Thermal decomposition behaviour of polyacrylamidomethyltrimethyl ammonium chloride in red mud separa

J. Cent. South Univ. Technol. (2008) 15: 808- -813包SpringerDOI: 10.1007/s11771-008-0149-xThermal decomposition behaviour of polyacrylamidomethyltrimethylammonium chloride in red mud separation processHU Hui-ping(胡慧萍), ZHANG Kun-yu(张琨瑜), ZHANG Li-juan(张丽娟), CHEN Qi-yuan(陈启元)(School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)Abstract: In order to provide references for selecting highly efficient red mud flocculants, the behaviour of polyacryl-amidomethyltrimethyl ammonium chloride (PATAC) in red mud separation process was investigated. PATAC was employed as aflocculant for red mud separation from the caustic aluminate liquor at 95 C. The used red mud was generated from Chinese diasporebauxite in Bayer process of alumina production. And the changes of PATAC before or after being treated in caustic solution at 95 Cwere studied by thermogravimetry (TG) and Fourier transform infrared (FTIR) spectral analysis. The results show that PATAC failsin effectively flocculating red mud and PATAC is readily converted to a quaternary ammonium hydroxide (PATAH) in causticsolution. PATAH can be decomposed to a new polymer (HPATAH) even at 95 C. Furthermore, there is an intramolecular hydrogenbond formed in the HPATAH polymer chain with two functional groups of- -CH2- OH and- CONH2. Therefore, the poorflocculation property of PATAC for red mud separation can be attributed to the thermal decomposition of PATAC in the caustic redmud slurry at 95 C and the formation of intramolecular hydrogen bond in the polymer chain of HPATAH during the thermaldecomposition, which causes the absorbable functional groups of PATAC to decrease greatly.Key words: alumina; red mud; polyacrylamidomethyltrimethyl ammonium chloride (PATAC); settlement; thermal decomposition;intramolecular hydrogen bondpolydiallyl dimethyl ammonium chloride was helpful for1 Introductionthe removal of organic impurities in aluminate liquor.LU et al4) showed that the application of cationicRed mud is an alkaline leaching waste of bauxite.flocculants in red mud separation could reform theRed mud separation from aluminate liquor is a criticalclarification degree of supermatant aluminate liquorstep in the process of alumina production, and is oftenrather than improve red mud settling rate, especially inenhanced by the addition of various flocculants tothe strong caustic soda liquor of Bayer process.promote the growth of red mud aggregation. TheCationic poly quaternary ammonium salts couldaluminate liquor overflowing the settler often contains anflocculate negatively charged solid particles effectivelyunacceptable concentration of suspended substance evenby not only polymer bridging but also chargeafter red mud separation, which results in a highneutralization during the flocculation process!. However,supernatant turbidity. Poly quaternary ammonium salts, aSHUBIN6] found a decrease in the adsorbed amount ofnew type of cationic polymers with good solubility, highcationic polyacrylamide onto negatively chargedcationic density, high effectiveness, no poison and lowcolloidal silica from basic electrolyte solutions with theprice, are used extensively in paper manufacturing,increase of pH value. Red mud particles in aluminatextile finishing， petroleum industry, mining， dailyproduction process also brought negative surface chargeschemical industry and water treatment". Because thereon their surface'7. In this work, in order to analyze theare some organic impurities such as humate in aluminatebehaviour of polyacrylamidomethyltrimethyl ammoniumliquor, ROE and MALITO2I and KOSESTER3I disposedchloride (PATAC) in red mud separation process andaluminate liquor containing red mud generated from aprovide references for selecting highly efficient red mudgibbsite bauxite by employing polydiallyl dimethylflocculants，PATAC， a cationicallymodifiedammonium chloride, a cationic quaternary ammoniumpolyacrylamide, was employed as a red mud flocculantsalt, to remove a portion of organic impurities present into improve the clarity of supernatant aluminate liquor.aluminate liquor. Their studies demonstrated thatAnd then the changes of PATAC before or after being中国煤化工Foundation item: Projc(2005CB623702) supported by the Major State Basic Research and DevelopReceived date: 2008- 01 - 25; Accepted date: 2008 04-15YHCNMH GCorresponding author: HU Hui-ping, Professor; Tel: +86- -731- 8877364; E-mail: phhuiping@hotmail.com.J. Cent. South Univ. Technol. (2008) 15: 808- -813809treated in caustic solution at 95 °C were investigated bywhere v is the red mud settling rate, m/h; h| is the .thermogravimetry (TG) and Fourier transform infraredinitial interface height, m; h2 is the interface height at the(FTIR) spectral analysis.fifth minute of settling test, m.The red mud slurry was allowed to settle for 30 min.2 ExperimentalAfter that, about 15 mL supernatant liquid was pipettedoff and transferred to a glass cuvette to determine the2.1 Materialsturbidity, which was measured in nephelometric turbidityThe red mud slurry used in this study was preparedunits (NTU) using a turbidity meter (WGZ-3, China).from Bayer digestion of diaspore bauxite by a bauxiterefinery at Zhengzhou Institute of China Aluminium Co.2.3 Alkali treatment of PATACLtd (CHALCO). The average grain size of red mudA caustic solution containing 0.3% PATAC wasparticles was measured by a Malvern laser granulometerprepared by dissolving 0.6 g PATAC in 200 mL 85 g/L(Mastersize 2000, U.K.) and found to be about 10 pum.NaOH solution. In the caustic solution， PATACThe red mud slurry contains 70 g/L red mud solids, 154contacted sufficiently with NaOH at 30 C and 150 r/ming/L Al2O3 and 160 g/L Na2O.for 24 h to obtain a solution containing the derivative ofPATAC used as a red mud flocculant in this studyPATAC, which is abbreviated as PATAH. After that,was provided by a chemical company in China. PATACPATAH solution was precipitated with acetone.was obtained by Mannich reaction and thenSubsequently, the precipitated PATAH was extractedquaternization of polyacrylamide. Its chemical structurewith acetone for 48 h by a Soxhlet's apparatus to removeis shown in Fig.1. The relative molecular mass andthe residual water in PATAH, and then dried underpositive charge density of PATAC are 3X10*-8X10~vacuum at 50 °C for 48 h. The dried sample was kept in aand 40%, respectively. As anotherflocculant, sodiumdesiccator for analysis later.polyacrylate (SPA) with an average relative molecularmass of about 3X 10“was purchased from Tianjin2.4 Thermalgravimetric analysisKermel Development Centre of Chemical Reagents inThe thermal behaviours of PATAC and PATAHChina. Both PATAC and SPA were dissolved in distilledwere investigated by thermalgravimetry (TG). TG waswater to form a 0.05% (mass fraction) flocculantperformed on a METTLER TOLEDO thermoanalyzersolution.(TGA/SDTA851c，Switzerland) using highly pure N2(99.999%) as purge gas. And the flow rate of N2 was 70l2 H出2 HmL/min.10-15 mg PATAC and PATAH samples wereheated from 25 to 300 C at a heating rate of 5 C/min.H2N」，o\用CHThe changes in the mass or the mass differentialdifference with temperature were recorded to obtain TGH2 1-CH3OHcurves and differential thermagravimetric (DTG)CH3analysis curves of PATAC or PATAH.JmFig.1 Chemical structure of PATAC2.5 Thermal treatment of PATAC and PATAH2.2 Settling tests of red mudThe prepared PATAC and PATAH samples wereRed mud settling tests were carried out in aheated to 95 "C, and then kept at that temperature for5 hclear-sided glass water bath maintained at 95 °C. Ain a tubular furnace (China) purged by highly pure N2well-mixed sample of the boiled red mud slurry was(99.999%) with a flow rate of 70 mL/min. After beingportioned out into six 150-mL graduated cylinders with acooled to ambient temperature, the treated PATAC anddiameter of 30 mm and a length of 220 mm, and thenPATAH, which were abbreviated as correspondingthese cylinders were immediately placed into the waterHPATAC and HPATAH, were taken out and kept in abath. Following the addition of PATAC or SPA, red mudslurry in the cylinder was mixed by several plunges. Theadded PATAC or SPA was calculated in the g/t unit2.6 Fourier transform infrared spectroscopybased on the mass of dry red mud solids in the cylinder.PATAC, PATAH, HPATAC and HPATAH wereSubsequently, the height of the interface was recordedanalyzed中国煤化工ured(FTIR)with settling time. Settling rates were calculatedspectroscopy.CNMH Celles) wereaccording to Eqn.(1):recorded onINICULet r 11K specurophotometerv=60(h.-h2)/5(1)(AVATAR-360, America)..810J. Cent. South Univ. Technol. (2008) 15: 808- -813role as well as polymer bridgingo. In other words,3 Results and discussionelectrostatic attraction between oppositely chargedpolymers and fine particles can enhance the absorption3.1 Red mud settling performance of PATAC and action of cationic flocculants onto negatively charged redSPAmud surface. Therefore, PATAC should produce a betterThe settling rate and supernatant turbidity of redsettling property for red mud than SPA. In fact, red mudmud slury treated by different types and differentsettling performance of PATAC is much worse than thatdosages of flocculants are listed in Table 1.of SPA in this study. Maybe, a series of chemicalchanges appear in PATAC when it is employed inTable 1 Settling rate and supernatant turbidity contrast forcaustic solution at 95 C.PATAC and SPASetting rate/Supernatant3.2 Thermal behaviour of PATAC and PATAHSample(m:h7)turbidity/NTUTo investigate the changes of PATAC in caustic red50 g/t PATAC0.40700nud slurry at 95 C, PATAC was treated by alkali100 g/t PATAC0.5272solution. Then both PATAC and its derivative after alkali200 g/t PATAC0.60550treatment (i.e. PATAH) were analyzed by TG. Fig.2shows the TG and DTG curves of PATAC and PATAH.300 g/t PATAC0.7096100 g/t SPA1.2671(a)70.02100T0It can be seen from Table 1 that with 100 g/tflocculants, the settling rate (0.52 m/h) of red mud slurry90DTG-0.02treated by PATAC is much lower than that by SPA (1.26-0.04m/h) and the supernatant turbidity (672 NTU) of red mudslurry treated by PATAC is higher than that by SPA (371-0.06g 70NTU). When the dosage of PATAC is 300 g/t, red mud-0.08.settling rate and supernatant turbidity are 0.70 m/h and60596 NTU, respectively. It is concluded that red mud-0.102settling performance of PATAC cannot be improved5(-0.12obviously, even if its dosage is increased to 300 g/t.The flocculation of fine particles in suspension may50 100 150 200 250 300occur by polymer bridging and charge neutralization'Temperature/CThe minerals commonly existing in red mud includesodium aluminosilicate hydrate, hydrogarnet, katoite and(b) 40.01other complex oxysalt, which makes red mud surface9Gbear lots of hydroxyl groups!'. Due to the deprotonationof hydroxyl groups on red mud particles in causticR 90-0.01solution， red mud has a negative surface charge' 1However, the anionic polymers (e.g. SPA) can adsorb on，85these negatively charged red mud particles by polymer2 8(bridging. This effect has also been observed in-0.03flocculating negatively charged kaolinite dispersion!' ” !75DTG/In this case, polymer bridging mechanism is of primaryimportancel, and bridging absorption is formed mostly70-by hydrogen bonding between red mud surface hydroxyl0 100 150 200 250 300-0.05groups and such anionic functional groups as- COOgroups in polymer chain or by an ion bridge such as CaFig.2 TG and DTG curves of PATAC (a) and PATAH (b)and Mg2, which can result in local coulombic forcebetween anionic particle surface and negatively chargedFig.2 shows that PATAC has a highercarboxylate groups in anionic polymers2. Indeed, SPAdecomposition temperature (267 °C) at the maximumis a highly efficient flocculant for red mud settlement.中国煤化工'ecompositionWhen red mud particles are flocculated by cationictemperature (9lysis rate. LUpolymers (e.g. PATAC) with a high relative molecularet al3) reportMHC N M H Gmperature ofmass, charge neutralization can also play an importantorganic quaternary ammonium salts was above 190 °C in.J. Cent. South Univ. Technol. (2008) 15: 808- -813811the air. GAO et al14] also found that phenolic resin3.3 FTIR spectral analysiscontaining quaternary ammonium hydrochloride groupsFor the further study on the decomposition reactionwas transformed into phenolic resin containingof PATAC and PATAH, FTIR analysis was carried out.quaternary ammonium hydroxide groups by KOHFTIR spectra of PATAC， HPATAC, PATAH andsolution, and thermal decomposition temperature (120 C )HPATAH are shown in Fig.3.of quaternary ammonium hydroxide groups was muchIn Fig.3, PATAC, HPATAC and PATAH exhibitlower than that (224 °C) of quaternary ammoniumthe similar characteristic absorption peaks. The spectrahydrochloride groups. From ambient temperature to 95clearly mark the presence of amide groups and"C, the total mass losses of PATAC and PATAH arequaternary ammonium groups. The band at 3 427 cm3.2% and 10.2%, respectively. These results show thatPATAC is thermostable at 95 C, but its derivative inarises from the asymmetric stretching vibration of theNaOH solution, i.e. PATAH, has a much lower thermalNH2 groups, the band at 1 668 cm from stretchingstability. PATAH can be decomposed to a great extent atvibration of the C= =O bondl6l, the band at 1 169 cm-'95 °C due to its nature. As a quaternary ammonium salt,from in-plane bending vibration of the NH2 groups'"7,PATAC is readily converted to a quaternary ammonium and the band at 620 cm- from out-of-plane rockinghydroxide in caustic solution by Eqn.(2)"5]. Therefore,vibration of the NH2 groupsl8l. The CH2- -Nt bendingPATAH is a quaternary ammonium hydroxide. Anvibration occurs at 953 cmand the band at 1 455quaternary ammonium hydroxide is easily decomposedcm-' is the- CH2一 bending vibration2oat a higher temperature. The decomposition reaction isIn addition to the similar absorption peaks of amidesummarized in Eqn.(3)I'5]. As a result, PATAH isand quaternary ammonium groups, a new absorptionunstable under a heating condition.peak appears at 2 957 cm ! in the spectrum of HPATAH,「2出]NaOHwhich maybe results from the stretching vibration of anassociated hydroxyl group in 一CH2- 0H18]. It is12N 0oPCH3concluded that an intramolecular hydrogen bond (asdescribed in Fig.4) composed of一CH2一 OH andC-N-CH3CrH2CONH2 from HPATAH can be constructed whenHPATAH exists in alkali solution.PATAC2yk2 H3.4 Discussion on red mud settling performance of(2From the results as discussed above, PATAC is .H2Nreadily converted to PATAH in caustic solution and广-CH2OHPATAH at 95 °C can be decomposed to HPATAH, inJmwhich a- CH2一 OH ~一CONH2 intramolecularhydrogen bond will be formed when HPATAH exists inPATAHalkali solution. Red mud separation process duringH2 Halumina production is conducted usually in strong causticaluminate liquor at a high temperature from95 to 110 C.HRed mud settling tests in this study are done at 95 °C andn|the alkali content in red mud slury is also quite high. Insuch a red mud system, PATAC is probably converted toHO-CH3HPATAH. Furthermore, PATAC used in this study bearsd240% of positive charge density, so the content of amidegroups in HPATAH is almost equal to that of alcoholichydroxyl groups. Once the intramolecular hydrogen bond,H2H+N(CH33↑(3)where one amide group is bonded with one alcoholichydroxyl group, is formed in the polymer chain ofo-HPATAH, HPATAH will have few free absorbable」，functional gro中国煤化工much worse-0Hflocculation prcNMHGsult,redmudsettling performance oI TA1 aL Is rauner poor.HPATAH.812J. Cent. South Univ. Technol. (2008) 15: 808- -813HPATAHMPATAHHPATACPATAC5001 0001 500200025003 0003500 4 000Wavenumber/cm 1Fig.3 FTIR spectra of PATAC, HPATAC, PATAH and HPATAHce0 ce0g0z0H2NNH, CH2CH2 .CH2Fig.4 Intramolecular hydrogen bond formed in polymer chain of HPATAH4 ConclusionsReferences1) Although cationic PATAC can absorb on[1] LIU Li-hua, GONG Zhu-qing， ZHENG Ya-jie. Synthesis andstructure characterization of diethyldiallylammonium chloride []. Jnegatively charged red mud surface by not only polymerCent South Univ Technol, 2003, 10(4): 347- -351.bridging but also charge neutralization, anionic SPA is[2] ROE W J, MALITO J T. Purification of Bayer process liquors: US4578255 [P]. 1986- 08 -27.more effective for caustic red mud separation at 95 (3] KOESTER G E. Process for making alumina: EP 0211338A2 [P].than cationic PATAC instead.1987-12- 29.2) At the same flocculant dosage, settling rate of redLU Hong-mei, ZHONG Hong, ZHANG Lei, YU Jia-geng, ZENGXiang-hui. The present situation and prospect of flocculants in redmud slurry treated by SPA is much higher than that by .mud setting []. Light Met, 2000(9): 23 - 26. (in Chinese)PATAC and the supernatant liquor of red mud slurry[5] BIGGS S, HABGOOD M, JAMESON G J, YAN Y D. Aggregatetreated by SPA is also clearer than that by PATAC.structures formed via a bridging flocculation mechanism []. ChemEng J, 2000, 80(1): 13-22.3) PATAC is unstable in caustic solution at a higher6] SHUBIN V. Adsorption of cationic polyacrylamide ontotemperature and can be decomposed to HPATAH withmonodisperse colloidal silica from aqucous electrolyte solutions [J].alcoholic hydroxyl groups.J Colloid Interf Sci, 1997, 191(2): 372- -377.4) HPATAH has two functional groups of - CH2-7] CHVEDOV D, OSTAP S, LE T. Surface properties of red mudparticles from potentiometric tirtion []. Colloid Surface A, 2001,OH and- CONH2 to form an intramolecular hydrogen182(2): 131-141bond in caustic solution, which can greatly decrease theNASSER M中国煤化Icrylamide chargeeffective absorbable functional groups in HPATAH anddensity and mMHCNMHnd sedimentationf Technol, 2006,result in a poor red mud settling performance of PATAC.52(2): 241-252..J. Cent. South Univ. Technol. (2008) 15: 808- -813813[9] APAK R, TOTEM E, H0G0L M, HIZAL J. Heavy metal cation2004, 20(1): 65- 67. (in Chinese)retention by unconventional sorbents (red muds and fly ashes) [].15] WEI Jun-jie. Organic chemistry [M]. Beijing: Higher EducationWat Res, 1998, 32(2): 430 440.Press, 2003: 290 -298. (in Chinese)[10] NASSER M S, JAMES A E. Effect of polyacrylamide polymers on16] DENG Y J, DIXON J B, WHITEG N, LOEPPERT R H,JUOASR.floc size and theological behaviour of kaolinite suspensions [J].Bonding between polyacrylamide and smectite [J]. Colloid Surface A,Colloid Surface A, 2007, 301(): 311-322.22006, 281();82-17] HASINE K, SAADET 0,MURAT O. Modified polyacrylamidethe effect of polymer type on flocculation, rheology and dewateringhydrogels and their application in removal of heavy metal ions [].behaviour of kaolinite dispersions []. Int J Miner Process, 2003,Polymer, 2003, 44(6): 1785-1793.71(4): 247-268.18] LU Yong-quan, DENG Zhen-hua. Practical infrared spectrum[12] ERSOY B. Effect of pH and polymer charge density on setling rateanalysis [M]. Beiing: Electronic Industry Press, 1989. (in Chinese)and turbidity of natural stone suspensions []. Int J Miner Process,19] VASSILEV K, STAMENOVA R, TSVETANOV C. Epoxidation of2005, 75(): 207 -216.styrene with hydrogen peroxide in the presence of polymer-supported[13] LU Gui-qian, WU Dingcai, FU Ruo-wen. Studies on the synthesisquatermary ammonium salts and peroxo complexes of W(VI) [J].and antibacterial activities of polymeric quatermary ammonium saltsReact Funct Polym, 2000, 46(2): 165-173.from dimethylaminoethyl methacrylate [J]. React Funct Polym, 2007,20] HU Hui-ping, HUANG Ke-long, PAN Chun-yue. Study on syntheses .67(4): 355-366.nd characterizations of polymethylphenethylsilane， polymethyl-[14] GAO Yu, ZHAN Xue-gui, XIE Hong-quan, GAN Zhi-wei. Synthesiscyclohexylsilane and their copolymers []. J Cent South Univand properties of water soluble photosensitive quaternary ammoniumTechnol, 2000, 7(2): 92- 96.hydroxide groups containing phenolic resin [J] Polym Mater Sci Eng,(Edited by ZHAO Jun)中国煤化工MHCNMH G.