Ore selection and sequencing

A block model of the ore body is used to plan a mine. The first requirement is to identify ore to be extracted and discard waste, so as to meet target grade, generally in multiple analytes. Commonly, cut-off values are set for each analyte, so as to distinguish ore from waste. It will be shown that, if more than one analyte is important, this procedure is wasteful of ore and a composite cut-off function is preferable. A second requirement is to sequence the ore extraction so that the variability in ore grade is controlled: failure to do so will result either in low-quality ore being marketed, or excessive re-handling being required to blend the ore to reduce the grade variability. A third requirement is that the extraction sequence should be such as to limit the amount of movement of equipment to enhance the productivity of the equipment. The overall objective, to optimise the Net Present Value of the mined ore, is simply stated but complex in realisation, since so many factors, such as target grade, grade variability, equipment choice, equipment movement, and downstream blending all have alternatives which can be traded off against each other, and all of which contribute to the costs and benefits making up the total Net Present Value. There exist commercial packages for ore selection and mine planning. However, they are often of a black box nature (i.e. not transparent), and users are sometimes not fully aware of the criteria being applied. In particular, the treatment of quality as a multidimensional vector may be problematic. The purpose of this paper is to discuss some of the issues involved and to suggest an alternative set of approaches, which should complement existing commercial treatment. The discussion will be illustrated specifically by reference to the mining of iron ore, but the issues are relevant to a wide variety of mining situations. © 2012 Institute of Materials, Minerals and Mining and The AusIMM.