Method in connection with the production of paper or cardboard

Details Method in connection with the production of paper or cardboard Method in connection with the production of paper or cardboard

2001-09-06

2003-02-11

Fortuna, Jose A. (Department: 1731)

Paper making and fiber liberation

Processes and products

With measuring, inspecting and/or testing

C162S130000, C162S191000, C162S004000, C162S055000

Reexamination Certificate

active

06517680

ABSTRACT:

TECHNICAL AREA
The present invention relates to a method in connection with the manufacturing of paper and cardboard. The method aims to level out quality variations and to raise the quality level through converting excess waste for manufacturing, so-called broke. The converted broke is returned in a more controlled way to the paper of cardboard, as compared to conventional techniques.
BASIS FOR THE TECHNIQUE
When manufacturing paper and cardboard from cellulose fibres, great emphasis is placed on the fibre composition for different products in order to utilise the fibres' properties in the best way. So, for example, paper and cardboard are manufactured from different stock compositions consisting of different proportions of different fibre and pulp types depending on which properties are desired in the final product. Even if a given paper or cardboard quality has a given nominal fibre composition, there is a natural variation in the fibre raw materials included as different wood species have different fibre characteristics, see Table 1, and there are also natural property variations for the fibres in wood with regard to length, width, fibre wall thickness, etc. The fibre length distribution for a Swedish softwood pulp spans from fractions of millimeters up to 6-7 mm. For birch pulp, the corresponding value is from fractions of millimeters up to approx. 3-4 mm. This means that fresh cellulose pulps demonstrate major non-homogeneities in fibre property distributions.
TABLE 1
Fibre dimensions for several different timber types
Southern state
pine Loblolly
pine, U.S.A.
Swedish pine
spring
summer
spring
summer
Euca-
wood
wood
wood
wood
lyptus
Birch
Average length,
3.0
3.5
2.7
3.0
1.0
1.1
mm
Average width,
45
35
35
25
16
22
&mgr;m
Lumen diameter,
32
12
30
10
10
16
&mgr;m
No. of fibres
1
1.5
13
8
per gram
timber, x107
Source: (Norman B. “Pappersteknik”, 1991, Stockholm, Institute for Paper Engineering, Kungliga Tekniska Hogskölan)
When manufacturing paper and cardboard, there is always produced a quantity of excess material, so-called broke. This broke can comprise edge strips, widths on rolls of final product which results in that the whole machine width cannot be used to the full, second-rate quality, etc. From the mentioned examples of factors which result in broke, it is understood that the broke quantity varies over time. If one manufactures a product which complies with the quality specification and utilises the whole machine width, there will be small quantities of broke. When for some reason there are problems with complying to the quality specification, e.g. at a change of quality on the machine, and the full machine width cannot be used, the quantities of broke will become larger.
Table 2 shows a calculation example of how a varying broke quantity changes the fibre composition in a three-ply product for different broke mixes. The example is based on the broke being returned directly to the inner ply.
TABLE 2
Share of the total basis weight of fibre composition for top ply,
inner ply and bottom ply in the finished product,
both nominally and for the three different broke mixes.
Broke mix
% of total weight
Nominal
0
10
20
Top ply (%)
25
25
27.5
30
Inner ply (%)
50
50
45
40
Bottom ply (%)
25
25
27.5
30
All paper and cardboard qualities contain larger or smaller nominal quantities of broke. Only in cases where the whole quantity of broke can be used directly, with or without subsequent processing, quality variations due to nominal fibre composition in the paper or cardboard product is avoided, i.e. if e.g. 15% of the production results in broke when a quality which nominally should contain 15% broke of the production is manufactured at the same time. In all other cases, the broke will contribute to a quality variation by a varying fibre composition which deviates from the nominal fibre composition, due to the fact that there is a surplus of broke which must be stored, a lack of broke, stored broke of a different fibre composition compared to the quality which is presently being manufactured, etc. Broke quantities varying over time will sooner or later result in quality variations for all paper and cardboard qualities.
According to known technique, the broke can be managed in different ways depending on the broke quantity. Conventionally, the disintegrated broke is recycled, either directly, after storage in a vat/tower or after storage as a roll, to the paper or an inner ply for multi-ply paper and multi-ply cardboard. In the ideal case, the broke is broken up in water, where appropriate subsequently processed through beating or refining, and re-used in the production directly together with the originally included cellulose pulps. The varying broke quantities will however often result in broke having to be stored. This storage can be performed in two ways. One way is that the disintegrated broke is stored in a storage vat/tower after the disintegration and possible subsequent processing. The other way is to have rolls of a outclassed quality in stock, which is disintegrated and where appropriate subsequently processed for use as needed.
In the case of multi-ply paper or cardboard, it is not only varying broke quantities that cause problems. By returning the broke to an inner ply in the cardboard, the inner ply to which the disintegrated broke is added will be brought to contain pulp with fibres that originate from an outer ply. The multi-ply paper or cardboard will thereby, calculated on its total weight, contain a larger proportion than nominally of fibres of the type which is intended to be used in an outer ply of the paper or cardboard and a smaller proportion than nominally of fibres of the type intended to be used in an inner ply of the paper of cardboard, see examples in Table 2.
It is known that the fibres in a fibre flow can be fractionated by means of a screen or hydrocyclone, a screen being used to fractionate fibres primarily according to fibre length, while a hydrocyclone is used to fractionate fibres with different thicknesses and thereby different fibre flexibilities. Studies have shown that with the aid of size fractionating (screen), it is possible to separate out a large share of short fibres from a fibre flow; Fredlund M. et al., “Förbättrade kvalitetsegenskaper hos kartong genom fraktionering”, STFI-rapport TF 23, 1996, Stockholm, STFI; Grundström K-J, “STFIs silteknik höjer kvaliteten vid kommersiell drift”, STFI Industrikontakt, 1995, no. 1, p. 7-8. It has also been documented that by using a hydrocyclone one can separate flexible fibres from more stiff fibres; Wood J. R. and Karnis A., “Distribution of fibre specific surface of papermaking pulps”, Pulp&Paper Canada 80 (1979):4, p. 73-78, Bliss T., “Secondary fibre fractionation using centrifugal cleaners”, Tappi Pulping Conference, 1984, 217 pp; Paavilainen L., “The possibility of fractionating softwood sulphate pulp according to cell wall thickness”, Appita 45 (1992):5, p. 319-326. In U.S. Pat No. 5,002,633 there is described a fractionating process which aims to separate the longest fibres from short fibres, fillers, contaminants, etc., from a pulp for re-use of the longest fibres in paper manufacturing.
Moreover, it is known to combine different fractionation equipment in fractionation systems for different purposes. In U.S. Pat No. 5,403,445, recycled fibres for manufacturing of paper with more than 70% recycled fibres are fractionated, and in U.S. Pat No. 5,061,345 a series of screens is used to separate out fibres from filler. In some fractionating systems, the aim is to separate fibres with different properties in order to be able to use the fibre fractions in different plies. This is described in U.S. Pat No. 5,147,505 where the fibres in a pulp are separated according to coarseness and the rougher fibres are used in one ply and the more slender fibres are used in another ply. In EP 0653516 A1, it is mentioned in a similar way that softwood fibres are separated into a fraction with thick-walled fibres which are used in one ply and a fraction with thin-walled fibres which are used in another

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