CA2194881A1

CA2194881A1 - Method for bleaching wood pulp using hydrogen peroxide

Info

Publication number: CA2194881A1
Application number: CA002194881A
Authority: CA
Inventors: Gustavo L. Bottan
Original assignee: Individual
Current assignee: Beloit Technologies Inc
Priority date: 1994-07-11
Filing date: 1995-06-28
Publication date: 1996-01-25
Also published as: BR9508390A; EP0770158B1; ZA955190B; JPH09508184A; WO1996001922A1; EP0770158A1; FI970106A0; FI970106A; ATE172504T1; DE69505536D1

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.

Description

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 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.

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.