Arrastra

An arrastra (or arastra) is a primitive mill for grinding and pulverizing (typically) gold or silver ore. Its simplest form is two or more flat-bottomed drag stones placed in a circular pit paved with flat stones, and connected to a center post by a long arm.

The word "arrastra" comes from the Spanish language arrastrar, meaning to drag along the ground.

[2] Arrastras were suitable for use in small or remote mines, since they could be built from local materials and required little investment capital.

At cleanup, the gold amalgam was carefully recovered from the low places and crevices in the arrastra floor.

The amalgam was then heated in a distillation retort to recover the gold, and the mercury was saved for reuse.