Removal of Silicon from High-Level Waste Streams via Ferric Flocculation

Abstract:

The presence of silicate from glass-forming frit in the recycle waste from the Defense Waste Processing Facility produces wastes that, when combined with the traditional aluminate-bearing wastes stored in Savannah River Site’s tank farms, can produce insoluble sodium aluminosilicates. Currently, aluminum-bearing wastes and silicon-bearing wastes are processed in separate evaporators. This, however, limits operational flexibility. Therefore, treatment to remove silicon has been proposed to allow greater flexibility for processing these wastes in the site’s evaporators. The use of a ferric precipitation (flocculation) to remove the silicon has been tested using waste simulants.

Ferric precipitation following ferric nitrate addition to two different alkaline DWPF recycle waste simulants was effective at removing silicon to levels below concern for processing in any of the Savannah River Site’s evaporators. Removal of silicon was rapid with removal complete in 2h. Elevated temperatures were tested and found to be not required. Capacities of the resultant iron flocculent were ~0.1 g of silicon per gram of iron at a final silicon concentration of 50mg/L. Moreover, the silicon removal obeys a classical Freundlich adsorption isotherm.