Bauxite and Alumina
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Alumina (aluminium oxide Al2O3) is a fine white material similar in appearance to salt. While alumina is also used in abrasive, ceramics and refectory industries, Queensland Alumina Limited produces only smelter-grade alumina for the owners reduction plants in Australia and overseas.
The QAL process was designed to refine bauxite located at Weipa in North Queensland. The extensive deposits of this ore were formed by weathering of sands and rocks millions of years ago, increasing the alumina content as other more soluble elements were removed.
Bauxite occurs close to the surface in seams varying from one meter to nine meters, formed as small reddish pebbles (pisolites). The ore is shipped to Gladstone following "beneficiation" to remove low-grade material, and blending to provide a consistent grade.
The Bayer Process
The Bayer Process - an economical method of producing aluminium oxide - was
discovered by an Austrian chemist Karl Bayer and patented in 1887.
The process
dissolves the aluminium component of bauxite ore in sodium hydroxide (caustic
soda); removes impurities from the solution; and precipitates alumina
tri hydrate which is then calcined to aluminium oxide.
A Bayer Process plant is
principally a device for heating and cooling a large re circulating stream of
caustic soda solution. Bauxite is added at the high
temperature point, red mud is separated at an intermediate temperature, and
alumina is precipitated at the low temperature point in the cycle.
Bauxite usually
consist of two forms of alumina - a mon hydrate form Boehmite (Al2O3.H2O) and
a tri hydrate form Gibbsite (Al2O3.3H2O).
QAL uses the Bayer Process to refine
two grades of Weipa bauxite, the bulk of which is "monohydrate" grade
bauxite.
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Boehmite requires elevated
temperatures (above 200°C) to dissolve readily
in 10% sodium hydroxide solution.
The tri hydrate grade bauxite is mainly Gibbsite
which dissolves readily in 10% sodium hydroxide solution at temperatures below
150°C.
Consequently,
monohydrate bauxite undergoes high temperature extraction under pressure in
digesters, while tri hydrate grade material is added as a "sweetening
bauxite" to the flash tanks where temperatures are less than 200°C.
The
design of the plant meets the requirement of smelters of coarse or sandy alumina
for reduction to aluminium. The recovery rate is about one tonne of
alumina per 2.2 tonnes of bauxite.
From the plants, million-tonne bauxite stockpile
to the A-frame alumina storage sheds is a processing journey of about 2.5 days.
The QAL plant circulates some
550 million liters of caustic soda solution through four distinct stages, the
functions of which are detailed in this process
description.
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Process:
1. DIGESTION OF BAUXITE
Grinding:
Pisolitic, monohydrate-grade bauxite sized to a maximum of 20mm, is ground
in 10 mills (each with one compartment of rods and one of balls) to allow better
solid liquid contact during digestion. Recycled caustic soda solution is added
to produce a pump able slurry, and lime is introduced for phosphate control
and mud conditioning.
Desilication:
The silica component of the bauxite is chemically attacked by caustic soda,
causing alumina and soda losses by combining to form solid desilication products.
To de silicate the slurry prior to digestion, it is heated and held at atmospheric
pressure in pre-treatment tanks, reducing the build-up of scale in tanks
and pipes. Most desilication products pass out with the mud waste as sodium
aluminium silicate compounds.
Digestion:
The plant has three digestion units. The monohydrate slurry is pumped by
high pressure pumps through two agitated, vertical digester vessels operating
in series. Mixed with steam and caustic solution, alumina in the bauxite
forms a concentrated sodium aluminate solution leaving un dissolved impurities,
principally inert iron and titanium oxides and silica compounds. Reaction
conditions to extract the monohydrate alumina are about 250°C and a pressure
about 3500 kPa, achieved by steam generated at 5000 kPa in coal-fired boilers.
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Under these conditions, the chemical reactions are rapid:-
2NaOH + Al2O3.3H2O --> 2NaAlO2 + 4H2O
2NaOH + Al2O3.H2O --> 2NaAlO2 + 2H2O
By sizing the vessel to optimum holding time, about 97% of the total available
alumina is extracted and the silica content of liquor is reduced.
Heat Recovery:
After digestion about 30% of the bauxite mass remains in suspension as a thin
red mud slurry of silicates, and oxides of iron and titanium. The mud-laden
liquor leaving the digestion vessel is flash-cooled to atmospheric boiling
point by flowing through a series of flash vessels which operate at successively
lower pressures.
The flash steam generated is used to preheat incoming caustic liquor in tubular heat exchangers located parallel to the flash tank line. Condensate from the heat exchangers is used for boiler feed water and washing waste mud.
Sweetening:
The tri hydrate bauxite has separate grinding and pre-treatment facilities.
During the pass through the flash tanks, this additional bauxite slurry with
high tri hydrate alumina content is injected to maximise the alumina content
of the liquor stream. This occurs in the appropriate flash vessels when the
slurry from the digesters has been cooled to less than 200°C.
2. CLARIFICATION OF THE LIQUOR STREAM
Settlers:
Most red mud waste solids are settled from the liquor stream in single deck
40 meter diameter settling tanks. Flocculants are added to the settler feed
stream to improve the rate of mud settling and achieve good clarity in the
overflow liquor.
Washers:
The mud is washed with fresh water in counter-current washing trains to recover
the soda and alumina content in the mud before being pumped to large disposal
dams on Boyne Island.
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Slaked lime is added to dilute caustic liquor in the washing process to remove carbonate (Na2CO3) which forms by reaction with compounds in bauxite and also from the atmosphere and which reduces the effectiveness of liquor to dissolve alumina. Lime regenerates caustic soda, allowing the insoluble calcium carbonate to be removed with the waste mud.
Na2CO3 + Ca(OH)2 --> CaCO3 + 2NaOH
Filters:
Settlers overflow liquor containing traces of fine mud is filtered in Kelly-type
constant pressure filters using polypropylene filter cloth. Slaked lime slurry
is used to produce a filter cake. Mud particles are held on the filter leaves
for removal and treatment in the mud washers when filters are sequentially
taken off line.
Heat Interchange:
With all solids removed, the pregnant liquor leaving the filter area, contains
alumina in clear supersaturated solution. It is cooled by flash evaporation,
the steam given off being used to heat spent liquor returning to digestion.
3. PRECIPITATION OF ALUMINA HYDRATE
Crystallisation:
Dissolved alumina is recovered from the liquor by precipitation of crystals.
Alumina precipitates as the tri hydrate Al2O3 .3H2O in a reaction which is the
reverse of the digestion of tri hydrate -
2NaAlO2 + 4H2O --> Al2O3.3H2O + 2 NaOH
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The cooled pregnant liquor flows to rows of precipitation tanks which are seeded with crystalline tri hydrate alumina, usually of an intermediate or fine particle size to promote crystal growth. Each precipitation tank is agitated, with a holding time of about three hours. During the 25-30 hours pass through precipitation, alumina of various crystal sizes is produced. The entry temperature and the temperature gradient across the row, seed rate and caustic concentration are control variables used to achieve the required particle size distribution in the product.
As correct particle size is important to smelter operations, sizing is carefully controlled. The QAL precipitation plant was designed to operate on a continuous basis to produce "sandy" or coarse alumina.
Classification:
The finished mix of crystal sizes is settled from the liquor stream and separated
into three size ranges in three stages "gravity" classification
tanks. The primary classifiers collect the coarse fraction which becomes
the product hydrate. The intermediate and fine crystals from the secondary
and tertiary classifiers are washed and returned to the precipitation tanks
as seed.
Spent Liquor:
Spent caustic liquor essentially free from solid overflows from the tertiary
classifiers and is returned through an evaporation stage where it is re concentrated,
heated and recycled to dissolve more alumina in the digesters. Fresh caustic
soda is added to the stream to make up for process losses.
4. CALCINATION OF ALUMINA
Washing:
A slurry of coarse hydrate (Al2O3.3H2O) from the primary thickeners is pumped
to hydrate storage tanks and is filtered and washed on horizontal-table vacuum
filters to remove process liquor.
Calcining:
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The resulting filter cake is fed to a series of calcining units - an 1800 tonnes a day circulating fluidised bed calciner or one of nine rotary kilns each 100m long and 4m in diameter. The feed material is calcined to remove both free moisture and chemically-combined water. Firing-zone temperatures above 1100°C are used, achieved by firing with natural gas. The circulating fluidised bed calciner is more energy efficient than the older rotary kilns. Product sandy alumina particles are 90%+ 45 µm (microns) in size.
Cooling:
Rotary or satellite coolers are used to cool the calcined alumina from the
rotary kilns, and to pre-heat secondary combustion air for the kilns. Fluidised-bed
coolers further reduce alumina temperature to less than 90°C before it
is discharged on to conveyor belts which carry it to storage buildings where
it is stockpiled for shipment.
Reference:
Web site http://www.qal.com.au/
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