Conversion of Coarse Gibbsite Remaining in Hanford Nuclear Waste Tank Heels to Solid Sodium Aluminate [NaAl(OH)4·1.5H2O]

The Hanford Site near Richland, WA, has 177 high-level nuclear waste tanks containing 56 million gallons of waste derived from nuclear fuel reprocessing. The site is removing waste from old single-shell tanks, but some gibbsite [γ-Al(OH) 3 ] has been left behind by the current retrieval technology and would remain in the tanks when grouted on site, if not removed first. This study develops a new process to remove this gibbsite by converting it into water-soluble NaAl(OH) 4 ·1.5H 2 O in highly concentrated NaOH(aq). Gibbsite was incubated in NaOH(aq) solutions of 8, 11, or 19.4 M concentration at 25 or 50 °C for up to 312 h. Inductively coupled plasma atomic emission spectroscopy and polarized light microscopy were used to monitor the reaction. Gibbsite rapidly converted into NaAl(OH) 4 ·1.5H 2 O at all NaOH concentrations and temperatures for nonradioactive and real waste gibbsite samples. NaAl(OH) 4 ·1.5H 2 O initially crystallized with a needle-shaped morphology, but subsequently transformed into a rectangular plate. These results demonstrate that gibbsite can be readily converted into NaAl(OH) 4 ·1.5H 2 O in concentrated NaOH at ambient temperatures, suggesting that this is a promising technology for removing gibbsite from high-level waste tank heels.