bauxite minerals processing

7/25/2019 Bauxite Minerals Processing

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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.

bauxite minerals processing

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