CA2189724A1 - Method of pulping - Google Patents

Abstract

A method for the bleaching of pulp. The pulp is first bleached with chlorine dioxide. Further bleaching is then carried out at alkaline pH with hydrogen peroxide in the presence of a chelating agent. The resulting pulp has low organic halogen, enhanced brightness and good strength.

Description

' A METHOD OF PULPING

This invention relates to a method of bleaching pulp .

Biomass is the most important source of cellulose for pulp and paper making. The most popular biomass is wood.

Wood comprises three major organic components, cellulose, hemicellulose and lignin. Cellulose and hemicellulose account for about 70-80% of the biomass and lignin accounts for about 20-35~. The remaining, minor organic components are primarily extractives. The cellulose and hemicellulose are chains of glucose units formed in linear fashion; lignin is an aromatic block polymer.

In the tree, lignin acts to bond the cellulose together to form a rigid structure that can stand up to the growing environment. In papemaking pulping is carried out to remove the lignin from the wood to liberate the cellulose, which is the principal ingredient of paper. This process requires strong agents to destroy the majority of the lignin. Residual lignin is removed from the pulp by bleaching, which functions to complete the pulping process and to lighten the colour of the pulp .

A variety of bleaching agents have been used in the pulping industry. These include chlorine tidentified by C) chlorine dioxide (identified by D) oxygen (identified by O) and hydrogen peroxide (identified by P). In addition the industry uses the abbreviation E for caustic extraction. Thus a conventional, prior art bleaching process may be defined as CEDED.

Existing bleaching techniques achieve certain brightness levels in pulps required for papermaking.

However the pulping art has run into contoversy over its effects on the environment. In particular the use of chlorine and chlorine dioxide for bleaching of pulp has produced some unfavourable results by introducing dioxin 5 or furan into the environment. As a result a great deal of effort has been expended in the pulp industry to discover environmentally sound bleaching methods that do not have an adverse effect on the environment. In general this has meant that chlorine compounds are not now used.

Recent developments to avoid the problems associated with chlorine have been the use of oxygen, ozone and hydrogen peroxide as the bleaching agent. A process in which hydrogen peroxide is the main bleaching agent, and 15 which includes a chelation treatment stage, is described in U.S. Patents 5,310,458 and 5,415,734. A different type of chelant in this technology, aminoalkane diphosphonic acid, is described in WO Patent 95/12029.
This chelant was added either in a pretreatment stage or 20 during a peroxide bleaching stage.

Ozone delignification coupled with chelation pretreatment is described in U.S. Patent 5,441,603. In this Patent the chelating agent used was selected from diethylenetriaminepentaacetic acid (DTPA) 25 ethylenediaminotriacetic acid (EDTA) and oxalic acid.
Chelation pretreatment was conducted at a pH in the range 1-4.

U.S. Patent 5,411,635 describes a treatment that combines ozone and peroxymonosulphate. A mixture of 30 agents is added at an acidic pH in the initial delignification steps after kraft pulping.

2 ~ B9724 _ - 3 -Oxygen delignification with ozone and peracetic acid added and, therefore, at an acidic pH is described in U.S. Patent 5,387,317.

In all of the above either the brightness of the product was insufficient, the strength was too low or the cost of production was too high. It is therefore clear that there is still substantial work to be done in uncovering economical and safe totally chlorine free (TCF) pulping.

To avoid the difficulties in TCF pulping elemental chlorine free (ECF) pulping using chlorine dioxide in the place of free chlorine was introduced. Chlorine dioxide was reported to give much less dioxin and furan both in the pulp and the effluents from the mill. A process of reducing the amount of halogenated organic compounds in the spent liquor of ECF bleaching was described in U.S.
Patent 5,143,580. The process described in this Patent uses chelation followed by a peroxygen stage at the beginning of the ECF sequence.

U.S. Patents 5,149,442 and 5,143,580 describes the reduction of halogenated organic compounds in spent bleach liquor.

However ECF pulp continues to have high organo-chlorine (OX) content in the solid-pulp phase. Mill data for the OX contents of ECF pulp typically shows a mean of 136 parts per million (ppm) with a standard deviation of 1.44 ppm for interior mill spruce-pine-fir (SPF) furnishes and a mean of 181 ppm with a standard deviation of 7.34 ppm for a coastal mill using various furnishes.

Some European countries believe that the OX content of a pulp should be below 30 ppm if the pulp is to qualify as TCF pulp. 30 Ppm OX corresponds to the detection limit of most OX measuring equipment. It is thus beneficial for the ECF pulp to have an OX content as low as possible.

Several methods of reducing the level of organic halogens in pulp and in effluent have been introduced. A
process for the reduction of organic halogens (OX) in pulp and adsorbable organic halogens (AOX) in effluent, and including an ozone stage, is described in U.S. Patent 4,959,124. U.S. Patent 5,389,201 also describes the use of ozone to reduce consumption of chlorine containing chemicals. Joncourt et al, Reduction of the Formation of AOX During Chlorine Dioxide Bleaching, TAPPI Pulping Conference, Oct. 1995/149-152, described two methods for drastic reduction of AOX in chlorine dioxide bleaching.
The method involves the addition of dimethylsulphoxide (DMSO) and splitting of chlorine dioxide charges. Both methods increase mills' operating costs significantly and introduce hazardous chemicals. Reeves et al, Impact of Sequence Position for Pressurized (PO) Stage in ECF
Bleaching, TAPPI Pulping Conference, Oct. 1995/263-280 examine various positions of PO stage (pressurized peroxide, temperature less than 100~C) in an ECF sequence.
They determined that the optimum position depends on the mill priorities, low cost, ClO2 generation limit reached, future TCF. Considerable capital expenditure is required to implement PO stage in the mill. The same issue was addressed by Devenyns et al, Optimal Use of Hydrogen Peroxide to Design Low AOX ECF Sequences, TAPPI Pulping Conference, Oct. 1995/281-288 with AOX reduction being the main goal. Chirat et al, Other Ways to Use Ozone in the Bleaching Sequence, TAPPI Pulping Conference, Oct.
1995/415-419 examine an ozone stage and its position effect in TCF and ECF bleaching, as well as the DZ stage with partial chlorine dioxide substitution by ozone.
This was shown to be very economical in the first stage of bleaching (D1oo). Stevens et al, The Effect of Lignin -Content on the Performance of a Hydrogen Peroxide Brightening Stage in an ECF Sequence, TAPPI Pulping Conference, Oct. 1995/421-439 investigated peroxide bleaching of pulps with different lignin contents and the split of peroxide function between delignifying and brightening with the latter being more important for low lignin content pulps under the conditions studied.

A discussion of the benefits of caustic extraction with oxygen and peroxide added (Eop) stage by Hill et al;
An Evaluation of Pressurized Hydrogen Peroxide Systems for Delignification and Bleaching, TAPPI Pulping Conference, Oct. 1995/789-805 showed some benefits of such an approach in terms of reducing chemical consumption. Caro's acid (peroxymonosulphuric acid) can be beneficial both in standard ECF sequence and in ozone enhanced ECF as discussed by Arnold et al, The Degox~
Process - Laboratory and Mill Experiences with Peroxymonosulphuric Acid, TAPPI Pulping Conference, Oct.
1995/897-902. Laskeeva et al, Several Multistage Bleaching Sequences for Softwood Kraft Pulp Using Hydrogen Peroxide in Place of Chlorination; Izv. VUZ, Lesnoi Zh. No. 6:90-95 (1982) [Russ.] examine the use of peroxide in one or two stages of sequences containing chlorine as a bleaching chemical, that is the process was not ECF. Delefosse, ECF or TCF Pulp? Pap. Carton Cellul.
43, no. 1/2:18,21-23 (January/February 1994) [England]
describe differences between ECF and TCF bleaching indicating that less than 1 kg/ton of pulp of AOX in the effluent the toxicity of the ECF efflent is very similar to TCF effluent indicating the need for developing ECF
methods producing low levels of halogenated compound.
Hamilton et al, Improvements in ECF Bleaching; Use of Activated Oxygen Species and Xylanase, IPST, Technical Paper Series, 559 (Atlanta, GA):19p.(March 1995) [England] investigated free bleaching with xylanase followed by ECF sequences containing chlorine dioxide 21 ~9724 reinforced with hydrogen peroxide, dimethyldioxirane and nitrylamine. They reported a 21% decrease in chlorine dioxide charges when pre-treatment with nitrylamine-activated hydrogen peroxide was used. Jean et al, Mill Trial Experiences with Xylanase:AOX and Chemical Reductions-Annual Meeting (80th CPPA):A229-233, Feb. 1-2, 1994 [England] also experimented with xylanase in ECF
sequence as a way of reducing AOX levels. Malinen et al, ECF Bleaching of Oxygen-Delignified Softwood Pulp with the Minimum Charges of Chlorine Dioxide Pulping Conf.(Atlanta)Proc.(Book3):925-932(TAPPI;Nov.1-3, 1993)[England] described ECF bleaching with xylanase pre-treatment in which two extraction stages of DEDED
sequence were reinforced with hydrogen peroxide. They obtained 15-20% reduction in chlorine dioxide usage at 15% higher cost.

The present invention shows improvement in reducing the OX content of ECF pulp while enhancing the brightness and preserving the strength of the pulp. The present invention differs from the prior art in that it is used at the end of an ECF sequence and uses a mixture of hydrogen peroxide and chelating agent at atmospheric pressure with alkaline conditions. No other oxidants such as oxygen, ozone or per-acids are added. No separate chelation stage is necessary.

Accordingly, in its broadest aspect, the present invention is a method for the bleaching of pulp comprising bleaching the pulp with chlorine dioxide; then further bleaching the pulp at alkaline pH with hydrogen peroxide in the presence of a chelating agent.

The method may be carried out in one stage or a plurality of stage, for example, two. The pH is maintained alkaline by the use of a base, for example, sodium hydroxide. Preferably the pH is in the range of about 10.5 to about 12.

The concentration of hydrogen peroxide is in the range 0.1 to 3% by weight of the dry pulp. In general there is little advantage in going beyond 3% by weight.

Prefered chelating agents include diethylenetriaminepentaacetic acid (DTPA) or its sodium salt, diethylenetriaminepenta (methylenephosphonic acid) or its sodium salt (DTMPA) and 1-hydroxyethylidene-1,1-diphosphic acid (HEDP). The use of a chelant isessential in maintaining the high peroxide residuals that are needed to preserve pulp strength. In the present invention there is no need for the use of magnesium sulphate for SPF furnish.

The temperature for the peroxide stage is preferably maintained at about 80~C for the best results. However the invention is effective at temperatures as low as 45~C.
The optimum range of peroxide residuals is 40-80%.

The present invention allows ECF pulps to be bleached to a brightness of 88% plus ISO (International Standards Organization) and good strength with as little as 0.7% total hydrogen peroxide charge. Significant brightness increases were achieved with as little as 0.1%
hydrogen peroxide charge.

The OX (organic chlorine) content was substantially reduced, well below 100 ppm in the final pulp. Typical furnishes used were 100% SPF (spruce-pine-fir) or 60%
hemlock mixed with 40% cedar.

The invention is illustrated in the following examples. In the drawings referred to in the Examples:

._ Figure 1 is a graph showing the effects of the chelant DTPA on peroxide residuals;

Figure 2 is a graph relating DTPA addition on pulp strength;

Figure 3 is a graph showing the effect of various chelants on peroxide residuals;

Figure 4 is a graph showing the effect of various chelants on pulp strength;

Figure 5 is a graph illustrating the effect of temperature on brightness and strength;

Figure 6 is a graph illustrating the effect of hydrogen peroxide charge on brightness and strength.

EXAMPLE 1: THE PEROXIDE CHARGE AND THE USE OF CHELANT

This experiment focused on comparing a range of total peroxide additions in the presence and absence of DTPA as a chelant. The chelant is mainly for the protection of cellulose during the bleaching action of peroxide and hence pulp strength preservation. It also stabilizes peroxide, which makes its action more effective due to the longer period of high peroxide concentration. The total peroxide (H2O2) charge was 3% of the pulp dry weight. This 3% chemical was added in two stages; one with 1.5/1.5 and the other 2.0/1.0 split.
The first number is the first stage charge and the second number is the second stage addition.

A 50 grams of spruce-pine-fir (SPF) kraft pulp that was ECF bleached to 84.1% ISO brightness in DEoD sequence with OX content of 111.9 ppm was used for this experiment. Figure 1 shows the effect of chelant on the g final peroxide residual (%) measured in the bleaching filtrate and brightness of pulp under the same 3% H2O2 addition but distributed differently in two stages. With and without the addition of chelant in the bleaching process the brightness values of pulps were similar in the range of 88.5 to 88.9% ISO. This is an increase of about 3.5 to 4% from the control of non peroxide treatment. This evidence suggests that the peroxide can be successfully applied to the ECF pulp to obtain a significant brightness increase. However, with the addition of DTPA the residual peroxide was about 80%, the same for both 1.5/1.5 and 2.0/1.0 peroxide charges. This percentage peroxide residual was much smaller when chelant was not added. The peroxide residue was greater for 2.0/1.0 charge with about 20% while that for the 1.5/1.5 charge was about 5%. Since the pulp brightness is similar, the big difference in peroxide residuals could be explained by the fast peroxide decomposition in the absence of chelant and thus the generated radicals not having enough time to react with lignin.

Figure 2 shows the relationship between the chelant addition and wet zero-span pulp strength. The advantage of the addition of the chelant is further substantiated by the pulp strength. The starting pulp had 13.6 km wet zero-span strength. With the chelant DTPA, the strength value for the different stages of peroxide was the same at about 13 km. Without the chelant, the 1.5/1.5 H2O2 charge gave about 11 km strength while the strength for the 2.0/1.0 H2O2 charge was about 9.8 km. Apparently, the first charge of larger amount of peroxide in the first stage had greater reduction effect on the pulp strength.
Since the original, DED, pulp had 13.6 km strength and DEDED pulp bleached with chlorine dioxide in the last stage had a zero span strength of 13.0 km, the addition of chelant did preserve the strength of pulp to maintain it at the level obtained during standard DEDED sequence.

This example demonstrates that with the peroxide and chelant addition according to the invention the ECF pulp brightness could increase significantly and the strength of the pulp be preserved.

EXAMPLE 2: THE REDUCTION OF ORGANIC HALOGEN CONTENT

Three experiments below illustrate the reduction of organic halogen in pulp.

In the first experiment the total peroxide (H202) charge was 1. 5% of the pulp dry weight. Chelant (DTPA) was added at 0.1% charge in the first stage only. In the second stage peroxide was added and caustic at 1.2%
charge. No magnesium sulphate was added. The pulp was SPF (spruce-pine-fir) bleached to 84.1% ISO brightness using DEopD sequence.

In the second experiment the total peroxide (H202) charge was 2% of the pulp dry weight. This 2~ chemical was added in two stages; first - O. 5% and second 1. 5%.
Caustic charges were 0.6% and 1.0% respectively and 0.1%
DTPA was added in each stage. No magnesium sulphate was added. The pulp was SPF bleached to 81.4% ISO brightness using DEopD sequence.

Figure 3 shows the effect of various chelants on peroxide residuals.

Figure 4 shows the effect of chelant on the strength of the pulp. Both chelants exemplified showed similar effectiveness in retaining the pulp strength while the pulp without the chelant added had significantly lower strength at both levels of peroxide charges.

In the third experiment the total peroxide (H202) charge was 0.7% of the pulp dry weight. This chemical . .

was added in one stage. Caustic charge was also 0.7% and 0.1% DTPA was added. Magnesium sulphate charge was 0.1%.
The pulp was 60% hemlock/40% cedar mixture bleached to 83.5% brightness using DEopD sequence.
Bleacbing stage ~p ~unple Avclage OX ill DED control Im~R~. ~ 3 3 ppm Brightnes~ofe~dpulp ~
OXineDdpulp ~ ppm 86.1ppm % OX ~ S.4%
Table 1: OX Reduction for Various Pulps The OX reduction averaged 33.6% and yielded pulp with significantly lower OX than the average mill ECF.
The mill data for the OX content of ECF pulp showed a mean of 136 ppm with a standard deviation of 1.44 ppm for spruce-pine-fir furnishes (interior mill) and a mean of 181 ppm with a standard deviation of 7.34 ppm for coastal mill pulps (various furnishes).

Examples 1 and 2 demonstrate that a procedure derived by the addition of chelant to peroxide can significantly increase the brightness of pulp, reduce the organic chlorine content (OX) and preserve the strength of pulp.

EXAMPLE 3: THE EFFECT OF CHELANTS FOR DIFFERENT H2O2 CHARGES

Besides DTPA, another chelant, DTMPA, was introduced in this experiment. Pulp samples were bleached with addition of chelants and with no chelant added. The total peroxide charges were 1.5% and 3%. In all cases 1.5% hydrogen peroxide was added in the second stage.
Hence, the first stage of first series of experiments (1.5% total H2O2) was either Q stage with 0.1% DTPA added or neutral soak with no chelant added.

21:8~:724 Figure 3 shows that with the addition of chelants the peroxide residuals were higher than those of non-chelant treatments at both peroxide charging levels.
Within the chelant application, DTMPA had lower peroxide residuals than DTPA, which explains slightly higher brightness obtained in experiments with DTMPA added -89.1% ISO versus 88.6% ISO and 89.7% ISO versus 88.7%
ISO. The difference in the peroxide charge did not significantly affect the brightness of the pulp. It means that as long as sufficient peroxide is present the brightness can be enhanced from the control of 84.1% to the 88% + level.

Figure 4 shows graphically the effect of various chelants on pulp strength. Example 3 again substantiates the discovery of Example 2, namely the effectiveness of peroxide application to ECF pulp for increasing brightness while the chelant stabilized the reactions and thus served to preserve the strength of pulp. The chelant can be DTPA, DTMPA, ATMP(aminotri(methylenephosphonic acid)), HEDP or any other chelant capable of chelating the metals in the pulp.

EXAMPLE 4. THE EFFECT OF TEMPERATURE

Bleaching experiments were conducted using QP
sequence with 0.1% DTPA charge in Q stage and 1.5%
H2O2/1.2% NaOH/0.1% DTPA/0.05% MgSO4 charge in P stage at various temperatures. SPF furnish bleached to 84.1% ISO
brightness in a DEoD sequence was used. Figure 5 shows that at temperatures ranging from 45~ to 85~ the bleaching results are very similar with high brightness and strength achieved.

This example shows, surprisingly, that contrary to conventional peroxide bleaching, which is usually conducted at 75~C+, temperatures as low as 45~ are sufficient in the present invention.

EXAMPLE 5. THE EFFECT OF LOW HYDROGEN PEROXIDE CHARGES

Bleaching experiments were conducted using one peroxide, P, stage with 0. 1% DTPA charge, 0.2% NaOH
charge and varying peroxide charges at 85~C. SPF furnish bleached to 84.1% ISO brightness in a DEoD sequence was used. The bleaching results presented in Figure 6 indicate that a charge of hydrogen peroxide as low as 0.1% still produced significant improvement in brightness.

This indicates that a range of hydrogen peroxide addition from 0.1% to 3% on OD pulp provides effective bleaching action. Higher amounts of hydrogen peroxide can be used but that would not be economically justified in most cases.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.

Claims (10)

1. A method for the bleaching of pulp comprising:
bleaching the pulp with chlorine dioxide; then further bleaching the pulp at alkaline pH with hydrogen peroxide in the presence of a chelating agent.

2. A method as claimed in claim 1 in which the further bleaching is carried out in one stage.

3. A method as claimed in claim 1 in which the further bleaching is carried out in a plurality of stages.

4. A method as claimed in claim 3 in which there are two further stages.

5. A method as claimed in claim 1 in which the pH is maintained alkaline by the presence of sodium hydroxide.

6. A method as claimed in claim 1 in which the pH is in the range of about 10.5 to about 12.

7. A method as claimed in claim 1 in which the concentration of hydrogen peroxide is in the range 0.1 to 3 percent by weight of the dry pulp.

8. A method as claimed in claim 1 in which the chelating agent is selected from diethylene triamine pentaacetic acid or its sodium salt (DTPA);
diethylenetriaminepenta(methylenephosphonic acid) or its sodium salt (DTMPA); and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP).

9. A method as claimed in claim 1 in which the temperature during the further bleaching is at least 45°C.

10. A method as claimed in claim 9 in which the temperature is at least 80°C.