Gravity Separators

Gravity separators classify dry, granular, free-flowing particles based on their specific gravity.

(Copyright Oliver Manufacturing Co., Inc., Rocky Ford, CO)

General Information

Gravity separators, or air tables, separate particles with very close specific gravities.

The particles are fed onto a large, slotted deck that is tilted slightly, as shown here. The deck vibrates and fans force air upward. Denser particles travel to the high side of the deck, while lighter particles roll off the low side. Hoods are sometimes used to protect surrounding areas from dust.

(Copyright Oliver Manufacturing Co., Inc., Rocky Ford, CO)

Equipment Design

During operation, material flows through the feeder, onto the vibrating deck. The vibrator shakes the material at a frequency of 200 to 600 cycles per minute, while fans force air upward through the particles. A diagram of a laboratory scale gravity separator is shown below with its key parts labeled.

(Copyright Oliver Manufacturing Co., Inc., Rocky Ford, CO)

The air flow and vibration cause the bed of material to fluidize . As this happens, gravity causes the denser particles to fall into riffles on the deck, while lighter particles land on top of the dense ones.

The vibrating motion of the deck causes the heavy particles to move upward between the riffles, toward gates on the higher side of the bed. The lighter particles roll down, over the heavy particles, and are collected on the low side of the deck in the discharge hopper.

The deck schematic below shows an overhead view of ideal deck operation. The particles fall into layers in the stratifying zone, then continue across the table to be separated. The size of the stratifying zone depends on the difficulty of the separation.

(Copyright Oliver Manufacturing Co., Inc., Rocky Ford, CO)

A set of controls are used to adjust the deck tilt, vibration frequency, air flow rate, and feed flow rate. This allows the unit to efficiently separate many different types of materials.

Usage Examples

Gravity separators are most often used in seed and grain operations. Stones, damaged or unfilled seed and other impurities can easily be removed, increasing the percentage of material that will germinate. The video below shows stones being removed from woodchips. Often they are used in series, when a pure finished product is desired.

(Video copyright Oliver Manufacturing Co., Inc., Rocky Ford, CO)

Gravity separators are also used in applications such as coffee bean production and insulation production.



  • Extremely effective separation method for smooth particles of similar size
  • Not effective for particles of varying size or non-flowing materials
  • Clean air source required


Oliver Manufacturing Company, Inc. , Rocky Ford, CO


Orr, Clyde. Particulate Technology . New York: The MacMillan Company, 1966. Print.

Rumpf, Hans. Particle Technology . Trans. F. A. Bull. New York: Chapman and Hall, 1990.      Print.

Seville, Jonathan; Ugur Tuzun and Roland Clift. Processing of Particulate Solids . New York:      Chapman and Hall, 1997. Print.


Michael Fein

Kelsey Kaplan