Excessive Impurities in Recycled Plastics? Optimization Solutions for Cleaning Equipment and Screening Systems

Excessive impurities in recycled plastics (such as soil, metals, foreign plastics, residual ink, etc.) are a core bottleneck restricting their high-value utilization. They directly lead to intensified equipment wear during extrusion molding and a surge in product defect rates (e.g., black spots, pinholes, and substandard mechanical properties). To solve this problem, we need to start from "targeted impurity removal" and achieve an increase in impurity removal rate from 60%-70% to over 95% through the upgrading of cleaning equipment and combined optimization of screening systems. The following is a specific optimization plan:

I. Analysis of Impurity Types and Characteristics in Recycled Plastics (Premise for Optimization)

Impurities in recycled plastics from different sources vary significantly, requiring targeted treatment logic:

II. Layered Optimization Plan for Cleaning Equipment

The cleaning system needs to achieve gradient impurity removal from "rough to fine" and design multi-stage cleaning units based on the characteristics of different impurities:

(1) Rough Cleaning Stage: Removing Adherent Impurities (Soil/Sand)

Core Goal: Peel off over 70% of surface loose impurities to reduce the load of subsequent fine cleaning.

1. Upgrading of Screw Scrubbing Machine

• Traditional Problem: Single-screw structure has insufficient friction, resulting in >30% soil residue;

• Optimized Design:

• Adopt a "twin-screw reverse scrubbing" structure, reducing the distance between screw blades from 50mm to 30mm to increase friction intensity between materials;

• Install wear-resistant rubber bumps (5mm in height) on the cylinder wall to enhance scraping effect on soil in film olds;

• Effect: Soil removal rate increased from 60% to 85%, sand residue <0.5%.

2. Optimization of High-Pressure Spraying Parameters

• Pressure and Angle: For agricultural films (0.01-0.02mm thick), use 10-12MPa pressure and 45° inclined nozzles (to avoid film damage); for thick-walled waste (e.g., PE bottle flakes), increase to 15-18MPa with vertical spraying to peel deep impurities;

• Water Temperature Control: Heat water to 40-50℃ in winter (ambient temperature <10℃) to reduce soil viscosity (peeling efficiency increased by 20%).

(2) Fine Cleaning Stage: Removing Stubborn Impurities (Ink/Adhesives)

Core Goal: Decompose or peel off chemically bonded impurities to achieve over 90% surface cleanliness.

1. Improvement of Hot Alkaline Cleaning Tank

• Traditional Problem: Single-tank soaking results in <50% ink peeling rate (especially for OPP film printing layers);

• Optimized Design:

• Adopt a "three-tank series" structure: 1st tank (80℃, 2% NaOH solution) to swell ink; 2nd tank (60℃, 1% surfactant) to emulsify adhesives; 3rd tank (room temperature clean water) to neutralize residual alkali;

• Add an ultrasonic device (20-30kHz) to break ink particles through cavitation effect (peeling rate increased to 85%);

• Application: Waste materials with high ink content such as express bags and food packaging films.

2. Parameter Matching of Friction Washer

• Rotational Speed and Blade Design: For tough PE films, use 800rpm speed + serrated blades (to increase shearing force); for brittle PP films, reduce to 500rpm + arc blades (to avoid excessive film fragmentation);

• Material Filling Rate: Control at 60%-70% of the tank volume (excessive filling causes entanglement, insufficient filling leads to inadequate friction), ensuring each film is rubbed >50 times per minute on average.

(3) Dehydration Stage: Simultaneous Removal of Lightweight Impurities (Fibers/Straw)

Core Goal: Separate lightweight impurities during dehydration to reduce drying load.

1. Upgrading of Centrifugal Dehydrator

• Traditional Problem: Only dehydrates, with fibrous impurities entering the drying process along with materials;

• Optimized Design:

• Install a "cyclone separation channel" on the top of the machine cover; airflow generated by centrifugation carries lightweight fibers (density <0.3g/cm³) into the separator, which are intercepted by the filter screen;

• Add guide grooves (3mm deep) on the inner wall to make dehydrated materials slide down along the grooves, avoiding secondary mixing with fibers;

• Effect: Fiber impurity removal rate increased from 30% to 75%, drying time shortened by 15%.

III. Combined Optimization Strategy for Screening Systems

The screening system needs to form "complementary impurity removal" with cleaning equipment to accurately separate impurities that are difficult to remove by cleaning (e.g., metals, foreign plastics):

(1) Combination of Multi-Stage Screening Equipment

1. Magnetic Separation + Eddy Current Separation: Removing Metal Impurities

• Primary Magnetic Separation: Use a high-intensity magnetic roller (surface magnetic field strength 12000Gs) to adsorb ferromagnetic impurities (iron nails, steel wires) with a removal rate ≥99%;

• Secondary Eddy Current Separation: For non-magnetic metals (copper, aluminum fragments), generate eddy current force through a high-frequency alternating magnetic field to throw metal impurities away from the material flow (separation efficiency ≥95%), suitable for recycled plastics mixed with electronic waste.

2. Air Separation + Density Separation: Separating Lightweight and Heavy Impurities

• Air Separation Parameters: For straw/foam (density <0.1g/cm³), use a horizontal air separator with 15m/s wind speed; for stones/glass (density >2.5g/cm³), use vertical drop air separation (utilizing gravity difference);

• Water-Medium Density Separation: For impurities with similar densities (e.g., PE and PVC with a density difference of  0.1g/cm³), adjust brine concentration (1.05-1.1g/cm³) to make PE float and PVC sink (separation purity ≥98%).

3. Photoelectric Sorting: Identifying Foreign Plastics and Color Sorting

• Near-Infrared (NIR) Sorting: Identify materials such as PE/PVC/PP through spectroscopy, enabling simultaneous separation of multiple materials for mixed waste (accuracy ≥95%) with a single machine processing capacity of 1-2 tons/hour;

• Color Sorter: Identify black impurities (e.g., charred particles) and off-color plastics, and reject them through high-speed air valves (response time <0.1s), suitable for recycled plastics requiring high whiteness (e.g., food-grade packaging reuse).

(2) Linked Control of Screening Systems

1. Flow Matching: Ensure stable material flow into the screening system after cleaning (fluctuation ≤±5%) to avoid reduced sorting accuracy due to overload (e.g., the recognition rate of photoelectric sorting decreases by 20% with flow overload);

2. Feedback Regulation: Install impurity detectors (e.g., metal detectors) at the screening outlet to real-time feedback impurity residue, and automatically adjust parameters such as magnetic separation intensity and wind speed (e.g., when metal residue exceeds the limit, the magnetic roller speed increases by 10%).

IV. Targeted Optimization Cases for Different Raw Materials

(1) Agricultural Film Recycled Plastics (High Soil/Straw Impurities)

• Optimization Plan: Twin-screw scrubbing machine (rough cleaning) → hot alkaline cleaning tank → centrifugal dehydration + cyclone separation (removing straw) → magnetic separation + air separation (removing metals/stones);

• Effect: Total impurity removal rate increased from 65% to 92%, soil residue <0.05%, applicable to injection-grade recycled PP production.

(2) Packaging Film Recycled Plastics (High Ink/Label Impurities)

• Optimization Plan: Friction washer (800rpm) → three-tank ultrasonic deinking → NIR sorting (separating PVC labels) → color sorter (removing black spots);

• Effect: Ink residue rate reduced from 25% to 3%, foreign plastic removal rate ≥98%, applicable to film-blowing grade recycled PE production (number of black spots on films <1 per m²).

V. Core Indicators and Benefits of the Optimization Plan

The core logic of impurity treatment in recycled plastics is "differentiated measures"—solving physically attached and chemically bonded impurities through cleaning equipment, and separating impurities with differences in density, material, and magnetism through screening systems. The two work together to form a "full-link impurity removal closed loop." For high-impurity recycled plastics, only 10%-15% of equipment upgrading costs need to be added to upgrade the recycled plastic grade from Class III to Class II or even Class I, significantly expanding its application scenarios (e.g., from roadbed filling to industrial product raw materials), achieving a win-win situation of environmental benefits and economic value.