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Article Flocculation of Pyrite Fines in Aqueous Suspensions with Corn Starch to Eliminate Mechanical Entrainment in Flotation

Wei Ge, Hongqiang Li *, Yanzeng Ren, Feiyu Zhao and Shaoxian Song

Received: 2 September 2015 ; Accepted: 23 September 2015 ; Published: 10 October 2015 Academic Editor: Kota Hanumantha Rao

School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China; [email protected] (W.G.); [email protected] (Y.R.); [email protected] (F.Z.); [email protected] (S.S.) * Correspondence: [email protected]; Tel.:+86-150-7109-4162

Abstract: The hydrophilic flocculation of pyrite fines in aqueous suspensions with corn starch was studied by measuring particle size distribution, microscopy observation and micro-flotation. Furthermore, the interaction of corn starch with pyrite was investigated by determining the density and based on measurements and X-ray photoelectron spectrometer (XPS) analysis in this work. The results of the particle size distribution measurement show that corn starch can effectively aggregate pyrite fines, and the pyrite floccules (flocs) are sensitive to mechanical stirring. The micro-flotation results suggest that the mechanical entrainment of pyrite fines in flotation can be effectively eliminated through the formation of large-size flocs. The zeta potential of pyrite particles decreases with the addition of corn starch. The XPS results prove that carboxyl groups are generated on the digested corn starch, and both iron hydroxyl compounds and ferrous disulfide on the pyrite surface can chemically interact with the corn starch digested by .

Keywords: hydrophilic flocculation; pyrite fines; corn starch; mechanical entrainment; adsorption

1. Introduction In froth flotation, mineral particles are floated through two methods: true flotation and mechanical entrainment. The former is a selective , by which hydrophobic mineral particles are attached to bubbles and transferred into the froth product. The latter is a non-selective process, where both hydrophilic and hydrophobic particles can be carried into the froth product through water in the triangle zone among air bubbles [1,2]. The entrainment is the primary mechanism, by which liberated and dispersed gangue fines (minus 30 µm) enter the froth products [3], which leads to a poor concentrate in grade. It was reported that the formation of hydrophilic flocs of gangue mineral fines before the flotation was an effective method to eliminate the entrainment [4–7]. Pyrite is a common mineral impurity in sulfide mineral flotation. Many reports have investigated the depression of pyrite with particle size larger than 30 µm in flotation; they indicate that several depressants such as dextrin, carboxy methyl cellulose (CMC), starch and tannins can significantly depress pyrite in the corresponding conditions because hydrophilic surfaces form on the pyrite [8–14]. However, if the sulfide ore is finely disseminated (the grain size is smaller than 30 µm), the pyrite is easily recovered using mechanical entrainment. Thus, it is difficult to remove pyrite from other valuable sulfide minerals in flotation even under good depressing conditions. Few works report on reducing the entrainment of pyrite fines using hydrophilic flocculation.

Minerals 2015, 5, 654–664; doi:10.3390/min5040515 www.mdpi.com/journal/minerals Minerals 2015, 5, 654–664

In this work, the hydrophilic flocculation of pyrite fines in aqueous suspensions was studied with corn starch as a flocculant using flocculation tests, microscopy observation, and micro-flotation tests.Minerals The 2015 interaction, 5, page–page of corn starch with pyrite was investigated in the adsorption test, zeta potentialtests. measurementThe interaction and of corn X-ray starch photoelectron with pyrite spectrometer was investigated analysis. in the The adsorption objective test, is tozeta better understandpotential the measurement flocculation and behavior X-ray photoelectron and the interaction spectrometer mechanism analysis. between The objective pyrite andis to corn better starch, and tounderstand discover the a potentialflocculation method behavior to and avoid the interaction the mechanical mechanism entrainment between pyrite of pyrite and finescorn starch, in sulfide mineraland flotation.to discover a potential method to avoid the mechanical entrainment of pyrite fines in sulfide mineral flotation. 2. Experimental 2. Experimental 2.1. Materials 2.1. Materials The pyrite sample used in this study was collected from Daye iron mine, Hubei province, China. The pyrite sample used in this study was collected from Daye iron mine, Hubei province, It was analyzed with chemical analysis and X-ray diffraction and showed approximately 96% FeS2 in China. It was analyzed with chemical analysis and X-ray diffraction and showed approximately purity. The sample was ground and sieved to obtain the minus 30 µm fraction, which was maintained 96% FeS2 in purity. The sample was ground and sieved to obtain the minus 30 μm fraction, which in a sealed plastic bottle and stored in a refrigerator. Figure1 illustrates the particle size distribution; was maintained in a sealed plastic bottle and stored in a refrigerator. Figure 1 illustrates the particle the d50size (particle distribution; size the at 50% d50 cumulative(particle size undersize)at 50% cumulative is approximately undersize) is 8.5 approximatelyµm, and d80 8.5 (particle μm, and size at 80% cumulatived80 (particle undersize)size at 80% cumulative is approximately undersize) 17.5 is µapproximatelym. 17.5 μm.

Figure 1. Size distribution of pyrite fines. Figure 1. Size distribution of pyrite fines. Corn starch used as a flocculant in this study was from Aladdin Industrial Co. (Shanghai, CornChina). starch Its molecular used as aweight flocculant of 150,000 in this was study determined was from using Aladdin the dynamic Industrial light Co. scattering (Shanghai, China).technique. Its molecular Hydrochloric weight acid of 150,000 and sodium was determined hydroxide, usingwhich thewere dynamic used to light adjust scattering the pH, technique.were Hydrochloricobtained acidfrom andthe sodiumXinyang hydroxide,chemical reagent which an wered Sinopharm used to adjust Chemical the pH,Reagent were Co. obtained (Shanghai, from the XinyangChina), chemical respectively. reagent All andchemicals Sinopharm were of Chemicalanalytical purity. Reagent Co. (Shanghai, China), respectively. All chemicalsThe water were ofin analyticalthis work was purity. produced using a Millipore Milli-Q Direct 8 system (Millipore SAS, Molsheim, France) with the resistivity of 18.25 Mٷcm. The water in this work was produced using a Millipore Milli-Q Direct 8 water purification system The corn starch stock was prepared by mixing 20 mL of 2.5 mol/L NaOH solution with (Millipore SAS, Molsheim, France) with the resistivity of 18.25 M٨ cm. 0.1 g corn starch in 50 mL water and subsequently diluted to 100 mL. The corn starch stock solution was prepared by mixing 20 mL of 2.5 mol/L NaOH solution with 0.1 g2.2. corn Methods starch in 50 mL water and subsequently diluted to 100 mL.

2.2. Methods2.2.1. Hydrophilic Flocculation Test

2.2.1. HydrophilicThe hydrophilic Flocculation flocculation Test of pyrite fines in aqueous suspensions was conducted in a stirring tank (Xing Rui experimental equipment business department, Nantong, China) of 42-mm inner Thediameter hydrophilic and 100-mm flocculation height with of pyritea blade fines impeller. in aqueous First, 1 suspensionsg of pyrite and was 100 conducted mL of water in were a stirring tankmixed (Xing while Rui experimentalHCl or NaOH equipmentsolution was businessadded for department,the pH adjustment. Nantong, Then, China) the of 42-mm was inner diameterconditioned and 100-mm at a given height stirring with speed a blade for a gi impeller.ven time in First, the presence 1 g of pyrite of corn and starch. 100 mL of water were mixed while HCl or NaOH solution was added for the pH adjustment. Then, the suspension was conditioned at a given stirring speed for a given time in the presence of corn starch. 2 655 Minerals 2015, 5, 654–664

2.2.2. Particle Size Analysis A Malvern Mastersizer 2000 (Malvern Instruments, Worcestershire, UK) was used to determine the size distributions of the original and flocculated pyrite particles in this study. d50 (Mean floc size) was reported in this paper. A peristaltic pump was used to circulate the flocs through the size analyzer instead of the fan impellers to protect the flocs from breakage during the measurements. The speed of the peristaltic pump was fixed at 280 rev/min. Each test was performed in triplicate, and the error was within ˘2.5 µm. The arithmetic average result of the three tests was reported in this paper.

2.2.3. Observation of Pyrite Flocs Hydrophilic flocs were observed using a Leica DMLP optical microscope (Leica, Wetzlar, Germany), which was equipped with a digital camera. The sample was prepared by dropping the suspensions of pyrite flocs onto a glass plate. During the observation, the flocs were imaged.

2.2.4. Micro-Flotation Tests Micro-flotation was performed in a Hallimond flotation tube (Xing Rui experimental equipment business department, Nantong, China) with nitrogen as the bubble source. A pyrite suspension was transferred into the tube and subsequently diluted to 100 mL. Then, flotation was performed for 4 min at a nitrogen gas flow rate of 15 mL/min. During the flotation, no collector or frother was used. The floated and non-floated products were dried. The recovery was obtained using the total weight of two products divided by the floated product weight. Each test was performed in triplicate. The arithmetic average result of the three tests was reported in this paper.

2.2.5. Adsorption Density Determination The adsorption densities were determined using the batch depletion method, and the specific surface area of the 3.169 m2/g pyrite sample was measured using a BET Monosorb unit (Gold Aipu Technology Co., Ltd, Beijing, China). 0.5 g of pyrite powder was taken to make 50 mL of solution by adding the desired amounts of 10´3 mol/L NaCl and corn starch solution of known concentration, and the pH was adjusted in 150-mL Erlenmeyer flasks. The suspensions were subsequently agitated for 2 h in a shaking bath at 25 ˝C. After equilibration, the solution pH was again recorded. The solution was subsequently centrifuged for 30 min at approximately 1720 g using a Thermo Scientific Sorvall ST 16 (Thermo Scientific, Waltham, MA, USA). Then, the supernatant liquid was analyzed to obtain the equilibrium concentration of corn starch that remained in the solution using a Thermo Scientific spectrometer (Thermo Scientific) with the phenol-sulfuric acid method [15]. Each test was performed in triplicate. The arithmetic average result of the three tests was reported in this paper.

2.2.6. Zeta Potential Measurements Zeta potential measurements were conducted using a Malvern Zetasizer Zeta-Nano (Malvern Instruments). In the measurements, all and reagents were prepared with stock solutions of 10´3 mol/L NaCl to maintain a constant . 1 g of the pyrite fines was dispersed in 1 L NaCl solution. For each measurement, 10 mL of this solution was siphoned out and diluted to 100 mL; the pH adjustment and reagent additions were also performed. The conditioned sample was placed in the sample cell of ZetaPALS, and the zeta potentials of the suspended particles were recorded.

2.2.7. X-Ray Photoelectron Spectrometer (XPS) Studies The XPS analyses were performed using a VG Multilab 2000 X-ray photoelectron spectrometer (Thermo Scientific) and an Al Kα X-ray source with a solution of 0.47 eV. The sample of pyrite, which was treated with corn starch, was prepared by mixing 0.5 g of pyrite with 100 mL of 50 mg/L corn

656 Minerals 2015, 5, 654–664

Minerals 2015, 5, page–page starch solution at pH 7.3. The treated pyrite sample was filtered and washed with deionized water afterMineralscorn stirring starch 2015 with, 5solution, page–page a magnetic at pH bar7.3. atThe 25 treated˝C. All pyrite samples sample were was dried filtered in aand vacuum washed desiccator. with deionized water after stirring with a magnetic bar at 25 °C. All samples were dried in a vacuum desiccator. 3. Resultscorn starch and Discussionsolution at pH 7.3. The treated pyrite sample was filtered and washed with deionized water3. Results after and stirring Discussion with a magnetic bar at 25 °C. All samples were dried in a vacuum desiccator. 3.1. Flocculation of Pyrite Fines 3.3.1. Results Flocculation and Discussion of Pyrite Fines Figure2 shows the effect of pH on the size of the pyrite flocs with corn starch. As observed, the flocs3.1. size FlocculationFigure significantly 2 shows of Pyrite increases the Fineseffect withof pH the on presencethe size of ofthe corn pyrite starch. flocs with The sizecorn ofstarch. the pyriteAs observed, flocs increases the flocs size significantly increases with the presence of corn starch. The size of the pyrite flocs with the increase in pH: at pH 3.6, the size is approximately 40 µm, which is nearly five times larger increasesFigure with 2 shows the increase the effect in ofpH: pH at on pH the 3.6, size the of sizethe pyriteis approximately flocs with corn 40 μ starch.m, which As observed,is nearly fivethe µ µ thanflocstimes the original sizelarger significantly than particle the original sizeincreases (d50 particle with = 8 size them), (d50 presence and = 8 itμm), increasesof andcorn it starch.increases to approximately The to sizeapproximately of the 60 pyritem 60 at μ flocsm pH at 11.3. The resultsincreasespH 11.3. suggest The with results the that increase suggest corn starch in that pH: corn is at an pHstarch effective 3.6, is the an flocculantsizeeffective is approximately flocculant for pyrite for 40 finespyrite μm, infines which the in tested theis nearly tested pH five rangepH in aqueoustimesrange suspensions, largerin aqueous than suspensions,the and original the alkaline particleand the pulp sizealkaline (d50 is favorable pulp = 8 μism), favorable forand theit increasesfor formation the formation to approximately of pyrite of pyrite flocs. flocs. 60 μ m at pH 11.3. The results suggest that corn starch is an effective flocculant for pyrite fines in the tested pH range in aqueous suspensions, and the alkaline pulp is favorable for the formation of pyrite flocs.

Figure 2. Effect of pH on the mean size of pyrite flocs in the presence of corn starch. Figure 2. Effect of pH on the mean size of pyrite flocs in the presence of corn starch. The effectFigure of 2.the Effect stirring of pH speed on the onmean the size size of pyriteof pyrite flocs inflocs the presencewas studied, of corn and starch. the results are Theillustrated effect in of Figure the stirring 3. Figure speed 3 shows on thethat sizethe st ofirring pyrite strength flocs wasstrongly studied, affects andthe thesize resultsin the are illustratedpresenceThe in effectof Figure corn of 3 starch..the Figure stirring The3 sizesshowsspeed increase on that the the withsize stirring ofthe pyrite increase strength flocs in wasstirring strongly studied, speed affects and until the thea resultsmaximum size inare the presenceillustratedvalue of is corn reached in starch.Figure and The3. subsequently Figure sizes 3 increase shows decrease. that with the theThe st increaseirringcritical strength stirring in stirring stronglyspeed speed is affects400 until rev/min the a maximum size for in corn the value presence of corn starch. The sizes increase with the increase in stirring speed until a maximum is reachedstarch. andAt high subsequently stirring speeds, decrease. the decrease The in critical floc sizes stirring may be speed attributed is 400 to rev/minthe destruction for corn of the starch. value is reached and subsequently decrease. The critical stirring speed is 400 rev/min for corn At highpyrite stirring flocs. This speeds, result the suggests decrease that in the floc flocculated sizes may flocs be with attributed the polymers to the are destruction much weaker of the than pyrite starch.the hydrophobic At high stirring agglomeration speeds, the in termsdecrease of strength in floc sizes [16]. may be attributed to the destruction of the flocs.pyrite This flocs. result This suggests result suggests that the that flocculated the flocculated flocs flocs with with the th polymerse polymers are are much much weakerweaker than than the hydrophobicthe hydrophobic agglomeration agglomeration in terms in terms of strength of strength [16]. [16].

Figure 3. Effect of the stirring speed on the size of pyrite flocs with corn starch.

4 Figure 3. Effect of the stirring speed on the size of pyrite flocs with corn starch. Figure 3. Effect of the stirring speed on the size of pyrite flocs with corn starch. 4

657 Minerals 2015, 5, 654–664

Minerals 2015, 5, page–page The size of the pyrite flocs as a function of stirring time at the speed of 500 rev/min is shown Minerals 2015, 5, page–page in Figure4The. Pyritesize of finesthe pyrite are quicklyflocs as a flocculatedfunction of stir byring corn time starch, at the andspeed the of 500 size rev/min of the is pyrite shown flocs in is Figure 4. Pyrite fines are quickly flocculated by corn starch, and the size of the pyrite flocs is approximately 70 µm with 1 min of stirring. With the increase in stirring time, the size rapidly approximatelyThe size of 70the μ pyritem with flocs 1 min as a offunction stirring. of stirWithring the time increase at the inspeed stirring of 500 time, rev/min the sizeis shown rapidly in decreases to approximately 40 µm in 8 min; then it slightly decreases from 40 µm to approximately Figuredecreases 4. Pyriteto approximately fines are quickly 40 μm inflocculated 8 min; then by it cornslightly starch, decreases and the from size 40 ofμm the to approximatelypyrite flocs is 35 µmapproximately35 in μm 12 in min. 12 min. This 70 Thisμ resultm resultwith indicates 1indicates min of that stirring.that the the hydrophilic hydrophilicWith the increase flocculationflocculation in stirring with with corntime, corn starch starchthe size is issensitive rapidly sensitive to mechanicaldecreasesto mechanical agitation, to approximately agitation, and the and larger40 the μ mlarger flocsin 8 flocsmin; are are morethen more it likely slightly likely damaged damageddecreases than thanfrom smallersmaller 40 μm toones. approximately 35 μm in 12 min. This result indicates that the hydrophilic flocculation with corn starch is sensitive to mechanical agitation, and the larger flocs are more likely damaged than smaller ones.

Figure 4. Effect of the stirring time on the size of pyrite flocs with corn starch. Figure 4. Effect of the stirring time on the size of pyrite flocs with corn starch. Figure 5 illustratesFigure 4. Effect the effect of the ofstirring the flocculant time on the ad sizedition of pyrite on theflocs size with of corn pyrite starch. flocs. Corn starch Figureeffectively5 illustrates flocculates the the effect pyrite of thefines flocculant even at a additionlow addition on the of sizeapproximately of pyrite flocs.10 mg/L. Corn The starch effectivelymaximumFigure flocculates size 5 illustrates is approximately the the pyrite effect 56 of fines μ mthe at flocculant even40 mg/L at cornad adition lowstarch. on addition Howeve the sizer, ofof the pyrite approximately size beginsflocs. Corn to decrease starch 10 mg/L. effectively flocculates the pyrite fines even at a low addition of approximately 10 mg/L. The The maximumwhen the concentration size is approximately of corn starch 56 isµ overm at 40 40 mg/L. mg/L The corn decrease starch. in size However, at high addition the size can begins be to maximum size is approximately 56 μm at 40 mg/L corn starch. However, the size begins to decrease decreaseattributed when to the the concentration steric effect of of adsorbed corn starch floccula is overnts on 40 the mg/L. pyrite The surfaces. decrease This in result size at shows high additionthat whenan appropriate the concentration addition of corn starch is beneficialover 40 mg/L. to the The pyrite decrease flocculation. in size at high addition can be can beattributed attributed to the to thesteric steric effect effect of adsorbed of adsorbed floccula flocculantsnts on the onpyrite the surfaces. pyrite surfaces. This result This shows result that shows that anan appropriate addition addition of ofcorn corn starch starch is beneficial is beneficial to the to pyrite the pyrite flocculation. flocculation.

Figure 5. Effect of the flocculant addition on the size of pyrite flocs with corn starch.

3.2. MorphologyFigureFigure 5.of EffectPyrite 5. Effect ofFlocs of the the flocculant flocculantaddition addition on th thee size size of of pyrite pyrite flocs flocs with with corn corn starch. starch.

3.2. MorphologyThe optical of microscopy Pyrite Flocs image of pyrite flocs with the addition of 50 mg/L corn starch at pH 7.3 3.2. Morphologyis shown in ofFigure Pyrite 6. Flocs It is observed that the pyrite fines indeed form flocs in the presence of corn starch,The and optical the floc microscopy size is larger image than of pyrite 100 μ flocsm. However, with the theaddition floc has of 50a loosemg/L porouscorn starch structure at pH and 7.3 The optical microscopy image of pyrite flocs with the addition of 50 mg/L corn starch at pH 7.3 ishigh shown sensibility in Figure to the 6. stirringIt is observed time an thatd stirring the pyri speed,te fines as shownindeed in form Figure flocs 6. in the presence of corn is shownstarch, in and Figure the 6floc. It size is observedis larger than that 100 the μm. pyrite However, fines indeedthe floc has form a loose flocs porous in the presencestructure and of corn starch,high and sensibility the floc to size the isstirring larger time than and 100 stirringµm. speed, However,5 as shown the flocin Figure has a6. loose porous structure and high sensibility to the stirring time and stirring speed,5 as shown in Figure6.

658 Minerals 2015, 5, 654–664 Minerals 2015, 5, page–page

Minerals 2015, 5, page–page

Figure 6. OpticalOptical microscopy microscopy image of pyrite fl flocsocs in the presence of corn starch. Figure 6. Optical microscopy image of pyrite flocs in the presence of corn starch. 3.3. Flotation of Pyrite Flocs 3.3. Flotation of Pyrite Flocs The recovery of pyrite in with and without corn starch as a function of pH is TheThe recovery recovery of pyriteof pyrite in in froth froth flotation flotation withwith an andd without without corn corn starch starch as a as function a function of pH of is pH is illustrated in Figure 7. As observed, the recovery of pyrite with size of −74 + 48 μm without corn illustratedillustrated in Figure in Figure7. As 7. As observed, observed, the the recovery recovery ofof pyritepyrite with size size of of −74´ 74+ 48 + μ 48m µwithoutm without corn corn starchstarch is lessless is less thanthan than 3%3% 3%inin theinthe the tested tested tested pH pH pH range. range. range. However, HoHowever, thethe the recoveryrecovery recovery of of ofpyrite pyrite pyrite fines fines fines with with with the the sizethe size size of of´20 −20µofm μ−m20 is approximatelyisμm approximately is approximately 40%. 40%. 40%. This This This result result result suggests suggestssuggests that thatthat the the mechanicalthe mechanical mechanical entrainment entrainment entrainment leadsleads leads toto thethe to high the highrecovery highrecovery ofrecovery pyrite of pyrite of fines pyrite withoutfines fines without without corn starch.corn corn starch. starch. The recovery The recoveryrecovery of pyrite of of pyrite pyrite fines fines isfines strongly is strongly is strongly depressed depressed depressed in the inpresence thein presencethe of presence corn of starch cornof corn atstarch 50starch mg/L. at at 50 50 Itmg/L. mg/L. decreases ItIt decreasesdecreases from 40% fromfrom to 40% 5%40% atto to pH5% 5% at 7.5. pHat ThepH 7.5. decrease7.5.The Thedecrease decrease becomes becomessharperbecomes with sharper sharper the increasewith with the the inincrease pH,increase which in in pH, pH, corresponds which which corres topondsponds the increase to to the the increase inincrease floc in size, flocin asfloc size, shown size, as shown as in shown Figure in 2in. Figure 2. Clearly, the mechanical entrainment of pyrite fines in flotation can be weakened or FigureClearly, 2. the Clearly, mechanical the mechanical entrainment entrainment of pyrite fines of in pyrite flotation fines can in be flotation weakened can or be eliminated weakened using or eliminated using the formation of hydrophilic flocs. Larger flocs have lower recovery. The flotation eliminatedthe formation using of hydrophilic the formation flocs. of Largerhydrophilic flocs haveflocs. lowerLarger recovery. flocs have The lower flotation recovery. results The suggest flotation that resultsresults suggest suggest that that selectively selectively hydrophilic hydrophilic floccuflocculationlation isis an an effective effective method method to reduceto reduce the the selectivelyentrainment hydrophilic of pyrite flocculation fines in the separation is an effective of gale methodna from to pyrite, reduce if galena the entrainment cannot be depressed of pyrite by fines in entrainment of pyrite fines in the separation of galena from pyrite, if galena cannot be depressed by the separationthe digested of corn galena starch. from pyrite, if galena cannot be depressed by the digested corn starch. the digested corn starch.

Figure 7. Effect of pH on the recovery of pyrite fines in froth flotation with and without the flocculant. Figure 7. Effect of pH on the recovery of pyrite fines in froth flotation with and without the flocculant.

3.4. Adsorption Isotherm 3.4. AdsorptionFigure 7. Effect Isotherm of pH on the recovery of pyrite fines in froth flotation with and without the flocculant. The adsorption isotherm of corn starch onto pyrite at pH 7.5 is shown in Figure 8. The The adsorption isotherm of corn starch onto pyrite at pH 7.5 is shown in Figure8. The adsorbed 3.4. Adsorptionadsorbed amount Isotherm of corn starch exhibits a slow continual increase with the increase in flocculant amountconcentration of corn starch and exhibitsdoes not areach slow continualthe maximum increase adsorbed with amount the increase in the in tested flocculant concentration. concentration and doesTheHowever, notadsorption reach as shown the isotherm maximum in Figure of adsorbed 9,corn in the starch amounttested onto concentration in pyrite the tested at range, concentration.pH 7.5 the is isotherm shown However, compliesin Figure as with shown 8. The in adsorbedFigureLangmuir9, in amount the equation tested of corn concentration when starch we plotted exhibits range, the areciprocal the slow isotherm co ofntinual the complies adsorbed increase with amount with Langmuir versusthe increase the equation reciprocal in flocculant when of we concentrationplotted the reciprocal and does of thenot adsorbed reach the amount maximumversus6 adthesorbed reciprocal amount of thein equilibriumthe tested concentration. concentration However, as shown in Figure 9, in the tested concentration range, the isotherm complies with Langmuir equation when we plotted the reciprocal659 of the adsorbed amount versus the reciprocal of 6 Minerals 20152015,, 55,, 654–664page–page

the equilibrium concentration with R2 (correlation coefficient) of 0.992. The isotherm may follow withLangmuirianMineralsR2 (correlation 2015 behavior, 5, page–page coefficient) according of 0.992.to the The Giles isotherm classification may follow [17]. Langmuirian The calculated behavior qm (saturated according toadsorption the Giles classificationamount on [17a ].monolayer) The calculated and q mb (saturated(adsorption adsorption equilibrium amount constant) on a monolayer) values were and the equilibrium concentration with R2 (correlation coefficient) of 0.992. The isotherm may follow b16.7 (adsorption mg/m2 and equilibrium 0.0042 L/mg, constant) respectively. values were 16.7 mg/m2 and 0.0042 L/mg, respectively. Langmuirian behavior according to the Giles classification [17]. The calculated qm (saturated adsorption amount on a monolayer) and b (adsorption equilibrium constant) values were 16.7 mg/m2 and 0.0042 L/mg, respectively.

Figure 8. Adsorption isotherm of corn starch onto pyrite. FigureFigure 8. 8.Adsorption Adsorption isotherm isotherm of corn corn starch starch onto onto pyrite. pyrite.

FigureFigure 9. Plot 9. Plot of the of the reciprocal reciprocal of of adsorption adsorptiondensity density versus reciprocalreciprocal ofof equilibrium equilibrium concentration. concentration. Figure 9. Plot of the reciprocal of adsorption density versus reciprocal of equilibrium concentration. 3.5. Effect of Corn Starch on the Zeta Potential of Pyrite 3.5. Effect of Corn Starch on the Zeta Potential of Pyrite 3.5. Effect Theof Corn zeta Starch potential on theof pyriteZeta Potential as a function of Pyrite of pH without and with corn starch is shown in TheFigure zeta 10. potentialPyrite shows of an pyrite isoelectric as a functionpoint at approximately of pH without pH 3, and and with the addition corn starch of corn is starch shown in FigureThedoes 10 .zetanot Pyrite significantly potential shows of anchange pyrite isoelectric its as isoelectric a point function atpoint, approximately of whichpH without may be pH attributedand 3, and with the tocorn additionthe causticstarch of digestedis corn shown starch in doesFigure notcorn 10. significantly starch Pyrite with shows an change isoelectrican isoelectric its point at point,approximat at approximately whichely may pH be3 [18].pH attributed 3,The and addition tothe the addition causticof corn of digestedstarch corn atstarch corn starchdoes 20not with mg/L significantly an dramatically isoelectric change decreases point its at isoelectric approximatelythe zeta potential point, pH ofwhich pyrite 3 [18 ].may in Thealkaline be addition attributed condition, of cornto which the starch caustic is favorable at 20digested mg/L dramaticallycorn forstarch the flocculation with decreases an isoelectric of the pyrite zeta fines potentialpoint by theat approximatdecrease of pyrite in in electricalely alkaline pH repulsion3 condition,[18]. The force. addition which is of favorable corn starch for the at flocculation20 mg/L dramaticallyThe of effect pyrite of fines corndecreases starch by the theaddition decrease zeta onpotential in the electrical zeta of potential pyrite repulsion in of alkaline pyrite force. is condition,shown in Figure which 11. is Cornfavorable starch can significantly decrease the pyrite zeta potential from approximately −19 to approximately for theThe flocculation effect of corn of starchpyrite additionfines by the on thedecrease zeta potential in electrical of pyrite repulsion is shown force. in Figure 11. Corn starch −6 mV with its concentration less than 1 mg/L. Above 2 mg/L, the zeta potential of pyrite remains can significantlyThe effect of decrease corn starch the pyrite addition zeta on potential the zeta from potential approximately of pyrite´ is19 shown to approximately in Figure 11.´ 6Corn mV stable. The decrease in zeta potential of pyrite may result from the compression effect of the electric starch can significantly decrease the pyrite zeta potential from approximately −19 to approximately withdouble its concentration layer for high less than concentration 1 mg/L. from Above the 2preparation mg/L, the of zeta the starch potential solution. of pyrite remains stable. The−6 mV decrease with its in concentration zeta potential ofless pyrite than may1 mg/L. result Ab fromove 2 the mg/L, compression the zeta effectpotential of the of electricpyrite remains double layerstable. for The high decrease ion concentration in zeta potential from theof pyrite preparation may7 result of the from starch the solution. compression effect of the electric double layer for high ion concentration from the preparation of the starch solution.

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Figure 10. Zeta potential of pyrite as a function of pH without and with corn starch.

Figure 11. Effect of the corn starch addition on the zeta potential of pyrite. Figure 11. Effect of the corn starch addition on the zeta potential of pyrite. 3.6. Interaction Mechanism between Pyrite and Digested Corn Starch 3.6. Interaction Mechanism between Pyrite and Digested Corn Starch XPS analysis was used to examine the pyrite surface before and after the corn starch treatment. The XPSsurface analysis species was were used identified to examine based the pyrite on de surfaceconvoluted before peaks and afterof the the photoelectron corn starch treatment. binding The surfacesurface speciesspecies were were identified identified based based on deconvolutedon deconvoluted peaks peaks of the of photoelectron the photoelectron binding binding energy energy spectra. Comparing the XPS spectra of pyrite before and after the treatment with corn spectra.starch, any Comparing obvious thebinding XPS spectraenergy ofshifts pyrite (more before than and 0.1 after eV) thecan treatment be used withto infer corn the starch, chemical any obviousstarch, any binding obvious energy binding shifts energy (more shifts than 0.1(more eV) than can 0.1 be usedeV) can to inferbe used the chemicalto infer the interactions chemical interactions between pyrite and corn starch. betweenFigure pyrite 12 shows and corn the starch. C 1s binding energy spectrum of corn starch after adsorption on pyrite, Figure 12 shows the C 1s binding energy spectrum of corn starch after adsorption on pyrite, and the peak fitting results are shown in Table 1. The C 1s spectrum consists of six binding energy andpeaks the at peak283.91, fitting 284.61, results 285.23, are 286.11, shown 287.51 in Table an1.d The288.7 C 0 1s eV. spectrum The peaks consists at 284.61, of six 286.11, binding 287.51 energy eV peaks at 283.91, 284.61, 285.23, 286.11, 287.51 an andd 288.7 0 eV. The peaks at 284.61, 286.11, 287.51 eV were assigned to C–C, C–H, C–O (alcohol) and O–C–O (acetal), respectively [19]. The presence of werea 288.70 assigned eV XPS to C–C,signal C–H, was C–O assigned (alcohol) to andO=C–O O–C–O (carboxyl (acetal), and respectively ester functions), [19]. The which presence is ofnot a 288.70a 288.70 eV XPSeV XPS signal signal was assignedwas assigned to O=C–O to O=C–O (carboxyl (carboxyl and ester and functions), ester functions), which is not which contained is not in contained in natural corn starch. This peak indicates that the carboxyl was indeed generated in corn naturalstarch because corn starch. of the This treatment peak indicates of the that sodium the carboxyl hydroxide was indeedsolution, generated which is in consistent corn starch with because the ofstarch the treatmentbecause of of the the sodiumtreatment hydroxide of the sodium solution, hy whichdroxide is consistentsolution, which with the is previousconsistent study with [ 15the]. previous study [15]. The peaks at 283.91 and 285.23 eV are unknown because the structural change Theof corn peaks starch at 283.91 that was and digested 285.23 eV with are sodium unknown hydroxide because theis not structural clear. change of corn starch that was digestedof corn starch with sodiumthat was hydroxide digested with is not sodium clear. hydroxide is not clear.

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FigureFigure 12. 12.C 1sC 1s spectrum spectrum of of corn corn starchstarch after after adsorption adsorption on on the the pyrite pyrite surface. surface. Figure 12. C 1s spectrum of corn starch after adsorption on the pyrite surface. Table 1. X-ray photoelectron spectrometer (XPS) C 1s peak fitting for corn starch after Table 1. X-ray photoelectron spectrometer (XPS) C 1s peak fitting for corn starch after adsorption on adsorption on the pyrite surface. theTable pyrite 1. surface. X-ray photoelectron spectrometer (XPS) C 1s peak fitting for corn starch after adsorption on the pyrite surface. C Chemical State C–C/C–H C–OO=C–OOC–C–O Unknown Unknown Chemical State C ChemicalBE State(eV) C–C/C–H 284.61 C–O 286.11 O=C–O 288.70 287.51 O–C–O 293.91 Unknown 285.23 Unknown C–C/C–H C–OO=C–OO–C–O Unknown Unknown BE (eV) 284.61 286.11 288.70 287.51 293.91 285.23 The BEFe 2p3/2(eV) spectrum 284.61 of pyrite before 286.11 and after 288.70 the corn 287.51 starch treatment 293.91 is shown 285.23 in Figure 13, and the peak fitting results are listed in Table 2. As observed in Table 2, the peak at 707.30 eV was Theassigned Fe 2p3/2 to FeS spectrum spectrum2. The peaks of of pyrite pyriteat 708.73, before before 710.15, and and 711.80after after the theand corn corn 713.88 starch starch eV belongtreatment to ferrous is shown oxide, in Figure iron 1313,, and the oxide, peak iron fitting fitting hydroxide, results and are listedferric insulfate, Table 2respectively 2.. AsAs observedobserved [20], ininwhich TableTable 2originate ,2, the the peak peak from at at 707.30the 707.30 partial eV eV was was oxidation of the pyrite surface. After the treatment of pyrite by corn starch, the binding energies of assigned to FeS2.. The The peaks peaks at at 708.73, 708.73, 710.15, 710.15, 711.80 711.80 and and 713.88 713.88 eV eV belong belong to to ferrous ferrous oxide, iron three peaks obviously decrease. The shifts of 0.61, 0.2 and 0.19 eV for iron disulfide, ferrous oxide, ironiron hydroxide, hydroxide, and and ferric ferric , sulfate, respectively respectively [20], which[20], which originate originate from the from partial the oxidation partial hydroxide, and ferric sulfate, respectively, attest their chemical interactions with corn starch. Iron oxidationof the pyrite of the surface. pyrite After surface. the treatment After the oftreatment pyrite by of corn pyrite starch, by corn the bindingstarch, the energies binding of threeenergies peaks of oxide and ferrous oxide on the surface of pyrite may not have participated in the chemical obviously decrease. The shifts of 0.61, 0.2 and 0.19 eV for iron disulfide, ferrous hydroxide, and ferric threeinteraction peaks obviously with corn starchdecrease. because The their shifts change of 0.61,in binding 0.2 andenergy 0.19 is lesseV than for 0.1iron eV. disulfide, ferrous hydroxide,sulfate, respectively, and ferric attest sulfate, their respectively, chemical interactions attest their with chemical corn starch. interactions Iron oxide with andcorn ferrous starch. oxide Iron oxideon the surfaceand ferrous of pyrite oxide may on not the have surface participated of pyrite in the may chemical not have interaction participated with corn in starchthe chemical because interactiontheir change with in binding corn starch energy because is less their than change 0.1 eV. in binding energy is less than 0.1 eV.

Figure 13. Cont.

9 Figure 13. Cont.Cont.

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Figure 13. Fe 2p3/2 spectra spectra of of pyrite pyrite before before ( (aa)) and and after after ( (bb)) the the adsorption adsorption of of corn corn starch. starch.

Table 2.2. XPS Fe 2p3/2 peak peak fitting fitting for for pyrite pyrite before before and after the treatment with corn starch. Fe Species Fe Species FeS2 FeO Fe2O3 FeOOH Fe2(SO4)3 FeS2 FeO Fe2O3 FeOOH Fe2(SO4)3 BE (eV) a 707.30 708.73 710.15 711.8 713.88 BE (eV) a 707.30 708.73 710.15 711.8 713.88 BE (eV) b 706.69 708.69 710.09 711.6 713.69 BE (eV) b 706.69 708.69 710.09 711.6 713.69 ShiftShift (eV) 0.61 0.04 0.06 0.2 0.19 a b ( :( apyrite;: pyrite; :b pyrite: pyrite treated treated byby corn corn starch). starch).

In the interaction mechanism of polysaccharides on the pyrite surface, polysaccharides interact with hydroxylated metallic sites on thethe mineralmineral surfacesurface [[11,21].11,21]. According to this mechanism, the interaction mechanismmechanism ofof thethe adsorptionadsorption of of corn corn starch starch on on the the pyrite pyrite surface surface may may be be the the interaction interaction of ofhydroxyl hydroxyl or carboxylor carboxyl groups groups on theon cornthe corn starch starch with with the iron the hydroxyliron hydroxyl species species that form that onform the on pyrite the pyritesurface. surface. In addition, In addition, iron atoms iron atoms in iron in disulfide iron disu andlfide ferric and ferric sulfate sulfate interact interact with with the corn the corn starch. starch.

4. Conclusions Conclusions (1) TheThe mechanicalmechanical entrainment entrainment of of pyrite pyrite fines fines in flotation in flotation can be can effectively be effectively eliminated eliminated by adding by addingan appropriate an appropriate amount ofamount corn starch, of corn which starch, may bewhich attributed may tobe the attributed formation to of the hydrophilic formation flocs of hydrophilicof fines bridged flocs by of corn fines starch bridged molecules. by corn The starch flocculation molecules. is notablyThe flocculation sensitive tois mechanicalnotably sensitive stirring. to mechanical(2) The stirring. XPS studies provided good evidence of the presence of carboxyl groups on pyrite surfaces,(2) The which XPS suggests studies a provided chemical interactiongood evidence between of the pyrite presence and corn of starchcarbox digestedyl groups with on sodium pyrite surfaces,hydroxide which (or chemical suggests adsorption a chemical of corninteraction starch between on pyrite), pyrite which and is alsocorn supportedstarch digested by the with zeta sodiumpotential hydroxide results. (or chemical adsorption of corn starch on pyrite), which is also supported by the zeta potential results. Acknowledgments: The financial support of this work from the National Natural Science Foundation of Acknowledgments:China (project No. 51474167)The financial and thesupport Fundamental of this Researchwork from Funds thefor National the Central Natural Universities Science ofFoundation China are gratefully acknowledged. of China (project No. 51474167) and the Fundamental Research Funds for the Central Universities of China are gratefullyAuthor Contributions: acknowledged.Hongqiang Li, Shaoxian Song and Wei Ge conceived and designed the experiments. Wei Ge, Yanzeng Ren and Feiyu Zhao prepared the samples and performed the experiments. Hongqiang Li and AuthorWei Ge analyzedContributions: the data. Hongqiang Wei Ge,Hongqiang Li, Shaoxian Li Song and Shaoxian and Wei Song Ge conceived contributed and to designed the writing the and experiments. revising of Weithe paper. Ge, Yanzeng Ren and Feiyu Zhao prepared the samples and performed the experiments. Hongqiang Li andConflicts Wei ofGe Interest:analyzedThe the authors data. Wei declare Ge,no Hongqiang conflict of interest.Li and Shaoxian Song contributed to the writing and revising of the paper.

Conflicts of Interest: The authors declare no conflict of interest.

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