AU7217091A

AU7217091A - Bleaching of paper pulp

Info

Publication number: AU7217091A
Application number: AU72170/91A
Authority: AU
Inventors: Roger Anthony Cook
Original assignee: Paper Australia Pty Ltd
Current assignee: Paper Australia Pty Ltd
Priority date: 1990-02-19
Filing date: 1991-02-11
Publication date: 1991-09-03
Anticipated expiration: 2011-02-11
Also published as: AU642971B2

Description

BLEACHING OF PAPER PULP

This invention relates to improvements in bleaching of pulp. In particular it relates to modifications to conventional bleaching processes which use chlorine and/or chlorine-containing substitutes. These chlorine-containing substitutes include hypochlorites (sodium or calcium) and chlorine dioxide.

Bleaching processes which use chlorine-containing chemicals present the problem of discharge of effluents containing organochlorine compounds, it is known that the amount of organochlorine compounds discharged can be reduced if the quantity of chlorine-containing bleaching chemicals is reduced via pretreatment of the pulp with oxygen and/or peroxides such as hydrogen peroxide. The process then involves the steps of (i) treating unbleached pulp with oxygen and/or hydrogen peroxide (ii) washing the pulp (iii) treating the washed pulp in conventional chlorine-containing sequences to complete the bleaching treatment.

These conventional sequences are carried out in a number of stages with a washing step between stages.

It is the normal practice to follow an oxidative delignification pre-treatraent using oxygen and/or hydrogen peroxide with a washing process to remove the products of reaction so that they do not consume the bleaching chemicals used in subsequent stages of the process. It has been believed that if these reaction products are not removed the consumption of the bleaching chemicals would be virtually the same as for a process without oxidative delignification. The reasoning for this is that there has been no removal from the system of the lignin-type and other chemical consuming species.

Although it is known that such oxidative pretreatment reduces chlorine-containing chemical usage, existing plants which use "chlorination" treatment have been reluctant to introduce this process because of the prohibitive cost of modifying the plant to incorporate an oxidation (oxygen and/or hydrogen peroxide) reactor and the associated washing equipment. Further, the effluent from the washing step can increase the load on the chemical recovery circuit requiring reduction in pulp production capacity or further capital expenditure to install extra recovery capacity.

To this end the present invention provides a method of bleaching paper pulp in which unbleached pulp is treated with oxygen and/or hydrogen peroxide and is subsequently contacted with a bleaching agent.

This invention is predicated on the surprising discovery that the washing step prior to application of the bleaching agent is not essential in achieving a significant reduction of the amount of bleaching chemical used.

The bleaching chemical may be selected from chlorine, hypochlorite or chlorine dioxide or a mixture thereof, and the stage using this chemical or mixture is referred to as a "chlorination" stage. The improvement proposed by this invention is an inexpensive modification to a conventional bleaching plant providing a substantial reduction in bleaching chemical usage and hence organochlorine discharge in the effluent for the same whitening result. This process is applicable to hardwoods such as eucalypts and also with softwoods. It is estimated that savings in chemical usage in the "chlorination" stage with this modification will be about 50% of the savings in chemical usage of a conventional hydrostatic oxidative delignification and washing stage but at vastly reduced capital cost.

The capital expenditure associated with installing and the cost of operating a washing plant is avoided. Not only is the washing plant not required but there is no increase in liquor recovery capacity requirements. When oxygen is employed for a hydrostatic oxygen delignification stage the oxygen may be injected through a sparger or mixer into the pulp as it is conveyed through a pipeline at medium consistency. The pressure required must be sufficient to exceed the pressure at the point of injection. Generally the bleaching conditions for the oxygen or hydrogen peroxide hydrostatic pretreatment are addition of 5 to 15 kg of sodium hydroxide and 5 to 15 kg oxygen and/or hydrogen per tonne of unbleached pulp.

The other conditions for either oxygen or hydrogen peroxide pre-bleaching are - Temperature 60 to 100°C

Time 3 minutes to 1 hour

Caustic 0.5 to 1.5% by weight on fibre to maintain pH at 10 to 11. This treatment is followed by at least one "chlorination" bleaching step without any washing step between. Example 1

In the laboratory, three samples of a kraft pulp were selected, two of which were subjected to an oxygen treatment under alkaline conditions in a manner known to simulate treatment equivalent to treatment in a full scale pulp mill. One of these treated samples was then washed to remove the liquor (designated "washed"). The other treated sample was left unwashed (designated "unwashed"). The third untreated sample was designated "control".

These three samples were then subjected to a multistage bleaching process as follows:

1. Bleaching with calcium hypochlorite followed by chlorine without washing between chemical additions.

2. Washing

3. Alkaline extraction with oxygen reinforcement

4. Washing 5. Bleaching with chlorine dioxide

6. Bleaching with calcium hypochlorite The treatment given to each of the three samples via steps 2. to 6. was identical. The chemical dose given in step 1. was adjusted so that the final brightness for each sample was approximately the same. The chemical dose for step 1. and the final brightness are tabulated below.

Bleach Number Designation Chemical Dose Brightness as % active %ISO chlorine

Example 2

Another sample of the same pulp as in Example 1 was pretreated in a similar fashion and not washed. This sample and another control sample were then bleched in another stagewise process.

1. Bleached with chlorine dioxide and chlorine in a sequential process without washing between chemical addition.

2. Washing

3. Alkaline extraction with oxygen and hydrogen peroxide reinforcement. 4. Washing

5. Chlorine dioxide bleaching

6. Washing

As previously, steps 2. to 6. were identical for the two samples of pulp and the active chlorine dose in the first step adjusted to give approximately the same brightness. The ratio of chlorine dioxide to chlorine in step 1. was constant (60/40 D to C).

Example 3

Another sample of the same pulp as in Example 1 was pretreated in a similar fashion and not washed. This sample and another control sample were then bleached in another stagewise process.

1, Bleached with chlorine dioxide 2 . Washing 3, Alkaline extraction with oxygen and hydrogen peroxide reinforcement.

4. Washing 5. Chlorine dioxide bleaching 6 Washing

As previously, steps 2. to 6. were identical for the two samples of pulp and the active chlorine dose in the first step adjusted to give the same brightness

Example 4

During a trial operation at a kraft pulp mill, caustic soda and hydrogen peroxide were added to the brownstock and then the pulp was bleached through the normal stagewise process. The quantity of active chlorine required to attain the same final brightness was reduced by approximately 30%. At the same time a reduction in organochlorine discharge was achieved.

Example 5

As in Example 4, a mill trial was run using a combination of oxygen and hydrogen peroxide together with caustic soda on the unbleached pulp. The measured response was that the effects of the oxygen and peroxide were additive with respect to reduced active chlorine use and effluent discharge.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of bleaching paper pulp comprising initially treating the unbleached pulp with oxygen and/or hydrogen peroxide and subsequently contacting the treated pulp with a bleaching agent without an intervening washing step.

2. A method according to claim 1 wherein the initial treatment stage is carried out for a period of 3 to 60 minutes at a temperature between 60°C to 100°C and at a pH between 10 and 11.

3. A method according to claim 1 or 2 in which the bleaching agent is chlorine and/or chlorine dioxide and/or hypochlorite.