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Measurement of Solubilities in the Ternary System NaCl + CaCl<sub>2</sub> + H<sub>2</sub>O and KCl + CaCl<sub>2</sub> + H<sub>2</sub>O at 50℃
Measurement of Solubilities in the Ternary System NaCl + CaCl2 + H2O and KCl + CaCl2 + H2O at 50℃
Journal of the Korean Chemical Society. 2010. Jun, 54(3): 269-274
Copyright © 2010, The Korean Chemical Society
  • Received : December 29, 2009
  • Accepted : March 17, 2010
  • Published : June 20, 2010
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About the Authors
Ji-min Yang
School of Chemistry & Resources Environment , Linyi Normal University, Linyi 276005, China
Yangjm7702@126.com
Guang-yue Hou
Changchun Institute of Applied chemistry of Chinese Academy of Sciences, Changchun, 130022, China
Tian-rong Ding
School of Chemistry & Resources Environment , Linyi Normal University, Linyi 276005, China
Peng Kou
School of Chemistry & Resources Environment , Linyi Normal University, Linyi 276005, China

Abstract
The solubility and the physicochemical property (refractive index) in the NaCl-CaCl 2 -H 2 O and KCl-CaCl 2 -H2O systems were determined at 50 ℃ and the phase diagrams and the diagrams of physicochemical property vs composition were plotted. One invariant point, two univariant curves, and two crystallization zones, corresponding to sodium Chloride (or potassium chloride), dihydrate (CaCl 2 ·2H 2 O) showed up in the phase diagrams of the ternary systems. The mixing parameters θ M,Ca and Ψ M,Ca,Cl (M = Na or K) and equilibrium constant K sp were evaluated in NaCl-CaCl 2 -H 2 O and KCl-CaCl 2 -H2O systems by least-squares optimization procedure, in which the single-salt Pitzer parameters of NaCl, KCl and CaCl 2 β (0) , β (1) , β (2) and C ф were directly calculated from the literature. The results obtained were in good agreement with the experimental data.
Keywords
INTRODUCTION
The prediction of the solubility in aqueous electrolyte solutions is important for a variety of applications in the chemical and geochemical processes, seawater systems, and evaporation as well as desalination. Salt solubility data are important as a tool for the design and simulation of unit operations such as drowning-out crystallization or liquidliquid extraction. The investigation of the thermodynamics and phase diagram of the system is of theoretical and practical importance. 1 , 2 In the salt lakes of western China, these alkali metal salts coexist with other minerals containing boron, calcium, magnesium, and chloride. 3 To scientifically exploit these natural resources preliminary investigation of sodium, potassium and calcium salt solution chemistry is necessary. Thermodynamic properties of the ternary systems (NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O) are of essential importance in the extraction of sodium or potassium from natural salt brine mainly containing sodium, potassium, lithium, magnesium and calcium.
Pitzer’s ion-interaction model 4 and its extended Harvie and Weare model 5 - 7 are very reliable for predicting the mineral solubility in natural water system with high ionic strengths over the wide temperature range from 0 to 300 ℃. 8 - 11 The solubilities of calcium chloride and magnesium chloride were determined by prutton, 12 and calculating the activity coefficient and osmotic coefficients of electrolytes in seawater and synthetic salt lake brines were calculated by Song, P. S. 13 , 14 Li Ya-hong 15 - 17 reported. The solubility in the ternary HCl-LiCl-H 2 O, HCl-MgCl 2 -H 2 O and LiCl-MgCl 2 -H 2 O systems at 273 and 293 K, using the ion-interaction Pitzer model and predicting the solubility isotherm of the NaCl-RbCl-H 2 O, KCl-CsCl-H 2 O and KBr-CsBr-H 2 O systems at 298 K. Some other systems were also reported, such as NaCH 3 COO + NaCl + H 2 O, 18 NaCl and KNO 3 , 19 LiCl-MgCl 2 -H 2 O system. 20 However, the application of the Pitzer model for predicting the component solubility of salt lake brine systems of NaCl + CaCl 2 + H 2 O and KCl+CaCl 2 +H 2 O at 50 ℃, has never been reported. Therefore, the solubility evaluation for the NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O systems at 50 ℃ was performed in our lab.
In this paper, the solubility of the ternary systems NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O was elaborately measured at 50 ℃ and an empirically Pitzer ion-interaction model was established based on those solubility data. The physicochemical properties (refractive index) of the equilibrium solutions were determined and a study on the prediction of the solubility was also performed.
EXPERIMENTAL SECTION
- Apparatus and Reagents
A thermostatic shaker (model HS-4) whose temperature could be controlled to 0.02 K was used for the measurement of phase equilibrium. The chemicals used were of analytical grade and obtained from either the Tianjin Chemical Reagent Manufactory or the Shanghai Chemical Plant: calcium chloride (CaCl 2 ·6H 2 O, 99.5 mass %), sodium chloride NaCl, g 99.8 mass %) and potassium chloride (KCl, g 99.5 mass %). Doubly deionized water was used to prepare the series of saturated solutions.
- Experimental method
Various mixtures of salt and water were made by starting with a ground 250 cm 3 Erlenmeyer flask containing only one salt and water and in each subsequent run more of the second salt was added to the solution and solid left from the previous run. The flask was immersed in the thermostat and the solution and solid in the flask were stirred with a magnetic stirrer. Each sample was stirred at a specific constant temperature for 72 h, and then kept static for about 6 h. A sample of the saturated solution was then taken with a pipette. The sample was transferred to a weighed 30 cm 3 ground quartz beaker with cover. The salt concentration in pure saturated salt aqueous solution and the total salt concentration in the three component solutions were determined by evaporation to dryness, fusing, and weighing. The wet residuals were analyzed in the same way as for the solution. The composition of the solid phase in the wet residues was identified by the method of Schreinemaker.
- Analytical method
The Cl ion concentration in the liquids and their corresponding wet residues of the solid phases were analyzed by titration with a standard solution of AgNO 3 in the presence of three drops of 0.1% (w/v) KCrO 4 as an indicator (precision within ± 0. 2 mass %. The Ca 2+ ion concentration was determined by titration with EDTA standard solution in the presence of indicator of Eriochrome Black-T 21 (uncertainty of ± 0.2%). An Abbe refractometer (model WZS-1) was used to measure the refractive index (n D ) with an accuracy of ± 0.0001.
RUSULT AND DISCUSSION
The experimental data on the solubilities and refractive index of the ternary systems NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O at 50 ℃ are presented in 1 and 2 , respectively. The electrolyte concentration values of the liquid phase in the equilibrium solution are expressed in mass fraction. According to the experimental data in 1 and 2 , the equilibrium phase diagrams of the systems NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O are plotted, as shown in . 1 and 2 at 50 ℃, respectively. The solid phases in equilibrium with saturated solution are CaCl 2 ·2H 2 O and MCl in the MCl (M = Na and K) + CaCl 2 + H 2 O system at 50 ℃.
In . 1 and 2 with solid lines, points A and B are the solubilities of the single-salts of sodium chloride (or potassium chloride) and calcium chloride dihydrate. Point C is a eutectic point of sodium chloride(or potassium chloride) and calcium chloride dihydrate (NaCl + CaCl 2 ·2H 2 O or KCl + CaCl 2 ·2H 2 O). There are two saturated curves corresponding to curves AC and BC, indicating the saturation of single salts. The phase diagram consists of two crystallization regions corresponding to the large area of NaCl (or KCl) and the relative small area of CaCl 2 ·2H 2 O. Obviously, the system belongs to the simple eutectic type, and neither double salts nor solid solutions are found.
On the basis of experimental data in 1 and 2 , relationship of the solution physicochemical property with the concentration of calcium chloride is shown in . 3 and 4 . It can be found that the refractive indexes of the aqueous solutions in the ternary system, changed gradually and regularly with the content change of calcium chloride.
Solubility data of the NaCl + CaCl2+ H2O system at 50 ℃
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Solubility data of the NaCl + CaCl2+ H2O system at 50 ℃
Solubility data of the KCl+ CaCl2+ H2O system at 50 ℃
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Solubility data of the KCl+ CaCl2+ H2O system at 50 ℃
In . 1 and 2 with solid lines, points A and B are the solubilities of the single-salts of sodium chloride (or potassium chloride) and calcium chloride dihydrate. Point C is a eutectic point of sodium chloride (or potassium chloride) and calcium chloride dihydrate (NaCl + CaCl 2 ·2H 2 O or KCl + CaCl 2 ·2H 2 O). There are two saturated curves corresponding to curves AC and BC, indicating the saturation of single salts. The phase diagram consists of two crystallization regions corresponding to the large area of NaCl (or KCl) and the relative small area of CaCl 2 ·2H 2 O. Obviously, the system belongs to the simple eutectic type, and neither double salts nor solid solutions are found.
On the basis of experimental data in 1 and 2 , relationship of the solution physicochemical property with the concentration of calcium chloride is shown in . 3 and 4 . It can be found that the refractive indexes of the aqueous solutions in the ternary system, changed gradually and regularly with the content change of calcium chloride.
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The phase diagram of the ternary system NaCl + CaCl2 + H2O at 50 ℃.
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The phase diagram of the ternary system KCl + CaCl2 + H2O at 50 ℃.
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Physicochemical property vs composition in the ternary system (NaCl-CaCl2-H2O) at 50 ℃.
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Physicochemical property vs composition in the ternary system (KCl-CaCl2-H2O) at 50 ℃.
- Calculation of the standard solubility product KMX
The solubility of hydrated salt in concentrated electrolyte solutions can be calculated from thermodynamic considerations provided that equilibrium constants are known and activity coefficients can be obtained. For a hydrated salt M νM X νX·ν0 H 2 O, the solubility equilibrium constant K sp , at a de finite temperature and pressure for the dissolution reaction
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is expressed by
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where m i and γ i represent the concentration expressed as molality and activity coefficient of the ions, respectively. The water activity a W is related to the osmotic coefficient
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Here, M w = 0.018015 kg․mol -1 is the molecular mass of water; ф is the osmotic coefficient.
- Parameterization
Pitzer’s binary parameters (β (0) , β (1) , β (2) and C Ф ) for pure electrolytes of CaCl 2 + NaCl + H 2 O and CaCl 2 + KCl + H 2 O systems at 50 ℃ were available in the literature. The parameters of single-salt CaCl 2 were fitted from osmotic coefficients by least-square method. The parameters of single-salts NaCl and KCl were calculated by using the expressions of the temperature dependency of the ion interaction parameters. These values are listed in 3 .
For the CaCl 2 + NaCl + H 2 O and CaCl 2 + KCl + H 2 O systems, the expression for the osmotic coefficient is
Pitzer’s binary parameters for single electrolytes at 50 ℃
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Pitzer’s binary parameters for single electrolytes at 50 ℃
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where M is Na and K.
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and
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The constants a1 and a2 are normally 1.4 and 2.0 mol 1/2 ·kg -1/2 respectively, θ M, Ca and Ψ Ca, M,Cl are the mixing parameters.
The activity coefficients of MCl (M = Na or K) and CaCl 2 in the ternary mixture are
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The function F for the mixtures is given by:
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Here
Calculated logarithm of the solubility equilibrium constant
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Calculated logarithm of the solubility equilibrium constant
Pitzer mixing parameters for systems investigated at 50 ℃
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Pitzer mixing parameters for systems investigated at 50 ℃
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These equations contain the additional functions, which were described: 24
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where b = 1.2 mol.Kg -1 . Value of the Debye-Hückel coefficient A φ is 0.408 kg 1/2 .mol -1/2 at 50 ℃. 25
For θ M,Ca and ψ Ca,M,Cl measurements of the activities of the complex salts in the system containing high concentrations of MCl (M = Na or K) and CaCl 2 are not available, hence the evaluation of the mixing parameters θ M,Ca and ψ Ca,M,Cl relied on solubility data and solubility product of hydrated solid at 50 ℃ was obtained by a least-squares optimization procedure on the solubility data of corresponding ternary system. All of the parameters used in our calculations are listed in 4 and 5 .
- Calculated Solubilities
The solubility data and the relevant physicochemical property data of the MCl (M = Na or K) + CaCl 2 + H 2 O system at 50 ℃ were measured, and the results are shown in 1 and 2 , respectively. Using the chemical equilibrium model and the above parameters, the calculated results of the solubility are shown in . 5 and 6 at 50 ℃, respectively. It is shown that the predicted values using the chemical equilibrium model agree well with experimental values. For the MCl (M = Na or K) + CaCl 2 + H 2 O system with high ionic strengths, this agreement indicates that the parameters obtained in this work are reliable and that the chemical equilibrium model of Harvie is capable of predicting equilibiria in the system studied.
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Calculated and experimental solution isotherms of NaCl + CaCl2 + H2O at 50 ℃.
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Calculated and experimental solution isotherms of KCl + CaCl2 + H2O at 50 ℃.
CONCLUSION
The experimental solubility data and the relevant physicochemical property data of the aqueous systems NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O at 50 ℃ were determined. Based on the solubility data measured, the isothermal phase diagram and diagram of physicochemical property vs composition are constructed. The single-salt Pitzer parameters of sodium chloride, potassium chloride and calcium chloride were calculated, and the mixing ion-interaction parameters of the NaCl + CaCl 2 + H 2 O and KCl + CaCl 2 + H 2 O systems could be fitted satisfactorily. The solubility product of NaCl, KCl and CaCl 2 ·2H 2 O have been calculated. The calculated results agree well with the experimental values. This study could be useful for solubility prediction for more complicated systems and supply a theoretical basis for the exaction of sodium, potassium and calcium from salt lake brine.
Acknowledgements
The project was supported by the National Nature science Foundation of Shandong Province, China (Grant No. Y2007B47) and the Science Foundation of linyi Normal University, China (Grant No.XJS04064, HX07202).
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