The Wet gas Sulphuric Acid (WSA) process enables in-mill production of concentrated commercial-grade H2SO4 to significantly reduce the cost of purchased chemicals.

needed to make up for sodium losses that occur when sulphur is discharged with recovery boiler fly ash, which consists mainly of

Na2SO4 and Na2CO3. Beginning in 2010, Andritz worked on the development of alternative process concepts and investigated them at pilot scale in several kraft pulp mills. Today, it offers tailor-made solutions for Lignin Recovery, customised for each application out of proven components.

The system can be configured as one stage without washing, two stages with acid washing or two stages with acid washing and drying. The exact configuration is based on the system infrastruc- ture of the mill and the end use of the lignin to be produced. The process is designed to remove lignin from black liquor at 35-45% dry solids having a typical pH of 12-13. After filtra- tion and washing, the dry solids content of lignin is typically 60- 62% before the drying stage, and 95% after. In its most complete form, the Lignin Recovery process consists of three sub-processes.


The first is precipitation in which the pH of black liquor from the evaporation plant is decreased with carbon dioxide (CO2) in the acidification reactor to precipitate the lignin from the black liquor. The resulting slurry is fed to a membrane filter press that effi-

ciently separates the precipitated lignin and lignin-lean black liquor.

Acid wash

This is followed by the acid wash when impurities (mostly sodium) are leached from the compressed precipitated lignin in a dilution wash with sulphuric acid. Displace- ment washing is used to remove the sodium sulphate formed dur- ing the acid wash. Counter-current washing minimises the consump- tion of fresh water, as well as the amount of filtrate recycled to the evaporation plant. The displace- ment wash is carried out with pressurised hot water from the black liquor cooler and without additional chemicals to ensure that no new sulphate molecules are formed. A second membrane filter press is utilised to dewater the slurry.


Finally, the moist lignin cake is fed through a disintegrator for proper size reduction and to increase the surface area before the dryer. The heat source for drying is mainly the flue gas from a lime kiln or recovery boiler, which also acts as an inert drying media.

While producing high-quality lignin, the amount of sulphur in the chemical recovery cycle is increased considerably due to the

large amount of sulphuric acid consumed in the acid washing step. This means that the excess sulphur must be dumped in order not to increase the sulphidity of white liquor. When sulphur is dumped in the form of recovery boiler fly ash, the amount of NaOH required as a 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 a Wet gas Sulphuric Acid (WSA) process. In the WSA process, commercial- grade sulphuric acid is produced on-site from CNCGs by catalytic conversion and condensation. There are over 130 references for the WSA technology operating world- wide on a variety of sulphurous gas streams, but none in the pulping industry. Up to 99.9% of the sul- phur in CNCGs can be converted to concentrated sulphuric acid. Depending on the downstream use of the lignin, Andritz can tailor the processes to achieve a mill’s exact goals. As with many of the company’s solutions, it can deliver a complete plant – process design and main equipment (presses, crushers, dryer, bag filter and lime kiln burner), as well as automa- tion and erection – from a single source.

More information from

15 Spring 2018 15

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24