AU660326B2 - Process for improving the selectivity of the delignification of a chemical paper pulp - Google Patents

Abstract

A process for improving the selectivity of delignification of a chemical paper pulp, in which the pulp is treated with an aqueous solution of an organic peroxy acid at 2 - 47 degrees C., followed by a water wash and a supplementary stage of alkaline extraction in the presence of hydrogen peroxide.

Description

AUSTRALIA

Patents Act 660 726 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Interox America Actual Inventor(s): Roy T. Hill Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: PROCESS FOR IMPROVING THE SELECTIVITY OF THE DELIGNIFICATION OF A CHEMICAL PAPER PULP Our Ref 305940 POF Code: 1659/185949 The following statement is a full description of this invention, including the best method of performing it known to applicant 6006 TITLE OF THE INVENTION PROCESS FOR IMPROVING THE SELECTIVITY OF THE DELIGNIFICATION OF A CHEMICAL PAPER PULP BACKGROUND OF THE INVENTION Field of the Invention: The invention relates to a process for the delignification of a chemical paper pulp. It relates especially to a process for improving the selectivity of such a delignification operation.

Discussion of the Background: It is known to treat unbleached chemical paper pulps obtained by cooking cellulosic materials in the presence of chemical reagents by means of a sequence of delignifying and bleaching treatment steps involving the use of oxidizing chemical products. The objective of the first step of a conventional sequence for bleaching chemical pulp is to complete the delignification of the unbleached pulp as it occurs after the cooking operation. This first delignifying step is traditionally carried out by treating the unbleached pulp with chlorine in an acid medium or with a chlorine/chlorine dioxide combination, mixed or sequentially, -2so as to react with the residual lignin in the pulp and give rise to chlorolignins, which may be extracted from the pulp by solubilization of these chlorolignins in an alkaline medium in a subsequent treatment step.

For various reasons, it proves useful, in some situations, to be able to replace this first delignifying step by a treatment which.no longer employs a chlorinated reagent.

The treatment of chemical pulps by a first step involving peracetic acid at temperatures above 50 0 C and pH values between 3 and 9 have already been proposed (Bailey C.W. and Dence "Peroxyacetic Acid Bleaching of Chemical Pulps", Tappi, January 1966, Vol. 49, No. 1, pages 9 to 15). In this known process, it is, however, apparent that the peracetic acid treatment gives rise to pulps having inferior viscosity and mechanical properties to the pulps delignified by a traditional step involving chlorine in an acid medium, as a result of a lower selectivity of delignification which manifests itself in a more pronounced attack of the cellulose chains.

SUMMARY OF THE INVENTION The objective of the invention is to remedy the drawbacks of the known processes by providing a process which achieves an effective delignification of the unbleached chemical .ulp accompanied by a high selectivity which enables pulps possessing high intrinsic qualities to be obtained.

-3- To this end, the invention relates to a process for improving the selectivity of the delignification of a chemical paper pulp by means of an organic peroxy acid, according to which the unbleached pulp originating from the cooking operation is treated with an aqueous solution of this organic peroxy acid at a temperature of between 2 and 47 0

C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention, chemical paper pulp is understood to denote pulps which have undergone a delignifying treatment in the presence of chemical reagents such as sodium sulphide in an alkaline medium (kraft or sulphate cooking), sulphur dioxide or a metal salt of sulphurous acid in an acid medium (sulphite cooking). The semi-chemical pulps such as those in which the cooking has been carried out using a sulphurous acid salt in a neutral medium (neutral sulphite cooking, also known as NSSC cooking) or alternatively those obtained by treatment with caustic soda in the cold state followed by a mechanical defibering may also be bleached and/or delignified by the process according to the invention.

The latter is directed especially towards pulps which have undergone a kraft cooking and whose residual lignin content after cooking lies in the range of kappa numbers between 8 and 40 (or alternatively having a permanganate number between 5 and 30) depending on the type of wood species from which they originate and the efficacy of the cooking process. All types of wood used for the produc,ion of chemical pulps are suitable for carrying out the process of the invention, and especially those used for kraft pulps, namely softwoods such as, for example, the various species of pine and fir and hardwoods such as, for example eucalyptus, beech, oak and hornbeam.

According to the invention, the organic peroxy acid is selected from aliphatic peroxycarboxylic acids containing a single percarboxyl group and a saturated linear or branched alkyl chain of less than 11 carbon atoms. Aliphatic peroxycarboxylic acids having a saturated linear alkyl chain containing less than 6 carbon atoms are preferred. Examples of such peroxy acids are performic acid, peracetic acid, perpropanoic acid, n-perbutanoic acid and n-perpentanoic acid.

Peracetic acid is especially preferred on account of its efficacy and its good biodegradable properties.

In a variant of the process according to the invention, the organic peroxy acid is selected from diperoxycarboxylic acids containing a linear or branched alkyl chain of less than 16 carlbon atoms and two percarboxyl groups substituted on carbon atoms located at the alpha,omega-position with respect to one another. Examples of such peroxy acids are 1,6-diperoxyhexanedioic acid, 1,8-diperoxyoctanedioic acid, 1,10-diperoxydecanedioic acid and 1,12-diperoxydodecanedioic acid.

In another variant of the process according to the invention, the organic peroxy acid is selected from aromatic peroxy acids containing at least one percarboxyl group per benzene ring. Preferably, aromatic peroxy acids which contain only a single percarboxyl group per benzene ring will be chosen. An example of such an acid is peroxybenzoic acid.

Another variant of the process according to the invention consists in choosing an organic peroxy acid substituted with one or more halogen atoms or with any other functional organic substituent. Any other functional organic substituent is understood to denote a functional group such as a carbonyl group (ketone, aldehyde or carboxylic acid), an alcohol group, groups containing nitrogen such as nitrile, nitro, amine and amide groups, and groups containing sulphur such as sulpho and mercapto groups.

The peroxy acid can be employed without discrimination in the state of an aqueous solution of peroxy acid, or alternatively in the form of an ammonium, alkali metal or alkaline earth metal salt of this peroxy acid.

Preferably, the treatment with the organic peroxy acid is performed at a temperature of between 35 and 45 0 C. Generally, the treatment is performed with the organic peroxy acid at atmospheric pressure. The period of this treatment depends on the temperature and the wood species which has been used for preparing the pulp, as well as the efficacy of the prior cooking. Periods of between approximately 120 minutes and approximately 240 minutes are very suitable, and it is possible even to exceed this limit by approximately 100% without risking substantial degradation of the mechanical resistance properties of the pulp. When the treatment temperature is high, that is to say above 45 0 C, it is possible to use shorter periods of between approximately 45 minutes and 120 minutes. The treatment according to the invention may be carried out in any type of apparatus suitable for the treatment of paper pulp by means of acid neutral or alkaline reagents. The storage chest for the unbleached pulp, present in all bleaching installation and performing the role of a buffer reservoir between the wood cooking unit and the pulp bleaching unit, is especially suitable for carrying out the process according to the invention. The pulp can thus be treated therein during its storage without the need to invest in costly specialized equipment. The possibility of being able'safely to exceed the normal treatment period is especially well suited to the storage buffer role of the storage chest.

The consistency in the step of treatment with the organic peroxy acid will generally be chosen to be between 2 and of dry matter, and more often than not between 10 and High consistencies, for example those of between 20 and are preferred.

In the process according to the invention, the quantity of organic peroxy acid used is chosen in accordance with the -7residual lignin level in the pulp as well as with the average treatment period. Generally, quantities of between 0.5 and 4% by weight of peroxy acid relative to the dry pulp are suitable. More often than not, a quantity of peroxy acid not exceeding 3% by weight relative to the dry pulp, and greater than 1% of this weight, will be used.

It may be advantageous, as a variant, to precede the treatment with the organic peroxy acid by a step of decontaminating pretreatment by means of an acidic aqueous solution. The object of this step is to extract from the pulp the impurities present in the form of metal ions which prevent the bleaching and/or delignification operations from proceeding satisfactorily. All inorganic or organic acids used in aqueous solution, alone or mixed, are suitable.

Strong inorganic acids such as, for example, sulphuric acid or hydrochloric acid are very suitable.

'It is advantageous for the acidic decontaminating pretreatment to be carried out, in addition, in the presence of an agent which complexes metal ions. For this purpose, mixtures of the strong inorganic acids mentioned above with organic acids of the aminopolycarboxylic or aminopolyphosphonic acid class or their alkali metal salts are especially suitable. Examples of appropriate aminopolycarboxylic acids are diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, cyclohexanediaminetetraacetic acid and nitrilotriacetic acid.

Diethylenetriaminepentaacetic acid (DTPA) is preferred.

Examples of aminopolyphosphonic acids are diethylenetriaminepentamethylenephosphonic acid (DTMPA), ethylenediaminetetra(methylenephosphonic) acid, cyclohexanediaminetetramethylenephosphonic acid (CDTMPA) and nitrilotri(methylenephosphonic) acid. DTMPA is preferred.

The working conditions for the acidic decontaminating pretreatment are not critical. They must be determined in each particular case in accordance with the type of paper pulp and the apparatus in which the treatment is performed.

Generally speaking, the choice of acid and the quantity employed should be established so as to impose on the medium a pH below 7, for example between approximately 1 and approximately 6.5. Especially advantageous pH values are those between approximately 2.0 and approximately 5.0. The temperature and pressure are not critical, room temperature and atmospheric pressure generally being very suitable. The pretreatment period can vary to a wide extent depending on the type of equipment used, the choice of acid, the temperature and the pressure, for example from approximately 15 minutes to several hours.

It is also possible to replace the decontaminating pretreatment by incorporating one or more agents which complex metal ions in the peroxy acid delignification step itself.

These complexing agents are chosen from the same complexing agents as those described above for the decontaminating pretreatment step.

In another variant of the process according to the invention, it is possible, if it is desired to obtain high levels of brightness, to follow the treatment with a peroxy acid by a sequence of traditional bleaching steps involving or not involving chlorinated reagents. Examples of such steps are as follows: steps employing gaseous oxygen or ozone, steps employing alkaline hydrogen peroxide in the presence or absence of gaseous oxygen, steps employing chlorine dioxide or sodium hypochlorite, and alkaline extractions with caustic soda.

The examples which follow are given with the object of illustrating the invention without, however, limiting its scope.

Examples The measurements undertaken in the examples described hereafter were conducted according to the following standards: Kappa Index: TAPPI Method T236, Viscosity: TAPPI Method T230, OM89.

A sample of kraft pulp of Ponderosa pine with a kappa index of 28.4 and viscosity of 32.1 mPs was subjected to a delignification step with an aqueous solution of peroxyacetic acid followed by an alkali extraction step in the presence of hydrogen peroxide (Paa Ep sequence).

Four tests were conducted, two under the temperature and time conditions of the prior art, (noted as Examples 1R and 2R), and two under the conditions of the invention (noted as Examples 3 and Example 1R, in contrast with the three other examples, did not include washing with water of the pulp between the peroxyacetic acid treatment step and the alkali extraction step.

Hydrogen peroxide which is present in the alkali extraction step of Example 1R also comes from the residual hydrogen peroxide of the peroxyacetic acid step which preceded. In Example 4, based on the invention, the peroxyacetic acid step came after acid pretreatment with diethylene triamine pentaacetic acid or DTPA (Q step).

In each one of those tests, 2.0 g of peroxyacetic acid were implemented per 100 g of dry pulp. The peroxyacetic acid being used was a commercially sound aqueous solution and it contained hydrogen peroxide balanced with the peroxyacetic acid (2.6 g of H202 for 100 g of dry pulp).

The operative conditions and the results that were obtained are indicated in the following table.

-11- Ex. Step Chemical reagents, Time Temp- Kappa viscosity No. min erature Index Mps PAA H,2 NaOH DTPA_____ IR Paa 2.0 2.6 70 24.0 Ep 2.0 5.4 1 60 60 17.8 19.5 2R Paa 2.0 1 2.6 60 70 t 24.0 Ep 0.5 3.0 60 75 16.5 19.4 3 pap 2.0 2.6 240 38 22.9 Ep 0.5 3.0 60 75 1S.4 24.4 4 Q 0.3 15 Paa 2.0 2.6 2. 120 45 1 25.3 Ep 0.5 1 60 75 15.2 29.3 In relation to the unwashed pulp, the delignification and viscosity loss results after the alkali extraction" step are as follows: Example Delignification, Loss of viscosity,% 1R 39.2 2R 41.9 39.6 3 42.2 24.0 [4 46.5 8.7 Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims (11)

1. A process for improving the selectivity of the delignification of a chemical paper pulp by means of an organic peroxy acid, according to which the unbleached pulp originating from the cooking operation is treated with an aqueous solution of this organic peroxyacid, characterized in that the treatment is performed at a temperature of between 2 and 47 0 C.

2. A process according to claim 1, characterized in that the temperature of the organic peroxyacid treatment is between and 45 0 C.

3. A process according to either one of claims 1 or 2, characterized in that the organic peroxyacid is one member selected from the group consisting of aliphatic peroxycarboxylic acids containing a single percarboxyl group and a saturated linear or branched alkyl chain of less than 11 carbon atoms.

4. A process according to either one of claims 1 or 2, characterized in that the organic peroxyacid is one member selected from the group consisting of diperoxycarboxylic acids containing a linear or branched alkyl chain of less than 16 carbon atoms and two percarboxyl groups substituted on carbon atoms located at the alpha,omega-position with respect to one another.

5. A process according to either one of claims 1 or 2, characterized in that the organic peroxyacid is one member selected from the group consisting of aromatic peroxyacids containing one percarboxyl group per benzene ring.

6. A process according to any one of claims 1 to characterized in that the organic peroxyacid is employed in the form of one of its ammonium, alkali metal or alkaline earth metal salts.

7. A process according to any one of claims 1 to 6, characterized in that it is carried out in the storage chest for the uebleached pulp which feeds a bleaching unit.

8. A process according to any one of claims 1 to 7, characterized in that the treatment with the organic peroxyacid is preceded by a step of decontaminating 39 -12- pretreatment by means of an acidic aqueous solution.

9. A process according to claim 8, characterized in that the acidic aqueous solution contains at least one compound which sequesters metal ions.

10. A process according to any one of claims 1 to 7, characterized in that a compound which sequesters metal ions is used in the peroxy acid treatment.

11. A process substantially as hereinbefore described with reference to either one of Examples 3 or 4. DATED: 28th September, 1992 PHILLIPS ORMONDE FITZPATRICK Attorneys for: INTEROX AMERICA 39 -13- -14- ABSTRACT OF THE DISCLOSURE Process for improving the selectivity of the delignificatior of a chemical paper pulp by means of a peroxy acid, according to which the unbleached pulp originating from the cooking operation is treated with an aqueous solution of this organic peroxy acid and the treatment is-performed at a temperature of between 2 and 47 0 C.