Watching dawn break from Century’s processing plant. The yellow-covered machines are flotation cells that, using a chemical process, separated zinc and lead from waste material.
The processing of ore at Century began at the Run of Mine (ROM) pad where ore was fed into a mechanism called the crusher. Here the ore was crushed from approximately 300mm down to pieces of approximately 100mm. After crushing, the ore was carried along a conveyor into the semi-autogenous grinding (SAG) mill where it was broken down further. From here, it was fed into the ball mill where steel balls (about the same size as a baseball) ground the ore into the material about fifty five microns in size. A micron is 1/1000 of a millimetre.
Century concentrator and other infrastructure (2015). The conveyor that carried ore from the crusher is seen on the left.
The Century concentrator was 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 was ground using an ultrafine grinding process. This ground the material to approximately six microns which maximised the amount of zinc that was recovered.
Once processing in the flotation circuit was complete, the zinc concentrates were thickened into a slurry form which resembled a thick liquid or paste of about 37% solids. During this process, excess water was transferred to the process water clarifier and then onto the process water dam, where it was recycled back into the process. The zinc concentrate slurry was pumped to storage tanks in preparation for transfer to Karumba via Century’s 304 kilometre underground slurry pipeline.
Tailings produced during Century’s operations are contained within a tailings storage facility (TSF) on the Lawn Hill mine lease.
The facility design incorporates natural topography and constructed earthfill embankments to contain the tailings over an area of 360 hectares.
During operations, process water transferred to the TSF with tailings were removed and stored in a nearby dam for evaporation or re-use in site activities.
MMG continues to consider options for long-term management of the Century tailings following the completion of processing operations in the first quarter of 2016. This includes the assessment of proven capping techniques as part of a new-three partnership with the University of Queensland’s Sustainable Minerals Institute (SMI).
Thickening tanks were part of the processing infrastructure at Century.
The Century concentrator operated 24 hours a day, seven days a week with most employed working an 8-days-on, 6-days-off, 7-days-on, 7-days-off roster.
A key element of Century’s plant maintenance program involved temporarily suspending processing operations to allow certain works to be carried out on the processing plant. This is known in the industry as a “shutdown”. Century scheduled three shutdowns of varying lengths each year. These shutdowns were carefully managed with safety of Century and contracted employees the most important priority.
A team does a Job Safety Analysis during a 2015 shutdown in Century's processing plant.
Century concentrate pipeline
A 304-kilometre underground pipeline was used to transfer processed zinc and lead concentrates to Century’s Port operation at Karumba for shipping. The concentrates were transferred in slurry form, with water separating each batch.
A single pump station at Lawn Hill pressurised the concentrate slurry, providing the driving force to propel the product through to Karumba. It took about three days for a batch of concentrate to be transferred the 304 kilometres.
The pipeline was continuously monitored at Century’s Lawn Hill and Karumba sites, with 10 spools located at various points along the length of the pipeline measuring pressure and flow rates.
To ensure safe and reliable operations until the end of Century open pit production, and in addition to continuous maintenance, MMG undertook a program of maintenance in 2012 costing approximately $14 million (AUD). This included improvement works on the pipeline.
Following the completion of production, the pipeline was cleaned using a series of Pipeline Inspection Gauges (PIGs) and then dewatered and left in place, buried approximately one metre below the surface. A cathodic protection system continues to be active. This is a common anti-corrosion technology.
Built as part of mine development in the late 1990s, Century’s Karumba Port facility included a dewatering and drying circuit, maintenance workshop, concentrate storage shed and buildings.
After receipt of batches of zinc and lead slurry at Century’s Port facility, water was removed from the concentrate slurry by five pressure filters. Zinc concentrates also passed through a rotary dryer to remove any remaining water. Following this process, product was stockpiled in Century’s concentrate storage shed in preparation for shipping.
Century completed a $32 million three-year project to fully refurbish its concentrate storage shed in late 2012. The project involved replacing more than 11 kilometres of purlins and 300 roof bracing members within the shed structure. As a result of the project, the Karumba concentrate storage shed now holds a Category Four cyclone rating.
MMG was recognised with a High Commended Award at the Australian Institute of Project Management (IPM) for the project in 2013 (read more).
Like Lawn Hill, Century’s infrastructure at Karumba is being kept in a state of Operational Readiness in the event that a future growth option becomes feasible.
Karumba Port and MV Wunma
Century’s transfer vessel, the MV Wunma, was custom-built for the shallow waters of the Norman River channel and is used to transfer parcels of concentrate to export ships anchored in the Gulf of Carpentaria.
Pronounced ‘Wood-MA’, the transfer vessel’s name means storm bird in the Lardil language of Mornington Island.
Self-loading and discharging, the MV Wunma was specially built to navigate the shallow waters of the Norman River channel and Gulf of Carpentaria. It took about 12 hours for the vessel to load, transport and discharge concentrates and return to the Karumba Port. The vessel operated day and night, depending on tides and weather conditions.
Whilst in use, ongoing maintenance was conducted on the MV Wunma to ensure safe and reliable operations.
This program included dry-dock services every four years, which allowed crews to carry out essential maintenance to the underside and other parts of the vessel that could not be accessed while the MV Wunma was on water. Engineers also checked all underwater marine components of the vessel during dry-dock, including its propellers, shafts and bearings.
Four dry-dock services have been conducted on the MV Wunma since its commissioning in 1999. The last dry-dock service was undertaken in 2014, when the vessel was sailed to Singapore for Lloyds re-certification and dry-dock maintenance.
In line with MMG’s commitment to safety, Century partnered with the Queensland Government to install next generation lights for the Port of Karumba. Century provided $60,000 of funding for the high intensity LED lights at the entrance of the Norman River channel. Ownership of the lights has now passed to Maritime Safety Queensland (MSQ).
During the years of Century’s operations, MMG and previous owners of Century dredged the channel to allow shipping without the limitations of tidal movements. Dredging of the channel by MMG ended in line with the completion of shipping.
Century’s contribution to Karumba therefore provides ongoing benefits to commercial and recreational fishers, as well as the live cattle export trade.
The Wunma (right) transfers Century product to a larger ship for transport to smelters in Australia, North Asia and Europe.