WO1995035406A1

WO1995035406A1 - Delignification of chemical pulp with peroxide in the presence of a transition metal

Info

Publication number: WO1995035406A1
Application number: PCT/FI1995/000351
Authority: WO
Inventors: Jukka JÄKÄRÄ; Aarto Paren; Juha Patola
Original assignee: Kemira Chemicals Oy
Priority date: 1994-06-20
Filing date: 1995-06-19
Publication date: 1995-12-28
Also published as: FI942970A; FI942970A0; AU2739895A

Abstract

The invention relates to a process for the delignification of a chemical pulp, such as a sulfate or a sulfite pulp, in which process the pulp is treated with a peroxide in the presence of an activating Ti-, V- or Cr-group transition metal, such as molybdenum, vanadium or tungsten. The essential idea in the invention is that in the activated peroxide treatment the pulp is treated with a mixture of a peroxide and a peracid. The peroxide/peracid treatment may be part of a bleaching sequence which includes as potential other treatment steps, for example, a treatment with oxygen and a chelation for the removal of heavy metals, such as iron, manganse and/or copper.

Description

Delignification of chemical pulp with peroxide in the presence of a transition metal

The present invention relates to a process for the delignifica- tion of a chemical pulp, in which process the pulp is treated with a peroxide in the presence of an activating Ti-, V- or Cr- group transition metal. The said transition metals include Mo, V, Nb, Ta, Ti, Zr, Hf and W.

After the cooking, chemical pulp is brown, owing to residual lignin present in it. The pulp to be used for higher-grade papers is bleached after cooking in order to remove the lignin.

The bleaching chemical used has conventionally been chlorine, by means of which an effective bleaching is achieved and the quality of the paper obtained is high. However, owing to the environmental problems caused by chlorine, there has recently been to an increasing degree a shift to other bleaching chemi¬ cals, such as chlorine dioxide, oxygen, ozone, peroxides, and peracids. The overall objective has been to shift to bleaching which is completely free of chlorine chemicals in order to avoid the environmental hazards caused by chlorine chemicals, and chlorine residues in completed paper.

The bleaching process usually comprises a bleaching sequence made up of successive treatment steps, wherein oxidative steps which decompose lignin and alkaline washing steps alternate. By bleaching without chlorine chemicals, wherein the oxidants used are oxygen and alkaline peroxide, usually a pulp has been ob¬ tained which in its brightness, 83-87 % ISO, and in its strength is not of the level of pulp bleached with chlorine chemicals. When ozone has been used as the oxidant, a bright¬ ness above 88 % ISO has been achieved, but there has been the problem of the proneness of the process to disturbances. Thus there has been a need to find a system by means of which, with¬ out the use of chlorine chemicals, a fully bleached pulp stronger than previously and corresponding in quality to con¬ ventional pulps bleached with chlorine chemicals could be ob¬ tained through a process reliable in operation.

It is known that the delignification of chemical pulps can be promoted by treating the pulp with hydrogen peroxide in the presence of certain metals, such as Sn, Ti, V, W, Mo, Cr, Nb, Os and Se, or compounds thereof (1, 2, 3, 4, 5, 6, 7, 8).

Metal compounds which have been used in organic chemistry to activate hydrogen peroxide are listed in, for example, the book Catalytic Oxidations with Hydrogen Peroxide as Oxidant (G. Strukul, Kluwer Academic Publishers 1992), Chapter 1, "Intro¬ duction and Activation Principles," page 9.

In the said references, the above-mentioned metallic activators have been used mainly in the peroxide step after the cooking or after the oxygen step.

Also known is the increasing effect of peracids on the efficacy of delignification (9, 10, 11, 12).

Free peroxide in peracid delignification is quite ineffective as compared with peracid. In the presence of heavy metals (e.g. Fe, Mn, Cu), free peroxide is detrimental.

According to the present invention it has now been observed that the efficacy of peroxide delignification of a chemical pulp, activated with a Ti-, V- or Cr-group transition metal, can be increased by treating the pulp with a mixture of a per¬ oxide and a peracid. The invention increases the reactivity of free peroxide and thereby improves the efficacy and selectivity of delignification.

A peroxide suitable for the process according to the invention is hydrogen peroxide, and suitable peracids include peracetic acid, performic acid, and Caro's acid. The ratio of peroxide to peracid in the mixture may be approx. 20-90 molar %, preferably approx. 25-80 molar %, peroxide to approx. 80-10 molar %, pref¬ erably approx. 75-20 molar %, peracid.

The pH of the activated peroxide and peracid treatment may according to the invention be within the range 1-12, preferably 3-5.5, and the temperature within the range 30-120 °C, prefer¬ ably 80-100 °C.

The activating transition metal is according to the invention preferably molybdenum, which can be used as a suitable com¬ pound, for example as an Na molybdenate solution, which is fed into the pulp separate from the feed of the peroxide and per¬ acid. In the experiments, vanadium and tungsten were used in addition to molybdenum, with good results. It is, however, clear that any transition metals of the above-mentioned groups, known per se, which activate peroxide delignification, can be used in the invention.

Delignification according to the invention may be further pro¬ moted by means of a compound which contains silicon or phos¬ phorus, such as waterglass or phosphoric acid. The silicon or phosphorus compound used can advantageously be fed in the same alkaline solution as is the activating transition metal into the pulp being delignified. It is also possible to use a com¬ pound which contains simultaneously both the activating transi¬ tion metal, such as molybdenum, vanadium or tungsten, and the silicon or phosphorus. Silicomolybdenic acid type compounds can be mentioned as an example of such compounds.

In addition to the said silicon and phosphorus compounds, it is according to the invention possible to use in an activated peroxide and/or peracid treatment also other additives, such as acetic acid or other organic acids, which serve as a buffer to maintain the pH at the optimum level, as well as elements B, Ni, Co, Cr and Se or periodic acid, which in some cases in¬ crease the reactivity of the chemical combinations used.

Furthermore, it is preferable, before the peroxide and peracid treatment activated with a transition metal, to subject the pulp to be delignified to chelation for the removal of heavy metals, such as iron, manganese and/or copper, derived from the wood raw material. Thereby these heavy metals are prevented from catalyzing the decomposition of the peroxide and the per¬ acid, which would increase the consumption of these chemicals in bleaching. Suitable chelation chemicals include in par¬ ticular DTPA (diethylenetriaminepentaacetic acid), although other chelate-forming substances, such as EDTA (ethylene- diaminetetraacetic acid), DTMPA, organic acids, quaternary ammonium compounds, etc., are also possible.

The invention is suitable for all different chemical pulps, such as softwood and hardwood sulfate pulps, sulfite pulps, semialkaline pulps, and organosolv pulps such as alcohol pulps or milox.

The following examples include experiment series in which the effect of the various parameters of bleaching on the results obtained was investigated.

Example 1

A softwood sulfate pulp was subjected to a chelation pretreat- ment, a peroxide-promoted oxygen step (OP) , and further a sec¬ ond chelation pretreatment. In the first and second chelation pretreatments DTPA was used at a rate of 2 + 1 kg/one metric ton of pulp and in the OP step H₂0₂ at a rate of 10 kg/one metric ton of pulp. The kappa number of the obtained pulp was 8.0, brightness 60.5 % ISO, and viscosity 840 dπr/kg. The results of the delignification following the pretreatment are shown in Table 1. Table 1. Effect of the reaction conditions on activated peroxide/peracid delignification

As can be seen from the table, long reaction times (compare Ex¬ periments 1 and 2, 5 and 6, and 8 and 9) , a high temperature (compare Experiments 2 and 3, and 9 and 10), and a high consis¬ tency (compare Experiments 2 and 4, 6 and 7, and 9 and 11) are optimal for peroxide delignifications activated with molybdenum (or a corresponding metal).

Peracetic acid gives a better result than does performic acid, as can be observed by comparing experiments 1-4 and 8-11. Mo- activated peroxide/peracetic acid and peroxide/performic acid delignifications give better results than does Mo-activated peroxide delignification or peroxide/peracetic acid delig¬ nification without Mo activation, when the comparison is made using equivalent peroxide charges (compare Experiments 2, 6, 9 and 14, as well as 2, 6 and 12, 13.

Silicate increases the efficacy of peroxide/peracid delig¬ nification activated with Mo (or a corresponding metal), as can be observed by comparing Experiments 2 and 17.

A final pH of 4.9 gave a better result for Mo-activated peroxide/peracetic acid delignification than did pH values of 2.7 and 6.8. A final pH of 3-5.5 is the optimum for peroxide/- peracetic acid delignifications activated with Mo (or a cor¬ responding metal). The optimum pH is largely dependent on the peracid used and on the peroxide/peracid ratio.

The delignified pulps of Table 1 were chelated (1 kg DTPA/t) and washed before the subsequent alkaline peroxide bleaching (25 kg H₂0₂/t). Pulps which had after the activated peroxide/- peracid delignification a kappa number of 2.8 or lower became bleached to values of 88-90 % ISO with good pulp viscosity and strength properties. A better pulp quality (kappa, brightness, viscosity) after the delignification step was reflected in a better quality also in the final bleached pulp. Example 2

A softwood sulfate pulp was subjected to peroxide-promoted oxygen delignification (OP) and a chelation step (2 kg DTPA/one metric ton of pulp), The kappa number of the obtained pulp was 7.7, brightness 55.8 % ISO and viscosity 800 dm /kg. Table 2 shows the effect of and V metals on peroxide/peracid delig¬ nification.

Table 2

As can be seen from Table 2, W and V improve the efficacy of peroxide/peracid delignification (compare Experiments 1, 2 and 5). Silicate- and phosphorus modification (Experiments 3, 4 and 6) further improves the efficacy of metal-activated delignifi¬ cations.

The softwood pulps used in the examples of Tables 1 and 2 had been chelation-pretreated before the delignification experi¬ ment. The chelation pretreatment is not necessary, but by re¬ moving detrimental heavy metals, such as Fe, Mn and Cu, which decompose the bleaching chemical, it does improve the efficacy and selectivity of peroxide/peracetic acid delignification ac¬ tivated with Mo (or a corresponding metal).

As can be seen from Table 2, partial replacement of the per¬ oxide with a peracid improves W- and V-activated peroxide de¬ lignifications (compare Experiments 1 and 7, 2 and 8).

Example 3

A softwood sulfate pulp was subjected to chelation, an oxygen step and a second chelation step by using 1 kg of DTPA/one metric ton of pulp. The kappa number of the obtained pulp was

•a 7.7, brightness 55.8 % ISO, and viscosity 800 dm /kg. There¬ after, delignification was performed, the results of which are shown in accompanying Table 3.

Table 3

In Experiments 3, 4 and 5, some of the chemicals were added after 200 min, in connection with pH control (pH 5.2).

Molybdenum-activated peracetic acid/peroxide delignification gives a better final result than does molybdenum-activated peroxide delignification, as can be observed by comparing Ex¬ periments 1 and 2 in Table 3. A silicate-modified molybdenum- activated peracetic acid/peroxide delignification (Experiment No. 7) gives an even better result than do the above.

An increase of the temperature increases the efficacy of silicate-modified molybdenum-activated peracetic acid/peroxide delignification (compare Experiments 7 and 8). Example 4

A birch sulfate pulp was subjected to an oxygen delignification and to a chelation pretreatment by using 2 kg of DTPA/one metric ton of pulp. The kappa number of the obtained pulp was 10, brightness 52.7 % ISO, and viscosity 863 dm³/kg. The re¬ sults of Mo-activated peroxide and peroxide/peracid deligni¬ fications carried on this pulp are shown in Table 4.

Table 4

Q: 2 kg DTPA/t, 45 min, 70 °C, Cs 5 %, pH 5.5

EP: 25 kg H₂0₂/t, 210 min, 90 °C, Cs 12, final pH 10 As can be seen in Table 4, molybdenum-activated peroxide/- peracid delignification (Experiment No. 2) gives a better result than does molybdenum-activated peroxide delignification (Experiment No. 1). The brightness values after the subsequent alkaline peroxide step are also better than the reference.

In a bleaching sequence based on alkaline peroxide bleaching, the kappa number of bleached birch pulp often remains at a level of 3-4. By the processes described above, the kappa num¬ ber of birch sulfate pulp can be caused to drop lower, which means, among other things, reduced after-yellowing.

For an expert in the art it is clear that the various applica¬ tions of the invention are not limited to those presented above as examples; they may vary within the accompanying patent claims.

List of references

1. Latosh M.V. , Reznikov V.M., Alekseev A.D., "Method for oxidative delignification of plant raw materials," USSR pat. 699,064. Application filed on April 8, 1977.

2. Eckert R.C., "Delignification and bleaching process and solution for lignocellulosic pulp with peroxide in the presence of metal additives," CA pat. 1,129,161. Applica¬ tion filed on January 18, 1979.

3. Kempf A.W. , "Delignification and bleaching process and solution for lignocellulosic pulp with peroxide in the presence of metal additives," U.S. pat. 4,410,397. Appli¬ cation filed on December 24, 1980.

4. Kubelka V., Francis R.C., Dence C.W., "Delignification with acidic hydrogen peroxide activated by molybdate," Journal of Pulp and Paper Science: vol. 18, No. 3, May 1992, pp. J 108-114.

5. Weinstock I.A., Springer E.L.,.Minor J.L., Atalla R.H. , "Alternative pathways in non-chlorine bleaching," Non- chlorine bleaching conference, March 14-18, 1993. S. Caro¬ lina, USA.

6. Mounteer A.H. , Colodette J.L., Gomide J.L., Campos A.S., "Alternativas para branquamento sem cloro molecular," 0 Papel 53, No. 4, April 1992, pp. 25-35.

7. Sundman G.I.,"Ph.D. Dissertation, SUNY College Environment Science and Forestry, Syracuse, USA, 1988.

8. Ow S.S., Singh R.P., "Method of bleaching lignocellulosic material with peroxide catalyzed with a salt of a metal," U.S. pat. 4,661,205. Application filed on August 28, 1981. 9. Johnson D.C., "Lignin reactions in delignification with peroxyacetic acid," Symposium "Chemistry of Delignification with Oxygen, Ozone and Peroxides," Raleigh, North Carolina, May 27-29, 1975. Uni Publishers Co., LTD., Tokyo, Japan, 1980, pp. 217-228.

10. Devenyns J., Desprez F., Troughton N. , "Peracetic acid as a selective prebleaching agent: an effective option for the production of fully bleached TCF kraft pulps," Non Chlorine Bleaching Conference, HHI, South Carolina, USA, 1993, Ses¬ sion VIII papers: New Developments in Non-chlorine bleach¬ ing, 35.

11. Springer E.L., McSweeny J.D. , "Treatment of softwood kraft pulps with peroxymonosulfate prior to oxygen delignifica¬ tion," Tappi Pulping Conference, Boston, MA, USA, 1992, pp. 537-545.

12. Poppius K. , Hortling B., Sundqvist J., "Chlorine-free bleaching of chemical pulps - The potential of organic peroxyacids," Tappi Int. Symp. of Wood and Pulping Chem¬ istry, May 22-25, 1989, Raleigh, N.C., USA, pp. 145-150.

Claims

1. A process for the delignification of a chemical pulp, in which process the pulp is treated with a peroxide in the presence of an activating Ti-, V- or Cr-group transition metal, characterized in that the pulp is treated with a mixture of a peroxide and a peracid.

2. A process according to Claim 1, characterized in that the peroxide is hydrogen peroxide.

3. A process according to Claim 1 or 2, characterized in that the peracid is performic acid or peracetic acid.

4. A process according to any of the above claims, charac¬ terized in that the ratio of peroxide to peracid in the mixture is approx. 20-90 molar % peroxide to approx. 80-10 molar % peracid.

5. A process according to any of the above claims, charac¬ terized in that the activating metal is Mo, V, or W.

6. A process according to any of the above claims, charac¬ terized in that in the said activated peroxide/peracid treat¬ ment there is additionally present a silicon or phosphorus compound.

7. A process according to any of the above claims, charac¬ terized in that the pH of the treatment is within the range 1- 12, preferably 3-5.5, and the temperature is within the range 30-120 °C, preferably 80-100 °C.

8. A process according to any of the above claims, charac¬ terized in that before the above-mentioned peroxide/peracid treatment the pulp is chelated for the removal of heavy metals, such as Fe, Mn and/or Cu, derived from the wood raw material.

9. A process according to Claim 8, characterized in that the chelation chemical is DTPA.