Mechanism of Depression by Fe3+ During Hemimorphite Flotation

Article Mechanism of Depression by Fe3+ During Hemimorphite Flotation

Junbo Liu, Shuming Wen *, Qicheng Feng *, Qian Zhang, Yijie Wang, Yaowen Zhou and Wenlin Nie State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (J.L.); [email protected] (Q.Z.); [email protected] (Y.W.); [email protected] (Y.Z.); [email protected] (W.N.) * Correspondence: [email protected] (S.W.); [email protected] or [email protected] (Q.F.); Tel.: +86-13095315156 (S.W.); +86-15987190563 (Q.F.) Received: 20 July 2020; Accepted: 3 September 2020; Published: 8 September 2020

Abstract: Sulfide hemimorphite can be depressed by Fe3+ during flotation. In this study, the depression mechanism was studied by microflotation, inductively-coupled plasma mass spectrometry, local electrochemical impedance spectroscopy (LEIS), and X-ray photoelectron spectroscopy (XPS). Flotation test results suggested that sulfated hemimorphite can be depressed by Fe3+ across the entire pH range. LEIS, adsorption analysis, and XPS indicated that S species were adsorbed on hemimorphite as ZnS. The sulfide film was attenuated and no adsorbed Fe species were found after treatment with Fe3+. The results indicate that Fe3+ reacts with the ZnS film, which decreases the number S species, and this leads to hemimorphite depression.

Keywords: depression; Fe3+; surface; sulﬁdization; ﬂotation; hemimorphite

1. Introduction Zinc is the fourth most used metal after aluminum, iron, and copper. It is commonly used in alloys and batteries [1]. Zinc sulfide minerals, which are the main sources for zinc production, are commonly concentrated and recovered by froth flotation. Recently, zinc sulfide minerals have become depleted because of increasing exploitation [2]. Hemimorphite is a typical zinc oxide mineral, and its use as an alternative source of zinc metal has been developed to meet future demands [3]. Many techniques such as pyrometallurgy, hydrometallurgy, flotation, and gravity separation combined with flotation have been used to treat zinc oxide ores [4]. The sulfidization-xanthate flotation method is the most commonly used commercial method for the concentration and pretreatment of zinc oxide minerals [5,6]. Generally, sulfidization flotation performs well in the commercial processing of lead, copper, and zinc oxide minerals. Sulfidization-xanthate flotation is usually used for the separation and concentration of hemimorphite from other minerals. A key step in the sulfidization-xanthate flotation separation process is sulfation by Na2S[7]. Adjustment and control of sulfation is important for achieving good flotation indexes. Jia et al. [8]. studied the sulfation of hemimorphite. They reported that the formation of ZnS can be described by the following chemical reaction:

Zn4Si2O7(OH)2H2O(s) + xHS− Zn4 xSi2O7(OH)2 2xH2O(ZnS)x(s) + xHO− + xH2O (1) → − − S(II) enters the hemimorphite crystal lattice and forms a new zinc sulfide phase on the hemimorphite surface, but the freshly generated zinc sulfide film differs from natural sphalerite. Flotation and sulfation are inevitably affected by ions in solution. Previous research has shown that ions have important + effects on the flotation of oxide minerals. Shen et al. [9]. reported that NH4 causes dissolution of the

Minerals 2020, 10, 790; doi:10.3390/min10090790 www.mdpi.com/journal/minerals Minerals 2020, 10, 790 2 of 12

2 malachite surface; it acts as a sulfidization promoter and prevents depression of excess S − during sulfidization. Feng et al. [10] suggested that more lead sulfide species form on the cerussite surface when chloride ions are added prior to sulfidization. The addition of chloride ions prior to sulfidization significantly improves cerussite sulfidization, and this improves the flotation performance. Fe3+ is highly oxidized and can react with sulfide ores and films. The hydrophilic compound ferric hydroxide can be adsorbed on mineral surfaces. Grano et al. [11] reported that Fe3+ depresses chalcopyrite when Fe3+ in the form of iron oxyhydroxide is adsorbed on the chalcopyrite surface and that the extent of flotation depression by iron oxyhydroxide depended on pH, conditioning gas, and the presence of adsorbed collector on chalcopyrite. Yu et al. [12] reported that Fe3+ exists in various forms in solution and its adsorption on an arsenopyrite surface leads to oxidation of Fe, As, and S on the mineral surface, and oxidation increases with increasing pH. The oxide residues on the arsenopyrite surface decrease its floatability. This report shows that water interface and the mineral surface following treatment with Fe3+ have a big influence in sulfide ore flotation, and the experiments also should pay attention to the iron hydroxyl complexes which exist in aqueous solutions. Additionally, for another sulfide mineral pyrite, Jiang et al. [13] suggested that the presence of Fe2+ and Fe3+ ions in solution significantly depresses pyrite ore flotation in a weakly alkaline environment. They found that the complete or partial depression of pyrite is mainly attributed to the reactions between xanthate and ferrous/ferric ions to form the poorly hydrophobic ferric dihydroxy xanthate on the surfaces and in solution. The flotation results showed that ferrous ions depress the pyrite near this pH range more remarkably than ferric ions. Deng et al. [3] reported that Fe3+ can react with a sulfated smithsonite surface. This reaction accelerates the dissolution of smithsonite. This dissolution and the generated carbon dioxide weaken the linked surface layer of sulfide (ZnS) film. This results in easy removal of the film from the surface under high-speed stirring. This process not only decreases the hydrophobicity of the mineral surface but also removes large quantities of the mineral surface layers. The amount of information available in the literature on the sulfidization and flotation of hemimorphite is much lower than that available on other oxidized ores. In particular, there are few reports on the mode of action of iron ions on hemimorphite surfaces and the implications for sulfidization and flotation. But the Fe3+ is unavoidable ions in flotation process. The Fe3+ may go into pulp from grinding process, stirring process even though the flotation process. These Fe3+ may have some influence on the hemimorphite flotation. In this study, the depression of sulfated hemimorphite by Fe3+ and the depression mechanism were studied by microflotation, time-of-flight secondary-ion mass spectrometry (TOF-SIMS), local electrochemical impedance spectroscopy (LEIS), and X-ray photoelectron spectroscopy (XPS). The changes in the surface properties caused by Fe3+ addition and the subsequent effects on hemimorphite flotation and sulfidization were studied in detail.

2. Materials and Methods

2.1. Materials The pure hemimorphite samples used in all experiments were obtained from Guangxi Province, China. The samples were dry ground with an agate pestle. The ground hemimorphite was sieved with a standard screen to achieve a particle fraction of 74–+38 µm for the flotation tests and inductively − coupled plasma mass spectrometry (ICP-MS). The rest of the hemimorphite sample was ground to a particle fraction finer than 5 µm for XPS. The X-ray diffraction (XRD) pattern in Figure1 shows that only the diffraction peaks from hemimorphite were detected. This confirms that the hemimorphite samples used in the experiments were of high purity. Minerals 2020, 10, 790 3 of 12

hemimorphite Intensity,a,u

10 20 30 40 50 60 70 80 2θ (°)

FigureFigure 1.1. XRDXRD patternpattern ofof purepure hemimorphitehemimorphite samples.samples.

3+ Isopentyl sodiumsodium xanthate xanthate (NaIX) (NaIX) was was used used as theas collectorthe collector and FeCland3 FeClwas3 used was asused the Feas thesource. Fe3+ Sodiumsource. Sodium sulﬁde (NasulfideS 9H (NaO)2S·9H was2O) used was as used the as sulfurizing the sulfurizing agent. agent. The pulp The pHpulp was pH adjustedwas adjusted with 2 · 2 aqueouswith aqueous NaOH NaOH and Hand2SO H42.SO All4. reagentsAll reagents were were analytical analytical grade. grade. Deionized Deionized water water was was produced produced by usingby using a Milli-Q5O a Milli-Q5O system system (Millipore, (Millipore, Burlington, Burlington, MA, MA, USA). USA).

2.2. Flotation Tests

A small-scalesmall-scale flotationflotation apparatus (XFGCâⅡ) with mechanical agitation was used for the flotationflotation experiments. The The hemimorphite hemimorphite samples samples (2 (2 g) g) were were mixed mixed with with deionized deionized water water (40 (40mL) mL) and and the pH the pHwas wasadjusted adjusted to the to thedesired desired value value by adding by adding NaOH NaOH or H or2SO H24;SO the4; pH the pHwas wasdetermined determined with with a pH a pHmeter meter (PHS (PHS-3C,-3C, Nanjing Nanjing T-Bota T-Bota Scietech Scietech Instruments Instruments & Equipment & Equipment Co., Co.,Ltd., Ltd.,Nanjing, Nanjing, China). China). The 2 Thepulp pulp was wasthen then sequentially sequentially conditioned conditioned with with S2− (5 S −min),(5 min), NaIX NaIX (2 min), (2 min), and and terpenic terpenic oil (1 oil min) (1 min) at an at anagitation agitation rate rate of of1700 1700 r/min. r/min. A Aflotation flotation time time of of 2 2min min was was used used in in all all single single-mineral-mineral experiments. experiments. After flotation,flotation, the solid was collected by filtration,filtration, dried, and weighed to determine the flotationflotation recovery. All experiments were repeated in triplicate, triplicate, and and average average flotation flotation recoveries recoveries were were recorded. recorded.

2.3. LEIS 2.3. LEIS A small small layer layer of of the the hemimorphite hemimorphite sample sample was was adhered adhered to epoxy to epoxy resin resin to form to form a substrate. a substrate. The Thesubstrate substrate was waspolished polished by hand by hand withwith wet silicon wet silicon carbide carbide paper paper in the in grit the sequence grit sequence 240, 320, 240, 400, 320, 600, 400, 600,800, and 800, 1200. and 1200.The freshly The freshly polished polished hemimorphite hemimorphite substrate substrate was washed was washed by ultrasonication by ultrasonication in ethanol in ethanoland deionized and deionized water for water 5 min. for The 5 min. freshly The freshlyprepared prepared substrate substrate was immediately was immediately transferred transferred to a 1 toL aelectrochemical 1 L electrochemical cell for cell LEIS. for LEIS. Pt wire Pt wire was was used used as asthe the counter counter electrode electrode and and a ann Ag/AgCl Ag/AgCl electrode was usedused as as the the reference reference electrode. electrode. LEIS wasLEIS performed was performed with a scanning with a electrochemicalscanning electrochemical workstation (VersaSCAN,workstation (VersaSCAN AMETEK SI,, AMETEK PA, USA). SI, PA,USA). µ The probe was fixed ﬁxed approximately approximately 50 50 μmm above above the the sample sample surface surface and and was was moved moved across across a a designated 0.2 0.2 mm2 surface area in step sizes of 10 µm in the X and Y directions. designated 0.2 × 0.2× mm2 surface area in step sizes of 10 μm in the X and Y directions. Similar Similarexperiments experiments have been have described been described in the literature. in the literature. After LEIS, After imaging LEIS, of imaging the hemimorphite of the hemimorphite substrate substrate was performed in 5 mM Na S solution, and 5 mM Na S + 1 mM Fe3+ solution. was performed in 5 mM Na2S solution, and2 5 mM Na2S + 1 mM Fe3+ solution.2 The hemimorphite The hemimorphite substrate was ultrasonically washed in ethanol and deionized water for 15 min substrate was ultrasonically washed in ethanol and deionized water for 15 min after use. KNO3 was afteradded use. to the KNO solution3 was addedto improve to the the solution electrical to improve conductivity. the electrical conductivity. 2.4. XPS 2.4. XPS A beaker containing deionized water (40 mL) was charged with a hemimorphite sample (2 g). A beaker containing deionized water (40 mL) was charged with a 3hemimorphite+ sample (2 g). The pulp (deionized water, 5 mM Na2S solution, or 5 mM Na2S + 2mM Fe solution) was magnetically The pulp (deionized water, 5 mM Na2S solution, or 5 mM Na2S + 2mM Fe3+ solution) was magnetically stirred for 5 min at 25 °C; the temperature was controlled with a water bath. The solid was collected by filtration and dried at 25 °C in a vacuum oven.

Minerals 2020, 10, 790 4 of 12

stirred for 5 min at 25 ◦C; the temperature was controlled with a water bath. The solid was collected by ﬁltration and dried at 25 ◦C in a vacuum oven. XPS was performed with a PHI 5000 Versaprobe-II (PHI5000, ULVAC-PHI, Kanagawa, Japan) scanning XPS microprobe system, with a monochromatic Al X-ray source (200 W). Each analysis started with a survey scan from 0 to 1100 eV. The acquired data were processed using MultiPak software and 284.8 eV was used as the standard C 1s binding energy

2.5. ICP-MS Pure hemimorphite (0.5 g) of particle size 38–74 µm was conditioned in a beaker with deionized water. Then 0.01 M Na2S was added and the pulp was conditioned for 5 min. FeCl3 solution (if needed) was added and the mixture was conditioned for 2 min. After solid-liquid separation, the solids were dissolved in 0.1 M HNO3 solution. The liquid samples were centrifuged (TL-4.7W, SCI, Jiangsu, China) for 20 min at a rotation speed of 4000 r/min. The S concentration in the supernatant was determined by ICP-MS (7700x, Agilent Technologies, Santa Clara, CA, USA).

2.6. TOF-SIMS Bulk hemimorphite samples were acquired by slicing and polishing by artiﬁcial methods. The bulk samples were added to a 100 mL beaker containing the desired solutions. Dilute NaOH solution was used to adjust the pulp pH to 11. After reaction for 5 min, the samples were air dried for analysis by TOF-SIMS (ION-TOF GmbH, Münster, Germany). All samples were placed in the middle of an 7 electron gun and evacuation was performed for 12 h to give a vacuum of at least 10− Pa during the analysis. The following surface analysis conditions were used: primary ion source Bi3+; ion energy 15 keV; analysis current 0.45 pA; analysis area 250,000 µm2; and measurement time 60 s. The following depth-proﬁling analysis conditions were used: sputter ion Cs+; sputter energy 2 keV; sputter current 34.23 nA; analysis area 100 100 µm2; and sputter time 52 s. Negative secondary-ion mass spectra 2 × 2 were calibrated. C−,O −, OH−, and C −; positive secondary-ion mass spectra were calibrated using + + + + + H ,C ,C2H3 ,C3H5 , and C4H7 .

3. Results and Discussion

3.1. Flotation Test Results Flotation tests were performed at various pH values. The results (Figure2a) show that the hemimorphite recovery increased significantly across the pH range on addition of Na2S to solutions with 5 10 4 mol/L NaIX as the collector; The results (Figure2b) show that the hemimorphite recovery × − increased with the concentration of Na2S but decline when use Na2S over 5mM, we speculation that too much Na2S may contend with NaIX as the collector. This shows that sulfurization considerably improved the floatability of hemimorphite. But in experiment results shown in Figure2c, when we add Fe3+, the recovery of hemimorphite is in decline. This is consistent with our conjecture. Kai et al. [8] observed the formation of amorphous or poorly crystalline ZnS on hemimorphite surfaces after treatment with Na2S, and the ZnS film on the hemimorphite surface enabled easy adsorption of xanthate species on the mineral surface. However, after Fe3+ addition to the pulp, the flotation recovery of hemimorphite decreased at the same Na2S concentration, i.e., between that for hemimorphite before and after treatment with Na2S. Zhang et al. [14] reported that sphalerite (ZnS) floated readily at pH 8–11 in the presence of Fe2+, but not in the presence of Fe3+. This suggests that Fe3+ can modify the Zn–S surface and make it more hydrophilic. We conclude that hemimorphite treated with Na2S can be depressed by Fe3+. There are two reasons for this: (1) Generation of a hydrotropic substance such as Fe(OH)3 on the hemimorphite surface; and (2) A decrease in the amount of sulfide film. More research was needed to clarify the mechanism of depression by Fe3+ during hemimorphite flotation. Minerals 2020, 10, 790 5 of 12

80 60 (a) 5mM Na2S (b) 3+ 70 Na2S+0.5mM Fe 50 pH=10 3+ 3+ Na2S+1mM Fe Fe =0 60 40

50 Recovery/% 30

20 40 0 2 4 6 8 10 60 Concentration of Na2S/mM Recovery/% 30 50 (c) pH=10 20 40 Na2S=5 mM

10 Recovery/% 30

0 20 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 0.0 0.2 0.4 0.6 0.8 1.0 Concentration of Fe3+/mM pH value FigureFigure 2. 2. ((aa)) Hemimorphite Hemimorphite flotation flotation recovery asas aa functionfunction of of pH pH value value in in the the absence absence and and presence presence of 3+ 3+ 3+ 3+ ofFe Fe,(,b ()b Hemimorphite) Hemimorphite flotation flotation recovery recovery as a functionas a function of Na2 Sof without Na2S without Fe ,(c )Fe Hemimorphite, (c) Hemimorphite flotation flotationrecovery recovery as a function as a function of Fe3+. of Fe3+.

3.2.3.2. LEIS LEIS ElectrochemicalElectrochemical reactions reactions can affect affect the surface conductivity of hemimorphite. LEIS LEIS maps maps of of the the untreated hemimorphite substrate and those treated with Na S or Na S + Fe3+ at pH 11 are shown untreated hemimorphite substrate and those treated with Na2S2 or Na2S2 + Fe3+ at pH 11 are shown in Figurein Figure 3a3–a–c.c. The The LEIS LEIS map map of of the the untreated untreated hemimorphite hemimorphite is is predominantly red-orange,red-orange, whichwhich indicatesindicates that that the the average average impedance impedance of of the the untreated untreated hemimorphite sample is 250 846.3 Ω.Ω. The The LEIS LEIS map of the hemimorphite sample after treatment with Na S is mainly blue, which indicates that the map of the hemimorphite sample after treatment with Na22S is mainly blue, which indicates that the averageaverage impedance impedance of of the the hemimorphite hemimorphite sample sample is 199 is 946.3 199 Ω 946.3. The Ω. decrease The decrease in the average in the impedance average impedancesuggests that suggests the material that the has material good electrical has good conductivity electrical becauseconductivity of the because generation of the of speciesgeneration such of as sulfide on the hemimorphite sample surface by treatment with Na S. However, the chemical component species such as sulfide on the hemimorphite sample surface by2 treatment with Na2S. However, the 3+ chemicalcould not component be identified could by LEIS not be analysis. identified When by FeLEIS isanalysis. present When in the solutionFe3+ is present during in sulfuration the solution of duringhemimorphite, sulfuration the LEISof hemimorphite, map of the hemimorphite the LEIS map sample of the is predominantlyhemimorphite green-yellow.sample is predominantly This implies greenthat the-yellow. average This impedance implies that of this the sample average is 231impedance 002.4 Ω. Theof this average sample impedance is 231 002.4 of the Ω. hemimorphite The average 3+ impedancesamples was of lower the hemimorphite than that of the samples sample was in the lower solution than withoutthat of the Fe sample. These in results the solution suggest without that the 3+ Fegood3+. These electrical results conductivity suggest generatedthat the good in the electrical hemimorphite conductivity samples generated decreased whenin the Fe hemimorphitewas present samplesin the solution decreased or species when withFe3+ was poor present electrical in the conductivity solution or (e.g., species iron with oxide poor hydroxide) electrical were conductivity generated (e.g.,on the iron surface oxide of hydroxide) the hemimorphite were generated sample. on These the surface results, of which the hemimorphite show that a decrease sample. inThese the amountresults, whichof sulfide show species that a ordecrease an increase in the in amount the amount of sulfide of iron species oxide or hydroxide an increase on in the the hemimorphite amount of iron surface oxide 3+ hydroxidedecreases theon floatabilitythe hemimorphite of hemimorphite surface samplesdecreases compared the floatability with that of in hemimorphit a solution withoute samples Fe , comparedare in good with agreement that in a with solution the flotation without testFe3+ results., are in good agreement with the flotation test results.

Minerals 2020, 10, 790 6 of 12

Figure 3. LEIS maps of hemimorphite surfaces treated with (a) deionized water, (b) 5 mM Na2S, and Figure 3. LEIS maps of hemimorphite surfaces treated with (a) deionized water, (b) 5 mM Na2S, (c) 5 mM Na2S and 1mM Fe3+. 3+ and (c) 5 mM Na2S and 1mM Fe .

3.3. Adsorption Adsorption Test Test Results LEIS analysis ( (SectionSection 33.2.2)) identified identified the changes in in the the surface surface properties properties of hemimorphite and showed thatthat thethe averageaverage impedance impedance of of a hemimorphitea hemimorphite sample sample decreases decreases in a in solution a solution containing containing Fe3+ . FeThis3+. sectionThis section focuses focuses on variations on variations in the amountin the amount of S adsorbed of S adsorbed on the surface. on the Adsorptionsurface. Adsorption amounts amountswere determined were determined by ICP-MS; by ICP the-MS; results the results are shown are shown in Figure in Figure4. The 4. amount The amount of S adsorbedof S adsorbed for 3+ 6 2 3+ forthe the pulp pulp without without Fe Fewas3+ was 4.2 4.2 10× −10−mol6 mol/m/m .2. However,However, withwith additionaddition ofof FeFe3+, the amount of of × adsorbed SS decreaseddecreased significantly. significantly. The The amount amount of S adsorbedof S adsorbed on the hemimorphiteon the hemimorphite surface decreased surface 3+ 3+ decreasedwith increasing with increasing Fe concentration. Fe3+ concentration. This is because This is because Fe ions Fe react3+ ions with react S specieswith S species adsorbed adsorbed on the onhemimorphite the hemimorphite surface, surface, which decreaseswhich decr theeases amount the amount of S, and of the S, eandffect the of sulfidizationeffect of sulfidization is therefore is thereforeattenuated attenuated with increasing with increasing Fe3+ concentration. Fe3+ concentration. The amount The ofamount adsorbed of adsorbed Fe species Fe detected species detected was low, waswhich low, suggests which that suggests this process that doesthis notprocess lead todoes adsorption not lead of Feto species adsorption on the of hemimorphite Fe species surface.on the hemimorphite surface.

Minerals 2020, 10, 790 7 of 12

M) 40 -7

10 Adsorption amount of S/(10 Adsorption amount of

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 3+ The concentration of Fe /(mM) FigureFigure 4.4. Adsorption amount of S on the hemimorphitehemimorphite surface.surface.

MoreMore sensitivesensitive information information regarding regarding adsorption adsorption of iron andof iron sulfide and species sulfide on thespecies hemimorphite on the surfacehemimorphite was obtained surface bywas using obtained TOF-SIMS by using to perform TOF-SIMS surface to perform analysis surface and depth-profile analysis and analysis. depth- TOF-SIMSprofile analysis. is a sensitive TOF-SIMS and reliableis a sensitive method and for reliable identifying method and understandingfor identifying interactions and understanding between mineralsinteractions and between various minerals pulp components. and various pulp components. TheThe normalizednormalized TOF-SIMSTOF-SIMS peakpeak intensitiesintensities forfor thethe hemimorphitehemimorphite samplessamples treatedtreated withwith NaNa22SS inin solutionssolutions withwith andand withoutwithout FeFe33++ areare shown in Figure 55.. ReconstructedReconstructed 3D3D negative-depthnegative-depth profilesprofiles for for thethe hemimorphitehemimorphite samplessamples afterafter SS adsorptionadsorption areare shownshown inin FigureFigure6 6.. FigureFigure5 a5a shows shows that that the the range range 2 4 1 ofof thethe normalizednormalized ZnS−,, Zn Zn22S,S, S S−,− and, and S2− S peak− peak relative relative intensities intensities was was 10 10−4 to− 10to−1 10. This− . This implies implies that thatsulfide sulfide species species are adsorbed are adsorbed on the on hemimorphite the hemimorphite surface surface and mineral and mineral surface surface in the inform the Zn form–S. Zn–S.These Theseresults results are in are agreement in agreement with withthose those of previous of previous research. research. After After treatment treatment with with Fe3+ Fe, the3+, 2 thenormalized normalized TOF TOF-SIMS-SIMS intensities intensities of the of thepeaks peaks for forZnS ZnS−, Zn−,2S, Zn S2−S,, and S−, andS2− decreased S − decreased gradually gradually with withincreasing increasing Fe3+ Feconcentration.3+ concentration. This This indicates indicates that that the the amount amount of ofsulfide sulfide film film on on the the hemimorphite surfacesurface decreaseddecreased whenwhen FeFe33++ waswas added added to to the the solution. solution. TOF-SIMS positive-ion analysis was used to detect iron species adsorbed on the surface. The -1 10-1 10 + + 2+ 5mM Na S normalized peak 5mM Na Sintensities for Fe , FeOH, FeS , and Fe2S are shown2 in Figure 5b. The range(b) of the 2 (a) 3+ 3+ 5mM Na S+0.5mM Fe 5mM Na S+0.5mM Fe 2 + 2 + 2+ −7 −5 2 -2 3+ normalized Fe , FeOH,3+ FeS , and Fe S peak relative intensities10 was 5mM Na 102S+1mM to Fe 10 . The positive-ion peak 5mM Na2S+1mM Fe relative intensities increased with increasing concentration of Fe3+ in the pulp. However, the relative -2 -3 intensity10 of the peaks was too low to show that Fe species10 were adsorbed on the hemimorphite

-4 3+ surface. The results differ from those obtained in a study10 of Fe adsorption on smithsonite surfaces. Deng et al. found that Fe(III) greatly decreased adsorption of S2− on the smithsonite surface and Fe(III) -3 -5 Relative intensity 10 speciesRelative intensity 10were adsorbed on the smithsonite surface via both electrostatic and chemical interactions. This difference shows that the mechanism of depression10 by-6 iron ions of zinc oxide ores varies. The difference show that the depression mechanism of iron ions to zinc oxide ores is various. Meanwhile, 10-4 10-7 the implicationZnS -for minerals- sulfide- process2- is also different. + + + 2+ Zn2S S S Fe FeOH FeS Fe2S Figure 6 shows Secondarythe reconstructed ions 3D negative-depth profile for the Secondaryhemimorphite ions sample after

S− adsorption. A Zn-S film was generated on the hemimorphite surface by treatment with Na2S. FigureFigure 5. 5.Relative Relative intensities intensities of of ( a()a) positive positive secondary secondary ions ions and and ( b()b negative) negative secondary secondary ions ions on on the the However, the Zn-S film was attenuated in a solution containing 1 mM Fe3+. This shows that Fe3+ can hemimorphitehemimorphite surface surface layer. layer. decrease the adsorption of S species on the hemimorphite surface. These results are in good agreement with the ICP-MS results. The results show that the Zn-S film on the hemimorphite surface was attenuated in a solution containing Fe3+ because hemimorphite was depressed, not because Fe species were adsorbed on the hemimorphite surface. The chemical reaction at the mineral surface is unknown and needs to be studied further.

Figure 6. Reconstructed 3D negative-ion (ZnS−) depth-profiling images of hemimorphite samples treated with (a) deionized water, (b) 5 mM Na2S, and (c) 5 mM Na2S and 1mM Fe3+.

-1 10-1 10 5mM Na S 5mM Na S 2 (b) 2 (a) 3+ 3+ 5mM Na2S+0.5mM Fe 5mM Na2S+0.5mM Fe -2 3+ 3+ 10 5mM Na2S+1mM Fe 5mM Na2S+1mM Fe

-3 10-2 10

10-4

-3 -5 Relative intensity 10 Relative intensity 10

10-6

10-4 10-7 ZnS- - - 2- + + + 2+ Zn2S S S Fe FeOH FeS Fe2S Secondary ions Secondary ions MineralsFigure2020, 105. ,Relative 790 intensities of (a) positive secondary ions and (b) negative secondary ions on the8 of 12 hemimorphite surface layer.

− FigureFigure 6.6. ReconstructedReconstructed 3D3D negative-ionnegative-ion (ZnS(ZnS−)) depth-proﬁlingdepth-profiling images ofof hemimorphitehemimorphite samplessamples 33++ treatedtreated withwith ((aa)) deionizeddeionized water,water, ((bb)) 55 mMmM NaNa22S,S, andand ((cc)) 55 mMmM NaNa22S and 1mM Fe . .

TOF-SIMS positive-ion analysis was used to detect iron species adsorbed on the surface. + + 2+ The normalized peak intensities for Fe , FeOH, FeS , and Fe2S are shown in Figure5b. The range of the + + 2+ 7 5 normalized Fe , FeOH, FeS , and Fe2S peak relative intensities was 10− to 10− . The positive-ion peak relative intensities increased with increasing concentration of Fe3+ in the pulp. However, the relative intensity of the peaks was too low to show that Fe species were adsorbed on the hemimorphite surface. The results differ from those obtained in a study of Fe3+ adsorption on smithsonite surfaces. Deng et al. 2 found that Fe(III) greatly decreased adsorption of S − on the smithsonite surface and Fe(III) species were adsorbed on the smithsonite surface via both electrostatic and chemical interactions. This difference shows that the mechanism of depression by iron ions of zinc oxide ores varies. The difference show that the depression mechanism of iron ions to zinc oxide ores is various. Meanwhile, the implication for minerals sulfide process is also different. Figure6 shows the reconstructed 3D negative-depth profile for the hemimorphite sample after S− adsorption. A Zn-S film was generated on the hemimorphite surface by treatment with Na2S. However, the Zn-S film was attenuated in a solution containing 1 mM Fe3+. This shows that Fe3+ can decrease the adsorption of S species on the hemimorphite surface. These results are in good agreement with the ICP-MS results. The results show that the Zn-S film on the hemimorphite surface was attenuated in a solution containing Fe3+ because hemimorphite was depressed, not because Fe species were adsorbed on the hemimorphite surface. The chemical reaction at the mineral surface is unknown and needs to be studied further.

3.4. XPS Results XPS is a surface-sensitive technique and can be used to characterize a surface to a depth of less than 10 nm. This technique was used to identify the chemical states and contents of the elements on the surfaces of the untreated and treated hemimorphite samples, on the basis of the distinctive binding energies of the inner electrons of each element [15,16]. In this work, XPS was used to identify the 3+ differences between untreated hemimorphite samples and those treated with Na2S or Na2S + Fe , and to clarify the chemical reactions on the mineral surface. The full spectra of the treated and untreated hemimorphite samples over the binding energy range 1100–0 eV are shown in Figure7. The spectra show the presence of the expected elements from hemimorphite, such as Si, O, and Zn, but no other peaks were detected in the XP spectrum of the untreated mineral. This implies that the hemimorphite was pure. A new, weak peak appeared at 3+ 161.86 eV in the XP spectrum of hemimorphite treated with Na2S. In the solution containing Fe , the S 2p peak was clearly attenuated. The iron species sign was not identified by XPS; the same results were obtained for the Fe 2p high-resolution XP spectrum. This is in agreement with the adsorption test and TOF-SIMS results. The results suggest that Fe3+ cannot be adsorbed on the hemimorphite surface. However, Fe3+ can decrease the adsorption of S species on the hemimorphite surface. The high-resolution S 2p XP spectra are shown in Figure8. The spectrum of the sample with 3.4. XPS Results XPS is a surface-sensitive technique and can be used to characterize a surface to a depth of less than 10 nm. This technique was used to identify the chemical states and contents of the elements on the surfaces of the untreated and treated hemimorphite samples, on the basis of the distinctive binding energies of the inner electrons of each element [15,16]. In this work, XPS was used to identify the differences between untreated hemimorphite samples and those treated with Na2S or Na2S + Fe3+, and to clarify the chemical reactions on the mineral surface. The full spectra of the treated and untreated hemimorphite samples over the binding energy range 1100–0 eV are shown in Figure 7. The spectra show the presence of the expected elements from hemimorphite, such as Si, O, and Zn, but no other peaks were detected in the XP spectrum of the untreated mineral. This implies that the hemimorphite was pure. A new, weak peak appeared at 161.86 eV in the XP spectrum of hemimorphite treated with Na2S. In the solution containing Fe3+, the S 2p peak was clearly attenuated. The iron species sign was not identified by XPS; the same results were obtained for the Fe 2p high-resolution XP spectrum. This is in agreement with the adsorption test and TOF-SIMS results. The results suggest that Fe3+ cannot be adsorbed on the hemimorphite Minerals 2020, 10, 790 9 of 12 surface. However, Fe3+ can decrease the adsorption of S species on the hemimorphite surface. The high-resolution S 2p XP spectra are shown in Figure 8. The spectrum of the sample with Fe3+ shows Feno3 +S showssignals, no whereas S signals, the whereas latter (sulfated the latter (sulfatedwithout Fe without3+) spectrum Fe3+) spectrum features featuresS 2p3/2 S(161.56 2p3/2 (161.56eV) and eV) S 2− 2 and2p1/2 S 2p1(162.74/2 (162.74 eV) peaks. eV) peaks. These Thesepeaks peaksare ascribable are ascribable to S ; tono Sother−; no S other species S species were detected were detected on the onhemimorphite the hemimorphite surface. surface. The spectrum The spectrum of the sample of the sample with th withe solution the solution containing containing Fe3+ shows Fe3+ Sshows 2p3/2 2− 2 S(161.64 2p3/2 eV) (161.64 and eV)S 2p1/2 and S(162.82 2p1/2 eV) (162.82 peaks, eV) which peaks, are which also areascribable also ascribable to S [17,18 to S]; −no[17 other,18]; S no species other Swere species found were on found the hemimorphite on the hemimorphite surface. The surface. S 2p The peak S 2pwas peak clearly was less clearly intense less intensethan that than for that the forsample the samplewith no with added no addedFe3+. Table Fe3+ .1 Tablesummarizes1 summarizes the concentrations the concentrations of the ofvarious the various Si, Zn, Si, O, Zn, and O, S andspecies S species on the on treated the treated and untreated and untreated hemimorphite hemimorphite surfaces. surfaces. The data The in data Table in 1 Table show1 showthat th thate levels the levelsof O, Si, of O,and Si, S andin the S in untreated the untreated hemimorphite hemimorphite were were49.59, 49.59, 10.67, 10.67, and 39.74 and 39.74at%, at%,respectively. respectively. The Theconcentration concentration of S ofwas S wasdetermined determined to be to 8.20 be 8.20 at% at%from from the theS signals S signals detected detected after after treatment treatment with with 0.1 33++ 0.1mM mM Na Na2S. 2ComparedS. Compared to tothat that of ofthe the hemimorphite hemimorphite sample sample in in the the solution solution with FeFe ,, the the concentration concentration of SS inin thethe hemimorphitehemimorphite samplesample waswas lower,lower, namelynamely 5.185.18 at%.at%. These results are inin agreementagreement withwith thosethose reportedreported above.above. We cancan concludeconclude thatthat SS speciesspecies were adsorbedadsorbed onon thethe hemimorphite surfacesurface inin 22− 3+3+ thethe formform ofof SS −,, and and the the addition addition of of Fe Fe toto the the solution solution decreased decreased sulfide sulfide adsorption. adsorption.

100000 a 80000

60000

40000 S 2p 20000

0 100000 b 80000

60000

40000

20000 I tensi ty(arb.uni ts)

0 100000 c

80000

60000

40000

20000

0 1100 1000 900 800 700 600 500 400 300 200 100 0 Binding energy(eV)

Figure 7. XPS survey spectra of hemimorphite treated with (a) deionized water, (b) 5 mM Na2S 3+ (c) 5 mM Na2S + 1mM Fe . MineralsFigure2020, 7.10 ,XPS 790 survey spectra of hemimorphite treated with (a) deionized water, (b) 5 mM Na2S (c) 105 of 12 mM Na2S + 1mM Fe3+.

4500 a 4000 3500 3000 2500 2000 6000 b 161.56eV

5000

CPS 4000

3000

2000 4500 c 161.64eV 4000

3500

3000

2500

2000 166 165 164 163 162 161 160 159 158 Binding Energy(eV)

Figure 8. 8. S2pS2p XPS XPS spectra spectra of hemimorphite of hemimorphite treated treated with ( witha) deionized (a) deionized water, ( water,b) 5 mM (b Na) 52S, mM (c) 5 Na mM2S, 3+ (Nac) 52S mM+ 1mM Na2 FeS +3+.1mM Fe .

Table 1. Concentrations of various elements on the hemimorphite surfaces. Table 1. Concentrations of various elements on the hemimorphite surfaces. Atomic Concentration/% Samples Atomic Concentration/% Samples OO SiSi Zn ZnS S UntreatedUntreated 49.59 49.59 10.6710.67 39.74 39.740 0 Na S 45.73 8.86 37.22 8.20 2 2 3+ Na S 45.73 8.86 37.22 8.20 Na2S + Fe 47.63 9.08 38.11 5.18 Na2S + Fe3+ 47.63 9.08 38.11 5.18 Determined from the intensities of the O 1s, Si 2p, S 2p, and Zn 2p XPS spectral lines. Determined from the intensities of the O 1s, Si 2p, S 2p, and Zn 2p XPS spectral lines.

4. Conclusions 4. Conclusions The mechanism of sulfated hemimorphite depression by Fe3+ is a complicated process. Use of a The mechanism of sulfated hemimorphite depression by Fe3+ is a complicated process. Use of a combination of techniques is needed to understand the mechanism. This study examined changes combination of techniques is needed to understand the mechanism. This study examined changes caused by S adsorption at the hemimorphite/water interface and on the mineral surface and assessed caused by S adsorption at the hemimorphite/water interface and on the mineral surface and assessed the implications of these changes for hemimorphite flotation. The following conclusions can be the implications of these changes for hemimorphite flotation. The following conclusions can be drawn drawn from our analysis of the results of this study. from our analysis of the results of this study. (1) High hemimorphite floatability can be achieved by treatment with Na2S, but the flotation (1) High hemimorphite floatability can be achieved by treatment with Na2S, but the flotation recovery decreases with increasing concentration of Fe3+ in solution. The addition of Fe3+ to the recovery decreases with increasing concentration of Fe3+ in solution. The addition of Fe3+ to the solution leads to hemimorphite sample depression at all pH values. solution leads to hemimorphite sample depression at all pH values. (2) The electrochemical impedance of the hemimorphite sample decreased after treatment with (2) The electrochemical impedance of the hemimorphite sample decreased after treatment with 3+ Na2S. The sulfuration of hemimorphite in a solution containing Fe3+ improved the average impedance Na2S. The sulfuration of hemimorphite in a solution containing Fe improved the average impedance of the hemimorphite sample. This implies that Fe3+ attenuates the adsorption of high-conductivity of the hemimorphite sample. This implies that Fe3+ attenuates the adsorption of high-conductivity materials or leads to the generation of low-conductivity materials on the mineral surface. materials or leads to the generation of low-conductivity materials on the mineral surface. (3) Adsorption tests showed that Fe species were not adsorbed on the hemimorphite surface but (3) Adsorption tests showed that Fe species were not adsorbed on the hemimorphite surface but treatment with Fe3+ attenuated the adsorption of S species. Reconstructed 3D negative-depth profiles treatment with Fe3+ attenuated the adsorption of S species. Reconstructed 3D negative-depth profiles for the hemimorphite samples showed that the amount of sulfide film decreased significantly after for the hemimorphite samples showed that the amount of sulfide film decreased significantly after treatment with Fe3+. treatment with Fe3+.

Minerals 2020, 10, 790 11 of 12

(4) The results of XPS analysis were similar to those of the adsorption tests and suggested that 2 3+ S was adsorbed on the hemimorphite surface in the form S − and Fe addition to the pulp led to a decrease in sulfide film formation. The hemimorphite floatability, therefore, decreases after treatment with Fe3+.

Author Contributions: Conceptualization, J.L. and Q.F.; Methodology, J.L.; Software, Q.Z.; Validation, S.W., Q.F. and J.L.; Formal Analysis, Y.Z.; Investigation, Y.W.; Resources, J.L.; Data Curation, W.N.; Writing—original draft preparation, J.L.; Writing—review and editing, J.L.; Visualization, Q.F.; Supervision, Q.F.; Project Administration, S.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (No. 51804144), and Ten Thousand Talent Plans for Young Top-notch Talents of Yunnan Province (No. YNWR-QNBJ-2018-051). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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