Recovery of metal cations from lime softening sludge using Donnan dialysis

Advances in the application of ion exchange membrane processes for material separation provide new approaches for improving the sustainability of industrial operations. In this work, we report on the application of Donnan dialysis to recover metal cations from lime softening sludge, a byproduct of drinking water production. A cation exchange membrane, which preferentially allows the transport of cations, was used in the Donnan cell. Slurries containing lime softening sludge, gypsum, or calcite were placed on the feed side of the cell and an acidic solution was placed on the sweep side. A concentration gradient between the two sides of the cell extracts metal cations from the feed in exchange for hydrogen ions in the sweep side solution. The influence of acid type, starting pH, and time were evaluated. Although the limited solubility of calcite inhibited its study, significant calcium ion recovery was measured from the feed solutions with gypsum. The results were consistent with theoretical expectations and demonstrated that the pH of the sweep side solution controlled the extent and rate of cation recovery. For the treatment of the lime softening sludge, over 99% of the hydrogen ions were exchanged in the recovery of up to 20% of the calcium and 50% of the magnesium from the lime softening sludge. Over time, depletion of the sweep-side hydrogen ions resulted in the preferential recovery of magnesium from the softening sludge. Our results suggest the process exhibits promise in treating the sludge by recovering metal cations for reuse.