Gravity Separation

1. Introduction

Separation by density difference is a process that is as old as recorded history. Separation of gold by density difference dates back to at least 3000 BC as depicted in writings from ancient Egypt. The principle employed in gravity separation goes back further in time to the formation and weathering of the rocks and the releasing of the minerals they contain and the transport of the mineral grains by heavy rains. It is the driving force for the formation of the alluvial deposits of precious metals and gemstones that have been worked since beyond recorded history as they still are today. Archaeological excavations have discovered mineral concentration activities such as the lead–silver concentrating plant in Attica, Greece, dating from 300 to 400 BC. So gravity separation has a long history as a mineral concentration process.

Not all mineral combinations are amenable to this type of concentration technique. To determine the suitability of gravity separation processes to a particular ore type, a concentration criterion is commonly used. A concentration criterion (CC) can be defined as

where SG = specific gravity (or density), and the fluid is typically water or air.
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What is Shaking Table

Shaking tables, also known as wet tables, consist of a sloping deck with a riffled surface. A motor drives a small arm that shakes the table along its length, parallel to the riffle and rifle pattern. This longitudinal shaking motion consists of a slow forward stroke followed by rapid return strike. The riffles are arranged in such a manner that heavy material is trapped and conveyed parallel to the direction of the oscillation. Water is added to the top of the table perpendicular to the table motion. The heaviest and coarsest particles move to one end of the table while the lightest and finest particles tend to wash over the riffles and to the bottom edge. Intermediate points between these extremes provides recovery of the middling (intermediate size and density) particles.

Shaking tables find extensive use in concentrating gold but are also used in the recovery of tin and tungsten minerals. These devices are often used downstream of other gravity concentration equipment such as spirals, reicherts, jigs and centrifugal gravity concentrators for final cleaning prior to refining or sale of product.