25. LI, J. and van HEININGEN, A.R.P., Kinetics dium Sulfate-Potassium Sulfate Eutectic: Re- Thesis, McGill Univ. (1991). of Gasification of Black Liquor Char by actions of Some Sulfur Compounds, Inorg. 28. WYNNYCKYJ, J.R. and RSUKIN, W.J., An Steam, IEC Research 30(7):1594 (1991). Chem. 22(22):3243 (1983). Intrinsic Transport Model for Solid-Solid Re- 26. DEARNALEY, R.I., KERRIDGE, D.H. and 27. ZOU, X., Recovery of Kraft Black Liquor actions Involving a Gaseous Intermediate, ROGERS, D.J., Molten Lithium Sulfate-So- Including Direct Causticization, Ph.D. Metallurgical Trans. B 19B(2):73 (1988). Behaviour of Residual Lignin in Kraft Pulp During Bleaching D. LACHENAL, J.C. FERNANDES and P. FROMENT The behaviour of residual lignin in tive has been to replace chlorine, hypo- The approach described in this paper softwood kraft pulp during bleaching has chlorite and, if necessary, chlorine dioxide consists of analyzing the actual effect of the been investigated. Chlorine, chlorine diox- by oxygen-based reagents (oxygen, hydro- bleaching chemicals on the residual lignin. ide, oxygen, hydrogen peroxide and ozone gen peroxide, ozone, peracids), it is of prime Basically two factors strongly influence lig- treatments have been carried out on un- importance to have a better understanding of nin solubilization during bleaching. The first bleached pulp and the residual lignin has the reasons why the chlorine-containing one is the amount of hydrophilic groups been extracted by enzymatic hydrolysis of chemicals are such efficient bleaching (mainly phenolic and carboxylic) and the the carbohydrates. The main functional agents. The role of chlorine during bleaching second one is the size of the degraded lignin groups have been measured by I9F NMR was re-examined recently and a new mecha- macromolecules. Several experimental pro- spectroscopy and the molecular weight dis- nism has been found to explain its outstand- cedures had to be developed. First the resid- tribution has been studied. New information ing reactivity on lignin [2]. Chlorine dioxide ual lignin was extracted in an almost on the chemistry of the bleaching agents has bleaching has also been under investigation quantitative way after each bleaching stage. been obtained, which explains some of the in many places during recent years [3-61, Enzymatic hydrolysis of the carbohydrate differences observed in their bleaching ef- primarily to try to inhibit the side reactions. fraction was achieved [14-15] by using a fect. For example, chlorine has the capabil- Simultaneously, oxygen, peroxide very powerful enzyme mixture [IO]. Then a ity to depolymerize the residual lignin while and ozone bleaching chemistry has been the new technique based on the analysis of some forming new free phenolic groups, which subject of considerable research activity [7- fluoro derivatives by I9F NMR was applied makes it without any equivalent among the IO]. The merit of these studies has been to for the determination of the main functional bleaching agents. On the contrary, ozone explain why chlorine-free bleaching is fea- groups in the lignin structure [16-171. The does not cleave the residual lignin very effi- sible, where the problems are coming from molecular weight distribution of lignin was ciently. Consequently, it must be associated and how some of them could be solved. also measured by gel permeation chroma- with another chemical which has this capa- However, the understanding is still tography. bility, like oxygen or chlorine dioxide. fragmented and does not give satisfactory With these tools in hand there was no answers to many questions, especially those obstacle to obtaining new and informative !NTROD!JCTION which relatc to thc optimum order of appli- rzsults cjn the effect of the bleaching chenii- The recent concern [I] about the fate cation during bleaching, and to the respec- cals (chlorine, chlorine dioxide, oxygen, of chlorinated organic compounds down- tive delignification power of the various hydrogen peroxide and ozone) on residual stream from a kraft mill has generated reagents. The reason for this lack of informa- lignin and to improve our understanding of unprecedented research activity in the tion comes partly from the fact that the actual the bleaching process. bleaching area. Although the major objec- modifications brought about to the residual lignin during the bleaching stages have very D. Lachenal, J.C. Fernandes and P. Froment seldom been investigated [ 11-1 31. The reac- EXPERIMENTAL rS Ecole Franqaise de Papeterie tions of lignin model compounds or of kraft Bleaching stages B.P. 65 lignin cannot fully represent the behaviour An unbleached softwood (roughly 38402 Saint Martin d'Heres of the insoluble and relatively unreactive 50% Picea abies, 50% Pinus sylvestris) kraft Cedex residual lignin embedded in a cellulosic ma- pulp of kappa number 30 was used in this France trix. study. JOURNAL OF PULP AND PAPER SCIENCE: VOL. 21 NO. 5 MAY 1995 J173 The bleaching stages were carried out COOH groups in lignin was performed by covered by precipitation with water, washed under the following conditions: reaction with the 4-fluorophenyldiazo- with water and dried. I9F NMR spectra were - Chlorine bleaching (C): room tempera- methane according to: performed as before. Precision of the data is ture, 3.5% consistency, 60 min, 2 and 4% around 10%. C12 on pulp. After treatment the kappa RCOOH + F - C&,CHN2 numbers were 14.2 and 7.4, respectively. Molecular Weight Distribution - Chlorine dioxide bleaching (D): 70°C, --> RCO - O - CH,C,H,F of Lignin 10% consistency, 60 min, 2 and 5% ac- The lignin samples were dissolved in tive Cl2 on pulp. After treatment, the The 4-fluorophenyldiazomethane was DMF and their elution curves obtained by kappa numbers were 21.1 and 12.0, re- obtained from 4-fluorobenzylamine follow- HPLC (polystyrene gel: PL gel mixed D, spectively. ing the procedure described by Overberger 2 columns) with a UV detector (flow rate - Oxygen bleaching (0):1 OO°C, 10%con- and Anselme [ 181. 0.5 mL/min, 60°C). sistency, 60 min, 3% NaOH on pulp, The 4-fluorophenyldiazomethane was 0.5 MgS04, 7H20 on pulp, 5 bars oxy- recovered as a 0.5 moIL solution in ether Analysis of Carbohydrates gen. After treatment the kappa number and was used just after its preparation. The Lignin (100 mg) was treated with was 14.1. lignin previously treated with 4-fluoroben- 20 mL of a 2 mol/L solution of trifluoro- - Hydrogen peroxide bleaching (P): 90°C, zoic anhydride (400 mg) was dissolved in acetic acid at boiling temperature for 4 h. 10% consistency, 120 min, 2% NaOH on 60 mL of dioxane and 20 mL of the solution The hydrolysate was extracted twice with pulp, 3% H202 on pulp. After treatment of 4-fluorophenyldiazomethane. After 24 h ethyl ether and evaporated to dryness. The the kappa number was 20.0. at 40°C, the volume was reduced to 5 mL by residue was redissolved in 2 mLof water and - Ozone bleaching (Z): room temperature, evaporation under reduced pressure. The analyzed by HPLC (Column: Polysphere 35% consistency, 1.2% ozone consumed derivatized lignin was recovered by precipi- OH- Pb, Merck; solvent: water; flow rate: on pulp. Before treatment the pulp was tation with ether, washed with ether and 0.4 mL/min; temperature: 80°C). acidified to pH 2.5 with sulphuric acid. dried. After treatment the kappa number was 19F NMR spectra of the derivatives RESULTS AND DISCUSSION 15.0. were recorded at 188.226 MHz on a Bruker Characterization of AC 200 spectrometer as already described Residual Lignin Extraction of Residual Lignin [16-17]. The phenolic and alcoholic OH Extraction of lignin by the enzymatic Enzymatic hydrolysis was performed groups were measured after the first esterifi- treatment gave preliminary information on on the unbleached and on each treated pulp cation and the carboxyl groups after the sec- its solubility at pH 4.6. Table I shows the with a commercial cellulase-hemicellulase ond one. Precision of the data is around 10%. respective weight (in percent of the theoreti- preparation (ONOZUKA R-10, Yakult Phar- cal content of lignin in pulp) of the insoluble maceutical Industries, Tokyo Japan). Hy- Determination of and soluble fractions. Most of the residual drolysis was performed at pH 4.6 (acetic Carbonyl Groups lignin went into solution during enzymatic acid-sodium acetate buffer) and 37°C. Five The carbonyl groups present in lignin hydrolysis after oxygen, chlorine dioxide successive treatments of 72 h each were were reacted with the 4-fluorophenylhy- and hydrogen peroxide bleaching, indicat- carried out. The residue was dissolved in drazine according to: ing that these bleaching stages caused some dioxane-water (9: 1) and after evaporation depolymerization and created new hydro- \ solubilized in DMF and then precipitated / C = 0 + NH2 - NH - C,H,F philic groups. Solubilization was less after with ether. chlorination and very low after ozonation, \ Part of the lignin went into solution -> C = N - NH - C6H4F indicating that chlorine and ozone differ during the enzymatic hydrolysis. It was re- radically from the other chemicals in their covered by acidification to pH 2 with HCI, Lignin (100 mg) was dissolved in action on lignin. dissolution in DMF and precipitation with 5 mLofdioxane/DMF(l:l)andmixed with Table I1 gives the amount of the major ether. a solution of 4-fluorophenylhydrazine functional groups in residual lignin after The yield of lignin represented at least (100 mg of 4-fluorophenylhydrazine in each bleaching stage. The quantity is ex- 80% of the theoretical content in pulp calcu- 2 mL of DMF and 0.5 mL of orthophos- pressed as the number of groups per 200 g lated according to the formula: lignin con- phoric acid). The mixture was allowed to of lignin. The results show substantial differ- tent in pulp (in per cent) = 0.15* kappa react for three days in the dark at ambient ences between the bleaching chemicals.