Xiangao Quan,Huiyun Wang*

Materials and Product Engineering

Preparation of a Novel Coal Gangue-Polyacrylamide Hybrid Flocculant and Its Flocculation Performance☆

Xiangao Quan,Huiyun Wang*

School of Pharmaceutical Science,Jining Medical University,Shandong 276826,China

A R T I C L EI N F O

Article history:

Hybrid f l occulant

Polyacrylamide

Coal gangue

Intrinsic viscosity

Anovel fl occulantbasedonhybridcoalgangue-polyacrylamide(HCGPAM)hasbeenpreparedbyusingmodi fi ed coal gangue and polyacrylamide.Factors related to the preparation such as reaction time,temperature,concentration of the polymer monomer and ratio of initiators are investigated.The product is characterized by infrared spectra(FTIR),scanningelectronmicroscopy(SEM),X-raydiffraction(XRD),aswellasviscometry.The fl occulating tests on oil fi eld drilling wastewater show that the removal ef fi ciency is 85.5%and the light transmittance is 53.6%.The results indicate that the coal gangue could be used for the preparation of inorganic-organic hybrid fl occulant and the removal ef fi ciency is much higher than that of commercial polyacrylamide(PAM)or PAM/ coal gangue blend.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

As is known to all,f l occulation is an eff i cient and cost-effective methodfornaturalwaterandwastewatertreatment[1-3].Variousf l occulants have been developed for the coagulation and f l occulation purposes over the last decades,especially inorganic-based coagulants[4], organic-based f l occulants[5]and organic-inorganic hybrid f l occulant [6,7].Andthenovelhybridf l occulationsystemhasattractedgreatinterest from researchers in recent years[6-16].

The organic-inorganic hybrid f l occulants are products composed of two or more different types of components[11],such as inorganic salts/polymer f l occulants[6],inorganic f l occulants/polymer f l occulants [7]and microparticle/polymer f l occulants[8,14-16].It is reported that the microparticle systems give better f l occulation and drainage than conventional polymer f l occulation systems[17,18].Up to date,several kinds of microparticle/polymer f l occulation systems have been preparedbyusinginorganicsolidparticleswithhighspecif i careaandpolymer monomer[8,14-16].However,to the best of our knowledge,little has been reported on producing organic-inorganic hybrid f l occulant with coal gangue particles.

Coal gangue(CG),a by-product from the coal mining or coal washing,has become the largest solid waste and has increased every year as the coal industry expands[19,20].And such a large quantity of this solid waste will invade large area of farmland and cause water or soil pollution without treatment[19].Therefore,disposal of thecoal gangue becomes an increasing environmental,even economic problem.At present,the bene fi cial utilization of coal gangue includes power generation[21],construction material[22-27]and other places[28].However,the utilization ef fi ciency of the coal gangue remains much lower[29, 30].Coal gangue contains large amount of valuable mineral resources such as SiO2,Al2O3,Fe2O3and CaO.Some researchers have made attempts to prepare inorganic fl occulant by coal gangue[31,32].In these attempts,it is also observed that the solid waste has extremely high speci fi c surface area after being calcined,and leached by alkali and acid,and shows a potential application as raw material for preparation of organic-inorganic hybrid fl occulant.

In this work,a new hybrid coal gangue-polyacrylamide(HCGPAM) fl occulant is synthesized based on coal gangue through in situ polymerization for inorganic fl occulant/AM(acrylamide)mixture.The fl occulation effect on oil fi eld drilling wastewater is investigated and its fl occulation ef fi ciency is much higher than that of commercial polyacrylamide(PAM)or PAM/coal gangue blend.It is shown that coal gangue can be used as a material for preparation of inorganic-organic hybrid fl occulant,which is bene fi cial both for environment and economy.

2.Materials and Methods

2.1.Materials

The coal gangue used in this study was obtained from a coal mine (Yanzhou Mining Bureau,China)with the mass composition of SiO226.58%,Al2O35.35%,Fe2O314.38%,CaO 3.56%and MgO 1.26%.The waste drilling f l uid was supplied by Shengli Oil Field of China.COD of the waste drilling f l uid was 2.34×104mg·L?1with a pH value of 11.6.

http://dx.doi.org/10.1016/j.cjche.2014.06.032

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.Commercial available polyacrylamide(PAM)was purchased from a water treatment agent factory(Henan Gongyi,China).All the chemical reagents were of analytic grade and supplied by Shanghai Chemistry Reagent Co.Ltd.Distilled water was used to prepare all the solutions.

2.2.Methods

2.2.1.HCGPAM preparation

2.2.1.1.Modif i cation of coal gangue.The coal gangue was crushed into powder with an LG 500 crusher and sieved with a 100 mesh screen. Sodium carbonate was added into the coal gangue powder at a mass ratio of 6:5,followed by thorough grinding and blending.Then, the mixture was calcined in the muff l e furnace at 750°C for 2 h.After cooling to room temperature,the coal gangue was subject to alkali leaching with 15%sodium hydroxide and acid leaching with 15% hydrochloric acid under the condition of 100°C and 5 MPa.The leaching mother liquor and the washing liquor were used to prepare the inorganic polymer f l occulant and polysilicate ferro-aluminum sulfate(PSFA)in our previous work[32].The leached solid was dried at 100°C after being washed with distilled water,which was used to prepare the coal gangue-polyacrylamide hybrid f l occulant.

Fig.1.FTIR spectra of(a)modif i ed coal gangue,(b)PAM and(c)HCGPAM.

Fig.2.XRD patterns of the samples.

2.2.1.2.Preparation of coal gangue-polyacrylamide hybrid.First,8 g of acrylamide(AM)and 90 ml of distilled water were added to a 250 ml polymerization bottle.Then,3 g of modif i ed coal gangue powder was added to above solution under stirring for 30 min.After being vacuumized,the solution was purged with N2for 3 times to remove the oxygen completely,and different amounts of(NH4)2Ce(NO3)6as the initiator were injected into the polymerization bottle slowly.Afterthat,the bottle was kept in the thermostated water bath at 75°C (±0.5°C)for 6 h.The solution in the polymerization bottle was transferred to a 250 mL beaker and precipitated with acetone,and immersed in acetone for 12 h.It is f i nally dried in a vacuum oven at 35°C for 24 h.

2.2.2.Characterization of HCGPAM

2.2.2.1.Intrinsic viscosity[η].The intrinsic viscosity measurement was conductedbyusinganUbbelohdeviscometerafterthehybridf l occulant was being dispersed in distilled water at(30±0.02)°C.Flux-times were recorded with an accuracy of±0.05 s.Intrinsic viscosity[13] was obtained by extrapolation according to the following formula:

where c is the concentration of the solution,ηspis the specif i c viscosity of the solution,and kHis the Huggins coeff i cient.

2.2.2.2.FTIR spectroscopy.The samples of HAPAM,PAM,modif i ed coal gangue powder,and HCGPAM were analyzed by an FTIR spectrophotometer(IR Prestige-21,Japan)and potassium bromide pellet method. The spectra were recorded in the range of 4000-400 cm?1.

2.2.2.3.XRD measurement.Powder X-ray diffraction(XRD)patterns were recorded on an X-ray diffractometer(D/MAX-2500,Japan)with Cu K radiation in the 2θ range of 5°-65°at a scan rate of 4°per minute. The structure and layer spacing of samples were calculated by Bragg equation.

2.2.2.4.Scanning electron microscopy measurement.Scanning electron microscopy(TXA-840,Japan)was used to investigate the structure and morphology of modif i ed coal gangue and HCGPAM.

2.2.3.Flocculation experiments for HCGPAM

Flocculation experiments were performed on drilling wastewater. All f l occulation tests were conducted in 1.0 L beakers using a six-unit stirringsystem.500mloftestsamplewasplacedinabeakerandadjusted to a pH of 3-4 with diluted HCl.It was stirred at 100 r·min?1for 1 min after adding the f l occulant at room temperature,followed by slowly stirring at 50 r·min?1for 5 min and precipitating for 1 h.After that,supernatant sample was taken 2.0 cm below the surface of test wastewater fortransmittance measurementona T1901ultraviolet-visible spectrophotometer at 520 nm,and for chemical oxygen demand (COD)measurement by CODcranalysis method(GB/T11914-1989, China).

3.Results and Discussion

3.1.FTIR,XRD and SEM analyses

3.1.1.FTIR analysis

Fig.1presentstheFTIRspectraofmodif i edcoalgangue,PAMandthe hybrid f l occulant.In Fig.1(a)and(b),the band at 800-1200 cm?1is attributedtoSi-Ostretchingvibrations,andthebroadbandat3400cm?1and 3200 cm?1is due to the stretching mode of the free-NH2and the aggregated-NH2bond in the PAM.The band at 1650 cm?1is assigned tothef i rstovertoneofN-H bendingvibration.Thebands at1140cm?1and 3000 cm?1are assigned to C-O stretching and C-H stretching,respectively.The band at 1400 cm?1is due to C-N stretching vibration andthebandat1700cm?1isassignedtotheC=Ostretchingvibrations. Fig.1(c)shows that the IR spectra of HCGPAM display almost the same characteristic bandsaspurePAMexcept forthenuances of 800,700 and 560 cm?1.The band at 800-1200 cm?1is attributed to Si-O stretching vibrations and the band at700 and560 cm?1is assigned to the bending vibration of Si-O and Al-O which are belonged to the coal gangue. These results indicate that the HCGPAM is an organic-inorganic hybrid.

3.1.2.XRD analysis

Fig.2 shows the XRD spectra of coal gangue,calcined coal gangue, leached coal gangue and HCGPAM.The XRD patterns of coal gangue in Fig.2(a)show that the major mineralogical composition of raw coal gangue is quartz,kaolinite and muscovite.By comparison,it is found that the diffraction intensities of muscovite and kaolinite reduce and the diffraction intensities of quartz phase increase after being calcined at high temperature.This might be caused by the decomposition of kaolinite and muscovite at 750°C[33,34]in Fig.2(b).When the coal gangue is being calcined,the burning of the coal increases the relativecontentofquartz.Thediffractionintensityofquartzfurtherincreases evidentlyandthepeakofmuscoviteandkaolinitevanishescompletelyin Fig.2(c).It could be attributed to the reactions of the components(SiO2, Al2O3,Fe2O3,CaO,MgO)in the coal gangue with acid and alkali,whichincrease the relative content and porosity of quartz remarkably. Fig.2(d)is the XRD pattern for the hybrid f l occulant.It is observed that the characteristic peaks of quartz in the leached coal gangue decrease in intensity or even disappeared,whereas the characteristic peaks of PAM become more evident.This result indicates that the PAM has entered the interlayer spacing of the coal gangue.Compared to pure coal gangue, the diffraction peaks of the hybrids shift to lower diffraction angles slightly.It is also proved that PAM intercalates coal gangue successfully.

3.1.3.SEM analysis

The morphologies of the coal gangue,modif i ed coal gangue and HCGPAM are observed by SEM,and the results are presented in Fig.3. The modif i ed coal gangue has obvious gap,extremely high specif i c surfacearea,andshowsapotentialapplicationasrawmaterialforpreparation of organic-inorganic hybrid f l occulant.It is obvious that HCGPAM displays a very different surface morphology from modif i ed coal gangue.The surface of HCGPAM consists of certain sorts of caulif l ower head with a series of pleated ditches of different widths and depths, which makes it possess much coarser surface and larger specif i c surface areathanmodif i edcoalgangue.ItcanbeinferredthatHCGPAMisfavorable for combining with pollutants.

3.2.Effect of synthesis conditions on intrinsic viscosity of HCGPAM

Fig.5.Comparison of the f l occulation performances,modif i ed coal gangue(a),PAM(b), HCGPAM(c)and PAM/modif i ed coal gangue blend(d).

Fig.4.Dependence of intrinsic viscosity of HCGPAM on mass fraction of(a)AM,(b)initiator concentration,(c)reaction time and(d)reaction temperature.

3.2.1.Effect of acrylamide mass fraction on intrinsic viscosity

The effect of the mass fraction of AM on the intrinsic viscosity of HCGPAM is shown in Fig.4(a).The mass fraction of initiator,reaction of temperature and reaction time are 2.5%,75°C and 6 h,respectively. In Fig.4(a),it can be seen that the intrinsic viscosity of the HCGPAM increases from 1.0%to 8.0%with an increase of acrylamide mass fraction,but decreases with a further increase.Within the range of AM mass fraction from 1.0%to 8.0%,the increase in intrinsic viscosity may be due to an increase of the crosslinking reaction and molecular massof HCGPAM,which improves the absorbing and bridging capability. However,further increase in acrylamide mass fraction may cause a rapid rise in temperature and the polymerization reaction rate,which would leadto theincrease in theextentof chain transfer tothe polymer in free radical solution polymerization with the increase of the AM concentration.Thus,the optimized AM mass fraction(8.0%)is suitable for the production of HCGPAM.

Table 1Light transmittance and COD removal eff i ciency of the HCGPAM,PAM,modif i ed coal gangue and PAM/modif i ed coal gangue blend

3.2.2.Effect of initiator concentration on intrinsic viscosity

Ce(NH4)2(NO3)6wasusedasaninitiatorforthereactionsystem.The effect of the initiator concentration on the intrinsic viscosity of the HCGPAM was studied with varying concentrations of the initiator (from 1.0%to 7.0%)and reaction time of 6 h with a f i xed acrylamide mass fraction 8%at 75°C[Fig.4(b)].In Fig.4(b),it can be seen that the intrinsic viscosity increases as the Ce(NH4)2(NO3)6concentration increases from 0.5%to 2.5%,but decreases with further increases in the Ce(NH4)2(NO3)6concentration.The increasing concentration of the initiator leads to the increase of free radical concentration,thereafter the degree of polymerization.However,when the concentration of the initiator is higher than 2.5%,the intrinsic viscosity will decrease. The possible explanation is that the excessive initiator leads to the rate of termination free radicals and chain transfer,thus resulting in a decrease in the intrinsic viscosity.Therefore,the initiator concentration of2.5%isoptimalinthepreparationofHCGPAMaccordingtotheexperimental results.

3.2.3.Effect of reaction time on intrinsic viscosity

Reactiontimesfrom1hto8hwereselectedtostudytheeffectofreaction time on the intrinsic viscosity of HCGPAM[Fig.4(c)].The mass fraction of AM,mass fraction of initiator and reaction of temperature were8.0%,2.5%and75°C,respectively.Fig.4(c)showsthattheintrinsic viscosity increases with increasing reaction time between 2 h and 6 h, but decreases with further increase in reaction time.This indicates that the optimum reaction time for preparation of HCGPAM is 6 h.

Fig.6.Microstructure of f l oc formed by HCGPAM(magnif i ed 4000 times).

Fig.7.COD removal eff i ciency of drilling wastewater in coagulation process.

3.2.4.Effect of reaction temperature on intrinsic viscosity

The effect of reaction temperature on intrinsic viscosity was investigatedwithvaryingreactiontemperaturesfrom40°Cto80°C,following 6 h reaction time with initiator concentration of 2.5%and acrylamide mass fraction of 8.0%.The results are shown in Fig.4(d).It can be seen that the intrinsic viscosity increases as the reaction temperature increasesfrom 40°C to75°C,reachinga maximumvalueat75°C,butdecreases thereafter.As the reaction temperature increases,the reaction eff i ciency and the molecule weight of the reaction product increase. Thus,theintrinsicviscosityisimproved.Thedecreaseinintrinsicviscosity at temperatures higher than 75°C can be explained according to the general rules for free radical polymerization:a further increase in temperaturemayacceleratethedecomposition andconsumption oftheinitiators markedly,resulting in the lack of suff i cient initiators to sustain the corresponding polymerization reaction[22].

3.3.Flocculation study

3.3.1.Evaluation of the f l occulation performance

To evaluate the capability of treating drilling wastewater for HCGPAM,a comparative study of f l occulation performance on modif i ed coal gangue,PAM,HCGPAM and PAM/modif i ed coal gangue blend was conducted.The dosage for each of the four samples was 1.0 mg·ml?1. The drilling wastewater was diluted 2 times at pH 4.0.The results are showninFig.5andTable1.ItcanbeseenthattheCODremovaleff i ciency of the HCGPAM is much higher than that of any other three samples. Andthef l occulationrateofHCGPAMisfoundtobeveryhigh duringthe initial 3 min and the rising velocity of the f l ocs is about 0.21 cm·s?1. However,there is no distinct f l occulating performance occurred with the same amount of PAM,modif i ed coal gangue and PAM/modif i ed coal gangue blend within 30 min.Moreover,the f l ocs formed by HCGPAM have much higher density,which does not re-disperse even at stirring of 100 r·min?1for 2 min.The microstructure of the f l oc is shown in Fig.6.Flocs formed by HCGPAM are very dense,tightly wrapped together and hard to disperse.The structure of f l oc does not makelongchainofpolyacrylamidetostretchfully,whiletofurlparticles of sewage.The results indicate that HCGPAM is a strong and effective fl occulant for drilling wastewater.

3.3.2.Effect of HCGPAM dosage on f l occulating capability

The effect of HCGPAM dosage on f l occulation capability is studied with varying HCGPAM dosages from 0.2 to 2.5 mg·ml?1in drilling wastewater at pH 4.0(Fig.7).

It can be observed that the COD removal eff i ciency increases with the increasing dosage of HCGPAM and reaches a maximum at2.0 mg·ml?1,and thereafter,forms a plateau beyond 2.0 mg·ml?1. Thismightbeattributedtothecontentofsuspendedcolloidsinthedrillingwastewater[6]and theincreasingviscosity caused by theexcessive dosage of the hybrid f l occulant.

4.Conclusions

A novel coal gangue-polyacrylamide hybrid f l occulant(HCGPAM) has been synthesized by free radical aqueous polymerization of acrylamide in coal gangue suspended solution.And the optimal synthesis conditions are obtained:reaction time of 6 h,polymerization temperature at75°C,mass fraction of AM with8%,and mass fraction of initiator with 2.5%.HCGPAM exhibits much higher f l occulatingcapabilityin drillingwastewaterthanPAMorPAM/modif i edcoalgangueblendwiththe same dosage at room temperature.When the dosage of HCGPAM is 2.0 mg·ml?1with a stirring rate of 100 r·min?1and a stirring time of 1 min,the COD removal eff i ciency can reach 85.5%.

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21 August 2013

☆Supported by the National Key Technology R&D Program during the“11th Five-Year Plan”period(2008BAC43B02).

*Corresponding author.

E-mail address:wang_huiyun@126.com(H.Wang).

Received in revised form 24 February 2014

Accepted 31 March 2014

Available online 1 July 2014