Field OF THE INVENTION:
The present invention relates generally to development of process and system for enriching the iron content of steel slag rejects and specifically, it relates to treatment of steel slag rejects of size -6mm through Enhanced gravity separator (Centrifugal jig) to produce a concentrate with enriched iron content.
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 dicalcium silicate, wustite, metallic iron and calcium alumino ferrite along with some amount of magnesio ferrite. Alternative techniques to recover metallic content have been developed that includes the physical separation method like 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. The Chinese patent CN 101596488 titled "Stainless steel slag iron separation technique" describes the method to recover metallic values through Jigging. The Chinese patent CN101569875 titled

"Process for jigging iron from molten iron nickel slag" also mentions about recovery of metallic iron through jigging technique. The patents US 4772384, DE 3339026 Al and EP 1312415 Al all 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. 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.
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. 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 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. Recent research works carried out by Menad et.al, 2014 have targeted 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. In one of the research works by Semikina et.al, 2010, it is 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 have attempted modification of AI2O3 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 literature work and patents cited 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 literature work reports use of enhanced gravity separation technique to physically separate iron rich compounds from calcium and phosphorus rich compounds.
The objective of this invention is to provide an alternative and better method to enrich the iron content and simultaneously lower the P content adopting enhanced 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 physical separation route for processing steel
slag.
Another object of this invention is to propose a process for upgrading iron content and lower phosphorus content of steel slag.
Further object of this invention is to propose a techno-economic feasible process for utilizing the waste steel slag in iron ore sintering or peptization.
brief descripTlON OF THF 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 followed by; crushing the steel slag to less than 6mm, subjecting the crushed 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 grinding followed by enhanced gravity separation using centrifugal jig.

The Steel Slag comprises of 43.5% CaO, 4.8% MgO, 18.4% Fe, 14.7% Si02, 2.7% AI2O3 and 1.2% P. The presence of metallic iron restricts usage 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 steel slag triggered the need to develop 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 magnetic separation of metallic from steel slag, screening of non-metallic slag rejects at 0.5mm, ball mill grinding of -6+0.5mm material to less than 0.5mm. The below 0.5mm size material is then subjected to enhanced gravity separation adopting centrifugal Jig to upgrade iron content and lower the phosphorus 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, 4.8% MgO, 18.4% Fe, 14.7% Si02, 2.7% AI2O3 and 1.2% P
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 -0.5mm material screened out from the as-received material is named as natural -0.5mm
material. :
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% Si02, 43.9% CaO, 2.8% AI203, and 1.25% P while
the -0.5mm assayed 15.7%Fe,14.8% Si02,42.8% CaO, 2.6% AI203, and 1.13% P.
The size fraction -6+0.5mm is then subjected to ball mill grinding in a dry grinding mill to
entirely grind the material to -0.5mm size. The ball mill grinding media consists of steel
balls of different diameters. The grinding is done in a batch mode for 30 minutes of
grinding time. After 30 minutes the material is taken out and again screened on a screen

of 0.5mm size to screen out the oversize and undersize fractions. The oversize fraction +0.5mm is again subjected to ball mill grinding for 30 minutes, while the undersize -0.5mm material (named as generated) is mixed with the natural -0.5mm material. The process of grinding and screening is repeated until the whole material is ground to
-0.5mm size.
The composite -0.5mm material which is a mix of natural -0.5mm material and generated -0.5mm material assays 43.5% CaO, 1.2% P, 2.7% Al203,18.4% Fe, 14.7% Si02 and 4.8% MgO. This material is then subjected to enhanced gravity separation in a centrifugal jig. The centrifugal jig is a gravity separation unit that works on the principle of jigging as well as imparts centrifugal force to the particles, thereby separating denser particles from the less dense particles.
The separation of denser and lighter fractions results in a preferential separation of iron bearing minerals in the denser fraction, while the less dense material contains more of calcium bearing minerals (dicalcium silicate). Since most of the phosphorus is associated with dicalcium silicate phase, it is seen that there is a sharp separation of phosphorus in denser and lighter fractions. The yield of denser material ranges from 10% to 30%. The concentrate assay corresponding to 30% yield is 32.87%Fe, 27.9% CaO, 9.22% Si02 and
0.75% 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 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 Centrifilgal jig 4.8% MgO, 18.4% Fe, 14.7% Si02 and 1.2% P. The presence of free lime and metallic iron in steel slag rejects restricts usage in road making and cement making applic; Tailing respectively. However the presence of high quantity of iron oxide and metallic iron could be useful for application in iron ore sintering. This potentiality of steel slag triggered the need to develop a process to lower P from steel lag. 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 0.5mm to produce -6+0.5mm and -0.5mm material.
• Ball mill grinding of -6+0.5mm material to less than 0.5mm.
• The -0.5mm material is then subjected to enhanced gravity separation adopting centrifugal Jig to upgrade iron content and lower the phosphorus content.

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% Si02, 43,9% CaO, 2.8% AI2O3, and 1.25% P while the -0.5mm assayed 15.7%Fe,14.8% Si02, 42.8% CaO, 2.6% A!203, and 1.13% P. The screen size of 0.5mm is a uniqueness of this process and controls the enrichment of iron.
The size fraction -6+0.5mm is then subjected to wet or dry ball mill grinding in a grinding mill to entirely grind the material to -0.5mm size . The ball mill grinding media consists of steel balls of different diameters. The grinding is either done in a dry batch mode or wet continuous mode. The ball mill (unit operation 2 in Figure 2) output is pumped to hydrocyclone (unit operation 3 in Figure 2) via sump and pump arrangement (unit operation 5&4 in Figure 2).
During the dry batch mode of grinding the material is ground for 30 minutes of grinding time. After 30 minutes the material is taken out and again screened on a screen of 0.5mm size to screen out the oversize and undersize fractions. The oversize fraction +0.5mm is again subjected to dry ball mill grinding for 30 minutes, while the undersize -0.5mm material (named as generated) is mixed with the natural -0.5mm material. The process of grinding and screening is repeated until the whole material is ground to -
0.5mm size.
In wet continuous grinding, the -6+0.5mm material to be ground is fed to the ball mill in a slurry form such that the percent solids in the slurry ranges anywhere between 40% to 70%. The wet ball mill grinding is combined with hydrocyclone classification so as to continuously classify the ball mill output and produce +0.5mm and -0.5mm fractions.

The wet ball mill grinding feed rate is controlled such that the recirculating load of the grinding mill ranges from 150% to 300%. This implies that the oversize +0.5mm material produced is 150-300% of the weight of fresh feed. Further the wet ball mill grinding is done in presence of a suitable grinding aid (dispersant Sodium hexa meta phosphate) that helps to achieve a consistent grinding mill output measured in terms of evenness in size. The dosage of the grinding aid was kept between lOOg/ton to 500 g/ton of dry feed.
The natural -0.5mm and the generated -0.5mm material are blended in the as-obtained proportion. The composite -0.5mm material which is a mix of natural -0.5mm material and generated -0.5mm material assays 43.5% CaO, 1.2% P, 2.7% AI2O3,18.4% Fe, 14.7% S1O2 and 4.8% MgO. This material is then subjected to enhanced gravity separation in a centrifugal jig (unit operation 1 in Figure 2).
The centrifugal Jig is a device which, with the ability to vary the apparent gravitational field by up to 100 times the normal gravitational force, can facilitate major improvements to separation efficiencies, particularly for very fine mineral feeds and / or for the separation of mineral particulates with small density differences. The centrifugal Jig utilises the same parameters as a conventional jig but superimposes the additional feature of being able to vary the apparent gravitational field acting on fine particles across a ragging bed by spinning the jigging mechanism.
The centrifugal Jig is fed down a fixed central pipe and distributed over a ragging bed supported by a cylindrical shaped screen, which is spun coaxially with the rotor. The bed is pulsed and pressurised hutch water is introduced to fluidise the ragging bed and facilitate stratification / sorting. This results in particles of density greater or equal to that of the, ragging passing through the ragging bed via the mechanisms of hindered settling and interstitial trickling, which are enhanced by the apparent higher gravitational forces. The denser particles pass through the internal screen to

concentrate hutches and then through spigots to a concentrate launder, while the lighter mineral particles are discharged over a ragging retention ring into tailings
launder.
The key operating variables of a centrifugal Jig include:
• Spin speed
• Pulse rate
• Stroke length
• Ragging s.g.
• Ragging size
• Hutch water addition
Centrifugal Jig tests were conducted with representative steel slag sample ground to -0.5mm. The ragging size was kept at 0.6-0.8mm for -0,5mm sample. The tests were conducted by varying the parameters of pulse rate and spin speed. The stroke length and hutch water addition was kept uniform in all tests.
The separation of denser and lighter fractions results in a preferential separation of iron bearing minerals in the denser fraction, while the less dense material contains more of calcium bearing minerals (dicalcium silicate). Further since most of the phosphorus is associated with dicalcium silicate phase, there is a sharp gradient in terms of phosphorus partitioning in denser and less dense particles. The yield of denser material ranges from 10% to 30%. The concentrate assay corresponding to 30% yield is 32.87%Fe, 27.9% CaO, 9.22% Si02 and 0.75% 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.

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.


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-S40.
4 Lin L BaoYP, Wang,M. and Zhour, H.M. 2013. • Influence of AI203 modification on phosphorus enrichment in P bearing steemaking slag". Ironmaking and Steelmaking. pp 1-7

WE CLAIM: 1. A process for upgrading iron content and lowering phosphorus content of steel slag, the process comprising;
crushing the cooled steel slag to a size less than 6mm,
subjecting the crushed 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 -6+0.5mm fraction to the step of grinding to produce -0.5mm product,
sending the generated -0.5mm product and natural -0.5mm materia, and centrifugal jigging of composite -0.5mm to separate high density and low density matenal.
2. The process as claimed in Claim 1 wherein the steel slag is crushed to less than 6mm using steel balls.
3 The process as claimed in Claim 1 wherein the -6mm matenal is subjected to roll magnetic separation from which the non-magnetic fraction is obtained for further processing.

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 natural -6+0.5mm fraction and natural -0.5mm fraction.
5. The process as claimed in claim 1 and 2, wherein the natural -6+0.5mm non-magnetic fraction is subjected to ball mill grinding in wet or dry mode such that the final output from the ball mill is a material of size -0.5mm.

6. The process as claimed in claim 1 and 6, wherein the -0.5mm output of ball mill is blended with natural -0.5mm material to get a composite -0.5mm material.
7. The process as claimed in claim 1 and 7, wherein the -0.5mm material is subjected to centrifugal jigging, wherein the feed percent solids is maintained at 20% and feed rate is kept at 50 kg/hr.
8. The process as claimed in claim 1 and 8, wherein the concentrate of centrifugal jigging assaying 32.87% Fe, 9.22% SiO2, 27.9% CaO, 3.17% MgO and 0.75% P.

9. The process as claimed in claim 1, wherein the centrifugal jigging concentrate (denser material) and tailing (less dense) yield is 30% and 70% respectively