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The use of pellets and briquettes in
steel production
The use of specially engineered pellets is an alternative to the conventional methods of
desulphurising iron, reducing iron skimming losses and deoxidising steel, both at tap and in
secondary steelmaking. Tailormade compositions provide specific solutions for metallurgical
treatments and offer new opportunities in cost and time savings. A well-defined pellet shape
enables storage in flobins and other facilities in order to handle the material under safe conditions.
Author: Wolfgang Gitterle
ALMAMET GmbH
I
ron and steelmaking processes have developed over
many centuries and have become very sophisticated
procedures. The processes are technologically comparable
with any other highly challenging chemical process, and
involve almost all the elements in the periodic table.
As steelmaking costs continue to rise and environmental
restrictions increase, ALMAMET has installed a briquetting
facility for the production of materials that can be substituted
for costly additives, providing cost-saving solutions. This
article will give an overview of new materials that could help
reduce costs and support steelmakers in reducing handling
and processing costs of wastes and slags.
PELLETS FOR HOT METAL
DESULPHURISATION
r Fig 1 Pellets for hot metal desulphurisation
MILLENNIUM STEEL 2015
r Fig 2 Addition of desulphurising pellets at blast furnace tapping
34
An interesting approach for steelmakers without any hot
metal desulphurisation station could be the use of pellets
in torpedo cars or transfer ladles for desulphurisation.
These pellets contain desulphurising and deoxidising
reagents and can be fed into the empty torpedo car, just
before tapping the hot metal from the blast furnace (see
Figures 1 & 2). The pellets disintegrate and provide a large
metallurgically active surface with the following benefits:
` The additives neutralise the carried over acidic blast
furnace slag and increase its basicity. This new slag has
a high sulphur capacity and reduces the incubation
time in any following hot metal desulphurisation
process.
` The additives generate a low-viscosity, homogenous
slag which also aids desulphurisation.
` Any remaining oxygen from the blast furnace is
reduced, resulting in a higher efficiency in the hot metal
desulphurisation step. Gas forming components in the
pellets generate an insulating foamy slag, leading to a
reduction in temperature losses during transport to the
steel plant.
` Since the ‘new’ slag has a higher basicity there is less
mechanical and chemical refractory wear of the torpedo
lining.
RAW MATERIALS AND IRONMAKING
r Fig 4 Secondary steel treatment
with deoxidising pellets
` Desulphurisation continues during the transport time
to the steel plant. In some cases this pre-treatment
will negate the need for further desulphurisation
treatment for some steel grades, resulting in cost
savings due to significantly reduced temperature and
iron losses as well as savings in desulphuriser costs.
PELLETS FOR DEOXIDATION AT CONVERTER
TAPPING
The conventional method for deoxidising crude steel is the
addition of aluminum in various forms. This procedure is
well established and produces reliable results, however,
for some steel grades, the alumina inclusions formed can
be detrimental to both the casting process (eg, nozzle
blocking) and to steel properties (eg, fatigue resistance/
deep drawing).
An alternative deoxidation practice can be achieved with
the addition of briquetted materials containing calcium
carbide during tapping (see Figure 3). These pellets
comprise both deoxidising materials and slag conditioners,
which achieves both homogenous slags as well as
PELLETS FOR SECONDARY STEELMAKING
Aluminium is commonly used to deoxidise steel in
secondary metallurgy processes. Pellets can be used as an a
MILLENNIUM STEEL 2015
r Fig 3 Deoxidation with pellets at
converter tapping
efficient deoxidation, without an impact on the carbon
content of the killed steel if certain metallurgical rules are
followed. The key aspects of this technology are as follows:
` Evaluations on the use of calcium carbide-containing
pellets have been made and a much lower number
of inclusions was detected, resulting in much higher
quality steel. This improvement is of high importance for
the production of bearing steels and other steel grades
used in the automotive industry or other high quality
applications.
` The main deoxidising component of the briquettes is
calcium carbide, which forms CaO and gaseous CO upon
reaction with oxygen. Both components are beneficial in
terms of metallurgy and result in improved quality of the
steel. In addition to deoxidation, some desulphurisation
can also be detected due to the generated quicklime.
The slag has a high capacity for impurities, resulting in
cleaner steels.
` When using calcium carbide for deoxidation the
formation of a gaseous component has to be considered
as an excess use of calcium carbide-containing pellets
could lead to steel splashing during treatment. The
addition of other slag conditioners, choosing the right
size of pellets and avoiding too fine a material will avoid
these negative effects.
` The formation of a gaseous component during the
deoxidation process brings about a foaming effect in
the slag, resulting in an insulating slag. This is beneficial
for reducing temperature losses and achieving higher
temperatures in the subsequent secondary metallurgy
processes.
` To reach the required oxygen level, it is beneficial to use
pellets in the first step of deoxidation and to finalise the
treatment with aluminum. An excess addition of calcium
carbide, leading to a peak in carbon content, can be
avoided and the targeted oxygen level can be precisely
achieved.
` The addition of different strong deoxidisers support
the steel deoxidation process and makes the pellets
an efficient method to reach the desired oxygen
level in the steel in an efficient and reliable way.
The treatment time and temperature loss will remain
within reliable limits.
` Use of undersized material can result in an excessively
foamy slag, whereas treatment with too coarse, lumpy
calcium carbide can sometimes result in a very slow,
inhomogeneous reaction. Since pellets are a shaped and
standardised material, their use provides reliable results
with a good foamy slag and a reasonable reaction time.
35
RAW MATERIALS AND IRONMAKING
can be avoided. Regular control of the pellet ingredients
ensures consistent pellet quality and reliable treatment
results.
FEEDING SYSTEM
r Fig 5 Feeding system for pellets
Pellets and briquettes are shaped materials which are
much easier to handle than powdered products. As some
of the ingredients of the pellets described are considered
to be dangerous, it is mandatory to follow the rules for
packing, storage and transport. For instance, calcium
carbide-based pellets are packed in UN-certified drums
or containers. For trials and in cases of limited space, all
types of pellets can be delivered in 10kg steel drums,
with or without plastic bags.
For a better and safer handling of pellets, ALMAMET
has developed a feeding system which works with
containers, as shown in Figure 5. This installation
contains a dosing system which can be manipulated via
pneumatic equipment, and can be adapted with weigh
cells to ensure the correct yet rapid addition of pellets.
PELLETS FOR SLAG SKIMMING
MILLENNIUM STEEL 2015
r Fig 6 Spraying of SKIMMEX onto the ladle
surface
36
alternative and efficient deoxidiser, again to help avoid
nozzle clogging problems during continuous casting (see
Figure 4). Briquettes for secondary steelmaking therefore
contain mainly calcium carbide as deoxidiser together
with a slag forming component to ensure homogenous
foaming slag which reduces temperature losses during
casting. Since many different slag formers with a high
capacity for impurities can be added, a large number
of slag characteristics can be achieved for improved
metallurgical results.
Some steel grades require very low carbon content to
achieve the necessary properties. In this instance it has to
be ensured that no excessive use of calcium carbide-based
pellets occurs which might cause C-pickup.
While aluminum killed steels tend to clog the nozzles
during casting, calcium carbide killed steels are much
easier to cast, resulting in longer casting sequences,
and interruptions due to clogged tundish nozzles
One of the most important cost drivers of hot
metal desulphurisation is iron losses generated
during slag skimming. In cases where the ladle
surface has to be as clean as possible, a large number
of skimming strokes is necessary, leading to excessive
amounts of skimmed iron losses. To reduce this iron loss,
after a pre-skimming of the main slag quantity, SKIMMEX
pellets are sprayed on top of the ladle and form a kind of
glue which sticks together the remaining slag particles
(see Figure 6). This mixture of slag and SKIMMEX
can then be easily removed with a small number of
skimming strokes.
SKIMMEX is an engineered material with consistent
properties, making it much more reliable than other slag
coagulants.
CONCLUSIONS
Pellets are a cost-effective and metallurgically efficient
reagent engineered for the modern steelmaking industry.
Tailormade compositions enable specific solutions for
metallurgical treatments, offer new opportunities in
cost and time savings and are an effective means to
improve the profitability of the steelmaking process. A
well-defined pellet shape enables storage in flobins and
other facilities in order to handle the material under safe
conditions. MS
Wolfgang Gitterle is a desulphurisation specialist at
Almamet GmbH, Ainring, Germany.
CONTACT: [email protected]