GB2326170A

GB2326170A - Method of sizing fibres.

Info

Publication number: GB2326170A
Application number: GB9812813A
Authority: GB
Inventors: Michael James Jaycock; Richard Whalley
Original assignee: Raisio Chemicals UK Ltd
Current assignee: Ciba Specialty Chemicals RC GB Ltd
Priority date: 1997-06-13
Filing date: 1998-06-15
Publication date: 1998-12-16
Anticipated expiration: 2018-06-15
Also published as: GB9812813D0; GB2326170B

Abstract

Rosin and aluminium salt in solution are used to size a neutral dispersion of paper-making fibres. The process is characterised in that the concentration of aluminium is at most 0.8 % by weight expressed as Al<SP>3+</SP>, and may be less than 0.1 weight percent, and the aluminum salt is added to a flowing dispersion of the fibres either separately from or simultaneously with the rosin. The dispersion of fibres is prepared in tank 1 before passing to be mixed with the sizing agents in fan pump 5. The aluminium salt and rosin are diluted and mixed with clarified water in pipe 6 prior to their addition to the solution.

Description

SIZING OF FIBRES The present invention relates to the sizing of fibres used in the production of paper (which term as used herein is intended to cover paper, board and like materials).

Paper is produced by the wet-laying of fibres from an aqueous dispersion thereof (frequently referred to as the "furnish"). It is generally the case that the fibres must be sized before they are laid on the paper-making wire (or belt) since sizing improves the properties of the finished paper with regard to water repellency. One conventionally used sizing system comprises a rosin emulsion and a soluble aluminium salt. The rosin may be one which has been fortified by the Bewoid process. The soluble aluminium salt may, for example, be aluminium sulphate (e.g. in the form of paper-makers alum [Al2(SO4)3.16-18 H20], aluminium chloride, polyaluminium chloride or aluminium chlorohydrate. Typically the amount of rosin used is such as to provide 1 to 10% by weight of rosin on the fibre. The aluminium salt:rosin ratio is typically 2-3:1.

For many years, sizing with rosin and a soluble aluminium salt was effected under acid conditions such that the furnish (after addition of the rosin and alum) had a pH of 3.5 to 5.5. However, more recently, calcium carbonate has been incorporated in the furnish as a filler. The presence of calcium carbonate means that it is not possible to effect sizing under acid conditions since the calcium carbonate would react with the acid to produce calcium sulphate and carbon dioxide. Therefore the use of calcium carbonate as a filler has led to the development of so-called neutral sizing processes in which the pH of the furnish (containing calcium carbonate) after addition of the rosin and soluble aluminium salt is at least 6.0 and more typically in the range 6.8 to 7.5.

The use of the neutral sizing conditions does however give rise to disadvantages in that solid intractable aluminium containing deposits (believed to be of amorphous Al(OH)3 and, in some cases, also containing calcium sulpho-aluminate (satin white)) are produced within the paper-making plant. These deposits may be formed along pipes within which the admixture of the furnish, rosin and aluminium compounds flow (e.g. in the thin stock pipe or at the fan pump). These deposits may require that the plant be shut down for maintenance purposes and, apart from the economic disadvantages this entails, the deposits are moreover very difficult to remove.

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 a neutral dispersion of paper-making fibres using rosin and an aluminium salt wherein the aluminium salt is added as a solution to a flowing dispersion of the fibres either separately from or simultaneously with the rosin characterised in that the concentration of the aluminium in said solution as added to the dispersion is at most 0.8% by weight expressed as Al3+.

We have found, and this forms the basis of the present invention, that addition of the aluminium (to the fibres) in the form of a solution which contains at most 0.8% by weight aluminium (based on Al3+) substantially avoids problems formed by deposits. Preferably the aluminium solution contains substantially less than 0.8% by weight of aluminium. It is particularly preferred that the aluminium solution does not contain more than 0.4% by weight aluminium, preferably not more than 0.2% and even more preferably not more than 0.1%. the solution may, for example, contain 0.05% by weight aluminium on the same basis as defined above.

In the method of the invention, the dispersion of fibres to which the aluminium salt is to be added will be at a "neutral" pH, usually (but not necessarily) in the range 7-8. This pH level is usually attributable to the presence of calcium carbonate in the dispersion.

The aluminium solution to be added to the dispersion will have an acidic pH, typically about 1. The amount of aluminium solution added to the dispersion will generally be such that the pH of the mixture is still "neutral" typically 6-7.

In the method of the invention, the aluminium salt solution is added to a flowing dispersion of fibres, i.e. a fibre dispersion which is being moved/transported along a conduit as part of the paper-making process. The dispersion may, for example, be so-called thick stock moving along the thick-stock line of a paper-making process or the so-called "thin stock" moving along the thin stock line.

For all such flowing dispersions as used in paper-making processes, it is generally the case that the flow is laminar. In the method of the invention, a dilute aluminium solution is added to the flowing dispersion and will itself be the subject of laminar flow. Thus, for example, the aluminium solution may be added at the wall of a pipe along which the fibrous dispersion flows and the aluminium solution will "travel" along the inside wall of the pipe before thorough downstream mixing occurs (e.g. at a fan pump). At the interface between the aluminium solution and the bulk phase of the fibrous dispersion, the aluminium undergoes a pH transition typically from about 1 to about 6-7. Depending on the concentration of aluminium in solution and the pH of the solution, the aluminium may be present in a number of different forms. The relationship between aluminium concentration and pH is shown in a graph in an article by J.J.Morgan entitled "Applications and Limitations of Chemical Thermodynamics in Natural Water Systems" in Adv.Chem.Ser. Vol 37 pages 1-29 (1967).

It will be appreciated that the concentration of the aluminium solution as added to the fibrous dispersion is significantly less than that normally employed in paper-making processes. In order to compensate, the rate at which the aluminium solution is added to the fibrous dispersion is such that the total amount of aluminium delivered by the fibrous suspension is similar to that conventionally used in papermaking processes. However, given that the aluminium solution is subject to laminar flow within the fibrous dispersion, the concentration of the aluminium solution at its interface with the fibrous stock is such that the solubility limit of aluminium hydroxide is not exceeded. Therefore, the process of the present invention may employ similar total amounts of aluminium salt as in prior processes but deliver in a much more dilute form so as to avoid production of aluminium hydroxide deposits.

The solution of the aluminium salt of the required concentration may be produced by dilution of a solution of higher concentration. The dilution is preferably effected by introducing the more concentrated aluminium solution into a flowing water stream, preferably a turbulent stream and (alternatively or additionally) one which is free of fibrous components.

It is particularly preferred to effect dilution using so-called "clarified water" which is water drained from the paper-making wire and then treated to effect substantial removal of any fibrous material therein prior to being returned to the paper-making process.

The aluminium solution may be added to a fibre dispersion which is then subsequently diluted prior to formation of paper. Thus, for example, the aluminium solution may be added to the so-called "thick stock" of a paper-making process prior to dilution to form the "thin stock". In a preferred embodiment of the invention, the aluminium solution is added to so-called "thick stock" containing 3%-4% of fibres.

In this case, it is preferred that the amount of aluminium solution added to the thick stock is on the basis of one part by volume of aluminium salt solution to 30-50 parts by volume of the thick stock. It should however be appreciated that this is an average amount since specific amounts may vary with time to cater for fluctuations in the paper-making process.

Preferably the temperature of the aluminium salt solution is lower (e.g. 3"C to 10 C (preferably about 5"C) lower) than the fibrous suspension to which it is added.

We believe that this temperature difference may also contribute to the avoidance of solid, intractable Al(OH)3 deposits.

Any soluble aluminium salt may be used in the method of the present invention but it is particularly preferred to use aluminium sulphate (e.g. provided as paper-makers alum), aluminium chloride, aluminium chlorohydrate or polyaluminium chloride.

A further component used in the method of the present invention is rosin. This may be used in the form of a rosin emulsion. In one embodiment of the invention, the rosin emulsion may be incorporated into the solution of the aluminium salt which is then added to the fibrous dispersion. Alternatively, the rosin emulsion may be added separately to the fibrous dispersion.

The rosin used in the invention may be any rosin conventionally used in papermaking processes, e.g. one fortified by the Bewoid process. The rosin emulsion to be used in the process will generally have a concentration of 30 to 50% by weight solids.

Typically the amount of rosin used in the process of the invention is such as to provide an amount of 1 to 10% by weight of rosin on the fibres of the paper.

Typically the ratio of aluminium salt to rosin is 2-3:1.

The invention will be described by way of Example only with reference to the accompany drawings, in which: Fig. 1 schematically illustrates one embodiment of the present invention.

Referring to the drawing, a dispersion of fibres for use in paper-making is prepared by conventional means in a tank 1 from which the dispersion passes into a thick stock pipe 2 subsequent to which this stock is diluted to form thin stock. This dilution is effected in a pipe 4 with water which has been supplied along pipes 5 and 6. More particularly, the water in pipe 5 is "fresh" water which is supplied to the process whereas pipe 6 carries clarified water which has been returned from the papermaking process and which has been treated for substantial removal of fibrous material.

As indicated, dilution is effected in the pipe 4 with the thick stock and water being thoroughly mixed at a fan pump 5 from which the resultant thin stock is passed to the paper-making wire for production of paper in accordance with conventional techniques.

In accordance with the present invention, rosin and a soluble aluminium salt are added to the clarified water in pipe 6 and are thoroughly mixed therein by virtue of the turbulent nature of the clarified water.

Consequently, the clarified water entering pipe 5 contains the requisite amounts of rosin and aluminium salt to effect sizing of the fibres.

It will be appreciated that the process described is different from conventional sizing processes where the rosin and aluminium salt would be added directly to the fibrous suspension, e.g. in line 2 or line 4.

We have found that employing the technique illustrated in the drawing considerably reduces problems with deposit formation.

The invention is further described with reference to the following non-limiting Example Example The invention was applied to a paper-making process in which the aluminium solution was added to thick stock closely upstream of a fan pump at which dilution water was added to effect a 6-fold dilution of the thick stock.

The original thick stock about 3% by weight of fibres, a pH of about 7.5 and had a flow rate of about 33,000 litres per hour.

The aluminium solution was prepared by addition of a 47% by weight solution of paper-makers alum (Al2SO4. 1 6H20) to a turbulent clarified water stream in an amount such as to effect (on average over time) an 80 fold dilution of the aluminium.

A rosin dispersion (30% solids) was also added to the clarified water stream in equal volume to the 47% alum solution. The resultant mixture (water/aluminium salt/rosin emulsion) was supplied at about 800 litres per hour to the thick stock. The temperature of this mixture was 5"C less than the thick stock.

It was found that there were no deposits of intractable aluminium hydroxide.

Claims

1. A method of sizing a neutral dispersion of paper-making fibres using rosin and an aluminium salt wherein the aluminium salt is added as a solution to a flowing dispersion of the fibres either separately from or simultaneously with the rosin characterised in that the concentration of the aluminium in said solution as added to the dispersion is at most 0.8% by weight expressed as A13+.

2. A method as claimed in claim 1 wherein the aluminium salt solution contains a maximum of 0.4% by weight aluminium expressed as Al3

3. A method as claimed in claim 2 wherein the aluminium salt solution contains a maximum of 0.2% by weight aluminium expressed as All+.

4. A method as claimed in claim 3 wherein the aluminium salt solution contains a maximum of 0.1% by weight aluminium expressed as Al3+

5. A method as claimed in any one of claims 1 to 4 wherein the aluminium salt solution is produced by dilution of a solution of higher concentration.

6. A method as claimed in claim 5 wherein the dilution is effected by introducing the more concentrated aluminium solution into a flowing water stream.

7. A method as claimed in claim 6 wherein the flowing water stream is a turbulent stream.

8. A method as claimed in claim 6 or 7 wherein the flowing water stream is one which is substantially free of fibrous components.

9. A method as claimed in claim 8 wherein the flowing water stream is provided by "clarified water".

10. A method as claimed in any one of claims 1 to 9 wherein the temperature of the aluminium salt solution is lower than that of the fibrous suspension to which it is added.

11. A method as claimed in claim 10 wherein the temperature of the aluminium salt solution is 3"C to 1 00C lower than the fibrous suspension to which it is added.

12. A method as claimed in claim 11 wherein the temperature of the aluminium salt solution is about 50C lower than the fibrous suspension to which it is added.

13. A method as claimed in any one of claims 1 to 12 wherein the aluminium salt is provided by aluminium sulphate, aluminium chloride, aluminium chlorohydrate or poly-aluminium chloride.