Wiktionary
n. A device formerly used by gold-miners for grinding auriferous quartz, crudely resembling a mortar and pestle, with the crushing element dragged around by horse or mule.
Wikipedia
An Arrastra (or Arastra) is a primitive mill for grinding and pulverizing (typically) gold or silver ore. The simplest form of the arrastra 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. With a horse, mule or human providing power at the other end of the arm, the stones were dragged slowly around in a circle, crushing the ore. Some arrastras were powered by a water wheel; a few were powered by steam or gasoline engines, and even electricity.
Arrastras were widely used throughout the Mediterranean region since Phoenician times. The Spanish introduced the arrastra to the New World in the 16th century. The word "arrastra" comes from the Spanish language arrastre, meaning to drag along the ground. Arrastras were suitable for use in small or remote mines, since they could be built from local materials and required little investment capital.
For gold ore, the gold was typically recovered by amalgamation with quicksilver. The miner would add clean mercury to the ground ore, continue grinding, rinse out the fines, then add more ore and repeat the process. 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.
For silver ore, the patio process, invented in Mexico in 1554, was generally used to recover the silver from ore ground in the arrastra.
Usage examples of "arrastra".
No signs of mining, no arrastra, no rusted picks or shovels, no evidence of placer mining.
She learned about stamp mills and the more primitive arrastra, used by the Mexicans and South Americans to crush gold-bearing rock.
The gaunt-faced man had fallen into the pit of the arrastra, and Dismukes had suddenly started ahead, shoving the beam over him.
Dismukes might never have been harnessed to the beam of an arrastra and driven like a mule, and his awful tread-mill toil in the terrible heat under the lacerating lash was as if it had never been.