GB2292158A

GB2292158A - Method of sizing paper

Info

Publication number: GB2292158A
Application number: GB9415013A
Authority: GB
Inventors: Richard Whalley; Michael James Jaycock
Original assignee: Roe Lee Paper Chemicals Co Ltd
Current assignee: Ciba Specialty Chemicals RC GB Ltd
Priority date: 1994-07-26
Filing date: 1994-07-26
Publication date: 1996-02-14
Anticipated expiration: 2014-07-26
Also published as: GB9415013D0; GB2292158B

Abstract

In a method of sizing fibrous materials used in the production of paper in which a suspension of the fibres passes to a mixing device (2) in which the suspension is mixed with water to effect dilution thereof, a resin or rosin based sizing emulsion and a metal salt (e.g. alum) for effecting deposition of the rosin or resin on to the fibres are added as separate streams (3, 4) such that mixing of these streams is first effected by the action of the mixing device. In accordance with a second aspect of the invention, the metal salt is added after the sizing emulsion and within 10 metres upstream of the mixing device. The rosin size and alum may respectively be fed to lines conveying paper stock and backwater to the mixing device. The method is particularly suited to stocks containing chalk. <IMAGE>

Description

SIZING METHOD The present invention relates to a sizing method used in the production of paper.

The term paper as used herein is generic to paper, paperboard and like fibrous sheet materials which are generally (but not necessarily) of a cellulosic nature.

The process for manufacturing paper comprises preparing a suspension of fibres (usually cellulose fibres) which is treated with a resin or rosin based sizing emulsion and a metal salt (usually an aluminium salt) which is effective for causing the resin or rosin to be deposited on the fibres.

More particularly, the suspension is generally formed as so called thick stock having a solids content of 2-5%, depending on the type of paper being produced as well as the design of the machine on which the paper is being produced and the conditions lender which it is operated. The thick stock is then supplied to a mixing device (usually referred to as the fan pump) at which the suspension is mixed with water to effect dilution. Subsequently the dilute suspension (called 'thin stock') is passed to the papermaking wire or former (hereinafter referred to generally as a wire) on which the suspension is deposited. A vacuum is applied to that side of the wire opposite to the side on which the suspension is deposited so that water is drawn through the wire to leave a sheet which may then be dried and processed as required.

WO-A-91/02119 (Roe Lee) discloses a sizing technique which is particularly useful for the sizing of fibres in a papermaking stock which includes calcium carbonate (either added as a filler or present due to the recycling of calcium carbonate containing paper) such that the pH of the stock is above 6. This prior technique involves providing within the fibrous stock a freshly prepared mixture of discrete streams of a rosin emulsion and of an aluminium salt which is at an acidic pH less than that of the stock, and causing the freshly prepared mixture to be dispersed within the suspension whereby the mixture undergoes a pH transition which is effective to cause rosin to be deposited on the fibres.

The freshly prepared mixture may be produced in various ways.

Thus, for example, Fig. 4 of WO-A-91/02119 discloses a pipe arrangement in which rosin and alum (or other aluminium salt) are supplied under turbulent flow conditions and the resulting mixture formed at the centre of the pipe issues from an aperture in the wall thereof. In other arrangements shown in WO-A-91/02119, the rosin and aluminium salt are introduced along separate pipes which have their ends in close proximity to ensure mixing of the streams thus producing a mixture which may then be dispersed through the suspension.

The technique disclosed in WO-A-9i/02119 provides effective sizing but we have found occasionally the disadvantage that deposits are formed at the outlets of the pipe(s) introducing the rosin and aluminium salt and that these deposits cause operating difficulties in a paper mill.

It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages.

According to a first aspect of the present invention there is provided a method of sizing fibrous materials used in the production of paper in which a suspension of the fibres passes to a mixing device in which the suspension is mixed with water to effect dilution thereof wherein a resin or rosin based sizing emulsion and a metal salt for effecting deposition of the rosin or resin on the fibres are added as separate streams such that mixing of said streams is first effected by the action of the mixing device.

In the method of the invention, the sizing emulsion and the metal salt are therefore added as separate streams which only become mixed together by the action of the mixing device which, in addition to producing the dilute suspension, effects mixing of the sizing emulsion and metal salt throughout the suspension of fibrous material.

We have found that mixing of the sizing emulsion with the metal salt in this way avoids the deposit problems mentioned about whilst still effecting sizing of the fibres.

The mixing device referred to above will generally be the fan pump of a papermaking machine which effects dilution of a suspension of fibres with water. The suspension of fibres to be diluted is generally called "thick stock" (which usually contains 2-5% solids depending on the type of paper being produced and the machine design and conditions. The water with which the thick stock is diluted will generally be that returned from the papermaking wire (known in the art as "backwater"). The sizing emulsion will generally be a fortified rosin and may be a cationic or anionic emulsion. The metal salt (which will contain a hydrolysable metal ion) will generally be an aluminium compound, e.g. papermakers alum (hereinafter referred to for simplicity as alum), aluminium chloride or polyaluminium chloride.

Alternatively the metal salt may be an iron (III) compound, e.g. the chloride.

In one preferred embodiment of the first aspect of the invention, the method is applied to a papermaking process in which the flow of the fibrous suspension to the mixing device (i.e. the flow in the thick stock pipe) is designed to be substantially laminar or streamlined. In this case, sizing emulsion and the metal salt may be introduced into the thick stock pipe via separate addition conduits such that separate streams of the sizing emuls;.on and metal salt move along the thick stock pipe to the mixing device. Preferably the outlet of one conduit is on the opposite side of the centre line of the thick stock pipe to that of the other conduit.The transverse spacing of the outlets of the conduits across the thick stock pipe needs to be such that, under the flow conditions prevailing in these conduits and in the thick stock pipe, separate streams of sizing emulsion and metal salt are formed (without contact of the two streams) and carried by the substantially laminar flow conditions prevailing in the thick stock pipe as separate streams until they are mixed (together with the fibrous suspension) by the mixing device. In this embodiment, the ends of the addition conduits will typically be spaced from each other, across the width of the thick stock pipe, by a distance of 15-50% of the width of the thick stock pipe. Thus, purely by way of example, for a thick stock pipe having a diameter of 6" (15cm) the ends of the conduits may be spaced by 2" (5cm) (this distance may be increased proportionally if desired for larger stock pipes).

Further factors which need to be taken into account for this first embodiment of the invention are the positioning of the addition conduits in relation to the mixing device and the flow rates of the sizing emulsion and metal salt through their respective conduits.

This is to ensure that neither the sizing emulsion nor metal salt becomes dissipated within the fibre suspension such that mixing of the sizing emulsion and metal salt will take place before the mixing device. To achieve these aims, the conduits will usually be provided as close as practicable to the mixing device. Ideally this will be within a metre or so of the mixing device but may be further if machine design dictates. Furthermore the addition conduits will generally have a fluid flow passageway with a diameter of 1-5 cm.

Flow rates through these conduits will typically be in the range 20300 litres per hour.

If desired, the addition conduits (in addition to having their outlets transversely spaced across the pipe) may have their outlets longitudinally staggered along the thick stock pipe. This permits the easy inspection and cleaning of the conduits. The longitudinal separation of the two conduits is normally less than 50 cm. In this case, it is preferred that the downstream conduit is the one used for addition of the metal salt.

In an alternative embodiment of the invention, the sizing emulsion and metal salt are added one to the thick stock pipe and the other to the water supply to the mixing device. Once again, both additions are made relatively close to the mixing device (e.g. ideally within a metre or so but inlay be further away if machine design dictates). In this case however, it is possible for the additions to be made close to the centre lines of the thick stock pipe and the water pipe where (assuming laminar flow) the velocity is greatest so as to ensure rapid transport of the additions to the mixing device.

In this embodiment it is preferred that the metal salt addition is made to the water rather than the thick stock. This is particularly so if the thick stock is at a higher temperature than the water and the salt is alum which appears to be effected by high temperature.

The method of the first aspect of the invention may be used under a wide range of pH conditions (e.g. for acid sizing). The method is however particularly effective for sizing fibres contained in a papermaking stock which incorporates calcium carbonate such that the stock normally has a pH of at least 7, e.g. 7-8. Assuming that the metal salt is alum, the alum stream goes from an initial low pH to near neutral during mixing and sc;icient: hydrolysed s?ecies appear to be formed, and have a sufficiently long life, to make the sizing process work at near neutral pH.

The addition points proposed in the above two embodiments represent a compromise between the desirability of forming the required species for sizing as near the papermaking wire as possible and the requirement of thorough mixing. Because of the construction of the housing of the fan pumps normally used for mixing, it is not usually possible to make the additions of the sizing emulsion and metal salt to the fan pump itself, although such a method would work well.

A second aspect of the invention relates to a modification of the first aspect. In accordance with the second aspect there is provided a method of sizing fibrous materials used in the production of paper in which a suspension of fibres having a pIS of at least 7 passes to a mixing device in which the suspension is mixed with water to effect dilution thereof wherein a resin or rosin based sizing emulsion is added to the suspension prior to the addition of a metal salt for effecting deposition of the rosin or resin on the fibres, the metal salt being added to the suspension within 10 metres upstream of the mixing device.

The method of the second aspect of the invention is particularly applicable to the sizing of fibrous suspensions containing chalk, preferably such that the suspension has a pH of 7-8.

As described above for the first aspect of the invention, additions of sizing emulsion and metal salt may be made by longitudinally staggered conduits, the metal salt normally being added via the downstream conduit. For the first aspect of the invention, the longitudinal spacing of the two conduits is normally less than 50 cm.

In principle however (and this forms the basis for the second aspect of the invention) the size addition point could be moved back to an earlier point in the stock approach flow, further from the mixing device. This is because the character of the rosin emulsion, if for example it is a casein stabilised rosin emulsion manufactured by the Bewoid process, is usually not appreciably altered during the residence time in the papermaking machine. On the other hand, as explained earlier, the same is not true of the metal salt (e.g. papermakers alum) hence the choice of position of the metal salt addition point is close to the mixing device. In the context of the second aspect of the invention this will be within 10 metres of the mixing device, more preferably within 5 metres, e.g. within 4 metres.Ideally the metal salt addition will be within 3 metres, even more preferably within 2 metres, most preferably within 1 metre or so of the mixing device. For preference, the metal salt addition is made as close as practicable to the mixing device. The exact distance may therefore be dictated by machine design.

The method of the second aspect is particularly applicable to a papermaking process in which the flow of the fibrous suspension to the mixing device is designed to be substantially laminar or streamlined. The addition of the metal salt may be made in the manner described for the first aspect of the invention. Other features of the first aspect of the invention are applicable mutatis mutandis to the second aspect.

The invention will be further described by way of example only with reference to the accompanying drawings, in which: Fig. 1 schematically illustrates one embodiment of the first aspect of the invention; Figs. 2-4 are variations of the method shown in Fig. 1; and Fig. 5 schematically illustrates a further embodiment of the first aspect of the invention.

Fig. 1 illustrates a thick stock pipe 1 along which thick stock moves under substantially laminar flow conditions to a fan pump 2 where it is diluted with water fed as a separate supply (not shown).

Extending into the thick stock pipe 1 are addition conduits 3 and 4 having outlet ends which are positioned directly opposite to each other on opposite sides of the centre line of the thick stock pipe.

The outlets of conduits 3 and 4 are within 1 metre of the fan pump.

Rosin emulsion is fed along conduit 3 and alum solution is fed along conduit 4. The flow conditions are such that the rosin and alum exit their respective conduits 3 and 4 and travel as separate streams (as represented by arrows 5 and 6 respectively) to the fan pump 2 at which mixing of the rosin and alum (together with the fibrous stock) is effected.

The arrangement shown in Fig. 2 is similar to that of Fig. 1 save that the addition conduits are at tiiffercnt angles to those shown in Fig. 1.

In the arrangement shown in Fig. 3 the addition conduits again have their outlets on opposite sides of the centre line of the thick stock pipe but in this case the outlets are staggered in the longitudinal direction of the pipe. The upstream addition conduit is within 1 metre or so of the fan pump. The longitudinal spacing of the conduits is normally less than 50 cm. As in the case of Fig. 1, the rosin and alum travel as separate streams until they reach the fan pump.

Fig. 4 is similar to Fig. 3 save that the conduits are at different angles to those shown in Fig. 1.

In the method depicted in Fig. 5, a fan pump 10 is supplied with thick stock along a line 11 and with backwater along a line 12.

Laminar flow conditions prevail in each of lines 11 and 12. A rosin addition conduit 13 is provided in line 11 and an alum addition conduit 14 is provided in line 12. Each of conduits 13 and 14 extend to the centre of the line in which they are provided. Rosin and alum flows remain separate until mixed by the fan pump.

For the second aspect of the invention an arrangement as shown in any one of Figs. 1 to 4 may be used with the modification that the conduit 3 is omitted and the rosin emulsion is made further upstream.

The invention is further illustrated by the following nonlimiting examples.

Example 1 In a small paper mill on a machine making 1.4 - 1.8 tonne per hour of envelope grade paper from carbonate containing mixed waste paper, 30% rosin emulsion was added to the thick stock line (stock pH approx. 6.7) 1 metre before the fan pump, and 47% papermakers' alum was added to the backwater line 50cm before the fan pump, as illustrated in Fig. 5. Sizing was achieved with 0.9% dry basis addition of size and 1.896 dry basis addition of papermakers' alum. The resulting 1 minute Cobb test value was 40 g m-2.

Example 2 In a medium size mill also making envelope grade paper from waste paper the thick stock line leading to the primary fan pump was fitted with two pipes carrying separately rosin size and alum, each extending to 1 inch from the centre line of the pipe, using the arrangement shown in Fig. 2. This addition point is approximately 2 metres from the fan pump which was as close as practicable and 30% rosin emulsion and 47% papermakers' alum were added to give addition rates of 1.8% of rosin calculated on a dry basis, and 3.9% alum, also on a dry basis. The resulting Cobb was 25 g m-2 and the pH was 6.35.

Claims

1. A method of sizing fibrous materials used in the production of paper in which a suspension of the fibres passes to a mixing device in which the suspension is mixed with water to effect dilution thereof wherein a resin or rosin based sizing emulsion and a metal salt for effecting deposition of the rosin or resin on the fibres are added as separate streams such that mixing of said streams is first effected by the action of the mixing device.

2. A method as claimed in claim 1, wherein the sizing emulsion and the metal salt are introduced through separate conduits into a pipe in which the suspension is moving under substantially laminar flow conditions.

3. A method as claimed in claim 2, wherein the outlet of one of said conduits is on the opposite side of the centreline of said pipe than the outlet of the other conduit.

4. A method as claimed in claim 2, wherein the transverse spacing of the outlets is 15-50% of the width of said pipe.

5. A method as claimed in any one of claims 2 to 4, wherein the outlets of said conduits are longitudinally staggered along the length of said pipe.

6. A method as claimed in claim 1, wherein the sizing emulsion and the metal salt are added one to the suspension and the other to the water supply to the mixing device.

7. A method as claimed in claim 7, wherein the sizing emulsion is added to the suspension and the metal salt is added to the water supply.

8. A method as claimed in claim 7 or 8, wherein said additions are made at or near the centre lines of the pipe carrying the fibrous suspension and the pipe carrying the water supply to the mixing device.

9. A method as claimed in any one of claims 1 to 8, wherein all additions are made within 1 metre or so of the mixing device.

10. A method as claimed in any one of claims 1 to 9, wherein the sizing emulsion comprises a fortified resin.

11. A method as claimed in any one of claims 1 to 10, wherein the sizing emulsion is an anionic resin emulsion.

12. A method as claimed in any one of claims 1 to 10, wherein the sizing emulsion is a cationic rosin emulsion.

13. A method as claimed in any one of claims 1 to 12, wherein the metal salt is an aluminium salt.

14. A method as claimed in claim 14, wherein the metal salt is alum, aluminium chloride or polyaluminium chloride.

15. A method as claimed in any one of claims 1 to 14, wherein the dispersion contains chalk such that the pH of the dispersion is at least 7.

16. A method of sizing fibrous materials used in the production of paper in which a suspension of fibres having a pH of at least 7 passes to a mixing device in which the suspension is mixed with water to effect dilution thereof wherein a resin or rosin based sizing emulsion is added to the suspension prior to the addition of a metal salt for effecting deposition of the rosin or resin on the fibres, the metal salt being added to the suspension within 10 metres upstream of the mixing device.

17. A method as claimed in claim 16 wherein the metal salt addition is made within 5 metres of the mixing device.

18. A method as claimed in claim i7 wherein the metal salt addition is made within 4 metres of the mixing device.

19. A method as claimed in claim 18 wherein the metal salt addition is made within 3 metres of the mixing device.

20. A method as claimed in claim 19 wherein the metal salt addition is made within 2 metres of the mixing device.

21. A method as claimed in claim 20 wherein the metal salt addition is made within 1 metres of the mixing device.