Method and Apparatus for Measuring Contaminating Impurities in Pulp Slurry

1. Field of the Invention

The present invention relates to a method for immediate and high-precision detection of impurities which contaminate raw pulp slurry during raw pulp processing, as well as an apparatus therefor.

2. Description of the Related Art

Pulp used in paper making is subject to contamination by impurities such as dirt, pitch, or ink or toner residue. These impurities cause trouble during the paper making process and their contamination lowers the quality of the product. Control of impurities is therefore necessary during the raw pulp processing. Usually, the raw material is periodically sampled to prepare a hand-made sheet, and the production process is controlled by the operator's visual determining of the contaminating impurities. The present applicant has already proposed a dirt measuring device using hand-made sheets (Japanese Patent Application No. 4-117668). There have also been developed an on-line measuring device for use during raw material processing (Japanese Unexamined Patent Publication No. 63-295946) and a method for measuring slurry pulp (Japanese Unexamined Patent Publication No. 61-272639).

Although the above-mentioned dirt measuring device using hand-made sheets provides highly precise measurements, time is required to prepare the hand-made sheets, leading to increased risk of damage to the sheets because of the delay for control during the raw material processing. On the other hand, in the case of the on-line measuring device used during the raw material processing, the setting location is limited to being on the drum-washer filter, and the poor condition of the measuring surface of the raw pulp on the drum-washer filter produces a crude measuring limit, making it impossible to obtain a satisfactory resolution. Furthermore, in the method for measuring slurry pulp, it is necessary to prepare a constant predetermined-dilution raw pulp concentration and flow rate for the measurement, and thus time is required for pretreatment prior to the measurement.

The object of the present invention is to improve the weaknesses of the conventional methods described above, by providing a method and apparatus capable of detecting impurities in raw pulp slurry at a high precision by taking samples on-line.

The method of measuring impurities according to the present invention is characterized by using wire having a mesh capable of passing through the moisture and trapping the fibrous portions of raw pulp slurry at various concentrations, at the time of raw pulp processing during the step of pulp production or paper making, and suctioning the pulp slurry from the back side of the wire or pressing the wire itself against the pulp slurry, to prepare a highly dense wet mat concentrated on the front of the wire, and then pressing a flat transparent plate on the surface of the wet mat, detecting the surface of the resulting highly dense, highly smooth wet mat with a CCD camera in a catoptric system Via a glees plate, and creating an image therefrom to measure the area which contains impurities with respect to the surface area of the wet mat.

In addition, the apparatus for measuring contaminating impurities in pulp slurry according to the present invention is characterized by having an elongated container having a rectangular or circular cross-section at least a portion of the bottom of which is formed of a flat glass plate which permits visual observation of the interior thereof, and a pulp slurry intake conduit connected to the top of the container and a pulp slurry discharge conduit connected to the bottom via a bulb, with flat wire having a mesh which passes through the moisture in the pulp slurry and is capable of trapping the fiber portion set so as to be vertically movable while in close contact with the side wall of the container; and by being provided with a CCD camera directed from outside of the container toward the above-mentioned plate, as well as a computer for processing the image from the CCD camera in order to measure the area of impurities per unit area of the wet mat.

The transparent plate used here may be a synthetic resin plate, but a glass plate is also suitable.

FIG. 1 is an illustration showing a measuring method according to an embodiment of the present invention.

FIG. 2 is a timing chart for the measuring apparatus according to the embodiment.

FIG. 3 is a drawing showing the correlation between the results of the method according to the embodiment and a conventional dry sheet measuring method.

FIG. 4 is an illustration showing a measuring method according to another embodiment of the present invention.

The measuring method of the present invention is carried out by preparing a wet mat in a container and making the measurement with this wet mat pressed against a flat transparent plate, and therefore although the image observed over the transparent plate is that of the wet mat, the mat is clearly seen in the same state as after drying. Furthermore, since the measurement is made while in the state of a wet mat, the preparation thereof is simple, and the prepared mat may also be easily returned to the pulp slurry state.

The measuring apparatus shown in FIG. 1 has an elongated container at least a portion of the bottom of which is formed of a flat glass plate which permits visual observation of the interior thereof, and a pulp slurry intake conduit and a pulp slurry discharge conduit are connected thereto; thus, as shown in the timing chart in FIG. 2, in this apparatus, first the drainage lid cylinder 2 is switched ON to close the lid, the raw pulp intake valve is switched ON to introduce the pulp slurry into the container, and the continuous raw pulp pressurizing cylinder 1 is switched ON to move the wire through the pulp slurry and prepare the wet mat. In this state, the condition of the surface of the wet mat is measured, after which the raw pulp-pressurizing cylinder 1 is switched OFF and released while the drainage lid cylinder 2 is simultaneously switched OFF to open the lid, and the wet mat returns to pulp slurry and is discharged from the drainage valve. Then, the wash water on-off valve is turned ON (opened) and OFF (closed) to introduce a constant amount of water into the container to wash the container. Since one measurement may be made by one such cycle, measurement is possible on-line during the paper making process.

The method and apparatus of the present invention will now be explained with reference to the drawings. FIG. 1 is an illustration showing an embodiment of the present invention. The pulp slurry to be measured is fed from a line pipe or a tank (neither shown) during the paper making step and, by the operation of a bulb, is taken into an elongated container 3 for measurement. This container 3 has a raw pulp intake opening 4 and an overflow outlet 6 for excess sample at the upper end, and it has an on-off sample discharge opening 7 and a wash opening 8 at the lower end, while the bottom is provided with an observation window consisting of a transparent plate 10 to allow observation of the raw pulp from the bottom. In addition, the inside of the Container 3 is provided with mesh wire 13 in close movable contact with the inner wall of the container due to the movement of the piston of a raw pulp-pressurizing cylinder 1. Furthermore, a CCD camera 11 is positioned opposite the above-mentioned transparent plate 10, and it is connected to a CPU 12 which processes the image from the CCD camera 11 and measures the area of impurities per unit area of the wet mat. In addition, illumination 14 is positioned facing the transparent plate 10 for a satisfactory photographic image of the CCD camera 11, while the CCD camera 11 is connected to the CPU 12 via an impurity counting board 15, and a monitor TV 17 and host computer 16 are also connected to the CPU 12. In cases where the image of the wet mat is detected while moving the CCD camera 11, the camera 11 is mounted on an XY stage 18 and controlled by an XY stage controller 19 which controls its movement in the XY direction. In the figure, 20 indicates a power source for the camera.

A method of measurement using the above-mentioned apparatus will now be explained. First, after pulp slurry is introduced into the container 3, pressure is applied to the piston of the raw pulp-pressurizing cylinder 1 to push the mesh wire 13, and the pulp fiber is compressed at the bottom of the container 3 to form a wet mat. The surface of the wet mat prepared in this manner is then detected by the CCD camera through the transparent plate 10 at the bottom of the container. The CCD camera 11 measures the impurities by a catoptric system, but the lens system may be set to match the resolution of measurement, allowing easy detection of impurities on the order of .phi.10.mu.m.

Also, if the CCD camera is set on an XY stage, a large measurement area may be taken even when a fine resolution of measurement makes the area of a single measurement smaller, as the measurement may be made while moving the XY stage. After the measurement the piston is raised, the discharge opening is opened, and wash water is introduced into the cell (i.e., elongated container) to discharge the pulp. By continuously performing this process, continuous on-line measurement of impurities may be made during the raw pulp processing. The results of the measurement are obtained by sending the image signal taken by the CCD camera 11 to the impurity counting board 15 at high speed for processing, from which it is passed through the CPU 12 at the measuring device end and sent the monitoring host computer. The host computer 16 gives the operating instructions for each of the measuring sections while displaying the results of measurement, i.e. the changes in contamination by dirt, on a trend screen.

FIG. 3 is a drawing showing the correlation between the results of actually measuring impurities in raw pulp slurry by the measuring method described above and the results of measuring impurities in the same raw pulp slurry by the method in Japanese Patent Application No. 4-117668 using a hand-made dry sheet. The measurement using a dry sheet and the method of the present invention are thus seen to correlate very well. Also, Table 1 gives a comparison between the results of measuring impurities based on the density and volume of the raw pulp measured. The bottom area of the cell or elongated container used in this case was 100 mm.times.100 mm, but as long as there is a constant amount or more of pulp which accumulates at least to a degree where the surface of the wire cannot be seen, so that a uniform wet sheet is thus formed, then it is possible to always detect a constant degree of contamination by impurities per measured area, without depending on the concentration or amount of the slurry itself.

The measuring system in FIG. 4 will now be explained as a separate embodiment. This measuring system has a suction box 22 whose suction side is formed into a mesh wire structure, and the mesh wire 23 passes through moisture of the pulp slurry while trapping the fiber portion, thus forming a wet mat on the surface of the mesh wire 23.

The wet mat formed by suction while the suction box 22 is directly immersed in the pulp slurry in the raw pulp tank 21 is then moved by a vertical cylinder mechanism 24 and rotating frame 25 which manipulate the box, to the position indicated by dotted lines, where it is pressed against a smooth transparent observation window 26. The smooth surface of the wet sheet obtained in this manner is then measured through the observation window 26 by a catoptric system which includes a CCD camera 27. The imaging and display of results is carried out in the same manner as the system described above relating to FIG. 1. After the measurement, the box 22 is raised and wash water is sent to the inside of the box and the measuring surface to wash the sheet on the surface of the box and the measuring surface. By repeating this process, continuous on-line measurement of impurities may be made during the raw pulp processing. In addition, more measurements may be made by mounting a plurality of suction boxes on the rotating frame and conducting simultaneous sampling and measurement.

The mesh wire used in the preceding example is 200 mesh wire, which is for measurement of foreign matter of .phi.10.mu.m, but the mesh may be selected depending on the size of the foreign matter to be measured. Also, since stable results are achieved only if the pulp concentration of the wet mat used for measurement is at least 20 wt %, the pressure of the pressurizing cylinder must be adjusted so as to adequately produce this concentration.

According to the present invention, highly precise control of impurities in pulp slurry is made possible on-line during the paper making process, allowing real-time evaluation of raw pulp; as a result, unusable raw pulp may be removed prior to or during the paper making step, thus preventing the production of defective paper.