sPeCIAL ArABAL - ALUMINIUM-Nachrichten – ALU-WEB.DE

ALUMINIUM INdUstry IN the GULf
© Fives Solios
high performance of ‘eolios’ pitch fume
treatment system at Qatalum paste plant
RTO plant at Qatalum
Since the introduction of paste-coolers
in the anode manufacturing process, the
amount of volatile compounds has drastically
increased; in parallel, standards of
emission levels became more stringent.
Consequently, the technologies to treat
such emissions must cope with more concentrated
streams at inlet and lower emission
requirements at outlet. Solios has
developed the ‘Eolios’ system which is an
optimised combination of a dry scrubber
based on the adsorption of PAH on coke
fines, and a RTO (Regenerative Thermal
Oxidiser) using oxidation technology.
This combination minimises the energy
consumption while maximising the treatment
efficiency.
never reached before for such installation.
With 1 mg/Nm 3 of PAH 16
(according to the NS 9815 Norwegian
Standard), it can be considered
a new benchmark in the Gulf.
Coal tar pitch – the source
of PAhs emissions
PAHs are the main components of
coal tar pitch, which is used as a binder
for the fabrication of anodes. Coal
tar pitch is a solid material at ambient
temperature; to be used in the manufacture
of anodes it must be heated to
become liquid and to be mixed with
aggregate (calcined petroleum coke)
to obtain a homogeneous paste: this
step is done at approx. 160-200 °C.
At such range of temperature, the
coal tar pitch releases a high amount
of Coal Tar Pitch Volatiles (CTPV),
which are mainly composed of Polycyclic
Aromatic Hydrocarbons (PAH). Some
of these molecules are known or suspected
to be highly carcinogenic. The PAHs can be
divided into two groups: the light and heavy
fractions. This classification depends on the
number of aromatic rings present in the PAH
molecule. When the PAH is composed of, at
least, three aromatic rings, we can say that it
is a heavy PAH. Depending upon their molecular
weights, PAHs exist either under solid
form (particulate PAHs) or gaseous form (gaseous
PAHs), or both.
Paste mixer
Coke + Pitch
The addition of water to cool down the anode
paste lowers the partial pressure of the vapour
generated by the paste, which displaces the
equilibrium towards production of the light
fraction; so all the PAHs with a low partial
pressure (the light fractions) are released in
gaseous form.
Consequently, the addition of water into
the paste-cooler has drastically increased the
amount of light PAHs. This phenomenon was
confirmed in the Deschambault paste plant
before and after the installation of a pastecooler:
the PAH emissions increased by a factor
of 6.
Pitch fumes are generated in all equipment
where coal tar pitch is used at high temperature
(160-200 °C). It was observed that the
pitch volatiles concentration is doubled when
the temperature of the liquid pitch increases
by 20 °C. So the mixing temperature is also a
parameter with impact on the PAH emissions.
Conventional coke dry scrubbing system
The dry scrubber system is derived from the
potline alumina dry injection scrubbing technology.
It consists in injection of coke fines in
a gas stream loaded by pitch fumes. An adsorption
phenomena occurs between the coke
and the pitch fumes: the large specific area
(6,000 to 7,000 Blaine) and opposite electrical
charge of the coke fines allow adsorption
of pollutants.
The fines fraction of coke readily available
for the preparation of the anode paste is
injected counter-currently to the fume-laden
stream. Turbulence and highly efficient contact
between the pitch fumes and aerosols are
The first Eolios system was installed at the Alcoa
Moesjen anode plant in Norway in 2007
to treat the higher concentration streams coming
from the paste-cooler. Solios observed destruction
efficiency from 97 to 99% on PAH.
In 2010 a second Eolios system was implemented
at Qatalum. In January this year, Solios
successfully completed the performance
tests for the pitch fume treatment system at
the Qatalum green anode plant. With a production
capacity of 60 tph, it is the world’s
largest single line green anode plant.
The PAH sampling and analysis, done by a
third party, demonstrate a level of emissions
+ Water
Paste cooler
Anode former area
Coke fines injection
Fig. 1: Dry scrubbing process
54 ALUMINIUM · 9/2013