The Mining & Rehabilitation Process
The stages of bauxite mining from clearing and conserving topsoil to returning habitat for wildlife.
1. Pre-Mining Surveys
Pre-Mining Surveys are conducted in all new mining areas, to provide information on fauna and vegetation, to map the extent of dieback disease, and to identify any significant Noongar heritage sites. If rare or protected species or significant sites are present, they are avoided or management plans are developed to minimise the impact of mining on them.
2. Exploration Drilling
Exploration drilling is undertaken to identify the specific location of ore bodies within the mine lease that are suitable for mining. Drilling starts at wide spacing (120 m) and then focuses in on identified ore bodies.
3. Mine Planning
All Mine plans take into account any significant vegetation, fauna or heritage sites identified during the pre-mining surveys as well as avoiding the spread of dieback disease. A number of mine plans are produced, including a conceptual 25 year mine plan, a ten year mine schedule and a detailed five year mine plan. The Five Year Mine Plan and the Mining and Management Program (MMP) is submitted to the Mining and Management Program Liaison Group (MMPLG) each year for Ministerial approval. The MMP summarises the major environmental management programs that will be undertaken at the mine, with emphasis on issues relevant to the next five years.
4. Salvage of Marketable Timber
Any marketable timber within Alcoa’s mine lease belongs to the State government. Before clearing, the Forest Products Commission takes any marketable timber from areas where mining is planned each year.
5. Clearing & Utilisation of Wood Residue
Once all marketable timber has been taken, all remaining timber is cleared form the areas to be mined and positioned in rows. In previous years the wood residue from clearing was burnt. Residue waste from clearing is now being utilised by external organisations for charcoal production and used as fauna habitats in the rehabilitated mine areas. This reduces the amount of wood residue burnt. Alcoa is aiming to stop burning wood residue completely by identifying markets for all of the residue material.
6. Topsoil & Overburden Removal
The soil profile is made up of a number of layers. The topsoil layer contains a large store of seed and nutrients that is vital to the success of the forest rehabilitation and is about 15cm thick. The overburden layer is 20-80cm of gravely sub-soil material sitting above a solid concrete-like layer called the caprock. These layers are removed separately using scrapers prior to mining.
7. Breaking of Caprock
Scrapers and small excavators are used to remove the remaining overburden and expose the caprock. The caprock is broken by blasting.
8. Mining Ore
Once the caprock layer has been broken, the bauxite is ready to be mined. An excavator or loader is used to load the bauxite onto haul trucks for transport to the crusher. Several pits are usually mined simultaneously in order to supply the refinery with a consistent grade of ore.
9. Crushing & Conveying to the Refineries
The crusher is used to break the ore down to a smaller size suitable for transport along a conveyor belt to the refineries.
10. Rehabilitation of Mined Areas & Roads
Each year, mine pits that have had the ore removed and haul roads no longer needed are rehabilitated. The long term objective of Alcoa’s mine rehabilitation is to establish a self-sustaining jarrah forest ecosystem, planned to enhance or maintain conservation, timber, water, recreation and other forest values. Alcoa’s rehabilitation process has been developed and improved over the past 35 years, and currently involves the following processes:
Large rocks are buried, vertical pit faces are flattened down and the pit floor is smoothed to blend the mined area into the surrounding landscape.
Pre-ripping breaks up the compaction of the pit floor caused by heavy rubber-tyred mining equipment. This helps water and roots to penetrate through the soil profile.
13. Soil Return
The overburden and topsoil layers are returned. Wherever possible, fresh topsoil is directly returned to landscaped areas from pits that have been recently cleared. This maximises the topsoil seed store, which is important for maximising the number of plant species in rehabilitated areas.
14. Final Contour Ripping & Seeding
Final surface ripping is undertaken on contour to increase the soil’s water storage capacity. This contour ripping is undertaken with a multi-tine. Contour ripping creates mounds in the rehabilitation which are very important for erosion control.
Attached to the dozer that performs the contour ripping is a mechanical seeding machine which spreads the rehabilitation seed mix. This seed mix has been specifically formulated by Alcoa’s Marrinup Nursery and contains 50 – 80 plant species.
15. Recalcitrant Planting
Despite a large amount of research, there are some species that Alcoa is unable to establish from the seed in the topsoil or applied seed mix. These plants include many grasses and sedges that produce little viable seed. Alcoa grows plants of these species through tissue cultures (cloning) or cuttings at the Marrinup Nursery, and plants them by hand in the rehabilitated areas.
To improve the establishment and early growth of trees and understorey in revegetated areas, fertiliser is applied by helicopter to the newly rehabilitated areas in August each year.
17. Ongoing Monitoring & Management of Rehabilitated Areas
Every March, when the rehabilitation is 9 months old, the previous year’s rehabilitation is monitored to check that the number of established plants meets targets agreed by the DEC and Alcoa, and to identify any areas which need further treatment to control weeds or repair erosion damage.
At 15 months of age, the rehabilitations botanical species richness (number of different plant species) is monitored against internal and government standards.
18. Relinquishment of Mined Regions to the State
The MMPLG in consultation with the community and other stakeholders have developed a set of Completion Criteria for rehabilitation areas. Due to improvement in rehabilitation standards and techniques, two sets of completion criteria exist for pre-1988 and post – 1988 rehabilitation. These criteria were developed to allow government agencies to assess whether rehabilitation is of a satisfactory standard so that Alcoa can hand the land back to the DEC for future management.