Studies of Sedimentation Rates of Non-Transition Alkaline- Earth Metal Carbonates in Aqueous Medium

Abstract

Rate data of sedimentation are useful in designing sedimentation equipment and in modeling operations such as filtration, fluidization, two-phase flow operations, and in environmental engineering. This study was thus embarked on to obtain empirical data on the gravitational sedimentation rates of magnesium and calcium carbonates (non-transition alkaline-earth metal carbonates) by determiningthe effect of reactant concentration and size of settling vessel of these carbonates, in chemical precipitation,by a modified approach of the 'isolation method'.The results showed that the sedimentation behaviour varied with the ionic concentrations of the chemical species involved in precipitation. Initial settling characteristics also showed that the nature of the precipitates and the medium in which the precipitation occurred affected the sedimentation rates of the metal carbonates.When the metal ion concentration was varied,-0.87 and -1.1 were obtained as sedimentation orders and 9.92 x 10 -4 cm s -1 M -1 and 6.82 x 10 -4 cm s -1 M -1 as sedimentation rate constants for magnesium carbonate and calcium carbonate respectively. On the other hand,when the carbonate ion concentration was varied, -0.79 and -1.8 were obtained as sedimentation orders and 9.87 x 10 -4 cm s -1 M -1 and 6.92 x 10 -5 cm s -1 M -1 as sedimentation rate constants for magnesium carbonate and calcium carbonate respectively.Sedimentation rate constants of MgCO3 and CaCO3 also showed close dependence on their densities (r 2 = 0.9963). The results of the sedimentation rates measured in cylinders of varying sizes showed that the sedimentation rate of the precipitates correlated positively with the internal diameter of the cylinders.