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.