CRYSTAL-GROWTH AND AGGLOMERATION OF CALCIUM SULFITE HEMIHYDRATE CRYSTALS

Crystal growth and agglomeration of calcium sulfite hemihydrate crysta ls from solution were studied by reacting Ca(OH)2 with NaHSO3 in a pH- stat semibatch crystallizer. Single platelet crystals and agglomerates of platelet crystals were produced in the pH range from 5.80 to 6.80. The crystallization mechanism changed from primary nucleation to crys tal growth in the progressive precipitation. Using the titration curve s, the growth rate was calculated from the titration rate at the final stage of operation. The crystal growth rates of calcium sulfite hemih ydrate crystals were found to obey the parabolic rate law in the low s upersaturation range. Another point to be noted is that the precipitat es of calcium sulfite hemihydrate in agitated suspensions have a tende ncy to form agglomerates. It was found that the degree of agglomeratio n is a weak function of relative supersaturation and magma density, wh ile the pH value is a key factor that affects the degree of agglomerat ion. Addition of EDTA also has an effect on the agglomeration of calci um sulfite hemihydrates.