PROCESS FOR BLEACHING CHEMICAL PULPS WITH LOW ORGANIC HALOGEN COMPOUNDS CONTENT

Abstract: The present invention refers to a process for bleaching chemical pulps comprising a step of adding chlorine dioxide in a pH in the range of about 4.5 to 5.5 as the first bleaching stage. The process of the present invention provides for obtaining contents of organic halogen compounds in the final pulp product lower than 30 ppm and for a reduction in the H2O2 needed for the subsequent stages.

Description: PROCESS FOR BLEACHING CHEMICAL PULPS WITH LOW ORGANIC HALOGEN COMPOUNDS CONTENT Field of the Invention The present invention refers to an enhanced process for bleaching pulps that al- lows 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 al- ready been studied by Suss 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 H 2 O 2 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 OCI 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 diox- ide An example of such processes is presented in patent EP 500,813 that discloses a process for bleaching hgno-cellulosic pulps with reduced formation of organic halogen compounds carried out by adding CIO 2 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 CIO 2 treatment e 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 man- ner 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 the OX content specially in low consistency pulps, and that can also reduce the amount of chemicals necessary to achieve the final OX reduc¬ Summary of the Invention The present invention refers to a process for bleaching low consistency chemical pulps comprising a step of adding chlorine dioxide in a pH in the range of about 4 5 to 5.5 as the first bleaching stage 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 CIO 2 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 proc- ess 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 low consistency chemical pulps when a step of adding chlorine dioxide is carried out as the first bleaching stage, said addition of chlorine dioxide being effected in a pH range of about 4 5 to 5 5 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 values 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 CIO 2 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 CIO 2 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 CIO 2 charges up to 10 kg/tsa without any disadvantageous production of organic halogen compounds In a preferred embodiment of the invention, the CIO 2 is added to the pulp in a range of 6 kg/tsa to 8 kg/tsa, more preferable at about 8 kg/tsa 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 de- oxidation step followed by the conventional subsequent extraction steps based on the use of sodium hydroxide and hydrogen peroxide Thus, in a bleaching stage comprising, for example, 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 gams 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 H 2 O 2 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 H 2 O 2 added to the pulp in a process according to the invention, wherein the gain in the reduction of the H 2 O 2 employed may be of up 40% while maintaining the low content of OX and low brightness reversion H 2 O 2 reductions to values lower than 35 kg/tsa 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 H 2 O 2 needed and, consequently, of the enhanced results in the final product Therefore, the addition of CIO 2 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 H 2 O 2 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 CIO 2 to the OX content did not vary even when increasing the CIO 2 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 CIO 2 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 of CIO 2 in the DO In order to obtain the desired brightness levels in the Eop stage without nega- tively 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 CIO 2 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

Claims: Claims 1- A process for bleaching low consistency chemical pulps characterized by comprising the step of adding chlorine dioxide in a pH in the range of about 4.5 to 5.5 as the first bleaching stage. 2- A process according to claim 1 , characterized in that the chlorine dioxide is added as a charge of up to 10 Kg/tsa. 3- A process according to claim 2, characterized in that the chlorine dioxide is added as a charge of about 6 Kg/tsa to about 8 Kg/tsa. 4- A process according to claim 3, characterized in that the chlorine dioxide is added as a charge of about 8 kg/tsa. 5- A process according to any one of the preceding claims, characterized in that the final pulp product comprise an organic halogen compounds content lower than 30 ppm. 6- A process according to claim 5, characterized in that the final pulp product comprise n organic halogen compounds content of about 15 ppm. 7- A process according to any one of the preceding claims, characterized by comprising the sequential steps DO-Eop-Q-(Po-P)

Inventors: PASQUALI SILVIA MARIA, TEDESCHI DE ARAUJO GUILHERME A, PASQUALI, SILVIA, MARIA, TEDESCHI DE ARAUJO, GUILHERME, ANTONIO
Applicants: ARACRUZ CELULOSE, PASQUALI SILVIA MARIA, TEDESCHI DE ARAUJO GUILHERME A, ARACRUZ CELULOSE S.A., PASQUALI, SILVIA, MARIA, TEDESCHI DE ARAUJO, GUILHERME, ANTONIO
Application year: 2000
Main classification: D21C9
IPC codes: D21C9