Pyrolysis of hydrazine hydrate waste salt: Thermal behaviors and transformation characteristics of organics under aerobic/anaerobic conditions

Abstract

The production of large quantities of industrial waste salts is becoming an issue of increasing concern with the adoption of the “zero liquid discharge” process for wastewater treatment. Recovery of waste salts as a useful resource after purification provides the best means of solving this problem. In this study, pyrolysis was studied as a purification technique to treat waste salt generated during hydrazine hydrate production (N2H4 WS) within the temperature range of 25–600 °C under aerobic and anaerobic conditions. Aerobic pyrolysis achieved 99.3% organic removal at a temperature that was 50 °C lower than was that achieved by anaerobic pyrolysis (600 °C). The formation of strong fluorescent species at 400 °C during anaerobic pyrolysis was detected using fluorescence excitation emission matrix (FEEM). These species were confirmed to be heterocyclic-N compounds, including pyridinic N and pyrrolic N, that were formed through cyclization reactions, as revealed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS). Harmful gases such as HCN and NH3 were released during anaerobic pyrolysis, and this may have been partially associated with the decomposition of heterocyclic-N compounds. Moreover, aerobic pyrolysis effectively reduced CO2 emissions by 8.7% based on energy consumption calculations. Therefore, aerobic pyrolysis is preferable for the purification of N2H4 WS owing to its low decomposition temperature, minimal release of harmful gaseous compounds, and low carbon emissions.