ESSC & DUPLEX 2019

A large variety of surface treatment processes is applied in stainless steel pickling. Mixed acid containing hydrofluoric and nitric acid is used as a standard treatment for hot and cold rolled material. The metal-fluorides and -nitrates from the waste acid can be pyrohydrolyzed by the Andritz Pyromars process. The recovered acid is recycled into the pickling process and the metal oxides are re-used in the steel making process. In the CAPL (Cold rolled annealing and pickling line) an electrolytic process using Sodium Sulphate is used. Due to the pH-neutral solution the metals precipitate as metal hydroxides, but Chromium VI stays in solution (as Chromic Acid salts). The Neolyte Recovery process uses an acidification and a Sodium Thiosulphate addition to reduce Cr VI to a harmless Cr III hydroxide. Additionally the new ZEMAP process (Zero Effluent Mixed Acid Pickling) recovers the valuables from the rinse water from the stainless steel pickling line. A neutralization process using ammonia prior to a multi stage evaporation unit produces a highly concentrated Metal-Fluoride/Nitrate stream that can be recovered by the a.m. Pyromars process. The clean condensate can be reused in the rinsing process. Overall, Nitrate-free effluents can be achieved.