Is progress in energy-efficient comminution doomed?

There is clearly a lot happening in comminution at the moment (see also my posting Where is comminution going).

Progress, or the lack of it, in energy-efficient comminution will be the theme of the keynote lecture at next year’s Comminution ’14 conference in Cape Town. It will be presented by Prof. Tim Napier-Munn, former Director and now Consultant at Australia’s JKMRC. Tim is also a Director on the Board of the Coalition for Eco-Efficient Comminution (CEEC).

Tim’s abstract for the keynote is shown below. It is thought provoking, and we invite your comments, and hopefully your input to next year’s conference.

We are constantly told that comminution is the largest single user of energy on the minesite, and even that it consumes a significant proportion of the world’s electrical power. These two facts are indisputably true; comminution is very inefficient in its use of energy. It would therefore seem that improving comminution energy efficiency should occupy our industry’s capacity for creative thinking more than many of the issues we are asked to confront, especially in a world driven increasingly by the mantra of sustainability. So it is surprising that it barely rates as an issue when companies consider the development of a new mining project or look for operational efficiencies.

The 2012 CEEC Roadmap for Eco-Efficient Comminution taught us that although significant improvements can be made by applying what is already known, the size of the prize is not well understood within the industry. Also current project valuation and discounting practices tend not to identify comminution energy as a significant factor in project optimisation. Other than external regulatory forces, therefore, the motivation for improvement is weak.

There is good theoretical evidence that there is little we can do about it anyway; the physics is against us. In the 1970s Professor Klaus Schonert showed that the most energy-efficient way to break a rock was to place it between two opposed platens and load it until it fractured in tension. This simple mechanism seriously limits the options regarding real innovation in the comminution process. To support this view a glance at a drawing of one of to-day’s cone crushers will show that in all material respects it is identical to a cone crusher manufactured in the 1930s.

The same applies to jaw and gyratory crushers. This is good evidence that the best way to crush a rock was worked out a long time ago and the only improvements since then have been in scale, materials and process control, all of them important but peripheral to the main game.

Schonert’s work led to his invention of the HPGR as a way of increasing the rate at which the crushing action could be usefully applied. Despite intensive research, and early claims of high energy efficiencies, HPGRs have still only succeeded in niche applications in mineral processing after 30 years of trying. AG/SAG mills have dominated grinding because of their capital efficiency and operability rather than their energy efficiency. Stirred mills have been a genuine innovation in fine grinding but even they are prodigious consumers of specific energy. Novel flowsheets, especially those with more effective coarse gangue rejection, have been shown to be capable of significant reductions in overall energy consumption but they are by definition limited by the inherent efficiency of the unit operations they link together. So how are we to achieve the nirvana of paradigm change, say a 50-90% reduction in energy? And do we need to anyway?

We still do not know enough about the physics of the fracture of heterogeneous brittle materials such as mineral ores. Comminution science is really a branch of materials science, but materials scientists are only interested in the fracture event itself (read ‘failure event’), not the nature of the products of fracture as we are. The main job of comminution is mineral liberation, and recent research has taught us that liberation is a product of the texture of the ore and to only a limited extent the fracture mechanism. However we now know that the random fracture assumed by the early liberation models for mathematical convenience is not always the prevailing form. Non-random fracture leading to selective liberation of minerals along grain boundaries can also occur, and it is clearly in our interest to promote such liberation. How is this best done?

Other research has taught us much about what goes on in crushers, tumbling mills and stirred mills. But successful research does not necessarily lead to successful innovation. Innovation is a messy and expensive process with no guaranteed outcomes. How can we translate good research into good technology more effectively?

We really have no excuse for not achieving substantial progress in understanding the comminution process more completely, with the impressive array of experimental tools now available to us including computationally intensive modelling, automated quantitative mineralogy, tomography, breakage testing devices and the like. However capturing this knowledge in a useful form and transferring the lessons to industrial practice is as difficult as it ever was.

In deciding whether there is any hope for significant improvement in comminution energy efficiency, the Comminution ’14 presentation will consider the key technical and cultural impediments to progress, and speculate about how the innovation process may yet provide the long-sought paradigm change.