Abstract
The crystallization rates of NaOHH·3.5H2O from a 39 wt.% sodium hydroxide aqueous solution were measured under various conditions. These serve to obtain some fundamental information on a process in which potassium is concentrated in the mother liquor by separation of crystal NaOHH·3.5H2O from a potassium-containing sodium hydroxide solution. A 39 wt.% sodium hydroxide solution was cooled to various temperatures, and then the crystallization was caused by introducing a very small crystal of NaOHH·3.5H2O as seed into the solution. The initial supercooling degree ranged from 2.45°C to 5.35°C. Some relationships such as crystallization rate, supercooling degree, amount of deposited crystal versus time were obtained.
The results showed that:
1. The crystallization rate increased in the early crystallization period, but decreased constantly after the maximum value.
The maximum enlarged with an increase in the initial supercooling degree.
2. A power of supercooling degree, n, in the equation expressing crystallization rate can be obtained by n=-(2/3) {ΔTmax. Vmax./Wvmax.(dΔT/dt) vmax.}
3. When the crystallization rate of NaOH·3.5H2O was not influenced by newly formed crystal nuclei, the rate made a linear change to the supercooling degree.
4. A supercooling degree, under which the amount of newly formed crystal nuclei was too little to affect the crystallization rate, was found to exist, and paralleled to about 60% of the initial supercooling degree.
5. The number of formed crystal particles increased exponentially with an enlargement of the supercooling degree, and showed a increase of 2.6 times per one degree centigrade of the initial supercooling degree.
6. The crystallization rate of NaOH·3.5H2O from a 39 wt.% sodium hydroxide aqueous solution was expressed by the following equation under the negligible formation of the crystal nuclei.
V= {γs/ (ρ·γv) 2/3} ·K·W2/3.N1/3·ΔT
