Antisolvent crystallization of NaCl using the minute-bubble technique – Effects of different antisolvent types

• NaCl crystallization is enhanced by local supersaturation around minute-bubbles.
• A decrease of the zeta potential is caused by accumulation of solute to gas-liquid interface.
• The minimizing effect of the NaCl size distribution with minute-bubble supply depends on alcohol type.
• The accumulation degree near minute-bubble surfaces might change by alcohol type.

To develop a crystallization technique that enables the control of the crystal size distribution, antisolvent crystallization of sodium chloride (NaCl) under a continuous supply of N2 minute-bubbles was performed. The effects of the additive volume ratio of ethanol (EtOH) on the molar yield and size distribution of the NaCl crystals and the effects of the antisolvent type on crystallization phenomena of NaCl were examined. The initial concentration of NaCl in the saturated solution was set at 5.54 mol/l, and EtOH was added as an antisolvent to the saturated NaCl solution, where the added volume ratio of EtOH was in the range of 5 to 50 vol% (as EtOH/NaCl system). As a comparison, the antisolvent crystallization phenomenon of NaCl in a MeOH/NaCl system was also investigated. N2 minute-bubbles with an average bubble size of 40 µm were continuously supplied to the NaCl supersaturated solution using a self-supporting bubble generator, and NaCl was crystallized. Consequently, the production enhancement and crystal size minimization of NaCl were caused by the residence of minute-bubbles because of the acceleration of nucleation and the inhibition of crystal coalescence. Moreover, the results indicated that the enhancement effect of NaCl crystal production and the minimizing effect of average crystal size depended on the additive volume and the type of alcohol as antisolvent.