Utilization of minute-bubble technique on crystallization: Application to antisolvent crystallization of sodium chloride

A crystallization technique to accelerate the nucleation of NaCl crystals utilizing local supersaturation regions around the gas-liquid interfaces of nitrogen (N2) minute-bubbles was developed. The effects of N2 bubble supply and minimizing bubble diameter on antisolvent crystallization phenomena of NaCl were examined. The initial concentration of NaCl in the saturated solution was set at 5.50 mol/l, and ethanol (EtOH) was added as an antisolvent to the saturated NaCl solution where the added volume ratio of EtOH became 10 vol%. N2 minute-bubbles with an average bubble diameter (dbbl) of 40 μm were continuously supplied to the suspended solution using a self-supporting bubble generator, and NaCl was crystallized. Furthermore, dbbl was varied in the range of 100–2000 μm by changing the hole size (5–120 μm) on the gas dispersion plate using a dispersing-type bubble generator. Consequently, the average size of NaCl crystals while supplying N2 minute-bubbles was about 7 μm at 1 min crystallization and almost same profile in size distribution was obtained independent of the crystallization time. When minute-bubbles were not supplied, the average size was shifted up to 13 μm and the size distribution width was expanded in comparison with that obtained under supplying N2 minute-bubbles. The supply of minute-bubbles at a dbbl of 40 μm into antisolvent crystallization of NaCl is effective in obtaining fine crystals with narrow distribution width.

► We studied antisolvent crystallization of NaCl utilizing N2 minute-bubble supply.
► Effect of temperature rising by ethanol addition on NaCl crystallization is small.
► Production rate of crystals is increased gradually with the lowering bubble diameter.
► Nucleation of NaCl crystals accelerates with the lowering bubble diameter.