The invention, more precisely, refers to the manufacture of various types of mechanical pulps, such as refiner mechanical pulp (RMP), thermo- mechanical pulp (TMP), chemi-mechanical pulp (CMP) and chemi-thermo- mechanical pulp (CTMP). The starting material can be wood chips, one-year plants as wheat, straw, bagasse or more or less worked pulp.

The known working of the fibrous material in the refiner most often is carried out in an ineffective way. A very essential part of the energy input is used for transporting the fibrous material through the refiner, whereby friction losses and heat losses are caused, which do not result in the changes of the fibrous material required for developing the pulp quality and for making the refining effective. This implies, that the energy consumption is higher than it need to be for achieving the desired mechanical working, i.e. desired pulp quality.

The present invention offers a solution of this problem, in that the staytime of the material in the feed zone is shortened, and the material substantially without mechanical working passes through the feed zone to the radially outside located refining zone. The staytime should be less than 2,5 sec, preferably less than 1 sec.

The ingoing material is by means of a central feeding device fed in and accelerated outward without material build-up in the feed zone. Thereby the material density in the feed zone is restricted to at maximum 10 kg/m3, preferably to at maximum 1 kg/m3. In this way, the contact of the material with the refining means in the feed zone is reduced, and thereby also the energy consumption inthe form of friction heat is reduced. Any proper mechanical working in the feed zone, thus, shall not take place, and the energy consumption in this zone preferably should be less than 5% of the total energy consumption.

The energy input, instead, shall be transferred to the refining zone, where the relative speed between the refining elements shall be high and preferably exceed 50 m/sec already in the inner portion of the refining zone.

The characterizing features of the invention are apparent from the attached claims.

The invention is described in greater detail in the following, with reference to an embodiment thereof illustrated in the accompanying drawings, in which Fig. 1 is a cross-section through a refiner for treating fibrous material according to the invention; Fig. 2 shows a feed device according to the invention; Fig. 3 shows a diagram from comparing experiments where the freeness is stated as a function of the energy consumption.

The refiner shown in Fig. 1 comprises a refiner housing 10, in which a stationary refining means 11 and an opposed rotary refiner means 12 attached on a rotary shaft 13 are provided. The refining means 11,12 are provided with refining elements 14 and, respectively, 15 which between themselves form a refining zone 16 in a refining gap 17. The refining gap 17 comprises also an inside located feed zone 18. The stationary refining means 11 is formed with a central feed opening 19 for the material to be worked. A screw feeder 20 for the material is connected to the feed opening 19. The refiner housing 10 is provided with an outlet 21 for the material passing through the refining gap where the material is worked to pulp.

On the rotary refining means a central feeding device 22 is located which is formed with a front axial screw 23 and at least one substantially radial rear wing 24 on a rear wall 25 in the device 22. The diameter of the axial screw 23 corresponds to the diameter of the feed opening 19. The rear wing or wings 24 extend into the refining gap 17 through the feed zone 18 out to the refining zone 16.

The feeding device 22 can be formed with an axial screw 23 and wings 24 as one unit or as separate parts, which are attached individually on the rotary refining means 12.

The number of wings 24 preferably is 2-4, and they can be radial or formed with their radially outer ends curved from the rotation direction of the device.

The material to be treated is advanced to the refiner by means of the screw feeder 20. The design of the feeding device 22 with a front axial screw 23 implies, that the transfer of the material from the screw feeder 20 to the refiner is ensured in that the material is prevented from rebounding out into the screw feeder 20. The rear wing or wings 24, furthermore, imply that the material, which is fed by the axial screw 23 between the refining means 11,12, rapidly passes through the feed zone 18 to the radially outside located refining zone 16 where the working of the material to pulp takes place. The wings 24, thus, accelerate the material outward without material build-up in the feed zone 18, in that the material is locked up in a space, which first is defined by the axial screw 23 and thereafter by the rear wall 25 of the feed device 22, an opposed substantially smooth portion on the stationary refining means 11 and wings 24. The material thus locked is subjected to an increasing centrifugal force, which throws the material outward to the refining zone. The material density in the feed zone 18 can thereby be restricted to at maximum 10 kg/m3, preferably at maximum 1 kg/m3. The contact of the material with the refining means in the feed zone, and thereby the energy consumption in the form of friction heat is reduced. No proper mechanical working, thus, takes place in the feed zone 18, but it takes place in the refining zone 16. The energy consumption in the feed zone preferably is less than 5% of the total energy consumption.

In the refining zone 16, where substantially the entire energy input occurs, the relative speed between the refining elements 14,15 must be high and preferably exceed 50 m/sec already in the inner portion of the refining zone.

EXAMPLE A refiner of the type shown in Fig. 1 was operated partially with a conven- tional feeding device and partially with a feeding device according to the invention for the manufacture of tissue pulp. See the following Table.

Regarding the load on the screw feeder 20, it was observed that it was about 40% lower with the feeding device according to the invention, which indicates that this feeding device effectively draws in the material into the refiner and moves it out to the refining zone.

TABLE Conventional Feed according to the feed invention Production ton/hour 7,2 7,2 7,2 7,7 7,7 7,7 Spec. energy kWh/hour 1417 1333 1250 1013 974 974 CSF ml 469 542 612 514 574 596 Tensile index kNm/kg 15,6 17,3 15,3 19,0 16,5 16,1 Tear index Nm2'kg 5,87 6,48 5,68 6,22 6,17 6,27 It can be stated that the quality of the produced pulp was substantially equivalent according to both alternatives.

The specific energy consumption, however, was reduced considerably by using the feeding device according to the invention.

For a corresponding freeness value, a reduction of the energy consumption by about 25% was observed. See Fig. 3.

The invention, of course, is not restricted to the embodiment shown, but can Claims 1. A method for mechanical working of lignocellulosic fibrous material in a refiner with opposed refining means rotating relative to each other, provided with refining elements, which between themselves form a refining zone in the form of a refining gap for working the material, where the material is fed between the refining means to a feed zone located radially inside the refining zone close to the centre, c h a r a c t e r i z e d in that the material fed is accelerated outward without material build-up through the feed zone without mechanical working, that the material density in the feed zone is at maximum 10 kg/m3, and that the mechanical working is carried out in the refining zone.

2. A method as defined in claim 1, c h a r a c t e r i z e d in that the material density in the feed zone is at maximum 1 kg/m3.

3. A method as defined in claim 1 or 2, c h a r a c t e r i z e d in that the material at its entering the refining zone is subjected to working by the refining elements with a relative speed exceeding 50 m/sec.

4. A feeding device intended for refiners with two opposed refining means (11,12), of which one is stationary (11) and one is rotary (12), which are provided with refining elements (14,15), which between themselves form a refining gap (17) with a refining zone (16) for working lignocellulosic fibrous material, c h a r a c t e r i z e d in that the feeding device (22) is formed with a front axial screw (23), which transforms to at least one substantially radial rear wing (24), that the feeding device (22) is intended to be placed centrally on the rotary refining means (12) directly in front of a feed opening (19) in the stationary refining means (11), and that the diameter of the axial screw (23) corresponds to the diameter of the feed opening (19), and the rear wing (24) is arranged to extend into the refining gap (17) between the refining means (11,12) out to the refining zone (16).

5. A device as defined in claim 4, c h a r a c t e r i z e d in that it is formed with 2-4 substantially radial rear wings (24).

6. A device as defined in claim 4 or 5, c h a r a c t e r i z e d in that the radially outer end of the wings (24) is curved from the intended direction of rotation of the device.