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.
See more of this Group/Topical: Process Development Division