JPH11152693A - Bleaching of chemical paper pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for bleaching a chemical pulp containing at least one treating stage using chlorine dioxide capable of protecting environment and providing a strongly bleached pulp. SOLUTION: In the method for bleaching chemical paper pulp on a series of treating stages containing at least one stage for preparing a reaction mixture for treating pulp by using chlorine dioxide solution, a reaction mixture is kept under oxygen pressure. In this case, the bleaching is carried out under the following conditions: (1) oxygen pressure is kept in 3-10 bar, (2) chlorine dioxide is used in a ratio of 0.05-5 wt.% based on dried weight of pulp, (3) the temperature of the reaction mixture is kept to 40-90 deg.C and (4) the viscosity of pulp is kept to 5-15%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a chemical paper pulp (papi).
er chimiques), especially
A method for bleaching chemical paper pulp comprising at least one chlorine dioxide treatment step.

[0002]

BACKGROUND OF THE INVENTION Chemical paper pulp or chemical pulp is obtained by cuissoning a lignocellulosic material, especially wood, with one or more chemicals. Chemical pulps are classified, for example, into: kraft pulp or sulphate pulp, sulphite pulp or hydrogen sulphite pulp, semi-chemical pulp or natural sulphite pulp, pulp after cooking with a common solvent, such as pulp obtained by an organosolve process ( Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, A18, 199
1 year, 568-569). Suitable for all the following woods: 1) Softwood, such as pine mates and firs, 2) Hardwood, such as birch, poplar, beech or eucalyptus.

[0003] The chemical pulp obtained by steaming is usually
It is subjected to delignification and / or bleaching. The delignification resulting from the cooking is completed in the first stage and bleached in the next stage. The first delignification stage is usually treated with chlorine gas, chlorine dioxide or oxygen. In conventional chlorine dioxide treatment, pulp is reacted with aqueous chlorine dioxide at normal pressure and acidic pH. pH is 2-3 in the initial stage,
At the end stage, it is set to 4-5. Operating conditions different from this general operating condition have been the subject of much research.

In US Pat. No. 5,474,654, after steaming,
At 9090 ° C. a gaseous mixture containing chlorine dioxide, oxygen and steam is applied to the pulp. The pH of this acidic medium is 1-6. U.S. Pat. No. 3,655,505 discloses a method of bleaching in two successive stages. In this method, first 20
Temperature 1 for pulp with ~ 60% consistency
Gaseous chlorine dioxide diluted with an inert gas such as air or water vapor at 5 to 100 ° C is allowed to act for 20 seconds to 60 minutes, then washed, and then treated with hydrogen peroxide. Swedish Patent No. 9303037 describes a method comprising at least one step of treating with chlorine dioxide. This processing is 0.
It is carried out at 90-100 ° C. under a pressure of 1-10 bar, but there is no mention of the type of dilution gas or the pH of the medium. on the other hand,
Attempts have also been made to use other oxidizing agents in place of the chlorination reactants to reduce the emission of chlorination by-products into the environment. Report by CL Forber, "Replacement of chlorine and chlorine dioxide in kraft pulp bleaching" (Emerg
ing Technologies or Laboratory Curiosities, TAPPI
In Press, Atlanta, Georgia, 1993), there are some compounds that have sufficient bleaching capacity to replace chlorine in delignification, but have efficiency, selectivity, and cost-effective reactions such as chlorine dioxide. He concluded that there was no object, and it would not be possible to replace chlorine dioxide in bleaching. By simply using chlorine dioxide instead of chlorine in the bleaching process, the amount of organic chlorine derivatives (AOX: Adsorbable Organic Halogen) in the wastewater discharged from paper mills is greatly reduced, but it is important to protect the natural environment. It is still not enough to meet the demand.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for bleaching chemical pulp which comprises at least one treatment step with chlorine dioxide so that the environment is protected and a strongly bleached pulp is obtained. Is to provide.

[0006]

SUMMARY OF THE INVENTION The present invention is a process for bleaching chemical paper pulp in a series of processing steps that includes at least one step of forming a reaction mixture for processing the pulp using a chlorine dioxide solution. A method is provided wherein the mixture is kept under oxygen pressure.

[0007]

DETAILED DESCRIPTION OF THE INVENTION Applicants have surprisingly found that operating under oxygen pressure reduces chlorine dioxide consumption and improves the brightness of chemical pulp. The processing step of the present invention can be used at any stage of the bleaching stage of the chemical pulp, but is preferably performed at the end of the series of processing operations. This step of the invention can be introduced into conventional CEDED sequences with molecular chlorine, but in general, ECF (Element
ary Chlorine Free) is preferably used in the bleaching method. The pH of the reaction mixture according to the invention is generally between 1 and 10, preferably between 7 and 10, more preferably between 7 and 9. Oxygen pressure can vary over a wide range, but in general 0.1-100 bar,
Preferably it is 3 to 10 bar. The oxygen present is pure oxygen or oxygen supplied by air. The reaction temperature is generally between 25 and 150C, preferably between 40 and 90C. The chlorine dioxide treatment lasts 2 minutes to 5 hours. This time is 30 minutes ~
Preferably 2 hours.

[0008] The consistency of the pulp is expressed as a percentage of the dry weight relative to the total weight of the pulp. The consistency during treatment with chlorine dioxide is generally between 1 and 30%, preferably between 5 and 15%. Chlorine dioxide is generally used in a proportion of 0.05 to 5% by weight, based on the dry weight of the pulp, and is advantageously used in a proportion of 0.2 to 3% by weight, based on the dry weight of the pulp. The chlorine dioxide is preferably used in the form of an aqueous solution having a concentration of 7 to 15 g / l. The present invention will be more clearly understood from the following examples. In the following, D represents a general treatment step with chlorine dioxide and E represents an extraction step with sodium hydroxide. Chlorine dioxide is expressed as a percentage by weight relative to the dry weight of the pulp.

Experimental Part The concentration of the aqueous solution of chlorine dioxide is 7 g / l. The whiteness was measured according to NF standard T12030. Degree of Polymeriza
was measured according to Tappi standard 230om-89. The general operation pulp is charged into a 1.5 liter Teflon-coated autoclave equipped with a stirring function and a temperature control function, and the autoclave is sealed. Add a small amount of water with stirring and adjust the pH of the mixture with buffer solutions (acetate buffer, phosphate buffer and borate buffer) if necessary to give the desired consistency . Thereafter, the temperature of the reaction mixture is brought to 55 ° C. Then, after introducing the chlorine dioxide solution using a closed dropping funnel, the reactor is pressurized with oxygen or nitrogen. After reacting for 30 minutes,
Stop heating. Subsequently, the autoclave is degassed and the bleach is separated from the pulp. Thereafter, the amount of residual chlorine dioxide in the bleaching solution is determined by iodine titration. Finally, the pulp is washed several times with deionized water and then subjected to various measurements (whiteness, degree of polymerization).

[0010]

EXAMPLE 1-9 Examples 1-9 shown in Table 1 is softwood pulp (pate de
This pulp was previously cooked for a long time using kraft (Kappa number after steaming KN = 24.1, DP =
(1530, whiteness = 29.1%), prebleached by DEDE sequence, polymerization degree is 1370, ISO whiteness is 69.9%. The operating conditions of the first stage D performed at atmospheric pressure are as follows: Pulp consistency = 10% Temperature of reaction mixture = 55 ° C. pH of reaction mixture = 2.5 Amount of chlorine dioxide = 2% Treatment time = 1 hour

The second D stage of prebleaching was performed under the same conditions as the first D stage, except that the amount of chlorine dioxide used was reduced to 1%. After the completion of the D stage, the pulp was dehydrated and dried, and then treated in the E stage. The operating conditions of stage E are as follows: Pulp consistency = 10% Temperature of reaction mixture = 70 ° C. Amount of sodium hydroxide = 2% Atmospheric pressure Treatment time = 1 hour 0.5% in Examples 1-9 The operation was carried out at a pulp consistency of 10% in the presence of chlorine dioxide. Examples 1 to 3 were performed under atmospheric pressure.

Examples 10 to 13 Examples 10 to 13 shown in Table 1 were carried out by using softwood kraft pulp, and this pulp was previously cooked for a long time (kappa number KN = 24.1) and preliminary bleaching by the DE sequence. The polymerization degree is 1420, the ISO whiteness is 43.1%, and the kappa number is 5.7. The operating conditions for the prebleaching are the same as those of the first two steps of the sequence used in Examples 1-9. With a pulp consistency of 10% (Examples 10-13), 1% (Examples 10, 12) and 1.6%
The treatment was performed using the chlorine dioxide of Examples 11 and 13.

[0013]

[Table 1]

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Elvesti France 94500 Champigny-sur-Marne Boulevard de Polinggi 10

Claims (7)

[Claims] 1. A method for bleaching chemical paper pulp in a series of processing steps comprising at least one step of preparing a reaction mixture for processing pulp using a chlorine dioxide solution, wherein the reaction mixture is maintained under oxygen pressure. A method characterized by the following. 2. The method according to claim 1, wherein the pH of the reaction mixture is 1 to 10. 3. The method according to claim 2, wherein the pH of the reaction mixture is 7-10. 4. The method according to claim 1, wherein the oxygen pressure is between 3 and 10 bar.
The method according to any one of claims 3 to 7. 5. The chlorine dioxide is added to the dry weight of the pulp.
5. The method according to claim 1, wherein the method is used in a proportion of 0.05 to 5% by weight. 6. The process according to claim 1, wherein the temperature of the reaction mixture is between 40 and 90 ° C. 7. The process according to claim 1, wherein the consistency of the pulp is from 5 to 15%.