Milling circuit selection for Nkomati 375ktpm concentrator


DRAMP was approached by the ARM/Lion Ore JV during July 2006 to conduct a feasibility study and Control Budget Estimate for a new Greenfield’s project for their NKOMATI Ni mine 45km east of Machadadorp. The scope included the evaluation and review of previous studies as well as a techno-economic trade off on four possible comminution circuits ranging from conventional crushing and ball milling to fully autogenous milling. The following document encapsulates the thought process, decisions and the references used in selecting the final comminution route for the new 375ktpm MMZ concentrator.

Apart from the pilot test work performed, design simulations from a number of sources, performance results from a recently commissioned 100ktpm MMZ concentrator, together with an understanding of the geology and mineralogy of the ore body and possible variations in the characteristics of the ore body over the life of mine was assessed.

The results from all the above mentioned work indicated that it would be possible to treat the MMZ ore at the desired throughputs to the desired grind sizes in a fully autogenous grinding (ABC-type) circuit. Autogenous milling is generally considered as being one of the higher risk milling circuits. This is mainly due to the fact that the ore, which is utilised as the grinding media, could be variable in terms of hardness and mineralogy / geology. Nevertheless, it is a very popular route as the cost benefits, more specifically the operating cost, are normally very attractive. This is especially the case when large low grade ore bodies are evaluated. In North / South America, autogenous and semi-autogenous grinding circuits have been successfully employed during the past 20 years on large scale low grade operations.

Taking all the above factors into account, the DRA/Nkomati project team recommended an autogenous circuit with the option to add steel to both the primary and secondary mills (if required) for the new operation in order to create the opportunity for significant operating cost savings. One has to bear in mind that all the new design simulations and decisions are based on previous work and potential unknown variations within the ore body will never be fully understood until the pit is operational for some time.