Influence of Cooling Rate on Crystallization of Borax in Stirred Batch Crystallizer

Abstract

The aim of this work was to determine the influence of cooling rate on the nucleation and crystal growth of borax decahydrate. All experiments were carried out in baffled stirred crystallizer with the volume of 2.14 dm3. Mother liquor saturated at 30 °C was cooled down at 3 different cooling rates in the range from the 4 to 9 °C/h. In order to analyze and compare the influence of fluid flow pattern in the crystallizer on product properties, examinations were performed using two different impeller types; radial straight blade turbine (SBT) and axial pitched blade turbine (PBT). Mixing with both of impellers was carried out at constant impeller speed of 300 rpm. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over process time were determined by optical microscope and image analysis software. The nucleation and crystal growth behaviors for borax–water system in a batch cooling crystallizer were estimated according to Mersmann’s criteria. From the results obtained, it was found that cooling rate has great impact on nucleation and crystal growth kinetics. Moreover, in all system investigated nucleation was heterogeneous and crystal growth integration limited, its rates differed with cooling rates. The changes of those parameters with cooling rates in the systems with radial and axial impeller were similar. Nucleation and crystal growth reflected on the crystal size distribution as well. Increasing cooling rate crystal mean size decreases while crystal agglomeration increases, indicating the importance of cooling rate of batch crystallizer in the production of the crystals of desired properties.