INFLUENCE OF MAGNESIUM ADDITION ON HYDROGEN PEROXIDE BLEACHING OF THE ACID TREATED PINEWOOD KRAFT PULP

Adam Wójciak
Institute of Chemical Wood Technology, The August Cieszkowski Agricultural University of Poznan, Poland

ABSTRACT

The effects of preliminary acid treatment and magnesium sulfate addition during hydrogen peroxide kraft pulp bleaching was evaluated. The magnesium retention level is similar after the acid treatment and after chelation. Independently of differences in native magnesium contents in pulp the addition of magnesium sulfate stabilizes hydrogen peroxide and limits its ability for delignification. Magnesium charge should be optimized for each type of pulp. Too low or too high level of magnesium causes the decrease of brightness and limits delignification. While the excess of magnesium does not influence negatively the degree of polymerization of cellulose.

Key words: bleaching, hydrogen peroxide, magnesium sulfate, kraft pulp.

INTRODUCTION

Aspects of environment protection as well as related legislation support the practical promotion of the technology of chlorine free bleaching of kraft pulps (TCF-Totally Chlorine Free). Hydrogen peroxide is one of the basic reagent applied in different schemes of TCF bleaching [7,16]. The merits of hydrogen peroxide are the ease of use and high reactivity with respect to chromophoric groups of residual lignin which are responsible for coloring cellulose pulps [17,19]. However, bleaching with the peroxide (P) firstly requires preliminary treatment removing heavy metals from pulps. The metals are responsible for catalyzing the decomposition of the peroxide anion in the alkaline medium leading to the formation of hydroxyl radicals. This in turn, results in the decrease of cellulose polymerization degree as well as the strength properties of pulps [10].

Two methods are known for the removal of metals from cellulose pulps: chelation (Q) and acid treatment at pH of 1.5 - 3 (A) [11]. Chelation effectively removes heavy metals. However, it has some disadvantages, i.e. relatively high costs of the treatment and possible unfavorable influence on the environment (accumulation of poorly decomposing chelates in water basins and increase of NO_x emission in case of chelates burning). Acid treatment is cheaper and makes it possible to recycle the acid in production processes. However, Bouchard et al. [2] suggest that not only heavy metals are transferred to the acid solution but also alkaline earth metals, including magnesium which may act as a natural stabilizer of the peroxide during the treatment. That is why the addition of magnesium ions to the pulps is required after the acid treatment in order to prevent cellulose from undesirable decrease of viscosity. It was proved that the combination of the optimized acid treatment (A) and addition of magnesium ions at the (P) stage may be an alternative to chelation [2,12]. The efficiency of the peroxide stabilization by magnesium ions at the presence of poorly removable catalytic active metals depends among others, on the amount of the added magnesium compound. The improper dosage of magnesium sulfate may cause the decrease of brightness as well as kappa number [2]. A negative influence of the magnesium excess on kraft pulps viscosity was also reported. However, experimental results were not given [2,10]. It should be mentioned that the majority of papers reporting the action of magnesium compounds during bleaching concern high yield pulps [6,1] or kraft pulps oxygen delignification stage [4,15]. Only few data on magnesium operation during peroxide bleaching of kraft pulps are available.

The present paper reports results of the study of oxygen delignified kraft pulp bleaching with the use of hydrogen peroxide, taking into consideration the estimation of the influence of the magnesium sulfate charge on indices of the technological process.

SCOPE AND METHODS

An oxygen delignified pinewood kraft pulp (O) was provided by International Paper Kwidzyń (Poland) mill. Conditions of oxygen delignification were as follows – pulp concentration 28%, oxygen pressure in high-pressure reactor 4-4.5 bars, stabiliser MgSO₄ × 7H₂O in quantity of 0.2% vs. o.d. pulp, NaOH 2% vs. o.d. pulp. The oryginal kappa number of this pulp was 16, brightness 37.43% and viscosity 883 cm³*g^-1. Pulps used in the study were prepared in form of soft sheets. All experiments on pulp bleaching were made in zip-lock PET bags maintained in a constant temperature water bath. The bags were hand mixed at 15 min intervals.

In order to determine if native magnesium influences the delignification value, two other pinewood kraft pulps were used: laboratory pulp with kappa number 30 obtained after cooking the chips in Hägglund autoclave heated in glyceryne bath and pulp with kappa number 58 produced by continous cooking method at Frantschach Swiecie mill (Poland).

Acid treatment and chelation
Acid stage (A) was carried out at 8% pulp concentration for 30 min and for one hour (two variants) at 50ºC. The pH of the treatment (between 1 and 4) was adjusted with sulfuric acid.

Chelation stage (Q) was done for one hour at 60ºC and at 5% pulp concentration, using ethylenediaminetetraacetic acid (EDTA) in quantity of 0.5% vs. o.d. pulp (pH of suspension 4.33). After the treatments, the pulps were washed with distilled water to neutral pH and dried at room temperature.

Hydrogen peroxide bleaching
The influence of native magnesium content in kraft pulp on delignification value was defined by using bleaching conditions allowing the total consumption of hydrogen peroxide (100%). The 5% solution of hydrogen peroxide was used in this study and pH 11 of the bleach slurry was adjusted with NaOH. Investigations were carried out at temperature 80°C and 10% pulp concentration. The treatment time was controlled by iodometric titration.

Bleaching experiments of the oxygen delignified pulps (after acid treatment or chelation stage) were made by charging 2% hydrogen peroxide vs. o.d. pulp at 90°C and at 10% pulp concentration. The pulps were treated over a range of times from 10 min up to 240 min. Pulp slurry pHs (10.8) were adjusted with NaOH. The influence of magnesium sulphate charge on chemical and technological indices of the process was defined by charging 0.05/ 0.1/ 0.2/ 0.3/ 0.5 % MgSO₄ × 7 H₂O vs. o.d pulp, maintaining the above mentioned parameters of pH, treatment time, temperature and pulp concentration. The sequence of reagents addition was as follows: water, pH controller (NaOH), magnesium sulphate (stabiliser), hydrogen peroxide.

After completing the bleaching process, pulps were washed with distilled water to neutral reaction and dried at room temperature.

All chemical charges used in the study are expressed as percent on pulp, oven dry basis (o.d. pulp).

Because hydrogen peroxide treatment (P), acid treatment (A) and chelation (Q) were carried out in Institute of Chemical Wood Technology, Agricultural University of Poznań and the listed stages are the elements of the technological cycle started with oxygen delignification (O) operated in industrial conditions, this paper also uses symbol (O) to indicate integrity of the above mentioned stages of bleaching.

Test methods
Metal ion concentration in the pulps after mineralization in CEM 2000 or MDS-2000 apparatus were determined by AAS method (apparatus Hewlett Packard), and by ICP method (apparatus Varian VISTA-MPX). Reaction of hydrogen peroxide solutions (pH) was determined by pH meter CP-401 Elmetron with automatic temperature compensation at constant solution temperature 22°C and ± 0.01 pH precision. Hydrogen peroxide consumption was determined with ± 0.34% accuracy by iodometric titration using saturated ammonium molybdate as a catalyst. Brightness was determined by Zeiss leucometer [13]. Pulp yield was defined on the basis of moisture content measurements by drying samples to constant weight. Degree of delignification (kappa number) was determined according to PN-P-50095-01/A1 test method [15]. Pulp viscosity was determined by viscosimetric measurements with Cuen solution (PN-92/P-50101/01 test method) [14].

Analytically pure chemical reagents were used in the investigations.

Test results were evaluated by the following indices:

• delignification value, indicating decrease of kappa number vs. its initial value

• delignification selectivity, indicating decrease of lignin content (0.13 as the conversion coefficient of kappa number to lignin content in pulp was used) vs. yield loss of pulp

RESULTS AND DISCUSSION

Acid treatment
The heavy metal content in cellulose pulps varies. The reasons of the variation are mainly related to the differentiation of natural conditions of tree growth used for pulps production (e.g. profile of soil inorganic compounds, tree species, and environment contamination). Moreover, different metals may be added to the pulp with water or reagents [10].

The specificity of raw material origin as well as production technology in different pulp and paper mills makes it necessary to determine basic parameters of the acid treatment for each type of pulp [2]. Because cellulose pulps behave similarly to a cation exchanger during the treatment, the pH of the acid solution is one of the most important factors influencing the efficiency of the metal removal [5,11]. Indeed, the high temperature of the solution speeds up the dissolution of metals contained in the pulp, but also cellulose hydrolysis. Therefore, the experiments reported in this study were limited to one level of temperature only (i.e. 50°C). For the same reason, the treatment time did not exceed one hour.

In the case of the investigated oxygen delignified kraft pulp, the author determined the most frequently occurring heavy metals which exhibit strong catalytic activity towards hydrogen peroxide (Fe, Pb, Cu, Mn) [18]. Furthermore, the content of “native” magnesium as well as that added to pulp during oxygen deliginification stage as the inhibitor of cellulose depolymerization was determined. The investigated pulp was characterized by the average level of iron content and relatively low content of copper and lead (Table 1). Manganese, which decomposes hydrogen peroxide even at low concentration, appeared in trace amounts [10].

Taking into account both the amount as well as the mechanism of interaction with the peroxide, magnesium and iron were the most important metals determined in the investigated kraft pulp. Fig. 1 presents the influence of pH of the acid solution on the effectiveness of the metal removal. The obtained results show that in the entire investigated range of the pH (1 - 4) more magnesium was removed than iron. The decrease of the magnesium content of 58.5% already at pH 4 was probably caused by the fact that apart from the native magnesium also the magnesium added to the pulp at the oxygen stage was removed. The relatively short period of the metal contact with the pulp during the oxygen delignification may be the reason for its weaker bonding with cellulose and residual lignin contained in the pulp. The hypothesis is supported by the results of investigations on the native magnesium removal during the acid treatment of the laboratory obtained pinewood kraft pulp as well as industrial kraft pulp produced by the continuous cooking method. After the treatment at the temperature of 50°C the amount of magnesium removed from the investigated pulps was lower in comparison with the pulp after oxygen delignification and respectively equaled 48.7% (30 min of treatment) and 53% (60 min of treatment) for the laboratory pulp as well as 20.9% (30 min of treatment) and 30% (60 min of treatment) for the industrial pulp (Fig. 2).

The percent iron removal after the acid treatment (pH ranging from 4 to 1) increased from 42% to 58% respectively, at the same time the distinct rise in the amount of the removed iron was found only for pH 1 (Fig. 1). Iron alongside manganese and copper, is one of the most active catalysts of the peroxide decomposition [10,18]. In the form of Fe⁺³ it is capable to create stable complexes with chemical components of pulps [10]. However, the total removal of iron from pulps by the acid treatment is difficult and may be accompanied by lowering of pulp quality (degree of polymerization). The viscosity of the cellulose pulp subjected to the acid treatment was not changed in the range of pH from 4 to 2. However, a certain decrease was found for pH 1 which indicates the beginning of hydrolytic processes (Fig. 1).

Table 1 presents the comparison of efficiency of the acid treatment as well as typical chelation. After 30 min of the acid treatment the level of the obtained magnesium retention was similar to that after chelation. However, the level of iron removal was higher. The lengthening of the time of acid treatment may even cause the removal of higher amounts of metals. However, even 1 hour treatment (pH 2.5) does not create a danger of the decrease of pulp viscosity. The results of the study do not quite support reports showing high selectivity of the chelate action against alkaline earth metals [2]. Chelate removed over 56% of magnesium from the pulp. The differences in the efficiency of lead and copper removal in relation to chelation and acid treatment are more difficult to interpret bearing in mind the little content of the metals in the pulp. The differences are probably related to specificity of bonds of the metals with cellulose or residual lignin of the pulp (weaker, stronger) and not only with the action of the acid or chelate. It should be mentioned that the heavy metal distribution is not uniform. It is difficult to analyze the results of the investigations in the case of metals which occur close to trace amounts (see lead in Table 1).

Conditions of hydrogen peroxide stabilization with the use of magnesium sulfate
The significant amount of magnesium removed during the acid treatment indicates the need for the addition of the metal again during bleaching in order to ensure the optimal conditions of the reaction of the peroxide with the residual lignin. Because the amount of the native magnesium (i.e. related to the conditions of trees growth) in cellulose pulps is diversified, it is also required to explain the magnesium influence on the efficiency of hydrogen peroxide performance. Fig. 3 presents results of the investigations on the delignification of kraft pulps containing different amounts of native magnesium. Because the pulps were characterized by the diversified initial kappa number the investigations were conducted in the conditions of 100% consumption of the peroxide. It was found that, independently of the differences in the native magnesium content in the pulps, the addition of the same magnesium sulfate charges restricted comparably the delignification ability of the hydrogen peroxide – ca. 5% for both investigated pulps. The results show the significance of the proper dosing of magnesium sulfate regardless of the native magnesium content in the kraft pulp.

Figs. 4 and 5 present the influence of the magnesium sulfate charge on changes of the basic technological factors of bleaching, i.e. hydrogen peroxide consumption, yield, kappa number and brightness. The increase of the magnesium sulfate charge increased stability of the peroxide. The limitation of the peroxide consumption is correlated with the distinct increase of the pulp yield (Fig. 4). Other relationship were found between the introduced amount of magnesium sulfate and pulp kappa number as well as brightness (Fig. 5). Unlike the results of studies made by Bouchard et al. [2], the change in the amount of the stabilizer added to the pulp had stronger influence on delignification than on brightness. It should be related to the delignifying conditions of the process (temperature of 90°C). The lowest kappa number (the highest delignification) was found for the magnesium sulfate charge ranging from 0.1 to 0.2% as related to oven dry mass. Both for lower (0.05%) and higher amounts (0.3 and 0.5%) of the added magnesium, was found the decrease of the delignification. The interpretation of the results may be assisted by plots of bleaching kinetics. Fig. 6 presents the influence of the treatment time on the consumption of hydrogen peroxide as well as on delignification. In the case of the small charge of magnesium sulfate (0.05%), the peroxide decomposes very quickly. The consumption exceeded 50% just after 30 min of bleaching. Too fast decomposition of the peroxide prevented the effective removal of very strongly bounded residual lignin. Residual lignin of oxygen delignified kraft pulps in comparison with the lignin from typical kraft cooking differs with molecular weight and the lack of hydroxyl groups [9] and it is difficult to remove this lignin during bleaching. In such conditions, the remaining amount of the peroxide is insufficient for the higher degree of delignification. The peroxide was found to behave differently when too high amount of magnesium sulfate was added. The magnesium cations limited the decomposition of the peroxide so effectively that the peroxide consumption reached 70% only after 3 h of bleaching. Even in the optimum pH conditions of the bleaching slurry (i.e. 10.8) the peroxide consumption was too low to obtain low kappa number. Although the decomposition of the peroxide was the fastest during the first hour of bleaching the highest rate of delignification was found in last two hours of the process. It should be noticed that such a run of delignification at the peroxide stage is independent of the applied magnesium sulfate charges.

For the point of view of the maximum brightness the range of the optimum charge of magnesium sulfate differs slightly from the range determined for the kappa number (Fig. 5). The differences are probably related to disparate run of delignification and process of chromophoric groups’ removal. The highest brightness was obtained for magnesium sulfate charges ranging from 0.1 to 0.3% as related to oven dry pulp. However, the measured brightness differences do not exceed 0.3% for the magnesium sulfate introduced to the pulp in the range from 0.1 to 0.3%. For the case of the little dose (0.05%) the explanation of lower brightness may be related to the similar causes as in the case of delignification, i.e. too fast decomposition of the peroxide. The introduction of too high amount of magnesium (0.5%) to the pulp is not only related to excessive protection action towards the peroxide (Fig. 4) but also to form a temporary excess of hydroxide ions near cellulose fibers. The phenomenon caused by magnesium hydroxide precipitation was already noticed by Brown and Abbot [3] in their investigations on bleaching of mechanical pulps. The similar behavior was found for oxygen delignified pinewood kraft pulps during peroxide bleaching (Fig. 7). Pulp darkening develops in the initial phase of bleaching however not at once. The effect of darkening appears after ca. 0.5 h of the treatment. The amount of the precipitating hydroxide has no significant influence on pH changes of bleaching slurry and the alkali excess phenomenon, which is limited to fiber surface, should appear rather at first minutes of the process. The observed pulp darkening may be yet related to the dynamics of superficial fibers delignification – this phenomenon is currently studied.

The authors of studies discussing the problem of influence of magnesium sulfate charge on effects of kraft pulps bleaching pay attention on the fact that the excess of magnesium sulfate may cause the decrease of viscosity and therefore strength properties [2,10,12]. Fig. 8 presents the relation between viscosimetric degree of polymerization (DP), kappa number and the amount of magnesium sulfate introduced to cellulose pulp. The degree of polymerization of cellulose is related to the amount of dosed magnesium sulfate by the similar relation as kappa number. The degree of polymerization of cellulose pulps decreases with the decrease of kappa number, which has the highest values for magnesium sulfate charging from 0.1 to 0.2% of oven dry pulp. This range corresponds to the consumption of hydrogen peroxide ranging from 80 to 90% which is recognized as the optimum amount in peroxide bleaching [16]. Such a level of peroxide consumption ensures its effective action towards residual lignin. However, hydrogen peroxide in alkaline medium is not characterized by the high selectivity and that is why it may cause the decrease of degree of polymerization of cellulose. The addition of magnesium sulfate excess (0.5%) effectively protects the peroxide against its decomposition which limits both delignification and the depolymerization of cellulose.

The comparison of indices of selectivity of delignification and delignification value has the fundamental importance in determining the optimum charge of magnesium sulfate. The results presented in Table 2 show that in the case of the investigated pulp the best indices are obtained after dosing 0.2% of magnesium sulfate as related to oven dry pulp. However, it should be mentioned that in the industrial practice the optimum charging of magnesium sulfate should be determined for each type of cellulose pulp on account of the specific profile of metal content in pulps.

CONCLUSION

1. The acid treatment removes heavy metals from oxygen delignified pinewood kraft pulp in equally effective way as chelation. The magnesium retention level is similar after the acid treatment and after chelation.

2. The acid treatment removes from oxygen delignified pinewood kraft pulp magnesium which was introduced at the alkaline-oxygen stage as well as partially native magnesium. Independently of differences in native magnesium contents in pulp the addition of magnesium sulfate stabilizes hydrogen peroxide and limits its ability for delignification.

3. Magnesium charge should be optimized for each type of pulp. Too low or too high level of magnesium causes the decrease of brightness and limits delignification. While the excess of magnesium does not influence negatively the degree of polymerization of cellulose.

ACKNOWLEDGEMENT

Financial support for this work was provided by the State Committee for Scientific Research (KBN), Poland.

REFERENCES

1. Abbot J., Brown D. G., Hobbs G. C, Jewell I. J., Wright P. J., 1992. The influence of manganase and magnesium on alkaline peroxide bleaching of radiata pine thermomechanical pulp. Appita, 2, 109-120.

2. Bouchard J., Nugent H. M., Berry R. M., 1995. A comparison between acid treatment and chelation prior to hydrogen peroxide bleaching of kraft pulps. J. Pulp Paper Sci., 6, 208-208.

3. Brown D. G., Abbot J., 1994. Magnesium as a stabilizer for peroxide bleaching of mechanical pulp. Appita, 3, 211-220.

4. Brown G., Dawe R., 1996. Effect of metal ions on oxygen delignification of kraft pulp, Proc. TAPPI Int. Pulp Bleaching Conf., 383-388.

5. Bryant P. S., Edwards L. L., 1996. Cation exchange of metals on kraft pulp. J. Pulp Paper Sci., 1, 37-42.

6. Colodette J. L., Rothenberg S., Dence C. W., 1989. Factors affecting hydrogen peroxide stability in the brightening of mechanical and chemimechanical pulps. Part III: hydrogen peroxide stability in the presence of magnesium and combinations of stabilizers, J. Pulp Paper Sci., 2, 45-50.

7. Desprez F., Hoyos M., Devenyns J., Troughton N. A., 1995. Blanchiment des pates chimiques sans produit chlore (TCF). II – optimisation des stades au peroxide d’hydrogene alcalin. Sequence TCF de l’avenir. [Chlorine free bleaching of the kraft pulps. II – Optimization of the alkaline hydrogen peroxide stage. Future sequences TCF]. Revue A.T.I.P., 49, 42-46 [in French].

8. Järrehult B., Samuelson O., 1993. Influence of metal compounds on the oxygen-alkali treatment of kraft pulp and cellobiitol. Nordic Pulp Paper Res. J., 3, 307-336.

9. Johansson E., Ljunggren S., 1993. The reactivity of lignin model compounds and the influence of metal ions during bleaching with oxygen and hydrogen peroxide, Proc. 7th Int. Symp. Wood Pulping Chemistry, Beijing, 1, 180-187.

10. Lapierre L., Berry R., Bouchard J., 2000. The effects of the order of chemical addition on the peroxide bleaching of an oxygen-delignified softwood kraft pulp. Holzforschung, 3, 279-286.

11. Lapierre L., Bouchard J., Berry R. M., Van Lierop B., 1995. Chelation prior to hydrogen peroxide bleaching of kraft pulps: an overview. J. Pulp Paper Sci. 8, 268-273.

12. Lapierre L., Bouchard J., Paleologou M., Berry R., 1996. The limits of metal removal from kraft pulp by acid treatment, Proc. TAPPI Int. Pulp Bleaching Conf., 515-517.

13. Modrzejewski K., Olszewski J., Rutkowski J., 1985. Metody badań w przemysle celulozowo-papierniczym [Methods of the studies in pulp and paper industry]. ŁódŸ Technical University [in Polish].

14. PN-92/P-50101/01:1993. Oznaczenie lepkosci granicznej [Determination of limiting viscosity number]. [in Polish].

15. PN-P-50095-01/A1:1996. Oznaczenia stopnia roztworzenia włóknistych mas celulozowych [Determination of the delignification degree] [in Polish].

16. Rutkowski J., 1994. Nadtlenek wodowu – efektywny, przyjazny dla œrodowiska reagent bielšcy umacnia swojš pozycję w przemyœle celulozowo-papierniczym [Hydrogen peroxide – effective, environmentally friendly bleaching agent reinforce its position in pulp and paper industry]. Przegl. pap., 11, 524-529 [in Polish].

17. Rutkowski J., Perlińska-Sipa K., 1998. Bielenie mas celulozowych siarczanowych bezchlorowymi srodkami tlenowymi [Bleaching of kraft pulp using chlorine-free oxygen agents]. Fol. For. Pol. B, 29, 83-103 [in Polish].

18. Rutkowski J., Wandelt P., Kopania E., 2001. Wpływ kationów metali ciężkich I okreslonych stabilizatorów nadtlenku wodoru na kinetykę jego rozkładu w srodowisku alkalicznym [Influence of heavy metals and stabilizers on kinetics of hydrogen peroxide decomposition in alkaline medium]1. Przegl. pap., 57:379-382 [in Polish].

19. Wójciak A., 2002. The effect of pH of hydrogen peroxide solution on kraft pine pulp delignification. Fol. For. Pol. B, 33, 33-45.

20. Wójciak A., Sikorski M., Gonzalez-Moreno R., Bourdelande J.L., Wilkinson F., 2002. The use of diffuse-reflectance laser-flash photolysis to study the photochemistry of the kraft pulp treated with hydrogen peroxide under alkaline and acidic conditions. Wood Sci. Technol., 36, 187-195.

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.