Effects of Elementary-Minute Crystal Ratio of NaCl on Crystalite Growth Rate in Supersaturated Solution

Abstract

The objective of this study is to investigate the adequate crystallization conditions that are suitable for the acceleration of crystallite growth of sodium chloride (NaCl) owing to the production of elementary-minute crystals under specified conditions such as higher suspension density and supersaturation. A mixing-type evaporation crystallizer that is easy to atably manipulate the vaporization condition of water was chosen as the experimental apparatus. NaCl crystals were grown by means of semi-batch type reaction in which the saturated NaCl solution was continuously supplied into the brine solution (evaporated seawater) including a specified weight of seed and fine crystals, and relation between the initial weight ratio of elementary-minute crystals based on seed crytsls (ξ) and NaCl crystallite growth were examined. A separable-type flask (1L) was used as the crystallization vessel. A propeller-type mixing impeller with four blades was installed in the vessel, and the mixing rate was set at 300min^-1. The evaporated steam was condensed using the Liebig condenser and trapped within the beaker. The total salt concentration of brine (evaporated seawater) was 250g/kg and the ratio of NaCl to the total salt weight was set at 90wt%, and the evaporated solution (mother solution) was prepared at 4.7times concentration of brine. The vaporization rate of water was 0.011min^-1, and NaCl concentration was maintained at 175g/kg. Initial weight ratio of seed crystals ([ω_s]₀) was changed in the range from 9.1 to 23.1wt% by the addition of seed crystals that have an average size of 324μm. ξ was changed in the range of 0.002-0.094 by the addition of minute crystals (average particle size: 8μm). Furthermore, an antisolvent (Et-OH) was added into the crystallizer, and elementary-minute crystals were generated in the suspension. Time changes in the physicochemical properties of both solution (mother solution weight, concentration of NaCl) and solid products (total weight, shape observation by a digital microscope, size distribution, and identification by XRD) were measured. Experimental results indicated the following 1) the crystallite growth of NaCl was accelerated owing to the production of elementary-minute crystals at 9.1wt% in [ω_s]₀, 2) when minute crystals (average particle size: 8μm) were supplied into the crystallizer at [ω_s]₀ of 16.7wt%, no acceleration of the crystallite growth was observed regardless of ξ, 3) when antisolvent (Et-OH) were added into the crystallizer at [ω_s]₀ of 16.7wt%, average crystal growth rate showed a maximum value at 0.024 in ξ, 4) under these experimental conditions, the deterioration of crystal purity was not observed accompanying the acceleration of crystal growth rate.