The top surface of hot rolled steel sheet oxidizes at high temperatures and forms various iron oxides. It is necessary to remove the iron oxide scale prior to cold rolling of the sheet. Pickling process is used in steel plants for oxide removal. A pickling line has 3-4 tanks containing acid with varying concentrations. Typically, hydrochloric acid is used in pickling tanks. Concentration range of acid lies between 5 to 20 % in various acid tanks. Pickling tank is 15 meters long and 2 feet deep.
To effectively remove iron oxide scale during pickling, the acid temperature is maintained close to 80oC. Acid fumes are generated due to high temperature which are effectively captured through a hood and suction system installed on top of each pickling tank. Some acid fumes still manage to escape from the cracks and small openings present in the hood (refer Figure). Apparently, the suction system of the acid fume scrubbing system should be powerful enough to suck these escaping acid fumes however in the present scenario, acid fumes are observed escaping from the hood.
These acid fumes, if not neutralized or captured, make the overall environment near the pickling tanks acidic. Presence of escaped acidic fumes in the environment is one of the prime cause of rusting of coils which needs to be salvaged and treated as product defect. The acid fumes also lead to corrosion of building structures nearby the pickling line (refer Figure) and leads to recurrent cost for replacement.
Solution is being sought for ways/methods of cleaning the air by removal of acidic fumes.