Recycling of dross in Aluminium smelter

1. Srihari Chodagiri

2. 
Brief on Sohar Aluminium and about Aluminium dross generation in
Casthouse

Ways of handling the hazardous dross
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Project background

Project implementation and results

Continuous improvements

Conclusion

3. 
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Joint venture between Oman
government and Rio Tinto Alcan
Started primary aluminium
production in 2009
State of the art AP-35 technology
used for Smelting
Production of 360,000 tonnes of
pure aluminium annually
World longest pot line and highest
designed capacity for casting lines
in it’s time

4. 
Two standard ingot casting lines(30 T/hr) using 4
holding/tilting furnaces of 80T each – Main source
of dross
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Metal siphoned into furnaces but dross formation
inevitable
Lowest melt loss in RTA smelters through use of
best practices
One sow casting line (25 T/hr) – Does not form
dross (Dross on metal crucible sinks into bottom
mixing with bath which is removed during crucibles
cleaning later)

5. 
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Produced as aluminium by-product due to reoxidation of aluminium when liquid metal comes in
contact with air
Essentially consists of Al2O3, Bath, carbides, oxides,
nitrides, fluorides, dirt and other impurities
Classified as class 4.3 hazardous material by U.S
Department of Transportation(DOT), UN 3170
Mainly formed in the furnaces due to reaction of
molten aluminium metal with air inside the furnaces
Skimmed into bins before starting the casting –
around 100 tonnes of dross gets generated every
month

6. 
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Dross consists of rich mixture of metal,
oxidised aluminium and bath that can be
entirely made use of.
Thermiting is burning at very high
temperatures (due to chemical reaction) –
temperatures can reach to the order of
1500 C
Thermiting needs to be stopped as soon
as possible to preserve the metallic
content in dross

7. 
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Different methods of cooling are used
like dross pressing, inert gas cooling,
dumping on open ground and cooling
down, etc.
Metal particles can be collected and
can be charged back into furnace. But
fumes from wet dross are very harmful
to the people
Segregated material can then be given
to dross processor who could further
extract metal and mix the remaining
material into refractory

8. 
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Early 2009, Sohar Aluminium started facing
problem on how to deal with the dross pile up
which crossed 500 tonnes of this hazardous
material accumulated on landfill
Containers stuffed for onward transportation
have been struck at Port terminals for clearance
With no foreseeable dross processor available in
country and with increased realization that
transportation across international boundaries is
becoming almost impossible to deal with,
management decided to set up a cross functional
team to come up with solution to this
burgeoning problem.

9. 
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Based on team members’ experience, it was
planned to initiate processing of the Casthouse
dross internally in smelter making use of the
available equipment in bath plant.
Bath treatment plant in Carbon is designed to
recycle the cover bath on spent anodes material,
bath/dross mixture from crucible cleaning and
tapped bath from pots
Alumina is mixed into this recycled material
which is processed at the rate of 150 tonnes
every shift
Anode
Block
Comparative analysis on bath/dross mixture
removed from crucible cleaning and the dross
from furnaces was done at Laboratory revealed no
significant difference in the elements present.
Anode
Block
Tapped Bath
(Electrolyte)
Anode Block
Liquid
Aluminium
Cathode
Cell
Cover
Bath

10. Project implementation and results(contd)
Casthouse
dross

11. 
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Initial trials done with dross lumps of
pressed dross after breaking them with
crust breaker and results were found
encouraging.
But several operational problems surfaced
with the use of this pressed drosswhere it
was found getting increasingly difficult to
process at rotary tumbler and that it
needed smaller chunks of dross.
Also, it is observed in Casthouse that
thermiting keeps taking place on a large
number of dross bins reducing the value of
dross material while dross pressing
operation for one bin is taking place.
As well many a times, thermiting does not
stop even after completion of dross
pressing.

12. 
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Project team decided to explore new ways of dross
treatment to be able to stop thermiting as quickly
possible as well as to maintain dross lump size to
be small
Based on experience of team members,
suffocating the dross by cutting down the
oxygen/air would be an effective way.
◦ 6 new bins with lids matching were made for trials

13. 
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Upon further trials, it has been identified that treatment
using inert gas is effective to stop thermiting as well keep
the dross lump size to be small
Several trials were carried out to calculate the inert gas
requirements, time and temperature studies for cooling, etc
At the same time, analysis of this dross material is carried
out in lab in comparison to bath/dross material from
crucible cleaning and no abnormalities were found. Given
that Sohar Aluminium produces only pure Aluminium with
no alloying elements added, no detrimental impurities are
observed, with onus on effective thermiting control
Metal Skimmin
Material
Siphon
crucible
g
Bath crucible
Composition
cleaning(
cleaning( station
cleaning) (%)
(%)
%)
%)
(%)
Al2O3
6.42
15.8
7.91
1.9
Bath
83.23
50.91
62.53
97.8
Metal Portion
9.18
32.38
29.15
0.0
Carbon
0.6
0.42
0
0.05
Elemental
Composition (%)
Si
0.024
0.137
0.188
0.02
Fe
0.025
1.375
0.176
0.06
75.4
15.4
8.8
0.22
Dross,
N2
Cooled
(%)
62.2
12.2
24.9
0.72
0.02
0.07
0.04
0.03
Dross,
Ar+N2
Cooled (%)
Treated in
Dross
press
69.5
17.7
11.5
1.28
0.03
0.03

14. 
This treated dross is continuously supplied to bath plant where it is fed into
rotary tumbler mixed with other recycle material which then gets separated
at drum magnetic separator where the metallic balls are collected into a bin

15. 
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These balls obtained at bath plant are
taken for segregation manually from
which the metal balls are directly brought
back to Casthouse for metal recovery.
This way of handling the dross has been
found to be effectively working with no
detrimental effects observed in this
process of bath recycling circuit.
Only instances reported were when
improperly cooled down dross resulted in
ignition of recycle material on floor
attributed to thermiting present in dross
and this is addressed with the
improvements made for dross treatment
in Casthouse

16. 
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The recovered metallic balls were
directly brought back and charged to
furnaces for immediate metal
recovery.
The balance dross material which is
quite low as compared to other
additives like tapped bath, alumina is
mixed in the bath circuit for recycling

17. 
Several improvements were put in place in
Casthouse to effectively stop the dross
thermiting
◦ Developing chambers to put bins inside for
cooling instead of lids
◦ Installing 1000 litre liquid argon tank for
continuous inert gas supply

18. 
Redesigning of the skimming bins to match
furnace height. This has resulted in many
advantages
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Reduced the number of skimmed bins
Eliminated the need to empty the bin inside Casthouse
in bigger skip
Quick cooling due to optimized enclosure inside the
cooling chamber
Automating the dross treatment initiation,
operators only needed to press start.

19. 
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Emptying of dross inside Casthouse has
completely been eliminated with these
improvements put in place and the cooled down
dross bins were directly taken to bath plant for
immediate processing
This has resulted in the improving cleanliness in
Casthouse to a great extent.

20. 
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The project was able to completely address the issue of dross
accumulation. In fact, as outcome of the project, company was able to
realize the benefits of this internal processing to completely make use
of the dross material
Several new projects have been taken to carry out recycling of waste in
many other areas that lead to several cost improvements
I would like to sign off saying this new methodology adopted quite
clearly proves, as could be seen from the results of the last 4 year
period, that dross could be entirely made use of provided
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there is good control on thermiting,
there is siphoning of metal for less dross generation
there is no alloying of elements
it gets thoroughly mixed in the bath circuit.