Precipitation of kraft lignin under alkaline conditions

82 Pages·2000·0.96 MB·English

by  Jonas Sundin

TRITA-PMT REPORT 2000:8
ISSN 1104-7003
ISRN KTH/PMT/R--00/8--SE
Precipitation of Kraft Lignin under
Alkaline Conditions
Jonas Sundin
Royal Institute of Technology
Department of Pulp and Paper Chemistry and Technology
Stockholm 2000

Precipitation of Kraft Lignin under
Alkaline Conditions
Jonas Sundin
Doctoral Thesis
Royal Institute of Technology
Department of Pulp and Paper Chemistry and Technology
Stockholm 2000
Division of Pulp Technology
Department of Pulp and Paper Chemistry and Technology
Royal Institute of Technology
SE-100 44 Stockholm
Sweden
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i
Stockholm framläggs till offentlig granskning för avläggande av teknologie
doktorsexamen, fredagen den 1 december 2000. Avhandlingen försvaras på svenska.
TRITA-PMT REPORT 2000:8
ISSN 1104-7003
ISRN KTH/PMT/R--00/8--SE
Stockholm 2000 KTH Högskoletryckeriet
To my dear wife Maria,
and to my wonderful children
Richard, Jonathan and Linnea

Abstract
In this work, the influence of metal cations on the removal of lignin during kraft pulp
washing has been studied. The interaction of metal cations with kraft lignin in alkaline
solutions has been elucidated by, among other things, the application of ion-selective
electrodes. The effect of the interaction of metal cations with kraft lignin in pulp
washing, as well as its consequences in oxygen delignification and hydrogen peroxide
bleaching stages are reported. An attempt is made to relate the laboratory findings to
industrial process conditions.
The high molecular mass fraction (>1000 g/mol) of kraft lignin was precipitated by
metal cations (Na+, Ca2+, Mg2+, Al3+) at pH 9, but only calcium and magnesium ions
caused any noticeable precipitation at pH 11-13. Since only the high molecular mass
fraction of the lignin was precipitated, the precipitation can be regarded as the
coagulation of a colloid. The critical coagulation concentration (ccc), that is, the
concentration of the metal cation when the coagulation occurs, was about 3 mmol/L
for Ca2+ at pH 11 and ambient temperature. The ccc(Ca2+) was the same for birch and
spruce kraft lignin but higher for dissolved lignin after oxygen delignification.
A method was developed to estimate the free calcium ion concentration in lignin
solutions during titration with calcium ions by the use of ion-selective electrodes. An
end point was detected for the reaction between calcium ions and lignin. At calcium
ion concentrations below the end point, almost all the added Ca2+ was bound to the
lignin. The maximum content of calcium ions bound to the lignin was found to be
about 30 Ca2+/100 C-9 units in the kraft lignin. This was confirmed by two other
methods.
The precipitation of kraft lignin by calcium ions during washing of a laboratory-
cooked kraft pulp resulted in a pulp with a kappa number 15 units higher than a pulp
washed in the absence of calcium ions. The degree of swelling of the fibres in these
pulps was constant. The corresponding difference in kappa number for laboratory
washing of an industrial unbleached softwood kraft pulp was 5 kappa number units
for a pulp produced for bleached products and 10 kappa number units for a pulp
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produced for unbleached products (sackpaper grade).
The precipitated lignin was darker than the other residual lignin in the pulp. Pulps
containing calcium-precipitated lignin yielded results on oxygen delignification
similar to those of pulps without precipitated lignin. In an alkaline hydrogen peroxide
stage, the precipitated lignin seemed to be more reactive than the other part of the
residual lignin. This is probably due to a higher content of phenolic groups in the
precipitated lignin.
An attempt to estimate the effect of precipitation of lignin by calcium ions during
industrial brownstock washing in a mill, where the calcium and magnesium
concentration in a number of filtered liquors was determined, resulted in 1-2 units
increase in kappa number for a pulp produced for bleached products in a fibre line
with an oxygen stage. The estimate of the effect of magnesium ions is uncertain, but it
may be considerably higher due to the higher concentrations of magnesium in process
liquors.
Keywords: Alkali lignin, Black liquors, Bleaching, Calcium, Coagulation, Colloids,
Kraft pulp, Magnesium, Metals, Mills, Precipitation, Washing
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Contents
INTRODUCTION.........................................................................................................1
Washing of kraft pulps............................................................................................2
Chemical processes and operations in pulp washing..................................3
Technical aspects of pulp washing..............................................................6
Lignin......................................................................................................................6
Determination of metal ions in pulp process liquors...............................................8
Objectives..............................................................................................................10
RESULTS AND DISCUSSION..................................................................................11
Precipitation of kraft lignin by metal cations in alkaline solutions.......................11
Influence of different metal cations..........................................................11
Influence of molecular mass and functional groups.................................16
Influence of different lignins.....................................................................19
Precipitation of kraft lignin by metal cations during pulp washing......................22
Influence of pH, [Ca2+] and temperature..................................................22
Fibre swelling............................................................................................24
Influence of wood species.........................................................................27
Influence of lignin concentration..............................................................28
Influence of precipitation of lignin during pulp washing on oxygen delignification
and pulp bleaching.................................................................................................31
Oxygen delignification..............................................................................31
Chromofore content in kraft and oxygen-delignified pulps......................34
Final bleaching..........................................................................................39
Studies on the interaction between kraft lignin and calcium ions using an ion-
selective electrode.................................................................................................42
Determination of end points......................................................................42
Determination of free calcium ion concentration during titrations...........45
Concentration of metal ions in industrial brownstock washing and its influence on
lignin precipitation................................................................................................51
CONCLUSIONS..........................................................................................................57
ACKNOWLEDGMENT..............................................................................................60
LITERATURE.............................................................................................................62
APPENDICES.............................................................................................................70
Definition of technical terms and abbreviations....................................................70
Nomenclature in bleaching stages.........................................................................72
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List of papers
This thesis is a summary of the following publications, referred to in the text by the
roman numerals I-VI.
I Sundin, J. and Hartler, N., “Precipitation of kraft lignin by metal cations in
alkaline solutions”, Nord. Pulp Pap. Res. J. 15(4):312-318 (2000).

II Sundin, J. and Hartler, N., “Precipitation of kraft lignin by metal cations during
pulp washing”, Nord. Pulp Pap. Res. J. 15(4):320-325 (2000).

III Sundin, J., “Influence of precipitation of lignin during pulp washing on oxygen
delignification and pulp bleaching”, Submitted to Nord. Pulp Pap. Res. J.
(2000).

IV Sundin, J. and Svedberg, M., “Studies on the interaction between kraft lignin
and calcium ions using an ion-selective electrode. Determination of end points”,
Submitted to J. Pulp Pap. Sci. (2000).

V Sundin, J. and Svedberg, M., “Studies on the interaction between kraft lignin
and calcium ions using an ion-selective electrode. Determination of free calcium
ion concentration during titrations”, Submitted to J. Pulp Pap. Sci. (2000).

VI Sundin, J. and Hartler, N., “Concentration of metal ions in brownstock washing
and its influence on lignin precipitation”, Submitted to Nord. Pulp Pap. Res. J.
(2000).
These publications are appended at the end of this thesis.
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