Sink-Float Tank Engineering: Physics of PET Density Separation

In the recycling industry, PET bottles (Polyethylene Terephthalate bottles) are both a high-value resource and a significant technical challenge. To the layperson, a discarded beverage bottle appears to be a single material. However, to an engineer, it is a complex "composite product." This seemingly simple object is actually composed of at least three different polymers that are chemically and physically incompatible with each other.

Understanding this "composite" nature is the key to understanding why an advanced PET bottle washing line is designed the way it is.

Bottle Body: This is the most valuable part, made of PET.
Cap and Neck Ring: Usually made of PP.
Label: Usually made of PE or paper, and affixed to the bottle body with adhesive.

The Challenge: These materials must never be mixed in the melting stage. If PP or PE contaminants enter the extruder along with the PET flakes, they will carbonize or degrade first, causing the final (rPET) flakes to be severely compromised in physical properties—becoming cloudy, brittle, and plummeting in commercial value.

Therefore, the entry barrier to the rPET market is purity. High-quality rPET flakes require a Total Impurity of less than 320 parts per million (320ppm), while advanced production lines (like Boxin Machinery's) aim to reduce PP/PE contamination to 50ppm, or even meet the stringent standard of below 15ppm for food-grade applications.

This brings us to the core thesis of this article: the true value of a high-performance PET washing line lies not in "washing," but in its separation capability.

The Specific Gravity of PET Flakes (Sink-Float Principle)

Before we dive deep into multi-million dollar mechanical engineering, we must first grasp a simple yet elegant physical principle: Specific Gravity.

Specific gravity, or relative density, is a dimensionless value that measures the density of a substance in comparison to a reference substance (usually water). As requested, we can explain this concept in the simplest terms:

The density (specific gravity) of water is defined as 1.0 g/cm³.
Any substance with a specific gravity greater than 1.0 will sink in water.
Any substance with a specific gravity less than 1.0 will float in water.

Fortunately for the PET recycling industry, the three main plastics that make up a PET bottle have distinctly different specific gravity "fingerprints," as if they were specifically designed for physical separation.

Recycled Material Density Comparison Table

Material	Specific Gravity (g/cm³) (Average)	Behavior in Water (1.0 g/cm³)
PET (Bottle Body)	$~$1.38	Sinks
PP (Cap/Neck Ring)	$~$0.90	Floats
PE (Label)	$~$0.93 - 0.96	Floats
Water (Separation Medium)	1.0	Baseline

In theory, we just need to wait for the PET to sink and the PP/PE to float. However, between theory and large-scale industrial production, there lies a gulf called "engineering."

The Engineering Marvel of the Float-Sink Tank

A. Breaking Perceptions: Why It Is Absolutely Not Just a "Big Tub of Water"

Given the physical principles in the second part, a common and costly misconception is that recyclers only need a "big tub of water." This assumption is the root cause of why many recycling projects fail, producing rPET flakes that do not meet purity standards.

An efficient Float-Sink Tank is a precision-engineered fluid dynamics device. Its primary and most difficult task is to control the flow of water.

B. The Greatest Enemy: Turbulent Flow

Many separation failures occur not because the density principle fails, but because the water flow is chaotic.

In an industrial line, materials (crushed flakes) are often pumped or flushed into the separation tank at a high flow rate. This high-speed, chaotic flow creates a high "Reynolds number", resulting in Turbulent Flow. Turbulence is characterized by unpredictable Eddies and intense Mixing.

And "Mixing" is the exact opposite of "Separation."

Imagine: a 5mm square PE label fragment (density 0.95 g/cm³). Its own buoyancy is extremely weak. In a chaotic body of water dominated by turbulence, its weak "viscous force" (the force making it float) can easily be overcome by the powerful "inertial force" (the eddies).

The direct consequence: the PP/PE fragments that should have floated are forcibly dragged to the bottom by the artificial turbulence, mixing with the clean PET flakes. This leads to catastrophic separation failure, and the final rPET flake purity will be far from meeting the PPM-level requirements.

Therefore, a poorly designed "big water tub" full of turbulence actually acts as a mixer, not a separator.

C. The Real Solution: Laminar Flow and Mechanical Removal

Achieving Physical Separation: Only in a calm "Laminar Flow" environment can gravity become the dominant force. The slight density differences are allowed to manifest; PE fragments have enough time and space to overcome water resistance and float, while PET flakes can sink steadily without interference.
Mechanical Removal: After achieving calm separation, the system must be able to continuously remove both separated material streams without disturbing the tranquility of the water.
Bottom (Sunk Material): The bottom of the tank is designed with precise inclination angles. Clean, high-density PET flakes settle here. A slowly rotating Screw Conveyor (or Auger) continuously "digs" these PET flakes out from the bottom of the tank, sending them to the next process.
Top (Floating Material): The floating PP/PE contaminants gather on the water's surface. Rotating Paddles or Skimmers gently push these contaminants into a separate collection trough, where they are collected and sold as by-products like "cap material."

Only at this point does the float-sink tank transform from a passive "tub" into an active, dynamic, and precise separation machine.

Deconstructing the Complete PET Bottle Washing Line: From Bales to Pure Flakes

The float-sink tank is the heart of the line, but it does not exist in isolation. It is a critical link in an automated production line precisely coordinated by a dozen or more modules. The following is a typical hot wash line configuration from (Boxin Machinery), detailing the complete journey from compressed bales to pure flakes.

Table 2: PET Washing Line Process Flow and Function

Stage	Key Equipment	Core Objective
Stage 1: Pre-Processing	De-baler, Trommel Screen, Label Remover	Break up materials, remove small contaminants, remove 95%+ of labels
Stage 2: Crushing & Physical Separation	Wet Granulator, Float-Sink Tank	Reduce size, separate PP/PE using specific gravity
Stage 3: Deep Cleaning	Hot Wash System, High-Speed Friction Washer	Remove glue, grease, and stubborn stains
Stage 4: Rinsing & Drying	Rinsing Tank, Centrifugal Dryer, Hot Air Drying	Remove chemical residues, reduce moisture to <1%
Stage 5: Quality Control	Air Classifier (Dust Removal), Packaging System	Remove fine dust, weigh and package

A. Stage 1: Pre-Processing

De-baler: Recycled PET bottles typically arrive at the factory in high-density compressed bales. The de-baler must break these hard bricks into free-flowing single bottles. (Boxin Machinery) utilizes a "four-axis structure" for high-pressure bales to ensure efficient bale breaking.
Label Remover: This is an often-overlooked but crucial step. Before crushing, the bottles enter a label remover.
- Working Principle: It uses high-speed rotating alloy blades and mechanical friction to cut openings in the labels and forcibly tear them off.
- Air Separation: An integrated air separator immediately vacuums away the lighter label paper and PE film, while the heavier bottles continue forward.
- Why it's Necessary: Why remove labels before crushing, and then also have a float-sink tank (which also removes labels)? This "redundant" design is a mark of efficiency. Explanation: Once the labels and bottles are both cut into small pieces, they become extremely difficult to separate. The dry label remover (which can remove 95-98% of labels) removes the labels when they are largest and easiest to separate. The subsequent float-sink tank is a finishing step, used to handle the residual 2-5% of label fragments and (more importantly) the PP/PE caps.

B. Stage 2: Crushing & Physical Separation

Wet Granulator: The pre-processed bottles enter the granulator and are cut into uniform "flakes," with the size typically controlled by a screen to 12mm. Water is continuously sprayed into the chamber during crushing for three purposes: to use the impact force of the water for pre-washing, to reduce the heat generated by friction, and to lubricate the blades to reduce wear.
Float-Sink Tank: The crushed mixed material (PET flakes, PP/PE cap and label fragments) enters the core separation unit detailed in Part 3. Only the high-density PET flakes are discharged from the bottom to enter the next stage.

C. Stage 3: Deep Cleaning

Physical separation cannot solve the problem of glue, grease, juice residues, and stubborn dirt. These contaminants require chemical and thermal solutions.

Hot Washing: This is the second key step to achieving high-purity rPET. The PET flakes are fed into one or more reactor tanks, where they are mixed with chemical agents (typically NaOH caustic soda and surfactants/detergents) in hot water at 80°C to 90°C.
(Boxin's) Synergy: Boxin Machinery optimizes this into a "three-force synergy" system:
1. Thermal Force: High temperature softens adhesives and grease.
2. Chemical Force: Caustic soda and detergents dissolve organic contaminants through saponification.
3. Mechanical Force: High-speed stirring paddles inside the tank continuously scrub the flakes, stripping off the softened dirt.
High-Speed Friction Washer: After coming out of the hot wash tank, the flakes enter a high-speed friction washer. It uses high-speed rotating blades and a water stream to powerfully "scrub" the surface of the flakes, removing the final residues that were softened during the hot wash but still adhering.

D. Stage 4: Rinsing & Drying

Rinsing: The purpose of this stage is to completely remove all residual chemical agents and suspended dirt separated during the hot wash. This usually includes one or more cold water rinsing tanks, and often employs a second or third float-sink separation design to ensure the final flakes are chemically neutral.
Drying: Drying is a two-step process with the goal of reducing the moisture content to below 1%.
1. Centrifugal Dryer: The flakes first enter a vertical or horizontal dewatering machine, which uses centrifugal force from high-speed rotation to fling off over 98% of the surface moisture.
2. Thermal Drying: The flakes then pass through a hot air pipe drying system. Hot air (approx. 120°C) evaporates the last remaining moisture in the flakes, ensuring the moisture content reaches the (<1%) standard.

E. Stage 5: Quality Control & Packaging

Dust Removal / Air Classifier: As a final quality control step, the dry flakes pass through an air classification system (a Z-shaped Air Classifier). The airflow precisely blows away extremely light dust, film shreds, and fine label particles, while the heavier, pure PET flakes fall down.
Silo & Packaging: The final pure rPET flakes are pneumatically conveyed to a large storage Silo, and then fed through an automatic weighing and packaging system to be bagged in jumbo bags, ready for sale.

Why Engineering Science Dictates the Final Value of rPET

Why is all this complex engineering—from laminar flow design to three-force synergy hot washing and multi-stage rinsing—so critical?

The answer is simple: because it directly determines the final grade and selling price of the rPET flakes.

Fiber-Grade: Used to produce carpets, apparel filling, or staple fibers. This requires high purity but has some tolerance for minute color specks and contaminants.
Sheet-Grade: Used for extruding new packaging sheets. The requirements are higher.
Food-Grade (Bottle-to-Bottle): This is the highest challenge and the highest-value application in recycling. It not only demands extremely high contaminant control (at the <15 ppm level) but also typically requires post-processing like SSP (Solid State Polycondensation) to restore the PET's intrinsic viscosity.

A "cheap" production line may end up producing flakes with 500ppm or even higher PP/PE contamination. Cold wash lines are only suitable for producing low-end products for PET strapping, which have extremely low market competitiveness.

Ultimately, purchasing a pet bottle washing line is not buying a pile of stainless steel machines; it is buying a repeatable, verifiable capability for physical and chemical separation.

Theory is hollow, but performance is permanent. A (Boxin Machinery) customer case in Kazakhstan provides the final proof: a PET washing line delivered 15 years ago (in 2009) is still running continuously under "high load" today, stably producing rPET flakes that meet the processing requirements of fiber factories.

This is the true value of precision engineering—it doesn't just work on day one; it continues to create value for the client every single day for the next 15 years.