EP0441113B1 - Process for the chlorine-free bleaching of cellulosic pulp - Google Patents

Abstract

The process comprises three stages, namely an oxygen bleach which is optionally combined with a hydrogen peroxide bleach (EOP stage), an ozone bleach (Z stage) and a peroxide bleach (P stage). According to the invention, the residual oxygen content of the exit air from the Z stage is completely reacted in the EOP stage, and the effluent of the Z stage is used for dilution between the EOP and Z stages and/or for ash removal from the pulp after bleaching. It is advantageous in this case to control the temperature, the NaOH supply and the O2/H2O2 ratio in the EOP stage, so that the oxygen consumption at a constant conversion is adjusted to the residual oxygen content of the exit air from the Z stage.

Description

The present invention relates to a process for chlorine-free bleaching of cellulose for the production of synthetic fibers with ozone according to a three-stage process, consisting of an oxygen bleach combined with a hydrogen peroxide bleach (EOP stage), an ozone bleach (Z stage) and a peroxide bleach ( P level).

Since chlorine is a toxic gas that is difficult to control, and chlorine-containing bleaches produce chlorinated wastewater constituents that can no longer be tolerated in the future, processes have been developed for bleaching cellulose that use O₂, H₂O or O₃ in various combinations and under different conditions as environmentally friendly Use alternative.

A method of the type mentioned at the outset is known from Example 24 (which relates to Examples 10, 2 and 1), US Pat. No. 4,216,054. Here, Douglas fir pulp is bleached with a relatively poor result (whiteness 74.6).

CA-A 11 03 409 describes a bleaching process which uses three ozone stages in series and introduces the ozone-containing residual gas from the 3rd ozone bleaching stage into the 1st ozone bleaching stage in order to utilize the remaining ozone content. The residual gas generated in the 1st ozone bleaching stage is no longer used.

It is an object of the present invention to improve the method of the type mentioned at the outset in such a way that it can be carried out more cost-effectively, in an environmentally friendly manner and in an energy-saving manner.

This object is achieved according to the invention in a process of the type mentioned at the outset in that the residual oxygen content of the exhaust air from the Z stage is completely reacted in the EOP stage and that the waste water from the Z stage, the pH of which is preferably below 3 , is used for dilution between the EOP and Z stages and / or for ash removal from the pulp after bleaching.

It has been surprisingly found that it is possible in a bleaching process of the type mentioned at the outset to select the process parameters so that the residual oxygen content of the exhaust air from the Z stage is completely reacted in the EOP stage. The chemical recycling according to the invention is thereby possible and the above-mentioned object is achieved.

When choosing the process parameters, note the following:
In the EOP stage, the kappa number of the pulp has to be reduced to such an extent that one single ozone stage is sufficient in the following. Do not bleach too much per ozone level, otherwise the quality of the pulp will suffer. The ozone requirement in the O stage (Z stage) is regulated by the temperature setting in the EOP stage so that the total amount of residual gas can be used in the EOP stage. Because of the low use of ozone, the temperature in the Z stage can be selected to be relatively high, so that the energy expenditure for cooling and reheating the pulp between the stages can be kept low. The activation via the temperature and the amount of NaOH in the EOP stage is chosen so that the total amount of residual gas from the O₃ stage is reacted here.

In the method according to the invention, the EOP and the Z stage can be regarded as one unit from the chemical balance. The Z level can be an LC (low consistency) level, i.e. below 4 mass% ATS, or as MC (medium consistency) level, i.e. in a range of 5-20% by mass ATS, preferably 7-15% ATS, according to A 2494/89.

The second option is of course particularly advantageous if the EOP stage and the P stage are also used as the MC stage, because then of course there is no need for dilution before the Z stage and sealing before the P stage to a much lesser extent.

There are various process parameters that are suitable for coordinating the oxygen requirement of the EOP stage with the residual oxygen content of the exhaust air of the Z stage. However, it is preferred according to one embodiment of the invention that the temperature, the NaOH use and the O₂ / H₂O₂ ratio in the EOP stage are controlled so that the oxygen consumption with constant conversion to the residual oxygen content of the exhaust air of the Z- Level is matched.

It is expedient if the temperature of the EOP stage is 70-100.degree. C., preferably 75.degree. C., if the NaOH concentration of the EOP stage is between 2.5-5% by mass, based on pulp, bleached dry and if there is at most as much H₂O₂ as O₂, based on the mass, in the mixture O₂ - H₂O₂ of the EOP stage
Furthermore, it is expedient if the Z stage is operated with an O₂ / O₃ mixture with a maximum of 10% by mass O₃ and the temperature of the Z stage is 40-70 ° C, preferably 50-60 ° C.

The bleaching sequence EOP - Z - P begins with the introduction of the exhaust gas from the Z stage into a mixture of cellulose for the production of synthetic fibers NaOH and H₂O₂.

Due to the set process parameters, a 50% conversion of the O₂ used is achieved with the pulp. This makes it possible to reduce the kappa number for pulp for the production of synthetic fibers. The kappa value of the pulp for the production of synthetic fibers, when it is introduced into the Z stage, should be a maximum of 2.0, preferably 1.8-2.5.

This Kappa number makes it possible to manage with a Z-level for synthetic fiber pulps. Paper pulps, on the other hand, are known to consume far more ozone.

If the Z stage is operated as an LC process, 0.5 - 10% O₃ in O₂ are sufficient. The small amount of O₃ allows temperatures between 50 - 60 ° C, so that the temperature is unchanged compared to the previous stage and no energy losses occur.

Both exhaust gas and wastewater are reacted completely.

The P stage, which follows the Z stage, is also adapted to the temperature of the preliminary stage, it runs between 60 - 70 ° C and requires less H₂O₂ than usual.

The following examples are intended to explain the invention in more detail.

example 1

Assorted beech pulp for the production of synthetic fibers unbleached with a kappa number of 5.2, a viscosity of 24.5 mPas and a white content Elrepho of 55.6 is subjected to a three-stage bleaching process.

1st bleaching stage (EOP)

The raw pulp with a consistency of 15% and a temperature of 75 ° C was mixed with sodium hydroxide in an amount of
33 kg / t cellulose bleached dry (corresponding to 366 l NaOH solution with 90 g / l) and
10.8 l of 50% H₂O₂ added
and led to an MC pump (center consistency pump). On the pressure side of the MC pump, 5.5 Nm³ of exhaust gas per ton of cellulose were bleached atroot the O₃ stage, which were compressed to 7 bar, via a frit. With a share of approx. 95% O₂ in the gas, this corresponds to an O₂ addition of 7.5 kg O₂ per t cellulose.

The stock suspension was then treated in a "high-shear" MC mixer. Another 5.5 Nm³ of exhaust gas from the Z stage were fed into this mixer.

This mixture was passed through a pre-reaction tube with a residence time of 3 min into an upward bleaching tower which allowed a reaction time of 2 h 45 min.

Subsequently, the pulp was freed from the adhering liquor on two vacuum drum filters and a screw press. The filtrate of the screw press (3.3 m³ / t cellulose) was used in countercurrent with other 3.3 m³ fresh water via washing pipes on the filters for washing.

The organic accompanying freight could thus be reduced to 3 kg OTS per t cellulose. The whiteness was 75.4%; the kappa number 1.89; the viscosity 24.0 mPas.

2nd bleaching stage (Z)

After the screw press, the pulp was diluted to a consistency of 2% with return water from the vacuum drum filter after the Z stage and a pH of 3 was established with sulfuric acid. In an OZON bleaching plant of known design (Waagner-Biro), the oxygen suspension of 2% consistency was conveyed in a circuit via an injector. Step by step, 1.45 g of OZONE per kg of pulp were introduced via this injector. The residence time in the reactor was 20 min, the temperature 51 ° C.

The pulp was then washed again. The acidic filtrate was used to ash the pulp before the last filter.

The pulp analysis values were:
Whiteness 78.3; Kappa number 0.97; Viscosity 22.0.

3rd bleaching stage (P)

The 3rd stage is carried out with the addition of 4 kg NaOH / t pulp and 7 kg H₂O₂ (50%) at 65 ° C and 13% consistency with a residence time of 4 h.

Subsequently, washing was carried out on two vacuum drum filters. The Z-stage filtrate was acidified before the second filter in order to reduce the ash content of the pulp to 0.06%. Finished material analyzes: Whiteness 89.7% viscosity 21.8 mPas Kappa number 0.76

Examples 2 and 3

A continuous experimental apparatus for bleaching O₃ was operated in the laboratory, O₃-containing oxygen being introduced into a 2% pulp suspension via an injector. The exhaust gas was collected in a pressure vessel, compressed by pressing in water and used for the batch tests of the 1st bleaching stage (EOP) in a stirred laboratory autoclave.

The 3rd bleaching stage (P) was also carried out batchwise.

Example 4

Pulp after the 1st bleaching stage of Example 1 was removed and further processed in the laboratory.
(White content 75.4%, kappa number 1.89, viscosity 24.0 mPas)
This was pressed to 20% consistency and diluted to 11% consistency with dilute sulfuric acid, so that the pH was 2.9.

In a "high-shear" mixer, this pulp suspension was fluidized and compressed O₃-containing oxygen gas was pressed.

The mixing time was 15 s, the reaction time 180 s, the O₃ pressure 5.1 bar, the temperature 50 ° C.

The specific O₃ use was 1.50 g O₃ / kg cellulose bleached, the O₃ consumption at 1.40 g.

The analysis data of the pulp obtained were: White content 80.4% viscosity 21.4 mPas Kappa number 0.92
This pulp was subjected to laboratory bleaching (P level): temperature 64 ° C Dwell time 4 h Consistency 13% NaOH use 0.38% H₂O₂ insert 0.38% Finished material data Kappa number 0.72 White content 90.0% viscosity 20.8 mPas

Claims (7)

1. Method for the chlorine-free bleaching of cellulose for the manufacture of artificial fibres, with ozone according to a three-stage method, consisting of acid bleaching, ozone bleaching (Z-stage) and peroxide bleaching (P-stage), in which the spent air of the Z-stage is supplied to the acid bleaching, characterised in that the acid bleaching is combined with a hydrogen peroxide bleaching (EOP stage), that the remaining acid content of the spent air of the Z- stage is completely reacted and that the effluent from the Z-stage, the pH value of which preferably lies below 3, is used for dilution between EOP- and Z-stages and/or for the removal of ash from the cellulose after bleaching.
2. Method according to claim 1, characterised in that the temperature, the use of NaOH and the O₂/H₂O₂ ratio are controlled in the EOP-stage such that the consumption of acid during constant conversion is checked against the remaining acid content of the spent air of the Z-stage.
3. Method according to claim 2, characterised in that the temperature of the EOP-stage amounts to 70-100°C, preferably 75°C.
4. Method according to claim 2 or 3, characterised in that the NaOH concentration of the EOP-stage amounts to between 2.5-5% by mass, with respect to cellulose, atro bleached.
5. Method according to one of claims 1 to 4, characterised in that in the O₂-H₂O₂ mixture in the EOP-stage, at most as much H₂O₂ as O₂, with respect to mass, is present.
6. Method according to one of claims 1 to 5, characterised in that the Z-stage is carried out with a O₂/O₃-mixture which contains in maximum 10% by mass O₃.
7. Method according to one of claims 1 to 6, characterised in that the temperature of the Z-stage amounts to 40-70°C, preferably 50-60°C.