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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.1 Concepción mar. 2003

http://dx.doi.org/10.4067/S0717-97072003000100006 

MODIFICATION OF A MILL DEopDD SEQUENCE: IMPROVEMENT IN THE D
STAGE FOR COMBINATION OF OZONE (Z) AND CHLORINE DIOXIDE (D) ON THE (ZD)EopDD SEQUENCE OF SOFTWOOD KRAFT-OXYGEN PULP

H. MILLAR, J. RUIZ, J. FREER AND J. BAEZA

Laboratorio de Recursos Renovables, Facultad de Ciencias Químicas, Universidad de Concepción,
Casilla 160-C, Concepción Chile. jbaeza@udec.cl
(Received: August 22, 2002 - Scceptrd: october 20, 2002)

ABSTRACT

In this study, the (ZD)EopDD bleaching sequence for a softwood pulp was compared with the conventional ECF reference sequence DEopDD used in the mill. A Pinus radiata kraft-oxygen pulp was treated with a (ZD) stage at 0.3% ozone and 0.5% chlorine dioxide charges. The best result was reached with the (ZD) process when O3 and ClO2 were added sequentially at high consistency. The use of ozone allowed a 43% reduction in the use of chlorine dioxide. With the (ZD)EopDD sequence a 90%ISO brightness bleached pulp was obtained, having acceptable mechanical properties, which are comparable to that generated by the DEopDD sequence.

Keywords: Kraft pulp, ECF bleaching, ozone, chlorine dioxide.

INTRODUCTION

Ozone (Z) and chlorine dioxide (D) are gases at room temperature and pressure. Ozone is more soluble in water than oxygen, at 20 °C its solubility is ~ 11.5 times higher. In solution at pH<7 ozone is present as molecular O3. However, with increasing pH ozone undergoes spontaneous decomposition. At 25 °C chlorine dioxide is about 23 times more concentrated in aqueous solution than in the gas-phase with it is in equilibrium. Chlorine dioxide is stable over a wide pH range, but it decomposes at elevated pH (pH>10) (1).

When O3 or ClO2 are used to bleach pulp, additional species need to be considered to fully understand the bleaching process. For example, the fact that ozone decomposes in pulp at a pH>7 have important consequences when designing pulp bleaching strategies. Thus, not only ozone, but also hydroxyl radical, peroxide and superoxide ions are important species that play a role in the lignin degradation and in the carbohydrate stability in the bleached pulp (2,3,4). Because of its high oxidation potential (-2.07 ev) ozone should offer a better alternative than ClO2, but it has been recognized that ozone, as a strong oxidant, tends to degrade cellulose (2,4).

Because ozone is pH compatible with chlorine dioxide, its application in elemental chlorine free (ECF) bleaching sequences can be simple and in many cases requires low capital investment, being this the major motivation to install (ZD) or (DZ) technologies. The implementation of these technologies could represent an attractive way of decreasing the chlorine dioxide charge and thereby the discharge in Adsorbable Organic Halides (AOX).

In the DZ application mode, the reaction mechanisms of ozone and of chlorine dioxide are complementary, i.e. chlorine dioxide causes lignin cleavage and ozone oxidizes these lignin fragments giving rise to a large amount of hydrophilic carboxylic groups (5,6). These combined actions increase overall lignin solubility, but the ozone treatment before the alkaline extraction with oxygen and hidrogen peroxide (Eop) stage has the inconvenience that the carbonyl groups generated trigger alkali hydrolysis of carbohydrates leading to viscosity losses.

In the ZD mode, the carbonyl groups generated in the ozone stage are partially destroyed by chlorine dioxide before Eop. In addition, the formation of unwanted by-products, such as ClO3-, is prevented when the chlorine dioxide application follows the ozone treatment. The oxygenated pulps tend to respond better to the ZD process, but the conventional pulps to DZ (5).

The purpose of this article is to show that exist conditions under which most efficient delignification is achieved with a minimal loss in pulp viscosity by using ozone. The DEopDD mill sequence is used as reference and ozone performance is evaluated in the (ZD) stage of the (ZD)EopDD bleaching sequence using softwood kraft-oxygen pulps.

EXPERIMENTAL

Raw material
The pulps used in this study are Pinus radiata kraft-oxygen delignified from the mill with kappa number of 13.9, brightness 35.4 %ISO and viscosity 20.6 cps.

Bleaching
Chlorine dioxide stages (D) were performed in polyethylene bags at 10% consistency. Ozone was generated from O2 by electric discharge at high voltage on a generator OZOCAV. In all experiments, ozone bleaching at high consistency and room temperature was performed in a rotating round-bottomed flask equipped with a gas inlet system. The residual ozone leaving the reactor was passed through a spectrophotometer Genesys 5, and determined at 254 nm in a flow cell connected in series with the reactor. The bleaching stages containing ozone (Z and ZD) were carried out with 50 g.o.d. pulp samples in the reactor described above. The (ZD) stage was carried out as follows (8,9,10,11,): the Z phase was performed at a pulp consistency of 2% and the pH adjusted at 2.1, by adding either sulfuric acid or NaOH. The pulp was pressed out after 1 hour to obtain 35-40% consistency. These impregnated pulps were fluffed and ozonized. After ozone consumption by the pulp, which took 4-5 min, the pulp without washing was passed to a polyethylene bag, and chlorine dioxide was introduced into the pulp at 10% consistency, staying at 70°C for 45 min.

The oxidative extraction stage (Eop) was performed in a Parr stainless reactor at 80°C, 10% pulp consistency with 3.0 bars of oxygen and 0.5% hydrogen peroxide. Magnesium sulfate was added at 0.5% over dry pulp when required.

Pulp characterization
Viscosity, brightness, kappa number and strength properties of the pulps were performed according to the Tappi Standard methods, brightness was measured with a Technibrite Micro TB - 1 C. For Pinus radiata kraft-oxygen pulps, the conventional ECF reference sequence DEopDD used in the mill was compared to (ZD)EopDD sequence. AOX in pulps was analyzed according to DIN N° 34809-14 using a EUROGLASS ECS 1000.

RESULTS AND DISCUSSION

Effect of ozone charge
Increasing the charge of ozone from 0.5% to 1.0%, the kappa number decreases from 8.53 to 3.56 and the brightness increases from 45.70 to 62.54%, but the viscosity drops from 14.06 to 10.16 cps. Due that ozone attacks all constituents of wood pulp, high degrees of delignification are not obtainable without a loss in viscosity (3). The experiment reported at high consistency also showed this trend, this is reflecting by the kappa number and viscosity values. Table 1 summarizes these effects of ozone charge on these parameters. With a 0.5% ozone charge (the optimal doses applied to the softwood kraft-oxygen pulp used in this study) the efficiency and selectivity were 10.8 and 1.23, respectively. At high ozone charges (1.0%), improvement in the efficiency (10.4) is not observed, and a significant depolymerization is produced, which is evaluated as viscosity loss. According to the literature, approximately 1% ozone charge is the maximum that can be applied to a softwood kraft pulp without serious fiber degradation (2,3).


Table 1. Effects of ozone charge on a Pinus radiata kraft-oxygen pulp.

% Z

Brightness (%)

ISO 3688

Kappa

TAPPI 236

Viscosity (cps)

TAPPI 230

Selectivity(a)

Efficiency(b)

0.0

0.5

0.8

1.0

35.45

45.70

55.66

62.54

13.93

8.53

5.31

3.56

20.56

14.06

11.56

10.16

 

1.23

1.41

1.47

 

10.8

10.8

10.4

(a)[(D kappa/initial kappa))/(D viscosity/initial viscosity)]. (b) [(D kappa) /(%O3)].

Effect of the Eop stage used in the (ZD)EopDD sequence
According to the results shown in Figure 1 as an example, the pulp obtained by the reference sequence has better strength properties than those of the pulp bleached by the proposed sequence. The mechanical physical behavior of the pulps obtained after applying the stages 0.5% D, 0.3 and 0.5 Z, and 0.3/0.5 ZD independently were similar, allowing to conclude that the biggest damage in the fibre seems to be produced in the Eop stage. An ozone stage induces the formation of carbonyl and, to a lesser extent, carboxyl groups along the cellulose chains, which are only partially destroyed by chlorine dioxide. The carbonyl groups fragilize the cellulose towards an alkaline medium, such as the Eop stage used after (ZD) on the sequence. It has been shown that these groups are responsible for the rapid depolymerization of the cellulose during a subsequent peroxide stage (12,4).


Fig. 1. Tear versus tensile for (ZD)EopDD and DEopDD stages

Improvement of Eop stage by addition of magnesium ion
The results of Figure 2 show that the magnesium sulfate addition (0.5% over dry pulp) at Eop stage improved the physical properties of (ZD)EopDD stage.


Fig. 2. Effect magnesium ion on tear versus tensile for (ZD)EopDD stages

The interaction of metals in oxygen bleaching has been widely studied (13,14,15). Additionally, related aspects from peroxide bleaching of mechanical pulp have been provided by Brown and Abbot (13 ). Oxygen and peroxide bleaching are generally considered different technologies, but it has been demonstrated an stretching relationship between them. That is, during oxygen bleaching, hydrogen peroxide is generated and during peroxide bleaching, oxygen is evolved (15). The role of Mg++ has been clarified by Samuelson an Ojteg (16), Mg++ complexation with the secondary hydroxyl groups of carbohydrates is one of the viscosity and mechanical properties preservation modes. Samuelson´s group identifies deactivation of catalytically important transition metals as another deactivation mode. Another possible deactivation mode may involve complexation of otherwise free ionizable groups, such that they are unavailable to participate in undesirable secondary reactions (such those peeling reaction of carbohydrates). Reduction of the carbonyl groups was found to be an efficient way to make the (ZD)Eop process very attractive. Other alternative is the use of sodium borohydride, this reactive reduces these groups very efficiently and makes the cellulose quite stable during an Eop stage, this stage should be applied after (ZD) and before Eop at charges ranging between 0.05% and 0.1% on pulp (4). The high capital required to retrofit borohydride indicates that mill application is not feasible, it is thought that an efficient and cheap elimination of the carbonyl groups is the key to the development, for combination of ozone and chlorine dioxide, bleaching processes for softwood kraft pulp.

Savings chemical
The requirements of ClO2, ozone, hydrogen peroxide and sodium hydroxide to reach 90%ISO brightness with the ECF reference and (ZD) implemented bleaching sequence are summarized in Table 2.


Table II. Chemical consumption (% over dry pulp) for each bleaching sequence

 

Sequence

ClO2 (%)

Do D1 D2 Total

O3 (%)

H2O2 (%)

NaOH (%)

D0EopD1D2

(ZD0)EopD1D2

1.52 0.76 0.28 2.56

0.50 0.75 0.20 1.45

0.00

0.30

0.50

0.50

0.17

0.17

The savings expressed as kgClO2/kgO3 and brightness are presented in Table 3.


Table III. Comparison of final brightness and saving of chemical

 

Sequence

Saving

kgClO2/kgO3

Brightness

(%ISO)

D0EopD1D2

(ZD0)EopD1D2

-

3.70

90.50

90.20

At a comparable brightness with the reference sequence, the replacement ratio of chlorine dioxide by ozone in the sequence under study was 3.70, showing that the savings are really significant, and additionally, the AOX content in pulp is reduced from 624 to 145 mg/kg, i.e. 4.3 times (Table 4).


Table IV. Comparison of AOX levels in pulp for each stage

Stage

ClO2 (%)

O3 (%)

AOX levels (mg/kg)

Z

D

(ZD)

unbleached

---

1.0

0.5

---

---

1.0

0.5

---

N.D.

624

145

N.D.

N.D. non detectable

The reduction reactions of ozone and chlorine dioxide proceed by equation I and II, respectively, such a complete reduction occurs when ozone and chlorine dioxide react with the residual lignin, therefore, it is recommended that the maximum oxidative power of ozone, 6e per mol, is utilized in (OXE) calculations (7). The same principle is also suggested for chlorine dioxide where 5 electrons per mol are transferred in pulp bleaching.

O3 + 6H+ + 6e ------ 3H2O (I)

ClO2 + 4H+ + 5e ------- Cl- + 2H2O (II)

These facts mean that the oxidation equivalent (OXE) of ozone and of chlorine dioxide is 125 and 74, respectively. Thus 1 kg of O3 should theoretically displace 1.7 kg of ClO2, this saving is minor than the observed in our study of 3.7. For certain type of pulps, the replacement ratio of ozone for chlorine dioxide can reach values up to 4 (5,8,10).

Strength properties
The strength properties of the (ZD)EopD treated pulps were similar to those of the reference DEopD bleached pulps. The results for drainability, porosity, tensile, and tear, up to PFI beating of 8000 are shown in Figure 3. According to these results, (ZD) stage could replace a Do stage very advantageously in a ECF sequence of the DoEopD1 type.



CONCLUSIONS

Ozone as lignin removing agent cannot be easily used in a bleaching process because of simultaneous degradation of cellulose, but combining ClO2 and O3 in a sequential (ZD) process proposed in this study gave the most promising results and should contribute to the acceptance of ozone in kraft pulp bleaching. In ECF bleaching sequence, chlorine dioxide can be reduced by replacing part of this chemical bleaching by ozone. The (ZD) stage, in which ozone is added before chlorine dioxide without washing, were particularly efficient since, 1kg ozone could save up to 3.7 of chlorine dioxide in a full bleaching with (ZD)EopDD sequence, thus application of (ZD) stage could then reduce the chemical costs significantly and an reduction of AOX content in the effluent. Finally the sequence (ZD)EopDD produced bleached pulps with strength properties similar to those of the DEopDD one.

REFERENCES

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