This is classic recycling and results in a clean, pelletised product that’s ready for use. This method is used for post-consumer waste. Such material can be heavily contaminated with organic matter, residual contents, soil, etc.
The first washing stage prior to granulation is not common but is used to ensure complete cleanliness of very contaminated input material. This often entails handwashing with a water jet. Modern equipment has two or three washing stations for post-consumer waste.
Shredders / nibblers are used to shred large components and lumps into smaller, more manageable pieces. There are various pieces of equipment performing the same function, namely nibblers, munchers, shredders, choppers, guillotines and even band saws…
A granulator is used to reduce the size of the incoming material into flakes. Granulation is done in machines with fast revolving cutter blades of various widths. Wet granulation could also be used for very contaminated incoming material.
The granulate is transported pneumatically or by screw conveyor, or in containers by hand, to a high-speed agitator. The granules are thoroughly washed and wet in the agitator. Wet, but still very dirty, flakes are transported to the settling tanks by screw conveyor or open belts.
The granulate is pushed forward slowly in the settling tank by slow-turning paddles. These paddles can be solid over their whole length or in the form of multiple fingers. Dirt that has been separated in the agitator is allowed to settle into the cones on the bottom of the settling tank. Sodden, separated paper labels will sink. Accumulated debris in the cones can be flushed out periodically or is taken out continuously with a screw conveyor in the bottom of the settling tank. To prevent clean flakes from accumulating at the exit end, one or two paddles can be fitted just before the exit conveyor. (Paddles are most often found only on the entrance side of the tank.)
For materials with densities of more than one, rotary washers are used. The separation function is then not part of the washing stage and sorting of incoming materials has to be thorough.
A wire mesh conveyor at the exit of the settling tank removes the washed flakes from the tank and at the same time allows for some drainage – the water flowing back into the tank. Further drainage can be done at this point by running the flakes over a vibrating screen in a continuous process. Batch de-watering can also be done by means of a centrifuge, or two.
Before extrusion, the granulate must be dried properly to prevent the formation of blown pellets that will cause voids in the final product. A number of drying methods can be employed depending on the cost factor and the material to be dried:
– A very popular method of drying is the use of a revolving inclined cylinder with internal vanes to tumble the granules in hot air;
– Using hot air to pneumatically transport the granulate to the next section of the plant or to silos that also have hot air circulation facilities;
– Transport through a circulating hot air drying box or room;
– Agglomerators, or
– Screw presses.
Silos of various types are used for intermediate storage. Their main function is blending –of different incoming materials before further processing, and blending of finished product before bagging or sometimes for drying. The size of the blending silo will determine the size of a homogenous batch – at least one ton, or forty bags.
Pelletising is done via an extruder. At this stage additives such as pigment, UV stabilisers, heat stabilisers, processing aids, etc. can be added. Cascade or vented extruders are used where volatiles like moisture and printing inks need to be removed from the recyclate.
A magnet is used to remove ferrous metal contaminants. This can be a separate manual process or a magnetic screen in the throat of the feed hopper. On larger machines, one or more metal detectors are fitted on the conveyors into the pelletiser.
The die consists of a multi-hole plate through which the molten plastic is extruded in the form of continuous strings. A metal screen pack is fitted before the die to improve the homogeneity of the melt and to filter out further contaminants. Automatic screen changers are used for post-consumer materials as screens need to be changed often. The degree of contamination permitted depends upon the nature and quality of the intended final product.
Die face cutting
The pellets are cut whilst still hot. The die-plate takes the task of extruding the strands and acts as a cutting plate. Centre- or off-center blades cut the strands on exit. Water conveys and cools the pellets. A cyclone separates the pellets, and the residual heat of the material provides final drying. The pellets are passed over a vibratory screen where the oversized pellets and fines are separated from the rest and bagged separately. Off-spec material can, in some cases, be reworked. The accepted particle size material is blown into the feed hopper at the bagging station.
Most of the smaller recyclers are using strand cutters. In spite of the larger space requirements and its high noise level, strand pelletising is fairly cost effective and popular in South Africa. Most probably this is because it is simple to operate and therefore suitable for use by unskilled personnel. It can also be used with great success for the harder materials like PS and PP. Using strand cutters makes inspection for unwanted inclusions in the strings easier – they break!
The melt is forced through a spaghetti die. The strands pass through a cooling trough filled with water, one or multiple passes. Take-off rollers pull the strands from the die at a constant speed. Draining is done by transporting the strands over an inclined trough that allows water to run back into the water bath. Upturned broom heads or old T-shirts are used to effectively wipe off excess water. The strands are finally dried with a high velocity stream of air from an air wipe. (The lack of a proper air-wipe often results in wet pellets and subsequent voids for the converter!)
A strand cutter or dicer pelletises the strands. Both the stationary and rotary blades need regular attention. The evidence of a blunt dicer creates the impression of poor quality material.
Newer technology includes blower fans and vibratory screens. Most locally run operations have only a catch bin next to the strand cutter. Pellets of all sizes and shapes are bagged together for the less fussy customer.
There were 105 recyclers in the 2014 survey that recycle by using Method (i) – granulate, wash and pelletise. Some of the recyclers (four) add enhancing additives to the material during the extrusion process that classify them as compounders. A total of 203 106 tons of plastics waste (71 %) was recycled by the granulate, wash and pelletise recycling method.