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Ultrafine Grinding 06
Prof. Hylke Glass, Camborne School of Mines, UK
What is ultrafine grinding and what role can ultrafine grinding play in the minerals industry now or in the future? This theme attracted representatives from equipment manufacturers, suppliers of grinding media, mining companies, consultants and academics to a conference in the seaside resort of Falmouth in Cornwall, UK (12-13th June 2006). The conference was attended by 64 delegates from 24 countries spread across all five continents.
While the concept of grinding minerals hardly requires an introduction, it emerged that the definition of ultrafine particles depends on the application: it can relate both to particles in the low µm size range and to particles which have been considerably reduced in size. Traditional barriers to ultrafine grinding are the relatively high energy requirement and a perceived reduction in efficiency during further processing. However, technological advances in ultrafine grinding may be welcomed by the mining sector, where the necessity to process increasingly complex ores is presently coupled to buoyant commodity prices.
While conventional milling techniques may produce an ultrafine grind, the required energy rises sharply as the product size, often characterized in terms of the 80 % passing size, d80, decreases. Ultrafine grinding techniques are those techniques which are more energy-efficient than conventional milling techniques in the sub 100 µm range. Other advantages over conventional techniques are the option to avoid contamination of the product with ferrous chips, for example from steel balls used as grinding media, lower wear for ultrafine applications, the ability to contain dust and manage heat generation. On the downside, D. Yan states that ultrafine grinding techniques are less predictable than conventional ball mills in terms of energy requirement and grinding performance while throughput may be limited.
During ultrafine grinding, particle breakage occurs by familiar mechanisms: impact or attrition by shear or a combination of these. Classic examples of impact techniques are jetmills where particles are either accelerated against a surface or against other particles moving in the opposite direction. While impact techniques are not suitable for breaking relatively tough materials such as quartz, modified jetmills may be useful for some tough applications. J. Roth (PMT-Jetmill) presented the spiral jetmill as an alternative to classic jetmill designs. The feed is injected into the milling chamber through tangential nozzles while the product is recovered through the classifier rotor located in the middle of the milling chamber. Intense shear between the particles in the milling chamber can produce delamination, preserving or even increasing the aspect ratio of particles. This is beneficial, for example, when talc is ground to enable separation of quartz prior to its application as a reinforcing filler in plastics. In another example, B.G. Kim added that it is attractive to preserve the flaky shape of graphite particles during ultrafine grinding in view of their high conductance.
A. Mc Veigh (Hicom International) discussed application of the Hicom mill for enhancing the recovery of kaolin from china clay waste minerals (silica and mica). Kaolin is commonly used a filler in paper, with platy kaolin enabling the production of lighter and smoother paper. In the Hicom mill, grinding is induced by the inability of the feed to follow the nutating motion of the milling chamber. Product particles escape through openings in the grinding chamber. The Hicom mill can be operated with or without the addition of grinding media. Using relatively fine grinding media, the Hicom mill achieved better delamination than a sand mill currently used by a china clay producer.
Evidence of high-intensity grinding in a Hicom mill was also observed by M. Thornhill, who ground potassium feldspar concentrates with a view to increasing the availability of potassium and assessing their potential as a fertilizer. Trials suggested that ultrafine grinding irreversibly increased the leachability of potassium, although a lower limit to the particle size was observed: very fine particles tended to agglomerate, reducing the specific surface area and the associated leachability of potassium. Comparison with ultrafine grinding of a potassium-rich nepheline syenite concentrate suggest that the effect of mechanical activation achieved by ultrafine grinding is mineral-specific. Although mechanical activation is a difficult concept, E. van der Ven pointed out that ultrafine metal powders can be hazardous on account of their extreme reactivity.
A. Mizitov (Oy Microworld) described the grinding action in a uniRim mill as the equivalent of a pestle-and-mortar. Grinding in the uniRim occurs between the cooled mantle of the grinding vessel and grinding blocks attached to a central rotor. Grinding also occurs well away from the rotor in a planetary mill, which consists of independently-rotating canisters located at the end of arms extending from a central rotor. The combination of centrifugal acceleration of particles towards the rim of the canisters and the even higher angular velocity of the canisters creates a high-intensity grinding environment. E. Kuznetzov (Cyclotec) provided a video to suggest that previous issues such as heat and dust generation, mechanical integrity and discontinuous discharge had been resolved. Scale-up to 100 t/hr is expected to be feasible.
M.K. Abd El-Rahman (CMRDI) studied ultrafine grinding of a variety of minerals in a planetary mill as a function of the rotation speed of the canisters and the presence and size of grinding media in the canister. The advantage of higher rotation speed levelled off at higher speeds, possibly due to cake formation on the canister mantle, while degradation of grinding media increased. Relatively fine grinding media initially led to reduced particle size reduction but, for prolonged grinding, enabled attainment of a smaller product size. This is thought to be due to the smaller interstitial space between media particles, which are generally much larger than the size of product particles. Perhaps surprisingly, a relatively low grinding media-to-feed ratio appeared to increase the initial rate of particle size reduction. For further improvement of grinding performance, the use of chemical additives acting as dispersants was deemed promising.
While stirred media mills stem from a fairly basic design, they have been subject to extensive further development and refinement. For example, the MaxxMill developed by Maschinenfabrik Gustav Eirich consists of a vertical rotating vessel which contains one or more eccentrically-positioned rotors and flow deflectors. Because the MaxxMill operates in dry mode, the smallest particles can be sucked out and classified in an auxilliary cyclone. Particles above a maximum tolerated size are returned to the mill, enabling control of the product size. S. Gerl (Gustav Eirich) presented options to integrate the MaxxMill into processes.
Maelgwyn Minerals Services' M. Battersby introduced the Deswik Turbomicronizer (TM) Mill, a high-speed stirred media mill with a series of impellers along the central shaft. Having evolved from a horizontal design, the vertical Deswik TM Mill is less sensitive to failures of bearing seals and blocking of screens. Feed material is introduced as a slurry at the bottom of the mill, moving upwards in a helix-type flow pattern along a hard-wearing polyurethane resin mill lining. Grinding media are recycled internally to the base of the mill.
Given the technological advances, the focus is turning on the reliability and availability of ultrafine grinding technology in a mining environment. Stirred media mills are starting to feature in large-scale commercial mining applications. Following K. Barns' introduction of the five key process technologies marketed by Xstrata Technology, the application of IsaMill for ultrafine grinding was discussed by D. Curry. Lead/zinc ore at the Mc Arthur River mine in Northern Territories, Australia, requires grinding the ore to below 7 ?m in order to achieve sufficient liberation of galena and sphalerite. Compared to conventional grinding, ultrafine grinding was reported to increase the zinc recovery by 10 %. The IsaMill, which has been developed in partnership with Netzsch Feinmahltechnik, is a high-intensity (up to 300 kW/m3) horizontal disk mill with 8 grinding chambers in series. While media are present to enhance the grinding process, media is retained in the mill without the use of screens. The product is reported to have a narrow particle size distribution, which is considered to be a key factor in maintaining efficient downstream flotation. Using d80 as a parameter, IsaMill may be scaled up, as witnessed by a 3 MW unit being commissioned for AngloPlatinum at Potgietersrust, South Africa.
Anglo Platinum already operates a 2.6 MW unit at Rustenburg, South Africa, for tailings re-treatment, aiming to recover Platinum Group Metals (PGM). C. Rule (Anglo Platinum) explained that ultimate recovery of PGMs locked up in silicates depends on enhanced liberation brought about by ultrafine grinding. A notable improvement in PGM recovery is observed when grinding more tailings below 75 ?m. Anglo Platimum's experience with IsaMill suggests that the wear of mill lining and media consumption were lower than predicted for this application and that the product size distribution and energy consumption were within specification.
The generally advanced nature of stirred media mills was confirmed by the laboratory-scale investigation of two ultrafine grinding techniques by J. Parry. Aiming to liberate fine galena in coarse sphalerite present in ore from Red Dog mine, both the IsaMill and the Stirred Media Detritor (from Metso Minerals) achieved comparable results, as measured with the Mineral Liberation Analyser (from JKTech).
Besides developments in the design and scale-up of stirred media mills, a significant technological breakthrough was achieved with the introduction of a new class of grinding media. Previously, typical grinding media, such as the ore itself, slag, silica sand or river pebbles, suffered from lack of quality, notably inconsistent size and competence. According to P. Hassall (St Gobain-Zirpro), the advent of manufactured media lead to substantial improvement of the media quality through a uniform chemical composition and a high hardness, sphericity, roundness, density, and competency. As a result, the grinding efficiency increases and the energy consumption is lowered. The modern media are typically manufactured ceramic materials such as aluminium oxide, yttrium-, cerium- and magnesium-zirconia oxides and various silicates. B. Clermont (Magotteaux International) described that Keramax MT1 media, containing a mixture of oxides, lowered the energy consumption because there was less sliding friction between hard, spherical media particles. G.A. Graves (Zircoa) emphasised that the smooth surface of media particles, a narrow media size distribution, a high hardness and fracture toughness are vital to achieve energy consumption reduction. Image analysis may be used to monitor the shape and size of media particles. It should be noted that determination of the optimum size of media particles for a given application requires careful consideration. While an application may require media with various media particle sizes, there may be a case for staged milling and ultrafine grinding with optimized media sizes.
While ultrafine grinding can improve recovery and reduce downstream reagent requirements, the effect of extra particle size reduction should be balanced by the cost of additional grinding energy. For example, the Deswik TM Mill reduced a feed with d90 of 89 µm to 20 µm with 10 kWh/t or 12 µm with 16 kWh/t. Ultimately, the economics will decide whether the application of ultrafine grinding will continue to grow.
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