TITLE:-
An optimized process to upgrade iron content and lower alumina content of steel
slag rejects.
FIELD OF THE INVENTION;-
The present invention relates generally to development of process and system
for upgrading the iron content of steel slag rejects and specifically, it relates to
treatment of steel slag rejects of size -6mm through gravity Jigging process to
produce concentrate with enriched iron content along with low alumina content.
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. The steel
slag generated from refining of hot metal contains metallic iron, wustite, and
some calcium bearings minerals. Most of the metallic iron and wustite is
recovered by processing the steel slag through magnetic separation. The
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 volume
consists of iron bearing minerals like calcium ferrite, wustite, metallic iron and
calcium alumo ferrite along with calcium bearings minerals. Alternative
techniques to recover metallic content have been developed that includes the
physical separation method of jigging. The 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 by (i)
crushing the slag and screening to recover slag having a diameter of less than
about 20 mm and (ii) further separating the recovered slag in a first fraction of
more than about 2 mm and a second fraction of less than about 2 mm. The first
fraction is separated into a metal concentrate fraction, an aggregate fraction a
coarse middlings fraction and a fine middlings fraction. The second fraction is
separated into a metal concentrate fraction, an aggregate fraction, and a
middlings fraction. The coarse middlings of the first fraction are re-crushed and
fed into step ii), whereas the middlings from the second fraction and the fine
middlings from the first fraction are ground and separated into a metal
concentrate fraction and an aggregate fraction. The Chinese patent no. CN
101596488 titled "Stainless steel slag iron separation technique" describes the
method to recover metallic values through Jigging. The Chinese patent no
CN101569875 titled "Process for jigging iron from molten iron nickel slag" also
mentions about recovery of metallic iron through jigging technique. The patents
no US 4772384, DE 3339026 and EP 1312415-A all describe about the recovery
of metallic content slag through Jigging technique. None of the patents mentions
about the lowering of AI2O3 content of Steel slag rejects obtained after metallic
recovery.
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 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. This has resulted in
accumulation of huge quantity of steel slag 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 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%
AI203 and about 15-18% Si02. The presence of more than about 20% Fe
makes it a good source of metallic iron for sinter making. However the presence
of deleterious impurities like Al2O3 which is present to the tune of 3% limits the
recycling application. The iron ore sintering process demands a AI2O3
specification of less than 2% which if achieved would result in bulk recycling of
steel slag within the steel plant itself.
The objective of this invention if to provide a method to lower alumina and
simultaneously upgrade the iron content of steel slag rejects. This enables
increased percentage recycling of steel slag as a source of iron.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a process for upgrading iron content and
lower alumina content of waste steel slag.
Another object of this invention is to propose a process for upgrading iron
content and lower alumina content of waste steel slag which is acceptable for
recycling in iron ore sintering.
Further object of this invention is to propose a process for utilizing the waste
steel slag instead of discarding the same.
BRIEF DESCRIPTION OF THE INVENTION:-
According to this invention there is provided a process for upgrading iron content
and lower alumina content of waste steel slag comprising cooling the steel slag;

crushing the steel slag to less than 6mm, subjecting the enriched steel slag to the
step of magnetic separation, screening the non-magnetic fraction to obtain -6 +
0.5 mm and -0.5 mm fractions, subjecting the screened non-magnetic fraction to
the step of jigging and spiral concentration to separate high density and low
density material.
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 being discarded
after recovery of metallic iron. The Steel Slag comprises of 43.5% CaO, 4.8%
MgO, 18.4% Fe, 14.7% SiO2, 2.7% AI2O3 and 1.2% P. The presence of free lime
and metallic iron restricts usage in road making and cement making application.
However the presence of high quantity of iron oxide and metallic iron could be
useful for application in iron ore sintering. The desired specifications of such flux
require Al2O3 less than 2%. This potentiality of steel slag triggered the need to
develop process to lower alumina (AI2O3) from Steel Slag to less than 2%.
The process involves air cooling of the steel slag, stage wise crushing to -6mm
followed by magnetic separation of metallic from steel slag, screening of non-
metallic slag rejects at 0.5mm, Jigging of -6+0.5mm fraction and spiral
concentration of -0.5mm fraction to upgrade iron content.
The magnetic separation process comprises of stage crushing of the steel slag
followed by recovery of metallic iron. The non-magnetic yield is 80% with an
assay of 43.5% CaO, 1.2% P, 2.7% Al2O3, 18.4% Fe, 14.7% SiO2 and 4.8%
MgO.
The non-magnetic fraction of magnetic separation is then screened at 0.5mm to
obtain two fractions i.e. -6+0.5mm and -0.5mm (process controlling and unique).

The weight wise distribution of -6+0.5mm and -0.5mm is 59.5% and 40.5%
respectively. The -6+0.5mm 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% AI2O3, and 1.13% P.
The non-magnetic fraction -6+0.5mm is then subjected for underbid air pulsated
jigging. Jigging is a technique of gravity separation where the material is fluidized
in water with the help of air. During fluidization the heavier and lighter particles
rise and fall with differential acceleration owing to their specific gravity and size.
This results in segregation of lighter and heavier particles as different layers. The
heavier particles consists of iron rich particles and assay 27.2% Fe, 0.9% AI2O3,
10.6% SiO2 and 35.1% CaO. The yield of Jig concentrate (heavy) and tailing
(light) is 36% and 64% respectively.
The non-magnetic fraction of -0.5mm is subjected for spiral concentration which
is a type of gravity separator. This results in separation of concentrate rich in iron
values and low in alumina content. The yield of spiral concentrate is 22% with an
assay of 36.0% Fe, 11.6% SiO2, 30.5% CaO, and 1.89% Al2O3.
The combined yield of concentrate rich in iron values from Jigging and Spiral
concentration is 30% with an average assay of 29.8% Fe, 1.19% Al2O3, 10.89%
SiO2 and 33.75% CaO.
The final combined concentrate is much below the limit of 2.0% Al2O3 acceptable
for recycling in iron ore sintering.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows that process flow sheet to recover iron rich concentrate from
steel slag rejects.

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 being discarded
after recovery of metallic iron. The Steel Slag rejects after metallic recovery
comprises of 43.5% CaO, 4.8% MgO, 18.4% Fe, 14.7% SiO2, 2.7% Al2O3 and
1.2% P. The presence of free lime and metallic iron in steel slag rejects restricts
usage in road making and cement making application respectively. However the
presence of high quantity of iron oxide and metallic iron could be useful for
application in iron ore sintering. The desired specifications of such flux require
Al2O3 less than 2%. This potentiality of steel slag triggered the need to develop a
process to lower alumina (AI2O3) from Steel Slag to less than 2%.
The process of recovering iron rich and low alumina concentrate consists of
cooling of steel slag followed by crushing to less than 6mm. The crushed -6mm
material is then subjected for magnetic separation to recover metallic iron. The
non-magnetic fraction is then screened at 0.5mm to obtain -6+0.5mm and -
0.5mm fractions. The -6+0.5mm fraction is subjected to under bed air pulsated
Jigging to separate heavy and light fractions. The heavy fraction is an iron rich
fraction with low alumina while the lighter fraction contains less iron with high
alumina. The -0.5mm fraction is subjected for spiral concentration to separate
high density and low density material.
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 0.5mm to separate the material into -6+0.5mm and -0.5mm fractions. The

weight wise distribution of -6+0.5mm and -0.5mm is 59.5% and 40.5%
respectively. The -6+0.5mm 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% AI2O3, and 1.13% P. The screen size of 0.5mm is a uniqueness of
this process and controls the enrichment of iron in both -6+0.5mm and -0.5mm
fraction.
The -6+0.5mm fraction of non-magnetic obtained after screening is then
subjected to under bed air pulsated Jigging. The Jigging machine is capable of
treating upto 50 Kg material in one batch. The fluidization water flow rate is
maintained at 25 litres per minute. The air pressure is maintained at 0.3 bar. The
material is treated for a jigging time of 15 minutes. During fluidization the heavier
and lighter particles rise and fall with differential acceleration owing to their
specific gravity and size. This results in segregation of lighter and heavier
particles as different layers. The heavier particles consists of iron rich particles
and assay 27.2% Fe, 0.9% Al2O3, 10.6% SiO2 and 35.1% CaO. The yield of Jig
concentrate (heavy) and tailing (light) is 36% and 64% respectively.
The non-magnetic fraction -0.5mm is subjected for spiral concentration which is a
type of gravity separator. The spiral concentrator used is a high gradient spiral.
The feed % solids is maintained at 20%. The spiral feed rate is maintained at 100
kg/hr of dry solids. The concentrate splitter position is set at 25% of the total
opening. This results in separation of concentrate rich in iron values and low in
alumina content. The yield of spiral concentrate is 22% with an assay of 36.0%
Fe, 11.6% SiO2, 30.5% CaO, and 1.89% Al2O3.
The concentrate obtained from Jigging and Spiral concentration is blended to get
a composite concentrate which assays 29.8% Fe, 1.19% Al2O3, 10.89% SiO2 and
33.75% CaO. The yield of the composite concentrate is 30%.

WE CLAIM:
1. A process for upgrading iron content and lower alumina content of waste steel
slag comprising cooling the steel slag;
crushing the steel slag to less than 6mm,
subjecting the enriched steel slag to the step of magnetic separation,
screening the non-magnetic fraction to obtain -6 + 0.5 mm and -0.5 mm fractions,
subjecting the screened non-magnetic fraction to the step of jigging and spiral
concentration to separate high density and low density material.
2. The process as claimed in claim 1 wherein the said slag is crushed to less than
6mm using steel balls.
3. The process as claimed in claim 1 wherein the said non-magnetic fraction is
obtained from roll magnetic separator.
4. The process as claimed in claim 1, wherein the -6mm non-magnetic fraction
obtained after magnetic recovery is further screened at 0.5mm using circular
vibratory screen to generate the -6+0.5mm and -0.5mm fractions.
5. The process as claimed in claim 1 and 2, wherein the -6+0.5mm non-magnetic
fraction is subjected for jigging operation and the said operation is carried out
with a water flow rate of minimum 25 Ipm, jigging time of minimum 15 minutes
and with an air pressure of 0.3 bar.

6. The process as claimed in claim 1 and 3, wherein the concentrate (heavier) and
tailings (lighter) fractions of jigging are separately collected such that the yield of
concentrate and tailings is 36% and 64% respectively.
7. The process as claimed in claim 1 and 2, wherein the -0.5mm non-magnetic
fraction is subjected for spiral concentration using high gradient spirals.
8. The process as claimed in claim 1 and 5, wherein the spiral feed percent solids is
maintained at 20%, splitter position is set at 25% of the total opening and spiral
feed rate is kept at 100 kg/hr. The spiral concentrate and tailings are separately
collected to get a yield of 22% and 78% respectively.
9. The process as claimed in claim 1, 4 and 6, wherein the concentrate of jigging
and the concentrate of spiral concentration are blended together in as obtained
proportion to get a composite concentrate assaying 29.8% Fe, 1.19% Al2O3,
10.89% SiO2 and 33.75% CaO. The yield of the composite concentrate is 30%.

ABSTRACT

A process for upgrading iron content and lower alumina content of waste steel
slag comprising cooling the steel slag; crushing the steel slag to less than 6mm,
subjecting the enriched steel slag to the step of magnetic separation, screening
the non-magnetic fraction to obtain -6 + 0.5 mm and -0.5 mm fractions,
subjecting the screened non-magnetic fraction to the step of jigging and spiral
concentration to separate high density and low density material.