Waste

There are three main types of waste produced at Cannington:

• tailings,
• waste rock,
• other waste materials.

Read on to find out how each of these is managed.

Tailings

Tailings is the finely powdered gangue material left over after the valuable silver, lead and zinc bearing ore minerals have been extracted in the processing plant.

About 50% of the tailings is pumped to the tailings dam for permanent storage. At the end of the mine's life the tailings dam will be rehabilitated. (See the section on Land to find out how).

The tailings dam.

The other 50% of the tailings is mixed with cement to produce a paste. The paste is then poured through boreholes into the underground mine to backfill mined-out stopes. The advantages of this are that it:

• makes good use of a waste product by returning it to where it came from originally,
• stabilises the rocks underground,
• allows miners to safely remove the pillars of ore bearing rock between stopes, thus extracting more ore from the mine,
• prevents local ground-water from flowing into the empty mined out stopes. As well as flooding parts of the mine, this could lower the local water table, which is undesirable.

The head frame with the paste plant behind it.

Waste rock

Waste rock is non-ore bearing rock removed from the underground mine during construction of the main access decline, the hoisting, ventilation and other shafts, and the drives and tunnels constructed underground to provide access to the ore bearing stopes. The amount of waste rock produced in an underground operation is very small compared with an open cut mine.

At Cannington, waste rock is

• used for construction purposes around the mine site (eg roads both on the surface and underground, tailings dam walls, the 1.5 m high pad to prevent flooding etc),
• incorporated along with some of the tailings into the paste used to backfill mined out stopes underground,
• stockpiled for later use and/or rehabilitation.

Part of a waste rock dump.

Some waste rock contains unwanted sulfide minerals, such as pyrite or pyrrhotite (both iron-bearing sulfide minerals), arsenopyrite (arsenic iron sulfide) or chalcopyrite (copper iron sulfide). These minerals have the potential to form acidic water known as acid mine drainage (AMD). AMD forms when sulfide minerals are exposed to both water and oxygen from the atmosphere. The sulfide minerals may be oxidised and may form strongly acidic solutions. This is highly undesirable as acidic solutions can transport dissolved toxic metals (e.g. lead, arsenic, cadmium, mercury etc), and could cause environmental damage if allowed to enter rivers, streams or ground water systems. At Cannington, waste rock with the potential to form acid is stockpiled in areas away from any water. Any AMD that does form, ends up in the retention ponds and therefore does not leave the mine site.

Other waste materials

BHP Cannington has an extensive recycling program. Materials such as scrap metals (e.g. copper, aluminium, steel etc), paper and cardboard, and waste oil and oil drums are all collected and removed from the mine site for recycling. In addition, kitchen waste from the administration block and the village is composted in a worm farm. The worm castings are then used in the nursery to raise seedlings of native plants to be used in rehabilitation of disturbed areas around the site.

Other waste materials are disposed of carefully, so as to minimise any environmental impact. For example:

• Oily rags and filters are incinerated in an "enviroburner",
• Waste wood is incinerated in a fire pit,
• Used hydraulic hoses are washed and then disposed of in empty stopes along with the backfill paste made from tailings, waste rock and cement,
• Waste metal which has been in contact with lead (e.g. drill rods and casings) is disposed of in the tailings dam.