FIELD OF THE INVENTION
The present invention relates generally to development of process and
system for enriching the iron content in steel slag rejects and specifically, it
relates to treatment of steel slag of size -6mm through reduction roasting-
magnetic separation technique to produce a concentrate with enriched iron
content. More particularly, the invention relates to a process for separating
iron values of steel slag by reduction roasting-magnetic separation technique.
BACKGROUND AND PRIOR ART
Steel slag is a by-product of steel making process which is carried out to
refine the pig iron. The steel making process is an oxidation process where
the hot metal impurities like C, Si and P are oxidised and subsequently
slagged out with the help of fluxing additives like lime and magnesia.
The steel slag generated from refining of hot metal contains metallic iron,
wustite, and some calcium bearings minerals. Most of the metallic iron is
recovered by processing the steel slag through a series of magnetic
separation. The magnetic 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 75% of the total steel slag volume, consists of iron bearing
minerals like wustite, calcium alumino ferrite and metallic iron along with
some amount of magnesio ferrite.
The steel slag non-metallic rejects normally comprises of about 45-50% CaO,
3-5% free lime, 2-3% MgO, 12-20% Fe, 1.5-2% P, 2-3% Al2O3 and about

15-18% SiO2. The slag is mostly discarded owing to its poor chemical and
physical properties. However the presence of high CaO in slag makes it
suitable for recycling application provided the phosphorus can be lowered to
less than 1%.
Alternative techniques to recover metallic content have been developed that
includes the physical separation method like jigging.
Canadian patent number CA2418020 C describes the process of recovering
metallic content through separation of steel slag into a metal concentrate
fraction and an aggregate fraction.
Chinese patent CN 101596488 titled “Stainless steel slag iron separation
technique” describes the method to recover metallic values through Jigging.
Chinese patent CN101569875 titled “Process for jigging iron from molten iron
nickel slag” also mentions about recovery of metallic iron through jigging
technique.
US 4772384, DE 3339026 A1 and EP 1312415 A1 patents describe about the
recovery of metallic content slag through Jigging technique.
The non-magnetic fraction of steel slag is mostly discarded since it does not
finds bulk application within or outside the steel plants. Research initiatives
have been taken to develop processes for utilization of steel slag in cement
manufacturing.
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.

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.
European patent EP0494218 A1 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.
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 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 rejects
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% Al2O3
and about 15-18% SiO2. The presence of more than about 20% Fe makes it
a good source of metallic iron for sinter making. However the presence of
impurities like P limits the usage in recycling application. The iron ore

sintering process demands a P specification of less than 1% which if achieved
would result in bulk recycling of steel slag within the steel plant itself.
Menad et.al, 2014 discussed the recovery of low phosphorus and iron rich
compounds from LD slag through low intensity (LIMS) and high intensity
magnetic separation (HIMS) techniques. These processes recover
ferromagnetic and paramagnetic particles at LIMS and HIMS respectively.
Wu et.al 2014, have reported successful recovery of high phosphorus and
calcium rich compounds in non-magnetic fraction adopting low intensity
magnetic separation technique.
Semikina et.al, 2010, reported that iron oxides contained in steel slag can
be converted to magnetite through controlled oxidation of LD slag at higher
temperatures. The converted magnetite can then be recovered through low
intensity magnetic separation techniques.
Lin et.al, 2013 attempted modification of Al2O3 for subsequent enrichment
of phosphorus in calcium silicate phase. The objective was to recover low
phosphorus and iron rich compounds in magnetic fraction.
Most of the non-patent literature work and prior patents referred above
suggest either the use of magnetic separation technique to separate low
phosphorus and high phosphorus compounds or methods to utilize steel slag
in cement making. None of the prior art reports use of enhanced gravity
separation technique to physically separate iron rich compounds from calcium
and phosphorus rich compounds.

gravity separation techniques. The concentrate thus produced from this
technique can be easily utilized in iron ore pelletization or sintering process.
This will enable increased recycling of steel slag within the steel plant.
OBJECTS OF THE INVENTION
An object of this invention is to propose a process for separating iron values
of steel slag by reduction roasting-magnetic separation technique, which
allows easier separation of iron oxide from steel slag.
Another object of this invention is to propose a process for separating iron
values of steel slag by reduction roasting-magnetic separation technique,
which enables upgrading iron content and lower phosphorus content of steel
slag.
Further object of this invention is to a process for separating iron values of
steel slag by reduction roasting-magnetic separation technique, which makes
it possible to utilize the waste steel slag in iron ore sintering or pelletization.
SUMMARY OF THE INVENTION:
According to this invention there is provided a process for upgrading iron
content of waste steel slag comprising the steps of cooling the steel slag
followed by crushing the steel slag to less than 6mm, subjecting the crushed
steel slag to magnetic separation, screening the non-magnetic fraction at
3mm to obtain -6+3 mm, and -3mm fractions, subjecting the -6+3mm
fraction to crushing so as to produce -3mm material, blending the crushed -
3mm material and the natural -3mm material as-obtained proportion and
subjected to reduction roasting using coal as a reductant cum fuel, subjecting
the reduced mass to grinding to below 0.5mm and subjecting the ground

mass to wet low intensity magnetic separation to separate magnetic fraction
from non-magnetic fraction.
The steel slag rejects after metallic recovery normally comprises about 45-
50% CaO, 3-5% free lime, 2-3% MgO, 12-20% Fe, 1.5-2% P, 2-3% Al2O3
and about 15-18% SiO2. The presence of metallic iron restricts usage of the
slag in cement making while the presence of free lime restricts usage in road
making and civil engineering applications. However the presence of high
quantity of iron oxide and metallic iron could be useful for application in iron
ore sintering. This potentiality of waste steel slag triggered the need to
develop a process to enrich iron content and lower the P content of steel
slag. The desired specifications of such flux require P less than 1%.
The process involves air cooling of the steel slag, stage wise crushing to -
6mm followed by dry magnetic separation of metallic from steel slag,
screening of non-metallic slag rejects at 3mm, crushing of -6+3mm material
to less than 3mm. The natural -3mm material and the crushed -3mm size
material are both mixed together and then subjected to reduction roasting
using coal as a reductant cum fuel. The reduction roasted -3mm material is
then ground to -0.5mm and subjected to wet low intensity magnetic
separation to upgrade the iron content of the magnetic concentrate. The
magnetic concentrate containing high iron content compared to non-metallic
steel slag also contains comparatively lower phosphorus content of less than
1%.
The dry magnetic separation process involves stage crushing of the steel slag
followed by recovery of metallic iron. The weight % yield of non-magnetic
material in this process is 80% with a typical assay of 43.5% CaO, 4.8%
MgO, 18.4% Fe, 14.7% SiO2, 2.7% Al2O3 and 1.2% P.

The non-magnetic fraction of dry magnetic separation is then screened at
3mm to obtain two fractions i.e. -6+3mm and -3mm (process controlling and
unique). The -3mm material screened out from the as-received material is
named as natural -3mm material.
The weight wise distribution of -6+3mm and -3mm is 75.5% and 24.5%
respectively. The -6+3mm assayed 20.2%Fe,14.6% SiO2, 43.9% CaO, 2.8%
Al2O3, and 1.25% P while the -0.5mm assayed 15.7%Fe,14.8% SiO2,
42.8% CaO, 2.6% Al2O3, and 1.13% P.
The size fraction -6+3mm is then subjected to crushing in a roll crusher to
entirely crush the material to -3mm size. The roll crushing is done in a
continuous closed circuit mode such that the crushed material is continuously
screened with recirculation of +3mm material and removal of -3mm material.
The oversize fraction +3mm is again subjected to crushing in roll crusher,
while the undersize -3mm material (named as generated) is mixed with the
natural -3mm material. The process of crushing and screening is repeated
until the whole material is ground to -3mm size.
The composite -3mm material which is a mix of natural -3mm material and
generated -3mm material assays 43.5% CaO, 1.2% P, 2.7% Al2O3, 18.4%
Fe, 14.7% SiO2 and 4.8% MgO. This material is then subjected to reduction
roasting in a furnace using coal as a reductant cum fuel. The size of the coal
used is -0.5mm. Reduction roasting is carried out at temperature between
500-900°C with a coal to slag ratio ranging between 1:9 to 2:8. The calcium
ferrite phase present in steel slag i.e 2CaO.Fe2O3 gets reduced to CaO+ Fe
in the presence of reducing atmosphere (CO).

The separation of iron from rest of the slag is done through a wet low
intensity magnetic separator. However before separating the same the
reduction roasted material is crushed to less than 0.5mm so that most of the
iron (Fe) gets liberated at this finer size. The preferential separation of iron
bearing material from calcium bearing matrix phase is done by subjecting the
ground reduction roasted -0.5mm material to a wet low intensity magnetic
separation at magnetic field intensity between 500-1500 gauss. The magnetic
iron gets attracted to the magnetic drum and is scarped at the other end of
the magnetic separator while the non-magnetic material containing mostly
matrix phase of calcium oxide and other phases like dicalcium silicate is
collected at the tailing collection end of the magnetic separator.
Since most of the phosphorus is associated with dicalcium silicate phase, it is
seen that there is a sharp separation of phosphorus in magnetic and non-
magnetic fractions. The yield of magnetic concentrate ranges from 30% to
45%. The concentrate assay corresponding to 30% yield is 59.76 %Fe, 12.5
% CaO, 4.17% SiO2 and 0.39% P.
The final concentrate is much below the limit of 1.0% P, which is normally
acceptable for recycling in iron ore sintering.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 and 2 shows the process flow sheet to recover iron rich concentrate
from non-magnetic steel slag.
DETAILED DESCRIPTION OF THE INVENTION
The Iron and steel plant generates solid wastes like steel slag that don’t find
bulk application owing to its poor physical and chemical properties. The steel
slag which is being generated at a rate of about 100 kg/ton of steel is

discarded after recovery of metallic iron. The Steel Slag rejects after metallic
recovery comprises of 45-50% CaO, 3-5% free lime, 2-3% MgO, 12-20% Fe,
1.5-2% P, 2-3% Al2O3 and 15-18% SiO2. The presence of free lime and
metallic iron in steel slag rejects restricts usage in road making and cement
making application respectively. The presence of high quantity of iron oxide
and metallic iron could be useful for application in iron ore sintering. However
the presence of 1.2-1.8% P is a deterrent for use in iron ore sintering
process. The potentiality of steel slag for use as a sinter feed triggered the
need to develop a process to lower P from steel slag. The desired
specifications of such flux require P less than 1%.
The process of treatment of molten LD slag comprises
. Pouring the molten LD slag in a slag pit.
• Spraying of water over the top of molten LD slag.
. Allowing the slag to cool for a period of 24-30 hours.
. Transferring of solidified LD slag to open storage yards and balling of
LD slag to crush it to less than 300m sized material (Figure 1).
• Stage wise crushing to 80mm and 6mm followed by magnetic
separation of magnetic iron from -80mm and -6mm material.
• Separation of non-magnetic material and screening of non-metallic
slag rejects at 3mm to produce -6+3mm and -3mm material.
. Crushing of -6+3mm material to less than 3mm.
. Blending of natural -3mm and crushed -3mm material in the as
obtained proportion.
. The composite -3mm material is then subjected to reduction roasting
using coal as a reductant cum fuel.
• The reduction roasted -3mm material is further crushed to below
0.5mm and then subjected to wet low intensity magnetic separation to
separate magnetic concentrate from non-magnetic tailing.

The air cooled steel slag is initially crushed to less than 6mm using steel balls,
Gyratory crusher and Jaw crusher. The -6mm material is then subjected to
recovery of magnetic material using roll magnetic separator. The non-
magnetic fraction obtained from roll magnetic separation is then subjected to
dry screening at 3mm to separate the material into -6+3mm and -3mm
fractions. The weight wise distribution of -6+3mm and -3mm is 75.5% and
24.5% respectively. The -6+3mm assayed 19.2%Fe, 15.6% SiO2, 44.4%
CaO, 2.6% Al2O3, and 1.4% P while the -3mm assayed 17.3%Fe, 15.2%
SiO2, 43.6% CaO, 2.8% Al2O3, and 1.35% P. The screen size of 3mm is a
uniqueness of this process and controls the enrichment of iron.
The size fraction -6+3mm is then subjected to dry crushing in a roll crusher
in a closed circuit in order to entirely crush the material to -3mm size. The roll
crusher consists of two steel rolls moving in opposite direction through which
the feed material is passed thereby subjecting the material to the
compression between the rolls and hence resulting in crushing of the
material. The roll crushing of -6+3mm is always done in dry mode (2-4%
moisture is desirable) since the presence of excessive moisture will affect the
crushing process and result in an inferior performance as reflected from the
product size distribution. Ideally in one pass of roll crushing operation the
feed material consisting of entirely +3mm material should result in a product
with a d50 of 2mm. Therefore the entire material should be crushed down to
less than 3mm within 3 passes of roll crushing.
During the dry batch mode of crushing the material is allowed to be crushed
in the roll crusher for a single pass and then the material is screened on a
screen of 3mm to screen out the oversize and undersize fractions. The
oversize fraction +3mm is again subjected to dry roll crushing for single pass,
while the undersize -3mm material (named as generated) is mixed with the

natural -3mm material. The process of crushing and screening is repeated
until the whole material is crushed to -3mm size.
The natural -3mm and the generated -3mm material are blended in the as-
obtained proportion. The composite -3mm material assays 43.5% CaO, 1.2%
P, 2.7% Al2O3, 18.4% Fe, 14.7% SiO2 and 4.8% MgO. This material is then
subjected to reduction roasting using -0.5mm sized coal as a reductant cum
fuel.
Roasting is a step of the processing of certain ores. More specifically, roasting
is a metallurgical process involving gas–solid reactions at elevated
temperatures with the goal of purifying the metal component(s). Often before
roasting, the ore has already been partially purified, e.g. by froth floatation,
magnetic separation or gravity separation. The concentrate is mixed with
other materials to facilitate the process. The technology is useful but is also a
serious source of air pollution.
Roasting consists of thermal gas–solid reactions, which can include oxidation,
reduction, chlorination, sulfation, and pyrohydrolysis. In roasting, the ore or
ore concentrate is treated with very hot air. This process is applied to both
oxide and sulphide minerals. During roasting of steel slag the iron containing
phases i.e dicalcium ferrite and wustite gets reduced to iron. The dicalcium
ferrite (2CaO.Fe2O3) in presence of reducing gas carbon monoxide gets
decomposed to calcium oxide and iron, while the wustite (FeO) gets directly
reduced to Fe. This reduction roasting of slag can also be termed as
magnetizing roasting since the conversion of dicalcium ferrite phase and the
non-magnetic wustite phase results in the formation of iron which is ferro
magnetic. The objective is to convert into iron so that during the further
physical separation process the metallic iron can be recovered easily at low
magnetic intensities. The balanced equations for the reduction roasting of
steel slag are:

2 CaO.Fe2O3 + 3CO → 2CaO + 2Fe+3CO2 ………………………… (1)
FeO + CO → Fe + CO2 (2)
The key operating variables of a reduction roasting process include:
• Size of the steel slag
• Size of the reductant (Coal)
• Time of reduction
• Temperature of reduction
• Ratio of coal to slag
Reduction roasting tests were conducted with representative steel slag
sample crushed to -3mm. The roasting temperature range was kept between
500-900°C while the reduction time was varied between 30 minutes to 90
minutes. The coal to steel slag ratio was kept between 1:9 to 2:8. The size of
the steel slag and coal sample was kept same in all the tests.
The reduction roasting of the steel slag sample resulted in conversion of
ferrite phase and wustite phase to metallic iron, while the non-magnetic
phase of calcium oxide and dicalcium silicate remained as the matrix. The
separation of the matrix phase and the magnetic phase was carried out
through low intensity magnetic separation. The reduction roasted material
was crushed to less than 0.5mm so as to liberate the reduced iron particles.
The size of 0.5mm is very unique and controls the recovery and grade of the
magnetic concentrate. Low intensity magnetic separation was carried out in
an induced wet low intensity drum magnetic separator at intensities ranging
between 500-1500 gauss. The magnetic concentrate contained mostly
reduced iron particles and some amount of unliberated ferrite and wustite.
Also some quantity of unliberated dicalcium silicate reported in the
concentrate. The non-magnetic tailing contained calcium oxide and dicalcium
silicate minerals. Further since most of the phosphorus is associated with

dicalcium silicate phase, there is a sharp gradient in terms of phosphorus
partitioning in magnetic and non-magnetic fractions. The yield of magnetic
concentrate ranges from 30% to 45%. The concentrate assay corresponding
to 30% yield is 59.76 %Fe, 12.5 % CaO, 4.17% SiO2 and 0.39% P.
The final concentrate is much below the limit of 1.0% P, which is normally
acceptable for recycling in iron ore sintering. The following example shows
typical concentrate and tailing grade and yield values.

In one more example shown below in Table 2, it is seen that the yield of
concentrate increases but at the same time the grade of concentrate
measured in terms of Fe value decreases.

In yet another example shown in Table 3, it is seen that there is not much
change in the yield of concentrate when compared with yield value of
example 1. However the grade of the concentrate is slightly inferior compared
to that obtained in example 1.


ADVANTAGES OF THE PROPOSED PROCESS
The centrifugal jigging of LD slag will recover a concentrate (product) that
will contain relatively higher proportion of iron values compared to the feed.
Also the phosphorus content of the product will be less than 1%. This
product can be directly recycled in iron ore sintering process, where in the
product will serve the following purpose
1. It will add iron values thereby reducing the specific iron ore consumption in
iron ore sintering.
2. It will add calcium oxide values thereby helping in reducing the flux
(Limestone) consumption.
In addition to the above benefits the addition of low phosphorus high iron LD
slag concentrate helps in achieving the desired properties of sinter like
Tumbler Index and Abrasion Index.
The invented process is capable of treating the air cooled slag containing 30-
60% CaO, 1.2-1.8% P, 1.5-4% Al2O3, 12-30% Fe, 9-18% SiO2 and 2.5-6%
MgO to produce a final product containing 38-59.8% Fe, 4.2-8.9% SiO2,
12.5-31% CaO, 1.65-3.2% MgO and 0.39-0.92% P. The yield of the low
phosphorus iron rich product is 30-45% 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 slow cooling of
steel slag, crushing and magnetic separation, re-crushing of non-metallic
steel slag to less than 3mm, reduction roasting of -3mm steel slag using coal

as a reductant cum fuel followed by grinding of reduction roasted concentrate
to -0.5mm and wet low intensity magnetic separation of reduction roasted -
0.5mm material. The final concentrate containing 0.39-0.92% P is well below
the acceptable P specifications (below 1%) required for fluxing application in
iron ore sintering/pelletization. The presence of iron and calcium oxide help in
reducing the specific consumption of iron ore and limestone in iron ore
sintering.

NON-PATENT CITATIONS
References
1. Menad, N. , Kanara, N. and Save, M. 2014 “Recovery of high grade
compounds from LD slag by enhanced magnetic separation
techniques”. International Journal of Mineral Processing, 126, pp-1-9

2. Wu, X.R., Yang, G.M., Li, L.S., Lu, H.H., Wu, Z.J. and Shen, X.M. 2014.
“Wet magnetic separation of phosphorus containing phase from
modified BOF slag”. Ironmaking and Steel making.Vol 31, No 5, pp-
335-341
3. Semykina, A., Shatokha, V. and Setharaman, S. 2010. “Innovative
approach to recovery of iron from Steelmaking slags”. Ironmaking and
Steelmaking. Vol 37, No.7, pp-536-540.
4. Lin, L., Bao,Y.P., Wang,M. and Zhour, H.M. 2013. “ Influence of Al2O3
modification on phosphorus enrichment in P bearing steemaking slag”.
Ironmaking and Steelmaking. pp 1-7

WE CLAIM:
1. A process for upgrading iron content of steel slag, the process
comprising steps of:
crushing steel slag to size of a -6mm fractions;
magnetically separating the -6mm fractions into a -6 mm magnetic and
a -6 mm non-magnetic fractions;
screening the -6 mm non-magnetic fractions at 3mm to obtain a
+3mm fractions and a first -3mm fractions;
crushing the +3mm fractions to generate a second -3mm fractions;
blending the first -3mm fractions and the second -3mm fractions to
form a composite;
reduction roasting of the composite;
crushing the composite to below 0.5mm; and
magnetically separating the composite to separate magnetic and non-
magnetic fractions.
2. The process as claimed in claim 1, wherein the steel slag is crushed to
-6mm fractions by means of steel balls.
3. The process as claimed in claim 2, wherein the magnetic separation of
-6mm fractions is done in a of roll magnetic separator.
4. The process as claimed in claim 1, wherein the screening of the -6mm
non-magnetic fractions at 3mm to obtain a +3mm fractions and a first
-3mm fractions is done by means of a circular vibratory screen.
5. The process as claimed in claim 1, wherein the crushing of the +3mm
fractions to generate a second -3mm fractions is done by means of a
roll crusher.

6. The process as claimed in claim 1, wherein the reduction roasting-
magnetic separation is done by using coal acting as a reductant and
fuel and a low intensity magnetic separator respectively.
7. The process as claimed in claim 1, wherein the reduction roasted
concentrate is subjected to crushing to below 0.5mm followed by
magnetic separation of -0.5mm to separate magnetic and non-
magentic material.