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An Introduction to Gravity Concentration

Gravity concentration is the separating of particles by their relative densities (specific gravity). Gravity concentrating has been used to process minerals for a long time, Agricola in De Re Metallic (1556) reported the use of gravity concentrating (jigging) by the Egyptians around 1000BC (3000 years ago). Gravity concentrating is still used in mining for separating the values from the waste; whether gold form rock, or coal from waste, or gems from the host rock.

PARTICLE SETTLING
The basic principle behind gravity concentrating is related to how a particle will settle in a fluid. By Stokes law, with no other factors being considered (free settling), particles of the same size in a fluid will achieve a settling velocity dependent on two times their relative densities in relation to the fluid density. Also from Stokes law particles of the same relative density will achieve a settling velocity dependent on the square of their size.

While simple settling defines the action of a single particle in a fluid, in actually practice there are many particles of differing density and size. In addition there are boundaries to the fluid which also impact the settling. These add drag to the particles, the drag forces of the fluid, other particles, and the vessel the settling is occurring in, as defined by Reynolds number for various conditions, effect the settling.

Settling is more complicated because of these other particles, and is the interrelation of three factors; hindered settling, differential acceleration, and consolidation trickling. In the simplest processes (such as in riffles or stream beds) the main actions are hindered settling and consolidation trickling. Jigging, tables, and even pans add differential acceleration.

In hindered settling, larger and heavier (denser – high specific gravity) particles will settle sooner than smaller or lighter particles, and move farther. Part of this comes from the ability of the larger heavier particles to push their way through the other particles, and part comes from relative fluid flow rates needed to keep particles in suspension.

In the differential acceleration, larger particles, having a bigger area to work on, move farther and faster than smaller particles with a similar density. Heavier (denser) particles (higher specific gravity) will start moving slower on an acceleration cycle and faster on a settling cycle than particles of the same size, they will also tend to move less on acceleration (inertia) and farther on the settling (again inertia).

In the final stages of settling, the consolidation trickling portion, the larger particles have stopped moving but the smaller particles will continue to move downward through the larger particles. This difference in movement will cause the heavier particles to end up or concentrate to the bottom of a bed, and the bed to have layers, with the smallest particles on the bottom. This is shown in Figure 1.

Figure 1 – Particle Settling Actions



In the above picture (figure 1) the shaded particles are heavier (higher specific gravity) than the open particles.

Figure 2 – Mixture of Wood and Steel Balls



A mixture of wood and steel balls of various sizes before applying gravity concentration. The wood balls have holes to help identify them from the steel balls.

Figure 3 – Results of Particle Settling



The same mixture after gravity concentration, showing the various layers. Layers of similar size and density materials have been picked off to help show the layering.
The above describes what happens with particles of the same shape, but particles are not all the same shape. Round and cubical particles will behave as described, but flatter particles distort this simple process.

Two particles of the same size and density will act differently if one is flatter than the other. The flatter particle will act as if it had a lower density, if the flat side is oriented in the direction of the fluid movement, and heavier if it is oriented opposite to the movement. Think of skipping rocks.

SETTLING SUMMARY
Hindered settling – how a particle moves downward through a mixture of water and other particles depends on it’s size, shape and density.

Differential acceleration – how quickly a particle moves depends on it’s size, shape and density.

Consolidation trickling – during final stages of settling, the larger particles have stopped moving but the smaller particles will continue to move downward through the larger particles.

In simple processes (jigging, tables, spirals, and sluices) the fluid is water (or in some cases air). In dense media the fluid is a mixture of magnetite, ferrosilicon or similar. Some operations have used dense liquids (salt solutions being the original, and now organic chemicals). Centrifuges and cyclones also increase the force by adding centripetal force greater than just gravity acceleration.