Solid–Liquid Phase Equilibrium in Aqueous Quaternary System Li⁺,Rb⁺,Mg²⁺//Borate–H₂O at T = 323 K

Abstract

The stable phase equilibria, densities, and refractive indices of the aqueous quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K were determined by the isothermal dissolution method. It was found that the phase diagram consists of one quaternary invariant point, two isothermal dissolution curves, and three fields of crystallization corresponding to three single salts Li₂B₄O₇ · 3H₂O, MgB₄O₇ · 9H₂O, and RbB₅O₈ · 4H₂O, respectively. The size of crystallization regions of salt decreased in the order MgB₄O₇ · 9H₂O > RbB₅O₈ · 4H₂O > Li₂B₄O₇ · 3H₂O. The comparison of phase diagrams of the system at 298–348 K shown that the crystalline form of salts lithium borate, magnesium borate, and rubidium borate did not change at 298–348 K. The crystalline area of MgB₄O₇ · 9H₂O is the largest than that of 298 and 348 K. The crystalline area of RbB₅O₈ · 4H₂O is the smallest than that of 298 and 348 K, while the crystalline are of Li₂B₄O₇ · 3H₂O is larger than that of 298 K, and smaller than that of 348 K.

1 INTRODUCTION

Phase equilibrium of borate containing system is important because there are various forms of boron such as B(OH)₃, ${\text{BO}}_{2}^{ - }$, ${\text{B}}({\text{OH}})_{4}^{ - }$, ${{{\text{B}}}_{{\text{4}}}}{{{\text{O}}}_{{\text{5}}}}({\text{OH}})_{4}^{{2 - }}$ appeared in the solution [1]. Therefore the solubility isotherms and thermodynamics properties of the borates in pure liquids within multiple temperatures are of significant importance for extracting borates from salt lake and underground brines. For this reason, the phase equilibria of the aqueous systems or subsystems composed of lithium, potassium, sodium, magnesium, rubidium, and borate were done by previous research [2–11].

As for the system Li⁺,Rb⁺,Mg²⁺//borate–H₂O studied in this article, the corresponding phase diagrams at 298 and 348 K have been completed by our research group [12, 13], while the solid-liquid equilibrium of system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K has not been carried out. Consequently, on the basis of results for ternary subsystems, the phase equilibrium of system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K is reported in detail.

2 EXPERIMENTAL

2.1 Materials

All solutions were prepared using the doubly deionized water, which was obtained using a Millipore water system with an electrical conductivity less than 5.5 × 10^–6 S m^–1. Lithium tetraborate anhydrous (Li₂B₄O₇), magnesium oxide (MgO), and boric acid (H₃BO₃) were obtained from Chengdu Kelong Chemical Reagent Plant with purity of 99.5%. Rubidium carbonate (Rb₂CO₃) was obtained from Jiangxi Dongpeng New Materials Co., Ltd. with purity of 99.5%. Rubidium pentaborate (RbB₅O₆(OH)₄ · 2H₂O) and Hungtsaoite (MgB₄O₅(OH)₄ · 7H₂O) with purity higher than 99.0% were synthesized in our laboratory [14, 15].

2.2 Methods and Apparatus

The isothermal dissolution method was applied to the phase equilibrium experiments [2]. A series of samples with different amounts of Li₂B₄O₇, MgB₄O₇ · 9H₂O, RbB₅O₈ · 4H₂O, and H₂O were put into 100 mL tightly sealed bottles, immersed in the THZ-82 type thermostatic shaker with 120 rpm speed at (323 ± 0.2) K, once the content of liquid phase became constant, it indicated that the equilibrium had been reached. And then, the liquid and solid phases were separated by filtration at 323 K. The densities of the solution at equilibrium were determined by the gravity bottle method [16], and the refractive indices of the equilibrated solution were measured with the Abbe refractometer (WYA type, Shanghai Precision & Scientific Instrument Co., Ltd.); meanwhile a certain amount solution was diluted to a final volume of 250 mL for analysis the compositions. The wet solids were dried at 323 K and characterized using X-ray diffraction (DX-2700 type, Dandong Fangyuan Instrument Co., Ltd.).

3 ANALYTICAL METHODS [17, 18]

Li⁺: ICP-OES, with standard uncertainty of 0.50%;

Rb⁺: sodium tetraphenylborate (STPB)—hexadecyltrimethylammonium bromide (C₁₉H₄₂BrN) back titration method, with standard uncertainty of 0.50%;

Borate: neutralization titration in the presence of mannitol with standard uncertainty of 0.30%;

Mg²⁺: titration with EDTA standard solution, standard uncertainty of 0.50%.

4 RESULTS AND DISCUSSION

The experimental data for the aqueous quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K are given in Table 1. Commonly, ${{{\text{B}}}_{{\text{4}}}}{\text{O}}_{7}^{{2 - }}$ represents all kinds of possibly existing forms of boron ions in the solution. Thus, the crystalline form of salt was described as RbB₅O₈ · 4H₂O and ${{{\text{B}}}_{{\text{4}}}}{\text{O}}_{7}^{{2 - }}$ is used to express the different boric species in solution, therefore, Rb₂B₄O₇ was indicated in Table 1.

Table 1.   The solid–liquid equilibrium and physicochemical properties (Density and Refractive index) of the quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K and pressure p = 0.1 MPa^a

The phase diagram of system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K was plotted as shown in Fig. 1. The phase diagram consists of one quaternary invariant point, three isothermal dissolution curves, and three crystalline zones. The composition of the solid phase in point E was analyzed by the X-ray diffraction method and the XRD pattern was given in Fig. 2. The identification is performed by comparison of the diffraction pattern to databases showing that salts Li₂B₄O₇ · 3H₂O (PDF no. 50-0564), RbB₅O₈ · 4H₂O (PDF no. 43-0415), and MgB₄O₇ · 4H₂O (PDF no. 34-1288) coexist in the invariant point E. The liquid composition of point E is w(Li₂B₄O₇) = 3.63%, w(Rb₂B₄O₇) = 4.87%, w(MgB₄O₇) = 0.25%, and w(H₂O) = 91.25%.

Fig. 1.

Phase diagram of the quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K.

Fig. 2.

X-ray diffraction pattern of the invariant point E in the quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K.

Among the three crystalline regions, the crystallization areas of the salts are in the following order: MgB₄O₇ · 9H₂O > RbB₅O₈ · 4H₂O > Li₂B₄O₇ · 3H₂O, which means that the solubility of Li₂B₄O₇ is the largest.

The comparison of the phase diagram (Fig. 3) of system Li⁺,Rb⁺,Mg²⁺//borate–H₂O between 298 [12], 323 and 348 K [13] shows that (1) the crystalline form of salts lithium borate, magnesium borate, and rubidium borate did not change at 298–348 K; (2) the crystalline of MgB₄O₇ · 9H₂O is the largest than that of 298 and 348 K, the crystalline area of RbB₅O₈ · 4H₂O is the smallest than that of 298 and 348 K, while the crystalline are of Li₂B₄O₇ · 3H₂O is larger than that of 298 K, while smaller than that of 348 K.

Fig. 3.

Phase diagram of quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 298, 323, and 348 K ($\blacksquare$ 298 K [12], 323 K, $\blacktriangle$ 348 K [13]).

Figures 4–6 were constructed with J(Rb₂B₄O₇) as the abscissa and J(H₂O), the density, or the refractive index of the solution at equilibrium as the ordinate. As Fig. 4 shows, the water content decreases on the AE and BE curve with J(Rb₂B₄O₇) incrementally until it reaches the smallest value at point E; while, on the curve CE, the water content reduces with J(Rb₂B₄O₇) decrement until it reaches the smallest value at point E. Figures 5 and 6 show that on the monovariant curve AE, the values of density and refractive index of the solution at equilibrium are positively correlated with J(Rb₂B₄O₇), the refractive indices increase along with J(Rb₂B₄O₇) decrease on the curves BE and CE, and reach the maximum values at invariant point E.

Fig. 4.

Water-content diagram of Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K.

Fig. 5.

Densities vs composition diagram of Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K.

Fig. 6.

Refractive indices vs. composition diagram of Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K.

5 CONCLUSIONS

The solubilities and physicochemical properties (density and refractive index) of the aqueous quaternary system Li⁺,Rb⁺,Mg²⁺//borate–H₂O at 323 K were investigated by using the isothermal dissolution method. The solid phases of the quaternary system were Li₂B₄O₇ · 3H₂O, MgB₄O₇ · 9H₂O, and RbB₅O₈ · 4H₂O. It is found that MgB₄O₇ · 9H₂O salt contains almost of the crystallization field. Comparisons between the stable phase diagrams at 298, 323, and 348 K show that the crystalline form of three salts was not affected by temperature from 298 to 348 K, when the temperature is 323 K, the crystalline are of MgB₄O₇ · 9H₂O is the largest than that of 298 and 348 K, the crystalline area of RbB₅O₈ · 4H₂O is the smallest, the crystalline are of Li₂B₄O₇ · 3H₂O is larger than that of 298 K, while smaller than that of 348 K.