WO1988009842A1 - A method of reducing chlorophenol compounds in the chlorine bleaching of pulp - Google Patents

Abstract

A method of reducing the chlorophenol compounds detrimental to the environment which are normally produced during the bleaching of fibrous cellulose pulp with chlorine. In order to obtain said reduction the invention suggests the combination of a) performing the chlorine bleaching in a first chlorine stage and a second chlorine stage with an intermediate washing stage but without any intermediate alkali stage, and b) dividing the charge of chlorine so that at most 0.14xK, preferably (0.06-0.12)xK, is added in the first chlorine stage, calculated as a percentage of the active chlorine on dry, well washed pulp, wherein K is the kappa number of the incoming pulp, the remaining amount of chlorine of a normal total chlorine requirement being added in the second chlorine stage.

Description

A method of reducing chlorophenol compounds in the chlorine bleaching of pulp

The present invention relates to a method of reducing the chloro¬ phenol compounds detrimental to the environment which are normally produced during the bleaching of fibrous cellulose pulp.

When bleaching cellulose pulp with chlorine, chlorinated organic material is formed which includes, amongst other substances, chlo¬ rinated phenol compounds. Although chlorinated phenol compounds constitute only a minor portion of the total amount of organic ma¬ terial containing bound chlorine (T0C1), that is about 50-100 g/ton out of a total of 4-6 kg/ton, the chlorinated phenol com- pounds are responsible for a considerable share of the negative effects to the environment which have been observed in the waste from the bleach plants. The production of chlorinated phenol com¬ pounds and T0C1 is linearly dependent on the lignin content in the pulp and the charge of chlorine. Methods known so far for reducing the production of chlorophenol compounds and T0C1 have consisted of lowering the lignin content of the pulp prior to chlorine bleaching by increasing the duration of the pulping or by perform¬ ing an initial delignification with the aid of oxygen gas. Another method has been to reduce the charge of chlorine by replacing chlorine by chlorine dioxide. However, these various methods have not proved entirely satisfactory and also entail high chemical costs.

The article "Rapid chlorination at ambient temperature with re¬ peated chlorination and extraction", by R.M. Berry, B.I. Fleming, pages 236-243, of "Paperi ja Puu - Papper och Tra" - 3/1987, de¬ scribes a method of bleaching pulp with chlorine in which the chlorination is performed in two or more chlorine stages with an alkali stage between two chlorine stages. The object of the present invention is to provide an improved method of bleaching cellulose pulp, which results in a reduction of the chlorophenol compounds damaging to the environment released in the waste water from the bleach plant.

This is achieved according to the present invention by carrying out a combination of the following measures: a) performing the chlorine bleaching in a first chlorine stage and a second chlorine stage with an intermediate washing stage but without any intermediate alkali stage, and b) dividing the charge of chlorine so that at most 0.14xK, prefer¬ ably (0.06-0.12)xK, is added in the first chlorine stage, cal¬ culated as percentage of the active chlorine on dry, well washed pulp, wherein is the kappa number of the incoming pulp, the remaining amount of chlorine of a normal total chlo- rine requirement being added in the second chlorine stage.

According to a preferred embodiment the two chlorine stages are carried out during a total time of at most 90 minutes, preferably between 10 and 60 minutes, and the duration of the first chlorine stage is considerably shorter than that of the second chlorine stage. The first chlorine stage is suitably carried out for at most ten minutes, preferably between 1 and 4 minutes. The process conditions in the first chlorine stage are suitably adjusted so that the pulp discharged contains a relatively low content of res¬ idue chlorine.

According to a particularly preferred embodiment of the invention, the chlorine charging in each chlorine stage is divided into at least two portions of chlorine, said portions being added to the pulp at different successive points along the flow line of the pulp.

The bleaching is suitably performed on a pulp having a concentra¬ tion of 2-14%, preferably 3.5-12% and the temperature during bleaching is maintained within the interval 10°-70°C, preferably 30°-60°C. According to yet another preferred embodiment, the chlorine bleaching is performed in the presence of a small amount of chlo¬ rine dioxide in order to protect the pulp from attack by the chlo¬ rine. Naturally it is within the scope of the invention to perform the bleaching with chlorine and chlorine dioxide in any conven¬ tional proportions, wherein the chlorine charging being divided in accordance with the present invention in order to achieve the de¬ sired result, i.e. a reduction in the chlorophenol compounds det¬ rimental to the environment.

As a result of the method according to the invention, considerably smaller amounts of chlorinated phenol compounds are produced, both in the effluent from the chlorine stages and in the liquor from a subsequent conventional alkali stage. This result has consequently a positive effect on the environment and is achieved by means of the combined measures proposed according to the invention, which in general comprise chlorination in two stages with a washing ope¬ ration therebetween, and a controlled, low dosage of chlorine in the first chlorine stage. The washing operation removes organic substances released from the pulp. These substances contribute di- rectly and also indirectly through autocatalysis, to the produc¬ tion of chlorinated phenol compounds which then accompany the ef¬ fluent from a conventional chlorine stage of long duration. Fur¬ thermore, a reduction is also obtained in the amount of chlorinat¬ ed phenol compounds, primarily guaiacols, released from the pulp in the alkali stage following the second chlorine stage in the process. This is probably because the washing reduces the amount of catalytic substances, particularly those of quinone structure, in the pulp. The first chlorine stage is thus interrupted by the washing stage, .this being commenced after the initial rapid reac-^' tion between chlorine and pulp has taken place, but before the amount of chlorocatechols starts to increase. If chlorination is carried out normally in one stage, the amount of chlorinated phe¬ nol compounds increases linearly with the bleaching time. The amount of chlorinated phenols is also dependent on the pH value during bleaching. A higher pH value, achieved by the addition of alkal to the pulp, will give a lower content of chlorinated phe¬ nol compounds if bleaching is carried out for the same length of time.

The production of chlorocatechols passes its highest value after a short period of time of chlorination, but it has been found that the great increase in the amount of chlorocatechols appears only if a relatively large dose of chlorine is added in the first chlo¬ rine stage. By performing the first stage in accordance with the invention, with a low charge of chlorine, i.e. below 0.14x , and preferably within the interval 0.06xK-0.12xl _s the formation of chlorocatechols is controlled to a low level.

The production of chloroguaiacols in the alkali stage is substan¬ tially proportional to the chlorine charge in the first chlorine stage. Thus, by using a low charge of chlorine in accordance with the present invention, in the first stage, which is then followed by a washing stage, a lower total amount of chloroguaiacols is ob¬ tained.

The method according to the invention can result in a reduction of about 50% of the amount of chlorocatechols in the effluents from the chlorination and the amount of chloroguaiacols can be about

30% lower in the liquor from the alkali stage, as compared with conventional chlorination in a single chlorine stage in which the total amount of chlorine used and the total time of chlorination are otherwise the same.

Another important advantage achieved by the method according to the invention, is that the properties of the pulp are improved. Comparative experiments have shown that the viscosity of the pulp after two-stage chlorination according to the invention increased by about 25% as compared with a single chlorine stage. The method according to the invention thus results in less decomposition of the cellulose and a better yield is thus obtained. Moreover, it has been found that the method according to the invention gives a pulp having a kappa number after the alkali stage which is about 20% lower than in the comparison experiment without an intermedi¬ ate washing stage. The pulp is thus more efficiently delignified by the method according to the invention, which in turn facil - tates the subsequent bleaching process so that the cost of chemi¬ cals and equipment can be reduced.

The invention is further illustrated in the following with refer¬ ence to the drawing showing a flow diagram of a bleaching sequence according to the invention, and by way of the following example:

Example

A sulphate pulp having a kappa number of 30 and a concentration of 3.5% was treated in two separate chlorine stages with a bleaching agent containing chlorine and consisting of a mixture of chlorine and chlorine dioxide, the chlorine dioxide content amounting to 15% calculated as active chlorine. The total dosage of bleaching agent amounted to 0.22xK, i.e.0.22x30 = 6.6% active chlorine cal¬ culated on dry, well washed pulp, corresponding to 66 kg active chlorine per ton of pulp. The total charge of the bleaching agent was divided so that in the first chlorine stage the charge was O.lOxK and in the second chlorine stage 0.12xK active chlorine calculated as a percentage of dry, well washed pulp. The addition of chlorine was thus divided so that the charge in the first chlo¬ rine stage was 0.085xK and in the second chlorine stage 0.102xK, calculated as a percentage of dry, well washed pulp. After 1 min- ute, the effect of the chlorine on the pulp in the first chlorine stage was interrupted by a washing stage. This comprised washing with clean water and was performed by repeatedly diluting and thickening the pulp. After this washing stage the pulp was immedi¬ ately subjected to the second chlorine stage for a period of 60 minutes. The temperature in both chlorine stages was 25°C and the pH-value was 1.8. The pulp was then washed before being treated with alkali in an amount of 30 kg NaOH per ton dry pulp. Examina¬ tion of the effluents from the two chlorine stages showed that the total amount of chlorinated phenol compounds in the effluents de¬ creased about 50% and the amount of chlorinated phenol compounds in the liquor from the alkali stage decreased about 30% compared with a reference bleaching performed simultaneously in one stage, where the total time and total amount of chlorine used were other¬ wise the same.

The chlorophenol analysis performed at the experiments is shown in the following table.

Tabl e

Chlorinations (C) performed on 3.5% pulp concentration at 25°C. Alkali stage (E) performed on 10% pulp concentration at 60°C using 3% NaOH.

Charge of active chlorine: Reference = 0.21xK; Cl = O.lOxK; C2 =

0.12xK.

PH = chl orophenol , CA = chl orocatechol , GU = chl oroguaiacol ,

VA - chl orovanil l n.

Concentration of chlorophenol s in ug/1.

Chlorination in two stages Reference Intermediate wash after 1 minute for Cl

Compound Cl C2 C1+C2

2,4-di-PH 150 231 22 179 2,4,6-tri-PH 76 280 13 240

Total 226 511 35 40 419

4,5-di-CA 178 62 86 23 40

3,4-di-CA 194 5 10

3,4,5-tri-CA 481 17 291 45

3,4,5,6-tertiary-CA 424 13 86 22

Total 1277 92 468 100 568 46

4,5-di-GU 512 303

3,4,5-tri-GU 870 704

3,4,6-tri-GU 102 62

4,5,6-tri-GU 166 110

3,4,5,6-terti.ary-GU 307 238

Total 1957 1417

6-VA 82 1113 46 42 736 5,6-di-VA 19 342 28 229

Total 101 1455 74 42 116 965

Final total 1604 4015 724 2847 In accordance with the present invention, the flow diagram shown includes two separate chlorine stages 1 and 2 and an intermediate washing stage. The pulp is supplied from a storage tower to the chlorine stage 1 via a chlorine mixer, said chlorine stage 1 being performed in a chlorination reactor. Chlorination of the pulp is thus initiated as soon as the chlorine is added in the chlorine mixer. The first chlorination reactor may alternatively be re¬ placed by a pipe connection of suitable length between the first chlorine mixer and the washing device. In this case, the chlorina- tion commenced in the chlorine mixer will continue as the pulp is pumped through this pipe connection. The washing device may con¬ sist of one or more washing filters or diffusers.

The alkali stage may be performed in the presence of oxygen gas or peroxide and may be followed by further conventional bleaching stages employed after a prechlorination.

According to another particularly advantageous embodiment of the invention, the chlorine charging in each chlorine stage is divided into at least two chlorine portions, said portions being added to the pulp at different successive points along the flow line of the pulp. These points may be represented by an equivalent number of chlorine mixers arranged at suitable distances, one after the oth¬ er, giving sufficient dwell time between two chlorine mixers for a considerable amount of the chlorine" portion to react with the lig¬ nin. Alternati ely, the chlorine charge may be spread in this way in the first chlorine stage only. Dividing the first or both chlo¬ rine charges into two or more chlorine portions gives a further reduction of the chlorophenols in the effluents from the chlorine stages and the alkali stage.

Claims

£IAI_i£

1. A method of reducing the chlorophenol compounds detrimental to the environment which are normally produced during the bleach¬ ing of fibrous cellulose pulp with chlorine, characterized by the combination of a) performing the chlorine bleaching in a first chlorine stage and a second chlorine stage with an intermediate washing stage but without any intermediate alkal stage, and b) dividing the charge of chlorine so that at most 0.14xK, prefer¬ ably (0.06-0.12)xK, is added in the first chlorine stage, cal- culated as a percentage of the active chlorine on dry, well washed pulp, wherein is the kappa number of the incoming pulp, the remaining amount of chlorine of a normal total chlo¬ rine requirement being added in the second chlorine stage.

2. A method as claimed in claim 1, characterized in that the two chlorine stages are carried out during a total time of at most 90 minutes, preferably between 10 and 60 minutes, and that the du¬ ration of the first chlorine stage is considerably shorter than that of the second chlorine stage.

3. A method as claimed in claim 1 or 2, characterized in that the first chlorine stage is carried out during a time of at most ten minutes, preferably between 1 and 4 minutes.

4. A method as claimed in any of the preceding claims, charac¬ terized in that the process conditions in the first chlorine stage are adjusted so that the pulp discharged contains a relatively low content of residue chlorine.

5. A method as claimed in any of the preceding claims, charac¬ terized in that the chlorine charging in each chlorine stage is divided into at least two chlorine portions, said portions being added to the pulp at different successive points along the flow line of the pulp.

6. A method as claimed in any of the preceding claims, charac¬ terized in that the bleaching is performed on a pulp having a con¬ centration of 2-14%, preferably 3.5-12%.

7. A method as claimed in any of the preceding claims, charac- terized in that the temperature during bleaching is maintained within the interval 10°-70°C, preferably 30°-60°C.

8. A method as claimed in any of the preceding claims, charac¬ terized in that the chlorine bleaching is performed in the pres¬ ence of a small amount of chlorine dioxide in order to protect the pulp from attack by the chlorine.

9. A method as claimed in any of the preceding claims, charac¬ terized in that the second chlorine stage is followed by an alkali stage.

10. A method as claimed in claim 9, characterized in that the alkali stage is performed in the presence of oxygen gas or perox¬ ide.

11. A method as claimed in claim 9 or 10, characterized in that the pulp is finally bleached in conventional manner after the al¬ kali stage.