Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With chemical or physical modification of liberated fiber
Reexamination Certificate
1997-02-20
2002-08-20
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With chemical or physical modification of liberated fiber
C162S181200, C162S181400, C162S182000, C162S183000
Reexamination Certificate
active
06436232
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a procedure for adding a filler into a pulp based on cellulose fibres, as defined in the preamble of claim
1
.
“Pulp based on cellulose fibres” in this context refers to pulps used in paper and pulp industry, produced by chemical or mechanical methods from plants or plant parts containing lignocellulose, such as wood or plants with a herbaceous stalk, from which the lignin has been removed or in which the lignin is partly or completely preserved, such as cellulose, wood pulp, refiner mechanical pulp, mixtures of these, fine material originating from these and/or derivatives of these. “Paper” refers to different kinds of paper and cardboard, coated or uncoated, produced with a paper and cardboard machine.
BACKGROUND OF THE INVENTION
Today, the trend of development of paper products is increasingly determined by the buyers of these products and by legislative measures. The buyers of printing paper want to economize on postage costs and reduce the amount of waste produced. Further, waste processing charges depending on weight have been imposed on packing materials. Generally, it seems that energy and disutility taxes are being added as an extra imposition to the price of paper products. For these reasons, paper buyers want paper products having a lower grammage which still meet high quality requirements.
Because of the general trend of development described above, there is a need to produce high-quality paper using a reduced amount of raw material. When the grammage of paper is reduced, its density becomes a critical property. In many applications, an even more critical property is the stiffness of paper, which is heavily reduced as the density is increased. This leads to a need to alter the structure of paper so as to reduce its density to a minimum. This imposes further requirements on the raw materials of paper and on paper production processes.
For paper-based communication to remain competitive with respect to electric communication, the impression quality of paper products should be further improved. Considering the strong tendency towards reducing the grammage of paper, gradual and slow development of different kinds of paper is not sufficient in this situation, but instead more intensive development of paper quality is necessary.
During several years, investigations have been made into the use of fillers to fill the pores and cavities in chemical pulp fibre. According to the investigations, the advantages include a better filler retention in paper manufacture, the possibility to increase the filler content of paper, reduced soiling and wear of the wire and reduced linting of paper. The use of titanium dioxide in this connection has been reported by Scallan et al. Patent specifications U.S. Pat. Nos. 22,583,548 and 3,029,181 describe methods by which calcium carbonate is precipitated in and on the fibres using two salts having a good water-solubility, e.g. calcium chloride and sodium carbonate. The method has the drawback that it produces a soluble by-product which has to be washed off before the fibres are used for paper production. This increases the amount of water needed, which is why the method is not very viable. Another drawback with these methods are the chemical changes that take place on the surface of the chemical pulp fibre, which involve a significant reduction in the strength values of the paper when such fibres are used in paper manufacture.
Specification JA 62-162098 describes a procedure in which carbon dioxide is added into a hydrous slurry of chemical pulp and calcium hydroxide, with the result that calcium carbonate is precipitated. The method has the drawback that the treatment is performed at a low consistency of chemical pulp. In this case a significant proportion of the carbonate is precipitated in the bulk solution and on the surface of the fibres instead of inside the fibres, resulting in a rather low paper strength. In addition, at a low chemical pulp consistency, the amount of water needed and also the volume of the crystallizing reactors needed on an industrial scale are high, which is uneconomic.
Today, the target is to reduce the amount of water used, the final aim being closed circulation. Because of this, the implementation of the above-described procedure at a low chemical pulp consistency is questionable.
Specification U.S. Pat. No. 5,223,090 describes a method in which the precipitation of calcium carbonate with carbon hydroxide is performed in a pressurized disc refiner in a medium-consistency chemical pulp suspension (consistency values 5-15% by weight). Paper produced by this method has better strength properties as compared with earlier filling methods. A significant drawback with this method is fast wear or refiner discs, because calcium carbonate and its raw material, calcium hydroxide, cause heavy wear. Moreover, the procedure comprises before the precipitation of the carbonate a low-consistency stage during which the calcium hydroxide is mixed with the chemical pulp. Therefore, the amount of water needed is in fact not at all smaller than in earlier methods, which limits the applicability of the method in production.
Precipitation of calcium carbonate with carbon dioxide at a high chemical pulp consistency has been subject to certain limitations due to the fact that if the consistency exceeds 2%, effective mixing of chemical pulp suspensions becomes more complex and more difficult. This is because the cellulose fibres in the water tend to form floccules in which fibres are hitched together. This phenomenon has been widely investigated since the 1950s, and it has been established that flocculation is a mechanical effect which always occurs when the fibre consistency in the suspension exceeds a critical value. For pulp fibres, this limit consistency is very low, below 0.1%.
SUMMARY OF THE INVENTION
The object of the present invention is to eliminate the drawbacks described above. A specific object of the invention is to present a new procedure for adding a filler to a pulp based on cellulose fibres so that the addition can be performed in a controlled manner in a medium-consistency suspension.
A further object of the invention is to present a new procedure for adding a filler to a pulp based on cellulose fibres so that a better filler retention is achieved and the filling agents are not washed away with the water during the paper production process. A further object of the invention is to present a new procedure for adding a filler to a pulp based on cellulose fibres so that the flexural strength of paper manufactured from the pulp is higher than when commercial fillers are used. A further object of the invention is to eliminate problems in the handling of the process water that are caused by fillers washed away with the water from the process. A specific object of the invention is to present a procedure for adding a filler into a pulp so that the procedure allows the use of a higher filler content in the paper than before so that a good retention is also achieved.
As for the features characteristic of the invention, reference is made to the claims.
The invention is based on comprehensive investigations. During the investigations it was established that the tendency of a fibre suspension to flocculate depends on many factors, but the most important factor is the consistency of the suspension. In medium-consistency fibre suspensions, the fibres are normally heavily flocculated. Flocculation can be reduced by influencing the state of flux of the suspension. It was found in the investigations that in a sufficiently intensive state of flux the suspension behaves like a Newtonian fluid in a turbulent state. The transition into such flux is hereinafter referred to as fluidization of a suspension.
The power required for fluidization is generally below 5 kW/l and it is indicated by the torque and the speed of rotation of the rotor together. In earlier investigations by Gullichsen et al it has been established that pulp consistency has an effect on the torque required for fluidizatio
Gullichsen Johan
Halinen Esa
Leskelä Markku
Silenius Petri
Altera Law Group LLC
M-Real Oyj.
Nguyen Dean T.
LandOfFree
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