Polystyrene is a plastic material having many uses and is particularly useful, when in a foam form, as a material for making food containers. It is a significant constituent of municipal solid waste and is particularly prevalent in waste from fast food restaurants, cafeterias and similar food service establishments. Such establishments commonly use trays, containers and cups made of polystyrene foam (PSF).
Fast food restaurant and cafeteria waste in addition to containing large amounts of PSF also commonly comprises a large proportion of paper products (such as bags, cups and napkins), wax-coated paper products, condiment containers, plastic eating utensils and food morsels.
Recycled polystyrene foam can be used to manufacture a wide variety of products such as plant pots, plastic desk organizers, foam construction insulation etc. and is therefore, a valuable product. Further, polystyrene foam, like most plastic, takes a very long time to degrade when placed in landfills. Therefore, it is desirable to recycle polystyrene foam to reduce the amount of such plastic in municipal landfills.
PROCESS DETAILS
1.Milling said waste particles to reduce said particles to be within a predetermined size range.
2.Mixing said milled waste particle with a liquid to form a mixture of said waste particles and said liquid.
3.Agitating said mixture so as to fiberize any paper waste particles and other waste particles which can be fiberized.
4.Separating and recovering said milled foamed polystyrene waste particles from said mixture.
PROCESS
The method comprises of an apparatus for recovering polystyrene foam from a stream of solid waste derived from a variety of sources, but in particular from fast food restaurant and cafeteria waste. The method and apparatus is highly adaptable for recycling of other materials. The sequence of steps, arrangement of apparatus and adaptation of the method and apparatus for various types of waste is highly dependent on the overall contents of the solid waste as well as the material which is to be separated and recycled.
In addition, entire steps can be eliminated from the process depending on the condition and contents of the waste. For instance, certain types of waste may be introduced into the process at an intermediate step while other types of waste which require additional separation steps may be introduced at the first stage. Therefore, an almost infinite number of adaptations of the present invention are possible depending on the contents of the waste. The discussion below is directed towards a method and apparatus for recycling PSF from fast food restaurant and cafeteria waste.
Waste is delivered to the apparatus primarily in plastic garbage bags. If the waste is from a fast food restaurant or cafeteria, the plastic bags largely contain PSF containers, paper products, plastic eating utensils and food. The present invention comprises two process lines. The primary line recovers polystyrene foam (PSF) from the stream of waste. The secondary line converts the remaining waste to densified refuse derived fuel (d-rdf).
The plastic bags are introduced continuously into a flail mill. The flail mill comprises a chamber having rotating hammers therein for tearing open the bags, liberating the waste within the bags, and reducing the waste particle size to a preferred range.
The waste is continuously delivered from the output of the flail mill to a rotating cylindrical trommel screen. Smaller particles of waste pass through the screen into an undersized waste receiving area, while the larger waste passes through the trommel onto the next stage. The bulk of the PSF will pass through to the trommel output without falling through the screen. At the output of the trommel, plastic liners are manually removed from the stream of waste.
FLAIL MILL(Helylpatterson Inc.)
The waste is then continuously fed into an air classifier. In a preferred embodiment, the air classifier is a large rotating cylinder which uses an air stream to carry lighter materials, such as napkins and PSF through the cylinder, while the heavier waste, primarily wax coated containers and remaining food, drops out of one end of the classifier and is removed from the primary stream of waste. The light fraction output of the air classifier, which comprises primarily PSF and paper, is fed into a plenum chamber where the air velocity is reduced allowing the light fraction waste to settle to the bottom of the chamber.
The light fraction waste is then continuously delivered to a reversible conveyor which can bring the waste either to a batch pulper or a continuous pulper. Either of the pulpers essentially mixes the waste with water and vigorously agitates the mixture so as to break down the paper waste to paper pulp. The water and paper pulp is then separated from the PSF by passing the mixture through a screen which allows the water and paper pulp to pass through but retains the larger PSF waste particles. The PSF is air conveyed from the screen to a granulator. The particle size is further reduced in the granulator.
The PSF is then dried and prepared for shipment. The paper pulp and water which passes through the screen is pumped to a static sieve where the water and paper are partially separated. The water is then recovered and recycled back into the apparatus. The paper pulp is further fed through a de-watering screw where additional water is removed and recycled.
The de-watered paper pulp is then collected in containers and can be sold for recycling. The undersized waste removed in the rotating trommel and the "heavy" waste removed in the air classifier is combined and delivered to a shredder where the size of the waste is further reduced. The output of the shredder is sent to a pellet mill, which compresses the waste and extrudes it into small pellets. To ensure that the pellets do not readily crumble, they are air cooled in a pellet cooler before storage.
The waste is then continuously fed into an air classifier. In a preferred embodiment, the air classifier is a large rotating cylinder which uses an air stream to carry lighter materials, such as napkins and PSF through the cylinder, while the heavier waste, primarily wax coated containers and remaining food, drops out of one end of the classifier and is removed from the primary stream of waste. The light fraction output of the air classifier, which comprises primarily PSF and paper, is fed into a plenum chamber where the air velocity is reduced allowing the light fraction waste to settle to the bottom of the chamber. 