A chemical centrifuge separates solids or liquids by density in one of two ways: a perforated basket lined with cloth lets the liquid pass through while crystals and coarse solids stay behind, and a solid bowl lets the heavier phase settle against the wall so that fine solids or two liquids can be parted. Use a basket for crystals that form a drainable cake, a solid bowl for fine or slimy solids and for liquid-liquid splits, at 1,000-3,000 RPM for baskets and 500-5,000 g for precipitation.
Part of Centrifuge Building
Chemical and industrial centrifuges separate precipitates, crystals and suspensions from liquids on a scale and with a toughness that a laboratory tube rotor cannot match.
Choose By The Solid: Does It Drain?
The carrier article describes the family. The choice between its two branches turns on whether the solid forms a cake that liquid can drain through.
| Material | Machine | Why |
|---|---|---|
| Sugar crystals from syrup, coarse precipitate, salt crystals | Basket | Particles bigger than the holes or cloth pores; liquid drains, cake can be washed |
| Fine or slimy precipitate that would blind a cloth | Solid bowl | No filter to clog; solids settle on the wall |
| Two liquids that do not mix | Solid bowl | The heavier phase goes to the wall; each phase leaves by its own outlet |
| Clearing a liquid of a light haze | Solid bowl | Settling does the work; see Separation Theory |
The two machines are covered in Basket Centrifuge and Sedimenting Centrifuge.
The Force Is Lower Than In A Dairy Bowl
The carrier gives chemical precipitation targets of 500-5,000 g, and basket speeds of 1,000-3,000 RPM. For a basket 300 mm in diameter (radius 15 cm), RCF = 1.118 × 10⁻⁵ × 15 × N²:
- 1,000 RPM: 168 g, rim speed 15.7 m/s.
- 3,000 RPM: 1,509 g, rim speed 47.1 m/s.
The same radius at 5,000 g would need N = √(5,000 ÷ (1.118 × 10⁻⁵ × 15)) = 5,461 RPM, a rim speed of 86 m/s, which is more demanding than a perforated basket should be asked to bear; for high g use a small solid bowl. The reason is in Centrifugal Force Calculations: the maximum g falls as the radius grows.
Why Chemistry Adds Hazards
Chemical work adds three things the milk separator does not have: a corrosive or flammable liquid, a heavy cake of solids that loads the rotor unevenly, and a need to wash and empty the machine between batches. The materials of construction must resist the liquid; stainless steel resists many liquids but not all (see Stainless Steel Production), and the right choice depends on the liquid. Crystals recovered after cooling a hot solution, as in Recrystallization, are a natural basket job.
What You Need
- A rotor of the right type, a casing around it to catch the liquid, and a lid
- A drive that can start gently; see Speed Multiplication and Belt Drives
- A heavy base with vibration isolation; see Vibration Isolation
- A way to see the speed; count revolutions against a clock
Common Mistakes
- Choosing a basket for a slimy solid. The cloth blinds and the machine floods and goes out of balance.
- Loading unevenly. A cake that builds on one side is an imbalance that grows during the run.
- Using the dairy rotor’s speed for a bigger basket. The stress goes as the square of the speed and radius.
- Feeding a liquid the lining cannot resist.
Safety
Chemicals plus a spinning rotor
A chemical centrifuge adds splash, fumes and fire to the usual rotor hazards. Corrosive or flammable liquid sprayed from a damaged casing can burn or ignite; keep the machine in a ventilated space away from open flame and never use a flame to test for vapor. The rotor can also burst; see Safety Considerations.
What’s Next
- Learn next: Basket Centrifuge — filtering crystals
- Combine with: Sedimenting Centrifuge — settling fine solids and parting liquids