What were arrastras used for in the patio process?

In the context of the patio process, arrastras were used as the initial stage for preparing silver ore.

These arrastras were shallow, circular pits paved with stone. To process the ore, large blocks of stone were attached by beams to a central rotating post and dragged around the pit by mule power. This mechanical action pulverized the raw ore, reducing it to a fine mud or pulp.1

Once this grinding process was complete, the resulting fine material was moved to a courtyard or patio to undergo the chemical stages of the patio process, which involved mixing the ore with mercury, salt, and copper sulfate to isolate the silver.1