AU766612B2 - Method for treatment of wood pulp for improved brightness and colour stability - Google Patents

Description

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AUSTRALIA

Patents Act 1990 Carter Holt Harvey Limited

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Method for treatment of wood pulp for improved brightness and colour stability The following statement is a full description of this invention including the best method of performing it known to us:t r-~-r4~ns, t,~trA~? TECHINICAL FIELD The invention comprises a process for the treatment of wood pulp to improve the brightness stability and colour stability of products forned from the unbleached pulp.

BACKGROUND

For linerboard and other products produced from unbleached wood pulp such as 00 kraft pulp, colour and brightness can be key qualities in determinig value to a customer. Manufacturers of packaging, who are primary users of linerboard, o 15 generally require a product of consistent brightness and colour.

Unbleached linerboard is generally produced from pulp having a high lignin content.

eo*o The higher the lignin content, the higher the yield. However, high lignin containing pulps have a dark colour. Lighter coloured, lower lignin content unbleached pulp can be used for the top layer of linerboard sheets for packaging intended to have a 0 0 reasonable quality printed outer surface.

Colour stability can be a problem with linerboard and other products produced from :unbleached pulp. The product may tend to darken under exposure to light, and in 0. 25 particular ultraviolet light The product colour also changes to become more red in shade. This darkening and reddening process is referred to as 'reversion" and if significant is poorly regarded in the market.

In the manufacture of bleached paper, it is kniown to use oxygen and alkali to delignify the pulp prior to bleaching. The process involves exposure of the pulp for a reasonably prolonged period, of generally an hour or more, to caustic conditions (an alkali charge in excess of 10%) with an oxygen chargc of 10-20 kg/ton pulp, or more, at a temperature generally of the order of 100'C or more. Such oxygen deligniiication is used to reduce the Kappa number of the pulp generally by about I- *11WI.

The resulting pulp may then be bleached to high brightness with conventional bleaching processes and with complete removal of the ligrdn, to produce a product having high brightness and colour stability. In general such an oxygen delignification process is considered not appropriate in the manufacture of unbleached linerboard, since it is desirable to maintain the pulp yield as high as possible, while achieving an acceptable linerboard brightness and colour.

Pulp produced under anosic conditions, such as Krafit pulp washed under ano~cc conditions, is brighter than pulp produced in the presence of oxygen.

STATENrDT OF MNENT'ION In broad terms in one aspect the invention comprises a process for treating wood pulp produced under otherwise substantially anoxic conditions to improve brightness stability and colour stability in products produced from the pulp without subsequent bleaching, comprising treating the pulp with oxygen or an oxygen containing gas at a pulp PH maintained between 10.5 and 12 for a time insufficient reduce the Kappa number of the pulp by more than 10% or the lignin content by more than and preferably insufficient to reduce the Kappa number of the pulp by more than or the lignin content by more than Typically the wood pulp will have a Kappa number in the range 25-60.

Preferably the wood pulp has a consistency of about 25 Preferably the pH is maintained by adding an alkali charge to the pulp of less than by dry weight of the pulp Preferably the process is performed at a temperature of 85 *C to 100 IC and most preferably at a temperature of about 85 QC.

The treatment time may be about 30 minutes or less, such as for ex-ample about minutes, or less.

2 In broad terms in another aspect the invention comprises a process for treating Wood Pulp to improve brightness stability and colour stability in products produced from the pulp without subsequent bleaching, comprising treating pulp of Kappa number in the range 25-60 and consistency of about 10%/ with oxygen or an oxygen containing gas at a pulp pH maintained between 11. 2 and 11.5 by adding an alkali charge to the pulp of less than 0.50% by dry weight of the pulp, at a temperature of about 85 OC, for a time of about 5 minutes or less, insufficient to reduce the Kappa number of the pulp by more than 10% or the lignin content by mnore than 6%.

Preferably the treatment is carried out after washing or partial washing of the pulp.

Preferably the process is performed at a pressure no greater than 10 bar.

Preferably the process includes a step of terminating the reaction by reducing the 15 pH of the pulp, most preferably with C0 2 BRIEF DESCRIPTON OF FIGURES The process of the invention is further described with reference to the accompanying figures in which: Figure 1 is a schematic representation of the process of the present invention in one preferred form, Figures 2a, 2b and 2c are graphs of redness, yellowness and brightness for unexposed and UV exposed brightness pads formed from pulp treated as referred to in the following experimental section, and Figures 3a, 3b and 3c are graphs showing the difference in colour reversion between pulp treated according to the process of the invention and untreated pulp, which are also referred to in the following experimental section.

DETAILED DESCRIPTION In the process of the invention a small amount of alkali such as NaOH is added to the pulp under carefully controlled conditions, after washing and typically in a repulper, followed by oxygen loading. If the alkaline conditions and oxygen treatment are maintained for only a short period, before neutralisation for example by treatment with carbon dioxide, the lignin content of the pulp is minimally affected while at the same time linerboard or other products subsequently produced from the pulp have significantly improved brightness stability and colour stability.

The final stages in the processing of kraft pulp typically involve washing of the pulp.

We have found that when this washing process is carried out substantially under anoxic conditions the resulting pulp is brighter and more yellow than pulp produced and washed in the presence of oxygen, but also that linerboard products formed S. 15 from this brighter pulp have poor brightness and colour stability, with severe brightness and colour reversion upon exposure to light. We have found that this colour reversion in products produced from pulp washed under anoxic conditions is due to photo-oxidation of lignin-related chemical structures in the pulp, probably stilbene or similar chemical structures that are potent leuco-chromophores. These readily degrade in the presence of oxygen. We have also found that alkalinity is also an important factor. In the process of the invention the pulp washing is carried out in a closed plant and closely controlled alkaline conditions and in the presence of oxygen, employed to degrade the precursor compounds to colourless products, and substantially prevent chromophore formation. Any chromophores formed tend also 25 to be readily degraded under the same conditions. The process is summarised in the following diagram: colourless 02 lignin-related chromophores structures UV light (red coloured) (leuco-chromophores OH 02 -fOH Scolourless degradation products Referring to Figure 1, processed pulp is washed in a washer 1, or is partially washed, and a low charge of a suitable alkali such as NaOH is then added to the washed pulp to give a pH of between 10. 5 and 12 and most preferably about 11.2 to 11.5, in a repulper 2. The amount of the alkali charge is less than about 0.5% by dry weight of the pulp, or about 2-4kg of NaOH-/air dry tonne of pulp- The pulp is treated with oxygen.

In the preferred form process the washed alkaline pulp is then pumped through a first medium consistency pump 3, immediately after which oxygen is injected into the pipeline at a rate (or charge) of about 2 kg/ton or less, and the pulp and oxygen then pass through a medium consistency mixer into pipeline 4 or alternatively a retention tank or vessel, where it is retained for a predetermined short period of less than one hour, preferably less than 30 minutes, and typically of the order of 1S minutes. Treatment of the pulp is carried out at a temperature of not greater than V....100 0 C and preferably in the range 85- 1000C.

The pulp is then pumped by a second medium consistency pump 5 into a storage tower. The time and temperature for which the pulp is treate'd should be insufficient to reduce the Kappa number of the pulp by more than 10% and most preferably not by more than or the lignin content of the pulp by more than 6%, and most preferably by not more than 3%.

Preferably the pulp is neutralised to reduce pulp pH before storage or further 25 processing. C0 2 gas or acid available may be introduced to the pulp at the second pump 5 to neutralise the pulp, at a rate of about 4 kg/air dry tonne of pulp for example. Storing the pulp at high PH may cause adverse changes to the pulp, such as further delignification.

While in the preferred form process the alkali/oxygen treatment is carried out after washing of the pulp and prior to storage, the treatment could be applied at different locations in any particular fibreline conflguration. For example in a multi-washer fibre line the treatment of the invention may be applied before the last washer, thereby enabling the added NaOH to be recovered in the final washer filtrate. The .4 .44 process may even be effective when applied before pulp washing, although this would not be an optimal treatment configuration since most of the oxygen would react with sulphide ion in the weak black liquor.

The process of the invention is also applicable to pulp of different Kappa numbers, although the advantages provided by the process may not be so commercially significant for pulps having higher Kappa numbers, having a darker starting colour and for which colour reversion is of less significance. Preferably the process is carried out with pulp having a Kappa number in the range 25-60.

The invention is further illustrated by the following description of experimental work and examples: Experimental In the following investigations Kappa number was measured to reflect pulp yield.

However, measurement of actual lignin content indicates that the process of the present invention may result in some ligiz being oxidised but not removed. Thus, a reduction in Kappa number of 10%/ corresponds with a reduction in lignin content of only about 6%.

The colour of linerboard can be measured by the optical parameters referred to as a' (a measure of redness) and b* (a measure of Yellowness). In addition L- is a measure of the brightness of a product. An alternative is to use ISO brightness 25 instead of Colour reversion can be quantified by measuring brightness, a* and b* before and after exposure of a product such as paper or linerboard to strong ultraviolet light for a predetermined period. It is characterised by increasing redness and decreasing yellowness and brightness.

Oxygen or air was bubbled through a pulp in ailkali at 1% consistency and at two different pH levels. Samples were removed at different times, washed and adjusted to pH 5 and formed into brightness pads.

.ank-r.~ After 5 minutes reaction, at either of the two pH's, the pulp showed little visible photo reversion.

Pulp was reacted with pressurised oxygen (0.7 MPa) at 10% consi3tency with levels of applied NaOH from 0 to 1% on pulp, in Stalsvets oil-bath reactors at 85°C for minutes. The pulp was then diluted to approximately 5% consistency and adjusted to pH 6 with sulphuric acid (but not washed).

As well as being able to control the extent of colour reversion, it was found that by varying the NaOH charge it was possible to also vary the colour and brightness of the pulp, as shown in Graphs a, b and c. The results shown in Graphs a, b and c (Figure 2) demonstrate: the marked reversion, as shown by the increase in redness (aw) and the decrease in yellowness and brightness under controlled conditions, with fresh pulp and in the absence of NaOH or oxygen treatment; and the marked reduction in this reversion by 02 treatment and a trend towards further reduction in reversion with increasing NaOH charge.

At NaOH charges above 0. 5% significant delignification occurred.

Using a similar test system, and with a 1% NaOH charge, the effects of simulated **high density (HD) storage on colour and reversion behaviour were investigated.

0 25 Treated, neutralised pulp was stored at 80°C for 8 hours. There was no adverse change in the pulp's colour or reversion behaviour other than a small drop in brightness. Therefore, the colour stabilisation achieved by the process of the invention should not be reversed or destabilised by HD storage. A similar conclusion was drawn following a study of the effects of HD storage on alkali/oxygen treated and neutralised pulp treated in a high shear mixer/reactor.

A high shear mixer/reactor was employed to simulate the alkali/oxygen process.

Pulp was dewatered to greater than 10% consistency, brought up to a temperature of 85 0 C and treated with NaOH and oxygen at various charges for a reaction time of minutes. The reaction was stopped by the injection of sulphuric acid into the reactor. The reactor was then depressu-ised, the pulp removed and diluted to 6% consistency, the pH measured and adjusted to 6. Conventional means were employed to form hand sheets, the optical properties of which were measured before and after 30 minutes UV exposure. The results are shown in Table 1.

Table I original Run Run Run Run Run Run Pulp 55 53 40 42 54 52 NaOH Charge -0.2 0.2 0.45 0.45 0.6 0.6 Oxygen Charge(% 0.5 3.1 1.2 1.21 0.5 3.1 Oxygen Pressure (kPa) 1125 800 1350 350 125 800 ISO Brightness 28.02 23.33 23.15 23.19 23.61 23.38 23.26 Reverted Brightness 20.98 21.49 21.58 20.75 21-61 21.18 21.16 Reversion -7.04 -1.84 -1.57 -2.44 -2.01 -2.2 -2.1 a'2.22 5.7 5.76 4.94 5.17 4.86 5.77 Reverted a* 6.96 5.12 4.95 5.03 4.91 4.98 4.91 Reversion 4.74 -0.58 -0.81 0.09 -0.26 0.12 -0.86 b*20.93 18.31 18.51 19.72 20.12 19.08 19.12 Reverted b* 17.58 18.8 19.01 19.62 20.25 19.05 19.56 Reversion -3.35 0.49 0.5 -0.1 0.13 -0.03 0.44 Kappa number 42.2 40.2 139.6 142.7 140.3 140.3 139.1 An alkali charge as low as gives substantial inhibition of colour reversion.

Only a small reduction in Kappa number was observed.

Example of Process Alkali in the form of NaOH, and oxygen were added to pulp alter exiting from the wash 1 (see Figure 1) and this treated pulp was retained at a temperature of for about 5 minutcs prior to the addition of CO 2 to neturalise the alkali.

4 ~t-Jt~*~Wfl.4rU.#4t4*~ 44 a a* a Graphs a, b and c (Figure 3) show the difference in colour reversion between pulp taken from the washer prior to alkali/ oxygen treatment anid treated pulp. A difference value of zero in these graphs indicates that the treated pulp reverted to the same extent as the original pulp.

For ISO brightness, e* and b* values there was a clear trend between treatment pH and the effectiveness of the treatment. The results indicate that a pH above 10.5 is required before significant treatment effect is observed. A pH of 11.2 to 11.5 obtained a consistent result. Typical results at this target pH of 11.2 to 11.5 are shown in Tables II and Ill.

Table 13 Property Original After After pulp Treatment Storage Brightness ISO) 29.0 27.7 27.8 ar2.34 3.24 3.15 b*22.75 21.20 21.73 Pulp brightness was reduced by only 1.3% ISO after treatment, with no substantial change after HD storage.

Table 133 Change in Property (under UV light exposure) Property Original After After Pulp Treatment Storage Brightness ISO) -5.6 -2.9 -2.7 a* +3.11 +0.76 +0.50 b--3.06 -1.21 -0.94 Thus, colour reversion was reduced by treatment, with some further reduction after storage.

Pulp samples taken from the second medium consistency pump 5 and stored in black plastic bags for 8 hours in an 80*C waterbath, to simulate effects of high density storage, showed no significant change in pulp brightness during storage.

Reversion in brightness and colour was less after storage than immediately after treatment. At the NaOH charge of 4 to 5 kg/adt, storage reduced the average brightness reversion from 2.9 to 2.7% ISO, whilst average a reversion was reduced from 0.76 to 0.5 and average b* reversion was reduced from 1.21 to 0.94.

Kappa number was measured on samples taken before and after treatment to investigate the effects of treatment on lignin content. There was a small reduction in Kappa number of about 2 to 4 units when the reaction pH was at the higher end of the pH range observed. No significant change in lignin content occurred during subsequent storage (incubation of a sample in a black plastic bag for 8 hourm in an 0 C waterbath), confirming that the extent of neutralisation achieved was adequate to stop further delignification during storage.

In summary, a pulp of sufficient yield having satisfactory brightness and colour characteristics, and with acceptable levels of colour reversion, can be achieved with oxygen/alkali treatment sufficient to obtain a pH above about 11 for a period of about 5 minutes at a temperature of 85°C. This result can be achieved with a NaOH charge of about 4 kg/adt and an 02 charge of about 2 kg/adt, without any substantial effect on lignin content.

The process of the invention enables the control of colour, and particularly colour reversion in the processing of unbleached pulp for the manufacture of linerboard.

Whilst the method is described with particular reference to the manufacture of unbleached kraft linerboard, the invention would be equally applicable to the manufacture of other paper grades in which unbleached kraft pulp is used.

'WiviiQ -W 4"W J.

The foregoing describes the invention including preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof, as defined in the accompanying claims.

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Claims (10)

1. A process for treating wood pulp produced under otherwise substantially mnoxic conditions to improve brightness stability and colour stability in products produced from the pulp without subsequent bleaching, comprising treating the pulp with oxygen or an oxygen containing gas at a pulp pH maintained between 10. 5 and 12 for a time insufficient to reduce the Kappa number of the pulp by more than or the lignin content by more than 6%.

2. A process according to claimn 1 insufficient to reduce the Kappa number of the pulp by more than S% or the lignin content by more than 3%.

3. A process according to claim 1 or 2 wherein the wood pulp has a K~appa number in the range 25-60. 15 4. A process according to claimn 1 to 3 wherein the wood pulp has a ***consistency of about A process according to claim i to 4 wherein the pH is maintained by adding an alkali charge to the pulp of less than 0.5% by dry weight of the pulp.

6. A process according to claims 1 to 5 performed at a temperature of 85 0 C to 11C. A process according to claims 1 to 5 performed at a temperature of about 25 8511C.

8. A process according to claims 1 to 7 wherein the treatment time is about minutes or less.

9. A process according to claims 1 to 7 wherein the treatment time is about minutes or less. 12 A process for treating wood pulp to improve brightness stability and colour stability in products produced from the pulp without subsequent bleaching, comprising treating pulp of Kappa number in the range 25-60 and consistency of about 10% with oxygen or an oxygen containing gas at a pulp pH maintained between 11. 2 and 11. 5 by adding an alkali charge to the pulp of less than 0. 5% by dry weight of the pulp, at a temperature of about 85 for a time of about minutes or less, insufficient to reduce the Kappa number of the pulp by more than or the lignin content by more than 6%.

11. A process according to any one of claims 1 to 10 wherein the treatment is carried out after washing or partial washing of the pulp. A process according to claim 1 to 11 performed at a pressure no greater than 10 bar.

13. A process according to any one of claims 1 to 12 including the step of terminating the treatment by reducing the pH of the pulp. A process according to claim 13 wherein CO 2 is used to reduce the pH4 of the pulp. A process for treating wood pulp comprising: *.washing or partially washing pulp produced by a substantially anoxic 25 process, during or after washing adding an alkali charge to the pulp so as to bring the pulp to a pH between 10.5 and 12, mixing oxygen or an oxygen containing gas with the pulp, maintaining oxygenated alkaline pulp mix at a temperature for a period of time insufficient to reduce the Kappa number of the pulp by more than 10%/ or the lignin content by more than but effective to increase the brightness stability and colour stability in products produced from the pulp without subsequent bleaching of the pulp, and treating the pulp to reduce the pH.

16. A process for treating wood pulp produced under otherwise substantially ano.xic conditions without subsequent bleaching, substantially as described herein.

17. Linerboard or other product produced by the use of a process of any one of claims 1 to 16. Dated: this 25th day of June 1999 CARTER HOLT HARVEY LIMITED Patent Attorneys for the Applicant: 00* F.B. RICE CO. 4 rx otaft