Tag Archives: carbon in leach

Wood tradeoff study puts HPGR, POx processing ahead at De Grey’s Mallina gold project

Prefeasibility process engineer Wood Australia has completed a comprehensive nine-month trade-off study into the preferred process flowsheet design for De Grey Mining’s Mallina gold project, selecting a process flowsheet that incorporates high pressure grinding roll (HPGR) and pressure oxidation (POx) technology.

The project, which includes the Hemi Deposit, is in the Pilbara region of Western Australia.

In late 2019, De Grey made a large scale, high value, near surface gold discovery at Hemi – an intrusion-hosted form of gold mineralisation new to the Pilbara region that, the company says, shows a scale of mineralisation not previously encountered in the Mallina Basin. The company currently has 6.9 Moz of measured and indicated resources declared at the project, with plans to turn this into an open-pit mining operation down the line.

The findings have come out ahead of the full PFS, which is due for announcement in the September quarter of this year.

The trade-off study has confirmed that Hemi mineralisation has excellent recovery and is amenable to HPGR and POx processing technologies, the company said. The process route chosen has demonstrated advantages relative to other processing technologies including:

  • Proven and accepted technology for the scale of operation and style of mineralisation;
  • Lower capital and operating costs;
  • Higher gold recovery between 93% and 95% depending on average feed grades;
  • Circa-20% lower energy consumption;
  • Circa-25% lower reagent (lime) consumption;
  • Circa-25% lower CO2 emissions;
  • No heat addition required to sustain – or cooling to control – the POx process; and
  • Robust and proven equipment.

The flowsheet selections have the benefit of lower carbon emissions through lower energy requirements and lower greenhouse gas emissions as a result of improved neutralisation and consequent lower lime consumption, De Grey added.
“Further studies are in progress as part of the PFS on the carbon intensity and greenhouse gas emissions for the Mallina Gold Project development,” it said.

The currently envisaged flowsheet at Hemi sees ore crushed and ground in the comminution circuit before being fed to the flotation circuit. Any gravity recoverable gold will be recovered prior to flotation with the use of, for example, a Knelson or Falcon concentrator. Test work has shown gravity recoverable gold is present in the Hemi and regional mineralisation, and the extent of gravity-recoverable gold will continue to be assessed through test work.

The flotation circuit will process gold bearing sulphides in Hemi ore producing a “low mass pull” gold-rich sulphide concentrate. The POx circuit is designed to receive the gold-rich sulphide concentrate from the flotation circuit. The POx circuit will have a throughput of 8% (800,000 t/y) of the throughput of the comminution circuit.

The POx circuit will convert the sulphide concentrate to a gold bearing residue amenable to standard carbon in leach (CIL) processing. The underflow from flotation is also amenable to standard CIL processing. Both streams will enter the CIL circuit followed by electrowinning to produce gold bars on site. This has numerous benefits for the POx circuit including lower capital and operating costs compared with projects that require whole of ore treatment through POx.

Tailings from the CIL circuit will be pumped to a tailing storage facility.

Hemi mineralisation has two significant and attractive mineralogical characteristics that lead to the overall expected metallurgical recovery of between 93% and 95% at expected mined grades, namely:

  • The majority of gold at Hemi can be floated into a low mass pull concentrate that recovers very high levels of sulphides and gold ahead of oxidation; and
  • The flotation tail contains gold that can be successfully recovered using standard CIL processing.

The crushing circuit will include a primary gyratory crusher, a secondary cone crusher and. The secondary cone crusher will operate in closed circuit with a sizing screen while the HPGR will operate in closed circuit with wet sizing screens to produce a nominal less than 7 mm mill feed. The grinding circuit will consist of two ball mills with conventional pinion drives each with their own classification circuit.

The oxidation circuit throughput for Hemi is proposed to be 800,000 t/y, or 8% of the proposed comminution circuit throughput of 10 Mt/y. The oxidation circuit will treat the gold-bearing sulphide concentrate generated by the flotation circuit. The POx circuit will consist of flotation concentrate thickening and storage, POx utilising autoclave technology and neutralisation in association with the flotation tail prior to co-leaching in a CIL circuit.

The oxidation circuit will be designed to have sufficient storage capacity prior to the autoclave to allow for maintenance shutdowns without the need for a mill shutdown. This decouples the comminution and oxidation circuits ensuring that each circuit does not impact on the availability of the other.

Kışladağ and Weir Minerals on the HPGR-backed gold recovery trail

The story that led to the installation of a Weir Minerals Enduron® high pressure grinding roll (HPGR) at Eldorado Gold’s Kışladağ heap leach operation in Turkey is a fascinating read, as well as a great example of the benefits of using such technology.

Back in March 2018, Eldorado announced it would suspend mining to evaluate processing options for the operation. This decision followed extensive laboratory tests that indicated gold recoveries would continue to trend downwards around the base of the open pit where mining was underway.

This suspension, plus further engineering and test work, led to the company advancing the potential development of a mill project. The transition away from its heap leaching roots to a possible mill and carbon-in-leach (CIL) process would have added significant capital costs and shortened the mine life significantly.

Before making this significant capital decision, Eldorado paused to take time to undertake the necessary technical work and sought the technical collaboration of Weir Minerals on a solution.

“It was a very challenging period,” George Burns, President & CEO of Eldorado Gold Corporation, told IM. “Kışladağ is a cornerstone asset in our portfolio. It required collaboration with our geologists, metallurgists, site teams and technology providers, including Weir, working together on a solution.”

(Credit: Eldorado Gold)

In late 2018, results showed increased recoveries from an extended leach cycle. Following a deeper understanding of the geometallurgical drivers of the sulphide component of the deposit, improved heap leach recoveries were realised by optimising the metallurgical conditions. With these improved recoveries confirmed, the heap leach plan was revised in early 2019. Eldorado announced plans to resume mining, crushing, stacking and heap leaching at Kışladağ, and suspended plans to build the processing plant.

The decision came following metallurgical test work on the material placed on the heap leach pad in 2018. Gold recovery had increasingly exceeded expectations throughout the year, providing a new, positive heap leaching outlook beyond the near term. At the same time, Eldorado worked with Weir Minerals on the potential use of an HPGR at the operation.

HPGR creates a finer particle size, which helps to liberate the gold particles, resulting in increased recoveries.

An improved understanding of the leaching process on its low-grade ore and the potential of this cost-effective grinding addition provided it with the confidence to continue heap leaching beyond the short-term time horizon previously envisaged.

“Our collaboration with Weir on this effort is an excellent example of how Eldorado collaborates with technology leaders and seeks out solutions,” Burns said. “We believe this is a strength of the company’s core business values. We are agile and flexible ‒ a good example in both business and operations to find innovative, technical solutions and demonstrate prudent capital discipline. Ultimately, the solution was the best technical and economic decision for Eldorado.”

Testing, testing, testing

“The relationship between Weir Minerals’ and Kışladağ goes back to 2013,” Bjorn Dierx, Global Product Manager Enduron HPGR, told IM.

The OEM had been provided with samples on several occasions to investigate different flowsheet possibilities.

“The benefits of HPGR in heap leaching recovery were known by the site’s crew,” he said.

Enduron technology was tested 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.

All the time, the mine operator, Tuprag, Eldorado Gold’s subsidiary in Turkey, was evaluating the impact on gold recovery.

Although HPGR in closed circuit with screening was also tested, most of the test campaigns were focused on a configuration with so-called ‘Partial Product Recycling’ (PPR), according to Serhat Onol, Weir Minerals Senior Process Engineer.

“This system includes splitter plates underneath the rolls which ‘cut’ a proportion of the product discharge and reverts this back to the HPGR head feed,” he explained. “This recycle stream can be adjusted online to adapt the product grind towards the downstream requirements.”

PPR is not an option for every application but for Tuprag – due to the feed and desired product size – all specifications showed it was the best route.

“The hybrid solution with screening serves to increase the flexibility and to control the circulating load to the HPGR,” Onol said.

With capacity rates of around 4,200 t/h at Kışladağ, the screening area requirement was reasonably high, he explained.

“The hybrid solution uses screening only for the recirculating stream: the centre product of the HPGR reports to downstream leaching, whereas the rest is recirculated,” he said. This not only removes the fines re-entering the HPGR and, thus, reduces the circulating load, but also avoids the over-grinding of fines before leaching.

The PPR option, itself, is a very flexible operation, with the adjustable discharge splitter plates providing the best circuit flexibility in terms of throughput and product size, according to Onol.

(Credit: Eldorado Gold)

Great expectations

With the main driver of the HPGR installation being an improved gold recovery scenario, leading to an increased heap leach life, the pressure has been on Weir Minerals to come up with the goods.

The final flowsheet, which includes a 2.4 m diameter by 2.2 m long Enduron HPGR with the capacity to process 4,200 t/h, is much simpler than the existing circuit, according to Dierx, with less equipment to maintain, control and monitor.

“A single Enduron HPGR will replace all of the five existing tertiary crushers of which liners would have an average lifetime of circa-one month with a crusher availability of 85%,” he said. “The HPGR tyres have a wear life close to 18 months, with the HPGR achieving a high asset availability of more than 95%.

“Additionally, as a result of the HPGR combined with the hybrid PPR system, the existing four tertiary screens are also being decommissioned.”

Not only has the HPGR alleviated the use of this equipment, it is also set to boost that bottom line gold recovery.

“During testing, it was determined that the circuit configuration and HPGR operating conditions, such as pressure, have a direct influence on the gold recovery,” Dierx said. “The current expectation is that the average recovery increase after the commissioning of the Enduron solution will be approximately 4%.”

This could bring gold recovery to approximately 56%, as was declared in a 2020 press release from Eldorado that highlighted a 15-year mine life at Kışladağ with an average annual production of 160,000 oz.

In action

One of the largest brownfield HPGRs Weir Minerals has ever installed has just processed its first material at Kışladağ, which is testament to the hurdles both the Weir Minerals and Tuprag teams overcame during the height of the COVID-19 pandemic.

“Despite the challenges we faced during the pandemic, we were able to fully install the machine, including pre-commissioning, in only 22 weeks,” Dierx said.

(Credit: Eldorado Gold)

This was achieved by minimising the amount of work carried out on site via pre-site testing and modular HPGR assembly.

“As the available footprint in existing sites is limited, the unique Enduron design (length:diameter ratio) not only improves the grinding efficiency but also allows for easier implementation with less civil requirements,” Dierx said.

The machine has the potential to be digitally connected to the Weir Synertrex® IoT platform where operators can benefit from direct access to maintenance specialists, who will be monitoring performance and provide necessary operating guidance.

And the Weir Minerals team is confident more Enduron HPGR installations will follow the one at Kışladağ.

“We are very proud of the product’s recognition by our customers as the Enduron HPGR has been selected for all major greenfield HPGR projects in the hard-rock space,” Dierx said. “Despite all the key Enduron differentiators, it is not only merely about the product but particularly the wider experience across Weir Minerals in both the upstream and downstream processes via our wide product portfolio.

“Particularly in these brownfield applications, the system is not operating in a vacuum and every process change influences how the overall circuit works. This requires tacit knowledge, which Weir Minerals holds across their global teams.”

Eldorado’s Burns concluded: “We are pleased to implement a solution that increases gold recovery and supports a 15-year mine life at Kışladağ, allowing Eldorado to continue to provide employment and economic opportunity in the region, as well as provide a solid foundation for future growth.

“Kışladağ has been the cornerstone asset of Eldorado for over a decade, producing over 3 Moz of gold and generating significant value for all stakeholders during that period. This project is a testament to our exceptional team and technology partners working together to execute in challenging circumstances during the pandemic.”

Cora Gold adds SENET, CSA Global and Epoch Resources to Sanankoro DFS team

Cora Gold Ltd has made key appointments related to the definitive feasibility study (DFS) it is carrying out on the Sanankoro gold project in southern Mali, bringing SENET, CSA Global and Epoch Resources into the study team, as well as naming Russell Bradford as Project Manager.

SENET, a DRA Global group company, has been appointed as independent project manager to oversee the critical elements of the DFS, while CSA Global, a member of the ERM group of companies, will be the geological and mining consultant, tasked with managing the updated mineral resource estimate and mining study. Epoch Resources has been appointed to oversee the tailings storage facility of the DFS.

The DFS will build upon the January 2020 scoping study, which outlined average annual production from Sanankoro of 45,632 oz.

Following positive metallurgical test work results in the second half of 2020, in addition to more recent positive drilling results, the company says it is likely it will look to focus on a conventional gravity/carbon in leach processing route at Sanankoro to allow higher recoveries.

Bert Monro, CEO of Cora Gold, said: “2021 has seen significant activity at Sanankoro with exceptional results reported from our largest ever drill campaign at the project. These results will support an updated mineral resource estimate in the coming months, which, in turn, will be used as the basis for our DFS aimed at outlining the optimum route to develop Sanankoro into a new gold mine in Mali.

“Last year’s scoping study highlighted the potential high returns for Sanankoro and fuelled our confidence in Sanankoro’s strong fundamentals, and the company looks forward to publishing the DFS in the first half of 2022.”

Outotec addresses carbon losses in CIP, CIL gold circuits

Outotec has released a fully automated in-situ measurement system that, it says, can minimise gold solution losses in carbon-in-pulp (CIP) and carbon-in-leach (CIL) circuits.

Outotec CarbonSense is a system that is directly immersed in the slurry, eliminating the need for sample transfer and ensuring optimal carbon management, according to the company. It is designed to operate continuously in the agitated area of the reactor to ensure a highly representative analysis. It also features a reference probe to ensure reliable measurement even in changing process conditions.

As part of the solution, a single connection cabinet can support up to eight measurement assemblies, along with fibre-optic and copper media connections to the plant network, according to Outotec. “The system supports virtually any network topology, including star and ring configurations,” it said. “Maintenance is simplified with all power supplies and fuses located in one place.”

The portable calibration unit uses authentic slurry and carbon to ensure the best possible results and reliable measurement performance, Outotec said. No recalibration is needed when slurry properties change since the measurement is based on the difference between the measurement probe and the reference probe.

Continuous and reliable carbon concentration measurements minimise gold losses by enabling accurate control over carbon transfer between reactors, according to Outotec, with the Outotec ACT Carbon Management application designed to take full advantage of the measurement data.

The system is delivered as a ready-to-install package including measurement assemblies, connection cabinets and analysis software and licences.

The key benefits of Outotec CarbonSense, according to the company, are:

  • Enables continuous, representative, and reliable carbon measurement;
  • Supports development of efficient strategies for advanced CIP/CIL control;
  • Fully automatic operation;
  • Simple to install, operate, and maintain with no moving parts or sample transfer; and
  • Increases safety by helping to avoid blockages and other sampling issues.

Africa first for Gekko’s Carbon Scout technology

Gekko Systems says Endeavour Mining’s Ity gold operation, in Cote d’Ivoire, has become the first mine in Africa to install a Gekko Carbon Scout.

The self-contained Carbon Scout device collects slurry samples from carbon-in-pulp (CIP) and carbon-in-leach (CIL) tanks to determine the distribution of the activated carbon in the pulp for each tank, to an accuracy of ± 0.5 grams of carbon per litre of pulp, according to Gekko.

Gekko, following a collaboration agreement signed with Curtin University for the development and commercialisation of a cutting-edge carbon management technology, went commercial with the technology in 2017. At that point, the company said the product promised to quickly become critical for gold process plant optimisation and for minimising soluble gold losses on tails.

By advancing the accuracy, regularity and consistency of sampling, the self-contained, ground-level unit will improve measurements in CIL and CIP circuits, Gekko said, adding that the Carbon Scout will also greatly improve the safety of operations by reducing operator exposure to cyanide and other hazards.

As recently as November, Endeavour announced it had completed a planned CIL plant expansion at Ity, upping capacity from 4 Mt/y to 5 Mt/y. This followed the commissioning of the 4 Mt/y plant in April 2019.

Xtract Resources and Mutapa enter hard-rock processing collaboration pact

Xtract Resources has signed up to a collaboration agreement with Mutapa Mining and Processing (MMP) that could see the building of a carbon in leach (CIL) plant and the subsequent processing of hard-rock ore from its Manica deposit in Mozambique.

As part of this agreement, MMP, in partnership with Omnia – a company Xstract previously signed a joint venture and collaboration agreement with in 2018, will be appointed as independent mineral processing contractor on the various gold-bearing mineral deposits on the Manica concession.

MMP has agreed to finance and conduct the mining operations at Manica, including all capital and operating costs, with Xtract receiving between 20% and 23% (depending on the prevailing gold price) of the hard-rock after tax operating cash flow.

At the same time as this, Xtract intends to exploit gravity-recoverable gold during the CIL plant-build phase, assuming suitable material is available.

Colin Bird, Executive Chairman of Xtract, said: “This agreement allows for the extraction of CIL-amenable resources to be commenced in the mid-term whilst alluvial operations continue. The income to Xtract, once commercial levels of production are achieved, is expected to be significant and continuing for in excess of six years.”

While alluvial operations continue and the CIL plant plan is finalised, alluvial gold will still be exploited through an agreement Xtract has in place with Huafei Gold Resources.

And a concession exploration programme will be initiated with a view to enlarging the currently known resource by drilling at depth and testing areas not yet investigated, according to Bird.

MMP is currently the owner of a 42,000 t/mth hard-rock processing plant, which includes crushing, milling and gravity recovery circuits and furnace, for mining and mineral processing, located in the Manica region of Mozambique, according to Xtract. This plant has already had over $11 million invested to date and, as Xtract is aware, represents the only sophisticated hard-rock processing capacity in the Manica region, the company said.

“The MMP plant is the key reason supporting the rationale of agreeing the collaboration agreement, as it reduces both capital expenditure requirement and the time to production of the Manica project,” Xtract said.

MMP has undertaken to commence construction of the CIL plant and to provide Xtract with a schedule of purchases and an activity chart indicating initial production and a target date for commercial production of no later than July 1, 2020, Xtract said. Commercial production is defined in the agreement as the installation of a processing plant with throughput capacity of not less than 29,000 t/mth (being 70% of the planned 42,000 t/mth throughput).

Endeavour Mining pours first gold from Ity CIL project in Côte d’Ivoire

Endeavour Mining Corp says it has poured first gold from the Ity CIL project in Côte d’Ivoire, ahead of schedule and under budget.

The first pour yielded around 1,800 oz of gold, while some 135,160 t of ore has been processed since being first introduced into the CIL processing plant on February 20.

The initial estimated budget was $412 million, with first gold pour coming four months ahead of the previous schedule.

Endeavour said commercial production was expected to be declared early in the June quarter with performance trial testing soon to commence, as the crushing, milling and CIL circuits have quickly attained a stable nameplate capacity of 4 Mt/y.

Following the performance tests already conducted, Endeavour launched optimisation and de-bottlenecking work which is expected to increase the plant nameplate capacity by 1 Mt/y to 5 Mt/y, at a minimal cost of $10-15 million.

The volumetric upsize work mainly comprises an upgrade in pipes and pumps, and a second 50-t oxygen plant with no additional mining fleet required. “These plant upgrades are expected to be completed during scheduled plant maintenance shut-downs throughout the next six months,” the company said.

Sébastien de Montessus, President and CEO, said: “As we approach commercial production, I would like to acknowledge the hard work of our in-house construction team for successfully delivering the Ity CIL project ahead of schedule and under-budget, with an exceptional safety record of over 8.5 million hours without a lost time injury.

“This first gold pour and the remarkably quick ramp-up period is a transformational event for Endeavour as the Ity CIL project will quickly contribute to group’s cash generation potential.

“Given its current 15-year mine life and strong exploration potential, our ability to increase the plant size by 1 Mt/y to 5 Mt/y for minimal additional capex represents a very compelling investment and is in line with our focus on capital allocation efficiency and return on capital employed criteria. With this upgrade, Ity has the potential to produce circa 300,000 oz of gold per annum at a low all-in sustaining cost (AISC).”

The Ity CIL project capex spend is tracking under-budget compared to the initial guidance of $412 million.

“As construction is tracking ahead of schedule and below budget, Endeavour decided to conduct additional works such as the construction of a fuel farm, building exploration facilities, and an additional $7 million of crop compensation and resettlement related to prospective exploration grounds,” Endeavour said. “Due to these additional works, and the $10-15 million required for the above-stated plant upgrade to 5 Mt/y, the total project capex spend is expected to amount to circa-$420 million.”

An $11 million exploration programme totalling around 71,000 m of drilling has been planned for 2019, with the aim of delineating additional resources at the Le Plaque target, and testing other targets such as Floleu, Daapleu SW and Samuel.

As previously guided, Ity is expected to produce 160,000-200,000 oz in 2019 at an AISC of $525-590/oz, with the bottom-end of production guidance corresponding to the 4 Mt/y nameplate capacity while the top-end factors in upsides such as an earlier start date, an expedited ramp-up and the plant producing above its nameplate.