DOWNLOAD RECYCLING METHODS

Recycling consists essentially of:

  • Sorting into materials and colours,
  • Reducing the size,
  • Cleaning the sorted materials and
  • Processing the material into a format that can be directly fed into the converter’s equipment.

Within these parameters widely differing methods can be used. The most common methods are illustrated below.

Click on each title to view more information on each method.

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.)

method1-process

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.

Strand-cutting
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.

method1-process2

This is also a popular recycling method and similar to the first one but without the washing process. It is the preferred method for clean material, e.g. ex-factory waste in general as well as clean post-industrial material like wrappings and dust covers. Although there is no formal washing stage, a hand wash is sometimes done on items such as crates and pipes or a simple rinsing process for films.

Larger items like lumps, pipes and crates can be shredded before granulation.

Agglomeration is used to densify film waste.

The agglomerator works on the principle that a high-speed set of blades not only cuts up the feed material but also heats it to just below melting point. A measured amount of cold water is introduced into the hot mix. The water flashes off leaving dry amorphous particles, which are then discharged. Warm agglomerate can be fed directly into the pelletising extruder. Agglomerators are being phased out as they consume a lot of energy and are being replaced with in-line presses, squeezers or continuous agglomerators – all performing the same function but called different names.

This is the second most popular recycling method in South Africa and 61 recyclers granulate and pelletise with an optional agglomeration process – Method (ii). A total of 45 410 tons (16 %) were recycled by this method.

Methods (i) and (ii) are the preferred recycling methods for PE-LD/LLD and PE-HD as well as PP films. These two methods represent 92.6 % of all tonnages recycled in South Africa in 2014. There is a perception amongst the larger recyclers that without pelletising equipment, a recycling operation should not be regarded as recycling in this survey.

Granulation only is used for hard plastics, e.g. PE-HD, PP, ABS, rigid PVC, PS, PMMA and other engineering plastics. It is also used for injection moulded scrap, PE LD/LLD lumps and plasticised PVC, e.g. cable stripping. A shredder can be used prior to granulation.

The input material is usually clean enough to eliminate washing. Where pre-washing is done, it is an unsophisticated hand-wash or high-pressure water jet wash. Dust can be wiped off. Aspiration and de‑dusting can be added to increase the value of the granulate.

Where film and film off-cuts are granulated, agglomerators are used to compact the fluffy flakes for direct conversion into film or pipe without having to pelletise. Additives like Carbon black or other pigments can be added during agglomeration.

A number of collectors have tried to increase the value of their collected materials by buying granulators to size-reduce selected hard plastics before selling them to recyclers for pelletising. Such organisations were considered collectors in this survey and their tonnages have not been included. However, certain granulated materials can be converted with great success. Where companies supplied granulated flakes directly to converters, they were considered recyclers. There were 39 recyclers in South Africa that granulated only and a further 2 recyclers granulate and agglomerate. Together, they kept 13 004 tons (4,6 %) off the landfill. Most of the engineering plastics are in this category.

Some flakes and agglomerate is sold to other recyclers who then pelletise it. The polywood manufacturers also prefer agglomerate. (Care was taken not to double-count recycling on this score and a number of recyclers were not included in the survey because their sole customer base was other recyclers.)

This is the one area in recycling where the skills shortages are most evident.

This isgranulator similar to the previous recycling method but with additional washing. Harder post‑consumer materials like crates and bottles are recycled with this method. The recyclers of PET post-consumer bottles in South Africa granulate and wash prior to baling for shipment overseas. This type of operation seldom operates automatic washing plants. It is more likely a home-built washer based on a domestic single-tub washing machine principle. The operator will load and unload the granulate by hand. In certain cases it is merely a hand wash prior to granulating the product. A couple of the PP recyclers use converted flow bins with water for washing. Operators will occasionally stir the materials. After granulation, the wet flakes are thrown on tarpaulins to dry.

Seven recyclers that granulate and wash in South Africa were included in the survey. Others were supplying pelletising recyclers with material. The washing stage can be prior to granulation by hand or after granulation in make-shift washing machines. Together, they represent only 2.1 % of the total tonnages, i.e. 6 034 tons.

Pulverising is an extension of the granulation process as trecyclatehe product would be shredded, granulated, sometimes washed, and pulverised into fine powders. Pulverised materials include rigid PVC for pipe and profile extrusion and PE-LLD powders for rotational moulding. A large percentage of pulverising done in South Africa is toll recycling and was not included in the survey figures.
There were only three pulverizing recyclers in the survey and although their tonnages are included in the total, it cannot be published to protect their confidentiality.

No one method can be regarded as superior to the others as each of the above has a function in the waste recovery and reprocessing value chain. The decision is typically based on available funds rather than for any technical reason or suitability.

Number of Recyclers per recycling method in South Africa in 2014

recycling-methods-table

Employees

The labour intensity of the various recycling methods differs considerably. The granulation and pelletising processes employ the largest number of people but the productivity is better as the process is automated for efficiency and throughput. The granulation process is more labour intensive as the products are often large and bulky. Various types of cutters and saws are used to cut the products into smaller sizes. Recyclers that granulate, wash and pelletise with a few other optional processes, employ 74 % of the total labour force in the industry and achieve annual productivity levels of 49 tons per employee.

recycling-employees