Low grade ores- smelt, leach or concentrate?

Falling ore grades and more complex ore bodies anticipated in the future can be expected to lead to increased energy consumption and associated greenhouse gas emissions for primary metal production. Sustainability concerns have seen the mineral processing and metal production sector come under increasing pressure to address these issues, but choosing the most appropriate processing route for low grade ores is not always clear. A study was therefore undertaken by Norgate and Jahanshahi of Australia’s CSIRO, using life cycle assessment methodology to examine various alternative processing routes for extracting metal from low grade ores (down to 0.1% metal), particularly those of copper and nickel, in terms of their life cycle-based energy consumption (embodied energy) and greenhouse gas emissions. The results have been published in Minerals Engineering, Volume 23 Issue 2 (2010).

The processing routes examined included conventional concentrating and smelting, direct ore smelting, heap leaching, pressure leaching and in situ leaching. The results of the study indicated that the most appropriate route for processing low grade ores in terms of embodied energy and greenhouse gas emissions largely depends on the mineralogy of the ore deposit concerned. Provided no additional grinding of the ore is required as the ore grade falls then conventional concentrating and smelting is the preferred route for both copper and nickel where pyrometallurgical processing can be applied. However, if fine grinding is required (down to 5 μm) then the preferred routes are heap leaching and direct smelting, in that order for copper ores, and in situ leaching, slightly ahead of pressure acid leaching, for nickel ores.

The authors will be presenting a continuation of this work in a couple of weeks’ time at Nickel Processing ’10 in Falmouth, in a paper entitled “Assessing the sustainability of nickel laterite processing”. Given the anticipated expansion in nickel laterite processing, it is important that the various processing routes for nickel laterite ores, both existing and proposed, be assessed from a sustainability point of view. To this end, the authors have assessed a number of hydrometallurgical and pyrometallurgical processing routes for nickel laterite ores, using life cycle assessment (LCA) methodology, focussing in the first instance on energy consumption and greenhouse gas emissions. The results of this study will be presented at the conference, including a comparison of the various processing routes.