Alternative Enhanced Chemical Cleaning Basic Studies Results FY09
Due to the need to close waste storage tanks, chemical cleaning methods are needed for the effective removal of the heels. Oxalic acid is the preferred cleaning reagent for sludge heel dissolution, particularly for iron-based sludge, due to the strong complexing strength of the oxalate. However, the large quantity of oxalate added to the tank farm from oxalic acid based chemical cleaning has significant downstream impacts. Optimization of the oxalic acid cleaning process can potentially reduce the downstream impacts from chemical cleaning. To optimize oxalic acid usage, a detailed understanding of the chemistry of oxalic acid based sludge dissolution is required. Additionally, other acid systems may be required for specific waste components with low solubility in oxalic acid and as a means to reduce oxalic acid usage in general. Solubility tests were conducted using non-radioactive, pure metal phases known to be the primary phases present in High Level Waste sludge. The metal phases studied included the aluminum phases gibbsite and boehmite and the iron phases magnetite and hematite. Hematite and boehmite are expected to be the most difficult iron and aluminum phases to dissolve. These mineral phases have been identified in both SRS and Hanford High Level Waste sludge. Acids evaluated for dissolution included oxalic, nitric, and sulfuric acids. The results of the solubility tests indicate that oxalic and sulfuric acids are more effective for the dissolution of the primary sludge phases. For boehmite, elevated temperature will be required to promote effective phase dissolution in the acids studied. Literature reviews, thermodynamic modeling, and experimental results have all confirmed that pH control using a supplemental proton source (additional acid) is critical for minimization of oxalic acid usage during the dissolution of hematite. These results emphasize the importance of pH control in optimizing hematite dissolution in oxalic acid and may explain the somewhat limited success observed during recent attempts to remove sludge heels from SRS Tanks 5F and 6F using oxalic acid. Additionally, based on the results of the solubility tests conducted, the following conclusions can be drawn: (1) Hematite dissolution in oxalic acid is a stoichiometric process dependant upon the provision of sufficient oxalate molar equivalents to complex the iron and sufficient H{sup +} to promote the dissolution reaction. (2) The optimal utilization of oxalic acid for hematite dissolution requires an additional proton source, such as nitric acid, and a pH of {le} 1. In the absence of a supplemental proton source, greater than stoichiometric amounts of oxalate are required. (3) Magnetite is generally more soluble than hematite in all acids tested. (4) Gibbsite is generally more soluble than the boehmite form of aluminum in all acids tested. (5) The OLI Thermodynamic Model is a useful tool for the prediction of equilibrium iron concentrations, but predictions must be experimentally verified. The OLI model appears to over-predict the solubility of the iron and aluminum phases studied in mineral acids.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 978456
- Report Number(s):
- SRNL-STI-2009-00791; TRN: US1003097
- Country of Publication:
- United States
- Language:
- English
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