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ANDRITZ sustainable SulfoLoop™ sulfuric acid plant technology for on-site production of sulfuric acid in kraft mills

Producing commercial-quality sulfuric acid on site from concentrated non-condensable gases

The ANDRITZ SulfoLoop™ solution is based on Wet-gas Sulfuric Acid (WSA) technology for converting wet, sulfur-rich gases and elemental sulfur into sulfuric acid.


Softwood mills often have a sulfur surplus due to additional sulfur inputs such as a tall oil plant. The traditional way to manage sulfur surplus has been to dump recovery boiler fly ash. The ash consists mainly of sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3) – meaning that, in addition to dumping sulfur, valuable sodium is also dumped. The lost sodium is compensated by purchasing sodium hydroxide (NaOH). This can be expensive.

An alternative is the on-site production of sulfuric acid (H2SO4) to significantly reduce, or to even eliminate, the need for recovery boiler fly ash dumping. The SulfoLoop™ sulfuric acid plant produces concentrated commercial-quality H2SO4 from concentrated non-condensable gases (CNCG) using the Wet-gas Sulfuric Acid (WSA) process developed by Haldor Topsoe, which uses catalytic conversion and condensation. Up to 99.9% of the sulfur in CNCGs can be converted into concentrated sulfuric acid with the WSA process.

The sulfuric acid from WSA is commercial-grade since the liquefaction of H2SO4 is performed at 100% condensation. This is critical, especially for demanding applications such as ClO2 generation. Additional sulfur capacity can also be created by extracting more than just the surplus sulfur as CNCG, which can save money in the optimization of make-up chemicals – as the internal Na2SO4 stream can be used as sodium make-up instead of purchased NaOH.

There are over 150 references for the WSA technology operating worldwide on a variety of sulfurous gas streams as well as spent sulfuric acid. It is a very well-proven and robust technology.

SulfoLoop™ sulfuric acid plants are part of the ANDRITZ’s CircleToZero initiative, which includes new technologies aimed at optimizing pulp mill side streams. This global initiative for our pulp and paper customers brings together the continuous development and improvement of existing ANDRITZ technology solutions with an ambitious innovation goal for the future: To eliminate unused side streams, create new added-value products, and lay the foundation for zero emissions and zero waste production.

ANDRITZ SulfoLoop™ sulfuric acid plant (3D rendering)


ANDRITZ SulfoLoop™ sulfuric acid plant (3D rendering)


Benefits of ANDRITZ SulfoLoop™ sulfuric acid production

  • Environmental and economic savings
  • Helps manage and control the mill’s sodium sulfur (Na/S) balance
  • Avoids costs for dumping recovery boiler fly ash
  • Avoids costs for purchase of make-up chemicals

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