Located in north-west Queensland, Century is Australia’s largest open-cut zinc mine. A 304-kilometre underground pipeline is used to transfer processed zinc and lead concentrates to Century’s port operation at Karumba for shipping. A single pump station at Lawn Hill pressurises the concentrate slurry, providing the driving force to propel the product over 300 km through to Karumba.
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.
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 underground slurry pipeline.
Pump Industry spoke to MMG General Manager Queensland Operations Mark Adams about the pipeline; “At MMG Limited’s Century mine, we use an underground pipeline to transfer concentrates from its mining and processing operation at Lawn Hill to its Port facility at Karumba, on the Gulf of Carpentaria. About 30 centimeters in diameter, the pipeline is located between 800 to 150 millimetres underground.
“A single pump station at Lawn Hill pressurises the pipeline, propelling batches of zinc and lead concentrates the 304 kilometres to Karumba. Monitoring spools located at various lengths along the pipeline measure flow and pressure rates, with teams at Century’s Lawn Hill and Karumba sites continuously monitoring pipeline operations. It takes about three days for a batch of concentrate to be reach Karumba.
“Once received at Century’s Port facility at Karumba, the zinc and lead concentrates are dewatered and stockpiled prior to transfer to the MV Wunma, MMG’s transfer vessel custom-build for operations on the shallow Norman River channel and Gulf waters. The MV Wunma then transports concentrates to export ships anchored in a designated area offshore.
“In 2012, we undertook an approximately $14 million program of pipeline maintenance and improvement work to ensure the pipeline can be operated safely and reliably into the future. The program included replacement of a 1.2 kilometre section of pipeline and 10 monitoring spools, extensive testwork and analysis, integration of newly-available best-practice standards and updating of contingency and other emergency response plans.
About the mine
Century produces zinc and lead concentrates at its mining and processing operation at Lawn Hill, located about 250 kilometres north-west of Mount Isa.Concentrates are then transferred via a 304-kilometre underground pipeline to Century’s Port facility for shipping to smelters in Australia, Europe and Asia. Century is currently in a transitionary period as it prepares for the end of open cut zinc production in approximately 2016 while examining options to extend operations. ■
About the pipeline
- 300NB X70 grade steel pipe
- Pipewall thickness 8.4mm at lawn hill, 4.8 mm at Karumba
- HDPE liner nominally 7.5 mm
- 1.2km flanged strings
- Minimal depth of 750 mm
- Directional drilling under all major rivers
- 3 x Wirth triplex diaphragm pump 8 ¾ “ x 14”
- 1.13MW VSD motors
- Parallel arrangement
- Discharge pressure – 20MPa (max)
- The pipeline operates continually at appox 270 m3hr-300 m3/hr
Zinc and lead concentrates are pumped in batches and separated by water and are often in the pipeline at same time.
As part of the refurbishment, the pipeline was deslurried by pumping water only, then a complete dewater was completed using compressors and foam pigs
