FIELD OF THE INVENTION

The present invention relates to a sintering process with enhanced sinter output by utilising steel making slag for insulation in hearth layer in sinter plant. More particularly, the present invention is directed to a method for enhancing sinter productivity of sinter plant by utilising steel making slag in combination with sinter products for providing the hearth layer over the pallet to serve on one hand as an insulation layer to protect pallet boxes and on the other hand advantageously increase the sinter output and increasing sinter plant productivity over conventional process of sinter making while simultaneously recovering the valuable iron contents of steel slag and recycling the otherwise waste material (steel making slag).

BACKGROUND OF THE INVENTION

Sintering involves the heating of fine iron ore with flux and coke fines to produce a semi-molten mass that solidifies into porous pieces of sinter having the required size and strength for feeding into the blast furnace. Sintering is a process of agglomeration and produces a feed of extremely consistent quality in terms of its chemical composition, grain size distribution, reducibility and sinter strength. On the sinter machine, its surface is ignited, air being induced through the ignited layer and sintering proceeds in the vertical direction in the sinter strand's material bed. There is thus a need to protecting the grate bars of the pallets during the sintering process. The material being used in the hearth layer should act as an insulating layer to protect the pallet boxes. Presently 40mm thick layer of sinter product with a size range of 10-20mm is being recycled in hearth to protect the pallet boxes. This in turn reduces the sinter productivity by about 1.75%.

There has been thus a need in the art to find alternative materials for the insulating hearth layer, which would be cost effective and minimizes the use of good quality sinter product and improves sinter plant output and productivity as well as enhances pallet life.

It was found by way of the present investigation that steel making slag possesses good insulating property. It also contains some iron, which can be utilized if recovered. On the other hand, slag generated in steel melting shop is difficult to dispose of or to recycle and occupies a larger area of valuable and scarce space in today's steel plants. All steel plants are thus on the look out for the utilization of this waste slag for alternate applications.

The present invention is directed to advancements in the method of sinter making, which attempts to improve sinter productivity on one hand and utilization of steel slag on the other, the disposal of which is a problem in the steel industry. Thus the present invention is targeted at improving the sinter output and productivity, which leads to economic utilization and or recycling of wastes while simultaneously recovering the valuable iron content of the steel making slag.

OBJECTS OF THE INVENTION

Thus the basic object of the present invention is directed to providing for advancement in process of sintering involving hearth layer favouring increased sinter output and sinter plant productivity.

A further object of the present invention is directed to a process of sintering involving advancements in hearth layer in sinter plant for better insulation to protect the grate bars of the sinter pallets during the sintering process so that the steel making slag can be gainfully recycled which is otherwise considered a waste material causing disposal problem.

A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant for better insulation so as to recover valuable iron content in steel making slag.

A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant for better insulation whereby conventional use of good quality sinter product in hearth layer is partially substituted with steel making slag so that sinter output is substantially increased.

A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant for better insulation whereby it is possible to achieve effective waste (slag) recycling to avoid land contamination on disposal. A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant, which would enable effective and advantageous utilization of scarce space within steel plant and provide avenues for avoiding slag dumping.

A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant which ensure improving sinter production and increasing sinter yield and avoiding problems of slag dumping in steel plants and enabling its benefit utilization.

A still further object of the present invention is directed to a process of sintering involving advancement in hearth layer in sinter plant which ensure enhancing desired iron content of sinter and also enhance pallet life.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to a process for sintering in a sinter plant comprising:

preparation of raw sinter mix;

preparation of the hearth layer comprising a mix of sinter and steel making slag;

charging of the said hearth layer comprising a mix of sinter and steel making slag onto sinter pallets followed by charging of the said raw sinter mix;

subjecting the charge as above to heating thereby producing a semi-molten mass, which in turn solidifies into porous pieces of sinter having the required size and strength, suitable for feeding into blast furnace.

A further aspect of the present invention is directed to said process wherein said hearth layer comprises of steel making slag upto 15 % by wt. preferably a mix of 85% to 90% preferably about 90% sinter and 10 to 15% preferably about 10% steel making slag. Addition of higher percentage of steel making slag may be used but is limited by total phosphorus input to the blast furnace.

Another aspect of the present invention is directed to said process wherein said hearth layer comprising a mix of 10-20mm of said sinter and 12-20mm of steel making slag.

A still further aspect of the present invention is directed to said process wherein said raw sinter mix charge comprises:

Iron ore fines: 46 to 53% by wt.; Coke breeze 5.0 to 5.5 % by wt.; Solid waste 8.0 to 16 % by wt.; BF (sinter) return fines 20 to 30 % by wt.; Quick lime 3.5 to 5.0 % by wt; Limestone 2.0 to 3.5 % by wt.; Dolomite 5.0 to 7.0 % by wt.

Yet another aspect of the present invention is directed to said process wherein said steel making slag comprises of steel making slag obtained from the steel melting shop which is crushed and sieved to different size fractions preferably in the range of 12-20 mm for use in the hearth layer.

A still further aspect of the present invention is directed to said process wherein said preparation of raw sinter mix comprises blending the raw materials for sintering in required proportions using weigh feeders and homogenizing and nodulising in primary and secondary mixing drums with controlled addition of water such as to maintain a moisture level of about 7-8 %.

In said process said raw sinter mix is charged from a hopper onto the sinter pallet and the feeding rate depends on the capacity of sinter plant.

In the above process, the undersize sinter, which is not suitable for blast furnace is recycled in the process.

A further aspect of the present invention is directed to a process wherein a select quantity of 10-20 mm sinter produced is screened and re-circulated for mixing with the steel making slag for providing the hearth layer.

According to yet another aspect of the present invention, the said process comprises the following preferred operating parameters:

The ignition temperature: 1050±50°C.

The sintering temperature: 1300±100°C.
Draft rate (suction pressure): 9-11 kpa
Sinter machine speed/Travel speed of pallet boxes: 1.1-1.3 m/min
Thickness of hearth layer: approx. 40 mm
BTP (Burn through temp) - 350 +/- 75 °C
Fuel rate - 65 kg/ton of sinter charge
Raw mix moisture - 7-8%
Quick lime addition - 40-60 kg/ton of sinter charge

A still further aspect of the present invention is directed to said process wherein the steel making slag is used in the hearth layer to provide good insulation and as well as producing sinter with improved yield and iron content beyond that present in the sinter raw mix.

Yet another aspect of the present invention is directed to a hearth layer for use as good insulation in sintering in a sinter plant comprising mix of the sinter and the steel making slag.

A further aspect of the present invention is directed to said hearth layer which comprises of steel making slag upto 15% by wt. preferably a mix of 85 to 90% preferably about 90% sinter and 10 to 15% preferably about 10% steel making slag. Addition of higher percentage of steel making slag may be used but is limited by total phosphorus input to the blast furnace.

A still further aspect of the present invention is directed to said hearth layer wherein said raw sinter mix charge comprises:

Iron ore fines: 46 to 53% by wt.; Coke breeze 5.0 to 5.5% by wt.; Solid waste 8.0 to 16% by wt.; BF (sinter) return fines 20 to 30% by wt.; Quick lime 3.5 to 5.0% by wt.; Limestone 2.0 to 3.5% by wt.; Dolomite 5.0 to 7.0% by wt.

The objects and advantages of the present invention are described in greater details with reference to the following accompanying non-limiting illustrative drawings and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: is the schematic illustration of the conventional system for sinter making process using only sinter of 10-20 mm size fraction as hearth layer for insulation.

Figure 2: is the schematic illustration of the system for sinter making process according to the present invention using steel making slag of 12-20mm size fraction in desired proportion along with 10-20 mm size sinter as hearth layer for insulation.

Figure 3: is the schematic illustration of the detailed view of steel slag charging system marked "A" in Figure 2, showing the various components for providing steel making slag along with sinter as hearth layer for insulation in sinter plant according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to a cost effective and improved process for sinter production using steel making slag along with sinter product as hearth layer in sinter machine for insulation whereby sinter output is increased and at the same time valuable iron content of steel slag is recovered while also resolving slag accumulation and disposal related issues in a large steel plant.

As already described herein before, a hearth layer is provided as the bottom most layer over sinter machine bed as insulating layer to protect the grate bars of the sinter pallets during the sintering process. The sinter mix charge is fed onto the hearth layer. Conventionally, the hearth layer is composed of good quality sinter product of 10-20mm size fraction to ensure desired permeability and strength. Thus a significant part of the sinter produced is recycled back as input for hearth layer in sinter charge causing reduction in effective sinter output.

On the other hand steel making slag, which is considered a waste, has better insulation property and valuable inherent iron content. The present invention thus attempts to make use of steel slag in combination with sinter product to form hearth layer in sinter plant favouring two fold benefit: on one hand recycling of slag mitigating disposal problem and on the other hand increasing sinter product output as well as recovering iron content of slag. Minimum change in the charging system is required to implement this combination of hearth layer feeding process in sinter plant.

The above and other objects and advantages of the present invention are described hereinafter with reference to the accompanying drawings:

Reference is first invited to the accompanying Figure 1 which schematically illustrate the conventional feeding system of sinter product recycling to the hearth for sinter making process using only sinter of 10-20 mm size fraction as hearth layer for insulation.

It is clearly apparent from Figure 1 that the Sintering process begins with the preparation of raw mix from iron ores, fluxes (such as limestone, dolomite), solid waste, mill scale and sinter return fines (from Blast Furnace). These materials are mixed and granulated. Water is added in order to assist the raw mix in obtaining optimum permeability. On the sinter machine, its surface is ignited, air being induced through the ignited layer and sintering proceeds in the vertical direction in the sinter strand's material bed. Subsequently, the sinter is cooled, crushed and screened to required particle size. Undersized sinter that is not suitable for the blast furnace (<5mm) is recycled to the return fines bin. A certain quantity of 10-20mm is screened out and re-circulated to the sinter machine, which serves as a hearth layer, protecting the grate bars of the pallets during the sintering process. Such recycling of good quality sinter for hearth layer formation reduced the sinter productivity by about 1.75%.

To avoid this deficiency of the existing sinter making process using recycled good quality sinter as hearth layer of sinter charge, a modified system and method is proposed by way of the present invention wherein steel making slag is mixed in the desired proportion to build the hearth layer at the bottom of sinter charge whereby sinter output is increased and valuable iron content of slag is recovered. Accompanying Figure 2 shows schematically the system for sinter production making use of steel making slag as feed to form hearth layer for insulation of pallet boxes in sinter plant as partial substitution of good quality sinter product according to the present invention. It is apparent from the accompanying Figure 2 that the system comprises all the basic components of Figure 1 and additionally a steel slag charging system marked as "A". It is not required to have any changes in the normal charging system except this additional steel slag feeding system. In the conventional method, only product sinter was being charged from the sinter bin, which subsequently forms the hearth layer (40 mm thickness) and in the modified method, up to 15% and preferably about 10% of steel melt slag is mixed with product sinter and stored in the respective bin. This mixture (90% sinter + 10% steel making slag) is charged onto the pallet box as per normal procedure.

Accompanying Figure 3 shows schematically the details of steel making slag charging system as marked "A" in Figure 2 involving a feed system consisting of a hopper (for storing steel making slag), a vibro feeder and skirt sealing system for charging the steel making slag onto sinter pallet. The vibro feeder serves to facilitate the ease of flow of material and the skirt sealing system serves to avoid material spillage and reduce dust emission.

It is to be noted that in sinter plants, normally hearth layer is made of 10-20mm fraction of sinter, as it provides better permeability when compared to sinter fines (<10mm). The slag generated from steel melt shop is crushed and screened to various size fractions for various applications. One such size fraction is 12-20 mm. As the permeability and strength of 12-20mm size fraction is far better than 10-20mm size, it is being used as substitute for good quality sinter for hearth layer. The crushing and screening facilities are equipped to conform to feed the above desired size fractions to the sinter machine bed as hearth.

The various steps involved and parameters used in the modified method of sinter production ensuring higher sinter output substituting good quality sinter product with steel slag of desired size fraction according to an embodiment of the present invention are illustrated with the help of the following Example I:

EXAMPLE I:

1. Providing steel making slag having sufficient strength with a tumbling index of >75% for use as hearth layer; steel making slag preferably with higher iron content, lower silica and alumina contents, but not essential, is selected as input material for hearth layer.

2. The normal range of composition of steel making slag used in the process comprise of:

CaO: 35 - 45%; MgO: 5 - 8%; SiO2: 8 - 15%; FeO: 15 - 40%; MnO: below 2%; Al2O3: below 4%; P2O5: 1 - 2.5%; Basicity: 2.5 - 4.5;

3. Crushing the steel making slag of above composition and properties obtained from the steel melting shop and sieving to different size fractions so as to select and use the 12-20mm fraction for desired permeability for hearth layer.

4. Preparing the sinter charge mix by blending the raw materials for sintering in the required proportion with the help of weigh feeders; homogenizing and nodulising the materials in primary and secondary mixing drums, where water is added at a controlled rate to maintain a moisture level of about 7-8%.

5. (i) Feeding a hearth layer comprising of (90% sinter of size 10-20mm + 10% steel making slag of size 12-20 mm) charged onto the sinter pallets up to a layer thickness of 40mm before raw mix is charged.

(ii) Feeding the prepared rax mix of sinter charge from a hopper onto the sinter pallet at the desired feeding rate on top of hearth layer.

6. Carrying out sintering process using the following parameters:
The ignition temperature: 1050±50°C.
The sintering temperature: 1300±100°C.
Draft rate (suction pressure): 9-11 kpa
Sinter machine speed/Travel speed of pallet boxes: 1.1-1.3 m/min
Thickness of hearth layer: approx. 40 mm
BTP (Burn through temp) - 350 +/- 75°C
Fuel rate - 65 kg/ton of sinter charge
Raw mix moisture - 7-8%
Quick lime addition - 40-60 kg/ton of sinter charge

7. The sinter so produced is cooled, crushed and screened to required particle size.
Undersized sinter (<5mm) that is not suitable for the blast furnace is recycled to the return fines bin. A certain quantity of 10-20mm is screened out and re- circulated to the sinter machine where it (along with steel making slag) serves as a hearth layer. Higher size fraction sinter (>5mm) is sent to blast furnace charge. 8. With the introduction of steel-making slag in hearth layer in preferred proportion of 90% sinter and 10% steel slag, the daily sinter production figure showed an increase in output to approximately 1.75% (equivalent to steel making slag charged). Following Table 1 presents a comparison (sample data) on the yield of sinter obtained from the conventional sinter feed and modified sinter feed.

Table 1:

as hearth making slag percentage charge (tons) layer as hearth layer
1 3934 3355 - 85.28
2 3534 - 3074 86.98
3 3809 3238 - 85.01
4 3508 - 3044 86.77
5 3634 3089 - 85.00
6 3801 - 3288 86.50

It is observed from above data that there is an increase in sinter yield in the range of 1.5-1.8% with modified method involving introduction of steel making slag in hearth layer partially substituting sinter, as compared to the conventional method having only sinter charge as hearth layer.

It is thus possible by way of the present invention to providing a method of sinter production using steel making slag of selective size fraction along with good quality sinter in desired proportion as hearth layer for insulation to protect pallet boxes in sinter plant whereby it is possible to gainfully recycle steel slag mitigating disposal problem, recover valuable iron content in slag, increase sinter output by about 1.75% while also ensuring longer pallet life.

We Claim:

1. A process for sintering in a sinter plant comprising:

preparation of raw sinter mix;

preparation of the hearth layer comprising a mix of sinter and steel making slag; charging of the said hearth layer comprising a mix of sinter and steel making slag onto sinter pallets followed by charging of the said raw sinter mix;

subjecting the charge as above to heating thereby produce a semi-molten mass, which in turn solidifies into porous pieces of sinter having the required size and strength suitable for feeding into blast furnace.

2. A process as claimed in claim 1 wherein said hearth layer comprises of steel making slag upto 15% by wt. preferably a mix of 85 to 90% preferably about 90% sinter and 10 to 15% preferably about 10 % steel making slag.

3. A process as claimed in anyone of claims 1 or 2 wherein said hearth layer comprises of 12- 20 mm of said mix of sinter and the steel making slag.

4. A process as claimed in anyone of claims 1 to 3 wherein said raw sinter mix charge comprises:

Iron ore fines: 46 to 53% by wt.; Coke breeze 5.0 to 5.5% by wt.; Solid waste 8.0 to 16% by wt.; BF (sinter) return fines 20 to 30% by wt.; Quick lime 3.5 to 5.0% by wt.; Limestone 2.0 to 3.5% by wt.; Dolomite 5.0 to 7.0% by wt.

5. A process as claimed in anyone of claims 1 to 4 wherein said steel making slag comprises steel making slag obtained from the steel melting shop which is crushed and sieved to different size fractions preferably in the range of 12-20 mm for use in the hearth layer.

6. A process as claimed in anyone of claims 1 to 5 wherein said preparation of raw sinter mix comprises blending the raw materials for sintering in required proportions using weigh feeders and homogenizing and nodulising in primary and secondary mixing drums with controlled addition of water such as to maintain a moisture level of about 7-8 %.

7. A process as claimed in anyone of claims 1 to 6 wherein the raw mix is charged from a hopper onto the sinter pallet and the feeding rate depends on the capacity of sinter plant.

8. A process as claimed in anyone of claims 1 to 7 wherein the undersize sinter, which is not suitable for blast furnace is recycled in the process.

9. A process as claimed in anyone of claims 1 to 8 wherein a select quantity of 10-20 mm sinter produced is screened and re-circulated for mixing with the steel making slag for providing the hearth layer.

10. A process as claimed in anyone of claims 1 to 9 comprising preferred operating parameters including:

The ignition temperature: 1050±50°C.
The sintering temperature: 1300±100°C.
Draft rate (suction pressure): 9-11 kpa
Sinter machine speed/Travel speed of pallet boxes: 1.1-1.3 m/min
Thickness of hearth layer: approx. 40 mm
BTP (Burn through temp) - 350 +/- 75 °C
Fuel rate - 65 kg/ton of sinter charge
Raw mix moisture - 7-8%
Quick lime addition - 40-60 kg/ton of sinter charge

11. A process as claimed in anyone of claims 1 to 10 wherein the steel making slag is provided in the hearth layer to act as a good insulation and as well as producing sinter with improved yield and iron content beyond that present in the sinter raw mix.

12. A hearth layer for use as good insulation in sintering in a sinter plant comprising mix of the sinter and the steel making slag.

13. A hearth layer as claimed in claim 12 comprises of steel making slag upto 15% by wt. preferably a mix of 85 to 90% preferably about 90% sinter and 10 to 15% preferably about 10 % steel making slag.

14. A hearth layer as claimed in anyone of claims 12 or 13 wherein said raw sinter mix charge comprises:

Iron ore fines: 46 to 53% by wt.; Coke breeze 5.0 to 5.5% by wt.; Solid waste 8.0 to 16% by wt.; BF (sinter) return fines 20 to 30% by wt.; Quick lime 3.5 to 5.0% by wt.; Limestone 2.0 to 3.5% by wt.; Dolomite 5.0 to 7.0% by wt.

15. A process for sintering in a sinter plant and a hearth layer substantially as herein described and illustrated with reference to the accompanying examples and figures.