Method and system for concentrating waste sulphuric acids from nitration processes

A method for recovering dilute and contaminated waste sulfuric acids from nitration processes, where

• the waste acid from the nitration process which is taken off from the nitration process and, in addition to up to 80% by mass of sulfuric acid and water, contains nitric acid, HNO3, nitrosulfuric acid, HNO2 and nitro-organic compounds, namely DNT and MNT, as further constituents being subjected to at least partial purification in a stripping column to remove said further constituents and then being fed to a further concentration stage in one or more downstream evaporators operating at reduced pressure, and
• the production of a highly concentrated nitric acid required in the nitration process from a more dilute nitric acid by extractive rectification with the use of concentrated sulfuric acid as an extracting medium also being coordinated with the nitration process, the sulfuric acid used as extracting medium being diluted during the extractive rectification and being reconcentrated for reuse,
• wherein the waste acid from the nitration process (F1) is fed to the top of a first, indirectly heated stripping column (K1) and is separated into a top product containing nitric acid and nitro-organic compounds and a prepurified sulfuric acid in the bottom product and
• the prepurified sulfuric acid taken off from the bottom of the first stripping column (K1) is being fed to the top of a second further indirectly heated stripping column (K2) which is connected in series and into the bottom region or evaporator of which a dilute sulfuric acid (F3) from the system for achieving a high concentration of nitric acid by extractive rectification is fed so that, in the second stripping column (K2), the prepurified sulfuric acid from the stripping column (K1) is further purified and is preconcentrated together with the dilute sulfuric acid from the extractive rectification of nitric acid, after which the preconcentrated sulfuric acid flowing out of the evaporator of the second stripping column (K2) is being further concentrated in one or more evaporator stage(s) at reduced pressure.