Solubility of α‑Calcium Sulfate Hemihydrate in Ca−Mg−K ChlorideSalt Solution at (353.0 to 371.0) KXiaoqin Wu,*,†,‡ Kemiao Wang,† Zhiyang Xiong,† and Xiaoli Ye†

†College of Chemical Engineering and Technology and ‡Hubei Province Key Laboratory of Coal Conversion and New CarbonMaterials, Wuhan University of Science and Technology, Wuhan 430081, China

ABSTRACT: The solubility of α-calcium sulfate hemihydrate in CaCl2−MgCl2−KClsolution at temperatures from (353.0 to 371.0) K was measured by the dissolutionequilibrium method. The investigated concentration ranges of CaCl2, MgCl2, and KCl wereup to (6.01, 4.50, and 0.27) m, respectively. The calcium ion demonstrates the common ioneffect, which results in the decreased solubility of α-calcium sulfate hemihydrate in CaCl2solution. The solubility of α-calcium sulfate hemihydrate is augmented due to theassociation effect caused by the formation of stable MgSO4

0 in MgCl2 solution. Also it isshown that potassium chloride can increase the solubility of α-calcium sulfate hemihydrateslightly, which may be attributed to the salt effect. In the mixed salt solution, the solubilityof α-calcium sulfate hemihydrate decreases because of the dominating action of thecommon ion effect. Compared with the solubility of calcium sulfate dihydrate in the mixed solution, the solubility of α-calciumsulfate hemihydrate is lower than that of calcium sulfate dihydrate above 363.0 K, which is beneficial to the conversion of flue-gas-desulfurized gypsum into α-calcium sulfate hemihydrate.

1. INTRODUCTIONThere are three main forms of calcium sulfate: calcium sulfatedihydrate (DH), calcium sulfate hemihydrate (HH), andcalcium sulfate anhydrite (AH). In recent years, α-HH hasgained more application in the fields of precision instrumentmolds, ceramics, industrial arts, and architecture because of itssuperior workability and high strength.1 The knowledge ofsolubility and phase equilibrium of solid calcium sulfate inaqueous electrolyte solution is considered important in thegypsum product development. Much work had been carried outabout the solubility of calcium sulfate hydrate in differentaqueous solution systems.2−8 However, there has been noreport of the systemic solubility data of α-HH in alkali/alkalineearth metal chloride solutions in literature.α-HH is becoming important in the gypsum industry because

of its outstanding properties. Therefore, it is necessary tounderstand the basic properties of α-HH for its preparation andfurther utilization. α-HH is usually produced from DH by themethods of solid autoclaving or slurry autoclaving in industry.9

Another suitable method for the conversion of flue-gas-desulfurized gypsum (FGD gypsum) into α-HH is the saltsolution method at atmospheric pressure, which seems to bemore attractive due to the employment of atmospheric pressureand mild transformation temperature.10,11 Hence, the acquire-ment of solubility data of α-HH is of great significance for theconversion of FGD gypsum into α-HH in the salt solution atatmospheric pressure.We previously achieved the modification of FGD gypsum

and successfully prepared α-HH from DH in the hydrothermalmixed salt solution at atmospheric pressure.12,13 We alsoinvestigated the solubilities of DH in alkali/alkaline earth metalchloride solutions, that is, Ca−Mg−K chloride salt solution intemperature range of (348.0 to 371.0) K.14 The goal in our

current study was to determine the solubilities of α-HH in Ca−Mg−K chloride solution at the temperature from (353.0 to371.0) K. Moreover, by comparison of the solubilities of DHand α-HH, we can have a better understanding of theconversion of FGD gypsum into α-HH in the Ca−Mg−Kchloride salt solution at atmospheric pressure and bettercontrol over the conversion.

2. EXPERIMENTAL SYSTEM AND METHODS

2.1. Materials and Experimental Equipment. Generally,two ways were used to determine the solubility of substances.One was the precipitation method, and the other was theisothermal dissolution equilibrium method. However, the latterwas regarded more reliably because it avoided complicationsduring precipitation. Therefore, the isothermal dissolutionequilibrium method was employed to determine the solubilityof α-HH in the present work.Reagent-grade CaCl2, MgCl2·6H2O, and KCl were used to

prepare the salt solutions, and ultrapure water (conductivity<0.1 μs·cm−1, prepared by UPT-I-520T) was used in allexperiments. Self-made α-HH12 with 96.5 % (by weight) purityhas a volume mean particle size of 43 μm was used in solubilitymeasurement, which was determined by the differentialscanning calorimetry/thermo gravimetric analysis (TG-DSCanalysis) shown in Figure 1. The graph in Figure 1 shows acharacteristic exothermic peak at around 182.5 °C of α-HHwith combined water content of 6.10 % (by weight).

Received: July 9, 2012Accepted: November 30, 2012Published: December 10, 2012

Article

pubs.acs.org/jced

© 2012 American Chemical Society 48 dx.doi.org/10.1021/je300751h | J. Chem. Eng. Data 2013, 58, 48−54

The experimental device included a 500 mL four-neckedround-bottom flask for solubility determination which wasequipped with a spiral condenser for vapor reflux in the systemto keep constant boiling point. Heat was provided by an oil-bath thermostat keeping constant temperature regulated at ±0.1 K adjusted according to a precise mercury thermometer.Uniform agitation of (100 to 150) rpm for the suspension wassupplied by an electromagnetic stirrer.2.2. Procedure. First, the prepared salt solution was added

into the flask and then preheated to (353.0, 358.0, 363.0, 368.0,and 371.0) K according to experimental scheme that was keptconstantly for half an hour. Then, 6.00 g of excess solid α-HHwas quickly introduced into the salt solution. After that, themixture was kept at a constant temperature until the dissolutionequilibrium process completed. During experiment, a 10 mLslurry sample was taken at different intervals and immediatelyfiltered with a 0.45 μm cellulose filter. A 2.00 mL clear filtratewas quantified by pipet (Gilson, P5000) and diluted withultrapure water in a 100.00 mL volumetric flask. The solubilityof α-HH was expressed as CaSO4 g/100 mL solvent, based onthe measured S abundance from ICP analysis (IRIS AdvantageRadial, ThemoElemental). A calibrating solution was used as astandard for every set of analyses, to ensure that the analyseswere accurate to within ± 1 %. The densities of the saturatedfiltrate were determined using a portable density meter (DMA35N) from Anton Paar. The measured densities were uncertainto 0.001 g·cm−3. The residual solid phases were quickly washedwith boiling ultrapure water three times and then with 10 mLacetone once and dried at 333.0 K in an oven for (2 to 4) h.Then, the dried solid samples were stored in a glassware-desiccator and analyzed by DSC/TG (Netzsch STA 449 Luxx,Germany) analysis.2.3. Determination of Equilibration Time. The equili-

bration time between α-HH and salt solutions should bepredetermined. Much work has been done to determine theequilibration time of α-HH. Ling and Demopoulos7 indicatedthat the α-HH could reach solubility equilibrium in sulfuric acidsolution at 371.0 K within 5 min. Yang8 reported that 10 minwas adopted as equilibration time of α-HH at (353.0 to 358.0)K and 40 min for temperatures higher than 358.0 K in purewater. Li and Demopoulos5 indicated that the solubilityequilibrium between solid α-HH and HCl + CaCl2 solutionsat the temperature range of (283.0 to 353.0) K was establishedwithin 1 h.We also studied the equilibration time in our research. It has

been indicated in our previous study14 that the dissolutionequilibration time of DH was 4 h. In addition, the solubilitydata of α-HH vs time in pure water and the salt solution areshown in Figure 2, in which the error bars represent the

corresponding standard deviation. The stabilization of solubilityachieved in pure water was about 5 h at 368.0 K, as shown inFigure 2a. The equilibration time experiments at 353.0 Kcontinued for 84 h in pure water and in the mixed salt solution,shown in Figure 2b, which indicates that the solubility of α-HHwas stabilized after 12 h at 353.0 K. Therefore, 6 h was selectedas the equilibrium time of α-HH in the temperature from(363.0 to 371.0) K, and 12 h was selected at (353.0 to 358.0) Kto ensure the dissolution equilibrium.

2.4. Reproducibility. The solubility data of α-HH in purewater were determined to verify the reproducibility and theaccuracy of the adopted procedure in this paper. The result isshown in Table 1. The data collected in this work and literaturevalues8,15 are demonstrated in Figure 3. Three replicatemeasurements of α-HH solubility were carried out to get thefinal averaged solubility. The uncertainty of the measuredsolubility values is within ± 0.0063 g/100 mL, and the averagerelative deviation is 3.81 %. The solubility of α-HH decreaseswith temperature increasing, which is mostly consistent withthe tendency reported in the literatures.8,15 However, thesolubility determined in this work is higher than the data in theliterature, especially in the lower temperature range. Thedifference in properties of α-HH could be responsible for thisdiscrepancy. α-HH mentioned in the literature8 is a commercialproduct with 95 wt % purity prepared from natural gypsumusing the processes of solid autoclaving, while α-HH in thiswork with 96.5 wt % purity is produced from a industrialbyproduct gypsum (FGD gypsum) in the mixed salt solutionunder atmospheric pressure. The difference in raw materials,preparation method, and crystal growth condition would resultin different crystal habits and properties. Therefore, there maybe deviations in solubility to some degree. Also the deviations

Figure 1. TG-DSC curves for α-calcium sulfate hemihydrate.

Figure 2. Equilibration time tests of α-calcium sulfate hemihydrate (α-HH) in pure water and in salt solution: (a) in pure water at 368.0 K.(b) ■, in pure water at 353.0 K; □, in mixed salt solution (Ca, Mg, andK chloride, total concentration = 30 %) at 353.0 K.

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are in allowable range. In this work, the solubility of α-HH inwater can be correlated to the temperature as followingequation with a standard deviation of 0.0648 g/100 mL.

· = − + −−s T T/g 100 mL 9.6784 0.05603 0.005354CaSO1 2

4

(1)

3. RESULTS AND DISCUSSIONThe solubility data of α-HH in different salt solutions wereobtained in the temperature from (353.0 to 371.0) K underatmospheric pressure. Tables 2 through 5 show the solubilitydata in different chloride salt solutions. The relationship of thesolubility of α-HH vs overall ionic strength is displayed inFigures 4 to 7. Thermal analysis (DSC/TG) detected no phasechange under all of the above tested conditions.3.1. Solubility of α-HH in CaCl2 Solutions. Figure 4

shows the solubility of α-HH in CaCl2 solution as a function ofconcentration of CaCl2 in the temperature from (353.0 to371.0) K. The solubility data of α-HH are listed in Table 2. Theuncertainties for the values measured are 0.0004 mol·kg−1 forsolubility and 0.001 g·cm−3 for the density of saturated filtrate.It is clear that the solubility of α-HH decreases significantly

as the concentration of CaCl2 in solution increases. Thecorrelation of the solubility of α-HH with CaCl2 concentrationscould be attributed to the common ion effect of the calciumion. Similar results have been reported.5,14 The influence of

temperature on the solubility of α-HH depends on theconcentration of CaCl2 in solution. For instance, below theconcentration of 1.59 m, the solubility of α-HH varies obviouslywith temperature. The influence of temperature on thesolubility above 1.59 m is slight. Compared with the solubilityof DH, the solubility of α-HH is higher than DH in CaCl2solutions. According to the theory of crystallization, DH couldnot been transformed into α-HH in CaCl2 solutions in theinvestigated temperature range, which is consistent with theresults in our previous study.13

3.2. Solubility of α-HH in MgCl2 Solutions. Thesolubility of α-HH was measured in MgCl2 solution from(353.0 to 371.0) K. The solubility data of α-HH are given inTable 3. The uncertainties for the values measured are 0.0003mol·kg−1 for solubility and 0.001 g·cm−3 for the density ofsaturated filtrate.As shown in Figure 5, the solubility of α-HH increases by 1

to 5-fold in MgCl2 solution compared with the solubilities inpure water. The augmentation in solubility is because themagnesium cation associates the partial sulfate ion in the formof stable MgSO4

0 ion pairs. The association effect ofmagnesium has been reported in some other literature.16−20

However, the solubility of α-HH decreases when theconcentration of chloride magnesium solution increases further.The explanation for the decrease of solubility in moreconcentrated MgCl2 solution is due to the change in activitycoefficient. The thermodynamic equilibrium constant (Ksp) ofα-HH can be expressed as below:15

γ γ

γ γ

=

=

=

+ −

+ + − −

+ − + −

Ka a a

a

m m a

m m a

( )

( )( )

spCa SO H O

0.5

CaSO 0.5H O(s)

Ca Ca SO SO H O0.5

Ca SO Ca SO H O0.5

24

22

4 2

2 24

24

22

24

2 24

22 (2)

where γCa2+ and γSO42− are the activity coefficients of Ca2+ and

SO42−, mCa

2+ and mSO42− are the concentrations of Ca2+ and

SO42−, and aH2O is the activity of water. The activity of solid α-

HH is set equal to 1 because the solid salt is at its standardstate. According to Martynowics et al.,21 the solubility of α-HHcan be written as the following equation:

Table 1. Solubility of α-Calcium Sulfate Hemihydrate in Water

T/K 353.0 358.0 363.0 371.0 373.0 378.0 383.0

s/g·100 mL−1 0.2001 0.1946 0.1926 0.1755 0.1643 0.1456 0.1273

Figure 3. Solubility of α-calcium sulfate hemihydrate (α-HH) in waterat different temperatures: ■, data of Hulett and Allen15 and Yang etal.;8 ●, this work.

Table 2. Solubility of α-Calcium Sulfate Hemihydrate (1) in CaCl2 (2) + H2O (3)

353.0 K 358.0 K 363.0 K 368.0 K 371.0 K

m2 m1 ρsa m1 ρs

a m1 ρsa m1 ρs

a m1 ρsa

mol·kg−1 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3

0.4742 0.0097 1.032 0.0098 1.030 0.0115 1.028 0.0105 1.025 0.0100 1.0241.0010 0.0080 1.061 0.0085 1.059 0.0110 1.057 0.0100 1.054 0.0093 1.0521.5898 0.0073 1.121 0.0074 1.119 0.0078 1.117 0.0078 1.114 0.0077 1.1132.2523 0.0057 1.152 0.0055 1.150 0.0058 1.147 0.0055 1.144 0.0054 1.1423.8610 0.0020 1.260 0.0016 1.259 0.0020 1.257 0.0019 1.254 0.0024 1.2534.8510 0.0010 1.291 0.0011 1.289 0.0009 1.287 0.0007 1.285 0.0010 1.2836.0060 0.0007 1.318 0.0006 1.316 0.0006 1.314 0.0005 1.312 0.0007 1.310

aExperimental density of saturated filtrate.

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γ γ= =+ −

+ −

m mK

asolubility Ca SO

sp

Ca SO H O0.52

42

24

22 (3)

By defining B = γCa2+γSO42−aH2O

0.5, the value of B can be

estimated on the base of eq 3 combined with the solubility dataof α-HH in MgCl2 solution at a specific temperature.Corresponding to the maximum value of solubility of α-HH,the value of B has a minimum. Therefore, the defined variable Bdecreases first with solution ion strength and then increaseswith further addition of MgCl2.A similar relationship between the γCa2+γSO4

2− and the

solubility of calcium sulfate has been reported in KCl

solutions,8 HCl solutions,22 and H3PO4 solution.23 Thesolubility of α-HH is lower than that of DH in theconcentration range of (0.67 to 1.17) mol·kg−1 (namely, (6to 10) % by mass) when the temperature higher than 363.0 K,which indicates DH could be transformed into α-HH.13

3.3. Solubility of α-HH in KCl Solutions. The solubilitydata of α-HH determined in potassium chloride solution from(353.0 to 371.0) K are listed in Table 4. The uncertainties forthe values are 0.0003 mol·kg−1 for solubility and 0.001 g·cm−3

for density of saturated filtrate. The curves of the solubility ofα-HH and DH vs KCl concentrations at different temperaturesare plotted in Figure 6. The solubility of α-HH increases withKCl concentration increasing in the whole investigatedconcentration range due to the salt effect. After KCl was

Table 3. Solubility of α-Calcium Sulfate Hemihydrate (1) in MgCl2 (2) + H2O (3)

353.0 K 358.0 K 363.0 K 368.0 K 371.0 K

m2 m1 ρsa m1 ρs

a m1 ρsa m1 ρs

a m1 ρsa

mol·kg−1 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3

0.1061 0.0383 0.997 0.0385 0.995 0.0388 0.993 0.0371 0.991 0.0364 0.9890.2144 0.0494 1.006 0.0549 1.004 0.0625 1.002 0.0584 1.000 0.0597 0.9980.3249 0.0576 1.015 0.0609 1.013 0.0665 1.011 0.0566 1.009 0.0621 1.0070.4376 0.0652 1.023 0.0676 1.021 0.0759 1.019 0.0740 1.017 0.0685 1.0150.5528 0.0672 1.031 0.0680 1.029 0.0760 1.026 0.0715 1.024 0.0661 1.0230.6704 0.0703 1.037 0.0704 1.035 0.0772 1.033 0.0703 1.031 0.0731 1.0290.9134 0.0704 1.069 0.0706 1.067 0.0789 1.065 0.0754 1.063 0.0795 1.0621.1671 0.0695 1.086 0.0702 1.085 0.0752 1.083 0.0750 1.081 0.0781 1.0801.8536 0.0629 1.108 0.0643 1.106 0.0794 1.104 0.0777 1.102 0.0825 1.1002.6259 0.0444 1.154 0.0433 1.152 0.0537 1.150 0.0536 1.148 0.0520 1.1463.5012 0.0380 1.202 0.0358 1.200 0.0349 1.198 0.0362 1.196 0.0349 1.1944.5015 0.0181 1.258 0.0187 1.256 0.0193 1.254 0.0188 1.252 0.0184 1.250

aExperimental density of saturated filtrate.

Table 4. Solubility of α-Calcium Sulfate Hemihydrate (1) in KCl (2) + H2O (3)

353.0 K 358.0 K 363.0 K 368.0 K 371.0 K

m2 m1 ρsa m1 ρs

a m1 ρsa m1 ρs

a m1 ρsa

mol·kg−1 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3

0.0269 0.0172 0.989 0.0172 0.987 0.0178 0.985 0.0173 0.983 0.0168 0.9810.0539 0.0179 0.991 0.0180 0.989 0.0237 0.986 0.0182 0.984 0.0179 0.9830.0810 0.0202 0.993 0.0197 0.990 0.0266 0.988 0.0203 0.986 0.0205 0.9840.1082 0.0229 0.994 0.0224 0.992 0.0289 0.990 0.0210 0.988 0.0229 0.9860.1355 0.0237 0.996 0.0236 0.994 0.0312 0.992 0.0256 0.990 0.0245 0.9880.2043 0.0282 1.000 0.0266 0.998 0.0319 0.996 0.0273 0.994 0.0252 0.9920.2737 0.0293 1.005 0.0289 1.003 0.0355 1.000 0.0301 0.998 0.0312 0.997

aExperimental density of saturated filtrate.

Table 5. Solubility of α-Calcium Sulfate Hemihydrate (1) in CaCl2 + MgCl2 + KCl (2) + H2O (3)

353.0 K 358.0 K 363.0 K 368.0 K 371.0 K

m2 m1 ρsa m1 ρs

a m1 ρsa m1 ρs

a m1 ρsa

mol·kg−1 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3 mol·kg−1 g·cm−3

0.4819 0.0119 1.031 0.0117 1.029 0.0117 1.027 0.0117 1.025 0.0114 1.0230.9860 0.0103 1.083 0.0099 1.081 0.0119 1.079 0.0112 1.077 0.0101 1.0761.5215 0.0095 1.121 0.0094 1.119 0.0089 1.117 0.0083 1.115 0.0080 1.1142.0843 0.0088 1.146 0.0064 1.144 0.0063 1.142 0.0054 1.140 0.0045 1.1392.6185 0.0047 1.179 0.0041 1.178 0.0040 1.176 0.0039 1.174 0.0034 1.1723.3088 0.0023 1.220 0.0025 1.218 0.0016 1.216 0.0010 1.214 0.0019 1.2134.0256 0.0015 1.257 0.0013 1.255 0.0005 1.253 0.0006 1.251 0.0003 1.2505.0578 0.0006 1.305 0.0005 1.303 0.0004 1.302 0.0005 1.300 0.0002 1.299

aExperimental density of saturated filtrate.

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added, the total molar concentration of ions increases, and thestrong mutual action of calcium ions and sulfate ions results inactivities of SO4

2− and Ca2+ decreasing. Therefore, precipitationin the form of CaSO4·0.5H2O is restrained, and the dissolutionof solid α-HH is improved. Then the solubility of α-HHincreases, which is consistent with the change trend of α-HH inKCl solution when concentration lower than 2 %.8 As shown inFigure 6, the solubility of DH is lower than that of α-HH whenthe temperature is below 371.0 K, and DH is the stable phase.This is consistent with our previous result.13

Clearly, there were different solubilities between our previouswork14 and the present work with the same residual α-HH solidat 371.0 K in KCl solution. In fact, the residual α-HH solid wasformed by dissolution and recrystallization from DH in ourprevious work. This transformation was a chemical process.While in present work, the solubility determination of α-HHwas only a physical dissolution process. Moreover, thedissolution equilibrium time of DH needs about 4 h, and thedissolution equilibrium time of α-HH needs 6 h at 371.0 K inKCl solution. Therefore, the transformation of α-HH from DHin our previous research14 was almost under the condition of asaturated solution of DH. So the solubility of calcium sulfateobtained in our previous work can represent DH even with theresidual α-HH solid. The physical dissolution of α-HH can onlyreach to its dissolving limitation to obtain the solubility of α-HH in the present work.

3.4. Solubility of α-HH in Ca−Mg−KCl MixedSolutions (Ratio by Weight, 25:5:1). The solubility dataof α-HH in the mixed chloride salt solution are exhibited inTable 5. The uncertainties for the values measured are 0.0005mol·kg−1 for solubility and 0.001 g·cm−3 for density. Thevariations of solubility of α-HH with concentrations of themixed salt solution at different temperatures are illustrated inFigure 7. Figure 7 shows that the solubility of α-HH decreaseswith the mixed salt concentration over the whole investigatedconcentrations. In the mixed salt solution, the coexistence ofthe common ion effect by calcium ion, the association effect bymagnesium ion, and the salt effect by potassium chlorideinfluences the solubility of α-HH. The common ion effect issuperior to the other two effects in the mixed salt solutionbecause of the largest portion of calcium chloride. Then thesolubility of α-HH decreases with the mixed salt concentrationincreasing but is still higher than in single calcium chloridesolution due to the promoting action of the other two effects.The solubility of α-HH inclines to decrease when temperatureincreases. When the concentration of the mixed salt solutionexceeds 2.619 mol·kg−1 (about 21.8 % by weight), and thetemperature exceeds 363.0 K, the solubility of α-HH is alwayslower than that of DH. So α-HH is the thermodynamical stablephase under these conditions13 according to the crystallizationtheory.24 This result may explain why DH could be convertedinto α-HH when the temperature is higher than 363.0 K andtotal salt concentration exceeds 20 %.12,14

3.5. Density of Calcium Sulfate-Saturated Solution.Although the chloride salt concentration and temperature havea complex influence on the solubility of calcium sulfate in

Figure 4. Solubility of α-calcium sulfate hemihydrate (α-HH) andcalcium sulfate dihydrate (DH) in calcium chloride solution: α-HH:■, 353.0 K; ●, 358.0 K; ▲, 363.0 K; ★, 371.0 K; DH:14 ○, 358.0 K;△, 363.0 K; ☆, 371.0 K.

Figure 5. Solubility of α-HH and DH in magnesium chloride solution:α-HH: ●, 358.0 K; ▲, 363.0 K; ★, 371.0 K; DH:14 ○, 358.0 K; △,363.0 K; ☆, 371.0 K.

Figure 6. Solubility of α-HH and DH in potassium chloride solution:α-HH: ■, 353.0 K; ●, 358.0 K; ▼, 368.0 K; ◆, 371.0 K; DH:14 □,348.0 K; ○, 353.0 K; ◇, 371.0 K.

Figure 7. Solubility of α-HH and DH in mixed salt solution: α-HH: ■,353.0 K; ●, 358.0 K; ▲, 363.0 K; ★, 371.0 K; DH:14 □, 353.0 K; ○,358.0 K; △, 363.0 K; ☆, 371.0 K.

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CaCl2−MgCl2−KCl−H2O system, their impact on the densityof the calcium sulfate-saturated solution is systematic, as isshown in Figure 8. The density of the calcium sulfate-saturated

solutions increases systematically with the mixed saltconcentration increasing. For a given mixed salt concentration,the density decreases in a nearly linear manner withtemperature increasing in the whole range of the mixed saltconcentrations. A very similar dependence of the density onchloride salt solution concentration and temperature is found insingle CaCl2 solution, MgCl2 solution, and KCl solution, asshown in Tables 2 to 4.That is, the density of calcium sulfate-saturated is influenced

predominantly by the concentration of the chloride saltsolution and temperature and is nearly not affected by thedissolved calcium sulfate. A similar relationship between thedensity of calcium sulfate-saturated and the salt solutionconcentration as well as temperature was reported by Dutrizacand Kuiper.25,26

4. CONCLUSIONSSolubilities of the α-HH in CaCl2−MgCl2−KCl solution from(353.0 to 371.0) K were investigated. It has been found that thesolubility of α-HH is influenced by three actions such as thecommon ion effect by calcium ion, the association effect bymagnesium ion, and the salt effect by potassium chloride. Ofthe investigated system, the common ion effect influences thesolubility of α-HH very much, the association effect less, andthe salt effect the least. The solubility of α-HH decreases withthe mixed salt concentration increasing because of the strongestcommon ion effect of calcium ion.In the mixed salt solution, the solubility of α-HH is lower

when the concentration of the mixed salt solution exceeds2.619 mol·kg−1 and the temperature exceeds 363.0 K, whichconfirms the feasibility of conversion into α-HH from DH.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +86-13657202189. Fax: +86-27-86556906. E-mail:leafwu205@yahoo.com.cn.FundingThis work was supported by the Hi-Tech Research andDevelopment Program (863 Program) of China (No.2006AA06Z385).

NotesThe authors declare no competing financial interest.

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Figure 8. Density of calcium sulfate-saturated CaCl2−MgCl2−KClsolutions at different temperatures: ■, 0.4819 mol·kg−1; □, 0.9860mol·kg−1; ●, 1.5215 mol·kg−1; ○, 2.0843 mol·kg−1; ▲, 2.6185mol·kg−1; △, 3.3088 mol·kg−1; ◆, 4.0256 mol·kg−1; ◇, 5.0578mol·kg−1.

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