When loading nitrogen tetroxide before launching the space shuttle, fugitive N2O4 is absorbed into caustic solutions in a two stage scrubber solution. The second stage of the scrubber has sodium sulfite added to the alkaline scrubbing solution to act as an oxidant scavenger, to improve scrubbing efficiency. The N2O4 oxidant reacts with water and with the sulfite to form acids (neutralized by the alkalinity in the solution) and sulfates. The residual caustic concentration makes the spent solution a characteristic hazardous waste. This proposal addresses an approach that uses freeze crystallization to crystallize sodium nitrate and sodium sulfate salts from the spent scrubbing solution, to remove part of the water in the spent solution as pure ice, and thus to concentrate and purify the residual hydroxide alkalinity so that it can be reused in the scrubbing operation. To improve the efficiency in removing the nitrate and sulfate salts, nitrites and sulfites will be oxidized by adding air. Testing here will determine whether this needs to be a pressurized reaction, or if atmospheric conditions are sufficient. The hydroxide can be crystallized from the treating solution if necessary to recover it at higher concentrations or in ultra-high purity.
Keywords: freeze crystallization nitrogen tetroxide sodium nitrite oxidation waste minimization
