Waste

• Waste rock - rock containing little or no gold or silver, removed from the underground workings during construction of the decline, tunnels and drives.
• Leach residue - residual material from which gold and silver have been extracted during the cyanide leaching process.
• Other waste materials - paper and cardboard, glass and plastic containers, wood, aluminium cans, used oils and grease from mine machinery.

Read on to find out how these waste materials are managed to minimise their environmental impact.

Waste rock

There are two types of waste rock produced at Henty,

• acid-forming waste and
• clean waste

Acid forming waste contains sulfide minerals, but little or no gold or silver. When the sulfide minerals are exposed to air and water at the surface, the sulfides oxidise and form acidic water. If this acidic water enters the soil, streams and rivers, it could cause significant environmental damage. For this reason, one of the State Government's environmental regulations governing the Henty operation is that all acid-forming waste rock must be returned underground before the mine closes. Acid-forming waste is stored temporarily in an old Hydro Tasmania quarry, and is progressively being incorporated, along with leach residue into a special cemented aggregate material which is used to back-fill mined out stopes. Henty was the first mine in Australia to utilise this technique.

Environmental damage caused by acid water drainage at a mine in South Australia (photo: AMEEF).

Clean waste is waste rock that contains no acid-forming sulfide minerals, and therefore does not produce acid water. Currently, all clean waste is being used to construct a new tailings storage facility on the mine site. Previously, clean waste was used to rehabilitate old Hydro Tasmania quarries on the lease.

Leach residue

Leach residue is the residual material left after gold and silver have been extracted by cyanide leaching from the powdered ore. It contains unwanted solid finely ground rock material, non-valuable sulfide minerals and water containing cyanide and other chemicals. Before storage, it is treated to remove cyanide solution left-over from the leaching stage.

The resultant muddy slurry is very nearly cyanide-free and is pumped along plastic pipes to the leach residue ponds for long-term storage. Leakage from these pipes is undesirable because the leach residue contains acid forming sulfide minerals and other chemicals. The plastic pipes through which the leach residue is pumped are therefore surrounded by sheathes of impermeable geotextile (synthetic impermeable material), so that any material that leaks from the pipes is contained within the outer sheath.

The pipes used to transfer leach residue to the leach residue ponds are seen running along the side of this road. Note how the pipes are enclosed in a special impermeable sheath to prevent leakage of material into the environment if the inner pipes are damaged.

Some solid leach residue is also mixed with concrete and acid-forming waste and used to back-fill mined out stopes underground. This increases the stability of the underground mine, and removes potentially acid-forming materials from the surface back underground where they came from in the first place.

At the end of the mine's life, the leach residue ponds will be revegetated with local native plant species.

Other waste

Efforts are made at Henty to store other materials, such as milk containers, aluminium cans, plastic and glass bottles, paper and cardboard, and used oil for recycling.