Thursday, 14 March 2013

Where is comminution going?

Last year at Comminution ’12 I opened the conference by discussing the quiet revolution which has taken place in comminution.

For most of last century there had been little development in comminution machines and circuits, cone crushing being followed by ball milling, rod mills sometimes acting as tertiary crushers. In the latter part of the century, however, autogenous and semi-autogenous milling became more prominent, and then high-pressure grinding rolls began to play an ever increasing role, as did stirred mills, the latter allowing the exploitation of ultra-fine grained ores which had hitherto been impossible to economically treat.

This revolution continues, so that comminution circuits today are very much different to those of the mid-20th century with their parallel lines of small ball mill-cyclones.

Typical 20th century ball mill circuit
The rod mill is almost an obsolete device, and I asked the question at Comminution ’12 whether ball mills would play a significant role in comminution circuits, or would they be superseded by SAG mills. Chris Rule, of Anglo Platinum felt that rod mills will play an insignificant role, as they are severely limited in terms of size, and ball mills may play a diminishing role as the upper feed size range of stirred mills increases.

This is interesting as last month at the SME Meeting in Denver I was discussing this with someone, I can’t recall who it was, who had heard that many operations were having to increase the proportion of steel balls in their SAG mills in order to improve performance, effectively converting them slowly back to ball mills.

So what are your opinions on this? How do you see future comminution machines and circuits evolving. Does the future lie with HPGR and stirred mills? These are all subjects which I hope will be discussed further at next year’s Comminution ’14.

15 comments:

  1. The answer to your question will depend not only on the successful applications of new technologies, but also on throughput, hardness variability, water availability and the commonly underestimated "resistance to change". There is no “one size fits all”. Certain traditional technologies might very well be the best option for smaller scale applications. Water scarcity (and access to alternative water sources) should be one of the drivers. Breaking more rock for less energy, to exactly the (economically) right particle size and not beyond - that is where we finally need to get.

    Romke Kuyvenhoven, Gecamin, Chile, via LinkedIn Minerals Engineers

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    1. I appreciate, Romke, that individual circuits will be designed according to the factors that you mention, but I am looking more to the general trends in comminution this century. Will there be a move away from tumbling mills to HPGR and stirred mills, in the same way that, at the beginning of last century, stamp mills were gradually superseded by tumbling mills?

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  2. Dear Barry Wills,

    The future is certainly for energy efficient size reduction machines. We commissioned one roller press to work in tandem with a wet grinding ball mill in the year 2003 at Jhamarkotra Phosphate concentrator, India. The energy consumption details may be seen here:
    https://www.researchgate.net/publication/234128538_Energy_Consumption_in_a_Phosphate_Concentrator_Jhamarkotra_India?ev=prf_pub and

    https://www.researchgate.net/profile/Dmr_Sekhar2/?ev=hdr_xprf

    Thanks,
    DMR Sekhar

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  3. Generically speaking there will be growing importance for processes requiring less power and water. Both are becoming scarce resources.From that point of view single stage or double stage crushing followed by HPGR with single stage ball milling would be attractive alternative.But it is not possible to generalise, as for high tonnages the circuit may be limiting and AG/SAG circuit after single stage crushing may be suitable.
    In fact one of the reasons for the advent of AG/SAG mills has been the capacity limitation of 3 stage crushing circuits & ball milling , particularly for hard ores, even though power consumption wise it is the most efficient. Also the wear pattern of HPGR for hard and abrasive ores is yet to be established and is one of the limiting factors when compared to conventional circuits with grinding mills.Rod mills are however already a vanishing entity and their use is now very limited.
    To be practical ultimately it is the cost per t of product that will decide.This is the reason why all available technologies of size reduction co-exist.
    Thanks
    Dr. A.Bandyopadhyay (Bandyo), India, via LinkedIn

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  4. You raised a real issue; for me it is exposing the dearth of good fundamental (relevant) work on this issue. I wish you were present during my talk at IMPC. This was one of the questions I raised.

    There are literally millions of papers on comminution, very high sounding; but:
    1) We still use Bond's (a great man for me) to estimate the power and
    design a mill.
    2) We still have not identified a single parameter which we can measure and press a button on a computer to know whether conventional circuit/sag mill/HPRP to be selected.; still an empirical measurement and highly time consuming process to take a definite route with confidence.
    3) 50 years back we were told (Great Gaudin, if I am right) that only one percent of the energy is usefully spent in comminution. Even to ay we do not know whether we moved from number and if so how much.
    4) The points I am driving at are "all these so called mathematical (while they call cyclone model is empirical) have not thrown new light on the subject.
    Best wishes, Rao
    Prof. (Dr.)T.C.Rao (India)

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  5. We must think to basic regulations which we are following since 100 years ago,
    Why the shape of mill must be like current rotary mills (cylinder)? Why we must rotate one heavy unit to make movement in our balls, rods or bigger size particles? why we generate huge noise? ....
    First we can look to the basic phenomenon in size reduction which is almost including three main factor:
    Impact, Pressure and Attrition

    The most energy-efficient is Impact, so I think the future will be belonging to Impact!! The Scientists will think how they can use Impact phenomena to break fine/ultra fine particles?
    For example I think everybody accept that if we shoot even one very small particle with enough speed to a solid/tight plate, it will break. So how we can shoot a lot of fine particles by an especial equipment to that plate same time to break all? how we can modify such ideas to be applicable in larger scale?
    Abbas Tabatabayi, Katanga Mining Co., DR Congo

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  6. I agree with the insight that energy and water will be scarce and expensive enough to promote the birth and growth of alternatives to the current rotary mills. Anyway, as a first measure, rotary mill design should be critically reviewed for segregation that seem to encapsulate larger particles in the core of the tumbling mass, where the fracture promoting impacts are largely absent. Residence time distribution measurement of mineral particles as a function of particle size may bring useful information about the extent of segregation. A non-uniform diameter of the mill shell and changes in lifter design may be useful in changing segregation patterns.
    Waldo Valderrama, Santa Maria University, Chile, via LinkedIn

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  7. HPGR may be seen as the extension of the crusher focusing of stress to the grinding size range. Their effectiveness is limited by the stress level approaching the roller surface material strength. A mean to reduce such stress level is superimposing an ultrasonic field, whose effect is to initiate crack propagation at a lower stress, but at the cost of producing a coarser product. We need to find other non-mechanical means of augmenting the stress in the bed, like microwave pulses or simultaneous electrodischarge incorporated in a second generation HPGR.
    Waldo Valderrama, Santa Maria University, Chile, via LinkedIn

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  8. There has been research work on this topic that was conducted at the University of British Columbia in Vancouver and was reported in the SAG 2011 conference.

    A PILOT-SCALE EXAMINATION OF A HIGH PRESSURE GRINDING ROLL / STIRRED MILL COMMINUTION CIRCUIT
    J.A. Drozdiak and B. Klein, S. Nadolski A. Bamber

    “In this paper we examine, through pilot-scale testing, the possibility of operating a circuit comprised of two stages of HPGR comminution, followed by grinding through a horizontal stirred mill. In order to assess whether the novel circuit design could achieve the reduced energy requirements indicated in the literature, two more-established circuits, a cone crusher / ball mill and an HPGR / ball mill, were examined using a combination of testing and flowsheet simulation. The results showed that, based solely on the specific energy requirements for comminution, the HPGR / stirred mill circuit achieved a reduction of 9.2% and 16.7% over the HPGR / ball mill and cone crusher / ball mill circuits, respectively.”

    http://www.isamill.com/EN/Downloads/Downloaded%20Technical%20Papers/A%20Pilot-Scale%20Examination%20of%20a%20High%20Pressure%20Grinding%20Roll-Stirred%20Mill%20Comminution%20Circuit.pdf

    Regards
    Michale Young, Xstrata Technology, Australia

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  9. Interesting discussion!

    Regarding water consumption/scarcity as a future driver for development of comminution technology: cyclones, gravity separation, leaching and flotation all require water. So to what extent will reducing water consumption in comminution be a driver for further technological advances if it is just required to be added later on? Furthermore, will the impact of reduced comminution water consumption outweigh the potential for increased dust generation and associated health issues?

    I very much appreciate that ore pre-treatment has been mentioned. Whether it be microwaves, high voltage pulses or some other as yet unidentified technology, I think there is a lot of scope in this area ( thoughI may be biased as I work on high voltage breakage). Significant energy saving and liberation improvement have been demonstrated, so 'all' that remains is to do this at economic energy inputs (what is economic?) and large throughputs, and most importantly, to get these technologies accepted by operators.

    I also think it is interesting no-one has mentioned pre-concentration so far. There is a clear potential to sorting material and only milling that portion of an ore that has financial value. Has the industry lost faith due to low tonnages achieved so far or is this still an actively developing area?
    Klaas Peter van der Wielen, selFrag AG, Switzerland, via LinkedIn

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    1. Hi Klaas, first of all congratulations on your new job. Might see you in Switzerland sometime- my favourite Alpine country! Sorting is very much an actively developing area (see MEI blog posting http://min-eng.blogspot.co.uk/2013/01/what-is-future-for-electronic-sorting.html ). It would be great to see you in Cornwall again in June for Physical Separation '13 ( http://www.min-eng.com/physicalseparation13)- you will see from the blog posting that sorting will be an important feature at the conference.
      Barry

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    2. Hi Klaas, good to see you at the CSM Annual Dinner last weekend. I like your comments on ore pre-treatment, energy savings and liberation improvements. We at Grinding Solutions regularly look at new methods of improving fracture in crushing and grinding applications, the benefits of in grain boundary fractures are enormous in many subsequent concentration processes, especially leaching. HPGR is often mentions but don't forget the VSI's they can also give significant benefits. We specialize in ultra fine grinding which are quite energy intensive so correct management of the process is crucial to ensure the lowest possible energy consumption for the maximum possible gain, time spent on control of these processes can save thousands in operating costs. Keep up the good work at seFrag and catch-up next time you are over in Cornwall. Nick Wilshaw Grinding Solutions Ltd. Cornwall, UK.

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    3. Thanks Nick. I am pleased that you and Klaas mentioned liberation, as many people seem to think that comminution is all about breaking rocks in the most efficient way. I hope that future research and circuits will also be aimed a promoting grain boundary fracture and hence enhanced liberation. By doing so, grinding can be carried out to coarser sized products, reducing energy consumption and improving the efficiency of downstream processes, by increasing concentrate grades and recoveries.

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    4. Hi Barry, as a supplement to your comment: Grain boundary fracture, or liberating minerals at or very near their in-situ grain size should be the ultimate goal for comminution, but it does not necessarily go hand-in-hand with improved performance of downstream processes. Being able to liberate sulphides at 300 microns is great, but this is quite coarse for flotation circuits so further grinding may still be required and you end up losing all the benefits of the coarser liberation. Alternatively, if you liberate coarse enough you can move away from flotation to, for instance, gravity concentration again for sulphides but this requires quite a paradigm shift in process design and operation.

      Of course this argument only holds for coarse-grained ores, but it does illustrate that if this discussion moves towards liberation, it may need to encompass more than just the future of comminution.
      Klaas Peter van der Wielen

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  10. Certainly the trend is to reduce the grinding cost. Hence, application of HPRG & stirred mills are gaining importance. However, when clayey, moist ores with differential hardness of rocks occur to reduce the capital cost of crusher- screens SAG mills are still open. The usage of rod mills has been slowly dispensed with modifications in tumbling mills like- cylindrical pebble mills. Crush more and grind less coupled with pre-concentrating devices[ if possible] is the current philosophy. The age old concept of rougher concentrate regrinding and regrinding of middlings for improvement in grade and recovery may still continue if a proper G-R curve is to obtained and if viability supports it.
    BP Ravi, VSK University, India

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