Thermal Decomposition of Nitrated Tributyl Phosphate

Extended contact between heated mixtures of trin-butyl phosphate (TBP) and aqueous solutions of nitric acid and/or heavy metal nitrate salts at elevated temperatures can lead to exothermic reactions of explosive violence. Most solvent extraction operations (e.g., Purex) are conducted at ambient conditions without heating TBP and have been performed safely for decades, but several explosions involving TBP have occurred in the United States, Canada, and the former Soviet Union. This investigation was undertaken to characterize the products of thermal decomposition of both single- and two-phase mixtures of TBP, nitric acid, and water under a variety of conditions. The data indicate that the extent of reaction and the rate of gaseous product formation are affected by the presence of Zr⁴⁺, distillation compared with reflux conditions, temperature, water/HNO₃ and HNO₃/TBP ratios, and whether the decomposition occurs under constant pressure or constant volume conditions. Higher reaction temperatures accelerate the rate of decomposition, but the extent of decomposition, as measured by the quantity of gaseous products, was greater at lower temperatures when the decomposition was performed under distillation conditions. Higher gas production occurs under reflux conditions, lower H₂O/HNO₃ ratios, and when a separate water-HNO₃ phase is initially present. The major gaseous products include N₂, CO, CO₂, NO, and N₂O. Measurable amounts of NO₂ were not present in the final product mixture, although an orange color suggesting the presence of NO₂ was observed in the early stages of decomposition. The major liquid products were dibutyl phosphoric acid, butyl nitrate, and water. Small amounts of C₁-C₄ carboxylic acids were also present. Because of the small sample sizes that were employed and the isothermal conditions of the decomposition, runaway reactions were not observed. Some possible reaction pathways are considered.