FIELD OF THE INVENTION
The present invention relates generally to development of process and system
for recycling steel slag within or outside the steel plant. Specifically, it relates to
development of method for separation of highly concentrated phases of
phosphorus from lowly concentrated or non-phosphorus bearing phases of steel
slag.
BACKGROUND OF THE INVENTION
Slag is a by-product of pyro metallurgical process which is mostly carried out to
reduce and melt ore minerals or is carried out to refine molten metal. Blast
Furnace Slag and Steel Slag are the two by-products of an integrated steel plant
that contribute almost 90% of the total waste generated during the steel making
processes. While Blast Furnace Slag finds direct application in cement industries
as a substitute for limestone for manufacturing Slag Cement, the Steel Slag
which is generated at a rate of about 100-125 kg/ton of crude steel is subjected
for metallic recovery. The metallic recovery process consists of balling of the
cooled slag to crush the cooled slag to less than 300mm, magnetic separation of
the coarse metallic, secondary crushing of non-metallic of first stage to less than
80mm followed by magnetic separation and finally tertiary crushing of secondary
stage non-metallic to less than 6mm followed by magnetic separation. The non-
magnetic fraction of size less than 6mm which constitutes about 80% of the total
Steel Slag consists of calcium bearing minerals like calcium silicate, dicalcium
silicate(C2S), free lime, magnesium silicate, calcium alumino silicates, calcium
alumino ferrite, phosphorus in the form of phosphate and traces of other
minerals.
The steel slag rejects of metallic recovery process are mostly discarded since
they do not find bulk application within or outside the steel plants. Research

initiatives have been taken to develop processes for utilization of steel slag in
cement manufacturing. The US patent 6491751B1 entitled "Method for
manufacturing cement using a raw material mix including finely ground steel
slag" describes the utilization of steel slag as one of the raw material mix in
cement manufacture. The method of manufacturing such cement consists of
preparing a raw material mix that consists of less than five percent steel slag,
supplying the mixture of raw materials to a raw mill, comminuting the mixture of
raw materials including steel slag to finely ground particulate matter and thereby
forming a more homogenous mixture of the raw materials including the steel
slag; supplying the finely ground particulate matter to the feed end of the kiln;
and supplying heat energy to the kiln such that, as the finely ground particulate
matter moves from the feed end toward the discharge end, chemical reactions
occur within the finely ground particulate matter to form cement clinker.
Research initiatives concentrating on bulk utilization of steel slag have been able
to develop processes that include utilization of steel slag as asphalt aggregates in
road making, railway ballast or other construction applications. The European
patent EP0494218 Al entitled "Reuse of by-products from the manufacture of
steel" describes the utilization of crushed steel slag as an asphalt mix for road
making application which comprises of bitumen, steel slag material and stones.
In one of the examples, the proportion of crushed steel slag of size 0-2mm has
been described as 34% by volume of the total mix.
Though researchers have been able to utilize the steel slag as a feed constituent
for cement manufacturing but the quantity of steel slag in total feed mix is very
negligible when compared to actual generation of steel slag. This has resulted in
accumulation of huge quantity of steel slag and thus causing an environment
concern. The application in road making as asphalt aggregate has the potential
to consume bulk quantity of steel slag but it has been observed that the

presence of free lime results in delayed swelling of the slag aggregates and thus
causing volumetric expansion of the road which subsequently results in cracks on
the roads.
The research work carried out so far has mostly concentrated on utilization of
steel slag in cement manufacturing or as road aggregates; very little work has
been carried out in developing solutions for recycling of steel slag within the
steel plant. The steel slag rejects after metallic recovery normally comprises of
about 45-50% CaO, 3-5% free lime, 2-3% MgO, 12-20% Fe, 1.5-2% P, 2-3%
AI2O3 and about 15-18% SiO2. The presence of more than 50% of calcium
bearing minerals in steel slag makes it a good source of flux for sinter making.
However, the presence of 1.5% phosphorus limits the proportion that can be
recycled in sinter making. The iron ore sintering process demands a phosphorus
specification of less than 1% P which if achieved would result in bulk recycling of
steel slag within the steel plant itself.
OBJECTS OF THE INVENTION
An object of this invention is to provide a method to lower phosphorus content of
slag produced from basic oxygen furnace process of steel making.
Another object of the invention is to develop a method to reduce the
phosphorous content in the steel slag to less than 1%.
Still another object of the invention is to increase percentage recycling of steel
slag.
Another object of the invention is to prepare flux feed for iron ore sintering
operations from steel slag.
A further object of the invention is to develop a method to reduce the
environmental waste from basic oxygen furnace process of steel making.

SUMMARY OF THE INVENTION
The Steel Plant generates by products in form of fuel gas (Blast furnace, Coke
Oven, LD gas) and Blast Furnace granulated slag which are utilized as fuel
source and slag cement feed respectively. During the Steel making process
considerable quantity of steel slag is also generated (@ 125 kg/ton of crude
steel) that does not finds any application due to poor physical/chemical
properties. The Steel Slag comprises of 45% Calcium Oxide (CaO), 4%
Magnesium Oxide (MgO), 20% Iron (Fe), 4% Free lime, 15.5% Silicon dioxide
(SiO2), 2.5% Aluminium trioxide (Al2O3)and 1.8% Phosphorous (P). The presence
of free lime and iron oxide restricts the usage in road making and cement
making applications. However, the presence of high quantity of CaO in Steel Slag
has the potential to act as a flux for Iron ore sintering. The desired specifications
of such flux require <1% P. This fluxing potentiality of steel slag triggered the
need to develop process to lower phosphorus (P) from Steel Slag to less than
1%.
The process involves air cooling of the steel slag, magnetic separation of metallic
from steel slag, grinding to less than 0.5mm followed by gravity separation and
finally regrinding to less than 0.075mm followed by physico-chemical separation
stage to lower the phosphorus from the non-magnetic fraction.
The magnetic separation process comprises of stage crushing of the steel slag
followed of recovery of metallic iron. The non-magnetic yield is 80% with an
assay of 52% CaO, 1.85% P, 2.7% Al2O3, 9% Fe, 17.5% SiO2 and 4.8% MgO.
The non-magnetic fraction of magnetic separation is then ground to 100%
passing 0.5mm and subjected to gravity separation using Spiral Concentrator.
The yield of Spiral tailings is 85% of the total spiral feed and it assays 54% CaO,
1.83% P, 2.75% Al2O3, 4.2% Fe, 19% SiO2 and 5.4% MgO.

The physico-chemical process comprises of ball mill grinding of the -0.5mm
(unique and process controlling) spiral tailings to 100% passing 0.075mm and
froth flotation of < 0.075mm spiral tailings using unsaturated or saturated fatty
acid or their salts or a combination of any two as collector to float phosphorus.
The flotation is conducted at pH 9.2-9.5 using Methyl isobutyl carbinol (MIBC) as
frother. The concentration of collector or their combination is very important and
is maintained at minimum 500 ppm. The concentration of frother is also an
important factor and is maintained at minimum lOOppm. The yield of un-floated
product obtained after flotation is 79% of the flotation feed and it assays 57.5%
CaO, 2.1% Al2O3, 0.8% P, 18% SiO2, 6% MgO and 2.2% Fe. The yield of float
product is 21% and it assays 5.7% P, 40.83% CaO, 22.76% SiO2, 5.2% Al2O3,
11.7% Fe and 3.14% MgO.
Overall, the results indicate that it is possible to lower the phosphorus from steel
slag to less than 1% by adopting processes involving gravity separation and froth
flotation. The yield of concentrate is 79% with a phosphorus rejection of about
57% and CaO recovery of about 80%.
The final concentrate containing 0.8% P is well below the acceptable P
specifications required for fluxing application in iron ore sintering. The presence
of 57% CaO equally compares with the CaO available from limestone which is
mostly used as a fluxing agent in iron ore sintering.
The consumption of limestone in sintering reaches a maximum of 12% of the
total feed. A typical 6 mtpa sinter plant consumes about 0.72 mtpa limestone
while a typical 10 mtpa steel plant generates 1.25 mtpa steel slag rejects. The
above process on commercialization will be able to recycle about 0.6 mtpa as a
substitute for limestone during sintering.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1(a)- Shows an activity chart for crushing and magnetic separation of
steel slag.
Figure 1(b) - Shows an activity chart depicting spiral concentration and froth
floatation
Figure 2 - Shows a graphical representation of LD slag feed composition
obtained through sample analysis
Figure 3 - Shows a trend on effect of magnetic separation process, gravity
separation, and froth flotation, of Fe, P, and CaO.
DETAILED DESCRIPTION OF THE INVENTION
An Integrated Iron and Steel Plant working on Blast Furnace-Basic Oxygen
Furnace route generates about 400 kg of solid waste per ton of steel produced.
Major share of this (70-80%) consists of blast furnace slag and basic oxygen
furnace slag (steel slag). While blast furnace slag is entirely consumed as a
feedstock in cement industries, steel slag which is generated at a rate of 100-125
kg/ton of steel is discarded.owing to its poor physical and chemical properties.
The steel slag primarily comprises of 48% CaO, 4% MgO, 22% FeO, 4% Free
lime, 15.5% SiO2, 2.5% AI2O3 and 1.8% P. The presence of free lime is a major
deterrent for usage in road making or any building application while iron oxide
restricts usage in cement making. The presence of high quantity of CaO (50%) in
steel slag however has the potential to act as a flux for Iron ore sintering. The
desired specifications of such flux require <1% P. This fluxing potential of steel
slag triggered the need to develop process to lower phosphorus (P) from steel
slag to less than 1%.

The process of recovering a phosphorus free fraction from steel slag involves air
cooling of the steel slag, stage wise crushing of the steel slag to -6mm, magnetic
separation of metallic iron from steel slag, grinding of the non-magnetic slag to -
0.5mm that ensures liberation of metallic iron and iron oxide, gravity separation
of -0.5mm to recover iron rich concentrate and iron deficient tailings, grinding of
gravity separation tailings .to -0.075mm to ensure liberation of phosphorus
bearing phase of slag and froth-flotation of -0.075mm tailings.
The air cooled steel slag is initially crushed to less than 300mm using giant steel
balls, the -300m crushed steel slag is then subjected for magnetic separation to
recover metallic iron, the non-magnetic -300 mm steel slag is again crushed to
less than 80mm using Gyratory crusher and then subjected to recovery of
magnetic material using roll magnetic separator. The non-magnetic fraction from
roll magnetic separator is further crushed to less than 6mm using jaw crusher
and then subjected to roll magnetic separator to recover magnetic material. The
total magnetic yield from all magnetic separation processes is about 20%. The
non-magnetic yield is 80% with a chemical composition of 52% CaO, 1.85% P,
2.7% Al2O3, 9% Fe, 17.5% SiO2 and 4.8% MgO.
The -6mm non-magnetic fraction of magnetic separation is then ground to 100%
passing 0.5mm using dry closed circuit roll crusher and screen. The grinding size
of -0.5mm ensures liberation of metallic iron and iron oxide bearing phase of
slag. The roll crusher setting is progressively set at 1.5mm, 1mm and 0.5mm in
stages to avoid overgrinding of sample. The screen used is circular vibratory
screen with a screen opening size of 0.5mm. The oversize +0.5mm of screen is
recirculated back to the crusher for further crushing while the -0.5mm steel slag
generated after roll crushing is then subjected to gravity separation using Spiral
Concentrator. The Spiral concentrator used is high gradient spiral with operating
parameters of 0.5 tonne per hour solid feed rate and with 20% feed solids in

slurry. The concentrate splitters were adjusted at 20% opening of the total
opening. The yield of Spiral tailings is 85% of the total spiral feed and it assays
54% CaO, 1.83% P, 2.75% Al2O3, 4.2% Fe, 19% SiO2 and 5.4% MgO
The Spiral Concentrator tailing containing maximum 30% solids is allowed to
naturally settle. The tailing is allowed to settle for 30 minutes or more followed
by which the overflow water is decanted. The spiral concentrator tailing is then
diluted with water/ further filtered to remove water to maintain 60% solids. This
is done to ensure a dilution ratio (wt of water/wt of material) of 0.67 is achieved
for the settled material (Spiral Tailings). The dilution ratio is exactly maintained
at 0.67 to maximize the liberation of phosphorus in ball mill grinding. The Spiral
tailings containing 60% solids is further ground to 100% passing 0.075mm in
wet condition using a closed circuit wet ball mill and screen. The size -0.075mm
to which the entire steel slag is ground controls the liberation of the phosphorus
bearing phases of slag. Therefore the invention relies upon the parameters i.e.
water/slag ratio of 0.67 and grinding size of -0.075mm. The screen used is a
circular vibratory screen with a screen opening size of 0.075mm. The oversize
+0.075mm of circular screen is recirculated back to ball mill for regrinding. The -
0.075mm material of ball mill is filtered using vacuum filter to maintain 85%
solids and 15% water. The -0.075mm material containing 85% solids is then
diluted by adding water. The % solids of 10%, 11% or at the most 12% are
maintained. The -0.075mm material containing 10-12% solids is then
conditioned for 3-5 minutes in a laboratory flotation cell of 2.5 litre capacity and
at an impeller speed of 1100 rpm. The 10% solids containing suspension is then
maintained at pH between 9.2-9.5 using NaOH. The suspension is agaoin
conditioned for 1 minute. To this suspension saturated or unsaturated fatty acid
or their salts or a combination of any two is added as collector to float the
phosphorus bearing phase. Fatty acids derived from either vegetable oils or from
animal fats or a combination of both is used. The collector dosage of minimum of

500 ppm is added. The suspension is again conditioned for 3-5 minutes and after
conditioning 100ppm of MIBC is added. After adding MIBC the suspension is
again conditioned for 2 minutes. The conditioned of suspension is uniformly done
at impeller speed of 1100 rpm. After conditioning of suspension, normal air is
allowed to flow through the suspension. The air flow rate is maintained at
minimum 2 Ipm. Immediately as the air is allowed to flow, froth starts to appear
over the top of suspension. This froth is collected for 2 minutes. The air flow to
the suspension is continued till 2 minutes and then stopped. The suspension
level in the flotation cell is maintained by adding tap water at a rate of 0.25 litre
per minute. The water addition is started immediately after starting the air flow
and the same is stopped immediately after stopping the air flow. The float
product and the un-floated product are separately collected, dried and weighed.
The yield of un-floated product obtained after flotation is 79% of the flotation
feed and it assays 57.5% CaO, 2.1% Al2O3, 0.8% P, 18% SiO2, 6% MgO and
2.2% Fe. The yield of float product is 21% of the flotation feed and it assays
5.7% P, 40.83% CaO, 22.76% SiO2, 5.2% Al2O3, 11.7% Fe and 3.14% MgO.
Overall, the results indicate that the phosphorus from steel slag is reduced to
less than 1% by adopting processes involving magnetic separation, gravity
separation and froth flotation. The yield of phosphorus deficient concentrate is
79% with a consequent phosphorus rejection of about 66% in the flotation
tailings.
The final concentrate containing 0.8% P is well below the acceptable
specifications required for fluxing application in iron ore sintering. The presence
of 57% CaO equally compares with the CaO available from limestone which is
mostly used as a fluxing agent in iron ore sintering.

WE CLAIM
1. A method for reducing phosphorus content of steel slag generated from
steel making process, the method comprising:
air cooling steel slag;
crushing steel slag to size ≤ 6mm and separating metallic
components by magnetic separation;
grinding non-magnetic fraction of steel slag to a size ≤0.5mm and
subjecting to gravity separation;
grinding spiral tailings to size ≤ 0.075mm in wet condition;
vacuum filtering ≤ 0.075 mm size material and diluting the solid
filtrate with water; and
froth flotation of the suspension at ph range 9.2-9.5.
2. A method as claimed in claim 1, wherein steel slag comprises (wt %) of
48% CaO, 4% MgO, 22% FeO, 4% Free lime, 15.5% SiO2, 2.5% Al2O3
and 1.8% P.
3. A method as claimed in claim 1, wherein steel slag after air cooling is
crushed to size ≤300mm and subjected to magnetic recovery.
4. A method as claimed in claim 3, wherein non-magnetic fraction of steel
slag is crushed to size ≤80mm and subjected to magnetic separation.
5. A method as claimed in claim 4, wherein non-magnetic fraction of steel
slag is crushed to size ≤6mm and subjected to magnetic separation.
6. A method according to claim 1 and claim 5, characterised in that the ≤
6mm non-magnetic fraction obtained after magnetic recovery is further

grounded to size ≤ 0.5mm using closed circuit roll crusher and circular
vibratory screen.
7. A method as claimed in claim 1, wherein ≤ 0.5mm non-magnetic fraction
is subjected to gravity separation using spiral concentrator.
8. A method as claimed in claim 1, wherein the tailings after gravity
separation step are grounded to size ≤0.075mm using closed circuit wet
ball mill grinding and 0.075mm circular vibratory screen.
9. A method as claimed in claim 1, wherein tailing size ≤0.075mm are
vacuum filtered to increase the solid concentration to 85 wt. %.
10. A method as claimed in claim 9, wherein the filtrate is further diluted with
water to prepare a suspension of concentration range 10-12 wt.%.
11. A method as claimed in claim 1 and 10, wherein fatty acid derived from
either vegetable oils or from animal fats or a combination of both can be
used as a collector in froth flotation.
12. A method as claimed in claim 1, wherein Methyl isobutyl carbinol (MIBC) is
used as frother in the froth floatation step.
13. A method as claimed in claim 1, wherein the un-floated product obtained
after froth flotation comprises less than 1% phosphorous.

ABSTRACT

The present invention relates to development of a process to lower the
phosphorus content of steel slag. It involves air cooling of the steel slag,
crushing and magnetic separation of metallic from steel slag, grinding and
gravity separation and finally physico-chemical separation stage to lower the
phosphorus from the non-magnetic fraction.
The invented process is capable of treating the air cooled slag containing 45%
CaO, 4% MgO, 20% Fe, 4% Free lime, 15.5% SiO2, 2.5% AI2O3 and 1.8% P to
produce a final product containing 57.5% CaO, 2.1% Al2O3, 0.8% P, 18% SiO2,
6% MgO and 2.2% Fe. The final yield of the phosphorus deficient product is
47.4% of the total slag feed.
Overall with the invented process it is possible to lower the phosphorus from
steel slag to less than 1% by adopting processes involving gravity separation and
froth flotation. The final concentrate containing 0.8% P is well below the
acceptable P specifications required for fluxing application in iron ore sintering.
The presence of 57% CaO equally compares with the CaO available from
limestone which is mostly used as a fluxing agent in iron ore sintering.