Investigation of Alternative Approaches for Cleaning Mott Porous Metal Filters

Abstract

SummaryThe Department of Energy selected Caustic Side Solvent Extraction (CSSX) as the preferred cesium removal technology for Savannah River Site (SRS) waste. As a pretreatment step for the CSSX flowsheet, the incoming salt solution that contains entrained sludge is contacted with monosodium titanate (MST) to adsorb strontium and select actinides. The resulting slurry is filtered to remove the sludge and MST. Filter fouling occurs during this process. At times, personnel can increase the filtrate rate by backpulsing or scouring. At other times, the filtrate rate drops significantly and only chemical cleaning will restore filter performance. The current baseline technology for filter cleaning uses 0.5 M oxalic acid. The Salt Processing Project (SPP) at SRS, through the Tanks Focus Area, requested an evaluation of other cleaning agents to determine their effectiveness at removing trapped sludge and MST solids compared with the baseline oxalic acid method.A review of the technical literature identified compounds that appear effective at dissolving solid compounds. Consultation with the SPP management team, engineering personnel, and researchers led to a selection of oxalic acid, nitric acid, citric acid, and ascorbic acid for testing. Tests used simulated waste and actual waste as follows. Personnel placed simulated or actual SRS High Level Waste sludge and MST in a beaker. They added the selected cleaning agents, stirred the beakers, and collected supernate samples periodically analyzing for dissolved metals.The conclusions from this work follow.• 0.5 M oxalic acid and 4.0 M nitric acid proved most effective at dissolving iron, aluminum, and titanium. The 0.5 M oxalic acid solution dissolved 23 -44 % of the iron, 11 -41 % of the aluminum, and 98 -100 % of the titanium calculated to be present in the solids used in the tests. The 4 M nitric acid solution dissolved 15 -32 % of the iron, 13 -47 % of the aluminum, and 67 -100% of the titanium calculated to be present in the solids used in the tests. The citric acid solution dissolved 1 % of the iron, 7 % of the aluminum, and 17 % of the titanium. The ascorbic acid solution dissolved 8 % of the iron, 5 % of the aluminum, and 2 % of the titanium.• 0.5 M oxalic acid proved more effective at dissolving iron, titanium, and silicon.• 4 M nitric acid proved more effective at dissolving aluminum.• Decreasing the concentration of oxalic acid or nitric acid decreased the amount of solids dissolved. Decreasing the oxalic acid concentration to 0.25 M reduced the amount of iron (from 23 to 10 %), aluminum (from 15 to 10 %), and titanium (from 100 to 56 %) dissolved. Decreasing the nitric acid concentration from 4 M to 0.5 M decreased the amount of iron (from 18 to 3 %), aluminum (from 19 to 9 %), and titanium (from 89 to 7 %) dissolved.Based on these studies, the SPP should maintain 0.5 M oxalic acid as the baseline cleaning technology for the Salt Waste Processing Facility, and the Alpha Removal Project should use 0.5 M oxalic acid as their baseline cleaning solution.

Filtration Understanding: FY10 Testing Results and Filtration Model Update

“…A review of a Savannah River National Laboratory (SRNL) report (Poirier and Fink 2002) on porous stainless steel filter cleaning led to the selection of oxalic acid as a cleaning agent. When applied to bench-scale filtration at PNNL, it was found that cleaning with 0.5 M oxalic acid solutions leached much of the irreversible (i.e., not removable by backpulsing) foulants and resulted in CWF values comparable to pre-test-values, or at least a fivefold or more increase over the CWF achieved after nitric acid cleaning alone.…”

Bench-Scale Filtration Testing in Support of the Pretreatment Engineering Platform (PEP)

“…Since it was hypothesized that iron hydroxide particles caused a substantial proportion of the fouling, a mixture of 0.5 M oxalic acid was introduced to the fouled filter element. The idea to use a dilute solution of oxalic acid was derived from a report describing Mott filter element cleaning for various wastes and simulants (Poirier 2002).…”

Waste Tank Heel Chemical Cleaning Summary

“…Poirier and Fink conducted a series of tests investigating various cleaning agents for porous metal filters. 30 Among the agents tested were oxalic acid, nitric acid, citric acid, and ascorbic acid. The tests involved placing simulated SRS High Level Waste Tank 40H sludge (5 g) and MST (5 g) in a beaker and adding the respective cleaning agent.…”

“…The use of nitric acid as a sludge dissolution agent has been investigated by Poirier and Fink. 30 The results of a comparative study were previously described in Section 4.1. Figure 11 demonstrates that 4.0 M nitric acid performed comparably ( in terms of the amount of aluminum, iron, manganese, and silicon that dissolved from the sludge) to 0.5 M oxalic acid, while 0.5 M nitric acid was less effective.…”

“…The conclusions from the tests were identical to the oxalic acid conclusions reported by Poirier and Fink. 30 • Increasing the nitric acid concentration enhanced sludge dissolution.…”