Thursday, 6 May 2010

Maybe rod mills are at the dusk of their usefulness?

My last blog posting has already had two interesting comments, and I think a lively discussion is building up on LinkedIn, on the Minerals Engineering and Grinding Mills Technology groups.

One of the comments, by Dave Almond, Global Product Manager, Process Control Systems at FLSmidth Automation, USA, I highlight here:

This is an interesting question. Although new rod mills were being installed in PGM's as recently as 8 years ago (Northam Platinum) and are still installed in phosphates I do think they are at the dusk of their usefulness. There are limits to the length of the rods manufactured and consequently rod mill sizes are limited to fairly small dimensions (~15' diameter) and thus capacity is limited and not really suited to the low grade, high capacity operations being built today. Pity in some ways as rod/ball circuits were more energy efficient than the new SABC circuits of today.

I am less inclined to agree that ball mills have reached the end of their usefulness. Once again mainly due to the economies of scale needed to make many new projects viable and stirred mills also have limited capacity making multiple lines necessary which in turn brings back the old headaches of maintenance intensity and associated costs. Ball mills are now as large as 28' diameter (yes, the first ones have been ordered for a copper project in Peru) making single line, high throughput grinding lines still very practical.

HPGRs are also still limited on throughput resulting in the same issues as for stirred mills. They may eventually get there but the large HPGRs installed today already have grinding rolls exceeding 100 tonnes in mass! These are not small components to change out!

I guess to answer your question, my perspective is that SAB circuits will still be around for some years to come. Rod mills are about done. HPGRs are interesting but not a silver bullet or that innovative given that roller crushers were used in the same applications years ago and the Randol conference in Perth (2005) seemed to support that HPGRs worked better at medium pressures than high pressures. These machines all have their place under the right conditions (Mogalakwena North in SA and Boddington in Australia seem to be good examples). What is a little worrying is that many minerals processing professional consultants I have talked with claim they are beginning to be concerned that they are being asked to justify selections of equipment types based on trends/fashion rather than good scientific investigations.

I would be interested to hear other perspectives on this subject as it is an interesting one.

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