METHOD FOR BLEACHING WOOD PULP USING HYDROGEN PEROXIDE

PROCEDE DE BLANCHIMENT DE PATE DE BOIS AU PEROXYDE D'HYDROGENE

English Abstract

A method for bleaching wood pulp using a hydrogen peroxide solution as a
bleaching agent, utilizes a two phase reaction, viz. a short high temperature
phase followed by a longer second phase to consume virtually all the hydrogen
peroxide remaining after the short phase. The method includes the steps of
introducing pulp (10), at a consistency of 10-18 %, to a mixer (100) in which
the pulp is heated to a temperature between 80 and 100 ~C; adding sufficient
sodium hydroxide (15) to bring the pulp to a pH of 9.5-11.5; adding sufficient
hydrogen peroxide (35) to provide a peroxide residual after the first reactor
column; passing the pulp through a reactor column (400) at a rate providing a
reaction time on the column of between 5 to 60 minutes; introducing the pulp
to a mixer (500) and reactivating residual hydrogen peroxide by bringing the
pulp to a pH of at least 9; and depositing the pulp in a reaction tower (600)
and allowing the reaction to proceed for 1-4 hours until a substantial portion
of the residual hydrogen peroxide has been consumed.

French Abstract

L'invention concerne un procédé de blanchiment de pâte de bois à l'aide d'une solution de peroxyde d'hydrogène utilisée comme réactif de blanchiment. Le procédé fait appel à une réaction en deux phases, à savoir une phase à températue élevée courte et une seconde phase plus longue destinée à consumer pratiquement tout le peroxyde d'hydrogène résiduel après la phase courte. Le procédé consiste à introduire la pâte (10) dont la concentration varie entre 10 % et 18 % dans un mixeur (100) dans lequel la pâte est chauffée à une température comprise entre 80 et 100 ~C; à ajouter une quantité suffisante d'hydroxyde de sodium (15) pour amener la pâte à un pH compris entre 9,5 et 11,5; à ajouter une quantité suffisante de peroxyde d'hydrogène (35) pour obtenir un résidu de peroxyde après passage dans la première colonne de réacteur; à passer la pâte dans une colonne de réacteur (400) à une vitesse permettant d'obtenir un temps de réaction dans la colonne compris entre 5 et 60 minutes; à introduire la pâte dans un mixeur (500) et à réactiver le peroxyde d'hydrogène résiduel en amenant la pâte à un pH d'au moins 9; et à déposer la pâte dans une tour de réaction (600) et à laisser la réaction se dérouler pendant 1 à 4 heures jusqu'à ce qu'une partie substantielle du peroxyde d'hydrogène soit consumée.

Claims

What is claimed is:
1. A method for bleaching wood pulp (10) using a hydrogen peroxide
solution (35) as a bleaching reagent, comprising the steps of:
introducing pulp (10), at a consistency of 10% - 18%, oven
dried basis, to a mixer (100);
heating said pulp (10) to a temperature between 80 and 100
degrees Celsius;
adding sufficient alkali (15) to bring said pulp (10) to a pH of
greater than 8.5;
adding sufficient hydrogen peroxide (35) with agitation in a
mixer (300) to assure that there will be a significant
residual of peroxide after the first reactor column (400);
passing said pulp (10) through a reactor column (400) at a rate
which provides a reaction time in said column less than
60 minutes;
introducing said pulp (10) to a mixing means (500) and
reactivating residual hydrogen peroxide by adding
sufficient alkali (55) to bring the pulp (10) to a pH of at
least 9; and
depositing said pulp (10) in a reactor tower (600) and allowing
the reaction to proceed for 1-4 hours until a substantial
portion of the residual hydrogen peroxide has been
consumed.
2. The method of claim 1, wherein sufficient alkali (55) is added to bring said pulp
to a pH of 9.5 - 10.5.
3. The method of claim 1, wherein said pulp (10) is passed through a reactor
column (400) at a rate which provides a reaction time in said column
between 10 and 15 minutes.

Description

WO 96/01922 PCT/US95/08152
21 948~1
METHOD FOR BLEACHING WOOD PULP USING HYDROGEN
PEROXIDE
BACKGROUND OF THE INVENIION
This invention relates generally to wood pulp bleaching processes and
more particularly to bl~ching of medium consistency pulp using hydrogen
peroxide solution in a two-phase ble~ching stage.
Hydrogen peroxide is widely used in the bri~htening of mt~h~ni~
5 semi-mech~ni~l, semi-chemical, and recycled pulps. More recently, it has
also been used in chemical pulp bleaching to aid in its delignification.
Oxidative extraction stages using peroxide in addition to oxygen are currently
used to reduce the amount of chlorine necessary in the first stage of pulp
bleaching. Hydrogen peroxide is also used in the final stages of pulp bl~rhing
10 to achieve a high brightnçss, stable bl~ched pulp.
For mechanical, semi-mechanical, and semi-chemical pulps, hydrogen
peroxide has been primarily used in high concictency systems, where the pulp
slurry is dewatered to about 30% consistency and passed through a fluffer or
high consistency mixer in which the peroxide solution is added. The pulp then
15 falls by gravity to a reaction tower usually sized for several hours retention.
The bleaching is performed in one stage such as described above or in two
stages in series. In the latter case, one of those stages may be pel~oll,.ed at
medium consictency.
In chemical pulps, hydrogen peroxide has been primarily used in
20 medium consistency systems, in which the pulp slurry, from a previous stage,
is dewatered in a thick~nçr or washer to about 10-14% con~ictency. The
peroxide solution is added together with alkali at the repulper (discharge from
the thickener or washer) or before the medium consistency tower in a medium
consistency pump or mixer, usually in combination with other oxidative
25 chemicals like oxygen.

WO 96/01922 2 ! 9 4 8 8 1 PCT/US9S/08152
There are several ways to bleach mechanical pulps. Of these, the hot
peroxide system is of interest because it pursues the same brightnecc
development as the conventional high consistency systems, but does so at
medium concictency (10-14%). This is achieved by increasing the te~nl)el~lure
of the pulp to 85oC. and lowering the pH to about 9.5 to 10.5, which is
differentiated from conventional high consistency peroxide system. Rec~llse of
the faster reaction, the retention time is reduced from hours to minutes (15-30
min.) and no silicate is required to stabilize the peroxide solution. The
peroxide charge remains about the same as that of the conventional systems.
Reactivation of residual peroxide has been proposed for use in the
bleaching of mech~niç~l pulps. This development allows reactivation of the
non-concumed peroxide (after the first reaction stage tower) by increasing the
~lk~linity of the pulp suspension. The aim of this is to çlimin~te expensive
dewatering equipment which is used after the blç~hing tower to recover the
residual peroxide by recirculating the filtrate from the dewatering equipment
back to the point of addition of the fresh peroxide (usually at a mixer before
the bleaching tower). This proposal becomes economically important when
compared with a conventional two stage peroxide bl~ching system which
requires expensive dewatering equipment between stages.
High consistency systems are claimed to be the best way to develop
highest brightness with lowest peroxide consumption. Its disadvantage resides
in the large investment cost required because of the expensive dewaterin
equipment (to 30% concictency), the expensive mixer-fluffer, expense of long
retention time tower and expensive high concictçncy tower discharge
equipment. Furthermore, any attempt to recycle residual peroxide to reduce
chemical consumption will require a second dewatering equipment after the
tower, which is similar to the one used ahead of it.
The medium consistency systems do not provide sufficient brightness

WO 96/01922 PCT/US9S/08152
2 ~ 9488 1
increases and are said to consume more peroxide and require extremely long
retention time for consumption of the peroxide. For more pronounced
d~lignific~tion or brightness effects peroxide must be applied in several towers,
i.e., in chemical bleaching together with oxygen in the first extraction stage,
5 alone or with other chemic~lc in the second extraction stage, and alone prior to
the ble~ched high density tower. Residual peroxide is not recovered in these
systems.
The foregoing illustrates limitations known to exist in present pulp
bleaching processes, and it would be advantageous to provide an alternative
10 directed to overcoming one or more of those limitations. Accordingly, a
suitable alternative is provided including features more fully disclosed
hereinafter.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a method of bleaching wood pulp
15 using a hydrogen peroxide solution as a ble~ching reagent, incllldes the steps of
introducing pulp, at a consistency of 10%-18% (oven dried basis), to a mixer;
heating the pulp to a lel-,pel~ture between 80 and 100 degrees Celsius; adding
sufficient sodium hydroxide to bring the pulp to a pH of 9.5-11.5; adding
sufficient hydrogen peroxide to assure that there will be a cignific~nt residual20 of peroxide after the first reaction tower; passing the pulp through a reactor
column at a rate which provides a reaction time in the column of between 5
and 60 minutes; introducing the pulp to a mixer and reactivating residual
hydrogen peroxide by adding sufficient sodium hydroxide to bring the pulp to a
pH of at least 9; and depositing the pulp in a reaction tower and allowing the
25 reaction to proceed for 1-4 hours until substantially all the residual hydrogen
peroxide has been concumed.
The foregoing and other aspects of the invention will become appal~ellt

WO 96/01922 2 ! 9 4 8 8 1 PCT/US95108152
from the following det~iled description, when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a sch~m~tic view illustrating a process sequence yielding a
5 preferred embodiment of the method of the present invention.
DETATLED DESCRIPTION
Referring to the figure, pulp from a conventional washer or thick~ner is
discharged through pipe 10 into mixer 100, where steam for heating the pulp
and alkali for adjusting the pH of the pulp is added through pipes 16 and 15
10 respectively. The alkali may be sodium hydroxide, white liquor, or other
alkali sources. The pulp is heated to more than the conventional 600C.,
preferably to 85-95oC., and adjusted to a pH of greater than 8.5, preferably
9.5-10.5. The heated and pH adjusted pulp is discharged from mixer 100
through pipe 20 to a conventional medium consistency pump 200 which pumps
the pulp through pipe 30 to a mixer 300. Hydrogen peroxide solution is added
to the mixer 300 through pipe 35 in a quantity sl-fficient to assure substantialresidual will be maintained at the end of the first phase of the reaction. As inconventional peroxide ble~ching systems, the addition of m~gn~sium
compounds for protection of cellulose viscosity, as well as sequestrants (such
20 as SiO2 and/or chelants {such as EDTA or DTPA}) may also be added with
the alkali solution through pipe 15, the peroxide solution through pipe 35, or
separately through pipes 16 and 36.
The pulp, which has been heated to the desired reaction ~I~pel~ture,
and adjusted to the desired pH, is pumped through pipe 40 into the upflow
25 reaction column 400, which is sized for the retention time desired for the first
phase of the reaction. According to the present invention, the upflow tube is

WO 96/01922 PCTIUS9S/081~2
2 1 94~ t
sized to assure a pulp retention time of 5-55 minutes, preferably 10-20
minutes, after which retention, the pulp retains substantial residual peroxide.
- The pulp is discharged through pipe 50 to an applopl;ate mixing device 500,
where additional alkali is added through pipe 55 to increase the pH of the pulp
S to 9.5-11.5, preferably 10.5-11Ø This alkali (sodium hydroxide) addition
reactivates the residual hydrogen peroxide so that, when discharged through
pipe 60 into reaction vessel 600, the pulp slurry will continue to brighten due
to the peroxide reaction during retention in reaction vessel 600. Typical
reaction vessels are sized for pulp retention times of 1-4 hours.

Representative Drawing