WO1995033884A1

WO1995033884A1 - Ozone bleaching

Info

Publication number: WO1995033884A1
Application number: PCT/CA1995/000311
Authority: WO
Inventors: Marco Solinas; Victor Chan; Thomas Howard Murphy; Zheng Tan
Original assignee: Macmillan Bloedel Limited
Priority date: 1994-06-02
Filing date: 1995-05-26
Publication date: 1995-12-14
Also published as: CA2190503A1

Abstract

Brown stock is ozone bleached in aqueous medium containing between 3 and 7 % based on the weight of the medium of a protector selected from the group comprising polyhydric alcohols, water soluble ethers of said polyhydric alcohols and mixtures thereof, and the medium, after bleaching, is separated from the bleached pulp and preferably delivered to the pulp mill recovery system.

Description

OZONE BLEACHING Field of the Invention

The present invention relates to bleaching of pulps, more particularly, the present invention relates to bleaching of pulp using ozone as the bleaching agent. Background of the Present Invention

There have recently been many developments in the bleaching of pulp for the manufacture of paper to eliminate chlorine from the bleaching sequence either as elemental chlorine or preferably, to produce a bleached pulp using a totally chlorine free (TCF) process to totally eliminate chlorinated organic compounds from the effluent. Processes are being advocated for completely closing the mill, i.e. to produce a bleached pulp using a totally effluent free (TEF) process wherein all of what normally would be considered as effluent from the mill is retained in the system, e.g. is fed to the recovery system.

The use of an ozone stage to eliminate elemental chlorine and in some cases produce a TCF pulp generally results in a relatively weak pulp compared with a conventionally made bleached pulp. However, processes have been invented that use ozone in a protective medium to produce a bleached pulp having significantly improved strength characteristics relative to conventionally ozone bleached pulps (see U.S. patent applications 08/056,496 filed May 3, 1993 by Solinas et al., 08/024,056 and 08/024,058, both filed March 1, 1993 by van Heiningen et al.).

The processes described in these patent applications of Solinas et al. and van Heiningen et al. use concentrations of reagents in the bleaching medium that are at levels wherein both from an economic and ecological point of view, the protecting chemical or reagent must be reused. These processes require a special recovery system for stripping the protecting chemical from the pulp and from the aqueous medium which increases both the capital and operating costs of the process. Brief Description of the Present Invention

It is an object of the present invention to provide a relatively inexpensive ozone bleaching process while protecting the pulp from degradation during the bleaching stage.

Broadly, the present invention relates to a method of bleaching pulp comprising surrounding a pulp in an aqueous medium containing between 3 to 7% based on the weight of the medium of protector selected from the group polyhydric alcohols, water soluble ethers of a polyhydric alcohols and mixtures thereof and adjusting the pH of said pulp in said surrounding medium to a pH in the range of 1.5 to 3.5 to provide a bleachable pulp, bleaching said bleachable pulp with ozone to consume an amount of between 0.5 to 4% ozone based on the dry weight of fibres in said pulp and form a bleached pulp, separating said polyhydric alcohol from said bleached pulp.

Preferably, said aqueous medium will contain between 4 and 6% of said protector. Preferably, said protector will be a polyhydric alcohol and will be selected from a group consisting of ethylene glycol, polypropylene glycol, 1,4 butanediol glycerol, pentaerythritol and diethylene glycol.

More preferably, the polyhydric alcohol will be ethylene glycol.

Preferably, said protector ether are selected from a group consisting of butoxyethanol, ethoxyethanol, methoxyethanol, 2-(2-butoxyethoxy) ethanol, 2-(2- ethoxyethoxy) ethanol and 2-(2-methoxyethoxy) ethanol.

Preferably, said ozone bleaching will be carried out at a temperature of less than 5°C.

Brief Description of the Drawings Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which;

Figure 1 is a schematic illustration of a flow diagram suitable for the present invention. Figure 2 is a plot of viscosity versus permanganate number (P no.) after the extraction stage following the ozone stage, showing the effects on ozone bleaching using ethylene glycol in the surrounding medium at different temperatures compared with the control.

Figure 3 is a plot of P no. after the extraction stage following the ozone stage versus corresponding viscosity as obtained by ozone bleaching wherein the surrounding aqueous medium contained various polyhydric alcohols all compared with a control. Figure 4 shows the amount of ozone consumed to obtain a selected P. no. when the ozone bleaching is carried out using an aqueous medium containing different polyhydric alcohols.

Figure 5 is a plot of viscosity versus P. no. after the extraction stage following the ozone stage, showing the effects of water soluble ethers of polyhydric alcohols on ozone bleaching. Description of the Preferred Embodiments

As illustrated in Figure 1, the pulp to be bleached is introduced as indicated by line 10 and is mixed in vessel 12 with a suitable acid introduced as indicated by line 14 (normally a mineral acid such as sulphuric acid) and with a suitable protector selected from the group consisting of polyhydric alcohols and water soluble ether of polyhydric alcohols introduced as indicated by line 16. The conditioned pulp containing the required amount of acid, i.e. at the desired pH which will normally be in the range about 1.5 to 3.5 and carrying the required amount of protector and in the surrounding aqueous medium (as will be described hereinbelow) is fed via line 18 to the thickener 20 wherein the consistency of the pulp is increased to that desired for application of ozone in the ozone bleaching stage (Z stage) 22. In the illustrated arrangement, the pulp from thickener 20 is taken via line 24 and introduced into the ozone stage 22 to which is also introduced ozone as indicated via line 26.

As above indicated, the pulp in the line 24 will be at the desired consistency and will contain the protector at a concentration in the aqueous medium of less than 7% by weight, generally between 3 and 7% and preferably between 4 and 6% based on the weight of alcohol in the aqueous medium. By operating the Z stage 22 at high consistency, i.e. in the range of about 30 to 50% consistency of pulp in medium, and by maintaining the concentration of the protector in the medium below about 7%, the amount of protector in the pulp can be maintained relatively low specifically to an amount that may require no added provisions for recovery, for example, it may simply be fed to the recovery system of the pulp mill in the conventional manner together with other recovered flows as will be described below. In any event, liquor or medium separated from the pulp in the thickener 20 is returned via line 28 to the vessel 12 to be reused in providing the required amounts of acid and protector to the incoming pulp (which will normally be at a consistency in the order of about 30%) i.e. so that only make-up acid and protector need be added via lines 14 and 16 respectively.

The pulp bleached in the ozone stage (Z stage) 22 carried via a line 30 is preferably washed as indicated at 32 and then delivered to an extraction stage (E stage) 34. Preferably, caustic will be added as indicated at 36 to the extraction stage so that the pH of the extraction stage is increased to an alkaline pH, preferably in the order of 10 - 11 at the end of the extraction. Other chemicals such as oxygen or peroxide, if desired, may also be added to the E stage in the conventional manner. The extracted pulp may then be washed as indicated at 38 and further bleached in stages (B) 40 as required to obtain the desired brightness of the pulp.

The bleached pulp leaves the further bleaching stages 40 via line 42 and is washed by the washing stage 44 and the bleached pulp leaves the system via line 46. The effluent from washer 44 is delivered to washer 38 via line 48 and used as a washing fluid in the washer 38 and the effluent from washer 38 is carried via line 50 either to washer 32 as indicated by dotted line 52 and the effluent from washer 32 returned to the brown stock washers via line 54 or passes directly via line 50A to the brown stock washers.

Preferably, the effluent in line 50A and/or 54 will be directed to and used as washing fluid in the brown stock indicated at 56 then sent to a set of multiple effect evaporators 58 and finally any residual will enter to the recovery furnace and function in part as a fuel. As above indicated, it is important that the amount of protector (polyhydric alcohol and/or ether) in the system be maintained relatively low for economic reasons to permit the system to operate within economic constraints without requiring the reuse of the protector which would require additional separating equipment to isolate the protector (polyhydric alcohol and/or ether) for recovery. The protectors used with the present invention will generally be polyhydric alcohols selected from the group consisting of ethylene glycol, propylene glycol, 1,4 butanediol glycerol, pentaerythritol and diethylene glycol and water soluble ethers of these polyhydric alcohols, e.g. mono-substituted alkyl ethers of the polyhdric alcohols. Preferably, the ethers are selected from the group comprising 2- butoxyethanol, 2-(2-butoxethoxy) ethanol and 2-(2-ethoxyethoxy) ethanol.

The process may be carried out using the normally used temperature in the Z stage, but it is preferred to maintain the temperature of the ozone stage relatively low, i.e. about freezing temperature, e.g. -2° to +2°C.

It is being found that a viscosity gain of up to about 7.5 cp may be obtained for example, using ethylene glycol at a temperature of 1°C over the viscosity attainable when the medium used is water substantially free of alcohols. Examples

The brown stock used for these examples had a kappa number of 27.4 ml and a viscosity of 28.1 cp.

The medium surrounding the pulp during bleaching i.e. in the Z stage was in all cases, adjusted to contain 5% of the specific polyhydric alcohol used. pH in the Z stage was 2.25 for each of the examples and the consistency of the pulp in the Z stage was 40%.

The results obtained are indicated in Figures 2 to 4 inclusive.

Figure 2 shows the results obtained using ethylene glycol in the medium surrounding the pulp and operating the Z stage at temperature of 1°C compared with the conventional temperature used in ozone bleaching of 20°C. A viscosity improvement after the extraction stage in the order of 5 cp was obtained at 20°C and when the same process was repeated but using a temperature of 1°C in the Z stage 22, a further improvement of 2 cp was obtained, i.e. operating with 5% ethylene glycol in the Z stage at a temperature of 1°C an improvement in viscosity of the bleached pulp of about 7 cp is attained relative to a conventional ozone system operating with water as the surrounding medium and at a temperature of 20°C.

Various polyhydric alcohols were tested as shown in Figure 3. It is apparent that, of the six different polyhydric alcohols used (all at 5%) a significant improvement in viscosity of the bleached pulp (after extraction) was obtained with some of the polyhydric alcohols being slightly better than the others and with the ethylene glycol providing the best results. The results plotted in Figure 3 are from tests all of which were carried out using a temperature 1°C in the Z stage. To determine the change in ozone requirements to obtain a given permanganate number (P. no.) in the pulp after the extraction stage, tests were carried out, the results of which are plotted in Figure 4, to determine the amount of ozone necessary to reach a selected P. no. for the various polyhydric alcohols used. It will be apparent from an examination of Figure 4, that for a given P. no. the amount of ozone that must be consumed is essentially the same regardless of the alcohol used.

The example of the water soluble alkyl ethers of the polyhydric alcohols is shown in Figure 5, using butoxyethanol, 2-(2-butoxyethoxy) ethanol and 2-(2- ethoxyethoxy) ethanol. It is being found that a viscosity gains of up to about 8.5 cp over the control may be obtained for example, using butoxyethanol at a temperature of 1°C.

It is apparent from the above, that effective results significantly better than those obtained using water as the medium surrounding the pulp in the Z stage are obtainable using small amounts of polyhydric alcohol (or its ether) in the medium surrounding the pulp during the ozone bleaching stage and that a relatively inexpensive process for obtaining ozone bleached pulp, i.e. to reduce the requirements of chlorine, and aid in the closing of the mill is provided.

Having described the invention, modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of bleaching pulp with ozone comprising mixing said pulp with an aqueous medium containing a protector selected from the group consisting of polyhydric alcohols, water soluble ethers of polyhydric alcohols and mixtures thereof, and adjusting the pH of said pulp and said medium to a pH in the range of 1.5 to 3.5 to provide a bleachable pulp, bleaching said bleachable pulp with ozone to consume an amount of between 0.5 to 4% ozone based on the dry weight of said pulp and form a bleached pulp, said aqueous medium in said bleachable pulp containing 3 to 7% of said protector based on the weight of said medium, and separating said protector from said bleached pulp.

2. A method of bleaching pulp as defined in claim 1 wherein said aqueous medium contains between 4 and 6% of said protector.

3. A method of bleaching pulp as defined in claim 1 or 2 wherein said protector is a polyhydric alcohol selected from a group consisting of ethylene glycol, propylene glycol, 1,4 butanediol, glycerol, pentaerythritol and diethylene glycol.

4. A method of bleaching pulp as defined in claim 3 wherein said polyhydric alcohol is ethylene glycol.

5. A method of bleaching pulp as defined in claim 1 or 2 wherein said protector is a water soluble ether of a polyhydric alcohol selected from a group consisting of ethylene glycol, propylene glycol, 1,4 butanediol, glycerol, pentaerythritol and diethylene glycol.

6. A method of bleaching pulp as defined in claims 1 or 2 wherein said ether is selected from the group consisting of 2-butoxyethanol, 2-(2-butoxyethoxy) ethanol and 2-(2-ethoxyethoxy) ethanol.

7. A method of bleaching pulp as defined in any one of claims 1 to 6 inclusive wherein said ozone bleaching stage is operated at a temperature of less than 5°C.

8. A method of bleaching pulp as defined in any one of claims 1 to 7 inclusive wherein said separating comprise washing said medium containing said polyhydric alcohol from said bleached pulp and delivering said polyhydric alcohol to a recovery system of a pulp mill.