Method of and apparatus for extraction of gold from placer gravel

In a method of extracting precious metal from a precious metal bearing ore deposit, the improvement comprising the steps of:

• providing a rotatably mounted cylindrical drum having a substantially horizontally disposed axis, said drum having an input end, an output end, and an internal peripheral surface, said input end being substantially closed about an opening adapted for flowing a slurry of precious metal ore into said drum, the internal peripheral surface of said drum having a plurality of spaced annular riffles of predetermined height, and at the output end having a plurality of longitudinally circumferentially spaced riffles of a height less than said annular riffles,
• flowing a slurry of precious metal bearing ore through said opening in said input end at a predetermined flow rate while
• simultaneously rotating said drum at a predetermined speed about its axis whereby, as the tank rotates, the slurry is caused to flow continuously in helical fashion over the longitudinal and annular riffles causing settling of precious metal fines, the fines being retained adjacent the inner peripheral surface of said drum due to centrifugal force caused by said drum rotation, and non-precious metal bearing material not affected by the same centrifugal force being flushed from the outlet of said drum.