GOLD PROCESSING
Looking upstream
Reduced cyanide consumption is just one of the benefits that comes with employing Weir Minerals ’ Enduron ® high pressure grinding roll ( HPGR ) technology in gold applications , according to Frank van der Meer , Weir Minerals Process Support Engineer , and Serhat Onol , Weir Minerals Senior Process Engineer .
This technology has come into the space more and more in recent years as operators have looked for comminution technology that can improve gold recoveries , while reducing a mine ’ s environmental footprint .
In gold processing applications where the ore is recovered by heap leaching , a single , large Enduron high pressure grinding roll ( HPGR ) can , in some cases , replace five or six tertiary crushers , van der Meer and Onol claim .
“ In brownfield projects , this significantly reduces the footprint of the plant , lowers energy consumption , reduces maintenance requirements and ensures plant availability remains high ,” they explained . “ In greenfield applications , where optimal flowsheets can be designed from the outset , the efficiency gains and improved sustainability outcomes are often even greater .”
Both greenfield and brownfield operations are taking this technology on , realising gold recovery improvements of as much as 10 % compared with traditional crushing circuits , they said .
Two examples are IAMGOLD ’ s Côté Gold Project in north-eastern Ontario , which plans to install an Enduron HPGR with rolls measuring 2.4 m x 2.4 m for mine start up in 2023 – becoming , in the process , the largest HPGR in Canada and the largest in the world in a gold hard-rock application – and Eldorado Gold ’ s operating Kisladag mine in Turkey , which has just commissioned an Enduron HPGR with rolls measuring 2.4 m x 2.2 m to improve recoveries at the heap leach operation .
The Enduron HPGR reduces particles by compressing and grinding the feed between two counter rotating , parallel rollers . This forces the rocks against each other and compresses the feed ’ s density to 85 % of its solid volume . This compression is achieved by applying high pressures of up to nearly 300 Mpa , exceeding the compressive strength of the feed material .
The interparticle compression is particularly advantageous in gold processing because it creates weaknesses and cracks in the particles ; leaching liquids are then channelled into the fissures , aiding the effective liberation of the gold ore , van der Meer and Onol explained .
In flowsheets where the HPGR performs tertiary grinding immediately prior to recovery – eradicating the need for a ball mill – a partial product recycling system ( PPR ) streamlines the process . The PPR consists of plates under the rolls that return oversized product back to the hopper ; it can be adjusted to meet the changing quantity of fines and product gradation required for heap leaching , they said . This helps maintain effective leaching percolation and heap stability . For Eldorado Gold ’ s Kisladag mine , a PPR system with the Enduron HPGR was the perfect choice , according to Onol .
“ Multiple test campaigns were conducted for Tuprag ( Eldorado Gold ’ s Kisladag operating subsidiary ),” Onol said . “ The recovery improvement was always the focus .
“ We have tested the Enduron HPGR technology at the quaternary crushing stage before heap leaching , in the quaternary crushing stage before ball milling and at the tertiary crushing directly before heap leaching
“ Although HPGR in closed circuit with screening was also tested , most of the test campaigns were focused on PPR . PPR is not an option for every application but for Tuprag – due to the feed size and the desired product size – all specifications showed that the PPR was the best option : simple , easy , and flexible ,” he said . “ The hybrid solution with screening serves to increase the flexibility and to control the circulating load to the HPGR .”
Such a circuit , designed around the effectiveness of the Enduron HPGR , can deliver up to 11 % additional gold extraction , according to van der Meer and Onol .
In applications where the Enduron HPGR is followed by tumbling mills , the induced micro cracks generally result in a reduction of the Bond Work Index . For most ores , the reduction is in the range of 10-25 %.
“ The benefits are most observable when processing hard ores , like gold , because the energy required to reduce each tonne with conventional crushers is particularly high ,” they
In gold processing applications where the ore is recovered by heap leaching , a single , large Enduron HPGR can , in some cases , replace five or six tertiary crushers , Frank van der Meer and Serhat Onol say ( pictured is the recent HPGR installation at Eldorado Gold ’ s Kisladag mine in Turkey )
said . “ The Enduron HPGR offers miners an effective way to increase production while keeping energy costs low and improving profitability .”
These advantages are on top of the power consumption reductions that come with using Enduron HPGR technology when compared with traditional grinding circuits .
Weir Minerals says power consumption savings of as much as 40 % have been observed , with each installation of an Enduron HPGR unit also estimated to save 12,000 t / y of CO 2 – the same as removing 3,600 cars from the road . van der Meer and Onol explained : “ The exact magnitude of the energy saving will vary depending on the operation , however , one of the most significant factors is the size of the desired final output and the hardness of the ore . Again , harder ores crushed to smaller sizes at higher volumes offer the greatest potential for energy reduction .”
One of the features critical to this improved performance is the Enduron HPGR ’ s “ unique ” roller bearing design , which allows for skewing alongside effective self-adjusting cheek plates , reducing wear and promoting better grinding , according to van der Meer and Onol .
This has , the company says , resulted in diminished premature bearing failures , while maximising grinding efficiency . And , when working in conjunction with the spring-loaded lateral walls , the function reduces the quantity of
28 International Mining | NOVEMBER / DECEMBER 2021