Digital Phase Diagram of K2SO4-KOH-H2O and Crystal Growth of K2SO4 in the Strong Alkali Solution

Potassium sulfate has always been an important and superior potash fertilizer. In this paper, we investigate the crystallization process of potassium sulfate from the strong alkali solution that is involved in the clean production process of raw alunite mineral. The research focuses on the digital phase diagram for a ternary system of K₂SO₄-KOH-H₂O and the crystal growth of K₂SO₄ in the strong alkali solution.

Firstly, the phase equilibrium data for the ternary system of K₂SO₄-KOH-H₂O are measured from 40°C to 80°C, where the contents of potassium and sulfur in the aqueous solutions are detected by ICP¨COES and the composition of solid phases are tested by X-ray diffraction, respectively. Based on the measured phase equilibrium data, origin is used to present the 3D colormap and ternary contour drawings, as shown in Figure 1. With the interpolation and fitting results obtained from the software, the saturation temperature of a given K₂SO₄-KOH-H₂O content can be predicted from the smooth surface of the ternary phase diagram. It is found that the K₂SO₄ solubility decreases with the increase of the KOH concentration in the aqueous solution, and the K₂SO₄ solubility becomes smaller in the strong alkali solution which is beneficial to separate K₂SO₄ from the KOH solution.

 

Figure 1. Solubility of K₂SO₄ in KOH solution and digital phase diagram of K₂SO₄-KOH-H₂O at different temperatures

Figure 2. Crystal growth of K₂SO₄ in the strong KOH solution measured using a reflection microscopic technique at 313K

The supersolubility and metastable zone width of K₂SO₄ in the concentrated KOH solution are investigated by focused beam reflectance measurement (FBRM) and ultrasonic sensor online monitoring for different operating conditions, which will provide the important information for the design and optimization of K₂SO₄ crystallization process. For the system of K₂SO₄-KOH-H₂O, the FBRM and the ultrasonic sensor can provide a high sensitivity for nucleation detection, which detects changes related to the amount of grown nuclei and changes in solution concentration, respectively. Furthermore, the crystal growth rates of K₂SO₄ in the strong KOH solution are measured using a reflection microscopic technique, where the saturated solution is poured into a thermo-controlled microscopic cell with a volume of 5 mL and a diameter of 3.5cm. Experiments are conducted by decreasing the solution to different temperatures to create different supersaturation levels, and then the crystal growth rates of K₂SO₄ are calculated by measuring the changes in the dimensions of the crystals with the analysis Image Processing Program (Olympus). Figure 2 shows the crystal growth process of potassium sulfate in the strong KOH solution measured under the microscope at 313K. The crystal shape of K₂SO₄ is a typical rectangular shape, which is convenient for easy identification and measurements of specific crystallographic faces. The influence of impurities from the alkaline solution on the crystal shape will be investigated using the reflection microscopic technique. Moreover, the two-dimensional population balance equation is proposed for the calculation of the crystal growth rate of K₂SO₄ with an acceptable accuracy. From this study of crystal growth, a given content of solution can be designed with a desired growth mechanism in order to effectively separate K₂SO₄ from the strong KOH solution by crystallization.