Monday, 18 May 2020

Comminution: developments and thoughts over the last decade

Comminution '20 was the first of the MEI Conferences to be postponed until 2021. We were expecting a record turnout, with news of all the recent developments taking place in comminution.
As a substitute I have thrown together edited highlights of some of the developments and thoughts on comminution which took place during the past decade. This is by no means a comprehensive review, it is intended to illicit discussion from comminution specialists from around the world, many of whom would have been in Cape Town last month.

Grinding is evolving and changing fast, with innovations in high pressure grinding rolls and stirred mills threatening to make the tumbling mill, which has been a stalwart for well over a century, obsolete. At the final panel discussion at Comminution '14 (posting of 5 May 2014), Tim Napier-Munn said that in terms of the future of comminution "we really have to get rid of tumbling mills". Are rod mills now finally obsolete? Ball mills would have dominated comminution conferences little over a decade ago, but they are mentioned only rarely now.
SAG mills are still of major importance, but 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question at Comminution ’12 whether ball mills would play a significant role in comminution circuits, or would they be superseded by SAG mills.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 would 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. Stirred mills, unheard of in mineral processing a few decades ago, are now increasingly used for ultrafine grinding applications.  At the SAG '15 conference in Vancouver, Chris showed how ISAMill™ technology has progressed from the original Mount Isa Mines ultrafine grinding applications. Larger ceramic media is now pushing the boundaries of feed size and can offer advantages in grinding efficiencies, product size distribution and internal wear.
At the 2012 SME Meeting in Seattle I was discussing SAG mills with someone 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! There was a lot of debate on the blog posting of 25 August 2014 which asked "Where are SAG mills going?".  An article from Weir Minerals suggested that there was an increase in demand for larger cone crushers that are matched with large high pressure grinding rolls for customers who want to replace SAG mills in order to increase efficiency. Utilising cone crushers and HPGRs allows ore to be processed from 250mm to 50mm in cone crushers, then down to less than 6mm from HPGRs.
In his keynote lecture at Comminution '18, Holger Lieberwirth asked whether SAG mills will still be relevant in 50 years’ time. Maybe they will be replaced by circuits containing only High Pressure Grinding Mills, which are crushing ever finer, and stirred mills, adopted for untrafine grinding, but whose upper particle size limit is being pushed towards coarser sizes. At SAG '15 in Vancouver, Paul Staples of Ausenco, Australia, asked whether SAG mills are losing market confidence. Although a mature technology, he said that a number of recent projects were not achieving nameplate capacity, but at Comminution '18 John Starkey, of Starkey & Associates, Canada, a company well known in SAG mill design, showed how single stage AG/SAG milling has the potential to reduce operating costs and increase profitability significantly when properly designed, installed, operated and maintained.
Mining is energy intensive, and grinding is responsible for consuming about 40% of the energy in the whole mining chain. Inefficiency in grinding has long been an outstanding problem, in particular when production of fines and ultra-fines are considered. Unlike milling, crushers are much more energy efficient, therefore it is logical to push the comminution process towards the crushing stage for energy efficiency, said Hamid Manouchehri of Sandwik, Sweden, at Comminution '18. Furthermore, crushing is done dry which reduces water consumption and related potential water contamination. Hamid said that finer crushing could be achieved through design of new crushing chambers, introducing more energy and higher rotation speeds in the crushing chamber, etc.  At the same conference Hakan Benzer, of Hacettepe University, Turkey, explained how novel energy efficient comminution circuit flowsheets incorporating energy efficient dry comminution technologies such as HPGR, Vertimill etc. have the potential to result in significant energy savings.
At Comminution '16 Gerard Van Wyk of ThyssenKrupp Industrial Solutions, Germany, asked if dry final grinding with HPGRs could be the next step ahead in mineral comminution? Historically, HPGRs have been used mainly as tertiary crushers in mineral applications for the production of ball mill feed. In the cement industry, however, HPGR systems have been successfully applied for grinding limestone, clinker and slag to a final product fineness (P80) of between 30 and 90 µm without the need for downstream ball milling. The total energy consumption of HPGR finish grinding systems in the cement industry has been found to be 30 to 50 % lower than in ball mill systems. This leads to the question of whether the same methodology can be adopted in the mineral industry. Such a step would require the use of dry rather than wet grinding systems.
The comminution circuit is usually made up of comminution devices operated in closed circuit with different types of classifier. The closed circuit arrangement can have separate comminution and classification devices linked through pump-sump arrangements or integrated comminution-classifier systems. It is well documented that the choice and operation of the classifier have a major influence on the performance of the comminution circuit as a whole. An inefficient classifier can increase the energy consumption of the comminution circuit and in most cases also compromise the quality of the product reporting to downstream processes, leading to losses in recovery of the valuable mineral.
Although Prof. Alban Lynch has been involved with hydrocyclones for very many years, in his conversation with me (MEI Blog 11th August 2014)  he said that "the way they are used now is an absolute nonsense, with circulating loads in some cases of well above 200%. The future is high frequency screens.....it is very clear that these screens are so much better than hydrocyclones."
By classifying by size-only, screens, compared to hydrocyclones, give a sharper separation with multidensity feeds and reduce overgrinding of the dense minerals. Derrick Corporation is the leader in this field and at Comminution '18 Nic Barkhuysen, of Derrick Solutions International, South Africa, said that replacing the ubiquitous cyclone cluster with Stack Sizer screens creates additional capacity, improved mineral recovery and a simultaneous reduction in power consumption.
At Comminution '16 Elizma Ford, of Mintek, South Africa, evaluated the potential throughput benefit of adopting Derrick fine screening technology and  concluded that it is becoming apparent that the ability of these machines to accurately classify by size only at efficiencies in the mid 90% range, as fine as 45 micron, has resulted in a paradigm shift in milling circuits, replacing hydrocyclones in the closing of secondary and tertiary circuits. At Comminution '18 Martyn Hay, of Eurus Mineral Consultants, South Africa, also emphasised that over the past decade there have been a number of success stories where cyclones have been replaced by wet screening resulting in improved grinding efficiency, higher throughput, lower operating work index, better liberation and increased recovery in downstream flotation. He highlighted that inefficiencies in classification efficiency account for the majority of metal loss from the milling/flotation process as well as excessive mill power draw.
The last major comminution conference before the Coronavirus pandemic was the European Symposium on Comminution and Classification, held in Leeds, UK last September. In his plenary lecture, Malcolm Powell, of the University of Queensland's JKMRC, and a regular contributor to MEI's comminution conferences, said that it is high time to dramatically upgrade historic empirical comminution models, that are based on back-fitted breakage rates, to mechanistic models. He presented an approach to embracing the available computational power and the progress in understanding of comminution systems to rewrite models to be predictive and reliable with respect to the range of conditions to be encountered in the current and future devices we use in industry. Underpinning such an approach is the need for appropriate measurement of breakage properties that include mineral association, that respond to the range of conditions encountered in comminution equipment for mineral processing.
Simulating comminution processes is one of the most complex tasks in mineral processing research and the Discrete Element Method (DEM) is one of the most widely used tools. DEM has provided the ability to resolve the complex phenomena experienced by ore within comminution devices such as tumbling mills. The new developments in DEM techniques and the corresponding increase in computational power has made it more feasible to study the movement of individual ore particles as they traverse a tumbling mill. Modelling of energy distribution in tumbling mills is also being increasingly investigated using positron emission particle tracking, a technique now really proving its worth in understanding comminution and flotation processes, as is coupled DEM and SPH (smoothed particle hydrodynamics).
At the Comminution '14 panel discussion (MEI Blog 5 May 2014) Wolfgang Peukert of University of Erlangen-Nuremberg, Germany, said that much could be gained if the science of comminution and industry could come closer together. It is apparent that modelling now gives us a greater understanding of what is going on in a milling circuit, and there is a lot to be gained from detailed modelling. The models, however, must be checked by reality to give us a reliable toolbox to assess what is happening, particularly with complex multi-phase particles which can be characterised to assess liberation, the balance between strength of grain boundaries and strength of grains.
An interesting debate on liberation was on the blog posting of 16 January 2014, where I asked the question "Is anyone researching liberation enhancement". Prior to this, Frank Shi, of Australia's JKMRC gave an interesting paper at the European Symposium on Comminution and Classification in Germany in 2013. He outlined the programme of work on electrical comminution by high voltage pulses which has led to a number of publications in Minerals Engineering over the last few years. Pre-weakening ore particles and preferential liberation of minerals at coarse sizes are the two major outcomes that may have potential benefits for the mineral industry. He described a novel particle pre-weakening characterisation method by single-particle/single pulse, developed in collaboration with the Swiss company SELFRAG AG.  Dr. Shi discussed the emerging challenges to bring electrical comminution to the mineral industry, including scale-up for industrial application, hybrid circuit design, maximisation of pulse-induced cracks and study of the downstream processing effects.
Gregor Borg, of PMS GmbH, Germany and Martin Luther University Halle-Wittenberg, Germany, showed at Physical Separation '19 how the innovative VeRo Liberator® applies a mechanical high-velocity comminution principle, where numerous hammer tools rotate clockwise and anti-clockwise on three levels around a vertical shaft-in-shaft (hollow shaft) system. The resulting high-frequency, high-velocity impacts cause a highly turbulent particle flow and trigger fracture nucleation and fracture propagation preferentially at and along mineral boundaries. Breakage of coarser particles occurs from the high-velocity stimulation of bulk ore particles, where the elasticity and compressibility modules control differential particle behaviour. The improved breakage behaviour results in reduced energy consumption and very high degrees of particle liberation in the relatively coarse fraction of the product.
Grinding of complex massive sulphide ores consumes vast amounts of energy, and extremely fine mineral dissemination leads to relatively low concentrate grades, and high metal losses, not only in the flotation tailings, but into the ‘wrong’ concentrates, penalties often being imposed for the presence of zinc and lead in copper concentrates. Way back in 2013 (MEI Blog 3rd June 2013) I asked "is there a technique currently available that could eliminate the comminution step in the treatment of these important sources of base metals?" Well, yes there is, and not only could it remove the comminution stage, but also the difficult and inefficient flotation stage! It may seem economically impossible, but it has been proven at pilot stage to be viable. Noel Warner, Emeritus Professor of Minerals Engineering at the University of Birmingham, UK, has often talked passionately of the process that he and his team at Birmingham developed for the treatment of polymetallic massive sulphide deposits. The process was direct ore smelting, but despite its attractions, the process has never been used at full scale, but it was suggested (MEI Blog 3rd June 2013) that it should be looked at more closely.
Alan Muir, Vice President Metallurgy at AngloGold Ashanti, South Africa also asked whether comminution could be eliminated from the mining process in his keynote lecture at Comminution '14, as he felt that current comminution activities were rapidly becoming unsustainable. He suggested that comminution might be removed from the gold mining process completely by moving directly to in-situ liberation and leaching.
I have no doubt that comminution and concentration techniques will continue to evolve, but will there be a time when they lose the battle, when the remaining ores are so finely disseminated and intergrown that they can no longer be treated by physical methods? Is no mineral processing the future of mining, and will the future be direct hydrometallurgical and pyrometallurgical routes?
Hopefully much food for thought here, so let's have your views please.

18 comments:

  1. Nice summary of the last years, Barry, thanks!
    Daniel Parvaz, Lightning Machines Science and Communication, UK

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  2. A great insight into evolution and development of Comminution with respect to change in Ore characteristics.
    Jyotirmaya Sahoo, Arcelor Mittal Nippon Steel India Ltd

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  3. A great summary of the evolution in comminution process...
    Daniel Jordán, Process Superintendent LBUG Project, Chile

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  4. Really good Barry, please keep us informed!
    Marco Vera, Manager Recovery Innovation Process Technologies, Australia

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  5. Very eloquent elaboration of the comminution developments in the last decade, the discussion on the efficiency of the screen circuit over the hydrocyclones is indeed one that needs further discussion in terms of data collection from operating plants that have replaced either one of the operating units. The other notable mention of the Ultrafine grindinding mills involving ceramic grinding media is one that is worth pursuing. I was personally involved in the installation and commissioning of the two units in one of the platinum mines in RSA and the initial results were very positive.

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  6. It's also important to consider pre-concentration technologies to reduce the volume of material that requires grinding and the implications of finer grinding for tailings dewatering and storage.

    Jason Maguire, University of Queensland

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    1. I agree Jason. I see pre-concentration as a way that existing operations can look to reduce their comminution energy requirements without significantly changing their circuits or equipment. While not every site will be able to benefit from pre-concentration, I am convinced that many can. Also, tangible downstream processing benefits to be had as well.

      Michael Myllynen, Magotteaux.

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  7. No doubt that replacing classifying cyclones with screens has been one of the important developments in comminution efficiency and flotation performance with ball mills and hopefully will become easier to sell soon.
    Roger Strickland, Tablelands Mining Group, Australia

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  8. Nice round-up and I hope you are all keeping well. You referred to the 2012 conference and I also remember talking you about the need for comminution to change, so that it is not simply about blanket size reduction. Rather we need the amount of comminution that is suited to the required purpose, i.e. if you want to implement coarse size based preconcentration (and the ore is amenable), how do you ensure the comminution generates a size distribution that will maximize the value from the sorting.
    Ted Bearman, Bear Rock Solutions Pty. Ltd., Australia

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    1. Hi Ted. All well here thanks, and I hope you and Claire are coping with the crisis. Thanks for mentioning preconcentration (and you too Jason). This is something I want to include in a future review article on physical separation methods

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    2. Thanks, we are keeping well. I look forward to your next post.
      Ted

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  9. Barry,
    I appreciate such summaries, periodically, so that people do not reinvent the wheel.
    I have some thoughts . comments as a holistic perspective

    a) I firmly believe that "comminution" should never be looked in
    isolation--blasting and subsequent beneficiation should be in focus
    while talking on crushing/grinding machines.
    2)I still believe discussions on which machines to be used for
    comminution will remain a discussion point unless we have more data on
    the material(ore characteristics-including whether the ore is
    friable/W.I./ hardness/grain size distribution i.e. more PhDs on the
    relation between ore characteristics and breakage relationship is well established.
    d)Screens or cyclones are not a big issue. I go to screens only when
    it gets established that all values would be in either screen oversize or
    undersize. I am sure we will never have such clear phenomena; we will
    always have gangue in products from a screen.
    Screens have limited design/operating variables to easily and quickly
    change,.If tonnage to plant or ore characteristics change, while in
    cyclones one can shut off a few cyclones in a battery/ can change flow
    rate/spigot diameter and so on.
    ONLY A HOLISTIC UNDERSTANDING AND CLEAR KNOWLEDGE OF EACH UNIT
    OPERATION should be the starting point.



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  10. Good summary Barry,

    I will add to the summary:
    - Testwork: The past decade has seen a growing number of projects valuing the benefit of conducting proper variability ore characterization testwork and Geometallurgy approach in the design of comminution circuit. The cost of variability/Geometallurgy testwork is matching the cost of pilot testwork as known in the past using 30 t of ore.
    Subsequently, there has been development of simplified testwork for standard tests such the Bond Ball Work Index to reduce the sample requirement, cost and duration per test.
    - Influence of "External" factors: The Mine to Mill concept, Pre-concentration (ore sorting, Dense Media separation, mag sep), "Grade engineering" and Coarser flotation have influenced probably more this decade than the previous, the design of comminution circuits.
    - Equipment size: While this decade has seen the growing in size of many equipment ( Metso 3 m HPGR, MP2500 cone crusher,... ), the size of the SAG/AG mill did not increase significantly
    - Circuit trends: Single Stage SAG mill and pre-crushing before the SAG mill topics have been studied extensively in addition to HPGR (tradeoff SAG vs HPGR), Fine milling.
    - Control of comminution circuit and equipment: Many improvements overall + the addition of new ones such as the MillSlicer

    Regards,

    Johnny T. Kalala

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  11. SAG mills were adopted to solve a particular set of problems in the Industry, so it is probably reasonable to ask if those problems remain relevant in the present and future -- and whether or not alternative technologies will rediscover the problems that are long since solved.

    Example problems:
    - dry processes like crushing & screening make a lot of dust; wet milling is preferred for industrial hygiene.
    - simpler flowsheets, a lot of crushers & conveyors were replaced with a single SAG mill -- but then the industry started adding crushers and conveyors around and before SAG mills, so do we still have simpler flowsheets?
    - SAG mills require less labour and were easier to automate than crushing plants. Show me a conveyor that doesn't spill and I'll show you a conveyor that isn't running.
    - Cold weather climates benefit from an indoor SAG mill; no need for fine ore bins which have a nasty habit of freezing into ice-pops in -30°C temperatures. As my Peruvian taxi driver once commented: "what do you mean, minus temperature? How is that even possible?"
    - Capital costs tended to be lower for SAG circuits, at the expense of higher power & consumables operating costs. Give an MBA the option of saving CAPEX or OPEX and I'm sure you can guess the answer.
    - Tumbling mills were easier to scale up than competing technologies in the 1970s/1980s. The 21st century hasn't resulted in bigger rod mills, but it certainly has given us bigger crushers and large alternative (stirred) mills.

    Final thought: I think it is fair to say that the industry knows how to size SAG mill and ball mill circuits -- even though as Paul Staples said, you can still screw it up if you try hard enough. There is value in a McNulty Type 1 curve associated with boring, reliable, and old- fashioned tumbling mill technology.

    Alex Doll, Alex G Doll Consulting Ltd., Canada

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  12. Great roundup Barry! Very interesting
    Carly Leonida, UK
    Freelance technical writer and editor, owner/author of The Intelligent Miner, European editor at Mining Media Intl

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  13. Thanks for a great summary Barry.
    Do you know that Loesche is interested in marketing their technology in the Minerals space?
    Regards,
    John

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    1. Thanks John, and yes, very much aware of that. Loesche has been a major contributor to MEI's comminution conferences, and was a sponsor of the last two ('16 and '18).

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  14. Mahmut Camalan5 June 2020 at 13:34

    Dr.Wills, thank you very much. This is a useful review. I hope that we can see this review on a future paper. I would like to raise some additional points:

    1. There seems to be two distinct sets of studies: (i) The studies focusing on particle breakage phenomena, (ii) the studies focusing on empirical or semi-empirical models of size-reduction processes. They are rarely connected with each other. For understanding size-reduction processes and developing permanent models, these two groups must be evaluated together. I feel that the only fundamental information that we currently have on size-reduction operations is still the grinding laws. However, fragmentation patterns and impact mechanics are widely studied in applied physics, astronomy, etc. These studies may provide insight on the fundamentals of size-reduction processes.

    2. There are some evidence regarding the positive effects of ultrasonic cavitation on grain-boundary cracking of materials in water. Could we perhaps use this cavitation technique to a flowing slurry as a pretreatment or direct breakage method ?

    3. Laser spalling is acknowledged in petroleum science to fracture rocks for (i) oil production, marble cutting (ii) and (iii) concrete waste spalling. It requires very short processing times and small energy inputs. Could we arrange lasers for pretreatment or breakage of rocks during gravitational flow of dry ore streams ?

    Regards

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