In-Depth Analysis of HDPE Recycling Process Challenges: Equipment Configuration Differences for Blow-Molding Grade Milk Bottles, Mixed-Color Materials, and Chemical Drums

Primary Contaminants:

Organic Residues: Fats, proteins, and sugars. 

Label Systems: Paper labels (prone to pulping), PP self-adhesive labels, and PVC shrink films.

Adhesives: Hot melt and water-based adhesives. 

Aluminum Foil: Most milk bottles utilize aluminum foil seal liners. 

Caps: The recycling process requires a mono-material PE stream, whereas most bottle caps are made of PP.

Primary Contaminants:

Chemical Contamination: Residual liquids from detergents and shampoos generate significant foam, affecting pH levels.

Physical Impurities: Include laminated labels, PP self-adhesive labels, heat shrink sleeves (PET/PVC), and aluminum foil sealing liners.

Adhesives: Hot-melt adhesives and water-based glues

Caps: The recycling process targets mono-material PE, whereas most caps are made of PP.

Special Feedstock (Pump Bottles): The pump assembly consists of PP, stainless steel springs, and silicone rubber gaskets, all of which must be removed.

Physical Properties: Characterized by thick walls, high strength, and extreme toughness. These materials tend to tumble and idle within the crushing chamber; therefore, a shredder is usually configured for primary size reduction.

Primary Contaminants:

Hazardous Chemicals: Residues may include hazardous waste such as pesticides, strong acids/alkalis, organic solvents, and lubricants.

Physical Characteristics: Solid structure with uneven wall thickness, often featuring reinforcing ribs. Prolonged outdoor exposure frequently results in a surface photo-oxidative degradation layer and brittle texture.

Primary Contaminants:

Inert Impurities: Mud, dust, nails (in pallets), and metal hinges.

Polymer Impurities: PP (handles, clips) and rubber anti-slip pads. Due to the minimal density difference between PP and HDPE (0.90–0.96), traditional sink-float separation is ineffective.

Single Shaft Shredder machine

Double Shaft Shredder machine

Working Principle

High-speed rotor cutting + Hydraulic ram + Fixed screen

Two low-speed, high-torque shafts intermeshing and tearing

Output Size

Application Scenarios

Energy Efficiency Analysis

Wet Granulator Machine

Dry Granulator Machine 

Working Principle

High-speed rotor drives rotary and stationary knives to generate shear force, with continuous circulating water injection into the crushing chamber.

High-speed rotor drives rotary and stationary knives to generate shear force; no water injection.

Output Size

Uniform, determined by screen aperture (e.g., 12mm).

Application Scenarios

Recyclables suitable for sale after simple washing.

Friction Washer

The high-speed friction washer utilizes centrifugal force and paddle impact to remove stubborn contaminants.

RPM Specifications: Typically above 1000–1200 RPM.

Working Principle: Materials are subjected to high-speed centrifugal force and friction between the screen and paddles. Contaminated water, paper pulp, and sediment are discharged through the screen, while the plastic advances.

Sink-Float Tank

Utilizes water density (1.0 g/cm³) for physical separation.

• HDPE (Density 0.93–0.97): Floats.

• Contaminants (PET, PVC, Metal, Sand/Mud, Nylon): Sink.

• PP (Density 0.90–0.91): Floats together with HDPE.

  • Note: PP and HDPE cannot be separated via water flotation and require subsequent optical sorting.

Hot Washer System

This is a critical step for removing grease, adhesives, and odors, particularly for milk bottles and household chemical containers.

Chemical Regimen:

• Sodium Hydroxide (NaOH / Caustic Soda): The core agent. It functions by converting water-insoluble fats (triglycerides) into water-soluble soap and glycerol.

• Surfactant (Detergent): Reduces surface tension and emulsifies mineral oils, preventing detached contaminants from re-adhering to the flakes (anti-redeposition).

• Residence Time: Ensures the material undergoes at least 10–20 minutes of effective soaking and agitation.

Centrifugal Dryer

• Application: Rigid HDPE flakes (bottle flakes).

• Principle: High-speed rotation (~1500 RPM) generates centrifugal force to dewater the material.

• Limitations: Effective only for removing surface free water; difficult to extract moisture from microscopic cracks. Typically reduces moisture content to 3–5%.

Hot Air Drying

• Application: Secondary drying following centrifugal dewatering.

• Principle: Utilizes hot air (via electric heating or steam heat exchange) to evaporate residual moisture within the transport piping.

• Energy-Saving Design: When sourcing, prioritize systems featuring hot air recirculation or heat exchangers to minimize energy consumption.

Air Classifier (Zig-Zag / Air Classifier)

Utilizes aerodynamic principles to remove light-density impurities.

• Principle: Within a "Z-shaped" channel, an upward airflow carries away lighter materials such as labels, dust, and film fragments, while the heavier rigid HDPE flakes descend.

• Positioning: Typically installed after the dryer, serving as the final physical purification stage for removing residual labels and fines.

Optical Sorter

NIR (Near-Infrared) Sorting: Sorts materials based on polymer spectral signatures.

• Function: Ejects residual contaminants such as PET, PVC, and ABS. This is critical for guaranteeing the Intrinsic Viscosity (IV) value and processing stability of the recycled material.

• Cap Separation: Although PP and HDPE have similar densities, NIR can accurately distinguish between them. If the client requires pure HDPE material, NIR is the only solution.

Vis (Visible Light/Color) Sorting: Utilizes CCD cameras for color rejection.

• Function: Removes off-color fragments (red, blue, green, yellow) from natural/white HDPE streams. Off-color flakes cause the pelletized granules to appear grey or exhibit color deviation, significantly reducing the selling price.

• (Note: A single color sorter is typically configured to retain or reject a single color category.)