Attritor Ball Mills Used in Grinding

Stirred Media Ball Mills Used in Dry Fine and Ultra-Fine Grinding Grinding is one of the fundamental steps in size reduction processes across different industries, including mining, ore processing, cement production, and others. It is typically used in process flowsheets to liberate minerals, separate them, and micronize them in ore processing. Conventional grinding is typically performed by rotating drum (tumbling, horizontal) mills. These can be rod mills, suitable for relatively coarse grinding, and ball mills, which have grinding capacity practically down to finer sizes. In both types, a rotating drum containing grinding media in the form of rods or balls is used. As the mill rotates, the charge (material to be ground) is lifted and dropped, with impact and attrition forces grinding it to finer particle sizes. Conventional grinding with ball mills can reduce particles to approximately 100 microns. When economically viable, finer and ultra-fine grinding is performed using ball mills or stirred media mills. While the definition of fine and ultra-fine grinding varies, fine grinding is generally accepted to occur at P80 values of 100 microns (the sieve size below which 80 percent by weight of particles pass) and ultra-fine grinding at P80 values below 20 microns. Mills used in ultra-fine grinding were previously used in numerous industries, including pharmaceuticals, paints, clay, paint and pigment production; today, in the mining sector, these mills are increasingly being used to liberate gold ore, magnetite ore, copper-lead-zinc deposits, and platinum ore particles with grain liberation sizes of 15 microns or smaller. Ball mills are traditionally used for fine grinding of minerals. In these cases, smaller ball sizes are selected compared to those in conventional grinding to increase contact area and reduce collision intensity, and lower mill speeds are used. However, the efficiency of these mills decreases when grinding below 75 microns and is rarely economical around 30 microns. Stirred media mills are increasingly replacing ball mills for fine and ultra-fine grinding; they typically allow approximately 30-40 percent less power consumption compared to ball mills when grinding to the same product size. It is known that the specific energy consumption in a stirred mill is lower for the production of ultra-fine particles compared with conventional drum mills. Compared with conventional grinding, stirred mills have less material loss; since there is no oxidation, there is better heat transfer and easier handling of toxic emissions, so air purification equipment is not required. Closed-circuit and smaller media drum mills can be used to achieve finer grinding, but this option is limited due to effects on kinetic energy. As the grinding media size decreases, the effect on kinetic energy decreases and energy transfer during particle and grinding media (ball) contact decreases. These limitations are met by ultra-fine grinding mills with rotating stirrers and fixed mill tanks. Because they have finer products, they act on a larger surface area. Additionally, the grinding media (ball) size used in ultra-fine grinding (2–3 mm) is much smaller than that of conventional grinding (12–100 mm).

Why Dry Grinding?

Stirred media mills typically operate as a wet process, because water promotes mixing and distributes product particles to suppress reagglomeration. In dry grinding, reagglomeration can be reduced somewhat by the addition of grinding aids with lower surface energy, but even so, this leads to a much higher practical grinding limit compared with wet grinding and ultimately results in significant efficiency losses. However, in some cases dry grinding may be necessary. Dry grinding can sometimes eliminate drying or wastewater treatment costs that offset the loss in grinding efficiency, and may actually be required in arid regions where water supply is difficult. Dry grinding may also be preferred for chemical reasons; for example, stirred media mills stand out in the efficient grinding of cement clinker, particularly for special fine cements. Again, dry grinding is widely used in the preparation of mineral fillers for sectors such as paints and plastics. Dry grinding exhibits much less moderate wear compared with wet grinding. For the same size distribution, dry grinding requires 15-50 percent more energy than wet grinding regardless of mill type. Wet grinding can produce finer particles at the same energy level as dry grinding. However, the particle size distribution after wet grinding is much more limited than in the dry state. For the same mineral, the Fe content in the pulp of the subsequent process is significantly lower following dry grinding because this is associated with distinctly less media and less liner wear compared with wet grinding.

Stirred Media Mills

Efficient grinding of fine and ultra-fine particles requires small grinding media to increase surface area and the resulting collision frequency; however, ball mills are fundamentally limited in terms of momentum that can be applied to such small balls by gravitational force, which sets a practical lower limit. Stirred media mills overcome this limitation by applying momentum to the media via a shaft; this allows the use of grinding media at millimeter scales while still applying high collision intensities, depending on shaft (stirring) speed and optionally. Stirred mills can have high power densities ranging from 40 to 300 kW/m³ (compared with approximately 20 kW/m³ for ball mills) and can also significantly reduce facility footprint requirements for a given production volume. The first example of using a grinding tank and grinding media (gravel, sand, balls) for size reduction was in 1922 when Szegvari stirred a tank filled with pebbles to rapidly disperse a sulfur suspension. This was developed in 1928 by Klein and Szegvari into the first 'stirred ball mill.' This prototype was transformed into the Attritor mill, a vertical pinned mill with low stirring speeds (3–5 m/s) that has been produced in various forms by Union Process since 1946. In subsequent years, several other stirred media mill designs were developed, the most common types used in the mining industry are introduced below. Gravity-Based Stirred Mills In these mills, media motion is provided by rotation of a shaft; the speed is not sufficient to completely suspend the grinding media, and therefore at the top of the shaft, the media cascades downward under gravity. This settling allows the media to remain in close contact for efficient grinding and results in a load zone on top of the media from which the product can be discharged without requiring screens. To enable settling, the grinding media is relatively large; diameters typically range from at least 12 mm to several centimeters. The first gravity-based stirred mill, the Tower Mill (3 m/s), was developed by Japan Tower Mill Company in the 1950s, and in the 1980s, Metso developed the Vertimill, an adaptation of this technology. Early applications included wet grinding of limestone used for neutralization, and in the 1980s and 1990s, gravity-based stirred mills were adopted in the mining industry. [caption id="attachment_159018" align="aligncenter"] Figure 1. Gravity-Based Stirred Mill[/caption] Stirred Media Detritor (SMD), consists of a "vertical" octagonal tank and a high-speed (11 m/s), multi-layer pinned shaft that fluidizes the grinding media (ball diameters typically on the order of a few millimeters). This mill is similar to the attritor mill, but is actually derived from the high-speed sand grinder invented by Du Pont in 1948 for pigment grinding using Ottawa sand as the grinding media. In the 1960s, the British China Clay company (now Imerys) developed the SMD, initially using sand as the media, but in recent years sand has been replaced by small ceramic (alumina) balls. For the past 45 years, SMDs have been used particularly in kaolin and calcium carbonate grinding applications, producing particles down to 1-2 micron product size. Today Metso also manufactures these mills for use in ultra-fine grinding of metallic ores. [caption id="attachment_159019" align="aligncenter"] Figure 2. SMD Mill[/caption] IsaMill–Developed by Mount Isa Mines Ltd. and Netzsch and commercialized in 1999 for ultra-fine grinding of ore, is a "horizontal" disk mill. It consists of a horizontal cylindrical grinding tank in which the stirrer is a series of perforated disks attached to a central shaft. As in the SMD, it is considered a high-speed 'fluid' mill and it is possible to achieve very fine product sizes; P80 values as low as 7 microns are practical, made possible by relatively fine grinding media balls (4-8 mm). IsaMills have similar ultra-fine grinding efficiencies to SMDs when similar operating conditions (stirring speeds and grinding media diameters) are considered. [caption id="attachment_159020" align="aligncenter"] Figure 3. IsaMill[/caption]   References 1. A.Gupta, D.S. Yan, Stirred mills-ultrafine grinding, A. Gupta, D.S. Yan (Eds.), Mineral Processing Design and Operations: An Introduction (Second Edition), Elsevier (2016). 2. A.Jankovic, Variables affecting the fine grinding of minerals using stirred mills, Miner. Eng., 16 (2003), pp. 337-345. 3. A.Jankovic, W.Valery, E. Davis, Cement grinding optimization, Miner. Eng., 17 (2004), pp. 1075-1081. 4. A.Kwade, J. Schwedes, Chapter 6-Wet grinding in stirred media mills, A.D. Salman, M. Ghadiri, M.J. Hounslow (Eds.), Handbook of Powder Technology: Volume 12 – Particle Breakage, Elsevier BV (2007). 5. Á.Rácz, K.Bohács, F.Kristály, É.Gregus, G.Mucsi, Comparison of wet and dry stirred media milling from energetic and mechanochemical point of view, XXIX International Mineral Processing Congress, Moscow, Russia, 15-21st September (2018). 6. A.Szegvari, M. Yang, Union Process Inc., Attritor Grinding and Dispersing Equipment, Available online at: https://unionprocess.com/tech_papers/Attritor_-Grinding-and- Dispersing-Equipment.pdf (2019). 7. A.G.Doll, Fine Grinding, a Refresher, 49th Annual Canadian Mineral Processors Operators Conference, Ottawa, Canada (2017). 8. A.K.Chatterjee, Cement Production Technology: Principles and Practice (1st Edition), CRC Press (2018). 9. B.A.Wills, J.A.Finch, Chapter 7-Grinding mills, B.A. Wills, J.A. Finch (Eds.), Wills'Mineral Processing Technology (Eighth Edition)-An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery, Butterworth-Heinemann (2016). 10. B.D.Burford, E. Niva, Comparing energy efficiency in grinding mills, Metallurgical Plant Design and Operating Strategies (MetPlant 2008), Perth, Australia, 18–19 August (2008). 11. Chapter 10. Stirred mills—Ultrafine grinding. In Mineral Processing Design and Operations, 2nd ed.; Gupta, A.; Yan, D. (Eds.) Elsevier: Amsterdam, The Netherlands, 2016; pp. 287–316. 12. G.Mucsi, A review on mechanical activation and mechanical alloying in stirred media mill, Chem. Eng. Res. Des., 148 (2019), pp. 460-474. 13. H.Cho, J.Kwon, K.Kim, M.Mun, Optimum choice of the make-up ball sizes for maximum throughput in tumbling ball mills, Powder Technol., 246 (2013), pp. 625-634. 14. H.de Bakker, Energy use of fine grinding in mineral processing, Metall. Mater. Trans. E, 1E (2014), pp. 8-19. 15. L. Taylor, D. Skuse, S. Blackburn, R. Greenwood, Stirred media mills in the mining industry: Material grindability, energy-size relationships, and operating conditions, Powder Technology, Volume 369, 2020, 1-16. 16. M.R.Pryor, Chapter 7: Dry grinding, M.R. Pryor (Ed.), Mineral Processing (Third Edition), Springer Science & Business Media (2012). 17. O.Altun, H.Benzer, A.Toprak, U.Enderle, Utilization of grinding aids in dry horizontal stirred milling, Powder Technol., 286 (2015), pp. 610-615. 18. S.C.Chelgani, M.Parian, P.S.Parapari, Y.Ghorbani, J.Rosenkranz, A comparative study on the effects of dry and wet grinding on mineral flotation separation - a review, J. Mater. Res. Technol., 8 (5) (2019), pp. 5004-5011. 19. T.Yokoyama, Y.Inoue, Chapter 10-Selection of fine grinding mills, A.D. Salman, M. Ghadiri, M.J. Hounslow (Eds.), Handbook of Powder Technology (Volume 12), Elsevier Science B.V. (2007). 20. Union Process, The Evolution of Milling Technology [brochure], Available online at https://unionprocess.com/pdfs/up-corporate-brochure.pdf (2012).   Prof. Dr. Öner Yusuf Toraman Niğde Ömer Halisdemir University Mining Engineering Department