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New concepts for upgrading pulp mill waste streams to value-added chemicals

Final Report Summary - WACHEUP (New concepts for upgrading pulp mill waste streams to value-added chemicals)

The 'New concepts for upgrading pulp mill waste streams to value-added chemicals' (WACHEUP) project investigated how to upgrade residual products from pulp and cork manufacture to create bio-based chemicals. Many positive results emerged from the project.

Separation and updrading of lignin

Lignin with good quality, i.e. low sodium content, was successfully precipitated from black liquor. The filtration resistance for both hardwood and softwood lignin could be improved through a lower precipitation pH.
Precipitated HW lignin showed to be more difficult to filter than SW lignin, however, which was presumed to be due to the higher hemicellulose content. In order to improve the filtration resistance, three different pre-treatment techniques for decreasing the hemicellulose content were investigated: heat-treatment, ultrafiltration and a combination of ultrafiltration and nanofiltration. All of the three pre-treatment techniques tested were successful in decreasing the filtration resistance of the thereafter precipitated HW lignin. The molecular mass was, as expected, lower for lignin precipitated from UF permeate. The molecular mass was lower for the heat treated lignin as well, indicating a degradation of lignin into smaller parts during the heat treatment, in addition to the degradation of hemicelluloses.

The initial separation of lignin from hemicelluloses in HW black liquor was successfully performed with membrane technique: UF using a 15 kDa ceramic membrane at low transmembrane pressure and high cross-flow velocity.

Hemicelluloses are preferentially retained, whereas lignin to a large amount passes through the membrane.

The combination of ultrafiltration and nanofiltration resulted in a hemicellulose-poor permeate with an increased lignin concentration. The filtration resistance of the finally precipitated lignin was still low despite the fact that not only lignin, but also (smaller) hemicellulose molecules were concentrated in the nanofiltration permeate.

Lignin from UF permeate contain more phenolic hydroxyl groups than retentate lignin, which is an advantage if the lignin is to be used as raw material for phenols. The phenolic content could also be increased by a lower precipitation pH.

The sulphur content is also higher in permeate, which might be a drawback for some product applications. Washing of the lignin did not decrease the sulphur content, indicating that the sulphur is covalently bound to the lignin and cannot be washed out with acidified wash water.

The developed method for separation of hemicelluloses before the lignin precipitation gives a solution to the problem with the dewatering of hardwood (especially birch) lignin. It will also produce a lignin with thermal properties suitable for spinning – the first process step in the production of carbon fibres. The results provide a valuable starting-point for the design and optimization of future plants designed to extract and refine lignin from various liquors in kraft pulp mills.

Birch and eucalyptus black liquors

In the case of eucalyptus cooking, the concentration of lignin was always higher than that of xylan, throughout the cook, and the highest xylan concentration was observed at a much later cooking phase. Moreover was the xylan concentration in the spent eucalyptus liquors much lower that that of the corresponding birch liquors.

This was partly due to the fact that the eucalyptus wood chips contained only half as much xylan (around 15 %) as the birch wood chips (around 30 %).

An ultrafiltration / diafiltration process sequence was developed for the isolation and upgrading of xylans from the spent black liquors. Xylan is isolated through ultrafiltration with a ceramic membrane at elevated temperature. Diafiltration is applied in order to improve the purity of the xylan.

Black liquor from the initial heating-up phase in birch kraft cooking exhibits a high xylan and low lignin content. An optimal point of xylan extraction was identified: after ca 80 minutes (i.e. in the heating-up phase). At this point, the xylan concentration had reached its maximum, while the concentration of lignin was still low.

Hemicelluloses from mechanical pulping

Effluents from TMP mills are a potential rich source of AcGGM. Conventional ultrafiltration techniques can provide a concentrate containing about 70 % of GGM.

The spruce TMP waters were a richer source of AcGGM than the CTMP waters. The pilot-scale isolation of AcGGM at the TMP mill was successful and about 20 kg technical grade AcGGM concentrate was produced and purified. Removal of lignin with XAD sorption followed by precipitation with ethanol addition increased the hemicellulose purity to over 90 %.

The physicochemical properties of AcGGM were assessed. Acid hydrolysis was extensive at pH 1 at 90 degrees Celsius, but was insignificant at human stomach conditions (pH 2, 37 degrees Celsius), a valuable property in some pharma and food applications.

Carboxymethylation of GGM was rather straight-forward, but did not provide any advantages over the original AcGGM regarding stabilisation of TMP pitch. However, one CM-GGM preparate gave superb stabilisation of oil-water emulsions, implying that CM-GGM preparates have promising applications in food and feed applications, as well as in non-food applications such as cosmetics.

The results have raised an industrial interest in recovering AcGGM and in finding high-value applications for this natural polysaccharide.

Extraction of bark / cork compounds

Cork oak samples have lower extractives contents (6-9 %) and higher contents of carbohydrates and lignin (23-27 % and 33-38 %, respectively) than those found for birch outer bark (40, 6 and 9 %, respectively). Suberin contents accounted for around 30 % of cork, 11 % of industrial cork powder and 45 % of birch outer bark.

A conclusion is that industrial cork by-products can be considered as abundant sources of triterpenic compounds, particularly betulinic acid and friedeline, which are known to have promising applications as bioactive substances. Lipophilic extracts of cork and cork by-products were quite rich in triterpenic compounds. Industrial cork powder (ICP) is particularly rich in betulinic acid (around12 g/kg), whereas in natural cork (QsC) the major component was cerine (4.6 g/kg), followed by friedeline and betulinic acid. This can be partially due to the fact that ICP is mainly composed by the outer (ONC) and inner (INC) fractions of cork planks, whose composition is quantitatively different from that of the whole cork, and also because of the boiling treatment and the natural variability of cork.

Depolymerisation of bark / cork suberin

It was soon noticed that suberin matrix is recalcitrant to enzymes. Various pretreatments were tested and the traditional steam explosure was found to be efficient in opening up the suberin structure prior to enzyme treatment.

Natural polymers, birch outer bark and cork suberin and apple cutin were treated with various esterase preparations in different conditions. Coprinus cutinase CcCUT1 and the commercial Optimyze preparations provided the most promising suberin depolymerisation results.

The amount of enzyme had a clear effect on the degree of suberin / cutin hydrolysis. Addition of HFBII hydrophobin improved clearly the hydrolytic effect of CcCUT1 on suberin and cutin while having no advantage for the efficiency of Optimyze. Both fatty acid monomers and oligomers were released by the enzymes.

Depolymerisation of bark / cork suberin

It was soon noticed that suberin matrix is recalcitrant to enzymes. Various pretreatments were tested and the traditional steam explosure was found to be efficient in opening up the suberin structure prior to enzyme treatment.

Natural polymers, birch outer bark and cork suberin and apple cutin were treated with various esterase preparations in different conditions. Coprinus cutinase CcCUT1 and the commercial Optimyze preparations provided the most promising suberin depolymerisation results. The amount of enzyme had a clear effect on the degree of suberin / cutin hydrolysis. Addition of HFBII hydrophobin improved clearly the hydrolytic effect of CcCUT1 on suberin and cutin while having no advantage for the efficiency of Optimyze. Both fatty acid monomers and oligomers were released by the enzymes.

Valorisation of bark/cork suberin

It was shown that suberin monomers, especially HEPA, are useful polyester precursors. This opens up a new research / development field for new renewable materials based on bark suberin. The synthesized polymers show unique characteristics, differing from the original suberin macrostructure. Cellulose can be surface-modified (grafted) by use of HEPA in combination with catalysed ring-opening of the epoxy group.

It was shown that it is possible to refine suberin into reactive co-polyesters by use of 'green' enzyme catalysis without the use of organic solvents. HEPA was successfully co-polymerised with caprolactone into epoxy-functionalised linear polyesters and cyclic oligomers (polymer precursors).

Suberin-based hemi esters were prepared from suberin with e.g. maleic, succinic, phthalic and acetic anhydrides.

Some of the synthesis products showed very good surface-tension decreasing properties in their salt form, making them interesting as e.g. components in cosmetics and potential monomers for oligo / polyesters to be used as bindes and lubricants. The surface activity of birch suberin acid Na-salt has nearly identical values with the commodity surfactant, sodium dodecyl sulphate (SDS). The worldwide usage of anionic surfactants is about 4 Mt/a and alkyl sulphates count for about 10 % of this market.

The prepared cork-polyethylene composites showed high dimensional stability and show promise to be developed into novel 'green' composite materials with tailored properties. In fact, it was proven that the incorporation of cork as reinforcements of thermoplastic matrices can be achieved without the incompatibility problems that are usually present when hydrophilic ligno-cellulosic materials are used. The results open up for new lines of research for the natural-based composites field, where the combination of the properties of the thermoplastic and cork can be substantially exploited due to their significant complementarities.

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