Closure studies and revegetation trials at the Ranger Mine Site have been progressing almost as long as the project has been in operation.
Ecosystem re-establishment has been a central focus of research for more than 30 years. Over that period, a large body of information has been accumulated on the technical aspects of successful ecosystem rehabilitation.
The management of mine tailings is one of the key aspects of closure.
The depth of Pit 1 measured approximately 170 metres below surface. A staged backfill of Pit 1 commenced in 1996 with the tailings being deposited into the pit over an eight year period.
Following the initial settlement of the tailings, 7,700 prefabricated vertical wicks were installed over Pit 1 to drain water from within the tailings. The wicks consist of a corrugated plastic drainage core which is wrapped in a non-woven geotextile.
A preload cap of waste rock was placed on top of the tailings to load them and facilitate water expression through the wicks.
The method is designed to shorten the flow path of the pore water from tailings and expedite the tailings consolidation process.
A laterite cap placed on top of the preload was completed in January 2016 to convert the process water catchment to pond water catchment.
Pond water is of a much higher quality than process water and therefore needs less treatment before it is released to the external environment.
The surface area of Pit 1 measures 39.3 hectares, following settlement of the surface, waste rock will be used to create the final landforms before revegetation takes place.
Soon after mining ceased in Pit 3 in November 2012, the backfilling of 31 million tonnes of waste rock commenced.
The approach the ERA team has taken in the rehabilitation of Pit 3 is quite innovative and cutting edge in terms of uranium mining.
The back filling of Pit 3 was not simply a material movement exercise to fill the void. It was an engineered backfill (known as an underfill) which was constructed in a specific manner with select materials to provide space for brine injection.
The drainage system will serve as long-term storage of brine from the Brine Concentrator operations. Tailings from the mill and the tailings storage facility are deposited directly into Pit 3 through pipelines.
Once the tailings are transferred, interspersed layers of clay and waste rock will encapsulate the tailings and control the movement of contaminants.
ERA has utilised a comprehensive solute egress model to predict solute egress pathways and concentrations from the rehabilitated site to optimise the design and to ensure no detrimental impact to the surrounding environment over a 10,000 year period.
Following settlement of the surface, waste rock will be used to create the final landforms before revegetation takes place.