EP0426652B1 - Process for the chlorine-free bleaching of pulp - Google Patents

Description

The invention relates to a method for bleaching cellulose, in particular cellulose for the production of synthetic fibers, e.g. B. hardwood pulps, with initial cap values of 15-1, preferably 4-1, or of pulps for the production of paper, for example softwood pulps, with initial cap values up to 30, preferably up to 10, by means of ozone, in which a pulp suspension at a temperature of 15-80 ° C, preferably 40-70 ° C, and at a pH of 1-8, preferably 2-3, with an ozone-containing gas with vigorous stirring or mixing in contact, the ozone-containing gas 20-300g / m ³ , preferably 50-150 g / m ³ , contains ozone and at most 2% by mass, preferably 0.05-0.5% by mass, of ozone, based on dry cellulose, is used.

Numerous patents have already been granted with regard to processes for chlorine-free, ie environmentally friendly bleaching of pulp, which can be processed into paper or fibers, by means of ozone; they differ primarily in terms of consistency and reaction conditions.

In principle, two different process technologies are dropaged: the high consistency (HC) and the low consistency (LC) technology.

HC ozone bleaching is carried out at consistencies above 25%, generally at 35 - 40%.

Since ozone bleaching is normally not used alone, but in combination with other bleaching stages and conventional bleaching is hardly used in such high density ranges, expensive drainage systems have to be invested. The reaction of ozone with the pulp is a two-phase reaction that takes place quickly and completely.

In addition to the investment costs for the drainage systems, the inhomogeneous and cellulose-damaging ozone attack in the HC area is also disadvantageous, especially with low kappa starting values. (With regard to the meaning and definition of kappa, reference is made to US Pat. No. 4,229,252, column 2.) It is therefore even advised in the literature to carry out HC ozone bleaching at kappa values below 10. (Lindholm C.-A. "Effect of pulp consistency and pH in ozonbleaching", Part 4, Paperi ja Puu - Paper and Ti mber 2/1989; Lindholm CA "Effect of pulp consistency and pH in ozonbleaching", Part 2, 1987 Int. Oxygen Delignification Conference, San Diego, June 7-11, 1987, Proceedings, p. 155; Lindholm C.-A. "Effect of pulp consistency and pH in ozonbleaching", Part 3, Nordic Pulp and Paper Research Journal, No . 1/1988.)

The cellulose damage becomes even worse if the cellulose has already been bleached with oxygen before HC ozone bleaching.

According to the state of the art, only LC ozone bleaching is an alternative if you want to avoid the use of chlorine-containing, environmentally harmful compounds. Compared to HC ozone bleaching, however, more ozone is used, the procedure is more complicated, and a lot of mixing energy is required. Furthermore, the reaction volumes are larger and the amount of contaminants is higher.

Low consistency (LC) in cellulose usually means consistencies of up to 5 or 6%. With ozone bleaching, however, there is a unanimous opinion that usable results can only be achieved with material densities of up to 1%, at most 2%.

For example, only a consistency of up to 0.7% is claimed in US-A-4,216,054. However, such a material density range means high demands on a closed water balance and thus additional investments. In this patent, the LC technology is systematically examined for kraft pulps and it is found that the reaction of the ozone with the pulp is inhibited by two barriers: the transition of the ozone from the gas phase to the liquid phase and the transition from the liquid phase to the solid Phase, ie to the fiber. From a minimum mixing power of 11 kW / m ³ , according to this patent specification only the second transition is decisive.

A method of the type mentioned is known from US-A-4 080 249. The stirring energy is preferably at most 18 kWh / t cellulose suspension. The size of the bubbles of the ozone-containing gas should not exceed 3 mm. In this patent, only material densities between 1 and 2% are described in all examples, so it is clearly an LC method. However, this patent claims - obviously to avoid circumvention - a material density range of up to 10%; according to claim 6, however, fabric densities of less than 3% are preferred; Obviously, only unsatisfactory results could be achieved at higher values.

The same also applies to US-A4 372 812. Here, ozone bleaching is generally referred to as a consistency between 1 and 40%. In the exemplary embodiment, however, work is again only carried out in the LC range, namely with a material density of 1% (see Table 1 of this patent specification). As such, this patent is concerned with a multi-stage bleaching process in which ozone is used in one or more stages, and not with ozone bleaching itself.

The object of the invention is to use the method mentioned at the beginning in the medium consistency range (MC), and of course good results are to be achieved in comparison with LC technology len.

This object is achieved according to the invention by a method of the type mentioned at the outset in that the pulp suspension has a consistency of 3-20% by mass, preferably 5-20% by mass, in particular 7-15% by mass, and in that the ozone-containing gas contains a Pressure of 1.1-15 bar, preferably 1.1-10 bar, is introduced into the pulp suspension.

The medium consistency range offers the advantage that the reaction vessels can be made smaller in comparison to the LC technology, but that on the other hand no expensive dewatering devices are necessary as with the HC technology. By applying pressure with vigorous stirring or mixing at the same time, surprisingly good results are now also achieved in the MC range: there is a homogeneous and gentle, but nevertheless efficient reaction of the pulp with ozone. The required mixing energy is lower than with LC. The reaction of the ozone with the pulp is more homogeneous than with HC. The cellulose damage - measured by the viscosity and the DP distribution - even at very low kappa values - is significantly less than with HC and at least comparable to LC. The specific ozone consumption (0 ₃ - consumption per eliminated kappa point) is significantly lower than with LC. Existing systems are relatively easy to convert: apart from pH-controlled acidification (which would also be required for LC and HC), only the investment of an MC pump and an MC mixer is required. Waste water recirculation and reuse of the reaction waste gases with a residual ozone content is possible, so that the process works in an environmentally friendly manner. Overall, the process is very economical (required mixing energy, amount of ozone used and necessary devices).

A further advantage is obtained if the process according to the invention is used as an ozone step in a multistage bleaching process. Because most stages that do not use ozone bleaching (e.g. oxygen bleaching) mostly work in the MC range, the pulp consistency no longer needs to be changed between the individual stages, so that the overall process becomes more economical.

It is already known from AT-A-380 496 to carry out the ozone bleaching with pressure. In this process, the pulp suspension in the LC range (2.5 - 4.5% consistency) is brought into intensive contact with an ozone-containing gas under pressure (in the exemplary embodiment 4 bar). The pulp is then dewatered to 10-30% consistency, keeping it at the same pressure and temperature both during dewatering and for at least 20 minutes afterwards. According to this patent specification, a subsequent reaction then takes place, but this is due to the intimate contact of the LC pulp with the ozone-containing gas (see page 3, lines 41-45 of the patent specification). In contrast, it has been found according to the present invention that MC pulp can also be successfully treated directly with an ozone-containing gas, provided that this gas is under pressure and at the same time is stirred or mixed vigorously. This eliminates the need to dilute and dewater the pulp suspension as provided for in AT-A-380 496 (see page 3, lines 19-20 and 35-36).

In order to achieve optimal results, it is expedient if the volume ratio gas: liquid is 1: 0.5-1: 8, preferably 1: 1-1: 6.

Cooled compressors, preferably water ring pumps, are preferably used to compress the ozone-containing gas.

A high-shear mixer is preferably used for vigorous stirring or mixing.

High-shear mixers are known and are currently used for a wide variety of applications, for example for the production of dispersions for paints (DE-A 24 06 430), for the production of PVC resin powder (US-A-3 775 359) and for the preparation of semi-solid emulsions (US-A-3 635 834). It is also known to use them for cellulose suspensions (JP-A 6 309 9389).

A high-shear mixer has plates with elevations that are spaced a certain distance apart. This means that there is no grinding, but an intimate mixing.

It is expedient if the process is carried out several times in succession, an alkaline extraction being carried out in between if necessary. The alkaline extraction can be carried out using oxygen or peroxide. In practice, this multiple procedure can be ensured simply by removing part of the pulp behind the reactor and feeding it back to the high-shear mixer, so that a cycle results.

It is preferred that the process be carried out after an oxygen and / or peroxide enhanced extraction, optionally followed by an alkaline peroxide stage, and / or after that a peroxide stage or an alkaline extraction follows. Oxygen can also be used in the peroxide stage.

Finally, it is also expedient if the wastewater filtrate obtained in the treatment with ozone is at least partially supplied to the cellulose suspension before it is brought into contact with the ozone-containing gas, whereby together with the wastewater filtrate also the acid required for the desired pH, in particular Sulfuric acid. Since the wastewater filtrate is acidic, acid can be saved in this way; at the same time the wastewater filtrate is put to sensible use, so that it does not have to be disposed of or discharged in an environmentally harmful way.

If softwood pulps with initial kappa values up to 30 or up to 10 are subjected to the process according to the invention, kappa values below 10 or below 5 are achieved.

If hardwood pulps with initial kappa values of 15 to 1 or from 4 to 1 are subjected to the process according to the invention, kappa values of 12 to 0.5 or 1.5 to 0.5 are achieved. An initial whiteness of 50-80% or 70-80% is increased to at least 65-90% or 75-90%.

In the process according to the invention, it is advantageous that the molecular weight distribution of synthetic fiber pulps can be determined. In addition, the viscosity, DP distribution and reactivity, measured by the filter value, required for the further use of the cellulose can be obtained by varying the pH value, the amount of ozone used and the temperature.

The invention is explained in more detail with reference to FIGS. 1a and 1b. The figures show systems which are suitable for carrying out the method according to the invention.

1 denotes the cellulose feed for ozone bleaching from one of the preceding bleaching stages, such as oxygen and / or peroxide bleaching, with subsequent alkaline extraction. Acidic wastewater filtrate 14, which is obtained at the end of the ozone bleaching stage, is added to the pulp suspension in order to adjust the consistency and to use the residual acid content. At the same time, a pH-regulating acid addition 2 is carried out, which determines the pH. The pulp suspension is conveyed by an MC pump 3 into an MC mixer 4, which is preferably a high-shear mixer.

Gas 7 containing ozone is compressed in a compressor 8 and introduced into the MC mixer 4 under pressure. An intimate, rapid mixing of the suspension and the ozone-containing gas 7 takes place in the MC mixer 4.

The reaction is continued under pressure in a reactor which is designed as a reaction tube 5. At the end of the reaction tube 5, a return 9 (in the form of a tube and a pump) of the reaction mixture is provided in order, if necessary, to subject the pulp suspension several times to the process according to the invention.

1a and 1b, the integration of the fumigated stock suspension in conventional bleaching towers 10 is shown. According to the invention, this is not absolutely necessary. A distinction is made between upward and downward tower. In the upward tower (Fig. 1a), the pressurized gas stock suspension with or without throttle 6 is conveyed into the bleaching tower 10, where a subsequent reaction can still take place. The pressure has been released in the area of the material discharge, and the exhaust gas is discharged via an exhaust gas disposal 11. The relaxed pulp suspension is mixed with dilution water 12 and discharged from the bleaching tower 10 to the washing filter 13. The resulting waste water filtrate 14 is fed to the pulp feed 1 via a waste water return line 15.

When using a downward tower (FIG. 1b), the fumigated stock suspension coming from the reaction tube 5 is passed over the throttle 6 and then expanded to atmospheric pressure in a degassing device 16. The material discharge into the bleaching tower 10 also takes place by adding dilution water 12. The low-ozone exhaust gas is also discharged via an exhaust gas system, e.g. a catalytic or thermal ozone depletion device.

The oxygen obtained in the waste gas disposal 11 can be used in an oxygen bleaching stage and the excess oxygen can be returned to the ozone generator after corresponding cleaning steps. If the oxygen is not used in an oxygen bleaching stage, it can be completely recycled after the necessary cleaning steps.

Waste water and exhaust gas recirculation can also save a lot of energy, especially at high process temperatures.

The residence time of the pulp suspension in the reaction tube 5 and in the bleach tube 10 is in any case less than 3 h, normally less than 1 h, preferably even less than 5 min.

In the following examples, a beech synthetic fiber pulp or spruce paper pulp is subjected to ozone bleaching according to the invention after a conventional oxygen extraction reinforced with peroxide.

Er wird einer Ozonbleiche mit folgenden Parametern unterworfen:

Drehzahl des high-shear-Mischers : 1700/min Der gebleichte Zellstoff weist folgende Kennzahlen auf:

EXAMPLE 1: After the peroxide-enhanced oxygen extraction (EOP stage), the pulp has the following characteristics:

It is subjected to ozone bleaching with the following parameters:

Speed of the high-shear mixer: 1700 / min The bleached pulp has the following key figures:

Drehzahl des high-shear-Mischers : 3,200/min Der gebleichte Zellstoff weist folgende Kennzahlen auf:

EXAMPLE 2: The pulp has the same characteristics as in Example 1 with the following exception: unwashed kappa: 2.9 It is subjected to ozone bleaching with the following parameters:

Speed of the high-shear mixer: 3,200 / min The bleached pulp has the following key figures:

Es wird einer Ozonbleiche mit folgenden Parametern unterworfen:

Drehzahl des high-shear-Mischers : 3200/min Der gebleichte Zellstoff weist folgende Kennzahlen auf:EXAMPLE 3: The pulp has the following key figures:

It is subjected to ozone bleaching with the following parameters:

Speed of the high-shear mixer: 3200 / min. The bleached pulp has the following key figures:

Drehzahl des high-shear-Mischers : 3200/min Der gebleichte Zellstoff weist folgende Kennzahlen auf:

Die Unterschiede in den Stoffeigenschaften zwischen Beispiel 3 und 4 sind ausschließlich auf den geänderten pH-Wert und die geänderte Temperatur zurückzuführen. Der pH-Wert ist also zur Viskositätseinstellung geeignet.EXAMPLE 4 The same pulp as in Example 3 is subjected to ozone bleaching with the following parameters:

Speed of the high-shear mixer: 3200 / min. The bleached pulp has the following key figures:

The differences in the material properties between Examples 3 and 4 are exclusively due to the changed pH value and the changed temperature. The pH value is therefore suitable for adjusting the viscosity.

The following Examples 5 and 6 relate to spruce paper sulfite pulps. The following test standards for substance parameters were used:

EXAMPLE 5

BleicheThe raw material had the following key figures:

Bleach

The bleaching was carried out using the sequence: EOP-Z ₁ -PE ₁ -Z ₂ -PE ₂ (EOP = alkaline oxygen treatment reinforced with peroxide; Z = ozone treatment; PE = alkaline peroxide treatment).

a) The EOP stage for a pulp batch was carried out in an MC mixer according to the following conditions:

Dabei wurden folgende Zellstoffparameter erhalten:

Mit diesem EOP-vorgebleichten Stoff wurde die restliche Sequenz Z₁-PE₁-Z₂-PE₂ auf drei verschiedene Arten V₁, V₂, V₃ durchgeführt.

The following pulp parameters were obtained:

The remaining sequence Z ₁ -PE ₁ -Z ₂ -PE _{2 was} carried out with this EOP-pre-bleached substance in three different ways V ₁ , V ₂ , V ₃ .

b) Ozone level - 1 (Z ₁ )

The parameters of the first ozone bleaching and the characteristics of the pulp after the first ozone bleaching were as follows:

c) PE _i level

The parameters of the first alkaline peroxide treatment and the characteristics of the pulp after the treatment were as follows:

d) ozone level - 2 (Z ₂ )

The parameters of the second ozone bleaching and the characteristics of the pulp after the second ozone bleaching were as follows:

e) PE ₂ level

The parameters of the second alkaline peroxide treatment and the characteristics of the pulp after the treatment were as follows:

Despite the exceptionally high degree of whiteness (> 90%) and the low viscosity values, the strength values correspond to those of the standard bleached pulps. ("Standard bleaching" means the sequence C-PE-H-H. "C" means chlorine bleach and "H" hypochlorite bleach.)

EXAMPLE 6

1. a) Die EOP-Bleiche erfolgte wie bei Beispiel 5.
2. b) Ozonbleiche (Z)

The same raw material as in Example 5 (spruce paper sulfite pulp) was subjected to the bleaching sequence EOP-Z-PE for lower demands on the degree of whiteness. The conditions (V4, V5) of the final bleaching (PE) were varied with the aim of achieving whiteness levels greater than 85% with the highest possible strength values.

1. a) The EOP bleaching was carried out as in Example 5.
2. b) ozone bleaching (Z)

The parameters of ozone bleaching and the characteristics of the pulp after ozone bleaching were as follows:

c) PE level

The parameters of the alkaline peroxide treatment and the characteristics of the pulp after the treatment were as follows:

The strength values of the three-stage bleached cellulose essentially correspond to that of the five-stage bleached cellulose. This is an indication that the strength properties of the pulps are not impaired when sequentially using small specific amounts of ozone, but very high degrees of whiteness can nevertheless be achieved.

EXAMPLE 7

A spruce kraft pulp was subjected to two different bleaching sequences according to the invention after conventional oxygen extraction. After the oxygen extraction, the pulp had the following key figures:

The bleaching was carried out on the one hand using the sequence Z ₁ - EO - Z ₂ - P, on the other hand using the sequence Z ₁ - EOP - Z ₂ - P. (Z = ozone treatment; EO = alkaline oxygen treatment; EOP = alkaline oxygen treatment reinforced with peroxide ; P = peroxide treatment.)

a) The ozone treatment (Z ₁ and Z ₂ ) were carried out under the following conditions:

b) The EO or EOP stage was carried out under the following conditions:

c) The peroxide treatment (P) was carried out under the following conditions:

Der Zellstoff hatte nach den einzelnen Stufen jeweils folgende Eigenschaften:

The cellulose had the following properties after each stage:

EXAMPLE 8: (Comparative experiment) For comparison with Example 7, the same pulp was subjected to chlorine bleaching with the bleaching sequence Z ₁ -EO-Z ₂ -D. (D = chlorine dioxide bleach).

The process conditions of the Z ₁ stage, the EO stage and the Z ₂ stage were identical to those of Example 7. The reaction conditions of the D stage were as follows:

The cellulose had the following properties after each stage:

Examples 7 and 8 show that there are no differences in the strength properties between unbleached or chlorine-bleached cellulose on the one hand and, according to the invention, chlorine-free bleached cellulose on the other hand.

Claims (12)

1. A process for chlorine-free bleaching of pulps, particularly of pulps for the manufacture of artificial fibres with initial kappa values of 15-1 or of pulps for the manufacture of paper with initial kappa values of up to 30, by means of ozone, in which a pulp suspension at a temperature of 15-80°C, and at a pH value of 1-8 is brought into contact with an ozone-containing gas with vigorous agitation or mixing, in which the ozone-containing gas contains 20-300 g/m³ ozone and in which at most 2% by mass ozone, relative to absolutely dry pulp, is used, characterised in that the pulp suspension has a stock density of 3-20% by mass and that the ozone-containing gas is introduced into the pulp suspension at a pressure of 1.1-15 bar, preferably 1,1-10 bar.

2. A process according to Claim 1, characterised in that the pulp suspension has a stock density of 5-20% by mass, preferably 7-15% by mass.

3. A process according to Claim 1 or 2, characterised in that 0.05-0.5% by mass ozone, relative to absolutely dry pulp, is used.

4. A process according to one of Claims 1-3, characterised in that the pulp suspension is brought into contact with the ozone-containing gas at a pH value of 2-3.

5. A process according to one of Claims 1-4, characterised in that the pulp suspension is brought into contact with the ozone-containing gas at a temperature of40-70°C.

6. A process according to one of Claims 1-5, characterised in that the gas:liquid volume ratio amounts to 1:0.5 - 1:8, preferably 1:1 - 1:6.

7. A process according to one of Claims 1-6, characterised in that cooled compressors, preferably water ring pumps, are used for compressing the ozone-containing gas.

8. A process according to one of Claims 1-7, characterised in that a high-shear mixer is used for the vigorous agitation or mixing.

9. A process according to one of Claims 1-8, characterised in that it is carried out several times in succession, an alkaline extraction being carried out, if necessary, in between.

10. A process according to one of Claims 1-9, characterised in that it is carried out after an oxygen- and/or peroxide-reinforced extraction, if necessary followed by an alkaline peroxide stage.

11. A process according to one of Claims 1-10, characterised in that after it there follows a peroxide stage or an alkaline extraction.

12. A process according to one of Claims 1-11, characterised in that the waste water filtrate arising during the treatment with ozone is fed at least in part to the pulp suspension before the latter is brought into contact with the ozone-containing gas, the acid required for the desired pH value, particularly sulphuric acid, also being fed together with the waste water filtrate.

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