Process for the preparation of nitrobenzene by adiabatic nitration

1. A process for the continuous preparation of nitrobenzene comprising

adiabatically nitrating benzene with a mixture of sulfuric acid and nitric acid in which the sum of the concentrations in the reaction zone of the metal ions which form sparingly soluble metal sulfates is less than 900 mg/l, based on the volume of the aqueous phase containing sulfuric acid;

separating the reaction mixture obtained in the nitration step into an aqueous phase and an organic phase;

evaporating water out of the aqueous phase;

recycling at least a portion of the resultant concentrated sulfuric acid into the nitration step;

and

additionally introducing sulfuric acid having a concentration of greater than 65% into the nitration step.

2. The process according to claim 1 , in which the reaction zone for adiabatically nitrating benzene has at least two dispersing elements.

3. The process according to claim 1 , wherein said nitric acid employed in the nitrating step contains less than 10 mg/l of metal ions which form sparingly soluble metal sulfates.

4. The process according to claim 1 , wherein said sulfuric acid which is additionally introduced into the nitration step contains less than 100 mg/l of metal ions which form sparingly soluble metal sulfates.

5. The process according to claim 1 , wherein said organic phase obtained in the phase separation is used to heat the benzene which is fed to the reaction zone.

6. The process according to claim 1 , in which the metal ions which form sparingly soluble metal sulfates are Al, Ca, Cr, Mg, Mn, Fe, Co, Ni, Cu, Sr, Cd and/or Ba and the total content of these metal ions in the reaction zone is less than 800 mg/l, based on the volume of the aqueous phase containing sulfuric acid.