ALTERNATIVE AND ENHANCED CHEMICAL CLEANING: BASIC STUDIES RESULTS FY2010

Abstract

In an effort to develop and optimize chemical cleaning methods for the removal of sludge heels from High Level Waste tanks, solubility tests have been conducted using nonradioactive, pure metal phases. The metal phases studied included the aluminum phase gibbsite and the iron phases hematite, maghemite, goethite, lepidocrocite, magnetite, and wustite. Many of these mineral phases have been identified in radioactive, High Level Waste sludge at the Savannah River and Hanford Sites. Acids evaluated for dissolution included oxalic, nitric, and sulfuric acids and a variety of other complexing organic acids. The results of the solubility tests indicate that mixtures of oxalic acid with either nitric or sulfuric acid are the most effective cleaning solutions for the dissolution of the primary metal phases in sludge waste. Based on the results, optimized conditions for hematite dissolution in oxalic acid were selected using nitric or sulfuric acid as a supplemental proton source. Electrochemical corrosion studies were also conducted (reported separately; Wiersma, 2010) with oxalic/mineral acid mixtures to evaluate the effects of these solutions on waste tank integrity. The following specific conclusions can be drawn from the test results: (1) Oxalic acid was shown to be superior to all of the other organic acids evaluated in promoting the dissolution of the primary sludge phases. (2) All iron phases showed similar solubility trends in oxalic acid versus pH, with hematite exhibiting the lowest solubility and the slowest dissolution. (3) Greater than 90% hematite dissolution occurred in oxalic/nitric acid mixtures within one week for two hematite sources and within three weeks for a third hematite sample with a larger average particle size. This dissolution rate appears acceptable for waste tank cleaning applications. (4) Stoichiometric dissolution of iron phases in oxalic acid (based on the oxalate concentration) and the formation of the preferred 1:1 Fe to oxalate complex is possible with the addition of a supplemental hydrogen ion source (HNO{sub 3} or H{sub 2}SO{sub 4}) and pH control. (5) Sulfuric acid is nearly twice as effective as nitric acid (on a molar basis) at promoting hematite dissolution in oxalic acid solutions, most likely due to the fact that it is diprotic. (6) The greater the oxalic acid concentration, the greater the demand for supplemental H{sup +} to promote optimal dissolution. Minimum mineral acid concentrations required for optimal oxalic acid utilization based on hematite solubility tests are provided. (7) Corrosion studies conducted (reported elsewhere) with 1 wt.% oxalic acid revealed that carbon steel corrosion rates are manageable at lower mineral acid concentrations (0.1 M HNO{sub 3} and 0.05 M H{sub 2}SO{sub 4}) and lower temperatures (45 C). (8) Proposed conditions for waste tank heel dissolution based on the solubility and corrosion test results are 0.5 wt.% oxalic acid and 0.18 M HNO{sub 3} or 0.09 M H{sub 2}SO{sub 4} at 50 C. (9) The OLI Thermodynamic Model appears to over-predict the solubility of the iron phases studied in oxalic acid and oxalic/nitric acid mixtures. The predictions show better agreement with experimental results at higher pH and in sulfuric/oxalic acid mixtures. (10) Oxalic, nitric, and sulfuric acids are effective at quickly dissolving gibbsite ({ge}86% dissolution in 2 weeks), with oxalic/sulfuric acid mixtures being particularly effective. (11) Limited dissolution tests conducted with carbon steel coupons revealed that the presence of metallic iron can, in some cases, result in dramatically different results. Additional studies in this area are recommended. Based on the current results, the optimal approach for the removal of sludge heels for HLW tanks would include the following steps: (1) removal of the maximum possible amount of heel materials by mechanical means; (2) neutralization and acidification of the heel using dilute mineral acid (This step should promote significant dissolution of certain metal hydroxides and salts, including gibbsite.); and (3) dissolution of the residual heel material at 50 C using an acid mixture containing 0.5 wt.% oxalic acid and 0.18 M nitric acid (This step should dissolve the iron phases.)