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ALUMINIUM SMELTING INDUSTRY
History of intensive mixing for carbon paste
B. Hohl, Eirich
Carbon paste is used in many different
fields of the heavy industries, for instance,
as anode or cathode blocks for primary
aluminium smelting, as graphite electrodes
for electric arc furnaces, as carbon bricks
for refractory linings, as Soederberg electrodes
for reduction furnaces, etc.
Over many decades, slowly running
batch mixers have been the only useful
aggregates for the preparation of such
products. For anode paste a continuous
preparation process became established
later which was based on one or two continuous
kneaders arranged downstream.
In the seventies of the last century, the
intensive mixer started on a triumphal
march through this industry. Starting
with individual machines for continuous
remixing and cooling of anode paste as
well as batchwise preparation of various
carbon bodies, the intensive mixer constantly
opened up new fields of application
in the carbon sector. Due to its special
benefits, such as high efficiency and
an attractive cost/performance ratio, the
intensive mixer can be found everywhere
in the carbon industry today.
This paper describes the most important
characteristics and applications, from
the beginnings until today.
Anodes for primary aluminium smelting
The world’s production of primary aluminium
reached approximately 44 million tonnes
in 2012, of which about 90% (39.6m t) were
produced in modern factories with so-called
prebaked anodes. The remaining 4.4 million
tonnes come from older works with Soederberg
technology. The requirement for anode
paste is therefore about 21.7 million tonnes
for prebaked anodes and 2.5 million tonnes
for Soederberg anodes.
Due to the high throughput rates together
with the constant formulas, continuous preparation
systems are used almost exclusively for
carbon anode paste preparation. More than
every second prebaked anode is produced
from partially or completely intensively prepared
paste (Eirich remixer-cooler, resp.
Eirich Mixing Cascade EMC). So, the use of
intensive mixers for the preparation of anode
paste has become state-of-the-art in the primary
aluminium industry.
Remixing and cooling of anode paste
Already in the 1970s, individual intensive
mixers were used as continuously operating
coolers for anode paste. In the Netherlands
and in Bahrain for instance, the breakthrough
of this technology began around 1990, followed
by two more machines in France and
Australia. Preceding this were extensive
test series at the Pechiney works in Sabart
(France). At that time, the customer was looking
for a paste cooler of high performance and
efficiency. With the continuously operating
Eirich intensive mixer, Pechiney found a machine
which not only coped reliably with this
task definition but additionally achieved excellent
homogenisation of the paste. By direct
addition of cooling water and its immediate
evaporation, the mixer achieved paste cooling
capacities never reached before.
On top of that is the effect of a more or less
‘cost-free’ homogeniser: the relatively long retention
time of 4-5 minutes, with the intensive
mixing effect and the additionally introduced
mixing energy of approx. 4 kWh/t, together
generate a considerably improved paste quality
compared to single-step preparation [2].
All these factors are the reason that in
the last 15 years hardly any single-step paste
preparation lines were built. In the same period,
numerous existing plants were retrofitted
with Eirich intensive remixer-coolers. The
installation of an additional machine into an
existing building often posed a great challenge
to the engineers. In the end, they always
found a solution to integrate the machine.
The effect of the Eirich cooler becomes
especially apparent when retrofitting into existing
lines, because here the improvement in
preparation quality is easy to prove.
Advantages
• Thanks to the cooler operating in a second
mixing stage, the hot mixing temperature
is freely adjustable independent of the
forming temperature
• Long retention time and intensive energy
input provide excellent mixing of the
paste
• Thus, essentially more stable paste quality;
parameter variations are reduced to less
than 50% of previous scatter range
• Agglomerate-free paste with constant
temperature
• Additional mixing energy raises paste
quality
• Higher green and baked anode density
improve anode strength
• Lower electric resistivity of the anode
improves electrolysis efficiency
• Clearly reduced porosity and optimised
pore structure improve anode life
• Lower chemical reactivity reduces anode
burn
• Possibility of increasing the performance
of the complete preparation system to a
certain extent.
High-performance remixer-coolers have been
installed recently, for instance at Alcoa Mosjoen
/ Norway, Qingtongxia/China and Emal
1+2 / Abu Dhabi.
All-intensive preparation of anode paste
The successful use of the intensive mixing
principle for paste cooling was the initiation
for the development of the Eirich Mixing Cascade
(EMC). Two series-connected intensive
mixers perform both hot mixing of coke and
binder pitch, plus subsequent remixing and
cooling. The special advantages of the Eirich
intensive mixer, such as low capex and opex,
short standstill periods on the occasion of
wear-related repairs, long retention time, com-
Fig 1: Eirich intensive mixing principle
Fig 2: Paste quality improvement thanks to the
second mixing level
© Eirich
40 ALUMINIUM · 1-2/2013