm mixing speed, 21  C, 2 mL/min organic, 6 mL/min aqueous, initial aqueous pH: 1.19 (a) and 1.95 (b). Calculations using mass balance.

remaining REEs can be recovered at higher pH using solvent extraction. Given the lower concentration of REEs in the feed, it may be practical to recover lanthanum, cerium and neodymium as a group for mischmetal production. This avoids the numerous solvent extraction and stripping stages needed to achieve individual separation. Alternatively, selective precipitation is an option; there is a lot of research on this topic due to the high interest in recovering REEs from REE-iron permanent magnet leachates (Ferron and Henry, 2015; Tunsu et al., 2018). A flowsheet to process Covas-type sulfuric acid leachate, which comprises the steps

5. Conclusions Tailings from past and current mining activities can be potential sources of critical raw materials. In the ENVIREE Project, various European and South African sites were screened to find suitable materials for urban mining. Samples from New Kankberg (Sweden) and Covas (Portugal) contained not only REEs but also other critical elements such as phosphate and tungsten. The former had the highest content of REEs among the investigated European sites, over 10 g/kg after beneficiation. The contents of tungsten in the latter reached 24 g/kg after beneficiation with gravimetric techniques and magnetic separation. Using gravity separation, the REEs in Covas tailings were concentrated 5.5 times (recovery above 70%) and tungsten was concentrated 2.5 times (50% recovery). This can be easily implemented on-site and requires mainly water and power, which translates into very low CAPEX and OPEX. Tungsten can be selectively leached from the contained REEs using basic solutions, followed by acidic leaching of REEs. Nonetheless, the high amounts of iron, phosphate and copper in such tailings can be problematic in the hydrometallurgical recovery of REEs. They are leached alongside REEs and can negatively affect the effectivity of subsequent solvent extraction processing. This study assesses how solvent extraction separation of REEs from simple model leachates is affected by high amounts of phosphate, iron and copper ions. We wanted to see how extraction of REEs with solvating (Cyanex 923, TODGA) and acidic extractants (DEHPA and Cyanex 572) is affected for various common mineral acids. The goal was to find mitigation paths for eventual hindrances during separation of REEs. In general, extraction of REEs from different acids decreased in the order: nitric acid, hydrochloric acid, sulfuric acid. Solvating extractants gave satisfactory results in nitric

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acid media but showed limited potential in other acids. Acidic extractants were able to extract REEs from all acids, a notable economic advantage for sulfuric acid leachates. Selectivity was significantly better with acidic extractants, which allowed for easier recovery of individual REEs. Extraction of REEs was not hindered by the high amounts of phosphate in solution. Phosphate extraction was possible with Cyanex 923. Copper didn't interfere with the recovery of REEs. Iron, on the other hand, was extracted together with the REEs in most systems, negatively affecting selectivity. Extraction of iron occurred much slower in time. A low phase contact time is recommended as first mitigation method, e.g., extraction at non-equilibrium conditions. Aside from pH control, co-extraction of iron with DEHPA can be reduced by blending the extractant with Cyanex 923. A mixed 1 mol/L DEHPA and 0.235 mol/L Cyanex 923 solution in kerosene notably reduced the amounts of iron extracted and led to increased extraction efficiency of all REEs. Selective striping of REEs from iron was possible with nitric acid or diluted hydrochloric acid. Extraction of REEs from Covas sulfuric acid model leachate was tested with the aforementioned organic phase in a counter-current mixer-settler system comprising 5 separation stages. It was possible to completely and selectively recover dysprosium from the other REEs at low pH. Further recovery of neodymium, cerium and lanthanum was done at higher pH. While lanthanum can be isolated from cerium and neodymium, individual separation of these two REEs is more difficult. They can be recovered easier as a group and used for production of mischmetal. Acknowledgements ERA-MIN and VINNOVA, Sweden, are acknowledged for financial support (project 2014-06227). ADEME, France, is also acknowledge for financial support (decision 1402C0142). Boliden AB, Sweden, is acknowledged for supplying samples. The authors would like to thank Nils-Johan Bolin for his input. References pez, F.A., 1996. The extraction of mineral acids by the phosphine Alguacil, F.J., Lo oxide Cyanex 923. Hydrometallurgy 42 (2), 245e255. Ansari, S.A., Murali, M.S., Pathak, P.N., Manchanda, V.K., 2004. Separation of iron from cobalt in nitrate medium using Cyanex-923 as extractant. J. Radioanal. Nucl. Chem. 262 (2), 469e472. Apichaibukol, A., Sasaki, Y., Morita, Y., 2004. Effect of DTPA on the extractions of actinides(III) and lanthanides(III) from nitrate solution into todga/n-dodecane. Solvent Extr. Ion Exch. 22 (6), 997e1011. Binnemans, K., Jones, P.T., Blanpain, B., Van Gerven, T., Yang, Y.X., Walton, A., et al., 2013. Recycling of rare earths: a critical review. J. Clean. Prod. 51, 1e22. Binnemans, K., Pontikes, Y., Jones, P.T., Van Gerven, T., Blanpain, B., 2013. Recovery of rare earths from industrial waste residues: a concise review. In: Proceedings of the 3rd International Slag Valorisation Symposium: the Transition to Sustainable Materials Management, pp. 191e205. Burlakovs, J., Jani, Y., Kriipsalu, M., Vincevica-Gaile, Z., Kaczala, F., Celma, G., et al., 2018. On the way to ‘zero waste’ management: recovery potential of elements, including rare earth elements, from fine fraction of waste. J. Clean. Prod. 186, 81e90. Dziwinski, E., Szymanowski, J., 1998. COMPOSITION OF CYANEX® 923, CYANEX® 925, CYANEX® 921 AND TOPO. Solvent Extr. Ion Exch. 16 (6), 1515e1525. European Commission, 2011. Tackling the Challenges in Commodity Markets and on Raw Materials. European Commission, 2017. Study on the review of the list of critical raw materials. In: Criticality Assessments. Ferron, C.J., Henry, P., 2015. A review of the recycling of rare earth metals. Can. Metall. Q. 54 (4), 388e394. Frohlich, P., Lorenz, T., Martin, G., Brett, B., Bertau, M., 2017. Valuable metalsrecovery processes, current trends, and recycling strategies. Angew. Chem. Int. Ed. 56 (10), 2544e2580. Gupta, B., Malik, P., Deep, A., 2003. Solvent extraction and separation of tervalent lanthanides and yttrium using Cyanex 923. Solvent Extr. Ion Exch. 21 (2), 239e258. Guyonnet, D., Planchon, M., Rollat, A., Escalon, V., Tuduri, J., Charles, N., et al., 2015. Material flow analysis applied to rare earth elements in Europe. J. Clean. Prod. 107, 215e228.

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