Claims:1. A process for minimizing the slag crumbling and volumetric instability in steel making process comprising the primarily converting liquid iron from a blast furnace to steel and then involving the secondary metallurgical steps of adding alloying agents and lowering dissolved gases,
- removing or altering chemically to ensure high quality of steel, slag formed during the process is being drained after finish of continuous casting crumbles into powder during cooling, wherein the slag obtained is treated with flyash to minimize slag crumbling and volumetric instability by reacting with the free lime present in the slag before arcing in the ladle furnace to avoid formation of 2CaO.SiO2 and proper dissolution in the slag thereby easy removal of the slag from the slag pot and transportation to slag yard for further utilization of slag for other purposes.

2. A process as claimed in claim 1, wherein the slag mainly consists of ?-2CaO.SiO2, Mayenite(Ca12Al14O33) and Merwenite (Ca3Mg(SiO4)2)along with other phases.

3. A process as claimed in claim 1, wherein the flyash consists of SiO2, Mullite (3Al2O3-2SiO2),Silmanlite(Al2SiO5) and quartzite.

4. A process as claimed in claim 1, wherein fly ash mainly consists of 45-55 % SiO2,15-30% Al2O3, 0.7-1% MgO, 0.8-1% CaO.

5. A process as claimed in claim 1, wherein fly ash used is in size range from 5µm to 90 µm

6. A process as claimed in claim 1, wherein 40-200 Kg of fly ash/ ton of slag is added in ladle at ladle furnace.

7. A process as claimed in claim 1, wherein basicity of crumbled steel slag is in range of 1.8 to 2.3.

8. A process as claimed in claim 1, wherein the basicity of the slag after treatment with fly ash is in range of 1.1 to 2.0.

9. A process as claimed in claim 1, wherein particle size range of crumbled steel slag is from 40 µm to 100 µm.

10. A process as claimed in claim 1, wherein crumbled steel slag has maximum of?-C2S phase.

11. A process claimed in claim 1, wherein there is very less C2S, free lime, free magnesia in slag treated with fly ash.

12. A process as claimed in claim 1,wherein the slag composition before and after treatment with fly ash is in the rangeCaO:45-55%,MgO:8-12%, SiO2:12-20%, FeO:0.5-2.0%,Al2O3:6-15%,P2O5:0.1-0.5%,MnO:0.1-1.0%&CaO:40-50%, MgO:6-10%, SiO2:15-30%, FeO:0.5-2.0%, Al2O3:10-25%, P2O5:0.1-0.5%, MnO:0.1-1.0% respectively.

13. A process claimed in claim 1, wherein the treated slag can be used for construction applications.
, Description:

FIELD OF INVENTION:
The present invention relates to the process for minimizing crumbling of steel slag obtained after finish of continuous casting operations which is drained into slag pot. This drained slag is crumbling into fine powder during cooling mainly due to volumetric instability. The main phases that leads to such volumetric instability are C2S (2CaO.SiO2), Free CaO and Free MgO. Slag crumbling can be minimized either by preventing volumetric instability of the phases during cooling or by preventing the formation of these phases in slag. Slag crumbling can be minimized by treating slag with fly ash in the ladle furnace after the finish of secondary metallurgy operations. Minimization of steel slag crumbling helps in attaining better working conditions and hence leads to process improvement. Slag obtained after stabilization treatment is in 100% compacted state and hence can be easily removed from slag pots and transported to slag yard.

PRIOR ART
US4179279 teaches about the process for treating slag from known converter and electric furnaces to improve the structural stability of such slag and render such slag suitable such as a landfill and construction material as well as a substitute for base material or crushed stone for road construction, comprising the steps of:(1) providing molten slag produced from a conventional converter or electric furnace in which the furnace is used in conventional steel production and wherein the molten slag when cooled contains Free CaO and 2CaO.SiO2 which render such cooled slag unsuitable for landfill purposes due to swelling and structural instability of the Free CaO and 2CaO.SiO2 therein,
(2) adding red mud obtained as a by-product from the aluminum industry to the molten slag thereby creating a thermo-chemical reaction between the red mud and the molten slag to transform a composition of said cooled slag containing the Free CaO and 2CaO.SiO2 to a relatively non-swelling and non-crumbling composition, said red mud accounting for 5 to 20 percent of said molten slag after the addition of said red mud, said red mud comprising as a major component Fe2O3 and as a lesser component Na2O, said red mud melting at a temperature lower than the melting temperature of said slag, and
(3) Utilizing the heat value of the molten slag to melt said red mud into said molten slag and cause said thermo-chemical reaction, whereby the thus processed cooled slag has structural ability rendering it suitable as a landfill.
BACK GROUND OF INVENTION:
Liquid iron (obtained from Blast furnace), iron ore, limestone and scrap are the raw materials used in steel making process for producing steel. In steelmaking, impurities such as nitrogen, silicon, sulfur, phosphorus and excess carbon are removed from the liquid iron, and alloying elements in the form of Ferro alloys are added to produce different grades of steel.
Modern steelmaking processes can be broken into two categories: primary and secondary steelmaking. Primary steelmaking involves converting liquid iron from a blast furnace and steel scrap into steel via basic oxygen steelmaking or melting scrap steel and/or direct reduced iron (DRI) in an electric arc furnace. Secondary steelmaking involves refining of the crude steel before casting and the various operations are normally carried out in ladles. In secondary metallurgy, alloying elements are added, dissolved gases in the steel are lowered, and inclusions are removed or altered chemically to ensure that high-quality steel is produced after casting. During the primary and secondary steel making process slag will be formed as a byproduct.
The leftover Steel slag obtained after secondary metallurgy which is drained after finish of continuous casting crumbles into fine powder during cooling.
The fine powder of slag -
• Takes up a large volume as it does not have wetting and compaction characteristics.
• Causes difficulties in handling and stocking.
• Creates pollution and dust problems in terms of the environment and thus resulting in additional costs for the plant.
The dusting of slag is an environment problem due to formation of very fine particles that are easily scattered at slag pits. The disintegration also prevents the cooled slag from being utilized for other purposes.
Due to the presence of unstable phases in its mineralogy, steel slags can show volumetric instability, caused mainly by the presence of free CaO, free MgO and C2S. In the presence of water, and free lime hydrates, it forms portlandite (Ca(OH)2). Portlandite has a lower density than CaO and hence, hydration of free CaO results in volume increase. CaO can hydrate almost completely in a few days with a volume increase as high as100%.
However, residual lime can be embedded in small pockets in gravel-size steel slag particles. Lime pockets may not hydrate at all if they are not given access to water through the fractures extending to them. If there are fractures in the slag particles extending to these lime pockets, then hydration can progress. Other expansive compounds, such as free MgO, may also be present in steel slag. Unlike CaO, free MgO hydrates at a much slower rate, causing significant volume changes happens for months or even years, the possibility of volumetric expansion due to hydration of MgO increases as well.

Another compound that causes volumetric expansion is dicalcium silicate (C2S) phase. The C2S phase is commonly present in all types of steel slags and in particular, is abundant typically as the main phase in ladle slags. C2S exists in four well-defined polymorphs: a, a', ß, and ?. a-C2S is stable at high temperatures (>630°C). At temperatures below 500°C, ß-C2S starts transforming into ?-C2S. This transformation produces volumetric expansion of up to 12-14%. If the steel slag cooling process is slow, crystals break, resulting in a significant amount of dust. This phase conversion and the associated dusting are typical for ladle slags. For this reason, ladle slags are commonly called “self-dusting” or “falling” slags
Existing practices for minimization of slag crumbling:
The slag obtained after continuous casting is being drained into slag pots and no particular practice is being followed to minimize crumbling of this slag. The slag after draining is left for air cooling. The present process of the invention helps in minimizing crumbling of steel slag and also in effective utilization of industrial solid waste i.e fly ash.

SUMMARY OF THE INVENTION

The present invention relates to minimization of crumbling of steel slag. This is done by addition of fly ash to the slag. SiO2 and Al2O3 present in fly ash helps in minimizing crumbling of steel slag. This drained slag is crumbling into fine powder during cooling mainly due to volumetric instability. The main phases that leads to such volumetric instability are C2S (2CaO.SiO2), Free CaO and Free MgO. Slag crumbling can be minimized either by preventing volumetric instability of the phases during cooling or by preventing the formation of these phases in slag. Slag crumbling can be minimized by treating slag with fly ash in the ladle furnace after the finish of secondary metallurgy operations. Fly ash used for the process is solid waste obtained during combustion of coal in Thermal power plant of, RINL-Visakhapatnam Steel Plant. Fly ash contains around 45-55 % SiO2, 15-30 % Alumina which helps in minimizing crumbling of steel slag. The phases such as SiO2 and Al2O3 present in fly ash helps in preventing C2S formation and also react with free CaO and free MgO to form calcium aluminates and other stable phases. Due to the presence of high melting point phases in Fly ash, extra heat source is required for its dissolution. Hence it should be added before arcing in the ladle for proper dissolution in the slag. So that it will change the slag composition and helps in its stabilization.
Minimization of slag crumbling is very important in steel industries because the crumbled steel slag causes environmental pollution and also difficulties in slag handling and storage.
Steel de-oxidation is done either by using Si source or Al source for killing steel. When Si killing is applied for de-oxidation, excess CaO and MgO are added to achieve required basicity of slag. Slag crumbling is more prevalent in Si killed steels. In Al-killed steel Al2O3 present in slag acts as a slag stabilizer and hence slag crumbling is less compared to Si-killed steel. Typical composition of steel slag after secondary steel making process in Si-Killed and Al-killed steels are shown in Table 1.

Table 1: Typical composition of steel slag after secondary metallurgy
Type CaO MgO SiO2 Al2O3 FeO MnO P2O5 Basicity
Si killed steels 47-50 12-14 25-30 6-10 1-2 0.2-1.0 0.1-0.5 1.6-1.9
Al killed steels 48-52 8-10 12-20 17-25 0.5-1.5 0.1-0.5 0.10-0.2 1.8-2.4

Minimization of slag crumbling is important phenomena in steel industry mainly due to problems caused by crumbled steel slag. It helps in attaining better working conditions and hence leads to process improvement. Slag obtained after stabilization treatment is in 100% compacted state and hence it can be easily removed from slag pots and transported to slag yard.

DETAILED DESCRIPTION OF THE INVENTION

To see the effect of fly ash additions in steel slag, initially lab scale experiments were conducted. Accordingly, crumbled steel slag and fly ash samples were collected and characterization studies were done. The Composition of as received crumbled steel slag used for experiments, typical fly ash and fly ash used for experiments are shown inTable 2,
Table 3, Table 4 respectively.
Table 2: Composition of as received crumbled steel slag used for experiments
CaO SiO2 Al2O3 MgO TiO2 FeO MnO P2O5 S Basicity
53.5 23.6 9.9 9.58 0.73 1.43 0.4 0.05 0.276 2.3

Table 3: Typical composition of fly ash
SiO2 Al2O3 Fe2O3 MgO CaO TiO2 P2O5 K2O Na2O
50-55 35-37 6.5-7.5 0.7-1 0.8-1 1-2 <0.59 1-1.2 trace

Table 4: Composition of as received Fly Ash used for experiments
SiO2 Al2O3 Fe2O3 MgO CaO TiO2 K2O Na2O LOI
59 33.85 4.85 0.55 0.57 1.2 0.175 0.13 0.9

The main phases present in raw slag were ?-2CaO.SiO2, Mayenite (Ca12Al14O33) and Merwenite (Ca3Mg(SiO4)2) as shown in Figure 1 regarding XRD of as received crumbled steel slag. The main phases present in fly ash were SiO2, Mullite, Silmantite and quartzite as shown in Figure 2 regarding XRD of as received Fly Ash.
Sieve analysis of crumbled slag is shown in Table 5. From that we can find the average particle size is 53 µm. The same were represented in graphical manner in Figure 3 regarding Mesh No Vs % weight retained and Figure 4 regarding Mesh No Vs Cumulative weight Typical size analysis of fly ash is shown in Table:6.
Table 5: Sieve analysis of crumbled LHF slag

Sl.No BSS mesh No Size µm Weight retained on each sieve % retained on each sieve Cumulative weight
1. 120 125 5 2.5 5
2. 150 106 25 12.5 30
3. 240 63 32 16 62
4. 300 53 109 54.5 172
5. 350 45 4 2 176
6. 400 38 3 1.5 179
7. Pan --- 16 8 199

Table 6: Size analysis of fly ash

Particle size distribution + 212 µm +90 µm + 45 µm -5 µm
% 0 0.37-16.8 3.38 -51.74 5.78-38.96

Experimental procedure:
To study the high temperature properties, the slag was re-melted to replicate actual site conditions. Muffle furnace having achievable temperature of 1700°C was used for conducting experiments. Using graphite crucibles, raw slag was re-melted in Muffle furnace and crumbling properties were observed during air cooling. It was observed that the slag after few minutes started to crack initially and slowly all the slag got crumbled into fine powder.
Laboratory scale experiments were carried out at different wt.% of Fly ash ranging from 6.0 wt. % to 12.0 wt. % (6.0 wt. %, 8.0 wt. %, 12.0 wt. %). This idea was generated in order to utilize fly ash, a solid industrial waste. Addition of fly ash to steel slag minimizes the crumbling problem, as it contains around 50% SiO2; it reacts with free lime in slag and avoids formation of 2CaO.SiO2. Addition of fly ash reduces basicity of the slag also. Slag treated with fly ash did not crumble and a compacted slag was obtained even after cooling. Stabilized steel slag is obtained with slag treated with 6-20 wt.% fly ash. pH of the slag was measured before and after treatment, the results are shown in
Table 7. From the results it was observedthat pH of the slag is decreasing after addition of fly ash.

Table 7: Variation of pH of LHF slag after Fly ash additions
Sl.No % of fly ash pH of slag before and after fly ash addition
1. 0 10.77
2. 6 10
3. 8 10.22
4. 12 10.6

Treated slag samples were collected and performed for characterization studies using XRF and XRD. The XRF results obtained for slag tretaed with fly ash is showin in Table 8. As fly ash stabilizes steel slag by modifying slag chemistry, graphs were plotted for % fly ash addition Vs basicity,CaO and SiO2 content in slag are shown in Figure 5 regarding Effect of fly ash additions on Basicity of steel slag and Figure 6 regarding Effect of fly ash additions on CaO and SiO2 content of steel slag respectevely.

Table 8. Analysis of slag treated with fly ash obtained from XRF analysis
Sl.No Wt% fly ash CaO % SiO2 % Al2O3 % MgO % TiO2 % FeO % MnO % P2O5 % S % Basicity
CaO/SiO2
1. 0 53.5 23.6 9.9 9.58 0.73 1.43 0.4 0.05 0.276 2.3
2. 6 41.7 30.7 15 9.5 0.86 1.2 0.05 0.09 0.27 1.3
3. 8 40.5 31.1 15.5 9.57 0.88 1.4 0.04 0.09 0.26 1.3
4. 12 41.5 34.6 10.8 10.61 0.98 0.6 0.09 0.06 0.15 1.2

The obatined XRF results were used to plot on terenary diagrams of CaO-SiO2-Al2O3 and CaO-SiO2-MgO to see the stability of the slag. It can be seen that slag before treatment is in the red coloured portion in the ternary diagram i.e fell in maximim C2S zone where as salg treated with fly ash fell in minimum C2S zone.

Slags after treatment with fly ash i.e stabilized slags fell in 0-20 % C2S region in ternary diagramas shown inFigure 7 regarding Ternary plotting of slag treated with fly ash and untreated-crumbled slag of Visakhapatnam Steel Plant based on slag chemistry.
This shows that fly ash additions helped in prevention of dicalcium silicate formation and rather other phases are formed which pose no volume expansion problem during cooling of slag. By this process overall chemistry of slag is modified and hence the steel slag is stabilized. With increase in addition of fly ash, SiO2 content in the slag increased from 23.6% to 34.6% and basicity decreased from 2.3 to 1.3. This method of slag stabilization with fly ash helps in effective utilization of steel slag for construction applications.
In slag treated with fly ash, there was minor dicalcium silicate formation and main phases present were Gehlenite (Ca2Al2SiO7), Hartrurite (Ca3SiO5), Akermanite (Ca2MgSi2O7) and Merwenite (Ca3Mg(SiO4)2).
Various researchers critically reviewed minimization of slag crumbling by various methods. By their research, they defined the compositional disintegrating limits for slag based on the stability field of C2S in the CaO-MgO-SiO2-Al2O3 system with an adjustment for the sulfur content (S) in the slag as shown in equations 1 & 2below.
CaO + 0.8 MgO = 1.20 SiO2 + 0.39 Al2O3 + 1.75 S -------- (1)
CaO = 0.93 SiO2 + 0.55 Al2O3 + 1.75 S -------- (2)
Slag which falls within these composition limits does not disintegrate and is stable. From the analysis it can be observed that there is huge difference in composition limits for untreated slag, whereas for treated slag, the difference is very less and is around 4-5 units.
Results of treated slag are compared with these equations and are given in Table 9:
Table 9.Comparison of composition limits for treated slag
Sl.No Wt% Fly Ash CaO 0.93 SiO2 + 0.55 Al2O3 + 1.75 S CaO + 0.8 MgO 1.20 SiO2 + 0.39 Al2O3 + 1.75 S
1. 0 53.5 27.8 61.16 32.66
2. 6 41.7 37.2 49.3 43.1
3. 8 40.5 37.9 48.1 43.8
4. 12 41.5 38.3 49.9 45.9

Figure 8 regarding XRD analysis of slag treated with 6 Wt % Fly Ash, Figure 9 regarding XRD analysis of slag treated with 8 Wt % Fly Ash and Figure 10 regarding XRD analysis of slag treated with 8 Wt % Fly Ash show the XRD analysis of slag treated with different fly ash additions. From the XRD peaks it can be seen that there is no C2S formation in slag treated with fly ash and hence stabilized slag is obtained. Sharp peaks are not present in XRD of slags treated with fly ash indicative of amorphous phases.

From the XRD peaks it was observed that slag treated with fly ash shows the presence of amorphous phases.As there is less di-calcium silicate formation in slag treated with fly ash, it is stabilized and compact slag is obtained after cooling. Major phases present in raw slag and treated slag observed from XRD analysis are shown in Table:10.
.
Table 10: Major phases present in raw slag and treated slag observed from XRD analysis
Sl.No Wt% fly ash Major phases present
1. 0 ß- 2CaO.SiO2 , ?-2CaO.SiO2 , Gehlenite (Ca2Al2SiO7 ) , Merwenite (Ca3Mg(SiO4)2), Periclase (MgO) , lime (CaO)
2. 6 Gehlenite (Ca2Al2SiO7 ), Hartrurite (Ca3SiO5), Akermanite (Ca2MgSi2O7), Merwenite (Ca3Mg(SiO4)2 )
3. 8 Gehlenite (Ca2Al2SiO7 ), Hartrurite (Ca3SiO5), Akermanite (Ca2MgSi2O7), Merwenite (Ca3Mg(SiO4)2 )
4. 12 Gehlenite (Ca2Al2SiO7 ), Merwenite (Ca3Mg(SiO4)2), Harturite (Ca3SiO5)

After optimizing the fly ash addition in lab scale, industrial trials were done at ladle furnace. From the lab scale trials it was observed that 6-20 Wt % fly ash is required to stabilize steel slag. The fly ash quantity varies based on the composition of slag, quantity of slag and type of de-oxidation practice employed. Conventionally borates are being used by many plants to minimize crumbling. Minimum 1.0 Wt.% B2O3 is required to stabilize slag. When borate addition is done along with fly ash, borate requirement comes down drastically. Also fly ash being solid waste generated in steel plants, it can be utilized effectively for stabilizing steel slag. Fly ash can be added in the ladle during secondary metallurgy operations. The present process thus minimizes crumbling of slag during cooling. 60-120 kg of fly ash per ton of steel slag is added into ladle at Ladle furnace.