CRYSTAL BREAK-UP IN DILUTE TURBULENTLY AGITATED SUSPENSIONS

An analysis of particle attrition in dilute stirred suspensions is developed to predict both the impact attrition rate as a consequence of crystal collisions and the turbulent attrition rate caused by the liquid motion. Detailed attrition experiments designed for model discrimination were carried out in a 1.51 vessel fitted with four wall baffles and agitated by means of stainless steel and silicone rubber (RTV) coated turbines, respectively, for dilute suspensions ( < 3.2% v/v) in the unit power input range 0.6-1.5 W/kg and parent particle size range 100-1000 mum, using potassium sulphate and potash alum crystals in saturated ethanol solutions respectively. The experimental results are consistent with predictions of attrition fragments arising from crystal-impeller collisions, with a strong effect of impeller hardness on the average attrition rate, together with turbulent fluid drag-induced particle attrition. It is also predicted, however, that the total attrition rate decreases on vessel scale-up at constant power input, with an increasing proportion of fine particles being due to turbulence, arising from the independence of turbulent forces of the vessel scale and a decrease in the corresponding collisional impact forces.