FI115470B - Process for the treatment of chemical cellulose material - Google Patents

Description

115470

The present invention relates to a process for bleaching or delignifying a chemical cellulosic material, wherein the pulp 5 is treated with a chelating agent to eliminate the adverse effects of the heavy metals in the pulp.

Chemical pulp is increasingly bleached using bleaching techniques that do not use elemental chlorine or chlorine compounds. Em. bleaching is called ECF (elementary chlorine free) bleaching and totally chlorine-free bleaching is called TCF (total chlorine free) bleaching. Especially, TCF bleaching is usually preceded by oxygen ligation. After oxygen delignification, the pulp can be bleached with chlorine-free chemicals such as ozone, hydrogen peroxide in acidic or alkaline solutions under non-pressurized or pressurized conditions. Useful bleaching methods also include bleaching with peroxy compounds (such as peracetic acid, Caronic acid, or mixtures of peracids), an oxygen peroxide enhanced step, and a peroxide enhanced: oxygenated time step.

»:» · · I «· ·: Such bleaching steps are usually preceded by bonding of heavy metals. Metals can be removed by acid washing. This; 25 is often disadvantageous because at least some of the following steps | The t incremental steps are performed under alkaline conditions. If <· · heavy metals are removed at low pH, ··· # acid must be used first to achieve low pH and then !!! in an alkali to neutralize the acid. In addition, Mg and Ca a-. * Ions, which are preferred for bleaching, are removed in the acid wash. Acid washing can also reduce the strength of the pulp.

* »; ·. ·. Peroxy compounds such as peracetic acid and hydrogen peroxide are highly susceptible to the catalytic effects of heavy metals. 35 Applicant FI Patent Applications 94 2968, 94 2969 and 94 2970 disclose transition metal activated bleaching with peroxy compounds in acidic conditions. For successful bleaching, heavy metals must be bound before and / or during bleaching.

Chelating agents for metal ions, for example polyaminocarboxylic acids, are used for binding heavy metals by bleaching with peroxy compounds. These include, in particular, ethylene diamine tetraacetic acid and its salts (EDTA) and diethylene triamine pentaacetic acid and its salts (DTPA). The most harmful ions for bleaching 10 are manganese (Mn), iron (Fe) and copper (Cu). Other heavy metals such as chromium ions (Cr), etc. also adversely affect both the consumption of peroxy compounds and often the bleaching effect by reducing e.g. the viscosity of the bleached pulp. Harmful heavy metals 15 are derived from pulp, treatment water and pulp treatment equipment.

In particular, chelation of iron and manganese is advantageous for bleaching. Conversely, chelation of alkaline earth metals such as calcium and magnesium is a disadvantage for bleaching.

»· • ·» · »·

In chemical pulp bleaching, chelation cannot generally be performed under alkaline conditions, since iron is then precipitated; 25 is purchased in the form of hydroxides, oxides, and oxyhydroxides which can •. *: *. acutely catalyze the decomposition of peroxy compounds. As a result, the chelation prior to peroxy bleaching is carried out under acidic conditions. Here, in particular, the removal of manganese has proved to be important because it does not form as stable 'I * 30 complexes as iron, and because especially in the chemical pulp, large amounts of manganese are often present. In P. · Bryant and L.L. Edwards, Tappi Pulping Conference 1993, pp. 43-55, "Manganese removal in closed kraft mill bleach II >>; plants" states that manganese can best be removed at 35 pH 4.5-6.5 before manganese binds to the pulp. too strongly.

3, 115470

In modern chemical pulp bleaching processes, oxygen bleaching precedes bleaching. In this process, magnesium ions have been added to the pulp, which retention in the pulp is important for bleaching with peroxy compounds, e.g. hydrogen peroxide. According to Ar-5, the best Mg / Mn ratio is achieved at pH 4.5-5.0. Therefore, in practice, chelation must be carried out at well below pH 6, e.g. near pH 5.5. In the examples of the above-mentioned reference, chelation has been carried out at pH 5.3.

10

Powerful chelating agents are often poorly degraded in nature, such as DTPA or are completely non-degradable, such as EDTA. The increase in TCF bleaching has use of chelating agents. As a result, there has been an interest in replacing poorly biodegradable chelating agents with partially or totally biodegradable chelating agents. In addition, in order to avoid environmental stress, it would be desirable for chelating agents which are biodegradable to contain as little nitrogen as possible.

In the bleaching and delignification process of the invention, j can be used as a chelating agent e.g. biodegradable ethylenediamine dimeric succinic acid (EDDS) or 2,2'-iminodimeric succinic acid (ISA) and their alkali metal salts;

Applicants have surprisingly found that an equally good bleaching effect is achieved when some of the aforementioned nitrogen-containing chelating agents are replaced by biodegradable, nitrogen-free chelating agents such as hydroxycarboxylic acids.

v · Thus, e.g., commonly used effective chelating agents such as aminopolycarboxylic acid, e.g.

DTPA and EDTA, can be partially replaced by nitrogen-free biodegradable compounds.

4, 115470

It is quite surprising that the above hydroxycarboxylic acids can be successfully used as chelating agents in bleaching together with nitrogen-containing chelating agents. Hydroxycarboxylic acids do not readily bind heavy metals to te-5. Instead, they chelate well calcium and magnesium. In particular, citric acid has been used instead of phosphates in phosphate-free detergents, which must bind calcium and magnesium. The usefulness of the chelating method according to the invention is based specifically on the interaction of different types of chelating agents.

When DTPA is used as a chelating agent prior to oxygen bleaching or delignification, the most preferred pH for chelation is about 5 to 5.5. When replacing the amino-polycarboxylic acids of the above-mentioned.

With the aid of hydroxycarboxylic acids, the chelation may 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.

The process of the invention is advantageous because the nitrogen-containing chelating agent containing Si-I can be partially replaced by a non-nitrogen-containing biodegradable chelating agent. Most often, pulp: cooking is carried out under time conditions. When using chelating agents of the invention using chelating agents

• i I

; 25 may be carried out at a higher pH, less acid is required to achieve a pH which is favorable for chelation. Using the chelating method described in the invention, ···. also less alkali to raise the pH to the norm !! painted the chelate to the next alkaline peroxide step * 30.

Characteristic features of the invention will be apparent from the appended claims. exam requirements.

* «5 115470

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 and gluconic acid.

5 The harmful nitrogen load in the wastewater can thus be reduced.

The process of the invention can be applied to all known chemical pulps. These include alkaline and neutral sulphite pulps, soda pulps, sulphate pulps (kraft pulps), and oxygen delignified (oxygen soap) sulphate pulps. In addition, the method can be used for bleaching so-called. organosolve pulps containing alcohols or organic acids as a cooking solvent, e.g. Milox soup in ants-15. The chelating method according to the invention can also be used when polysulfides or, for example, anthraquinone, are used as adjuvants in the sulfate cooking.

The treatment may be carried out on a variety of fibrous raw materials, such as softwood, hardwood or strip, straw, or other vegetable raw material for cooked pulp.

The chelating method according to the invention can be used in val; : when banding or delignifying the pulp under acidic conditions and / or pretreatment prior to the process described above. steps.

, ···. Adjustment of the pH of the acid chelation step can be accomplished with standard mineral acids such as sulfuric acid, sulfur dioxide, 4'- '' or its aqueous solution, carbon dioxide or organic *. "with acids such as formic and acetic acid.

J. ·. The nitrogen-containing chelating agent used in the process of the invention may be ethylenediamine-Ν, ′ -dimeric succinic acid (EDDS), its various isomers or its alkali metal salts such as sodium and potassium salts or its alkaline earth metal salts, such as calcium or magnesium salts. , 2,21-iminimeric 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 include polyethylenediaminetetraacetic acid (EDTA) or its salts, diethylenetriaminepentaacetic acid (DTPA) or its salts of polyaminocarboxylic acids.

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

Example 1.

To determine the chelation of heavy metals and alkaline earth metals, the oxygen-delignified chemical pulp was washed with aqueous solutions of DTPA and sodium salt of citric acid. The metal concentrations of the wash solution were 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-20 to the washing solutions is advantageous for bleaching. On the other hand, the transfer of calcium and magnesium to the washing liquids is disadvantageous for bleaching. In control experiments, the pulp was washed with DTPA solutions. Chelating agent concentrations and pH during washing are reported in Table 1.

: 25 • ♦ · 7 115470

table 1

Coniferous sulphate pulp Chelating conditions

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

Brightness 39.6% ISO consistency% 12

Chelate Dose, Filtrate metal concentrations in kg / ts pH Fe, ppm Mn, ppm Mg, ppm Ca, ppm

NasDTPA 1 6,71,2 2,9 4 17

Na5DTPA 2 6.5 2.0 2.8 17 48

NasDTPA 2 5,72,0 3,3 13 52

Na4EDTA 2 6,51,8 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 + 15, 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: “HjDTPA + Na3 Citrate 1 + 1 8.6 0.3 2.7 5 24

In Table 1, Na5DTPA represents the pentanedium salt of DTPA, '·' 10 Na4EDTA represents the tetrasodium salt of EDTA and the acid form of H5DTPA DTPA. However, the pH used determines how the chelating agents are dissociated, i.e. in what form they actually occur in the treatment.

115470 DTPA is generally dosed during the chelating step of softwood pulp at about 2 kg / ts. The chelation was clearly less when the DTPA dosage was reduced from 2.0 kg / ts to 1.0 kg / ts at pH 6.5.

5

The use of citric acid sodium salt (Na citrate) as a chelating agent in addition to DTPA significantly reduced the dosage of DTPA. Although the dosage of DTPA was reduced to 1.0 kg / ts, the chelation of the metals in these experiments was as complete as 2.0 kg / ts of DTPA alone. Chelation with a mixture of DTPA and Na citrate gave a better chelating result at pH 7.0 than with DTPA. It should be noted that even at pH 8.6, the manganese coiled very well. At such a high pH, iron has already been purchased for sa-15.

It can be seen that water washing is ineffective with respect to metal chelation. Similarly, Na citrate alone does not remove heavy metals. Na citrate only chelates alkaline earth alkali metals, which is undesirable for bleaching. At a dose of 1 kg / ts of DTPA, Sa- '* · was not sufficient to obtain a satisfactory * *: chelating result. These comparative experiments show that a good chelating result is achieved by the combined effect of DTPA and: Na citrate.

: 25 * »

Example 2. ··. Table 2 shows the results of washing experiments similar to those described in Example 1 when Na-citrate or Na-gluconate was used as chelating agent together with EDDS or ISA at pH 5.8-8.9. As a control, chelation was performed on the same mass of DTPA at different dosages at pH 6.5.

• ·> · 9 115470

Table 2

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, Filtrate metal concentrations in 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

NasDTPA 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. H4EDD S + Na Gluconate 1 + 1 6.7 1.8 2.6 9 29

Na4ISA 1.5 5.8 1.1 1.9 2 50: ·: · 'Na4ISA 1.5 8.9 0.0 1.3 25 26' 1 · Na4ISA + Na citrate 1 + 1 7.7 0 , 5 2.2 15 26 »9 • · · 10 When the DTPA dosage was reduced from 2.0 kg / ts, heavy metal chelation was incomplete. A coil · ... · conversion result comparable to the best chelating result achieved with DTPA was achieved when EDDS was used at 1.5 kg / ts. While good chelation results were obtained, even though the EDDS an- 15 increments were reduced by up to half of this, with the use of Na-citrate or Na-gluconate 1.0 kg / ts. Similarly, the dosage of ISA could be reduced without significantly worsening 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.

10 115470

The washing results cannot be directly deduced from the bleaching results. Therefore, the effect of different chelating methods on the quality of the bleached pulp was investigated.

Example 3

The oxygen delignified pulp was chelated and bleached with alkaline hydrogen peroxide. DTPA was used alone or in combination with Na citrate or Na gluconate as chelating agents. The results are summarized in Table 3. The bleaching result can be estimated from the peroxide consumption, the brightness achieved and the viscosity of the pulp. 1 * »« · 115470 ^ ro oi co LT) LO LT) O V - LD 00 ^ - 00

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12 115470

When no chelating agent was used or when the chelating agent was performed with Na citrate alone or at a lower dosage of DTPA (1.0 kg / ts), the pulp viscosity after bleaching was low. Similarly, peroxide consumption was high.

In the comparative test, the pulp was chelated with DTPA (2.0 kg / ts).

Bleaching results comparable to this experiment were achieved after chelation with DTPA and Na citrate, although the dosage of DTPA was reduced to 0.5 kg / ts. Similarly, excellent bleaching results were obtained for bleaching pulps subjected to chelation with Na-gluconate. When the pulps were chelated with mixtures of the above chelating agents, the viscosities of the pulps after bleaching remained pa-15 higher than in the control. In many experiments, the consumption of hydrogen peroxide in bleaching was also lower than in the comparative experiment.

• · * * • * «* '* *»

Claims (13)

A process for bleaching or delignifying chemical cellulose material with oxygen chemicals, in which the cellulose material is treated with a chelating agent to eliminate the deleterious effects of the heavy metals present in the pulp, characterized in that the chelating of the cellulosic material takes place with a mixture of contains nitrogen, and lactic acid, citric acid, tartaric acid or glycolic acid. 2. A process according to claim 1, characterized in that the chelating agent containing nitrogen is ethylenediamine-N, N'-dibaric acid, its alkali or alkaline earth metal salt, 2,21-iminodibaric acid, its alkali or alkaline earth metal salt, ethylenediamine diamine salt. , its alkali or alkaline earth metal salt or diethylenetriamine pentacetic acid, its alkali or alkaline earth metal salt. 20 3. A process according to claim 1 or 2, characterized in that the oxygen chemical bleaching or • 2 · delignification comprises bleaching with peracetic acid, with a blend with peroxides as a seed or as blends either alone or as a blend. mixture with peracids within the acidic pH range of transition metal catalyzed hydrogen peroxide bleaching, alkaline peroxide bleaching and combinations of all of these, and oxygen delignification, one peroxide enhanced acid alkaline phase and pressurized peroxide stage: and to these hearing any possible treatments with enzymes, ozone or chlorine dioxide. »» 2 f I Process according to any of the preceding claims 1-3, characterized in that the chelation treatment is carried out: either before and / or in connection with the oxygen chemical bleaching or delignification. 17 115470 Process according to claim 1 or 3-4, characterized in that the chelation treatment prior to bleaching or delignification takes place at a pH of 4-8, preferably at 5.0-7.5 and most preferably at 6.5-7.5. 5 Process according to claim 1 or 3-4, characterized in that the oxygen chemical bleaching or delignification takes place at a pH of 4-8, preferably at 4-6 and most advantageously at 4-5. 10 Process according to claim 1 or 4-5, characterized in that the chelation, which takes place before the bleaching or delignification, is done one or more times. 15 Method according to claims 1.3-4 or 6, characterized in that the oxygen chemical bleaching or delignification is done one or more times. 9. A process according to claims 1,3-4,6 or 8 characterized in that the oxygen chemistry, potassium bleaching or delignification of the cellulose pulp is done simultaneously with peroxide and peracid. Method according to claims 1.3-4.6 or 8-9 * 25 characterized in that the peroxide, peroxide treatment of the cellulose pulp is made with hydrogen peroxide, a mixture of hydrogen peroxide and oxygen gas, organic per or perhydro compounds. A method according to claims 1.3-4.6 or 8-9 characterized in that the peracidic treatment of the cellulose pulp is done with peracetic, permyric, perpropionic or ca- '; ronic acid or with transition metals activated peracetic, permyric, perpropionic or caronic acid or combinations of I I:. these. * 1 35 18 115470 Process according to any of claims 1-11 characterized in that chelating agent is used 0.1-5 kg / ton dry cellulose material. 5 Process according to any of claims 1-12, characterized in that a chelating agent containing nitrogen is replaced to 20-75% with lactic, tartaric or glyconic acid. »1 · •>: ·»! · »·> V 1 t *» * · * * »» * »•»