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question at Comminution ’12 whether ball mills would play a significant role in comminution circuits, or would they be superseded by SAG mills.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.

@barrywills