Sink-Float Separation Washing Tank

Sink-float separation washing tank

A sink-float tank separates plastics using nothing but water and the difference in density between two materials. There is no screen, no sensor, no consumable and nothing to calibrate. Material enters at one end, the fraction lighter than water travels along the surface and leaves at the other end, and the fraction heavier than water settles to the bottom and is taken out separately.

Because the principle is so simple, the useful thing to establish first is not how the tank works but what it can and cannot do. That boundary decides whether this machine solves your separation problem or only part of it.

What it separates, and what it cannot

What floats

PP, PE and EPS are all less dense than water and rise to the surface. On a film or woven-sack line these are the target material, and the tank is being used to lift them clear of everything else.

What sinks

Metal fragments and PET flake settle to the bottom. On a polyolefin line these are the contaminants you are trying to shed; on a PET line the roles reverse and the sinking fraction is the product. The tank does not care which is which — it separates by density and you decide which outlet is the product.

What this tank cannot separate

This is the part worth reading twice. PET and PVC have densities close enough that flotation cannot tell them apart. Both sink. Once both are in the heavy fraction there is no water-based route to separating them, which is why PVC has to be removed upstream — on a sorting belt, by hand or by detection, while the pieces are still large enough to identify.

The same logic applies to any pair of polymers whose densities overlap. A sink-float tank is decisive where the density gap is wide and useless where it is narrow. Plan the separation sequence around that fact rather than expecting the tank to make up for a weak sorting stage.

Inside the tank

Three components do the work, and each addresses a way that simple flotation fails in practice.

Paddle rollers along the length of the tank push floating material forward and, just as importantly, dunk it. Flake that arrives dry or with air trapped in a fold will float regardless of its polymer, so the paddles hold material under the surface long enough for it to wet out and show its true density.

A conveying screw in the tank floor moves the settled heavy fraction along the bottom towards the discharge. Without it, sunk material builds up in a bed, and once the bed is deep enough it starts trapping good material.

A valved discharge outlet lets you drain water and sludge through a hose. Where the heavy fraction is continuous rather than occasional, a lifting screw can be added to carry it out of the tank without stopping.

Stainless where the water touches

Every part in contact with water is 3 mm stainless steel. That is a deliberate specification rather than a finish: the tank runs wet permanently, often with detergent or caustic carried over from an upstream hot wash, and it holds a standing volume of water at all times. Carbon steel in that duty corrodes from the inside of the weld outwards, and by the time it is visible the tank is already thinning.

3 mm is also a structural choice. A tank of this length full of water is heavy, and the wetted plate is doing structural work as well as containment.

Four standard lengths, and how to choose between them

Standard lengths are 5 m, 6 m, 8 m and 11 m, and tanks are also built to suit a customer’s line layout and capacity.

The variable that length actually buys you is residence time — how long a piece of flake spends in the water before it reaches the outlet. Separation is not instantaneous. A flake has to wet out, release trapped air, and then travel far enough vertically to reach its fraction. Push material through too fast and heavy pieces leave over the weir before they have had time to sink.

So the selection logic runs from upstream throughput, not from floor space:

Higher throughput needs a longer tank to hold the same residence time, because more material per hour means faster flow along the same cross-section.

Contaminated or mixed feedstock needs a longer tank than clean single-polymer material, because the separation you are asking for is harder and marginal pieces need more time.

Film needs longer than rigid flake. Thin film traps air in folds and resists wetting far more stubbornly than a solid chip does.

A tank that is too short does not fail visibly. It quietly sends a percentage of your heavy contamination out with the product, and you find out at the pelletiser or from a customer complaint.

The optional dewatering unit at the tail

Material leaving a flotation tank is, by definition, soaking wet. A vertical lifting dewatering unit running at 850–1,200 rpm can be fitted at the tail: floating material that has been thoroughly rinsed along the length of the tank is lifted out, spun to throw off surface water, and passed to the next stage.

Fitting it here rather than treating dewatering as a separate machine downstream saves a transfer and a footprint. Whether that is the right choice depends on how much water the next stage tolerates — a friction washer does not care, a thermal dryer very much does.

Where it sits in a washing line

The tank goes after size reduction and after the first friction washing stage, and before final dewatering and drying.

That position is not arbitrary. Material has to be in flake form before flotation, because a whole bottle or a folded sheet traps enough air to float regardless of polymer. It also has to be reasonably clean, because dirt and label fragments in suspension cloud the water and settle into the heavy fraction, where they have to be removed along with the genuine contamination. A tank asked to do the friction washer’s job as well fills with sludge and needs its water changed constantly.

See it running in a line

Frequently asked questions