Solid–Liquid Phase Equilibrium in the Quaternary System KCl + KBO₂ + K₂SO₄ + H₂O at 298.15 K

Abstract

Solubilities, densities and refractive indices for the quaternary system KCl + KBO₂ + K₂SO₄ + H₂O at 298.15 K were determined using the isothermal dissolution equilibrium method. According to the experimental data, the diagrams of solubilities, densities, and refractive indices were plotted, respectively. The phase diagram consist of one invariant point, three isothermal dissolution curves, and three crystallization regions corresponding to potassium chloride (KCl), potassium metaborate hydrate (KBO₂⋅1.33H₂O), and potassium sulfate (K₂SO₄), respectively. The size of crystallization areas of salt is in the order K₂SO₄ > KCl > KBO₂⋅1.33H₂O, which indicates K₂SO₄ could be more easily separated from the system. The diagrams of solution density and refractive index changed regularly with changing concentration of KCl at 298.15 K.

INTRODUCTION

Potassium and boron are important chemical raw materials, as they are widely used in fertilizer, pigment, pharmaceutical, paper, detergent, glass, and ceramics industries [1]. As the available amounts of solid ores are decreasing, the development of liquid potassium and boron mineral resources has become an inevitable tendency in future. Liquid potassium and boron mineral resources are mainly distributed in salt lake brine, seawater, geothermal water, and oil-field water. There are abundant salt lake brine resources in the Qaidam Basin, China with high contents of lithium, potassium, boron, the valuable brine mostly belong to the MgSO₄ subtype brine as a seven-component system (Li⁺ + Na⁺ + K⁺ + Mg²⁺ + Cl^- + ${\text{SO}}_{4}^{{2 - }}$ + borate + H₂O), which is of great exploitation potential and strategic importance [2, 3].

It is well-known that phase diagrams and phase equilibria play an important role in guiding the comprehensive utilization of the valuable brine. To take full advantage of the brine resources, the phase equilibria of some subsystems containing potassium borate have been researched, such as (KBO₂ + H₂O) [4], (KBO₂ + KOH + H₂O) [4, 5], (LiBO₂ + NaBO₂/KBO₂ + H₂O) [6], (KCl/K₂SO₄ + KB₅O₈ + H₂O) [7], (LiB₅O₈/NaB₅O₈ + KB₅O₈ + H₂O) [8], (K₂O + CaO + B₂O₃ + H₂O) [9]. Although the phase equilibrium and phase diagrams of potassium borate-containing systems over a wide temperature range have been previously reported, research has mainly focused on ternary systems, the solubility data of more complex systems is seriously insufficient. The quarternary system (KCl + KBO₂ + K₂SO₄ + H₂O) is actually part of (K₂O + B₂O₃ + K₂SO₄ + KCl + H₂O) with fixed K₂O/B₂O₃ ratio, which has significant value for separating and purifying potassium and boron salts from the brines. In this paper, the solubilities, densities, and refractive indices of the quaternary systerm (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K are presented.

EXPERIMENTAL

Apparatus and Reagents

A thermostat with magnetic stirring (XHC-500-6, Beijing Fortunejoy Sci. Technol. Co. Ltd.) was used to maintain the temperature with a precision of 0.01 K in the equilibrium experiments. The refractive index (n_D) was measured using an Abbe refractometer (Abbemat 550, Anton Paar Co. Ltd., Austria) with an uncertainty within ±0.0001. Density (ρ) was determined using Anton Paar Digital vibrating-tube densimeter (DMA 4500, Anton Paar Co. Ltd., Austria) with an uncertainty of less than ±0.15 mg cm⁻³.

The reagents used in this study were of analytical grade and obtained from the Sinopharm Chemical Reagent Co. Ltd.: KCl (mass fraction >99.0) and K₂SO₄ (mass fraction >99.0). The potassium metaborate were synthesized in the lab [10], and the purity is 99.0 in mass fraction. Doubly deionized water (DDW, k < 1 × 10^–4 S m^–1, pH 6.60) were employed to prepare the series of samples and for chemical analysis.

Method

The isothermal dissolution equilibrium method was adopted in this work [11, 12]. On the basis of the phase equilibrium composition, salts and water of appropriate quantity were mixed together and loaded into a series of equilibrium bottles, and then placed in the magnetic stirring thermostatic bath and the temperature was set at set at 298.15 ± 0.1 K with 150 rpm to accelerate the achievement of equilibrium. At intervals, a 5.0 mL sample of the clarified solution was taken out for chemical analysis. When the difference between the concentrations of the two samples taken from the same sample was within ±0.3% in mass fraction, it indicated that the equilibrium had been achieved. Generally, it took approximately 14 days to reach the equilibrium state for the two systems. Then, liquid supernatant was taken out for quantitative chemical analysis, and the solid phases were also sampled and identified by X-ray diffractometry.

The ${\text{BO}}_{2}^{ - }$ ion concentration was measured in triplicate by alkali titration method with sodium hydroxide standard solution in the presence of mannitol and double indicator of methyl red and phenolphthalein and the relative standard uncertainty of the analytical results is no more than ±0.003. The Cl^‒ ion was determined by volumetric titration with AgNO₃ standard solution in the presence of potassium chromate as indicator with relative standard uncertainty of no more than ±0.003. The K⁺ ion concentration was determined in triplicate by the gravimetric method with sodium tetraphenylborate. The ${\text{SO}}_{4}^{{2 - }}$ ion was determined via gravimetric method with barium chloride; the relative standard uncertainty for the analytical results in triplicate was within ±0.005 in mass fraction [13].

RESULTS AND DISCUSSION

Phase Equilibrium of Quaternary System at 298.15 K

The experimental data on the solubilities, densities, and refractive indices of the quaternary system (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K are presented in Table 1. There are three boundary subternary systems in this quaternary system, which are (KCl + KBO₂ + H₂O), (KBO₂ + K₂SO₄ + H₂O), and (KCl + K₂SO₄ + H₂O). A series of studies have been carried out on the stable phase equilibrium of these three ternary subsystems at different temperatures and relevant experimental data and phase diagrams have been obtained [7, 14, 15]. Solubilities of the invariant points for the three subternary systems are shown in Table 2. Among them, the ternary systems (KCl + KBO₂ + H₂O) and (KBO₂ + K₂SO₄ + H₂O) have not solubility data at 298.15 K in the literature; therefore, the solubilities of ternary system containing K₂B₄O₇ and KB₅O₈ were listed for comparison in Table 2. The solid phases of potassium borate were showed K₂B₄O₇⋅4H₂O and KB₅O₈⋅4H₂O, and the solubility of K₂B₄O₇ and KB₅O₈ is much less than KBO₂ [7, 14]. For the (KCl + K₂SO₄ + H₂O) system, the solid phases and solubilities in the invariant point obtained in this work are in accordance with the literature [15].

Table 1.   Solubilities of the quaternary system KCl + KBO₂ + K₂SO₄ + H₂O at 298.15 K and 0.1 MPa

Table 2.   Comparison of the solubility data in this work and the literature at the invariant points of the boundary subternary systems at 298.15 K

In Table 1, w is the mass fraction, J(b) is the Jänecke index values of the component b, with J(KCl) + J(K₂SO₄) + J(KBO₂) = 100. The calculating formula for J(b) is

$$J{\text{(}}b{\text{) }} = \frac{{w{\text{(}}b{\text{)}}}}{{w{\text{(KCl)}} + w{\text{(}}{{{\text{K}}}_{{\text{2}}}}{\text{S}}{{{\text{O}}}_{{\text{4}}}}{\text{)}} + w{\text{(KB}}{{{\text{O}}}_{{\text{2}}}}{\text{)}}}} \times 100.$$

(1)

The calculating formula for water concentration is

$$J{\text{(}}{{{\text{H}}}_{{\text{2}}}}{\text{O)}} = \frac{{w({{{\text{H}}}_{{\text{2}}}}{\text{O}})}}{{w{\text{(KCl)}} + w{\text{(}}{{{\text{K}}}_{{\text{2}}}}{\text{S}}{{{\text{O}}}_{{\text{4}}}}{\text{)}} + w{\text{(KB}}{{{\text{O}}}_{{\text{2}}}}{\text{)}}}} \times 100.$$

(2)

According to the experimental data of the Jänecke index J(b), the dry-salt phase diagram of this system at 298.15 K were plotted with solid line in Fig. 1. In Fig. 1, the stable phase diagram of the quaternary system at 298.15 K consists of one invariant points, three isothermal dissolution equilibrium curves and three crystalline fields corresponding to potassium chloride (KCl), potassium metaborate hydrate (KBO₂⋅1.33H₂O), potassium sulfate (K₂SO₄). The crystallization field decreases in order of K₂SO₄ > KCl > KBO₂⋅1.33H₂O, and the solubility of the corresponding salt increases sequentially. The five isothermal dissolution equilibrium curves correspond to AE (KBO₂⋅1.33H₂O + KCl), BE (K₂SO₄ + KCl), and CE (KBO₂⋅1.33H₂O + K₂SO₄), respectively. The invariant point E is composed of liquid + KCl + K₂SO₄ + KBO₂⋅1.33H₂O, with w(KCl) = 5.00, w(K₂SO₄) = 0.11, w(KBO₂) = 39.03.

Fig. 1.

Phase diagram of the quaternary system KCl + KBO₂ + K₂SO₄ + H₂O at 298.15 K and 0.1 MPa; (points) liquid phase; (lines) isotherm curve; E, invariant point; A, co-saturated point of (KCl + KBO₂ + H₂O); B, co-saturated point of (KCl + K₂SO₄ + H₂O); C, co-saturated point of (KBO₂ + K₂SO₄ + H₂O).

In order to fully reflect the existence of a certain point in the system, the diagram of the water-content vs composition was plotted in Fig. 2 according to its dry salt composition. It can be seen that the water content of quaternary system (KCl + KBO₂ + K₂SO₄ + H₂O) changes regularly with the change of KCl concentration at 298.15 K, and the water content maximizes at point E. Regarding isothermal dissolution equilibrium curve CE, the value of J(H₂O) decreases from 140.27 to 126.55 with the increase of J(KCl), and increases gradually on the curve EB.

Fig. 2.

Water-phase diagram of the quaternary system KCl + KBO₂ + K₂SO₄ + H₂O at 298.15 K and 0.1 MPa; (points) liquid phase; (lines) isotherm curve; E, invariant point; A, co-saturated point of (KCl + KBO₂ + H₂O); B, co-saturated point of (KCl + K₂SO₄ + H₂O); C, co-saturated point of (KBO₂ + K₂SO₄ + H₂O).

The solid phase was identified by X-ray diffraction (XRD) as shown in Fig. 3. The obtained X-ray diffraction pattern matches well with the standard diffraction pattern. It shows that salts KBO₂⋅1.33H₂O, K₂SO₄, and KCl coexist in this quaternary systerm (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K.

Fig. 3.

X-ray diffractive diagram of the solid phase in (KCl + KBO₂ + K₂SO₄ + H₂O) system; XRD diagram of (a) KBO₂⋅1.33H₂O; (b) KBO₂⋅1.33H₂O + K₂SO₄ + KCl.

The solid phase of potassium borate in (KCl + K₂B₄O₇ + H₂O) and (K₂SO₄ + K₂B₄O₇ + H₂O) systems was showed K₂B₄O₇⋅4H₂O, and the solubility of K₂B₄O₇ is much less than KBO₂.

The phase equilibria of the quaternary system (KCl + K₂B₄O₇ + K₂SO₄ + H₂O) at 298.15 K has been reported [14]. For comparison purposes, it is shown in Fig. 4 along with data of (KCl + K₂SO₄ + KBO₂ + H₂O) system measured at 298.15 K. The diagrams show that there are remarkable differences in the reported phase field sizes and solid phase types. (1) There are three crystallization phase region present in (KCl + K₂B₄O₇ + K₂SO₄ + H₂O) system: KCl, K₂SO₄⋅H₂O, and K₂B₄O₇⋅4H₂O; (2) potassium borate of the formula K₂B₄O₇⋅4H₂O is formed in (KCl + K₂SO₄ + K₂B₄O₇ + H₂O) system, while KBO₂⋅1.33H₂O generated in (KCl + K₂SO₄ + KBO₂ + H₂O) system, and the solubility of K₂B₄O₇ is much less than KBO₂; (3) the area of crystallization region of KCl and K₂SO₄⋅H₂O decreased, while the K₂B₄O₇⋅4H₂O increased obviously in (KCl + K₂SO₄ + K₂B₄O₇ + H₂O) system.

Fig. 4.

A comparison of phase diagram for the quaternary system: (1) (KCl + KBO₂ + K₂SO₄ + H₂O) with (2) (KCl + K₂SO₄ + K₂B₄O₇ + H₂O) at 298.15 K [14].

Based on the experimental data in Table 1, the diagrams for solution densities and refractive indices versus composition in the quaternary system (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K were drawn and are shown in Fig. 5. It was found that the solution densities and refractive indices changed regularly with the increasing of KCl concentration, which first increased slightly with the increasing concentration of KCl in the solubility isotherm curves of AE and CE, and reached the maximum value of 1.55085 g cm^–3 and 1.4264 at the invariant point E, and then decreased sharply with the increasing concentration of KCl in the solubility isotherm curve of EB.

Fig. 5.

Diagram of solution densities and refractive indices in the ternary system (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K; (a) density vs. composition; (b) refractive index vs. composition.

CONCLUSIONS

Solid–liquid phase equilibria for the quaternary system of (KCl + KBO₂ + K₂SO₄ + H₂O) at 298.15 K were studied using the isothermal dissolution equilibrium method, and the solubilities, densities, refractive indices were obtained for the first time. There are one invariant point, three univariant solubility curves and three crystallization regions corresponding to KCl, KBO₂⋅1.33H₂O, K₂SO₄, respectively. The size of crystallization areas of salt is in the order K₂SO₄ > KCl > KBO₂⋅1.33H₂O, which indicates K₂SO₄ could be more easily separated from the system. There are remarkable differences in the reported phase field sizes and solid phase types in comparison with the (KCl + K₂B₄O₇ + K₂SO₄ + H₂O) system, and the solubility of K₂B₄O₇ is much less than KBO₂. This phase diagram can be used to guide the separating process of KBO₂, KCl, K₂SO₄ from the high concentration potassium-containing salt lake brine.