GB1588140A

GB1588140A - Method of preparing bleached pulp

Info

Publication number: GB1588140A
Application number: GB250177A
Authority: GB
Original assignee: Courtaulds PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1977-01-21
Filing date: 1977-01-21
Publication date: 1981-04-15

Description

(54) METHOD OF PREPARING BLEACHED PULP (71) We, COURTAULDS LIMITED, a British Company, of 18 Hanover Square, London W1A 2BB, England, 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 state ment This invention relates to the production of bleached wood pulp.

In known processes wood is pulped, for example by stone grinding logs of wood, or by thermomechanically pulping wood chips, and the wood pulp in dilute slurry form is contacted with a bleaching agent, usually hydrogen peroxide, in a separate step.

According to the invention a process for the preparation of bleached wood pulp comprises pulping wood chips in a single pulping step in a thermomechanical pulping machine and introducing from 0 5 to 5 per cent by weight of hydrogen peroxide, based on the dry wood solids in the pulp, into the pulping machine to bleach the wood during the pulping process.

In a thermomechanical pulping process wood chips are first pre-heated then confined between two opposed grinding surfaces in the thermomechanical pulping machine. The opposed grinding surfaces are generally two rotating discs in which case the thermomechanical pulping machine is called a double disc refiner. The pre-heated wood chips - are introduced at or near the centre of the pulping machine and have to pass outwards through the grinding zone to the outlet. The heat generated by the grinding process converts water present in the wood chips to steam.

Water is added during the thermomechanical pulping, generally at such a rate'as to keep the moisture content of the wood chips approximately constant. The wood being pulped is thus constrained within a steam zone at a temperature of 100"C or above.

Our preferred procedure for thermomechanical pulping comprises a pre-heating step using live steam at a temperature of 105 135 C, most preferably 105-115 C, and a pulping step in a double disc refiner at a temperature lower than that used for preheating and in the range 100-1 100C.

In the process according to the invention, the hydrogen peroxide is preferably added to the water which is fed to the thermomechanical pulping machine during pulping so that it is introduced in aqueous solution.

Conventional double disc refiners have an inlet for the water to reach the centre of the machine and if the hydrogen peroxide is added through this inlet it contacts the wood chips before they are pulped and remains in contact as the wood chips are forced outwards between the opposed grinding surfaces and thereby pulped.

The choice of hydrogen peroxide as the bleaching agent has advantages both on the grounds of cost and because it increases the absorbency of the pulp which is desired for many end uses. The amount of hydrogen peroxide used is preferably about 2 per cent by weight based on dry wood solids. The concentration of hydrogen peroxide fed to the thermomechanical pulping machine varies according to the proportion of water and wood chips used in the pulping process, but is generally in the range of from 0 5 to 10 per cent by weight. The pH of the hydrogen peroxide solution added to the wood chips is preferably 9 to 12, for example about 11.

The pH falls during the bleaching. Buffering agents are preferably used in the hydrogen peroxide solution, for example caustic soda can be used in conjunction with sodium silicate and/or sodium phosphate. As an example the hydrogen peroxide solution can contain about 2 per cent by weight sodium hydroxide and about 4 per cent sodium silicate. Heavy metal sequestering agents can be used to prevent the catalysed oxidation of hydrogen peroxide to oxygen although we have found in general that the hydrogen peroxide bleaches the pulp efficiently despite the high temperature in the refiner.

The invention will now be described by way of example with reference to the single Figure of the accompanying drawing which is a diagrammatic sectional side elevation of an apparatus suitable for producing bleached wood pulp by the process according to the invention.

The apparatus comprises generally a preheater 1 and a double disc refiner 2.

Wood chips are fed to the pre-heater 1 through an inlet 11. The pre-heater is in the form of a screw conveyor. The residence time of the wood chips in the pre-heater can be controlled by controlling the speed of the screw conveyor and is preferably from 0.5-3 minutes.

Steam is fed to the pre-heater through an inlet 15.

Wood chips emerging from the pre-heater 1 pass through a rotary valve 17 and a chamber 18 to a feeder 19 which has a screw conveyor.

It feeds the wood chips to the double disc refiner 2.

The double disc refiner 2 comprises two contrarotating discs 31 and 32 mounted on shafts 33 and 34 respectively which rotate about a common axis. The discs 31 and 32 have opposed grinding surfaces 35 and 36, respectively, seperated by a narrow gap 41. Wood chips are fed by the feeder 19 to a chamber 37 behind the disc 31. From this chamber the wood chips pass through two inlet ports 38 and 39 in the disc 31 to the central region 40 of the double disc refiner.

To pass out of the refiner 2, the wood chips must pass from the central region 40 through the gap 41 to the rim 42 of the discs. The wood chips are ground to pulp in the gap 41.

A 2 per cent by weight aqueous solution of hydrogen peroxide, is supplied to the double disc refiner through an inlet 43 feeding a chamber 44. The hydrogen peroxide solution passes to the central region 40 of the double disc refiner through ports 45 and 46 in the disc 32. The heat generated in the grinding causes some of the water in the wood chips and added through the ports 45 and 46 to be released as steam. The steam can vent back through the ports 38 and 39, the chamber 37 and the feeder 19 to the chamber 18. The chamber 18 is provided with a vent pipe 22 controlled by a valve 23 so that when excess pressure of steam builds up in the chamber 18 it can be vented out of the system. The temperature in the double disc refiner 2 is thus prevented from becoming too high.

Substantially all the hydrogen peroxide added through inlet 43 is thus constrained to pass through the grinding zone 41 of the double disc refiner with the wood chips as they are pulled between the grinding surfaces 35 and 36. The hydrogen peroxide bleaches the wood as it is being pulped.

We have found that the whiteness of the bleached pulp produced according to the invention is at least as good as is achieved in conventional processes using a time much longer than the one second or less which it takes for bleaching to be completed in the double disc refiner. We have found that the absorbency is also as is achieved with conventional processes.

Typical uses for the bleached pulp products are in paper-making and as an absorbent material (fluffed pulp) in absorbent disposable sanitary products. One preferred subsequent process is to dry the pulp directly by contacting it with a gas at a temperature of 300-600"C in or at the entry of a high turbulence mixer through which the pulp and hot gas pass and which exerts a shearing action on the pulp to separate fibre bundles in the pulp as described and claimed in our Specification No. 1,564,202 (Application No.

51255/75). The dried pulp can be used in the dry forming of paper or board or can be consolidated to form sheets of fluffing pulp.

Alternatively, the pulp can be screened by a wet process and can then be further treated by known procsses, for example for use in either wet or dry paper-making or as fluffing pulp.

WHAT WE CLAIM IS: 1. A process for the preparation of bleached wood pulp comprising pulping wood chips in a single pulping step in a thermomechanical pulping machine and introducing from 0 5 to 5 per cent by weight of hydrogen peroxide, based on the dry wood solids in the pulp, into the pulping machine to bleach the wood during the pulping process.

2. A process according to claim 1 in which preheated wood chips are pulped at 100 tollO C. in a double disc refiner.

3. A process according to claim 2 in which the hydrogen peroxide in aqueous solution is introduced through an inlet whereby it contacts the wood chips at the centre of the double disc refiner.

4. A process according to any of claims 1 to 3, in which the hydrogen peroxide is introduced as a 0 5 to 10 per cent by weight aqueous solution of pH 9 to 12.

5. A process for the preparation of bleached wood pulp substantially as hereinbefore described with reference to the accompanying drawing.

6. Bleached wood pulp produced by the process of any of claims 1 to 5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

way of example with reference to the single Figure of the accompanying drawing which is a diagrammatic sectional side elevation of an apparatus suitable for producing bleached wood pulp by the process according to the invention.

The apparatus comprises generally a preheater 1 and a double disc refiner 2.

Wood chips are fed to the pre-heater 1 through an inlet 11. The pre-heater is in the form of a screw conveyor. The residence time of the wood chips in the pre-heater can be controlled by controlling the speed of the screw conveyor and is preferably from 0.5-3 minutes.

Steam is fed to the pre-heater through an inlet 15.

Wood chips emerging from the pre-heater 1 pass through a rotary valve 17 and a chamber 18 to a feeder 19 which has a screw conveyor.

It feeds the wood chips to the double disc refiner 2.

The double disc refiner 2 comprises two contrarotating discs 31 and 32 mounted on shafts 33 and 34 respectively which rotate about a common axis. The discs 31 and 32 have opposed grinding surfaces 35 and 36, respectively, seperated by a narrow gap 41. Wood chips are fed by the feeder 19 to a chamber 37 behind the disc 31. From this chamber the wood chips pass through two inlet ports 38 and 39 in the disc 31 to the central region 40 of the double disc refiner.

To pass out of the refiner 2, the wood chips must pass from the central region 40 through the gap 41 to the rim 42 of the discs. The wood chips are ground to pulp in the gap 41.

A 2 per cent by weight aqueous solution of hydrogen peroxide, is supplied to the double disc refiner through an inlet 43 feeding a chamber 44. The hydrogen peroxide solution passes to the central region 40 of the double disc refiner through ports 45 and 46 in the disc 32. The heat generated in the grinding causes some of the water in the wood chips and added through the ports 45 and 46 to be released as steam. The steam can vent back through the ports 38 and 39, the chamber 37 and the feeder 19 to the chamber 18. The chamber 18 is provided with a vent pipe 22 controlled by a valve 23 so that when excess pressure of steam builds up in the chamber 18 it can be vented out of the system. The temperature in the double disc refiner 2 is thus prevented from becoming too high.

Substantially all the hydrogen peroxide added through inlet 43 is thus constrained to pass through the grinding zone 41 of the double disc refiner with the wood chips as they are pulled between the grinding surfaces

35 and 36. The hydrogen peroxide bleaches the wood as it is being pulped.

We have found that the whiteness of the bleached pulp produced according to the invention is at least as good as is achieved in conventional processes using a time much longer than the one second or less which it takes for bleaching to be completed in the double disc refiner. We have found that the absorbency is also as is achieved with conventional processes.

Typical uses for the bleached pulp products are in paper-making and as an absorbent material (fluffed pulp) in absorbent disposable sanitary products. One preferred subsequent process is to dry the pulp directly by contacting it with a gas at a temperature of 300-600"C in or at the entry of a high turbulence mixer through which the pulp and hot gas pass and which exerts a shearing action on the pulp to separate fibre bundles in the pulp as described and claimed in our Specification No. 1,564,202 (Application No.

51255/75). The dried pulp can be used in the dry forming of paper or board or can be consolidated to form sheets of fluffing pulp.

Alternatively, the pulp can be screened by a wet process and can then be further treated by known procsses, for example for use in either wet or dry paper-making or as fluffing pulp.

WHAT WE CLAIM IS: 1. A process for the preparation of bleached wood pulp comprising pulping wood chips in a single pulping step in a thermomechanical pulping machine and introducing from 0 5 to 5 per cent by weight of hydrogen peroxide, based on the dry wood solids in the pulp, into the pulping machine to bleach the wood during the pulping process.
2. A process according to claim 1 in which preheated wood chips are pulped at 100 tollO C. in a double disc refiner.
3. A process according to claim 2 in which the hydrogen peroxide in aqueous solution is introduced through an inlet whereby it contacts the wood chips at the centre of the double disc refiner.
4. A process according to any of claims 1 to 3, in which the hydrogen peroxide is introduced as a 0 5 to 10 per cent by weight aqueous solution of pH 9 to 12.
5. A process for the preparation of bleached wood pulp substantially as hereinbefore described with reference to the accompanying drawing.
6. Bleached wood pulp produced by the process of any of claims 1 to 5.