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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.4 Concepción dic. 2004

http://dx.doi.org/10.4067/S0717-97072004000400016 

 

Chil. Chem. Soc., 49, N 4 (2004): págs: 355-360

HEXENURONIC ACID REMOVAL FROM UNBLEACHED KRAFT EUCALYPTUS PULP BY PEROXYMONOSULFURIC ACID

 

XIMENA PETIT-BREUILH*, CLAUDIO ZAROR, ROBERTO MELO

Laboratorio de Productos Forestales, Departamento de Ingeniería Química Universidad de Concepción
*Universidad Católica de Temuco - Chile - E-mail: xpetit@uct.cl


ABSTRACT

The use of peroxymonosulphuric (Ps) acid to remove hexenuronic acid (HexA) present in unbleached eucalyptus kraft pulps was evaluated here. HexA is formed during pulping and is held responsible for reagent consumption during bleaching. Laboratory experiments, using oxygen predelignified eucalyptus kraft pulps (kappa number around 9), treated at 20-110C and 0.2-1.0%Ps, were conducted here,

Experimental results show that Ps effectively removed both HexA and lignin, even under mild conditions. Selectivity towards HexA increased at higher temperature and lower Ps loads. However, at lower temperatures and higher Ps concentrations, cellulose was also attacked.

As a result of Ps pre-treatment, ClO2 savings in pulp bleaching in the range 40-79% were determined here. Moreover, all Ps pretreated pulps yielded higher brightness than untreated pulps. Furthermore, lower AOX emissions occurred as a result of lower ClO2 loads. However, moderate Ps pre-treatment conditions were necessary to obtain a bleached pulp with acceptable mechanical properties. Results reported here show that HexA removal by Ps offers an interesting option for industrial implementation, both from an economic and environmental point of view.


INTRODUCTION

During the last decades, bleached kraft cellulose production has undergone important technological innovations, in order to reduce environmental impacts, particularly, due to the generation of toxic organic chlorinated compounds. Consequently, efforts have been made to reduce the chlorine load in bleaching, and important modifications in both pulping and bleaching processes have been introduced [1].

It is known that the presence of hexenuronic acid (HexA) in the unbleached pulp increases the consumption of bleaching chemicals, such as ClO2 and ozone, leading to higher production costs, and affecting product quality and environmental performance. As illustrated in Figure 1-A, HexA is formed during alkaline pulping, due to beta elimination of methylglucuronic acid, which are randomly distributed on both sides of xylan main chain, both in hardwood and softwood pulps [2]. Such reaction rapidly takes place during the heating phase at early stages of pulping, between 110 y 150C at pH 12 ­13, [3]. HexA rapidly reacts with permanganate, thus affecting the kappa number determination. Indeed, about 3-7 kappa units are reported to be accounted for by the presence of HexA in hardwood pulps [4,5]. Various authors have reported HexA reduction in unbleached pulps by modification of pulping conditions [3,6,7,8,9]. However, HexA groups protect the xylan chains from terminal depolymerization, and thus their presence avoids a reduction in performance in the alkaline stages [10]. Removal of these groups during pulping is complex, since the quality of the pulp is reduced and there are more problems with the process [7]. HexA removal after pulping has been successfully used. In general, the HexA groups suffer electrophilic and nucleophilic attacks, and could be removed by strong acids.


 
Fig. 1. Formation and reactivity of Hexenuronic Acid

A. Formation of hexenuronic acid during kraft pulping

 
B. Funcionality of hexenuronic acid groups attached to xylan [10]

Hexenuronic acids contain functional groups of enol-ether and unsaturated carboxylic acids (figure 1-B), which are inert under slightly acid conditions; however in strongly acid solutions enol-ether groups suffer rapid hydrolysis, generating aldehydes, cetones and alcohols. Acid pretreatment at 90C is reported to remove HexA, with some lignin reduction (viz. around 4.3 kappa units) [11]. Unfortunately, such treatment has shown to reduce the ozone stage efficiency in TCF bleaching .

In the search for low cost and selective agents for HexA removal, peroximonosulphuric acid (Ps) has been identified as a potential alternative at industrial scale since it does not require pressurization. Ps is a peracid derived from hydrogen peroxide when a hydrogen atom is replaced by a SO3H group. Unfortunately, there is little published information on its HexA removal capacity. In this context, the aim of this work is to present experimental information on the Ps HexA removal capacity and its effect on bleaching chemical consumption and environmental performance.

EXPERIMENTAL

Pulp preparation

Oxygen predelignified Eucalyptus globulus pulp with kappa 9.7 was used here. Eucalyptus globulus chips were placed in a 20 L rotatory retort, and mixed with white liquor at a 4:1 solid:liquid ratio. The white liquor featured 25% sulphidity and 10.5% effective alkali (based on dry wood weight). Heating was provided by electric resistance at an average rate around 1.5C/min. The reactor was kept at 170C over 1 h, before quenching and discharge. The pulp was thoroughly washed with distilled water, and then treated with oxygen in a 5 L stainless steel pressure reactor at 3 kg O2/cm2 and 90C, for 50 min, in the presence of 2% NaOH and 0.05% MgSO4, at 10% consistency. Predelignified pulp was washed with distilled water, pelletised and stored at 4°C.

Preparation of peroximonosulphuric acid (Ps)

A 2:1 molar ratio H2SO4 and H2O2 solution was prepared and allowed to rest for 2 h. The peracid concentration was determined by the method proposed by Greenspan and McKellar (1948) [12]. A 150 mL 5% H2SO4 solution is added to a 500 mL beaker, in an ice bath to set temperature as close to 0C as posible. This solution is titrated with Ce(SO4)2. 4H2O 0.1 N using ferroin as indicator; titration ends when the pink colour dissapears. Then, a 10 mL 10% KI solution is added and a second titration is carried out, with 0.1 N sodium thiosulphate, using starch as indicator. The amount of peroximonosulphuric acid is estimated as:

where OA corresponds to active oxygen, w is the sample dry weight, V and N are volume and normal concentration, respectively. Subindex A and B correspond to ceric sulphate and sodium thiosulphate, respectively.

Peroximonosulphuric acid treatment

200 g oxygen treated pulp samples were fed into a 5 L reactor and treated with peroximonosulphuric acid (Ps) at a set load (0.2-1.0 % w/w, based on dried pulp weight), 10% consistency, and controlled temperature (20-110°C), for 1 h. After Ps treatment, the pulp was washed with distilled water, characterized and stored for further use.

Bleaching sequence

A sequence (D0 - E - D1) was applied to pulp from E. globulus. Bleaching conditions are shown in Table 1.


Table 1. Bleaching Conditions

Variable Bleaching stage

  D0 E D1

Kappa Factor 0.22 - 0.09
Consistency (%) 10 10 10
Time (min) 40 60 180
Temperature (C) 70 70 70
NaOH (%) - 2 -
Pressure (kg O2 /cm2) - 2 -

Analytical methods

The content of hexenuronic acid (HexA) in the pulp was determined using the spectrophotometric method reported by Chai et al (2001) [13]. The pulp was hydrolized using a sodium acetate and mercury chloride solution at 65C for 30 min. Absorbance was measured at 260 nm y 290 nm. The HexA content is calculated as:

where CHexA is the HexA molar fraction in the pulp expressed as (mol HexA /g dry pulp); A260 and A290 are absorbance values determined at 260 y 290 nm, respectively; V is hydrolysis solution volume in mL , and w is the pulp dry weight (g).

The lignin content was measured as klason lignin (Tappi 222 om 83). Pulp samples were characterised on the basis of kappa number and brightness (ISO%), according to Tappi UM246 and T 525 om-92, respectively. Tensile index, tear index and burst index of bleached pulp samples were also determined by the standard Tappi Methods: T494 om-88, T496 cm-85 and T403 om-91, respectively [14].

The intrinsic pulp viscosity was measured according to the standard Scan CM 15:88 [15].

The selectivity of Ps treatment on HexA removal was evaluated as the ratio between the fall in viscosity and the fall in the kappa index.

RESULTS AND DISCUSSION

Peroximonosulphuric Treatment

Figures 2-6 show experimental results of peroxymonosulfuric acid (Ps) treatment at 0.2-1.0 % acid concentration and 20-110C, over 1 h. As seen in Figure 2, greater HexA reductions were attained as temperature increased. At temperature levels above 80C, HexA removal sharply increased, from 30-40% to nearly 100% at 110C. The Ps concentration also affected the extent of HexA removal and, at a given temperature, HexA removal increased with Ps concentration.


 
Fig. 2. Effect of the different treatments used to remove HexA.

Figure 3 shows the relationship between HexA removal and the reduction in kappa number as a result of Ps treatment. It must be mentioned that HexA accounted for 60% of total kappa number in the untreated pulp, with one kappa unit being equivalent to 10.9 mmol of HexA/kg of pulp. This value is similar to those reported in the literature [5,16].


 
Fig. 3. Correlation between HexA removed and kappa reduction due to Ps treatment (R2=0.978).

After 100% HexA removal, the pulp reached a kappa value around 2, showing that part of initial lignin was also attacked by Ps. Indeed, as seen in Figure 4, around 10 mmol of HexA were removed per 2.9 g of lignin removal.


 
Fig. 4. Correlation between HexA and lignin removed (R2=0.936)

Results on the extent of HexA removal as a function of time are presented in Figure 5. Initial removal rates in the range 14-28 (mmol HexA/min) were found at 70-90°C. Preliminary experiments conducted under batch conditions showed that the HexA degradation rate followed a linear relationship with the product of remaining HexA and Ps concentrations (R2 = 0.95). Such experiments were carried out at 70°C with HexA and Ps initial concentration around 0.2 g/L and 1 g/L. On this basis, a second order model is used here to describe the HexA degradation rate,

where k is the apparent second order rate constant, and [HexA] and [Ps] are the HexA and Ps molar concentrations in the reactor, respectively. Matlab software was used to obtain kinetic parameters using experimental data reported here. Apparent second order rate constants for HexA removal by Ps treatment were estimated around 5, 11, 22 (mol-1 min-1 l), at 70, 80 and 90°C, respectively, corresponding to an activation energy of 80 kJ/mol.


 
Fig. 5. HexA removal rate.

Ps treatment also affected the cellulose degree of polymerization of unbleached pulp, as shown by reductions in intrinsic viscosity up to 50%, under severe conditions. Figure 6 shows the ratio between intrinsic viscosity and kappa reductions, as a function of the extent of HexA removal by Ps treatment (using 0.32% initial Ps, and temperature in the range 60-110°C). At lower values of HexA removal, greater reductions in intrinsic viscosity as compared with kappa reduction were found. Under such conditions, the cellulose chain was affected at a greater extent than lignin. As treatment temperature increased and HexA removal increased, greater selectivity towards kappa reduction was detected.


 
Fig.6. Selectivity of peroxymonosulphuric acid at 0.32% (dpb)

Bleaching Sequences D0-E-D1

Table 2 summarises results obtained after ClO2 bleaching of Ps treated pulps. The bleaching sequence is described in the Materials and Methods section. Bleached pulps were characterised on the basis of brightness, viscosity, tensile index, tear index and burst index.

As the extent of HexA and lignin removal during Ps treatment increased, the ClO2 requirements to attain acceptable brightness decreased. Indeed, ClO2 load reductions in the 45-79 % were determined here, yielding bleached pulps with 90-92ISO brightness. All Ps treated pulps achieved higher brightness levels than untreated pulps. However, in the case of severe Ps treatment, final bleached pulps showed poor viscosity and physical properties. In particular, high Ps concentration and temperature drastically affected the final pulp viscosity and physical properties. Pulp physical properties are known to rapidly deteriorate when viscosity levels drop below 10-11 cp (ie. [h] 629-668 mL /g) [17].


Acceptable bleached pulp qualities were obtained in the case of Ps treatment within the range 0.2%-0.6% Ps and 93-100°C. Under these conditions, between 45% and 62% ClO2 savings could be achieved, with a proportional reduction in AOX generation.

As seen in Figure 7, the reduction in ClO2 consumption associated with HexA removal by Ps pretreatment, leads to significant reductions in chlorinated organic contaminants (AOX) in bleaching effluents. The removal of all HexA before bleaching led to 55% reduction in AOX content in bleaching effluents, with consequent environmental gains.


 
Fig. 7. Relationship between AOX and HexA removal (R2=0.971)

CONCLUSIONS

Peroximonosulphuric acid effectively removed hexenuronic acid (HexA) from eucalyptus pulps. Even under mild conditions, substantial reductions in HexA and lignin content were observed. However, cellulose may also be attacked during treatment, leading to significant reductions in intrinsic viscosity. Greater selectivity towards HexA and lignin occurred at higher temperature and lower Ps concentrations.

As a result of Ps pre-treatment, important savings in ClO2 and higher brightness in subsequent ECF bleaching were obtained. Moreover, lower AOX generation occurred as a result of lower ClO2 loads. However, moderate Ps pre-treatment conditions were necessary to obtain a bleached pulp with acceptable mechanical properties.

Results reported here show that HexA removal by Ps offers an interesting option for industrial implementation, both from an economic and environmental point of view.

 

ACKNOWLEDGMENTS

This work was supported by a Doctoral Thesis Support Grant Conicyt provided by Conicyt-Chile, to whom the authors are grateful.

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(Received: September 30, 2004 - Accepted: October 28, 2004)

 

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