Optimizing Permanganic Acid Production: Effects of Temperature on Stability.

In the nuclear industry, the decontamination of nuclear metallic structures is an essential process to reduce radiation exposure during maintenance or dismantling. The oxide layer, such as chromium (III) oxide (Cr2O3), formed on stainless steel and nickel-based alloys, contributes significantly to surface radioactivity by trapping radioactive contaminants. To address this, permanganic acid (HMnO4) has proven to be a promising oxidizing agent for dissolving these oxide layers-particularly chromium oxide-on stainless steel and nickel-based alloys. In this study, HMnO4 was synthesized via ion exchange using AmberLite IRN97 H resin and potassium permanganate (KMnO4). The optimized process yielded a highly acidic solution (pH~1.6) with potassium concentrations below 0.1 ppm, indicating near-complete exchange efficiency. Dissolution kinetics were investigated at HMnO4 concentrations ranging from 240 to 1920 ppm and temperatures from 30 °C to 80 °C. At a constant temperature, increasing HMnO4 concentration significantly improved Cr dissolution, with up to 31% of total chromium solubilized after 33 h. Lower temperatures favored higher dissolution efficiency, likely due to improved thermal stability of HMnO4. For durations shorter than 4 h, the influence of temperature was limited compared to the effect of acid concentration. To assess post-treatment options, HMnO4 decomposition was studied using oxalic acid (H2C2O4) at 80 °C. Results showed that a minimum H2C2O4/HMnO4 molar ratio above 2.75 was necessary to achieve effective reduction while preventing MnO2 precipitation. However, even under strongly acidic conditions and with a large excess of reductant, Mn2+ yields remained below 55%, suggesting that thermal degradation of oxalic acid and possible formation of undetected manganese species limited the reduction process.