Spodumene Flotation Mechanism

Article Spodumene Flotation Mechanism

Lev Filippov 1,2 , Saeed Farrokhpay 1,* , Lichau Lyo 1 and Inna Filippova 1

1 Université de Lorraine, CNRS, GeoRessources, Nancy F-54000, France; lev.ﬁ[email protected] (L.F.); [email protected] (L.L.); inna.ﬁ[email protected] (I.F.) 2 National University of Science and Technology MISIS, Moscow 119049, Russia * Correspondence: [email protected]; Tel.: +33372744535

Received: 23 April 2019; Accepted: 11 June 2019; Published: 21 June 2019

Abstract: Fine and coarse fractions of spodumene were obtained from a pegmatite ore and their flotation was investigated under different conditions. In particular, the optimum pH and collector dosage were studied. It was found that the best flotation performance occurs at pH 10 using 250 mg/L of sodium oleate. It was also observed that upon the addition of CaCl2, spodumene flotation recovery increases to about 90%. In addition, poor floatability was found for spodumene when Na2CO3 was used as a pH regulator (compared to NaOH).The zeta potential data confirmed the adsorption of oleate on the spodumene surface. It was found that activation of spodumene by calcium ions makes the surface charge less negative due to the adsorption of Ca2+ on the surface. The crystallographic properties of spodumene were analyzed. The adsorption of sodium oleate was attributed to the chemisorption of oleate to the exposed Al and Si sites generated after breakage of the Al–O and Si–O bonds on the mineral surface. It was observed that the {110} planes are the most favorable for the adsorption of oleate. The {110} plane is the weakest plane, and spodumene has the highest tendency to cleave along this plane. The XRD data revealed that fine spodumene particles have more {110} planes than the coarser fraction, which may explain why the former has better floatability.

Keywords: lithium; spodumene; ﬂotation; sodium oleate

1. Introduction Lithium is widely used in batteries, ceramics, glass, lubricants, refrigeration and the nuclear and optoelectronics industries. With the development of computers, digital cameras, mobile phones and other electronic devices, the battery industry has become the largest consumer of lithium. The lithium market has grown dramatically in recent years. The total global consumption of lithium almost doubled in 2016 compared to 2008, due mainly to the development of the lithium-ion battery market [1]. It is expected that the lithium demand to be increased at an average annual growth rate of 10% over the next decade [2], and the supply of lithium should increase accordingly. Around 150 different minerals contain lithium and approximately 30 of these have a significant quantity of lithium. However, only five of these minerals have economic value. They include spodumene (LiAlSi2O6) from the pyroxene sub-group, lepidolite (KLiAl1,5Li1,5(Si3Al10)(F)2) and zinnwaldite (KLiFeAl(AlSi3)O10(OH,F)2) from the mica family, petalite (LiAlSi4O10, tectosilicate) and a lithium phosphate-amblygonite ((Li,Na)AlPO4(F,OH)). Spodumene contains approximately 8% Li2O which is the most abundant lithium-containing mineral. Due to the similarity of the physical-chemical properties of lithium minerals (including spodumene) and their associated gangue minerals (e.g., quartz, feldspar, mica), it is often challenging to separate lithium minerals [3]. The typical beneficiation methods consist of flotation, hand sorting, heavy media separation, thermal treatment and magnetic separation. Flotation is the most widely used beneficiation method for lithium minerals, including spodumene. While sodium oleate is often used as a collector in spodumene flotation, recent studies have suggested

Minerals 2019, 9, 372; doi:10.3390/min9060372 www.mdpi.com/journal/minerals Minerals 2019, 9, 372 2 of 11

Minerals 2019, 9, x FOR PEER REVIEW 2 of 12 new collector mixtures for this purpose [4,5]. The characteristic surface crystal chemistry of spodumene haschemistry been determined of spodumene to be has the been main determined contributing to factorbe the in main spodumene contributing selective factor flotation in spodumene with an oleateselective collector flotation when with other an oleate aluminosilicates, collector when such other as aluminosilicates, muscovite, quartz such and as feldspar, muscovite, are presentquartz and [6]. Investigationsfeldspar, are present of the crystallography[6]. Investigations of of spodumene the crystallography have become of spodumene more important. have become As shown more in Figureimportant.1, each As ofshown the individual in Figure tetrahedrons1, each of the in indi a single-chainvidual tetrahedrons of silicates in facesa single-chain the same of direction. silicates Thesefaces the chains same of direction. silicate tetrahedrons These chains are of bound silicate together tetrahedrons laterally are through bound ionictogether bonding laterally with through Li and Al in octahedral coordinates, leading to a unit cell formula of Li Al Si(O ) . The Al3+ sites on the ionic bonding with Li and Al in octahedral coordinates, leading to4 a unit4 cell3 8formula of Li4Al4Si(O3)8. spodumeneThe Al3+sites surface on the havespodumene specific surface interactions have with specific different interactions chemical with reagents, different as ischemical discussed reagents, further inas thisis discussed paper. further in this paper.

Figure 1. The crystallographic structure of spodumene [[7].7].

Moon and Fuerstenau [[6]6] studied the surface chemistrychemistry of spodumene when oleate is present in order to understand the crystallographic differen differencece between spodumene and other aluminosilicates. They found that the surface Al site is the most favo favorablerable site for the selective selective chemisorption chemisorption of of oleate. oleate. In fact,fact, Al sitessites inin otherother aluminosilicatesaluminosilicates areare deeplydeeply buriedburied insideinside thethe crystalcrystal unit,unit, makingmaking themthem unavailable for for oleate oleate adsorption. adsorption. They They also also calculated calculated the the broken broken ionic ionic bond bond strength strength per per unit unit area area for forthe the{110}, {110}, {010}, {010}, {001} {001}and {100} and {100}cleavage cleavage planes planes as 2.521 as × 2.5211014, 3.15510 14× ,10 3.15514, 4.16510 ×14 10,14 4.165 and 4.5701014 × and1014 × × × 4.570valence/cm10142, valencerespectively./cm2, This respectively. indicates that This cleavage indicates occu thatrs along cleavage the weakest occurs along{110} plane. the weakest Zhu et al. {110} [8] × plane.also reported Zhu et al.that, [8] alsowhile reported the four that, spodumene while the foursurfaces spodumene ({110}, {010}, surfaces {001} ({110}, and {010}, {100}) {001} are andnaturally {100}) arehydrophilic, naturally the hydrophilic, preferential the adsorp preferentialtion of adsorption sodium oleate of sodium on the oleate{100} and on the {110} {100} surfaces and {110} makes surfaces them makeshydrophobic. them hydrophobic. The spodumene surface surface generally generally has has a anegative negative zeta zeta potential potential in solutions in solutions of pH of > pH 3 [9].> 3 This [9]. Thisindicates indicates that thatthe theseparation separation of ofspodumene spodumene coul couldd be be possible possible using using cationic cationic collectors. Amine collectors are the main cationic collectors used in spodumene flotation,flotation, and are normally applied in reverse flotationflotation toto floatfloat thethe ganguegangue mineralsminerals (e.g.,(e.g., quartz,quartz, micamica oror feldspar).feldspar). The performanceperformance of sodium oleateoleate andand lauryllauryl amineamine asas collectors collectors in in spodumene spodumene flotation flotation has has been been investigated investigated [10 [10]] and and it wasit was found found that that lauryl lauryl amine amine achieves achieves a a higher higher recovery recovery of of spodumene spodumene thanthan sodiumsodium oleate (i.e., 90% vs. 50%, respectively). Although Although the the clean clean surface surface of of spodumene spodumene is is negatively charged at pH > 3, there areare normallynormally severalseveral cationiccationic ionsions suspendedsuspended inin thethesolution, solution,such such as as Li Li++ andand AlAl33++, whichwhich cancan attach to the clean surfacesurface andand easilyeasily formform aa bondbond withwith thethe anionicanionic collectors.collectors. This is probably the reason for thethe widewide utilizationutilization ofof anionicanionic collectorscollectors inin spodumenespodumene flotation.flotation. As stated above,above, oleicoleic acid is thethe mostmost frequentlyfrequently usedused anionicanionic collectorcollector inin spodumenespodumene flotation.flotation. Oleic acid performs best under slightly alkalinealkaline conditionsconditions (pH(pH= = 8–9). As oleic acid forms RCOO − ionsions at at a a strong strong alkaline alkaline pH, pH, and ROOH at acidic and neutral pH levels, both exist under a slightly alkaline pH and tend to form an ion-molecule complex. The RCOO− groupgroup will will react react with with the positive Al ions on the spodumene surface generated through chemical bonding. Meanwhile, the presence of molecular oleic acid willwill increase the negative value of the charge of RCOO−, which strengthens the chemical bonding and increases its hydrophobicity [11].

Minerals 2019, 9, 372 3 of 11

increase the negative value of the charge of RCOO−, which strengthens the chemical bonding and increases its hydrophobicity [11]. It is acknowledged that many studies have been carried out to improve spodumene flotation. However, most of the published papers focus on using different reagents, and the flotation mechanism is not yet fully understood. In this study, the flotation behavior of spodumene (obtained from a pegmatite ore) with different particle sizes was studied to determine the optimum flotation particle size range. This provides a theoretical basis for the selective grinding of spodumene. In addition, the optimum flotation conditions of spodumene is determined in order to improve the efficiency of flotation. Finally, the mechanism of spodumene flotation is discussed in conjunction with its surface charge behavior and its crystallography. The results of this study could have a significant effect on future productions of spodumene, and therefore could be of economic importance.

2. Materials and Experimental Methods Spodumene was prepared from Länttä pegmatite ore deposit in Finland. The main minerals in the ore were identified as spodumene, quartz, muscovite, albite and potassium feldspar. The lumpy spodumene-rich pegmatite ore particles were manually selected from the batch rocks. The ore was crushed to less than 2 mm. Due to the density difference between spodumene and the gangue minerals, heavy media separation was performed to separate the spodumene. The gangue minerals present in the ore have densities of about 2.70 g/cm3 while the density of spodumene is about 3.20 g/cm3 [12]. The dense media used in this study was a mixture of bromoform and tetra-bromoform with a density 3 of 2.95 g/cm . The heavy fraction was dried at 50 ◦C and then ground to less than 700 µm. Another stage of heavy media separation was conducted to remove fine gangue particles and impurities. Finally, the desired size fractions were obtained after grinding and sieving. XRD measurements were conducted to analyze the composition of the prepared spodumene samples and extract structural information. The measurements were performed on a Bruker D8 ADVANCE diffractometer with CuKα radiation (wavelength = 17.8897 nm). The XRD data were processed using DIFFFRAC.EVA provided by Bruker. Scanning electron microscopy (SEM) was used to scan surface of the samples. Various signals generated by the interactions between the electrons and atoms contain information on the sample topography and composition. The machine used was a Hitachi S-4800 (Tokyo, Japan), which features a resolution down to 1 nm at 15 kV with acceleration tension between 0.1 and 30 kV. Zeta potential measurements were performed using a Zetameter IV (CAD Instruments, France). Generally, particles have different movements in the presence of an electric field. Thus, the surface charge can be calculated according to this movement when combined with knowledge of the electric field magnitude, mobility velocity and direction. Using an agate mortar, 1 g of spodumene was ground to below 5 µm and stored in a glass bottle. The amount of sample used for each test was 50 mg with 200 mL of NaNO3 electrolyte solution. Each sample was stirred for 3 min to obtain a stable dispersion. The pH regulators were dilute solutions of HCl or NaOH. After the desired pH was obtained, the collector was added to the solution when needed. Finally, the suspension was injected into the apparatus using a syringe to test the electrophoretic mobility of the particles. Each test was repeated at least three times. Flotation tests were performed using a Hallimond tube. One g of spodumene sample was suspended in 300 mL of deionized water and the reagents were then added and rotated with a magnetic stirrer for 30 s. Nitrogen gas was introduced at the volume rate of 60 mL/min. Sodium oleate (NaOl) was used as a collector (250 mg/L, as reported by Liu et al. [9]). The optimum collector dosage was also reconfirmed in the initial tests since the spodumene recovery increased as the NaOl dosage increased up to 250 mg/L, and was unchanged after that. Flotation tests were carried out for 10 min with agitation at 500 rpm. Minerals 2019, 9, 372 4 of 11

Minerals 2019, 9, x FOR PEER REVIEW 4 of 12 3. Results and Discussion recovery is a bit higher for almost all pH ranges for the finer fraction (i.e., the 40–80 μm compared to the 80–150MineralsIn the μ initial2019m fraction),, 9, x tests, FOR PEER the as REVIEW spodumeneis further discussed flotation later recovery in this increased paper. as the pH was raised from4 of 12 2 to 10, and then stayed more or less unchanged up to pH 12 (Figure2). Figure2 also shows that the flotation recoveryrecovery is a is bit a higherbit higher for for almost almost all all pH pH rangesranges for the the finer finer fraction fraction (i.e., (i.e., thethe 40–80 40–80 μm comparedµm compared to to 80-150 μm 40-80 μm the 80–150the 80–150µm μ fraction),m fraction), as as is furtheris further discussed discussed later in in this this paper. paper. 100 90 80-150 μm 40-80 μm 80 100 70 90 80 60 70 50 60 40

Recovery % Recovery 50 30 40 Recovery % Recovery 20 30 10 20 0 10 0246810120 024681012 pH pH Figure 2. Effect of pH on spodumene flotation for the coarse and fine fractions. FigureFigure 2. E2.ff Effectect of of pH pH on on spodumene spodumene flotation flotation for for the the coarse coarse and and fine fine fractions. fractions. As shown in Figure 3, the isoelectric point (i.e.p.) of spodumene is at approximately pH 2, which As shownAs shown in Figure in Figure3, the3, the isoelectric isoelectric point point (i.e.p.) (i.e.p.) of of spodumene spodumene is isat atapproximately approximately pH 2, pH which 2, which is in agreement with the previously reported data (e.g., Xu et al. [10]). The zeta potential becomes is inis agreement in agreement with with the the previously previously reported reported datadata (e.g., (e.g., Xu Xu et et al. al. [10]). [10]). The The zeta zeta potential potential becomes becomes more negative as the pH increases up to pH 10, at which point it becomes less negative (Figure 2). moremore negative negative as theas the pH pH increases increases up up toto pHpH 10, at at which which point point it itbecomes becomes less less negative negative (Figure (Figure 2). 2). This is similar to the flotation recovery results seen at different pH levels. In fact, the maximum ThisThis is similar is similar to theto the flotation flotation recovery recovery results results seenseen at different different pH pH levels. levels. In fact, In fact, the maximum the maximum flotationflotation recovery recovery was was obtained obtained at atpH pH 10, 10, when when spod spodumene is is highly highly negatively negatively charged. charged. At this At thispH, pH, flotation recovery was obtained at pH 10, when spodumene is highly negatively charged. At this pH, the spodumenethe spodumene surface surface is capable is capable of ofthe the maximum maximum numb number ofof interactions interactions with with the the positively positively charged charged the spodumene surface is capable of the maximum number of interactions with the positively charged sites. sites.It should It should be noted be noted that that the the zeta zeta potential potential ofof thethe cleaned spodumene spodumene surface surface is different is different from from sites. It should be noted that the zeta potential of the cleaned spodumene surface is different from that ofthat the of non-cleaned the non-cleaned surface surface (Figure (Figure 3). 3). Therefor Therefore,e, contaminants can can affect affect the the surface surface charge charge of of that of the non-cleaned surface (Figure3). Therefore, contaminants can a ffect the surface charge of spodumenespodumene and, and, consequently, consequently, its its interactions interactions with with otherother reagents.reagents. It It is istherefore therefore necessary necessary to clean to clean spodumenethe spodumene and, consequently, surface before its flotation. interactions The withzeta potential other reagents. data for Itthe is thereforeacid-cleaned necessary sample are to clean the spodumene surface before flotation. The zeta potential data for the acid-cleaned sample are the spodumenesimilar to those surface reported before in the flotation. literature The (e.g., zeta Xu potential et al. [13]). data for the acid-cleaned sample are similar similar to those reported in the literature (e.g., Xu et al. [13]). to those reported in the literature (e.g., Xu et al. [13]). Non acid cleaned sample Acid cleaned sample Non acid cleaned sample Acid cleaned sample 20 20 0 0 024681012 -20 024681012 -20 -40

-40 -60

-80

-60Zeta Potential (mv)

-80-100 Zeta Potential (mv) -120 -100 pH . -120 Figure 3. Comparison of the zeta potentials ofpH non-cleaned and cleaned spodumene. . Figure 3. Comparison of the zeta potentials of non-cleaned and cleaned spodumene.

Figure 3. Comparison of the zeta potentials of non-cleaned and cleaned spodumene.

Minerals 2019, 9, 372 5 of 11 Minerals 2019, 9, x FOR PEER REVIEW 5 of 12 Minerals 2019, 9, x FOR PEER REVIEW 5 of 12 When the collector (i.e., NaOl) is added, the zeta potential of spodumene becomes more negative When the collector (i.e., NaOl) is added, the zeta potential of spodumene becomes more negative Whenover thea wide collector pH range (i.e., NaOl) (Figure is added,4), demonstrating the zeta potential that collector of spodumene adsorption becomes indeed more occurs. negative The zeta overover a potential wide pH pH of range the range spodumene (Figure (Figure 4), 4particles demonstrating), demonstrating becomes that even thatcollector mo collectorre negative adsorption adsorption when indeed the dosage indeedoccurs. of NaOl occurs.The zeta increases The zeta potentialpotential(Figure of of the the 4). spodumene spodumeneThe ion-molecule particles particles complexes becomes becomes even react evenmo withre morenegative the exposed negative when Al the when sites dosage on the the of dosage NaOl spodumene increases of NaOl surface, increases (Figure(Figurereducing 44).). The The ion-moleculethe ion-molecule positive charge complexes complexes on the react spodumene react with with the surf exposed theace exposed and Al therefore sites Al on sites thedecreasing spodumene on the the spodumene zeta surface, potential. surface, reducingreducing the the positive positive charge charge on onthe the spodumene spodumene surface surface and therefore and therefore decreasing decreasing the zeta thepotential. zeta potential. 20 20 0 0 024681012 024681012-20 -20 -40 -40 No NaOl -60 No NaOl -60 10-6 NaOl -6 -80 10 NaOl -80 Zeta potential (mV) 10-4 NaOl Zeta potential (mV) 10-4 NaOl -100 -100 -120 -120 pH pH

Figure 4. Effects of NaOl on the zeta potential of spodumene under different pH conditions. Figure 4. Effects of NaOl on the zeta potential of spodumene under different pH conditions. Figure 4. Effects of NaOl on the zeta potential of spodumene under different pH conditions. Based on the initial results, the highest recovery for spodumene flotation is still less than 70% Based on the initial results, the highest recovery for spodumene flotation is still less than 70% (Figure2). The flotation recovery may be improved by adding an activator. Common activators for Based(Figure on 2).the The initial flotation results, recovery the highest may recoverybe improved for spodumene by adding an flotation activator. is still Common less than activators 70% for 3+ 2+ spodumene(Figurespodumene 2). The flotation flotation flotation arerecovery are metallic metallic may ions be ions improved (e.g., (e.g., Fe Feby3+ oradding or CaCa2+ an))[ [14]. activator.14]. Figure Figure Common 5 5shows shows activators that that by by activating for activating 3+ 2+ spodumenespodumenespodumene flotation using using calcium are calciummetallic ions, theions, ions recovery the(e.g., recovery Fe of spodumeneor ofCa sp) odumene[14]. is Figure increased is increased5 shows significantly thatsignificantly by toactivating 90%. to In90%. addition In , whilespodumeneaddition, the optimum using while calcium recoverythe optimum ions, was the recovery initiallyrecovery was obtainedof initiallyspodumene at obtained approximately is increased at approximately significantly pH 10 (Figure pH to10 90%.2(Figure), by In activating 2), by spodumeneaddition,activating while using the spodumene optimum calcium using ionsrecovery the calcium recovery was ionsinitially the peak recoveobtained shiftsry towardspeak at approximately shifts a neutraltowards pH pH.a neutral 10 This (Figure pH. makes 2),This by the makes process activating spodumene using calcium ions the recovery peak shifts towards a neutral pH. This makes easierthe tocontrol process in easier industrial to control practice. in industrialIn addition practice., the pH In rangeaddition, for the maximumpH range for flotation the maximum spodumene the process easier to control in industrial practice. In addition, the pH range for the maximum recoveryflotation after spodumene activation (i.e.,recovery pHof after approximately activation (i.e., 8) pH matches of approximately well with the 8) matches values reported well with by the other flotationvalues spodumene reported recoveryby other researchersafter activation [10,11]. (i.e., pH of approximately 8) matches well with the researchersvalues reported [10 ,by11 ].other researchers [10,11].

80-150 μm 40-80 μm 80-150 μm 40-80 μm 100 100 90 90 80 80 70 70 60 60 50 50 40 40 % Recovery 30 Recovery % Recovery 30 20 20 10 10 0 0 024681012 024681012 pH pH Figure 5.FigureThe effect 5. The ofeffect calcium of calcium activation activation on on the the coarse coarse and and fine fractions fractions (NaOl: (NaOl: 250250 mg/L, mg CaCl/L, CaCl2: 120 2mg/L).: 120 mg/L). Figure 5. The effect of calcium activation on the coarse and fine fractions (NaOl: 250 mg/L, CaCl2: 120 mg/L). The i.e.p.The i.e.p. of spodumene of spodumene is atis at approximately approximately pHpH 2, as discussed, and and its its surface surface is negatively is negatively The i.e.p. of spodumene is at approximately pH 2, as discussed, and its surface is negatively chargedcharged at pH at> pH2. > After 2. After the the addition addition of of calcium calcium ions, ions, thethe zeta potential potential of of the the spodumene spodumene particles particles charged at pH > 2. After the addition of calcium ions, the zeta potential of the spodumene particles becomes less negative (Figure 6), indicating the adsorption of Ca2+2 +on the negative sites of the becomes less negative (Figure6), indicating the adsorption of2+ Ca on the negative sites of the becomes less negative (Figure 6), indicating the adsorption of Ca on the negative sites of the spodumene surface due to electrostatic attraction. As stated, when the collector (i.e., NaOl) is added, the zeta potential of spodumene becomes more negative over a wide pH range. The ion-molecule complexes react with the exposed Al sites, reducing the positive charge on the spodumene surface, and therefore shifting the zeta potential to less negative values. Adding Ca2+ activator followed Minerals 2019, 9, x FOR PEER REVIEW 6 of 12

spodumene surface due to electrostatic attraction. As stated, when the collector (i.e., NaOl) is added, Mineralsthe2019 zeta, 9 ,potential 372 of spodumene becomes more negative over a wide pH range. The ion-molecule6 of 11 complexes react with the exposed Al sites, reducing the positive charge on the spodumene surface, and therefore shifting the zeta potential to less negative values. Adding Ca2+ activator followed by by NaOlNaOl decreases decreases thethe quantityquantity of of negative negative charge charge on on the the surface surface and and results results in a inreduction a reduction in the in the electrostaticelectrostatic repulsion repulsion between between the the collector collectorand and Al sites and and an an increase increase in the in theinteraction interaction efficiency, eﬃciency, 2+ whichwhich can improvecan improve the the ﬂotation flotation of of spodumene. spodumene. CaCa2+ adsorbsadsorbs on on the the negative negative SiO SiO– site− siteof the of silica the silica tetrahedrons,tetrahedrons, and and thus thus increases increases the the density density of of available available adsorption sites. sites. Moreover, Moreover, Ca2+ Ca can2+ adsorbcan adsorb on theon other the other planes planes of spodumene, of spodumene, allowing allowing for for denserdenser adsorption layer layer formation. formation.

Spodumene Spodumene + Ca(II) Spodumene + Ca(II) + NaOL

0 024681012 -20

-40

-60

Zeta potential (mv) -80

-100 pH

Figure 6. Comparison of the zeta potential of the spodumene surface with or without NaOl and CaCl Figure 6. Comparison of the zeta potential of the spodumene surface with or without NaOl and CaCl2 2 (250 and(250 120and mg120/ L,mg/L, respectively). respectively).

It is evidentIt is evident that that the the pulp pulp pH pH plays plays a a very very importantimportant role role in in spodumene spodumene flotation. flotation. The Themain main regulatorsregulators used used in spodumenein spodumene flotation flotation are generally generally NaOH NaOH and and Na2 NaCO23.CO A comparison3. A comparison between between the the effeffectsects of of these these twotwo pHpH regulators is is shown shown in in Figure Figure 7.7 It. Itcan can be be seen seen that that NaOH NaOH results results in much in much higherhigher spodumene spodumene recovery recovery (than (than Na Na2CO2CO33)) whenwhen used to to control control the the pH pH, with, with or without or without activation. activation. This suggestsThis suggeststhat that NaOH NaOH is ais more a more suitable suitable pH pH regulatorregulator in in spodumene spodumene flotation. flotation. Figure Figure 7 also7 also shows shows that when Na2CO3 is used as a pH regulator, the flotation recovery of the activated spodumene that when Na2CO3 is used as a pH regulator, the flotation recovery of the activated spodumene decreasesdecreases as the as pH the increasespH increases from from 8 to 8 11. to 11. Meanwhile, Meanwhile, the the recovery recovery of of non-activated non-activated spodumenespodumene also also drops when the pH increases. This is not the case when NaOH is used, indicating that an drops when the pH increases. This is not the case when NaOH is used, indicating that an increased increased dosage of Na2CO3 has a negative effect on spodumene recovery (dropping from 50% to 20%). dosage of Na CO has a negative effect on spodumene recovery (dropping from 50% to 20%). On the On the other2 hand,3 activated spodumene has a much higher recovery than the non-activated sample other hand, activated spodumene has a much higher recovery than the non-activated sample when when either NaOH or Na2CO3 is used. This suggests that CaCl2 has an activating effect on spodumene eitherin NaOH flotation, or even Na2CO if a 3depressantis used. This (i.e., suggestsNa2CO3 in that the current CaCl2 hascase) an is present. activating effect on spodumene in Minerals 2019, 9, x FOR PEER REVIEW 7 of 12 flotation, even if a depressant (i.e., Na2CO3 in the current case) is present.

100 90 80 70 60 50 40 Recovery % Recovery 30 20 10 0 7 8 9 10 11 12 pH

Figure 7. The inﬂuence of pH regulators (: NaOH, : Na2CO3) on spodumene ﬂotation recovery. TheFigure results 7. The for influence the activated of pH regulators and non-activated (■: NaOH, spodumene ●: Na2CO3) areon spodumene represented flotation by solid recovery. and dashed The lines,results respectively. for the activated and non-activated spodumene are represented by solid and dashed lines, respectively.

Many metallic ions are the effective components of flotation reagents (activators or depressants). The hydration equilibrium of inorganic ions is the basis for studying flotation. Metal ions in the hydrolysis reaction occurring in solution generate a variety of complexes. The concentration of each component can be obtained according to the solution equilibria: 𝐶𝑎𝑂𝐻 𝐶𝑎 𝑂𝐻 ⇋ 𝐶𝑎𝑂𝐻 𝛽 10. 𝐶𝑎𝑂𝐻 𝐶𝑎𝑂𝐻 𝐶𝑎 2𝑂𝐻 ⇋𝐶𝑎𝑂𝐻 𝛽 10. 𝐶𝑎𝑂𝐻

𝐶 𝐶𝑎 𝐶𝑎 𝐶𝑎𝑂𝐻

𝐶𝑎 𝐶/1 𝛽𝑂𝐻 𝛽𝑂𝐻

𝐶𝑎𝑂𝐻 𝛽𝐶𝑎 𝑂𝐻

𝐶𝑎𝑂𝐻𝛽𝐶𝑎 𝑂𝐻

The solubility of Ca(OH)2 is 3.55 mmol/L and the initial concentration of CaCl2 was 1.08 mmol/L. Therefore, there is no precipitation of Ca(OH)2 over the entire pH range. Chemical analysis of the flotation solution revealed that when CaCl2 is added to the slurry, it is mainly in the form of Ca2+ at pH < 12.5. The concentration of Ca(OH)+ increases with the pH and at pH > 12.5, Ca(OH)2(𝑎𝑞𝑢𝑒𝑜𝑢𝑠) gradually dominates. Therefore, over the pH range of the flotation when the maximum spodumene recovery was obtained, the calcium was predominantly in the form of Ca2+ ions. A typical structure of spodumene is shown in Figure 8. Spodumene is composed of a series of single-chain silica tetrahedrons along the y‐axis which are bonded together by sharing an oxygen between two neighboring tetrahedrons [6].

Minerals 2019, 9, 372 7 of 11

Many metallic ions are the effective components of flotation reagents (activators or depressants). The hydration equilibrium of inorganic ions is the basis for studying flotation. Metal ions in the hydrolysis reaction occurring in solution generate a variety of complexes. The concentration of each component can be obtained according to the solution equilibria:

[CaOH+] Ca2+ + OH CaOH+ β = = 101.4 − 1 2+ [Ca ][OH−]

[Ca(OH) ] 2+ 2(aq) 2.77 Ca + 2OH− Ca(OH) (aqueous) β2 = = 10 2 2+ 2 [Ca ][OH−] h 2+i h 2+i CT = Ca + Ca + [Ca(OH)2(aqueous)]

h 2+i 2 Ca = CT/ 1 + β1[OH−] + β2[OH−]

h +i h 2+i CaOH = β1 Ca [OH−]

h 2+i 2 [Ca(OH)2(aqueous)] = β2 Ca [OH−]

The solubility of Ca(OH)2 is 3.55 mmol/L and the initial concentration of CaCl2 was 1.08 mmol/L. Therefore, there is no precipitation of Ca(OH)2 over the entire pH range. Chemical analysis of the 2+ ﬂotation solution revealed that when CaCl2 is added to the slurry, it is mainly in the form of Ca at + pH < 12.5. The concentration of Ca(OH) increases with the pH and at pH > 12.5, Ca(OH)2(aqueous) gradually dominates. Therefore, over the pH range of the ﬂotation when the maximum spodumene recovery was obtained, the calcium was predominantly in the form of Ca2+ ions. A typical structure of spodumene is shown in Figure8. Spodumene is composed of a series of single-chain silica tetrahedrons along the y-axis which are bonded together by sharing an oxygen between two neighboring tetrahedrons [6]. Minerals 2019, 9, x FOR PEER REVIEW 8 of 12

FigureFigure 8. 8.The The crystallographic crystallographic structure structure of of spodumene. spodumene.

In ﬂotation,In flotation, the adsorptionthe adsorption of the of collectorthe collector is mainly is mainly determined determined by theby generationthe generation of new of new surfaces duringsurfaces grinding. during For grinding. spodumene, For spodumene, the liberated the libera surfacested surfaces mainly mainly consist consist ofbroken of broken Si–O Si–O and and Al–O Al–O bonds. The lengths of these three bonds vary; the Li–O bond is the longest followed by Al–O bonds. The lengths of these three bonds vary; the Li–O bond is the longest followed by Al–O and and Si–O [15]. However, the strength of the Li–O bond is not high enough to form a bond, as shown Si–O [15]. However, the strength of the Li–O bond is not high enough to form a bond, as shown in in Figure 8. The Al–O bonds are longer than the Si–O bonds, so it is easier to break these bonds and Figureexpose8. The more Al–O Al bondsions, which are longer causes thanthe adsorption the Si–O bonds,of NaOl. so This it is interaction easier to breakoccurs theseover a bonds long and exposedistance more Alcompared ions, which to covalent causes bonding. the adsorption of NaOl. This interaction occurs over a long distance comparedThe to covalenttotal broken bonding. bond strengths per unit cell area were calculated as 9.372 × 1018, 6.352 × 1018, The4.734 total × 1018 broken and 7.524 bond × 10 strengths18 valence/m per2 for unit the cell {100}, area {010}, were {110} calculated and {001}as planes, 9.372 respectively.1018, 6.352 The 1018, × × 4.734{110}10 plane18 and is 7.524the weakest1018 crystallinevalence/ mplane,2 for indica the {100},ting that {010}, cleavage {110} occurs and {001} along planes, the {110} respectively. plane × × The {110}during plane the comminution is the weakest process. crystalline Each plane,crystalline indicating plane has that a cleavagedistinct type occurs and alongnumber the of {110} Al–O plane duringbonds, the comminutionwhich leads to variance process. in Eachthe total crystalline bond strength. plane In has terms a distinct of density type of Al and sites number in the unit of Al–O cell area, the crystalline planes are ranked in the order of {001} > {110} > {100} > {010}. The Al sites on bonds, which leads to variance in the total bond strength. In terms of density of Al sites in the unit the surfaces provide a high affinity for oleate molecules through complexation and the broken Al–O bonds are responsible for adsorbing the oleic acid ions [16]. The {110} plane is still more favorable to chemisorption with oleate than the {001} plane because it has two broken Al–O bonds which provide two unsatisfied coordinates, whereas only one exists in the {001} plane. Therefore, the chemical properties of the surface, such as the surface potential and hydrophobicity, are dominated by the {110} crystalline planes. Recently, Zhu et al. [8] also reported that the adsorption of sodium oleate on the spodumene mineral occurs at the {100} and {110} surfaces. The XRD patterns of the spodumene samples are shown in Figure 9. The relative intensities of the four crystalline planes ({100}, {010}, {110} and {001}) are listed in Table 1. It should be noted that the relative intensities only represent the relative amounts of phases in one sample. Therefore, a conversion should be developed in order to compare the same phase between different samples. The ratio of the amount of desired crystalline planes ({110}) to the amount of the three undesired crystalline planes ({100}, {010} and {001}) defines the relative quantity of desired planes in each fraction. Higher values of this ratio indicate the presence of more {110} planes, more exposed Al sites and more collectors adsorbed on the surface (in the current case, this ratio is higher for the 40–80 μm fraction than the 80–150 μm fraction). This indicates that the 40–80 μm fraction is preferable for collector adsorption.

Minerals 2019, 9, 372 8 of 11 cell area, the crystalline planes are ranked in the order of {001} > {110} > {100} > {010}. The Al sites on the surfaces provide a high affinity for oleate molecules through complexation and the broken Al–O bonds are responsible for adsorbing the oleic acid ions [16]. The {110} plane is still more favorable to chemisorption with oleate than the {001} plane because it has two broken Al–O bonds which provide two unsatisfied coordinates, whereas only one exists in the {001} plane. Therefore, the chemical properties of the surface, such as the surface potential and hydrophobicity, are dominated by the {110} crystalline planes. Recently, Zhu et al. [8] also reported that the adsorption of sodium oleate on the spodumene mineral occurs at the {100} and {110} surfaces. The XRD patterns of the spodumene samples are shown in Figure9. The relative intensities of the four crystalline planes ({100}, {010}, {110} and {001}) are listed in Table1. It should be noted that the relative intensities only represent the relative amounts of phases in one sample. Therefore, a conversion should be developed in order to compare the same phase between different samples. The ratio of the amount of desired crystalline planes ({110}) to the amount of the three undesired crystalline planes ({100}, {010} and {001}) defines the relative quantity of desired planes in each fraction. Higher values of this ratio indicate the presence of more {110} planes, more exposed Al sites and more collectors adsorbed on the surface (in the current case, this ratio is higher for the 40–80 µm fraction than the Minerals 2019, 9, x FOR PEER REVIEW 9 of 12 80–150 µm fraction). This indicates that the 40–80 µm fraction is preferable for collector adsorption.

FigureFigure 9. The 9. The XRD XRD patterns patterns of the of spodumene the spodumene samples. samples. Table 1. Relative intensities of the main planes of the ﬁne and coarse fractions of spodumene. Table 1. Relative intensities of the main planes of the fine and coarse fractions of spodumene.

Crystalline Planes (h,k,l)40–80 40–8080–150µm 80–150 µm Crystalline Planes (h,k,l) {110} ( 1, 1,0)µm 56.9µm 72.1 {110} (−1,−−1,0)− 56.9 72.1 {110} ( 2, 2,0) 9.3 6.6 {110} (−2,−−2,0)− 9.3 6.6 {110}{110} (− (3,−3,3,0)3,0) 7.9 7.9 4.4 4.4 {100} (−−2,0,0)− 44.4 21.3 {100} ( 2,0,0) 44.4 21.3 {100} (−4,0,0)− 6.4 4.7 {100}{010} (0, (−2,0)4,0,0) 33.7 6.4 78.3 4.7 − {010}{001} (0,0, (0,−2)2,0) 8.5 33.7 4.1 78.3 The ratio of {110} to {100} + {001} + − {001} (0,0, 2)0.80 8.50.76 4.1 {001} − FigureThe 10 ratio shows of {110}that to{110} {100} is +the{001} main+ {001} cleavage plane of spodumene.0.80 A total of one 0.76hundred {110} planes were randomly selected in the images and their aspect ratios were calculated using ImageJ software. The histograms of the aspect ratios presented in Figure 11 show that the {110} planes fromFigure the 40–80 10 showsμm fraction that {110}exhibit is a the smaller main aspect cleavage ratio plane than those of spodumene. in the coarse A fraction. total of oneThis hundredis in {110} planesagreement were with randomly the results selected reported in the by imagesXu et al. and [10], their who aspect proposed ratios a were grinding calculated method using for ImageJ software.spodumene The that histograms results in various of the aspectparticle ratios sizes. presentedSpodumene in is Figure a very 11anisotropic show that mineral. the {110} Due planes to the from the 40–80high tendencyµm fraction of spodumene exhibit a smaller to generate aspect a cleavage ratio than plane, those the inmonoclinic the coarse structure fraction. consists This is of in the agreement with{110} theplanes results as the reported side plane by and Xu the et {001} al. planes [10], who as the proposed basal plane a [6]. grinding When the method particles for are spodumene slightly that crushed, the {110} planes are the most likely to break, so the quantity as well as the relative surface area of the {110} planes increase more quickly than those of the {001} planes. The quantity of {001} planes starts to increase as the particle size decreases (Table 1). The higher {001} planes adsorb less NaOl, which could be responsible for the decreased floatability of the coarser fraction (Figure 2). It should be noted that the very fine fraction (less than 40 μm) was not considered in this work due to the complexity which may arise due to entrainment. Minerals 2019, 9, 372 9 of 11

results in various particle sizes. Spodumene is a very anisotropic mineral. Due to the high tendency of spodumene to generate a cleavage plane, the monoclinic structure consists of the {110} planes as the side plane and the {001} planes as the basal plane [6]. When the particles are slightly crushed, the {110} planes are the most likely to break, so the quantity as well as the relative surface area of the {110} planes increase more quickly than those of the {001} planes. The quantity of {001} planes starts to increase as the particle size decreases (Table1). The higher {001} planes adsorb less NaOl, which could be responsible for the decreased ﬂoatability of the coarser fraction (Figure2). It should be noted that the very ﬁne fraction (less than 40 µm) was not considered in this work due to the complexity which may arise due to entrainment. MineralsMinerals 2019, 9 2019, x FOR, 9, x PEER FOR PEER REVIEW REVIEW 1010 of 12

Figure 10. Representative SEM images for the 80–150 μm (top) and 40-80 μm (bottom) fractions (The scale of the top and bottom images are 600 and 200 μm, respectively). FigureFigure 10. 10. RepresentativeRepresentative SEM SEM images images for the for 80–150 the 80–150 μm (top)µm and (top 40-80) and μm 40–80 (bottom)µm fractions (bottom (The) fractions scale of the top and bottom images are 600 and 200 μm, respectively). (The scale of the top and bottom images are 600 and 200 µm, respectively).

30 25

25 20

20 15

Frequency 10 15 5 Frequency 10 0 5 0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 7.0 7.5 8.0

0 Aspect ratio 0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 7.0 7.5 8.0 Figure 11. Distribution of the aspect ratios for the {110} planes in the 40–80 µm (grey) and 80–150 µm Figure 11. Distribution of the aspect ratios forAspect the {110} ratio planes in the 40–80 μm (grey) and 80–150 μm (black) fractions. (black) fractions.

Figure4. Conclusions 11. Distribution of the aspect ratios for the {110} planes in the 40–80 μm (grey) and 80–150 μm (black) fractions.

4. Conclusions

Minerals 2019, 9, 372 10 of 11

4. Conclusions The optimum pH for spodumene flotation was found to be approximately 10, and the optimum dosage of sodium oleate was found to be 250 mg/L. After the addition of CaCl2, spodumene flotation recovery increased considerably to 90%. The effects of pH regulators were also investigated and it was found that spodumene had poor floatability when Na2CO3 was used as a pH regulator compared to NaOH. The i.e.p. of spodumene shifted to a lower pH when NaOl was added, which confirmed the adsorption of oleate to the mineral surface. Activating spodumene with calcium ions makes the mineral surface charge less negative due to the adsorption of Ca2+ on the surface. To understand the flotation mechanism of spodumene, its crystallographic properties were analyzed. The adsorption of NaOl was attributed to the chemisorption of oleate to the exposed Al and Si sites, which were generated after breaking the Al–O and Si–O bonds on the mineral surface. Through the calculation of the strength of the broken bonds on the spodumene surface, it was found that the {110} planes are the most favorable for the adsorption of oleate. The {110} plane is the weakest plane and spodumene has the highest tendency to cleave along the {110} plane. The XRD patterns of spodumene revealed that the 40–80 µm fraction has more {001} planes than the 80–150 µm fraction, which may explain why the former has better floatability. The results obtained in this work suggest that the comminution process (e.g., crushing and grinding) can affect the floatability of spodumene by changing its crystallography. Therefore, further investigations into more suitable grinding methods which could increase the floatability of spodumene are required.

Author Contributions: Methodology, L.F., L.L.; project administration, L.F., S.F.; investigation, L.L., I.F., supervision, L.F., I.F.; validation, L.F., I.F., S.F.; formal analysis, L.L., I.F.; original draft preparation, L.L; writing—review and editing, S.F., L.F.; funding acquisition, L.F. Funding: This research was funded by Labex Ressources21 (Investissements d’Avenir grant agreement no. ANR-10-LABX-21).

Acknowledgments: Financial support from Labex Ressources21 (ANR, n◦ ANR-10-LABX-21 “Strategic Metals in the 21st Century”) is acknowledged. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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