Precipitation and Deposition of Aluminum Containing Species in Tank Wastes

Aluminum-containing phases represent the most prevalent solids that can appear or disappear during the processing of radioactive tank wastes. Processes such as sludge washing and leaching are designed to dissolve Al-containing phases and, thereby, minimize the volume of high-level waste glass required to encapsulate radioactive sludges. Unfortunately, waste-processing steps that include evaporation can involve solutions that are supersaturated with respect to cementitious aluminosilicates that result in unwanted precipitation and scale formation. Of all the constituents of tank waste, limited solubility cementitious aluminosilicates have the greatest potential for clogging pipes and transfer lines, fouling highly radioactive components such as ion exchangers, and completely shutting down processing operations. For instance, deposit buildup and clogged drain lines experienced during the tank waste volume-reduction process at Savannah River Site (SRS) required an evaporator to b e shut down in October 1999. The Waste Processing Technology Section (WPTS) of Westinghouse Savannah River Company (WSRC) at SRS is now collaborating with team members from Pacific Northwest National Laboratory (PNNL) to verify the thermodynamic stability of aluminosilicate compounds under waste tank conditions in an attempt to solve the deposition and clogging problems. The primary goals of this study are to understand the (1) the major factors controlling precipitation, heterogeneous nucleation and growth phenomena, of relatively insoluble aluminosilicates, (2) role of organics for inhibiting aluminosilicate formation, and (3) to develop a predictive tool to control precipitation, scale formation, and cementation under tank waste processing conditions. The results obtained from this will provide crucial information for (1) avoiding problematical sludge processing steps, and (2) identifying and developing effective technologies to process retrieved sludges and supernatants before ultimate vitrification of wastes