Air-cooled blast furnace slag
In the UK many tens of millions of tonnes of blastfurnace slag
(BFS) have been used as both unbound and bound aggregates in construction
since the turn of the last century, most notably in asphalt and
concrete. Blastfurnace slag also provides a diverse range of products
such as filter media for water sewage treatment, as a cement replacement
material in concrete and it has been widely used in major land
reclamation schemes such as Seal Sands on Teesside. Blastfurnace
slag is a good example of a by-product from one industry providing
raw materials for another.
The production of BFS is unavoidable and is controlled within
specific parameters during the iron making process, with approximately
one third of a tonne being produced for every tonne of iron. Clearly,
therefore, it makes sense to utilise it fully where possible,
not only to avoid the creation of unsightly slag heaps, but also
to reduce the need to extract finite primary minerals. The introduction
in April 2002 of taxation on primary aggregates sold for construction
purposes does not apply to by-products such as BFS.
initially in liquid form, BFS may be aircooled in pits then crushed
and screened like naturally occurring aggregates, or it may be
vitrified or expanded by quenching rapidly with water to produce
granulated slag respectively. It is important to distinguish between
the relative chemical consistency of BFS compared to 'old bank'
slags which are still in use today but which can present difficulties
due to their inherent variability. This is not to say the latter
cannot be utilised but measures do need to be taken to mitigate
against the risk of potential volumetrical expansion. However,
BFS is not expansive, it is volumetrically stable and is therefore
ideally suited as bulk fills, cappings and sub-bases, as well
as in higher specification applications.
Approximately 3 Million tonnes is produced annually, shared between
the three 'Corus' (previously British Steel) operations at Redcar,
Scunthorpe and Port Talbot . The majority, typically 75%, is granulated
then ground to make ground granulated blastfurnace slag (ggbs)
which is sold into the cement market. The remainder is processed
into aggregates for construction and other uses. European or British
standards apply to all applications.
Iron production is via a continuous batch process with the iron
and slag produced in the blastfurnace at a temperature around
1500ºC. The furnace is fed repeatedly with materials in a sequence
of a 'coke dump', followed by a 'ferrous dump'. The 'ferrous dump'
typically consists of a pre-blend of mainly sinter, but also graded
lump iron ore and iron ore pellets, also known as acid pellets.
Sinter typically consists of 10% lime derived from calcined dolomitic
limestone and crushed fine ores which consist of 60% Fe ( 90% Fe2O3 ), silica and alumina depending on the source. Australian
ores can be high in alumina whereas Brazilian ores tend to be
low. Approximately 10-20% of sinter is recycled materials from
the iron and steel works. The acid pellets consist of metal iron
ore balls, approximately 1cm in diameter which have been fired
with bentonite as a binder. The 'ferrous dump' pre-blend, also
known as the 'burden', is introduced into the top of the blastfurnace
via a hopper with an airlock to prevent gas escape. The burden
uniformity controls the BFS composition.
Most iron ores in use today contain about 90 per cent Fe2O3,
or Fe3O4 and are pre-refined to this standard. The remaining percentage
is usually earthy material such as sand or clay. The furnace temperature
and pressure is controlled to help the 'burden' descend through
the furnace without restricting the rise of gases to allow the
necessary reactions to take place. The coke (carbon) essentially
performs three functions; it burns and maintains the furnace temperature,
it reduces the iron oxide to iron (2Fe2O3 +3C 4Fe + 3CO2) and
provides a physical support for the burden which is porous to
allow hot gases to permeate to the top of the furnace. The iron
ore is essentially 'reduced' via a chemical reduction process
which involves the magnesium and calcium oxides present in the
sinter combining with the alumina and silica in the iron ore.
The combined oxides form blastfurnace slag, which has a lower
density than molten iron and hence floats as a separate layer
on top of the iron. The molten slag and iron collect at the 'hearth'
at the bottom of the furnace. The iron and slag are 'tapped' (removed)
together at regular intervals, held in a trough to allow separation,
then the slag is allowed to run into open air pits where it cools
in layers. The cooling is facilitated by water spraying which
causes the slag to crystallise and crack, aiding excavation from
the pit. The cooling conditions affect gas liberation (i.e. escaping
gases become trapped in the slag), which in turn affects the slag
porosity and density. The molten iron (bottom layer in the trough)
is also tapped allowing it to run into torpedo cars which transport
it to the steel plant where it is converted into a range of steel
grades ranging from low carbon to high carbon depending on the
end use.
Blastfurnace slag is chemically and mineralogically as consistent
as naturally occurring aggregates, comprising primarily the silicates
and aluminosilicates of calcium and magnesium together with other
compounds of sulfur, iron, manganese and other trace elements.
A typical chemical analysis is 41% CaO, 35% SiO2, 14% Al2O3, 7%
MgO and 0.8% S. In terms of its mineralogy, BFS is usually melilite
(solid solution series of gehlenite, 2CaO.Al2O3.SiO2, and akermanite,
2CaO.MgO.2SiO2) with a small amount of calcium sulphide (oldhamite)
<1%. Sometimes merwinite (3CaO.MgO.2SiO2) is also present and
more rarely dicalcium silicate 2CaO.SiO2.
Air cooled BFS is excavated from the open air pits next to the
blastfurnace and transported to the crushing and screening plant
where it is initially processed prior to being naturally weathered
in controlled stockpiles.
After completing an appropriate period of weathering, BFS is
processed further by crushing and screening where it is converted
into the standard range of aggregate sizes. The physical properties
of BFS make it ideally suited for use as an aggregate demonstrating
good shape with a rough surface texture providing good frictional
properties and good adhesion to both bituminous and cementitious
binders.
Air cooled Blastfurnace slag primarily provides quality controlled
aggregates for use in construction. It can be used in most applications
that would otherwise require the use of natural aggregates.
The principal applications include aggregates for use in asphalts
and surface dressings in accordance with BS EN 13043 and aggregates
for use in unbound mixtures, BS EN 13242, covering sub-bases,
cappings and fills.
The hard, stable, vesicular nature of air-cooled blastfurnace
slag provides an excellent medium for percolating filter beds
in sewage treatment works where its high surface area maximises
biological activity.
The hydraulic self-cementing nature of granulated BFS makes it
ideal as a pavior bedding material and also as a slow cementing
binder in Slag Bound Materials (SBM) which are increasingly being
used as alternative road bases and for surfacing pedestrianised
areas.
BFS also acts as a source of lime, silica and alumina in mineral
wool production for thermal insulation.
For more information contact John Bullock
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