USE OF SOL-GEL SYSTEMS FOR SOLID LIQUID SEPARATION/

A unique approach using sol-gel technology is presented for separating and recovering particulates and colloids from caustic waste slurries. The approach involves the addition of an alkali silicate and an organ ic gelling agent directly to the waste stream to immobilize particulat es that range from macro sizes to submicron colloids. The particulates and colloids become trapped within a silica network that remains poro us during the early stages of the sol-gel process. The freshly gelled monolith undergoes a process of syneresis, whereby the water and solub le salts are ejected from the monolith as it contracts. The approach h as been illustrated by removal of ultrafine particulates from a Hanfor d Tank Waste simulant. Initial laboratory tests have shown that it is possible to produce silica monoliths in the presence of 4 M hydroxide. Analysis of the mother liquor produced during syneresis indicated qua ntitative recovery of the particulates within the monolith. The partit ioning of ions between the silica gel and the mother liquor during syn eresis correlates directly with the lyotropic series. Salt recoveries from the mother liquor in excess of 90% can be achieved. With a capabi lity of recovering >99.999% of all particulates, including colloids, t he process is more efficient than membrane filtration. This approach p roduces a rock-hard silica monolith that can be used directly as a fee dstock to a-glass melter or can be consolidated to near theoretical de nsity by sintering.