Claims:We Claim:
1. A process for sintering involving improved productivity comprising:
following primary mixing limited to below 3mm fines carrying out step of secondary mixing and nodulizing during last stage of wet mixing/granulation involving return fines of size 3-5 mm to improve sinter productivity.

2. A process as claimed in claim 1 wherein said combination of size fractions involved for primary mixing and secondary mixing and nodulizing (wet mixing) is carried out to increase the void of sinter bed and to increase the size of the granules for improved productivity.

3. A process as claimed in anyone of claims 1 or 2 wherein the void of sinter bed is increased leading to air filtration velocity through sinter bed in the level of 8.22 to 9.87 m/s and the size of the granules increased from 2 to 15 % for desired improved productivity in the level of 0.5 to 18.4 %.

4. A process as claimed in anyone of claims 1 to 3 wherein the size fraction of the return fines below 5 mm are screened into two fractions i.e. below 3 mm and above 3 mm (3-5 mm) where below 3 mm are added using conventional route whereas return fines above 3 mm (3-5mm) are separated and added in secondary mixing and nodulizing drum.

5. A process as claimed in anyone of claims 1 to 4 wherein total amount of return fines used in the sinter mix ingredients is in the range of 15-70% by wt. preferably 35% by wt. and out of total return fines, 5-70% by wt. preferably 35% by wt. of it are separated and added in a secondary mixing and nodulizing drum in the last stage of granulation/wet mixing for a duration of 5-90 seconds preferably 30 seconds to get mixed with the remaining sinter mix ingredients which are already added in the primary mixing and nodulizing drum for initially dry mixing and subsequently wet mixing/granulation.

6. A process as claimed in anyone of claims 1 to 5 wherein sinter mix ingredients comprising sinter grade iron ore (< 10 mm), limestone (< 5mm), dolomite (< 5 mm), calcined lime (< 1mm) , return fines (< 5 mm) and coal/coke breeze (< 5 mm).

7. A process as claimed in anyone of claims 1 to 6 wherein adding of return fines (3-5 mm) in last stage of granulation/wet mixing enables increased air filtration velocity inside sinter bed.

8. A process as claimed in anyone of claims 1 to 7 wherein the moisture content of the sinter mix obtained after mixing with sinter return fines with remaining sinter ingredients is controlled in the range of 6 to 10% by wt. preferably 7.5% by wt.

9. A process as claimed in anyone of claims 1 to 8 wherein dry form of returns are fines added in the last stage of granulation/ wet mixing to get mixed with the remaining sinter ingredients.

10. A process as claimed in anyone of claims 1 to 9 wherein the return fines is a sinter product obtained from iron ore sintering process which are characterized as a dry fused brittle materials of size fraction below 5 mm generated internally within the sinter plant as well as during transportation to blast furnace.

11. A process as claimed in anyone of claims 1 to 10 wherein by increasing the fraction of return fines in the last stage of granulation/wet mixing from 5-70% by wt. preferably 35% by wt. of total return fines, the sinter productivity has increased from 0.5 to 18.4 % preferably 9.18%.

12. A process as claimed in anyone of claims 1 to 11 wherein the total sinter mix ingredients used for sinter making consists of return fines 15-70% by wt. preferably 35%, limestone 5-12% by wt. preferably 6%, dolomite 5-12% by wt. preferably 6%, calcined lime 1-12% by wt. preferably 2%, coal/coke 2-9% by wt. preferably 4% and rest balance iron ore as sinter grade fines.

Dated this the 28th day of August, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION
The present invention relates to a method of sintering of iron ore comprising selective mixing and granulation involving sinter return fines in desired size range to improve sinter quality and productivity. This method uses return fines, which comprises of iron ore sinter of size fractions below 5 mm generated within sinter plant (known as Sinter Return Fines, SRF) and fines generated during handling and transportation to blast furnace (known as Blast Furnace Return Fines, BFRF) to improve the sinter bed permeability and consequently enhancing sinter productivity and quality. In the present invention, the return fines (SRF & BFRF) are screened into two different size fractions, return fines whose size fractions below 3 mm are added along with the remaining sinter mix ingredients is added in the primary mixing and nodulizing drum whereas return fines whose size fraction in the range of 3-5 mm are separated from the total return fines (wt.%) and added in secondary mixing and nodulizing drum during the last stage of wet mixing/granulation for desired duration to get mixed with the remaining sinter mix ingredients obtained after primary mixing and nodulizing drum and subsequently subjecting the whole blend to sintering process. More particularly, the present invention is directed to provide a novel process to improve sinter productivity by selective mixing the sinter green mix with return fine of size fraction 3-5 mm to improve the sinter bed permeability.

BACKGROUND OF THE INVENTION
In conventional Dwight-Lloyd iron ore sintering process, the iron ore whose size fraction below 8 mm is agglomerated via incipient fusion in presence of other raw material constituents like limestone, dolomite, calcined lime, return fines, solid fuel (coke-breeze/coal) and steel plant wastes. The heat energy for sintering is given to the top layer of the sinter bed by gas firing. This top layer heat helps in the combustion of the solid fuel distributed in the raw mix and subsequently temperature of the sinter bed rises to 1300°C and above resulting in the formation of slag phase which is a complex compound of gangue and flux which binds the iron ore particles to form strong agglomerates called iron ore sinter. This sinter cake is broken and screened to the required size possessing optimum cold and high strength properties is send to the blast furnace for iron making. Sinter is being used as major (60-70%) burden material for blast furnace iron making which puts working pressure onthe sinter plant to produce sinter of consistent quality with high productivity. In order to improve sinter productivity, various studies/investigations have been carried out as discussed below:

Chinese patent CN105200229A entitled “Mixing and granulating method for improving sintering” focused on improving the sinter bed permeability using following methods:
• Screening the sintered return fines into three different size fractions to obtained sizes of 0-1 mm, 1-3 mm and 3-5 mm respectively.
• All these size fractions of sintered return fines to be pre wetted with water.
• Then 0-1 mm size fraction of pre wetted sintered return fines to be directly added to the primary mixing machine to perform stirring and mixing.
• Remaining pre wetted sintered return fines size fraction (1-3mm & 3-5 mm) to be added to the secondary cylindrical mixing machine to perform pelletizing.

An advantage of this method is that it will result in the increase of production efficiency of secondary mixing machine and control over grain size of materials. However, this patent requires several modifications to achieve the above objectives like three sieve sizes, additional pre wetting system to wet the sintered return fines and arrangements to be made to charge these sintered return fines to primary and secondary mixing machine respectively.

But in the proposed invention, these conditions are overcome by using one sieve size, no pre wetting of sintered return fines are required and no separate system for addition of sintered return fines in the primary mixing machine.

Paper published in ISIJ International, Vol. 53 (2013), No. 1, pp. 34–40 titled “Improvement of Sinter Productivity by Adding Return Fine on Raw Materials after Granulation Stage” proposed method to improve sinter productivity as described below:
• Sinter raw materials including return fine(X mass% to sinter mixture) are mixed and granulated together with water in mixer. Return fine (X mass %) from bypass return fine bin is added after the mixer to form final mix. In this paper, return fine added after granulation stage is named as “bypass return fine”.
• It uses return fines as a dry particle which is added on granulated raw materials to improve sinter bed permeability.
• In experiments, proportion of the bypass return fine added after granulation stage was varied in 5% increments from 0% to 15%.
• Size fraction of the bypass return fine added in the ratio defined as the ratio of return fine (+1mm) to return fine (-1 mm).
• Experiments performed under above conditions showed that with the increasing ratio of bypass return fine, productivity increased.

Chinese patent CN103757202A entitled“ Sintering method with part of returns fine being sintered without pelletization being pelletized” aim to improve sintering air bed permeability using following methods:
• The sinter raw material ingredients (mass percentage) comprises:40-80% iron ore powder, 5-50% return fines, 0-10% lime, 0-10% dolomite,0-10% limestone,3-8% coke powder.
• Prior to mixing step, 3-50% by wt. of return fines removed for subsequent processes use.
• Mixing of remaining sintering raw materials into drum mixer, water is added to wet the sintering raw materials.
• Wetted sinter raw materials conveyed to secondary drum mixer for granulation. Then the remaining return fines are added to the granulation to obtain the final sintered material.

Paper published in J. Min. Metall. Sect. B-Metall. 47 (2) B (2011) 113 – 123 titled “Prediction of size distribution of iron ore granules and permeability of its bed” predicted that relation between the size distribution of the granules and permeability of sinter bed. In the paper, size of the particles between 0.7-3 mm are suitable for granulation process and improve the permeability of the bed. Therefore, particles below 3 mm can be utilized to form granules which can improve the permeability of the sinter bed. In this regard, proposed invention will utilize the return fine of size fraction below 3 mm in the early part of the granulation to get mixed and acts as nuclei for granules formation to improve the permeability of the sinter bed.

Therefore, it is evident from the above prior art that return fines are treated in a different fashion to improve sinter productivity. As in patent CN105200229A, different size fractions of sinter return fines are wetted with water and distributed between primary mixing machine and secondary cylindrical mixing machine. Again in patent CN103757202A, sinter return fines are added after secondary mixing machine without participating in the steps of material mixing.

In the proposed invention, the limitations of the above patents and papers have been overcome through various modification and methodology adopted. In particular, size fractions of the return fines have been dealt in-depth. The mixing time of the separated return fine are also discussed in this invention. Thus with new invention, the sinter bed permeability will be improved and consequently sinter productivity.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to a process for improving sinter productivity by adding sinter return fines of 3-5 mm in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation, wherein the return fines added varies from 0-70 wt % of the total return fines (preferably 35 wt %).

A further object of the present invention is directed to process for improving the sinter productivity by selective mixing of return fines of size fraction 3-5 mm in the last stage of wet mixing/granulation for duration of 0-90 seconds (preferably 30 seconds) along with the sinter mix ingredients, viz., iron ores (<10 mm), limestone (<5 mm), dolomite (<5 mm), solid fuel (coal/coke breeze, < 5mm), calcined lime (<1 mm) and remaining balance of return fines (<3 mm).

A still further object of the present invention is directed to a process to expedite the sintering process by increasing the permeability of the sinter bed and thereby allowing better heat transfer and slag formation leading to increase in sinter quality and productivity.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a process for sintering involving improved productivity comprising:
following primary mixing limited to below 3mm fines carrying out step of secondary mixing and nodulizing during last stage of wet mixing/granulation involving return fines of size 3-5 mm to improve sinter productivity.
A further aspect of the present invention is directed to a process wherein said combination of size fractions involved for primary mixing and secondary mixing and nodulizing (wet mixing) is carried out to increase the void of sinter bed and to increase the size of the granules for improved productivity.
A still further aspect of the present invention is directed to said process wherein the void of sinter bed is increased leading to air filtration velocity through sinter bed in the level of 8.22 to 9.87m/s and the size of the granules increased from 2 to 15% for desired improved productivity in the level of 0.5 to 18.37 %.
A still further aspect of the present invention is directed to said process wherein the size fraction of the return fines below 5 mm are screened into two fractions i.e. below 3 mm and above 3 mm (3-5 mm) where below 3 mm are added using conventional route whereas return fines above 3 mm (3-5mm) are separated and added in secondary mixing and nodulizing drum.
A still further aspect of the present invention is directed to said process wherein total amount of return fines used in the sinter mix ingredients is in the range of 15-70% by wt. preferably 35% by wt. and out of total return fines, 5-70% by wt. preferably 35% by wt. of it are separated and added in a secondary mixing and nodulizing drum in the last stage of granulation/wet mixing for a duration of 5-90 seconds preferably 30 seconds to get mixed with the remaining sinter mix ingredients which are already added in the primary mixing and nodulizing drum for initially dry mixing and subsequently wet mixing/granulation.
Another aspect of the present invention is directed to said process wherein sinter mix ingredients comprising sinter grade iron ore (< 10 mm), limestone (< 5mm), dolomite (< 5 mm), calcined lime (< 1mm) , return fines (< 5 mm) and coal/coke breeze (< 5 mm).
Yet another aspect of the present invention is directed to said process wherein adding of return fines (3-5 mm) in last stage of granulation/wet mixing enables increased air filtration velocity inside sinter bed.
A further aspect of the present invention is directed to said process wherein the moisture content of the sinter mix obtained after mixing with sinter return fines with remaining sinter ingredients is controlled in the range of 6 to 10% by wt. preferably 7.5% by wt.
A still further aspect of the present invention is directed to said process wherein dry form of returns are fines added in the last stage of granulation/ wet mixing to get mixed with the remaining sinter ingredients.
A still further aspect of the present invention is directed to said process wherein the return fines is a sinter product obtained from iron ore sintering process which are characterized as a dry fused brittle materials of size fraction below 5 mm generated internally within the sinter plant as well as during transportation to blast furnace.
A still further aspect of the present invention is directed to said process wherein by increasing the fraction of return fines in the last stage of granulation/wet mixing from 0-70% by wt. preferably 35% by wt. of total return fines, the sinter productivity has increased from 0 to 18.4 % preferably 9.18%.
A still further aspect of the present invention is directed to said process wherein the total sinter mix ingredients used for sinter making consists of return fines 15-70% by wt. preferably 35%, limestone 5-12% by wt. preferably 6%, dolomite 5-12% by wt. preferably 6%, calcined lime 1-12% by wt. preferably 2%, coal/coke 2-9% by wt. preferably 4% and rest balance iron ore as sinter grade fines.
The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Schematic diagram to illustrate the conventional sintering process.

Figure 2: schematic process flow diagram for the modified sintering process according to present invention involving return fines bin (as marked with reference No.6) for the addition of return fines in the last stage of secondary mixing and nodulizing drum to get mixed with remaining sinter ingredients.

Figure 3: shows graphically the effect of return fines addition (3-5 mm) at 0, 5,25, 35, 50 & 70% of total return finesin secondary mixing and nodulizing drum using return fines bin during last stage of wet mixing/granulation on sinter productivity under present invention as illustrated in Example 1.

Figure 4: Effect of return fines addition (3-5mm) at 0,5, 25, 35, 50 & 70% of total return finesin secondary mixing and nodulizing drum using return fines bin during last stage of wet mixing/ granulation on sinter tumbler index under present invention as illustrated in Example1.

Figure 5: Effect of return fines addition (3-5 mm) at 0, 5,25, 35, 50 & 70% of total return finesin secondary mixing and nodulizing drum using return fines bin during last stage of wet mixing/granulation on air filtration velocity under present invention as illustrated in Example1.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to a process for improving sinter productivity by adding return fines of 3-5 mm in the secondary (wet) mixing and nodulizing drum for duration of 0-90 seconds preferably 30 seconds to get mixed with sinter raw mix. The raw sinter-mix used for sinter making consists of return fines 15-70% preferably 35%, limestone 5-12% preferably 6%, dolomite 5-12% preferably 6%, calcined lime 1-12% preferably 2%, coal/coke 2-9% preferably 4% and balance iron ore as sinter grade fines. The size of the individual components of sinter mix ingredients used for sinter making are return fines (<5 mm), iron ore sinter grade size fraction (<10 mm), limestone (<5 mm), dolomite (<5 mm), solid fuel (coal/coke breeze, < 5mm), and calcined lime fines (<1 mm).

The return fines of below 3 mm size are added in the primary mixing and nodulizing drum along with other sinter mix ingredients includes limestone, dolomite, calcined lime, coke/coal and rest sinter grade iron ore to get mixed and added to secondary mixing and nodulizing drum to form granulated sinter mix. Return fines of 3-5 mm (0-70%, preferably 35% of total return fine) is added in the secondary mixing and nodulizing drum to get mixed with the granulated sinter mix for duration of 90 seconds preferably 30 seconds. The moisture content of the final sinter blend discharged from the secondary mixing and nodulizing drum was controlled in the range of 5 to 10% preferably 7.5%.

The blending of raw mix is carried out using mixing and nodulizing drum with blending conditions maintained for dry mixing is 3-10 minutes preferably 5 minutes in the primary mixing and nodulizing drum and wet mixing/ granulation is 3-10 minutes preferably 5 minutes in the secondary mixing and nodulizing drum. Further, the ignition holding time is maintained from 45 to 180 seconds preferably 90 seconds and negative suction pressure maintained after ignition from 600 to 1700 mm Water Column preferably 1200 mm water column. The addition of return fines enables controlling sinter bed permeability and overall distribution of heat and combustion zone movement in the sinter bed avoiding resistance of air flow inside bed.

The present invention relates to a process for sintering of raw mix prepared by selective mixing of return fines with raw mix to improve sinter productivity and sinter quality as compared to conventional sintering process.

Under conventional sintering, iron ore fines (< 10 mm) along with the other sinter mix ingredients (limestone, dolomite, calcined lime, sinter return fines, coke breeze and other metallurgical wastes) are mixed in two steps: dry mixing and wet mixing. In dry mixing, all the sinter mix ingredients are homogenously mixed in the primary mixing and nodulizing drum and in wet mixing; water is added to the secondary mixing and nodulizing drum which facilitates the nuclei formation which is required for the optimum granules size. These granules are discharged from the secondary mixing and nodulizing drum and are charged into the sinter bed machine for sinter making. The schematic flow process chart as shown in accompanying Figure 1 pictorially represent and explains the conventional sintering process of iron ore, wherein following components have been shown:
1. Raw materials Bin (Iron ore, return fines, coal/coke breeze, limestone, dolomite, calcined lime and metallurgical wastes)
2. Primary Mixing and Nodulizing Drum
3. Water Spraying Nozzle
4. Water Spraying Nozzle
5. Secondary Mixing and Nodulizing Drum
6. Sintering Machine

In the present invention, return fines of size fraction below 5 mm are screened to get the size fraction between 3-5 mm which is separated from the total return fine, whose amount varies from 5-70% by wt. of total return fine preferably 35% by wt. or 1-25% by wt. of the total mix preferably 11.9% by wt and are added in the secondary mixing and nodulizing drum using return fine bin to get mixed with sinter mix ingredients in the final stage of wet mixing/granulation to form granules. The schematic of the new process is shown in accompanying Figure2, wherein following components have been shown:
1. Raw materials Bin (Iron ore, return fines, coal/coke breeze, limestone, dolomite, calcined lime and metallurgical wastes)
2. Primary Mixing and Nodulizing Drum
3. Water Spraying Nozzle
4. Water Spraying Nozzle
5. Secondary Mixing and Nodulizing Drum
6. Return Fines bin
7. Sintering Machine

The present invention thus targets to utilize return fines of size fraction 3-5mm to improve sinter bed permeability and consequently sinter productivity. The present invention consists of two major steps for the green mix preparation:

Step1: The raw material fractions which include limestone, dolomite, calcined lime sinter grade iron ore, return fines of size fraction below 3 mm and coke/coal are mixed in a primary mixing and nodulizing drum initially for dry mixing and subsequently added in secondary mixing and nodulizing drum for wet mixing by the addition of water to form granules.

Step2: The balance proportions of return fines of size fraction 3-5 mm are added in the secondary mixing and nodulizing drum in the last stage of wet mixing/granulation.

The details of the methodology in the implementation of the process according to present invention and the respective parameters used are as follows:
Methodology:

• Sinter mix used under present invention includes iron ore, return fine, limestone, dolomite, calcined lime and coke/coal.
• Approximately 90 kg of sinter mix is prepared using primary and secondary mixing and nodulizing drum.
• Sinter mix added in primary mixing and nodulizing drum includes iron ore, limestone, dolomite, calcined lime, coal/coke & return fine (< 3mm).
• Return fine (3-5 mm) are added in secondary mixing and nodulizing drum.
• Sequence of blending condition
? Dry mixing in primary mixing and nodulizing drum @ 20 rpm for 300 seconds to homogenize the sinter mix.
? Wet mixing in secondary mixing and nodulizing drum @ 20 rpm for 300 seconds to agglomerate the sinter mix in presence of moisture.
? Return fine (3-5mm) addition in the last stage of secondary mixing and nodulizing drum @ 5-20 rpm for 30 seconds to get distributed on sinter mix.
• Sintered the material in pot grate with ignition conditions as under:
? Ignition holding time 90 seconds.
? Suction pressure after ignition (-) 1200 mm H2O.

The resulting properties of the sinter product produced according to present invention and improvement in productivity and yield were established through the following experimental trials:
Example 1:

Under this example, effect of adding return fine of size fraction 3-5 mm which varies from 0, 5, 25, 35, 50 & 70% by wt. of total return fines in a secondary mixing and nodulizing drum for duration of 30 seconds in the last stage of wet mixing/ granulation are examined and compared with normal mixing are studied.

Five experiments were performed under pilot scale pot sinter to understand the impact of adding return fines in secondary mixing and nodulizing drum on sinter productivity and quality. In this experiment, the size fractions of return fines (<5 mm) are screened with 3 mm sieve size. The return fines whose size fraction above 3 mm are separated and added in the secondary mixing and nodulizing drum according to the increased proportions of 0,5,25,35,50&70% by wt. of total return fines as discussed in above invention whereas return fines whose size fraction below 3 mm are charged thru conventional route i.e. primary mixing and nodulizing drum. All other operational parameters are remained constant throughout the experiment. The chemical analysis and raw materials size fractions used for experimentation are given in Table1 & Table2. Table3 shows raw materials proportion used for experimentation under present invention. Experiment No.1 is conventional sinter making in which all sinter mix ingredients are mixed in a primary mixing and nodulizing drum for dry mixing and wet mixing/granulation in secondary mixing and nodulizing drum.

Table 1: Chemical analysis (wt. %) used under present invention

Blend Fe(T) FeO SiO2 Al2O3 CaO MgO LOI
Iron Ore-1 57.58 -- 9.03 4.18 0.22 0.11 3.44
Iron Ore-2 62.01 -- 2.81 3.67 0.05 0.06 4.03
Return fines 53.59 6.15 6.15 3.25 11.95 2.12 --
Limestone 0.89 -- 2.05 0.47 50.49 2.73 42.59
Dolomite 0.79 -- 5.22 0.67 29.05 19.49 43.97
Calcined Lime 0.31 -- 0.81 0.28 93.94 1.49 3.05
Ash VM S Fixed Carbon
Coke breeze/Coal 12.6 3.18 0.42 84.23

Table 2: Size fraction of raw materials (wt. %) used under present invention

Size fraction -10+8 mm - 8 +6.3 mm -6.3+ 5 mm -5 +3.15 mm -3.15 + 1 mm -1 +0.5 mm -0.5+0.15 mm -0.15 mm
Iron Ore-1 11.69 7.79 4.76 11.69 17.75 6.49 9.96 29.87
Iron Ore-2 12.36 7.81 5.21 12.15 15.83 6.58 9.83 30.23

-5 +3 mm -3 +2 mm -2 +1mm -1+0.5 mm -0.5 mm
Return fines 58.86 1.53 5.00 19.58 15.03

-5 +3.15 mm -3.15 +1 mm -1
mm
Limestone 0 51.04 42.71
Dolomite 0 50.13 42.61

-5 +3.15 mm -3.15 +1 mm -1+0.5 mm -0.5 mm
Coke breeze
/Coal 4.94 31.99 23.86 39.21

-1 mm
Calcined Lime 100

Table 3: Raw materials proportions (wt. %) used under present invention
Experiment No. 1 2 3 4 5 6
Return Fines addition in secondary MND as % of total return Fines 0 5 25 35 50 70
Iron ore-1 14.19 14.19 14.19 14.19 14.19 14.19
Iron ore-2 33.11 33.11 33.11 33.11 33.11 33.11
Total return fines 34 34 34 34 34 34
Return Fines addition in Primary MND 34 32.3 25.5 22.1 17 10.2
Return Fines addition in secondary MND 0 1.7 8.5 11.9 17 23.8
Limestone 7.1 7.1 7.1 7.1 7.1 7.1
Dolomite 5.4 5.4 5.4 5.4 5.4 5.4
Calcined Lime 2 2 2 2 2 2
Coal/Coke 4.2 4.2 4.2 4.2 4.2 4.2
Total 100 100 100 100 100 100

*MND: Mixing and Nodulizing Drum
Expt.1: Sintering of raw mix under conventional route.
Expt. 2: Sintering of raw mix by adding return fines (3-5mm) at5% by wt. of total return fines in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation for duration of 30 seconds.
Expt.3: Sintering of raw mix by adding return fines (3-5mm) at25% by wt. of total return fines in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation for duration of 30 seconds.
Expt.4: Sintering of raw mix by adding return fines (3-5mm) at 35% by wt. of total return fines in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation for duration of 30 seconds.
Expt.5: Sintering of raw mix by adding return fines (3-5mm) at 50% by wt. of total return fines in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation for duration of 30 seconds.
Expt.6: Sintering of raw mix by adding return fines (3-5mm) at 70% by wt. of total return fines in the secondary mixing and nodulizing drum during last stage of wet mixing/granulation for duration of 30 seconds.

The improvement in sinter quality and productivity with addition of increased proportions of return fines (3-5mm) in secondary mixing and nodulizing drum which varied from 5 to 70% by wt. of total return fines are shown below in Table 4. It is clear from table 4, that adding return fines of size fraction 3-5 mm in secondary mixing and nodulizing drum during last stage of wet mixing/granulation improves sinter bed permeability. It is due to the increase in the void of sinter bed, as the mixing of 2-3 mm size of green mix granules with the return fines of size 3-5 mm leading to the overall increase in the size of the granules which is finally discharged from the secondary mixing and nodulizing drum. This increased granules size improves the sinter bed permeability which is also evident from the results of air filtration velocity as shown in Figure 5. As the proportions of return fines of size fraction 3-5 mm are increased in the secondary mixing and nodulizing drum, the velocity of air passing thru the sinter bed also increases like in conventional sinter making (Expt1) the velocity of air is 8.22 m/s and as the fractions of return fines (3-5 mm) increased to 70% by wt. of total return fines (Expt 6), air velocity passing thru sinter bed increased to 9.87 m/s. This increase in passage of air thru sinter bed increases sinter productivity as shown in Figure 3. However, the sinter quality (Tumbler Index) also increases with the usage of return fines but attained maximum value during usage of return fines (3-5mm) at 35% total return fines as shown in Figure 4. After that it decreases, which can be explained from the results of air filtration velocity as the air velocity increases beyond 8.50 m/s, residence time of heat distribution in sinter bed starts decreases therefore slag formation and melt formation getting affected thus the tumbler index also decreasing.

Table 4: Pot sinter test Results
Experiment No. 1 2 3 4 5 6
Sinter Productivity (+ 5 mm, t/m2/h) 1.96 1.97 2.03 2.14 2.21 2.32
Sinter Tumbler Index (+6.3mm, %) 65.8 65.82 66 67.5 66.2 65.95
Air Filtration Velocity (m/s) 8.22 8.23 8.27 8.5 9.1 9.87
Sinter Product Yield (+ 5 mm, %) 77.95 77.97 78.7 79.4 78.15 78.73
Sinter Fines generation (-5 mm, %) 22.05 22.0 21.3 20.6 21.85 21.27

It is thus possible by way of the present invention to provide a process for iron ore sintering by selectively mixing sinter return fines of selective size range partly in primary dry mixing stage and partly during secondary wet mixing and nodulizing stage and subjecting thus obtained sinter mix to sintering with selected ignition condition in sintering furnace to improve permeability and improved sinter quality and productivity.