Experimental and Predictive Study Using Cryoscopy and

Experimental and Predictive Study UsingCryoscopy and Calculation Code Modelingof Seawater at Different Concentrations

and Ice Water-Salts Precipitation

Fatima Zahra Karmil1,2(&), Sara Mountadar1,2, Abdelkader Hayani1,Anouar Rich2, Mostapha Siniti2, and Mohammed Mountadar1

1 Laboratory of Water and Environment, Faculty of Sciences,Chouaib Doukkali University, PB: 20, 24000 El Jadida, Morocco

[email protected] Team of Thermodynamics Surface and Catalysis, Faculty of Sciences,

Chouaib Doukkali University, PB: 20, 24000 El Jadida, Morocco

Abstract. This study aims to determine the effect of composition on the pre-cipitation of sea-salts mineral and ice water recuperation on the MoroccanAtlantic Ocean. The main objective of this study is the determination of theoptimal conditions of freezing process through the experimental liquid-solidequilibrium of seawater at different concentrations. Hence, the physicochemicalparameters of freezing were evaluated through the calculation codes Frezchemand PhreeqcI3 to quantify the effect of the ionic composition of seawater onsalinity and the precipitation of ice water and salts. Moreover, the theoreticalresults obtained were validated by experiment. The experimental results carriedout under the determined optimal conditions for different compositions were ingood agreement with the theory results obtained. Furthermore, the temperatureeffect on the liquid-solid equilibrium is demonstrated and evaluated by calcu-lation code modeling. In addition, for each type of salt precipitated for differentcomposition of seawater at different temperature are depending mainly of thesolubility variation. The quantity of ice water recuperated is correlated with Ca2+

and SO42− concentration and the experimental results obtained by Freezing

seawater process and by calculation code confirmed a negative impact in rela-tion with ice water quality and salts precipitation quantity.

Keywords: Seawater � Freezing � Precipitation � Salts � Ice water

1 Introduction

Freezing is one of the efficient processes for separating dissolved brines from seawater.It allows the study of marine freezing systems. It has a similar effect to that of evapo-ration technologies [1]. It depends on operating conditions such as temperature (freezingand air), air movement, and salinity, with H2O in ice form. It requires well-definedprecipitation operations to obtain the extraction of specific mineral salts at differenttemperatures and composition, which makes this process relatively complex [2].

© Springer Nature Switzerland AG 2020M. Ezziyyani (Ed.): AI2SD 2019, AISC 1104, pp. 319–336, 2020.https://doi.org/10.1007/978-3-030-36671-1_28

As a raison, the sea ice is a complicated and multi-component system where hismicrostructure is permeated by brine channels and pockets that contain concentratedseawater-derived brine which are present in different concentrations and forms. Coolingthe sea ice results in further formation of pure ice within these pockets as thermalequilibrium is attained, resulting in a smaller volume of increasingly concentratedresidual brine [3]. The ionic composition and the freezing temperature are two relatedfactors. Whose brine composition is conservative but begins to deviate from its con-servative behaviour when there is an enough reduction in temperature when the brinebecomes supersaturated compared to the hydrated polymorphs of CaCO3, Na2SO4,CaSO4 and NaCl. At this time, each mineral precipitated depends on the temperature ofthe sea ice, each one having a specific temperature at the beginning of its precipitation.Many approaches have been proposed as paradigms for this process. As a result, thelower the ice temperature, the more concentrated the brine becomes as the purer waterfreezes to maintain the thermal equilibrium [3–6].

Several studies were done in order to understand the experimental and theoreticalapproach of freezing process and deduce the effect of temperature and initial compositionon the precipitation of sea-salts. Gitterman have been conceived and characterized thesequential initiation of precipitation of brine in the development of mirabilite(Na2SO4.10H2O)at−7.3 °C, gypsum(CaSO4.2H2O)at−15°C,hydrohalite (NaCl.2H2O)at −22.9 °C, sylvite (KCl) at −33 °C, and finally MgCl2.12H2O at −36.2 °C. This expe-rience was confronted by other studies which show that the mirabilite forming at−8.2 °C,hydrohalite at−22.9 °C, sylvite andMgCl2.12H2O at−36.0 °C, and a eutectic of−53.8 °Cwhich occurs upon the precipitation of antarcticite (CaCl2.6H2O) [5, 7]. All whose resultswere made at different freezing temperature and salinity conditions and were confirmedmainly by a set of theoretical calculations.Moreover, precipitationdepends not onlyon thefreezing temperature but also on the kinetic modalities, the nature of the equilibriumestablished between the precipitated phases and the ionic composition [8, 9].

The thermodynamic study quantifies the effect of the ionic composition and salinityof seawater on the precipitated mineral phases as a function of composition at differenttemperatures of freezing. The theoretical sequence of fractional crystallization wasdeveloped for the determination of solubility as well as salt saturation points. Pitzercalculation of liquid-solid equilibrium was based on the ion interaction model that waswell developed by Marion et al.

In the present work, the physicochemical parameters of liquid-solid equilibrium ofsolutions diluted and concentrated at different temperatures on El Jadida Bay werestudied. Knowledge of these properties is essential for studying in the first step thepossibility to separate salts and ice water by freezing seawater and secondly, thedetermination the optimum conditions for the precipitation and crystallization at dif-ferent composition of seawaters and at different temperatures. Furthermore, the dif-ferent results obtained experimentally and by theory is discussed for to explain somedifferences in particularly the precipitation salts and volumes of ice water crystalize byusing the Pitzer model.

320 F. Z. Karmil et al.

2 Materials and Methods

2.1 Sample Pre-treatment of Raw Seawater and Preparation at DifferentConcentrations

Seawater solutions were filtrated and concentrated by reduction of volume by evapo-ration process at different percentages (10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%and 90%). As well as the other solutions were diluted for the same volume. After that,the salinity of solution was determined and compared with chlorinity, conductivity anddry residue at different temperature.

The Fig. 1 shows the experimental design used to determine the liquid-solidequilibrium on lowering the melting temperature of seawater at different concentrationswhich the solution is cooled slightly until the solid appears. The temperature of thesolution is then monitored over time. Figure 2 summarizes the temperature profileduring crystallization. It consists of four main steps:

In the initial phase (1), a metastable state subcooling (also called supercooling orsupercooling) of the solution is carried out up to the minimum temperature of nucle-ation Tn, supercooling is a transient cooling below the solidification equilibriumtemperature that occurs before any crystallization. It is due to the delay in the startingrate of crystal formation compared to the cooling rate.

(2) the so-called recalescence step in which the temperature of the solution rises tothe equilibrium temperature Tf corresponding to the melting temperature.

(3) the equilibrium between phase in the presence of solid and liquid water, thesolution remains at the melting temperature until the sample is completely solidifiedand cooling of the solid (4).

2.2 Chemical Analysis

Chemical analysis was applied to determine the composition of solution at each stageof concentration and dilution process of seawaters. Potentiometric titration was used todetermine Ca2+, Mg2+ and Cl− ions. The Ca2+ and Mg2+ ions concentrations were

Fig. 1. Experimental design used to studythe liquid-solid equilibrium of seawater

Fig. 2. Temperature profile during crystal-lization of a pure product

Experimental and Predictive Study Using Cryoscopy 321

determined by EDTA complexmetric titration. The Cl− ion concentration was deter-mined argentometrically by AgNO3. Thermogravimetric method was used for thedetermination of SO4

2− with BaCl2. The K+ ion with sodium tetraphenylborate and of

Na+ ion was analysed by flame spectrometer respectively. The accuracy of theseanalyses was about 0.1–0.2% [2, 10, 11].

2.3 Prediction of Liquid-Solid Equilibrium in Seawater

The liquid-solid equilibrium of seawater were evaluated by thermodynamic ion-interaction Pitzer model by using the Frezchem calculation code for different con-centration considering only the six major elements of seawater (Na+, Ca2+, Mg2+, K+,Cl− and SO4

2−). A thermodynamic ion-interaction was used the Frezchem (FREEZINGCHEMISTRY) calculation code applied by Marion et al. [2, 12–23].

Pitzer ApproachThe Pitzer model [24–29] was based on Debye-Huckel extended law in which a set ofterms have been added to determine the ionic strength (c) of binary and ternary systemsas a function of temperature at a pressure of 1.01 bar for cations (M), anions (X), andaqueous neutral species (N) [2, 12–23]:

ln cMð Þ ¼ Z2MFþ

Xma 2BMa þ ZCMað Þþ

Xmc 2UMc þ

XmaWMca

� �

þXX

mama0WMaa0 þ ZM �XX

mcmaCca þ 2X

mnknM

þXX

mnmafnMa

ð1Þ

ln cXð Þ ¼ Z2XFþ

Xmc 2BcX þ ZCcXð Þþ

Xma 2UXa þ

XmcWcXa

� �

þXX

mcmc0Wcc0X þ ZXj j �XX

mcmaCca þ 2X

mn knX þXX

mnmcfncX

ð2Þ

ln cNð Þ ¼X

mc 2BkNcð ÞþX

ma 2kNað ÞXX

mcmafNca ð3Þ

Where B, C, U, W, k and f are the interaction parameters of the Pitzer equation, miis the molar concentration, with F and Z are the functions of the equation. In theseequations, the Pitzer interaction parameters and the F function are temperaturedependent. The indices c, a, and n refer to cations, anions, and neutral species,respectively. For c0 and a0 are referred to cations and anions that differ from c and a.Water activity (aw) at P = 1.01 bar is given by:

aw ¼ exp � ;Pmi

55;50844

� �ð4Þ


Where ; is the osmotic coefficient of which:

/� 1ð Þ ¼ 2Pmi

f� A/I3=2

1þ bI1=2þ

XXmcma B/

ca þ ZCca� �þ

XXmcmc0 ðU/

cc0

þX

maWcc0aÞþXX

mama0 U/aa0 þ

XmcWcaa0

� �

þXX

mnmcknc þXX

mnmaknc þXXX

mnmcmafn; c; agð5Þ

Also, the temperature dependent as a function of changes in the parameters of thePitzer equation and the solubility range of each product studied:

P ¼ a1 þ a2T þ a3T2 þ a4T

3 þ a5T

þ a6ln Tð Þ ð6Þ

Where P is the Pitzer parameter, Ks is the product of solubility, T is the absolutetemperature and a is the coefficient of interaction [2, 25–29].

The equilibrium constant of the dissolution reaction of an ionic compound in wateris called the solubility product Ks. When equilibrium is reached, the solution is said tobe saturated:

MmXnsolide , mMzMþaq þ nXzX�

aq ð7Þ

KS ¼ amManX

aMmXn Sð ÞWith aMmXn Sð Þ ¼ 1 ð8Þ

Then:

KS ¼ amManX ð9Þ

Thermodynamic Simulation of the Precipitation of Sea SaltsThermodynamic simulation of ionic interactions was performed using PHREEQCcomputer program, version 3, of major elements Na+, K+, Ca2+, Mg2+, Cl−, and SO4

2−

to different temperatures by using PhreeqCI3-pitzer data. Due to the system com-plexity, the saturation index (SI) of precipitated solid phases, under experimentalconditions, was calculated according to the following relationship:

SI¼ log10 IAP=Kspð Þ ð10Þ

Where IAP is the ionic activity product and Ksp is the solubility product.The calculations were carried out on the seawater compositions of El Jadida coast.

The saturation index is used to determine the water is saturated, undersaturated, orsupersaturated with respect to the state of the mineral [2, 10].


3 Results and Discussions

3.1 Physicochemical Analysis of the Different Samples at DifferentConcentrations of Seawater

The different samples prepared from El Jadida bay seawater and their analyses by usingchlorinity, conductivity and dry residue at 180 °C at different concentrations aregrouped in Table 1. The obtained results in Table 2 was compared with other data fromliterature.

The sum of the ions masses of each composition allows us to determine the relativesalinity. Thus, the salinity of the seawater of El Jadida Bay is estimated at 36.35 g/kgon the Rabat Atlantic Ocean at 36.12 g/kg and for synthetic similar solution fromliterature on 36.33 g/kg for the raw sample.

Moreover, salinity increases or decreases with the concentration or dilution stage ofdifferent seawaters. For each physicochemical parameter, it is noted that it has an ele-vation proportional to evaporation rates for concentrated solution and diminution fordiluted solutions [30]. Indeed, the distribution average of the major ions is practically the

Table 1. Salinities estimated by different technics of seawater at different concentrations fromEl Jadida bay

Salinity of diluted Seawater El Jadida (g/kg)

Dilution (%) 90% 80% 70% 60% 50% 40% 30% 20% 10%

Solid residue at 180 °C 8.76 10.84 15.07 16.95 19.42 19.87 21.01 24.94 33.65Chlorinity 8.97 10.54 15.39 17.95 19.23 20.52 21.8 25.65 34.63Conductivity 8.14 9.76 12.42 18.32 21.45 25.56 28.4 29.48 34.74

Salinity of concentrated Seawater El Jadida (g/kg)Concentration (%) 10% 20% 30% 40% 50% 60% 70% 80% 90%Solid residue at 180 °C 51.28 58.19 71.7 88.36 97.47 104.99 135.45 150.31 160.8Chlorinity 51.31 56.44 63.49 76.96 86.58 96.2 127.62 141.09 152.63Conductivity 54.32 60.95 72.97 86.22 103.64 104.94 124.6 126.68 138.06

Table 2. Composition of raw seawater from El Jadida bay compared with literatures data

Elements Composition (Molality (mol/kg) and Concentration (g/kg))

This work: El Jadida bay Rabat Coast (Rich 2011) [24] Marion et Grant (1999) [24]

mol/kg g/kg mol/kg g/kg mol/kg g/kg

Na+ 0.4378 10.0694 0.46984 10.80632 0.48695 11.1998

K+ 0.0101 0.3948 0.02744 1.107016 0.01063 0.41457Ca2+ 0.0330 1.32 0.00688 0.275888 0.00953 0.382153Mg2+ 0.0620 1.50722 0.05356 1.301508 0.05516 1.340388

Cl− 0.5408 19.1984 0.56348 20.00354 0.56818 20.17039SO4

2− 0.0402 3.8616 0.02734 2.627374 0.02939 2.82437

Salinity (g/kg) 36.3514 36.1216 36.3316


same percentages (Cl− (50%), Na+ (28%), SO42−(12%),Mg2+ (4%), K+ (2%), Ca2+ (3%))

[31]. However, the dry solids mass for diluted and concentrated solutions of marinewaters is depending of temperature (Fig. 3a, b). The mass of the dry solid residue extractof diluted and concentrated solutions decreases with increasing temperature, and remainsconstant for T superior than 180 °C.More the salinity increases more the hygroscopic rate(humidity) due to hydration increases. Indeed, the water loss rate between raw seawatersalinity and dry residue at 180 °C is probably related to the degree of dehydration ofhydrated mineral salts. (MgCl2.6H2O), (CaSO4.2H2O) and (MgSO4.7H2O). Also,bischofite (MgCl2.6H2O) dehydrates at 116 °C. Then, gypsum (CaSO4.2H2O) turns intoplaster (CaSO4.1/2H2O) at 128 °C.After that, the epsomite (MgSO4.7H2O) loses six of itsseven water molecules at 150 °C and at the end the rest of some forms in low moisture at180 °C [29]. These results confirm the dry residue at 180 °C.

100 120 140 160 180 2000

0.1

0.2

0.3

0.4

Temperature(c°)

mas

s(g)

Solid residue mass (g) as a function of temperature (°C) for diluted solutions 90%

80%70%60%50%40%30%20%10%

(a)

100 120 140 160 180 2000

0.5

1

1.5

2

Temperature (c°)

∆ m (g

/kg)

Solid residue mass (g) as a function of temperature (°C) for concentrated solutions 10%

20%

40%

50%

60%

70%

80%

90%

(b)

Fig. 3. (a) Variation of mass solid residue at different temperatures of El Jadida bay for dilutedseawater. (b) Variation of mass solid residue at different temperatures of El Jadida bay forconcentrated seawater


3.2 Temperature Profile of Liquid-Solid Equilibrium Duringthe Freezing of El Jadida Seawaters at Different Salinity

Figure 4a, b summarizes the temperature profiles during freezing of the differentdiluted and concentrated compositions of El Jadida seawater. The temperature profilesclearly show a decreasing of liquid-solid equilibrium temperature when the salinity ofseawaters increase and the different steps of freezing of different forms solids phases arethe same described in materials and methods.

(a)

(b)

Fig. 4. (a) Temperature profiles of liquid-solid equilibrium of El Jadida seawater for dilutedcompositions. (b) Temperature profiles of liquid-solid equilibrium of El Jadida seawater forconcentrated compositions


3.3 Effect of the Freezing of Seawater from El Jadida at DifferentTemperatures on the Ice Water and Salts Precipitation

In general, the solubility of a salt in water varies with temperature according to theequilibrium law. The decrease in temperature will cause a decrease in solubility.Therefore, the solubility of many solids phases decreases with digression of temper-ature. Consequently, we observed for different composition many precipitations atdifferent liquid-solid equilibrium [24].

The First Stage of PrecipitationFirst, gypsum or calcium sulphate (CaSO4.2H2O) is the first mineralogical phaseprecipitated in El Jadida seawater compared by other seawater composition.

For El Jadida the calcium sulphate precipitation is carried out between 0 °C and−6.7 °C at −22.3 °C a second precipitation was started of this phase until a maximummass evaluated at 4,55 g for each liter of seawater. This result was compared withRabat seawater studied by Rich (2011) and synthetic solution prepared by Marion andGrant (1999). Their results indicate that the gypsum precipitation was performed at−23.5 with 0.909 g/l for Rabat seawater and at −22 °C for synthetic solution with1.297 g/l (Fig. 5). This difference of temperature precipitation is explained by theenrichment in calcium and sulphate in El Jadida seawater. In some studies, according tomain paradigm derived from the Gitterman path these experience that describe thesequence of minerals that precipitate during freezing process. The gypsum precipitationbegins between −12 °C and −14 °C [3–5].

0

1

2

3

4

5

-40-35-30-25-20-15-10-50

mas

s for

1 li

ter (

g)

Temperature (°C)

Gypsum (CaSO4.2H2O) precipated as a function of Freezing Temperature (°C)

Gypsum (This Work)

Gypsum (Marion.1999)

Gypsum (Rich.2011)

Fig. 5. Gypsum Precipitation for different compositions at different temperatures


The Second Stage of PrecipitationThe mirabilite (Na2SO4.10H2O) is the second solid phase precipitated after gypsum(Fig. 6) in different composition of different seawater. Therefore, the mirabilite isprecipitated between −6.9 °C and −23 °C. The maximum mass obtained is evaluated at5,82 g for each liter of seawater and started to decrease. This result was compared withRabat coast studied by Rich (2011) and synthetic solution prepared by Marion andGrant. their findings show that the mirabilite precipitation was performed at −6.3 °Cwith a maximum mass varied between 8.44 g/l for synthetic solution and 8.52 g forRabat (Fig. 6).

As highlighted by two main paradigms derived from the Gitterman path and theRinger-Nelson-Thompson (RNT) these experiences describe the sequence of mineralsthat precipitate during freezing process. The mirabilite begins to precipitate at −7.3 andaccording to Ringer–Nelson–Thompson (1954) at −8.2 [3–5].

The Third Stage of PrecipitationFor El Jadida seawater the hydrohalite (NaCl.2H2O) precipitation is carried out at−23.4 °C with a maximum mass of 27.45 g for each liter of seawater. The findingsobtained by literature show that the hydrohalite precipitation was performed at −22.9 °Cfor synthetic solution with a maximum mass of 28.45 g and for synthetic solution and27.4574 g at −23.05 °C for Rabat coast (Fig. 7). Some studies show that after mirabiliteprecipitation, Hydrohalite began to precipitate at −22.9 °C according to Ringer–Nelson–Thompson and Gitterman [4, 5].

0

1

2

3

4

5

6

7

8

9

10

-40-35-30-25-20-15-10-50

mas

s for

1 li

ter (

g)

Temperature (°C)

Mirabilite (Na2SO4.12H2O) precipitated as a function of Freezing Temperature (°C)

Mirabilite (This Work)

Mirabilite (Marion.1999)

Mirabilite (Rich.2011)

Fig. 6. Precipitation of Mirabilite for different compositions at different temperatures


The Last Stage of PrecipitationFinally, K-Mg salts are the last precipitates. For El Jadida seawater the K-Mg saltsprecipitation is carried out by the precipitation of Sylvite (KCl) at −27.9 °C with amaximum mass of 0.752 g for each liter of seawater. The findings obtained by liter-ature show that the sylvite precipitation was performed at −34 °C for synthetic solutionwith a maximum mass of 0.792 g and 0.750 g at −27.03 °C for Rabat. Then,MgSO4.12H2O at −35.3 °C with m = 3.73 g for El Jadida bay and at −35.23 °C forRabat coast and synthetic solution with m = 6.683 g. Finally, MgCl2.12H2O at the laststage presented by the eutectic temperature. For El Jadida it was at −36.1 °C withm = 10.3 g followed by the Rabat area at −23.08 °C with m = 6.7 g and for thesynthetic solution at −36.2 with m = 10.14 g (Fig. 8).

0

5

10

15

20

25

30

-40-35-30-25-20-15-10-50

mas

s of 1

liter

(g)

Temperature (°C)

Hydrohalite (NaCl.2H2O) precipitated as a function of Freezing Temperature (°C)

Hydrohalite (This Work)

Hydrohalite (marion.1999)

Hydrohalite (Rich.2011)

Fig. 7. Precipitation of Hydrohalite for different compositions at different temperatures

0

2

4

6

8

10

12

-40-35-30-25-20-15-10-50

mas

s for

1 li

ter (

g)

Temperature (°C)

K-Mg seasalts precipitated as a function of Freezing Temperature (°C)

MgSO4.12H2O (This Work)

MgSO4.12H2O (Marion.1999)

MgSO4.12H2O (Rich.2011)

MgCl2.12H2O (This Work)

MgCl2.12H2O (Marion.1999)

MgCl2.12H2O (Rich.2011)

KCl (This Work)

KCl (Marion.1999)

KCl (Rich.2011)

Fig. 8. Precipitation of K-Mg Salts for different compositions at different temperatures


The different precipitated K-Mg salts have been extensively studied by differentauthors [4, 5]. They confirm that precipitation of K-Mg salts is carried out at veryadvanced stages of seawater concentration by freezing. The present work is charac-terized by gypsum precipitation followed with mirabilite (Na2SO4.10H2O) thenhydrohalite (NaCl.2H2O) and at the end K-Mg salts. This is explained not only by theeffect of the different temperatures on the ionic composition. But also, by the kineticnucleation and crystallization factors used to define the stability during the equilibriumbetween the precipitated mineral phases.

Water RecuperationFigure 9 shows the quantity of water recovered during the freezing process for thedifferent compositions. In the beginning, there is a strong agreement between thequantity recovered for the different solutions studied and the same quantity during thefreezing process. However, before the first precipitated phases more than 65% purewater without impurities can be recovered in a lower temperature range of −6 °C forseawater from Rabat (Rich 2011) and the synthetic solution obtained by Marion et al.(1999). Contrary to El Jadida bay less than 10% pure water is obtained with gypsumimpurities. therefore, gypsum is the limiting factor of the quantity recovered from pureice in the first phase of liquid - solid equilibrium. Also, we observed in Fig. 10 theincreasing of mass of solid phases at liquid-solid equilibrium with increasing salinitiesof seawaters were estimated by Frezchem/Pitzer calculation code. However, the dif-ferent masses obtained depending also of the temperature for ice and mirabilte solidphases.

Water recuperation

0

200

400

600

800

1000

-40-35-30-25-20-15-10-50

mas

s for

1lit

er (g

)

Temperature (°C)

Ice (H2O) formed as function of Freezing Temperaure (°C)

Ice (This Work)

Ice (Marion1999)

Ice (Rich 2011)

This work

Rich (2011);Marrion and Grant (1999)

Fig. 9. Quantity and quality of ice water recovered by using calculation code


Furthermore, we are chosen that different temperature for studying the precipitationof mirabilite and gypsum for minimizing the impurity of ice in the freezing process.The main objective will be to decrease the mass of ice with minimum of impurities.The Fig. 11 shows that the quantity of gypsum increases with increasing concentrationsof Ca2+ and SO4

2− for the different temperatures. Also, the mass of pure solid icedecreases when the mass of gypsum increases for all temperatures studied.

Fig. 10. Evolution of the mass of major solid phases as a function of initial salinity at differenttemperatures

Fig. 11. Evolution of the mass of the solid phases of ice and gypsum as a function of the mass ofCa2+ and SO4

2− at −7 °C, −8 °C and −9 °C for different seawater compositions


For this reason, a pretreatment is necessary for reducing the quantity of sulphatesand calcium to limit the precipitation of gypsum and mirabilite during freezingdesalination process for increasing the quantity of ice water recovered.

3.4 Comparison Between Experimental Salinities and Datafrom the Literature of the Liquid-Solid Equilibrium of Seawater

The evolution of the experimental and calculated salinity of the liquid-solid equilibriumof seawater shows a good agreement between experimental salinities and calculatedsalinities is only valid at high temperatures (Fig. 12). The model predicts the presenceof ice below −1.9 °C for El Jadida seawater and −1.93 °C, −1.89 °C and −1.84 °C forthe maximum, average and minimum composition of Rabat seawater respectively. Theagreement between experimental salinities and calculated salinities is only valid fordifferent seawaters at high temperatures [0 °C; −4 °C].

Also, we observed the same evolution of crystallisation temperature of ice below−1.9 °C for El Jadida seawater and −1.93 °C, −1.89 °C and −1.84 °C for the maxi-mum, average and minimum compositions of Rabat seawater respectively. This min-imum difference is related to the composition of seawaters. In effect, more the salinityincreases with increasing of freeze temperature more the minority species increasing inseawater, this phenomenon is not considered in the model or this variation influencedthe liquid-solid equilibrium. In general, the agreement between the model and theexperimental measurements becomes less good below −4.7 °C. However, the modelpredicts the precipitation of gypsum (CaSO4.2H2O) in El Jadida seawater at 0 °Ccompared with Rabat seawaters at −22 °C. The precipitation of Na2SO4.10H2O isconstated at −6.2 °C for El Jadida seawater and −5.77 °C, −6.49 °C and −7.55 °Crespectively for the maximum, average and minimum composition of Rabat seawater.

-18 -16 -14 -12 -10 -8 -6 -4 -2 0 0 20406080100120140160180200220240260

Temperature (°C)

Salin

ity (g

/Kg)

Experimental and predectve salinity as a function of temperature (°C) average composition of Rabat Coast

Minimum composition of Rabat Coast

Maximum composition of Rabat Coast

Experemental data (Rich.2011)

This Work

Na2SO4.10H2

H2O(Ice)CaSO4.2H2O

Fig. 12. Validation of Experimental and predictive study


However, we observed a difference of quantity of mirabilite between El Jadida andRabat seawaters. This difference is caused by excessive concentrations of calcium andsulphate in El Jadida seawater where we are constated the precipitation of gypsum ininitial freezing consequently, the El Jadida seawater coming poor in sulphate ion thanwe are recovered minimum mass of mirabilite compared with the results of Rabatseawaters.

3.5 Effect of Temperature on the Quantity and Quality Precipitated SaltDuring Freezing Process

According to phreeqcI3-Pitzer data applied to different seawaters, the correlationbetween the volume of seawaters reduction and the global salt concentrations at dif-ferent temperatures does not follow a simple law. In effect, the quantity of salt pre-cipitated with the reduction of seawater volume is depending in general of theconcentration of all ions at the different temperatures. The freezing of seawater causes adecrease in the volume of water which is accompanied by an increase in the concen-tration of solutes (mineral salts in our case). If the concentration exceeds the solubilityof the solutes, then they precipitate. The precipitation of mineral salts decreases whenthe temperature of the solution increases according to the general law of liquid-solidequilibrium.

The results obtained by application of calculation code under the different tem-peratures at different reduction of volumes is shows in Fig. 13.

-2

0

2

4

6

0 20 40 60 80 100

Satu

ratio

n in

dex

Volume of freezing (%)

Saturation index of Gypsum as a function of Freezing Volume (%)

T=0°C T=-5°CT=-10°C T=-15°CT=-20°C

-2

-1

0

1

2

0 20 40 60 80 100Satu

ratio

n In

dex


Saturation index of Mirabilite as a function of Freezing volume (%)

T=0°C T=-5°CT=-10°C T=-15°CT=-20°C

-4

-2

0

2

4

0 20 40 60 80 100Satu

ratio

nind

ex


Saturation index of Hydrohalite as a function of Freezing volume (%)

T=0°C T=-5°CT=-10°C T=-15°CT=-20°C

-4

-2

0

2

4

0 20 40 60 80 100Satu

ratio

n in

dex


Saturation index of K-Mg Sea-Salts as a function of Freezing volume (%)

T=0°C T=-5°CT=-10°C T=-15°CT=-20°C

Fig. 13. Saturation index at different temperatures for El Jadida seawater


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