Claims:We Claim:
1. A method of dephosphorization of Basic Oxygen Furnace (BOF) Slag for producing recyclable low phosphorous content slag comprising

subjecting said BOF slag to re-melting through selective thermal treatment cycle including heating, melting and cooling for separating slag into phases comprising of a phosphorous enriched non-magnetic phase and a substantially phosphorous free magnetic phase and selective dissolution of phosphorous in said non-Magnetic phase.

2. A method as claimed in claim 1 wherein said non-magnetic phase is obtained as a major crystalline phase of C2S phase with majority of the phosperous content of said slag.
wherein phosphorous gets dissolved into Non-Magnetic Di-calcium silicate Phase( C2S);

subjecting the thus obtained melted slag having said phosphorous enriched non-magnetic phase and substantially phosphorous free magnetic phase from furnace to water quenching; and

carrying out magnetic separation to thereby obtain the substantially phosphorous free magnetic phase as the recyclable low phosphorous content slag.

3. A method as claimed in anyone of claims 1 or 2 wherein said magnetic separation is carried out in gravity magnetic separator using a carrier media to form slurry; and

applying selective magnetic field to said slurry prepared with crushed slag in gravity magnetic separator, to obtain said phosphorus rich non magnetic part which is discarded and another phosphorus lean magnetic part/product suitable for recycling in iron and steel industry.

4. A method as claimed in anyone of claims 1 to 3 comprising

providing BOF slag with high phosphorous content duly characterized for composition and phase distribution;

subjecting said BOF slag to re-melting through selective thermal treatment cycle comprising heating, melting, cooling with respective dwelling time in steps in a furnace sequentially maintained for selective dissolution of phosphorous in Non-Magnetic phases whereby phosphorous gets dissolved into Non-Magnetic Di-calcium silicate Phase( C2S);

subjecting the thus obtained melted slag from furnace to water quenching;

preparing said treated slag for magnetic separation in gravity magnetic separator using a carrier media to form slurry; and
applying selective magnetic field to said slurry prepared with crushed slag in gravity magnetic separator, to obtain two products one with phosphorus rich non magnetic part which is discarded and another phosphorus lean magnetic part/product which becomes suitable for recycling in iron and steel industry.

5. A method as claimed in anyone of claims 1 to 4 comprising

(i) Providing BOF slag having composition comprising
Components Fe(T) SiO2 Al2O3 CaO MgO P2O5
Amount (wt%) 15.23 13.20 0.71 47.62 6.89 2.47

and phase distribution comprising
Phase Name Crystal Structure Orientation Wt., %
Calcium Silicate, Ca2SiO4 Monoclinic a 90 – ?9 32.9
Calcium Silicate, Ca2SiO4 Orthorhombic a 90-? 6.5
Fayalite, Fe2SiO4 Orthorhombic a 90-?90 18.30
Akermanite, Ca2MgSi2O7 Tetragonal a 90-? 11.5
Wollastonite,CaSiO3 Monoclinic a 90-?90 9.5
Calcium Iron Phosphate, Ca19Fe2(PO4)14 Rhombohedral a 21.3

(ii) Subjecting said slag to selective thermal treatment cycle involving heating, melting, cooling with selective dwelling time comprising
(a)first increasing the temperature from room temperature to 10000C at the rate of 60C/min and then keeping at this temperature for 15 min;
(b)further increasing the temperature to 12000C at the rate of 70C/min with dwell time 15 min,
(c) further increasing the temperature to 13000C at the rate of 50C/min with dwell time 15 min,
(d) next increasing the temperature to 14000C at the rate of 40C/min with dwell time 15 min,
(e) finally increasing the temperature to 15000C at the rate of 30C/min with dwell time 10 min, and
(iii) after holding for about 10min at the melting temperature of 15000C, cooling the slag to 14000C at a cooling rate of 10C/min, and holding for 240 min at this temperature before quenching in water;
(iv) crushing remelted slag to very fine size and mixing with water at desired ratio which is stirred continuously to make a homogenous slurry.
(v) feeding the homogenous slurry into the wet magnetic separator operated under selected magnetic field for separation of magnetic particles/product with lower phosphorous content from phosphorous rich phase collected in non-magnetic product.
(vi) collecting and drying the phosphorous lean magnetic product for recycling.

6. A method as claimed in anyone of claims 1 to 5 wherein the magnetic separator is operated at 14A current, 60V DC voltage and magnetic field strength of 10,500 Gauss (1.05T).

7. A method as claimed in anyone of claims 1 to 6 wherein the magnetic and non magnetic product slurries are allowed to settle down for 24 hours and then dried in an oven for around one and half hours for complete removal of moisture.

8. A method as claimed in anyone of claims 1 to 7 wherein the remelted slag is crushed to fine size in the range of 80 to 100 microns and is mixed with water to form slurry in the concentration rage of 20 to 25 wt% before magnetic separation.

9. A method as claimed in anyone of claims 1 to 8 wherein the temperature and time of thermal treatment cycle is selected based on composition of BOF slag.

10. A method as claimed in anyone of claims 1 to 9 wherein said thermal treatment cycle applied to BOF slag enable rearranging distribution of its constituents which is utilized for selective dissolution of P2O5 in calcium di silicate (C2S) phase.

11. A method as claimed in anyone of claims 1 to 10 wherein amount of phosphorous present in said magnetic particles product obtained from the process after magnetic separation of treated slag is in the range of 1.70 to 1.90 wt%, preferably <1.70 wt% making it suitable for recycling in iron and steel industry.

Dated this the 20th day of February, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION

The present invention relates to a method of dephosphorization of BOF slag. More particularly, the present invention is directed to a method of treating slag obtained from any steel plant containing high phosphorous for lowering the phosphorous content in slag involving a thermal treatment cycle for BOF slag based on mineralogy of the slag which can rearrange distribution of its constituents. This thermal cycle includes heating of slag from room temperature to above its melting point with several dwelling period at different stage. This redistribution can be utilize for selective dissolution of P2O5 in calcium di silicate (C2S) phase. This slag is melted, Heat treated, cooled and crushed. The phosphorous content of the C2S is 6.39wt-% P2O5, when remelted, which is nearly three times more than that of the original slag. A special heating, cooling and dwell time cycle in tubular furnace is maintained for selective dissolution of phosphorous in Non-Magnetic phases. Phosphorous gets dissolved itself into Non-Magnetic phases (Di-calcium silicate Phase, C2S) which were also validated after SEM analysis. And thereafter, Melted Slag from tubular furnace was water quenched and prepared for magnetic separation in gravity magnetic separator using a carrier media. Applying selective magnetic field to the slurry prepared with crushed slag in gravity magnetic separator, two products one with phosphorus rich and another phosphorus lean product are obtained whereby phosphorous rich phase was collected in non-magnetic sample. Adoption of this method can result in maximum recycling of slag in an iron and steel industry such as in sinter making and iron making to replace lime and recover iron and CaO.

BACKGROUND OF THE INVENTION

Large quantities of steel slags are generated by the various steelmaking processes; of which basic oxygen furnace slag accounts larger fraction. Steel and steel slag annual output of 2015 in India reached around 89.58 million tons and 11 million tons respectively. However, the current utilization rate of steel slag in India is only 20 % i.e. 2.20 million tons, far behind the developed countries like USA, Australia and European Countries etc. of which the rates have been 70, 70 and 80% respectively. However, with steel production on the rise, slag production is also expected to increase manifold. In contrast with other nations, most slag produced in India, especially steel slag, is mostly discarded; however, this is increasingly becoming a problem due to paucity of land. These BOF slags are for recycled into two main ways; external reuse for road construction and hydraulic engineering, fertiliser etc and internal recycling within integrated steelworks as feed to the sinter plant or to BOF to utilise useful elements such as Fe, Mn, CaO, MgO. Due to presence of high amount of Ca, it can be used as flux in blast furnace, but presence of high amount of phosphorus in the LD slag makes them unsuitable for industrial application as it is a harmful element for high quality steel products which limits the use of BOF slag in sinter making and BF to less than 50kgs/thm.

Basic oxygen furnace slag (BOF slag) is an unavoidable by-product of steelmaking process of basic oxygen furnace. It has very complex mineral phases as well mineral composition. The main mineral constituents of BOF slag are CaO, FeO and SiO2 and MgO. Due to its high metallic value (FeO: 16-20%) and lime content (CaO: 45-50%), it is possible to use in sinter making and iron making to replace lime and recover iron and CaO. But it also contains P2O5 around 2 – 3% which is too high for recycling as it increases the recycling load i.e Phosphorous content in product.

Because steel quality requirements have increased, the need for lower phosphorous levels has led to a reduction in the quantity of steel slag that is recycled as BOF slag contains about 1-3% P2O5 which is too high for recycling in steel making processes. Hot metal is the main source of phosphorous in steel making as input material. After treatment of these input materials almost all Phosphorous gets oxidized and goes to slag as P2O5.

A typical chemical analysis of slag is as follows:
Table 1:
FeO SiO2 CaO MgO MnO Al2O3 TiO2 P2O5 S Basicity
16.16 16.6 50.05 7.66 1.09 2.44 1.03 3.18 0.14 3.02

When this chemistry of slag will be recycled the phosphorous content will build up as the process progress. Detrimental effects of phosphorus in steel include various forms of embrittlement which reduce the toughness and ductility. The most familiar example in this category is the classic phenomenon of temper embrittlement in heat-treated low-alloy steels resulting from segregation of phosphorus and other impurities at prior austenite grain boundaries. Thus there has been a persistent need for effective de-phosphorization of BOF slag to desired level to enable its recycling in steel making process to recover iron and lime.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to a method of dephosphorization of BOF Slag using mineral processing technique involving magnetic separation to enable its recycling in steel making process.

A further object of the present invention is directed to a method of dephosphorization of BOF Slag favouring internal recycling of the treated slag within integrated steelworks as feed to the sinter plant or to BOF to utilize/recover useful elements such as Fe, Mn, CaO, MgO.

A still further object of the present invention is directed to a method of dephosphorization of BOF slag involving a thermal treatment cycle for BOF slag based on mineralogy of the slag which can rearrange distribution of its constituents.

A still further object of the present invention is directed to a method of dephosphorization of BOF slag wherein a special heating, cooling and dwell time cycle in tubular furnace is maintained for selective dissolution of phosphorous in Non-Magnetic phases.

A still further object of the present invention is directed to a method of dephosphorization of BOF slag wherein slag sample is melted, heat treated, cooled and crushed to form slurry in a carrier medium for subjecting to magnetic separation under selective magnetic field for separation of high phosphorous bearing Non-Magnetic phases from recyclable magnetic phase.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a method of dephosphorization of Basic Oxygen Furnace (BOF) Slag for producing recyclable low phosphorous content slag comprising

subjecting said BOF slag to re-melting through selective thermal treatment cycle including heating, melting and cooling for separating slag into phases comprising of a phosphorous enriched non-magnetic phase and a substantially phosphorous free magnetic phase and selective dissolution of phosphorous in said non-Magnetic phase.

A further aspect of the present invention is directed to said method wherein said non-magnetic phase is obtained as a major crystalline phase of C2S phase with majority of the phosphorous content of said slag;
wherein phosphorous gets dissolved into Non-Magnetic Di-calcium silicate Phase( C2S);

subjecting the thus obtained melted slag having said phosphorous enriched non-magnetic phase and substantially phosphorous free magnetic phase from furnace to water quenching; and

carrying out magnetic separation to thereby obtain the substantially phosphorous free magnetic phase as the recyclable low phosphorous content slag.

A still further aspect of the present invention is directed to said method wherein said magnetic separation is carried out in gravity magnetic separator using a carrier media to form slurry; and

applying selective magnetic field to said slurry prepared with crushed slag in gravity magnetic separator, to obtain said phosphorus rich non magnetic part which is discarded and another phosphorus lean magnetic part/product suitable for recycling in iron and steel industry.

Another aspect of the present invention is directed to said method comprising

providing BOF slag with high phosphorous content duly characterized for composition and phase distribution;

subjecting said BOF slag to re-melting through selective thermal treatment cycle comprising heating, melting, cooling with respective dwelling time in steps in a furnace sequentially maintained for selective dissolution of phosphorous in Non-Magnetic phases whereby phosphorous gets dissolved into Non-Magnetic Di-calcium silicate Phase( C2S);

subjecting the thus obtained melted slag from furnace to water quenching;

preparing said treated slag for magnetic separation in gravity magnetic separator using a carrier media to form slurry; and

applying selective magnetic field to said slurry prepared with crushed slag in gravity magnetic separator, to obtain two products one with phosphorus rich non magnetic part which is discarded and another phosphorus lean magnetic part/product which becomes suitable for recycling in iron and steel industry.

Yet another aspect of the present invention is directed to said method comprising

(i) Providing BOF slag having composition comprising
Components Fe(T) SiO2 Al2O3 CaO MgO P2O5
Amount (wt%) 15.23 13.20 0.71 47.62 6.89 2.47

and phase distribution comprising
Phase Name Crystal Structure Orientation Wt., %
Calcium Silicate, Ca2SiO4 Monoclinic a 90 – ?9 32.9
Calcium Silicate, Ca2SiO4 Orthorhombic a 90-? 6.5
Fayalite, Fe2SiO4 Orthorhombic a 90-?90 18.30
Akermanite, Ca2MgSi2O7 Tetragonal a 90-? 11.5
Wollastonite,CaSiO3 Monoclinic a 90-?90 9.5
Calcium Iron Phosphate, Ca19Fe2(PO4)14 Rhombohedral a 21.3

(ii) Subjecting said slag to selective thermal treatment cycle involving heating, melting, cooling with selective dwelling time comprising
(a)first increasing the temperature from room temperature to 10000C at the rate of 60C/min and then keeping at this temperature for 15 min;
(b)further increasing the temperature to 12000C at the rate of 70C/min with dwell time 15 min,
(c) further increasing the temperature to 13000C at the rate of 50C/min with dwell time 15 min,
(d) next increasing the temperature to 14000C at the rate of 40C/min with dwell time 15 min,
(e) finally increasing the temperature to 15000C at the rate of 30C/min with dwell time 10 min, and
(iii) after holding for about 10min at the melting temperature of 15000C, cooling the slag to 14000C at a cooling rate of 10C/min, and holding for 240 min at this temperature before quenching in water;
(iv) crushing remelted slag to very fine size and mixing with water at desired ratio which is stirred continuously to make a homogenous slurry.
(v) feeding the homogenous slurry into the wet magnetic separator operated under selected magnetic field for separation of magnetic particles/product with lower phosphorous content from phosphorous rich phase collected in non-magnetic product.
(vi) collecting and drying the phosphorous lean magnetic product for recycling.

A further aspect of the present invention is directed to said method wherein the magnetic separator is operated at 14A current, 60V DC voltage and magnetic field strength of 10,500 Gauss (1.05T).

A still further aspect of the present invention is directed to said method wherein the magnetic and non magnetic product slurries are allowed to settle down for 24 hours and then dried in an oven for around one and half hours for complete removal of moisture.

A still further aspect of the present invention is directed to said method wherein the remelted slag is crushed to fine size in the range of 80 to 100 microns and is mixed with water to form slurry in the concentration rage of 20 to 25wt% before magnetic separation.

A still further aspect of the present invention is directed to said method wherein the temperature and time of thermal treatment cycle is selected based on composition of BOF slag.

Another aspect of the present invention is directed to said method as claimed in anyone of claims 1 to 9 wherein said thermal treatment cycle applied to BOF slag enable rearranging distribution of its constituents which is utilized for selective dissolution of P2O5 in calcium di silicate (C2S) phase.
Yet another aspect of the present invention is directed to said method wherein amount of phosphorous present in said magnetic particles product obtained from the process after magnetic separation of treated slag is in the range of 1.70 to 1.90 preferably < 1.70 wt% making it suitable for recycling in iron and steel industry.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative Figures and Examples.

BRIEF DESCRIPTION OF THE ACCOMPNAYING FIGURES
Figure 1: EPMA mapping image of a typical dephosphorization slag (FetO = 19%, CaO/SiO2 = 4.4%, P2O5 = 2.8%) of a steel making company.
Figure 2: illustrates the SEM images showing distribution of various elements in slag matrix.
Figure 3: illustrates the melting behavior of slag sample at different temperatures.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPNAYING FIGURES

The present invention is directed to a method of dephosphorization of BOF Slag using mineral processing technique involving magnetic separation wherein a special heating, cooling and dwell time cycle in tubular furnace is maintained for selective dissolution of phosphorous in Non-Magnetic phases enabling its recycling in steel making process.

In general, slag can be divided into three major phases in its solidified micro-structure: FeO-free phosphorus-rich phase (Ca3P2O5-Ca2SiO4), a phosphorus-free manganese-enriched phase (mangano-wustite phase), and a phosphorus-free matrix phase (FeO-CaO-SiO2) containing minor part of manganese.
The main components of slag are CaO, FeO, SiO2, and P2O5. Accompanying Figure 1 shows EPMA mapping image of a typical Dephosphorization slag (FetO = 19%, CaO/SiO2 = 4.4%, P2O5 = 2.8%) of a steel making company. Figure 1 shows the structure and phosphorus distribution among the phases precipitated in a slag, as observed by energy-dispersion type electron-probe microanalysis (EPMA).The white parts in upper right-hand picture have high concentrations of phosphorus but no FeO. It is known that phosphorus in slag generally shows remarkable segregation as solid solutions of calcium phosphate and di-calcium silicate, which depends on the total slag composition. Studies showed that C2S phase is the major crystalline phase, which concentrates almost all phosphorous and was termed as the phosphorous concentrating phase. The phosphorous content of the C2S is 6.39wt-% P2O5, when remelted, which is nearly three times more than that of the original slag. This phosphorus enriched crystal phase generally contains approximately 3 to 6.5 mass % depending on the operating conditions. The black parts in this picture consist mainly of FeO–CaO–SiO2, an almost phosphorus-free phase.

It has been revealed on experimentation that the efficiency of separation of separation of P2O5 from slag in C2S phase depends on following factors:
1. Difference in density between dicalcium silicate and remaining liquid, the higher the difference easier the separation.
2. Viscosity of liquid slag which becomes high with low FeO content blowing of oxygen helps in improving the separation of P2O5 from slag.
3. Size of crucible, the more is the depth better is the separation.
4. Slag composition which determines the viscosity, the lower is the basicity clearer the separation. Fluorspar helps in improving the fluidity.
5. Starting temperature of cooling, the higher it is better is the separation ratio of P205. The difference between liquidus and starting temperature of crystallization should be at least 10000C for best results.
6. Cooling rate, the separation is better if the cooling rate is below 2°C. Floating speed of di-calcium silicate particles is affected by particle size. The growth of particle during its ascent in liquid bath increases as the cooling rate decreases.
7. Effect of blowing oxygen, suspended Fe and FeO get oxidized to higher oxides like FeO and Fe203. As the oxidation reaction is exothermic, the temperature of slag increases which helps separation of P-rich phase.

Based on above observations, a method of dephosphorization of BOF slag has been developed under the present invention wherein Basic Oxygen Furnace slag was collected from applicant’s Rourkela Steel Plant. X-ray fluorescence (XRF), X-ray powder diffraction (XRD) and Scanning electron microscope (SEM) analysis was carried out for characterizing different property of Basic Oxygen Furnace (BOF) slag which are presented below.
XRF analysis was done in order to investigate the amount of several components present in the slag sample. For this, sample is first crushed with the help of ball mill hammer (ball made up of tungsten carbide), then the powder obtained is compacted in an aluminium cup with boric acid at the bottom as a binder. Compaction is done with the help of hydraulic press operated at 20 ton press for 1 min. This compacted powder is called as pellet having diameter of 40mm commonly known as count. The sample is now put in XRF machine (manufactured by Rigaku Japan) and the composition of slag obtained are as follows:
Table 2: Chemical composition of BOF slag (RSP)
Components Fe(T) SiO2 Al2O3 CaO MgO P2O5
Amount (wt%) 15.23 13.20 0.71 47.62 6.89 2.47

X-ray Powder Diffraction Analysis
The above components are present in various phases in the slag. Therefore XRD analysis is carried out for phase quantification and the results obtained are as follows:.
Table 3: XRD analysis of BOF slag (RSP)
Phase Name Crystal Structure Orientation Wt., %
Calcium Silicate, Ca2SiO4 Monoclinic a 90 – ?9 32.9
Calcium Silicate, Ca2SiO4 Orthorhombic a 90-? 6.5
Fayalite, Fe2SiO4 Orthorhombic a 90-?90 18.30
Akermanite, Ca2MgSi2O7 Tetragonal a 90-? 11.5
Wollastonite,CaSiO3 Monoclinic a 90-?90 9.5
Calcium Iron Phosphate, Ca19Fe2(PO4)14 Rhombohedral a 21.3

Scanning Electron Microscope Analysis for Elemental Distribution Of Slag Sample:

Scanning Electron Microscope was used for investigating elemental distribution of BOF slag sample. For this, the sample is powdered and it is mounted with the help of copper powder in a pellet size container. Since the slag is non-conducting therefore it is coated with the help of a conducting material. Generally thin plating of gold is done for this purpose. Then SEM is carried out to see the distribution of elements over an area and at different location of sample. Accompanying Figure 2 illustrates the SEM images showing distribution of various elements in slag matrix.

Melting Characteristics Investigation:
Melting characteristics has been investigated by Hot stage microscope. Hot-stage microscopy (HSM) is the combination of microscopy and thermal analysis to enable the study of materials as a function of temperature and time. It revealed valuable information about the compound with regard to melting point or range and other transformations during heating. Accompanying Figure 3 illustrates the melting behavior of slag sample at different temperatures.

Hot stage microscope shows that as the temperature is increasing the specimen changes its shape. The change in shape is due to getting plastic or fluid property at higher temperature. This change in shape with respect to temperature and time varies for different composition of slag. The slag used in this experiment has got flowing property at 14080C.
After conducting the different characterization studies as given above to investigate the properties of slag and its possible utilization, a special heating, cooling and dwell time cycle in tubular furnace was maintained for selective dissolution of phosphorous in Non-Magnetic phases. Phosphorous gets dissolved itself into Non-Magnetic phases (Di-calcium silicate Phase, C2S) which were also validated after SEM analysis. And thereafter, Melted Slag from tubular furnace was water quenched and prepared for magnetic separation in gravity magnetic separator using a carrier media. Applying magnetic field to the crushed slag in gravity magnetic separator, two products one with phosphorus rich and another phosphorus lean product was obtained and analyzed.
The method of slag treatment for dissolution of phosphorous in C2S phase and its magnetic separation according to the present invention is illustrated hereunder with reference to following example 1:
Example 1:
Under this example, the slag sample is subjected to selective melting, heating, cooling and quenching steps comprising:
(vii) 250gm of the slag sample is taken. It is then placed into alumina crucible and melted at a high temperature in a tubular furnace. The temperature is first increased from room temperature to 10000C at the rate of 60C/min and then it is kept at this temperature for 15 min. Further temperature is increased to 12000C, 13000C, 14000C, 15000C at the rate of 70C/min, 50C/min, 40C/min, and 30C/min with dwell time 15 min, 15 min, 15 min, 10min respectively. After holding for about 10min at the melting temperature of 15000C, the slag is cooled to 14000C at a cooling rate of 10C/min, and held for 240 min at this temperature before quenching in water. The heating cooling cycle followed is presented in the following Table 4:

Table 4: Heating and Cooling Cycle applied on slag sample
Initial Temp, 0C Final Temp, 0C Rate, 0C/min Dwell time, min
Room temp 1000 6 15
1000 1200 7 15
1200 1300 5 15
1300 1400 4 15
1400 1500 3 10
1500 1400 1 240
1400 Water quench

(viii) SEM analysis of treated slag was carried out to verify redistribution of phases in the sample. Accompanying Figure 4 illustrates the SEM images showing elemental distribution of Phosphorous, Iron and Oxygen in treated Slag.
(ix) The quenched and dried slag sample was then crushed to very fine size and 50 gms of the re-melted slag sample was taken in a beaker containing 800ml water. It was stirred continuously to make the slurry homogenous. When the slurry became homogeneous it was fed into the wet magnetic separator. The magnetic separator was operated at 14A current, 60V DC voltage and 10,500 Gauss (1.05T). The slurry was poured at the funnel shaped inlet. All magnetic materials adheres to the inside wall of the separator and all non-magnetic materials are collected in a container through the outlet. Some water was also poured additionally so that the passage is cleared. When the magnetic separator was turned off, all the magnetic materials which adhered to the wall gets removed. Water was again poured to take this material out through the outlet. By this process we got two slurries, one with magnetic particles and another with non-magnetic particles. These slurries are allowed to settle down for 24 hours and then dried in an oven for around one and half hours for complete removal of moisture. Then XRF analysis of these two products i.e. magnetic and non magnetic part, were done separately. The results are shown in following Table 5:

Table 5: Composition of Magnetic and Non magnetic product sample obtained after magnetic separation of treated slag
Product Nature Components Fe(T) SiO2 CaO P2O5
Slag Amount (%) 15.23 13.20 47.62 2.47
Magnetic Part Amount (%) 18.69 13.73 47.17 1.73
Non Magnetic Part Amount (%) 14.24 12.55 48.98 3.37

From the above results it is clearly observed that phosphorous rich phase was collected in non-magnetic sample. This is also the part with greater content of calcium oxide. The amount of phosphorous present in non-magnetic sample is 3.37 % which is much higher than the magnetic sample. Thus the magnetic sample with lower phosphorous content can be recycled with advantage in steel plant processes.

In a separate example, the behavior of slag sample was investigated without melting/reheating steps as illustrated in following example 2:

Example 2:

A separate experiment as in example 1 was also carried out involving same steps but without any melting of slag subjecting to selected thermal cycle, and result found was encouraging as phosphorus level did not get much affected by only magnetic separation which is revealed in the results presented in Table 6.

Table 6: Composition of Magnetic and Non magnetic product sample obtained after magnetic separation of normal slag.

Product Nature Components Fe(T) SiO2 CaO P2O5
Slag Amount (%) 15.23 13.20 47.62 2.47
Magnetic Part Amount (%) 16.54 13.86 45.75 2.39
Non Magnetic Part Amount (%) 13.86 11.47 48.32 2.54

The above result goes to confirm the inventive finding in the method steps involving selective thermal cycle and parameters as followed under example 1 directed to separating the phosphorous rich C2S phase from slag and separating as non magnetic part in magnetic separation.

It is thus possible by way of the present invention to provide a method for dephosphorization of BOF slag. In this method slag from any steel plant with high phosphorous can be treated for lowering the phosphorous content in slag. A thermal treatment cycle has been identified for BOF slag which can rearrange distribution of its constituents. This thermal cycle includes heating of slag from room temperature to above its melting point with several dwelling period at different stage. This redistribution can be utilized for selective dissolution of P2O5 in calcium di silicate (C2S) phase. Adoption of this method can advantageously result in maximum recycling of slag in an iron and steel industry.