The controlled growth of calcium sulfate dihydrate (gypsum) in aqueous solution using the inhibition effect of a bubble column evaporator

• Novel approach to control precipitation using a bubble column evaporator (BCE).
• Inhibition of precipitation observed within a BCE compared to normal stirring.
• Characterization of CaSO4 precipitates deposited onto 23 nm silica spheres in a BCE.
• The BCE process may be used for the production of fine particles of controlled size.

Chemical precipitation is a widely used method to synthesize sparingly soluble salts for inorganic materials production. A bubble column evaporator (BCE) process was used to precipitate calcium sulfate (as CaSO4·2H2O) from the reaction of calcium chloride and potassium sulfate solutions, at different degrees of supersaturation. In the BCE system, fine bubbles are continuously produced as a warm dry gas is pumped into a column of solution through a porous sinter. This process was used here as the basis for a new approach to control precipitation with uniform mixing and continuous concentration through rapid water vapor transfer. In this study it was found that the BCE process, compared to simple stirring and even the quiescent solution, had a significant inhibition effect on the induction of precipitation and reduced the growth rate of the precipitate. This precipitation control facilitates the collection of particles over a wide range from nanometer to micrometer size. In the BCE process, the early stage of CaSO4 precipitated onto the surface of nano-sized silica spheres was characterized using dynamic light scattering, zeta potential analysis and atomic force microscopy imaging, which indicated that a coating was produced on the silica surface. The inhibition effect observed with the BCE process suggests that it could be used for controlled precipitate growth and also for the de-watering or concentration of industrial wastewater prior to disposal.

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