Leaching Characteristics of Hanford Ferrocyanide Wastes

Abstract

A series of leach tests were performed on actual Hanford Site tank wastes in support of the Hanford Tank Waste Treatment and Immobilization Plant (WTP). The samples were targeted composite slurries of high-level tank waste materials representing major complex, radioactive, tank waste mixtures at the Hanford Site. Using a filtration/leaching apparatus, sample solids were concentrated, caustic leached, and washed under conditions representative of those planned for the Pretreatment Facility in the WTP. Caustic leaching was performed to assess the mobilization of aluminum (as gibbsite, Al[OH]3, and boehmite AlO[OH]), phosphates [PO4 3−], chromium [Cr3+], and, to a lesser extent, oxalates [C2O4 2−]). Ferrocyanide waste released solid phase 137Cs during caustic leaching; this was antithetical to the other Hanford waste types studied. Previous testing on ferrocyanide tank waste focused on the aging of the ferrocyanide salt complex and its thermal compatibilities with nitrites and nitrates. Few studies, however, examined cesium mobilization in the waste. Careful consideration should be given to the pretreatment of ferrocyanide wastes in light of this new observed behavior, given the fact that previous testing on simulants indicates a vastly different cesium mobility in this waste form. The discourse of this work will address the overall ferrocyanide leaching characteristics as well as the behavior of the 137Cs during leaching.

Speciation of aluminum phases at the Hanford Site

“…14 Since that time, samples have been retrieved from more than 80 of the tanks and subsequently characterized to provide information on the aluminum composition present. 6,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, many these tanks represent a mix of different waste types. The goal was to identify tanks that could reasonably represent the specific waste groupings.…”

Gibbsite Solubility in Hanford Nuclear Waste Approached from above and below Saturation

“…9−13 Boehmite is widespread in tanks at both Hanford and Savannah River. 10,11,14 precipitates at high temperatures, but both precipitation and dissolution kinetics are extremely slow below the highest current tank temperature of 45 °C. 15,16 Consequently, boehmite would not be expected to equilibrate with the liquid phase of the tanks over a reasonable time frame at Hanford.…”

“…Some solid phases containing aluminum that have been identified in the waste are gibbsite (Al­(OH) 3 ), boehmite (AlOOH), Dawsonite (NaAl­(OH) 2 CO 3 ), and sodium aluminosilicates. − Boehmite is widespread in tanks at both Hanford and Savannah River. ,, Boehmite dissolves and precipitates at high temperatures, but both precipitation and dissolution kinetics are extremely slow below the highest current tank temperature of 45 °C. , Consequently, boehmite would not be expected to equilibrate with the liquid phase of the tanks over a reasonable time frame at Hanford. Dawsonite has also been found in some tanks at Hanford, and may equilibrate rapidly with the waste, but it is not clear how widespread the phase is across all of the tanks at the Hanford and Savannah River sites.…”

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

“…NaOH will eventually be used to dissolve gibbsite and boehmite from the bulk tank waste at elevated temperatures. ,, Gibbsite dissolution, however, is reported to be slow at NaOH concentrations below 5 M at ambient temperatures. , It is hypothesized that reasonable rates can be achieved at ambient temperatures using highly concentrated NaOH(aq), from which sodium aluminate readily precipitates. − It is further hypothesized that gibbsite rapidly dissolves in highly concentrated NaOH(aq) and recrystallizes as sodium aluminate. − If the NaOH concentration were high enough, sodium aluminate might precipitate even as gibbsite was still dissolving through a metathesis reaction. The high water solubility of sodium aluminate suggests that the sodium aluminate solid could be easily redissolved in water after metathesis is complete. , …”

“…The second purpose is to show that this is also true of gibbsite from a real nuclear waste heel sample. Most previous works have dissolved gibbsite at much lower NaOH concentrations using temperature to accelerate this admittedly very slow reaction to practical time frames. ,− Given the obstacles in temperature control in >50-year-old buried waste tanks at Hanford, this study examines the possibility of using high NaOH concentration rather than temperature to achieve reasonable rates. Similarly, other researchers have crystallized NaAl(OH) 4 ·1.5H 2 O by evaporating sodium aluminate solutions. ,,− This is the first study that crystallizes NaAl(OH) 4 ·1.5H 2 O while simultaneously dissolving gibbsite in concentrated NaOH(aq) and demonstrates that this reaction happens over a reasonable period of time.…”