Multiscale Flotation Testing for the Recovery of REE-Bearing Fluorapatite from a Finnish Carbonatite Complex Deposit Using Conventional Collectors and Lignin Nanoparticles

Abstract

Apatite and rare earth elements (REEs) are vital to the European Union’s economic growth and resource security, given their essential roles in fertilizers, green technologies, and high-tech applications. To meet rising demand and reduce reliance on imports, the exploitation of domestic deposits has become increasingly important. This study investigates the beneficiation potential of ore from a carbonatite complex (Finland), focusing on the recovery of fluorapatite concentrate through froth flotation. This research addresses two key objectives: evaluating the potential for REE enrichment alongside fluorapatite concentration using conventional anionic and amine-based reagents, and assessing separation efficiency when partially substituting the most effective conventional collectors with bio-based organosolv lignin nanoparticles. Adequate recovery rates for apatite and REEs were achieved using common anionic collectors, such as hydroxamate and sarcosine, yielding P grades of 23.4% and 21.5%, and recoveries of 96.4% and 89.2%, respectively. Importantly, concentrate quality remained stable with up to a 30% reduction in conventional collectors and the addition of organosolv lignin. Bench-scale trials further validated the approach, demonstrating that lanthanum and cerium recoveries exceeded 71%, alongside satisfactory apatite recovery. Lignin nanoparticles were observed to interact with both minerals; however, the interaction was more pronounced in the case of phlogopite, which exhibited a markedly greater increase in surface hydrophilicity following treatment, suggesting a stronger affinity or surface modification effect, which was beneficial to the performance of the separation process.

1. Introduction

Rare earth elements (REEs) are considered valuable minerals that have a variety of applications in different industries such as tech, automotive, and industrial etch [1]. They consist of the 15 lanthanide series elements (La, Pr, Ce, Eu, Pm, Ho, Nd, Gd, Yb, Dy, Sm, Er, Tm, Tb, and Lu) with the addition of Sc and Y [2]. With an increasing demand for REEs for the clean energy market and advanced materials, alternative sources of minerals should be implemented. As of 2023, China, with an annual production of around 240,000 metric tons, holds of 68.57% of the annual production share of REEs, followed by the US at 12.29% and Myanmar at 20.86%, while Europe does not produce yet, despite the fact that REEs were recognized by the European Commission in 2017 as one of the vital raw materials (CRMs) to the European economy [3,4]. There are, however, few identified REE deposits in Europe, with those associated with Mesoproterozoic rift-related magmatism in Greenland and Sweden and with Neoproterozoic to Palaeozoic carbonatites across Greenland and the Fennoscandian Shield being the most important ones, but none of them are currently under exploitation [5,6].

Apatite, a group of phosphate minerals, can incorporate substantial amounts of REEs into its crystalline structure, and has been identified as an important host for rare earth elements and a secondary source of them [7,8,9,10]. This is due to enrichment of REE within apatite, which can be performed by two processes: the first is the substitution of REE³⁺ and Na⁺ for Ca²⁺ within the apatite, while the second is P⁵⁺ substituted with REE³⁺ and Si⁴⁺, with the end member of the second substitution being britholite, a mineral heavily enriched in REE [11,12]. As a consequence, the REE content can reach as high as 0.1% to 3.5%, as shown by Owens et al. in a list presenting REE-hosting apatite deposits [8].

The carbonatite complex, located in North Savo, Finland, is a typical case of an REE-enriched apatite deposit. The complex comprises intermixed carbonatite and glimmerite, with almost all rocks constituting phosphate ore with an apatite content of about ≈10% [13,14]. The mine production is 11 Mt/y, and ore reserves have been estimated to be 234 Mt at an average grade of 4 wt. %, and have been identified as a potential source of REE and F [13,14]. REE content in apatite has been estimated to be 0.3 wt. %–0.4 wt. %, while the fluorine content for apatite ranges from 2.3% to 3.5% [15,16].

Further to the primary sources of REEs, the wide range of uses and the rising global demand have led to exploration of alternative sources beyond regular mining, with recovery from industrial wastes and byproducts like acid mine drainage and mine tailings being considered viable secondary sources of REEs (Vaziri Hassas et al., 2020) (Patil et al., 2023) [17,18].

Being identified as an important source of REEs, various beneficiation studies deal with the production of a valuable concentrate from ore deposits or mining tailings [17]. For the production of bastnäsite (Ce,La(FCO₃)), monazite (Ce,La(PO₄)), and xenotime (YPO₄) concentrates, which are the REE-bearing minerals that have been extracted on a commercial scale, gravity, magnetic, electrostatic, and flotation separation are commonly applied, with the latter being the most significant one worldwide [17,18,19]. The most commonly used collectors for REE mineral flotation are either hydroxamic or fatty acids [18]. Initially, fatty (carboxylic) acid was the preferred collector for bastnasite flotation due to its wide availability and low price; however, considerable amounts of depressants are needed to achieve a high grade and recovery in the concentrate [20]. Regarding monazite and xenotime, which are found typically together in heavy metal sand deposits, the collectors that are used are similar to those for bastnasite (fatty acid and hydroxamate collectors), due to surface similarities they share [17]. On the other hand, apatite flotation is commonly performed using either fatty acids like oleic acid [21,22] and sodium oleate [23] or anionic surfactants like alkyl sulfate [24], alkyl sulfonate [25], and alkyl hydroxamates [26].

Lignin is a polymer structuring the cell walls of plants, exhibiting a complex structure with various functional groups, including phenolic hydroxyl, aliphatic hydroxyls, and carboxyl acids, while when extracted using organic solvents (organosolv), it results in a less chemically modified and more uniform product compared to other lignin types [27]. Recent studies have shown that organosolv lignin nanoparticles can be beneficial to flotation by interacting with mineral surfaces, probably through the many functional groups [28]. Other studies have shown that lignin is beneficial to flotation because it enhances the separation efficiency through depressing calcite in a scheelite–calcite system [29] and molybdenite in a molybdenite–chalcopyrite system [30]. The use of lignin in the flotation process is of particular interest because of the environmental benefits that arise from the fact that it is a biodegradable, natural, and renewable biopolymer of low toxicity, as well as its abundance and cost-efficiency.

The potential use of organosolv lignin micro- and nanoparticles has been investigated by Hruzova et al. in a Cu-Ni sulfide ore (Maurliden, Sweden) and a complex Cu-Pb-Zn ore (Kristineberg, Sweden), leading to improvement in the Zn grade and recovery, Ni grade, and Pb recovery [31]. In another study dealing with Kupfershiefer copper ore (Poland), it was shown that lignin nanoparticles can replace maltodextrin in final concentrate selective flotation, where they can provide the selective separation of copper and total organic carbon [32]. Angelopoulos et al. use organosolv lignin nanoparticles to partially replace sodium isopropyl xanthate (SIPX) in the flotation of sphalerite and pyrite/arsenopyrite from mixed sulfide ore, achieving a higher sphalerite grade and higher pyrite/arsenopyrite grade and recovery by 50% replacement of SIPX by lignin [33]. Recently, Bazar et al. investigated the flotation of iron oxide apatite ore tailings using a combination of a tall oil fatty acid-based collector (TOFA) and organosolv lignin nanoparticles, focusing on the identification of synergy [34]. The study highlights a synergistic effect between OLP and the TOFA collector, suggesting that lignin might interact with TOFA, either by enhancing its adsorption onto apatite or by surface modification, leading to a higher P₂O₅ grade and recovery.

This article presents an investigation on the beneficiation potential of ore originating from a Finnish carbonatite complex, targeting the recovery of fluorapatite concentrate through froth flotation tests with a dual aim: the identification of the possibility to achieve enrichment of REEs in parallel to the concentration of fluorapatite by evaluating the performance of different conventional reagents in this process, but also the determination of the separation performance under the reduction in best-performance collectors and the addition of lignin, on application levels extending from lab to bench scale.

2. Materials and Methods

2.1. Ore Characterization

Ore originates carbonatite ore deposits, located in central Finland. It is an open-pit deposit rich in fluorapatite, calcite, and phlogopite but also in REE, which is attributed to the re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in REEs [14]. The chemical composition of the feed and flotation products was analyzed via X-ray fluorescence. The sample was homogenized and dried overnight at 105 °C, and subsequently subjected to chemical analysis on an Energy-Dispersive X-Ray Fluorescence instrument Xepos (SPECTRO A.I. GmbH, Enschede, The Netherlands). For REE analysis determination, sodium peroxide plus sodium hydroxide digestion was used and measurement was performed with the ICP-MS technique. Quantitative mineralogical studies were carried out by using a mineral liberation analyzer (MLA) and an electron probe micro analyzer (EPMA). The MLA equipment consists of the standard modern SEM (Quanta 600, formerly FEI, now Thermo Fisher Scientific Inc., Waltham, MA, USA) with an energy-dispersive X-ray analyzer (EDAX 72, Mahwah, NJ, USA). The XMOD-STD and XBSE methods were used for analyzing the modal mineralogy and the mineral liberation, respectively. The EPMA was used for determining the chemical compositions of apatite in the sample.

As for the SEM images, a thin polished cut of raw material was prepared by impregnating approx. 1 gr of dry matter with epoxy resin in a cylindric mold, and subsequent cutting and surface smoothing for morphological observation on a SEM (JSM-IT500LV, JEOL, Tokyo, Japan) under accelerating voltage 20 kV, probe current 1.5 nA, and working distance 12 mm. Local element identification was performed by an energy-dispersive sensor type Ultim Max 100 (Oxford Instruments, Abingdon, UK).

2.2. Lignin Production and Properties

The lignin used for preparation of lignin nanoparticles was extracted by organosolv pretreatment of spruce wood chips [35]. The spruce wood chips were pretreated in 60% v/v ethanol in water solution at 183 °C for 1 h. The extracted lignin was dried and dissolved in 75% v/v ethanol/water solution. Subsequently, the 5% w/v lignin in the ethanol/water solution was homogenized at 750 bar by using a pressure homogenizer (APV-2000, SPX FLOW, Charlotte, NC, USA). The homogenized liquid was further diluted by deionized water, which led to the formation of nanoparticles. To obtain nanoparticles as a dry powder, the sample was freeze-dried. The preparation process yielded nanoparticles that were smaller than 500 nm [36].

The morphology of the lignin nanoparticles was observed by SEM (FEI Magellan 400 field emission XHR-SEM, formerly FEI, now Thermo Fisher Scientific Inc., MA, USA). The samples were placed on conductive carbon tapes prior to the analysis and the images were taken at a low accelerating voltage of 3 kV and a beam current of 6.3 pA. For the purpose of the flotation trails, the dry powder of nanoparticles was dispersed in deionized water at a concentration of 1% w/v and sonicated by ultrasound for 5 min. Subsequently, the sample was mixed before use to prevent sedimentation and increase the homogeneity of the dispersion in the flotation trials. Macroscopic and microscopic views of the produced lignin nanoparticles are depicted in Figure 1a,b, respectively.

2.3. Flotation Tests

The research approach that is followed in this study consists of 3 consecutive steps, depicted in Figure 2; after characterization of the raw material and size control, flotation experiments were executed at the lab scale using conventional reagents, aiming to identify their efficiency in the concentration of both apatite and REEs of interest, namely cerium, lanthanum, and yttrium. After evaluation of the results, two flotation experiments with best-performance reagents were carried out again, applying partial substitution with OLN to different degrees. Finally, process upscaling was carried out in a 13 L flotation cell, where flotation scenarios of a sole conventional and mixed collector with lignin were run and their performance was compared and discussed. This experimental design is favorable as it allows a systematic, stepwise evaluation of flotation reagents, starting with lab-scale tests to establish the baseline performance of conventional reagents, while partial replacement of top-performing reagents with green alternatives is focused on balancing efficacy and sustainability. This progressive design minimizes risk, reduces material usage in initial trials, and ensures resource efficiency as the process moves toward full-scale application.

As for the collectors used in the flotation trials, their trade name, formula, and structure are presented in

Table 1. Trade name, formula, and structure of the conventional collectors used [39].

Trade Name and Formula	Molecular Structure of the Functional Group
Aero 6494© (Syensqo SA, Brussels, Belgium) anionic, alkyl hydroxamate-based collector
Sodium Oleate (NaOL) (Merck KGaA, Darmstadt, Germany), anionic, sodium salt of oleic acid
Berol A3© (Nouryon, Amsterdam, Netherlands), sarcosine, a carboxylic acid coupled to a methylated nitrogen

, and the applied flotation conditions for each trial are tabulated in

Table 2. Lab-scale flotation tests’ reagents and conditions.

Number	Reagents (g/t)						pH	Time (m)		Cell Size (L)	Pulp Density (g/L)
	Hydroxamate (Aero 6494®)	Long-Chain Fatty Acid (NaOL)	Sarcosine (Berol A3®)	Organosolv Nanosized Lignin	Replacement **	Na2SiO3		Conditioning	Flotation
1 *	250					800	10	5	9	2.5	300
2	250					800	10	5	9	2.5	300
3		300					10.5	5	6	2.5	300
4		350				1400	10.5	5	9	2.5	300
5			300				11	5	9	2.5	300
6	150					800	10	5	12	2.5	300
7	300					800	10	5	9	2.5	300
8			240	60	20		11	5	9	2.5	300
9			210	90	30		11	5	9	2.5	300
10	210			90	30	800	10	5	9	2.5	300
11			180	120	40		11	5	9	2.5	300
12	200			200	50	800	10	5	9	2.5	300

* Heated at 55 °C, ** % replacement of conventional reagent.

. Their selection is based on relevant studies found in the bibliography regarding the flotation of REE-hosting apatite [8,18,26,37,38]. Lab-scale flotation trials were performed in triplicate.

The flotation experiments were carried out under an alkaline pH range between 10 and 11. The collector dosage was between 150 and 300 g/t and in most trials Na₂SiO₃ was used as a depressant of phlogopite. Nanosized lignin partially replaced conventional reagents in tests 8–12 without affecting other flotation conditions.

The steps of the conditioning and flotation procedure are depicted in Figure 3.

The potential association between minerals and reagents was investigated through Fourier-transform infrared spectroscopy (FTIR) with the transmission KBr pellet technique on a PerkinElmer Spectrum 100 FT-IR device (PerlinElmer U.S. LLC., Shelton, CT, USA). Hydrophobicity/hydrophilicity of pure minerals was evaluated using a Rame Hart contact angle goniometer Model 210 (Ramé-hart instrument Co, Succasunna, NJ, USA). Pure fluorapatite and phlogopite crystals were subjected to fine polishing using Al abrasive paper to create flat surfaces. Subsequently, the minerals were treated with different reagents, and the contact angle of distilled water droplets was measured fivefold with the goniometer.

3. Results and Discussion

3.1. Feed Properties

The chemical composition of the sample is presented in

Table 3. Chemical analysis of the feed.

Oxide	Content (wt. %)
SiO2	32.4
MgO	17.90
CaO	17.17
Al2O3	7.52
FeO	6.88
K2O	6.51
P2O5	3.75
Na2O3	0.40
C	3.04
La	0.01
Ce	0.019
Y	0.002

, while the mineralogical one is shown in

Table 4. Mineralogical composition of the ore.

Mineral	Content (wt. %)
K-feldspar	2.28
Phlogopite (KMg3(AlSi3O10)(OH)2)	57.37
Biotite (K(Mg,Fe)3(AlSi3)O10(OH)2)	3.18
Apatite (Ca5(PO4)3(OH, F, Cl)	8.87
Calcite (CaCO3)	16.68
Dolomite (CaMg(CO3)2	2.45
Magnetite (Fe3O4)	0.74

, as follows from the mineral liberation analysis. The sample consists of phlogopite in content exceeding 55%, followed by calcite and apatite at approx. 16.6% and 8.9%, respectively. The chemical analysis, which is in line with the mineralogical one, revealed the high content in the sample of silicon and magnesium, which are major components of phlogopite, calcium for calcite, and phosphorous for apatite. Such findings are confirmed by the micrographs presented in Figure 4 and the depicted local chemical analyses, which allowed the identification of the main mineralogical phases of the sample, namely phlogopite, apatite, magnetite, calcite, and biotite.

An XRD diagram of the feed is presented in Figure 5. The phases identified through XRD are phlogopite, fluorapatite, calcite, and dolomite, which present a high content in the sample. It has to be noted that despite the different chemical compositions, phlogopite and biotite present the same XRD peaks, making their discretization impossible through this method. Thus, the different analyses applied for the characterization of the sample are complementary, facilitating a deeper understanding of the sample composition, and not competitive.

Figure 6a depicts a sample micrograph with pseudo-color mapping of particles for easier distinguishing of the identified minerals, while Figure 6b shows grains classified according to the apatite liberation. The phlogopite phase dominates in the raw material, with apatite, calcite, and magnetite presence being considerable.

Focusing on the REEs, Figure 7a presents the grain size for apatite, as well as REE minerals, like parisite (Ca(Ce,La)₂(CO₃)3F₂), allanite ((Ce,Ca,Y,La)₂(Al,Fe⁺³)₃(SiO₄)₃(OH)), pyrochlorite ((Na,Ca)₂Nb₂O₆(OH,F)), monazite (Ce,La,Th)PO₄, zircon (ZrSiO₄), and ferrocolumbite (FeNb₂O₆). It has to be noted that the same phases have been identified by other researchers for this deposit [40]. By considering the size distribution of the liberated apatite grains, which present a D80 of 245 μm, and also that all other phases of interest present a lower D80 value, the grinding process was controlled accordingly. Thus, the raw material was grinded for 30 min in a laboratory rod mill under wet conditions, and the obtained material reached a D80 of 179 μm, and the size distribution is depicted in Table S1. Elutriation screening was applied for the determination of a −45 μm grade.

As for the association degree of apatite and REE minerals, presented in Figure 7b, a high degree of liberation was identified for allanite, pyrochlorite, apatite, and zircon, where the recognized free surfaces exceeded 80%. Parisite and monazite are equally associated with apatite, calcite, and biotite, while ferrocolumbite is solely associated with richterite.

The results of the mineralogical analysis of the sample are in line with other published studies that are focused on the geology of the carbonatite complex [14,40,41].

3.2. Lab-Scale Flotation with Conventional Collectors

Figure 8a presents the temporal evolution of P₂O₅ recovery in the concentrates obtained using different collectors, and Figure 8b the evolution of P₂O₅ grade and recovery. Among the presented flotation trials, higher flotation rates were achieved with hydroxamate that was dissolved in hot water (55 °C), while the phosphorus recovery reached almost 99% at 9 min of flotation. A sharp drop in P₂O₅ grade was identified during the flotation; while at the first stage the recovery reached almost 90% with a P₂O₅ grade of 30.6%, the grade dropped to 10% at the maximum recovery. Under ambient conditions and the same collector dosage, the flotation kinetics and the separation efficiency reduced significantly, with the recovery reaching 90.6% at a P₂O₅ grade of 12.6%.

Such findings are in line with several studies highlighting the beneficial effect of heating on the flotation rate, as well on the overall efficiency of hydroxamic collectors. This might be connected to a change in the solubility and activation energy of reagents, the partial dehydration of minerals’ surfaces and reagents’ molecules, and the removal of ultra fines from the surfaces of the mineral particles [42,43]. Undoubtedly, the increase in the temperature affects hydrodynamics by decreasing the viscosity of the water and increasing the rate of elutriation of the gangue back to the pulp [42,44]. However, this is not the case here, since flotation took place under ambient temperature conditions. It is worth noting the work of Pradip and Fuerstenau, who investigated the effect of temperature on the adsorption of hydroxamate on minerals like barite, calcite, and bastnaesite, and found increased values upon heating, suggesting an endothermic chemisorption mechanism [45]. A flotation test under elevated temperature confirmed this for monazite and bastnaesite [46,47], as well as apatite and malachite ore [42,48].

As for the use of hydroxamate under ambient conditions, an increase in the reagent dosage increases the flotation rate and the efficiency in general, which, however, is inadequate for a dosage below 250 g/t, where the P grade and recovery reach only 80.2% after 9 min of flotation with a P₂O₅ grade of 9.4%. The sarcosine collector presents the slowest rate in phosphorus collection; however, the selectivity is remarkable, with a considerable recovery of 89.2% achieved at a P₂O₅ grade of 20.1%. Sodium oleate showed poor performance in terms of selectivity; despite achieving a high P₂O₅ recovery (97.7%), the grade remained below 10%. The addition of waterglass slightly improved the grade, but the overall selectivity was still limited.

The combined concentrates’ contents in phosphorus, lanthanum, cerium, and yttrium for the aforementioned tests are presented in the graph of Figure 9. A comparison between the results obtained using different reagents reveals that the trend in La, Ce, and Y recovery follows that of P; the higher the recovery of apatite, the higher that of REEs, which confirms the characterization findings regarding the apatite hosting of REE minerals.

As shown in

Table 5. REE content in the concentrates obtained from the flotation of the ore using conventional reagents. For comparison purposes, the concentration of the elements in the feed are also given in the last row.

Flotation Experiment	REE Grade in Final Concentrate, %
	La	Ce	Y
Sodium Oleate (NaOl) [#3]	0.0192	0.0413	0.0031
NaOl and Na2SiO3 [#4]	0.0237	0.0550	0.0037
Sarcosine [#5]	0.0402	0.0926	0.0054
Hydroxamate (150 g/t) [#6]	0.0498	0.1046	0.0041
Hydroxamate (250 g/t) [#2]	0.0265	0.0627	0.0038
Hydroxamate (250 g/t, 55 °C) [#1]	0.0257	0.0550	0.0032
Hydroxamate (300 g/t) [#7]	0.0299	0.0595	0.0037
In the feed	0.0098	0.0190	0.0020

, in all cases the concentrates of La, Ce, and Y in the produced concentrates are significantly higher, indicating enrichment.

3.3. Synergy of Lignin with Conventional Reagents

Following the positive results obtained using sarcosine and hydroxamate collectors in terms of the achieved grade and recovery of apatite and the REE of interest (La, Y, and Ce) in the concentrate, more flotation trials were carried out applying reduction in collectors‘ dosages by 20%, 30%, and 40% for sarcosine and 30% and 50% for hydroxamate, and adding lignin nanoparticles, and the respective results are depicted in Figure 10.

More specifically, Figure 10a,b present apatite recovery versus time, and apatite grade–recovery, respectively, using solely sarcosine, as well as a sarcosine reduction of 20% and addition of lignin by 20%, 30%, and 40%. Flotation rate increases substantially when a 20% reduction in sarcosine is applied, while a further increase in the reduction ratio by 10% (30% lignin) results in the same flotation rates as with solely sarcosine. As far as the obtained concentrate grade, it is enhanced throughout the flotation and at the entire recovery range under 20% and 30% sarcosine reduction and lignin addition. When the reagent reduction exceeds 30%, further attenuation of the flotation rate is identified. The 40% reduction in sarcosine burdens flotation kinetics as well the P₂O₅ recovery and grade in the concentrate. Among all trials presented here, higher phosphorous recovery was achieved at a 20% reduction in sarcosine (93.6%). P, La, Ce, and Y recoveries of the combined concentrates are depicted in Figure 11. Except for Y, the 20% replacement of sarcosine by lignin lead to the improvement of P, Ce, and La. Indeed, for a 40% dosage reduction in the specific conventional collector, poor separation efficiency is achieved since the recovery of P as well as Ce, La, and Y are considerably low.

Figure 12a,b present apatite recovery versus time and grade, respectively, when solely hydroxamate was used as the collector, and for a 30% and 50% reduction in hydroxamate and addition of lignin. Enhancement in flotation kinetics occurred after a 30% reduction in hydroxamate and lignin addition, while the flotation rate was attenuated by a further increase in the lignin content. It is also noteworthy that, upon a 30% reduction in hydroxamate and addition of lignin, an increase in P₂O₅ recovery by 4% was achieved. However, when the hydroxamate dosage was reduced by 50% and lignin was added—resulting in a total hydroxamate dosage of 200 g/t, or 66% of the control—the phosphorus recovery reached only 86.9%. As shown in Figure 13, comparable recoveries of La, Ce, and Y were also obtained under these conditions. Nevertheless, a general decline in recovery for all target elements was observed at a 50% dosage reduction. Notably, the P₂O₅ grade in the concentrate increased following a 30% reduction in the conventional collector dosage combined with lignin addition.

The FTIR analysis allowed the investigation of the interaction between apatite and phlogopite, with hydroxamate and lignin nanoparticles. Figure 14 and Figure 15 depict the FTIR spectra of pure apatite and pure phlogopite samples, respectively, lignin nanoparticles, and a pure ore sample after treatment with solely hydroxamate (300 g/t), hydroxamate (210 g/t) and lignin nanoparticles (90 g/t), and solely lignin nanoparticles (300 g/t) at a pH of 10.

Possible interaction of lignin with the apatite and phlogopite surfaces should be indicated by new peaks, shifts, or changes in band shape in the following regions: ~3400–3700 cm⁻¹ at the O-H stretching band, indicating highly active sites for hydrogen bonding, ~1500–1600 cm⁻¹, denoting vibration of aromatic rings, and ~800–1100 cm⁻¹, showing two peaks attributed to the bending vibration of aromatic C-H bonds.

Several peaks were observed in solely apatite and samples treated with lignin at wavenumbers of 852 cm⁻¹, 950 cm⁻¹, 1097 cm⁻¹, 2079 cm⁻¹, and 3538 cm⁻¹, corresponding to C-H deformation of syringyl (S) units [49], symmetric and asymmetric vibration of the PO₄ group [50,51], and O-H vibrations [50], respectively. Possible interaction of the mineral surface and the lignin nanoparticles might be indicated by the peak at 1512 cm⁻¹, which is present only when apatite and lignin nanoparticles are combined, but not in the individual phases. The peak is attributed to C=C aromatic skeletal vibration [34,52] and was expected in the organic phase, though its appearance after apatite and lignin mixing indicates a change in the aromatic ring, possibly due to surface adsorption.

Also, new peaks appearing after using lignin in the collector mixture at 1214 cm⁻¹ and 1326 cm⁻¹ are attributed to guaiacyl absorbance and C=O bending of the syringyl unit, with both being structural components of lignin [53]. Possible interaction of apatite and hydroxamate and/or lignin nanoparticles should be accompanied by a change in the intensity or shifting of peaks associated with hydroxyl (-OH), carbonyl (C=O), or phosphate ${PO}_4^{-3}$ functional groups. The broad peak at 1000–1150 cm⁻¹ associated with asymmetric stretching of ${PO}_4^{-3}$ [50,51] attenuated after the addition of hydroxamate, but remained strong when hydroxamate and lignin were combined. Also, the O-H stretching band at 3538 cm⁻¹ appeared slightly enhanced after the treatment of apatite with hydroxamate and lignin nanoparticles, which could indicate hydrogen bonding [50].

Regarding the phlogopite sample, the broad peak at the 3200–3700 cm⁻¹ region is attributed to the O-H stretching band [54]. Compared to pure phlogopite and pure lignin, this band appears broader and slightly shifted in the samples that were treated with lignin and a combination of lignin and hydroxamate, which indicates hydrogen bonding interaction, probably between the hydroxyl groups of lignin and of phlogopite. Regarding the peak in the 1500–1600 cm⁻¹ region, the shifts are attributed to C=C aromatic skeletal vibrations of lignin [34]. Interestingly, the band persists in phlogopite samples treated either with lignin or the hydroxamate—lignin mixture, but are not present in pure phlogopite and pure lignin, suggesting shifting or activation due to their interaction. The band reflects a change in the electronic environment of the aromatic ring, denoting the formation of non-covalent interactions, like electrostatic attraction, Van der Waals force, or π-π stacking between lignin and phlogopite [55,56]. Regarding the intensive peak at 1011 cm⁻¹, it is attributed to Si–O–Si stretching of tetrahedral silicate layers of phlogopite [54]. The peak appeared sharp, symmetric, and intensive in pure phlogopite, while for treatment with hydroxamate an intensity reduction was observed. In addition to this, in the case of phlogopite treatment with lignin, the band becomes broader and more asymmetric, indicating some sort of interaction. Detailed characterization of the FTIR spectra for pure minerals (apatite and phlogopite samples), lignin, and minerals treated with hydroxamate, lignin, and their mixture are tabulated in Tables S1 and S2.

The contact angles of apatite and phlogopite when exposed to hydroxamate and a combination of hydroxamate and lignin under a pH of 10 are presented in Figure 16. After being treated with hydroxamate (300 g/t), apatite and phlogopite presented contact angle values of 79.6° and 53.8°, respectively. After mineral treatment with hydroxamate (210 g/t) and lignin (90 g/t), apatite presents a contact angle value of 75.2°, which is slightly lower compared to the case of treatment with solely Aero. This is not the case for phlogopite, which presented a significantly reduced contact angle value of 36.9° in the case of lignin addition. The results suggest that the adsorption of lignin on phlogopite was greater than on apatite, enhancing the hydrophilicity of the mineral, thus inhibiting its flotation.

3.4. Separation Efficiency

Further evaluation of the flotation results was performed using two flotation indexes. The selectivity index (SI) is calculated from Equation (1) [57]:

$$SI=\sqrt{{\displaystyle \frac{R_{v,c}\times R_{g,t}}{\left(100-R_{v,c}\right)\times \left(100-R_{g,t}\right)}}}$$

(1)

where R_v,c and R_g,t are the recovery of the valuable in the concentrate and of the gangue in the tailings, respectively. An SI value over 1 denotes selectivity in flotation of the valuable over the gangue; however, higher values are desirable. In our case, phlogopite is considered the gangue mineral and Mg is the tracer element.

The separation efficiency (SE) is calculated by [58,59]

$$SE=R_{v,c}-R_{g,c}$$

(2)

where R_g,t is the recovery of gangue in the concentrate. An SE value over 80% reflects excellent separation.

Table 6. P and Mg distribution in concentrates and tailings, and SI and SE values for all laboratory flotation tests.

Number	Reagents	Collector Dosage Reduction (%)	P rec. in Conc. (%)	Mg rec. in Conc. (%)	Mg rec. in Tail (%)	SI	SE
1	Hydroxamate diluted at 55 °C	-	98.7	13.6	86.4	5.2	85.1
2	Hydroxamate	-	90.6	13.7	86.3	0.7	76.9
3	Fatty acid	-	97.7	38.3	61.7	0.9	59.4
4	Fatty acid, Na2SiO3	-	97	46.2	53.8	0.5	50.8
5	Sarcosine	-	89.2	6.7	93.3	1.3	82.5
6	Hydroxamate, Na2SiO3	-	80.2	59.3	40.7	0.0	20.9
7	Hydroxamate, Na2SiO3	-	91.5	8.9	91.1	1.2	82.6
8	Sarcosine, lignin	20	93.6	4.8	95.2	3.1	88.8
9	Sarcosine, lignin	30	86.7	5.5	94.5	1.2	81.2
10	Hydroxamate, lignin, Na2SiO3	30	95.4	18.6	81.4	1.0	76.8
11	Sarcosine, lignin	40	75.1	4.7	95.3	0.7	70.4
12	Hydroxamate, lignin, Na2SiO3	50	86.9	5.9	94.1	1.2	81

tabulates the recoveries of phosphorous in the concentrate, and of magnesium in the concentrate and tailings, as well as the selectivity index and separation efficiency for all lab-scale flotation tests (conditions are presented in Table 2).

As far as the selectivity index, the highest value is reached when hydroxamate is used as the collector, which has previously been diluted in aqueous solution preheated to 55 °C. A 80/20 v/v sarcosine/lignin collector mixture performed very well too, which further to the high selective index (3.1) exhibits the highest separation efficiency (88.8%). When the replacement ratio of both sarcosine and hydroxamate by lignin exceeds 30%, the flotation performance declines, which is reflected by low phosphorous and/high magnesium recoveries in the concentrate or by a low selectivity index and separation efficiency.

3.5. Bench-Scale Flotation Tests

Figure 17a,b present grade–recovery curves for P, La, and Ce, respectively, using 4 and 13 L flotation cells, and solely sarcosine and a sarcosine/lignin 80/20 mixture. Satisfactory phosphorous recoveries of 95.3% and 96.5% were observed when solely sarcosine and sarcosine/lignin were used in a 4 L cell, respectively. The respective P grade was found to be 17.4% and 19.1%. Better flotation results are obtained by process upscaling in a 13 L cell; in this case, the P grade and recovery reached 24.2% and 94.5%, respectively. As for the lanthanum and cerium content, the achieved cumulative Ce and La grade and recovery for solely sarcosine are 0.12% and 73.3%, respectively, and for sarcosine/lignin are 0.13% and 77.8% in a 4 L cell. Thus, the partial replacement of sarcosine by lignin allows an increase in the recovery by about 4.5% at the same grade. Upon an increase in the flotation cell size, the La and Ce grade in the concentrate increases to 0.15% at the expense of recovery, which falls to 71.2%.

4. Conclusions

This study presented the results of flotation tests for the recovery of REE-hosting apatite from ore originating from a carbonatite deposit, Finland, using conventional anionic and amine-based collectors, but also natural organosolv lignin nanoparticles individually or in a mixture to identify synergies. The ore originates from a carbonatite deposit located in central Finland, which exhibits a fluorapatite content of about 8.9%, also with an overall content on L, Ce, and Y of about 0.03%. Apatite hosts most REE minerals, which was confirmed by the flotation tests, which showed that the concentration of apatite implies the concentration of L, Ce, and Y.

Adequate recovery of apatite and REEs was achieved using common anionic collectors hydroxamate and sarcosine, reaching a P grade of 23.4 and 21.5% and recovery of 96.4% and 89.2%, at a collector dosage of 250 and 300 g/t and pH of 10 and 11, respectively. The reduction in conventional collectors by up to 30% and the addition of lignin nanoparticles does not burden the flotation process and does not deteriorate the quality of the concentrate; in both sarcosine and hydroxamate, after reduction in the collectors by 30% and the addition of lignin nanoparticles, the P recovery reached 86.7% and 95.4%, respectively. Bench-scale flotation tests in a 13 L flotation cell confirmed the lab-scale results for sarcosine; a 20% reduction in sarcosine and the addition of lignin nanoparticles allowed the concentrate to be obtained with a P recovery of 94.5% and La and Ce recovery of 71.5%.

The FT-IR spectra of phosphate (PO₄³⁻) and hydroxyl (OH⁻) groups in pure apatite demonstrated spectral modifications upon treatment with lignin, suggesting the occurrence of interactions, likely through hydrogen bonding. These interactions were found to be more pronounced in the case of phlogopite, as evidenced by notable alterations in the O–H stretching region, indicative of hydrogen bonding or possible coordination mechanisms. Furthermore, the emergence of characteristic aromatic ring vibrations in the phlogopite spectrum supports the adsorption of lignin moieties. These spectroscopic findings are consistent with contact angle measurements, which revealed increased hydrophilicity of both apatite and phlogopite upon lignin treatment—more significantly in the latter—thereby facilitating improved selective separation during flotation.