THIOKRAFT: Sustainable desulfurization system for kraft pulp mills

Desulfurization system THIOKRAFT provides a revolutionary biological process that removes more than 75% of sulfur from green liquor in kraft pulp mills and recovers it as solid elemental sulfur. The process is efficient, stable, and designed for harsh conditions, converting sulfide into high-purity sulfur and sodium carbonate using autotrophic bacteria—with no toxic sludge and minimal chemical use.

THIOKRAFT is purpose-built for kraft pulp mills. THIOKRAFT benefits all relate to kraft pulping:

	It stabilizes sodium/sulfur balance, reducing sodium replacement costs
	It enables biorefinery concepts (e.g. lignin extraction)
	Its sustainable practice improves environmental performance with a waste-free sulfur purge and lower chemical need.

Make your pulp mill operations more efficient and sustainable with a THIOKRAFT implementation.

The new THIOKRAFT is a more sustainable and cost-effective desulfurization system for kraft pulp mills. This is why.

ESP disadvantages

Traditionally, kraft pulp mills use electrostatic precipitators (ESPs) to remove particulate matter from the flue gases, including dust, from the recovery boiler. The dust collected in the ESP is a byproduct of the chemical recovery process and contains a complex mixture of materials, including sodium sulfate, other salts, and organic matter. This kind of leftover ash is considered waste. However, this method removes a lot of sodium along with the sulfur. That’s a problem because − to keep the kraft pulp process running smoothly and cost-effectively − the balance between sodium and sulfur must be carefully maintained. For this purpose, the mill then needs to buy and add expensive make-up chemicals like NaOH (sodium hydroxide).

THIOKRAFT benefits

THIOKRAFT offers a smarter and flexible solution. It allows the kraft mill to adjust sulfur removal without disturbing the overall chemical balance for kraft pulping. THIOKRAFT biologically treats just the right portion of the green liquor to separate elemental sulfur and return sodium(bi)carbonate. The amount treated is flexible, depending on the Na/S balance. If sulfur is too high, it treats more green liquor. If sodium is low, it treats less—so sodium counterion isn’t lost. This approach makes the process efficient and cost-effective.

THIOKRAFT removes sulfide from kraft pulping by biologically converting the sulfide in green liquor to elemental sulfur. Since every kraft pulp mill produces green liquor as part of its standard operation, and it is the most suitable point for selective sulfur removal, using the new THIOKRAFT desulfurization system is both logical and widely applicable.

• Keeps sodium in: Recovers sodium as carbonate for reuse in the mill
• High sulfur yield: 70-80% conversion of sulfide, produces up to 99.8% pure elemental sulfur after washing
• Robust system: Uses aerobic bacteria tolerant to high salt and pH conditions
• Low maintenance: Operates continuously and withstands planned and unplanned mill shutdowns without reseeding

Make your pulp mill operations more efficient and sustainable with a THIOKRAFT implementation.

1. Feed Preparation
   Green liquor is cooled and partially diluted (if needed) before entering the system.
2. Bioreactor
   Salt- and alkali-tolerant bacteria convert sulfide (NaHS) to elemental sulfur using air and CO₂:
   • 2 NaHS + O₂ + CO₂ → Na₂CO₃ + 2 S + H₂O
   • pH is maintained around 9–10 using CO₂ buffering (e.g. by flue gas addition)
   • Bacteria operate optimally between 25°C and 30°C
3. Sulfur Separator
   Gravity settling separates the hydrophilic sulfur, which can be washed and dewatered further.
4. Optional upgrading
   Filtered sulfur cake (up to 99.8% pure) can be melted and used as feedstock for sulfuric acid production.

THIOKRAFT creates a sulfur cake of >99% purity when washed in a filter press. This can be reused or sold. The molten sulfur is very pure with practically no contaminants (ash content of 12 ppmwt (parts per million by weight). This level of purity is suitable for use in sulfuric acid plants, which demand high-quality sulfur.

See how THIOKRAFT can strengthen your pulp mill operations.