2002 Volume 88 Issue 4 Pages 202-209
In order to control NOx gas emissions on pickling both stainless steel and titanium metal with nitric-hydrofluoric acid, a new control system has been investigated in laboratory and tested at a pickling plant, which doses hydrogen peroxide into the pickling bath automatically. The system includes pairs of platinum electrodes and electrolyzers, pumps for injecting hydrogen peroxide solution into the pickling bath, and their controllers. Results of the tests at the pickling plant showed that the NOx gas concentration can be lowered to 50200 ppm and consumption of the nitric acid was reduced apparently by dosing hydrogen peroxide so as to oxidize most of nitrous acid on pickling stainless steel, or as to oxidize whole nitrous acid and go to slight excess on pickling titanium metal. It was also verified that the electrolytic current measured in the pickling bath was of use as index which showed the concentration of remaining nitrous acid or hydrogen peroxide excessively added. The amounts of consumed hydrogen peroxide through pickling using this system was about 0.92 or 0.82 mole for one mole of SUS304 or titanium metal, respectively.