How Does the Separation Floating Tank Sorting Bottles and Caps?

In the PET bottle recycling and washing line, the rinsing tank (Floating Washer) is a key equipment used to separate light impurities (such as labels, bottle caps, foreign matter, etc.) in PET bottles. It can be used in conjunction with the entire washing line or alone. It mainly uses the density difference of different materials to complete the separation of light impurities, ensuring that the PET bottle flakes maintain high purity in subsequent production processes.

I. Working principle of the rinsing tank

There are usually two ways of sorting in the rinsing tank: water flow and air flow floating sorting

(1) Water tank design

• The bottom of the rinsing tank is usually equipped with a high-pressure water flow nozzle, which uses the impact force of the water flow to make the bottle roll, and uses the flow rate of the water flow to drive light impurities (such as labels, bottle caps) to float to the water surface.

• PET bottles have a high density (1.34-1.40 g/cm³) and remain at the bottom of the water tank, while labels and bottle caps have a low density, especially PVC, PE, and paper labels, which will float on the water surface.

(2) Separation process

• When the PET bottle enters the rinsing tank, the impact of the water flow causes the label and cap to fall off and float to the surface of the water with the water flow.

• The PET bottle continues to sink, and the water flow carries away the light impurities (labels, bottle caps, foreign objects, etc.) floating on the water surface.

II. Discharge methods of labels and bottle caps

(1) Floating object discharge system

• Water flow assistance: light impurities will overflow along the edge of the rinsing tank with the water flow and be discharged through the drainage system.

• Air flow assistance: Some rinsing tanks are equipped with wind systems, which further help floating objects to converge to the slot through air flow and eventually be discharged.

(2) Filter or screen

• In some designs, the outlet of the rinsing tank may be equipped with a screen or filter to intercept floating labels and bottle caps to prevent them from being discharged with the water flow, thereby ensuring the purity of the recycled PET material.

III. Factors affecting separation efficiency

The separation efficiency of the rinse tank can usually reach more than 90%, especially for the separation of light labels (such as PVC, BOPP labels) and bottle caps.

o Water flow rate: The appropriate water flow rate can help light impurities to be effectively separated, but too strong a water flow may cause the PET bottle to be washed away by the water flow.

o Material of labels and caps: The density difference of different labels and bottle caps determines their floating behavior in water.

o Water temperature and chemical additives: Using warm water or adding an appropriate amount of chemical additives (such as surfactants) can effectively reduce the surface tension of water, help labels to fall off the bottle more easily, and improve separation efficiency.

IV. Methods to improve separation efficiency

In order to achieve a separation efficiency close to 100%, the following measures can be taken:

• Accurately control the water flow rate and water level: The water flow rate should be high enough so that the labels and caps can fall off and float quickly, but avoid too fast a water flow to carry away the PET bottle flakes.

• Adjust the shape of the water tank and the diversion design: Optimize the structure of the water tank so that the water flow can cover the entire tank body, avoid local uneven water flow, and ensure the complete separation of floating objects.

• Use wind or airflow to assist sorting: Add a wind system in the rinse tank to help floating objects such as labels to gather on the water surface through airflow, thereby increasing separation efficiency.

• Multi-stage sorting: Use multiple rinse tanks for graded cleaning. The water flow, stirring and floating object separation mechanism of each tank can gradually improve the removal rate of labels and bottle caps.

V. Methods for achieving 100% separation efficiency

Although the rinse tank can efficiently separate labels and caps, there are still some challenges to achieving 100% separation, especially for labels with strong adhesion or small bottle caps. To this end, the following methods can further optimize the separation effect:

(1) Multi-stage sorting

• The separation efficiency can be improved by setting up multiple rinse tanks and adding additional water flow and airflow to assist separation at different stages.

(2) Subsequent fine sorting

• After rinsing, post-processing methods such as near-infrared sorting (NIR) or electrostatic sorting can be combined to further remove any remaining impurities, such as labels or bottle caps that have not completely fallen off.

(3) Monitoring and automated control

• Introducing an automated monitoring system to adjust parameters such as water flow and temperature in real time to ensure that the separation effect during the rinsing process is always maintained at the best state.