The present invention relates to a method and an apparatus for separating contaminants from paper stock and defibering undefibered waste paper in the field of industries using waste paper pulp as stock such as paper pulp and fiberboard industries.
Screening is generally composed of coarse and fine screening stages.
In the coarse screening stage, relatively large contaminants are removed, using a screen plate with holes usable for relatively high consistency (2 to 4%) of stock slurry in order to reduce in quantity the contaminants to be transferred to the fine screening stage.
In the fine screening stage, fine contaminants not removable by the above-mentioned hole screen plate are removed, using a screen plate with slots suitable for relatively low consistency (0.5 to 2%) of stock slurry so as to facilitate passing of the stock through the screen.
Generally, efficiency or ratio of removing contaminants in a screen is closely related with reject ratio. Increase and decrease of reject ratio lead to enhancement and lowering of contaminant removal ratio, respectively. Attempt to reduce the reject ratio in an ordinary screen will tend to cause plugging of the screen plate or plugging of a reject valve due to increased reject consistency. Even if such plugging may be averted, extreme reduction of the reject ratio would worsen the effect of removing contaminants as shown in FIG. 1, failing to obtain good screening effect. Increase of the reject ratio to a certain extent is therefore required for obtaining pulp with less quantity of contaminants. However, increase of the reject ratio means reduction of yield.
Generally, in order to overcome this problem in a screen stage, a reject ratio of 20 to 25% is selected, over which the curve shown in FIG. 1 becomes dull and the contaminant removal ratio is less affected, and reject is re-processed by a so-called "multiple cascade flow" system to reduce the reject ratio in the whole of the system. In a typical cascade flow employed, reject of a primary screen is processed by a secondary screen and the accept stock is brought to accept of the primary screen. Reject of the secondary screen is processed by a tertiary screen and the accept stock is returned to the feed stock of the secondary screen. Only reject of the tertiary screen is discharged out of the system. Generally, stock slurry consistency in a screen becomes higher than the consistency of the feed stock and therefore the feed stock used for the cascade manner is required to be diluted with water into appropriate consistency for the screen.
On the other hand, paper stock to be fed to a screening stage is in the form of defibered suspension of waste paper in water by a defibrator, usually called a pulper. Defibering performance of the pulper is not in linear relationship to defibration time period (motive power). In comparison with initial defibering performance, subsequent defibering performance is decreased. That is, defibering efficiency is satisfactory up to a certain level of defibration [i.e., defibered stock/(defibered stock+undefibered stock)] and higher motive power is required for defibration over the level. In order to defiber the stock which has been defibered to the certain level, a device generally called "secondary defibrator" is widely used. Typical secondary defibrators are a closed pulper type defibrator and a high-speed defibrator. Such secondary defibrators also have defibering performance which is not in linear relationship to motive power and are effective for use at a zone or portion of the system where undefibered waste paper is accumulated.
To defiber undefibered waste paper is very significant for improvement of production yield since the undefibered waste paper shows the same behavior as contaminants to be removed in screening stages.
In FIG. 2 which is a flow sheet of a conventionally used screening process for waste paper stock pulp slurry, reference symbol a represents a tank to receive waste paper stock slurry which has been defibered by a pulper (not shown). In a coarse screening stage A, reference symbols b, c and d represent primary, secondary and tertiary coarse screening screens, using hole screen plates, respectively; g, a high-speed defibrator for defibering reject of the primary coarse screening; and e, f and m, tanks. In a fine screening stage B, reference symbols h, i, k, and 1 represent primary, secondary, tertiary and quaternary fine screenings, using slot screens, respectively; j, a high-speed defibrator for defibering reject of the secondary fine screening screen; and n, o and p, tanks. In FIG. 2, solid lines represent pulp lines and dotted lines, lines of reject including undefibered waste paper.
In FIG. 2, usual screens with hole screen plates are used in the coarse screening stage A. Reject of the primary screen b is processed by the high-speed defibrator g to defiber undefibered waste paper accumulated in the reject. In the fine screening stage B, a quaternary cascade system with slot screens is used and the reject of the secondary screen is processed by the high-speed defibrator j.
In FIG. 2, nine apparatuses with screens, seven tanks with agitators and seven pumps are required. For automatic operation, various instruments are further required such as pressure control for each screen and level control for each tank.
Instead of defibering waste paper, the waste paper may be ground by a refiner. Such grinding is however directed to crushing not only the undefibered waste paper but also contaminants such as plastics and is different from the defibration in which contaminants such as plastics and wooden pieces are passed without crushing, and therefore has a deteriorated degree of screening compared with the defibration. Also, the stock slurry consistency in the grinding is as high as 15 to 25% while in the defibration, the stock must be diluted to have the consistency of 1 to 4% because of the above difference.
As described above, the more the number of screens for cascade is increased, the more the degree of screening and production yield can be enhanced, but the more the scale and cost of the facilities increase.
To solve the above problems, there have been various proposals to provide a system in which a screening section is combined with a defibering section or with a grinding section.
For example, Japanese Patent 1st Publication No. 62-90391 (JP-A-62-90391) proposes "a screening apparatus with reject reducing means" which processes pulps with vegetable fiber of 6 to 15% in consistency. A grinding zone is provided adjacent to a screen with a cylindrical screen plate and the reject is decreased in quantity by grinding the reject of the screen into pulpiness. However, when this apparatus is used for waste paper pulp, there arise the following problems:
(1) Unlike vegetable fiber pulp, waste paper pulp includes not only the undefibered waster paper but also contaminants such as plastics and metal pieces. If these contaminants are ground and mingled in the accept, the product quality is decreased. PA0 (2) A consistency suitable for the grinding is 15 to 25%. In the case of waste paper pulp, if the reject of the screen is condensed to this range of consistency, plugging tends to occur in the screen. If meshes of the screen are enlarged for prevention of such plugging, then the contaminant removal ratio is reduced. PA0 (3) After the grinding, contaminants remain in the pulp. To remove them, another screen is required.
On the other hand, the inventors have made various experiments to find that, when waste paper pulp slurry is screened, reject not passing through a screen is accumulated more and more and its consistency is increased as the slurry flows through a screening section, deteriorating the separation effect, and that the separation effect may be improved if such condensed reject is diluted in the screen.
To solve the above problems, it is an object of the present invention to provide a method and an apparatus for screening waste paper in which a single screen has screening and defibering sections and reject after the defibration is diluted and re-separated, thereby increasing contaminant removal ratio and production yield and achieving space- and cost-saving and simple system control.
To attain the above object, an apparatus according to a first aspect of the present invention comprises
a cylindrical casing having a stock inlet at one end thereof, a reject outlet at the other end thereof and an accept stock outlet between the ends thereof, PA1 a cylindrical screen plate concentrically fixed to define a space between an inner surface of said casing and said screen plate, PA1 an annular defibration stator concentrically disposed adjacent to an end of said screen plate near said reject outlet, PA1 a rotor rotated around an axis of said casing, PA1 said casing partitioned into an inlet chamber communicated with said stock inlet and with a space inside said screen plate, an accept chamber outside said screen plate and communicated with said accept stock outlet and a reject chamber communicated with said reject outlet, PA1 said rotor having scraper blades faced to said screen plate for preventing plugging of the screen, a defibration rotor faced to said defibration stator and a dilution chamber opened to said reject chamber, PA1 dilution openings extending through a peripheral wall of said dilution chamber and spaced apart from each other in a circumferential direction so as to pass dilution water toward said screen plate between axial ends of said screen plate, PA1 facing surfaces of said defibration stator and said defibration rotor being divergent toward said reject chamber, and PA1 a dilution water nozzle in said casing adjacent to said dilution chamber for feeding dilution water to said dilution chamber. PA1 a cylindrical casing having a stock inlet at one end thereof, a reject outlet at the other end thereof and a plurality of accept stock outlets between the ends thereof, PA1 cylindrical front and rear screen plates concentrically fixed to define a space between an inner surface of said casing and said screen plates, PA1 an annular defibration stator concentrically disposed between said front and rear screen plates, PA1 a rotor rotated around an axis of said casing, PA1 said casing partitioned into an inlet chamber communicated with said stock inlet and with a space inside said front screen plate, accept stock chambers disposed outside said screen plates and communicated with said accept stock outlets and a reject chamber communicated with said reject outlet, PA1 said rotor having scraper blades faced to said screen plates for preventing plugging of the screen, a defibration rotor faced to said defibration stator and a dilution chamber opened to said reject chamber, PA1 dilution openings extending through a peripheral wall of said dilution chamber and spaced apart from each other in a circumferential direction so as to pass dilution water toward said screen plates and PA1 a dilution water nozzle in said casing for feeding dilution water to said dilution chamber.
An apparatus according to a second aspect of the present invention comprises
In the first aspect of the present invention, waste paper stock pulp slurry containing undefibered waste paper is introduced into the screen to separate the slurry into a high quality stock passing through the screen plate and a reject not passing through the screen plate. Said high quality stock is sent to a next stage as accept. Said reject is passed through the gap of the defibrator comprising said defibration stator and said defibration rotor adjacent to said screen plate so that undefibered waste paper in said reject is defibered and the reject is increased in pressure and is discharged into the reject chamber where it is diluted with dilution water and circulated through the dilution chamber to said screen, the reject being partly discharged out of the system.
In the second aspect of the present invention, waste paper stock pulp slurry containing undefibered waste paper is introduced into a front screening section to separate the slurry into a high quality stock passing through the front screen plate and a reject not passing through the front screen plate. Said high quality stock is sent to a next stage as accept. Said reject is passed through the gap of the defibrator comprising said defibration stator and said defibration rotor adjacent to said front screen plate so that undefibered waste paper in said reject is defibered. The slurry thus processed for defibration is sent to the rear screening section disposed adjacent to said defibrator and is separated into a high quality stock passing through the rear screen plate and a reject:not passing through the rear screen plate. Said high quality stock is sent to a next stage as accept. Said reject is discharged out of the system through the reject outlet. Dilution water is supplied to the front and rear screening sections through the rotor.
Embodiments of the present invention will be described in conjunction with the accompanying drawings.