St Ives — 2021 Technical Report

The Lefroy carbon-in-pulp processing facility treats open pit and underground ore through a conventional crushing, grinding, gravity, leaching, and adsorption circuit, with a supporting heap leach legacy operation.

Report context

This report section forms part of the St Ives 2021 Technical Report, which describes the recovery methods and process plant design for the Lefroy operation. The 4.7 Mtpa Lefroy CIP process plant is the central processing facility for the St Ives operation, and the report provides a detailed description of the installed flow sheet, recent plant performance, and processing risks.

Processing route

Crushing and grinding

Ore from open pit and underground operations is hauled from local surface ore pads to the run-of-mine pad at Lefroy via dedicated on-road and off-road multiple trailer road trains operated by a specialist contractor. Ore is fed to the primary crusher either by loader or direct tipping from road trains, with blending managed according to grade and physical characteristics to optimise throughput and recovery.

Primary crushing is carried out using a 54/77 gyratory crusher, with crushed product transferred to a coarse ore stockpile. Grinding is completed using a 13 MW single stage variable speed 36' x 20' SAG mill with a 315 kW pebble crusher in closed circuit. The target grind size is 80% passing 125 µm. Classification is performed by ten 20" hydro-cyclones, with overflow reporting to the leaching circuit and underflow returned to the mill, with a split directed to the gravity recovery circuit.

Gravity recovery

The gravity recovery circuit comprises a gravity screen, two Knelson QS40 concentrators, and an In-line Leach Reactor. A portion of cyclone underflow slurry at a nominal 280 t/hour is fed to the two Knelson concentrators after screening. Concentrate is collected and subjected to intensive cyanidation in an In-line Leach Reactor (ILR 3000BA), with tailings recycled to the milling circuit.

Leaching and adsorption

Cyclone overflow is leached in five 3,400 m³ sequential leach tanks in series. Gold adsorption is achieved using a six-stage carousel AAC Pump Cell CIP circuit. Loaded carbon is processed through a 5 t split AARL elution, with electrowinning and smelting to produce crude doré. Gold wool from both the gravity and CIP electrowinning circuits is calcined and further refined through smelting.

Gravity gold production averages approximately 36.4% of total gold produced, with the leach circuit contributing approximately 56.0%, for an overall processing reconciled recovery of 94.6% for 2021. Gravity doré bars are approximately 90% to 95% gold, while CIP doré bars range from 80% to 90% gold depending on ore feed characteristics. Doré is dispatched to the Perth Mint for refining into gold bullion.

Tailings management

Tailings from the CIP section is thickened to a target slurry density of 55% solids w/w before being pumped to an in-pit tailings storage facility via two-stage pumps. When required, hypersaline water from the North Orchin bore is injected into the tailings slurry at the tailings pump box to maintain total dissolved salts above 50,000 ppm. A cyanide detoxification circuit using the INCO method (sodium metabisulphite and oxygen) was commissioned in 2019.

Heap leach legacy

The heap leach facility was closed at the end of 2012, with the associated wet plant decommissioned in May 2016 following irrigation of historic cells. Crushing, agglomeration, stacking, and wet plant equipment was subsequently sold. The heap leach drainage system and water collection ponds continue to be maintained in accordance with government regulations and International Cyanide Management Code requirements. Spent heap leach material is flushed and categorised to Class I landfill criteria for causeway construction.

Plant modifications and requirements

Commissioned in 2005, the Lefroy plant has undergone several key changes and upgrades since original installation, including:

• Upgrade of pre-existing gravity concentrators to late model Knelson concentrators
• Decommissioning of the sulphides recovery and regrind circuit
• Installation of a partial INCO cyanide destruction circuit using sodium metabisulphite and oxygen
• Relocation and upgrade of the main plant control room
• Replacement of linear trash screens with vibrating screens

Key work associated with the 2021 life of mine plan includes replacement of the gravity concentrate ILR. A structural refurbishment program is ongoing to maintain structural steel and concrete and ensure compliance with the International Cyanide Management Code.

Estimated key process plant requirements for the Mineral Reserve life of mine plan are based on the 2022 Budget LoM, prorated based on plant feed mass. The number of plant employees required is in the range of 40 to 50.

Key reported parameters

Technical qualifications

Metallurgical testing and scale-up

Laboratory test results are scaled up to estimate industrial processing performance, with the testing regime designed to represent the installed facility based on years of operating experience. Gravity and leach recoveries achieved in the laboratory are assumed to be achievable at plant scale, with overall results model-fitted to a bounded head grade relationship. Hardness properties are applied to the Morrell Total Power method to estimate grinding mill throughputs.

However, inherent uncertainty remains in predicting industrial performance from small-scale laboratory tests due to differences between laboratory methods and full-scale processes, and potential errors in testing. A key limitation is the practice of ore blending, which can limit direct comparison of plant performance with laboratory results over the longer term. No pilot-plant testing is carried out prior to reserve declaration, with metallurgical properties based on bench scale test results only. The sample requirements and cost for pilot testing are considered prohibitive; however, given the relative simplicity of the facility and its operating history, pilot plant testing was not considered required for estimation of plant modifying factors for the 2021 reserves.

Deleterious elements

Routine metallurgical testwork includes multi-element ICP-MS head analysis to check for potentially deleterious elements including mercury, arsenic, organic carbon, antimony, tellurium, and base metals. This assessment is carried out on a limited number of metallurgical composite samples and is not typically undertaken on individual exploration samples. The relatively low number of metallurgical samples initially checked for deleterious elements means some inherent risk remains of unexpectedly encountering such species during subsequent mining and processing operations.

Major equipment failure risk

The plant consists of a series of dedicated unit processes with inherent risk of catastrophic failure of key equipment items such as crushers, grinding mills, or leach/CIP tanks. Risk minimisation activities include a dedicated on-site maintenance department undertaking condition monitoring and preventative maintenance, selected critical maintenance spares, contingency operational plans (including contract/mobile crushing plant and leach/CIP tank by-passing), fire suppression systems, and insurance. Some inherent risk and uncertainty associated with catastrophic failure of processing equipment remains outside the control of the Qualified Person.

Operating cost estimation

The estimation of future processing costs is required as input into cut-off-grade calculations and economic assessments. Cost estimation requires assumptions concerning consumables consumption rates, unit prices, and inflation rates. Consumables, commodity pricing, and inflation are subject to external influences outside the control or predictive capability of the Qualified Person. Operational decisions made by plant management, or unexpected variances in ore nature, could unexpectedly impact reagent and consumables usage rates.

Source: St Ives CPR, 2021. Sections 17.1, 17.3, 17.3.2, 17.3.4, 17.3.5, and 17.3.6.