Claims:We Claim:
1. A modified steel slag adapted for use as construction aggregates, wherein the modified steel slag comprising:
basicity less than 2%,
MgO less than 7%,
di-calcium silicate phase between 50-55 %,
Akermanite phase between 5-8%, and
volume expansion of less than 1%.
2. The modified steel slag as claimed in claim 1, wherein the modified steel slag comprising:
CaO (Wt %) 35 - 40
FeO (Wt %) 22 - 26
SiO2 (Wt %) 18 -22
MgO (Wt %) 6 - 7
Basicity (%) < 2
Dicalcium Silicate Phase (Wt %) 50 - 55
Akermanite Phase (Wt %) 5 -8
Free Lime (Wt %) 0.5 -1.5
Free MgO (Wt %) < 0.5
Volume Expansion (%) 0.9%

3. A process of producing the modified steel slag as claimed in anyone of claim 1 or 2 comprising the steps of:
charging a molten steel slag in a slag pot, wherein said molten steel slag comprising:
CaO (Wt %) 43 – 47
FeO (Wt %) 24 - 27
SiO2 (Wt %) 10 - 13
MgO (Wt %) 8 -10
Basicity (%) 4 - 4.5
Dicalcium Silicate Phase (Wt %) 40 - 45
Akermanite Phase (Wt %) 0 - 3
Free Lime (Wt %) 4 - 6
Free MgO (Wt %) 1 - 3
Volume Expansion (%) 4.5 %

charging quartzite in the range of 10 to 15 % by weight of the molten steel slag through a dip lance;
charging carbon and aluminum source in the range of 3 to 5 % by weight of the molten steel slag;
blowing oxygen gas in the slag pot; and
air cooling the molten steel slag to produce a modified steel slag;
wherein the modified steel slag produced by the process has basicity less than 2%, MgO less than 7%, di-calcium silicate phase between 50-55 %, and Akermanite phase between 5-8% and volume expansion of less than 1%.
4. The process of producing the modified steel slag as claimed in claim 3, wherein the molten steel slag is a decarburization slag, hot metal pretreatment slag and/or electric furnace slag.
5. The process of producing the modified steel slag as claimed in claim 3, wherein the molten steel slag has temperature greater than 1500oC.

6. The process of producing the modified steel slag as claimed in claim 3, wherein the quartzite size is < 10mm.

7. The process of producing the modified steel slag as claimed in claim 3, wherein the carbon and aluminum source is added to increase the temperature of the molten steel slag in the slag pot.

8. The process producing the modified steel slag as claimed in claim 3, wherein the carbon source is carbon containing briquettes.

9. The process of producing the modified steel slag as claimed in anyone of claims 4, wherein the modified steel slag is dumped in a slag pit for metallic separation after cooling and pulverized to get uniform particle size.

Dated this the 14th day of March, 2020 Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION
The present invention relates to modified steel slag adapted for use as construction aggregate and a process to manufacture the same by modifying the steel slag for direct use in civil construction. More particularly, the present invention is directed to a process to stabilize steel slag in the slag pot when the slag is in a molten state in a simple and cost effective way. In this method molten steel slag coming from the steel making shop is directly modified/treated to have lower expansive phases for intended use in construction aggregates. The basic aspect of the present disclosure is directed to develop a process to stabilize both the free CaO and MgO present in steel slag, reduce basicity and formation of di-calcium silicate, Akermanite for aggregate applications for use as civil construction material and hence avoiding crack formation.
BACKGROUND OF THE INVENTION
Steel industries generate huge amount of iron and steel slag through iron and steel making processes. The dumping of slags is a major concern for the steel industry as well as a very serious environmental concern. As steel production is increasing to meet the market demand of steel, substantial efforts and investments have been made into research over the years to develop the slag processing technologies which enable re-use of slag either directly or through enlargement of slag-based products. Due to rapid suburbanization and rising environmental concerns over sand quarrying and rock crushing, the construction industry is also facing enormous encounter to meet the accumulative demand of natural coarse and fine aggregates. Owing to the fact that the chemical and physical properties of iron and steel process slags counterpart the properties of natural aggregates, it is found that one of the major application areas where large quantities of slag can be consumed is in road and civil constructions as replacement of natural aggregates. However, slags must be processed to match the requirements for use as aggregates. In spite of holding good mechanical properties, direct use of nonmetallic component of this processed slags as aggregates has not been successful due to its expansion characteristics as steel slag shows volumetric instability in the presence of water. Several expansive reactions are the reasons for steel slag expansion. The primary reason for volumetric expansion and structural instability of steel slags is the presence of free lime (CaO - [CaO + H2O ? Ca(OH)2]) and magnesium oxide (MgO - [MgO + H2O ? Mg(OH)2]) in its mineralogy which forms low density hydroxides in the presence of water and temperature. The swelling nature of steel slag is detrimental to almost all civil engineering applications. Hence, the main challenge to the researchers has been to develop a treatment process to decrease the volume instability caused by the expansive components of steel slag by changing its chemical and/or mineralogical properties.
Presently available techniques stabilize the solidified slag, externally by natural ageing, water ageing and steam ageing. Natural ageing involves exposure of slag to natural air and rain. In water ageing, the slag heaps are sprinkled with water, whereas steam ageing involves low to high pressure steam passed through the slag heaps. In these processes solidified metallic separated slag is subjected to air, water and steam for converting the expansive components into stabilized hydroxide and carbonate compounds.

JP2007297693 disclosed a method wherein molten steelmaking slag is charged into a reaction vessel in which molten iron is held, and a SiO2-containing modifier and a reducing carbon source are added to the molten steelmaking slag charged in the reaction vessel, and the steelmaking slag. By adding a SiO2 -containing modifier to the molten steelmaking slag, the unreacted free CaO in the steelmaking slag is hatched, and the reaction between the hatched free CaO and SiO2, etc. Thus, free CaO can be reduced. Therefore, volume expansion due to the hydration reaction of free CaO (Ca + 2H2O ? Ca (OH)2 + H2) can be prevented. However, the treatment does not target free MgO which also causes volume expansion is still available in free form. Further, the treatment takes longer duration and cannot be carried out on all types of slags. The temperature at which it is carried is only 1350oC. Low temperature of steel slag cannot maintain high fluidity.

Therefore, the present invention directed to solve such problems of prior art, and provides a method for treating steelmaking slag generated during the refining process of a steelmaking process, wherein the steelmaking slag is reformed and reduced while maintaining the molten state of the slag. In this disclosure slag is processed in molten state, online in the steel making process, in the existing slag pot to reduce the expansive compounds before solidification. No other separate treatment is required.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to a modified steel slag adapted for use as construction aggregate and a process to produce the same by modifying steel slag by reducing the volume expansion of the steel slag.

A further object of the present invention is directed to develop a process to produce construction aggregate by modifying steel slag so as to avoid undesirable cracks in the building materials when steel slag is added.

A still further object of the present disclosure is to develop a process to reduce the free MgO and CaO present in steel slag.

A still further object of the present invention is to use the modified steel slag as an aggregate for building material.

Another object of the present invention is to develop a slag modifier for steelmaking slag to stabilize both the free CaO and MgO present in steel slag.

Yet another object of the present invention is to provide an easy and techno-economic solution that finishes expansive characteristics of steel slag which can be adopted universally.

A further object of the present disclosure is to reduce the hydration and expansion of steel slag to make it suitable for use as construction aggregate.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a modified steel slag adapted for use as construction aggregates, the modified steel slag comprising basicity less than 2%, MgO less than 7%, di-calcium silicate phase between 50-55 %, Akermanite phase between 5-8% and volume expansion of less than 1%.
A still further aspect of the present invention is directed to said modified steel slag comprising:
CaO (Wt %) 35 - 40
FeO (Wt %) 22 - 26
SiO2 (Wt %) 18 -22
MgO (Wt %) 6 - 7
Basicity (%) < 2
Dicalcium Silicate Phase (Wt %) 50 - 55
Akermanite Phase (Wt %) 5 -8
Free Lime (Wt %) 0.5 -1.5
Free MgO (Wt %) < 0.5
Volume Expansion (%) 0.9%

A further aspect of the present invention is directed to a process of producing said modified steel slag comprising the steps of:
charging a molten steel slag in a slag pot, wherein said steel slag comprising;
CaO (Wt %) 43 – 47
FeO (Wt %) 24 - 27
SiO2 (Wt %) 10 - 13
MgO (Wt %) 8 -10
Basicity (%) 4 - 4.5
Dicalcium Silicate Phase (Wt %) 40 - 45
Akermanite Phase (Wt %) 0 - 3
Free Lime (Wt %) 4 - 6
Free MgO (Wt %) 1 - 3
Volume Expansion (%) 4.5 %

charging quartzite in the range of 10 to 15 % by weight of the molten steel slag through a dip lance;
charging carbon and aluminum source in the range of 3 to 5 % by weight of the molten steel slag;
blowing oxygen gas in the slag pot; and
air cooling the molten steel slag to produce a modified steel slag;
wherein the modified steel slag produced by the process has basicity less than 2%, MgO less than 7%, di-calcium silicate phase between 50-55 %, and Akermanite phase between 5-8% and volume expansion of less than 1%.

A still further aspect of the present invention is directed to said process wherein the molten steel slag is a decarburization slag, hot metal pretreatment slag and/or electric furnace slag.

A still further aspect of the present invention is directed to said process of producing the modified steel slag wherein the molten steel slag has temperature greater than 1500oC.

A still further aspect of the present invention is directed to said process of producing the modified steel slag wherein the quartzite size is < 10mm.

A still further aspect of the present invention is directed to said process of modification of steel slag, wherein the carbon and aluminum source is added to increase the temperature of the molten steel slag in the slag pot.

Another aspect of the present invention is directed to said process of modification of steel slag wherein the carbon source is carbon containing briquettes.

Yet another aspect of the present invention is directed to said process of producing the modified steel slag wherein the modified steel slag is dumped in a slag pit for metallic separation after cooling and pulverized to get uniform particle size.

Other features and aspects of the present invention will be apparent from the following description with reference to the accompanying non limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Embodiments are illustrated by way of example and are not limited in the accompanying figures.
FIG. 1 illustrates equipment for modification of molten steel slag;
FIG. 2 illustrates the flow chart of a process for modification of molten steel slag;

Person skilled in the art would appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present disclosure relates to a modified steel slag adapted for use as construction aggregate and a process of modification of molten steel slag accordingly. The process comprising the steps of firstly charging a molten steel slag in a slag pot. The second step includes charging quartzite in the range of 10 to 15% by weight of the molten steel slag through a dip lance. The third step includes charging carbon and aluminum source in the range of 3 to 5 % by weight of the molten steel slag. The fourth step includes blowing oxygen gas in the slag pot. The last step includes air cooling the molten steel slag to produce a modified steel slag. The modified steel slag produced by the process has basicity less than 2%, reduced MgO less than 7%, di-calcium silicate phase between 50-55 %, Akermanite phase between 5-8% and volume expansion of less than 1%.

According to the present invention, a process for production of construction aggregates by modification of molten steel slag is disclosed. This present disclosure relates to a process to stabilize steel slag in a very economic method in the slag pot when the slag is in a molten state. In this method molten steel slag coming from the steel making shop is directly modified to have lower expansive phases when used in construction aggregates. The basic aspect of the present disclosure is directed to develop a process to reduce the volume expansion, MgO, basicity and increase in di-calcium silicate, Akermanite for aggregate applications for use in building material. The process comprising the steps of:
i) charging a molten steel slag in a slag pot;
ii) charging quartzite in the range of 10 to 15 % by weight of the molten steel slag through a dip lance;
iii) charging carbon and aluminum source in the range of 3 to 5 % by weight of the molten steel slag;
iv) blowing oxygen gas in the slag pot; and
v) air cooling the molten steel slag to produce a modified steel slag.

In the present disclosure, steel slag is not particularly limited. For example, decarburization slag, hot metal pretreatment slag, electric furnace slag etc. can also be used. Hereinafter, this point will be described in more detail.
Accompanying FIG. 1 illustrates an equipment 100 for modification of molten steel slag according to present invention. The process of steel slag modification is carried out in a convectional slag pot 102 generally used to carry the molten steel slag 104 from the tapping position to the slag yard where it is dumped. The quartzite 106 is injected by dip lance 108 inside the molten steel slag 104. The oxygen lance 110 is used for blowing oxygen 112. The oxygen lance 110 is vertical or slant. A material feeding chute 114 is provided for addition of carbon and aluminum source 116. Preferably, the carbon source 116 is carbon containing briquettes. The aluminum source 116 is aluminum shots.
Accompanying FIG.2 illustrates the flow chart of a process for modification of molten steel slag 200. Step 202 includes charging a molten steel slag in a slag pot. Step 204 includes charging quartzite 106 in the range of 10 to 15% by weight of the molten steel slag 104. The amount of quartzite 106 is chosen so that the free lime content of the steel slag is largely bound, in the form of di-calcium silicate. Due to its high di-calcium silicate content, the steel slag prepared according to the present disclosure can be processed excellently into building materials without generating cracks. Free MgO and CaO can be reduced by the reaction of SiO2. Therefore, volume expansion due to the hydration reaction of free MgO and CaO can be prevented. The free CaO of steel slag reacts with SiO2 and converts calcium into calcium silicates. The free CaO and MgO reacts with SiO2 and converts calcium into Akermanite. The quartzite 106 has a size <10 mm is added through the dip lance 108. The dip lance 108 has either single hole or multiple hole. The dip lance 108 is dipped into the molten steel slag 104 to reduce the basicity. The dip lance 108 is used for better distribution of particles and more reaction sites.
The quartzite 106 is added in the range of 10-15 % of the slag weight. The added quartzite 106 increases the combined di-calcium silicate and Akermanite phases and reduce the free CaO and MgO.
Step 206 includes charging carbon and aluminum source 116 in the range of 3 to 5 % by weight of molten steel slag 104 in a slag pot 102. The carbon containing briquettes 116 provide heat, which can accelerate the mixing of molten steel slag 104 and quartzite 106. The machine feeding chute 114 stored and add carbon containing briquettes into the molten steel slag. Optionally, the aluminum shots are also added to the molten steel slag by machine feeding chute 114. The carbon containing briquettes and aluminum shots 116 increases the temperature of the molten steel slag 104 by oxidation reaction.
Step 208 includes blowing oxygen 112 in the slag pot 102. The slag pot 102 is stirred by blowing oxygen 112 preferably for 10 minutes to mix the molten steel slag 104, quartzite 106 and carbon source 116 evenly. Blowing oxygen 112 generates heat to maintain the higher temperature in the slag pot. The oxygen 112 stirring can burn the carbon source 116, so that the molten steel slag 104 temperature can be efficiently maintained simultaneously with the stirring. The oxygen 112 is blown onto the molten steel slag 104 surface by an oxygen lance 116. The oxygen 112 also increases the temperature by oxidation of iron. Based on the initial molten steel slag 104 temperature the ratio of carbon and aluminum source 116 and oxygen 112 blown can be varied which results in exothermic reactions.
In an embodiment, oxidizing agents are added to achieve the temperature > 1500oC for better mixing and higher fluidity in the slag modification process 200. The process 200 has to be completed in 3 to 4 minutes.
Step 210 includes air cooling to produce modified steel slag 104. In an embodiment, after cooling of steel slag, pulverization is done to get uniform particle size. After the treatment, the slag pot 102 is lifted and modified steel slag is dumped in the slag pit from where it is taken for metallic separation after cooling.
The process of treating steel slag according to present invention is low in cost and simple. The reaction kinetic conditions in slag pot are excellent, so that the purpose of dissolving free calcium oxide and magnesium oxide is achieved in a short time.
The steel slag obtained from the present disclosure can be used for high-grade applications such as upper roadbed materials, concrete aggregates, and stone materials.
In an embodiment, the slag is transported from tapping position to slag yard in the slag pot. A process for modification of molten steel slag in the slag pot is disclosed wherein the process station is created below the converter at the tapping position or between tapping point and slag yard where the steel slag is dumped to make the process online.

Example: Comparison of Unmodified and Modified Steel Slag

Table 1 shows the components, volume expansion and basicity of molten steel slag without modification and after modification by the present process of modification.

Table 1:

Test Standard Unmodified Steel Slag Modified Steel Slag
CaO (Wt %) XRF 43 – 47 35 - 40
FeO (Wt %) XRF 24 - 27 22 - 26
SiO2 (Wt %) XRF 10 - 13 18 -22
MgO (Wt %) XRF 8 -10 6 - 7
Basicity (%) XRF 4 - 4.5 < 2
Dicalcium Silicate Phase (Wt %) XRD 40 - 45 50 - 55
Akermanite Phase (Wt %) XRD 0 - 3 5 -8
Free Lime (Wt %) JIS 5015 4 - 6 0.5 -1.5
Free MgO (Wt %) JIS 5015 1 - 3 < 0.5
Volume Expansion (%) IS-383 4.5 % 0.9%

As shown in Table 1, the modified steel slag produced by the process had basicity less than 2%, reduced MgO less than 7%, di-calcium silicate phase between 50-55 %, Akermanite phase between 5-8% and volume expansion of less than 1%.
It is thus possible by way of the present invention to provide steel slag adapted for use as construction aggregate and a process to manufacture the same by modifying the molten steel slag for direct use in civil construction. More particularly, the present invention is directed to a process to stabilize steel slag in the slag pot when the slag is in a molten state in a simple and cost effective way. In this method molten steel slag coming from the steel making shop is directly modified/treated to have lower expansive phases for intended use in construction aggregates having basicity less than 2%, reduced MgO less than 7%, di-calcium silicate phase between 50-55 %, Akermanite phase between 5-8% and volume expansion of less than 1%, making it suitable for applications such as upper roadbed materials, concrete aggregates, and stone materials.