(213b) Behavior of Mercury During Chemical Processing In the Defense Waste Processing Facility

The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies High Level Waste (HLW) for disposal. DWPF has been processing radioactive waste since March 1996 to produce over 3,000 canisters filled with HLW glass. Sludge waste consists of insoluble metal hydroxides (primarily iron, aluminum, calcium, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate).

The pretreatment process in the Chemical Processing Cell (CPC) Sludge Receipt and Adjustment Tank (SRAT) adds nitric and formic acids to the sludge to lower pH, destroy nitrite and carbonate, and chemically reduce mercury and manganese. Acidification of the sludge also allows higher solids concentrations to be processed by lowering the yield stress of the sludge. During these processes, hydrogen can be produced by noble metal catalyzed decomposition of excess formic acid. The ratio of nitric to formic acid is adjusted to target a glass pool with 20% of the iron having a valence of Fe²⁺.

The feed to the SRAT contains mercury as mercuric oxide (HgO). The reduction by formic acid converts the HgO to elemental mercury which is subsequently stripped off by boiling. The maximum concentration of mercury in allowed in the product is about 0.8 wt% of the total solids to reduce deposition of mercury compounds in the melter offgas system. The theoretical stripping ratio, based on vapor pressures, is 1g mercury evaporated per 250g water evaporated; early studies showed that the actual ratio was about 1:750. Recent experience in DWPF has shown that the ratio is often much greater than 1:750 resulting in extremely long stripping times. Also, recovery of mercury in the offgas system in a decanter has been poor.

Laboratory studies were performed to improve the understanding of the mechanisms of mercury reduction, stripping, and recovery. The effects of other redox species, chlorides, mercury drop size, and slurry concentration on reduction and stripping were examined. Recovery of mercury was studied to determine the effect of non-condensable gases, and other species condensed with mercury that may chemically react or inhibit condensation and coalescence.