FIELD OF THE INVENTION
The present invention relates to a system for stabilization of Steel Making (BOF) slag
through hydration treatment and method for such stabilization. More particularly, the
present invention is directed to provide a system for accelerated primary stabilization of
BOF slag using steam as medium of stabilization through hydration, by introduction of
saturated steam at predetermined pressure and temperature into a pressure vessel such as
to induce penetration of steam into the slag sample and thereby stabilize the slag. The
system and method to stabilize Steel Making (BOF) slag according to the present invention
thus render the slag suitable for further use/reuse for variety of applications avoiding the
conventional technical limitation of expansion of BOF slag.
BACKGROUND OF THE INVENTION
In the basic oxygen process, hot liquid blast furnace metal, scrap, and fluxes, which consist
of lime (CaO) and dolomite lime (CaO, MgO) are charged to converter. High-pressure
oxygen is injected which combines with and removes the impurities in the charge. These
impurities consist of carbon as gaseous carbon monoxide, and silicon, manganese,
phosphorus and some iron as liquid oxides, which combine with lime and dolomite (flux) to
form the steel slag. At the end of the refining operation, the liquid steel is tapped into a
ladJe while the steel slag is tapped into a separate slag pot where it solidifies upon cooling.
Generation of BOF slag varies between 100-160 kg/ton crude steel depending on many
factors. BOF slag is recycled in Steel Melting shop, Sinter Plant and Blast Furnaces but due
to 'Phosphorus' limitations in steel production, total BOF slag recycling inside steel plant is
not feasible. Free calcium and magnesium oxides are not completely consumed in the steel
slag. Un-slaked lime and magnesia hydrates with contact of moisture and render steel slag
expansive in nature (volume changes up to 10 percent or more). The free lime hydrates
rapidly and can cause large volume changes over a relatively short period of time (weeks),
while magnesia hydrates much more slowly and contributes to long-term expansion that
may take years to develop. This potentially expansive nature could cause difficulties with
products containing steel slag. Hence, part of this is either recycled or reused and rest
portion is dumped. This causes not only environment degradation but occupies land for long
time.

Most Indian steel plants report use of BOF slag for a range of ex-process applications such
as internal road making, rail ballast, storage yard, etc. While some Indian plants report upto
60 percent use of BOF slag, often that includes landfilling and therefore cannot be termed
as gainful reuse. Moreover, utilization statistics is generally bundled together as total
utilization, specific information on various routes of utilization is difficult to obtain.
Although steel slag has excellent engineering properties its uses have been limited by its
expansion characteristics. The use of steelmaking slag in road construction has fallen into
disfavor in some parts of the world because the free lime present in steelmaking slag can
subsequently hydrate, causing expansion and disintegration of the roadbed.
Free calcium and magnesium oxides are not completely consumed in the steel slag, and
there is a general agreement in technical literature that the hydration of un-slacked lime
and magnesia in contact with moisture is largely responsible for the expansive moisture of
most steel slag. The free lime hydrates rapidly and can cause large volume changes over a
relatively short period of time, while magnesia hydrates much more slowly and contribute to
long term expansion that may take years to develop. The BOF slag can be used as
aggregate base for various road projects if this challenge is addressed adequately.
One of the most common applications for steel slag, in many countries ( but not commonly
reported in Indian steel industry ) is the construction of unpaved parking lots & road
shoulder material. Physically, natural aggregates are unable to provide a surface that will
resist polishing; therefore, they easily become slippery when wet. Where friction is an
important safety consideration in pavement design, properties of Steel Slag qualify it as a
premier surface aggregate for skid resistant applications and also is considered as suitable
aggregate in Hot Mix Asphalt (HMA) surface mixture applications.
A method of treatment of steel-making slag in about one minute by immersing the slag in
water and by applying ultrasonic waves to the slag contained in a small container, such as a
glass beaker has been disclosed in Japanese Laid-Open Patent Publication No. Hei 5-17182.
However, since the slag layer conveyed on a belt conveyor through water is required to be
made thicker when the method is applied on an industrial scale, it is difficult to put the
method into practical use.

A method of charging high-temperature slag having been just discharged from a steel
making smelter and having coagulated into a pressure vessel and sprinkling water or the
like to perform cooling, aging and crushing simultaneously has been disclosed in Japanese
Laid-Open Patent Publication No. Hei 6-127984. In this method, since solids at an extremely
high temperature of about 1000° C. must be transferred, the transfer means are required to
be heat resistant. Furthermore, since uncrushed large lumps of slag are required to be
charged into the pressure vessel, the sealing surfaces of the lids and the walls of the
pressure vessel are scratched easily, thereby causing a problem of unsmooth operation.
Japanese Laid-Open Patent Publication No. 61-101441 discloses that steel-making slag has
been conventionally left in natural conditions for three to six months, and proposes a
method purportedly capable of significantly reducing the period by blowing high-
temperature gas including steam into slag.
As per available literature, Nippon steel has also been working on the development of
application techniques of steel slag to asphalt mix aggregate. The use of steel slag in this
application has been made possible by making full use of the physical properties of steel
slag while controlling its expansion characteristics and by coating the surface of steel slag
by asphalt emulsion. With these preventive measures against expansion, laboratory results
have shown that crushed steel slag of any mixing ratio exhibits superior properties to
crushed stone.
Therefore steel slag, if processed suitably for stabilization, has favourable mechanical
properties for aggregate use, including good abrasion resistance, good soundnesss
characteristics, and high bearing strength. This necessitates development of a system and
process that would stabilize steelmaking slag for enhanced applicability of BOF slag.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to provide a system for
stabilization of Steel Making (BOF) slag through hydration treatment and a convenient user
friendly method for such stabilization of slag sample to render the slag suitable for further
use/reuse.

A further object of the present invention is directed to provide a system and method for
accelerated primary stabilization of BOF slag using steam as medium of stabilization through
hydration.
A still further object of the present invention is directed to provide a system and method for
stabilization of Steel Making (BOF) slag through hydration treatment wherein BOF slag
crushed to desired size would be treated by introduction of saturated steam at
predetermined pressure and temperature into a pressure vessel such as to induce
penetration of steam into the slag sample and thereby stabilize the slag.
A still further object of the present invention is directed to provide a system and method for
stabilization of Steel Making (BOF) slag through hydration treatment wherein Percent
reduction in Calcium oxide as well as change in pH and alkalinity with time is used as
marker for completion of hydration of free lime present in BOF slag.
A still further object of the present invention is directed to provide a system and method for
stabilization of Steel Making (BOF) slag through hydration treatment wherein wire net
baskets of various sizes made of stainless steel used for keeping samples for better
penetration of saturated steam.
A still further object of the present invention is directed to provide a system and method for
accelerated stabilization of Steel Making (BOF) slag through hydration treatment using
saturated steam wherein the time of exposure of sample including heating time which in
turn is connected to pressure development, stabilization time and pressure release time
would be substantially less and maximum upto 3.5hrs.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a system for stabilization of Steel
Making (BOF) slag through hydration treatment to render the slag suitable for further
use/reuse comprising:
a pressurized vessel for housing charged slag sample for treatment;
means for introducing saturated steam into said pressure vessel;
means for measuring saturated steam pressure in said pressure vessel;

means for controlling the introduction of said saturated steam into said pressure
vessel at predetermined pressure and temperature such as to induce penetration of
steam into the sample and thereby stabilize the slag.
A further aspect of the present invention is directed to said system comprising means for
charging slag sample into said vessel and discharging treated slag sample from the vessel
preferably involving net basket on a condensate collection tray.
A still further aspect of the present invention is directed to said system comprising
mechanism for paddle lifting for putting and removing of slag in and from the vessel.
A still further aspect of the present invention is directed to said system wherein said net
basket resting on a condensate collection tray are removably placeable and taken out from
said vessel.
A still further aspect of the present invention is directed to said system wherein said
pressurized vessel is adapted to withstand predetermined pressure preferably upto 6 kg/cm2
and preferably obtained of stainless steel.
A still further aspect of the present invention is directed to said system comprising a
mechanism of heating through heating coils.
Yet another aspect of the present invention is directed to said system comprising a digital
pressure controller and a manual pressure gauge to measure saturated steam pressure.
A further aspect of the present invention is directed to said system comprising means to
monitor the temperature difference between the upper and lower portion of said pressure
vessel preferably comprising two thermocouples installed at the upper and lower portion of
the vessel.
Another aspect of the present invention is directed to said system wherein said net basket
comprises wire net basket of various sizes made of stainless steel and said collection tray is
also of stainless steel.

A still further aspect of the present invention is directed to said system comprising multiple
level of detachable net baskets for various size ranges.
Yet another aspect of the present invention is directed to a method of BOF slag stabilization
involving the system as described above comprising:
providing the sample siag to be treated in said detachable net basket on condensate
collection tray inside the pressure vessel;
maintaining the interior of the pressure vessel in predetermined saturated steam condition
such as to induce penetration of steam into the sample kept inside the net basket;
stopping the steam generation after desired time and exhausting the generated steam by
opening steam exhaust valve to cool it down;
after cooling, taking out the detachable net basket having the treated sample condensate
collection tray having sample mixed condensate and stand outside the vessel.
A further aspect of the present invention is directed to said method of BOF slag stabilization
involving the system comprising;
charging steel-making slag crushed to the extent that the slag is having a predetermined
diameter which constitutes 90% or more of the total amount into said vessel where
primarily in-situ generated steam is used as media for said slag stabilizing free lime which
can sustain a pressure higher than atmosphere ;
handling of slag & steam-water inside the said vessel through batch discharging of hot
condensate for maintenance of slag temperature as well as removal of insoluble calcium
oxide from the slag surface along with discharged hot condensate.
A still further aspect of the present invention is directed to said method where air dried slag
is crushed to have the predetermined diameter charged preferably having diameter of 15
mm or less constituting 90% or more of the slag charge into said sealable pressure vessel,
where the interior of the pressure vessel is retained at the high temperature and
pressurized steam atmosphere, where high temperature is generated and pressurized
saturated steam condition is attained within the vessel itself.
A still further aspect of the present invention is directed to said method wherein the
progress of the hydration reaction is monitored through surrogate parameters of pH and

alkalinity which when stabilized can be used as indicator of end point of reaction and
therefore can be used as an indicator of stabilization of BOF slag involving the stabilization
of free lime .
A still further aspect of the present invention is directed to said method comprising
operating parameters involving:
Time of treatment : 21/2 - 31/2 hours
Ratio of heating, Treatment & stabilization time: 1:4:1
Pressure range: 3-6 kg/cm2
Size Range: 80% slags of size < 15 mm
Saturation pH indicator : 11.5-12.3
Saturation alkalinity : 600-700 mg/l
Saturation hardness of stabilizing media: 1100-1300 mg/l
Slag Volume/apparatus total volume: 10-15%
A still further aspect of the present invention is directed to said method wherein the time of
exposure including heating time, stabilization time and pressure release time comprise
maximum upto 3.5hrs which in turn is connected to pressure development.
The objects and advantages of the present invention are described hereunder in greater
details with reference to the following accompanying non limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the schematic view of the system/apparatus for stabilization of Steel Making
(BOF) slag through hydration treatment using saturated steam at predetermined pressure
and temperature involving a pressurized vessel according to the present invention showing
its different components.
Figure 2 & 3: is the photographic image of two different views of the outside appearance of
the apparatus used for stabilization of Steel Making (BOF) slag through hydration treatment
according to the present invention.

Figure 4: is the photographic image of the apparatus used for stabilization of Steel Making
(BOF) slag through hydration treatment according to the present invention along with
display/control module.
Figure 5: is the graphical illustration of the change in pH with time as an indicator of
completion of hydration treatment of slag according to the present invention.
Figure 6: is the graphical illustration of the change in alkalinity with time as an indicator of
completion of hydration treatment of slag according to the present invention.
Figure 7: is the graphical presentation of apparent reduction in Free lime as surrogate
measure of stabilization of slag sample.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention is directed to provide a system and method for accelerated primary
stabilization of BOF slag using steam as medium of stabilization through hydration, by
introduction of saturated steam at predetermined pressure and temperature into a pressure
vessel such as to induce penetration of steam into the slag sample and thereby stabilize the
slag.
BOF slag stabilization by various methods were tried in the laboratory. Experiments using a
number of variables have been carried out on different sizes of BOF slag pH, alkalinity
values was taken as marker parameters for completion of hydration. To accomplish the
purpose of accelerated primary stabilization of BOF slag using steam as medium of
stabilization through hydration, a prototype system/apparatus made of stainless steel was
designed and fabricated.
Details of the system/apparatus used for experimentation:
The purpose of the system/apparatus developed according to the present invention is to
stabilize Steel Making (BOF) slag to render the slag suitable for further use/reuse for
applications where the major technical limitation of utilization of BOF slag is expansion of
slag. The working principle of the apparatus developed according to the present invention, is
that of a pressurized vessel. The interior of pressurized vessel is kept in saturated steam

condition at the predetermined pressure upto 6 kg/cm2 and temperature ( saturated steam
condition ) to induce penetration of steam into the sample kept inside the vessel in net
basket resting on a 'condensate collection tray'. After few hours, steam generation is stopped
and generated steam in the vessel is exhausted by opening steam exhaust valve to cool it
down. After cooling, the detachable net basket (having sample), condensate collection tray
(having sample mixed condensate) and stand is taken outside the vessel as treated sample
and their cleaning for next experiment.
For stabilization of BOF slag by steam, a steam stabilization apparatus was designed and
fabricated to withstand a pressure of 6 kg/cm2. The apparatus was made of stainless steel.
A digital pressure control as well as a manual pr. gauge was provided to measure saturated
steam pressure. To monitor the temperature difference (should not be more than 5°C), two
thermocouples were installed at the upper and lower portion of the vessel. Wire net baskets
of various sizes made of stainless steel used for keeping samples for better penetration of
saturated steam. To collect condensate and precipitate, a collection tray of SS was provided
under it.
Accompanying Figure 1 shows the schematic view of the system/apparatus for stabilization
of Steel Making (BOF) slag through hydration treatment using saturated steam at
predetermined pressure and temperature involving a pressurized vessel according to the
present invention showing its different components.
Accompanying Figure 2 & 3 show the photographic images of two different views of the
outside appearance of the apparatus used for stabilization of Steel Making (BOF) slag
through hydration treatment according to the present invention.
Accompanying Figure 4 is the photographic image of the apparatus used for stabilization of
Steel Making (BOF) slag through hydration treatment according to the present invention
along with display/control module.
The detailed specification of the system/apparatus developed according to the present
invention for stabilization of BOF slag sample using saturated steam are as given in the
following Table 1.

The above described apparatus has the means for charging slag into the pressure vessel
and discharging slag from said pressure vessel., involving a mechanism of paddle lifting for
putting and removal of slag in the vessel. The apparatus of the present invention further
comprises a mechanism of heating through heating coil. Another advantageous feature of
the invented apparatus is multiple level of detachable net baskets for various size ranges.
Experimental process steps:
Process and operating conditions for treatment of slag for accelerated stabilization are as
follows:

The present invention provides a method for steel-making slag stabilization where air dried
slag is crushed to have the predetermined diameter charged into a sealable pressure vessel,
where the interior of the pressure vessel is retained at the high temperature and
pressurized steam atmosphere, where high temperature is generated and pressurized
saturated steam condition is attained within the vessel itself. .
BOF slag sample of different sizes viz (-5) mm, (5-15) mm & (16-45) mm were treated at
various process and operating conditions. After treating with steam, pH and alkalinity of
samples were measured. Samples were dried and analyzed by XRF to see the percentage of
Calcium oxide (CaO) present in the slag sample. Percent reduction in Calcium oxide is used
as marker for completion of hydration of free lime present in BOF slag. pH and alkalinity
change are also used as additional marker.
The method also involves charging steel-making slag crushed to the extent that the steel
making slag crushed in such a manner that the slag having diameter of a 15 mm or less
which constitutes 90% or more of the total amount into an apparatus where primarily in-
situ generated steam is used as media for stabilizing free lime which can sustain a pressure
higher than atmosphere .
The process step and mechanism of handling of slag and steam-water inside the said
apparatus is implemented through batch discharging of hot condensate for maintenance of
slag temperature as well as removal of insoluble calcium oxide from the slag surface along
with discharged hot condensate.
The above mentioned process induces a hydration reaction. The progress of the hydration
reaction is monitored through surrogate parameters of pH and alkalinity which when
stabilized can be used as indicator of end point of reaction and therefore can be used as an
indicator of stabilization of BOF slag (through stabilization of free lime ).
The method of BOF slag stabilization involving the system according to the present
invention has carried out wherein the following parameters were maintained:
Time of treatment : 21/2 - 31/2 hours
Ratio of heating, Treatment & stabilization time: 1:4:1

Pressure range: 3-5 kg/cm2
Size Range: 80% slags of size < 15 mm
Saturation pH indicator : 11.5-12.3
Saturation alkalinity : 600-700 mg/l
Saturation hardness of stabilizing media:1100-1300 mg/l
Slag Volume/apparatus total volume: 10-15%
The time of exposure including heating time, stabilization time and pressure release time
maximum upto 3.5hrs, which in turn is connected to pressure development.
Results:
The relevant supporting results, are presented in accompanying Figs 5,6 & 7. Percent
reduction in Calcium oxide is used as marker for completion of hydration of free lime
present in BOF slag pH and alkalinity change are also used as additional marker.
Accompanying Figure 5 is the graphical illustration of the change in pH with time as an
indicator of completion of hydration treatment of slag according to the present invention.
Accompanying Figure 6 is the graphical illustration of the change in alkalinity with time as
an indicator of completion of hydration treatment of slag according to the present invention.
Accompanying Figure 7 is the graphical presentation of apparent reduction in Free lime as
surrogate measure of stabilization of slag sample.
It is thus possible by way of the present invention to developing a system and method for
accelerated primary stabilization of BOF slag using steam as medium of stabilization through
hydration, by introduction of saturated steam at predetermined pressure and temperature
into a pressure vessel such as to induce penetration of steam into the slag sample and
thereby stabilize the slag and thus render the slag suitable for further use/reuse for variety
of applications avoiding the conventional technical limitation of expansion of BOF slag.

We Claim:
1. A system for stabilization of Steel Making (BOF) slag through hydration treatment to
render the slag suitable for further use/reuse comprising:
a pressurized vessel for housing charged slag sample for treatment;
means for introducing saturated steam into said pressure vessel;
means for measuring saturated steam pressure in said pressure vessel;
means for controlling the introduction of said saturated steam into said pressure
vessel at predetermined pressure and temperature such as to induce penetration of
steam into the sample and thereby stabilize the slag.
2. A system as claimed in claim 1 comprising means for charging slag sample into said
vessel and discharging treated slag sample from the vessel preferably involving net
basket on a condensate collection tray.
3. A system as claimed in anyone of claims 1 or 2 comprising mechanism for paddle
lifting for putting and removing of slag in and from the vessel.
4. A system as claimed in anyone of claims 1 to 3 wherein said net basket resting on a
condensate collection tray are removably placeable and taken out from said vessel.
5. A system as claimed in anyone of claims 1 to 3 wherein said pressurized vessel is
adapted to withstand predetermined pressure preferably upto 6 kg/cm2 and
preferably obtained of stainless steel.
6. A system as claimed in anyone of claims 1 to 5 comprising a mechanism of heating
through heating coils.
7. A system as claimed in anyone of claims 1 to 6 comprising a digital pressure
controller and a manual pressure gauge to measure saturated steam pressure.
8. A system as claimed in anyone of claims 1 to 7 comprising means to monitor the
temperature difference between the upper and lower portion of said pressure vessel

preferably comprising two thermocouples installed at the upper and lower portion of
the vessel.
9. A system as claimed in anyone of claims 1 to 8 wherein said net basket comprises
wire net basket of various sizes made of stainless steel and said collection tray is also of
stainless steel.
10. A system as claimed in anyone of claims 1 to 9 comprising multiple level of
detachable net baskets for various size ranges.
11. A method of BOF slag stabilization involving the system as claimed in anyone of
claims 1 to 10 comprising:
providing the sample slag to be treated in said detachable net basket on condensate
collection tray inside the pressure vessel;
maintaining the interior of the pressure vessel in predetermined saturated steam
condition such as to induce penetration of steam into the sample kept inside the net
basket;
stopping the steam generation after desired time and exhausting the generated steam
by opening steam exhaust valve to cool it down;
after cooling, taking out the detachable net basket having the treated sample
condensate collection tray having sample mixed condensate and stand outside the
vessel.
12. A method of BOF slag stabilization involving the system as claimed in claim 11
comprising;
charging steel-making slag crushed to the extent that the slag is having a
predetermined diameter which constitutes 90% or more of the total amount into said
vessel where primarily in-situ generated steam is used as media for said slag stabilizing
free lime which can sustain a pressure higher than atmosphere ;
handling of slag & steam-water inside the said vessel through batch discharging of hot
condensate for maintenance of slag temperature as well as removal of insoluble calcium
oxide from the slag surface along with discharged hot condensate.

13. A method as claimed in anyone of claims 11 to 12 where air dried slag is crushed to
have the predetermined diameter charged preferably having diameter of 15 mm or less
constituting 90% or more of the slag charge into said sealable pressure vessel, where
the interior of the pressure vessel is retained at the high temperature and pressurized
steam atmosphere, where high temperature is generated and pressurized saturated
steam condition is attained within the vessel itself.
14. A method as claimed in anyone of claims 11 to 13 wherein the progress of the
hydration reaction is monitored through surrogate parameters of pH and alkalinity which
when stabilized can be used as indicator of end point of reaction and therefore can be
used as an indicator of stabilization of BOF slag involving the stabilization of free lime.
15. A method as claimed in anyone of claims 11 to 14 comprising operating parameters
involving:
Time of treatment : 21/2 - 31/2 hours
Ratio of heating, Treatment & stabilization time: 1:4:1
Pressure range: 3-6 kg/cm2
Size Range: 80% slags of size < 15 mm
Saturation pH indicator : 11.5-12.3
Saturation alkalinity : 600-700 mg/l
Saturation hardness of stabilizing media:1100-1300 mg/l
Slag Volume/apparatus total volume: 10-15%
16. A method as claimed in anyone of claims 11 to 15 wherein the time of exposure
including heating time, stabilization time and pressure release time comprise maximum
upto 3.5hrs which in turn is connected to pressure development.

ABSTRACT

The present invention relates to a system for stabilization of Steel Making (BOF) slag
through hydration treatment using saturated steam and method for such stabilization. More
particularly, the present invention is directed to provide a system for accelerated primary
stabilization of BOF slag using steam as medium of stabilization through hydration, by
introduction of saturated steam at predetermined pressure and temperature into a pressure
vessel such as to induce penetration of steam into the slag sample and thereby stabilize the
slag. The system and method to stabilize Steel Making (BOF) slag monitors the progress of
the hydration reaction through surrogate parameters of pH and alkalinity which when
stabilized can be used as indicator of end point of reaction and therefore can be used as an
indicator of stabilization of BOF slag involving the stabilization of free lime at reduced level
avoiding the conventional technical limitation of expansion of BOF slag and thus rendering
the slag suitable for further use/reuse for variety of applications.