Ancillary Equipment

Sink Float Separation Tank for PET, PE/PP and Mixed Plastic Flakes

A sink float separation tank — also called a floating tank or density separator — splits mixed plastic flakes by specific gravity in a controlled water bath. Rumtoo engineers the tank around your actual feed (PET bottles, HDPE milk jugs, PP caps, film, PVC contamination), target purity, water-reuse rate and sludge load, so the separator becomes a stable production stage rather than a bottleneck.

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Plastic floating separation tank for PET, PE and PP flakes

How Sink Float Tank Selection Should Actually Work

Tank volume alone never predicts real-world purity. The machine must be matched to the feed density spread, flake size, contamination load and water-reuse target — here is the selection path that actually works.

1

Map the feed density spread and target cut-point

List every polymer and contaminant in the feed with its density — PE 0.91–0.94, PP 0.85–0.91, PET 1.38–1.40, PVC 1.35–1.45, PS 1.04–1.06. If the nearest pair differs by less than 0.02 g/cm³, a plain water tank alone is not enough; salt brine, hydrocyclone or electrostatic stages must be added.

2

Choose tank length and retention time

Retention time of 60–120 seconds is the sweet spot: too short and heavy flakes do not settle; too long and floating flakes absorb water and behave ambiguously. Rumtoo sizes the tank so flakes travel at 0.05–0.10 m/s through the working length.

3

Size the water-reuse, filtration and sludge loop

Real operating cost is driven by make-up water and wastewater treatment, not by tank price. Rumtoo specifies a sedimentation tank, bag filter and dissolved-air flotation (DAF) loop so 70–85% of the bath water is reused and sludge is discharged as a cake rather than as liquid waste.

4

Integrate upstream label remover and downstream hot wash

The sink float tank is never the first step and never the last. Rumtoo lays out the full washing line — label remover, pre-wash, sink float, friction washer, hot wash, rinse, centrifugal dryer — so the sink float stage operates at stable load instead of chasing fluctuating contamination.

What Buyers Actually Need From a Sink Float Tank

The business reason to install a sink float tank is not the technology — it is a measurable jump in PE/PP and PET purity, a lower reject rate at the extruder, and a closed water loop that avoids wastewater permits and 30–50 m³/day of fresh water consumption.

High-Purity Density Separation

Typical one-pass purity is 98–99% for PET-from-PE/PP and PE/PP-from-PET, measured at the tank discharge. Twin-tank designs with brine adjustment push purity to 99.5%+ for bottle-to-bottle lines that feed food-grade rPET extrusion.

70–85% Water Reuse

With a filtration, DAF and buffer-tank loop, make-up water drops to 3–5 m³ per ton of flake — compared to 20–30 m³/ton for a once-through tank. That alone saves USD 30,000–80,000/year in water, discharge fees and softener chemicals on a 1000 kg/h line.

Removes Label, Film, Paper, Glue and Metal Fines

The tank is the single stage that removes paper, cardboard, PLA sleeves and residual film from PET flakes — and metal fines, PVC and silicone from PE/PP flakes — before they reach the extruder screw. Extruder filter-screen change intervals typically triple after a properly-sized sink float tank.

Handles 500–5000 kg/h With Same Footprint Logic

Rumtoo scales the tank length, paddle count and skimmer speed together. A 5000 kg/h bottle-to-bottle line fits in roughly 18 × 3 × 2.5 m — comparable to competing systems but with higher water-reuse and lower sludge volume.

Common Sink Float Problems and Real Fixes

Most poor-separation reports trace back to five production conditions — not the tank itself. This table shows what actually goes wrong in running lines and how Rumtoo corrects it during commissioning.

Problem

PET pellets float instead of sink — extruder rejects rPET for PVC contamination.

Rumtoo Solution

Rumtoo adds a short de-foaming baffle and lowers paddle speed; most "PET floating" cases are actually entrained air in thin bottle-wall flakes. Persistent cases get a brine dosing line that lifts water density to 1.05 g/cm³ for a guaranteed PET-PS separation.

Problem

Water turns gray, filters clog within two shifts and discharge fails limits.

Rumtoo Solution

Rumtoo sizes the sedimentation tank, bag filter and DAF loop so fine label, paper and glue particles are removed continuously. Sludge is de-watered to 60–70% solids in a screw press instead of being pumped out as liquid.

Problem

Paddle agitators foul with film and rope — tank shuts down twice a week for cleanout.

Rumtoo Solution

Rumtoo specifies an upstream trommel and film-rope separator so ribbons and long strands never reach the tank. Paddle geometry is adapted (cut-blade vs flat paddle) to the dominant flake shape in your feed.

Problem

Winter start-up produces black sludge and foul odor from caustic carry-over.

Rumtoo Solution

Rumtoo isolates the hot-wash caustic loop from the sink float bath with a rinse buffer tank and specifies a weekly tank drain/scrub SOP. A pH and turbidity sensor warns before biological growth crosses odor thresholds.

Machine Reference and Density Separation Principle

Mixed flakes enter a water bath where paddle agitators, baffles and a cross-flow carry low-density fractions (PE/PP, 0.90–0.94 g/cm³) over a weir to the top skimmer while high-density flakes (PET, PVC, PS, metals) sink and are discharged by a screw conveyor at the tank bottom.

Stainless steel sink float tank with paddle agitator and skimmer

Machine View: paddle agitator, weir skimmer and sinking screw conveyor

The tank body is built from 304 or 316L stainless steel to resist caustic wash carry-over. Paddle-style agitators keep the water bath moving so flakes cannot re-agglomerate; a surface skimmer lifts the floating fraction over an adjustable weir; a sinking screw conveyor drags heavy flakes out of the tank cone with minimum water loss.

  • Variable-speed paddle agitators tuned to flake size to avoid re-entrainment
  • Adjustable weir height for fine cut-point control between 0.95 and 1.05 g/cm³
  • Twin-stage design lets the sinking fraction pass through a second polishing tank

Process View: PE/PP float, PET/PVC sink in a plastic washing line

In a PET bottle-to-bottle line the sink float tank sits after label remover and pre-wash. HDPE and PP caps float; PET, PVC, silicone and metal fines sink. In an HDPE/PP rigid line the inverse holds: the tank separates PE/PP from PS/PET/PVC residue. Either way, the tank removes the wrong polymer before hot wash — which protects the downstream heat exchanger and extruder.

  • Separates PET/PVC/metal from PE/PP at 98–99% purity in one pass
  • Removes paper, cardboard, label snippets and film that still float after pre-wash
  • Water circuit feeds the filtration/flotation loop for 70–85% water reuse
Recycled PE and PP plastic flakes after sink float separation

Sink Float Separation Tank in Operation

Watch how the paddle agitator, weir skimmer and sinking screw conveyor separate mixed plastic flakes into pure PE/PP floating fraction and PET/PVC sinking fraction in a controlled water bath.

Typical Sink Float Tank Use Cases

A sink float tank earns its place in any plastic washing line where the target and contaminant polymers differ by at least 0.05 g/cm³ and the feed is already wet from pre-wash.

  • PET Bottle-to-Bottle Washing Lines

    Removes HDPE caps, PP rings, paper label fragments and PLA sleeves from PET flakes before hot wash. Critical stage for lines targeting food-grade rPET approval from EFSA, FDA or similar regulators.

  • HDPE Milk/Detergent Jug Recycling

    Separates PP caps, PET tamper bands, metal fines and dirt from HDPE flakes produced from milk, detergent and motor-oil bottles. Prepares HDPE flakes for pipe, crate and packaging compounders.

  • PP Rigid Packaging and Caps Regrind

    Splits PP from residual PET, PVC tamper bands and PS labels coming off food tubs, buckets and cap regrind so compounders receive a single-polymer feedstock.

  • Mixed Post-Consumer Plastic Sorting

    Acts as the first bulk density stage for mixed post-consumer curbside input, routing a clean PE/PP float fraction to pelletizing while high-density material is sent to NIR, electrostatic or manual picking.

  • Agricultural Film and Big-Bag Reclaim

    Paired with a pre-wash, the tank floats LDPE film fragments while soil fines, stones and metal wear particles sink — reducing extruder melt filter changes by 2–3×.

  • WEEE and Automotive Plastic Regrind

    Used as a rough density splitter ahead of electrostatic separation to remove metals, glass fragments and rubber from ABS/PS/HIPS-dominant WEEE and automotive shredder residue feeds.

Reference Configurations

These are sizing references. Actual purity, water reuse and sludge volume depend on feed composition, contamination level and the filtration loop selected — Rumtoo confirms the final specification after reviewing your material data.

ModelThroughputTank VolumePowerKey Features
RT-SF500300–500 kg/h2.5 m³3 kWCompact single-stage tank, 304 stainless, for small PET or HDPE lines
RT-SF1000800–1200 kg/h5 m³5.5 kWPaddle agitator + weir skimmer, bag filter loop, suits standard PET bottle washing lines
RT-SF20001500–2500 kg/h9 m³7.5 kWTwin-tank with brine adjustment, DAF loop, 70–80% water reuse
RT-SF50003500–5000 kg/h18 m³15 kWBottle-to-bottle grade twin-tank, 316L, DAF + screw press, 80–85% water reuse

Typical water make-up with a full reuse loop is 3–5 m³ per ton of clean flake. Energy consumption is 20–35 kWh per ton including agitators, pumps and filtration. Rumtoo sizes the unit after reviewing your feed analysis, contamination level and local water and discharge cost.

Sink Float Tank Checklist Before RFQ

Bring these data points to the RFQ and Rumtoo can size the tank, water-reuse loop and commissioning protocol in the first reply — avoiding weeks of back-and-forth.

  1. Polymer Mix and Density Spread

    List every polymer in the feed with its approximate share — PET, HDPE, PP, PVC, PS, PA, metals — so Rumtoo can confirm the cut-point target and decide whether plain water, salt brine, or a two-stage tank is required.

  2. Feed Flake Size and Shape

    Share the flake size distribution (typical working range is 6–30 mm), shape (flake vs film vs granule) and fines content below 4 mm. Particle size controls paddle speed and retention time.

  3. Contamination Load (Paper, Film, Fines, Sand, Metal)

    Specify the expected contamination per ton — paper label, PE/PP film, paper pulp, sand and metal fines. This determines filtration, sludge press sizing and tank cleaning frequency.

  4. Target Throughput and Shift Pattern

    Provide target kg/h, planned shift hours and scheduled cleaning windows. Rumtoo builds around a realistic effective uptime, not name-plate capacity.

  5. Water Source, Reuse Target and Discharge Limits

    Share local water cost, discharge cost and effluent limits (COD, TSS, pH). The water-reuse loop is sized for the economic break-even between make-up water cost and capex.

  6. Downstream Stages (Hot Wash, Friction, NIR, Electrostatic)

    Describe the full line — what comes before and after the tank. Rumtoo adjusts the cut-point, skimmer angle and discharge method so the tank integrates with existing hot-wash, friction and polishing stages.

Sink Float Tank vs Alternative Separation Methods

A sink float tank is the right answer when the density gap is clear and the feed is already wet. This table shows where it wins and where a hydrocyclone, electrostatic separator or NIR sorter fits better.

Decision CriteriaHydrocycloneNIR Optical SorterElectrostatic SeparatorRumtoo Sink Float Tank
PE/PP vs PET bulk separationGood, but high capexWorks if colors differNeeds pre-dryingProven at 98–99% in one pass
Food-grade bottle-to-bottle rPETWorks as polishing stageWorks upstream of washFinal PVC-ppm stageEssential bulk stage, add electrostatic for <50 ppm
Handles paper, label and film finesPoorNot designed for thisBlocks the fieldRemoves them continuously
Water consumptionVery high, high pressureNoneNone3–5 m³/ton with reuse loop
Energy per ton40–60 kWh + pump pressure10–15 kWh30–50 kWh20–35 kWh including filtration
Capex vs operating costHigh capex, high opexVery high capex, low opexMedium capex, low opexLow-medium capex, low opex

Frequently Asked Questions

What is a sink float separation tank and how does it separate plastic?

A sink float tank is a water bath with paddle agitators, a surface skimmer and a bottom screw conveyor. Flakes lighter than water (PE, PP) float and exit over the weir; flakes heavier than water (PET, PVC, PS, metals) sink and are discharged at the bottom. The tank typically holds 60–120 seconds of retention time and runs at 25–40 °C.

Can a sink float tank separate PET from PVC?

Not on its own. PET (1.38–1.40 g/cm³) and PVC (1.35–1.45 g/cm³) overlap. The sink float tank removes paper, label and PE/PP from PET, but residual PVC will still be in the sinking fraction. For food-grade rPET with <50 ppm PVC, Rumtoo adds an electrostatic separator or optical sorter as a polishing stage.

What is the typical water consumption and how high can reuse go?

Once-through operation consumes 20–30 m³ per ton of flake. With a sedimentation tank, bag filter and dissolved-air flotation loop, consumption drops to 3–5 m³ per ton — a 70–85% reuse rate. On a 1000 kg/h line running 16 h/day that is a saving of roughly 200–300 m³ per day of fresh water plus the discharge cost.

Do I need hot water or can the tank run at ambient temperature?

Sink float separation itself does not need heat — it runs at 25–40 °C. The tank often sits between a cold pre-wash and a hot caustic wash, so carry-over from the hot wash is what warms it. Running hotter than 45 °C weakens paddle bearings and accelerates biological growth, so Rumtoo installs a plate heat exchanger to bleed excess heat into the make-up water.

What is the smallest density difference the tank can separate reliably?

0.05 g/cm³ with plain water, 0.02 g/cm³ with salt-brine adjustment and a twin-tank cascade. Below 0.02 g/cm³ — for example PE/PP from ABS, or PE from EVA — a sink float tank cannot guarantee purity. Those cases need a hydrocyclone, froth flotation or electrostatic stage.

How do I manage the sludge produced by the tank?

Sludge is a mix of paper fibers, label glue, fines and tank scale. Rumtoo feeds the filtered sludge into a screw press that de-waters it to 60–70% solids so it can be handled as a cake instead of liquid waste. On a 1000 kg/h PET line sludge output is typically 20–40 kg/h dry, most of it paper and glue.

What detergent or caustic dosing is required?

The sink float tank itself runs on water only. Detergent and caustic live in the dedicated hot-wash tank downstream, usually 1–3% NaOH with 0.2–0.5% non-ionic surfactant for PET bottle flakes. Rumtoo isolates the two loops with a rinse buffer tank so caustic does not migrate into the sink float bath.

What maintenance does the machine actually need?

Daily: skimmer and weir cleaning, tank-wall inspection, sludge-press output check. Weekly: paddle shaft and bearing lubrication, tank partial drain and wall scrub, filter bag change. Monthly: full tank drain, inspect stainless welds and pipework, calibrate pH and turbidity sensors. Annually: replace paddle seals, inspect screw-conveyor flights, service DAF pump. The full schedule is part of the Rumtoo handover document.

How do I estimate ROI and payback period?

The two big levers are water cost and extruder stoppage cost. Moving a 1000 kg/h line from once-through to 80% reuse saves roughly USD 30,000–80,000/year in water plus discharge fees, and tripling the extruder melt-filter change interval typically saves another USD 40,000–120,000/year in downtime. Capex for a full sink float stage (tank + reuse loop) sits in the USD 40k–180k range depending on model. Rumtoo builds a case-specific payback model before you sign.

Which industries use this machine most?

PET bottle-to-bottle recyclers, HDPE milk/detergent jug processors, PP rigid packaging and cap recyclers, film and big-bag reclaim lines, mixed post-consumer sorting facilities, and WEEE/automotive plastic regrind plants that need a bulk density split ahead of electrostatic or NIR polishing.

Ready to Size a Sink Float Tank for Your PET or HDPE Line?

Send Rumtoo a feed analysis (polymer mix, flake size, contamination) and your target purity, throughput and water-reuse rate. We will return a sized tank, filtration loop, commissioning plan and a case-specific ROI model — usually within 24 hours.

Request Sink Float Tank Proposal
Expert response within 24 hours. No obligation quote.