Elimination of Ferric Ion Effect on Separation Between Kyanite and Quartz Using Citric Acid As Regulator

Article Elimination of Ferric Ion Effect on Separation between Kyanite and Quartz Using Citric Acid as Regulator

Yanping Niu 1, Ya Li 1, Haoran Sun 2,*, Chuanyao Sun 3, Wanzhong Yin 2 and Hongfeng Xu 4

1 Heilongjiang Province Geology Ore Test and Application Institute, Harbin 150000, China; [email protected] (Y.N.); [email protected] (Y.L.) 2 College of Resources and Civil Engineering, Northeastern University, Shenyang 110816, China; [email protected] 3 State Key Laboratory of Mineral Processing, BGRIMM Technology Group, Beijing 100160, China; [email protected] 4 Heilongjiang Mining Group Co., Ltd., Harbin 150000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-188-4250-4743

Abstract: Ferric ions produced during grinding inﬂuence the ﬂotation separation between kyanite and quartz adversely. In this study, citric acid was used as a regulator to eliminate the effect of ferric ions on the separation of kyanite from quartz with sodium oleate (NaOL) as a collector.

The microflotation test results indicated that the quartz was selectively activated by FeCl3 and maintained significant quartz recovery. However, the citric acid could selectively eliminate the effect of ferric ions on the quartz and minimally influenced the kyanite. Contact angle tests demonstrated that FeCl significantly increased the interaction between NaOL and quartz, resulting in the high 3 hydrophobicity of quartz, and the addition of citric acid made the quartz surface hydrophilic again but slightly influenced the kyanite. Fourier-transform infrared spectroscopy showed that FeCl Citation: Niu, Y.; Li, Y.; Sun, H.; Sun, 3 C.; Yin, W.; Xu, H. Elimination of facilitated NaOL adsorption onto the quartz surface, and the addition of citric acid eliminated the Ferric Ion Effect on Separation activation of FeCl3 on the quartz, resulting in the nonadsorption of NaOL onto the quartz surface. between Kyanite and Quartz Using However, the FeCl3 and citric acid exhibited a negligible effect on NaOL adsorption onto the kyanite Citric Acid as Regulator. Minerals surface. X-ray photoelectron spectroscopy analysis indicated that the citric acid eliminated FeCl3 2021, 11, 599. https://doi.org/ activation on the quartz. 10.3390/min11060599 Keywords: kyanite; quartz; citric acid; ferric ion; selective elimination Academic Editors: Zhiyong Gao, Wenjihao Hu, Peipei Wang, Kirsten Claire Corin and Ljudmilla Bokányi 1. Introduction Received: 6 May 2021 Kyanite is an essential metamorphic nesosilicate with the chemical formula Al SiO , Accepted: 31 May 2021 2 5 Published: 3 June 2021 and it is widely used in metallurgy, building materials, ceramics, and other industrial sectors [1,2]. It can also be used for preparing silicon–aluminum alloys and metal fibers [3].

Publisher’s Note: MDPI stays neutral However, with the extensive exploitation of kyanite resources, low-grade kyanite, rich in with regard to jurisdictional claims in quartz and muscovite impurities, has become relatively more abundant [4]. Thus, reducing published maps and institutional affil- the impurity contents (for example, quartz) is required to improve the quality of kyanite iations. concentration powder. Kyanite is produced in commercial grades using several mineral processing methods, such as hand sorting, gravity separation, magnetic separation, flotation, and roasting [5–8]. Because kyanite properties are becoming increasingly complex, flotation is widely applied for kyanite beneficiation. Many studies on the separation of kyanite and quartz were Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. focused on the development of collectors, such as anionic sodium hexadecanesulfonate [9], This article is an open access article dodecylamine hydrochloride [10], sodium dodecyl sulfate [11], and potassium oleate [12]. distributed under the terms and Nonetheless, despite the several novel collectors used in these experimental studies, sodium conditions of the Creative Commons oleate (NaOL) remains the most widely used collector for the direct flotation separation Attribution (CC BY) license (https:// between kyanite and quartz [13,14]. creativecommons.org/licenses/by/ The minerals are crushed and ground before the flotation process is started, and ferric 4.0/). ions released from the steel-grinding medium may activate gangue minerals [15]. Therefore,

Minerals 2021, 11, 599. https://doi.org/10.3390/min11060599 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 10

Minerals 2021, 11, 599 2 of 10 The minerals are crushed and ground before the flotation process is started, and ferric ions released from the steel-grinding medium may activate gangue minerals [15]. There- fore,the flotation the flotation separation separation of kyanite of kyanite from from quartz quartz in the in presencethe presence of NaOL of NaOL as a as collector a collector can cancause cause a large a large amount amount of quartz of quartz to float to togetherfloat together with kyanite, with kyanite, reducing reducing the grade the of grade kyanite of kyanitein the concentrate in the concentrate [16–18]. Thus,[16–18]. the Thus, activation the activation of quartz of by quartz ferric ion by shouldferric ion be eliminatedshould be eliminatedto enhance to flotation enhance separation flotation separation between kyanite between and kyanite quartz and efficiently. quartz efficiently. CitricCitric acid acid is is an environmentally friendly friendly organic organic chelating agent, agent, which has been widelywidely used in recent years.years. Many researchresearch studiesstudies havehaveshown shown that that citric citric acid acid can can form form a astable stable complex complex with with iron, iron, prevent prevent iron iron precipitation, precipitation, increase increase the the concentration concentration of of soluble solu- bleiron iron in water,in water, and and promote promote the the production production of of more more activeactive oxygenoxygen free radicals [19 [19––21]21].. Therefore,Therefore, inin this study,study, aa citric citric acid acid regulator regulator was was employed employed to to achieve achieve efficient efficient separation separa- tionthrough through flotation flotation and and increase incre thease differencethe difference in the in flotationthe flotation performance performance of kyanite of kyanite and andquartz quartz using using NaOL NaOL as a as collector a collector in the in presencethe presence of ferric of ferric ions. ions. The effectsThe effects of the of citric the citric acid acidand ferricand ferric ions onions the on flotation the flotation behavior behavior of these of two these minerals two minerals were investigated were investigated through throughexperimental experim microflotationental microflotation tests. Furthermore, tests. Furthermore, the effects the ofeffects the differentof the different adsorption ad- sorptionbehaviors behaviors and selective and reactionselective mechanisms reaction mechanisms of the citric of acid theand citric ferric acid ion and on ferric the flotation ion on theseparation flotation between separation the between two minerals the two were minerals determined were determined through contact through angle contact measure- angle measurments,ements, Fourier-transform Fourier-transform infrared infrared (FTIR) analysis, (FTIR) analysis, and X-ray and photoelectron X-ray photoelectron spectroscopy spectroscopy(XPS) measurements. (XPS) measurements.

2.2. Materials Materials and and Methods Methods 2.1.2.1. Materials Materials and and Reagents Reagents SeparateSeparate kyanite andand quartzquartz minerals minerals were were obtained obtained from from Henan Henan and and Hebei Hebei Provinces Prov- in China, respectively [22]. The X-ray powder diffraction patterns of the samples are shown inces in China, respectively [22]. The X-ray powder diffraction patterns of the samples are in Figure1, and the chemical analysis results are listed in Table1. The kyanite and quartz shown in Figure 1, and the chemical analysis results are listed in Table 1. The kyanite and purities were 98.60% and 99.31%, respectively. quartz purities were 98.60% and 99.31%, respectively.

150000

☆ ☆ Quartz 120000

90000

60000

Intensity(Counts) ☆ 30000 ☆ ☆ ☆ ☆☆ ☆ ☆ ☆ ☆ ☆ ☆☆ 0

10 20 30 40 50 60 70 80 90 o 2θ ( ) (a) (b)

FigureFigure 1. 1. XX-ray-ray powder powder diffraction diffraction patterns patterns of ( a)) kyanite and ( (b)) quartz quartz..

TableTable 1. Chemical composition analysis of mineral samples (wt%).

Minerals SiO2 Al2O3 TFe MgO CaO TiO2 Mn Loss onLoss Ignition on Minerals SiO Al O TFe MgO CaO TiO Mn Kyanite 36.522 60.402 3 0.81 0.18 — 0.0602 — Ignition0.51 Quartz 99.31 — 0.08 — — — — — Kyanite 36.52 60.40 0.81 0.18 — 0.060 — 0.51 Quartz 99.31 — 0.08 — — — — — The −0.074 + 0.045 mm fraction was used for the experimental flotation tests and contact angle analyses. Analytical grades of citric acid, NaOL, and FeCl3 were obtained from − AladdinThe Industrial0.074 + Corporation, 0.045 mm fraction Shanghai, was China used for, NaOH the experimental and HCl solutions ﬂotation were tests used and to adjustcontact the angle pH, analyses.and deionized Analytical water gradeswas used of citricfor all acid, experimental NaOL, and tests FeCl. 3 were obtained from Aladdin Industrial Corporation, Shanghai, China, NaOH and HCl solutions were

used to adjust the pH, and deionized water was used for all experimental tests.

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Minerals 2021, 11, 599 3 of 10 2.2. Microflotation Tests Microflotation tests were performed using an XFG flotation machine manufactured with2.2. a Microflotation 30 mL plexiglass Tests cell [22]. Deionized water (20 mL) and the sample (2.0 g) were placedMicroflotation in the microflotation tests were performedcell and mixed using an at XFGa stirring flotation speed machine of 1920 manufactured rpm. Figure 2 shows thewith flotation a 30 mL test plexiglass procedure cell [22 for]. Deionizedthe microflotation water (20 mL) test and (a themixture sample of (2.0 kyanite g) were and quartz in placed in the microflotation cell and mixed at a stirring speed of 1920 rpm. Figure2 shows a massthe flotation ratio of test 1:1). procedure First, forthe the slurry microflotation was stirred test (a for mixture 2 min of to kyanite ensure and that quartz the in minerals a were mixedmass ratiothoroughly. of 1:1). First, Subsequently, the slurry was NaOH stirred foror 2HCl min towas ensure then that added the minerals to adjust were the slurry pH formixed 2 min. thoroughly. In addition, Subsequently, FeCl3, NaOHcitric oraci HCld, and was thenNaOL added were to adjust then the added slurry pHsequentially, for with each2 min. addition In addition, accompanied FeCl3, citric by acid, pulp and stirring NaOL were for then3 min. added Next, sequentially, the froths with were each scraped man- uallyaddition with accompanied a plastic blade by pulp for stirring 3 min. for The 3 min. froth Next, products the froths and were scraping, scraped manually drying, and weigh- with a plastic blade for 3 min. The froth products and scraping, drying, and weighing to ingcalculate to calculate recovery. recovery Furthermore,. Further X-raymore, fluorescence X-ray analysisfluorescence was performed analysis to was determine performed to de- terminethe chemical the chemical compositions compositions of the artificially of the mixed artificially mineral concentrations. mixed mineral concentrations.

Sample 2 min Agitation 2 min HCl or NaOH

3 min FeCl3 3 min Citric acid 3 min NaOL

Flotation 3 min

Concentrate Tailing

FigureFigure 2. 2. ProcedureProcedure for for ﬂotation flotation tests. tests.

2.3.2.3. Contact Contact Angle Angle Tests Tests Contact angle measurements were detected using a OCA25 analyzer (Dataphysics). Then,Contact 2 g of the angle sample measurements were added to 20were mL detect of deionizeded using water, a andOC theA25 pH analyzer of the pulp (Dataphysics). Then,was adjusted.2 g of the Subsequently, sample were regulators added were to 20 added mL duringof deionized stirring onwater, a magnetic and the stirrer, pH of the pulp wasand adjusted. the solution Subsequently, was stirred before regulators 5 min. The were pulp added was adjusted during based stirring on the on ﬂotation a magnetic stirrer, andprocess. the solution Finally, the was solid stirred fraction be wasfor washede 5 min. three The times pulp with was distilled adjusted water andbased dried on at the flotation 313 K for detect. process. Finally, the solid fraction was washed three times with distilled water and dried at 2.4.313 FTIR K for Spectroscopy detect. Measurement FTIR was tested using a FTIR-650S spectrometer at 298 ± 2 K within 4000–400 cm−1 [23]. 2.4.The FTIR mineral Spectroscopy samples were Measurement ground to approximately 5 µm. The solution was prepared using 1 g of the sample and 20 mL of distilled water. The pulp was adjusted based on the flotation −1 process.FTIR Finally, was test the solided using fraction a wasFTIR washed-650S threespectrometer times with distilledat 298 water± 2 K and within dried at4000–400 cm [23]313. The K. For mineral the measurements, samples were a 1 mg ground sample wasto approximately mixed with 100 mg 5 μm. of spectrally The solution pure KBr was prepared usingand pulverized1 g of the using sample an agate and mortar 20 mL [24 of,25 distilled]. The resulting water. mixture The waspulp then was pressed adjusted into a based on the flotationthin specimen process. for analysis. Finally, the solid fraction was washed three times with distilled water and2.5. dried XPS Measurement at 313 K. For the measurements, a 1 mg sample was mixed with 100 mg of spectrallyXPS spectrapure KBr were and obtained pulverized using an Axis using Ultra an DLD agate Kratos mortar AXIS SUPRA[24,25]. spectrometer. The resulting mixture wasThe then binding pressed energy into scale a was thin corrected specimen using for a C1s analysis. peak at approximately 284.80 eV as the internal binding energy standard [26,27]. A 2 g sample was added to 20 mL of distilled 2.5.water XPS and Measu centrifugedrement at 2000 rpm, and the pulp was adjusted based on the ﬂotation process. Finally, the centrifuged solid mineral was washed three times with deionized waterXPS and spectra then dried were at 313obtained K for analysis. using an Axis Ultra DLD Kratos AXIS SUPRA spectrometer. The binding energy scale was corrected using a C1s peak at approximately 284.80 eV as the internal binding energy standard [26,27]. A 2 g sample was added to 20 mL of distilled water and centrifuged at 2000 rpm, and the pulp was adjusted based on the flotation process. Finally, the centrifuged solid mineral was washed three times with deionized water and then dried at 313 K for analysis.

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3. Results 3.3.3.1. Results Results Microflotation Tests 3.1.3.1. Microflotation MicroflotationFlotation tests Tests Test weres performed using NaOL as the collector to observe the floatability of theFlotationFlotation untreated tests tests minerals. were were performed performed Several research using using NaOL NaOL studies as as thehave the collector collector illustrate to tod observe thatobserve kyanite the the floatability floatabilityand quartz ofofcan the the be untreated untreated efficiently minerals. minerals. separated Several Several in a researchweakly research alkaline studies studies haveenvironment, have illustrated illustrate andd that that th kyanite ekyanite NaOL and andconcentra- quartz quartz cancantion be beis efficiently approximatelyefficiently separated separated 140– in160 ain weakly mg/La weakly [2,3,9,22]. alkaline alkaline environment,The environment, flotation andtest and theresults NaOLthe atNaOL a concentration pH concentra- of 7.5 and istion150 approximately mg/Lis approximately of NaOL 140–160 at increasing140– mg/L160 mg/L [ 2the,3, 9[2,3,9,22].FeCl,22].3 concentration The The flotation flotation testare test presented results results at inat a pHaFigure pH of of 7.5 3 7.5. and and 150 mg/L of NaOL at increasing the FeCl concentration are presented in Figure3. 150 mg/L of NaOL at increasing the FeCl3 3 concentration are presented in Figure 3. 100

10090

9080

8070

7060

6050

5040

Recovery (%) 4030 Recovery (%) 3020 Kyanite Quartz 2010 Kyanite Quartz 100 0 5 10 15 20 25 0 FeCl concentration (mg/L) 0 5 3 10 15 20 25 FeCl concentration (mg/L) Figure 3. Effect of FeCl3 concentration3 on flotation behavior of single minerals.

FigureFigure 3. 3.Effect Effect of of FeCl FeCl3 3concentration concentration on on ﬂotation flotation behavior behavior of of single single minerals. minerals. Within the evaluated range of the FeCl3 concentration (Figure 3), the kyanite recovery

fluctuatedWithinWithin thebetween the evaluated evaluated 90% rangeand range 94%, of of the illustratthe FeCl FeCl3ingconcentration3 concentration that the kyanite (Figure (Figure exhibited3), 3), the the kyanite goodkyanite floatability. recovery recovery fluctuatedfluctuatedHowever, between whenbetween the 90% 90% FeC and andl3 content 94%, 94%, illustrating illustratwas increaseding that that the fromthe kyanite kyanite 0 to 20 exhibited exhibited mg/L, the good good quartz floatability. floatability. recovery However,However,increased when fromwhen the1.06% the FeCl FeC to3 l73.22%,content3 content demonstrating was was increased increased fromthat from a 0 certain to0 to 20 20 mg/L, amountmg/L, the the of quartz quartzquartz recovery recoverycould be increasedincreasedfloated [22 from from]. With 1.06% 1.06% an toincrease to 73.22%, 73.22%, in demonstrating thedemonstrating FeCl3 concentration, that that a a certain certain the amountdifference amount of of quartzin quartz floatability could could be be- be floated [22]. With an increase in the FeCl concentration, the difference in floatability be- floatedtween the [22 kya]. Withnite andan increase quartz decreased in the FeCl 3gradually,3 concentration, and the the difficulty difference in separating in floatability kyanite be- tweentweenfrom thequartz the kyanite kya increasednite and and quartz quartz progressively. decreased decreased These gradually, gradually, trends and and shown the the difficulty difficultythat FeCl in 3in separatinghad separating a selective kyanite kyanite acti- from quartz increased progressively. These trends shown that FeCl had a selective activa- fromvation quartz impact increased on quartz progressively. flotation, consistent These trends with shownthe results that FeClreported3 3 had in a selectivethe literature acti- tionvation[3,22]. impact impact on quartz on quartz flotation, flotation, consistent consistent with thewith results the results reported reported in the literature in the literature [3,22]. Flotation tests were performed using citric acid as a regulator to reduce the activation [3,22].Flotation tests were performed using citric acid as a regulator to reduce the activation effect of ferric ions on the quartz during ﬂotation. The ﬂotation test results for the increasing effectFlotation of ferric testsions onwere the performed quartz during using flotation. citric acid The as flotationa regulator test to results reduce for the the activation increas- citric acid concentration at a pH of 7.5, 40 mg/L NaOL, and 20 mg/L FeCl are shown in effecting citric of ferric acid ionsconcentration on the quartz at a duringpH of 7.5, flotation. 40 mg/L The NaOL, flotation and test 20 mg/Lresults FeCl3 for 3the are increas- shown Figure4. ingin Figure citric acid4. concentration at a pH of 7.5, 40 mg/L NaOL, and 20 mg/L FeCl3 are shown in Figure 4. 100 10090

9080

8070

7060

6050

Kyanite 5040 Quartz

Recovery (%) Kyanite 40 30 Quartz Recovery (%) 3020

2010

100 0 2 4 6 8 10 0 Citric acid concentration (mg/L) 0 2 4 6 8 10 Citric acid concentration (mg/L) Figure 4. Effect of citric acid concentration on ﬂotation behavior of single minerals.

Minerals 2021, 11, 599 5 of 10

Figure4 shows that within the range of the citric acid concentration investigated in this study, the kyanite recovery decreased from 94.32 to 87.41 mg/L. Furthermore, the quartz recovery trend was similar to the kyanite recovery trend; when the citric acid was increased from 0 to 10 mg/L, the quartz recovery rate decreased from 73.22% to 11.83%. This result indicated that the citric acid could cause the depression of both the kyanite and quartz. However, the decrease in the quartz recovery was approximately 8.9 times that of the kyanite, and the recovery difference between the kyanite and quartz was the maximum when an 8 mg/L citric acid was added. Therefore, the citric acid could weaken the ferric ion effect on quartz flotation and minimally influenced the kyanite. To further certificate the maximum flotation difference between the kyanite and quartz was measured at 8 mg/L citric acid content, the artificially mixed minerals (the mass ratio of kyanite and quartz is 1:1) was observed. The recoveries and grades of the minerals are listed in Table2.

Table 2. Mixed-mineral ﬂotation test results.

Grade (%) Recovery (wt%) Flotation Conditions Product Yield (%) Al2O3 SiO2 Kyanite Quartz NaOL: 150 mg/L; Concentrate 81.50 35.50 64.46 92.00 71.00 pH: 7.5; Tailings 18.50 13.60 86.39 8.00 29.00 FeCl3: 20 mg/L. Feed 100.00 31.45 59.26 100.00 100.00 NaOL: 150 mg/L; Concentrate 51.62 52.51 47.43 86.11 16.89 pH: 7.5; Tailings 48.38 9.08 90.91 13.89 83.11 FeCl3: 20 mg/L; Feed 100.00 31.45 59.26 100.00 100.00 Citric acid: 8 mg/L.

When citric acid was added, the yield of the concentrate decreased from 81.50% to 51.62%, the Al2O3 grade increased from 35.50% to 52.51%, and the grade of SiO2 decreased from 64.46% to 47.43% (Table2). Moreover, the recovery of kyanite had little change, but the recovery of quartz decreased from 71.00% to 16.89% compared to NaOL + FeCl3, confirming that the citric acid significantly weakened the effect of ferric ions on quartz flotation and enhanced the separation efficiency between the kyanite and quartz.

3.2. Contact Angle Tests After the flotation agents were adsorbed onto the mineral surfaces, their surface potentials changed, significantly influencing mineral floatability [23,28]. Under conditions similar to those of the flotation tests, contact angle measurements were conducted (Table3).

Table 3. Contact angles of mineral samples under different test conditions.

Contact Angles of Minerals (◦) Test Conditions Kyanite Quartz Natural minerals 23 14 NaOL: 150 mg/L 92 17 FeCl3: 20 mg/L; NaOL: 150 mg/L 97 72 FeCl : 20 mg/L; Citric acid: 8 mg/L; 3 91 22 NaOL: 150 mg/L

In previous studies [22,29,30], the contact angles of natural kyanite and quartz were 23◦ and 14◦, respectively, indicating that the minerals are hydrophilic. With only NaOL treatment, the contact angles of the kyanite and quartz obtained in this study were 92◦ and 17◦, respectively, indicating that NaOL could signiﬁcantly enhance kyanite hydrophobicity [3,19]. When FeCl3 and NaOL were added, the contact angles of quartz increased from ◦ ◦ 17 to 72 , demonstrating that FeCl3 signiﬁcantly improved the interaction between NaOL and quartz, resulting in high hydrophobicity of quartz [22]. When FeCl3, citric acid, and Minerals 2021, 11, 599 6 of 10

NaOL coexisted, the contact angles of kyanite and quartz were 91◦ and 22◦, respectively, suggesting that citric acid addition made the quartz surface hydrophilic; however, a slight inﬂuence on kyanite was observed, and the signiﬁcant recovery of kyanite was maintained.

3.3. FTIR Measurements The effects of the regulator on the NaOL adsorption onto the kyanite and quartz were investigated through FTIR measurements at a pH of 7.5 using 20 mg/L FeCl3, 8 mg/L citric acid, and 150 mg/L NaOL. The FTIR spectra of the reagents and samples are shown in Figures5 and6, respectively.

2919.8

2850.8

2919.5

2850.8

2919.5

2850.7

Absorbance

675.8

972.6

933.4

972.6

933.4

3200 3100 3000 2900 2800 2700 2600

2850.8

2919.8

2850.8 2919.8 -1 Wavenumber (cm-1)

Kyanite + FeCl3 +Citric acid + NaOL

675.3

972.5

933.1

2919.5

2850.8

2919.5

2850.8 Kyanite + FeCl3 + NaOL

675.8

972.1

933.7

Absorbance

2919.5

2850.7

2919.5

2850.7

675.2

972.4

675.2 Kyanite + NaOL 972.4

933.9

Kyanite

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 5. FTIR analysis of kyanite. Figure 5. FTIR analysis of kyanite.

2921.3

2850.8

778.5

Absorbance

778.5

Absorbance

693.6

3200 3100 3000 2900 2800 2700 2600 -1 Wavenumber (cm-1) Quartz + FeCl + Citric acid + NaOL Quartz + FeCl3 + Citric acid + NaOL

3 778.0

778.0

693.3

2921.3

2850.8

2921.3

2850.8 Quartz + FeCl + NaOL 3

778.4

Absorbance

693.7

Absorbance

693.7

Quartz + NaOL

778.0

693.7

Quartz 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure 6. FTIRFigure analysis 6. FTIR of quartz. analysis of quartz. The kyanite peak at approximately 675 cm−1 corresponded to the Si–O tetrahedron symmetrical bending vibration peak; however, the kyanite peaks at approximately 972 and 933 cm−1 were attributed to the Si–O asymmetric stretching vibration of the tetrahedron Minerals 2021, 11, 599 7 of 10

peaks [22] (Figure5). Figure6 shows that the characteristic peaks of quartz at approximately 778 and 693 cm−1 correspond to Si–O symmetrical bending vibrations and Si–O–Si asymmetrical bending vibrations, respectively [23,31]. When NaOL and kyanite coexist, the new peaks appearing at 2918.5 and 2850.7 cm−1 corresponded to the symmetrical of –CH3 and asymmetric stretching vibrations of –CH2 in NaOL, respectively [28,32] (Figure5). These results suggested that the NaOL was adsorbed onto the kyanite surface. For the kyanite treated with FeCl3 and NaOL, symmetrical vibrations of methyl –CH3 and asymmetric stretching vibrations of methylene –CH2 were detected, and the same peaks were detected in the kyanite treated with FeCl3, citric acid, and NaOL. These results indicated that the FeCl3 and citric acid regulators had negligible effects on the adsorption of NaOL on the kyanite surface. The FTIR spectra of natural quartz and quartz + NaOL were similar, indicating that NaOL was not adsorbed onto the quartz surface (Figure6). With the addition of FeCl 3 and NaOL, the new peaks appearing at 2921.3 and 2850.8 cm−1 corresponded to the symmetrical of –CH3 and asymmetric stretching vibrations of –CH2 in NaOL, respectively [28,32]. Hence, FeCl3 facilitated NaOL adsorption onto the quartz surface. For the quartz treated with FeCl3, citric acid, and NaOL, the characteristic peaks of quartz were somewhat equal to those of the quartz treated with only NaOL. This result indicated that the addition of citric acid eliminated the activation of FeCl3 on the quartz, resulting in nonadsorption of NaOL onto the quartz surface.

3.4. XPS Measurement XPS measurements were performed for kyanite and quartz at a pH of 7.5, 20 mg/L FeCl3, and 8 mg/L citric acid to identify the reaction mechanisms between the regulator and the minerals. The binding energy results for the kyanite and quartz are shown in Figures7 and8, respectively.

O 1s

Fe 2p: 709.87

Counts (s) Counts

710.5 710.0 709.5 709.0 708.5 708.0 707.5 Binding energy (eV)

Kyanite + FeCl + NaOL C 1s Si 2p Al 2p 3

Fe 2p

Counts(s) Kyanite + FeCl Si 2p 3 C 1s Al 2p

C 1s Si 2p Kyanite Al 2p

900 750 600 450 300 150 0 Binding energy (eV)

FigureFigure 7. Binding 7. Binding energy energy of kyanite of kyanite with and with without and without regulator regulator treatment. treatment.

The XPS measurements could not detect Fe on the natural kyanite and quartz surfaces O 1s (Figures7 andFe 2p: 710.648). In the presence of FeCl 3, Fe 2p peaks appeared on the kyanite and

quartz surfaces at 709.87 and 710.64 eV, respectively, suggesting that Fe species could

Counts (s)

precipitate on the surfaces [22 ]. For the kyanite and quartz treated with FeCl3 + citric acid,

the characteristic711.5 711.0 710.5 710.0 709.5 peaks709.0 708.5 of708.0 Fe 2p disappeared, indicating that the citric acid prevented the Binding energy (eV) C 1s Si 2p adsorptionQuartz + ofFeCl Fe+ species NaOL onto the kyanite and quartz surfaces. 3 The relative contents of surface elements of minerals with and without regulator

treatment were analyzed (Table4) to evaluate the effect of the citric acid on the mineral adsorption of ferricFe ions. 2p

Counts(s) Quartz + FeCl 3 C 1s Si 2p

Quartz C 1s Si 2p

900 750 600 450 300 150 0 Binding energy (eV)

Figure 8. Binding energy of quartz with and without regulator treatment.

O 1s

Fe 2p: 709.87

Counts (s) Counts

710.5 710.0 709.5 709.0 708.5 708.0 707.5 Binding energy (eV)

Kyanite + FeCl + NaOL C 1s Si 2p Al 2p 3

Fe 2p

Counts(s) Kyanite + FeCl Si 2p 3 C 1s Al 2p

C 1s Si 2p Kyanite Al 2p

900 750 600 450 300 150 0 Minerals 2021, 11, 599 Binding energy (eV) 8 of 10

Figure 7. Binding energy of kyanite with and without regulator treatment.

Fe 2p: 710.64 O 1s

Counts (s)

711.5 711.0 710.5 710.0 709.5 709.0 708.5 708.0 Binding energy (eV) C 1s Si 2p Quartz + FeCl + NaOL 3

Fe 2p

Counts(s) Quartz + FeCl 3 C 1s Si 2p

Quartz C 1s Si 2p

900 750 600 450 300 150 0 Binding energy (eV) Figure 8. Binding energy of quartz with and without regulator treatment. Table 4. Content of element of samples surface with and without regulator treatment. Figure 8. Binding energy of quartz with and without regulator treatment. Content of Element (%) Minerals Reagent C 1s O 1s Si 2p Al 2p Fe 2p None 14.51 56.34 12.92 16.24 — Kyanite FeCl3 17.20 53.48 12.01 15.73 1.57 FeCl3 + Citric acid 17.94 53.14 12.23 16.57 0.12 None 12.18 55.35 32.48 — Quartz FeCl3 18.34 52.67 26.97 2.02 FeCl3 + Citric acid 22.13 49.28 28.50 0.09

The atomic percentages of Fe 2p in the kyanite and quartz surfaces when they were treated with only FeCl3 were 1.57% and 2.02%, respectively (Table4). However, when citric acid was added after FeCl3, the atomic percentages of Fe 2p on the kyanite and quartz surfaces were 0.12% and 0.09%, respectively. These results showed that ferric ions in the solution were adsorbed onto the kyanite and quartz surfaces, and the citric acid could decrease the ferric ion content adsorbed onto the kyanite and quartz surfaces.

4. Conclusions Based on the above results, the following conclusions are drawn. 1. The addition of citric acid weakened the effect of ferric ions on quartz flotation and had minimal influence on kyanite. 2. In the presence of NaOL and FeCl3, the addition of citric acid made the quartz surface hydrophilic again but also slightly deceased the hydrophobicity of the kyanite. Thus, the citric acid maintained the significant recovery of kyanite. 3. Citric acid could eliminate the activation of FeCl3 on the quartz, resulting in the nonadsorption of NaOL onto the quartz surface. However, the FeCl3 and citric acid had a negligible effect on NaOL adsorption onto the kyanite surface. 4. Citric acid decreased the ferric ion content adsorbed onto the kyanite and quartz surfaces.

Author Contributions: Validation, Y.N.; data curation, Y.L.; methodology, H.S.; conceptualization, C.S. and W.Y.; writing—original draft preparation, H.X. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the State Key Laboratory of Mineral Processing of BGRIMM Technology Group, China, grant number: BGRIMM-KJSKL-2017-11 and the Fundamental Research Funds for the Central Universities, China, grant number: N2001029. Minerals 2021, 11, 599 9 of 10

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: The authors would like to express their sincere gratitude to the editors and the anonymous reviewers for their helpful remarks and constructive comments, which have improved the quality of the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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