Chiang Mai J. Sci. 2018; 45(4) 1921

Chiang Mai J. Sci. 2018; 45(4) : 1921-1932http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Stable and Metastable Phase Equilibria in the TernarySystem MgB

4O

7-MgSO

4-H

2O at 273 K

Tingting Zhang [a], Wenyao Zhang [a], Ruizhi Cui [b], Yunyun Gao [b] and Shihua Sang* [a]

[a] College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology,

Chengdu 610059, P. R. China.

[b] Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059,

P. R. China.

* Author for correspondence; e-mail: sangshihua@sina.com.cn

Received: 19 September 2016

Accepted: 1 March 2017

ABSTRACT

The stable and metastable phase equilibria of the ternary system MgSO4-MgB

4O

7-

H2O at 273 K were measured respectively using isothermal solubility equilibrium method and

isothermal evaporation crystallization method. The solubilities and densities of equilibriumliquid phase in the system were determined. Based on the experimental data and thecorresponding equilibrium solid phase, the stable and metastable phase diagrams and theircorresponding density versus composition diagrams in this system were plotted, respectively.A comparative analysis of the stable and metastable phase diagrams were made, in additionto, compared with stable phase diagrams at 273 K and 298 K in the system. Experimentalresults show that both the stable and metastable phase diagrams are constituted by a eutecticpoint for (MgSO4

⋅7H2O+MgB

4O

7⋅9H

2O), two univariant solubility curves, two solid phase

crystalline regions corresponding to MgSO4⋅7H

2O and MgB

4O

7⋅9H

2O. Compared with the

stable phase diagram, the sizes of the crystalline regions are different in the metastable phasediagram. Both the stable and metastable phase equilibria in the system at 273 K are simplecosaturated type and there are no complex salt or solid solution were found.

Keywords: stable equilibrium, metastable equilibrium, solubility, MgSO4, MgB

4O

7

1. INTRODUCTION

A large of brine resources werediscovered, in Qinghai, China. The resourcesare extensively used in high-efficiencyagriculture, information technology, energy,non-ferrous materials, environmentalprotection and other industries [1]. QaidamBasin salt lakes are a subtype of magnesiumsulfate brines, which are famous for abundantsalt lakes with high concentrations of lithium,

potassium, magnesium, and boron resources.The brines mostly belong to the complexseven-component system of Li+, Na+, K+,Mg2+//Cl-, SO

42-, borate-H

2O [2].

In order to economically exploit thebrine and mineral resources, it is of greatimportance to research the phase diagramsfor salt lakes. As a part of the complex system,stable phase diagrams of some four-or

1922 Chiang Mai J. Sci. 2018; 45(4)

five-component subsystems have beenmeasured. [3-8]. However, salt lakes arenaturally formed complex multicomponentsystems under water and salt interaction.For studying on phase equilibria, it is notenough to only study stable phase equilibriain the natural evaporation crystallizationprocess of salt lake brine. Because there arethe phenomena of supersaturation of thebrines found in salt lakes. For example, somecontaining sulfate and borate systems [9-12]were reported. Therefore, metastable phaseequilibria research is essential to predict thecrystallized path of evaporation of the saltlake brine.

Although the industry program alwaysproceed at room temperature, the designof the complete process route is inseparablefrom the theoretical guidance of themulti-temperature water and salt system phasediagrams. Therefore, it is imperative to studythe phase equilibrium of water and saltsystem at different temperatures. Importantly,Qaidam Basin have a typical climateconditions, are generally windy, arid, littlerainfall, and great evaporating capacity.In recent years, based on this climate,Zeng and Sang [13-18] have studied somemetastable equilibria containing lithium,sodium, potassium, and boron at 273 K.

Recently, the complex system Li+, K+, Mg2+

//Cl-, SO42-, B

4O

72--H

2O and its subsystem

at 273 K were studied successively by ourgroup. As a subsystem, the stable andmetastable phase equilibria of the ternarysystem MgSO

4-MgB

4O

7-H

2O at 273 K were

measured in this paper, respectively. Moresignificantly, a comparison of the stable andmetastable phase equilibria is to predictaccurately the crystallized path. Therefore, thisstudy provides ternary system solubility datafor the subsystem of complex system Li+,K+, Mg2+//Cl-, SO

42-, B

4O

72--H

2O, and some

reference value for industrial developmentof inorganic saltsin salt lake.

2. MATERIALS AND METHODS

2.1 Reagents and InstrumentsThe water was doubly deionized for the

experimental samples preparation andquantitative analysis. It has an electricalconductivity less than 1 × 10-5 S⋅m-1 andpH≈6.60 at 298 K. The inorganic chemicals(MgSO

4 and other auxiliary reagents) used

in this work were all analytical purity gradeand provided by Chengdu Kelong ChemicalReagent Manufactory, China, tabulated inTable 1. MgB

4O

7⋅9H

2O was synthesized by

experimental methods in laboratory.

Table 1. Chemical sample descriptions.

Chemical name

MgSO4

MgO

H3BO

3

MgB4O

7⋅9H

2O

Source

Chengdu Kelong ChemicalReagent Manufactory

Chengdu Kelong ChemicalReagent Manufactory

Chengdu Kelong ChemicalReagent Manufactory

Synthesis

Initial MoleFractionPurity0.99

0.98

0.995

-

PurificationMethod

None

None

None

chemicalreaction

Final MoleFractionPurity0.99

0.98

0.995

0.99

AnalysisMethod

titration

-

-

titration

Chiang Mai J. Sci. 2018; 45(4) 1923

A UPT-II-20T type Ultra-pure water(supplied by SichuanYoupu Instrument co.,LTD) was applied to provide the deionizedwater. An AL104 type analytical balance of a 110 g capacity and 0.0001 g resolution(provided by the Mettler Toledo InstrumentsCo., Ltd.) was applied to determine the weightof samples. A SHH-250 type biochemicalincubator of temperature range at 258 333K and ±0.1 K resolution (supplied by theChongqing Inborn Experiment Instrumentco., LTD.) was employed to achieve thetemperature. A HY-5 type cyclotron oscillator(supplied by the Jintan Kexi Instrument co.,LTD) was applied to sample oscillation.An X-ray diffraction analyzer (DX-2700with Cu Kα radiation) was used to analyzethe phase composition of solid salt. Thepolyethylene containers (Length: 20 cm, width:14 cm and height: 8 cm) were applied tometastable evaporation experiment.

2.2 Experimental MethodsThe stable phase equilibria were studied

using isothermal solubility equilibriummethod [19]. The system points of the ternarysystems were prepared by adding the othersalt gradually on the basis of the binaryinvariant points at 273 K. Each sample wasadded doubly deionized water quantitativelyfor dissolving the prepared salts and loadedinto clean sealed glass bottles. However,it must ensure the solid phases were notdissolved entirely in the whole experimentprocess. Then, the glass bottles were placedin the cyclotron oscillator (HY-5), whichwas put into the incubator (SHH-250)to controlled the internal temperature to273 ± 0.1 K. The oscillator should be shookvigorously in order to sufficiently dissolvethe salts. The supernatant of samples weretaken out periodically for chemical analysis(Samples should be stationary at a constanttemperature, so that the salt could sink

completely). When the compositions ofsolution remain unchanged, the system hasreached the thermodynamic equilibriumstate. After equilibrium, we took out the liquidand solid phases separately. Then the liquidcompositions were measured quantitativelyby chemical methods, and the solidphases were determined by powder X-raydiffraction. The results show that it takesabout 40 days for the ternary system toreach phase equilibrium.

The metastable phase equilibria werestudied using isothermal evaporationcrystallization method [19]. An appropriatequantity of salts and DDW were mixedtogether to produce a series of artificialsynthesized brines. When the salts completelydissolved, the synthesized brines were placedin the open and clean polyethylene containers,which had been put into the biochemicalincubator (SHH-250) beforehand. Theexperiment was carried out in the conditionsof the internal temperature (273±0.1) K,wind velocity (4.0 to 4.4) m⋅s-1, a relativehumidity (30 to 35) %. Throughout thecourse of the experiment, the artificialsynthesized brines had been still leftunstirred, and the crystal behaviors of thesolid phase were observed periodically.When the new solid phases appeared inthe polyethylene containers, the 5.00 mLsupernatant was taken out with a pipettegun and then diluted to 100 mL in avolumetric flask, which was analyzed bychemical methods to obtain the compositionsof salts. At the same time, the wet residuemixtures were removed from the solutionand determined by powder X-ray diffraction.However, the remainder of the solutioncontinued to be evaporated and reached anew metastable equilibrium point.

The densities of saturated solution weredetermined by pycnometer method both in

1924 Chiang Mai J. Sci. 2018; 45(4)

the stable and metastable phase equilibriaof the aqueous ternary system. The specificmethod is that the 5.00 mL supernatantwere taken out with a pipette gun quickly,then, weighed it as fast as possible.

2.3 Analytical MethodsThe concentration of Mg2+ was evaluated

using the EDTA complex titration methodwhen the eriochrome black-T as an indicator,with a precision ±0.5% [20]. The concentrationof borate ion (B

4O

72-) was measured through

alkaline neutralization titration with the standardsolution of NaOH (C=0.01 mol⋅L-1), thephenolphthalein solution as an indicatorwhen the mannitol existed, with an uncertaintyof mass fraction within ±0.3% [21]. Theconcentration of SO

42- in liquid phase was

analyzed by ion balance combined withgravimetric methods in the presence ofbarium chloride, with an uncertainty of massfraction within ±0.5% [22]. The densities weremeasured with a density bottle methodwith an uncertainty of 0.0002 g⋅cm-3. Theequilibrium solid phases were analyzed by

the X-ray diffractometer.

3. RESULTS AND DISCUSSIONS

3.1 Synthesis of MgB4O

7⋅9H

2O

The MgB4O

7⋅9H

2O is one of equilibrium

solid phases in ternary system MgO-B2O

3-

H2O at 298 K. According to the new method

of synthesis MgB4O

7⋅9H

2O provided by

Jing [23], the purity is more than 99.0%,which can satisfy the need of the experimentstudy. We learned that the hungtsaoite(MgB

4O

7⋅9H

2O) was prepared by the

materials of MgO and H3BO

3, the mass

ratio of MgO, H3BO

3 and H

2O is 1:8:66.

Then, the mixed solution was stirred withan agitator, which was placed in thethermostatic water bath at 298 K, the solutionkeep stirred until the turbid solution hadclarified. Then, it had kept stirring afterremoved the insoluble. Finally, a mass ofwhite precipitate was found. It takes aboutthree days to obtain the MgB4

O7⋅9H

2O.

The X-ray diffraction pattern of thehungtsaoite (MgB

4O

7⋅9H

2O) is shown in

Figure 1.

Figure 1. X-ray diffraction pattern of the Hungtsaoite [MgB4O

7⋅9H

2O].

Chiang Mai J. Sci. 2018; 45(4) 1925

3.2 The Stable Phase Equilibria ofMgB

4O

7-MgSO

4-H

2O at 273 K

After chemical analysis, the solubilitiesof the salt and densities of equilibriumsolution in this system are listed in Table 2.The mass fraction of salt B is noted asw (B), and the density of solution is notedas ρ/g⋅cm-3. The stable phase diagram ofternary system MgB

4O

7-MgSO

4-H

2O is

plotted in Figure 2. On the basis of theexperimental data, Figure 3 is the partialenlarged diagram. The density compositiondiagram is presented in Figure 4, thecrystallization forms of the invariant pointE

1(MgB

4O

7⋅9H

2O+MgSO

4⋅7H

2O) solid

phase were determined with an X-raydiffraction analysis method and demonstratedin Figure 5.

Table 2. Solubilities and densities of the stable phase equilibrium for ternary systemMgB

4O

7-MgSO

4-H

2O at 273 K.

a Standard uncertainties u are u(T) = 0.50 K; ur (p) =0.9 KPa.; u

r (ρ) = 2.0⋅10-4 g/cm-3; u

r

(MgB4O

7) = 0.0050; u

r (MgSO

4) = 0.0020; w(B), mass fraction of B;

No

1, A1

234567891011

12,E1

13141516171829

20, B1

Composition of solution/%100⋅w(MgSO

4)

0.002.254.985.949.5912.8715.9117.9018.9019.7120.4520.8820.9021.1421.1521.1721.2121.2321.2721.08

100⋅w(MgB4O

7)

0.450.460.520.530.540.550.560.570.580.590.620.650.570.400.330.270.250.180.140.00

Densityρ/g⋅cm-3

1.0131.0321.0591.0681.1041.1391.1701.1921.2031.2121.2221.2281.2261.2231.2221.2221.2211.2211.2171.200

Solidphase

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O+MgSO

4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

1926 Chiang Mai J. Sci. 2018; 45(4)

Figure 2. Stable phase diagram of ternarysystem MgB

4O

7-MgSO

4-H

2O at 273 K.

Figure 3. Partial enlarged diagram of Figure2.

Figure 4. Density composition diagram of the stable phase equilibrium for ternary systemMgB

4O

7-MgSO

4-H

2O at 273 K.

Figure 5. X-ray diffraction pattern of the eutectic point E1 [MgB

4O

7⋅9H

2O + MgSO

4⋅7H

2O]

in the stable phase equilibrium for ternary system MgB4O

7-MgSO

4-H

2O at 273 K.

0 20 40 60 80 100

P

C

A

100

80

60

40

20

0 1

100⋅

w(M

gB4O

7)

W B1 D Q

100⋅w(MgSO4)

3

2

1

0

A1

100⋅

w(M

gB4O

7)

W B1

0 10 20 30 40

100⋅w(MgSO4)

0 5 10 15 20 25

100⋅w(MgSO4)

1.40

1.30

1.20

1.10

1.00

0.90

A1

ρ/g⋅

cm-3

Chiang Mai J. Sci. 2018; 45(4) 1927

According to the Table 2, Figures 2and 3, the stable phase diagram consists ofone invariant point E

1, which is saturated with

salts MgB4O

7⋅9H

2O and MgSO

4⋅7H

2O. The

composition of the equilibrium solution isw(MgB

4O

7)=0.65% and w(MgSO

4)=20.88%.

There are two univariant solubilitycurves (A

1E

1, B

1E

1), and two solid crystalline

phase regions (A1E

1C, B

1E

1D), which

corresponding to two single salts,hungchaoite (MgB

4O

7⋅9H

2O) and epsomite

(MgSO4⋅7H

2O), respectively. It is significant

difference that the solid crystalline phaseregions (A

1E

1C) of MgB

4O

7⋅9H

2O is larger

than the (B1E

1D) of MgSO

4⋅7H

2O in the

phase diagram, which means that thesalt MgB

4O

7⋅9H

2O has smaller solubility

among the coexisting salts. The area (CE1D)

is the common crystallization area ofMgB

4O

7⋅9H

2O and MgSO

4⋅7H

2O, and the

area (WA1E

1B

1W) is unsaturated single

liquid region. It is found that there are neithercomplex salt nor solid solution formed.Therefore, it belongs to a simple eutectic type.

From Table 2 and Figure 4, it is seenthat the solubility of magnesium sulfate ismuch greater than that of magnesiumborate among the coexisting salts at 273 K.Thus, the density is mainly affected by themagnesium sulfate content in the equilibriumsolution. On account of this, the densitiesversus composition is plotted according to

the w(MgSO4). It is found that the densities

of solution increase with the increasingof w(MgSO

4), at the invariant point E

1, the

density becomes the largest one.The stable phase equilibrium of ternary

system MgB4O

7-MgSO

4-H

2O at 298 K had

been reported by Song [24]. A part of theexperiment data for the ternary system at273 K and 298 K are listed in Table 3.On the basis of the data, the phase diagramsare plotted in Figure 6. From Figure 6,it can be seen that the crystallization formsof the salts are not changed with the increaseof temperature, are MgB4

O7⋅9H

2O and

MgSO4⋅7H

2O, whereas the crystallization

zones of salts have changed, the crystallizationzone of MgB

4O

7⋅9H

2O obviously increased

at 298 K. From Table 3, the solubilities ofMgB

4O

7 and MgSO

4 obviously increase with

increasing of temperature from 273 K to 298K. The mass fractions composition of thecorresponding liquid phase at 273 K arew(MgB

4O

7)=0.45%, w(MgSO

4)=21.08%, are

w(MgB4O

7)=0.64% and w(MgSO

4)=26.78%

at 298 K. The liquid compositions alsosignificantly increase at the eutectic point,the mass fractions are w(MgB

4O

7)=0.65%,

w(MgSO4)=20.88% at 273 K, are

w(MgB4O

7)=1.02% and w(MgSO

4)=26.54%

at 298 K, respectively. Therefore, thedecrease of temperature is beneficial to thecrystallization of magnesium borate.

Table 3. Solubilities of saturated solution in the stable phase equilibrium for ternary systemMgB

4O

7-MgSO

4-H

2O at 273 K and 298 K [24].

Temperature

237 K

298 K [24]

Composition of solution/%

100⋅w(MgSO4)

21.08

20.88

0

26.78

26.54

0

100⋅w(MgB4O

7)

0

0.65

0.45

0

1.02

0.64

Systems

MgSO4-H

2O

MgB4O

7-MgSO

4-H

2O

MgB4O

7-H

2O

MgSO4-H

2O

MgB4O

7-MgSO

4-H

2O

MgB4O

7-H

2O

Solid phase

MgSO4⋅7H

2O

MgB4O

7⋅9H

2O+ MgSO

4⋅7H

2O

MgB4O

7⋅9H

2O

MgSO4⋅7H

2O

MgB4O

7⋅9H

2O+ MgSO

4⋅7H

2O

MgB4O

7⋅9H

2O

1928 Chiang Mai J. Sci. 2018; 45(4)

Figure 6. Sable phase diagrams of ternarysystem MgB

4O

7-MgSO

4-H

2O at 273 K and

298 K [24].

3.3 The Metastable phase equilibria ofMgB

4O

7-MgSO

4-H

2O at 273 K

Similarly, the solubilities and densities ofthe metastable phase equilibrium for theternary system MgB

4O

7-MgSO

4-H

2O are

listed Table 4. According to the experimentaldata, the metastable phase diagram is shownin Figure 7, and Figure 8 is the partialenlarged diagram. The density compositiondiagram is presented in Figure 9, the X-raydiffraction pattern of the invariant point E

2

(MgB4O

7⋅9H

2O+MgSO

4⋅7H

2O) is given in

Figure 10.

Table 4. Solubilities and densities in the metastable phase equilibrium for ternary systemMgB

4O

7-MgSO

4-H

2O at 273 K.

No

1,A2

234567891011

12,E2

1314151617181920

21,B2

Compositionof solution/%100⋅w(MgSO

4)

0.003.924.675.707.079.7711.8014.9318.5819.3819.9820.4520.4420.4820.5920.7420.8720.9221.1921.4421.70

100⋅w(MgB4O

7)

0.530.580.620.650.690.700.720.780.840.900.991.031.020.910.870.760.670.500.280.140.00

Densityρ/g⋅cm-3

1.0181.0591.0681.0801.0951.1231.1461.1781.2101.2191.2241.2311.2301.2291.2281.2261.2261.2231.2211.2181.209

Solidphase

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O

MgB4O

7⋅9H

2O+MgSO

4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

MgSO4⋅7H

2O

a Standard uncertainties u are u(T) = 0.50 K; ur (p) =0.9 KPa.; u

r (ρ) = 2.0⋅10-4 g/cm-3;

ur (MgB

4O

7) = 0.0050; u

r (MgSO

4) = 0.0020; w(B), mass fraction of B;

100⋅w(MgSO4)

W B1 B

3

0 10 20 30 40

3

2

1

0

100⋅

w(M

gB4O

7)

A1

A3

Chiang Mai J. Sci. 2018; 45(4) 1929

Figure 7. Metastable phase diagram in theternary system MgB

4O

7-MgSO

4-H

2O at

273 K.

Figure 8. Partial enlarged diagram of theFigure 7.

Figure 9. Density composition diagram of the metastable phase equilibrium for ternary systemMgB

4O

7-MgSO

4-H

2O at 273 K.

Figure 10. X-ray diffraction pattern of the eutectic point E2 [MgB

4O

7⋅9H

2O + MgSO

4⋅7H

2O]

in the metastable phase equilibrium for ternary system MgB4O

7-MgSO

4-H

2O at 273 K.

100⋅

w(M

gB4O

7)

C

100⋅w(MgSO4)0 20 40 60 80 100

P100

80

60

40

20

0A2

W B2 D Q

0 5 10 15 20 25100⋅w(MgSO4)

1.40

1.30

1.20

1.10

1.00

0.90

A2

ρ/g⋅

cm-3

3

2

1

0

A2

100⋅

w(M

gB4O

7)

W B2

0 10 20 30100⋅w(MgSO4)

1930 Chiang Mai J. Sci. 2018; 45(4)

It is a focus to compare the stable withmetastable phase equilibrium for the ternarysystem MgB

4O

7-MgSO

4-H

2O at 273 K in this

paper. A comparison chart of the stableand metastable phase diagrams is presentedin Figure 11.

Figure 11. Comparison chart in the stable andmetastable phase diagrams.

There are some similarities in the stableand metastable phase equilibrium forthe ternary system MgB

4O

7-MgSO

4-H

2O at

273 K. From Tables 2 and 4, and Figure 11,it is found that the metstable phase diagrambelongs to a simple co-saturation type, thereare neither solid solutions nor double salts areformed. The reason may be that the latticetype, lattice parameters, and atomic structureof MgB

4O

7⋅9H

2O and MgSO

4⋅7H

2O are

quite different. The MgSO4⋅7H

2O is a

columnar crystal, but the MgB4O

7⋅9H

2O is a

powdery crystal. Therefore, it is difficultto form double salts and solid solutionbetween the two salts. Analogously, itconsists of one invariant point E

2

is comprised by MgB4O

7⋅9H

2O and

MgSO4⋅7H

2O, two univariant isothermal

metastable solubility curves (A2E

2, B

2E

2).

Two solid phase crystallization zones (A2E

2C,

B2E

2D) corresponding to MgB

4O

7⋅9H

2O

and MgSO4⋅7H

2O, respectively. The area

(CE2D) is the common crystallization area

of MgB4O

7⋅9H

2O and MgSO

4⋅7H

2O, and the

area (WA2E

2B

2W) is unsaturated single

liquid region. From the Figure 9, therelationship between density and compositionof magnesium sulfate in the metstable ternarysystem MgB

4O

7-MgSO

4H

2O at 273 K.

It is found that the densities are graduallyincreased regularly with increasing themagnesium sulfate concentration in thesolution, and becomes the largest one at theinvariant point E

2.

However, there are many differencesbetween the stable phase equilibrium andmetastable phase equilibrium. The singlesalts (MgB

4O

7⋅9H

2O and MgSO

4⋅7H

2O)

solubilities in the metastable phase diagramare greater than that the stable phase diagram.The mass fractions of single salts arew(MgB

4O

7)=0.53%, w(MgSO

4)=21.70%

in the metastable phase diagram, however,they are corresponding to w(MgB

4O

7)=0.45%,

w(MgSO4)= 21.08% in the stable phase

diagram. In the eutectic point, the massfractions of liquid phase are w(MgB

4O

7)=

1.03%, w(MgSO4)=20.45% in the metastablephase diagram. But, the mass fractions arew(MgB

4O

7)=0.65%, w(MgSO

4)= 20.88% in

the stable phase diagram. It can be seen thatthe solubility of MgB

4O

7 in the metastable

phase diagram are obviously increased thanthat the stable phase diagram, which illustratesthat there some supersaturated phenomenafor MgB

4O

7 at 273 K in the metastable phase

equilibrium. It may be due to the speed ofthe crystal formation, probably the speed ofnucleation and the speed of grow up ofcrystal is very slow for MgB

4O

7 in the solution,

so that the chemical potential of the salt(MgB

4O

7) in the liquid phase is greater

than the solid phase in the process ofevaporation[19].

4. CONCLUSIONS

The stable and metastable phase equilibriafor the ternary system MgB

4O

7-MgSO

4-H

2O

A2

W B1 B2

0 10 20 30 40100⋅w(MgSO4)

A1

100⋅

w(M

gB4O

7)

3

2

1

0

Chiang Mai J. Sci. 2018; 45(4) 1931

at 273 K were studied using isothermalsolubility equilibrium method and isothermalevaporation crystallization method,respectively. Based on the experimental data,the stable and metastable phase diagrams andthe density composition diagrams weredrawn. The results show that both the stableand metastable phase diagrams belong tothe simple co-saturation type. The solidphase crystalline region of MgB

4O

7⋅9H

2O is

larger than MgSO4⋅7H

2O, so that the salt

MgB4O

7⋅9H

2O has smaller solubility among

the coexisting salts. Besides, the densitiescomposition show a regular variation, thereare a maximum in the eutectic point both inthe stable and metastable phase equilibria.

Compared with the stable phase diagramin the ternary system MgB

4O

7-MgSO

4-H

2O

at 273 K, the crystalline region of MgSO4

and MgB4O

7 have changed, there is a

supersaturated phenomenon for MgB4O

7 in

the metastable phase equilibrium.Finally, the salts MgSO

4 and MgB

4O

7 have

the same crystallization forms (MgB4O

7⋅9H

2O

and MgSO4⋅7H

2O) in stable and metastable

phase equilibria for the aqueous ternarysystem MgB

4O

7-MgSO

4-H

2O at 273 K,

including the stable phase equilibrium at298 K. The solubilities of two salts in waterincreasing with the increase of temperature.

ACKNOWLEDGMENTS

This project was supported by theNational Natural Science Foundation of China(U1407108), scientific research and innovationteam in Universities of Sichuan ProvincialDepartment of Education (15TD0009)

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