Method for optimising material recovery in a chemical pulping process

Abstract

The invention relates to a method for optimising material recovery in a chemical pulping process. The method comprises treating of cellulosic raw material with cooking chemicals in a cooking step, where lignin is at least partly dissolved into cooking liquid phase and separated from fibres. The lignin is separated from the cooking liquid phase. The separated lignin is used for formation of a treatment composition comprising at the most 1 weight-% of aluminium. A bleaching liquid phase is treated with the treatment composition in order to remove organic humic substances from the bleaching liquid. Organic sludge is formed and it is used for energy production in a chemical recovery step.

Claims

1. A method for optimising material recovery in a chemical pulping process, the method comprising treating of cellulosic raw material with a liquid phase comprising cooking chemicals in a cooking step, where lignin and optionally other substances are at least partly dissolved into cooking liquid phase and separated from fibres, separating fibres from the said cooking liquid phase, which comprises used cooking chemicals and dissolved substances from wood, such as lignin, transferring fibres to a bleaching step, where the fibres are brought into contact with bleaching chemical(s), and separating the bleached fibres from the bleaching liquid phase, separating lignin from the cooking liquid phase, recovering the cooking chemicals from the cooking liquid phase in a chemical recovery step and circulating the recovered cooking chemicals back to the cooking step, characterised in using separated lignin for formation of a treatment composition comprising at the most 1 weight-% of aluminium, calculated on basis of the weight of dry lignin in the composition, treating the bleaching liquid phase with the treatment composition and removing of organic humic substances, preferably recalcitrant organic substances, from the bleaching liquid and forming organic sludge, using the organic sludge for energy production in the chemical recovery step.

2. The method according to claim 1, in wherein the separated lignin is cationised.

3. The method according to claim 2, wherein separated lignin is cationised by reacting lignin with a cationisation chemical and/or compound selected from glycidyltrimethylammonium chloride, 2,3-epoxypropyltrimethylammonium chloride and N-(3-chloro-2-hydroxypropyl)-trimethylammonium chloride.

4. The method according to claim 1, wherein the treatment composition is free of aluminium.

5. The method according to claim 1, wherein the treatment composition comprises aluminium at the most 0.7 weight-%, preferably at the most 0.5 weight-%, calculated on basis of the weight of dry lignin in the composition.

6. The method according to claim 1, wherein the aqueous bleaching liquid phase, which is treated with the treatment composition has a pH value under 7, preferably at the most 5, more preferably at the most 3.

7. The method according to claim 1, wherein the method is free of any pH adjustment steps.

8. The method according to claim 1, wherein the pH during the removal of organic humic substances changes at the most 2, preferably at the most 1, pH units.

9. The method according to claim 1, wherein the inorganic content of the organic sludge is less than 30 weight-%, preferably less than 10 weight-%.

10. The method according to claim 1, wherein untreated bleaching liquid phase has a COD value, which is over 1000 g/m.sup.3, preferably over 2000 g/m.sup.3.

11. The method according to claim 1, wherein the treatment composition is dosed in amount, which provides a lignin dose 0.05-2 g/g COD, preferably 0.1-1 g/g COD.

12. The method according to claim 1, wherein at least one flocculating agent, such as polyacrylamide, is added to bleaching liquid phase.

13. The method according to claim 12, wherein flocculating agent is added simultaneously or sequentially with the treatment composition.

Description

EXAMPLE

[0035] Coagulation and flocculation tests for COD removal from bleaching filtrate were carried out in a laboratory of a Scandinavian kraft mill. Tests were carried out with mixed fresh wastewater samples of 5.0 litres from alkaline bleaching filtrate line and 4.7 litres from acid bleaching filtrate lines. Temperature of both samples in experiments was ca. 60 C.

[0036] Tests were done with Jar test equipment Kemira Flocculator 2000 with batch sizes 500 ml. The Jar test equipment was operated in generic step-wise manner: [0037] fast mixing, e.g. 10 seconds at 350 rpm; coagulant addition in the beginning); [0038] slow stirring, e.g. 5 minutes at 40 rpm; flocculant addition at the end); and [0039] sedimentation, e.g. 15 minutes.

[0040] The coagulants used in the test procedure were

a) inorganic aluminium coagulant (Fennofloc A100, Kemira Oyj), dose 186 g Al/m.sup.3, and
b) experimental cationised lignin product: charge density 2.2 meq/g at pH 4, 1.5 meq/g at pH 7.5; dose 200 g lignin/m.sup.3, calculated as lignin dry solids.

[0041] Flocculant was

a) nonionic polymer from Fennopol N series (Kemira Oyj), dose 1.0 g DS/m.sup.3, with inorganic aluminium coagulant (Fennofloc A100, Kemira Oyj); and
b) cationic polymer from Fennopol K series (Kemira Oyj), dose 1.0 g DS/m.sup.3, with experimental cationised lignin product.

[0042] In the mill laboratory turbidity, pH and total COD.sub.cr were analysed from the supernatant, as well as UV-absorption at 245 nm and filtered COD.sub.cr were analysed from supernatant filtered with 0.45 m filter. Dissolved organic carbon, DOC, from filtered sample were later analysed with LC-OCD in laboratory. Total DOC is the sum of all dissolved organic carbon. Humic DOC is the subfraction of total DOC and it is determined with specific molecular size and UV-absorbance at 254 nm. Experimental results of treated and untreated samples are shown in Table 1. The test results indicate that soluble organic COD removal is possible with cationised lignin product.

TABLE-US-00001 TABLE 1 Jar test results. Reference sample, Reference for sample, Inorganic cationised Cationised for inorganic Al lignin lignin Al coagulant coagulant coagulant coagulant pH 5 4.2 5 5.2 Turbidity, NTU 38 16 102 18 UV-absorption 4.3 3.7 4.0 3.9 at 254 nm Total DOC.sub.filt, g/m.sup.3 850 530 770 Humic DOC.sub.filt, 430 180 390 g/m.sup.3 COD.sub.Cr, .sub.filt, g/m.sup.3 2220 1250 2100 1790 COD.sub.Cr, filt 44% 15% removed COD.sub.Cr, .sub.filt 1.55 removal g/g lignin

[0043] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.