Effects of Magnesium and Ferric Ions on Crystallization of Calcium Sulfate Dihydrate Under the Simul

CHEM. RES. CHINESE UNIVERSITIESAvailable online at www.sciencedirect.com2008, 24(6). 688- 693Article ID 1005- 9040(2008)-06-688-06ScienceDirectEffects of Magnesium and Ferric Ions on Crystallization ofCalcium Sulfate Dihydrate Under the SimulatedConditions of Wet Flue-gas DesulfurizationGAO Xiang , HUO Wang, ZHONG Yi, LUO Zhong yang, CEN Ke-fa, NI Mingjiang and CHEN Le -mingState Key Laboratory of Clean Energy Uriliation, Instiute ofThermal linergy and Power Engineering,Zhejiang Universiy, Hangzhou 310027 P. R ChinaAbstract The influences of magnesium and ferric ions in their different ratios on the rate of gypsum crystallizationwere studied under the conditions simnilar to those of wet fluc-gas desulfurization(WFGD). The results show thataddition of both Mg2* and Fe" incrcased induction time and decreased the growth efficiency up to 50% compared withthe baseline(without impurities) depending on the concentration and the type of impurity. The effects of Mg^" and Fe2+on the surface energy and the rate of nucleation were estimated by employing the classical nucleation theory. Thesurface energy decreased by 8% and 14% with the addition of 0.02 mol/L magnesium or ferric ions, respectively,compared to the baseline. Mg2+ and Fe' made the growth rate of the (020), (021) and (040) faces of gypsum crystal amuch grealer reducion, which leads to the formnation of needle crystals compared to the baseline which favors theformation of plate or flakes. Furthermore, an edgc detection program was developed to quantify the effects of impuritieson the filtration rate of gypsum product. The results show that the inhibition efficiency of the prescnce of 0.02 mol/LMg°* and Fet* on the fltration rate of gypsum crystal ranges from 22% to 39%。Keywords Calcium sulfatc dihydrate; Morphology; Wet fluc-gas desulfurization; Filtration rate1万京数据ctionsmaller the crystals which are more difficult to filter,The wet fluc-gas desulfurization(WFGD) processwhich would increase the operation cost. Therefore,is the most recognized and accepted solution to re-enhancing the formation of large and uniform gypsummove sulfur dioxide emissions from the coal-firedcrystals is desired in WFGD industry.A considerable amount of researches on the me-power plants. Nucleation and crystal growth of cal-chanism of calcium sulphate dihydrate crystal growthcium sulfate dihydrate represent an important step inboth in the presence and the absence of different typesWFGD, because they affect the scrubber solutionof chemical additives has been done. The formationcomposition, the SO2 absorption, the absorbent utili-conditions and the presence and the type of impuritieszation, the quality of byproduct and consequently thenot only influence agglomeration or dispersionoperation and economic benefits of the whole process.behavior but predominantly alter the nucleation, mor-Air is sparged to a slurry tank where the oxidation ofphology and size of calcium sulphate dihydrate crys-calcium sulfite to gypsum takes place. The separationtals'2- '!. Trivalent transition metal cations incorpo-of gypsum crystals as byproduct from the slurry inrated through the faces of the crystals a cause theWFGD is mostly performed by belt filers in which thecrystal morphology change'7). The most used absor-filtration characteristic of gypsum plays an importantbent in WFGD, limestone, contains about 0.2%- -2%role. Gypsum forms small needle-like crystals havingof both ferric and magnesium ions which influence thea monoclinic prismatic structure with water moleculescrstallization and properties of the byproductbetween the calcium and sulfate ions in the unit cell'l(gypsum). Some of additives can change the mor-which increases the load of the filter and causes it tophologys and retard the crystallization of gypsumplug. Moreover, the larger the number of nuclei, the*Corresponding author. E-mail: xgao@cmee. zju.cdu.cnReceived December 14, 2007; accepted January 29, 2008.中国煤化工Supported by the State 11.5 Support Plan(No.2006BAA01 B04) and.MYHC N M H GSuppont Pla ofChina(No.NCET-06-0513).Copyright。2008, Jilin University. Published by Elsevier Limited. All rights reserved.No.6GAO Xiang et al.689even at their low concentrations(10- -6 mmolL)!. Inand FeCl3, were also dissolved in the solution of cal- ,the presence of borax and different free sulfate con-cium chloride which was added to the reactor beforecentrations, crytallization kinetics of calcium sulfatethe addition of Na2SO4 As soon as the reactor systemdihydrate has been studiedlo. and the growth rate ofhad reached a slate of a constant temperature, thecalcium sulfate was also orelated. Diferent ordersaddition of Na2SO4 was performned by starting the pe-of kinetic expressions depending on supersaturationristaltic pump connecting to reactor by rube. Slurrylevels have been repored!2.Bs. However, all of thesamples from the crystallization reactor were alsoabove studies were based on simulated conditions oftaken Periodically from the outlet and immediatelyphosphoric acid production which involves the lowerfiltered(0.22 山, Mllipore) to separate the solution.supersaturation and higher temperature compared toThe filtrate was measured for calcium concentrationthe WFGD process. The present research aims atby EDTA titration and the solid crystal was characte-studying the effect of ionic impurities on the inductionrized by XRD(Rigaku-D/Max-2550pc), SEM(Hitachitime, nucleation rate, crystal size distribution andSEM-S-570), and determined for particle size distri-morphology of the formed gypsum crystals in simu-bution by a Malvern Mastersizer(2000).lating the process of WFGD..3 Calculation of the Supersaturation Level2 ExperimentalThe supersaturation level of calcium sulfate di-2.1 Experimental Apparatushydrates can be determined by considering the fol-lowing liquid-solid equilibrium in the reactor:The experimental apparatus consisted of a stiredbatch ecylindrical crystallization reactor, made of PVC,Caq>+SOaq>+2H2Om CaSO42H2O&;) (1)with a working volume of 2 L. The peristaltic pumpCa)+S0g)一→CaSO4;2H2Oaq)(2)was互方数齷speed digital pump capable of pumping2.5--10 mL/min depending on the desired value ofaca2+asoz-ai mca2+mso?-怪屁(3)residence time. The stiring is carried by a two-bladeKspstirrer with 450 r/min to keep the slurry in suspensionwhere Ksp is solubility product of gypsum and S is thewithout damage the crystal. Under those conditionssaturation ratio. The solubility of gypsum can be cal-the crystallization process was simulated in the typicalculated via the sum of the quantity of the free calciumslurry sump of WFGD.ions and the associated calcium sulphate species.Therefore, the total solubility is[awrmc2+mcso.* at(m'so至K<(mca+)(mso3-)y2gwhere y is the mean activity coefficient of cal-cium-sulphate, Ks is the equilibrium constant and aw isactivity of water. A variety of methods have been usedfor calculating calcium sulphate solubility in pure andFig.1 Experimental apparatus for measuring effectsmixed metal sulphate solutions. Because of the highof ions on erystallization of calcium sulfateion strength in WFGD, an extended Debye Huckeldihydrateequation which can accurately model gypsum solubil-a. Feed tank; b. peristalic pump; c. stirer; d. conductivity alyzer;ity up to 2 mol/L is adopted to calculate the activitye. reactor,f. water bath; g sample point.ceofficients!"4:2.2 Materials and Methodslov=-A|ZcaZso4|Two 1 L of solutions of calcium chloride and中国煤化工-aso.I+sodium sulfate were prepared from the reagents ofreagen grade and the deionized water was heated toYHCNMH Gosl(5).5the ideal temperature before starting the experiment.The calculated amounts of impurities, such as MgCl2(+pc[ZcaZs04|CHEM. RES. CHINESE UNIVERSITIESVol.24690where A is Debye-Huckel constant, z is the charge onof time from the formation of supersaturated solutionion, and I is ionic strength. The Bromley-Zemaitis ionuntil the appearance of the nuclei. The crystal growthinteraction parameters(Bcaso) for CaSO4 can be ob-eficiency(E) was determined from the value of tnd.tained in accordance with the research results ofcalculated with(1o) and without impurity ions() asAdamsl4. There are many methods in literature forfollows:the calculation of Ksp of calcium sulphate':1,16. In thisE=(to- t)lto(10)paper, the values of Ksp and Ks were taken from the3 Results and Discussionmodifed OLI database and they are 2.5X 10~ and 189,respectively.3.1 Effect of Mg* and Fe2* lons on InductionTime and Growth Efficiency2.4 Calculation of Surface Energy and NucleationCrysallization is composed of three consequentRatesteps: supersaturation, nucleation and crystal growth.As soon as the stable nuclei, ie. particles largerAs the solution is supersaturated, the Ca2* and so4-than the critical size, have been formed in the crystal-in solution start to form clusters and the subsequentlization reactor, they begin to grow into crystals ofgrowth of nuclei leads to a crystal. To study the effectvisible size. Assuming that the nuclei are similar inof lonic Contaminants on the crystal growth of gyp-shape and grow at the same velocity, the surfacesum, several experiments were carried out with addi-energy can be calculated with the aid of cassic ho-tion of Mg and Fe ions at different ratios.mogencous nucleation theory aslh7.B8:The efectst of the ionic contaminants on gypsumlgime= A+A/[Ts(gS)弓]6)crystallization were studied by measuring the conduc-where tind. is the time of induction period,An is an em-tivity of solution which can more precisely representpigigt; anandienioness and A2 depends on thethe starting time compared with the most used me-number of variables, and is given by:thods of turbidity21.22. The time that corresponds toAz=(m3VPN)(2.3R}this inflection is referred to the starting time for nuc-Assuming that particles smaller than re will dis-leation.solve and larger than re will continue to grow, the rateThe concentration of calcium vs. induction timeof nucleation, B, e.g., the number of nuclei formed peris plotted in Fig.2, which shows that the presence ofunit time and volume can be expressed by combiningeither Fe'+ or Mg* increases the induction period. Ithe Arrhenius reaction velocity equation with thecan be seen that the concentration of calcium decrea-crystal size which can be drawn from the literatureses very slowly at first and then sharply drops because[19]:of the formed nuclei combining to cluster,0Gcin=4TYs≌16πy3vz(8)3.033(aTInS)3B=FXexp[-nY3V2NAf(9)笪2.(RT)3In2S?2where η is a geometric(shape) factor of 16m/3 for the2spherical nucleus, f is a crrection factor(when homo-2.geneous nucleation takes place, f=l), V is the mole-1.cular volume(74.69 cm/mol for gypsum), T is the020.406080 100120t/minabsolute temperature(K), R is the gas constantFig2 Effects of Mg2* and Fe* on the induction time(J-mor-'.K~"), and ys is the surface energy(/m). Thea. Without impurty: b. 0.02 mol/LMg"*;c 0.02 mol/LFe'*.critical size r。is the minimum size of a stable nucleus,Moreover, addition of both 0.02 mol/L Mg* anda is the Bolzmann constant, N is the Avogadro's0.02 mol/L Fe'* also made the plots shifted towardsnumber(6.023X 102/mol) and F is a frequency con-long中国煤化工ts without impuritystant( 1030 nuclei/cm312%).ions.:OHCNMH GMg* and Fe*i unnununs VivviNj2.5 Calculation of Inhibition Eficiency(E)(7=313 K) is shown in Table I, which indicates thatThe induction period(ina), is defined as a periodthe presence of Fe2+ or Mg* increase the inductionNo.6GAO Xiang et al.691time of gypsum from 18.5% to 51% depending on theTable 2 Surface energy and nucleation rate withoutconcentration and type of impurity. The explanation isand with 0.02 mo/L Mg2* and Fe}+C(Mgy(Fey10*B/that the added impurity decreases the activity coeffi-(mol-Lhy(molL)_ energy(mJ-m" 2)(nucleicm's)cient which decreases supersaturation level of gypsum08.69578.300.027.98783.90in solution.7.51387.58Table 1 Inhibition eficiencies of ionic contaminants.027.73985.68c(Mg' 'Y(mol-L") c(Fe2 )V(mol:L")InducticInhibitiontime/s fficiency(%)nuclei are formed. These nuclei have a lower chance1135to grow to relatively large crystals compared with the18.50.05145528.2formed nuclei in the baseline. This result will be fur-172542.3ther justified by the size distribution and SEM photo187551.2in the following sections.3.2 Effect of Mg2* and Fe+ on Gypsum Crystal-3.3 Effect of Mg2* and Fe+ on the Size Distribu-lization Rate(Growth)tion of GypsumThe interfacial tension between the gypsumThe crystal size distribution of gypsum in thecrystals and the aqueous solution is a fundamentalpresence ofMg* and Fe'* at different ratios comparedparameter for understanding the nucleation. The ef-with the baseline(without impurity) is given in Fig.4fects of impurities on the crystallization rate were de-which shows the gypsum crystal in the absence oftermined indirectly by the measurement of the con-impurities will provide the largest crystals and highestcentrations of calcium and sulfate dissolved in thefiltration rate.reactor at the staring time for nucleation.. Plotting lgtind. against 1/(lgS)2 over a range ofhigne其aturation(17- -22) for a fixed temperaturegives a straight line with a slop of A2/T*. Referring tothe slopes of these lines as shown in Fig.3, the sufaceenergies can be calculated by combining Eqs.(6) and豆(7) at the fixed temperature.0.1101000Size distribution/um.dFig.4 Effects of Mg* and Fe+ on the size distribu-.s ttion of gypsuma. Without impurity, b. 0.02 molL Mg*; e. 0.02 molLMg* +0.02 molL Fe2*; d.0.02 molL Fe*..1 tFig.4 also reveals that the presence of both Mg2*and Fex* has a drastic effect on the filtration rate be-.9 L0.110.140.21cause it makes the size distribution of the formed1/(gS)2gypsum crystal shifted to a uniform distribution andFig.3 Relationship between lgnd and 1/(gS)2 with-decreases the filtration rate. The statistics of gypsumout and with ionic contaminantscrystal size are given in Table 3.a. Without ionie contaminants; b. Mg";c Mg”'+ Fel;d. Fe*.Table 3 also shows that the median and meanThe nucleation rate can be calculated accordingto the relation mentioned in Eq.(9) from the knowndiameters of gypsum crystals are higher in the absencesurface energy value and supersaturation level. Theof additives than those of gypsum crystals in theobtained results of the surface energies and nucleationpresence of Fe't or Mg2*. The high content of finerate are given in Table 2. It is clear that the surfacecrystals and the short mean diameter in the case ofenergy is slightly decreased by the addition of Mg^*gypsum crystals in the presence of both Fe'+ and Mg2and highly decreased by the addition ofFet*.may indicate that the formed gyDsum crystal growth isThe calculated value of surface energy aroundinhib中国煤化Iine. Moreover, the10 mJ/m? agrees with the reported ”value for theincreYHC N M H Gurace area) indi-CaSO42H2O8. A high nucleation rate in the case ofcates unat une impunty erecuvery retarded crystalboth Mg and Fe ions means that a larger number ofgrowth and decreased the filtration rate.692CHEM. RES. CHINESE UNIVERSITIESVol.24Table3 Effects of Mg2*, Fe+ or both Mg°* and Fe+reduction in the growth rate of the (020), (021) andon the size distribution of gypsum'0.02 molL 0.02 mol/L 0.02 mo/L.Fe"(040) faces, leading to the formation of needlesltemBaselineFe"Mg0.02 mol/L Mg”°compared with the baseline which favors the forma-16.26.49.27.1tion of plates or flakes. It can be explained by thed/μm26922226fact that Mg2* and Fet+ can poison special faces of106.938'54.7Dx/um120.614.82016.4gypsum crystals and thus inhibit the crystal growthPSA/(m'g)__ 0.150.230.190.20rate on those faces.中PSA: specifie surface area.Table4 Effet of Mg2* and Fe?+ on lttice parame-ters of gypsum crystals(peak area)3.4 Effect of Mg2* and Fe+ on the Structure ofFWHM (hkl)(020)(021)(040)Gypsum0.1190.[360.02 molL Mg”0.1330.1350.130Powder XRD patterns of the gypsum crystal0.02 molLFe"0.1620.1640.161samples in the absence of and in the presence of Mg0.02 mol/LMg*+0.02 mo/LFe'* 0.1380.145 .0.137and Fe'+ are given in Fig.5.3.5 Effect of Mg2* and Fe3+ on the Gypsum Mor-phologydThe gypsum morphology(size and shape of crys-tals) is one of the fundamental factors afecting thefiltration rate. Gypsum crystals can be formed indifferent morphologies, but the WFGD favors theuniformed plte-like crystals which have a higher125filtration rate and protect the cloth from blocking.万方数螈XRD patterns of gypsum crystalThe SEM images of gypsum crystals in the ab-a. Baseline, b. 0.02 mo/L Mg"; c.0.02 mo/LFe";d. 0.02 moVL Mg2 +0.02 molL Fe".sence of and in the presence of Mg"+ and Fe" areThe results show that the crystals formed argiven in Fig.6, which indicates that the presence im-gypsum, which indicates that the impurities also canpurities alter crystal morphology from plate like toblock kink sites without being absorbed, thereby ef-needle like and decrease the filtration rate.fectively retarding crystal growth.The reason is that both Mg2* and Fe'+ can maskThe quantitative analysis of the characteristicand prevent the growth of some planes of gypsum.gypsum diffraction peaks is given in Table 4, whichMoreover, the effect of Mg^* is less than the effect ofindicates that the peak areas decreased in the presenceFe*+, which is because that the charges of Mg2+ areof Mg2* and Fe'*. The values of FWHM show that theless than those ofFe+123.24.presence of both Mg* and Fe' caused a much greater[A)(B)DI K20m20pumFig.6 SEM images of crystals(A) Without imprity (B)0.02 moV/. Mg";(C)0.02 molL Fe": (D)0.02 mol/L Mg"*+0.02 mol/L Fe'*.However, it is important to be able to distinguishover the images. Fig.7 shows the images of Fig.6 afteras well as quantify the effects of impurities on theprocessing. The processed images indicate that: themorphology and filtration rate of gypsum product.larger, the crvstals. the lpse the numher of edges.Using Matlab 6.5 combined with an edge detection中国煤化工g 20 samples inprogram by the Image Processing Toolbox, the SEMeach (YHCNMHGaveragevalueofpictures from different cases can be detected and cal-edges in Matlab and the output results are given inculated in order to analyze the whole length of edgesTable 5.No.6GAO Xiang et al.693可Fig.7 Processed images(A) Without impurity, (B) 0.02 molL Mg2 ; (C) 0.02 molL Fe"; (D) 0.02 mo/L Mg* + 0.02 mo/L Fe'*.Table 5 Quantify the efects of impurity ongypsum and indicate that the impurities can blockthe morphology of gypsumkink sites without themselves being absorbed, therebym(Mg' 'Ymol m(Fe"YmolLength of edgesInhibition eficiencyeffectively retarding crystal growth and decreasing the(dimensionles)on fltration rate(%)021945filtration rate.。2684322.320.023058639.38398References_0.022839529.39The average lengths of edges in the presence of[1] Cetin E, Eroglu L, Ozkar s., J. Cryst. Growth, 2001, 231 (4) 559[2] Shall H. E, Rashad M. M, Abde-Aal E. A, Cryst. Res. TechnoL,Mg*[Fig.7(B)]，Fe'*[Fig.7(C)], and both of them2002, 37, 1264[Fig.7(D)] were detected as 26843, 30586, and 28395[3] Nyvt J, Urich J, Admistue in rstaliaion, VCH, Weinheim,compared to 21945 for the baseline[Fig.7(A)] which is1995a typical plate morphology. The presence of Fe has[4] Mahmoud M. H. H, Rashad M. M, Ibrahim L. A, et al, J. Coll.加mterf Scei, 2004, 270, 99the largest inhibition efficiency on the crystal growth[5] Alimi F. Gadni A. Desalination, 2004, 166, 427ang_the想st filtration rate for the byproduct which[6] Rashad M. M, Mahmoud M. H. H, Ibrahim 1. A, er al, J. Cryat.justifies the results of the effect of impurity on PSA.Growth, 2004, 267, 372Furthermore, the presence of Mg2* attached on the[7] Laix M. C., Robert K J, Miller M. C, Chem Res. Chinese Untver-special face of the crystal can inhibit the attachment ofsiries, 2004. 20(4), 411[8] Rashad M. M.,J. Cryst. Growth, 2004, 267,372Fe'+ on these faces and decreases the effect of Fett on[9] Hamdona s. K,J. Cryst Growth, 2007, 299, 146the morphology of gypsum. It is supported by the fact[10] Abdel-Aal E. A, Cryst. Res. Technol. 2004, 39, 313that the gypsum crystal growth is a surface controlled[1I] Jiang W. G, Langmuir, 2007, 23, 5070processl5l.[12] Smih B. R., Sweet F. J. Colloid and Interface Science, 1971, 37(3),614 Conclusions[13] Klepetsanis P. G, Koutsoukos P. G, J. Colloid and IterfaceScience, 1991, 143(2). 299The effects of Mg2* and Fe2+ on calcium sulfate[14] Adams F. J, PhD Thesis, University of Toronto, 2004dihydrate crystallization under simulated conditions of15] Adler M. s, Glater J, WcCutchan J, Jouonal of Chemical andWFGD have been studied. The above analyses showEngineering Dala, 1979. 24, 187that the presence of some metal ions in the solution[16] Raju K. U. G, Akonson C. Joumnal of Chemical and EngineeringData, 1990, 35, 361has a pronounced effect on the rate of gypsum crystal[17] He s., Oddo J. E, Tomnson M. B.. J. Collold Imerface Ssci, 1994,growth.162, 297The nucleation rate is increased by 6.7% or 12%[18] Lancia A., Musmara D., Prisciandaro M., AICHEJ, 199, 45, 390with the addition of 0.02 mol/L Mg2+ or Fe*+ ions,[19] Mulin J. W., Bter Worh-Heinemarn, 2001, 182[20] Myerson A. s. Butter Worih-Heinemarn USA, 1993respectively, compared with the baseline. In the pre-21] Felix Brandt D. B. J. Crysr. Growth, 2001, 233,837sence of Mg* and Fe", the median and mean diame-[22] Rashad M. M. Mahmoud M. H. H, AIbrahim L.et al, Cryst. Res.ters of gypsum crystals are less than those under theTechmol, 2005, 40, 741conditions of baseline. The developed edge detection23} Guo X. H, Li s. P., Hou W. G, et al, Chem. Res. Chinesefor the SEM pictures shows that Inhibition EfficiencyUniversities, 2003. 19(2).211on filtration rate is increased by 22% or 39% with the[24]中国煤化工Growh, 2070 29，146[25]JIE. A, Crys. Res. Tech-adition of 0.02 mol/L Mg2* or Fe}, respectively. TheMHCNMHGXRD results show that the crystals formed are still