Hydrolysis and Coagulation of Aluminum Salts in the Initial Stage of Mixing of Solutions

The turbulent mixing of a solution of a coagulant with water is studied in the initial process step, when, in the bulk of the system, regions of different liquids can still be individualized. The mixing is accompanied by the hydrolysis of metal salt and coagulation of the hydrolysis product (mixing is considered within a small vicinity of the interface between the liquids being mixed). It is shown that, in the course of time, a locking layer is formed, which prevents the diffusion transfer of the component. This layer exists throughout the initial process step and decomposes only at the end of this step. A model of aluminum salt hydrolysis is constructed, on the basis of which analytical expressions are obtained for pH and the concentrations of various hydrolysis products as functions of the concentration of the initial reactant solution, the water alkalinity, and the equilibrium constants for the hydrolysis reactions. The coagulation kinetics is analyzed. The limiting sizes of aggregates as functions of the properties of the liquids being mixed and the process parameters are determined. It is shown that, once the limiting sizes of aggregates are attained, the coagulation is suspended until the end of the initial mixing step; a new coagulation step then begins, in which the role of coagulation nuclei is played by the previously formed aggregates.