Dariusz Danielewicz Barbara Surma-Ślusarska Oxygen Delignification of High-kappa Number Pine Kraft Pulp Technical University of Łódź, Institute of Papermaking and Printing, ul. Wólczańska 223, Abstract Łódź, Poland The possibility of producing easy bleachable pulp (kappa number < 16) from high kappa number (68) pine kraft pulp through its oxygen delignification has been studied. The effect of the application of peroxy compounds in the oxygen delignification sequences of such pulps was also established. The yield and properties thus produced easy bleachable pulps were compared to the properties of pulp having similar bleaching susceptibility, produced in a conventional method (i.e. by wood pulping to kappa number approx. 30 and then oxygen delignification). The effect of factors such as: higher pH of medium, elevated temperature and presence of metal ions on stability of peracetic acid solution was also determined. The most important conclusion from carried out studies was finding that fibrous semi product produced in the process of oxygen delignification of pine kappa number 68 pulp with or without the use of peroxy compounds show higher yield (by approx. 2.5% on wood) compared with pulps produced by means of conventional method. Key words: high kappa number pine kraft pulp, oxygen delignification, degree of deligni- fication, peracetic acid, hydrogen peroxide, fibre properties. n Introduction reinforcement of the delignification ef- indices. Our earlier investigations proved fect by the addition of hydrogen peroxide that fibres of pulp with a kappa number The production costs of high-grade [6] or the use of catalytic agents [7] and of 68 are less damaged, longer, exhibit bleached pulps with high and stable enzymes [8]. higher coarseness and larger width, and are brightness and appropriate mechanical less susceptible to cross-deformation. By strength can be lowered above all by an For lignin activation, peracetic acid means of using atomic force microscopy increase in pulp yield [1] as well as an (CH3COOOH) [9], and inorganic per- (AFM) differences have also been noted optimisation of the bleaching process sulphate compounds such as Caro acid in the chemical composition of the fibre technology [2]. Approx. 80% of the pulps (H2SO5), 2KHSO5·KHSO4·K2SO4 surface layers between this pulp and pulp used at present for the production of high- (known as Oxone) [10] and sodium per- of kappa number 28 [12]. brightness, uncoated, non-wood printing oxysulphate (Na2SO5) can be used. Per- papers are manufactured by means of the acetic acid and Caro acid are produced on n Aim of work chemical method, the so-called sulphate a commercial scale, although their price method or kraft process, in which 50% of in comparison with the price of oxygen The aim of the work was to define the raw material is dissolved. The increase of and hydrogen peroxide is much higher. conditions for the oxygen delignification pulp yield in this process by one per cent Sodium peroxysulphate can be produced of high-kappa number pine kraft pulp (e.g. from 42% to 43%) decreases wood by a copper ion (Cu++)-catalysed oxida- that will enable easily bleachable semi- consumption by approx. 2.4%, and thus tion of sodium sulphate with oxygen in a product to be obtained for the production lowers the raw material costs, which have strongly alkaline medium [11]. of high-brightness, non-wood printing been showing a steady upward trend [3]. papers, and to determine the effects that At present, an oxygen delignification in According to literature, the amount of can be achieved by the application of alkaline medium (a method first used in lignin removed from pulp in a single- peroxy compounds in the oxygen delig- 1970) is commonly used for the further oxygen stage usually does not exceed nification of such pulps. delignification of kraft pulps before their bleaching by the ECF or TCF method. In 40 to 50% [5]. In order to achieve higher order to optimise these processes in regard delignification degrees of pulp without n Experimental its deterioration (i.e. decreased yield and to pulp yield and quality, various modifi- Pulps cations are investigated including an op- strength), it is suggested that preliminary tion which uses cooking at a relatively lignin activation is applied by means of For these experiments, the following high content of residual lignin in pulp chemicals with higher oxidation poten- pulps were used: high-yield pine kraft (corresponding to kappa number > 40), tial than oxygen, or that delignification pulp (kappa number 68) produced in the and then continuing the process of lignin should be divided into two stages. Oxy- laboratory at an average yield of 51.8%, removal by a more selective method of gen delignification of pulps with high and pine kraft pulp (kappa number 28) oxygen delignification [4]. The success of lignin content (kappa number > 40) is produced in the laboratory at an average such a solution will depend on profit and characterised by special features. On the yield of 45.4%. Pine wood chips were cost balance. Preliminary estimates seem one hand, the amount of lignin to be re- used as raw material for the production to support such modification [1]. moved is much higher than in the case of of these pulps. Pulping was carried out in conventional pulps with a kappa number a rotary Hatoo digester in 3l autoclaves. The attractiveness of oxygen delignifica- of 30; on the other hand, the residual The chips were preliminarily vacuum- tion of pulps also results from the pos- lignin in pulp of a high kappa number impregnated with cooking liquor of 30% sibility of applying different modifica- may be less altered chemically, and thus sulphidity which in the case of the high- tions to improve its effectiveness. These easier to remove. kappa number pulp contained 18.5% of include the preliminary or inter-stage active alkali (as NaOH) relative to o.d. activation of lignin [5], the removal of The replacement of part of the pulping wood, while this figure was 23% of ac- heavy metal ions participating in the for- process with oxygen delignification (which tive alkali relative to o.d. wood in the mation of hydroxyl radicals that take part is considered to be more selective) should case of conventional pulp. The heating in cellulose depolymerisation, and the result in an improvement of pulp strength times and pulping temperatures were FIBRES & TEXTILES in Eastern Europe April / Juni 2006, Vol. 14, No. 2 (56) 89 the same in both cases, that is, 90 min Determining the pulp’s properties a chelation stage (Q). The individual and 164 °C respectively. The pulping The pulping degree (kappa number) was series of experiments, whose results are time at maximum temperature was 90 determined by lignin oxidation with po- represented by the curves in Figure 4, min for high yield pulp, and 150 min tassium permanganate (PN-70/P50095) differed from each other in the initial for conventional pulp. After pulping, the [13]. The pulp brightness –was classified pH in the peracetic acid treatment, al- pulp was preliminarily washed with dis- according to International Standard ISO though a constant temperature of 70 °C tilled water on the wire, then by diffusion No 36688 (1997) ‘Pulp Measurement of was maintained. The amount of residual for 24 hours, and finally re-pulped and diffuse blue reflectance factor (ISO bright- active oxygen was determined after dif- screened. After having been separated ness)’ [14]. The fibre strength factor (FS- ferent reaction times within the range of from pulp, any knots were refined in a factor) was tested in a Zero-Span Trou- 5 to 120 minutes. The course of the laboratory impact mill and then added to bleShooter Tester TS-100, Pulmac [15]. curves in Figure 4 shows above all the screened pulp. Next, the treated pulp that the consumption of peracetic acid was centrifuged, and after determining its during the treatment of pine kraft pulp dryness it was stored in a refrigerator in n Discussion of results with a kappa number of 42.2 is faster closed polyethylene bags. in those experiments in which the pulp Defining pulp treatment conditions was not treated with a chelating agent, Delignification and bleaching with peracetic acid or/and when the treatment of pulp was chemicals As mentioned above, one promising performed at a higher pH (5.5 or 7). In method of lignin activation is the treat- all probability, the faster consumption Oxygen, distilled peracetic acid (CH3- -COOOH, Fluka Chemica), EDTA, hy- ment of pulp with peracids, above all of peracetic acid in these experiments drogen peroxide. with peracetic acid. The amount, tem- is the result of transition metal catalysed perature and pH are important param- decomposition of peracetic acid, or its Oxygen delignification eters of the pulp treatment process with spontaneous decomposition taking place this chemical. Considering this, prior at higher pH values. Thus, the obtained The weighed portion of pulp (40 g o.d.) to experiments with pulp, the effect of results confirm the earlier stated ten- was placed in a polyethylene bag, and initial pH on the stability of a distilled dency of peracetic acid decomposition then the following chemicals were add- peracetic acid solution at temperatures under the influence of heavy metal ions ed: 0.5% magnesium sulphate relative of 50 °C, 70 °C and 80 °C was studied. [17], and the higher stability of this agent to o.d. pulp (in the form of a 1% solu- This effect was evaluated on the basis of at lower pH [18]. tion), and then an appropriate amount of determining the active oxygen concentra- sodium hydroxide. The bagged content tion (A.O.) in a peracetic acid solution by Taking into consideration the results of was hand-mixed and then transferred in means of the titration method [16].