Gas chromatographic thermodynamics on hydration processes of magnesium chloride with low water

Vol. 13 No. 2Trans. Nonferrous Met. Soc. ChinaApr. 2003Article ID: 1003 - 6326(2003)02 - 0467 - 06.Gas chromatographic thermodynamics onhydration processes of magnesium chloride with low water”CHEN Jian-jun( 陈建军)'，CHEN Guan-cheng(陈关城)”，MA Pei-hua(马培华)”，BAO Ji-qing( 保积庆)，MA Yu-tao( 马玉涛)'，CHEN Feng-qiu( 陈丰秋)3(1. College of Chemical Engineering, Qinghai University, Xining 810016，China;2. Qinghai Institute of Salt Lakes，Chinese Academy of Sciences，Xining 810008，China;3. College of Chemical Engineering, Zhejiang University, Hangzhou 310027, China)Abstract: The dehydration and hydration processes of magnesium chloride hydrates were studied by means of fron-tal chromatography analysis ，calorimetry, thermogravimetry and chemical analysis. The mathematical imitation forthe adsorption isotherms of MgCl2●4H2O and MgCl2●2H2O at different temperatures indicates that BoltzmannFunction is the ideal equation to describe those adsorption isotherms. Its adsorption heat is- 13. 06 kJ/mol and-16. 11 kJ/mol，respectively. The adsorption equilibrium constants are also given. From the data obtained, there isa thermodynamical possibility to use partial dehydrated magnesium chloride hydrates as an absorbance to clean watervapor contained in bischophite dehydration equipment and let the protection gas HCI recycle in the fluid bed reactionsystem.Key words : magnesium chloride with low water; hydration heat; adsorption heat; differential scanning calorime-try; frontal chromatographyCLC number: O642. 13Document code: AINTRODUCTIONtained from MgCl2●5. 97H2O ( analytical pure ma-Magnesium chloride is a basic material forterial heated about 4 h at 40 C) after dehydrationin the fluid system-2]. The compositions of sampleschemical industry, and it has wide applications inare shown in Table 1.metallurgy, architectural material and textile in-dustry. It is also a material for fire- resistant prod-Table 1 Compositions of samplesucts，cryogen and compound fertilizer produc-( mass fraction，%)tions1l. The fully dehydrated magnesium chlorideSampleMgCl2MgOH2Ois a good material for electrolytic metal magnesiumproduction. Because magnesium chloride has aMgCl2. 5. 96H2O46.9653. 04strong adsorptive ability, it is also used as a dryingMgCl2●3. 98H2O 57.0142. 99agent. Many successful works have been done onthe dehydration of bischophite-2-10]. But in fact,MgCl2●1. 95H2O72.071.3726.56the dehydration processes of magnesium chloridehydrates are inseparably connected to the hydrationIn order to decide the experimental temperature,process. The latter one will influence the dehydra-thermogravimtric analysis for MgCl2 ●5. 97H2O ation process. Very few scientific research on itsdifferent temperature increasing rates was carried out.hydration process has been reported so farl1I .The results are shown in Fig. 1.The aims of this paper are to obtain basic dataAccording to thermogravimetric analysis，un-for hydration process of magnesium chloride withler the experimental conditions the dehydrationlow water and check the thermodynamic possibilitytemperature of those processes， from 6 moles wa-of using partial dehydrated magnesium chloride toclean the vapor in the dehydration tail gas and letter to 4，from 4 moles water to 2，and from 2moles of water to 1. is 121.7- 131.7 C，169. 2-protection gas recycle.191.6 C and 201. 1 -225.0 "C，respectively.2 EXPERIMENTALet。 reartions go towards hydra-tion中国煤化工temperature of thoseThe experimental materials are MgCl2●5. 97prod:MHCNMHGian121.7，169.2andH2O，MgCl2●3. 98H2O and MgCl2 ●1. 95H2O201.1 C，respectively. But relatively high aqueouswith a grain size of 30 - 80 mesh， which are ob-vapor partial pressure is needed at high experimen-①Received date; 2002 - 02 - 27; Accepted date: 2002 -07 -02CorresponderEe?MAssociate Prof. CHEN Jian-jun; Tel: + 86-971-5310441; E mail: chijqdhy@ 163. com●468.Trans. Nonferrous Met. Soc. ChinaApr. 2003HFNF凸U3DTGτHT2HTGTH70170 270370 470570 670|Temperature/CFig. 1 DTA and TG curves of dehydrationprocess of MgCl●6H2O at temperatureFig. 2 Experimental diagram for measurement ofincrease rate of 10 C/minadsorption isotherms of magnesium chlorideMode of equipment: SETARAM TG-DTA92，hydrates by means of frontal gasmade in France;chromatography analysisTest samples: 11. 83- 17.29 mg MgCl2●5. 96H2O;1- Cold trap; 2一Magnesium column;Reference sample: caleined Al2O3;3- Chromatography column; 4一Water bath;Holders for test and reference sample: quartz crucible;5- Agitator; 6- Spiral pipe; 7一Vaporizer;Temperature range: room temperature - 750 C;8- Gas Chromatography; Ti- Flow controller;Temperature increase rate: 5,10 and 15 C/min;T2-s- Cut-off valve; F- Flow indicatorCarrying gas: pure N2umn. This channel is used for adjusting the molartal temperature. Considering that maximum waterratio of nitrogen gas and aqueous vapor, so as tovapor partial pressure can be supplied at room tem-measure the amount of water vapor adsorbed byperature, we choose the adsorptive temperature formagnesium chloride hydrates under different par-MgCl2●4H2O and MgCl2●2H2Oas40-60 'C and .tial pressure of aqueous vapor.70-90 C.The gas chromatography used in this experi-The experimental equipment diagram foment is GC-9AM, made in Japan Hitach Company，measurement of hydration isotherms is shown inalong with a data processor, C-R4A. AnalyticalFig. 2.column: GDX301，d 3. 0 mmX 1.0 m; referenceHighly pure nitrogen， from pressure pan,column: GDX301, d 3. 2 mmX3.6 m; water bathgoes into gas chromatography and then flows outtemperature: 20.2 - 24.0 C; chromatographic col-from it. Before N2 goes into gas chromatographyumn temperature: 40 - 90 C; temperature of injec-once again， it has three different channels to gotor: 85 C; temperature of TCD: 100 C; totalthrough.pressure: 490 kPa; pressure of first chamber: 120Channel I . N2 goes through cut-off valve T2kPa; pressure of second chamber: 110 kPa; flowand gas chromatography. Because no aqueous vatrate of first chamber: 90 mL/mim; flow rate ofpor is carried by N2 in this channel, it is only usedsecond chamber: 45 mL/ mim; current of detector:before the equipment goes steady.80 mA. The magnesium chloride column， with aChannel II. After going through cut-off valveheight of about 2 cm，was filled under minus pres-T3 and spiral pipe 6, nitrogen gas will go throughsure in a dry glove box.vaporizer 7, which is put in a carefully temperaturecontrolled water bath，and carries aqueous vapor3 RESULTS AND DISCUSSIONfrom it. Then nitrogen gas goes through cold trap1 and magnesium chloride column 2. Magnesium3.1中国煤化工queous vapor and cali-chloride with low water adsorbs aqueous vaporcontained in the nitrogen gas. The surplus aqueousMHC N M H Gndense aqueous vaporvapor will go into chromatographic column and becarried by input nitrogen gas. The collected wateranalyzed at the column temperature.within a certain period of time was weighed to de-Channel II. Nitrogen gas goes through flowtermine the amount of input aqueous vapor.' Thecontroller T，flow indicator F, cold trap 1，mag-soap film gas meter is used for measuring gas flux.nesium cho&ls-olumn and chromatography col-From calibration curve, as shown in Fig. 3，itVol. 13 No. 2Gas chromatographic thermodynamics on hydration processes●469●where C is concentration of aqueous vapor in N2, .mol●mL-'; h is the height of chromatographypeak，μV; K is a constant of thermal conductivityZ之9.0cell, mol●μV-1●mL-，it is considered an un-changeable constant within experimental tempera-ture range3.2 Adsorption isothermsThe hydration processes for magnesium chlo-号6.5ride hydrates are expressed as:MgCl2●RH2O (s) +2H2O (g)台11121314151671819MgCl2●(R+2) H2O (s)(R=2, 4 )Height of chromatographic peak/103(2)In order to let a hydration process take placerelatively high and stable aqueous vapor is neededFig.3 Calibration curveand the frontal chromatography analysis was se-lected for this system.is seen that the height of chromatography peak isThere is a relationship between the absorbanceproportional to the concentration of water vapor.and the area of chromatographic curves-131.But the line does not go through the coordinate ori-r= KFc.A.gin and there is a positive intercept. Because thereU1 mis a temperature drop near the cold trap due to thewhere r is the absorbance of adsorbent over unitcold liquid nitrogen, the cold trap just likes amass adsorbate; Ac is the area of chromatography“pump”，which will make the flow flux increase.curve; K is the constant of TCD ( Thermal Con-On the other hand, due to water condensation inductivity Detector); m is the mass of adsorbate;the cold trap，a concentration grade would formF'c is the calibrated fluid rate of carrier gas; u1 is aand lead to an increase of flow flux. Based on themoving speed of data recording paper.above analysis， the calibration line should bemoved and let it go through coordinate origin.F'c=j●P二Pw.C●FcIn Eqn. (4) [14], po and pw are saturated aque-ous vapor pressure at experimental temperatureand under atmospheric pressure, respectively; j isa coefficient; Fc is the fluid rate of carrying gas;一4Tc and TR are chromatographic column tempera-ture and room temperature, respectively.The chromatographic adsorption peaks underM60 'C for MgCl2●3.98H2O are shown in Fig.4.The amount of water adsorbed by magnesium chlo-ride hydrates may be obtained from Eqns. (3) and(4) . The blank assay should be subtracted fromit.From chromatographic adsorption peaks, data102304050for adsorption isotherms are obtained and listed inTime/minTable 2，and plotted in Fig. 5 and Fig. 6 corre-spondingly.Fig.4 Gas chromatography curves for3.3 Adsorption isotherm equationMgCl2●3. 98H2O hydration at 60 'CThe Langmuir equation and B. E. T Equation areCurve 1: p(H2O)=1. 12，t(water bath)=20.2 C;not suitable for this system. But Boltzmann equationCurve2: p(H20)=1.53， t(water bath)=24.05 C;(5)中国煤化工vell and it is a good ad-Curve3: p(H20)=1. 46, t(water bath)=24.0 C;sorpthis system. .Curve 4: p(H2O)=1. 29, t(water bath)=20.03 C;MHCNMHGCurve5: p(H2O)=1.83, t(water bath)=24.0 C十U1+exp(B二B)The equation of the calibration line is given aswhere I is the absorbance, mol/kg; a, β，γ and θfollows:are simulative parameters，as shown in Table 3 andC=历方数据8.642X10-11Xh(1)Table 4. It is seen from Fig.5 and Fig.6 that the iso-●470●Trans. Nonferrous Met. Soc. ChinaApr. 2003therm data agree with Boltzman equation， only withfor adsorption isotherms are smaller than). 07，onlythe exception of data at 40 'C. The simulation errorswith the exception of isotherm at 40 C, 0. 393.Table 2 Data of adsorption isotherms for MgCl2●RH2O[r/(mol●kg-I)]MgCl2●1. 95H2OMgCl2●3. 98H2OSample No.p(H2O)/kPa90 C80 C70 C60 C55 C50 C45 C40 C1.123. 4911. 8613. 360. 891.942.733. 591.298.7413.2315. 512.032.743. 724.755. 371.4612.8314. 0817.732. 294. 155.025.66.6.291.5313. 2114. 4918. 312.614. 495.736. 207. 631.8314. 0014. 6819. 152. 824.916. 106.278.27Table 4 Parameter of hydration isotherms foIsotherm .ParameterBCEa- 7.258- 4.347- 12.951. 2971.201. 2540.12190.132 30. 092 3419.2414.7413. 981.1 1.21314151.61.71.81.9p(H2O)/kPa3.4 Adsorption heatsFig. 5 Adsorption isotherm of MgCl. 3.98H2OThe adsorption heat may be defined by Calu-to aqueous vaporsis- Clapeyron Equation:20 [raln(p/po)7raln(p/po)-Q= RT'-RaT21/T)(6)于15|Select r as 3. 63，4.47 and 13. 50,13. 31 molkg from Fig. 5 and Fig. 6 respectively， we obtainhe corresponding equilibrium partial pressure of a-1-70C●二80queous vapor, p. There is a linear relationship be-tween ln(p/po) and 1/T (see Fig. 7 and Fig. 8).5|The adsorption heat equals minus R times the slope1.1 1.213141.51.61.71.81.9of the line. .From Fig. 7 and Fig. 8, the adsorption heats ofmagnesium chloride hydrates were obtained andFig. 6 Adsorption isotherm of MgCl2. 1. 95H2Olisted in Table 5.-4.15 [-4.20●●- - B: Adsortance=4.47 mol/kgTable 3 Parameter of hydration isotherms for■- - C: Adsortance= 3.63 mol/kg-4.25Isotherm户-4.30DF昌-4.35--27.29-5.679-5.799--6.628-70456..40 t中国煤化工β1.2601.1431.2561.2350.8288.DYHCNMHG.0.1889 0. 12960. 13280.1241 0. 13153.043.083.123.163.208.6676.344 6. 2104.9602. 727T-1/10-3K-1Fig. 7 Diagram of ln(p/po) to 1/T forisotherms of MgCl2 ●3. 98H2OVol. 13 No. 2Gas chromatographic thermodynamics on hydration processes●471.would be higher if the experimental temperature islower than 70 C.-4.15Moscowtz[3] used partial dehydrated ( P. D)magnesium chloride hydrates mixture as an adsor--4.25bent to absorb water vapor contained in tail gas offluid bed system for bischophite dehydration. Ir-4.35入this way，protection gas， HCl(g)，mixed in tail量gas, can be circulated and it will not be harmful toour environment. But Moscowtz did this work on a●- B: Adsortance= 13.50 mol/kgsmall scale experiment.■一C: Adsortance= 13.31 molkgDehydration of MgCl2●6H2O may take place-4.55in two stage fluid bed system. After treatment in2.742.802.862.92the first fluid bed, about 4 mol of water is removedT-1/10-3K-1from MgCl2●6H2O. Meantime MgCl2●2H2O isyielded，which includes about 17. 7% of waterFig.8 Diagram of ln(p/po) to 1/T forwaiting for further treatment in the next fluid bed.isotherms of MgCl2 ●1. 95H2OThe highest absorbance of MgCl2 ●2H2O is34.5% at 70 °C under the experimental conditions.It would be much higher at lower temperatures.Table 5 Adsorption heats of magnesium chlorideSo，using P. D magnesium chloride as an adsorbentto treat aqueous vapor contained in the tail gas ofhydrates to aqueous vaporsecond fluid bed and let HCl circulate is possible inAbsorbanceAbsorption Heata thermodynamics view.Adsorbentr/(mol(kg 1)Q/(KJ●mol - 1)Usually，adsorption equilibrium constant isMgCl2●3. 98H2O3. 63一13.06obtained by static method, but the system men-tioned in this paper is not a static one. So, the reli-4.47-14.17ability of adsorption equilibrium constants obtainedMgCl2●1. 95H2O13.31一15.59in this work needs further confirmation.13. 50-16.613.6 Influence of grain size3.5 Adsorption equilibrium constantIn order to investigate the influence of sampleAdsorption equilibrium constant is definedgrain size on hydration process, we select two sam-asL15] :ples of MgCl2●1. 95H2O with the grain size of 30K。=(7)- 80 mesh and small than 80 mesh to check theirn"/newhere n° is molar quantity of aqueous vapor ad-absorbance under the same experimental conditionsas before. T he chromatography column tempera-sorbed by magnesium chloride hydrates when theture is 70 "C.system reaches adsorption equilibrium; m is theIt is found from Table 7 that the absorbancemass of magnesium chloride hydrates. Meantime ;decreases with the decrease of sample grain size,n" is the molar quantity of aqueous vapor containedand the average adsorption equilibrium time ex-in n。moles of carring gas. So, in equation (7)，n'/tends from 63 min to 97 min， which contradictsm equals absorbance T; n/n。is a molar ratio of a-with the general concept. In this case, the authorsqueous vapor and nitrogen ， which goes throughguess that when the sample grain size is decreased,gas chromatography.It is found from Table 6 that the adsorption e-the hydration tendency of MgCl2●1. 95H2O is aquilibrium constant for MgCl2●3. 98H2O is lessdominant process compared to the dehydration.From this result，the authors guess that the hydra-than those for MgCl2●1. 95H2O. As the tempera-tion process of MgCl2 ●2H2O is not a solid stateture increases, the adsorption equilibrium constantinner diffusion controlled process，but probably itsis decreased. The highest adsorption constant fordehydration process is.MgCl2●1.95H2Oat70Cis2338mol/kganditTable 6 Adsorption equilibrium constants中国煤化工ed on .magnesium chloride hydratesMYHCNMH G_MgCl2. 3. 98H2O .40 C45 C50 C55 C60 C70 C80 C90 C1 0107667445993342 3381 7921 709一●472●Trans. Nonferrous Met. Soc. ChinaApr. 2003Table 7 Influence of grain size on absorbance of MgCl2●1. 95H2O at 70 CGrain between 30 and 80 meshGrain small than 80 meshComparison parameters24513Equilibrium time/ min~80850151201151(Absorbence Fw/%24.0827. 9131.94 32. 9634. 4623. 7424.05 27. 1930.54 32. 18[8] Salyulev A B. Preparation of anhydrous magnesiumREFERENCESchloride[P]. URXXAF SU 1726380 A1 19920415.[9] Orekhova A I, Lelekova R P，Sverdlovsk G. Dehydra-[1] CHEN Jian-jun, MA Yu-tao. 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