This invention relates to the disintegration and refin¬ ing of lignocellulosic material in a disc refiner. The invention, more precisely, relates to a refining element for use in a refiner of the said type.

A disc refiner comprises two opposed refining discs, which are rotatable relative each other where one or both of the discs are rotary. A plurality of refining elements arranged on the refining discs form a pattern of bars and intermediate grooves. The refining discs are positioned so that the refining elements form a refin¬ ing gap, through which the fiber material is intended to pass outward from within whereby the disintegration is carried out by the bars oϊ the refining elements;

The refining elements. ^' in the inner portion of the refin¬ ing gap are formed with bars of a greater coarseness in order to carry out an initial disintegration and to feed the material outward to the outer portion of the refining gap where the disintegration and refining proper are taking place.

The refining of the material requires a large energy input, which is supplied by the rotation of the rotary refining disc or discs. The greatest part of the energy is transformed to heat, as a result of which large amounts of steam are generated due to the water content in the material. The steam, which substantially is gene* erated in the refining gap, has high pressure and flows both outward and inward in the refining gap.

To disintegrate and refine the fiber material by the bars of the refining elements, it is necessary in most cases to provide flow restrictions, so-called dams,

in the grooves between the bars. The material is forced by these dams to move upward out of the grooves and be worked between the bars in the refining gap. Unworked material is hereby prevented from passing outward through the refining gap. The said dams, however, obstruct the generated steam, which thereby disturbs the material flow, and have a negative effect on the capacity and operation stability of the refiner.

The aforesaid problems can be reduced by the present invention, which also offers additional advantages with regard to the quality of the refined material. Accord¬ ing to the invention, co-operating refining elements on opposed refining discs are provided with radial zones, in which the bars are arranged in a denser or sparser relationshi .

The characterizing features of the invention are appaϊ-ent from the attached claims.

The invention is described in greater detail in the following, with reference to the accompanying drawing illustrating an embodiment of the invention, in which drawing

Figs. 1 and 2 show two co-operating refining elements according to the invention.

The co-operating opposed refining elements 10,11 in Fig. 1 and, respectively, 2 carry a pattern of bars 12 and intermediate grooves 13, which extend substant¬ ially radially across the surface of the refining ele¬ ments. The co-operating refining elements 10,11 are intended to define between themselves a refining gap. An inner portion 14 of the refining elements is provid¬ ed with coarse bars 15, which are intended to carry out a first disintegration of the material and to feed

it outward in the refining gap. An outer portion 16 of the refining elements 10,11 is intended to form the refining gap proper where disintegration and refining of the material take place.

The outer portion 16 of the refining elements 10,11 is divided into a plurality of limited zones 17-20 and, respectively ,21-24.-which are located radially outside of each other. The bars in these zones 17-24 are arr¬ anged alternatingly in a denser or sparser relation¬ ship from one zone to an adjoining zone, counted in radial direction. The refining elements 10,11 are so formed, that a zone 18,20 and, respectively, 21,23 with a dense pattern of bars on a refining element is located directly in front of a zone 22,24 and, respect¬ ively, 17,19 with sparsely spaced bars on the opposed co-operating refining element. The grooves, further¬ more, preferably are shallower in the more densely patterned zones and deeper in the more sparsely patt¬ erned zones.

By this arrangement, the fiber flow through the refin¬ ing gap will be moved alternatingly over to the opposed refining element when the flow in a more sparsely patt¬ erned zone arrives at a more densely patterned zone. The demand for dams can hereby be reduced and in certain cases be eliminated entirely. The steam transport is hereby facilitated and the disturbing effect of the steam be decreased substantially. As to the size of the zones, in order not to obstruct the fiber flow, the more densely patterned zones must be somewhat narrower than the adjoining more sparsely patterned zones.

The different zones in the outer portion 16 of the defin¬ ing elements preferably are formed so that the width of

the bars substantially is equal while the spacing between the bars, i.e. the width of the grooves, varies.The .ratio between the groove width in a more densely patterned zone

18.20.21.23 and an adjoining more sparsely patterned zone

17.19.22.24 should be between 1:1,25 and 1:1,75, prefer¬ ably about 1,5. Suitable dimensions are a bar width of 1-3 mm and a groove width of 1-3 mm and a groove depth of 1-3 mm in a more densely patterned zone, and a bar width of 1-3 mm and a groove width of 1,5-5 mm in a more sparsely patterned zone.

As regards the groove depth, the ratio between the zones should be between 1:1 and 1:4. Suitable dimensions are a groove depth of 2-5 mm in a more densely patterned zone and 5-8 mm in an adjoining more sparsely patterned zone.

The number of zones in the outer portion of the refining elements can be varied, but the refining elements prefer¬ ably are formed with two more densely and two more sparse¬ ly patterned zones.

Dams can be excluded entirely or be reduced in number, for example to one dam per groove in a more densely patterned zone.

It was found possible with this type of refining element to produce pulp with a very low shives content, below 0,15% at a freeness of 150 ml CSF, and an increased tensile strength (tensile index) at a lower specific energy input. It was observed that the energy level was lowered by up to 20%.

It was further possible to reduce the long fiber fraction (+30 mesh according to BMcNett) by 10-15% where the reduction to its greatest part applies to the fraction

+16 mesh. This can in certain cases be particularly ad¬ vantageous, for example at the making of magazine paper

The invention, of course, is not restricted to the em¬ bodiments described above, but can be varied within the scope of the invention idea.