Gold Pinched Sluice Box
The Pinched Sluice: A high capacity concentrator that will catch fine grained gold is called the "pinched" sluice. It uses another concentrating approach to improve upon the sluice box.
Fig. 4.5: Plan and Sectional diagrams of a pinched sluice, showing the separation of high density material from low and medium density material at the end of the sluice.
Particles on the bed of a sluice box have a lifting force on them similar to aerodynamic force. The velocity of water is higher at the top of the particle than at the bottom. This velocity shear creates a lower pressure area at the top of the particle that "lifts" it up. The particle is then carried downstream until it again hits bottom.
This process is called saltation. It is repeated over and over until the particle leaves the sluice box. Particles of high specific gravity have lower lifting force per unit mass and tend to stay close to the bottom, producing a layered effect. If the pinched sluice has laminar flow, the layers become well defined, with virtually all the heavy minerals on the bottom.
When such a sluice is narrowed or "pinched" at the downstream end, the depth of the flow is increased making the layers easier to separate into concentrate and tailings. Particles larger than 1.65 mm (.06 inches) tend to roll rather than jump. This destroys the layering effect, making classification necessary to keep the layers stable. The pinched sluice must be operated with an even, dense feed. If this is done, very high recovery rates may be achieved.
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