Reduction of organically bound chlorine formed in chlorine dioxide bleaching

Abstract

A reduction in AOX levels is obtained when the process conditions in the chlorine dioxide stage are elevated to above 91° C. and extended to more than 90 minutes. A major reduction of AOX up to 50% has been shown without a corresponding increase in OCl. The chlorinated substances is degraded by the process conditions to harmless chloride ions, instead of being liberated into the effluent as AOX or bound to pulp as OCl. No oxygen gas or nitrogen gas are added during the first or any subsequent chlorine dioxide bleaching stage.

Description

TECHNICAL FIELD

The invention relates to the reduction of organically bound chlorine formed in chlorine dioxide bleaching without using oxygen gas or any other pressurized gas.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention is related to the formation of chlorinated organic matter in chlorine dioxide bleaching of kraft pulp, and how to reduce the amount of organically bound chlorine in pulp (OCl) and/or reduce the amount of organically bound chlorine compounds (measured as, e.g., AOX or TOCl) in the waste water.

The most efficient and inexpensive bleaching chemical so far known is elemental chlorine. The use of it has in most parts of the world come to an end during the last decade. The driving forces in this development have been environmental, expressed either as market demands or as environmental standards set by governments or a combination of the two. The negative environmental impact connected to the use of elemental chlorine is primarily the formation of chlorinated organic structures.

Following a massive introduction of oxygen delignification systems, the work needed in the subsequent bleaching could be significantly reduced and the ECF concept (Elemental Chlorine Free), i.e., bleaching without the use of any elemental chlorine or hypochlorite was introduced. The chemical normally replacing the elemental chlorine is chlorine dioxide, which had been used for final brightening of pulp and for obtaining a good cleanliness, e.g., due to its excellent capability of removal of extractives.

The chlorinated structures, e.g., formed in chlorine bleaching, are denoted AOX (adsorbable organic halogene compounds) when found in the bleach effluents and OCl (organically bound chlorine) when stuck in the pulp. The amount of both AOX and OCl were largely reduced upon conversion to ECF bleaching, but a zero level was not reached and in fact a significant amount of OCl is still found in ECF bleached pulps and AOX in the effluents from chlorine dioxide stages.

The levels are also significantly higher than those arising from TCF (Totally Chlorine Free) bleaching operations. This is due to the fact that when chlorine dioxide reacts with the lignin in pulp, hypochlorous acid in equilibrium with chlorine is formed, both of which are able to act as chlorinating agents. Also during manufacturing of chlorine dioxide at the mill site some elemental chlorine is produced, typically in the order of 1-4%, most often below 5% elemental chlorine, all dependent on the type of chlorine dioxide forming process used.

Considering AOX in effluents it is urgent to keep in mind that although the discharges per ton of pulp produced have decreased significantly when switching to ECF-bleaching, the mills have simultaneously grown too, meaning that the total AOX load to the specific recipient need not have changed very much and thus still constituting a potential problem. In FIG. 1 is shown how the total amount of AOX in effluents may be constant even tough the AOX level per BDt pulp have decreased over time, due to that production volumes have increased.

A pulp having been bleached using chlorine dioxide in an ECF sequence is still easily identified due to its content of OCl, which hinders it from being used in certain paper products or at certain markets. For several mills producing market pulp this is a crucial fact, since it means certain customers will not be interested in a high OCl pulp.

For various reasons, a massive conversion to TCF bleaching has so far not occurred, leaving the field open for innovative ways to approach the OCl and AOX problems in ECF-bleaching.

The obvious way, to reduce the overall charge of chlorine dioxide, has in several cases been entered upon in, what is often called “ECF-light” concepts, using a rather small charge of chlorine dioxide in the D-stage, often a charge factor of active chlorine as chlorine dioxide of below 1.

At the Tappi Pulping Conference Oct. 22-25, 1989, two papers where presented where solutions to the AOX problem was presented. Lowering of the delignification in the D-stage (or C- or C/D-stage), by using a lower charge factor of active chlorine as chlorine dioxide (i.e., kappa factor) was identified as methods for decreasing AOX, and where compensation for the lower delignification effect in D-stages is made by higher charges in other stages. One paper was presented by J. Basta, L. Holtinger, J. Hook and P. Lundgren with the title “LOW AOX, POSSIBIILITES AND CONSEQUNCES” (pp. 427-436), and the second paper was presented by H. Suss, W. Eul, N. Nimmerfroh and J. Meier, all from Degussa AG/Corp, with the title “ENVIRONMENTAL ASPECTS OF SHORT-SEQUENCE BLEACHING” (pp. 527-537). The main approach in these papers, when AOX-reduction is the objective in ECF-bleaching, is to decrease the use of chlorine dioxide at the expense of higher charges of hydrogen peroxide.

This approach is shown in EP,B,500813, where a charge factor of active chlorine as chlorine dioxide below 2.0 is used in the D_o stage (i.e., the first D-stage in a multiple sequence D-E-D . . . etc.), and where following P-stage (P=peroxide) use at least 3.0 kg of hydrogen peroxide per ton dry pulp, and having chlorine dioxide charges in following D-stages less or equal than the charge used in D0, i.e., from 20-100% of the D0 charge.

In addition to this approach it has been proposed the first chlorine dioxide stage be pH profiled by means of a short-term reaction at low pH followed by an increase to alkaline conditions (Ljunggren, S., Bergnor, E. and Kolar, J. (1994): Modified Modern ClO2-Bleaching, International Pulp Bleaching Conference (IPBC), Vancouver, Canada, Vol. 1: 169-176. and Ljunggren, S., Bergnor-Gidnert, E. and Kolar, J. (1996): Chlorine Dioxide Bleaching with a Two-step Low-to-High pH Profile, Tappi J. 79: 12, pp. 152-160.).

This approach has many similarities with the Ultim-O process (no washing between D0 and E). Although this approach indeed enabled significant reductions in the AOX discharges, the OCl content was less affected and most important, the need for alkali increased largely, making it less attractive.

Lately, a reductive alkaline post-treatment has been proposed as a way of significantly reducing the OCl content of a pulp, (see Ljunggren, S., Johansson, E. and Pettersson, B. (1998): Dechlorination of ODEDD Bleached Kraft Pulps, 5th European Workshop on Lignocellulosics and Pulp (EWLP), Aveiro, Portugal, pp. 437-440), which is a somewhat refined way of utilizing the well-known fact that an alkali extraction undoubtedly is a very efficient way for the removal of OCl. Although efficient, such a post-treatment of the pulp requires both additional washing equipment and additional bleaching towers, making also this approach less attractive for mill implementation.

Improvements in Chlorine Dioxide stages have been made for several purposes. In a paper presented by Lachenal, D. and Chirat, C. (1998): High Temperature Chlorine Dioxide Delignification: A Breakthrough in ECF Bleaching of Hardwood Kraft Pulps, Pulping Conference, Atlanta, U.S.A., Vol. 2:pp. 601-604.), a modification of the conventional D-stage is suggested. With the objective to make the D-stage more efficient, and reduce charges of chlorine dioxide, it is proposed to modify the conventional 45° C. D-stage to a high temperature (90-100° C.) D-stage having long retention time (1.5-4 hrs). An alternative modification achieving the same improvement was proposed where instead this high temperature is implemented after, “at the exit of”, the D-stage when the chlorine dioxide have been consumed, during which process position the high temperature could not affect the break-down process of chlorine dioxide in the D-stage. This paper also indicates that the change from chlorine to chlorine dioxide bleaching will solve the AOX-problem.

Attempts have been made to add pressurized oxygen gas at high concentrations, such as 85%, to reduce the AOX level as a result of a possible oxidizing effect upon the filtrate/liquid. One drawback of the oxygen gas addition is the fact that the gas content of the pulp increases and this in turn may decrease the washing efficiency in the subsequent washing stage since air bubbles in the pulp have a tendency to reduce the drainage capability of the pulp that makes the washing less effective. Additionally, residual chlorine gas is often a problem and adding oxygen gas to the process does not reduce the problems with the chlorine gas.

The main objective with the present invention is to reduce the total amount of chlorinated organic matter leaving a chlorine dioxide stage, and especially the total amount of AOX and OCl, where at least a substantial reduction in AOX levels is obtained, and this while being able to operate chlorine dioxide stages with higher charges of chlorine dioxide than “ECF light”.

Another objective is that the overall operating costs for pulp bleaching could be kept low if the delignification effect from chlorine dioxide is utilized in full in the first chlorine dioxide stage in the bleaching sequence, whereby charges of other more expensive bleaching chemicals, in cost per kg or per bleaching effect, could be kept at lower levels.

Another objective according to the invention is that an initial chlorine dioxide stage run at high temperature for long time is shown to be an efficient means of reducing the overall discharge of AOX by about 50 percent, presumably through a forced degradation of the chlorinated structures formed in the stage. This high reduction of AOX by about 50% at a given overall chlorine dioxide charge compared to operation of said initial chlorine dioxide stage at conventional conditions, i.e., some 60-70° C. and 20-60 minutes.

Moreover, a further addition of sulphamic acid to a final D-stage is presented as an efficient tool for reducing the total amount of AOX and OCl, with substantial decrease of the OCl content of an bleached pulp, preferably ECF bleached pulp, since sulphamic acid captures in situ formed elemental chlorine. Said substantial decrease amounting to about 50 percent in a final D-stage operating at similar charge of chlorine dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a total amount of AOX relative to the mill size and AOX/BDt;

FIG. 2 is a schematic illustration of possible substances from chlorinated structures; and

FIG. 3 is a schematic illustration of OCl levels over time.

DETAILED DESCRIPTION

The invention is based upon the origin of OCl and ways to decrease it, without necessarily reducing the use of chlorine dioxide and still reaching the same final brightness. The distribution of OCl in ECF-bleached pulp is playing an important role.

It is important to understand the correspondence between AOX and OCl. In FIG. 2, the three major possible faiths of a chlorinated substance in the pulp are summarized. Following a chlorination there are thus three alternatives, either that the chlorinated structure sticks to the final pulp becoming OCl, or that it is liberated during subsequent bleaching stages becoming AOX, or that the structure is substituted/degraded so that the chlorine atoms form harmless chloride ions.

Important to keep in mind is hence that there is no direct correspondence between AOX and OCl telling, e.g., that a high AOX discharge means a low OCl content in the pulp at a certain chlorine dioxide charge.

In a series of trials the standard ECF bleaching sequence of DEDED, using an overall chlorine dioxide charge of 29.6 kg a Cl/BDt, was used to bleach the oxygen delignified HW kraft pulp from the second series of trials (kappa 9.8) to full brightness (above 89 % ISO). 19.6 kg a Cl/BDt was used in D0, and 5 kg a Cl/BDt in each of D1 and D2. The charge factor of active chlorine as chlorine dioxide in D0 equaling (19.6/9.8=) 2.0. This standard sequence was compared with three modified sequences, D*EDED, DEDE(SD) and D*EDE(SD). D* denotes a D-stage run at high temperature (90° C.) and long time (120 min). “S” denotes the presence of sulphamic acid. E stages were performed according to above. D1 and D2 stages were performed at 75° C. and 120 min.

Kappa number, viscosity and ISO brightness were analyzed using the respective SCAN standards. In addition, SCAN standard CM 52:94 “Pulps, papers and boards—organic chlorine” was used to determine the content of OCl in the pulp after different stages. All bleaching experiments were performed at 10 percent pulp consistency in plastic bags, which after intense kneading were placed in heated water baths. The charge of sulphamic acid should be somewhat higher, i.e., on a molar basis, than the charge of active chlorine, in this investigation meaning 1.0 mmol sulphamic acid/BDt.

In those stages to which sulphamic acid addition was made, the charge of active chlorine was increased in order to compensate for the decreased oxidizing capacity of the stage when the reduction of chlorine dioxide to chloride ion is broken at the level of elemental chlorine. The oxidizing capacity of chlorine dioxide is decreased by 20 percent in the presence of sulphamic acid, which captures intermediately formed elemental chlorine, and following reaction pattern is developed with and without sulphamic acid.

In practice this means that 4 out of 5 electrons are used when chlorine dioxide bleaching in the presence of sulphamic acid is used and thus the charge of active chlorine to such stages were increased by 25 percent. This way, all the pulps were subjected to identical charges of “active” active chlorine.

Results from the 5-stage bleaching study on HW mill oxygen delignified kraft pulp are given in following Table 1.

TABLE 1
Trial	DEDED	D*EDED	DEDE (SD)	D*EDE (SD)
Final kappa	2.1	0.6	2.5	1.2
Final viscosity [ml/g]	975	937	939	911
ISO brightness [%]	89.4	89.9	89.4	89.5
Total a Cl charge	27	27	27	27
[kg/ADt]
Total Ocl [mg/kg]	152	158	88	116
Total AOX [kg/ADt]	0.41	0.23	0.39	0.21

From the results it is clear that the AOX discharge could be reduced with about 50 percent using D* instead of D as the first bleaching stage. It should be noted that this result is obtained when comparing sequences with identical overall charge of chlorine dioxide. In addition to this reduction of AOX, the value can be even further reduced when the chlorine dioxide saving effect of the D*, (as, e.g., noted by Lachenal, D. and Chirat, C. (1998): High Temperature Chlorine Dioxide Delignification: A Breakthrough in ECF Bleaching of Hardwood Kraft Pulps, Pulping Conference, Atlanta, U.S.A., Vol. 2: 601-604.) is taken into account, here instead recorded as a higher final brightness.

This finding was very unexpected. One would else have anticipated that if the AOX levels experienced a decrease, then the OCl would increase by a similar order. However, the findings showed that the AOX-levels were decreased without a similar order of increase in OCl.

The interpretation of the result should not be that less chlorination takes place or that less AOX is formed in a D*-stage than in a conventional D-stage. On the contrary, it seems appropriate to suppose that under the tough conditions of the D* stage, a substantial part of the AOX formed in the stage is further degraded to, e.g., harmless chloride ions. With this knowledge in mind it is interesting to compare D* with (AD), i.e., where A is performed as a hot acid treatment for long duration at, e.g., 90-100° C. and 120 min according to concepts like GB 1.062.734. In GB 1.062.734 this acid treatment at pH 2.25, temperature 100° C. and during 120 minutes was implemented in order to reduce brightness reversion.

The extreme A-stage was followed by a conventional D0-stage at some 60° C. without intermediate washing. In conformity with D*, an (AD) approach gives a potential to reduce the overall need for chlorine dioxide in the bleaching of especially HW kraft pulp, although D* has been shown to have a greater potential in this respect. However, in contradiction to D*, an (AD) approach will not enable any reduction of the AOX according to the mechanisms presented. Theoretically, D* can of course be utilized in any position in the bleaching sequence irrespective of the number of D-stages in the bleaching line. Although in general it is likely that the benefits of the stage primarily motivates its utilization in the D0 position, i.e., the first stage using chlorine dioxide.

From the results in Table 1 it is also clear that the presence of sulphamic acid in the final D-stage is an efficient means of reducing the OCl content of the pulp. Having the OCl pattern shown in FIG. 3 in mind, it is easily concluded that the largest effect to the lowest charge of sulphamic acid is obtained when sulphamic acid addition is made to the last D-stage, although a larger effect of course can be obtained using it in all D-stages. Although sulphamic acid already today is commonly used in pulp mills, e.g., for the removal of scales in machinery upon shutdowns, its use in continuous bleaching processes for obtaining low OCl pulp is new. The addition of sulphamic acid should be added in a continuous manner during the bleaching process in the chlorine dioxide stage, i.e., so that sulphamic acid is present during the consumption of chlorine dioxide in the chlorine dioxide stage. The sulphamic acid could be added to the pulp before, after or during addition of the chlorine dioxide in a chlorine dioxide mixer.

It should be added that the chlorine dioxide charge in a (SD) stage has to be increased by some 15-30%, typically 25 percent, in order to compensate for the reduced oxidizing power lost due to the capture of elemental chlorine by sulphamic acid. However, when utilized in D2-position, or in the final D-stage, this means a very moderate additional need for chlorine dioxide in this last stage.

The two concepts D* and (SD) could also be utilized in the same sequence, thus enabling the manufacture of a pulp with low OCl content at the same time as the AOX discharges are kept low, as shown in Table 1.

It can be concluded that a 50 percent reduction of the overall AOX discharge of a DEDED sequence can be obtained by using a D*-stage instead of a conventional D-stage in D0 position. The OCl content can also be fought and decreased by about 50 percent even in an existing bleaching line by changing the last D-stage to operation with sulphamic acid addition in a (SD)-stage.

An important feature of the present invention is that no oxygen gas or nitrogen gas is added to the process and it is sufficient to simply maintain the temperature and pressure at suitable levels for a sufficiently long retention time as long as the pressure is above a saturation pressure at the prevailing temperature to prevent the pulp from boiling. The minimum suitable retention time is 90 minutes and the maximum retention time is 300 minutes to achieve an AOX reduction so that the AOX level is less than 0.23 kg/ADt in the first chlorine dioxide stage (D1). It was surprising to note that the relatively long retention time of between 90-300 minutes did not result in a loss of pulp viscosity and other negative effects. The fact that the longer retention time is in the first chlorine-dioxide bleaching step has the surprising effect that the loss of pulp viscosity in the final treatment steps is minimal. A skilled person would normally hesitate to use a long retention time due to the loss of pulp properties such has pulp viscosity. There has been a goal in the conventional technologies to minimize the retention time at high temperature over 90-95C due to the loss of pulp strength and the formation of residual chlorine gas in the first chlorine-dioxide bleaching step that negatively affects the subsequent washing steps. It was surprising that the operation at 91C and above and a longer retention time over 90 min alone reduced the AOX level to as low as 0.23 kg/ADt and lower without the need for the addition of gas such as oxygen gas.

While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.

Claims

1. A process for reducing the amount of organically bound chlorine formed in chlorine dioxide bleaching of kraft pulp, comprising: providing several bleaching stages wherein at least one of the stages is a bleaching stage using chlorine dioxide as bleaching chemical to form a bleaching sequence; in a first chlorine dioxide bleaching stage, charging a first amount of chlorine dioxide to the pulp having a first kappa number so that a charge factor of the first amount of chlorine dioxide/first kappa number is above 0.5 and operating at a temperature above 91 C and a retention time longer than 90 minutes; reducing a resulting AOX content in an effluent from the bleaching sequence to a value below 0.23 kg/ADt; and adding no oxygen gas or nitrogen gas to the first chlorine dioxide bleaching stage.

2. The process according to claim 1 wherein the retention time is less than 300 minutes.

3. The process according to claim 1 wherein the first chlorine dioxide bleaching stage used during the bleaching sequence is operated at a temperature between 95° C. and 120° C. at a retention time of between 90 minutes and 300 minutes and the first chlorine dioxide bleaching stage is pressurized to a pressure exceeding the vapor saturation pressure for the temperature in the stage by at least 20%.

4. The process according to claim 1 wherein the charge factor is in the range 1.5-3.0.

5. The process according to claim 1 wherein the first chlorine dioxide bleaching stage has a pulp concentration between 7-25%.

6. The process according to claim 1 wherein the process comprises the steps of delignifying the pulp to a kappa number below 20 prior to bleaching the pulp in the first chlorine dioxide bleaching stage.

7. The process according to claim 6 wherein the method further comprises adding a sulphamic acid to at least one of the chlorine dioxide bleaching stages in the bleaching sequence to capture intermediately formed chlorine or hypochlorite to form a chlorosulphamic acid.

8. The process according to claim 7 wherein the method further comprises adding the sulphamic acid in a mmol amount that is greater than a mmol amount of active chlorine dioxide added during the first chlorine dioxide bleaching stage.

9. The process according to claim 7 wherein the method further comprises adding at least 1.0 mmol sulphamic acid/BDt of pulp.

10. The process according to claim 7 wherein at least 80% of all sulphamic acid added is added to a last chlorine dioxide bleaching stage.

11. The process according to claim 10 wherein the last chlorine stage is a D2 stage preceded by a D0 chlorine stage and a D1 chlorine stage and the method further having extraction stages E between the chlorine dioxide stages to form a D0-E-D1-E-D2 bleaching sequence.

12. The process according to claim 11 wherein a charge of chlorine dioxide, used in a chlorine stage including sulphamic acid, is at least 10% greater than a charge of chlorine dioxide used in a chlorine stage that is free from sulphamic acid.