aBSTRaCT MINETOMETaL is developing technology based on manipulating the ferric to ferrous iron ratio in acidic sulfate liquor to improve copper leaching from low-grade chalcopyrite ores that are not amenable to conventional concentrating and smelting. Despite many years of research copper leaching from these ores has proven very difficult with typically <50 per cent copper recovery being obtained. the technology being developed can be applied to heap leaching freshly mined ore, to existing heaps where secondary copper minerals have already been leached and to in situ leaching of primary sulfides. minetometal has undertaken an experimental program where low-grade copper ore with a mix of primary and secondary copper sulfide minerals has been leached in sulfuric acid liquor with controlled ratios of ferric to ferrous iron. the test work has been carried out on ground ore (p80><100 um) using bottle rolls and the program has also examined the effects of temperature, ph and total iron content. we have found that controlling the ferric ferrous iron ratio enables: . over 99 per cent of the copper present to be the leached in><7 days with no reduction in leach rate due to passivation. . the leaching of the chalcopyrite can be stopped by controlling the ferric ferrous iron ratio. . passivated ore requires further manipulation of the ferric ferrous iron ratio to restart the copper leaching. . manipulating the ferric ferrous iron ratio allows the sulfur present to be converted to either elemental sulfur or sulfate. . the preferred chalcopyrite leach reaction generates minimal heat and external heating is required to operate heap and or in situ operations at above ambient temperature. the work is continuing to better define the preferred operating window for the ferric ferrous sulfuric acid system.>