The remote Benambra mine site in Victoria’s East Gippsland mountains lies 23 km from the historic gold-mining village of Omeo, and close to the headwaters of the Tambo River.
From 1992 the mine was a busy producer of copper and zinc concentrates, but production ceased abruptly in 1996. By the beginning of 2006 the mine lay abandoned, a stark reminder of the hazards of the metal market.
At that point things began to change. Today the Benambra mine site is an outstanding example of Australian innovation in site rehabilitation and restoration.
The transformation owes a great deal to the skill of the Victorian Department of Primary Industries (DPI), Australian Tailings Consultants and Abigroup Contractors, and has won the department and the two companies the 2006 Excellence in Environmental Management Award, sponsored by CAPS Australia , in the Australian Mining Prospect Awards.
However it was not just the now vastly improved environment of the site that earned the esteem of the Australian Mining Prospect Awards judges. Rather it was the exceptional range of innovative and technically challenging techniques employed by the team in recovering the problematical site to industry best practice standard, while meeting the requirements of government, stakeholders and the local community.
“We believe we’ve set a new standard for rehabilitation through the creation of an environmentally self-sustaining, stable site,” says Peter Brecht, managing director of Abigroup. “And we were able to achieve this outcome in a spirit of cooperation with all the parties concerned.”
Mining operations at Benambra had ceased without the normal post-mining works to restore the environment, and in the second half of 2004 the site became a state government liability. It fell to DPI to carry out the rehabilitation of the mine.
DPI engaged Australian Tailings Consultants, who in turn called in various experts to assist in developing a plan to rehabilitate the site to a condition as near as possible to its pre-mining state. Abigroup won the contract to carry out the work.
The heart of the project was a tailings dam which during the mine’s productive period received hundreds of thousands of tonnes of sulfidic tailings by pipeline from the concentrate processing plant. At the time of commencement of the rehabilitation project the tailings, contaminated with heavy metals, were covered by water less than one metre deep, with tailings exposed to the air in some areas of the dam.
Sulfidic material elsewhere on the site had oxidised and formed sulphuric acid which rain water had spread, contaminating the soil.
The planned solution was to store the contaminated material in the tailings dam, and to create a permanent water body, at least two metres deep, covering the tailings.
“To achieve this, we first needed to level the tailings at the bottom of the dam. This was done by employing a dredge to redistribute the contaminated solids by progressively pumping them into a geotechnically horizontal plane,’ explains Peter Brecht.
“The tailings were then stabilised by mixing them with limestone sand, which helps prevent the potential resuspension and oxidation of the tailings if they are disturbed under the water body.
“Six hectares of jute matting were then laid above the tailings to provide an extra barrier to resuspension, and to help prevent the migration of dissolved oxygen from the water into the tailings, which would lead to acid generation.”
The tailings dam wall was strengthened by batter flattening to maximum credible earthquake standards.
By restoring the original creek courses in the catchment and removing diversion channels, water was channelled back into the tailings dam. Construction of a spillway to provide long-term geotechnical stability of the dam wall also permits controlled release of water to help maintain the required minimum depth over the tailings.
“Another innovation was the creation of an anaerobic wetland below the dam wall, to passively treat seepage from the base,” says Brecht.
“Upstream of the dam, passive systems have been installed to raise the natural acidity of the creek water closer to neutral prior to entering the tailings dam. This helps to reduce the level of metal content in the water body.”
Revegetation of the dam perimeter not only helped fulfill surface rehabilitation objectives, it also ensured a permanent supply of decomposing organic material to the dam, which helps minimise oxidation of the tailings while consuming oxygen in the water.Elsewhere on the site innovative methods were used to store and stabilise zinc-rich precipitates, described as having the consistency of whipped cream. They were mixed with recycled concrete to ensure they will not disperse.