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Alumina Refining
Bauxite Stockpiles
Bauxite is transported from the mine to the plant via an
overland conveyor system. It is stacked, blended and recovered
using rotating barrel reclaimers.
Grinding and Digestion
Bauxite is fed into a grinding mill together with caustic liquor
recovered from the process to produce bauxite slurry. The
ground bauxite slurry is then mixed with hot recycled caustic
liquor and passed through a series of digestors to extract
alumina. The slurry then passes through a series of flash tanks
which reduces the temperature by allowing steam to flash off.
Heat is recovered from this steam to reheat the caustic liquor
fed into the digestors.
Mud Separation and Filtration
Undissolved impurities such as sand and iron oxide are
separated from the alumina rich caustic liquor in large settling
tanks called thickeners. Polymer is added to assist the settling
to produce an alumina rich over-flow called pregnant liquor. The
pregnant liquor is pumped to Security Filtration where filters
remove any remaining fine red mud.
Precipitation
Pregnant liquor is then cooled and seeded with alumina hydrate
crystals to initiate the precipitation process. Hydrate crystalsform in the precipitation tanks under carefully controlled
conditions.
Classification and Filtration
Hydrate Classification separates the precipitation slurry into 3
sizes as product, fine seed or coarse seed. Each hydrate
fraction is filtered to remove liquor, which is recycled to
Evaporation. The product hydrate is washed and filtered again
to remove any remaining caustic, then conveyed to Calcination.
The fine and coarse seed is recycled back to Precipitation.
Calcination
Hydrate is passed through kilns, where temperatures up to
1100C drive off the chemically bound water to form alumina.
The alumina leaving the kilns is cooled and stored in silos prior
to shipment.
Evaporation
The Evaporation plant removes water from the recycled liquor
to produce strong caustic liquor suitable for re-use in Digestion
area. The water evaporated from the liquor is condensed and
re-used for washing in the process.
Mud Washing and Residue Disposal
To recover caustic, red mud waste is washed using recycled
condensate in a counter-current washing process before the
high-density mud is pumped to the Residue Disposal Area. Red
mud waste is dry stacked, an efficient means of disposal
requiring less area and improved rehabilitation, at the ResidueDisposal Area.
Water Treatment Facility
Waters that have become contaminated at the refinery are
collected and neutralised with seawater before being
discharged into the ocean.
Ancillary Services
The refinery is supported by a best in class, efficient co-
generation power station that produces steam and electricity for
the process and supplies electricity to the mining operations,
Nhulunbuy town site and nearby communities. The water supply
for the refinery, mine and Nhulunbuy is sourced from bores
located at the mine area.
Port Facilities
The port facilities established by Nabalco in Melville Bay are
used for handling alumina, bauxite, aluminium hydroxide,
general cargo and bulk liquids. Oil, liquid caustic soda and
petroleum products are pumped from vessels at the tanker jetty
to their relevant storage tanks.
Precipitation tanks
Process
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Process Flow diagram
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Bauxite Mining
Clearing
The bauxite deposit at Gove is found in approximately 3.5
metre thick upper layers of a plateau. Topsoil and overburden
are removed two years prior to mining and placed on previously
mined areas as part of the rehabilitation process.
Mining
Bauxite is mined using conventional open-pit mining
techniques. On average 135 hectares is mined per year with a
similar area rehabilitated. The ore is ripped by large bulldozers
and then loaded by front-end loaders into dump trucks for
transport to the crushing plant.
Crushing and Screening
The crushing plant reduces the size of the ore to below 25 mm,
at a rate of 1600 tonnes per hour. Crushed ore is transported to
the plant via a 19-kilometre overland conveyor system.
Rehabilitation
The mine rehabilitation program has proved highly successful.
The steps include:
timber clearing well in advance of mining, allowing
understorey species of plants to propagate;
topsoil and overburden stripped from pre-mined areas is
immediately placed on the adjacent mine floor. The seed
rich topsoil leads to the regeneration of ground cover and
grasses; mined-out areas are ripped along natural contours to
promote drainage, aeration and root penetration;
seed from native shrubs and trees indigenous to the mine
lease are collected annually;
seeds are sown with a single application of fertiliser
immediately prior to the wet season;
rehabilitated areas develop naturally however care is taken
to avoid bushfires until the trees are well developed.
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Yirrkala Business Enterprises employee,
Bitika Wunungmurra, seed collecting at the mine.