The main components of today’s typical hard rock mining flowsheet – comprising a series of ball mills paired with cyclones to grind target minerals to a size that suits the downstream recovery process – have remained largely unchanged for many decades. While high-pressure grinding rolls (HPGR) were first utilised in the minerals processing industry over thirty years ago, most concentrators still use tumbling mills for grinding duties.
Over that same period, the operating environment has changed significantly, which begs the question: why haven’t flowsheets?
Two of the most notable operational changes are: reduced ore grades and increased ore hardness (particularly in relation to base metals) and the increased cost in milling consumables, like power and steel grinding media.
These two things are directly correlated because processing harder ores requires higher grinding energies, which, when achieved with tumbling mills, typically consumes more steel media and liners, resulting in significantly higher milling costs.
Furthermore, traditional mill circuit flowsheets are inefficient, with 40-60% of the slurry returned to the mill for reprocessing. These recirculated loads obviously reduce the throughput and capacity of the mill.
Rethinking traditional flowsheets
Weir Minerals co-developed an innovative flowsheet at Fortescue Metal Group’s (FMG) Iron Bridge magnetite mine in Western Australia. It’s the world’s first large-scale magnetite flowsheet without tumbling mills; instead, Enduron® HPGRs and STM vertical stirred mills have been installed in their place. Weir Minerals believes the project is indicative of the direction the industry is heading in.
It's a flowsheet that delivers energy savings by ensuring that energy is not applied to gangue, which is achieved using coarse dry magnetic separation as inter-stage beneficiation between the Enduron® HPGRs.
This allows for the rejection of more than 20% of the barren material. By combining the micron-sized grinding with dry-air classification, water addition is minimised prior to feeding it into the highly efficient vertical stirred mills. This, in turn, prevents overgrinding by making use of internal classification to minimise mill retention time.
Coarse particle flotation
There’s the potential for further efficiency gains by implementing Eriez’s HydroFloat Coarse Particle Flotation (CPF) technology downstream of the grinding circuit. This permits the use of a coarser, rougher grind sizes, delivering the operator additional reductions in grinding energy-requirements.
The recovery or efficiency of conventional flotation is strongly related to the particle size of the ore. Conventional flotation only achieves high efficiency in a narrow interval in the middle of the range, typically 50 to 130 µm.
As a result, the tailings of these concentrators are enriched in valuable metal units in both the fine and coarse classes of the size distribution. Therefore, Eriez’s HydroFloat CPF represents a major step forward in terms of recovery, by extending flotation efficiency over a size range that is two to three times greater than conventional flotation machines’ limit.
CPF produces this result by combining features that facilitate particle collection and mass transfer, including fluidisation water to enhance lift, counter-current contacting, a plug-flow residence time distribution and a zero-order froth. This amounts to CPF allowing the effective flotation size-range for copper to be increased to approximately 400 µm.
As a result, the P80 is shifted to significantly coarser sizes. This, in turn, reduces the volume of fine feed in the distribution, thereby enhancing overall recovery, reducing energy or allowing increased throughput for the same energy, as well as creating safer, sand-like tailings for disposal. Notably, CPF reduces ball mill energy consumption alone by 30% to 50%.
There are obviously a lot of site, operational and application variables that need to be considered when developing flowsheets, which is why Weir Minerals prioritises working closely with the customer to understand their unique challenges. It can then leverage its extensive and diverse team of equipment experts, process engineers and testing capabilities to develop and implement the ideal solution based on the mine’s specific requirements.
The partnerships with STM Minerals and Eriez highlights Weir Minerals commitment to collaborate with other companies who share its vision to make mining more sustainable and provide its customers with solutions based on their needs, rather than simply selling them equipment from its portfolio.