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.