NZ237019A

NZ237019A - Bleaching paper pulp by initially treating with oxygen and/or hydrogen peroxide and subsequently treating with a bleaching agent without intervening washing steps

Info

Publication number: NZ237019A
Application number: NZ237019A
Authority: NZ
Inventors: Roger Anthony Cook
Original assignee: Amcor Ltd
Priority date: 1990-02-19
Filing date: 1991-02-05
Publication date: 1992-11-25
Also published as: PT96816B; ZA911062B; BR9106033A; ES2038954A6; WO1991012366A1; PT96816A

Description

New Zealand Paient Spedficaiion for Paient Number £37019 2 3 7 0 19 ? c, .. ,;ii- ; , ; (■ .. ;..03.l.C4/.!ul, Ik, a . .

'M ,■ V ... «... . ./iviti '4 NEW ZEALAND The Patents Act, 1953 COMPLETE SPECIFICATION ^CBlss, 'BLEACHING OF PAPER PULP" WE, AMCOR LIMITED, a company incorporated under the laws of the State of New South Wales, of 4 South Gate, South Melbourne, Victoria 3205, Australia, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 237 0 19 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-treatment 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 liynin-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 237 0 #> 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 5 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 10 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 15 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. 20 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 25 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 30 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. 35 When oxygen is employed for a hydrostatic oxygen delignification stage the oxygen may be injected through a 23 7 0 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 5 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 -10 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 15 "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 20 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 25 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 ^ 30 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 237019 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 IISO chlorine 89B233 Control 3.5 87.1 89B220 Unwashed in 87.4 89B221 Washed 1.6 87.6 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 . Chlorine dioxide bleaching (. 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). 237 0 19 Bleach Number Designation Chemical Dose Brightness as t active tiso chlorine 89B273 Control 4.25 87.8 89B263 Unwashed 3.25 87.6 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 . 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 Bleach Number Designation Chemical Dose Brightness as 1 active (ISO chlorine 89B287 Control 4.50 86.6 89B283b Unwashed 3.75 86.0 Example 4 During a trial operation at a kraft pulp mill, caustic soda and hydrogen peroxide were added to the

Claims

2 3 7 0 -7- 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. 237011) -8- VHATVE .CLAIM IS:

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.

4. A method according to claim 1 substantially as herein described or exemplified. Any newel feature er newel cambinatian ef faetyrea diooleoed he»e4wi AMCOR LIMITED By Their Atto/neys HENRY HUGHES/LIMITED