Process for recycling nitrogen oxides from nitrosyl sulfuric acid to produce concentrated or highly concentrated nitric acid and sulfuric acid

An apparatus for the recycling of a mixed acid comprising nitrosyl sulfuric acid and sulfuric acid to obtain nitric acid and sulfuric acid, the apparatus comprising the following components:

• a) a reaction vessel (2) for decomposition of a mixed acid comprising nitrosyl sulfuric acid and sulfuric acid, feed lines (4) and (6) for introducing the mixed acid and a dilute nitric acid, respectively, into the reaction vessel (2), and outlet lines (8) and (10) for, after decomposition, discharging gaseous and liquid phases from the reaction vessel (2),
• b) a stripping column (12) into which the liquid phase from reaction vessel (2) is introduced via line (10) and further comprising an inlet line (14) for introducing steam into the stripping column (12), and outlet lines (16) and (18) for discharging purified sulfuric acid and an evaporated mixture of concentrated nitric acid and nitrogen oxides, respectively,
• c) a cooler/condenser (22) into which the evaporated mixture of concentrated nitric acid and nitrogen oxides from stripping column (12) and, optionally, the gaseous phase from reaction vessel (2) are introduced via feed line (20), and further comprising and outlet lines (24) and (26) for discharging a nitric acid condensate and gaseous nitrogen oxides, respectively,
• d) an absorption column (28) into which nitrogen oxides obtained in cooler/condenser (22) are introduced via line (26), further comprising inlet line (30) and outlet lines (32) and (34) for discharging waste gas and dilute nitric acid, respectively, and
• optionally further comprising
• a feed line (36) connecting outlet line (34) and inlet line (6) for reintroducing dilute nitric acid into reaction vessel (2).