What Is Recycling? The Five Stages, and Which You Can Skip

Recycling is the process of collecting used materials, sorting them by type, breaking them down and turning them back into raw material for new products. For plastics that means a chain of mechanical stages — bale, sort, shred, crush, wash, dry and often pelletize — between the collection point and a finished pellet.

Almost every explanation of recycling stops at the bin. It tells you which items go in, and then the story ends. This one starts where the material arrives at a processing plant and follows it through the machines that actually do the work, because that is where the outcome is decided. Along the way it answers the two questions an operator asks that a household guide never does: which of those stages does my material actually need, and what does one tonne cost to put through them.

What Recycling Actually Means

Three words get used as if they were the same thing, and they are not.

Reuse means using an item again as it is — a returnable crate, a refilled drum. Nothing is broken down and no machine is involved. Recycling means the material is reduced back to a raw form and remade into something else. Downcycling is recycling where the output is worth less than the input: a clear bottle that becomes grey strapping band has been recycled, but it has also stepped down a rung and it will not step back up.

Within recycling itself there are two routes. Mechanical recycling — the subject of this article and of virtually every plant operating today — cuts, washes and melts the plastic without changing its chemistry. Chemical recycling breaks the polymer back into its building blocks. Mechanical is cheaper, established and limited by contamination. Chemical tolerates dirtier input but is capital-intensive and still a small share of what is processed worldwide.

The last distinction is the one that matters commercially. Recyclable is a property of the material: this bottle is made of something a plant can process. Recycled is a description of what actually happened to it. A recyclable bottle only becomes a recycled bottle if it is collected, sorted correctly, and reaches a plant equipped for its polymer, its colour and its level of contamination. Every one of those four conditions can fail independently, which is why national recycling rates are always lower than the share of packaging that is technically recyclable. The EPA’s material-specific data for plastics shows the size of that gap by resin.

What Arrives at a Processing Plant, and in What Shape

The first thing that decides how a plant is built is not the polymer. It is the physical shape the material turns up in, because that determines what has to happen before anything can be judged, cleaned or fed at a steady rate.

Four intake forms cover almost everything that comes through the gate.

Baled. Nothing about recycling is economic if you ship air, and a loose PET bottle is almost entirely air, so bottles and film usually arrive compressed into wire-strapped cubes of a few hundred kilograms. Baling solves transport and creates a new problem: once material is a solid block, nobody can see inside it. A bale can look immaculate on its outer face and contain PVC bottles, wet cardboard, sand, a length of steel banding or half a bag of household waste in the middle. Baled material is bought and sold on grade descriptions, and the gap between the description and the contents is one of the running frictions of the whole industry. A bale opener has to come before anything else.

Loose and bulky. Drums, crates, pipe and mouldings arrive whole. A whole drum will not enter a crusher — it has to be opened first, which is why rigid streams need a shredder in front of the line and PET bottle streams often do not.

Rolled or tangled. Agricultural film and liners arrive as rolls or as matted lumps carrying mud, sand and moisture that can weigh more than the plastic. This stream is the one where the intake shape changes the whole line design, not just the front end.

Clean and already granulated. Factory offcuts, runners and rejects come from a known process, in a known polymer, with no soil on them. They skip most of the line entirely — covered in the routing section below.

The practical consequence is that two plants running the same polymer at the same tonnage can have completely different machine lists, purely because one buys bales and the other buys loose drums.

The Sorting Line — How the Plant Tells PET From PVC

Sorting decides what the batch can eventually be sold as, and it is the last stage where a mistake can still be corrected. After this point, whatever is still in the stream stays in the stream.

It runs in layers. A rotary trommel screen takes out the small heavy debris first — stones, grit, loose caps, glass fragments. Then the material spreads onto a belt where several things happen in sequence. Optical sorters read the material as it passes and eject what does not belong. Magnets and a metal detector pull out ferrous and non-ferrous fragments, which are removed as much to protect the knives downstream as to clean the product.

And people still stand at the belt. Manual picking has not disappeared from any plant worth visiting, because human eyes catch what the sensors miss: dark bottles a near-infrared sorter reads poorly, unexpected objects, items whose label hides the polymer underneath, and anything that has to be judged rather than measured. The manual pick station is not a sign of a primitive plant. It is the stage that keeps the specification honest, and as the consumption figures further down show, it is also the single largest reason a PET line costs more to staff than a rigid one.

The Shredder and the Crusher — Two Machines That Get Confused

Public explanations of recycling almost always compress this into one word, usually “shredded”. There are two machines here, they do different jobs, and knowing which is which is the single most useful piece of vocabulary for anyone reading a quotation.

The shredder — low speed, high torque

A plastic shredder machine is a slow, heavy machine built for bulk. A twin-shaft unit turns its rotors at 8–15 rpm against the 72–74 rpm of a single-shaft machine, and that difference is the whole point: low speed buys torque, and torque is what lets it take a whole bale, a stack of drums or a length of pipe without stalling. Output is coarse and irregular — pieces, not flakes. The shredder’s job is to make bulky waste feedable, nothing more. The trade-offs between the two are worked through in how a recycling shredder is specified.

Not everything going through one is plastic. The same low-speed, high-torque principle handles scrap tyres, and a dedicated tire shredder is a common first machine for operators who process rubber alongside plastics.

The crusher — a screen decides the flake size

A crusher runs fast and cuts against a fixed bed knife. Underneath the rotor sits a perforated screen, and material stays in the chamber until it is small enough to pass through the holes. That screen is what sets the output size: for PET bottle processing it produces flake of roughly 12–20 mm, for rigid drums and crates 8–16 mm, and for film 60–100 mm. Those are not preferences. They are what the wash stages downstream are built around.

For bottles the crushing is usually done wet, with water already in the chamber. It suppresses dust and begins cleaning before the material reaches the washing tanks. A heavy-duty crusher with a force feeder is the standard unit for this stage.

So: a shredder makes waste feedable, a crusher makes flake, and a pelletizer — which does not appear until two stages later — is the only one of the three that melts anything. Which of them a given plant actually needs, and in what order, is covered in the machines a plastic recycling line actually needs.

The Wash — What 85–95 °C Alkali Actually Removes

Washing is the point at which recycling stops resembling waste handling and starts resembling process engineering. A plastic recycling washing line is not a tank. It is a sequence, and each step removes a different class of contamination.

Label and cap removal happens before crushing, while the bottles are still whole, because a whole label is one object a dry friction system can strip while a crushed label is thousands of fragments the same size as the flake. A well-set-up label remover takes off 80–90% of labels and caps at this stage.

Friction washing scrubs mechanically. A high-speed friction washer drives flake against screens and against itself, which is how surface dirt, dust and pulp residue come off — by abrasion, not chemistry.

Sink-float separation uses density and nothing else. In a sink-float separation tank, PET sinks while polyethylene and polypropylene from caps and label film float, and the two streams leave by different routes. It is elegant and cheap and it has one blind spot, discussed below.

Hot washing at 85–95 °C with 1–3% alkali is the chemical stage. Heat plus caustic is what releases the glue that held the labels on, saponifies oils and lifts beverage residue that friction alone leaves behind. It is also the most expensive stage in the line to run, because you are heating water continuously. Whether a given material genuinely needs it is a real decision rather than a default — see hot wash versus cold wash and when each is enough.

Dewatering and drying finish the job. A centrifugal dryer at around 1,500 rpm throws off free water and brings moisture down to 0.5–2%; a hot-air stage after it takes the flake below 1%. This is not cosmetic. Water in PET flake causes hydrolysis during extrusion, which shortens the polymer chains and permanently lowers the value of the material.

At this point the material is already a saleable commodity. Many plants stop right here and sell washed flake, which is a legitimate business rather than an unfinished one.

The Pellet — Where It Becomes Raw Material Again

The plants that continue send the flake to a plastic pelletizing machine, where three things happen that washing cannot do.

The flake is melted in an extruder. The melt is pushed through a filter screen — typically 40–120 mesh — that catches solid contamination too small to have been separated mechanically. Then the filtered melt is cut into pellets of uniform size and shape.

Why bother, when clean flake already sells? Because of what happens at the customer’s factory. A pellet is dense, free-flowing and dimensionally consistent, so it doses accurately through gravimetric feeders, blends predictably with virgin resin and does not bridge in a hopper. Flake is lighter, irregular and much harder to meter. Buyers pay a premium for pellets largely because pellets do not disrupt their process, and the price gap between the two forms is one of the central facts of the recycled materials market — laid out in plastic regrind and how buyers judge its value.

One warning that applies to the whole chain: pellet quality is decided upstream. Polymer purity, moisture, residual ink, heat history and filter performance all arrive at the extruder already fixed. A good pelletizer cannot rescue a dirty flake stream; it can only make dirty flake into consistent-looking dirty pellets.

One Batch, Five Different Routes

Everything above describes the full chain. Almost no material goes through all of it. The question that decides a machine list is not “what are the stages of recycling” but “which of these stages does my material actually need, and which can I skip”.

Across the five streams a plant is likely to see, the answer is different every time.

Baled PET bottles normally skip shredding — a bottle is already small enough for a crusher once the bale is opened — but need crushing, washing and drying without exception. Pelletizing is optional, because washed flake has its own market. The full sequence is set out in PET bottle recycling from bale to flake, and the machine that does it is the PET bottle-to-flake washing line.

HDPE drums and rigid items are the one stream that needs everything at the front. A whole drum has to be shredded before a crusher screen means anything at all, so shredding and crushing are both mandatory. See HDPE drum recycling and the rigid plastic washing line.

PP woven and jumbo bags need shredding to open the weave, but crushing is a maybe rather than a must, because raffia cut to length may already be the right size for the wash. What actually decides the difficulty here is the stitching, metal fragments and coatings, not the tonnage — see PP woven bag recycling and the soft plastic washing line.

Film inverts the usual assumption. Shredding is optional, but pelletizing is not, because nobody buys loose washed film at a useful price — it has to be densified into something a converter can dose. LDPE film recycling covers why this stream is the most expensive of the five to run.

Clean factory regrind is the only stream in the matrix that skips the wash. It never left a controlled environment, so there is nothing on it to remove; it goes straight from crushing to pelletizing. That single exception is worth more than it looks, because washing and drying together are where most of the energy and effectively all of the water go. Plastic regrind explains what a buyer will pay for it.

What the Line Rejects, and What That Costs You

We have followed what makes it through. Now turn round and look at what comes out of the same intake and does not, because yield loss is a line item and most feasibility calculations quietly assume it is zero.

The useful way to think about “not recyclable” is not as a moral category. It is a plain engineering statement: at some specific stage in the line, this material cannot be separated, cleaned, melted or sold without damaging the batch. Each item fails at an identifiable machine, and each failure has a different cost attached.

Two of those invert what most people assume. Compostable plastic is not a safer choice inside a recycling stream — it is one of the more troublesome contaminants a PET plant deals with. And dark-coloured plastic is not rejected at all; it is simply worth less, which is a commercial outcome rather than a technical failure.

The commercial point is that these losses are decided at purchase, not at the machine. A bale bought on an optimistic grade description carries its reject rate with it, and no amount of equipment recovers material that was never going to pass. The APR Design Guide is the reference most buyers and converters work from when they argue about what should have been in the bale.

What One Tonne Actually Consumes

The question after “can it be recycled” is “what does it cost to run”. The honest answer is that it depends far more on which material you chose than on how well you negotiated the machine price.

Put three lines side by side at the same throughput — roughly 1,500 kg/h — and the spread is wide enough to decide a project.

Four things in that table are worth stating plainly, because they rarely appear in a quotation.

A film line draws two to three times the power of a rigid line at the same tonnage. 128 against 253–400 kWh per tonne. The reason is physical: film has to be squeezed, then hot-air dried, then air-conveyed, where rigid flake is thrown dry in a centrifuge and moved on a belt. Buyers comparing the two on equipment price alone never see this layer.

A film line uses roughly three times the water. 5.7–7.1 against 13.3–20.0 m³ per tonne. In a water-scarce region this one row can end a project on its own, whatever the machine costs.

Scale cuts energy hard and barely touches water. A rigid line moving from 250 kg/h to 1,900 kg/h drops from 400 to 131 kWh per tonne, a fall of about two thirds. Over the same range water only moves from 16–20 down to 5.3–7.9 m³, and most of that is because the smallest tier was over-specified to begin with. Past the middle of the range the three lines all flatten out. Planning to dilute your water bill by building bigger does not work.

PET is the most labour-dense of the three. At the same 1,500 kg/h a PET line needs 5.3–6.7 people per tonne per hour against 2.1–3.6 on a rigid line. The extra people are all standing at the sorting belt described earlier. That single row is why PET projects are harder to justify in high labour-cost countries than the equipment price suggests.

One caveat on how to read these figures. Installed power is not metered consumption. It is the sum of every motor’s nameplate rating, and a running line draws materially less — typically 60–70% — because motors are sized with margin and not everything is loaded at once. The per-tonne figures above are derived from installed power divided by rated throughput, so they are correct for comparing lines against each other and too high if read as an electricity bill. What adds up in plastic recycling plant cost works through the rest of the layers.

Frequently Asked Questions