EP1180172B1

EP1180172B1 - Process for bleaching chemical pulps with low organic halogen compounds content

Info

Publication number: EP1180172B1
Application number: EP00926590.1A
Authority: EP
Inventors: Silvia Maria Pasquali; Guilherme Antonio Tedeschi De Araujo
Original assignee: Fibria Celulose SA
Current assignee: Fibria Celulose SA
Priority date: 1999-04-27
Filing date: 2000-04-27
Publication date: 2014-07-02
Anticipated expiration: 2020-04-27
Also published as: BR9901291B1; NO20015241D0; ES2501141T3; WO2000065148A1; CA2372069C; IS6132A; NO20015241L; CA2372069A1; ZA200108811B; NO331632B1; PT1180172E; EP1180172A1; BR9901291A

Description

Field of the Invention

The present invention refers to an enhanced process for bleaching pulps that allows the obtention of a low concentration of organic halogen compounds in the final cellulose pulp.

Background of the Invention

In view of increasing market demands, several studies have been carried out in an attempt to obtain pulps with properties similar to those known as TCF (total chlorine free), but that can reduce production costs. Several alternatives have been proposed for processes that yield pulps with good brightness properties without causing environment contamination and with low amounts of organic halogen compounds expressed by a low OX content ("organically bound halogen").
The decrease in the halogen compounds organically bound to the pulp have already been studied by Süss et al (Suss, Hans U., at al., 1991, TAPPI Pulping Conference, 705-71). This paper discloses the results achieved in the reduction of OX content in the pulp through a step of weak alkaline extraction in the presence of H₂O₂ carried out in the end of the bleaching process. On the other hand, Annergen et al (1990, TAPPI Pulping Conference, 185-194) teach that the removal of halogen compounds organically bound to the pulp is very difficult and that it would only be possible by the addition of an alkali due to their high molecular weight. That study also mentions that a partial removal of said compounds may occur by combining steps of severe washing and controlled chlorinating the pulp.
Another study about the removal of organic chlorine compounds from chemical pulps was disclosed by Gunnarsson et al (1996, JPPS. J457-J463) who described the mechanism of OX and AOX formation in the pulp and concluded that it would be possible to reduce the OCI levels in the pulp by working with a final pH value around 4 in the deoxidizing step and that variation in the amount of chlorine dioxide added in this phase would not yield an increasing concentration of OC1 in the final pulp product.
Further processes have also been proposed with the objective of reducing the organic halogen compounds during the treatment of chemical pulps by using chlorine dioxide. An example of such processes is presented in patent EP 500,813 that discloses a process for bleaching lingo-cellulosic pulps with reduced formation of organic halogen compounds carried out by adding ClO₂ in a first bleaching step, followed by a second step comprising the treatment of the pulp with hydrogen peroxide and at least one further step of treatment with chlorine dioxide.
Another example of a process for reducing the content of organic halogen compounds in chemical pulps in which chlorine dioxide is added is taught in US 5,030,324 . According to that process several steps comprising chlorine treatment are carried out, including a step wherein chlorine dioxide is necessarily added together with separate additions of chlorine, wherein the stage of ClO₂ treatment is effected at pulp low consistency.
Patent US 4,959,124 refers to a process for reducing organic halogen compounds in low consistency pulp by treating the pulp with chlorine dioxide in a first bleaching step necessarily followed by a stage of ozone treatment.
However, it was still desirable to develop a process for bleaching chemical pulps that could provide the removal of the organic halogen compounds in a more effective manner and with significant reduction of production costs.
The objective of the present invention is, therefore, to provide a pulp bleaching process that allows an efficient reduction in the OX content specially in low consistency pulps, and that can also reduce the amount of chemicals necessary to achieve the final OX reduction.

Summary of the Invention

The present invention refers to a process for bleaching chemical pulps, especially low consistency chemical pulps, characterised by comprising the sequential steps DO-Eop-Q-(PO-P) wherein the addition of chlorine dioxide is carried out only in the first bleaching stage as a charge of 6 kg/tsa to 8 kg/tsa (i.e. 6 kg/adt to 8 kg/adt) and at a pH in the range of about 4.5 to 5.5.

Brief Description of the Drawings

Figures 1 and 2 are graphics that respectively demonstrate the total amount of peroxide added to the pulp produced in accordance with the present invention and the amount of peroxide relative to the initial Kappa number.
Figure 3 illustrates the average concentration OX concentration in pulps treated in accordance with the present invention when compared to pulps treated by methods comprising different bleaching parameters.
Figure 4 shows the brightness increase obtained with the addition of ClO₂ in the DO stage.
Figure 5 presents a graphic that illustrates the relationship between the amount of chlorine dioxide added to the pulp and the OX concentration in the final product.
Figure 6 shows a graphic that illustrates the relationship of the pH value in the first bleaching step with the organic halogen compounds content.
Figures 7 and 8 are graphics that illustrate the OX profile in two different process conditions.

Detailed Description of the Invention

After several studies the present inventors found out that excellent results in the reduction of organic halogen compounds may be achieved in a process for bleaching previously delignified chemical pulps, especially low consistency chemical pulps, comprising the sequential steps DO-Eop-Q-(PO-P) when a step of adding chlorine dioxide is carried out once only - as the first bleaching stage - said addition of chlorine dioxide being effected in a pH range of from 4.5 to 5.5 and with a charge of from 6 to 8 kg/tsa (i.e. 6 to 8 kg/adt).
Contrary to the teachings of the processes known from the prior art according to which a reduction of pH values would lead to a substantial increase of OX formation, it has now been found that the use of an acid during the deoxidation step (DO) in order to provide a pH value in the range of about 4.5 to 5.5 decreases the concentration of organic halogen compounds. Although it was already recognized that a better efficiency in the use of ClO₂ could be obtained at low pH values, the consequent high formation of OX was always considered as a significant technical drawback in this kind of process, which drawback has now been overcome by the process of the present invention by establishing the optimum values of pH.
The addition of CIO₂ in the first bleaching stage together with the control of the pH in the range of 4.5 to 5.5 allows the use of ClO₂ charges up to 10 kg/tsa (i.e. up to kg / air dried ton) without any disadvantageous production of organic halogen compounds. In the invention, the ClO₂ is added to the pulp in a range of 6 kg/tsa to 8 kg/tsa, (i.e 6 kg / air dried ton to 8 kg / air dried ton), more preferably at about 8 kg/tsa (i.e 8 kg / air dried ton).
According to the process of the present invention it is possible to obtain organic halogen compounds concentrations in the final product (pulp) lower than about 30 ppm. Such a low OX formation is primarily due to the enhanced control of the pH during the deoxidation step followed by the conventional subsequent extraction steps based on the use of sodium hydroxide and hydrogen peroxide. Thus, in a bleaching stage comprising ( the sequential steps DO-Eop-Q-(PO-P), that is, treatment steps with (chlorine dioxide - hydrogen peroxide alkali - sequestering agents (chelating agents) - peroxide ozone peroxide), expressive gains in brightness and in the reduction of chemicals necessary to the removal of organic halogen compounds are obtained by working at low consistency conditions and with addition of chlorine dioxide in the first bleaching stage, and further controlling the pH in the ideal range of 4.5 to 5.5.
The graphic shown in Figure 3 indicates a more efficient control of the average OX content in the resulting product in accordance with the present invention, with said OX values being stable at levels of about 15 ppm. As can be concluded by comparing these values with those of pulps obtained by conventional processes, when the deoxidation step is not carried out under conditions in accordance with the present invention, it is sometimes possible to reach low OX contents but which are not stable.
Analysis of the obtained results showed that the process according to the invention decreases the need of H₂O₂ addition during the bleaching stage, which represents a significant technical and economic advantage never achieved by the prior art processes. Figure 1 illustrates the decrease of H₂O₂ added to the pulp in a process according to the invention, wherein the gain in the reduction of the H₂O₂ employed may be of up 40% while maintaining the low content of OX and low brightness reversion. H₂O₂ reductions to values lower than 35 kg/tsa (i.e., 35 kg/ adt) were achieved. On the other hand, Figure 2 shows the relationship between the peroxide added and the initial Kappa number which is also a good indicative of the reduction of the H₂O₂ needed and, consequently, of the enhanced results in the final product.
Therefore, the addition of ClO₂ in the first stage of the bleaching process carried out under the above mentioned conditions helps the removal of the organic charge, also contributing for the removal of lignin, as well as the increase of the H₂O₂ reactivity in the Eop stage, whereby providing enhanced brightness when compared to pulps treated with the same amount of peroxide within conventional processes. The pulps obtained by the process according to the present invention show brightness of about 90% ISO and all technical process parameters lead to low production costs and to a high recycling rates of effluents in the bleaching process.
Figure 5 presents the relationship of the added chlorine dioxide amount to the OX content formed in the pulp during the process of the invention. It can be seen that the ratio of ClO₂ to the OX content did not vary even when increasing the ClO₂ amount, which indicates that the latter acted for removing the OX present in the pulp after the first stage, thus maintaining the pulp quality in the desirable levels.
Figure 4 shows the potential brightness gain with the addition of ClO₂ in a stage prior to the step of adding hydrogen peroxide The performance of the Eop stage was substantially enhanced with the addition of 2 Kg/tsa (2 mg/adt) of ClO₂, in the DO.
In order to obtain the desired brightness levels in the Eop stage without negatively affecting the OX results, an optimum range for the pH was determined for the DO stage and, therefore, an ideal application range was established as indicated in Figure 6.
After some experiments, it was observed that an addition of ClO₂ in the first stage also provided an increasing in the number of washing/extraction stages, substantially decreasing the amount of OX present in the pulp. Moreover, Figures 7 and 8 show that the Eop stage has the best performance in the removal of the organic halogen compounds while stages such as PO (pressurized peroxide) do not present good capacity for removing said OX.

Claims

A process for bleaching chemical pulps characterized by comprising the sequential steps DO-Eop-Q-(PO-P) wherein the addition of chlorine dioxide is carried out only in the first bleaching stage as a charge of 6kg/tsa to 8kg/tsa (6kg/adt to 8kg/adt) and at a pH in the range of 4.5 to 5.5.
A process according to claim 1, characterized in that the chlorine dioxide is added as a charge of 8kg/tsa (8kg/adt).