(18l) Refinement of the Alkaline/Oxidative Leaching Process to Separate Chromium and Aluminum from Hanford High Level Nuclear Tank Waste

The Hanford Site Tank Farms
contain approximately 53 million gallons of radioactive solid and liquid waste
currently stored in a combination of 177 single and double-shelled tanks.  The
Office of River Protection (ORP) was established in the late 1990's to assist
in completing the DOE mission of site closure by providing management of the
immobilization, treatment, and final dispose of the high level waste (HLW) and
hazardous wastes.   The Waste Treatment Plant (WTP) is under construction at Hanford in order to pretreat and vitrify (i.e., immobilize/solidify) HLW for long-term
safe storage, transportation, and final disposal in a federal repository. 
Borosilicate waste glass has been chosen as the immobilization technology for
HLW processing.  The current Hanford WTP baseline design for HLW vitrification
is a joule-heated ceramic melter (JHM).  The baseline approach will result in
approximately 9,200 canisters of borosilicate HLW glass that will be stored
onsite for later transport and disposal at the Yucca Mountain repository.

One of the challenges in processing
the Hanford Waste is the undesirable effect of the chromium (Cr) content on the
joule heated melters and the HLW glass properties.  Cr loaded into glass at a
level greater than approximately 1.4 weight percent precipitates as spinels ([Fe,Ni,Mn][Cr,Fe]₂O₄)
and eskolaite (Cr₂O₃) crystals in the glass melt during
the vitrification process, These particles will potentially clog and/or short
circuit the joule heated melters and also destabilize the structure of the
glass, thus affecting the long term stability of the vitrified waste. 

The WTP pretreatment facility
is currently designed to separate waste into fractions of high level waste
(HLW) and low activity waste (LAW).  In the present design, Cr will be leached
out of the HLW vitrification facility feed stream into the LAW vitrification
feed stream through an 8 hour 0.1 M alkaline leach at 85 °C followed by a 6
hour permanganate (MnO₄^-) leach at 25 °C.  This process
has been observed to effectively mobilize Cr(III) as chromate (CrO₄^2-). 

ORP's strategy is to further
refine the baseline alkaline/oxidative leaching process by eliminating process
steps to reduce operating costs and shorten the process schedule. Alternative
oxidant processes which combine the alkaline and oxidative leaching steps are
currently being evaluated; these may reduce overall processing time by up to
three years.  The oxidant dose requirements are also being refined by
incorporating the inherent properties of the waste into the washing and leaching
mechanism.  Ionizing gamma radiation produced by radioactive constituents of
the waste contributes to the oxidation potential while some ions in the waste
are themselves a reservoir of oxidants.  The consideration of these waste
properties in the choice of oxidants and dose will provide the WTP operator
with a method with which to tailor an optimal treatment scheme for each unique waste
batch that is processed through the WTP.  The placement of the oxidative leach
step is also being evaluated - the process may be more effective applied in the
waste storage tanks rather than in the Pretreatment facility.