CS228128B2 - Method of bleaching of cellulose fibrous materials - Google Patents

Abstract

11 The advantages of the serial application of chlorine dioxide and chlorine without intermediate wash during the bleaching of wood pulp in the absence of significant amounts of dissolved organic materials are improved by the presence of dissolved chlorine in the chlorine dioxide solution in an amount of 6 to 10% of the total available chlorine of that solution.

Description

The present invention is directed to an improved bleaching operation.

U.S. Pat. No. 3,536,577 suggested bleaching cellulosic fibrous materials, and particularly wood pulp, using an aqueous solution of chlorine dioxide and chlorine at a moderate temperature of up to 35 ° C. In this process, chlorine dioxide accounts for about 20 to about 95% of the total usable chlorine in solution.

It has also been previously found that the characteristics of pulp bleached in this process can be improved when chlorine dioxide and chlorine are used sequentially, instead of being used as a mixture with each other without washing between the two bleaching applications of chemicals, as described in U.S. Patent Nos. 3,433,702 and 3,501,374.

It has now surprisingly been discovered that the removal efficiency of lignin from bleached pulp is further improved when part of the chlorine is introduced with chlorine dioxide in the first bleaching by application of chemicals.

According to the present invention, a bleaching process is provided for bleaching cellulosic fibrous materials in aqueous suspension using chlorine dioxide and chlorine using a chlorine dioxide solution followed by chlorine application, without an intermediate wash step, and wherein all chlorine dioxide is about 20 to about 90% Totally usable chlorine characterized in that the bleaching is carried out essentially in the absence of dissolved organic materials, wherein the aqueous chlorine dioxide solution contains chlorine in an amount of from 0 to 10%, preferably 7 to 8%, based on available chlorine. solution.

To distinguish the bleach-like process using the chemicals of the present invention from before. as described above, the application of an aqueous solution of chlorine dioxide and chlorine and then chlorine without washing between them is termed "serial bleaching" and the former process using chlorine dioxide and then chlorine without embedded washing is termed " gradual bleaching '.

The bleaching process of the present invention can be used for any cellulosic fibrous material, but is particularly suitable for bleaching wood pulp, preferably one that is produced by the kraft process, that is, pulp produced by boiling wood chips in caustic containing sodium hydroxide and sodium sulfide. as effective paper chemicals.

Serial bleaching using the chemicals of the present invention is preferably carried out. ... ... ... JU is the first bleaching stage of multi-stage bleaching and caustic extraction operations. However, serial bleaching can be carried out, for example, with subsequent removal of lignin from the pulp by treatment with oxygen and alkali.

The total bleaching step in the series bleaching process of the present invention is carried out using chlorine dioxide in the first stage in an amount of about 20 to about 90%, preferably about 30 to about 75%, of the total chlorine used in the step.

The term "totally usable chlorine" is used here in the normal sense used in the bleaching field and refers to the total bleaching power of the chemicals used. Chlorine dioxide has a bleaching force that is 2.63 times greater than that of chlorine based on the weight of the base.

As used herein, the term "bleaching stage" refers to a pulp bleaching operation performed between further pulp processing, usually washed. As used herein, the term "bleaching step" refers to the bleaching treatment of pulp carried out in the bleaching stage.

In the present invention, the first bleaching step is carried out using an aqueous solution of chlorine dioxide and chlorine which contains chlorine in an amount of 6 to 10% of the total usable chlorine, and preferably about 7 to 8% of the total usable chlorine. The aqueous solution of chlorine dioxide and chlorine for use in the first bleaching step of the series bleaching process of the present invention preferably contains about 8 to about 9 g of chlorine dioxide per liter and about 1.5 to about 1.8 g of chlorine per liter.

Solutions of such concentration arise from gaseous mixtures, chlorine dioxide, chlorine, and vapor formed by the reaction of chlorate ions with chloride ions in an acidic aqueous reaction medium while the reaction medium is maintained at its boiling point at a pressure below atmospheric pressure.

A process of this type is described in U.S. Patent No. 3,864,456 and is known as the ERCO R3 (trade mark) process, wherein sodium chlorate is reduced by added chloride ions in the presence of sulfuric acid, with a total acidity of about 2 to about 4.8. The process is preferably carried out in a non-partitioned reaction zone at a temperature of about 55 to about 85 ° C under reduced pressure of about 10,666 to about 39,997 Pa. Anhydrous neutral sodium sulfate is precipitated from the reaction medium as a solid by-product.

Examples of other processes that can be used to produce chlorine dioxide and chlorine are the ERCO R5 (trade mark) process described in Canadian Patent No. 913,328 and U.S. Patent 4,075,308, and the ERCO R6 (trade mark) process described in in Canadian Patent No. 913,328 and U.S. Patent No. 3,929,974, wherein sodium chlorate is reduced with hydrochloric acid to obtain all chloride ions and all hydrogen ions required for the process.

Another process for producing chlorine dioxide and chlorine that can be used to produce a gaseous stream to form a chlorine dioxide and chlorine solution is the ERCO R7 (trade mark) process described in U.S. Patent 4,086,329, wherein sulfuric acid is used. and the hydrochloric acid used in the reduction reaction is mainly produced by the internal reaction of chlorine with sulfur dioxide and water.

The gaseous mixture of sulfur dioxide, chlorine and steam is usually contacted after cooling - to condense a portion of the steam, preferably at a temperature of about 15 to about 55 ° C - with cold water, preferably at a temperature of about 3 to about 10 ° C. to dissolve all of the chlorine dioxide contained therein together with part of the chlorine.

The chlorine and chlorine solutions produced in this way containing chlorine in an amount of 6 to 10% of the total usable chlorine from the solution are distinctly different from the chlorine dioxide solutions obtained by other processes for the production of chlorine dioxide. In this case, where no chloride ions are added and substantially pure chlorine dioxide is formed, the aqueous solution contains a negligible chlorine concentration. In those processes where chloride ions are added as a reducing agent and chlorine is formed in addition to chlorine dioxide, as in the ERCO R2 (trademark) process of U.S. Patent No. 2,863,722, and where air is used to dilute gases, a solution The chlorine dioxide is produced by absorption in water containing some dissolved chlorine, but the concentration is significantly lower than the concentration in the chlorine dioxide / chlorine solution used in the present invention. Under typical production conditions, the partial gas pressures differ in an aqueous chlorine dioxide / chlorine solution containing 8 g of chlorine dioxide per liter and 1,7 g of chlorine per liter for the ERCO R3 or similar procedure and an aqueous solution of chlorine dioxide and chlorine containing 8 g of oxide of chlorine dioxide per liter and 0,5 g of chlorine dioxide per liter and 0,5 g of chlorine per liter for ERCO R2.

Relative to the chlorine dioxide solutions used in an earlier step-wise bleaching process containing a small amount of chlorine, there are earlier solutions where "technical chlorine dioxide solutions" can contain up to about 10% by weight of chlorine, corresponding to 4% of available chlorine. The authors are aware that values higher than 5.7% of usable chlorine are mentioned in the literature. A typical solution from the ERCO R2 process contains about 2.3% spent chlorine.

The difference in chlorine concentrations between the concentration achieved by the prior art and the concentration resulting from the present invention arises from differences in the partial pressure of chlorine dioxide and chlorine in the gaseous stream which is brought into contact with cold water.

It has been found that when an aqueous solution of chlorine dioxide and chlorine containing 6-10% of the total usable chlorine in solution is used in the first step of serial bleaching using chemicals, substantially in the absence of soluble organic material, improved removal is achieved lignin from the pulp, compared to an aqueous solution of chlorine dioxide, where all the chlorine present constitutes less than about 5% of the total usable chlorine from the solution, as explained above.

In conventional papermaking, the pulp is washed to remove as much of the spent liquor from the pulp as possible to reduce the amount of impurities dissolved therein, which otherwise use bleaching chemicals. A certain amount of black liquor can be 'transferred', which can lead to the presence of dissolved organic material in the pulp suspension. As used herein, the term "dissolved organic material" refers to organic material dissolved in the aqueous phase of a pulp slurry consuming bleaching chemicals and is determined as "total organic carbon" (COU).

The concentration of dissolved organic material from the source may be in the range of up to about 2% by weight of COU per pulp, although the concentration usually does not exceed about 1% by weight of COU per pulp. The present invention relates to bleaching of pulp in its suspension, which may contain some dissolved organic material.

In a preferred embodiment of the invention, the pulp suspension is subjected to a first bleaching step with a solution of chlorine dioxide and chlorine at a temperature of about 35 to about 70 ° C, preferably about 50 to about 65 ° C. The first bleaching step can be carried out in any acidic pH range and usually the pulp suspension has a pH of about 1 to about 6.

After a period of about 5 seconds to about 10 minutes and usually about 30 seconds to about 5 minutes has elapsed since the use of an aqueous solution of chlorine dioxide and chlorine, the pulp suspension is treated with an aqueous solution of chlorine. Treatment with chlorine may be carried out in any desired acidic pH, wherein the pulp suspension usually has a pH of about 0.7 to about 3. The bleaching method may be carried out in about 10 to about 60 minutes after the bleaching temperature of about 35 to about 70 ^Q C .

The total amount of usable chlorine used on the pulp in the first and second bleaching steps, i.e. in both chlorine dioxide and chlorine, is usually about 2 to about 10% by weight of the pulp.

Although the second series bleaching step of the present invention is usually carried out using chlorine alone, a small amount of chlorine dioxide with chlorine may be introduced to achieve a well-known protective effect on the pulp which counteracts the possibility of chlorination by chlorination.

The bleaching process of the present invention can be carried out under any desired humidity conditions, such as conventional bleaching, and usually about 2 to about 6% by weight, calculated on the pulp, as well as humidity conditions, such as conventional ambient humidity, typically about 6 to about 10% by weight of the pulp.

As mentioned above, the series bleaching step is preferably used as the first stage of multi-stage bleaching. When this is done and the bleaching step is complete, the pulp is washed and then subjected to caustic extraction, washed again and subjected to one or more bleaching and caustic extraction steps to obtain the desired light color and purity of the pulp. The caustic extraction is usually carried out using aqueous sodium hydroxide solution. The additional bleaching may be carried out with a bleaching agent selected from the group consisting of chlorine dioxide, chlorine, hypochlorite, peroxide and mixtures thereof.

For example, the pulp can be subjected to EDED sequences after an initial series bleaching step, wherein E is caustic extractions and D is a bleaching step using an aqueous solution of chlorine dioxide, washing after each chemical treatment step. The conditions used in such steps are the same as the conventional conditions. These operations can also be carried out as described in Canadian Patent No. 783,483, in which case the washing step can be omitted.

The invention is illustrated by the following examples and with reference to the accompanying drawings, in which:

Giant. 1 graphically represents the change in Ei Kappa values in the pulp as a function of the chlorine content of an aqueous solution of chlorine dioxide and chlorine using serial bleaching using chemicals at different levels of dissolved organic material.

Giant. 2 graphically represents similar changes to FIG. For different levels of chlorine equivalent in the bleaching stage.

Example 1

Wood pulp of Kappa number 34,2, К number 23,4 and viscosity 28,8 mPa. s undergoes bleaching in a number of attempts for. using 7.6% equivalent of chlorine for pulp for 30 minutes at 3.5% humidity and 60 ° C, washed and caustically extracted with 3.0% sodium hydroxide for pulp for 2 hours humidity 12% and temperature 70 ° C and washed again. In any case, the Kappa number is determined after processing. Chlorine dioxide and chlorine are used in the bleaching step in a proportion of 70% chlorine dioxide of total chlorine, 30% chlorine of total chlorine.

A number of experiments include tests performed in the absence of dissolved carbon and tests. carried out in the presence of dissolved organic carbon at different levels of% COU in the pulp. A number of experiments also include tests carried out using an aqueous solution containing all chlorine dioxide, one minute after a serial procedure applied to all chlorine in a bleaching step without intermediate washing, and tests using an initial application of the aqueous solution. of chlorine dioxide and chlorine containing all the chlorine dioxide and a part of the chlorine in different proportions, one minute after its application, removal of the chlorine residue without intermediate washing.

The results of a series of experiments are shown graphically, with the Kappa number Ei plotted against the ratio of chlorine in the chlorine dioxide solution for varying% COU values, as. is shown in the graph of FIG. 1. From the results shown in FIG. 1, it is clear that Kappa lignin removal efficacy of lignin removal is improved when serial bleaching is performed by chemical application using an initial aqueous solution of chlorine dioxide and a portion of chlorine. Compared with the initial application of an aqueous solution containing chlorine dioxide and chlorine, with a limiting chlorine

Claims (8)

1. A method of bleaching cellulosic fibrous material. of the material in aqueous suspension using chlorine dioxide and chlorine by applying a chlorine dioxide solution and then chlorine without introducing a wash step, and wherein the chlorine dioxide contains about 20 to about 90% of the total usable chlorine, characterized in that the bleaching is essentially, in the absence of dissolved organic materials, and the aqueous chlorine dioxide solution contains chlorine in an amount of from about 6 to about 10% of the usable chlorine from the solution. 2. The process of claim 1 wherein the first and second bleaching steps are carried out at a temperature of about 35 ° C to about 70 ° C. 3. The method of claim 2, wherein said method is used. a temperature of about 50 to about 65 ° C. 4. The method of any one of claims 1 to 3, wherein the second bleaching step using chlorine begins about 5 seconds to about 10 minutes after the start of the first bleaching step of chlorine dioxide and chlorine and the% COU level in the pulp. Improved efficiency is best at 0% COU and practically disappears at 3% COU. Improvements in efficacy are achieved in the range of 6 to 10% of the total usable chlorine. supplied C12, in an aqueous solution of chlorine dioxide and chlorine. Example 2 The procedure of Example 1 was repeated except that a series of tests at% COU were performed only at a chlorine equivalent level of 6.2%, calculated per grit. The Ei Kappa results obtained are plotted in Fig. 2. The results apparent from Fig. 2 confirm the good results illustrated in Fig. 1. It is clear from the disclosure that the present invention provides a method of serial bleaching using chemicals in the absence of a significant amount of To-z-discharged organic materials, in which improved lignite removal efficiency can be achieved by the presence of small amounts of chlorine dissolved in the oxide solution. of chlorine. Modifications are also possible within the scope of the invention. OF THE INVENTION and the total bleaching time of both the first and second steps is about 10 to about 60 minutes. 5. The method of claim 4, wherein the second bleaching step starts from about 30 seconds to about 5 minutes after the start of the first bleaching step. 6. A process according to any one of Claims 1 to 5 wherein the aqueous solution of chlorine dioxide contains chlorine in an amount of about 7 to 8% of the spent chlorine from the solution. 7. The process of any one of claims 1 to 6, wherein the chlorine dioxide solution comprises about 30 to about 75% total usable chlorine used in the first and second bleaching steps. 8. A process according to any one of Claims 1 to 7 wherein the chlorine dioxide and chlorine solution used in the first step has a concentration of about 8 to 9 grams of carbon dioxide per liter and about 1.5 to about 1.5 grams per liter. 1.8 g of chlorine per liter.