Chemical Treatments in Bleaching Stages Which Increase Pulp Brightness

The bleaching of pulp by chemical means is a well-known phenomena and is described in the text, Pulp and Paper Science and Technology, Volume I, Pulp, Edited by C. Earl Libby, McGraw-Hill Book Company, 1962, Chapter 13 entitled, "Bleaching." The type bleaching with which the invention is concerned may be considered as the chlorine bleaching which, under mill conditions, is a multi-stage process. These stages typically consist of a chlorination stage (C), alkaline extraction (E), hypo chlorite treatment (H), which is an alkaline process, and chlorine dioxide stage (D). Many other processes using stages are known which involve basically themes or variations of the stages mentioned above. Several typical multi-stage bleaching processes are described in Casey cited above. For a more detailed description of multi-stage chlorine bleaching processes, reference may be had to "Chemical Environment of Pulp in the Bleaching Process," by N. Liebergott, Pulp and Paper Magazine of Canada, Pulp and Paper Research Institute of Canada Technical Paper T10, pp. 80-84, and "Principles of Pulp Bleaching," Parts I and II, by Vernon B. Bodenheimer and J. O. Enloe, Southern Pulp and Paper Manufacturer, Vol. 39, Nos. 3 and 4, published by Patchen, Mingledorff & Associates, Inc., Atlanta, Ga., March, 1976, issue, pp. 29-39, and April, 1976, issue, pp. 30-39.

For some time it has been known that the compounds of iron and manganese which can be contaminants in the pulp bleaching system tend to reduce the amount of brightness of the finished pulp. These contaminants come into the paper and pulp system in a variety of ways. It is generally the practice of pulp bleaching operations to increase the concentration of the chemicals used in the bleaching process to increase brightness or maintain it in the finished pulp. It would be though that a method of preventing iron or manganese reduction of brightness in pulping operations would be by complexing these materials in the water prior to their entry into the bleaching process. This approach is entirely impractical since it is impossible to treat these waters economically. Also, in most multi-stage bleaching processes, substantial quantities of the water are recycled, thereby continuing to build up iron and manganese contamination in the system.

The present invention is predicated upon the discovery that small amounts of water-soluble chelating agents may be added to the pulp being bleached, most preferably after the last alkaline stage, under acidic conditions, to prevent iron and manganese from interfering with the final brightness of the pulp after bleaching is completed. By practicing the invention, it is possible to reduce the amount of normal bleaching chemicals used or by using the same amount, to increase the final brightness of the bleached pulp.

The invention is a method for increasing the final brightness of pulp contaminated with iron or manganese in a chlorine bleaching process of the type comprising one or more alkaline stages which comprises treating the pulp with at least 0.1 lbs. per ton of a water-soluble chelating agent.

The water-soluble chelating agents that may be used in the practice of the invention may be selected from any number of well-known commercially available chelants so long as they are capable of complexing with iron or manganese under the environment in which the invention is practiced. In addition to using single chelating agents, it is contemplated that one or more chelants may be formulated into a unitary product, which blended products oftentimes give superior results.

To illustrate the various types of chelants that may be employed, although the invention is not limited thereto, consideration should be given to the use of:

The phosphonate chelants comprise a large group of well-known phosphorus-containing materials. A most useful class of phosphonates are the hydroxy alkylidene diphosphonic acids having the formula: ##STR1## wherein X is OH or NH.sub.2, and R is an alkyl of 1 to 5 carbon atoms, water soluble salts of said diphosphonic compound, and a mixture of said diphosphonic compound and said water-soluble salts thereof.

These compounds and their use as chelants are described more fully in U.S. Pat. No. 3,149,151.

The preferred phosphonate of this group is 1-hydroxyethylidene 1, 1-diphosphonic acid.

Another useful group of phosphonates are the phosphonates prepared by reacting ammonia, a primary or secondary amine, with phosphorus acid and an aldehyde such as formaldehyde. Phosphonates of this type are disclosed in U.S. Pat. No. 3,288,846, the disclosure of which is incorporated herein by reference. A preferred phosphonate of the type disclosed in this patent is tris amino trimethylene phosphonic acid and the phosphonates prepared by reacting a polyamine such as hexamethylene diamine with formaldehyde and phosphorus acid.

Although not phosphonates, I can use the so-called water-soluble substituted hypophosphites of the type disclosed in U.S. Pat. No. 4,088,678 and British Pat. No. 1,521,440, the disclosures of which are incorporated herein by reference. A preferred material of this type is sodium phosphinico BIS (succinic acid).

Still another group of useful phosphonates are those described in U.S. Pat. No. 3,886,204, which compounds are generically 2-phosphono-butane-1,2,3,4-tetracarboxylic acid. These compounds have the general formula: ##STR2##

The preferred compound of this group useful in the practice of this invention is 1,2-phosphono-butane-1,2,4-tricarboxylic acid of the formula: ##STR3## in which R is hydrogen or lower alkyl, and R.sup.1 is hydrogen or methyl, and the alkali metal and ammonium salts thereof.

This group of compounds is illustrated by the well-known chelating materials, ethylene diamine tetra acetic acid (EDTA) and nitrilo tris triacetic acid (NTA). Other related compounds which have chelating activity are described in U.S. Pat. No. 2,396,938 and U.S. Pat. No. 2,240,957, the disclosure of which is incorporated by reference.

In this group of chelants, NTA is particularly useful in complexing manganese, thus rendering it useful when manganese ions prevail in the system to be treated.

These polymers encompass a large group of water soluble polymeric compounds. As indicated, these polymers should have a molecular weight of at least 1,000. When used herein the expression, "low molecular weight," encompasses polymers having a molecular weight range of from 1,000-40,000. Intermediate molecular weight polymers may be arbitrarily described as having molecular weights within the range of 40,000-500,000. High molecular weight polymers encompass all polymers having molecular weights greater than 500,000 and, in some instances, may be in excess of several million.

A class of water soluble polar addition polymers have, as a part of their molecular configuration, at least 5% by weight of side chain groups which will be either anionic or which, under conditions of alkaline hydrolysis, are capable of being converted into anionic functional groups.

These anionic polymers desirably contain as a functional side-chain group, carboxylic acid groups, carboxylic anhydride groups, carboxylic salt groups, carboxylic acid ester groups or carboxylic acid amide groups.

Surprisingly, very effective results have been obtained with the acrylic acid polymers or acrylic acid-methacrylic acid polymers which have molecular weights below 2000. These materials are particularly effective in complexing manganese in the systems treated in accordance with the invention.

This group encompasses a large number of chelants that may be either organic or inorganic although they generally may be characterized as being acidic. Typical compounds are citric acid, tartaric acid, and gluconic acid. Other chelants are the well-known molecularly dehydrated phosphates such as sodium hexametaphosphate, sodium pyrophosphate, and sodium tripolyphosphate.

The amount of chelating agent used will vary depending upon the bleaching system being treated, the particular chelant selected, and the point of application for the chelant. Generally as little as 0.1 pounds per ton of pulp in the system represents a minimal dosage that is effective. Preferably between 0.5-5 pounds per ton will give good results although more may be used.

It is to be understood that the chelating agent may be added at any point in the system although, as previously indicated, it is preferably added after the last alkaline stage to an acidic point in the system. These acidic points in the system generally have a pH range from 2-6 whereas in most instances, the pH is about 5. It is at these pHs that the chelating agents are most effective.

To illustrate the invention, the following are given by way of example.

Listed below are a variety of chelating compositions that were evaluated in treating pulp during its bleaching in a variety of stages. The results of these tests are presented in Tables I-VIII.