This article summarises the processing route and proposed upgrades for the Waihi District pre-feasibility study, based on decades of operating data and recent metallurgical testwork.
Report context
This article is derived from the NI 43-101 Technical Report – Waihi District Pre-feasibility Study, New Zealand, released 11 December 2024 (Document ID: STU-063-REP-002-0). The report presents a pre-feasibility study for the Waihi District, including the processing plant that has been operational since 1988.
Processing route
Existing flowsheet and operating history
The Waihi processing plant uses a conventional gold recovery flowsheet comprising five stages: comminution, leaching/adsorption, elution, electrowinning and smelting. The plant has been in continuous operation for 35 years. It underwent a major upgrade in 1999 to increase throughput capacity of open-pit mill feed and a minor upgrade in 2006 to introduce campaign treatment of underground ore.
Current throughput capacity is 1.25 million tonnes per annum on open-pit mill feed and 0.66 million tonnes per annum on underground ore. Open-pit and underground ores are treated on a campaign basis due to differences in ore hardness and gold liberation size, with open-pit ores being softer and requiring a coarser grind.
Comminution
The existing comminution circuit is designed to treat open-pit ore at 150 tonnes per hour to produce a target grind size (P80) of 75 µm, or underground ore at 80 tonnes per hour to produce a target grind size (P80) of 53 µm. Stockpiled ore is reclaimed by front end loader and fed onto a static grizzly with an aperture of 150 mm. Before entering the semi-autogenous (SAG) mill feed chute, ore is reduced in feed size via a jaw crusher to a P80 of 110–130 mm. The SAG mill uses 125 mm balls and operates typically with a 10% ball load, drawing between 2.1 and 2.5 MW of power.
The SAG mill is normally set up as a closed circuit but can be operated in open circuit if required. The SAG mill discharge is sized using a trommel; +12 mm oversize material is conveyed to a 30 kW cone crusher and recycled back to the SAG mill. Undersize slurry from the SAG trommel is pumped to two 500 mm diameter inclined Weir Warman Cavex cyclones. Cyclone underflow reports to the SAG mill feed chute, while cyclone overflow gravitates to the ball mill discharge hopper. Slurry is then pumped to a cyclone distributor consisting of fourteen 250 mm diameter Weir Warman Cavex cyclones. The cyclone underflow reports back to the ball mill for further grinding, and the cyclone overflow reports to the pre-leach thickener. The ball mill is always operated in closed circuit.
Leaching and adsorption
The pre-leach thickener increases slurry density from approximately 15% solids to approximately 37–40% solids before the leach/adsorption circuit. The circuit comprises five leach tanks of 700 m³ capacity and seven carbon-in-pulp (CIP) adsorption tanks of 300 m³ capacity. Total residence leach/adsorption time is 24 hours for open-pit ore and 48 hours for underground ore. Wedge wire cylindrical inter-stage screens with mechanical wipers are installed in each adsorption tank. A bleed stream is pumped from an adsorption tank to the previous tank, providing counter-current flow: slurry flows from adsorption tank 1 to 7 while carbon flows from tank 7 to 1. From adsorption tank 7, slurry passes over a carbon safety screen before barren tailings are pumped to the tailings storage facility.
Cyanide is delivered and mixed on-site via a sparging system to a concentration of 21% wt./vol. Cyanide is dosed into the first leach tank and maintained at 250–280 ppm. Oxygen is added to the first leach tank by a shear reactor to enhance leach kinetics and reduce cyanide consumption.
Elution, electrowinning and smelting
Loaded carbon from the adsorption circuit is fed into an elution column where carbon is washed at high temperature and pressure using the AARL process. The pregnant eluate is passed through electrowinning cells where gold and silver are electroplated onto stainless steel cathodes. After elution, barren carbon is reactivated and recycled to the adsorption tanks.
Cathodes are periodically harvested and rinsed to yield a gold and silver bearing sludge. The sludge is dried, mixed with fluxes and put into a furnace at 1200°C. The molten sludge is poured as bars of doré ready for shipment to the Mint.
Metallurgical accounting
Metallurgical accounting is based on tonnage of wet ore processed through the comminution circuit, totalized on a conveyor weightometer, and gold receipts from the Mint. Wet tonnes are converted to dry tonnes using a moisture factor derived from samples taken from the conveyor. Gold production is based on gold receipts from the Mint and changes to gold stocks in circuit, with gold stock takes taken monthly.
Proposed process plant upgrade
To align with the proposed mine schedule for the new WUG orebody, throughput capacity on underground ore will need to increase to 0.8 million tonnes per annum (100 tonnes per hour). Ausenco were engaged to design and cost the expansion options to pre-feasibility study level accuracy. OceanaGold provided the major inputs to the process design criteria and specified the major equipment.
Key elements of the proposed plant upgrade include:
- Installation of a primary crusher salvaged from OceanaGold's Reefton process plant
- Relocation of process water and elution water tanks that currently sit in the proposed crusher site
- Installation of a 1.8 MW tower mill for secondary grinding to replace the existing conventional 1.2 MW ball mill, with associated feed pumps and hopper
- A new classification circuit to operate with the new tower mill, including cyclones, feed pump and associated hopper
- Replacement of six current adsorption tank interstage screens with larger capacity Derrick units
- Replacement of six current adsorption circuit air lifts with centrifugal recessed impeller carbon advance pumps
- Upgrade of the tailings pumping system to pump to the new TSF3
- Containerized MCC to provide power to the primary crusher area
- Upgrades to existing switchroom(s) to support installation and associated new equipment
- HV/LV switchroom and stepdown transformers
Water treatment plant
The Waihi processing plant operates with a positive water balance. A water treatment plant (WTP) and a reverse osmosis (RO) plant treat various water sources before discharging into the local Ohinemuri River. The WTP has been in operation since 1988 with upgrades in 1999 and 2011. The RO plant was commissioned in 2008 but has not been used since 2012. The WTP has performed consistently with no recorded non-compliances with consent conditions.
The WTP incorporates four parallel streams: three dedicated to soluble metals removal only, and one with two phases of treatment including oxidation of cyanide followed by metals precipitation and removal. Lime and ferric chloride are added to all four water streams to facilitate metals precipitation. Cyanide oxidation uses a combination of hydrogen peroxide, copper sulphate and lime. Clarifiers remove suspended solids and metals; the resulting slurry is pumped to the tailings pond via a thickener. Carbon dioxide is added to clean water overflow to reduce pH to compliance limits.
Two polishing ponds hold treated water for approximately 16 hours prior to discharge, allowing time for testing. Water that meets discharge criteria is discharged to the Ohinemuri River; water that does not meet criteria is recycled back through the plant, used in processing, or pumped to the tailings storage facility. There are five operating regimes, each providing for a different combination of water requiring cyanide destruction versus metals removal only.
Proposed water treatment plant upgrade
GHD were engaged to design and cost an upgrade of the WTP to meet predicted demands of the WNP. The design capacity of the expanded WTP is double that of the existing infrastructure: 1,800 m³/hr for metals removal and 500 m³/hr for cyanide destruction and metals removal. This aligns with expected dewatering flows and the newly consented discharge regime (Regime E).
The upgrade study identified preferred treatment options:
- Metals removal: Two additional streams replicating the current metals precipitation treatment method, including mixing tanks, clarifiers, a thickener, and in-pipe CO₂ dosing
- Cyanide destruction: One additional 250 m³/hr stream replicating the current hydrogen peroxide oxidation process, including mixing tanks, chemical oxidation tanks, and a clarifier
The study also identified that lime storage would need to be doubled, and river discharge infrastructure would need upsizing to accommodate higher outflow rates.
Key reported parameters
| Parameter | Existing design/operating value | Proposed upgrade design value |
|---|---|---|
| Open-pit ore throughput rate | 1.25 Mt per annum (150 tph) | No change reported |
| Underground ore throughput rate | 0.66 Mt per annum (80 tph) | 0.8 Mt per annum (100 tph) |
| Open-pit ore target grind (P80) | 75 µm | 75 µm (testwork confirmed) |
| Underground ore target grind (P80) | 53 µm | 53 µm (testwork confirmed) |
| Target gold recovery | +90% | +90% (testwork confirmed) |
| SAG mill power draw | 2.1–2.5 MW | No change reported |
| Ball mill power | 1.2 MW (existing) | Replaced by 1.8 MW tower mill |
| Leach/adsorption residence time (open-pit) | 24 hours | No change reported |
| Leach/adsorption residence time (underground) | 48 hours | No change reported |
| Leach tank capacity | 700 m³ each | No change reported |
| Adsorption tank capacity | 300 m³ each | No change reported |
| Cyanide concentration in leach | 250–280 ppm | No change reported |
| WTP metals removal capacity | Not specified (existing) | 1,800 m³/hr (expanded) |
| WTP cyanide destruction capacity | Not specified (existing) | 500 m³/hr (expanded) |
| Process unit cost (historical, >40,000 t/month) | NZ$34–50/tonne milled (CPI adjusted) | Not reported |
*Note: Parameters are sourced from the pre-feasibility study report. Design values are from the proposed expansion; actual values are from historical operations; testwork values are from metallurgical testwork on MUG, WUG and WNP orebodies.*
Technical qualifications
The processing information in this article is based entirely on the NI 43-101 Technical Report – Waihi District Pre-feasibility Study, New Zealand. The report specifies that Ausenco were engaged to design and cost the expansion options to pre-feasibility study level accuracy. The proposed plant upgrade and water treatment plant upgrade are subject to further engineering design and costing. Metallurgical testwork results cited are for MUG, WUG and WNP orebodies and support ongoing use of the existing flowsheet. The study does not report overall project economics, ownership details, current operational status, or links to external sources beyond the technical report.
Source: NI 43-101 Technical Report – Waihi District Pre-feasibility Study, New Zealand, OceanaGold Corporation, Document ID: STU-063-REP-002-0, released 11 December 2024.
