Kennecott's Ammonia Leaching Plant was built in 1916 and was used to process the lower grade copper from the mines. A chemist named E.T. Stannard invented this advanced mining technology, and later became CEO of the Kennecott Corporation. In 1923, a floatation plant was added to the building where the "fines", ore less than 2 mm in diameter, were processed.
Source: McCarthy Kennicott Historical Museum