GB1603711A - Method and apparatus for preparing thermomechanical paper-making pulp and products thereof - Google Patents

Description

(54) A METHOD AND APPARATUS FOR PREPARING THERMOMECHANICAL PAPER-MAKING PULP AND PRODUCTS THEREOF (71) We, VALMET OY of Punanoikonkatu 2, 00130 Helsinki 13, Finland, a Finnish body corporate, 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:- This invention relates to a method and apparatus for preparing thermomechanical paper-making pulp, and to pulp when made by the method of the apparatus.

Thermomechanical paper-making pulp is generally produced by grinding woody material in a grinder, preferably utilizing mutually opposed grinding discs which rotate relative to each other, the material therein being heated so that the plasticisable constituents which are present, for example lignin or hemi-cellulose, become plasticized and softened, thereby causing the fibres of the woody material to separate at least partially.

The grinding is usually accomplished in two grinding steps, of which the primary grinding is carried out in a grinder subject ot internal overpressure. The pressure varies between 1.5 and 3.5 bar (absolute), and the temperature in the grinder is the saturated steam temperature corresponding to this pressure. The secondary grinder operates at atmospheric pressure.

The woody material, after washing with water, is fed by means of a plug screw or rotary seal into a preheater and heated with steam. The same pressure as in the primary grinder prevails in the preheater. From the preheater the material is conveyed by screws into the primary grinder, whence the fibres are blown (by aid of the steam generated therein) to the second grinding step. The steam and fibres are separated by means of a cyclone prior to the second grinding step.

From the secondary grinder the fibres are usually conducted into a pumping tank under the grinder, where the pulp stock is diluted to desired consistency, and then the stock is pumped to a screening step.

The softening of the lignin with the aid of heat permits the production of long, pliant and undamaged fibres. This way of producing thermomechanical pulp results in a yield which is almost equal in weight to the weight of the original woody material consumed. The difference between this thermomechanical pulp and chemical pulp is that in thermomechanical pulp the lignin remains in the pulp itself and is therefore present in the paper which is manufactured therefrom.

It is well known that woody materials often contain 20 to 30% by weight of lignin.

Lignin is a generic term for the celluloselike substances which line woody fibres, and includes lignose, lignon and lignin which is a polymeric aromatic substance. The proportion of lignin in the woody material is highest in the intercellular central lamellae.

When the lignin is heated and softened in the manner described above, the wood fibres can be detached from each other without great damage.

Recent studies have shown that thermomechanical pulp appears to comprise long pliant opened fibres, which are able to form a great number of bonds in a paper sheet.

The "Fines" present in the pulp consist mainly of long fibrils or parts of fibrils, which contribute to the establishment of bonds between long fibres. Up to quite recently, in the manufacture of newsprint, there has been used by weight about 75% of thermomechanical pulp together with 25% of chemically produced cellulose, which was added to improve the properties of the pulp. One of the drawbacks recognised is that in chemical defibration the pulp yield is only about one-half the weight of the woody material which is consumed.

By the use of thermo-mechanical pulp alone it has been possible to develop a light newsprint having a basis weight substantially lower than that of commonly used newsprint, which was of the order of about 50g per m2.

A further advantage of the use of thermomechanical pulp is that as raw material it is possible to utilize partly such materials as, for example, sawdust, which heretofore has been inappopriate for the production of paper pulp in general and of mechanical pulp in particular.

Thus, among the advantages of thermomechanical pulp, are the following: 1. It is possible to use small-size timber or brushwood, sawdust, and other timber dressing residues, as well as pine and birch which are not suitable for the production of groundwood in the production of thermomechanical pulp.

2. It is possible to use dry wood as the raw material, because the chipped wood may be wetted.

3. The process is easily controllable and pulp of uniforms quality can be produced.

4. The space requirements of the plant and number of operating personnel can be kept low.

5. When thermomechanical pulp is used in place of groundwood both the strength and the bulk of the paper improve.

6. The quantity of added cellulose may be reduced or even completely omitted.

7. The dewatering of the pulp may be improved and the speed of the papermaking machine may be increased.

However, despite all of the considerable advantages of thermomechanical pulp, there are several drawbacks in its use, including the following: 1. The lignin becomes plastic at high temperatures and this causes blackening during the calendering treatment, which is due to the fact that the paper becomes more transparent in spots, a change which the eye registers as a darker spot.

2. Paper made from thermomechanical pulp has a yellowish colour which becomes intensified by solar effect as the paper ages.

3. Although there has been remarkable development in recent times in mechanical pulp manufacture, many problems still exist which require solution. One of these problems is how to bleach the mechanical pulp and preserve the light colour obtained.

Mineral fillers have been used in the conventional production of paper, primarily because they improve the properties of the paper with respect to printing. The addition of filler is accomplished in one of two different ways, either by pulp filling or by coating.

In pulp filling the filler is added in the form of a suspension to the pulp slurry prior to the arrival of the slurry on to the papermaking machine. This is accomplished by introducing the filler, for example as a 30% to 40% by weight aqueous suspension, into a.

mixing tank preceding the high consistency slurry tank. Talc, for example, may be added in the form of an aqueous suspension on the intake side of the so-called headbox pump.

Depending upon the type of paper to made, the material quantities employed in pulp filling vary between about 2% and 40% calculated with respect to the weight of the completed paper. The most common filler contents are between 5 and 20% by weight.

Among the common fillers which are used are talc, clay (kaolin), and chalk.

Recently there has been an increase in the use of high quality fillers such as titanium oxide and zinc sulfide for the purpose of increasing the opacity of the paper. The use of fillers results in improvement in the opacity and lightness of colour of the paper as well as an increase in absorption of the printer's ink. Still further, such fillers improve the smoothness and burnishability of the paper surface.

Among the disadvantages of fillers is that they impair the strength of paper. Furthermore, the fillers, particularly talc, settle rapidly in the slurries and thus cause problems with respect to the maintaining of a homogeneous filler mixture. Among the drawbacks of conventional filler addition procedures is the poor adhesion of the fillers to the fibres, which manifests itself in a poor retention or leakage of fillers through a forming wire along with the water removed from the web. The poor adhesion has caused, in finished paper, disadvantageous distribution of the fillers and the shedding of dust from the paper.

According to the present invention, there is provided a method of manufacturing paper-making pulp thermomechanically, in which woody material is used as the raw material and is subjected to grinding in the presence of water to produce a pulp mixture, said woody material containing plasticisable constituents, such as lignin and/or hemicellulose, which are plasticised due to elevated temperature during the grinding process, characterized in that a substantially chemically inert and waterinsoluble filler material is introduced into the pulp mixture before or during the grinding process or immediately after the grinding process while said pulp constituents are still in a plasticised state, without the addition of any other materials, and the filler material is mixed with the pulp and adheres to the plasticised constituents of the pulp.

As the filler is in contact with the plasticisable constituents whilst these are in plasticised and softened condition, the filler adheres thereto, and this has the effect of improving the properties of the resulting thermo-mechanical pulp.

It is preferable to effect the method so that considerable quantities or proportions of the filler become bound to other substances in the pulp.

Consideration can be given to an increase in the lightness and colour and opacity of paper made from the pulp, and to reducing the "blackening' ' tendency of paper made from the pulp when the same is calendered at water contents higher than 10 to 11%.

It is possible to avoid undue asymmetry and dust-raising tendency of paper made from the pulp.

It is also possible to reduce the energy losses in the grinding process, it being known that in connection with the conventional production of thermo-mechanical pulp there is often an excess of heat, the dissipation of which has given rise to considerable problems.

As indicated above, the plasticisable constituents of woods are mainly lignin, and reference will generally be made below to lignin as meaning the constituents which undergo softening and plasticising as a result of the rise in temperature during the grinding. However, it must be noted that some woody materials contain constituents other than lignin, such as hemicellulose, which become plasticised like lignin. Thus, for example, hardwoods and bagasse contain substantial quantities of hemicellulose, which behaves like lignin as regards effecting the present invention.

For the purposes of the present invention the filler which is substantially inert and substantially insoluble in water may, for example, comprise clay, kaolin, talc, calcined gypsum, barium sulfate in cylindrical granular form known as blanc fixe, barium sulfate in rhombic form known as barytes, silica, titanium dioxide (anatase or rutile), and zinc sulfide, all of which may be used alone or in mixtures for the purposes of the present invention.

The amount of filler which is added varies depending upon the final paper desired, in the same manner as the addition of filler in ordinary paper-making processes. In general, the content of filler, analysed in terms of ash content, in the pulp slurry, will vary between 1% to 7% by weight. The filler may be introduced into the paper pulp in a quantity such that the plasticised constituents become incorporated in an opaque light coloured substance which increases the lightness of colour and opacity of the paper made from the pulp.

The aqueous pulp slurry which is subjected to the grinding in the production of the thermo-mechanical pulp of the present invention preferably has a solids content of 20% to 25% by weight. Thus, the filler content of the slurry may vary between about 0.2% and 18% by weight, although further variations may be made depending upon the type of filler, the purpose for which the filler is added, and the purpose for which the final paper is to be used. The particular filler or fillers selected from among those mentioned above and others can serve for improving various properties of the paper, including the printing properties, the opacity, the lightness of colour, the absorption of printer's ink, the surface smoothness and the feel of the paper. The particular filler or mixture of fillers is dependent upon which properties are of greatest importance, and of course also on price.

The plasticisable constituents are in the softened and plasticised condition as a result of the heat resulting at least partially from the friction during the grinding. Thus, the filler can be added before the pulp reaches the grinder, in the grinder itself, or immediately after leaving the grinder, but while the plasticisable constituents are still in plasticised and softened condition. Some or all of the filler is added after the pulp has left the grinder but while the plasticisable constituents are still in plasticised condition if the grinding process would be unduly impaired by the presence of some or all of the filler in the grinder.

How the invention may be put into practice appears from the following description with reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating a method of the present invention; and Figure 2 diagrammatically illustrates an apparatus for effecting an alternative form of the method in accordance with the invention.

In both Figures 1 and 2, the symbol Mln refers to the entry of pulp constituents and Mout refers to the exit of the pulp constituents.

The block 20 represents the stage of preheating of the pulp, while block 21 represents a first grinding stage, and block 22 represents a second grinding stage. The pulp from the second grinding stage 22 is conducted to a straining and purifying stage 23. The reject from this straining stage 23 is conducted to a reject grinding stage 25 from which it is returned through a conduit c to the straining and purifying stage 23. After the second grinding stage 22 a part of the pulp is returned through a conduit b to the input side of the second grinding stage 22.

At least one filler from block 24 is introduced into the pulp through pipes or conduits al, a2 a2 and a4.

Preferably the filler is introduced into the pulp at a point 26 between the first grinding stage 21 and the second grinding stage 22, through the conduit a,. The conduits a2 and a2 represented by dotted lines in Figure 1 indicate how one or more fillers may in part or in whole be introduced directly into the first grinder 21 through conduit a2 or at its input side through conduit a. Additionally or alternatively fillers may be introduced into the pulp immediately after the grinding stages 21 and 22, for example through the conduit a4; the plasticisable constituents of the pulp must be in softened plasticised condition at the time of the addition of the filler through the conduit a4.

As shown in Figure 2 a grinder 10 is utilized in preparing the thermomechanical pulp. The grinder 10 comprises two opposed metallic grinding discs 11 and 12 which are rotated in mutually opposite directions by means of electric motors 15 and 16, respectively. The pulp, for example in the form of an aqueous slurry of 2025% solids content, is introduced into the grinder 10 by means of a positive feed screw conveyor 13.

The selected fillers, for example talc or chalk in an amount of about 0.218% by weight of the slurry, is introduced into the screw conveyor 13 at 26, the filler being supplied from the block 24 through the conduit a,.

As shown in Figure 2, the pulp raw woody material, for example sawdust or wood chips, is fed into a preheater 20 through a rotary valve 29. In the preheater 20 the woody material is heated to about 1000--130"C. The preheater 20 includes a screw conveyor and the preheated woody material leaves the preheater 20 through a rotary valve 19 and is subsequently conducted through a steam separator 17 to the screw conveyor 13 by which the preheated woody material is positively fed into the grinder 10. The screw conveyor 13 is so arranged that the steam can be conducted in countercurrent with respect to the direction of movement of the woody material.

Fresh steam is introduced into the process as required through a control valve 28, and the pressure in the preheater 20 is controlled by means of a control valve 27.

The pressure in the steam separator 17 is controlled by a similar valve.

As shown in Figure 2, the woody material is subjected to powerful mechanical treatment in the gap A between the grinding discs 11 and 12, which rotate in opposite directions. This treatment takes place partly by contact with the rapidly rotating grinding discs, but mainly as a result of the mutual contact of woody particles with each other, the friction resulting therefrom causing heating of the particles. This heating of the particles results in the softening and plasticising of the lignin or hemicellulose or other plasticisable constituents of the wood.

The softening of the lignin occurs more easily in the case of wood having a higher water content.

The fillers are admixed with the plasticised constituents in the grinder 10 and adhere to the surface of the fibres at the softened lignin, creating from the lignin and the other plasticisable constituents of the wood an opaque, light-coloured mass. The fillers may be supplied in the form of an aqueous suspension thereof into the grinder 10.

Many fillers commonly used in papermaking may be used for the purposes of the present invention. One of the most suitable of such fillers is talc, which is a particularly soft mineral with a greasy feel, white and of flaky structure. Talc is particularly appropriate due to its slipperiness which results in reduced energy losses during the grinding process.

An advantage is that it is possible for some of the wood fibres to be preserved in intact condition better by the method of the present invention than in conventional processes.

In general, the method of the present invention may be applied in all cases in which thermomechanical pulp has previously been used, particularly in the case of such pulps as those in which fillers have already conventionally been used. This includes the use thereof for photogravure and offset printing paper. In addition, the present invention is suitable for the production of some pulp products in which fillers have not hitherto been used regularly but in which it is now possible to use fillers due to the advantages of the method of the present invention. such applications includes, for example, newsprint paper, particularly when it is desired to reduce the basis weight of the paper, which reduction is of considerable economic importance nowadays. This reduction becomes possible with the method of the present invention due to the improved opacity of the paper produced.

WHAT WE CLAIM IS: 1. A method of manufacturing papermaking pulp thermomechanically, in which woody material is used as the raw material and is subjected to grinding in the presence of water to produce a pulp mixture, said woody material containing plasticisable constituents, such as lignin and/or hemicellulose, which are plasticised due to elevated temperature during the grinding process, characterised in that a substantially chemically inert and water insoluble filler material is introduced into the pulp mixture before or during the grinding processor immediately after the grinding process while said pulp constituents are still in a plasticised state without the

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. through the conduit a,. The conduits a2 and a2 represented by dotted lines in Figure 1 indicate how one or more fillers may in part or in whole be introduced directly into the first grinder 21 through conduit a2 or at its input side through conduit a. Additionally or alternatively fillers may be introduced into the pulp immediately after the grinding stages 21 and 22, for example through the conduit a4; the plasticisable constituents of the pulp must be in softened plasticised condition at the time of the addition of the filler through the conduit a4. As shown in Figure 2 a grinder 10 is utilized in preparing the thermomechanical pulp. The grinder 10 comprises two opposed metallic grinding discs 11 and 12 which are rotated in mutually opposite directions by means of electric motors 15 and 16, respectively. The pulp, for example in the form of an aqueous slurry of 2025% solids content, is introduced into the grinder 10 by means of a positive feed screw conveyor 13. The selected fillers, for example talc or chalk in an amount of about 0.218% by weight of the slurry, is introduced into the screw conveyor 13 at 26, the filler being supplied from the block 24 through the conduit a,. As shown in Figure 2, the pulp raw woody material, for example sawdust or wood chips, is fed into a preheater 20 through a rotary valve 29. In the preheater 20 the woody material is heated to about 1000--130"C. The preheater 20 includes a screw conveyor and the preheated woody material leaves the preheater 20 through a rotary valve 19 and is subsequently conducted through a steam separator 17 to the screw conveyor 13 by which the preheated woody material is positively fed into the grinder 10. The screw conveyor 13 is so arranged that the steam can be conducted in countercurrent with respect to the direction of movement of the woody material. Fresh steam is introduced into the process as required through a control valve 28, and the pressure in the preheater 20 is controlled by means of a control valve 27. The pressure in the steam separator 17 is controlled by a similar valve. As shown in Figure 2, the woody material is subjected to powerful mechanical treatment in the gap A between the grinding discs 11 and 12, which rotate in opposite directions. This treatment takes place partly by contact with the rapidly rotating grinding discs, but mainly as a result of the mutual contact of woody particles with each other, the friction resulting therefrom causing heating of the particles. This heating of the particles results in the softening and plasticising of the lignin or hemicellulose or other plasticisable constituents of the wood. The softening of the lignin occurs more easily in the case of wood having a higher water content. The fillers are admixed with the plasticised constituents in the grinder 10 and adhere to the surface of the fibres at the softened lignin, creating from the lignin and the other plasticisable constituents of the wood an opaque, light-coloured mass. The fillers may be supplied in the form of an aqueous suspension thereof into the grinder 10. Many fillers commonly used in papermaking may be used for the purposes of the present invention. One of the most suitable of such fillers is talc, which is a particularly soft mineral with a greasy feel, white and of flaky structure. Talc is particularly appropriate due to its slipperiness which results in reduced energy losses during the grinding process. An advantage is that it is possible for some of the wood fibres to be preserved in intact condition better by the method of the present invention than in conventional processes. In general, the method of the present invention may be applied in all cases in which thermomechanical pulp has previously been used, particularly in the case of such pulps as those in which fillers have already conventionally been used. This includes the use thereof for photogravure and offset printing paper. In addition, the present invention is suitable for the production of some pulp products in which fillers have not hitherto been used regularly but in which it is now possible to use fillers due to the advantages of the method of the present invention. such applications includes, for example, newsprint paper, particularly when it is desired to reduce the basis weight of the paper, which reduction is of considerable economic importance nowadays. This reduction becomes possible with the method of the present invention due to the improved opacity of the paper produced. WHAT WE CLAIM IS:

1. A method of manufacturing papermaking pulp thermomechanically, in which woody material is used as the raw material and is subjected to grinding in the presence of water to produce a pulp mixture, said woody material containing plasticisable constituents, such as lignin and/or hemicellulose, which are plasticised due to elevated temperature during the grinding process, characterised in that a substantially chemically inert and water insoluble filler material is introduced into the pulp mixture before or during the grinding processor immediately after the grinding process while said pulp constituents are still in a plasticised state without the

addition of any other materials, said filler material being mixed with the pulp and adhering to the plasticised constituents of the pulp.

2. A method according to claim 1, wherein the filler is introduced into the paper pulp in a quantity such that the plasticised constituents become incorporated in an opaque light-coloured substance which increases the lightness of colour and opacity of the paper made from the pulp.

3. A method according to claim 1 or claim 2, wherein the filler is introduced before or during the grinding and comprises talc or other filler which is slippery and reduces the energy loss during the grinding.

4. A method according to claim 1, wherein the filler is fed into the paper pulp in the grinder.

5. A method according to claims 1, 2 or 3, wherein the grinding is effected in two stages and the filler is introduced after the first grinding step and before the second grinding step.

6. A method according to claim 5, wherein the plasticisable constituents are in a substantially plasticised condition as a result of the first grinding stage.

7. A method of preparing paper-making pulp, substantially as hereinbefore described with reference to the accompanying drawings.

8. An apparatus when used for preparing pulp by a method according to any one of the preceding claims and substantially as hereinbefore described with reference and illustrated in Figure 2 of the accompanying drawings.

9. Pulp when prepared by a method according to any one of claims l to 7 or by an apparatus according to claim 8.

10. Paper when made from a pulp according to claim 9.