WO1997045585A1

WO1997045585A1 - Process for the bleaching of high yield pulp or recycled paper pulp

Info

Publication number: WO1997045585A1
Application number: PCT/FI1997/000334
Authority: WO
Inventors: Jukka JÄKÄRÄ; Aarto Paren; Reijo Aksela; Ilkka Renvall
Original assignee: Kemira Chemicals Oy
Priority date: 1996-05-30
Filing date: 1997-05-30
Publication date: 1997-12-04
Also published as: FI970304A; FI105932B; AU2964597A; FI970304A0

Abstract

The invention relates to a process for the bleaching of high yield pulp or recycled paper pulp, in which process the pulp is treated with an oxidizing or reducing bleaching chemical and additionally the pulp is chelated in order to bind heavy metals, such as Fe, Mn and/or Cu, into a chelate complex. According to the invention the chelating chemical is N-bis-((1,2-dicarboxy-ethoxy)-ethyl)-amine, N-bis-((1,2-dicarboxy-ethoxy)-ethyl)-aspartic acid or N-tris-((1,2-dicarboxy-ethoxy)-ethyl)-amine, or an alkali metal or earth-alkali metal salt thereof. The chelation may constitute a step preceding the bleaching, or the chelating and the bleaching may be carried out simultaneously with the effective chemicals being combined in the same treatment solution.

Description

Process for the bleaching of high yield pulp or recycled paper pulp

The invention relates to a process for the bleaching of high yield pulp or recycled paper pulp, in which process the pulp is treated with an oxidizing or reducing bleaching chemical and additionally the pulp is chelated in order to bind heavy metals, such as Fe, Mn and/or Cu, into a chelate complex.

The bleaching of mechanical pulps, i.e. high yield pulps, differs from the bleaching of chemical pulps in that an effort is made to avoid the removal of lignin. The yield of pulp is thus maintained at a high level. The aim is to render the colored groups of lignin, i.e. chromophores, colorless by either oxidizing or reducing bleaching methods.

Mechanical pulps can be divided into two main categories: purely mechanical pulps, which include stone groundwood (SGW), pressure groundwood (PGW) and thermomechanical pulp (TMP), and chemimechanical pulps, which include low- sulfonated (chemithermomechanical pulp CTMP and BCTMP), chemically modified (OPCO) and high-sulfonated (CMP, UHYS) pulps. All of these pulps are within the scope of the invention. Mechanical pulps are used, for example, in the making of newsprint and magazine papers and porous paper grades.

Lignin-saving bleaching processes aiming at a high yield are also used on recycled paper fiber, in which the weight ratio of recycled papers based on mechanical pulp to those based on chemical pulp is typically approximately 70/30. If recycled paper is classified, a recycled batch may consist even of only paper made of chemical pulp, but even in this case, no lignin but only printing ink is removed. The scope of the invention thus covers, alongside mechanical pulps, also the bleaching of all types of recycled paper pulps.

One problem in the bleaching of mechanical pulps and recycled paper pulps consists of the heavy metals, mainly iron, manganese and copper, present in the pulp. These heavy metals enter the pulp in either the wood or the process waters and catalyze the decomposition of the bleaching chemicals so that the consumption of chemicals increases and the brightness of the pulp suffers. Heavy metals also catalyze, for example, alkali darkening, and thus they are a considerable disturbing factor in the pulp and paper making processes. The mitigation of the effect of detrimental heavy metals in connection with the bleaching of mechanical and recycled paper pulps may be carried out in a number of ways. The pulp may first be pretreated with a solution which contains chelating agents, whereupon a large proportion of the metals dissolved in the filtrate is removed along with the filtrate, whereafter the pulp is treated with a bleaching agent, such as hydrogen peroxide. Alternatively it is possible to add substances, such as waterglass and/or chelating agents which stabilize the bleaching agent, to the solution which contains the bleaching agent. The two procedures stated above may also be combined so that first a considerable proportion of the metals present in the pulp is removed by a separate chelating treatment, which is followed by the actual bleaching step, in which chelating agents are also present.

The oxidizing peroxide treatment of mechanical pulps and recycled paper fiber is carried out in alkaline conditions. Waterglass, which is an alkaline sodium silicate solution, is commonly used as an auxiliary agent in peroxide bleaching. The purpose of the waterglass is that the colloidal silicate formed from it binds detrimental heavy metals during the bleaching. In addition to waterglass, chelating agents are also commonly used. Chelating agents which can be used for the binding of heavy metals include polyaminocarboxylic acids, the most commonly used among them being ethylenediaminetetraacetic acid (EDTA) and diemylenetriamine- pentaacetic acid (DTP A). In addition to the above-mentioned substances it is also possible to use magnesium salts, such as magnesium sulfate, as stabilizers of hydrogen peroxide. When a high degree of brightness is the aim in the bleaching of mechanical pulps, it has become common to use chelating agents as a pretreatment at a lower pH than the one at which the actual hydrogen peroxide bleaching is carried out. The publication W.C. Fross et al, Tappi 1992 Pulping Conference, pp. 899-915 describes as an example a DTPA treatment at a pH of 5.5-6.0 as part of the peroxide bleaching of mechanical pulp.

Also in reducing bleaching processes, in which the bleaching chemical is, for example, dithionite or formamidine sulfinic acid (FAS), chelating agents can be used as auxiliary agents.

The heavy metal problems in the bleaching of mechanical pulps and recycled paper fiber are largely the same, and the same chelating agents and methods are used for solving the problems. For recycled paper fiber, hydrogen peroxide bleaching and die chelating associated therewith can, however, also be carried out in connection with the slushing of the recycled paper fiber. The said current chelating agents have the disadvantage that EDTA does not degrade in watercourses and DTPA degrades very poorly. Since the forest industry uses very large amounts of these non-biodegradable chelating agents, their concentration in watercourses is increasing continually. In watercourses these chelating agents may, for example, affect the transfer of heavy metals between the sediment and the aqueous phase. However, EDTA and DTPA have been found to be so effective and good chelating agents that discontinuing their use would cause considerable additional costs owing to extra consumption of the bleaching agent. It would be advantageous if a substitute were found for these agents, a substitute which would be biodegradable and/or would contain less nitrogen. A biodegradable chelating agent would bind to slurry at biological water treatment plants and would thereby be removed from the waste waters. If, on the other hand, the agent were for some reason released into a watercourse, it would rapidly degrade in it.

The said problem has now been solved by the novel bleaching process according to the invention for high yield pulp and recycled paper pulp, the process being characterized in that the stabilizing chelating chemical used is an N-bis- or N-tris- ((l,2-dicarboxy-ethoxy)-ethyl)-amine derivative which has been selected from a group consisting of N-bis-((l,2-dicarboxy-ethoxy)-ethyl)-amine, N-bis-((l,2- dicarboxy-ethoxy)-ethyl)-aspartic acid and N-tris-((l,2-dicarboxy-ethoxy)-ethyl)- amine, as well as the alkali metal and earth-alkali metal salts thereof.

The formula of the three amine derivatives (A, B and C) used according to the invention are:

N-bis((l,2-dicarboxy-ethoxy)-ethyl)-amine (A) is hereinafter referred to by using the acronym BCEEA, N-bis-((l,2-dicarboxy-ethoxy)-ethyl)-aspartic acid (B) by using the acronym BCEEAA and N-tris-((l,2-dicarboxy-ethoxy)-ethyl)-amine (C) by using the acronym TCEEA.

The process for the preparation of the said amine derivatives used as chelating agents, which are novel compounds, has been described in the applicant's patent application FI 962261. These compounds can be used as such in an acid form or as their alkali metal or earth-alkali metal salts.

The chelating agents used in accordance with the invention are assumed to be biodegradable. Their additional significant advantage over EDTA and DTPA is their lower nitrogen content and over phosphonates that they do not contain phosphorus.

The said chelating agents have been observed to be excellently suited for stabilizing highly alkaline peroxide solutions. On this basis they are well suited for the peroxide bleaching of mechanical pulps, in which an ability to stabilize peroxide solutions is often also required.

The chelating agents can be used both in a separate chelating treatment and as an auxiliary agent in bleaching in which chelating and bleaching agents are combined in the same treatment solution. It is also possible that a chelating pretreatment is followed by a combined chelating and bleaching treatment. The chelating agents are suitable for use not only in oxidizing peroxide bleaching but also in reducing dithionite or FAS bleaching.

The chelating agents to be used do not set limitations with respect to the pulp to be bieached but may be applied to bleaching of purely mechanical pulps. Chemimechanical pulps as well as recycled fibres.

In the chelation step the temperature may be 20-120 °C, preferably 50-100 °C. The pH of the chelation in a separate chelating treatment may be 4-8, preferably 5-7. When chelating agents are used during the bleaching step, the pH may be at the level normally used in the bleaching process concerned; the use of the chelating agent does not set any limitations in this respect. Waterglass can be added in the conventional manner to the pulp to be bleached.

The invention is described below with examples, which do not, however, in any way restrict the invention. It should be pointed out that the mixture of chelating agents BCEEA + BCEEAA according to the invention, used in the examples, contained 18 % BCEEA and 34 % BCEEAA, the balance being mainly water. The doses (kilograms/metric ton of pulp) in the examples have been calculated for all chelating agents as 100 % sodium salts.

Preparation Example 1

A disodium maleate solution was prepared by dissolving 29.4 g (0.3 mol) of maleic anhydride in 50 ml of water and by adding to the reaction mixture 50 g of a 48 % cautie solution (0.6 mol NaOH). During the adding the temperature of the reaction mixture was maintained at 70-90 °C. 17 g (0.05 mol) of lanthanum(III) nitrate, La(NO3)3 x 6 H2O was added to the reaction mixture together with diethanolamine (10.5 g, 0.1 mol). The reaction mixture was stirred at 85 °C under a reflux condenser for 48 h. The reaction mixture was cooled and was rendered acidic (pH 1.8) by means of a strong sulfuric acid. Thereafter the reaction mixture was reheated to 60 °C, and 10 g of oxalic acid and 50 ml of water were added, the mixture was stirred at 60 °C for 20 minutes, and the La(III) oxalate precipitate formed was removed from the hot solution by filtration. The filtrate was cooled, and any precipitate subsequently formed was removed by filtration. The remaining solution (40 ml), which contained 54 % water, was analyzed for organic compounds by means of ^C NMR spectra and by a mass spectrometer as silyl or methyl ester derivatives.

BCEEAA and BCEEA were identified from the ¹³C NMR spectrum. The unreacted starting substances were identified on the basis of reference spectra: diethanolamine and maleic acid, as well as oxalic acid which was used for precipitating the catalyst. Malic acid and fumaric acid formed as byproducts of the reaction; these were also identified on the basis of reference spectra.

On the basis of a quantitative ^C NMR analysis, the composition of the obtained reaction mixture was as follows: % by weight

BCEEAA 18.5

BCEEA 7.9 diemanolamine 1.2 maleic acid 2.2 malic acid 2.5 oxalic acid 0.3 fumaric acid 2.1 water 54.3

Na2SO4 11.0

The said analysis and the ascertaining of the molecular structures of the compounds BCEEA and BCEEAA by gas chromatography and mass spectrometry are described in the priority application FI-962261.

In order to isolate the compounds BCEEA and BCEEAA, a sample (13.25 g) of the reaction mixture obtained as described above was pretreated by adding to it 1.16 g of calcium carbonate. Thereupon the sulfate ions present in the sample precipitated as calcium sulfate.

The ion exchange resin used was a strong anion exchange resin (Bio-Rad AG 1 - X8, 200-400 mesh) in its formiate form. The sample was eluted through an ion exchange column with an eluent (1000 ml), the formic acid concentration of which was increased gradually so that the formic acid concentration of the eluent ranged from 0 to 2 moI/l. During the run, one hundred samples of 10-20 ml were collected and were analyzed by a liquid chromatograph. BCEEA and BCEEAA were isolated from the fractions. The ^C NMR spectra and GC-MS spectra of the reaction products were ascertained by comparing the spectrum data for the purified and isolated reaction products with the spectrum data for the reaction products identified from the reaction mixture. The spectrum data of the purified BCEEA and BCEEAA were found to be identical with those obtained from the reaction mixture.

Preparation Example 2

A magnesium maleate solution was prepared by dissolving 29.4 g (0.3 mol) of maleic anhydride in 50 ml of water and by adding to the reaction mixture 35.0 g of magnesium hydroxide (0.3 mol Mg(OH)2) slurried in 70 ml of water. During the adding the temperature of the reaction mixture was maintained at 70-90 °C. 17 g (0.05 mol) of lanthanum(III) nitrate, La(NO3)3 x 6 H2O was added to the reaction mixture together with diethanolamine (10.5 g, 0.1 mol). The pH of the reaction mixture was adjusted to a pH value of 1 1 by an addition of a 48 % sodium hydroxide solution. The reaction mixture was stirred at 85 °C under a reflux condenser for 10 hours. The reaction mixture was cooled and was rendered acidic (pH 1.8) by means of a strong sulfuric acid. Thereafter the reaction mixture was reheated to 60 °C, and 10 g of oxalic acid and 50 ml of water were added, the mixture was stirred at 60 °C for 20 minutes, and the formed precipitate was removed from the hot solution by filtration. The filtrate was cooled and any precipitate subsequently formed was removed by filtration. The remaining solution (42 ml), which contained 54 % water, was analyzed for organic compounds by means of ^C NMR spectra and mass spectrometer as silyl or methyl ester derivatives.

BCEEAA and BCEEA were identified from the ¹³C NMR spectra. The unreacted starting substances were identified on the basis of reference spectra: diethanolamine and maleic acid. Malic acid and fumaric acid formed as byproducts of the reaction; these were also identified on the basis of reference spectra.

The organic compound composition of the reaction product was as follows on the basis of a quantitative ^C NMR analysis:

% by weight

BCEEAA 13.8

BCEEA 4.5 diethanolamine 7.5 maleic acid 2.3 malic acid 1.3 fumaric acid 0.3

Preparation Example 3

TCEEA w^s prepared by the method described in Example 1 by using triethanol- arnine (1.0 mol) and maleic anhydride (3.4 mol) as the starting substances.

TCEEA was identified from the ^C NMR spectrum. The unreacted starting substances were identified on the basis of reference spectra: triethanolamine and maleic acid, as well as oxalic acid used for precipitating the catalyst. Malic acid and fumaric acid formed as byproducts of the reaction; these were also identified on the basis of reference spectra.

On the basis of a quantitative 1-*C NMR analysis, the composition of the reaction product was as follows.

molar %

TCEEA 46.3 triethanolamine 18.5 maleic acid 11.5 fumaric acid 3.2 malic acid 13.5 oxalic acid 6.6

Example 1

TMP (initial brightness 55 % ISO) was chelated and thereafter bleached by using DTPA (test batches 1 and 3) and a BCEEA + BCEEAA mixture (test batches 2 and 4) as the chelating agents. The reference was test batch 5, which was not chelated but was subjected directly to a hydrogen peroxide bleaching. The hydrogen peroxide step was performed on the chelated pulps by using two different doses of caustic. The results, presented in Table 1 , show that DTPA and BCEEA + BCEEAA yield a final brightness of more or less the same level, and that a clear improvement is achieved with each chelating agent as compared with unchelated pulp. An increasing of the caustic dose reduces the residual peroxide and somewhat improves the brightness.

Example 2

Stone groundwood was bleached with peroxide or dithionite. The initial brightness of the pulp was 60.3 % ISO. The measured Fe co ncentration before the treatment was 83 ppm and Mn concentration 140 ppm. In hydrogen peroxide bleaching, the chelation was first performed at a consistency of 3 %, whereafter the pulp was com¬ pressed to a consistency of 10 %, at which the peroxide bleaching was performed. In dithionite bleaching, the chelating and bleaching were performed at a consistency of 3 % so that first the chelating agent and 60 minutes thereafter the dithionite were added to the pulp. The chelation was performed at two pH levels, approximately 4.5-5.5 and approximately 6-6.5. The chelating agents in the test were EDTA, DTPA, a BCEEA + BCEEAA mixture, and TCEEA. The results, presented in Table 2, show that at the lower pH level (4.5-5.5) EDTA was the best in the dithionite step, although the other chelating agents yielded almost the same results. At the higher pH the results with all of the chelating agents were approximately of the same level, the BCEEA + BCEEAA mixture being barely the best. In peroxide bleaching, chelation at the lower pH level yielded a clearly lower brightness than did chelation at a pH of 6-6.5; the difference is in the order of approximately 2-3 % ISO. Chelation at a pH of 6-6.5 worked well with all of the chelating agents; DTPA worked best, but the others yielded nearly the same final result.

Example 3

In the test, TMP (initial brightness 65 % ISO) was bleached with hydrogen peroxide. Before the bleaching the pulp had been chelated carefully (3 kg/tp of DTPA, temperature 75 °C, time 30 min, pulp consistency 10 %, pH 5.5) in order to remove the metals present in it. Thereafter peroxide bleaching runs were carried out by adding iron and manganese separately to the bleaching solution in order to see the effect of chelating agents added at this stage in bleaching. DTPA, the BCEEA + BCEEAA mixture, and TCEEA were compared in the test. The results and the conditions are presented in Table 3. The results show that BCEEA + BCEEAA and DTPA yield results of approximately the same level in the presence both of iron and of manganese. Both the brightness and the amount of residual peroxide are of the same order. With TCEEA the amount of residual peroxide is of the same order as with the former, but brightness is somewhat lower. However, with all the chelating agents a clear improvement was achieved over the case in which no chelating agent was used.

Example 4

The procedure was as in Example 3, but the amount of waterglass in the bleaching was reduced from 19 kilograms to 13 kilograms per metric ton of pulp in order for the effect of the chelating agents to show more clearly. The results are shown in Table 5. The results show that BCEEA + BCEEAA yielded the best result in the presence both of iron and of manganese. TCEEA also yielded a good result in the presence of manganese. DTPA was somewhat weaker, but all of the chelating agents yielded a clear improvement over the case in which no chelating agent was used. Table 1

Chelating

Test batch 1 2 3 4 5

Time min. 30 30 30 30

Temperature °C 60 60 60 60

Consistency % 10 10 10 10 pH 6.5 6.5 6.5 6.5 -

DTPA kg/tp 1.5 - 1.5 -

BCEEA + BCEEAA kg/tp - 1.5 - 1.5 -

Bleaching

Time min. 90 90 90 90 90

Temperature °C 70 70 70 70 70

Consistency % 15 15 15 15 15 pH initial 10.2 10.1 10.5 10.6 10.6 pH final 7.8 7.8 9 9 9

H₂0₂ kg/tp 20 20 20 20 20

Waterglass kg/tp 20 20 20 20 20

NaOH kg/tp 15 15 25 25 25

DTPA kg/tp 1.5 - 1.5 - -

BCEEA + BCEEAA kg/tp - 1.5 - 1.5 -

Residual H2O2 kg/tp 6.5 5.7 3.9 3.1 0.5

Brightness % ISO 69.6 68.5 70 69.8 65.4

Table 2

Test 1 2 3 4 5 6 7 8

Time min 60 60 60 60 60 60 60 60

Temperature °C 55 55 55 55 55 55 55 55

Consistency % 3 3 3 3 3 3 3 3 pH initial 43 53 53 46 62 62 62 64 pH final 44 52 53 46 62 61 6 63

BCEEA + BCEEAA kg/tp 2 - - - 2 - - -

EDTA kg/tp - 2 - - - 2 - -

DTPA kg/tp - - > - - - 2 -

TCEEA kg/tp - - - 2 - - - 2

Mn in filtrate ppm 44 43 42 41 48 49 49 44

Mn in filtrate ppm 078 08 084 088 08 082 086 078

Dissolved Mn g/tp 142 139 136 133 155 158 158 142

Dissolved Mn g/tp 25 26 27 28 26 27 28 25

" ¹ ' ' ' ¹ . ' ' ' ' ' ' ' ' " ¹ ' ' ' ' ' '

Test la lb 2a 2b 3 a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8a 8b

Time min 30 60 30 60 30 60 30 60 30 60 30 60 30 60 30 60

Temperature °C 55 55 55 55 55 55 55 5f 5 55 55 55 55 55 55 55 55

Consistency % 3 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10 pH initial 94 98 97 97 10 99 99 99 pH final 44 76 48 78 47 79 43 78 52 8 52 8 52 8 56 8

NaOH kg/tp - 8 - 8 8 - 8 - 7 7 - 7 - -

Waterglass kg/tp - - - - - - - - 10 - 10 - 10 - -

Dithionite kg/tp 8 - 8 c ! - 8 - 8 8 - 8 - 8 -

H₂O₂ kg/tp - 10 - 10 - 10 - 10 - 10 - 10 - 10 - 10

Residual H₂O₂ - 25 - 15 - 26 - 19 - 32 - 31 - 39 - 23 kg/tp

Brightness % ISO 658 642 664 644 649 644 64 64 652 662 65 662 646 67 647 66

Table 3

Test 1 2 3 4 5 6 7 8

Time min 90 90 90 90 90 90 90 90

Temperature °C 65 65 65 65 65 65 65 65

Consistency % 25 25 25 25 25 25 25 25 pH-alku c 95 c 95 c 95 c 95 c 95 c 95 c 95 c 95

Fe addition ppm 160 160 160 160 - - - -

Mn addition ppm - - - - 80 80 80 80

H₂O₂ kg/tp 25 25 25 25 25 25 25 25

Watesglass kg/tp 19 19 19 19 19 19 19 19

NaOH kg/tp 23 23 23 23 23 23 23 23

DTPA kg/tp 3 - - - 3 - - -

BCEEA + BCEEAA kg/tp - 3 - - - 3 - -

TCEEA kg/tp - - 3 - - - 3 -

Residual H₂O₂ kg/tp 121 118 115 101 122 118 115 44

Brightness % ISO 773 772 764 751 795 794 783 755

Taulukko 4

Koe 1 2 3 4 5 6 7 8

Time min 90 90 90 90 90 90 90 90

Temperature °C 65 65 65 65 65 65 65 65

Consistency % 25 25 25 25 25 25 25 25 pH initial c 95 c 95 c 95 c 95 c 95 c 95 c 95 c 95

Fe addition ppm 160 160 160 160 - - - -

Mn addition ppm - - - - 80 80 80 80

H₂O₂ kg/tp 25 25 25 25 25 25 25 25

Waterglass kg/tp 13 13 13 13 13 13 13 13

NaOH kg/tp 23 23 23 23 23 23 23 23

DTPA kg/tp 3 - - - 3 - - -

BCEEA + BCEEAA kg/tp - 3 - - - 3 - -

TCEEA kg/tp - - 3 - - - 3 -

Residual H₂O₂ kg/tp 77 131 84 71 102 103 96 19

Brightness % ISO 763 764 756 741 772 774 773 739

Claims

1. A process for the bleaching of high yield pulp or recycled paper pulp, in which process the pulp is treated with an oxidizing or reducing bleaching chemical and additionally the pulp is chelated in order to bind heavy metals, such as Fe, Mn and/or Cu, into a chelate complex, characterized in that the chelating chemical used is an N-bis or N-tris-((l,2-dicarboxy-ethoxy)-ethyl)-amine derivative selected from a group consisting of N-bis-((l,2-dicarboxy-ethoxy)-ethyl)-amine, N-bis-((l,2- dicarboxy-ethoxy)-ethyl)-aspartic acid and N-tris-((l,2-dicarboxy-ethoxy)-ethyl)- amine, as well as alkali metal and earth alkali metal salts thereof.

2. A process according to Claim 1, characterized in that the pulp is first chelated with the said chelating chemical and thereafter a bleaching treatment is carried out on the pulp.

3. A process according to Claim 2, characterized in that the pH of the separate chelating step is within a range of 4-8, preferably within a range of 5-7, and that the temperature is within a range of 20-120 °C, preferably within a range of 50-100 °C.

4. A process according to Claim 1, characterized in that the bleaching and chelating of the pulp are carried out simultaneously with the chelating and bleaching chemicals combined in the same treatment solution.

5. A process according to Claims 2 and 4, characterized in that first a chelation and thereafter a combined chelating and bleaching treatment are performed on the pulp.

6. A process according to any of the above claims, characterized in that an oxidizing bleaching is carried out by using peroxide as the bleaching chemical.

7. A process according to Claim 6, characterized in that the bleaching is an alkaline hydrogen peroxide bleaching.

8. A process according to Claim 6 or 7, characterized in that waterglass is added to the pulp in the bleaching step.

9. A process according to any of Claims 1-5, characterized in that a reducing bleaching is carried out by using dithionite or formamidine sulfinic acid as the bleaching chemical.

10. A process according to any of the above claims, characterized in that the pulp to be bleached is a mechanical or chemimechanical pulp.