Sulphuric Acid Recycling

Spent sulphuric acid and other sulphate compounds, previously regarded as waste, are an increasingly important source of sulphur dioxide for sulphuric acid manufacture. The sulphuric acid may have been used as a catalyst or drying agent, or in the preparation of metal surfaces for plating.

The acid may be recovered, cleaned and concentrated, or it may be broken down chemically ("cracked").

Recovery and Concentration

Acid used as a catalyst is often contaminated with organic material. Recovery and concentration is possible in this case.

The acid is pre-heated and fed into a stripping column, which removes the organic material using steam. The acid is then vacuum distilled to concentrate it.

Important features in the process designed by Kvaerner Chemetics are:

  • Heat from the product acid is recovered and used for pre-heating the feed acid, reducing the energy needed in the process
  • Much of the liquid flow uses gravity rather than electric pumps, saving on energy and the cost of using acid-resistant pumps

Reaction vessels must be acid resistant, and these are made from glass or Teflon-lined steel.

diagram: sulphuric acid regeneration

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pre-concentration

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 stage.

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 power generation.

Regeneration by Cracking

In cases where the acid cannot be recovered by stripping and distillation, an alternative can be used. The SATCO process (Sulphuric Acid by Thermal Cracking Oxidation) breaks the acid down to produce sulphur dioxide, which is then used to produce more sulphuric acid by the Contact Process.

 

 

 

 

Feed Gas Cleaning

The gas stream is introduced into a drying and purification unit where any organic products, volatile metals, solid ash impurities and sulphuric acid mist are removed by cooling and by electrostatic filters. The remaining gases, which are sulphur dioxide, oxygen, nitrogen and steam, are dried in 98% sulphuric acid, before being reheated and entered into the contact process to make new sulphuric acid.

diagram: regeneration of spent sulphuric acid by the SATCO process

 

 

 

 

 

 

 

 

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