10 bauxite processing flowsheet case study

Fall 2022
1
Mineral Processing
(MINE 342)

COURSE CONTENT
1. Course Introduction
2. Safety in Mineral Processing plants
3. Sampling, Liberation and Metallurgical Balance
4. Size distribution (screening) and Classification
5. Comminution (Crushing and Grinding)
6. Sorting and Gravity Separation
7. Magnetic and Electrostatic Separation
8. Flotation
9. Dewatering and Tailing Disposal
10. Local Ore processing Flowsheets: Bauxite ore
11. Local Ore processing Flowsheets: Gold ores

OUTLINES
- What is bauxite?
- What is Alumina?
- The whole story?
- The Bayer process Story!
- Inputs and outputs balance (heat and
mass)

Bauxite is an ore and is the main source of aluminum.
This form of rock consists mostly of the minerals
gibbsite Al(OH)3, boehmite γ-AlO(OH), and diaspore α-
AlO(OH), attached with the two iron oxides goethite
and hematite, Quartz SiO2, and small amounts of
anatase TiO2.
Bauxite named after the village Les Baux in southern
France, where it was first recognized in 1821.
What is bauxite?
25
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12
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2011 Dr. Hussin Ahmed, hussien135@gmail.com

Diaspore, α-AlO(OH),
Sp. Gr. =3.4
Boehmite, γ-AlO(OH),
Sp. Gr. =3
Gibbsite, Al (OH)3,
Sp. Gr. =2.4
What is bauxite?
Valuable minerals
Bauxite = mixture of valuable materials + gangue is it really mixture????
(Gibbsite, Al(OH)3; and or Diaspore, α-AlO(OH); ; and or Boehmite, γ-AlO(OH)
26
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12
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2011
Dr. Hussin Ahmed, hussien135@gmail.com

What is bauxite?
Gangue (Goethite (FeO(OH)), and/or Hematite Fe2O3; and/or Anatase TiO2 )
Hematite Fe2O3
Sp. Gr. =4.9-5.3
Gangue minerals
Anatase TiO2
Sp. Gr. =3.9 – 4.2
Goethite (FeO(OH)),
Sp. Gr. =from 3.3 to 4.3
26
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12
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2011

What is Alumina?
Alumina, is aluminum oxide (Al2O3), a white or nearly colorless
crystalline substance that is used as a starting material for the
production of aluminum metal. It also serves as the raw material
for a broad range of advanced ceramic products and as an active
agent in chemical processing.

How to obtain Alumina?
Remarks
Method
Source
????
Aluminum metal
oxidation
Synthetic
Very expensive
Grinding
Corundum, Al2O3
Expensive & Limited
resources
remove Mg
Spinel, MgAl2O4
Limited resources
remove Fe
Hercynite, FeAl2O4
Limited resources
remove Mn
Galaxite, MnAl2O4
Working
leaching
Kaolinite Al2Si2O5(OH)4
This is our topic in this lecture
Gibbsite, Al(OH)3
Diaspore, AlO(OH)
Boehmite, AlO(OH)
25
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12
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2011 Dr. Hussin Ahmed, hussien135@gmail.com

DETAILS
Mineral
27 16 1 55 56 24 45 Mol. wt Al2O3,
%
Al O H Mn Fe Mg Ti
Gibbsite
Al(OH)3 1 3 3 78 65.38
Boehmite, γ-
AlO(OH) 1 2 1 60 85.00
Diaspore, α-
AlO(OH) 1 2 1 60 85.00
Corundum,
Al2O3 2 3 102 100.00
Spinel,
MgAl2O4 2 4 1 142 71.83
Hercynite,
FeAl2O4 2 4 1 174 58.62
Galaxite,
MnAl2O4 2 4 1 173 58.96
All gangue
minerals 0 variable 0
Alumina content in the different minerals

What we can conclude
from our detailed
calculations using Excel
?

Facts About Bauxite valuable minerals
Gibbsite has a maximum alumina content of 65.4%. It has a typical
metal hydroxide structure with hydrogen bonds. It is built up of double
layers of hydroxyl groups with aluminium ions occupying two-thirds
of the octahedral holes between the two layers. Aluminium hydroxide
is amphoteric.
Böhmite and diaspore both have a maximum alumina content of 85%3.
Of the bauxites currently being mined, the dominant form is gibbsite,
followed by a mixture of gibbsite and böhmite.
They dissolve in Acid, forming Al(H2O)6
3+ (hexaaquaaluminium(3+)).
They also dissolves in strong Alkali, forming Al(OH)4
-
(tetrahydroxidoaluminate(1-)).
The main impurities are compounds of iron, silicon and titanium4.

What is to be done?
You have bauxite to the right, you want alumina from it to the left

Refinery
Alumina
production
Smelting
Al production
Manufacturing
useful product
(Rolling, etc)
Mining
Bauxite production
The whole story
From Bauxite to Aluminum useful product
4th Story
3rd Story
2nd story
1st story

The whole story and more in line diagram

Simplified form of the first story

Inputs versus outputs of the first story

The four stories and their techniques
Many studies
Bauxite found
Bauxite mined

Simple description for Bayer process

What is the Bayer Process?
Refineries around the world use the Bayer chemical process to extract
alumina from bauxite. Karl Bayer, invented the process in 1888.
The alumina refining process consists of four main stages, as shown
below:
Dissolve alumina in a hot solution of caustic soda
Digestion
Remove of residues from the liquor (red mud)
Clarification
Remove of crystals of alumina hydrate from the caustic
soda solution of the liquor.
Precipitation
Remove water from crystals from the alumina hydrate
Calcination

Bayer process Chemistry
Digestion
Gibbsite: Al(OH)3 + Na+ + OH- ---> Al(OH)4
- + Na+
Bِ hmite and Diaspore: AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- +
Na+
Precipitation
Crystalline aluminium trihydroxide (Gibbsite), conveniently named
"hydrate", is then precipitated from the digestion liquor:
Al(OH)4
- + Na+ ---> Al(OH)3 + Na+ + OH-
Calcination "Hydrate", is calcined to form alumina for the
aluminium smelting process. In the calcination process water is
driven off to form alumina:
2Al(OH)3 ---> Al2O3 + 3H2O

 Gibbsite (Al2O3.3H2O)- Az Zabirah
Requires 145Degs.C to be digested or dissolved
 Boehmite (Al2O3.H2O)- Az Zabirah
Requires 250+Degs.C to be digested or dissolved
 Diaspore (Al2O3.H2O)- not in Az Zabirah
Requires 300+Degs.C to be digested or dissolved
Sciences interactions concluded:

3D Animation for Bayer process

Fe2O3 30-60wt%
Al2O3 10-20wt%
SiO2 3-50wt%
Na2O 2-10wt%
CaO 2-8wt%
TiO2 trace-25wt%
Red Mud is produced during the Bayer process for alumina production. It is the
insoluble product after bauxite digestion with sodium hydroxide at elevated
temperature and pressure. It is a mixture of compounds originally present in the
parent mineral, bauxite, and of compounds formed or introduced during the Bayer
cycle. It is disposed as a slurry having a solid concentration in the range of 10-30%,
pH in the range of 13 and high ionic strength.
A chemical analysis would reveal that RM contains silica, aluminum, iron, calcium,
titanium, as well as an array of minor constituents, namely: Na, K, Cr, V, Ni, Ba, Cu,
Mn, Pb, Zn etc. The variation in chemical composition between different RMs
worldwide is high. Typical values would account:
Red Mud Composition

26
Caustic: For extraction of Alumina from the Bauxite
Lime: Filter Aid, Impurities control and mud conditions
Flocculants: For Mud & Hydrate Settling
Acids: For Process Equipment Cleaning
Water: For washing
Steam/Power: For heating and transferring masses
Consumables: Other consumable filter cloth and grinding
media
Input Raw Materials in the Bayer Process

A line diagram for Bayer process

Animation Showing Bauxite milling

What after the mill?
A-Digestion Process
In digestion, we add heat and Caustic Soda to ground Bauxite, to dissolve
the Aluminum Bearing Minerals from the bauxite.
The purpose of Digestion is to dissolve the aluminum minerals from the
bauxite so that it can be separated in subsequent stages.
The following steps take place during the Digestion stage:
Adding concentrated caustic soda solution to the powdered bauxite
Using steam to heat the caustic soda and powdered bauxite to a high
temperature
Cooling the hot mixture before it goes on to clarification.

B-Clarification
Clarification is the removal of solid waste and wash solid waste
from the Green Liquor stream.
The liquor enter the settler as Blow off slurry contains dissolved
alumina and undissolved waste and exit as clear liquor at 104˚C
from the filter.
The filter recycle the fine mud to the settler ,so, we can extract the
washed residue.
The fine mud washed with countercurrent washing to remove the
solids.
Green Liquor is sent to PRECIPITATION
Mud is washed to recover caustic, then pumped to Residue
Disposal Area

C-Precipitation
Precipitation is the recovery of alumina from the Green Liquor
solution
Liquor has been cooled to 80ºC by this point, which encourages
crystallisation
Alumina re-crystallisation is triggered by “seeding”
– seed is fine alumina-hydrate, which provides a “growth surface” for
the new crystals. Without seeding the crystal growth would be very
slow
– Seed is mixed with incoming Green Liquor in large stirred tanks
– Alumina recrystallises into solid form, hence called ALUMINA
HYDRATE
– Tanks provide a long holding time (~30 hours) to give the alumina
time to grow. Precipitation is much slower than Digestion.

C-Precipitation (continued)
– Once alumina has recrystallised it is time to separate the slurry
into product crystals, seed crystals, and remaining SPENT liquor
– Spent Liquor is sent back to evaporation then to Digestion for
reuse.
– Product crystals require water to be removed by CALCINING
– Seed crystals are sent back to start of Precipitation to meet
incoming Green Liquor

Alumina Hydrate crystals viewed by Scanning Electron Microscope
(SEM)
~0.1mm (100mm)
.

D-Calcination
Calcination is basically “cooking” the product to remove
free water and chemically bonded water.
Alumina Hydrate from Precipitation is filtered to remove
moisture, then heated at 1100ºC
– Water vaporises and is vented as steam
– Remaining solids are almost 100% pure alumina
– The final product is a fine, dry, white powder, which is alumina.
Alumina is then cooled and stored then transferred to smelter.

Refinery Alumina Calcination Process

Product Quality Typical of SGA
Parameter Units Value
Al2O3 wt% 98.5
Na2O wt% 0.35
SiO2 wt% 0.008
Fe2O3 wt% 0.008
CaO wt% 0.05
MnO wt% 0.001
ZnO wt% 0.015
V2O5 wt% 0.004
P2O5 wt% 0.003
TiO2 wt% 0.003
LOI(300) wt% 1.5
LOI(300-1000) wt% 0.8
Alpha Alumina wt% 5
BET Specific Surface m2/g 70
+150um wt% 2
+75um wt% 50
-45um wt% 8
Chemical analyses of Final Produced Alumina

10 bauxite processing flowsheet case study

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