The Open Pit Mine
The Open Pit mine is 500 meter deep, below the highest point of surface topography and approximately 30 million tonnes (t) of ore per year is excavated. The material mined is a combination of gold bearing ore and barren waste.
Ore from the Pit is delivered to the crusher either as direct feed or via adjacent Run-of-Mine (ROM) Pads depending on metallurgical blending and material handling requirements.
Lower grade ore material that is not fed directly to the mill is stockpiled depending on cut-off grade applied. If the gold grades available in the Pit are lower than the stockpile grades, or if there is a shortage of direct feed ore, these stockpiles are recovered and fed to the mill.
Competent waste rock is disposed in stable dumps at Anawe and Kogai. Semi-competent and incompetent waste is disposed in two erodible dumps, Anawe and Anjolek.
Mining in the open pit is undertaken on 10 meter high benches. When the material is too hard to be free dug, the rock is drilled and blasted with a range of drill patterns to suit the litholog.
The Open Pit is a conventional truck and shovel operation and is serviced by a large modern mining fleet of 175t and 90t haul trucks, loaded by 200t and 120t shovels and excavators. There are also smaller 40t and 35t excavators for ancillary work.
The Underground Mine
The Porgera Underground Mine is a modern, trackless mine, producing approximately 1.4 million tonnes of ore per year from three primary production areas: North Zone, East Zone and AHD.
The underground (UG) operation mines the lodes at depth currently up 330 meter beneath the current pit floor. The Underground operation has a strike extent of approximately 1.6 kilometer (km). The primary access is via a twin decline system from surface with secondary internal accesses connecting the various mining areas. Mining is done by convention long hole open stoping. Drives are developed along the strike of the lenses at 15m vertical intervals with long-hole stoping used to extract the ore between these levels.
The voids generated by mining are back-filled with a combination of paste and co-paste disposal to enhance ground stability and allow maximum extraction of ore.
The UG mine operates on a modern trackless mining fleet consisting of CAT 2,900 loaders, 45t and 55t capacity haul trucks and supporting drills and auxiliary equipment.
The Processing Plant
Ore is processed in a complex minerals processing plant. The open-pit and underground mines supply both course gold and refractory ore to a processing plant that includes crushing, grinding, gravity recovery, flotation, oxidation (autoclave) and Carbon In Pulp (CIP) circuits to produce a gold dore product and a high grade pyrite concentrate for shipment to offsite refineries and smelters for final processing.
In 2016, 5.7 million tonnes of ore was delivered to the process plant.
Run-of-Mine (ROM) ore is crushed and ground in semi-autogenous (SAG) and ball mills, free gold is recovered in a gravity circuit and flotation is used to recover a sulphide concentrate. This is then oxidized using autoclaves, producing feed for conventional carbon-in-leach (CIL) cyanide leaching to recover the contained gold.
In January 2016, the Porgera mine commenced exporting of pyrite concentrate. A recent modification to the flotation circuit had enabled the production of a gold rich sulphide concentrate that is shipped to an offsite smelter for further gold extraction. The concentrate export enables lower grade stockpiled concentrate to be reclaimed and processed in the autoclave and CIL circuits oxidized on site.
The autoclaves operate at 1,725kPa pressure and 197°C to oxidize the pyrite (iron sulphide) into hematite (iron oxide) producing feed for the CIL circuits which recovers up to 95 per cent of the contained gold.
Lime used to neutralize the acid generated in the autoclaves is produced from limestone quarried from a quarry 15km from the mine. The limestone is burnt in two vertical kilns which use waste oil or diesel as fuel before the lime is trucked to the autoclave plant for addition to tanks to neutralize the acid.