The spent sulphuric acid is first concentrated to remove water
and so reduce the volume being handled. This means a smaller
furnace can be used, and less energy is needed for the second
The degree of concentration depends on the dissolved impurities
- concentration stops at the point these begin to precipitate.
Thermal Cracking in Regeneration Furnace
In the second stage, the concentrate is mixed with compressed
air and sprayed into a furnace heated to 1000 - 1200ºC,
cracking the H2SO4 molecule into sulphur
dioxide and steam.
The best results are obtained if the residence time (time spent
in the furnace) is about 3 seconds.
Although this reaction is endothermic, energy from other processes
in the plant may be used for pre-heating, reducing the energy
demand. Energy must in fact be removed after cracking, and this
can be used to pre-heat the spent acid, or to raise steam for