FI89514B - Foerfarande Foer att saenka energifoerbrukningen vid raffinering av fibermaterial - Google Patents

Description

1 89514

Method for reducing energy consumption during refining of fibrous material. - Förfarande föt att sanka energiför-bruknignen vid raffinering av fibermateria].

The present invention relates to the refining of fibrous material in devices with interlocking grinding surfaces. On the one hand, refining of lumpy fibrous material, for example refining chips into pulp according to TMP, CTMP, CMP or other methods for the production of high-yield celluloses or so-called mechanical pulp. More specifically, the invention relates to a certain combination of functions for reducing energy consumption in such refining, as current processes have very high energy exchange and can be in the order of about 1000 kWh per ton of pulp per refining stage, i.e. in two stages refining it can be about 2000 kWh per ton of pulp.

Only a small portion of this amount of energy is used, if necessary, for the fiberization work itself, while the vast majority is used to generate moisture in the fibrous material and possibly convert the added liquid to steam. The steam generated in the refiner slot takes up space, which in itself is not advantageous because the steam at the pressure in the refiner slot impedes the natural passage of the fibrous material through the refiner slot and also reduces the residence time of the fibrous material in the grinding area.

Therefore, the main object of the invention is to reduce the energy exchange during refining by carrying out a combination of operations, which are described in more detail below.

Although there are refiners in which both grinding surfaces are movable to achieve the desired kinkir movement in the refiner slot, only refiners 2 d 9 514 with a fixed part called a stator and a moving part inside it called a rotor will be considered in the following.

The first energy saving measure is to greatly reduce the grinding frequency by keeping the speed of the rotating part substantially the same as before, but by greatly reducing the number of cutting elements of the rotor and stator.

As a second measure, in part to compensate for the lower grinding frequency, the volume, i.e. the standing time between the grinding elements, is simultaneously increased. Because the use of a disc refiner in which there are only a radial grinding gap due to centrifugal forces acting on the fibrous material at given speeds, certain limitations on the diameter of the grinding zone are imposed, but instead it is possible to increase the grinding zone volume by forming a grinding gap or cylinder. In this way, the diameter can be kept within reasonable limits while increasing the length of the grinding zone.

As a third measure, which also partially compensates for the lower grinding frequency, as much of the steam formed in the grinding gap as possible is removed to reduce energy consumption already when the steam is located in the grinding gap to better adhere the grinding surfaces to the fibrous material.

Much research and development work has been done on the structure of the grinding members themselves, which in modern machines are usually replaceable when exposed to high stresses and high wear and tear. Canadian Patent CA 901857 discloses examples of plate-like as well as conical grinding parts with grooves or booms of different structures to achieve the most efficient grinding. Another structure is disclosed in U.S. Patent No. 3,815,834, which discloses disc refiners which clearly show that when the disc diameter is smaller, the splines are relatively large apart and that the distance decreases and decreases as one approaches the circumference of the disc. Grinding frequency means the number of revolutions per second of the rotor multiplied by the number of grooves on the circumference of the grinding surfaces. A rotor speed of 1500 to 100 rpm, with frequencies of the order of 6000 to 20,000 Hz, can be considered normal. It has been found by the invention that energy consumption can be considerably reduced by less than half under favorable conditions if the frequency is reduced to about 300 to 800 Hz, so that while maintaining the motor speed, the number of rotor and stator cutting elements is reduced from the usual 400 to 600 - 30.

In order to compensate for the above-mentioned reduced frequency, it is necessary to increase the volume, i.e. the standing time between the grinding elements, by constructing the refiner cylindrical or preferably somewhat conical. In this case, it is possible to extend the path of the fibrous material through the refiner, i.e. along the conical surface to a greater extent than was possible with the disc refiner, because as shown, it is not advantageous to increase the diameter of the grinding area too much.

In order to further prolong the residence or standing time, the steam generated in accordance with the invention is drawn continuously in the refiner according to the invention substantially over the entire length of the rotor. In this case, the steam separation becomes as complete as possible to reduce the compressibility of the chip-fiber-water-steam mixture, which helps to direct the frictional forces produced efficiently to the chips and fiber.

A problem in this field has long been how to efficiently draw steam away from the refining life of refiners. However, one example is disclosed in Canadian Patent CA 1,079,559, in which the grinding segment, in addition to the grooves, is provided with openings which conduct steam and the accompanying fibrous material from the grinding slot to the rear of the grinding segment itself.

According to the present invention, a relatively large part of the steam formed in the grinding slit leaves the grinding area and, moreover, is largely cleaned of the accompanying fibrous material. The steam then passes out of the refiner at a different location than the fibrous material through one or more steam removal channels. The amount of steam is then controlled by maintaining a suitable pressure difference between the removal of steam and the removal of fibrous material.

Further studies have shown that the amount of cutting element can be sharply reduced to typically about 20 to 60 and the frequency range can be sharply reduced to typically about 200 to 2000 Hz at conventional rotor speeds of about 1000 to 1800 rpm. In this case, the method is characterized in that the grinding frequency is reduced from a normal value of about 6000 to 20,000 Hz to about 200 to 2000 Hz by reducing the number of shear elements from a conventional value of about 400 to 600 to a typical value of about 20 to 60 while maintaining a normal speed of about 1000 to 1800 rpm.

i 5> 95.4 and that the residence time of the fibrous material in the grinding slot is increased to compensate for the amount of grinding impulse lost through frequency reduction by forming the grinding surfaces conical or cylindrical to allow the due to the energy exchange, the fibrous material is released from the following liquid, whereby the steam leaving the grinding gap is first released inside the refiner from substantially all the fibrous material following the steam and then discharged from the refiner through at least one steam outlet.

Claims (2)

A method for lowering energy consumption in refining a mixture or suspension of fibrous material 1, for example, moist wood chips in a refiner with inlet and outlet for fiber material, in which the material passes through a mill gap between each other with a speed of 1000 rpm - 1500 rpm moving conical or cylindrical shape surfaces with the ma 1 frequency being 6000 - 20 000 Hz, characterized in that the a) ma 1ing operation is performed with a drastically reduced grinding frequency fill about 300 - 800 Hz through a reduced (b) the grinding process is carried out with an increased maintenance time for the fiber material in the grinding plate to compensate for the grinding pulse number lost through the frequency reduction, and c. from the grinding column. 2. A method according to claim 1, characterized in that the meadow removed from the mill gap within the refiner is freed from substantially all the fiber material with the meadow.