Fine Grinding Technology for Mines and Concentrating Plants: "Stirred Mill" vs. Ball Mills
Fine Grinding at Mines: Stirred Mill vs. Ball Mill
An energy-efficient alternative to the ball mills used for material size reduction at mines.
Item ID: 479
Process Loads & Appliances--Industrial Processes
The horizontal stirred mill approach for grinding rocks, minerals, and industrial materials was developed in Australia, where it has been used for a number of years. Conventional ball mills rotate the mill body (a horizontal cylinder), tumbling both rock and metallic balls. Stirred mills use an internal auger to agitate the feedstock (ore which is fed into the mill as a slurry) and grinding media (sand, ceramic balls), which produces finer, more uniform grind sizes with reduced energy consumption.
Grinding power requirements are strongly related to product size (in microns), and stirred mill technology has been shown to use as little as two-thirds of the energy of conventional ball mills in coarse grinding applications. Horizontal stirred mills are a commercially available technology that had no market penetration in the northwest until BC Hydro funded a demonstration project at the Taseko Mines Ltd. Gibralter Mine in British Columbia. The vertical stirred mill employed saved about 4 million kWh/year in electrical energy, reducing annual operating costs by about $325,000. Larger horizontal stirred mills were later installed at the Embako mines in BC.
Ball mills are often used to grind feedstock into particles of 30 to 100 microns in size. Horizontal stirred mills can efficiently produce product down to 7 microns. The stirred mill was found to take up less floor space and was simple to install and maintain. It also is capable of producing a finer grind. Stirred mill technology may be suitable as a replacement or complement for ball mills used at mines to produce a finely ground product that is fed into separation and concentration processes such as flotation or leaching. They should definitely be considered for new mines and mine expansions. While originally developed for fine and ultra-fine grinding applications, efficient stirred mills are now being applied in some coarse grinding applications.
Baseline Description: Ball Mill
Manufacturer's Energy Savings Claims:
"Typical" Savings: 30%
Savings Range: From 25% to 37%
Manufacturers do not specify or guarantee savings for vertical or horizontal stirred mills. For large projects, it is likely that bench or laboratory tests would be conducted on samples of the ore to be processed to determine energy use to produce a product (finely ground particles) of a desired size. The stirred mill would then be optimized in terms of grinding media selection and rotational speed for the specified grinding duty. The baseline would generally be a ball mill using steel balls of a specified diameter.
A paper in Minerals Engineering ("Comparison of Energy Efficiency Between Ball Mills and Stirred Mills in Coarse Grinding", Vol. 22 (2009) 673-680) reported the energy use of ball mills versus vertical tower and horizontal stirred mills. Both coarse and fine grinding applications were investigated. The study examined seven ore samples with feed sizes ranging from 3.35 mm to 150 microns. The vertical stirred mill showed energy savings of 25%, 37%, and 27% relative to a ball mill in coarse grinding duty. For desired particle sizes less than 40 microns, horizontal stirred mills show increasing advantages over ball mills. Both ball and stirred mill specific energy (kWh/tonne) depend heavily on ore type, feedstock size, and ground particle size.
Best Estimate of Energy Savings:
"Typical" Savings: 30%
Low and High Energy Savings: 25% to 37%
Energy Savings Reliability: 2 - Concept validated
For any grinding application, specific energy consumption (kWh/tonne) depends on the material characteristics (size, shape, density) of the product being ground and the initial and desired or final particulate sizes. The finer the product requirements, the more energy efficient horizontal stirred mills are compared with vertical tower mills or ball mills. Both ball and stirred mill specific energy (kWh/tonne) depend heavily on ore type, feedstock size, and ground particle size.
Xstrata conducted lab test programs to determine the energy efficiency of the horizontal stirred mill (the IsaMill) when compared against full scale vertical mills. When a copper concentrate sized at 75 microns (P80) was supplied to both mills, the IsaMill matched the vertical mill product P80 of 48 microns with an energy consumption, including pumping requirements, of 3.74 kWh/tonne relative to the vertical mill consumption of 5.78 kWh/tonne. This represents a 35% reduction in consumed power and energy. Another series of regrinding tests consisted of feedstocks of three sizes. In these tests, the horizontal stirred mill was found to be 25% to 43% more efficient than the vertical mill.
It is difficult to specify an expected energy savings percentage due to design and operational differences. The IsaMill (high capacity horizontal stirred mill) is referred to as an enabling technology. Through significantly lowering the cost of fine grinding, many locations that had high grade but fine-grained ore became economical. Stirred mill technology energy efficiency is extremely dependent upon several variables including grinding media selection. The effeciency of these horizontal stirred mills is strongly dependent upon the significantly higher chance of media-particle collisions---which is in turn dependent upon grinding media material size. Net power (expressed as MW for a 250 tonne/hour gold bearing ore grinding operation) can vary from 1.9 MW up to 7.0 MW based upon whether ceramic balls (1.9 MW), alumina (3.3 MW), sand (2.8 MW), slag (4.4 MW), or river pebbles (7.0 MW) are used as the grinding media (from: B.D. Burford and L.W. Clark of Xstrata Technology, "IsaMill Technology Used in Efficient Grinding Circuits").
Also of importance is the ground product size. Ball mills may be able to efficiently reduce coarse aggregate to particles as small as 30 to 40 microns. Below 30 microns, the advantages of a horizontal stirred mill become dramatic and below a certain point, the horizontal stirred mill is the only choice as a ball mill cannot produce a 10 micron product at any practical power consumption. (Stirred mills can produce product down to 7 microns). A comparison chart for grinding a pyrite concentrate with a ball mill (9 mm balls) and a stirred mill (2 mm sand) shows a decrease in net energy from about 44 to 24 kWh/tonne when producing a 20 micron product and going from about 80 to 30 kWh/tonne when producing a 15 micron product. (from: J.D. Pease, M.F. Young, and D.C. Curry, Xtrata Technology, "Fine Grinding as Enabling Technology:---the IsaMill"). Power requirements increase rapidly at smaller product diameters. In fact, power requirements may double when the target grind size is 8, rather than 10 microns.
Energy Use of Emerging Technology:
Currently no data available.
Currently no data available.
Installed first cost per: ton
Manufacturers of large horizontal stirred mills were reluctant to provide equipment cost information as their competitors could potentially use this information to undercut them in the bidding process.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.