Use of Additives in Pulp Bleaching Processes to Preserve Pulp Strength

This invention relates to a process for the delignification and bleaching of cellulosic pulp produced by a chemical pulping process. Typically, processes of this type are conducted utilizing chemicals that, while increasing the brightness of the resulting pulp stock, cause cellulose degradation and, hence, a loss of pulp strength. It has been discovered, however, that through the use of certain amine additives, pulp strength can be preserved in hypochlorite bleaching processes.

The object of delignification and bleaching of cellulosic pulp is to produce pulp with high brightness, good brightness stability, and maximum pulp strength at minimum cost and with minimum environmental pollution. Unfortunately, however, achievement of or improvement in one of the above factors is often attained only at the expense of another of the important factors.

In an effort to achieve a suitable balance between the competing factors, bleach plants have resorted to multi-stage processes. A typical bleach plant pulp treatment comprises: (a) chlorination (C) of the pulp under acid conditions; (b) alkaline extraction (E) of the chlorinated lignin derivative from the pulp with aqueous sodium hydroxide; (c) oxidation (i.e. bleaching) with sodium or calcium hypochlorite (H) under alkaline conditions; (d) a second sodium hydroxide extraction (E); and, (e) a final bleach with chlorine dioxide (D).

Such a sequence is labeled CEHED and is commonly used for delignification and bleaching of kraft (i.e. sulfate) pulp. Similar sequences with fewer stages, such as CEH or CEHD are commonly used for sulfite pulp which generally contains less lignin and color bodies than does sulfate pulp. Many other such sequences have been proposed and used in the industry's continuing efforts to achieve a suitable balance of the competing factors for the various pulps, pulping processes, and end-use physical property requirements.

Regardless of the sequence used, the bulk of cellulose bleaching still is performed using some combination of chlorination (C) alkaline extraction (E), and hypochlorite bleaching (H) stages. The chlorination stage converts most ofthe colored lignin that remains after the initial pulping or digestion process to chlorinated lignin derivatives which are partially soluble in acidic chlorine solution and particularly soluble in alkaline extraction liquors. Such a stage is also referred to as the delignification stage. Although the net effect of such a stage (after alkaline extraction) is generally a darkening of the cellulosic pulp attributable to increased color development in the residual lignin material, a major amount of the lignin is removed in the chlorination-extraction sequence, facilitating efficient bleaching reactions in the later oxidation stages.

An extraction stage generally follows chlorination. Such a stage serves to remove the chlorinated lignin derivatives from the cellulose substrate, thus exposing for subsequent treatment the lignin material which was unaffected by the chlorination stage by virtue of the topochemical nature of the reaction. H. W. Gierts "Developments in Bleaching Processes," TAPPI, May 1951, volume 34, No. 5.

Hypochlorite treatments conducted under alkaline conditions, and chlorine dioxide treatments conducted at an acidic pH value, are primarily characterized by destructive oxidation of residual colored lignin to colorless degradation products. Such a stage is, therefore, primarily a bleaching stage, although some minor amount of chlorination followed by extraction of the alkaline or acidic bleaching liquors may occur simultaneously.

Of the competition between important factors, the tradeoff between brightness and pulp strength (often measured as pulp viscosity) has been of particular concern to the paper industry and has been primarily responsible for the proliferation of the various bleaching sequences. Such a tradeoff apparently results from a nonselective oxidation reaction. By the term "nonselective," it is meant that the oxidative action is not limited to lignin oxidation, but instead also involves destructive oxidation of the cellulosic material, thus reducing the length of the cellulosic molecules and, accordingly, reducing the strength and viscosity of the pulp. The exact extent of such cellulose oxidation depends upon reaction conditions such as temperature, pH, reaction time, and chlorine concentration, and upon the nature of the pulp being treated. For example, the brightness-strength tradeoff in delignification and bleaching is less pronounced for sulfite pulp than it is for sulfate pulp since less severe treatment is required for sulfite pulp than for sulfate pulp to achieve equivalent brightness characteristics.

While additives have been proposed in the past to reduce cellulose degradation during hypochlorite bleaching stages, none known by the applicants are as effective and cost efficient as those of the instant invention. Prior art materials, exemplified by sulfamic acid (such as that described in U.S. Pat. No. 3,177,111), which has performed but has drawbacks in the facts that it is a solid material, is difficult to feed into the hypochlorite bleaching stage, is very corrosive when used in the acid form and in the fact that salt solutions of the material are unstable.

While it has been proposed to add certain amines to the chlorination step to inhibit cellulose degradation (see U.S. Pat. No. 2,478,379) materials of this type have not been used heretofore in hypochlorite bleaching stages.

In either case, that is for sulfite or sulfate produced pulps, cellulose degradation occurs during the delignification and bleaching steps. While physical parameters such as the concentration of bleaching agent, temperature and time can be optimized for a given system, pulp degradation invariably occurs. As such, it would be a benefit to the art if a method could be obtained for maintaining pulp strength of chemically produced cellulosic pulp during the hypochlorite bleaching stage.

It is therefore an object of this invention to provide to the art a process for limiting cellulose degradation during the hypochlorite bleaching stage of pulp processing.

A further object of this invention is to provide to the art an improved process for the delignification and bleaching of chemically produced cellulosic pulp which encompasses maintaining in the aqueous slurry of the pulp during the hypochlorite bleaching stage from 0.01-0.5 weight percent based upon the dry weight of the fiber of an amine compound.

The present invention is applicable to all liquid-phase acidic chlorination processes for the delignification of wood pulp produced by chemical pulping processes. While chlorination is the general method employed industrially for the delignification of pulps of this type, bromine or other halogens can also be employed (see for example U.S. Pat. No. 4,096,029 which is hereby incorporated by reference). It will be readily seen that this invention also applies to those processes. The materials which are useful as viscosity preservation aids in hypochlorite bleaching according to this invention include alkyl amines containing 2-8 carbon atoms, alkyl diamines containing 2-8 carbons atoms, alkanol amines containing 2-8 carbon atoms, and alkyl oxyalkyl amines containing 1-6 carbon atoms in each alkyl group. Certain cyclic amines including aniline and cyclohexyl amine also work as viscosity preservation aids in accordance with this invention. Specific examples of amine materials found to preserve cellulose fibers during hypochlorite bleaching include: ethylamine, monoethanolamine, methoxypropylamine, n-butylamine, isopropylamine, 3 aminopropanol, 1,3 diaminopropane, diaminohydroxypropane, 2 amino-2 methylpropane, morpholine, 1-aminoethanol (acetaldehyde ammonia), hexamethylenediamine, aniline, 2-aminopropanediol, ethylenediamine, and triethylenetetramine.

The hypohalite bleaching stage when discussed in terms of this invention is generally performed at temperatures of from 30.degree.-80.degree. C., pH values of from 6 to 11 and consisting of treating an aqueous slurry containing approximately 5-15% cellulose fiber with an alkali metal or alkaline metal hypochlorite.

The materials useful in the practice of the invention are added to the hypochlorite bleaching stage at a level of from 0.01-0.5% by weight, based upon the dry weight of the pulp. Preferably the additives of this invention are added at a level from 0.02-0.1%, based upon the dry weight of the pulp.

When used in accordance with this invention the additives as described and claimed act to limit cellulose degradation occurring during the hypochlorite bleaching stage. The additives of this invention when used at the dosage levels directed generally have no effect on brightness. The additives, however, when used at high levels above about 0.2% by weight may cause some brightness loss.

In order to demonstrate the utility of this invention the following examples are presented.

Pulp that had previously undergone commercial chlorination and extraction stage treatments was utilized to ensure a constant baseline pulp characteristic. The post extraction (E) stage pulp was thoroughly washed with fresh water, thickened in a laundry type centrifuge, and homogenized in a Hobart mixer to ensure uniform moisture. The consistency of the post E pulp was 30.+-.2% and was checked before each experiment by determining the moisture loss at 105.degree. C.

The hypochlorite bleaching was conducted in plastic sealable bags that were placed in a constant temperature bath controlled to .+-.1.degree. F.

The bleaching bags were prepared by weighing the equivalent of 20 grams dry pulp from the post E stock into a heat sealable plastic bag. Bleaching liquor was prepared by adding 10-15 g/l sodium hypochlorite (as C1.sub.2), diluted from commercial 5.25% household bleach, 25 g/l NaOH, and amine treatments preapred in 10 g/l solutions; to a sufficient volume of fresh water so the consistency of the pulp in the bag would be 10%. After the addition of the bleaching liquor, the bags were sealed, and the contents mixed thoroughly by massaging the bags for one minute before placing them in a constant temperature bath. After the allotted time in the bath, the bags were cut open and a sufficient amount of spent bleaching liquor filtered through a stainless steel 100 mesh screen and collected for pH and residual hypochlorite analysis. The bleached pulp was washed with several aliquots of fresh water totaling 1 liter. The pulp was then available for further bleaching stages or to make handsheets for pulp viscosity of GE brightness analysis.

A sample of commercial post E softwood pulp having a pulp viscosity of 27.5 cps, permanganate number of 3.8, and 33 GE brightness was bleached with 2.5% sodium hypochlorite based on dry pulp as previously described for 60 minutes at 110.degree. F. The control hypo bleached pulp was degraded to a pulp viscosity of 8.6 cps with a 77.0 GE brightness. With no caustic buffer added to the bleaching bag, the final pH of the bleaching liquor was 7.3. The bleaching liquor had a pH over 12.0 at the start, but the caustic is consumed during bleaching as HCl and organic acids are formed. When 0.5 percent NaOH (based on pulp) was added, the hypo control bleached pulp had a 14.5 cps viscosity and a 75.4 GE brightness. The final liquor pH was 8.8.

The same commercial pulp used in Example 1 was bleached under the same conditions as above with the addition of one of several amine compounds to demonstrate the ability of these compounds to preserve pulp viscosity. The results of this work, including the use of additives listed in this invention, are found in Table I.

Many additional materials were tested for preservation ability during hypochlorite bleaching. Materials tested and found to stabilize the viscosity of softwood pulp included: triethylenetetramine, 1,3-diaminopropanol, isopropylamine, ethylenediamine, aniline, acetaldehyde ammonia, 2-aminopropanediol, hexamethylenediamine, hexamethylenetriamine, cyclo hexylamine, and diethylenetriamine.

Monoethanolamine, methoxypropylamine, and ethylamine were tested at different dosages to determine the effect of dosage on viscosity preservation. Results are shown in the attached Table II.

Another sample of commercial post-extraction pulp was obtained, which had a viscosity of 17.0 cps and 3.8 permanganate number. This pulp was bleached as previously described, but using calcium hypochlorite. Monoethanolamine was chosen as a representative amine treatment. The calcium hypochlorite was prepared by bubbling chlorine gas into a 6 percent aqueous calcium hydroxide solution until the chlorine gas started to rapidly lower the pH of the solution as it neutralized the hydroxide base. The calcium hypochlorite was analyzed and found to contain 58.0 g/l active Cl.sub.2 at pH 10.8.

The bleaching was done in larger plastic sealable bags containing 200 g of pulp dry weight so 400 grams each of control and treatable pulp could be obtained in order to process the pulp for strength testing. The bleaching was conducted with 2.5% calcium hypochlorite at 10 percent consistency for 60 minutes at 115.degree. F. The pairs of control and treated bags were combined and homogenized. Standard TAPPI beater runs (T 200 os-70) were performed on both the control and treated hypo-bleached pulp. TAPPI handsheets were prepared from samples withdrawn from the beater at various levels of freeness. The handsheets were conditioned under proper temperature and humidity conditions and tested for burst, tensile, and tear properties. The results of the viscosity and strength property improvement are shown in Table III.

A sample of commercial post E pulp that was a blend of hardwood and softwood was bleached with calcium hypochlorite in quantities sufficient for pulp strength testing as described in Example 4. Monoethanolamine was selected as a representative amine compound to demonstrate the improvement in pulp viscosity and pulp strength properties such as burst, tensile, and tear. The results of the pulp viscosity and handsheet testing are shown in Table IV.