As part of the Feasibility Study for the Las Cruces(Phi) copper deposit, Rio Tinto Technology carried out an in-depth evaluation of two hydrometallurgical processes. The two options examined were mild pressure leaching and atmospheric ferric sulphate leaching. Both were proved technically viable at pilot plant scale. The atmospheric ferric leach pilot plant was designed, constructed and operated by Rio Tinto Research & Technology Development (R&TD), Melbourne in order to evaluate areas of technical uncertainty, to generate process design data and establish the deportment of elements in the various process streams. This paper provides a review of the pilot plant operation and presents some of the results generated from this work. (C) 2000 Elsevier Science Ltd. All rights resented.