(223e) 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.