The main centrifugal concentrators used to improve the gold recovery are the Knudsen bowl, the Knelson Concentrator, the Kelsey centrifuge Jig, the Mozley Multi-Gravity Separator and the Falcon. The oldest form of gold recovery, the sluice, uses centrifugal forces developed by vortices downstream from riffles and its efficiency can be severely restrained if sluice geometry or flow rate are not within the appropriate range, as the centrifuge action of the vortices can completely disappear. Even when operating efficiently, its lower recovery limit is around 100 µm. The Knudsen bowl is probably the oldest centrifugal concentrator for gold recovery. Although the Knudsen bowl is rarely mentioned in the literature on gold recovery, references are more often not anecdotal and metallurgical performance in unavailable. Probably, applications are limited to alluvial operations.
The Knelson concentrator is one of the most successful gold centrifugal concentrator and is actually preferred especially to process circulating loads with grinding circuits. It uses a fluidized particulate bed as a gold trap. Centrifugal forces, a theoretical 60 g, are high enough to force all particles into the riffles until these reach about 37-42% solids by volume. Particles are then rejected as they are prevented to enter the riffles by particle collision. The High collision frequency maximizes the importance of the initial acceleration phase, which obviously favors very heavy particles, nearly irrespective of their size, at least down to 15 to 25 µm depending on the particle size distribution and specific gravity of the gangue. Because of the injection of fluid to create a bed, the Knelson may be compared to a packed column hydrosizer that produces separation based on solids specific gravity and much less on particle size.
The batch Falcon, like the Knudsen, is basically a rotating bowl in which a slurry fed from a central well. Upon initial feeding of the equipment, abed quickly grows, unselectively, and rapidly approaches its final profile. Material is then recovered selectively in a mode that is extremely size and specific gravity dependent. After a certain time, the recovery sites saturate and recovery drops dramatically. It finally stabilizes at a level near or at zero. The actual concentrate bed is covered by a layer of feed particles that saltate their way out of the recovery zone. This third layer and sometimes part of the concentrate bed can be eroded by the feed flow, leading to erratic performance. The three recovery phases are first, initial unselective recovery; second, selective recovery that can rapidly drop as the bed saturates; and third, stable and near zero recovery. Second recovery mode is acceptable, and then only inasmuch as concentrate bed saturation is not too rapid. Particles report to the bed in two ways. Fine particles are collected when they are lodge in capture sites created by the asperities of the concentrate bed. Coarse particles can partially bury themselves in the concentrate bed, which they can also erode away. Recovery at intermediate particle sizes is lowest, as they too coarse for most of the capture sites, and too fine to gain enough momentum to impact the concentrate bed.
The Continuous Falcon is similar in geometry to the batch unit. However, concentrate is removed through a slot that circles the bowl near the top section. The concentrate production rate is controlled by periodic removal using twin valves that are sequentially actuated. This yields a very high density concentrate, but the twin valve system limits yields to values of about 10-14% at the optimum feed arte. A version with nozzles replaces the outer valves. Its yields can be much higher, but some control over the removal rate is lost. It can be partially regained by adjusting rotating velocity. Falcon can yield good recoveries, but at low upgrading ratios.
The Kelsey Centrifuge jig is a jig spun around a rotating axis with a theoretical 60 g of acceleration. The feed access the rotation bowl through a central downcomer. The ragging sits on top of a screen whose opening is slightly larger than the largest heavies to be recovered. The concentrate is recovered in radial hutches at low density, typically around 5% solids. Water is pulsated as it is with regular jig. The Kelsey Jig is capable of very high recoveries and high grade concentrates. Some models include a distinct ragging and others rely on an Autogenous ragging of intermediate density particles.
The Mozley Multi-Gravity Separator is a wrapped around shaking table. It is operated at lower centrifugal forces than other centrifuges, typically 5 to 25 g. effective separation of cassiterite at sizes down 10 µm are reported. The rotating action of the drum provides a high gravity force which pins the heavy particles to the drum surface to be removed via the drum scrappers. The shaking motion combined with the selective use of water gives an excellent cleaning effect for maximum particle concentration.