Project Overview
Location: Waihi, North Island, New Zealand
Process Type: Conventional CIP Plant
Annual Production: 800,000 tonnes (increasing from 660,000)
Feed Grade: Not specified
Recovery Rate: Historical 85-95%
Final Product: Not specified
Project Stage: Plant Upgrade
Detailed Process Description
Comminution Circuit
The Waihi processing facility employs a sophisticated comminution circuit designed to handle both open-pit and underground ores. The circuit begins with a static grizzly (150mm aperture) followed by jaw crushing to achieve a P80 of 110-130mm. The SAG mill operates with a 10% ball load and draws between 2.1 and 2.5 MW of power. The facility is undergoing a significant upgrade with the replacement of the existing 1.2 MW ball mill with a new 1.8 MW tower mill for enhanced grinding efficiency.
Leaching and Adsorption
The leaching circuit consists of five leach tanks and seven carbon-in-pulp (CIP) adsorption tanks, providing a total residence time of 24 hours for open-pit ore and 48 hours for underground ore. The pre-leach thickener increases slurry density from approximately 15% to 37-40% solids. Cyanide concentration is maintained at 250-280 ppm, with oxygen addition in the first leach tank to enhance kinetics and reduce cyanide consumption.
Elution and Recovery
The facility utilizes the AARL elution process, operating at high temperature and pressure to strip precious metals from loaded carbon. The circuit includes carbon regeneration capabilities with a kiln operating at 650-750°C with a 20-minute residence time. The electrowinning circuit produces gold and silver-bearing sludge which is dried, mixed with fluxes, and smelted at 1200°C to produce doré bars.
Plant Upgrades
Current upgrades include installation of larger capacity Derrick interstage screens, replacement of air lifts with centrifugal recessed impeller carbon advance pumps, and enhancement of the tailings pumping system. The modifications will increase underground ore processing capacity from 0.66 Mt to 0.8 Mt per annum, while maintaining the ability to process 1.25 Mt per annum of open pit ore.
Environmental Controls
The operation includes a comprehensive water treatment plant (WTP) capable of handling 1,800 m³/hr for metals removal and 500 m³/hr for cyanide destruction and metals removal. The WTP incorporates both metals precipitation and cyanide oxidation processes, ensuring environmental compliance before discharge to the Ohinemuri River.
