ANDRITZ GROUP
PULP & PAPER products
Pulp production
Kraft pulp
Lignin Recovery

ANDRITZ Lignin Recovery technology for kraft mills

The benefits to installing Lignin Recovery technology in a kraft mill include the ability to debottleneck a recovery boiler and the potential cost savings from substituting a sustainable biofuel for purchased fossil fuels. Lignin is also being utilized as a sustainable, renewable raw material for advanced bioproducts such as composites or as a substitute for phenolic and aromatic compounds, which might create an additional revenue stream for a kraft mill.

Lignin as a sustainable, renewable biofuel

Lignin is a main component of wood that is separated from wood fiber during the kraft cooking process. As such, it becomes a sustainable, renewable biofuel that is combusted in the chemical recovery boiler as concentrated black liquor. However, there are occasions where a mill’s recovery boiler is the production bottleneck and, in these instances, it might be advantageous to remove and recover some of the lignin for use as a lime kiln fuel (for example) or as an additional source of revenue, since lignin is utilized as a raw material for advanced bioproducts.

The challenge in doing this is making lignin recovery cost-competitive. Lignin recovery introduces sulfur (in the form of sulfuric acid) into the chemical recovery cycle. This excess sulfur must be discharged or utilized to keep the cycle in balance. For mills with no internal sulfuric acid cycle in place, the excess sulfur is discharged with the recovery boiler fly ash and NaOH must be purchased to make up for the sodium losses that also occur with this discharge. This can be a significant cost. One alternative to make lignin recovery cost-competitive is to create an internal sulfuric acid production cycle such as the Wet gas Sulfuric Acid (WSA) process which eliminates the need to discharge sulfur.

ANDRITZ Lignin Recovery systems

ANDRITZ began pilot plant testing Lignin Recovery alternatives in 2010 and then proceeded to design a portable pilot plant unit that was transported to various mills. This helped solidify the design for a flexible system (one stage without washing, two stages with acid washing, or two stages with acid washing and drying).

Depending on the intended use of the lignin, ANDRITZ tailors the Lignin Recovery system to achieve a mill’s exact goals. As with many ANDRITZ solutions, the company can deliver a complete plant – with the process design; main equipment including presses, crushers, dryer, bag filter, and lime kiln burner; as well as automation, and erection – from one source.

In its most complete form, the Lignin Recovery system consists of these sub-processes:

Precipitation: where the lignin in black liquor from the evaporation plant is precipitated out of solution and the slurry is filtered to separate the precipitated lignin from the lignin-lean black liquor.
Acid Wash: where impurities (mostly sodium) are leached from the lignin with sulfuric acid in a dilution wash and the lignin is washed in a displacement washer with hot water to remove the sodium sulfate formed during the dilution wash.
Drying: where the moist lignin is reduced to proper size particles and then dried.

The ANDRITZ Lignin Recovery process is designed to remove lignin from black liquor at 35-45% dry solids having a typical pH of 12-13. After filtration and washing, the dry solids content of lignin is typically 60-62% before the drying stage, and 95% after.

Combination with WSA process

When producing high-quality lignin, the amount of sulfur in the chemical recovery cycle is considerably increased due to the large amount of sulfuric acid consumed in the acid washing sub-process. This means that the excess sulfur must be dumped in order not to increase the sulfidity of cooking liquor. When sulfur is dumped in the form of recovery boiler fly ash, the amount of NaOH required as make-up chemical is high (up to 20-30% of the total cost of produced lignin). For this reason, it makes sense to combine the Lignin Recovery system with the WSA process, which is an economic and compatible way to remove the excess sulfur from the recovery cycle and process sulfur compounds to recyclable concentrated sulfuric acid.