WO1997030207A1

WO1997030207A1 - Process for bleaching of chemical pulp

Info

Publication number: WO1997030207A1
Application number: PCT/FI1997/000105
Authority: WO
Inventors: Ilkka Renvall; Elias Suokas; Reijo Aksela; Aarto Paren
Original assignee: Kemira Chemicals Oy
Priority date: 1996-02-19
Filing date: 1997-02-19
Publication date: 1997-08-21
Also published as: FI960755L; FI960755A0; AU1797297A; FI115470B

Abstract

A process for the bleaching or delignification of a chemical pulp with oxygen chemicals, in which process the pulp is treated with a chelating agent in order to bind into a chelate complex the heavy metals present in the pulp. The chelating agent used is a nitrogen-containing chelating agent and a mixture of one or more compounds according to Formula (I): R1CnHm(OH)p(COOH)qR2, in which formula n is 1-8, m is 0-2n, p is 0-n, q is 0-2, R1 is COOH, and R2 is H, CH2OH or COOH.

Description

Process for bleaching of chemical pulp

The invention relates to a process for the bleaching or de¬ lignification of chemical pulp, wherein the pulp is treated with an acid chemical and a chelating agent in order to bind into a chelate complex any heavy metals present in the pulp.

Ever more commonly, bleaching processes using no elementary chlorine or chlorine compounds are used for the bleaching of chemical pulp. The former bleaching is called ECF (elementary chlorine free) bleaching and bleaching which is entirely free of chlorine is called TCF (total chlorine free) bleaching. Especially TCF bleaching is usually preceded by oxygen delig¬ nification. After oxygen delignification the pulp can be bleached with chlorine-free chemicals such as ozone or hydrogen peroxide in acid or alkaline solutions in non-pressurized or pressurized conditions. Usable bleaching processes also include bleaching with peroxy compounds (such as peracetic acid, caron acid, or mixtures of peracids), a peroxide-enhanced oxygen step, and a peroxide-enhanced oxygen-alkali step.

Such bleaching steps are most commonly preceded by the binding of heavy metals. The metals can be removed by an acid wash. This is often disadvantageous, since at least some of the sub¬ sequent bleaching steps are carried out in alkaline conditions. If heavy metals are removed at a low pH, it is necessary first to use an acid in order to reach a low pH and in the next step an alkali to neutralize the acid. Furthermore, the acid wash removes Mg and Ca ions, which are regarded as advantageous for bleaching. The acid wash may also reduce the strength of the pulp.

Peroxy compounds such as peracetic acid and hydrogen peroxide are highly susceptible to the catalytic action of heavy metals. The applicant's WO application publications 95/35406, 95/35407 and 95/35408 describe transition-metal activated bleaching with peroxy compounds in acid conditions. The success of the bleaching presupposes the binding of heavy metals before the bleaching and/or during the bleaching.

In the bleaching with peroxy compounds, heavy metals are bound by using agents which chelate metal ions, for example poly- aminocarboxylic acids. These include in particular ethylene diamine tetra-acetic acid and its salts (EDTA) and diethylene triamine penta-acetic acid and its salts (DTPA).

The ions the most detrimental in terms of bleaching are man¬ ganese (Mn), iron (Fe) and copper (Cu). Also other heavy metals, such as chromium ions (Cr), etc., have a detrimental effect, both on the consumption of peroxy compounds and often on the bleaching result, by reducing, for example, the vis¬ cosity of the bleached pulp. Detrimental heavy metals originate in the pulp, the treatment waters and the pulp-treatment apparatus.

In particular the chelating of iron and manganese is ad¬ vantageous for bleaching. In contrast, the chelating of earth- alkali metals such as calcium and magnesium is disadvantageous for bleaching.

In the bleaching of chemical pulp it is usually not possible to carry out chelating in alkaline conditions, because in such a case iron will precipitate in the form of hydroxides, oxides and oxyhydroxides, which strongly catalyze the breaking down of peroxy compounds. For this reason, the chelating preceding peroxy bleaching is carried out in acid conditions. Especially the removal of manganese has proved to be important in this case, since it does not form as stable complexes as does iron and since especially a chemical pulp often contains large amounts of manganese. In publication P.S. Bryant and L.L. Edwards, Tappi Pulping Conference 1993, pp. 43-55, "Manganese removal in closed kraft mill bleach plants" it is noted that manganese can best be removed at a pH of 4.5-6.5, before the manganese becomes too strongly bound to the pulp.

In modern processes for the bleaching of chemical pulp, the bleaching is preceded by oxygen delignification. In this process there are added to the pulp magnesium ions, the remain¬ ing of which in the pulp is important in a bleaching with peroxy compounds, for example hydrogen peroxide. According to the article, the best Mg/Mn ratio is reached at a pH of 4.5- 5.0. For this reason the chelating should in practice be car¬ ried at a pH clearly below 6, for example at a pH close to 5.5. In the examples of the above reference publication, the chelat¬ ing was carried out at a pH of 5.3.

Effective chelating agents are often poorly biodegradable, as is DTPA, or are completely non-biodegradable, as is EDTA. The increasing of TCF bleaching has increased the use of the said chelating agents. Therefore interest has arisen in replacing poorly biodegradable chelating agents either in part or entire¬ ly with biodegradable chelating agents. In order to avoid the environmental load it is additionally desirable that the bio¬ degradable chelating agents be preferably phosphorus-free and also contain as small an amount of nitrogen as possible.

In the bleaching and delignification process according to the invention it is possible to use as the chelating agent, for example, biodegradable ethylene diamine disuccinic acid (EDDS) or 2,2'-i inodisuccinic acid (ISA) and their alkali metal salts, which are per se previously known as compounds.

The applicants have observed, surprisingly, that an equally good bleaching result is achieved even if a portion of the above-mentioned nitrogen-containing chelating agents is re¬ placed with biodegradable nitrogen-free chelating agents, such as conventional carboxylic acids, hydroxycarboxylic acids, polyhydroxycarboxylic acids and hydroxypolycarboxylic acids. Thus, for example, generally used effective chelating agents, such as a inopolycarboxylic acids, e.g. DTPA and EDTA, may in part be replaced with nitrogen-free biodegradable compounds.

It is quite surprising that the above-mentioned hydroxy¬ carboxylic acids can be used successfully as chelating agents in bleaching, together with nitrogen-containing chelating agents. Hydroxycarboxylic acids do not bind heavy metals effec¬ tively. Instead, they chelate well calcium and magnesium. Especially citric acid has been used instead of phosphates in phosphate-free detergents and cleansing agents which must bind calcium and magnesium. The usability of the chelating process according to the invention is specifically based on the joint effect of chelating agents of different types.

When DTPA is used as a chelating agent before oxygen bleaching or delignification, the most preferred pH in terms of chelating is approx. 5-5.5. When aminopolycarboxylic acids are replaced with the above-mentioned hydroxycarboxylic acids, the chelating can be carried out at a higher pH. When chelating-agent mix¬ tures according to the invention are used, the usable pH range is pH 6-8, most preferably pH 6.5-7.5.

The procedure according to the invention is advantageous, since a nitrogen-containing chelating agent can in part be replaced with a biodegradable chelating agent which does not contain nitrogen. Most commonly the cooking is carried out in alkaline conditions. Since, when chelating agent mixtures according to the invention are used, the chelating can be carried out at a higher pH, less acid is needed for achieving a pH advantageous for the chelating. Also, when the chelating process described in the invention is used, less alkali is required for raising the pH to a level suitable for the alkaline peroxide step nor¬ mally following the chelating. The characteristics of the invention are stated in the accom¬ panying patent claims.

According to the invention, nitrogen-containing chelating agents may be replaced by compounds known per se, having the general Formula I

where n is 1-8, m is 0-2n, p is 0-n, g is 0-2,

R-^ is COOH, and

R₂ is H, CH₂OH or COOH.

The detrimental nitrogen load in effluents can thus be reduced. Conventional carboxylic acids, hydroxycarboxylic acids, poly¬ hydroxycarboxylic acids and hydroxypolycarboxylic acids accord¬ ing to Formula I, such as citric acid, tartaric acid, lactic acid, pimelic acid, glutamic acid, glucoheptonic acid, ascorbic acid, glycolic acid, glutaric acid, adipic acid, succinic acid or malonic acid, can be used as replacement chelating agents.

The process according to the invention can be used for all known chemical pulps. These include alkaline and neutral sul- fite pulps, soda pulps, sulfate pulps (kraft pulps) and oxygen- delignified (oxygen cooking) sulfate pulps. Furthermore, the process can be used in the bleaching of so-called organosolv pulps in which alcohols or organic acids have been used as the cooking solvent, for example Milox cooking in formic acid. The chelating process according to the invention may also be used when polysulfides or, for example, anthraquinone, have/has been used in sulfate cooking. The treatment can be carried out on pulp cooked from different fiber raw materials, such as softwood, hardwood or reed, straw or other raw material of vegetable origin.

The chelating process according to the invention can be used in the bleaching or delignification of pulp in acid conditions and/or as a pretreatment before the process steps described above.

The pH control of an acid chelating step can be carried out using conventional mineral acids, such as sulfuric acid, sulfur dioxide or an aqueous solution thereof, carbon dioxide, or organic acids such as formic acid and acetic acid.

In the process according to the invention, nitrogen-containing phosphorus-free chelating agents are preferably used, whereby environmental problems due to phosphorus are avoided. Such chelating agents include ethylenediamine-N,N'-disuccinic acid (EDDS) , its various isomers and its alkali metal salts, such as sodium and potassium salts, and its earth-alkali metal salts, such as calcium and magnesium salts, 2,2' -iminodisuccinic acid (ISA), its various isomers and its alkali metal salts, such as sodium and potassium salts, and its earth-alkali metal salts, such as calcium and magnesium salts. Usable chelating agents also include, among polyaminocarboxylic acids, ethylene diamine tetra-acetic acid (EDTA) and its salts, and diethylene triamine penta-acetic acid (DTPA) and its salts. The functioning of such nitrogen-containing chelating agents in the invention is shown in the embodiment examples below.

Chelating agents within the scope of the invention, but less recommendable, are those which in addition to nitrogen also contain phosphorus, such as polyaminomethylene phosphonic acids or biodegradable aminoalkane diphosphonic acids, the use of which in the bleaching of pulp is known per se from WO applica¬ tion publication 95/12029. Some specific examples of these chelating agents are, among aminoalkane phosphonic acids, 4- morpholinomethylene-l,l-diphosphonic acid (MMDPA) and its salts and, among aminopolymethylene phosphonic acids, aminotri- methylene phosphonic acid and its salts, ethylene diamine tet- ra ethylene phosphonic acid and its salts, and diethylene tri- amine pentamethylene phosphonic acid (DTMPA) and its salts.

The invention is described below in examples, which, however, do not limit the invention.

Example 1

To investigate the chelating of heavy metals and earth-alkali metals, an oxygen-delignified chemical pulp was washed with aqueous solutions containing DTPA and a sodium salt of citric acid. The metal contents of the washing solution were analyzed after the wash. Thus the transfer of iron (Fe), manganese (Mn), calcium (Ca) and magnesium (Mg) into the washing waters was investigated. The transfer of iron and manganese into the wash¬ ing solutions is advantageous for bleaching. In contrast, the transfer of calcium and magnesium into the washing solutions is disadvantageous for bleaching. In reference tests the pulp was washed with DTPA solutions. The chelating agent concentrations in kilograms per metric ton of pulp (kg/tp) and the pH during the wash are indicated in Table 1.

Table 1

Softwood sulfate pulp Chelating conditions Kappa number 16.9 Time (t) 60 min Viscosity 963 dm /kg Temperature (T) 70 °C Brightness 39.6 % ISO Consistency (CS) 12 %

Chelate Dose Metal contents in the filtrate (ppm) kg/tp pH Fe Mn Mg Ca

Na₅DTPA 1 6.7 1.2 2.9 4 17

Na₅DTPA 2 6.5 2.0 2.8 17 48

Na₅DTPA 2 5.7 2.0 3.3 13 52

Na EDTA 2 6.5 1.8 3.3 5 49

Water wash 6.0 0.3 0.3 6 22

Water wash 7.0 0.3 0.3 14 66

Na₃ citrate 1 6.3 0.0 0.6 9 26

H₅DTPA + Na₃ citrate 0.5+0.5 6.0 1.0 1.4 9 30

H₅DTPA + Na₃ citrate 0.5+1 5.8 1.2 1.6 12 39

H₅DTPA + Na₃ citrate 1+1 6.3 1.3 3.0 10 29

H₅DTPA + Na₃ citrate 1+1 7.0 2.1 3.9 13 48

H₅DTPA + Na₃ citrate 1+1 8.6 0.3 2.7 5 24

In Table 1, Na^DTPA stands for the pentasodium salt of DTPA, Na^EDTA stands for the tetrasodium salt of EDTA, and H5DTPA stands for the acid form of DTPA. However, the pH used will determine how the chelating agents are dissociated, i.e. in which form they actually appear in the treatment.

DTPA is usually dosed into softwood pulp in the chelating step at a rate of approx. 2 kg/tp. Chelating was clearly less when the DTPA dose was reduced from a rate of 2.0 kg/tp to a rate of 1.0 kg/tp at a pH of approx. 6.5.

When the sodium salt of citric acid (Na citrate) was used as a chelating agent alongside DTPA, it was possible to reduce the dose of DTPA significantly. Even though the dose of DTPA had been decreased to a rate of 1.0 kg/tp, the chelating of metals in these experiments was as complete as when DTPA was used alone at a rate of 2.0 kg/tp. Chelating with a mixture of DTPA and Na citrate yielded a better chelating result at a pH of 7.0 than when using DTPA. It is to be noted that even at a pH of 8.6 manganese chelated very well. At this high a pH iron has already precipitated.

It can be observed that a water wash has no effect as regards the chelating of metals. Likewise, Na citrate used alone does not remove heavy metals. Na citrate chelates only earth-alkali metals, which is not desirable for bleaching. Likewise, a DTPA dose of 1 kg/tp was not sufficient to produce a satisfactory chelating result. These reference experiments show that a good chelating result is achieved through the joint effect of DTPA and Na citrate.

Example 2

Table 2 shows the results of washing experiments similar to those described in Example 1 when the chelating agent used was Na citrate or Na gluconate together with EDDS or ISA at a pH of 5.8-8.9. As a reference experiment, chelating was carried out on the same pulp with DTPA at various doses at a pH of 6.5.

Table 2

Softwood sulfate pulp Chelating conditions Kappa number 16.9 Time 60 min Viscosity 963 dm /kg Temperature 70 °C Brightness 39.6 % ISO Consistency 12 % pH 6-7

Chelate Dose Metal contents in the filtrate (ppm) kg/tp PH Fe Mn Mg Ca

No chelate 0 6.0 0.3 0.3 6 22

Na₅DTPA 1 6.5 1.2 2.9 4 17

Na₅DTPA 2 6.5 2.0 2.8 17 48

H₄EDDS + Na₃ citrate 0.5+1 7.5 1.3 2.4 8 18

H₄EDDS + Na₃ citrate 0.75+1 5.7 2.2 1.6 17 58

H₄EDDS + Na₃ citrate 1+1 7.4 1.4 2.7 10 23

H₄EDDS 1.5 7.1 1.9 2.3 13 37

H EDDS + Na gluconate 1+1 6.7 1.8 2.6 9 29

Na^ISA 1.5 5.8 1.1 1.9 2 50

Na_AISA 1.5 8.9 0.0 1.3 25 26

Na₄ISA + Na citrate 1+1 7.7 0.5 2.2 15 26

When the dose of DTPA was reduced from 2.0 kg/tp, the chelating of heavy metals was incomplete. A chelating result comparable to the best chelating result achieved with DTPA was achieved when EDDS was used at a rate of 1.5 kg/tp. Equally good chelat¬ ing results were achieved even if EDDS dosing was reduced even to one-half of this, when Na citrate or Na gluconate at a rate of 1.0 kg/tp was used together with it. Likewise, it was pos¬ sible to reduce the dose of ISA without significant worsening of the chelating result when Na citrate was used as a chelating agent together with it. It is to be noted that in this experi¬ ment the pH of the pulp was considerably high.

A conclusion regarding the bleaching result cannot be made directly from the washing results. Therefore an investigation was made of the effect of the various chelating processes on the quality of bleached pulp.

Example 3

An oxygen-delignified pulp was chelated and bleached with an alkaline hydrogen peroxide. The chelating agents used were DTPA alone or together with Na citrate or Na gluconate. The results are compiled in Table 3. The bleaching result can be evaluated on the basis of peroxide consumption, the brightness achieved, and the viscosity of the pulp.

Table 3

Softwood sulfate pulp

Pulp before chelating and final bleaching Kappa 5.2

Viscosity 817 dm³/kg

Brightness 74.7 % ISO

t, min 60 60 60 60 60 60 60 60 60 60 60

T, C 75 75 75 75 75 75 75 75 75 75 75

CS, % 12 12 12 12 12 12 12 12 12 12 12 pH 5 6.6 5 6. 6. 6.5 6. 6. 6. 6.5 6.5

Na₅-DTPA, kg/tp 2 2 1 0 0 1 0. 1 1 0.5 0.5

Nac citrate, kg/tp 0 1 0.5 0.5

Na gluconate, kg/tp 0 0.5 0.5

P i 1 4. t t, min 180 180 180 180 180 180 180 1810 180 180 180

T, C 90 90 90 90 90 90 90 90 90 90 90

CS, % 12 12 12 12 12 12 12 12 12 12 12 pH, initial 10.4 10.4 10.4 10.4 10.4 10. 4 10.4 10.4 10.4 10.4 10.4 pH, final 10.3 10.2 10.1 10.3 10.3 10. 2 10.2 10.2 10.3 10.3 10.3

H₂0₂, kg/tp 20 20 20 20 20 20 20 20 20 20 20 Residual H₂0₂, kg/tp 17.5 14.4 8.2 3 3.7 16 15.6 16.3 15.8 15.8 17.2 Residual r C^, % 87.5 72 41 15 18.5 80 78 81.5 79 79 86 Residual NaOH, kg/tp 7.1 6.6 4.8 4.3 5 7 7.5 7.3 7.1 7.4 7.6

Kappa 3.9 3.7 3.6 3.5 3.5 3. 8 3.9 3.8 3.7 3.8 3.8

Viscosity, dπr /kg 787 736 606 573 572 793 766 806 769 764 814

Brightness, % ISO 83.9 85.1 84.9 86 85.4 85. 2 84.8 85.3 85.4 84.8 84.8

H2O2 consumption, kg/tp 2.5 5.6 11.8 17.0 16.3 4. 0 4.4 3.7 4.2 4.2 2.8

When no chelating agent was used, or when the chelating was carried out using only Na citrate or a lower dose of DTPA (1.0 kg/tp), the viscosity of the pulp after bleaching was low. Likewise, the consumption of peroxide was high.

In a reference experiment the pulp was chelated with DTPA (2.0 kg/tp). Bleaching results comparable to this experiment were achieved after a chelating which had been carried out with DTPA and Na citrate even if the dose of DTPA had been reduced to 0.5 kg/tp. Likewise, excellent bleaching results were ob¬ tained in the bleaching of pulps on which chelating had been carried out using Na gluconate. When the pulps had been chelated with mixtures of the above-mentioned chelating agents, the viscosities of the pulps after bleaching remained better than in the reference experiments. In several experiments the consumption of hydrogen peroxide in the bleaching was also lower than in the reference experiment.

Claims

1. A process for the bleaching or delignification of a chemical pulp with an oxygen chemical, in which process the pulp is treated with a chelating agent in order to bind into a chelate complex the heavy metals present in the pulp, charac¬ terized in that the chelating of the pulp is carried out using a mixture of a nitrogen-containing chelating agent and one or more compounds according to Formula I or a salt thereof

R_{l n}l OH)_p(COOH)_gR₂ (I)

where n is 1-8, m is 0-2n, p is 0-n, g is 0-2,

R₁ is COOH, and

R₂ is H, CH₂OH or COOH.

2. A process according to Claim 1, characterized in that the compound according to Formula I is lactic, citric, tar- taric, gluconic or glucoheptonic acid.

3. A process according to Claim 1 or 2, characterized in that the nitrogen-containing chelating agent is ethylene diamine-N,N' -disuccinic acid, its alkali metal or earth-alkali metal salt, 2,2'-iminodisuccinic acid, its alkali metal or earth-alkali metal salt, ethylene diamine tetra-acetic acid, its alkali metal or earth-alkali metal salt, or diethylene triamine penta-acetic acid or its alkali metal or earth-alkali metal salt.

4. A process according to any of the above claims, charac¬ terized in that the oxygen chemical bleaching or delignifica¬ tion comprises bleaching with a peracetic acid, bleaching with a peracid mixture, bleaching with peroxides as such or as mix- tures, either alone or as a mixture with peracids within the acid pH range of transition-metal catalyzed hydrogen peroxide bleaching, an alkaline peroxide bleaching and combinations of all of these, as well as oxygen delignification, a peroxide- enhanced oxygen-alkali step and pressurized peroxide steps, and combined with these optional treatments with enzymes, ozone or chlorine dioxide.

5. A process according to Claim 4, characterized in that the oxygen chemical bleaching or delignification of the pulp is carried out simultaneously with a peroxide and a peracid.

6. A process according to Claim 4 or 5, characterized in that the peroxide treatment of the pulp is carried out with hydrogen peroxide, with a mixture of hydrogen peroxide and oxygen gas, or with organic per- or perhydro-compounds.

7. A process according to any of Claims 4-6, characterized in that the peracid treatment of pulp is carried out with per¬ acetic, performic, perpropionic or caron acid or with a transition-metal activated peracetic, performic, perpropionic or caron acid or a combination of these.

8. A process according to any of the above claims, charac¬ terized in that the chelating treatment is carried out before an oxygen chemical bleaching or delignification or in connec¬ tion with these.

9. A process according to Claim 8, characterized in that the chelating pretreatment taking place before bleaching or de¬ lignification is carried out at a pH of 4-8, preferably 5.0- 7.5, and most preferably 6.5-7.5.

10. A process according to Claim 8 or 9, characterized in that the chelating taking place before bleaching or delig¬ nification is carried out once or several times.

11. A process according to Claim 8, characterized in that the combined chelating treatment and oxygen chemical bleaching or delignification is carried out at a pH of 4-8, preferably 4- 6, and most preferably 4-5.

12. A process according to any of the above claims, charac¬ terized in that the oxygen chemical bleaching or delignifica¬ tion is carried out once or several times.

13. A process according to any of the above claims, charac¬ terized in that the chelating agent is used at a rate of 0.1- 5 kg/metric ton of dry pulp.

14. A process according to any of the above claims, charac¬ terized in that the weight ratio of the compound according to Formula I to the nitrogen-containing chelating agent is between 1:4 and 3:1, preferably between 1:2 and 2:1.