Why HPGRs are revolutionising mill circuits
Chosen for their energy efficiency, long wear life and downstream advantages, High Pressure Grinding Rolls (HGPRs) are increasingly being used at the heart of comminution circuits.
Chosen for their energy efficiency, long wear life and downstream advantages, High Pressure Grinding Rolls (HGPRs) are increasingly being used at the heart of comminution circuits.
7th August 2019
Starting in the 1980’s, HPGR was first successfully implemented in the cement industry and is now used across a wide range of mining applications. Today, there are more than 200 machines operating in the mining industry around the globe, on more than 150 sites.
These energy-efficient grinding machines are mostly used for processing iron, gold, copper and diamond ores, although they’re increasingly being deployed for lithium, limestone and vanadium.
Despite having a significantly smaller footprint than conventional “tumbling” mills, HPGR’s are high-capacity crushing units delivering a high degree of size reduction, resulting from sophisticated inter-particle grinding.
High Pressure Grinding Rolls consist of two counter rotating, similar sized rolls, which synchronously operate at variable speeds, depending on the plant’s requirements, which force particles to crush one another and compress the feed’s density to 80% of its solid volume.
This compression mechanism introduces ‘micro-cracks’ in the grains, which in turn increases downstream milling capacity and improves mineral recovery.
As ore grades decline across the globe, miners are being pushed to dig deeper and process more competent ore bodies, requiring a more efficient comminution technology to remain profitable. The inter-particle grinding of a HPGR produces significantly more fines than conventional crushing, as the force is applied from multiple directions and to particles of all sizes, not just the coarser size fractions.
This means more product is at size with one pass and doesn’t require to recirculate back through the comminution device again, driving plant efficiency.
In total, HPGRs represent energy savings of as much as 40% compared to conventional circuits whilst reducing downstream milling requirements. As a single HPGR unit only uses 0.8 - 2.5 kWh per tonne of material, they not only saving you money but freeing up capacity for downstream milling.
The high pressure of HPGR’s “bed-breakage” creates micro-cracks in the ore, promoting improved mineral recovery at coarser size fractions. This means less leaching acid is required and percolation periods are shortened, reducing costs and maximising recovery in less time.
In the diamond industry, the HPGR acts as a selective grinder, protecting the valuable diamonds whilst preferentially weakening the surrounding waste rock (gangue).
Apart from mineral liberation improvements, recent studies have proven that the microcracks also significantly reduce the required grinding media in the subsequent milling stage. These direct energy savings are amplified by the reduced grinding material consumption and lower liner wear. Through the reduction of material and electrical waste, High Pressure Grinding Rolls are reducing the ecological impact of grinding.
In traditional cone crushing, and especially with hard and abrasive ores, the size reduction is achieved by direct contact between the ore and the liners resulting in significant wear and thus higher operational costs.
With HPGR, the indirect particle-on-particle grinding in combination with a “autogenous layer” which is formed in-between the studs, exposing the roll surface to less wear.
Additionally, by using high-quality tungsten carbide studs optimised and tailored for the specific application, operational lifespan can be maximised. Building on decades of operational experience, Enduron® roll surfaces have been improved, making use of varying stud hardness across the full width of the tyre that promote even wear by transferring pressure to the ‘operating gap’ and away from the edges. The “edge studs” are patented and have a unique interlocking system, keeping them in position even under the toughest conditions.
When tyres do reach the end of their effective lives, it wasn’t uncommon for replacement to take 72 hours or longer. However advanced frame designs such as the Enduron® HPGR now allow tyres to be exchanged within a single 24-hour shut down and innovative features like spring-loaded lateral walls ensure particles stay where they’ll receive the most pressure and cause the least abrasion.
The unique cylindrical roller bearing arrangement of the Enduron® HPGR removes the ever-present threat of HPGRs suffering from bearing failure, which often has a significant price tag and lead-time risk attached due to the global shortage of large engineered bearings.
HPGRs can handle a wide variety of ores and particle sizes, from fine pellet feed all the way up to coarse grinding of 75mm magnetite iron ore. Alongside their greater tolerance for ores of different hardness and variable roller speed, HPGRs promote greater flexibility in feed conditions and circuit design than traditional mills.
Additionally, thanks to Enduron® HPGRs unique bearing construction, the rolls can be skewed outside the bearing, eliminating bearing peak loads without compromising crushing conditions. This allows tramp metals or oversized materials present in mineral feeds to pass smoothly through without damaging the tyres.
Skewing also ensures that even pressure is applied across a segregated or uneven feed, whereas flanged roll designs only allow a symmetric operating gap, resulting in pressure loss and coarser product quality.
High Pressure Grinding Rolls are generally thought to only be effective in controlled “dry” grinding applications, however (depending on the material saturation level) they can readily process minerals well above 10% moisture. This gives HPGR’s the flexibility to fit into a wide array of applications with both wet and dry classification.
In a departure from traditional classification, Weir Minerals developed a complete dry air classification system which combines HPGR fine grinding and efficient classification without a single drop of water. As water becomes more challenging to secure around the globe, this development takes sustainable mining to the next level, making Enduron® HPGR an ideal choice for mines in arid environments without significant, or affordable access to groundwater.
For more information on how Enduron® High Pressure Grinding Rolls can future-proof your next mill circuit, increase efficiency, reduce your cost per tonne and maximise plant availability, click the link below.