FI115641B - Bleaching process for high yields - Google Patents

Description

115641

The present invention relates to a mechanical pulp bleaching process, wherein the pulp is either pretreated prior to peroxy-5 bleaching and / or, in the course of peroxide bleaching, treated with a chelating agent to eliminate the adverse effects of heavy metals in the pulp.

The bleaching of mechanical pulps, or high yield pulps, differs from the bleaching of chemical pulps in that lignin removal is avoided. The mass yield is maintained at a high level. Bleaching is intended to render the lignin-dyeing portions (chromophore groups) colorless.

The chelating agent used in the bleaching of mechanical pulps is different from the pre-treatment in the bleaching of chemical pulps. In the latter case, the complexes formed during chelation are removed by washing. In the case of mechanical pulps, the pre-treatment is not washed, but the pre-treated pulp is pressed to the bleaching pulp, whereby only a small part of the metal complexes is removed from the V. pulp.

• «• t ♦ · *; ·. Mechanical masses can be divided into two main classes, pure mechanical and chemimechanical masses. The masses can be “! 25 further subdivides into mechanical masses ;;; are grit (SGW), pressure grit (PGW), pulp (RMP), heat pulp · * (TMP) and others such as TRMP and PRMP. Correspondingly, the chemimechanical masses are the low sulfonated (chemothermic pulp CTMP and * ... * BCTMP), chemically modified (OPCO) and high sulfonated ... · 30 0 masses (CMP, UHYS).

* · ·

Mechanical pulps are used, for example, in the manufacture of newsprint / newsprint and magazine papers and porous paper grades (· paper). Certain highly bleached '' chemimechanical pulps (BCTMP grades) are also used in bleached printing papers to replace the chemical pulp.

2 115641

Hydrogen peroxide is used for the bleaching of mechanical pulps and recycled fiber containing paper fibers made from mechanical pulps. For bleaching to succeed, the bleaching conditions must be such that hydrogen peroxide does not decompose.

Heavy metals catalyze the decomposition of hydrogen peroxide and peroxy compounds. The most harmful ions for bleaching are manganese (Mn), iron (Fe) and copper (Cu). Other ras-10 annual metals, such as chromium ions (Cr), etc., also adversely affect the consumption of peroxy compounds. Harmful heavy metals originate from pulp, treatment water and pulp processing equipment.

Chelating agents for metal ions, for example polyaminocarboxylic acids, are used for binding heavy metals with peroxy compounds for bleaching. These include, in particular, ethylenediaminetetraacetic acid and its salts (EDTA) and diethylenetriaminepentaacetic acid and its salts (DTPA) and diethylenetriaminepentamethylenephosphonic acid (DTMPA) and its salt, lat.

* Bleaching of mechanical pulps and recycled fiber is ... performed generally with hydrogen peroxide at high pH. To stabilize the peroxide, magnesium sulfate is added during the bleaching step ;;; and water glass. Many hypotheses have been presented for the stabilizing effect of the above mentioned substances. The stabilizing effect of the water glass is probably due to the fact that the water glass, under alkaline conditions, oxidizes the hydroxides, oxyhydroxides and oxides formed by precipitation of iron, rendering the catalytic surface of these solids inactive.

· Mechanical pulps are manufactured in paper mills and not in separate mills like chemical pulps. In this case, the effluent from bleaching * * 35 becomes one of the other wastewater from the paper mill. Therefore, it would be desirable to use biodegradable chelating agents in the process of silencing.

In order to avoid environmental stress, it would be advantageous to replace the biodegradable chelating agents 5 with at least partially non-nitrogen biodegradable chelating agents.

Often, the bleaching of the recycled fiber can also be accomplished with reducing agents, such as dithionite, and peroxide bleaching is not always required. Hydrogen peroxide is used more in pulping pulp than in bleaching itself. Therefore, the treatment of the recycled fiber has the same problems regarding the stability of hydrogen peroxide.

EDTA and DTMPA are considered non-biodegradable. DTPA is poorly degraded in nature.

Previously, hydrogen peroxide bleaching of mechanical pulps was performed in a single step. Nowadays, bleaching is carried out in two steps because of the aim of higher pulp brightness and because e.g. BCTMP can be used in printing papers. Kak-:. ·. residual peroxide in bleaching from the side: -. back to step one. Thus, residual peroxide plays a very important role in the economics of bleaching.

25 * · ·

In bleaching processes for mechanical pulps, aiming for high luminosity, it has become common to pretreat pulp with chelating agents at a lower pH than where bleaching itself takes place. In W.C. Fross et al. Tappi 1992 Pulping 30 Conference, ss. 899-915, "Factors affecting mencanical pulp Brightening when peroxide liquor is recycled" is described as a precursor to DTPA treatment at pH 5.5-6.0.

4, 115641

Biodegradable complexing agents have been developed as detergent builders. At the same time, they must have a water softening effect, ie they must bind calcium and magnesium ions. One such sequestering agent is ethylenediamine disuccinic acid (EDDS). The compound has three stereoisomers.

Another known biodegradable complexing agent is 2,21-iminodisuccinic acid (ISA).

The use of the compound in alkaline detergents is disclosed in European Patent Application 509 382. The patent application mentions the use of 2,2'-iminodimeric succinic acid as a stabilizer for peroxide compounds, in particular in alkaline detergent compositions containing hydrogen peroxide and its derivatives.

15

In the bleaching and delignification processes developed by the applicant, biodegradable ethylenediamine dimersuccinic acid (EDDS) or 2,2'-iminodimeric succinic acid and their alkali metal salts have been successfully used as chelating agents.

:. ·. The object of the present invention is to eliminate the undesirable effects of ras metal in the bleaching of mechanical pulp.

... The aim is to provide a biodegradable chelating agent composition that provides good bleaching performance.

Applicants have surprisingly found that an equally good mechanical pulp bleaching result is achieved even though some of the above chelating agents (e.g. EDTA; DTPA; EDDS; ISA) are replaced by biodegradable chelating agents such as hydroxycarboxylic acids.

It is quite surprising that the aforementioned hydroxycarboxylic acids '·' can be successfully used as chelating agents in bleaching together with nitrogen-containing chelating agents. Heavy metals are not effectively bound by hydroxycarboxylic acids. Instead, they chelate well calcium and magnesium. For example, citric acid has been used instead of phosphates in phosphate-free detergents, which must bind calcium and magnesium. The usefulness of the chelating agent of the invention is specifically based on the interaction of different types of chelating agents.

When DTPA is used as a chelating agent prior to bleaching the mechanical pulp, the most preferred pH for chelation is from about 5 to about 5.5. When the amino-polycarboxylic acids are replaced by the aforementioned hydroxycarboxylic acids, the chelation can be carried out at a higher pH. In the use of mixtures of chelating agents according to the invention, a useful pH range is pH 6-8, most preferably pH 6.5-7.5.

15

The process of the invention is advantageous because the nitrogen-containing chelating agent can be partially replaced by a biodegradable non-nitrogen-containing chelating agent.

The characterizing features of the invention will be apparent from the appended claims.

According to the invention, nitrogen-containing chelating agents may be replaced by hydroxycarboxylic acids known per se, such as lactic acid, citric acid, tartaric acid or Glu-conic acid.

The harmful nitrogen load in the wastewater can thus be reduced. As the substituting chelating agents, conventional hydroxycarboxylic acids such as citric acid, tartaric acid, lactic acid and glyconic acid can be used.

The process according to the invention can be used in the pre-treatment of mechanical pulps and / or in actual bleaching.

35 The method is applicable to all mechanical pulps, such as ground, pressed, pulp, hot pulp, chemical pulp and chemomechanical pulp. The method is also suitable for bleaching recycled fiber and textile fiber. The process can be used for both single-step and two-step mechanical pulp peroxide bleaching. The treatment is also suitable for bleaching mechanical pulps where dithionite or formadine sulfinic acid bleaching is used at some point along with the peroxide. In the case of mechanical pulps which are sulphonated with sulphite or bi-sulphite, the compounds of the formula I can already be added to the impregnation solution, whereby the efficiency of the bleaching can be improved.

The treatment can be carried out on pulp obtained from various fibrous raw materials, such as softwood or hardwood.

15

Adjustment of the pH of the acid chelating step can be carried out with standard mineral acids such as sulfuric acid, sulfur dioxide or aqueous solution thereof, carbon dioxide or organic acids such as formic and acetic acid.

20

The nitrogen-containing: chelating agent used in the process of the invention may be ethylenediamine-Ν, ′ -dimeric succinic acid. po (EDDS), its various isomers or its alkali metal salts, such as ···. sodium and potassium salts thereof, or its alkaline earth metal salts such as calcium or magnesium salts, 2,2'-iminodimeric succinic acid (ISA), its various isomers or its alkali metal salts such as sodium and potassium salts or its alkaline earth metal salts such as calcium and magnesium salts. Useful chelating agents also include ethylenediamine tetraacetic acid (EDTA) or its salts and diethylenetriaminepentaacetic acid (DTPA) or its salts from polyaminocarboxylic acids.

The invention is illustrated by the following examples, which, however, do not limit the invention.

35

Initially, washing tests on the chemical pulp were performed to determine the transfer of heavy metals. The success of bleaching does not depend solely on the efficiency of the chelation of heavy metals, but the results of the chelation can be considered as indicative. Washing tests can be used to find the best pH range for the removal of metals during the pre-treatment step. Washing tests on the same mass at the same pH can be used to compare the chelating efficiency of metal ions of different chelating agents.

Example 1.

To determine the chelation of heavy metals and alkaline earth metals, the mechanical pulp was washed with aqueous solutions of mixtures of chelating agents. The metal silicon-15 content of the wash solution was analyzed after washing. The migration of iron (Fe), manganese (Mn), calcium (Ca) and magnesium (Mg) into the wash waters was thus investigated. The transfer of iron and manganese to the washing solutions is advantageous for bleaching. Conversely, the transfer of calcium and magnesium to the washing solutions is disadvantageous for bleaching. In control experiments, the pulp was washed with DTPA or EDTA solutions. Table 1 shows the concentrations of the coil agents and the pH during washing. 1 · · 8 115641

table 1

Coniferous sulphate pulp Chelating conditions

Number of tracks 16.9 Time, min. 60 5 Viscosity 963 dm3 / kg T, C 70

Brightness 39.6% ISO Consistency% 12 _pH_6-7_

Chelate Dose, Metal concentrations of filtrate __kg / ts pH Fe, ppm Mn, ppm Mg, ppm Ca, ppm

No Chelate 0 6.0 0.3 0.3 6 22

NasDTPA 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 H4EDDS + Na Gluconate 1 + 1 6.7 1.8 2.6 9 29

Na4ISA 1.5 5.8 1.1 1.9 1.9 50. · *. · 'Na4ISA 1.5 8.9 0.0 1.3 25 26':: Na4ISA + Na citrate 1 + 1 7.7 0.5 2.2 15 26 10 When the DTPA dosage was reduced from 2.0 kg / ts the chelation of heavy metals was incomplete. A chelating result comparable to the best chelating result achieved with DTPA was achieved when EDDS was used at 1.5 kg / ts. Even good chelation results were obtained even though the EDDS increase of 15 was reduced by up to half of this when with it; 'Na citrate or Na gluconate 1.0 kg / ts was used. Thus, the dose of ISA could be reduced without significantly reducing the chelating result when Na- ·· * citrate was used as the chelating agent. It should be noted that in this experiment the pH of the pulp was remarkably high.

9 115641

It can be seen from Table 1 that water washing is ineffective with respect to chelation of metals. Likewise, citric acid alone does not remove heavy metals. Citric acid only chelates alkaline earth metals, which is not desirable for bleaching. This indicates that a good chelating result is achieved as a result of the interaction of nitrogen-containing chelating agents and, for example, citric acid.

It should be noted that from the washing tests 10 described above, the bleaching result cannot be directly deduced. Therefore, the effect of the corresponding chelating agents on alkaline hydrogen peroxide bleaching was investigated. The residual peroxide content reflects the bleaching result relatively well. Therefore, not all of the tests determined the brightness of the pulp.

15

Example 2.

In order to find out how the different chelating agents would work alone when used in pre-treatment, comparative tests were first performed.

Commercial TMP with an initial brightness of 55% ISO was bleached with peroxide by first treating it with 10% consistency and 60 ° C for 30 minutes. at your place. The dosage of the chelates was 1.5 kg / tonne mass. You prepared-. . ·. The pH of the calf was 6.5. The peroxide bleaching was performed at 70 ° C in a heat, 1 °, and 15% consistency for 60 minutes. The dispensing of the water glass and • 1 9 lye is shown in Table 2 below.

• · 1 1 10 115641

Table 2

Chelation: T = 70 ° C, t = 30 min, CS = 10%, pH = 6-6.5

Chelating Hey

Phase P: Chelated DTPA EDDS ISA

T, C 70 70 70 70 h, min 60 60 60 60 CS,% 15 15 15 15 pH start 10.2 10.2 10.2 10.2 pH end n.8.5 n.8.5 n .8.5 n.8.5 H2O2, kg / tm 20 20 20 20

Water glass, kg / tm 20 20 20 20

NaOH, kg / tm 15 15 15 15 Residual H2O2, kg / t 0.5 7.5 4.8 7 Residual H2O2,% 2.5 37.5 24 35

Brightness,% ISO 65.4 69.6 69.2 69.3 5 It can be observed that very low residual peroxide exhibits poor bleaching performance.

• * 'Because residual peroxide is a good indicator of the success of bleaching' ··· 'and when it is fast compared to measuring the pulp lightness • ♦ • h * 10, the following experiments were performed only by comparing the residual peroxide concentrations.

Example 3.

The pulp was bleached by pretreating it at the same consistency, temperature and time as in Example 2. The dosage of the reel agents was 2 kg / tonne pulp. The pH of the pretreatment was about 6-6.5. The peroxide bleaching conditions are shown in Table 3. The final bleaching pH was about 8.5 in all. The residual peroxide was determined from the Val-20 trituration filtrate.

115641

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It can be seen that without chelation the amount of residual peroxide was quite small, whereas DTPA, EDDS and ISA alone or diluted with citric or gluconic acid gave good and almost the same result.

5

Example 4.

The pulp was bleached with hydrogen peroxide by pretreating it at 10% consistency at 60 ° C. In all experiments, chelating agents were dispensed at 2 kg / tonne for both pretreatment and bleaching. The pH of the pretreatment was 6-6.5. Peroxide blotting was performed at 70 ° C, 15% consistency for 60 min. The results of the experiments are shown in Table 4.

15 Table 4

Chelating n: Q__1 2 3 4 5 6 t '™ -11 30 30 30 30 30 30. . T'C 60 60 60 60 60 60 CS '% 10 10 10 10 10 10: PH 6-6.5 6-6.5 6-6.5 6-6.5 6-6.5 6-6.5 : DTPA, kg / ts 2 1 * ·> β EDDS, kg / ts 2 1 * ... Citric acid kg / ts 1

Na-gluconate kg / ts 111 • * · · ', · · Peroxide bleaching j, l, ···, t, mm 60 6Q 6Q g0 6Q 60 T, C 70 70 70 70 70 70 »* · PQ 9? •; '15 15 15 15 15 15 pH_flku 10.5 10.5 10.5 10.5 10.5 10.5 8.5 8.5 8.5 8.5 8.5 8.5 2 2' 9 // tS 25 25 25 25 25 25: 3 ^ 1 '/ k9 ^ tS 25 25 25 25 25 25 JL' kg / ts 20 20 20 20 20 20

:. ·, DTPA

: · 'EDDS 2 21 1 ·; ··; Citric acid in kg / ts χ

Na-gluconate kg / ts! ± Residue -H, 0,, / v „Residual HO% P 6'5 11 (8 10'4 9 9'9 8'9 -12 2 '_____ 26 47,2 41,6 36 39,6 35 , 6 13 115641

After bleaching, residual peroxide was determined from the solutions. Residual peroxide levels in experiments using mixtures of chelating agents were comparable to those obtained with DTPA or EDDS. The bleaching performance of mixtures of chelating agents was clearly better than that of the comparative test in which no chelating agents were used.

Example 5 10

The pulp was bleached with hydrogen peroxide at 70 ° C, 15% moisture for 60 min. The results of the experiments are shown in Table 5.

15

Table 5.

Direct peroxide bleaching t, min 60 60 60 60 T, C 70 70 70 70 ': CS,% 15 15 15 15 ΐ i · · pH start 10.5 10.5 10.5 10.5 pH end 9 8, Δ 8.8 8.7; H 2 O 2, kg / ts 25 25 25 25

Water glass, kg / ts 25 25 25 25

NaOH, kg / ts 20 20 20 20 ... DTPA 2 1 * ♦ EDDS 2 1

Na-Gluconate kg / ts 11 L t / v: Residual H 2 O 2, kg / tp 7.6 4.4 6.9 5 L Residual H 2 O 2,% 30.4 17.6 27.6 20 »20 When used blends of chelating agents, the bleaching result was better than without chelating agents and almost the same as when using DTPA or EDDS alone.

Claims (13)

115641 A process for bleaching a mechanical pulp with a peroxide comprising treating the pulp with a chelating agent 5 to remove the adverse effects of heavy metals in the pulp, characterized in that the pulping is carried out with a mixture of nitrogen-containing chelating agent and lactic acid, citric acid, tartaric acid or gluconic acid. Process according to Claim 1, characterized in that the nitrogen-containing chelating agent is ethylenediamine / N, N'-dimeric succinic acid, its alkali or alkaline earth metal salt, 2,21-iminodimeric succinic acid, its alkaline or alkaline earth metal salt, ethylenediaminetetraacetic acid, its alkali or alkaline earth metal salt, or diethylenetriamine pentaacetic acid, its alkali or alkaline earth metal salt. Process according to claim 1 or 2, characterized in that the peroxide bleaching comprises alkaline peroxide bleaching and combinations thereof with oxygen, peracids or ozone. Method according to one of the preceding claims 1 to 3, characterized in that the chelating treatment is carried out either before and / or during peroxide bleaching. Process according to claim 1 or 4, characterized in that the pre-bleaching chelating pretreatment is carried out at a pH of 4-8, preferably at a pH of 5.0-7.5, and preferably at a pH of 6.5-7. , 5 Process according to claim 1 or 4 to 5, characterized in that the chelation prior to bleaching is carried out one or more times. 35 115641 Process according to claim 1 or 3, characterized in that the peroxide bleaching is carried out at pH 6-12, preferably at pH 7-11 and most preferably at pH 8-10.5. Process according to claim 1 or 4, characterized in that the peroxide bleaching is carried out one or more times. 9. A process according to claim 1.3 or 7-8, characterized in that the peroxide bleaching of the pulp is carried out simultaneously with peroxide and peracid. Process according to claim 3 or 7-9, characterized in that hydrogen peroxide, a mixture of hydrogen peroxide and oxygen gas, an organic per or per hydro compound is used in the peroxide bleaching of the pulp. Process according to claim 1, 3 or 7-10, characterized in that peracetic, permic, perpropionic or caronic acid or permethacetic, permicural, perpropionic, a combination of these. * · 1. f A method according to any one of claims 1-10. * * 25 characterized in that the chelating agent is used in an amount of 0.1 to 1. 5 kg / ton dry mass. * «· I i A method according to any one of claims 1 to 11, characterized in that milk, lemon, wine or; The gluconic acid is replaced by 20-75% of the nitrogen-containing coiling agent. 1 * t) I * I * »I *» f 1 · * »* 115641