(642e) Dry Granulation of Blast Furnace Slag for Heat Recovery

Blast
furnace slag is known as by-product from the production of hot metal at
integrated steel and iron mills. Approximately 300 kg of this liquid slag with
a temperature of about 1500 °C is tapped together with each ton hot metal. This
means a worldwide annual production of about 400 million tons of blast furnace
slag. Further processed, water granulated blast furnace slag can be produced, if the liquid
slag is cooled down rapidly for solidification. State of the art is a closed system
which uses lots of water for rapid cooling and solidification to produce a
granulated blast furnace slag which has an amorphous structure. This product
normally is dried and grinded for use as a binder in the cement production due
to its latent hydraulic properties. The disadvantage of wet systems is on the
one hand, the recooling of the circulating water and
on the other hand the need for drying of the granulated blast furnace slag, due
to the residual moisture up to 20 mass percent. Furthermore the sensible heat
of about 1.5 GJ/t of slag is lost. [1]

Therefore
a new lab rig has been installed by Siemens VAI in cooperation with Montanuniversität Leoben at the
Chair of Thermal Processing Technology for research and development. In this
project there are the co-partners voestalpine Stahl
GmbH, ThyssenKrupp Steel Europe AG and FEhS-Institut für Baustoff-Forschung e.V. This project is funded by the German Federal Ministry
of Economics and Technology (BMWi).

Figure 1: Lab rig at the University of Leoben,
Chair of Thermal Processing Technology (Picture: Siemens VAI)

The
lab rig is based on the ?Rotating Cup? principle. The liquid slag will be
atomized by a cup and small particles spread away due to the occurring forces. First
tests involving the ?Rotating Cup? principle were performed in Redcar at British
Steel in the 90's. Further investigations took place at Vitkovice
in the Czech Republic between 2002 and 2004. These tests were part of the ZeroWaste project of Siemens VAI. Both research activities
did not focus on heat recovery due to the lack of political pressure at this
time. It could be shown that dry granulation for producing glassy blast furnace
slag is also feasible with the ?Rotating Cup?. [1]

Figure 2: Trials at Redcar (left) and Vitkovice (right) [1]

The
new lab rig at the University of Leoben has been
designed on basis of the results of the lab tests at FEhS-Institut
and the modelling of Siemens VAI to find the best operating parameters for
producing high quality granulated blast furnace slag and highest possible air
outlet temperatures. Therefore about 300 kg of slag is molten in the
?Flash-Reactor? for each trial at the University's workshop. The liquid slag is
tapped into special slag pots which are lined with refractory to prevent heat
losses. Afterwards the slag pot is manipulated into a tilting unit, installed
close to the granulator. The liquid slag feed into the granulator can be
measured and controlled via load cells. The liquid slag will be delivered to
the centre of the granulator passing a slag runner and a vertical refractory
pipe. After atomizing the slag droplets cool down rapidly during their flight
towards the granulator wall once spread away by the fast spinning cup. The
surface of the particle has to solidify during this short flight time before
falling into a developed fluidized bed made by granulated particles. [1]

Figure 3: ?Rotating Cup? principle [1]

Furthermore
the granulator wall is indirect cooled by a water jacket. Air will be
introduced at the bottom of the device. A special air distribution insures a
proper cooling of the particles, an agitated bed and no agglomeration. So a
maximum heat exchange rate can be reached. Hot off gas can be added to raise
the input air temperature for the granulation process. Hence a recirculation
system can be simulated which should be used in industrial application. Flow straighteners at the top hinder any carryover of the
particles, where the hot air leaves the granulator. [1]

Afterwards
the air is cooled down by a spray tower before leaving the lab rig passing a
droplet separator and the suction fan. The granulated slag particles can be
discharged at the bottom of the granulator after each trial.

Figure 4: Isometric
drawing of the lab rig

The
whole lab rig is fully automated. More than 90 sensors are recording the
process data like slag mass flow, different pressures, temperatures and other measurands. This helps analyzing each trial to find the
best operating parameters for getting a proper product while having a maximum
air outlet temperature. The main challenge considers ensuring
rapid slag freezing to reach the transformation point of the slag very fast.
Otherwise the product will not get fully glassy but partially crystalline which
is unsuitable for further use in the cement industry. Cooling with air is much
more difficult due to a substantially lower heat capacity compared with water
used in conventional devices. Furthermore the air heats up while streaming
through the agitated bed. So additionally the temperature difference worsens
the setting process.

First
campaigns showed promising results regarding glass content, grain size
distribution and particle shape. Also a significant off gas temperature
increase could be measured. With the obtained data, process optimization can be
done and a scale up for further investigations can be realized in the future.
Using dry slag granulation at integrated steel and iron mills will gain access
to the sensible heat of the blast furnace slag. The recovered energy can be
used for the production of steam or electricity or for other useful preheating
processes. An energy potential for recovering 20 MW thermal energy or
alternatively of about 6 MW electric power generation from a slag feed of one
ton per minute can be calculated. Without the need of an upstream drying system
for the blast furnace slag energy and as in consequence CO₂ can be
saved. In summary dry granulation of blast furnace slag allows an environmental
friendly and sustainable subsequent processing for one of the last big heat
recovery potentials at a modern blast furnace. [1]

References

[1]        McDonald, I.,
Long, E., Werner, A. & Most, D. (2010) Dry slag granulation - The
environmental friendly way to making cement. In: IOM3 conference on Waste
Recovery in Ironmaking and Steelmaking Processes.
London: United Kingdom