A sink-float separation tank is a wet density separation unit that exploits the density gap between the process medium and incoming solids, plus controlled hydrodynamics, to drive “light float, heavy sink” classification at scale. It is a core step in plastics recycling (for example, separating PP/PE from PET or PVC), resource recovery, minerals preconcentration, and end-of-life appliances or vehicles dismantling. Stability and cut-point sharpness directly influence downstream washing, sorting, and pelletizing yields and costs.
1) Working Principle: Why Do Some Particles Float and Others Sink?
- Medium density rule: A process medium of known density (water by default, optionally modified by additives) fills the tank. Particles with density below the medium float; particles above the medium sink.
- Flow-field control: Overflow weirs, baffles, bottom screw conveyors or scrapers, side guide plates, and optional aeration maintain a stable flow regime and suppress entrainment that would carry light particles downward or lift heavy particles.
- Residence time and energy: Particle size distribution, upflow velocity, agitation intensity, and tank geometry determine the separation window. Proper tuning improves discrimination near the cut-point.
Quick mental model: the tank acts like a “density gate.” Lighter-than-gate passes to the float product, heavier-than-gate to the sink product.
2) Key Components and Structural Design
- Infeed and pre-wetting zone: Minimizes agglomeration and bubble attachment. Often includes spray bars or a pre-wetting screw.
- Main separation chamber: Defines separation space and residence time. Tank depth and length shape the sharpness of the partition curve.
- Baffles and flow guides: Stabilize the surface and limit cross-currents.
- Discharge systems: Float product exits via scraper or overflow weir; sink product via bottom screw conveyor or elevator.
- Medium clarification loop: Settling box, screens, or centrifuges recover fines and maintain medium cleanliness.
- Instrumentation and control: Level, flow, turbidity, and density monitoring tied to VFDs and dosing for closed-loop control.
3) Typical Applications and Feed Mixes
- Plastics recycling:
- Coarse split of PP/PE (ρ≈0.90–0.96 g/cm³) versus PET, PVC, PS (ρ≈1.04–1.38 g/cm³).
- Decoupling mixed film/sheet streams and bottle flakes versus caps.
- Ferrous and nonferrous preconcentration: In higher-density media to cluster alloys or upgrade feed.
- Glass and ceramics: Density-based impurity removal.
- WEEE streams: Early-stage split of plastics and metals, or first-pass resin grouping.
Note: For near-density materials or surfaces contaminated by oils or trapped air, add pre-wash, hot wash, and surface-energy conditioning to avoid false float/sink.
4) Core Performance Metrics (for Selection and FAT/SAT)
- Partition sharpness (α or Ep): Misplacement around the cut-point. Lower is better.
- Throughput (t/h or kg/h): Function of cross-sectional area, residence time, and PSD.
- Yield and recovery of float/sink: Must align with downstream capacity and target purity.
- Medium and water consumption: Loop closure ratio, evaporation, and carry-over losses.
- Footprint and energy: kWh/t and specific system capacity.
- Maintainability: Access and replacement intervals for scrapers, seals, bearings, screws, and screens.
5) Comparison to Alternative Separation Methods
- Air classification: Sensitive to thin films and low-density, high-area particles but affected by humidity and shape; purity can fluctuate.
- Electrostatic separation: Enables resin-level splits among near-density materials, but requires low moisture and high surface cleanliness.
- Optical/NIR sorting: Good for color/material recognition, limited by heavy soiling and multilayers.
Typical integrated route: Shredding → Pre-wash/Hot wash → Sink-float separation → Friction wash → Dewatering → Optical/Electrostatic finishing → Pelletizing.
6) Engineering Guidelines and Setpoints (Experience Window)
- Particle size: 5–30 mm typical. Very fine particles risk carry-over; very large particles require longer residence.
- Solids-to-liquid ratio: 1:5 to 1:12 usually balances stability and footprint.
- Geometry: Deeper tanks improve sharpness; longer tanks stabilize product zoning, at the cost of space and energy.
- Level control: Adjustable overflow weirs and inlet head help match bulk density swings.
- Agitation and aeration: Mild aeration helps deagglomerate; excessive energy can cause misplacement.
- Online control: Recommend density meter, turbidity, flow, and conductivity tied to PLC feedback.
7) Quality and Compliance for Recycled Plastics
- Target purity: Food-grade rPET is extremely sensitive to PVC contamination (ppm-level). Follow sink-float with optical/electrostatic polishing.
- Regulatory alignment: Track EU/US/China standards for recycled contact materials and traceability.
- Water treatment: Close the loop when possible; manage COD, TSS, oils/fats, and microplastics.
8) Cost Model and ROI Levers
- CAPEX: Tank and drives, pumps, piping and valves, structures and platforms, power and controls.
- OPEX: Power, water/chemicals, spare parts, labor and maintenance, sludge and screen rejects handling.
- Value drivers:
- Higher purity lowers downstream washing and finishing loads.
- Higher recovery reduces loss of value-bearing fractions.
- Closed-loop water cuts consumption and wastewater fees.
9) RFQ/RFP Checklist
- Throughput and feed spectrum: Composition, PSD, moisture, and contamination types.
- Product specs: Purity, yield, and variability limits for both float and sink.
- Site and utilities: Footprint, headroom, floor loading, power, water, and drainage.
- Automation: Local/remote monitoring, data capture, and traceability.
- Maintenance: Wear parts list, spares stocking, downtime, and safety procedures.
10) Commissioning and Operations Best Practices
- Stage commissioning: Water loop first, then low-rate feed, then nameplate.
- Cut-point validation: Use density standards to map the partition curve and Ep.
- KPI trending: Log key metrics and outliers; adjust overflow and agitation weekly.
- Avoid false float: Hot wash to remove oils/waxes; manage surface tension and bubbles.
11) FAQ
- Q: Can PP and PE be fully separated in water?
- A: Both float in water; differentiate further via medium tuning, hot wash, and electrostatic or optical finishing.
- Q: How severe is PVC carryover into rPET?
- A: Extremely severe. It degrades polymer and color; reinforce polishing and online monitoring post sink-float.
- Q: Why does stability swing within the same lot?
- A: Usually due to feed contamination/PSD drift or level/velocity offsets; add online density and turbidity feedback.
12) Further Reading and Resources
- Process design should be evaluated with adjacent unit ops such as cold/hot wash, friction wash, dewatering, and finishing.
- External resource: Case studies and perspectives on plastics recycling flowsheets: Plastic bottle recycling (sink-float separation insights)
Closing Note
A sink-float separation tank is not a plug-and-play gadget but a system node that spans feed preparation, online control, and downstream finishing. Center the design on the use case and co-optimize setpoints across units to maximize purity and recovery while minimizing cost, strengthening Rumtoo’s topical authority in plastics recycling and resource recovery.