Tuesday, 4 May 2010

Are rod and ball mills set to become obsolete?

This was the provocative question I asked delegates at Comminution '10, during the final summing up of the conference.

At the beginning of the 20th century the advent of rod and ball mills led to the demise of stamp mills, and rod and ball mills then dominated 20th century grinding circuits.

Are we about to see a new revolution, with rod and ball mils superceded by circuits involving HPGR/SAG mills and stirred mills?  It was apparent at the conference that HPGR are being increasingly utilised, while the use of stirred mills, originally designed for ultrafine grinding, is extending slowly into the coarser grinding ranges.


  1. I hope not!

    I'm in the process of designing a rod-ball mill circuit for a new mill in North America (refurbished equipment).

    In my opinion the mining industry has always been slow to change to newer technologies as no one wants to be the first to try something new (and potentially fail).

    This is more so sen in grinding circuits where the capital expenditure is significant. Do we stick with proven technologies or take a risk on new unproven technologies that may give us efficiencies down the line.

    In true mining fashion I'm going to err on the side of caution!



  2. The problem with rod-ball technology is that it has limited capacity (in today's world) for a one line plant. And it requires relatively fine crushing to make it work.

    SAG (or its natural variant AG) grinding is universal in that it works on any ore, is usually done wet (environmental credit) and if designed properly, can give steady tonnage for maximum recovery. The only real criticism is the high energy consumption on hardest ores.

    HPGR does offer energy saving in certain cases but since it is dry and does not always work well, it should only be evaluated in terms of a valid trade off study with a properly sized SAG mill. Fortunately, the problem of being off JK's chart of operating hard ore plants has now been resloved by the introduction of SAGDesign testing which has been acknowledged as being equal to JK in measuring ore hardness. So even though there may be no operating plants to compare with, a valid SAG mill design can be done using SAGDesign technology.

    The potential for using HPGR is real and I thank you Barry for pointing this out. We can deal with that challenge.


  3. Ron Weissenberg6 May 2010 at 12:19

    Please can you assist with the contact details /parties involved in SAGDesign testing. We are running 3 ball mills on Mohs 6 - 7 hardness ore and need to look at alternative technologies.
    Regards/ Ron

  4. Certainly can Ron, but I do not want to put an email address on here without permission. Could you send me an email, and then I will send you the info.

  5. There are good discussions of this topic on the LinkedIn groups Minerals Engineers, and Grinding Mills Technology

  6. I am looking for a roll crusher for abrasive mineral to reduce from 12.7 mm to 2 - 3 mm not higher and to be able to work with 85% solids. the problem is to have a lining to resist and work with tin mineral (highly abrasive), please comment dear friends.thanks Jorge

  7. Barry is on the right track. In processing base metal ores, at least, the -1" + 100 mesh size range is like Death valley, to be traversed as quickly as possible. While machine sorting is emerging as a viable technology for +1" material, and flotation reigns supreme minus 100 mesh, there appears no waste discard technology available for the -1"+100 mesh size fraction, except for (under-appreciated) jigging. The move towards lower ore grades and higher tonnages (open pit mining) will be outpaced by increasing energy costs. This will drive the industry inevitably towards machine sorting in the pit and/or jigging in the Mills, so as to move and process only valuable ore "chunks".
    Quick now: How many jigging experts do you know?

    Gus van Weert
    University of British Columbia, Canada

  8. In addition to lower energy costs, stirred mills appear to be easier to install, hence a reduction in installation time and structural/civil costs can be expected.

    I believe Cadia-Newcrest are active in installing a Vertimill in a primary duty (after the SAG mill).

    Michael Braaksma
    Senior Process Engineer at Gekko Systems, Australia

  9. It's an interesting question, and I'm sure if you had a room full of metallurgists you would get a room full of differing opinion.

    From an energy perspective I can see a time where the primary grinding circuit may be HPGR/Tower mill, and regrinding will utilise the IsaMill or SMD. Using a Tower mill as a secondary grinding mill is not new. BHPB Cannington have an AG mill followed by a Tower mill.

    From an down stream processing perspective the HPGR/Tower mill option may actually produce a partical size distribution that is not as "random" as the SAG/ball combination, and this may have a positive flow on for recoveries. This is in addition to the reported improvements to liberation when using HPGR.

    However, how do we get over the conservatism of the mining industry and try a "new" path for grinding? Further, would the HPGR/Tower mill option be able to handle the high throughput rates of an Escondida or Freeport Indonesia?

    Chris Greet
    Manager Metallurgy - Minerals Processing Research at Magotteaux, Australia

  10. The HPGR definitely has merit but how about another fine crushing alternative - Vertical Shaft Impactors. They require a lot less infrastructure to install and depending on circuit configuration have a wide range of product sizes they can produce from coarse ~10mm down to 400microns product ranges. The other big advantage to my mind is that they can take a wet (though not too wet) feed.

    Using a lab VSI we've generated some data on some polymetallic ore showing a significant difference in the size distributions of the various sulphide minerals produced and the host rock, potentially preferential liberation? We believe the crusher is breaking minerals along grain boundries effectively liberating the sulphides. At this end of this day if you can minimise energy input into gangue mineral breakage and maximise the energy used for valuable liberation you've got yourself a pretty effective comminution system.......it would be great to see some further research on this technology.

    The circuit of the future: VSI - stirred mill with perhaps a jig or flash float cell in closed circuit with the VSI and classifier..........

    Ben Murphy, Gekko Systems, Australia

  11. The discussion continues on LinkedIn's Minerals Engineering Group. Some of the comments are shown below:

    From Nigel Ricketts
    Manager Process Engineering and Innovation at AMEC Minproc:

    An interesting discussion. I must outline up front that I am not an HPGR convert yet. I am still waiting for more empirical evidence from independent operators.

    However, I think the demise of ball mills may come about from a different direction, in particular improvements in ore sorting technologies. We waste a huge amount of power and steel by grinding gangue minerals. We also overgrind due to the non-selective grinding from a ball mill.

    We are looking at one of Gekko's jigs at the moment as an ore sorting device to reject barren waste in order to minimise the size of a subsequent ball mill which may end up being a stirred mill.

    Mike Daniel
    Process Consultant at CMD Consulting Pty Ltd:

    Nigel supposing you get the empirical evidence. On what basis will you decide the HPGR is a winner or not ? You wont have any empirical data to support a conventional circuit to make a comparison.

    Elizabeth Lewis-Gray :
    Chairman & CEO at Gekko Systems

    The concept of heavies waste rejection from milling circuits is an excellent one and has not been widely applied. The Gekko InLine Pressure Jig has both low operating costs and water consumption and is ideal for this application.

    It would appear that there is a strong cohort of communition experts in Brisbane including you guys, JK and Sustainable CRC. Is there a group that meet regularly to debate this topic? Communition methodology is critical to optimising gravity recovery and we would be keen to get involved in any discussions and data sharing on this topic.

    Nigel Ricketts
    Manager Process Engineering and Innovation at AMEC Minproc:

    Mike, it would be interesting in 2 years time to have the operating results from Boddington and compare wear rates and particle size distributions actually achieved to those predicted from the test work program.

    I still think we should be researching ways of weakening and cracking ore mineral interfaces with gangue via means other than brute force:
    * Can we use EM radiation of a wavelength tuned to the bonding energy between particular minerals?
    * Could we use vibration in combination with an HPGR to "squeeze and shake" at the same time? This could put the rock interface in tension and then hit it with a spike in amplitude of vibration to crack it.
    * Can we use shock temperature changes to exploit differential rates of thermal expansion? Grinding in liquid nitrogen would be lots of fun to try.

    How much do we even know about bonding and de-bonding of minerals to each other? 3M have spent billions of $ on how to join things together, but nature seems to do a very good job of this by geological forces.

    Ray Shaw :
    Minerals R&D Consulting

    Nigel, There is some interesting work around on using pulsed microwaves and/or high voltage electric pulses to weaken and break certain ores. Aled Jones of Nottingham University gace an excellent presentation on their work recently (google search would find it) and Selfrag have put out some results.
    Both have found some increased liberation in the 500um-1mm area which is the lower end of the valley of death. Suspect both still have some way to go to be truly comercial and question on separation remains.

    Also some work on floating coarser particles (Graeme Jameson, IWRI ??) which suggests is possible but I have no recent information on the actual stage of development by them and/or any others.

  12. Regarding the end of Rod/Ball mills; we are big fans of stirred mills, or rather, high intensity mills and see them as the best chance of reducing power costs and maximising recovery. The ability to use inert grinding media is probably as much value as the liberation. But I can’t see the end of ball mills just yet. HPGR’s are good but with their coarse tail, edge effect and flakes maybe requiring disagglomeration are not the best direct feed for a HIM, no matter what the size. Our experience is that they struggle with amounts of topsize/oversize material. For me it’s HPGR’s, ball mills for preparing the feed to HIM’s for more liberation and less energy use. And of course ultra fines flotation for more recovery.

    Michael Battersby, Maelgwyn Mineral Services, UK

    1. Mike,

      I hope you remember me - I gave you some clothes in Angola after yours were pinched - great times!
      Well, I am also looking at ball mills versus stirred mills for an iron ore project in Brazil.
      On the face of it, the HPGR/HIM would seem the best option, but I am going with HPGR/ball mill to 75 um just because it is more industry standard.
      Do you have any more info on HIM Vs ball mill capex and opex at a grind F80 of 2mm and a P80 of 45 um?

      Rob Riggir

  13. Gus, another physical separation method that is too often overlooked in base metal applications is DMS (Dense Media Separation). I have been involved with numerous projects that looked into the use of DMS for different reasons and many of them produced promising results. DMS generally works really well for stringer type mineralizations and even lower grade base metal ores often yield high mass rejections of ~50% at moderate metal losses of 1-5%. DMS is a standard process in the diamond industry, but for some unknown reason has not been accepted as a suitable treatment option for base metal ores. I predict that the industry will start to look into DMS and other forms of pre-concentration more closely in the future as more lower-grade deposits are being evaluated. The higher energy costs on a metal unit basis in low-grade deposits will increase the need to reject coarse waste prior to grinding.

    Oliver Peters, Metpro Management, Canada

  14. Oliver, You are correct, of course, any technology that rejects +100 mesh material that costs more to further process than the value of its contained payload (plus the costs of rejection) should be considered. DMS certainly qualifies, and is well established, of course. I should have mentioned it.

    Gus Van Weert, University of British Columbia, Canada

  15. To use DMS a density different must be exploited at coarse sizes, and DMS is widely used on Pb-Zn ores where the ore is usually a narrow replacement vein in a limestone or dolomite host. It was also a commom technique with tin ores in Cornwall, where the narrow mineralised lode was in a host of barren light country rock such as granite. Apart from these, exploitable density differences do not exist with most base metal ores.

  16. Going back to the HPGR/ Tower mill discussion, there seems to be a lot of evidence that the HPGR-stirred mill would offer a more energy efficient grinding circuit than a SAG/ball mill circuit. My query would be whether you will get the same liberation advantage from the HPGR when using a HPGR in combination with a stirred mill. I would have thought that you would need some sort of impact breakage after the HPGR to take advantage of the micro-cracking along the grain boundaries. Has there been any research comparing liberation in a HPGR/ball milling circuit with HPGR/stirred milling circuit?
    Chris Anderson, Hatch Africa, South Africa

  17. The HPGR technology has a future in many energy-usage optimized circuits, let alone circuits where particle micro cracking has shown significant improvements in downstream recoveries and grades. Their application is increasingly becoming a normal consideration in mineral circuits of the future.
    Winchester Maphosa
    Weir Minerals, South Africa

  18. I have come into this excellent discussion late. i need to go back through the previous contributions in more detail, but thought i would throw in a few comments now:
    HPGR seems to offer benefits and has won many advocates. My concern from the few installations I have seen is dust generation, but this can be solved.
    As lower grade ores are treated, the role of pre-concentration whether by DMS, high capacity jigs, or optical/radiation sorting should become more important, Why mill more than you need to?
    Regarding grain size. The finer particle size from stirred mills may be expected to reduce concentrate filtration rate, but there appears to be a trade-off with higher grade/higher proportion of hydrophobic particles and/or narrower size distribution. It would be interesting to hear more about the effects on tailings deposition and water recovery.
    Ian Townsend, Larox, UK


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