This invention relates to a method of controlling the manufacture of mechanical pulp in a refining process where cellulose-containing material in lumps, such as wood chips ^* , is refined. The chips prior to the refining can be treated with heat and/or chemicals for manufact¬ uring TMP (thermomechanical pulp) or CTMP (chemi-thermo- mechanical pulp). The refining is carried out in one or several steps by single- or double-disc refiners. These refiners are provided with opposed refiner discs rot¬ ating relative one another. The discs are provided with disc segments comprising bars and intermediate grooves. Opposed disc segments form a gap where the material is refined during its passage outward.

The properties of the manufactured pulp are influenced, besides by the quality of the wood chips, by a great number of system parameters. Among them can be mentioned the distance between the disc segments (gap), the load of the motor driving a rotary refiner disc, the pressure by which the refiner discs are pressed in the direction toward each other, the pressure at the feed-in of the chips, the pressure in the housing enclosing the discs, the supply of diluting water, the material flow through the refiner (the production), the material con¬ centration. Some of these parameters are depending on each other while other are substantially independent. For example, the motor load increases and so does the pressure by which the discs are pressed toward each other when the gap decreases.

It is impossible in practice to check and control all parameters influencing the properties of the pulp. It was found, however, that a desired pulp quality can be achieved with pretty high precision by controlling some especially important parameters, viz. the gap size, the ma ^* t?eria ^" l concentration and the production.

A great problem is that the measuring of the system parameters does not yield a direct measure of the pulp properties. For being able to determine the properties of the pulp, such as tensile strength, tearing resistance, dewatering capacity, shives content, fibre length etc., it is, of course, necessary to analyze the pulp and the paper made thereof. In a mill it takes normally several hours to obtain the results of such analysis, and sampling usually is carried out not more than 2-3 times per day.

It is, therefore, impossible to rapidly discover and compensate for such variations in the pulp properties which are due to system parameters, which have not been determined, or where there is no simple relation between the system parameter and the pulp properties.

One factor causing the relation between the measured system parameters and pulp properties to change in operation is the wear of the refiner discsegments. This implies that certain pulp properties can deteriorate although the measured system parameters remain unchanged. This implies in practice, that the system parameters must be adjusted on the basis of analysis results of a pulp, which had been manufactured several hours earlier. This is, of course, a great disadvantage.

The delay in obtaining the analysis results involves substantial disadvantages also in connection with the testing of and comparison between different refiner disc segment

It is desired, therefore, to be able during the refining process to measure such system parameters,which render ^* it possible to predict the pulp properties with greater accuracy than it has been heretofore possible.

The present invention offers a solution of this problem.

The invention implies that the vibrations arising in the refiner discs during the refining are utilized for calculating the pulp properties. The characterizing features of the invention become apparent from the attached claims.

The invention is described in greater detail in the follow¬ ing, with reference to embodiments and test results shown in the accompanying drawing, in which Fig. 1 shows a frequency analysis of the measured vibrations, Figs .2 show the agreement between measured and calculated tensile strength without and, respectively, with utilization of the vibrations in the refiner discs.

One property important for the quality of pulp is the tensile strength. This applies especially to mechanical pulp intended for papermaking.

By controlling and adjusting the three system parameters gap size, material concentration and production, it is possible with pretty good precision to maintain a desired pulp quality. Experiments carried out on mill scale, however, have shown that the pulp quality deteriorates with the time due to wear of the refiner discs, without the possibility of predicting this by control of the aforesaid system parameters.

By measuring the high-frequency vibrations arising in the refiner discs due to their relative rotation and their segment design, it is possible to calculate the vibration energy over the refiner disc segment. The frequency depending on the rotation speed of the discs and the design of the disc segments can amount to several thousands c.p.s. The meas¬ uring is carried out by means of an accelerometer attached to the disc, preferably to the rear side of a segment. In a single-disc refiner the accelerometer is attached on the stationary disc. It is also imaginable to attach accelerometers to both discs in a single- or double-disc refiner, in order to obtain additional information on the vibrations of the discs.

By including the vibration energy thus measured in the calculation of the pulp properties, it was found by surprise, that these properties can be predicted with much higher

precision. This applies especially to the strength propert¬ ies of the pulp (tensile strength). It was found possible, thus, to predict the reduction in tensile strength caused by wear of the disc segments.

This implies simultaneously that the vibration energy also can be utilized for determining the condition of the processing surfaces of the segments. The vibration energy, furthermore, can be utilized for comparing the efficiency of disc segments of different types.

Example

In a single-disc refiner an accelerometer was mounted in a hole drilled in the rear side of a disc segment in the stationary disc. The segments were designed with three zones comprising bars and grooves of different size.

The refining was carried out with pre-heated chips for the manufacture of TMP. The system parameters and pulp properties at two test runs were as follows: _{Test ψe} _ _t J _T

Production (ton bone-dry pulp/24 h) 70 80

Material concentration ( % ) 48 48

Gap size (mm) 0.46 0.38

Tensile index (Nm/g) 33 33

CSF (ml) 184 150 Tear index (mN m /g) 8.5 7.5

Specific energy (k h/ton pulp) 2150 2075

The signal from the accelerometer was simultaneously measured and analysed. The frequency range in question was 5 - 5 kc/s. In Fig. 1 a frequency analysis of this signal is shown. The signal can be divided into three differen areas corresponding to the three zones of the segments. In the inner zone comprising the eoacsest bars the frequencies 5.6-11.2 kc/s were noted. In the central zone 11.2-17.6 kc/s, and in the outer zone comprising the finest bars 17.6- 25 kc/s were noted. The vibration energy is represented by the surface beneath the frequency curve in Fig. 1.

After 800 operation hours new measurements of the system parameters and pulp properties were carried out. It was then found, that most of the measured pulp properties agreed well with the pulp properties, which were calculated by means of measured system parameters and results from previous tests. One exemption was the tensile strength, of which the measured values were lower than the calculated ones. In Fig. 2 the measured tensile index is shown as a function of the tensile index, which was calculated by means of measured values of production, gap size and material concentration. It shows that there is a heavy systematic error. The fully drawn line designates full agreement, and the dashed lines designate an acceptable error range.

By including in the calculation of the pulp properties the vibration energy obtained from the accelerometer signal, all measured pulp properties could be predicted with high precision. In Fig. 3 the measured tensile index is shown as a function of the calculated tensile index where the vibration energy has been utilized together with the adjusted production, gap size and material con¬ centration. The agreement there lies within the error range. No systematic errors could be stated.

The deterioration in the tensile strength of the pulp can be explained by the wear of the disc segments. Heretofore it has not been possible to find a controllable relation between the tensile strength and the wear of the segments. The present invention, thus, offers such a control possib¬ ility. By measuring the vibration energy according to the invention, thus, the condition of the disc segments can be determined, which also can be utilized for determining the time when the segments have to be exchanged. The invention can also be used for comparing different segment patterns and materials.

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