Four-type specific gravity separator

Abstract

Conventional sorting equipment requires a lot of equipment and a large space for sorting. The more equipment and processes required, the higher the cost of recycling. Also, because waste plastics are dropped from the top of the tank, there is a problem in that any waste plastics that get caught on floating plastics on the surface of the water are collected as floating plastics. [Solution] This four-type specific gravity separator includes a cylindrical water tank (21) for storing liquid therein, a vortex generating mechanism (22) for generating downward vortex currents in the liquid stored in the cylindrical water tank, a flow path (31) that spreads horizontally from the mid-height of a part of the side surface of the cylindrical water tank to the water surface and guides the water to the next process, and a slow-flow water tank (41) for storing the water that has flowed from the flow path.

Description

[Technical Field]

[0001] The present invention relates to a four-gravity separator that separates waste plastics into waste materials such as metal pieces and glass pieces and waste plastic raw materials by utilizing differences in specific gravity. [Background technology]

[0002] There are various types of plastics depending on their use, from thin and light items such as the film wrapped around PET bottles and packaging bags, to thick and heavy items such as lunch containers and automobile interior parts. Used plastic products are collected and recycled in accordance with the Containers and Packaging Recycling Law. Collected waste plastics (especially those made of thermoplastic resins) are crushed into flakes in a crusher, washed with water, and sorted by type of plastic.

[0003] Material recycling, which reuses plastic as a raw material for plastic products, requires highly pure raw materials. Collected waste plastics contain various other substances, such as metal fragments, in addition to plastics. These impurities must first be separated from the plastics.

[0004] Plastics with a specific gravity of less than 1 are called "light plastics (keipla)," while plastics with a specific gravity of more than 1 are called "heavy plastics (jupla)." Additionally, foam-type plastics, which contain air bubbles (foam) inside the plastic, have an even lower specific gravity than light plastics and are distinguished from them by being called "foams." Foams include expanded plastic pieces such as Styrofoam. Light plastics include PP (polypropylene) and PE (polyethylene). Heavy plastics include ABS (acrylonitrile butadiene styrene), PVC (polyvinyl chloride), and PET (polyethylene terephthalate). Table 1 shows the specific gravities of major plastics.

[0005] [Table 1]

[0006] The specific gravity of heavy materials such as metal fragments and glass fragments mixed in with waste plastics is much larger (heavier) than these plastic materials. For example, the specific gravity of glass is about 2.5, that of aluminum is about 2.7, and that of stainless steel is about 7.9. Therefore, it is possible to separate materials by specific gravity by using a fluid such as water to take advantage of the differences in specific gravity. Various gravity separators have been proposed so far.

[0007] Patent Document 1, "Sorting Equipment for Metal-Containing Waste Plastics," describes a technology for separating metal pieces, floating plastics, light plastics, and heavy plastics from waste plastics, and includes a washing and crushing machine having a rotary blade and an injector for injecting water to wash the crushed material in the main body of the machine in order to wash and crush the metal-containing waste plastics into predetermined shapes and sizes; a vortex-type cylindrical separating water tank having a propeller for stirring the water in a cylindrical water tank in order to separate the washed and crushed material washed and crushed by the washing and crushing machine into crushed metals, crushed floating plastics, and crushed light and heavy plastics having specific gravities intermediate between the crushed metals and crushed floating plastics by the vortex flow of the water and the difference in specific gravity; a funnel-shaped outlet formed at the bottom of the cylindrical water tank for discharging the settled crushed metals; and a discharge device attached to the periphery of the opening of the cylindrical water tank for taking out the separated crushed light and heavy plastics; and a vortex-type cylindrical separating water tank having a separation device in the vortex-type cylindrical separating water tank. A sorting facility for waste plastics containing metals is provided, which is equipped with a flow-through density separator having a plurality of paddles attached to the top surface of a box-shaped sorting tank and a discharger attached to the bottom of the box-shaped sorting tank for discharging the settled heavy plastic crushed material, in order to separate the separated light and heavy crushed plastics based on their difference in specific gravity into light crushed plastics with light specific gravity and heavy crushed plastics with heavy specific gravity that sink to the bottom of the water; and a vibration separator having a housing for containing the crushed plastics and a vibrator for vibrating the housing, in order to vibrate the heavy crushed plastics sorted by the flow-through density separator to separate the granular crushed plastics downward and the powdery crushed plastics upward, and is characterized in that the vortex-type cylindrical separating tank is located adjacent to the washing crusher, the flow-through density separator is located adjacent to the vortex-type cylindrical separating tank, and the vibration separator is located adjacent to the flow-through density separator in the same location. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-183734 Summary of the Invention [Problem to be solved by the invention]

[0009] However, with conventional gravity separation equipment, waste plastics are dropped into the tank from above, meaning that any waste plastics that end up on the floating plastics on the surface of the water are collected as floating plastics. Furthermore, the separation requires a large amount of equipment and a large space. The increased number of equipment and processes also increases the cost of recycling.

[0010] The present invention has been made to solve the above-mentioned problems, and aims to provide a four-type specific gravity separator that can reliably drop waste plastics into a vortex and perform four-type separation in a space-saving manner compared to conventional technology. [Means for solving the problem]

[0011] One aspect of the present invention is a cylindrical water tank (21) for storing liquid therein, and a device for generating a downward vortex in the liquid stored in the cylindrical water tank. , keeping light and heavy plastics in the water The vortex generating mechanism (22) and the water flowing from the cylindrical water tank , light and heavy plastics The cylindrical tank and the slow-flow tank are connected, and the bottom is inclined upward toward the slow-flow tank. Only water, light plastics, and heavy plastics are sent to the slow-flow tank. and a flow path (31) through which the particles are conveyed.

[0012] It is preferable that the apparatus further comprises an input device (10) having an input port (11) that is open at the top and a screw conveyor (12) whose one end is located above the water surface in the cylindrical water tank and connected to the bottom of the input port, and whose other end is inclined and connected to the side of the cylindrical water tank so as to be located below the water surface in the cylindrical water tank.

[0013] It is preferable that the device further comprises a conveying mechanism (32) that has a rotating shaft positioned above the water surface of the flow channel and blades attached to the rotating shaft with part of the blades below the water surface, and that conveys the waste plastics and water from the flow channel to the gentle-flow water tank.

[0014] It is preferable that the slow-flow water tank further be provided at its bottom with an agitation and transfer mechanism (44) having a rotating shaft arranged parallel to the bottom of the slow-flow water tank, screw blades spirally fixed to the rotating shaft, and scooping blades attached between each pitch of the screw blades.

[0015] It is preferable that the device further includes a suction mechanism (23) for sucking and recovering the foam above the water surface of the cylindrical water tank.

[0016] One aspect of the present invention is a method for producing a water tank having an open top and a cylindrical water tank at one end. of Inside Department The other end is connected to the bottom of the inlet, which is located above the water surface of the cylindrical tank. of Inside Department a cylindrical water tank for storing liquid therein; and a screw conveyor connected to the side of the cylindrical water tank, which is inclined so as to be located below the water surface of the cylindrical water tank. , keeping light and heavy plastics in the water a vortex current separating step in which the foams collected at the center of the water surface are collected by an vortex current separating machine (20) having a vortex current generating mechanism for generating a vortex current generated by the vortex current generating machine, a suction mechanism for sucking and collecting foams above the water surface of the cylindrical water tank, and a heavy object collecting port at the bottom of the cylindrical water tank, and and a slow-flow tank, the bottom of which slopes upward toward the slow-flow tank, and which sends only water, light plastics, and heavy plastics to the slow-flow tank. Water flowing from , light and heavy plastics and a slow-flow sorting process for sorting light plastics and heavy plastics using a slow-flow sorter (40) equipped with an agitation and transport mechanism, the slow-flow sorter (40) having a slow-flow tank that stores and drains the waste, a rotating shaft that is arranged at the bottom of the slow-flow tank parallel to the bottom surface of the tank, screw blades spirally fixed to the rotating shaft, and scooping blades attached between each pitch of the screw blades. [Effects of the Invention]

[0017] The four-type specific gravity separator of the present invention can separate waste plastics into four types: foams, heavy materials, light plastics, and heavy plastics. In addition, by providing a dumping machine, waste plastics can be dumped into the water reliably, allowing for more accurate sorting. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is an overall side view of a four-gravity separator according to the present invention. [Figure 2] FIG. 1 is an overall plan view of a four-gravity separator according to the present invention. [Figure 3] 1 is a cross-sectional side view of an entire four-gravity separator according to the present invention. [Figure 4] FIG. 2 is an enlarged cross-sectional side view of the charger and vortex separator of the present invention. [Figure 5] FIG. 2 is an enlarged cross-sectional side view of the conveyor and slow-flow separator of the present invention. [Figure 6] FIG. [Figure 7] FIG. 2 is a rear view of the four gravity separator of the present invention. [Figure 8] 1 is a schematic diagram illustrating four-gravity separation according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] An example of an embodiment of the four-gravity separator of the present invention will be described with reference to the drawings. The size of each part, such as the tank, is changed as appropriate depending on the amount of waste plastic to be processed and the composition of the materials contained therein, and their layout also differs.

[0020] <Overview> Fig. 1 is a side view of the four-gravity separator of the present invention. Fig. 2 is a plan view of the four-gravity separator from above. The side where waste plastics and water are input (left side of the drawing) is the front, and the side where the liquid is finally discharged (right side of the drawing) is the back.

[0021] The four-type specific gravity separator 1 is equipped with, from upstream to downstream, an injector 10, an eddy current separator 20, a conveyor 30, and a gentle current separator 40, and is mounted on a stand 51. Because the four-type specific gravity separator 1 continuously flows waste plastics and water, it is equipped with multiple viewing windows 52 so that the inside can be seen. In addition, guards 53 are provided on both sides of each conveying mechanism, which will be described later, to prevent entanglement.

[0022] In the present invention, waste plastics W are first put into an input device 10. Foams FP and heavy objects M are sorted and recovered from the waste plastics W input by the input device 10 into an eddy current separator 20. Light plastics LP and heavy plastics HP that are not recovered by the eddy current separator 20 are sent to the next process by a conveyor 30. Light plastics LP and heavy plastics HP are sorted and recovered by a slow current separator 40.

[0023] Liquids such as water required for the sorting process are continuously fed into the feeder 10 together with the waste plastics W, and are continuously discharged together with the light plastics LP by the slow-flow separator 40. It is also possible to pour liquids such as water into the vortex separator 20 from above or by connecting a water supply pipe, instead of feeding them into the feeder 10.

[0024] The mounting position of the feeder 10 on the vortex current separator 20 on a plane is not limited to the example shown in Figure 2 and can be changed as appropriate. Ideally, it would be placed downstream of the feeder 30 in the vortex rotation direction (near the top of the page, below), as this would lengthen the path, but if the vortex current separator 20 is small or the slow current separator 40 is large, the positions can be adjusted as appropriate according to the constraints of the installation location. The sizes of the vortex current separator 20 and the slow current separator 40 are determined by the amount and composition ratio of the waste plastics to be processed.

[0025] Figure 3 is a side cross-sectional view showing the inside of a four-type specific gravity separator. The area indicated by diagonal lines is underwater. The height of the water surface is determined by the height of the light plastic recovery port 43, from which the water is ultimately discharged. The upper end of the feeder 10 is higher than the water surface, and the lower end is connected to a height that is underwater. The entrance (upstream) side of the flow path 31 opens from the middle height of the cylindrical water tank 21 to the water surface, and the bottom slopes upward toward the gentle flow separator 40 (downstream). The connection part with the gentle flow separator 40 is slightly lower than the light plastic recovery port 43. In other words, it is configured to have a wide connection with the cylindrical water tank 21 and a narrow connection with the gentle flow tank 41.

[0026] <Feeding machine> FIG. 4 is an enlarged cross-sectional side view of the feeder and vortex separator. The feeder 10 comprises an inlet 11 and a screw conveyor 12. The inlet 11 is used to continuously feed waste plastics W into the four-type specific gravity separator 1, and is wide open at the top. The bottom of the inlet 11 is connected to the top of the screw conveyor 12. The screw conveyor 12 is tilted up and down, with its top end higher than the water surface and its bottom end completely submerged. This angle of inclination is not limited to the embodiment, and it is sufficient that the inlet 11 is above the water surface and the bottom end of the screw conveyor 12 is submerged.

[0027] The screw conveyor 12 rotates to move the waste plastics W downward, forcing them continuously into the water. This prevents clumps of waste plastics W from floating on the surface of the water because of their extremely small bulk density, or from sinking as clumps because of their extremely large bulk density, preventing proper processing. By allowing the dropped waste plastics time to stay in the flowing water, it is possible to prevent them from solidifying and floating or sinking without being sorted.

[0028] <Eddy current separator> The vortex separator 20 comprises a cylindrical water tank 21, a vortex generating mechanism 22, a suction mechanism 23, and a heavy load collection port 24. The cylindrical water tank 21 is provided with the vortex generating mechanism 22 in its center, the suction mechanism 23 at its top, and the heavy load collection port 24 at its bottom. The cylindrical water tank 21 is a tank for storing input waste plastics W and water. The inner planar shape is circular to form a vortex. The vortex generating mechanism 22 is provided in the center of the cylindrical water tank 21. In this embodiment, the vortex generating mechanism 22 is a screw propeller, which, when rotated, generates a downward vortex in the center of the cylindrical water tank 21 in a plan view. The vortex generating mechanism 22 is not limited to the shaft and blades shown in the embodiment, and may be any mechanism capable of generating a downward vortex, such as a rotating inner wall of the cylindrical water tank 21, as in a washing machine.

[0029] Foam FP, which has enough buoyancy (low specific gravity) to not be swallowed by the vortex, accumulates on the surface of the water, while heavy objects M, light plastic LP, and heavy plastic HP sink underwater, allowing the foam FP to be sorted out. A downward vortex current is generated in the central part of the cylindrical water tank 21 in plan view, and an upward water current is generated in the outer periphery by the water current pushed up from the bottom, causing heavy plastics HP, which have a specific gravity greater than 1 and would normally sink, to float in the water.

[0030] If heavy plastics HP with a high specific gravity (heavy) are included, they can be separated from the heavy objects M by increasing the rotation speed of the vortex generating mechanism 22. This is because the faster the water flow (the higher the rotation speed), the more the plastics float up rather than pooling at the bottom of the cylindrical water tank 21. Note that the rotation speed should be set at a speed sufficient to float up the heavy plastics HP so that the light plastics LP do not float up too much. The action of the vortex generating mechanism 22 also pushes the water outward, causing it to flow to the flow conveyor 30.

[0031] The suction mechanism 23 is installed at the top of the cylindrical water tank 21. The suction mechanism 23 uses a pump to suck up and recover the foam FP that has risen to the water surface. If the suction mechanism 23 is too close to the water surface, it will also suck in the water, so it is best to set it at a height some distance from the water surface so that it will suck up the foam FP when it has accumulated to a certain extent and rises above the water surface. The foam FP gathers at the center of the vortex, but the vortex does not necessarily occur in the center of the cylindrical water tank 21 depending on the size of the flow feeder 30, etc. Therefore, the suction mechanism 23 can be configured to change the location where it sucks up depending on the position of the vortex. The larger the flow feeder 30, the closer the vortex will be to the flow feeder 30.

[0032] The bottom of the cylindrical water tank 21 is funnel-shaped, allowing the settled heavy objects M to collect. A heavy object recovery port 24 is provided at the bottom of the cylindrical water tank 21, from which the accumulated heavy objects M can be recovered. A gate valve is provided at the heavy object recovery port 24, and by closing this valve, the accumulated heavy objects M can be recovered as needed without stopping the sorting process. Furthermore, if there is a large amount of heavy objects M, a screw conveyor can be provided to continuously discharge them.

[0033] <Flowing machine> FIG. 5 is an enlarged cross-sectional side view of the conveyor and the slow-flow separator. The conveyor 30 comprises a conveying path 31 and a transport mechanism 32. The upstream side of the conveying path 31 is connected to the cylindrical water tank 21 at about the middle height, through which light plastics LP and heavy plastics HP flow. The downstream side is connected to a gentle-flow water tank 41, and the bottom of the conveying path 31 is inclined, so that the area of ​​the connection port is smaller than that of the upstream side. This is to prevent a large amount of water from flowing in from the cylindrical water tank 21, and to enable the sorting of foams FP and heavy objects M.

[0034] FIG. 6 is a side view of the transport mechanism. A conveying mechanism 32 is installed above the water surface of the flow path 31. The conveying mechanism 32 is, for example, a paddle wheel. It has a rotating shaft above the water surface and is installed at a height where a portion of it is submerged. By rotating (counterclockwise on the page), it pushes the water near the water surface and sends the water and light and heavy plastics (LP and HP) to the next process. In other words, if light and heavy plastics (LP and HP) accumulate in the flow path 31, it pushes them into the slow-flow water tank 41. The shape of the conveying mechanism 32 is not limited to the illustrated example; it can be octagonal, cross-shaped, or circular, as shown in Figure 6. It is recommended that the conveying mechanism 32 be made of perforated metal or other material to prevent excessive water pressure. By bending the tips of the paddles attached to the drum, light and heavy plastics (LP and HP) can be conveyed even in reverse rotation (clockwise on the page). Reverse rotation allows the water to flow more slowly through the slow-flow sorter 40, resulting in a more gentle water flow.

[0035] <Slow flow separator> The slow-flow sorter 40 consists of a slow-flow water tank 41, a conveying mechanism 42, a light plastic recovery port 43, an agitation and transfer mechanism 44, and a heavy plastic recovery port 46. The conveying mechanism 42 is located above the water surface of the slow-flow water tank 41, and the light plastic recovery port 43 is located downstream of that. The agitation and transfer mechanism 44 is located at the bottom of the slow-flow water tank 41, and the heavy plastic recovery port 46 is located at the end.

[0036] The slow-flowing water tank 41 is a tank that stores the flowing water and light and heavy plastics (LP and HP). Because the water flows slowly in the slow-flowing water tank 41, the shape of the slow-flowing water tank 41 is not limited to the rectangular shape shown in the embodiment, but may be a flat circular shape or any other shape. Water continues to flow slowly into the slow-flowing water tank 41 from the conveyor 30, and water continues to flow slowly out of the light plastic recovery port 43. In other words, the water continues to flow slowly, and light plastics (LP) that float on the water and heavy plastics (HP) that sink are separated based on their difference in specific gravity.

[0037] If light plastics LP accumulate near the water surface, they are sent downstream (towards the light plastic recovery port 43) by the transport mechanism 42. Also, heavy plastics HP that have become caught in the light plastics LP and float along with them can be sunk by being pushed back into the water by the transport mechanism 42. In other words, sorting accuracy is improved.

[0038] The light plastic recovery port 43 is located at the top of the slow-flow water tank 41 and consists of a slope and side walls. The upper end of the light plastic recovery port 43 must be higher than the connection point with the transporter 30. This is because the height of the light plastic recovery port 43 determines the height of the water surface in the slow-flow water tank 41. Light plastic LP and water are continuously discharged from the light plastic recovery port 43. A net or the like can be installed here to recover the light plastic, or it can be transported to the next process together with the water.

[0039] FIG. 7 is a rear view of the four-gravity separator. As shown in Figure 7, the bottom of the slow-flow water tank 41 is V-shaped, with its lowest point being semicircular. The bottom of the slow-flow water tank 41 is equipped with an agitation and transfer mechanism 44. The agitation and transfer mechanism 44 is equipped with a screw blade spirally fixed to a rotating shaft and a scooping blade 45 located between each pitch of the screw blade. The settling heavy plastics (HP) are collected on one side of the bottom of the slow-flow water tank 41 by the agitation and transfer mechanism 44, while the heavy plastics (HP) are scooped up by the scooping blade 45, allowing the light plastics (LP) that have been caught between the heavy plastics (HP) and sunk to the bottom to float. This improves sorting accuracy.

[0040] The heavy plastics recovery port 46 is provided at one end of the bottom of the slow-flow water tank 41, and is configured to collect the heavy plastics HP sent by the agitation and transfer mechanism 44. In the embodiment, it is provided on the same (downstream) side as the light plastics recovery port 43, but this is not limiting and it may be provided on the (upstream) side of the flow transporter 30. Also, instead of placing the agitation and transfer mechanism 44 in the same direction as the water flow, it may be provided in a direction perpendicular to the water flow in a plan view, for example, and the heavy plastics recovery port 46 may be provided on the front (or back) side of the page. It is preferable to change the location as appropriate depending on the size of the treatment facility where the four-type specific gravity separator 1 is installed and the layout of the next process.

[0041] A gate valve is provided at the heavy plastic recovery port 46, and by closing this valve, the accumulated heavy plastic HP can be recovered appropriately without stopping the sorting process. Also, if there is a large amount of heavy plastic HP, a screw conveyor can be provided to continuously discharge it.

[0042] By using a heavy liquid with a specific gravity greater than 1 instead of water, it is possible to separate heavy plastic HPs with different specific gravities. For example, salt water with dissolved salt can be used as the heavy liquid. The composition of plastics contained in waste plastics is somewhat determined for each treatment facility, and this method is suitable for cases where multiple types of heavy plastic HPs are included but light plastics LPs are not.

[0043] Alkaline cleaning water can be used instead of water. By performing cleaning at the same time as sorting, processing in the next process can be made smoother and the entire processing facility can be made more compact.

[0044] Figure 8 is a schematic explanatory diagram showing the process of sorting waste plastics using a four-gravity separator 1. Waste plastics and water are put in (putting process), and then the waste plastics are separated into three types: foams, heavy objects, and light and heavy plastics using an eddy current separator (eddy current separation process), and then the waste plastics are separated into light and heavy plastics using a gentle current separator (gentle current separation process). [Industrial Applicability]

[0045] The four-gravity separator of the present invention can separate foams, heavy objects, light plastics, and heavy plastics from waste plastics in a space-saving and low-cost manner, allowing for efficient recycling of plastics. [Explanation of symbols]

[0046] 1. Four-type specific gravity separator 10 Loading machine 11 Inlet 12 Screw conveyor 20 Eddy current separator 21 Cylindrical Water Tank 22 Eddy current generation mechanism 23 Suction mechanism 24 Heavy object collection port 30 Streaming machine 31 Flow path 32 Conveyor mechanism (paddle wheel) 40 Slow Flow Sorter 41 Slow-flowing water tank 42 Conveyor mechanism (paddle wheel) 43 Light plastic collection port 44 Agitation transfer mechanism 45 Scooping Feather 46 Heavy plastic collection port 51 Mounting stand 52 Peephole 53 Guard W Waste plastics FP foam LP Light Plastic HP Heavy Plastic M Heavy objects

Claims

1. A cylindrical water tank that stores liquid inside, a feeder having an inlet that is open at the top, and a screw conveyor having one end located above the water surface inside the cylindrical water tank and connected to the bottom of the inlet, and the other end connected to the side of the cylindrical water tank at an angle so as to be located below the water surface inside the cylindrical water tank; a vortex generating mechanism that generates a downward vortex in the liquid stored in the cylindrical water tank, causing light plastics and heavy plastics to remain in the water; a slow-flow water tank that stores and drains the water, light plastics, and heavy plastics flowing from the cylindrical water tank; a flow path that connects the cylindrical water tank and the slow-flow water tank, has a bottom that slopes upward toward the slow-flow water tank, and sends only water, light plastics, and heavy plastics to the slow-flow water tank; Four-type specific gravity sorter.

2. The stirring and transferring mechanism further includes a rotating shaft disposed at the bottom of the slow-flowing water tank in parallel with the bottom surface of the slow-flowing water tank, a screw blade spirally fixed to the rotating shaft, and a scooping blade attached between each pitch of the screw blade. The four gravity separator according to claim 1.

3. The device further includes a conveying mechanism having a rotating shaft positioned above the water surface of the flow channel and a blade attached to the rotating shaft, part of which is below the water surface, for conveying waste plastics and water from the flow channel to the gentle-flow water tank. The four gravity separator according to claim 1 or 2.

4. Further provided with a suction mechanism for sucking and recovering foam above the water surface of the cylindrical water tank. The four gravity separator according to claim 1 or 2.

5. a step of using a feeder to feed waste plastics into water, the feeder having an inlet that is open at the top and a screw conveyor whose one end is located above the water surface inside the cylindrical water tank and connected to the bottom of the inlet, and whose other end is connected to the side of the cylindrical water tank at an angle so as to be located below the water surface inside the cylindrical water tank; an eddy current sorting process in which foams collected on the water surface are collected and heavy objects collected at the heavy object collection port are collected by an eddy current sorter having a cylindrical water tank for storing liquid therein, a vortex generating mechanism for generating downward vortices in the liquid stored in the cylindrical water tank to keep light plastics and heavy plastics underwater, a suction mechanism for sucking and collecting foams above the water surface of the cylindrical water tank, and a heavy object collection port at the bottom of the cylindrical water tank; a slow-flow tank that connects the cylindrical tank and the slow-flow tank, has a bottom that slopes upward toward the slow-flow tank, and stores and drains water, light plastics, and heavy plastics that flow from a flow path that sends only water, light plastics, and heavy plastics to the slow-flow tank; and a slow-flow sorting process that sorts light plastics and heavy plastics using a slow-flow sorter equipped with an agitation and transfer mechanism, the slow-flow sorter having a rotating shaft that is arranged at the bottom of the slow-flow tank and parallel to the bottom of the slow-flow tank, screw blades that are spirally fixed to the rotating shaft, and scooping blades that are attached between each pitch of the screw blades. Four-gravity sorting method.

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