Mineral Resource Estimate Update and Preliminary Economic Assessment of Kora North and Kora Deposits — Kainantu Project

This report documents the recovery methods and processing route assessment for the Kainantu Project, covering historical plant operations, current refurbishment status, and proposed upgrades for treatment of Kora ore.

Report context

This report, dated September 2018, provides the Mineral Resource Estimate Update and Preliminary Economic Assessment for the Kora North and Kora Deposits at the Kainantu Project. It draws on historical operating data from the Irumafimpa Mine processing between 2006 and 2008, current plant condition assessments, and scoping study findings for a proposed expansion to treat Kora ore. The processing information is based on the original plant design, operational experience, and engineering studies by Mincore Pty Ltd.

Processing route

Historical plant design

The Kainantu processing plant is located approximately 7 km from the opening of the 800 portal which accesses the Irumafimpa Mine. The plant was designed and constructed in 2005. Simple processing technology was used to treat Irumafimpa ore, consisting of crushing, screening, grinding, and flotation to separate sulphide-bearing material from non-mineralized host rock. The design throughput was 21 tonnes per hour (170,000 tpa). Approximately 10% of the ore was recovered as a high-grade gold-bearing flotation concentrate, with waste material pumped to an engineered tailings storage facility. The concentrate was packed in containers and trucked to Lae for shipping to smelters, including Japanese smelters, with specification generally acceptable to copper smelters seeking high gold and high sulphur feedstock.

Historical unit processes

The original Process Plant comprised the following unit processes:

• Ore Receiving and Crushing: ROM sizing reduction prior to reclamation for grinding. Screening and recycling was found to be problematical in previous operation and may be removed and replaced with a more suitable crusher operating in open circuit.
• Grinding and Classification: Crushed ore reclaimed and ground to the required size for flotation.
• Differential Flotation: Commencing with an Outokumpu Skimair Flash Flotation unit in the classification circuit, combined with Outokumpu tank cells treating grinding product, to recover a gold-bearing sulphide concentrate for export.
• Flotation tailings deposition in the tailings storage facility.
• On-site reagent storage and mixing facilities.
• Services for plant air and water distribution.

The plant treated ore from the Irumafimpa lodes over two separate periods between 2006 and 2008 (HPL and Barrick).

Current plant condition

The processing plant was placed on care and maintenance when processing ceased in December 2008. Following completion of studies on the redesign of the crushing circuit to handle wet clay rich mill feed, Mincore Pty Ltd was engaged by K92ML to undertake plant refurbishment and installation of a drum scrubber (Mincore, 2015). Refurbishment and repair commenced in March 2016.

A new main 415V switchboard was installed as part of the refurbishment. The process plant was enhanced by the addition of a drum scrubber commissioned in October 2016. The drum scrubber was installed to handle wet clay rich mill feed. The assay laboratory has been recommissioned under the control of Intertek.

Plant upgrade scoping study

In August 2016, Mincore completed a scoping study on requirements for upgrading the existing plant to allow treatment of Kora ore at a proposed rate of 400,000 tpa (Mincore, 2016). This was subsequently updated (Mincore 2018) following the in-progress installation of the gravity gold circuit and new lime preparation and dosing facility.

Mincore concluded that:

• There is sufficient crushing and milling power (comminution) to grind 50 tph to P80 of 106 μm.
• The current two stage crushing circuit is rated at 68 tph producing a product size P80 of 10–12 mm. However, the secondary crusher has been capacity limited during operations in 2018, and a new larger secondary crusher is proposed, with the existing crusher relegated to standby service. The new crusher will also provide a better crushed ore size distribution to maximise throughput of the downstream milling circuit.
• Additional flotation capacity is required to achieve acceptable residence times for each cell. There is sufficient space to install additional cells if future test work identifies a requirement for longer residence time.
• The existing concentrate thickener and filter is adequate for 400,000 tpa of Kora feed averaging 1.7% copper.
• The existing tailings line is adequate but a full pump upgrade will be required.

Proposed circuit modifications

Mincore suggested the following circuit modifications:

• Jaw crusher: Liner wear and capacity should be optimized by converting the existing pan feeder to grizzly finger feeder to bypass fines from jaw crusher.
• Secondary crushing: The current two stage crushing circuit is rated at 68 tph producing a product size P80 of 10–12 mm. The secondary crusher has been capacity limited during operations in 2018, and a new larger secondary crusher (J35/TC1000 or equivalent) is proposed, with the existing crusher relegated to standby service.
• Cyclone upgrade: The existing 150CVX cyclones will be upgraded to a 6 x 250CVX cyclone pack to meet the higher feed rate and recirculating load. Mass balance and cyclone selection is based on a 400 ktpa throughput (50 tph).
• Gravity circuit: A gravity concentrator is required in the milling circuit for recovery of any gravity gold. The gravity circuit will be installed to capture free gold that is liberated during the grinding process. One hundred percent of the primary mill discharge will be screened at 1 mm via a linear vibrating screen. Screen oversize will report directly back to the ball mill for further grinding. Screen undersize will gravitate to a Falcon centrifugal concentrator, which is automated to provide routine flushing of collected concentrate that will gravitate to a goldroom for upgrading via a Holman shaking table. The Holman table concentrate will be dried in an electric oven and then smelted in a gas fired barring furnace to produce Dore gold bars. Tails from the Holman table will report back to the primary cyclone feed pump for further processing. The gravity circuit is expected to recover a high proportion of the free gold, which is readily converted into Dore bars and refined for a significantly faster revenue/value realisation. The gravity circuit will reduce the amount of gold reporting to the flotation circuit, which should contribute to an overall improvement in recoveries.
• Classification arrangement: The Falcon concentrator tail will be pumped to a set of primary hydro cyclones for classification. The fine cyclone overflow will report to a Flotation Surge Tank. The cyclone underflow arrangement will remain as it currently is with a portion of the stream reporting to an existing Flash Flotation cell. The remainder of the cyclone underflow combines with the Flash Flotation tail and gravitates back into the ball mill feed chute for further grinding. The addition of the gravity screen will reduce the amount of coarse material the primary cyclones and the flash flotation cell receive, reducing wear rates and improving operating performance.
• Flotation reconfiguration: Reconfiguration of the current flotation circuit is based on preliminary test results generated by Barrick Gold indicating that Kora ore can produce a single high copper with gold concentrate. These test results were confirmed by operational results achieved during the treatment of the higher Cu-grade K2 ore lode in the first half of 2018. The existing rougher and cleaner circuits are proposed to be used for the new rougher duty since the current cleaner circuit comprises four cells which are exact duplicates of the four cells which make up the current rougher circuit. This will effectively double the capacity and provide satisfactory retention time with minimal modification. A new bank of Cleaner and Recleaner cells will need to be added and connected to the existing concentrate thickener and filter.
• Tailings: The existing tailings line is adequate but a full pump upgrade will be required.

Recommended test work

Mincore considered that test work is warranted to address:

• Further work to establish the most suitable grind size to achieve balanced liberation, recovery, concentrate grade and production rate. This has not been investigated by operations to date, and the current grind is P80 75 µm versus P80 106 µm design.
• Investigation of reagent type, addition rate and addition point, including use of new reagents (all at variable pH).
• Investigation of ore blending procedures by use of laboratory flotation tests at various grind sizes to optimise the grinding, flotation and filtering circuits.
• Variability test work to confirm comminution and flotation parameters.
• Barrick 2009 test work suggests that rougher mass pull balancing with cleaner cells will require consideration for high copper feed. Mincore suggested configuration of the 3rd and 4th Rougher Cells as Scavenger cells, and investigating the requirement for regrind of the rougher, and/or rougher scavenger concentrates. Mincore also suggested investigating opening and closed circuiting the cleaner tails, and undertaking locked cycle test work to confirm performance.
• Investigating reducing reagent consumption, particularly lime, by installation of a tailings thickener. Pumping power, water consumption and time would also be saved.

Key reported parameters

Technical qualifications

The following limitations and qualifications apply to the processing information presented in this report:

• The gravity circuit and gold room were still being installed at the time of the report. The expected Au recovery benefit from the gravity circuit of 0.5–1% may not be added to current recovery figures, as it has not been shown that the same recoveries for Cu and Au will be achieved at P80 106 μm at 400,000 tpa versus the current operation at P80 75 μm at 180,000 tpa. Parallel lab testwork is recommended with plant operation at both grind targets before and after commissioning of the gravity circuit.
• The flotation circuit reconfiguration was based on preliminary test results from Barrick Gold and confirmed by operational results from the first half of 2018. Further test work is required for reagent optimisation, ore blending procedures, variability confirmation, and locked cycle testing.
• The secondary crusher has been capacity limited during operations in 2018, and the proposed new crusher is subject to future implementation.
• Optimisation projects, such as tailings thickener installation, should be investigated during implementation of the expansion projects where results can be incorporated into the pre-commissioning phase.

Source: Mineral Resource Estimate Update and Preliminary Economic Assessment of Kora North and Kora Deposits , Kainantu Project, September 2018, Section 17: Recovery Methods.