GOLD PROCESSING
A full recovery
Paul Moore reviews HPGR-based comminution and leach reactor tech in gold processing
Record gold price highs are an important impetus for mine expansions and the commencement of several major projects , many of which are leveraging new technologies to optimise recovery , as well as to ensure their operations are as sustainable as possible , in line with stakeholder and shareholder expectations . For greenfield high tonnage gold projects , miners are increasingly opting for HPGR-based solutions over traditional tumbling mills to minimise their energy consumption , improve recovery and , ultimately , maximise the project ’ s economic viability .
As a case in point , Weir Group was recently awarded a £ 53 million contract to provide Barrick ’ s Reko Diq ’ s copper-gold project in Pakistan with a transformational flowsheet solution that includes ENDURON ® HPGRs and ENDURON ® ELITE vibrating screens .
A project that ’ s more mature , the Côté Gold Project in Ontario , Canada , provides some interesting insights about the benefits of HPGRs compared to traditional tumbling mills , as well as how to optimise their operation .
Bjorn Dierx , Weir Minerals , Global Product Manager , HPGR and Vertical Stirred Mills : “ Weir was involved with the project from early on and there are some key take-aways from each stage of the project – from the test work to scale up and flowsheet design and validation , through to operational observations during commissioning and ramp-up – that will streamline the implementation of HPGR-based solutions in other high tonnage gold projects .”
A comprehensive review of the samples drilled from 2009 to 2015 provided insight into the comminution characteristics and indicated a nominal Axb of about ~ 26 and a bond abrasion index of 0.55 . During additional composite work , bond abrasion was measured at an increased value of 0.68 .
Based on these metrics , the nature of the ore can be classified as highly competent and abrasive , which requires a robust and flexible processing plant to handle these geological feeding conditions . During the techno-economic evaluation of the comminution circuit , both SABC and HPGR-ball mill-based scenarios were compared prior to gravity concentration and cyanide leaching .
Due to overall energy limitation at the site , the trade-off was based on the configuration that could give the highest tonnage for the same amount of energy .
Comparative OPEX modelling between both circuits was conducted based on the conclusions derived from test work and equipment scale-up simulations and indicated a 15 % OPEX reduction in favour of the HPGR-based circuit . Taking into account other recent studies and the nature of the ore competency , these estimates now seem rather conservative .
The HPGR-based flowsheet was selected because it better delivered on the main success metric – ie to maximise tonnage within the available energy supply – and , as a result , the CAPEX trade-off was deemed unnecessary .
There were also advantages with regard to recovery since leaching kinetics are influenced by the particle permeability . Since HPGRs introduce microfractures in the gangue matrix , particularly in the coarser size fractions , with up to 60 % higher rock matrix fracturing compared to traditional crushing , this is another factor that will help deliver higher recoveries .
Notably , the importance of durable dynamic skew became apparent during the ramp-up . The moveable roller skewed almost permanently due to the high degree of fines combined with coarser harder particles being fed to the operating gap . Should skew be limited or completely disabled , this would either cause
Commissioning of the ENDURON ® HPGR at the Côté Gold Project
frequent machine trips or exert excessive and potentially destructive loads into the bearings .
Moreover , having a multidisciplinary Weir team integrated into Côté ’ s daily operation allowed them to navigate through teething issues and optimise the circuit beyond nameplate capacity . For example , shortly after ramp-up commenced , the team identified that redesigning the HPGR discharge chute with rock box style baffle plates would provide significant wear improvements .
The owners expect the Côté Gold operation to exit the year at approximately 90 % of nameplate throughput . Dierx : “ The Côté Gold Project reaffirmed the importance of roller skewing to accommodate load balancing and to maximise size reduction . The ENDURON ® HPGRs ’ bearing arrangement skews with the roller , ensuring the seals aren ’ t ‘ opened up ’ and thereby preventing contaminant ingress . This is achieved with the fully-controlled oil lubrication system , which ensures the lubrication remains clean and the bearings remain at a constant temperature .”
He adds : “ Moreover , the internal knuckle joint within the single side hydraulic cylinder aligns during roll skew , generating hydraulic press force , while the rubber thrust pad distributes the loads evenly across the bearing arrangement and avoids ‘ hard contact ’. Radial and axial loads are separated and distributed across both four-row cylindrical bearing configuration and axial thrust bearing .”
As miners increasingly adopt HPGR technology , Weir is also building service networks to ensure it can support this growing demand . For the Côté Gold project , this involved a long-term service agreement that included dedicated HPGR services at site .
Another example is Weir ’ s recently opened Port Hedland Service Centre , which is strategically located to support its customers in the Pilbara region of Western Australia . The centre features facilities for ENDURON ® HPGR servicing , including tyre roller assembly . In addition to Fortescue ’ s Iron Bridge Operation , it will support De Grey Mining ’ s flagship Hemi Gold Project among others , allowing the miners to benefit from product training in close proximity to the mine site .
Similarly , the Reko Diq site in Balochistan , Pakistan is very remote , so as part of the contract to supply ENDURON ® HPGRs , Weir will also resource the service centre on site to enable the miner to utilise the technology they need , rather than simply opting for legacy solutions they might be more familiar with .
Dierx concludes : “ As more high tonnage gold assets come online in the next few years , OEMs have an important role to play to ensure production is optimised and aligned with the industry ’ s shift towards more sustainable
54 International Mining | NOVEMBER / DECEMBER 2024