Phase Equilibria in the Ternary Systems Li2B4O7âMgB4O7âH2O and

Article pubs.acs.org/jced

Phase Equilibria in the Ternary Systems Li2B4O7−MgB4O7−H2O and K2B4O7−MgB4O7−H2O at 273 K Chao Fu,† Shi-hua Sang,*,†,‡ Mei-fang Zhou,† Qing-zhu Liu,† and Ting-ting Zhang† †

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, P. R. China Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, P. R. China

‡

ABSTRACT: Phase equilibria and phase diagram always play an important role to utilize the salt lake brine resources. In this paper, the phase equilibria in two ternary systems Li2B4O7− MgB4O7−H2O and K2B4O7−MgB4O7−H2O at 273 K were studied using the isothermal solubility equilibrium method. The densities and solubilities of the components in these systems listed above were measured. Based on the determined equilibrium data and the corresponding equilibrium solid phase, the phase diagram of the ternary systems were shown, respectively. The results show that the two ternary systems at 273 K are simple total saturated type and there are no complex salt and solid solution found. Besides, the phase diagrams of the two ternary systems are both constituted of a eutectic point, two univariant solubility curves, and two solid crystalline phase regions. The two equilibrium solid phases corresponding to the eutectic point of the Li2B4O7−MgB4O7−H2O ternary system are Li2B4O7·3H2O and MgB4O7·9H2O. In the K2B4O7−MgB4O7−H2O ternary system, K2B4O7·4H2O and MgB4O7·9H2O are the two equilibrium solid phases relevant to the eutectic point. Furthermore, the experimental results of densities were simply discussed, and the phase diagrams at different temperatures were compared.

1. INTRODUCTION There are many salt lake brines in China which are a kind of high salinity liquid mineral resources. Salt lakes in China with numerous types and large number are famous for their rich resources and are rich in rare elements.1 Because of the influence of geologic structure, topoclimate, ionic migration, and the characteristic of hydrochemistry, the modern salt lakes are usually provided with zonal aggregation.2 The Qinghai− Tibet Plateau, located in the west of China, is the main distribution area of salt lakes in the world. A subtype of sulfate salt lake brine is widely distributed in the area, which have distinct characteristics from other brine types, and is famous for the high concentrations of lithium, potassium, and boron.3 The subtype of sulfate salt lake brines belong to the sevencompound system Li+ + Na+ + K+ + Mg2+ + Cl− + SO42− + B4O72− + H2O.4 Sang et al. studied the equilibrium relationship about a series of ternary systems containing lithium borate or potassium borate at 288 K.5−7 The solubility diagram and physicochemical properties of solution in the ternary system Li2B4O7−K2B4O7− H2O at 25 °C were researched by Yu et al.8 Song et al. completed the study on the solubility diagram and physicochemical properties of solution in the quaternary interaction system Li+, Mg2+//SO42−, B4O72−−H2O and the subsystem of the ternary system Li+//SO42−, B4O72−−H2O and Mg2+//SO42−, B4O72−−H2O at 25 °C.9−11 Solubility calculations of systems LiCl−Li2B4O7−H2O and Li2B4O7−MgCl2− H2O at 298 K have been researched.12,13 Sun and Song et al. © 2016 American Chemical Society

carried out research on the phase equilibrium in a sulfate type pentabasic system with Li, Mg, and B coexisting.14 However, most studies on phase equilibrium of systems containing boron at 273 K are limited to metastable phase equilibrium. The metastable equilibrium solubilities of solutions in the quaternary system of K2B4O7−Na2B4O7−Li2B4O7−H2O at 273 K were researched by Sang et al.15 Zeng et al. carried out a sequence of researches on metastable equilibrium in the aqueous quaternary systems containing boron and sulfur at 273.15 K.16−18 Solubility and density measurements of concentrated Li2B4O7 + Na2B4O7 + K2B4O7 + Li2SO4 + Na2SO4 + K2SO4 + H2O solution at 273.15 K have also been reported by Zeng et al.19 The ternary systems Li2B4O7−MgB4O7−H2O and K2B4O7− MgB4O7−H2O are two significant subsystems to study the complex system of the salt lakes on the Qinghai, Tibet plateau of China. Phase equilibria in the ternary system Li2B4O7− MgB4O7−H2O at 323 K20 and the ternary system K2B4O7− MgB4O7−H2O at 298 K21 and 348 K22 have been reported, but research about the stable phase equilibrium of the two systems at 273 K has not been found. Based on our previous studies, the stable phase equilibrium about these two ternary systems were studied in this work. We used isothermal solubility equilibrium method to study the Received: July 9, 2015 Accepted: February 4, 2016 Published: February 17, 2016 1071

DOI: 10.1021/acs.jced.5b00570 J. Chem. Eng. Data 2016, 61, 1071−1077

Journal of Chemical & Engineering Data

Article

Table 1. Chemical Sample Descriptions chemical name

source

Li2B4O7 K2B4O7·4H2O MgB4O7·9H2O

Chengdu Kelong Chemical Reagent Manufactory Chengdu Kelong Chemical Reagent Manufactory synthesis

initial mole fraction purity

purification method

final mole fraction purity

analysis method

0.99 0.99

none none chemical reaction

0.99 0.99 0.99

titration titration titration

gradually to the invariant samples of relevant binary subsystems at 273 K. A portion of 50 mL of deionized water was used to dissolve the prepared salts to form the artificial synthesized brines in sealed glass bottles. Then, the bottles were placed in the vibrator (HY-5) which was placed in the incubator (SHH250) beforehand, and the sample temperature was maintained to (273 ± 0.1) K. The solution of one sample was taken out periodically, and the compositions were analyzed by chemical methods. When the concentrations of the solution compositions remain unchanged, it means the system has reached equilibration. It took about 40 days for the ternary systems to reach equilibration. After equilibrium, we took out the liquid and solid phases separately. Then the liquid compositions were measured quantitatively by chemical methods, and the solid phases were determined by powder X-ray diffraction. The invariant point was determined from the solution compositions and the X-ray diffraction photograph. For experimental details, Niu and Cheng24 can be referenced. 2.3. Analytical Methods. The concentration of borate ion (B4O72−) was measured through basic titration (0.05 mol·L−1 NaOH) using a phenolphthalein solution as the indicator when the mannitol existed (The expanded uncertainty (0.95 level of confidence) is U = 0.003). The concentration of potassium ion (K+) was measured using sodium tetraphenyl borate (STPB)− hexadecyl trimethylammonium bromide (CTAB) back-titration (The expanded uncertainty (0.95 level of confidence) is U = 0.005). The concentration of magnesium ion (Mg2+) and lithium ion (Li+) was measured by ICP-OES and calculated based on an ion balance (The expanded uncertainty (0.95 level of confidence) is U = 0.005). Each analysis for a sample was repeated thrice and took the average of three parallel determinations as the final results. The pycnometer method was used to measure the densities of the equilibrated solutions (The expanded uncertainty (0.95 level of confidence) is U = 0.0002 g·cm−3).25

equilibrium relationship of the ternary system lithium borate− magnesium borate−water and potassium borate−magnesium borate−water at 273 K. We measured the solubilities and densities of the equilibrated solutions at 273 K. This can further supplement and perfect the above ternary systems and provide basic solubility data to develop boron resource.

2. EXPERIMENTS 2.1. Reagents and Instruments. The chemicals used (K2B4O7·4H2O, Li2B4O7, and other auxiliary reagents) were of analytical purity grade and provided by Chengdu Kelong Chemical Reagent Manufactory, China, listed in the Table 1. All of the experimental solutions was prepared using deionized water (conductivity less than 1 × 10−5 S·m−1, pH = 6.6). MgB4O7·9H2O was laboratory synthetic. A new method to compound hungtsaoite (MgB4O7·9H2O) was provided by Jing Yan,23 which can meet the need of our experiment research. According to Jing Yan we obtained hungtsaoite using analytical reagent MgO and H3BO3 as raw materials and the purity of MgB4O7·9H2O was more than 99%. The X-ray diffraction photograph of the sample of hungtsaoite [MgB4O7·9H2O] prepared by us was shown in Figure 1.

Figure 1. X-ray diffraction photograph of the hungtsaoite [MgB4O7· 9H2O].

3. RESULTS AND DISCUSSION 3.1. The Li2B4O7−MgB4O7−H2O System at 273 K. The experimental results of phase equilibria in the ternary system Li2B4O7−MgB4O7−H2O at 273 K are presented in Table 2. The mass fraction of salt B is noted as w(B), and the density of equilibrated solution is noted as ρ. The phase diagram of ternary system Li2B4O7−MgB4O7−H2O at 273 K was plotted in Figure 2 based on the experimental data, and Figure 3 is the partial enlarged drawing. The X-ray diffraction photograph of the invariant point E in the ternary system is given in Figure 4. According to this figure, point E corresponds to the saturation point of Li2B4O7·3H2O and MgB4O7·9H2O. According to Table 2 and Figure 2, this ternary system belongs to a simple common saturation type that no complex salt and solid solution formed at the investigated temperature. The phase diagram is composed with one invariant point, two univariant curves, and two crystallization regions. The two crystallization regions are lithium borate trihydrate (Li2B4O7· 3H2O) and hungtsaoite (MgB4O7·9H2O). The larger one is the

A standard analytical balance of a 110 g capacity and 0.0001 g resolution (AL104, provided by the Mettler Toledo Instruments Co., Ltd.) was used to measure the solution density. A biochemical incubator of −15 to 60 °C temperature range and 0.1 °C resolution (SHH-250, supplied by the Chongqing Inborn Experiment Instrument Co., Ltd.) was employed to control the temperature. An oscillator (HY-5, supplied by the Jintan Kexi Instrument co., LTD) placed in the incubator (SHH-250) was employed for the sufficient reaction of the samples. An electro thermal blowing drybox (101-0AB, supplied by the Beijing Zhongxing Weiye Instrument co., LTD) was employed to dry the solid samples. 2.2. Experimental Method. The phase equilibria of the two ternary systems at 273 K were determined by the isothermal solution method. The system points for the ternary systems were compounded by adding the second component 1072



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Table 2. Solubilities and Densities of Solution in the Ternary System Li2B4O7−MgB4O7−H2O at 273 K solubilities and densities of solution in the ternary system Li2B4O7−MgB4O7−H2O at 273 K composition of solutiona,b

density

no.

w(Li2B4O7) × 100

w(MgB4O7) × 100

solid phase

ρ/g·cm−3

1,B 2 3 4 5 6,E 7 8 9 10 11 12 13 14,D

2.06 2.02 2.09 2.20 2.24 2.31 2.25 1.83 1.48 1.10 0.69 0.37 0.28 0.00

0.00 0.02 0.06 0.07 0.09 0.10 0.10 0.09 0.07 0.10 0.14 0.27 0.34 0.45

Li2B4O7·3H2O Li2B4O7·3H2O Li2B4O7·3H2O Li2B4O7·3H2O Li2B4O7·3H2O MgB4O7·9H2O + Li2B4O7·3H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O

1.0211 1.0208 1.0220 1.0233 1.0238 1.0247 1.0241 1.0196 1.0158 1.0122 1.0084 1.0064 1.0062 1.0045

a

w(B) is the mass fraction of component B, and the experimental pressure is 94.77 KPa. bThe expanded uncertainties (level of confidence 0.95) are U(w(MgB4O7)) = U(w(Li2B4O7)) = 0.005, U(T) = 0.1 K, U(ρ) = 0.0002 g·cm−3, and U(P) = 0.9 KPa.

Figure 3. Partial enlarged diagram of the ternary system Li2B4O7− MgB4O7−H2O at 273 K. Figure 2. Equilibrium phase diagram of the ternary system Li2B4O7− MgB4O7−H2O at 273 K.

Li2B4O7−MgB4O7−H2O at 273 K. It is seen from Table 2 and Figure 5 that when the w (Li2B4O7) increases the density of solution also increases. At the invariant point E, the density becomes the largest one, and the value is 1.0247 g·cm−3. The phase diagram of the ternary system Li2B4O7− MgB4O7−H2O has been studied at 323 K.22 The comparison between the phase equilibrium in the ternary systems at 323 and 273 K was shown in Figure 6. Although the salts in this system at the two temperatures (Li2B4O7·3H2O and MgB4O7· 9H2O) have the same crystallization forms, their crystallization fields are different. The crystallization fields of both the salt Li2B4O7·3H2O and MgB4O7·9H2O are larger at 323 K than that at 273 K because the solubility of the salts Li2B4O7 and MgB4O7 in water increase with the rising of temperature. 3.2. The K2B4O7−MgB4O7−H2O System at 273 K. The experimental data of phase equilibria in the ternary system K2B4O7−MgB4O7−H2O at 273 K are listed in Table 3. Based on the data in Table 3, the phase diagram of the system at 273

crystallization area of hungtsaoite (GDE), and the smaller crystallization area corresponds to lithium borate trihydrate (HBE). The area (HGE) is the crystallization area of Li2B4O7· 3H2O and MgB4O7·9H2O. In this system the solubility of salt MgB4O7 is much smaller than salt Li2B4O7, so it can be easily separated from solution. BE and DE are the two univariant curves. Point E is the invariant point for the system Li2B4O7− MgB4O7−H2O at 273 K. It saturates with salts Li2B4O7·3H2O + MgB4O7·9H2O, and the corresponding liquid phase is measured with the mass fraction compositions w(Li2B4O7) = 2.31%, w(MgB4O7) = 0.10%. In this system, the solubility of the salt Li2B4O7 is greater than that of the salt MgB4O7. Therefore, the concentration of Li2B4O7 is the main factor affecting the solution density. Figure 5 is the density vs composition diagram of the ternary system 1073



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Figure 4. X -ray diffraction photograph of the eutonic point E [MgB4O7·9H2O+Li2B4O7·3H2O] in the ternary system Li2B4O7−MgB4O7−H2O at 273 K.

K was shown in Figure 7 and Figure 8 is the partial enlarged drawing. Also given is the X-ray diffraction photograph of the invariant point (F) of the ternary system (Figure 9), where point F is the saturation point of K2B4O7·4H2O and MgB4O7· 9H2O. According to Table 3 and Figure 7, it can be seen that this ternary system is simple eutectic type and there are no double salt or solid solution found at 273 K. In the equilibrium phase diagram there are one invariant point, two univariant curves, and two crystallization fields (K2B4O7·4H2O and MgB4O7· 9H2O). In Figure 7, we can see that the larger crystallization field is the crystallizing zone of MgB4O7·9H2O (MNF), and the smaller one is the crystallization area of K2B4O7·4H2O (KLF). The area (KMF) is the crystallization area of K2B4O7·4H2O and MgB4O7·9H2O. The salt MgB4O7·9H2O has a much smaller solubility than the salt K2B4O7·4H2O. Therefore, the salt MgB4O7·9H2O is more easily saturated and crystallized from the solution. The two univariant curves in the system are curves LF and NF. Point F is the invariant point, saturated with salts K2B4O7· 4H2O and MgB4O7·9H2O. The mass fraction composition of liquid phase of invariant point F is w(K2B4O7) = 10.4%, w(MgB4O7) = 0.12%. In this system, the solubility of the salt K2B4O7 is greater than that of the salt MgB4O7. Therefore, the concentration of K2B4O7 is the main factor affecting the solution density. Figure 10 is the density vs composition diagram of the ternary system K2B4O7−MgB4O7−H2O at 273 K. According to Table 3 and Figure 10, we can see that the density of the solution increases with the increase of w(K2B4O7). The density reaches maximum at the invariant point F, and the value is 1.1176 g·cm−3. The phase diagram of the ternary system K2B4O7−MgB4O7− H2O has been studied at 298 K.23 A comparison between the phase equilibrium in the ternary systems at 273 and 298 K is shown in Figure 11. Compared with the two phase diagrams at different temperatures, it can be seen that the shapes of these phase diagrams are similar. In all of them there are one invariant point, two univariant curves, and two crystallization regions. In this ternary system the two crystallizations are

Figure 5. Density value vs composition of the ternary system Li2B4O7−MgB4O7−H2O at 273 K.

Figure 6. Equilibrium phase diagram of the ternary system Li2B4O7− MgB4O7−H2O at 323 K (Li et al., 2014) and 273 K. 1074



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Table 3. Solubilities and Densities of Solution in the Ternary System K2B4O7−MgB4O7−H2O at 273 K solubilities and densities of solution in the ternary system K2B4O7−MgB4O7−H2O at 273 K composition of solutiona,b

density

no.

w(K2B4O7) × 100

w(MgB4O7) × 100

solid phase

ρ/g·cm−3

1,L 2 3 4 5 6 7 8,F 9 10 11 12 13 14 15,N

7.23 7.65 8.09 8.64 8.96 9.19 10.20 10.40 8.30 7.57 6.77 5.46 3.12 1.94 0.00

0.00 0.02 0.06 0.08 0.09 0.10 0.11 0.12 0.09 0.08 0.08 0.08 0.13 0.21 0.45

K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O K2B4O7·4H2O MgB4O7·9H2O + K2B4O7·4H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O MgB4O7·9H2O

1.0780 1.0831 1.0887 1.0955 1.0996 1.1024 1.1149 1.1176 1.0916 1.0829 1.0736 1.0587 1.0337 1.0221 1.0045

a

w(B) is the mass fraction of component B, and the experimental pressure is 94.77 KPa. bThe expanded uncertainties (level of confidence 0.95) are U(w(MgB4O7)) = U(w(K2B4O7)) = 0.005, U(T) = 0.1 K, U(ρ) = 0.0002 g·cm−3, and U(P) = 0.9KPa.

Figure 7. Equilibrium phase diagram of the ternary system K2B4O7− MgB4O7−H2O at 273 K. Figure 8. Partial enlarged diagram of the ternary system K2B4O7− MgB4O7−H2O at 273 K.

K2B4O7·4H2O and MgB4O7·9H2O. It is easy to find that at the two temperatures the crystallization forms of salts in this system are same, but the crystallization fields are different. The crystallization field of the salt K2B4O7 and salt MgB4O7 at 273 K is larger than those at 298 K because the solubility of both them increases with the rising of temperature. Obviously, the solubility of the salt K2B4O7 in water at 298 K is much greater than at 273 K, so the crystallization field of the salt K2B4O7· 4H2O and MgB4O7·9H2O are larger at 298 K than at 273 K, and it is the same to the salt MgB4O7.

The salt K2B4O7 has a salting-out effect on MgB4O7, and it is nonlinear. The equilibrium solubilities of MgB4O7 change from smaller to bigger, because there are two kinds of reaction in this system: salting-out action and salt dissolution. At the beginning of adding K2B4O7, the common ion effect is greater than the hydrogen bond force, so the salting-out action is much stronger than salt dissolution. With the increase of K2B4O7 the hydrogen bonding force is gradually enhanced, then exceeds the common ion effect, and makes the B4O72− associate, so the solubility of MgB4O7 has an increase tendency. Compared to a higher temperature, the salts Li2B4O7·3H2O, K 2 B 4 O 7 ·4H 2 O, and MgB 4 O 7 ·9H 2 O all have the same crystallization forms. But their crystallization fields are larger at the higher temperature than those at 273 K, because the solubilities of the three salts in water increase with the rising of temperature.

4. CONCLUSIONS We studied the phase equilibria in the two ternary systems Li2B4O7−MgB4O7−H2O and K2B4O7−MgB4O7−H2O at 273 K using the isothermal solution saturation method. Based on the experimental data, we also plotted the phase diagrams and density diagrams of the two systems. According to the results, we can see that these two ternary systems belong to the simple cosaturation type; there were no solid solution and double salts formed. 1075



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Figure 9. X-ray diffraction photograph of the eutonic point F [MgB4O7·9H2O + K2B4O7·4H2O] of the ternary system K2B4O7−MgB4O7−H2O at 273 K.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This project was supported by the National Natural Science Foundation of China (U1407108), scientific research and innovation team in Universities of Sichuan Provincial Department of Education (15TD0009) and the youth science and technology innovation team of Sichuan Province, China (2013TD0005). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 10. Density value vs composition of the ternary system K2B4O7−MgB4O7−H2O at 273 K.

Figure 11. Equilibrium phase diagram of the ternary system K2B4O7− MgB4O7−H2O at 298 K (Jin et al., 2004) and 273 K.

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