{"id":4113,"date":"2026-05-26T08:45:40","date_gmt":"2026-05-26T08:45:40","guid":{"rendered":"https:\/\/kitech-recycling.com\/?p=4113"},"modified":"2026-05-26T08:45:40","modified_gmt":"2026-05-26T08:45:40","slug":"float-sink-separation-plastic-recycling","status":"publish","type":"post","link":"https:\/\/kitech-recycling.com\/es\/blog\/float-sink-separation-plastic-recycling\/","title":{"rendered":"Separaci\u00f3n flotador-sumidero en el reciclaje de pl\u00e1stico: c\u00f3mo la clasificaci\u00f3n por densidad limpia el PET y el PE"},"content":{"rendered":"

In any plastic recycling washing line, only one metric determines the market value of your output flakes: whether or not the target polymer floats in the liquid phase. Float-sink separation-or sink-float separation, also sometimes known as density-based sorting-is the most widely implemented mechanical separation step in the world today within the context of plastic recycling.<\/span><\/p>\n

Because it leverages the difference between a polymer\u2019s specific gravity and the density of the liquid in which it\u2019s dispersed, a well-designed, water-based sink-float separation tank is capable of producing a contaminant-free PET or HDPE flake with two to five minutes of residence time, zero chemical addition, minimal internal moving parts, and no optical sensors or lasers. The level of purity of your output-whether measured by part-per-million PVC content, low moisture level, or a very high polymer purity percentage-will decide if your flakes are fit for fiber spinning, safe for use in bottle-to-bottle food-grade recycling, or only suitable for general-purpose packaging products. In this guide, we will cover the basic physics, polymer density, engineering of the equipment, expected real-world purity levels, and position of float-sink separation within a complete plastic recycling solutions<\/a> line.<\/p>\n
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Quick Specs \u2014 Float-Sink Separation<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Separation principle<\/td>\nSpecific gravity differential in liquid medium<\/td>\n<\/tr>\n
Float fraction<\/td>\nPE (0.91\u20130.97 g\/cm\u00b3), PP (0.89\u20130.91 g\/cm\u00b3)<\/td>\n<\/tr>\n
Sink fraction<\/td>\nPET (1.33\u20131.45), PVC (1.38\u20131.65), PS (1.04\u20131.06)<\/td>\n<\/tr>\n
Standard medium<\/td>\nWater (1.00 g\/cm\u00b3 at 23\u00b0C per ASTM D792)<\/td>\n<\/tr>\n
Dense-media option<\/td>\nCaCl\u2082 \/ NaCl brine (1.05\u20131.35 g\/cm\u00b3)<\/td>\n<\/tr>\n
Industrial throughput range<\/td>\n250\u201310,000+ kg\/h<\/td>\n<\/tr>\n
Typical residence time<\/td>\n2\u20135 minutes (30 min for multi-stage PP\/PET trials)<\/td>\n<\/tr>\n
Fiber-grade PET target<\/td>\nPVC < 40 mg\/kg<\/td>\n<\/tr>\n
Density standard<\/td>\nISO 1183 \/ ASTM D792<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

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The Science Behind Float-Sink Separation \u2014 Archimedes’ Principle Applied to Polymer Sorting<\/h2>\n

What Is Float-Sink Separation in Plastic Recycling?<\/h3>\n

The Float-Sink Process Float-sink separation is the foundational wet method for plastic separation based on density \u2014 shredded plastic flakes are placed into a water-filled tank where polymers sort according to their specific gravity. It is the most widely deployed wet density separation technique for separation of plastics in post-consumer solid waste streams \u2014 raising the recycling rate of post-consumer packaging by enabling material to be diverted from being landfilled or sent to incineration. Materials with specific gravities less than 1.0 g\/cm\u00b3 -chiefly PP and PE-will float to the surface while those materials with specific gravities above 1.0 g\/cm\u00b3 -commonly PET, PVC, PS, and ABS in standard applications-will sink to the bottom where they may be removed using screw conveyors or other physical separation mechanisms such as paddle wheels.<\/p>\n

As per Archimedes\u2019 principle, each body submerged in a liquid experiences a buoyant force which is equal to the weight of the liquid displaced. Thus, when an object\u2019s density is less than the medium density, the buoyant force is great enough to overcome gravity and carry the object upward whereas when an object\u2019s density is greater than that of the medium, it will sink. Since water at 23\u00b0C has an average density of roughly 0.9975 g\/cm\u00b3, (based on ASTM D792 standard), it naturally serves as the dividing line between polyolefin (floaters) and other, denser plastic types in most of industrial applications.<\/p>\n

The physics of plastic densities can be precisely understood by means of density-measurement procedures such as the immersion method, known as ISO 1183 (Hungarian standards), or the displacement method ASTM D792, which uses water to make measurements. Each of the two standards define plastic density based on measurement at 23\u00b0C, which matches that of actual operating temperature in the float sink tanks that ensure the stability of separation.<\/p>\n

ISO 1183 reference on the Radwag\u2019s website states that, directly from the Archimedes law, calculation of plastic density after weighing a sample both in the air and dipped into distilled water gives difference caused by buoyant forces.<\/p>\n

One weakness has to be highlighted before you consider the implications for the process: the technique separates plastics by density only, not colour, molecular weight, grade or additive content. It mainly affects items at density borders. In a study from the Delft University of Technology (Hu, Fraunholz & Rem, 2010), it was observed that 2 mm of air at the edges of a 200 mm PET flake represents a 10 kg\/m error on average density – sufficient to transfer 2-5% of the PET input to the float fraction.<\/p>\n

So a Pre-Wetting of the surface is essential for a working plant.<\/p>\n

Polyolefin fractions – PP, LDPE, HDPE The Polyolefin densities are 0.89 g\/cm – 0.97 g\/cm, they always sink float very reliably in normal water. The sink-float method to recover polyolefins from post consumer packaging has also been proven in a research paper (Bauer et al., Montanuniversitaet Leoben, 2018): the used sample waste streams contained always more than 90 wt% polyolefins in the recovered lightweight fraction. If density alone can’t remove all materials because densities match (like PP & LDPE or with border-lines like PS density at 1.04-1.06 g\/cm), then another liquid medium or secondary step must follow as shown further below.<\/p>\n

<\/p>\n

Polymer Density Reference Table \u2014 Which Plastics Float, Which Sink (and When You Need Brine)<\/h2>\n

Use the table to compare ISO 1183 density with ASTM D792 density of twelve primary polymers by their behavior in pure water and 1.10 g\/cm CaCl solution. The density differences between polymer groups determine whether plain water, brine, or a secondary stage is required. Sorting plastics based on density starts with knowing the exact specific gravity of every polymer in your input stream \u2014 this table serves as a working guide for judging new feedstocks or designing your sorting media. Kitech’s plastic material comparison tool<\/a> provides an interactive calculator to compare separation feasibility for your specific input stream.<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Polymer<\/th>\nDensity (g\/cm\u00b3)
\nISO 1183 \/ ASTM D792<\/span><\/th>\n		Behavior in Water<\/th>\nBehavior in Brine (1.10 g\/cm\u00b3)<\/th>\nNotes<\/th>\n<\/tr>\n<\/thead>\n
PP (homopolymer)<\/td>\n0.90\u20130.91<\/td>\nFloats<\/td>\nFloats<\/td>\nCopolymer 0.89\u20130.91<\/td>\n<\/tr>\n
LDPE<\/td>\n0.91\u20130.94<\/td>\nFloats<\/td>\nFloats<\/td>\nFilm grades overlap with PP at 0.91<\/td>\n<\/tr>\n
HDPE<\/td>\n0.94\u20130.97<\/td>\nFloats<\/td>\nFloats<\/td>\nPipe grade at upper bound (0.96\u20130.97)<\/td>\n<\/tr>\n
PS (crystal)<\/td>\n1.04\u20131.05<\/td>\n\u26a0 Borderline sink<\/td>\nFloats<\/td>\nAir bubbles can cause PS to float without pre-wetting<\/td>\n<\/tr>\n
ABS (standard)<\/td>\n1.04\u20131.06<\/td>\n\u26a0 Borderline sink<\/td>\nFloats<\/td>\nFR grades reach 1.18\u20131.22; needs brine for WEEE<\/td>\n<\/tr>\n
HIPS<\/td>\n1.03\u20131.06<\/td>\n\u26a0 Borderline sink<\/td>\nFloats<\/td>\nCommon in WEEE housings<\/td>\n<\/tr>\n
PA6 (Nylon 6)<\/td>\n1.12\u20131.14<\/td>\nSinks<\/td>\nSinks<\/td>\nGF-reinforced grades up to 1.23<\/td>\n<\/tr>\n
PA66 (Nylon 66)<\/td>\n1.13\u20131.15<\/td>\nSinks<\/td>\nSinks<\/td>\nFR-modified grades up to 1.19<\/td>\n<\/tr>\n
PMMA (Acrylic)<\/td>\n1.17\u20131.20<\/td>\nSinks<\/td>\nSinks<\/td>\nImpact-modified grades may drop to 1.10<\/td>\n<\/tr>\n
PC (Polycarbonate)<\/td>\n1.19\u20131.22<\/td>\nSinks<\/td>\nSinks<\/td>\n30% GF grade: 1.40\u20131.43<\/td>\n<\/tr>\n
PET (bottle-grade)<\/td>\n1.33\u20131.45<\/td>\nSinks<\/td>\nSinks<\/td>\nPost-consumer recycled PET may drop to 1.33\u20131.37<\/td>\n<\/tr>\n
PVC (rigid, PVC-U)<\/td>\n1.38\u20131.41<\/td>\nSinks<\/td>\nSinks<\/td>\nPlasticized PVC: 1.30\u20131.70 (variable by plasticizer %)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

– water to plastic >0.05 g\/cm\u00b3; pure water not good for separations, unless pre-wetted. Source: ISO 1183; ASTM D792; Radwag metrology PDF, PMC8724085<\/a> (Meneses Quelal et al. 2022).<\/p>\n

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Engineering Note \u2014 The Brine Decision Rule<\/strong><\/p>\n

But as for most polymers, with density differences of less than 0.05g\/cm between your polymer of interest, using simple water for industrial-level sorting of plastics does not perform well. Some Polymers in this narrow density gap: ABS (1.04 – 1.06) PS (1.04 – 1.05) HIPS (1.03 – 1.06) The use of a CaCl solution with density around 1.05 – 1.10g\/cm raises the threshold, allowing these polymers to float over denser impurities (PET 1.33 and PVC 1.38). That\u2019s how it commonly works on our WEEE (Waste Electronic and Electrical Equipment) sorting lines with ABS and PS contained within e-waste casings.<\/p>\n

Also note that common NaCl brine is not soluble enough to reach the densities needed for some plastics to float (e.g. PVC, PA6). In order to reach high densities of separation for materials like PVC you will require a CaCl or similar solution or an even denser media such as silicones.<\/p>\n<\/div>\n

<\/p>\n

Sink-Float Tank and Sink Tank Configuration \u2014 Design, Sizing, and Throughput Parameters<\/h2>\n

What Is the Right Residence Time for Industrial Float-Sink Separation?<\/h3>\n

Standard industrial sink float tanks typically operate with 2-5 minutes Residence Time (RT) for Post-Consumer Rigid Plastics (i.e. PET, HDPE and PP closures), when the density difference to water is high (>0.1g\/cm) and a wash process removes contamination from the flakes. For smaller density difference plastics (i.e.<\/p>\n

PS, ABS, or blended plastic streams), residence time typically increases to 10\u201330 min. A 2025 TU Wien study (Lipp et al.) operated a flotation tank in plain water at 25\u00b0C to separate PP from PET label contamination at a 30-minute residence time, achieving a density cut above 1,000 kg\/m\u00b3. A 3-minute residence time is the standard design target for most PET or HDPE cleaning lines \u2014 long enough for reliable separation, short enough to keep tank volume and capital cost reasonable.<\/p>\n

Things that make it take longer: particle size under 10mm, too much contaminent attached (labels, food debris), lack of pre-wet, too slow water motion, below 15C, cool water (causes higher surface tension). There is an automatic throughput calculator on our website for you to properly calculate you needs for our separatory tank.<\/p>\n

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Engineering Note \u2014 Tank Volume Estimate<\/strong><\/p>\n

A conservative tank sizing formula:<\/p>\n

V = (Q_mass \u00f7 \u03c1_bulk_wet) \u00d7 t \u00d7 2.0<\/strong><\/p>\n

Where: Q_mass<\/em> = throughput (kg\/h); \u03c1_bulk_wet<\/em> \u2248 0.30 kg\/L (wet plastic flakes); t<\/em> = residence time (hours); 2.0<\/em> = safety factor for headspace and water volume.<\/p>\n

Example \u2014 1,000 kg\/h, 3 min residence time: V = (1,000 \u00f7 0.30) \u00d7 (3\u00f760) \u00d7 2.0 = 333 L \u00d7 0.05 \u00d7 2.0 = 33 L effective volume \u2192 tank gross size 0.35\u20130.5 m\u00b3 including water headspace (30\u201340% additional). A 2,000 kg\/h line doubles to 0.70\u20131.0 m\u00b3 minimum.<\/p>\n<\/div>\n

The tank geometry is of the same importance as volume of the separation tank. The separation process within the tank depends on laminar \u2014 not turbulent \u2014 flow pattern, which keeps the float and sink fractions from re-mixing. Residence times are calculated to match this flow pattern for each feedstock type \u2014 the velocity field inside the tank must remain laminar across the full cross-section, including the zones where solid particles from contaminated label debris accumulate. Even with large density differences, turbulence reduces separation efficiency to near zero.<\/p>\n

A well-designed sink float tank separates the incoming material stream at the water surface \u2014 lighter plastics rise, while heavy material (PET, PVC, glass flakes, sand) sinks to the bottom of the tank. Water enters from below and the floated fraction is extracted via weir or paddle-skimmer without disturbing the settled sink fraction below. Separate extraction of sink material is realized through bottom-valves driven by a screw or conveyor, resulting in the so-called \u201csink tank function\u201d \u2014 sink material collected independently from the float separation. A separate sink tank-body with separate extraction screw serves to allow sink-material out without interfering with float-skimming.<\/p>\n

<\/p>\n

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Vendor Evaluation Checklist \u2014 Sink-Float Separation Tank Machinery<\/p>\n