Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 180

softwoods (sawdust or thermomechanical pulp fibres) can be completely dissolved up to
a concentration of 8 wt% and the most efficient solvents tested were 1-butyl-3-methyl-
and 1-allyl-3-methylimidazolium chloride. These novel methods are judged as very
promising to yield relatively unmodified lignins that can be considered representative
for the overall composition in the wood sample. Ionic liquids could be expected to be
of great help in the future to increase the understanding of lignin biosynthesis, structural
characteristics and in investigations of lignins' interactions with other materials, as well
as in the development of usage and new products from lignin.

7.3.2 Isolation of Lignin from Spent Pulping Liquors

The main source of lignin readily available for use in larger scale comes from spent
pulping liquors after chemical liberation of wood fibres. By far the most important
pulping methods are kraft and sulfite cooking and only the lignin recovered from these
processes will be discussed in this section.

7.3.2.1 Kraft Lignin

Lignin isolated from the black liquor remaining after the chemical pulping of wood using
hydroxide and hydrogen sulphide is known as kraft lignin. This material accounts for
the overwhelming majority of lignin produced worldwide with up to 50 million tonnes
produced annually. Currently, by far the greatest use of kraft lignin is in fuel applications
where the burning of lignin produces more than sufficient energy for the powering of
the pulping plant. The chemical composition of kraft lignin is influenced by a number
of factors including wood species and cooking conditions but typical molecular weights
of the lignin polymers (and oligomers) are in the range of 1-5 kDa with a relatively
high polydispersity (M W / M n ∼ 3-4). The kraft process is relatively harsh on the native

lignin structure leading to significant depolymerisation. Furthermore, the nucleophilic
attack by the hydrogen sulphide ion on the β -O-4 ether linkages results in a significant
increase in the charge on the macromolecules with up to 13% of phenolic monomers
containing a carboxyl group (compared to MWL which is virtually uncharged). The
presence of the charged functional groups increases the solubility of lignin substantially
under alkaline conditions with the solution properties of kraft lignin to be discussed in
the next section.
Kraft lignin is usually isolated by precipitation of the spent liquor (black liquor) that
remains after cooking through acidification, where sulphuric acid, hydrochloric acid but
also carbon dioxide is used ( Ohman et al . 2007). During the acidification large amounts
of gaseous dihydrogen sulfide are released. Depending on the pH value to which the
black liquor is acidified, different composition and yield of the lignin is obtained. If the
liquor is brought to pH 2 in one step more or less all of the lignin, including the highly
charged low molecular fraction, is precipitated. Kraft lignin may also be separated in
relatively large scales by ultrafiltration of the black liquor and precipitated as above
(Wallberg et al . 2003). Thereafter the precipitated kraft lignin is washed and finally
filtered and dried. Further purification to remove extractives and carbohydrates is done
according to methods described elsewhere.