Effect of dyes and surfactants on the dynamic behaviour of liquid—liquid gravity settlers

Abstract

The dynamic response of the steady-state dispersion height in gravity settlers due to the pulse injection of aqueous solutions of dyes and surfactants into the mixer or settler inlet is experimentally investigated. Positive changes in dispersion height are observed when an anionic or nonionic surfactant solution is injected at the settler inlet. Ampholytic surfactants produce a small negative change and cationic surfactants have more complicated effects, first producing a positive change followed by a negative change, probably related to the decrease in interfacial tension with concentration combined with the variation in interfacial tension gradients within the dispersion as the surfactant passes through. In all cases an increase in surfactant concentration decreases the interfacial tension but does not significantly affect the pH. When dyes are injected, positive and negative changes in dispersion height are observed depending on whether the pH increased or decreased; the dispersion height returning to its original value after a short time when the injected dye has passed through the system. The magnitude of the effect depends on the concentration and is lower when the dye is injected at the settler inlet rather than in the mixer. By adjusting the concentration of mixtures of dyes producing positive or negative changes it is possible for the dispersion height to be unaffected by the injection. Dyes have a pronounced effect on the pH but only a small effect on the interfacial tension. This is also true for mixtures of dyes and surfactants. Batch experiments indicate that reducing the pH so the solution is acidic increases the rates of both binary and interfacial coalescence whereas increasing the pH, so the solution is alkaline, decreases the interfacial coalescence rate but has no effect on the binary coalescence. These observations substantiate the effect of acidic and alkaline dyes on the dynamic behaviour of continuous dispersions.