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.

Granulate – Wash & Dry – Pelletise ― Method (i)

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

Sor

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(i)Wash Shredding Granulate

Wash & Dry Pelletise

Bag

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(ii)Wash Shredding Granulate Agglomerate

Pelletise

(iii)Shredding Granulate Agglomerate

(iv)Shredding Granulate Wash Agglomerate

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

Material coming out of the shredder

Exit end of wash line showing screw conveyor and settling tank

Settling tank showing finger like paddles

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 cuttingThe 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-cuttingMost 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.

Die face cutter with centre blades

Strand pelletising

Vibrating screen

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.

Table 1: Number of recyclers per recycling method in South Africa in 2014

2009 2014No of

recyclersTonnage recycled

% of total

No of recyclers

Tonnage recycled

% of total

Granulate, Wash and Pelletise

96 166 573 73,0% 101 203 106 71.4

Granulate, Wash, Compound and Pelletise

3 12 210 5,4% 4 14 484 5.1

Granulate and/or Agglomerate and Pelletise

53 35 327 15,5% 61 45 410 16.0

Granulate, Wash and/or Agglomerate

5 1 980 0,9% 7 6 034 2.1

Granulate and/or Agglomerate

33 10 386 4,6% 41 13 004 4.6

Pulverise 3 1 582 3 N/A

Unknown 4 N/A

Total 193 228 058 221 284 520

Granulate and/or Agglomerate – Pelletise ― Method (ii)

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.

Granulate and/or Agglomerate ― Method (iii)

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.

Granulate – Wash and/or Agglomerate ― Method (iv)

This is 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

Pulverising is an extension of the granulation process as the 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

Schematic representation of a granulator

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.

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

Recyclate sold to the rotomoulding industry needs to be pulverised.

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Figure 1: Labour utilisation for various recycling processes

other optional processes, employ 74 % of the total labour force in the industry and achieve

annual productivity levels of 49 tons per employee.