JPH0134258B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for separating and removing plastics mixed in wood chips. Nowadays, plastics are used everywhere, but the current situation is that we are struggling to deal with their waste, which is referred to as plastic pollution. This problem is similar in the pulp and paper industry, where wood chips contain plastic bags,
Strings, name tags, fragments of containers, toys, other plastic contaminants, and rubber (these are collectively referred to as plastics) are often mixed together. Plastics have excellent chemical stability, so even when treated with chemicals under high temperature and high pressure during the pulping process, they remain intact in the pulp, with at most a degree of deformation due to heat. These mixed plastics are further broken down into fine pieces by various mechanical actions such as pumps and beaters during the pulping process, and can hardly be separated by conventional screens. Furthermore, even if cleaners that apply centrifugal force are used, the difference in specific gravity between pulp and foreign matter is small, and plastics usually have a smaller specific gravity than pulp, so it is difficult to use cleaners that consume a large amount of power. Separation efficiency is extremely low. If paper is made from pulp that contains such fine particles of foreign matter, the foreign matter will enter the paper layer and
Currently, paper manufacturers are struggling with measures to remove plastics mixed in with wood chips, as they cause dust, pinholes, holes, and even paper breaks. Currently, the methods for separating and removing plastics mixed in wood chips include manual separation, and flotation separation of plastics using the slight difference in specific gravity between wood chips and plastics in a fluid. Although methods are known, the former requires a lot of manpower and is inefficient.
In the latter case, the separation efficiency is not very good. Therefore, the inventor of the present invention conducted extensive research to find a way to increase separation efficiency without using human hands, and as a result, discovered that wood chips contain a large amount of water.
Among the dielectric properties, the dielectric loss tangent is extremely large, so the charge decay is extremely fast and it is difficult to charge, whereas plastics generally have a low dielectric loss tangent.
For this reason, we focused on the fact that charge decay is slow and therefore are extremely easy to charge, and succeeded in developing a separation method that takes advantage of these properties. The present invention is a separation method that utilizes the difference in charging characteristics between wood chips and plastics, and its structure is as follows. That is, first, wood chips mixed with plastics are subjected to corona discharge treatment to become electrically charged. In the first place, wood chips contain a large amount of water, so they have high electrical conductivity. Plastics, on the other hand, are insulators, and their electrical conductivity is orders of magnitude lower than that of wood chips. Therefore, the charge decay rate on corona discharge-treated wood chips is fast, and the observed surface potential decay rate is also fast.
On the other hand, the rate of charge decay on plastic treated with corona discharge using a corona discharge treatment machine is slow;
As a result, the decay rate of the observed surface potential also slows down, so when the potential is observed after corona discharge treatment, the absolute value of plastic is large, while the absolute value of wood chips is extremely small. For this reason, the present method attempts to efficiently separate and remove plastics by detecting the presence of plastics from the difference in surface potential observed through active charging through corona discharge treatment. The electrode section of the surface electrometer is installed in a position where corona discharge treatment is applied to the wood chips containing plastics that are being transferred on the belt conveyor, and the electric charge of the charged plastics is not attenuated. and the surface potential of the plastics is sufficiently large to be distinguishable from the surface potential of the wood chips. In this method, the plastics that are mixed in the wood chips that are being transferred (generally, they are not evenly mixed, but are often locally unevenly distributed) are subjected to electrical discharge treatment in advance. to charge the
The resulting surface potential is measured using a surface electrometer, and in this case, a comparator (potential comparator) is connected to the electrometer, and the output of this comparator is sent to a display device such as a lamp or alarm. At the same time, a timer is activated by the signal from the comparator. After the set time of the timer has elapsed, the belt conveyor is stopped by the output signal of the timer, or a plastics removing device, which will be described later, is activated. The timer is used to stop the belt conveyor after a certain time delay after the plastics mixed in the chips are detected by their surface potential, or to automatically remove the chips. It is used to operate the Hereinafter, specific means for separating and removing plastics from wood chips using this method will be generally listed. (1) The output of the surface electrometer is introduced into a comparator, and the output signal of the comparator is introduced into a display device such as an alarm. In such cases, if the surface potential detected by the electrometer exceeds a certain value, a lamp will light or an alarm will sound. On the other hand, since the output signal of the comparator is also supplied to a separately provided timer, the belt conveyor is stopped after a predetermined time delay through the timer. In other words, when plastics are mixed in wood chips, the presence of plastics is detected by a surface electrometer, and when the surface potential exceeds a certain value, a comparator is activated to sound an alarm and turn on the pilot. This method uses a timer to stop the belt conveyor after a certain period of time and manually removes only the plastic from the wood chips. (2) When the presence of plastic is detected by a comparator connected to an electrometer, the pilot lamp lights up, an alarm sounds, and a timer is activated. After a short delay, the wood chip remover moves on the conveyor belt and removes the plastic. To automatically remove wood chips contained in the belt conveyor in the lateral direction. (3) As a device for removing plastics from wood chips, this device is installed at the discharge end of a belt conveyor, and after it is moved automatically in a timely manner to remove the wood chips containing the plastic, Another method is to remove plastics by moving them to another location. Note that the comparator used in the present invention is operated so that the lower limit of the absolute value of the measured potential is set in advance and it automatically determines whether the measured value is greater than the lower limit, and is used as a type of potential comparison. It is a vessel. In addition, in order to effectively perform corona discharge treatment on a group of wood chips moving on a belt conveyor and to be able to effectively detect the surface potential of plastics, it is necessary to Preferably, it flows on a conveyor. Furthermore, in the case of plastics to which pigments have been added, the potential decay rate is fast, so in order to detect the chargeability of the plastic, the position where the corona discharge treatment is applied and the position where the surface potential is measured should be as close as possible. This is desirable. In other words, it is necessary to install the electrometer at a position where even if the electric charge caused by the corona discharge of the plastic attenuates, it can be observed at a magnitude that can be distinguished from the electric potential of the wood chips. Usually, the corona discharge treatment potential is from 1KV
Using a range of 50KV, the distance between the needle electrode of the corona discharge bar and the counter electrode of the belt conveyor is
The length should be between 3cm and 20cm. The potential and distance between the electrodes are not limited to the above-mentioned ranges, but can be appropriately selected so as to effectively charge the plastic. Plastics whose presence can be detected with a surface electrometer through corona discharge treatment include polystyrene, polyethylene, polyvinyl chloride, polycarbonate, polyethylene terephthalate, polytetrafluoroethylene, polypropylene, polyvinylidene chloride, polyvinyl fluoride, and polyvinyl chloride. Synthetic polymer compounds including vinylidene fluoride, epoxy resins, polyacrylic resins, polyamides, melamine resins, polybutadiene, etc. and their copolymers, as well as cellophane, rubbers, etc., have a charge decay rate that is lower than that of wood chips. It's slow. The size of plastics that can be detected is 1 mm ² or more, but
Even plastics with a size of mm ² or less, such as polystyrene, polyethylene, and polyester, which have extremely high charge density due to corona discharge treatment, can be sufficiently detected with an electrometer. It is also preferable that the electrode of the surface electrometer be placed as close as possible to the flowing layer of wood chips.
Install it in a location where it will not come into contact with wood chips. Of course, since the value of the surface potential differs depending on the electrode position of the electrometer, it is necessary to make corrections. Furthermore, extremely small plastic pieces can also be screened out beforehand using a coarse mesh wire conveyor. Generally, it is said that the charging efficiency of plastics is better when negatively charged, so it is preferable to set the needle electrode of the corona discharge bar to a negative potential for corona discharge. However, it is of course possible to use the opposite potential. The present invention will be specifically explained below based on the additional drawings. Example 1 As shown in FIG. 1, wood chips are continuously dropped from a storage container 1 containing wood chips onto a belt conveyor 2 with a mesh size of 32, for example, and the wood chips are scraped and leveled as shown by numeral 3. Level the chips into a single layer on the conveyor using a leveling rod. On the other hand, above the transfer surface of the conveyor 2, a needle-shaped corona discharge bar 5 connected to the high voltage power supply device 4 is provided,
The wood chips being transported are subjected to corona discharge treatment to become electrically charged. As the discharge bar, it is preferable to use an electrode in which a large number of needles are embedded in a rod-shaped insulator. Further, the metal belt conveyor is grounded as shown in the figure, and the conveyance speed is, for example, 3 m/min. Also, a surface electrometer 6 is placed behind the corona discharge bar 5 at a distance of about 30 cm.
The electrode section 7 is installed to measure the amount of potential charged on the chips. When using three scanning type electrode parts 7a to 7c as shown in FIG. 2, the guide rod 8
The three electrodes 7a to 7c are arranged at a position about 5 cm above the conveyor via the belt conveyor so as to each cover 1/3 of the width of the belt conveyor, and are arranged at right angles to the conveying direction of the conveyor. Scan back and forth at a speed of 7.8 m/min. Note that the scanning type electrode portions 7a~
7c may be arranged in two or three stages in the front and rear. The surface potential of the wood chips thus charged is detected and measured by a surface potentiometer 6. The output measured by the surface electrometer 6 is
The voltage is introduced into the comparator 9 shown in the figure, and the comparator detects whether or not the potential is higher in absolute value than the reference potential. For example, if the lower limit potential of the comparator 9 is set to -30 volts and a higher potential than this is detected in absolute value, the pilot lamp 10 is turned on by the detection signal, or the alarm 11 is sounded at the same time. let As mentioned above, when the detection signal of the comparator is emitted, it indicates that plastics are contained in the wood chips. In response to this signal, a separately installed timer 12 is activated, and the drive drum 2a of the conveyor 2 is stopped after a certain period of time has elapsed. Thereafter, the plastics contained in the chips on the belt conveyor are manually removed. Thus, the present invention also includes a method for automatically detecting plastics contained in wood chips and manually removing them. Example 2 Next, an example will be presented in which wood chips containing plastics are automatically and locally removed without manual intervention. In the apparatus shown in FIG. 1, a hopper 13 is attached to the discharge end of the belt conveyor 2,
Branch pipes 15a and 15b each having a distribution damper 14 are disposed in this hopper, and the damper 14
The signal of the timer 12 described above is supplied to the drive source 16 of the drive source 16 . Therefore, the damper 14 is normally set at the position shown by the chain line in the figure, so that the chips discharged from the conveyor 2 are stored in the receiver 17. If plastics are mixed in the wood chips, their presence is automatically detected as described above and timer 12 is activated.
is activated, and after a predetermined period of time has elapsed, the damper 14 is switched to the position shown by the solid line in the figure. In such a case, the chips containing plastics are stored in the plastic receiver 18 via the branch pipe 15b. Embodiment 3 As a method for removing plastics mixed in wood chips, it is also possible to use the means shown in FIGS. 3-4 or 5. In the former case, a scraper 19 that can be raised and lowered is installed diagonally above the belt conveyor 2 that transfers the chips, and the switching valve of the lifting cylinder 20 is operated by the above-mentioned detection signal and the output signal of the timer 12. This is an example of trying to force someone to do something. That is, the scraper 19 is normally raised as shown by the solid line in FIG. 2, and when the chips containing plastics reach a predetermined transfer position,
The scraper is lowered in response to each of the above signals and is automatically removed as shown by the arrow in FIG. In the case shown in FIG. 5, a reciprocating type plastic receiver 22 attached to the piston rod of a cylinder 21 is installed at the discharge end of the conveyor 2. In this case, an electromagnetic solenoid type spool valve 23 is switched,
Plastics are thereby automatically removed. Embodiment 4 Figure 6 shows an example of transformation when detecting the surface potential of wood chips and plastics mixed therein, using a non-scanning surface potentiometer model SSV- manufactured by Kawaguchi Electric Co., Ltd. 40 is placed 4 cm above the belt conveyor, the detection area of one electrode 7d is 7 to 8 cm wide, so if this is applied to a 60 cm wide belt conveyor, the detection area in the lateral direction It is sufficient to arrange eight electrodes, and in some cases, as shown in the figure, eight electrodes may be arranged in two rows, front and back.
The electrodes may be arranged in a staggered manner. Further, when a plurality of electrodes are used, when any one of the electrodes detects a potential higher than the set potential of the comparator 9, an output signal of the comparator is generated. Incidentally, to explain the corona discharge bar 5, the distance between the bar 5 and the belt conveyor 2 is preferably 7 to 8 cm, and the voltage is preferably 10 to 40 KV, for example. Furthermore, since the belt conveyor needs to play the role of a counter electrode for the corona discharge, it is necessary to use a conductive material and to ground it. The time from detecting the mixture of plastics as described above to activating the damper 14, scraper 19, reciprocating plastic receiver 22, etc. depends on the transfer speed of the belt conveyor 2;
This is related to the mounting position of the electrodes 7, 7a to 7b, and also depends on the distance to the end of the conveyor.
Take these into account and set as appropriate. The relative relationships in each of the above embodiments are listed below.

[Table] Next, when the voltage applied to the corona discharge bar is 6KV and the distance between the belt conveyor and the electrode is 2cm, the surface potential of the plastic is:
Although it varies depending on the size, shape, composition, etc. of the plastic, for example, the surface potential of plastic on a belt conveyor 2 cm away from the electrode (applied voltage of 6 KV to corona discharge bar, measured after 3 seconds of discharge treatment) values) are shown in Table 2.

[Table] If the wood chips contain a large amount of moisture, especially if the chips are pre-treated with steam to prevent the wood chips from catching fire due to corona discharge, the plastics will also become wet. It was confirmed that the surface potential of plastics can be observed to be large even when the plastics are subjected to corona discharge treatment, and that the object of the present invention can be fully achieved.

[Brief explanation of drawings]

Fig. 1 is a skeletal side view of an example of an apparatus for implementing this method, Fig. 2 is a skeletal cross-sectional view showing a scanning electrode in the same device, and Fig. 3 is a skeletal cross-sectional view showing a scanning electrode used in the above device. 4 is a plan view of a portion of the same as above; FIG. 5 is a vertical sectional view of a portion of another embodiment of the plastics removal device; FIG. 6 is a surface potential diagram. FIG. 3 is a layout diagram showing fixed electrodes for detection. 1: Wood chip storage device, 2: Metal belt conveyor, 3: Bar for raking, 4: High voltage power supply, 5: Corona discharge bar, 6: Surface electrometer,
7, 7a to 7d: electrode, 8: guide rod, 9: comparator, 10: lamp, 11: alarm, 12: timer, 13: hopper, 14: damper, 15a, 1
5b: branch pipe, 16: damper drive device, 17: wood chip receiver, 18: plastics receiver,
19: Scraper, 20, 21: Cylinder, 2
2: reciprocating plastic receptor, 23: spool valve.

Claims (1)

[Scope of Claims] 1 Wood chips mixed with plastics are subjected to electrical discharge treatment, and the presence of plastics is detected using an electrometer using the difference in chargeability (surface potential) between the wood chips and the plastics. A method characterized in that the plastics are detected and separated from wood chips. 2. Claim 1, wherein wood chips mixed with plastics are transported by a metal belt conveyor, and scanning or fixed surface potential detection electrodes are disposed above the conveyor.
The method described in section. 3 Introduce the output of the surface electrometer to the comparator,
The output signal of the comparator operates a display device such as a pilot lamp or an alarm, and
3. A method according to claim 1, wherein said output signal is fed to a timer, and a device for removing plastics is activated after a set time of said timer has elapsed. 4. A hopper having a branch pipe is disposed at the discharge end of the belt conveyor, and a damper for distribution is attached to the branch part of the branch pipe, according to claim 1, 2, or 3. the method of. 5. A method according to claim 1, 2 or 3, comprising a reciprocating plastic receiver attached to the discharge end of the belt conveyor.