1 IsaMill™ Technology Used in Efficient Grinding Circuits B.D. Burford1 and L.W. Clark2 High intensity stirred milling is now an industry accepted method to efficiently grind fine and coarse particles. In particular, the IsaMill™, which was invented for, and transformed the fine grinding industry, is now being included in many new comminution circuits in coarser applications. While comminution has always been regarded as important from a processing perspective, the pressure being applied by environmental concerns on all large scale power users, now make highly energy efficient processes more important than ever. The advantages that were developed in fine grinding in the early IsaMill™ installations have been carried over into coarse grinding applications. These advantages include a simple grinding circuit that operates in open circuit with a small footprint, the ability to offer sharp product size classification, as well as the use of inert media in a high energy intensive environment. This paper will examine the use of IsaMill™ technology in fine grinding (P80 below 15 micron), and examine the use of the technology in conventional grinding applications (P80 20 - 150 µm). Recent installations will be examined, including fine and coarse grinding applications, as well as the recent test work that was undertaken using an IsaMill™ in a primary grinding circuit, and the resulting circuit proposal for this site. While comminution has been relatively unchanged for the last century, the need to install energy efficient technology will promote further growth in IsaMill™ installations, and result in one of the biggest challenges to traditional comminution design. 1. Senior Process Engineer, Xstrata Technology, L4, 307 Queen Street, Brisbane 4000, Qld, Australia 2. Business Manager – Mineral Processing, Xstrata Technology, L4, 307 Queen Street, Brisbane 4000, Qld, Australia 2 INTRODUCTION and BACKGROUND The development of IsaMill™ technology was driven by the metallurgical requirements of Lead/Zinc deposits at Mount Isa in Queensland and McArthur River in the Northern Territory, both of which were controlled by Mount Isa Mines Limited (now Xstrata). The McArthur River deposit was discovered in 1955 but, despite the efforts of numerous mining companies, an economic method for treatment of the fine grained deposit to produce saleable Pb/Zn concentrates was not achieved in 25 years of investigations (Enderle et al, 1997; Pease et al, 2006). In 1989 it was determined that regrinding down to 80% passing 7 µm was necessary to achieve sufficient non sulphide gangue liberation to allow the production of a bulk concentrate. Figure 1 is a comparison of the relative grain sizes of McArthur River and Broken Hill ore and illustrates the complexity of the mineralogy at McArthur River. Figure 1: Comparison of McArthur River and Broken Hill Ore Grain Size (Grey square is 40 µm) In the case of Mount Isa, there was a gradual decrease in plant metallurgical performance from the mid 1980’s as a result of decreasing liberation size and increased amounts of refractory pyrite in the ore. Concentrate grade targets were reduced to maintain zinc recovery, however plant performance continued to deteriorate to such an extent that by the early 1990’s the zinc recovery had decreased from 70% to 50% (Young et al, 1997; Pease et al, 2005; Pease et al, 2006). Significant work was conducted at Mt Isa on projects investigating finer regrinding using conventional ball and tower mill technology however the power consumptions necessary to achieve the required fine liberation sizes made them uneconomic. Further, the high rate of steel media consumption contaminated the mineral surfaces with iron, resulting in poor flotation response post regrinding. A real need had arisen for a technology that could grind to ultrafine sizes in metallurgical operations economically and without serious contamination of mineral surfaces and pulp chemistry. However in 1990, there was no generally accepted technology for regrinding economically to such sizes in base metals. So testwork was undertaken at the time into high speed horizontal stirred mill technology, which was used in pigment and other industries. It was shown that such mills could grind down to the ultrafine sizes required for mineral liberation. Arising from these findings, a program of major mechanical modification of horizontal stirred mill technology was undertaken between Mount Isa Mines Limited and NETZSCH-Feinmahltechnik GmbH (Enderle et al, 1997), the manufacturer of stirred milling technology used for other industries. 3 After many prototypes of increasing capacities, the first full scale model was developed and installed at the Mount Isa Mines’ Lead Zinc Concentrator in 1994. The mill, the M3000 IsaMill™, was quickly installed in other circuits at this concentrator, and was installed in the McArthur River Concentrator in 1995 (Johnson et al, 1998). Later, in 1999, it was commercialised and sold outside of the Xstrata group. Since commercialisation of the IsaMill™, there are now over 50MW of installed IsaMills™ operating around the world, treating materials including copper/gold, lead/zinc and platinum. While the early installations treated only ultrafine sizes, the current mill installations are treating courser sized materials, once the domain of tower and ball mills. The need for energy efficient grinding circuits will only result in more IsaMill™ circuits being applied in the future. IsaMill™ OPERATION Grinding Mechanism The IsaMill™ is a horizontally stirred mill consisting of a series of 8 discs rotating around a shaft driven through a motor and gearbox. The discs operate at tip speeds of 21-23m/s resulting in high energy intensities of up to 300kW/m3. Figure 2 illustrates the layout of the IsaMill™. Figure 2: IsaMill™ Layout FEED PIPE MILL (SLURRY and DISCHARGE MEDIA) Gearbox Rotor SHELL SLIDES AWAY FOR MAINTENANCE Motor Shaft Grinding Bearings Discs The mill is filled with a suitable grinding media and the area between each disc is essentially an individual grinding chamber. As a result the mill is effectively 8 grinding chambers in series. The media is set in motion by the action of the grinding discs which radially accelerate the media towards the shell. Between the discs, where the media is not as subject to the high outwards acceleration of the disc face, the media is forced back in towards the shaft – creating a circulation of media between each set of discs. Minerals are ground as a result of the agitated media, the predominant mechanism being attrition grinding. The mechanism is best illustrated in Figure 3. 4 Figure 3: IsaMill Grinding Mechanism Shaft rotating at high speeds generating disk tip Multiple stages of grinding Impeller pumps liquid back into speeds of 21 to 23 m/s chamber to retain media Displacement Grinding Body Disks Product exit Grinding Shaft Chamber PRODUCT SEPARATOR ZONE Recirculating grinding patterns of media Media centrifuged to outside of Rotor occur between disks due to variation in grinding chamber by high centrifugal velocity profile across disks force generated inside mill As a result of having 8 chambers in series, short circuiting of mill feed to the discharge is virtually impossible. There is a very high probability of media-particle collision as a result of the high energy intensity and the 8 chambers in series. Media The key to the efficiency of the IsaMill™ is the ability to use fine media. While tower mills are typically limited to 10-12mm fresh media sizing, the IsaMill™ can use media as small as 1mm. This results in significantly more surface area per unit volume of media in the IsaMill™ than in a Tower Mill – a 2mm charge has 90 times more particles per unit volume compared to 12mm media. As a result, there is a significantly higher chance of media-particle collision, particularly at fine sizes. The IsaMill™ is able to use a range of media types. Typically, low cost, locally available media such as sand or smelter slag have been used, which provide good grinding performance at acceptable energy efficiency. However, the need for improved energy efficiency at many installations has resulted in the use of high quality, high density ceramics, designed specifically for stirred milling applications, such as Maggotteaux’s Keramax MT1. Media Retention Grinding media is retained in the mill without the need for screens, which is why IsaMills™ can use fine media. At the end of the mill is a patented product separator consisting of a rotor and displacement body (refer to Figure 3). The close distance between the last disc and the rotor disc centrifuges any coarse particles towards the outside of the mill. Ground product flows into the rotor area where it is essentially pumped back towards the feed end of the mill. This pumping action retains the media in the mill. The balance of the product (equivalent to the feed flowrate) exits the mill through the displacement body. This unique mechanism means that screens or cyclones are not required to retain media in the IsaMill™ which can therefore be operated in open circuit without cyclones, which simplifies the circuit, and reduces capital. 5 Energy Intensity The high tip speed of the IsaMill™ results in a high energy intensive environment. Energy intensity of the IsaMill™ is significantly higher than any other commercially available grinding equipment as illustrated in Table 1. Combining the energy intensity and the high grinding efficiency leads to a compact mill, able to be fitted into existing plants where floor space is limited. On the other hand, Tower Mills require a settlement zone at the top to separate the media from the slurry – this limits the agitation speed to a tip speed of 3m/s (compared to IsaMill™ at 21-23m/s) and therefore limits the energy intensity, while ball mills can only have a relatively low amount of ball loading before media empties from the mill. Table 1: Comparative Energy Intensity of Grinding Technologies Installed Power Mill Volume (m3) Power Intensity (kW) (kW/m3) Autogenous Mill 6400 353 18 Ball Mill 2600 126 21 Regrind Mill 740 39 19 Tower Mill 1000 12 42 IsaMill™ 3000 10 300 Product Size Distribution In open circuit operation, the IsaMill™ is able to produce a sharp product size distribution.