Gravity Separation or Hydraulic Wash

Gravity separation (also called hydraulic washing) exploits the DIFFERENCE IN SPECIFIC GRAVITY (density) between the valuable ore and the lighter gangue that surrounds it. The ore is first crushed to a finely powdered form to expose the ore particles from the rock matrix, and this powder is then treated with a rapidly flowing current of water.

Because the ore particles are denser than the gangue particles, they settle and stay behind (or are carried a shorter distance), while the lighter, less dense gangue particles are picked up and washed away by the running water. No chemical change occurs at all in this method -- it is a purely physical, mechanical separation based on relative weight. …