Weir recently completed a comprehensive study highlighting a significant opportunity to reduce energy use and emission in comminution by using a combination of three alternative technologies.

The study utilised the World Business Council for Sustainable Development’s (WBCSD) Avoided Emissions Guidance and then had it independently assured by SLR Consulting Limited.

The alternative technologies were evaluated against a conventional comminution circuit design for an archetypal Chilean copper mine processing 15Mt of ore per year. Each circuit was based on a ‘rock-to-recovery’ system boundary; that is, reducing rock directly from the mine to a size that enables mineral recovery.

The four configurations were:

Conventional comminution circuit based on a semi-autogenous grinding (SAG) mill and ball mill.
Weir’s Enduron® high-pressure grinding rolls (HPGR) replacing the SAG mill at the initial grinding stage.
Enduron® HPGR, plus Swiss Tower Mills’ (STM) vertical stirred mill replacing the ball mill.
Addition of an Eriez HydroFloat™ coarse particle flotation (CPF) unit.

The study demonstrated that replacing conventional technology with innovative solutions can cut energy use by 40%, while also avoiding 50% of CO₂e (carbon dioxide equivalent) emissions.

By adopting a systems-based approach to technology collaborations, the mining industry can scale up and clean up at the same time.

Since the study was published, Weir has developed and released further technology to enhance its transformational flowsheet offering. The HPGR discharge can now feed Weir’s Enduron® Elite screen, allowing miners to reduce their energy consumption even further.

Screening HPGR discharge typically requires a large screen because HPGR circuits operate with up to a 200% recirculation load, resulting in high screen feed rates. Moreover, because the cut size of typical HPGR screens range from 1-4mm, larger screens than those usually employed in typical mining application are required to ensure optimal performance.

Adapting to change

While HPGRs were first utilised in the minerals processing industry over 30 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, and the increased cost in milling consumables, such as power and steel grinding media.

These two things have a direct relationship because processing harder ores requires higher grinding energies, which, when achieved with tumbling mills, typically consumes more steel media, resulting in significantly higher milling costs.

Therefore, to reduce their milling costs, miners can implement more energy-efficient grinding technologies, like HPGRs and vertical stirred mills. Given that grinding power demand is inversely correlated to the final grind size, the ability to recover target metals at a coarser grind size may also significantly reduce operating costs.

Energy-efficient grinding

Enduron® HPGRs can adapt to changing feeding conditions instantly because the roll speed and grinding force can be changed while in operation. As a result, the most effective, minimal stress-intensity force is applied to the particles to ensure they break at the given throughput rate.

With grid and energy availability being more heavily scrutinised, comminution equipment that can be swiftly ramped up and down to maximise energy consumption when off-peak energy becomes available during the day can deliver additional cost savings to the owners.

There is also a growing demand for vertical stirred mills with higher installed power ratings. The largest mill that has been delivered has 6500 kW installed power.

The STM mill has a vertical mill chamber with grinding rotors (discs with castellations) installed on the central mill shaft. The mill feed is a slurry, pumped through the bottom of the mill and discharging from the top. The mill chamber is filled with ceramic grinding media to approximately 60% of its volume. As the ceramic media is inert, it doesn’t contaminate the recovery plant feed, further improving efficiency.

The grinding media bed moves only in the horizontal plane, which ensures energy isn’t wasted. The vertical mill arrangement, combined with bottom feed entry and top discharge, ensures coarse particles don’t short-circuit the mill.

The mill shell is internally equipped with shell liners and 360° stator rings between each two grinding rotors, generating numerous consecutive grinding chambers.

The target is that coarser particles, together with the ceramic media, are pushed by centrifugal forces to the periphery of the chamber into the high-intensity grinding zones; while finer particles tend to travel upwards through the centre of the mill, thereby avoiding any unnecessary grinding.

This is called selective grinding, where the energy from the mill motor is applied as much as possible to the coarser particles.

Combining these two proven technologies achieves the grinding circuit that delivers the highest recovery and the lowest possible energy consumption.

The Enduron® HPGR and screen prepare the feed for the single or double-stage STM vertical stirred mill grinding circuit.

Screening HPGR discharge

Weir’s new Enduron® Elite screen is our large, high-capacity, double-deck banana screen, featuring Weir’s new ETX exciter technology. With a deck measuring up to 4.3m wide and 10m long, the Enduron® Elite screen range is perfectly suited to screening the high-capacity conglomerated fines created by the HPGR.

The varying slope angles within the screen mean that the feed material is subjected to varying angles of throw throughout its journey along the deck. This benefits the changing PSD on the deck as material gets removed through the apertures.

The steep slope angle at the feed end ensures the fine material is rapidly stratified. This creates interparticle space within the material layer, allowing the fine material to easily reach the deck surface and stratify through the screen media panel apertures. As the deck angle flattens out towards discharge, the throw angle in relation to the deck becomes more vertical, and the material velocity slows down, which is ideally suited to near-size screening.

A typical banana screen will separate most P₅₀ material within the first third of the deck. The second third removes the P₈₀ material, while the last third is used for near-sized screening, which concentrates on the material closest to the cut-size (i.e. P₉₀ and above).