The Collaborative International Dictionary of English v.0.48:

pachuca tank \pa*chu"ca tank\ (Metallurgy)
   A high and narrow tank, with a central cylinder for the
   introduction of compressed air, used in the agitation and
   settling of pulp (pulverized ore and water) during treatment
   by the cyanide process; -- so named because, though
   originally devised in New Zealand, it was first practically
   introduced in Pachuca, Mexico.
   [Webster 1913 Suppl.]