Over the decades, with decreasing ore deposit grades, miners have turned to economies of scale and bulk mining to gain an advantage. This has resulted in an inherent dilution of feed to the mill. However, a point is being reached for most ore deposits where the mill feed grades for the selective mining unit (SMU) sizes are too low to be deemed economic. So what can we do to improve mill feed?

Mineral processers have long had to consider ‘mineral liberation’. The smaller the grain size, the more likely a particle will contain pure or near pure mineral, which can then be separated and retrieved by various methods. However, what now needs to be considered by both mineral processors and miners is ‘waste liberation’. At what size does a particle have insufficient mineralisation to exceed the cut-off grade and can no longer be deemed ‘ore’?

Ore deposits are found in many forms; they can be vein hosted hydrothermal deposits, with short-range spatial variation in grade, or porphyry deposits, which are more disseminated and smoothed in grade. Another way of considering a deposit’s disposition of grade is to refer to its heterogeneity. Heterogeneity is the degree to which waste and mineralised material are distinguished in a deposit. SRK has developed methods to assess the heterogeneity of deposits which can then be exploited to determine how waste can be liberated.

However, knowing deposit heterogeneity is only part of an assessment for improving mill feed. It is also important to have an understanding of the relative hardness of mineralised and gangue material, as well as the deportment of mineralised material to the coarser or finer size fraction in the mill feed.

Miners can play a role in improving mill feed. Armed with the knowledge of the deposit heterogeneity, material hardness, and mineral deportment, certain processes and technology can be introduced ahead of the mill to remove waste from the feed or to segregate the mill feed into different streams for alternate processing methods.

Methods being investigated and developed by SRK include:

• SMU size definition – refine the SMU size to maximise opportunities to segregate waste from ore at that scale

• Differential blasting of ore – target zones of gangue and mineralisation in an ore block to exploit differences in hardness and mineral deportment, and thereby create coarser waste particles

• Screening of mill feed – exploit mineral deportment to create different ore streams at different size fractions or to allow outright rejection of waste. This can be done on blasted ore or after crushing

• In-pit crushing and conveying (IPCC) – though regarded as a means to reduce haul trucks, it also presents opportunities to segregate conveyed material through mineral sensing and sorting

• Mineral sensing and sorting – exploit certain characteristics of conveyed material to identify opportunities for bulk or particle sorting. Such sorting can operate in preconcentration (waste rejection) or scavenger (mineral retrieval) mode