Gold settling velocity & Centrifuging Gold

The equation points out the importance of particle size in determining terminal velocity which occurs when drag and buoyancy forces balance out gravity. Initial settling velocity when drag forces can be neglected is independent of particle size, but can be reached rapidly for small particles. Free settling from zero initial velocity makes it almost virtually impossible to separate a fine and dense particle from a coarse, light one. Initially drag forces are negligible and the two particles accelerate at a rate dependent on their density alone. As the settling velocity increases, the drag forces increase proportionally, and very quickly reduce the acceleration rate of both particles more rapidly so for the fine heavy particle. Very soon, the velocity of the large light particle catches up to and then exceeds that of fine heavy.

If settling could be stopped at the exact time the two particles reach the same velocity, the difference in distance traveled would be in the range of few particles diameters.

Practically this means that very few separators operate in the free settling mode. The equipment try to do a separation that is based on size or a combination of size and density. To minimize the impact of particle size, feeds are sometimes narrowly sized. This is referred as size preparation.

Other aspect that gravimetric equipment consider is the particle-particle interaction with the double objective of maximizing the period of initial acceleration, which should follow a collision in which a particle is brought to a near complete stop, and making use of the ability that small particles have to settle between large ones (interstitial trickling), this can be done by operating at higher percent solids. As Spirals evolved over the past forty years, has made possible better operation with higher percent solids. It can be done by mechanically fostering interactions between particles. Jigs create a cyclical motion which is meant to create some initial acceleration and interstitial trickling for part of each cycle. Shaking Tabling equipment is meant to prevent much settling and bank on the differential action of wash water versus that of inertia and friction (between table deck and particles). For many years, this meant impractical and there was a mismatch between the need to go to a finer separation sizes, as ore bodies were getting leaner and liberation sizes finer and gravity separation which could not deal with particles below 100 µm very well.

Fine particle technology has evolved based on planar units such as Vanners, Slime Tables and Bartles-Mozley concentrators. Some applications have been limited in practice due to low capacities, mechanical and operating difficulties, and high capita and operating costs. Thus, more and more manufacturers have turned to centrifuge units.

The main centrifugal units are Knelson, Falcon, the Mozley Multi Gravity and the Kelsey Centrifuge jig. It is important to mention that centrifuges are not only the product of the past thirty years. For example, Knudsen bowls have been used for more that 60 years for gold recovery. Classification by Hydrocyclones make use of centrifugal forces, closer to gravity separation, dense medium cyclones, Dynawhirlpools and tri-flos are also centrifuge units, but whose primary separation principle remains dense medium. Specific to gold recovery, baffles in a sluice creates turbulent eddies that prevent the riffles from packing, but also a centrifuge force whose rotation axe is parallel to and slightly downstream from each baffle. Disappearance of the eddies invariably results in a dramatic drop of the sluice performance.

The basic principle of probably all centrifuge separators.