Amonyum pentaboratın kristalizasyon kinetiği / Crystallization kinetics of ammonium pentaborate

Abstract

Growth rate of ammonium pentaborate octahydrate has been measured both in pure sate and in the presence of sodiumchloride, borax and boric asid impurities as a fiinction of supersaturation. The effect of various particlesizes and different temperature range of 20 °C to 50 °C have been investigated on the crystals growth rate of ammonium pentaborateoctahydrate crystals in a laboratory-scale fluidized bed crystallizer. The values of mass transfer coefficient K, reaction rate constant, kr and reaction order, r were determined from the general mass transfer equation without assumption using an improved computer program which gives a high accuracy by using the date obtained both in pure state and presence of impurities. It was found that increasing the temperature and particle size cause increase the values of the kinetic parameters (Kg, kr, K ). Overall growth rate of ammonium pentaborate octahydrate is controlled by both diffusion and reaction steps at the temperature lower than 40 °C, whereas it is completely controlled by diffusion at the temperature higher than 40 °C. The presence of impurities in APBO solution increase the growth rate with respect to in pure state. It was found that the presence of sodium chloride, borax and boric acid decrease the reaction rate constant, kr while it increase the mass transfer coefficient, kd of APBO crystals. In pure aqueous solution, the crystal growth rate of APBO is mainly controlled by diffusion. However, both diffusion and integration steps affect the growth rate of APBO crystals in presence sodium chloride, borax and boric acid. KEY WORDS: Ammonium pentaborate, Mass transfer, Borates, Seed Crystals,