IsaMill technology is now in its 13th year since the first full scale development went into operation at Xstrata’s lead zinc concentrator in Mount Isa in 1995.
It was the first of 14 x M3000 IsaMills to be installed in Xstrata’s Mt Isa and McArthur River Mine (MRM) operations.
The initial mill was designed with a 3000L grinding chamber, powered by a 1.12 MW motor.
The duty of the IsaMills at the MRM concentrator was regrinding concentrate down to 7µm.
Today, the basics of the technology has remained the same, however the design has progressed.
The development of larger IsaMills, as well as ceramic media, has enabled the mill to treat much coarser feeds than it was originally designed for.
Feed sizes up to a F80 of 150µm or greater are now standard, and development work is being trialled at the MRM operation on coarser materials.
Operating duties for the technology include tertiary and secondary grinding applications, for ore types including platinum, copper, pyrite, nickel, as well as lead and zinc.
Testwork has been undertaken on magnetite as well, and has shown good grindability compared to some of the full scale installations that treat much harder ore types such as platinum and pyrite, that have energy requirements over 30 kWh/t to achieve the required grind.
Operators, either relying on grid power or running diesel generators, are facing increased power cost.
IsaMills can provide energy savings of a 1/3 to ½ compared to traditional grinding technologies, such as ball mills or tower mills.
This is due to the IsaMill having a horizontal packed bed of grinding media, with up to 8 grinding disc rotating through the bed at relatively high speeds.
This creates a high probability of high energy intensity media-particle collisions, resulting in attrition grinding, as well as limiting short circuiting, as the multiple disc act as independent grinding chambers.
This also eliminates the need to enclose the grinding circuit with cyclones, reducing operating and capital cost. A patented internal classifier at the end of the mill retains the oversize particles and media and pumps them back into the mill for further grinding, allowing the ground particles to pass out the mill.
The action of the classifier produces a narrow product sizing distribution, with minimal over grinding and under grinding in the mill, which assist in metallurgical response as well as slurry handling and filtration.
The other benefit of IsaMilling is the use of inert grinding media.
While initial IsaMill operations used sands and slags, most of the current operations now use ceramic media, that has been developed specifically for IsaMill operation by Magotteaux, called Keramax MT1.
Media sources such as sand and slag may make economic sense for use in IsaMills if located near the milling operation. However the use of ceramics provides media with high SG, consistent hardness and roundness, as well as other properties, which enables increased grinding efficiency, resulting in less installed power compared to similar applications using sand.
The energy impacted by the media is a function of the velocity, media size and the media SG, so ceramic media is important in coarse grinding applications due to the larger media size (up to 3.5mm), and higher SG, compared to sand (the SG of sand is 2.4 to 2.6, ceramic media SG is 3.7).
Inert grinding is the key to improving the metallurgy of downstream processes after IsaMills, as it enables the flotation or leaching of the liberated products to take place without being hindered by ferric ions.
Ferric ions are generated by steel media, which coats the newly created mineral surfaces, and effects the action of the leaching or flotation reagents.
Often other regents are required to minimise the effects of ferric ions.
This is further compounded by closed circuit operation in traditional milling operation, where the milled product undergoes cycloning and the underflow, which can be up to 200% to 300% of the new feed tonnage to the mill, is fed back to the mill for further contamination with the ferric irons. However with the use of inert grinding in IsaMills, in open circuit operation, results in no ferric ions being generated, and reagent usage is reduced, as only the new surfaces are dosed with reagent.
The open circuit operation also makes simpler circuit control and lower installation cost.
Another advantage of the IsaMill is the scalability of the mill from lab scale to full scale plants.
Importantly this has been verified between the full scale operations and the lab scale results.
The lab scale work is carried out using the 4L IsaMill, which is powered with a 2.2 kW motor.
15kg samples are required to be passed through the mill a number of times until the required grind size is achieved.
For each pass the power usage is measured directly from the motor, so a log size versus energy graph is plotted.
This information can then be used to design the full plant installation.
Importantly the power gained from each pass is measured directly from the motor to give an accurate measurement.
Also it is important is to recognise the power measurement for the IsaMill includes the power required for both grinding and classification, so when this power data is compared to other technologies, the power used for downstream classification must be added to the signature plot of the other technology so accurate comparisons can be made.
This article was written by Brenton Burford, senior IP coordinator for Xstrata Technology.