Reduction of nitrogen dioxide from etching vent gases by scrubbing with caustic sodium sulfide solution

Abstract

BACKGROUND Acids and nitrogen dioxide (NO2) in gases that are vented from Al-etching operations for the production of thin-film transistor-liquid crystal displays (TFT-LCD) are harmful to the environment. Scrubbing the gases using traditional caustic liquids with high salinities is ineffective in removing NO2. This work develops practices for improving the removal of NO2 by adding reducing agents to the scrubbing liquids. RESULTS Field results reveal that Na2SO3 is a less effective agent for reducing NO2 than is Na2S, because it has a stronger reduction ability. In this work, when the scrubber parameters were a packing height of 1.0–2.0 m, a liquid-to-gas ratio L/G of 0.48–2.64 L m−3 and a superficial gas velocity U of 0.66–2.08 m s−1, and for influent NO2 concentrations of several tenths of ppm, the NO2 removal depended more on the sulfide dose than on the mass-transfer rate of NO2 to the liquid. CONCLUSIONS A sodium sulfide dose of 1.53±0.22 kg/(kg NO2 removed) over influent NO2 at a concentration 30–50 ppm was obtained. This value was much higher than the stoichiometric value for the reduction of NO2 to nitrite. Sulfide addition using an ORP-controlled mode with an ORP of −400 ± 10 mV and a pH of 12.5 ± 0.1 in the scrubbing liquid is suggested for reducing chemical costs and obtaining the desired NO2 concentration of exhaust gas. © 2013 Society of Chemical Industry