Centrifugal concentrator

Centrifugal concentrators were originally developed to improve gold recovery from alluvial sands. Since about 1990, they have been used increasingly for hard rock mines, which is now the largest area of application. Centrifugal concentrators consist of a vertical rotation bowl with a series of concentric rings that trap the gold. A centrifugal force is applied on the ore particles, in such a way that this force is 60 (in the case of Knelson) to 300 (in the case of Falcon) times higher than the gravitational force. The rotor is accelerated and feed slurry is introduced to the concentrating cone through a stationary feed tube. Upon reaching the deflector pad at the bottom of the cone, the slurry is driven outward to the cone wall by the centrifugal acceleration. As slurry flows up the cone wall, the solids fill each ring to capacity creating the concentrating bed. The tailings product overflows the bowl and the gold becomes trapped in the rings. Some centrifuges have a smooth wall at the bottom of the bowl where stratification takes place. The high density gold is concentrated at the wall forcing and displacing lower density particles away from the wall. Compaction of the bed in the rings can be prevented by introducing pressurized fluidization water from behind the rings. This helps the high-density gold particles displace the lower density gangue particles causing the gold grade to increase in the concentrating rings with time. After a period of time, the feed is stopped and the rotor is shut off. The concentrate is flushed from the cone into the concentrate launder and can be upgraded further by panning.

Deya machinery is the main manufacturer of centrifugal concentrator in China, in the concentrator it has a ribbed rotating cone into which the pulp of 20 to 40% solids is fed and the concentrate is accumulated in the riffles. Compaction of the concentrate layer is avoided by injection of water in counter flow. This water fluidizes the concentrate bed and allows fine gold particles to penetrate into the concentrate layer.

For hard rock deposits, the ore must be ground before feeding to the centrifuge. The feed should also be screened to below about 1 mm to remove coarse material, which can be ground further and fed back to the separator. These centrifuges recover between 15% and 60% of the gold, depending on the mineralogy of the ore. In large scale mines the tailings
are usually processed by cyanide leaching.

There are many models of centrifugal concentrators with capacity up to 100 tonnes of ore/h. For a small-scale production, a centrifuge with 1-2 tonnes/h capacity is more than enough. This uses a motor with 1.5 hp and 80-140 liters/min of water.

Basic knowledge for miners
• Gold pans, sluices and centrifuges are all methods of “gravity concentration”
• Gravity concentration works because gold settles faster than other minerals (e.g., sand) in water
• Gravity concentration works best when the particles in the feed are close to the same size—screen the ore before concentrating, and regrind the oversize
• Using mercury in sluices and centrifuges does not improve recovery—indeed, gold is often lost along with the mercury. These practices release much mercury to the environment and should be banned in all countries.

• Panning
-Panning is an efficient and very low cost method of gravity concentration—unfortunately, miners can only process small amounts of ore in a day.
-There are many shapes and sizes of gold pans that are used by miners in the world

• Sluices
-Sluices are as efficient as panning, but a miner can process much more ore per day with a sluice
-Sluices work best when the water/slurry is fed at a constant rate
-Use an old diesel drum to feed the water at a constant rate
-Miners should experiment with different kinds of carpets and riffles
-Control the slurry flow velocity by adjusting the slope angle to be just steeper than the angle where the sand clogs the carpet
-Keep the sand out of the spaces in the carpet fibers in order to trap the gold
-Adjust the sluice width and slurry depth to fit the feed rate, kind of ore and grain size, the slurry density and trapping mechanism
-Most gold is captured in the first 1/3rd of the sluice
-Use short sections of sluice (consider using a zigzag configuration if possible) to limit acceleration of slurry
-If the slurry flowing over a sluice is deep, the pulp can speed up too much and lead to loss of gold; shallow slurry (e.g., 1 cm deep on a Nomad carpet) does not accelerate as much as deep slurry
-Use different trapping mechanisms and/or slope angles to capture coarse and fine gold
-Copper amalgamation plates are a form of sluice that should be banned in all countries

• Centrifuges
-Centrifuges can be simple and relatively inexpensive; these centrifuges recover about the same amount of gold as a copper amalgamating plate
-More sophisticated centrifuges (Knelson and Falcon) are more expensive and require a clean constant water supply to operate, however these centrifuges can recover more gold
than other forms of gravity concentration equipment
-Putting mercury in centrifuges leads to the loss of fine gold with the floured mercury