GB2035839A - A method of making paper pulp stock, and stock made by the method - Google Patents

Abstract

Logs 2 are fed through a chamber to be wet ground on a grinding stone 3 dipping into pulp 9 in a pulp pit 4. The pulp is stirred by a stirrer 15 in a pit extension 16 with a baffle 17 and overflows a weir 8 into a channel 10 for extraction through a pump 11. Above-atmospheric pressure in the chamber 12 is maintained by compressed air through a pipe 14 and valve 13 and steam through a pipe 19 and valve 18. Aqueous filler suspension and water are supplied as selected through jet pipes 5, 5a, 6 and 7. The heat generated by grinding causes lignin and other thermoplastic substances to become sticky and bind the filler material particles to the wood fibres. <IMAGE>

Description

SPECIFICATION A method of making paper pulp stock, and stock made by the method This invention relates to a method of making paper pulp stock, and stock made by the method.

The present invention is intended to improve the properties of paper pulp stock made by mechanically comminuting wood material, and to increase the retention grade of its filler content to the end of improving, without any chemical reaction, the optical and printing characteristics of the paper made from said pulp. The pulp is preferably made by comminuting or defibrating wood material which is initially in log form.

There are two main ways of mechanically defibrating or comminuting wood. The oldest way involves a grinding-process, wherein wet logs of suitable lengths are urged against the coarse-surfaced mantle of a cylindrical grinding stone which is being rotated. Water is also conducted in appropriate quantity on to the grinding surface, its presence being indispensable, as dry wood grinding would produce worthless wood flour. As a result of the process, virtually all of the wood material run through the process is converted into a fibre/water slurry, splinters and slivers excluded. Fibre material of this type is used for raw material of newsprint, for instance.

In a more recent mechanical pulp manufacturing method, the wood raw material is introduced into the process in the form of chopped wood, that is of wet chips, these being defibrated, for instance, between two disk-type grinding elements rotating against each other and having appropriate surface structures.

This method also results in a paper pulp slurry which is suitable, after screening, for use in paper-making.

It is well known that wood often contains between 20 and 30 per cent by dry weight of lignin, which is a high-molecular aromatic substance and resembles glue in that it binds the wood fibres together. The greater part of the lignin is dissolved in chemical cooking processes but remains on the fibres during mechanical defibration processes.

It is characteristic of both chemical and mechanical pulp manufacturing methods that all the constituents of the wood, that is the cellulose fibres and the lignin bonding them mutually, are present in the pulp fibre stock obtained, even if modified.

During mechanical grinding an appreciable amount of heat is generated, as mechanical energy is converted into heat, causing the pulp stock temperature to rise to a level sufficient to render the lignin soft and sticky.

These two circumstances are significant with a view to improving the properties of paper pulp stock made of wood by mechanical means for the purpose of the present invention. It is also well known that suitable chemically inert filler materials, usually of mineral origin, such as talc or china clay, are added to pulp stock.

The use of mineral filler materials in paper is practised to improve the appearance of the paper, but above all, for the reason that these additives improve the printing properties of the paper. There are two different ways in which the filler materials may be added: by admixture to the paper stock or by coating the paper web with filler material. When added to the pulp, the filler materials are added in suspension form to the pulp stock slurry prior to its entering the paper making machine. To accomplish this, the filler materials, made up to a 30 to 40 per cent aqueous suspension, are introduced into the mixing chest preceding the high consistency stock chest. Talc, for instance, may be added as an aqueous suspension on the intake side of the so-called headbox pump.

The quantities used when the filler materials are added to the paper stock vary from 2 to 40 per cent, calculated on the weight of the completed paper, depending on paper grade.

Commonest are filler contents between 5 and 20 per cent by weight. The commonest fillers used are talc, clay (kaolin), chalk, and equivalent. The use of high quality filler materials for appreciably increasing the opacity of the paper, such as titanium oxide and zinc sulphide, has recently increased. It may be observed that the use of filler materials is intended to improve the opacity and brightness of the paper, to add to its ink receptivity, and to improve the smoothness and finishing capacity.

A problem invariably encountered when using filler materials, is unsatisfactory retention to the fibres. It is certainly possible to admix filler material to the pulp stock slurry to produce a homogeneous aqueous suspension of fibres and filler particles. But when this suspension is conducted on to the paper-making machine wire, where the water is drawn off and the fibres combine to form a continuous web, what happens is that a considerable proportion of the filler material escapes with the water, and the part staying in the web tends to concentrate in one or other superficial part of the web, giving rise to so-called one-sidedness. It is possible to improve the retention and to eliminate the drawbacks mentioned, in part at least, by using certain additives. As a rule, however, such are comparatively expensive, and other disadvantages may be associated with them.According to the present invention there is employed a method of making paper pulp stock, including the step of comminuting wood material mechanically to produce fibres, characterised in that filler material particles are brought into connection with the fibres whilst the lignin or other thermoplastic material bound to the fibres, as a result Of the high temperature generated during the comminuting process, is soft plasticised and sticky and is able to enhance the adhesion of the filler material particles to the fibres.

Thus the present invention can improve the filler retention without using any extra retention aids, in particular in the case of paper pulp stock mechanically made from wood, where the fibres have retained their inherent lignin. As stated, lignin at a high enough temperature is soft and sticky, and the partly lignin-covered fibres are then able with ease to bind on their surface a greater amount of filler particles than usual.

However, the said temperature at which lignin displays this property of stickiness which is important in view of filler retention prevails only during or immediately after the comminuting or defibrating step. Therefore, in order to obtain the desired result, the filler materials must be conducted into contiguity with the fibres at a stage in which the temperature is high enough.

Although in the following lignin is mentioned with particular emphasis as the constituent of wood or equivalent which becomes plastic or sticky under a temperature increase from 100 to 1 70 Centigrade, for instance, it is to be noted that certain paper stocks have other constituents, hemicellulose for instance, which become plastic in like manner as lignin, from the viewpoint of the present invention.

Therefore, such constituents fall within the scope of the invention as well. For instance, hardwood pulps and paper stock made of bagasse contain substantial amounts of hemicellulose, which behaves in a manner substantially similar to the behaviour of lignin, with a view to the retention of filler materials.

It should be noted that the aim of the present invention is to improve the retention of filler materials by physical means, in other words making use of the ability of the lignin or other sticky substances in the fibres to bind, like glue, filler particles on the fibre surfaces at a suitable temperature.

Although steam or other means may be used to heat the pulp, for economy those filler materials which are to be used in the paper that is to be manufactured are brought into contiguity with the fibres at a stage such that in this stage, owing to the high temperature produced in connection with defibration, the lignin or other substance bound to the fibres is soft, plastic and sticky and is able to make the filler particles adhere to the fibre surfaces.

How the invention may be put into practice is described below with reference, by way of example, to the accompanying drawing, which is a schematic view of a grinding machine.

The illustrated grinding machine comprises a chamber 1 for the logs 2 to be ground, a grinding stone 3 rotating in the direction indicated by arrow, and a pulp pit 4. Known means such as a hydraulically loaded plunger (not depicted) for instance, are employed to urge the logs 2 against the surface of the stone 3. The water indispensable in the grinding process is directed on the surface of the stone 3 by means of the jet pipe 5. The pulp produced in the process collects as a fibre/water suspension 9 in the pit 4 under the stone 3 and runs over an overflow weir 8 into a channel 10, whence the stock is for warded by a pump 11 to a screening appa- ratus (not depicted) for the removal of splinters, etc., and further to the paper-making process.The grinding stone 3 and stock pit 4 are enclosed in a hermetically sealed chamber 12, where it is possible if required to maintain a pressure over atmospheric pressure, e.g. by the aid of compressed air or steam introduced through a pipe 1 4 provided with a valve 1 3.

The stock pit 4 comprises an extension part 16 with a mixer 15, and a baffle 17 guides the continuous stock flow entering the pit 4 to flow through the extension part 1 6 of the pit 4 towards the overflow weir 8. The apparatus also comprises a jet pipe 6, through which filler suspension is conducted on to the surface of the grinding stone 3 in the form of an aqueous suspension of approximately 30 to 40 per cent by weight. Another jet pipe 7 serves to supply filler in among the pulp on the surface of the stone 3. The introduction of filler material may be through one or both of the two jet pipes 5 or 6, depending on circumstances. To the pit 4 is connected a steam pipe 1 9 with a valve 1 8 for the heating of the pulp 9 in case additional heat is required in the process.The task of the mixer 1 5 is, on one hand, to maintain the pulp flow through the extension part 1 6 and, on the other hand, to provide for efficient mixing of the filler materials in the stock.

The power brought to the shaft of the grinding stone 3 is converted to heat in the grinding process, and the temperature of the fibre pulp produced may in the grinding zone rise to 100" to 1 30" Centrigrade, similarly as in the process wherein a disk grinder is used.

Hereby the lignin or equivalent in the fibres softens and becomes sticky.

It is however necessary to dilute the fibre stock obtained by grinding, to a consistency such that it can be pumped. The adding of cold dilution water, which may be accomplished through a jet pipe 5a, rapidly lowers the temperature of the pulp suspension far below 100 Centigrade.

If it is desired to utilise the capacity of lignin-containing fibres to bind filler material on their surface, the addition of filler must take place in connection with or immediately after the grinding. The filler suspension may be conducted on to the bare surface of the grinding stone 3 at a point before the grind ing zone. But if this should interfere with the grinding, it is also possible to add the filler immediately after the grinding zone, on to the pulp-covered surface of the grinding stone. In order to ensure adhesion of the filler particles, endeavours should be taken to prolong the dwelling time of the pulp in the pit 4, this being done by increasing the capacity of the pit 4 with the aid of the extension part 1 6.

The same end, i.e. positive fixation of the filler, is served by the mixer 1 5 placed within the extension part 1 6 and by the possibility of heating the pulp with the aid of steam.

It is well known that the production of high grade groundwood implies that the logs subjected to the process are wet and that enough water is present during the grinding process proper. Much heat is produced in the process, and the temperature rise past 100 Centigrade may result in fast evaporation of water and drying out of the material in the grinding zone. It is possible, with a view to preventing this, to effect the grinding process in a pressure-tight chamber in which pressure over atmospheric pressure is maintained, with the aid of compressed air, for instance. The higher pressure may also be produced by the aid of the steam used to heat the pulp in the pit 4 under the grinding stone 3.

In some grades of paper pulp stock heretofore no fillers have been used, but in these grades the use of fillers may now be commenced on account of the advantages afforded by the invention. These applications include newsprints, particularly when endeavours are made to reduce the base weight of newsprint, which is a general aspiration nowadays. This now becomes possible, especially owing to the improved opacity of the paper.

Claims (7)

1. A method of making paper pulp stock, including the step of comminuting wood material mechanically to produce fibres, characterised in that filler material particles are brought into connection with the fibres whilst the lignin or other thermoplastic material bound to the fibres, as a result of the high temperature generated during the comminuting process, is soft plasticised and sticky and is able to enhance the adhesion of the filler material particles to the fibres.

2. A method according to Claim 1, wherein some or all of the filler material particles are introduced prior to the comminuting process.

3. A method according to Claim 1 or 2, wherein some or all of the filler material particles are introduced during or immediately after the comminuting process.

4. A method according to Claim 1, 2 or 3, wherein the comminuting process is effected at a pressure above atmospheric pressure.

5. A method according to Claim 4, wherein the pressure above atmospheric pressure is created by means of steam introduced into the materials being treated.

6. A method according to Claim 1, 2, 3, 4, or 5, wherein the filler material particles are brought into connection with the fibres in a pulp stock tank in association with a machine which effects the comminuting process.

7. Paper pulp stock when made by a method according to any one of the preceding Claims.