Processing
Concentrator operations
Run of mine (ROM) pad and conveyor belt
The processing of ore at Century begins at the run of mine (ROM) pad where the ore is fed into a mechanism called the crusher. Here the ore is crushed from approximately 300mm down to pieces of approximately 100mm. After crushing, the ore is carried along a conveyor into the semi-autogenous grinding (SAG) mill where it is broken down further. From here, it is fed into the ball mill where steel balls grind the ore into the material about fifty five microns in size. A micron is 1/1000 of a millimetre.
Century concentrator
The Century concentrator is primarily a conventional grinding and froth flotation circuit, comprising one SAG Mill, two ball mills, 21 stirred mills and 79 flotation cells. Due to the particularly fine nature of the zinc in Century’s ore, and its silicate content, the ore must be ground using an ultrafine grinding process. This grinds the material to approximately six microns which maximises the amount of zinc that is recovered..
Once processing in the flotation circuit is complete, the zinc concentrates are thickened into a slurry form which resembles a thick liquid or paste, of about 37% solids. During this process, excess water is transferred to the process water clarifier and then onto the process water dam, where it is recycled back into the process. The zinc concentrate slurry itself is pumped to storage tanks in preparation for transfer to Karumba via Century’s 304-kilometre underground slurry pipeline.
Thickening tanks
The Century concentrator operates 24 hours a day, seven days a week with employees working an 8-days-on, 6-days-off, 7-days-on, 7-days-off roster.
Maintenance
Maintenance is carried out regularly on the plant with works carried out by Century’s Site and Asset Reliability team.
A key element of Century’s maintenance program involves temporarily suspending processing operations to allow extensive works to be carried out at the plant. This is important as some maintenance cannot be conducted safely while the plant is operational. This program of works, known in the resources industry as ‘shutdowns’, are scheduled approximately three times each year at Century.
Century concentrate pipeline
The zinc concentrate slurry is transported to Century’s Karumba Port via a 304-kilometre underground pipeline. Lead concentrates are also transported to the Karumba Port through the pipeline, separated from zinc concentrates by batches of water.
A single pump station at Lawn Hill pressurises the concentrate slurry, providing the driving force to propel the product the 304 kilometres to Karumba. It takes about three days for the product to reach Karumba. The first 80 kilometres of pipeline is uphill with the remaining 224 kilometres about one degree downhill. The pipeline itself is small, only about 30 centimetres in diameter.
The pipeline is continuously monitored from control rooms at both sites. Regular inspections and testing are also carried out along the length of the pipeline to ensure its integrity.
Karumba Port
At the Karumba Port, water is removed from the concentrate slurry by five pressure filters. The zinc concentrate then passes through a rotary dyer to remove any remaining water. The remaining concentrate product resembles powder. After drying, the concentrates are stockpiled in an 80,000-tonne capacity storage shed.
Karumba Port and MV Wunma
MV Wunma
Century concentrate is exported via ship using the transfer vessel, the MV Wunma, which was specially built to navigate the Norman River channel water and shallow Gulf of Carpentaria waters.
Self-loading and discharging, the MV Wunma is designed to transfer parcels of up to 5,000 tonnes of concentrate to export ships anchored in the Gulf of Carpentaria. Due to the size of the export ships, a number of trips may be necessary to fill an order of concentrate. On average, Century loads one full shipload of concentrate each week which departs the Gulf waters.
It takes about 12 hours for the vessel to load, transport and discharge the concentrates and return to the Karumba Port. The MV Wunma operates day and night, depending on tides and weather conditions.
Pronounced ‘Wood-MA’, the transfer vessel’s name means storm bird in the Lardil language of Mornington Island.
Staying safe on the water
Century’s crews do everything they can to minimise the impact of our operations on locals and tourists. However, in the interest of public safety, we ask that people boating on Gulf waters:
- Stay at least 50 metres from the MMG wharf
- Stay at least 25 metres from the MV Wunma at all times
- Do not anchor in the Norman River when the MV Wunma is moving
Thank you for your cooperation and assistance.