DESC:FIELD OF THE INVENTION

The present invention relates to a process for sintering of iron ore micro fines (below 150 micron particles) through micropelletization so that they can be used for commercial sintering process. More particularly, the present invention deals with the iron ore micro fines utilization in sinter making and to achieve better sinter productivity and quality by micro-pelletizing the fines in disc/drum pelletizer with optimum flux and fuel so as to achieve required strength wherein flux and fuel provide sufficient bonding and heat requirement during sintering. The sufficient green strength properties of micro pellets ensure minimum fines generation during handling, transportation and mixing with other green mix components which otherwise hampers the bed permeability, sinter productivity and quality. The carbonaceous fuel (< 1 mm size) is added at the final stage of granulation wherein the finely divided coal particles form a coating on the surface of the micro pellets/granules which enhance the uniform heat distribution throughout the bed.

BACKGROUND OF THE INVENTION
In conventional iron ore sintering (Dwight-Lloyd) process, the agglomeration of iron ore takes place via incipient fusion of iron ore particles in presence of other raw material constituents like limestone, dolomite, calcined lime, sinter return fines, solid fuel (coke breeze/coal) and steel plant wastes. The required heat energy for sintering is given from top layer gas firing followed by the combustion of solid fuel present in raw mix. During this process, the temperature at the sinter bed rises up to 1300°C resulting in the formation of molten slag in presence of flux whereby bonding of fines into lump particle takes place during solidification. Around 40-85 % of the sinter is used as burden material in most of the blast furnace of Asian countries. Extensive use of mechanized mining and rapid growing production of iron ores have resulted in generation of large amount of fines and microfines, which pose a challenge to sinter makers for direct use in conventional sintering process due to adverse impact on sinter bed permeability. The adverse impact of use of iron ore microfines on sintering performance and quality has been reported in the past.

US Patent 4,273,575 entitled “Process for transforming fines of iron or manganese into raw-material for sintering” reported that iron ore more than 40% of particles below 150 microns is detrimental to sintering leading to low yield due to obstruction to passage of gases necessary for coke combustion. They proposed preparation of a micro-pellet of < 6 mm in presence of lime and molasses which was cured in a closed environment filled with carbon di-oxide. Those hardened pellets can be transferred for sintering into the mix or drum. In brief process description, ore particles of size below 150 microns and humidity below 15% is mixed with agglomerates like lime, cement, bentonite, calcium chloride, silicate and fluorosilicate of sodium in proportions that vary in accordance with the characteristics of the ore in a conventional belt mixer and then mixed mixture are conveyed to pelletizer disc, cane molasses added to mixture through sprinkler in a varied proportion to form micro-pellets of diameter less than 6 mm, they are discharged to silos where they are hardened by subjecting them to controlled humidity and temperatures below 300º C, for a period of several days. These hardened spheres or micro-pellets are transported to a sintering plant where they formed into sinter and used in steel making furnaces.

US Patent 3,134,667 entitled “Pelletizing of Iron Ore for Sintering” proposed composite pelletization process where a clayey iron ore and iron ore concentrates were pelletized in a pelletizing drum along with carbonaceous fuel to form primary pellet, which encased within a hard surface coating comprised of a finely divided adhesive iron ore which is sprinkled onto the primary pellet while rotating in the drum. This surface coating prevents the disintegration of primary pellet through breaking away of the iron ore concentrates. The pellets formed comprised of three distinct layers: : an inside layer containing a coarse ore; a surrounding blended layer of iron ore concentrates and iron ore; and a surface coating of finely divided iron ore particles which form a tenacious outer shell.

European Patent Application EP 0 415 146 A1 entitled “Method for manufacturing agglomerates of sintered pellets” where iron ore containing 40 wt. % or more fine materials, flux, coarse coke and return fines are mixed along with 90% of the total water addition in a drum mixer for the stipulated time period. The mixture obtained from drum is again sent to disc pelletizer where balance 10% water is added for improved granulation/pelletization. The granulated mix is surface coated with the powdery coke into drum mixer. A method for coating the surfaces of the green pellets is disclosed in a Japanese Patent Publication Laid Open No. 149333/88.

There has been thus an unfulfilled need to developing a process for utilization of iron ore microfines in larger proportions in commercial sintering process to convert to pellets with sufficient green strength as pellet feed for sintering process while also ensuring desired sinter property and productivity.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to a process for sintering of iron ore microfines by effective utilization of iron ore microfines (below 150 micron) in sinter making with sustained sinter quality and plant productivity.

A further object of the present invention is directed to a process for sintering of iron ore microfines in order to convert a particle size of iron ore whose size fraction is below 150 micron to a pellet feed of size <6 mm and drop number > 7 using disc pelletizer, wherein ingredients used for disc pelletization are iron ore fines, calcined lime, limestone, dolomite, and coke breeze in varying proportions with water sprayed during pelletization for micro-pelletization.

A still further object of the present invention is directed to a process for sintering of iron ore microfines involving mixing homogeneously the balance sinter mix ingredients which includes sinter return fines (<5 mm), iron ore sinter grade size fraction (<8 mm), limestone, dolomite (<3 mm) and calcined lime fines (<1 mm).

A still further object of the invention is a process for sintering of iron ore microfines involving adding the micro pellets (<6 mm) formed from disc pelletizer to the mixed materials in the drum mixer to completely homogenize the pellet in the whole mixture.

A still further object of the present invention is directed to a process to add the remained proportions of coke breeze particles below 1 mm in the final stage of granulation for coating purpose.

A still further object of the present invention is directed to a process to expedite the sintering process though early carbon combustion and thereby better heat transfer and slag formation leading to increase in productivity.

A still further object of the present invention is directed to a process to effectively utilize the heat energy for the melt formation from the surface to the core of the micropellets/granules, which is achieved by incorporating the optimum flux and fuel into the micropellets which combust and react at later stage of melting which help in uniformly distributed slag.

A still further object of the present invention is directed to a process to provide an optimum flux and fuel addition which should facilitate effective heat utilization and melt formation with required melt viscosity.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a process for sintering involving iron ore micro fines < 150 microns in sintering comprising:
separately converting the iron ore micro fines < 150 microns into micropellets with fluxes and solid fuel;
thereafter including the said micro fines based micro pellets with green mix components and subjecting the thus obtained mix of said micro fines based micro pellets with green mix components to sintering.
A further aspect of the present invention is directed to a process wherein the iron ore micro fines < 150 microns used for generating said micro pellets is more than 25%.
A still further aspect of the present invention is directed to a process comprising binder or fluxes for pellet formation including calcined lime, slaked lime, limestone, bentonite and dolomite, carbonaceous materials selected from coal, coke breeze, charcoal as solid fuel.
Another aspect of the present invention is directed to said process wherein said pellet feed size are below 12 mm preferably below 6mm.
Yet another aspect of the present invention is directed to said process comprising adding solid fuel of less than 1 mm size in the final stage of granulation preferably in the drum mixer to coat the micropellets/granules surface.
A further aspect of the present invention is directed to said process wherein the iron ore micro fines (less than 150 microns) is used as major source of iron ore.
A still further aspect of the present invention is directed to said process wherein the above said micro pellets from the disc pelletizer is further processed in the drum mixer along with remaining green mix components.
A still further aspect of the present invention is directed to said process comprising the steps of:
i) separately converting said iron ore fines (< 150 microns) along with the finer fractions of lime, limestone, dolomite and coke breeze (< 1 mm each) to micro pellets in a disc pelletizer in the presence of water to generate micro pellets feed of size <6mm.

ii) thereafter mixing balance sintering raw materials fractions including return fines, limestone, dolomite, calcined lime and sinter grade iron ore (<8mm) in a drum mixer along with the said micro-pellets (< 6 mm) and granulating; and

iii) Adding finer size of the coke breeze (<1 mm) towards the final stage of granulation of step ii whereby the coke breeze coated mix favoured homogeneous distribution and participation of heat for the incipient fusion process of sinter making.

A still further aspect of the present invention is directed to said process as claimed in anyone of claims 1 to 8 wherein the moisture content of pellet was controlled in the range of 9 to 12% preferably 10.5% and drop number 7-10.

Another aspect of the present invention is directed to said process wherein selective inclusion of said iron ore fines based micro pellets enables controlling subsequent sintering stages with granulation efficiency based on sinter bed permeability and overall distribution of heat and combustion zone movement in the sinter bed avoiding resistance of air flow inside bed.

Yet another aspect of the present invention is directed to said process wherein the said micro fines as micro pellets and washed iron ore as sinter grade fines are mixed in the range of 40 to 90 % : 10 to 60 % preferably about 70% :30%.

A further aspect of the present invention is directed to said process wherein the blend composition used comprised micro-pellet: micro-fines: Washed iron ore of 40 to 90%: 0 to 10%: 10 to 60% preferably about 60:10:30.

A still further aspect of the present invention is directed to said process wherein the sinter mix included micro pellets up to 90 % by wt. of iron ore, balanced fractions of iron ore as sinter grade fines, return fines (depending on the in-plant generation of fines) preferably <40% by wt, limestone <10 % by wt., dolomite of <10 % by wt., calcined lime 1-5 % by wt. coal/coke > 3-7% by wt. of total blend.

A still further aspect of the present invention is directed to said process wherein the process of sintering involving said iron ore micro fines < 150 microns as micro pellets favour productivity and reduced time and yet achieve advanced sinter properties including Tumbler index (+6.3 mm) >65%, with significantly reduced generation of fines from 25 to 10 %.

A still further aspect of the present invention is directed to said process for sintering of iron ore microfines comprising selectively:
sinter mix blending is carried out using balling drum maintaining blending condition as mixing at 12 to 30 rpm preferably about 20 rpm for 120 to 600 seconds preferably about 300 seconds;

said sinter mix is agglomerated in presence of moisture using a drum mixer maintain agglomerating condition as mixing at 12 to 30 rpm preferably about 20 rpm for 180 to 720 seconds preferably about 480 seconds;

maintaining sinter mix ignition condition comprising:
Suction air flow volume before ignition: preferably about 2.0 m3/min,
Ignition holding time: 45 to 180 sec preferably about 60 sec,
Suction pressure after ignition: 700 to 1600 mm H2O preferably about -1200 mm H2O; and

said Disc pelletizer operating conditions comprising:
(a) Inclination Angle: 40 to 50 preferably about 44 Degree,
(b) Disc rotation: 15 to 30 preferably about 23 rpm,
(c) Moisture level: 9 to 12% preferably about 10.5%,
(d) Pelletizing duration: 5 to 20 min preferably about 10 min.

Yet another aspect of the present invention is directed to Sintered iron ore pellets obtained by the process as described above having superior sintering behavior, sinter quality and fines generation in comparison with the conventional process having 100% sinter grade fines as iron ore including: Tumbler index (+6.3 mm)> 65 %.

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

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

Figure 2: Schematic flow chart of the new sintering process according to present invention involving micro-pelletization of iron micro fines.

Figure 3: bar chart showing effect of fines on sintering process behavior and sinter quality under conventional sintering process.

Figure 4: bar chart showing effect of microfines addition through micropelletization on sintering behavior, sinter quality and fines generation - comparison with the conventional process having 100% sinter grade fines as iron ore.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention relates to a process for sintering of iron ore microfines (below 150 micron particles) through micropelletization so that they can be used for commercial sintering process, wherein better sinter productivity and quality is achieved by micro-pelletizing the iron ore micro fines in disc/drum pelletizer with optimum flux and fuel so as to achieve required strength wherein flux and fuel provide sufficient bonding and heat requirement during sintering.

Under conventional sintering, iron ore fines (< 8mm) are mixed in a drum mixer along with the other sinter mix ingredients (limestone, dolomite, calcined lime, sinter return fines, coke breeze and other metallurgical wastes). The micro fines in the iron ores used in sinter making are near about 24-25%. Two steps are followed for mixing the above mentioned raw materials: dry and wet mixing. During wet mixing water is added in to the drum which facilitates the nuclei formation for the optimum granules size. Those granules are charged into the sinter bed machine for sinter making. Beyond 25% of micro fines in iron ore affects the sinter plant productivity and quality. The schematic flow process chart as shown in accompanying Figure 1 pictorially represent and explains the conventional sintering process of iron ore.

In the micro pelltizing sintering process according to present invention, iron ore micro fines (< 150 microns particle size) mixed with limestone, dolomite, binder (calcined lime, bentonite, polygel) and converted into micro pellets (< 6 mm) in presence of required quantity of water spray. The schematic of the new process is shown in accompanying Figure 2.

The present invention thus targets to utilize 100% microfines (< 150 microns) through sintering with improved sinter quality and productivity. The present process consists of three major steps for the green mix preparation:

Step1: Iron ore fines (< 150 microns) along with the finer fractions of lime, limestone, dolomite and coke breeze (< 1 mm each) with varying proportions are conveyed to disc pelletizer where water is added to form pellet feed of size <6mm. The moisture content of pellet was 10.5% and drop number 7-10.

Step2: The balance raw materials fractions (return fines, limestone, dolomite, calcined lime) and sinter grade iron ore (<8mm) are mixed in a drum mixer along with the micro-pellets (< 6 mm) and granulated for the stipulated time period.

Step3: The finer size of the coke breeze (<1 mm) is added at the final stage of granulation of step 2. The coke breeze coated mix improves the homogeneous distribution and participation of heat for the incipient fusion process of sinter making.

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

Methodolgy:
(i) Approximately 90 kg of sinter mix blended using balling drum. Blending condition: mixing @ 20 rpm for 300 seconds.
(ii) One kg of each blended sinter mix sample was dried and analyzed for particle size distribution.
(iii) Agglomerated the sinter mix in presence of moisture using a drum mixer; agglomerating condition: mixing @ 20 rpm for 480 seconds.
(iv) Sintered the material in pot grate with ignition conditions as under:

Ignition conditions:

Ignition holding time 60 sec
Suction pressure after ignition -1200 mm H2O

Disc pelletizer operating conditions:
(a)Inclination Angle: 44 Degree
(b)Disc rotation: 23 rpm
(c)Moisture level: 10.5%
(d)Pelletizing duration:10 min

(v) Raw materials were screened to obtain the desired size as given below.

Material Size
Base Mix As received
Calcined lime - 0.212 mm
Coke breeze - 3.15 mm

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 iron ore microfines (< 150 microns) in sintering are studied.
Two experiments were performed to understand the impact of microfines in sinter making. The raw materials used for Experiment 1 is similar to the conventional sinter making in which sinter grade iron ore fines (SGF) having size fractions (< 8 mm) was taken as an iron ore whereas 50 % of SGF was replaced with 50% of pellet fine (< 150 microns size) in experiment 2. The raw materials fractions are given in Table 1.
Table 1: Raw materials proportions used under conventional sintering process
Expt. No. 1 2
Conventional
Total I/O 48 50.36
IOF fractions
(µ-fines : SGF) 0:100 50:50
Internal Return Fines 34.00 34.00
Limestone 5.30 2.25
Dolomite 5.50 6.20
Calcined Lime 3.00 3.00
Coal/Coke 4.20 4.20
TOTAL 100 100

Results: The sinter productivity, product yield and sinter quality deteriorates with increased fraction of micro fines in sinter raw mix for sintering. The sintering process behavior and sinter quality are presented in figure 3. The reasons attributed to this change in sintering behavior primarily due to decrease in granulation efficiency consequently affecting sinter bed permeability and thus overall disturbance on heat and combustion zone movement in the sinter bed due to high resistance of the air flow rate movement inside bed.

Example 2:

Under this example, effect of increased micro fines through micro pelletization and coke coating on sintering performance and quality according to the proposed process of the invention are studied:

To maximize micro fines addition up to 70%, the sintering parameters are optimized. The raw materials for the optimized proportions are given in Table 2. The solid fuel was increased from 4.2 to 5.0 of the total mix may be attributed due to the increase moisture requirement for pelletization. The iron ore characteristics also play major role in determining the flux and fuel requirement for sintering. In the present work, high loss on ignition (LOI) containing iron ore was used which require extra heat for LOI removal.

Table 2:

Sinter Raw Mix Proportions
*EXPERIMENTS Conventional Micro-pelletizing
1 3 4 5
Total I/O, kg 48.00 49.46 47.20 47.20
IOF fractions
(µ-pellets : SGF)
0:100
(µ-P : SG-1)
70:30
(µ-P : SG-1)
70:30
(µ-P : SG-2)
*60:10:30
(µ-P : µ-F : SG-2)
Return Fines (BF+SRF), kg 34.00 34.00 34.00 34.00
Limestone, kg 5.30 5.30 4.50 4.50
Dolomite, kg 5.50 5.50 5.50 5.50
Calcined Lime, kg 3.00 3.00 3.00 3.00
Coal/Coke, kg 4.20 5.00 5.00 5.00
TOTAL, kg 100 100 100 100

*60:10:30 :: micro-pellet : micro-fines : SG-2, sinter grade beneficiated (washed) iron ore (This mixture is made to understand the effect of generation of fines due to micro pellet addition at plant implementation stage.)

Results:
The pilot scale pot sinter trials having 70% micro fines as micro pellet and 30 % washed iron ore as sinter grade, a significant improvement in sinter properties and productivity achieved in comparison to the conventional sintering with 100% sinter grade iron ore. The generation of fines reduced significantly from 24.3 to 13.1 %. Such improvement in reduction of fines generation may be attributed due to the better melt formation with fluidity due to increased solid fuel consumption from 4.2 to 5.0 % of the total blend. The additional heat requirement is also for removal of LOI and higher moisture (10.5% in micro-pellets) as compared to 8% in conventional sinter mix. The laboratory test with the blend composition of iron ores with the ratio of (60:10:30 :: micro-pellet : micro-fines : Washed iron ore) is also conducted considering the fines generation during pelletization and granulation of micro pellets with remaining raw mix materials. The inbuilt micro fines helped the granulation behavior in presence of washed iron ore may be due to the increased wettability of washed iron ore in presence of micro fines. Accompanying Figure 4 shows the effect of micro fines addition through micro pelletization on sintering behavior, sinter quality and fines generation; and comparison with the conventional process having 100% sinter grade fines as iron ore

It is thus possible by way of the present invention to provide a process for sintering of iron ore microfines (below 150 micron particles) through micropelletization so that they can be used for commercial sintering process wherein iron ore microfines is utilized in sinter making to achieve better sinter productivity and quality by micro-pelletizing the fines in disc/drum pelletizer with optimum flux and fuel so as to achieve required strength wherein flux and fuel provide sufficient bonding and heat requirement during sintering. The sufficient green strength properties of micropellets ensure minimum fines generation during handling, transportation and mixing with other green mix components. The carbonaceous fuel (< 1 mm size) is added at the final stage of granulation wherein the finely divided coal particles form a coating on the surface of the micropellets/granules which enhance the uniform heat distribution throughout the bed.

,CLAIMS:We Claim:

1. A process for sintering involving iron ore micro fines < 150 microns in sintering comprising:
Separately converting the iron ore micro fines < 150 microns into micro pellets with fluxes and solid fuel;
Thereafter including the said micro fines based micro pellets with green mix components and subjecting the thus obtained mix of said micro fines based micro pellets with green mix components to sintering.
2. A process as claimed in claim 1 wherein the iron ore micro fines < 150 microns used for generating said micro pellets is more than 25% .
3. A process as claimed in anyone of claims 1 or 2 comprising binder or fluxes for pellet formation including calcined lime, slaked lime, limestone, bentonite and dolomite, carbonaceous materials selected from coal, coke breeze, charcoal as solid fuel.
4. A process as claimed in anyone of claims 1to 3 wherein said pellet feed size are below 12 mm preferably below 6mm.
5. A process as claimed in anyone of claims 1 to 4 comprising adding solid fuel of less than 1 mm size in the final stage of granulation preferably in the drum mixer to coat the micro pellets/granules surface.
6. A process as claimed in anyone of claims 1 to 5 wherein the iron ore micro fines (less than 150 microns) is used as major source of iron ore.
7. A process as claimed in anyone of claims 1 to 6 wherein the above said micro pellets from the disc pelletizer is further processed in the drum mixer along with remaining green mix components.
8. A process as claimed in anyone of claims 1 to 7 comprising the steps of:
i) separately converting said iron ore fines (< 150 microns) along with the finer fractions of lime, limestone, dolomite and coke breeze (< 1 mm each) to micro pellets in a disc pelletizer in the presence of water to generate micro pellets feed of size <6mm.

ii) thereafter mixing balance sintering raw materials fractions including return fines, limestone, dolomite, calcined lime and sinter grade iron ore (<8mm) in a drum mixer along with the said micro-pellets (< 6 mm) and granulating; and

iii) Adding finer size of the coke breeze (<1 mm) towards the final stage of granulation of step ii whereby the coke breeze coated mix favoured homogeneous distribution and participation of heat for the incipient fusion process of sinter making.

9. A process as claimed in anyone of claims 1 to 8 wherein the moisture content of pellet was controlled in the range of 9 to 12% preferably 10.5% and drop number 7-10.

10. A process as claimed in anyone of claims 1 to 9 wherein selective inclusion of said iron ore fines based micro pellets enables controlling subsequent sintering stages with granulation efficiency based on sinter bed permeability and overall distribution of heat and combustion zone movement in the sinter bed avoiding resistance of air flow inside bed.

11. A process as claimed in anyone of claims 1 to 10 wherein the said micro fines as micro pellets and washed iron ore as sinter grade fines are mixed in the range of 40 to 90 % : 10 to 60 % preferably about 70% :30%.

12. A process as claimed in anyone of claims 1 to 11wherein the blend composition used comprised micro-pellet: micro-fines: Washed iron ore of 40 to 90%. : 0 to 10%: 10 to 60% preferably about 60:10:30.

13. A process as claimed in anyone of claims 1 to 12 wherein the sinter mix included micro pellets up to 90 % by wt. of iron ore, balanced fractions of iron ore as sinter grade fines, return fines (depending on the in-plant generation of fines) preferably <40% by wt, limestone as per the remaining CaO requirement (after calcined lime addition) of blast furnace preferably <10 % by wt., dolomite as per the MgO requirement of blast furnace preferably <10 % by wt., calcined lime 1-5 % by wt. coal/coke > 3-7% by wt. of total blend.

14. A process as claimed in anyone of claims 1 to 13 wherein the process of sintering involving said iron ore micro fines < 150 microns as micro pellets favour productivity and reduced time and yet achieve advanced sinter properties including Tumbler index (+6.3 mm) >65%, with significantly reduced generation of fines from 25 to 10 %.

15. A process as claimed in anyone of claims 1 to 14 for sintering of iron ore micro fines comprising selectively: sinter mix blending is carried out using balling drum maintaining blending condition as mixing at 12 to 30 rpm preferably about 20 rpm for 120 to 600 seconds preferably about 300 seconds;

said sinter mix is agglomerated in presence of moisture using a drum mixer maintain agglomerating condition as mixing @ 12 to 30 rpm preferably about 20 rpm for 180 to 720 seconds preferably about 480 seconds;

maintaining sinter mix ignition condition comprising:
Suction air flow volume before ignition: preferably about 2.0 m3/min,
Ignition holding time: 45 to 180 sec preferably about 60 sec,
Suction pressure after ignition: 700 to 1600 mm H2O preferably about -1200 mm H2O; and

said Disc pelletizer operating conditions comprising:
(a) Inclination Angle: 40 to 50 preferably about 44 Degree,
(b) Disc rotation: 15 to 30 preferably about 23 rpm,
(c) Moisture level: 9 to 12% preferably about 10.5%,
(d) Pelletizing duration: 5 to 20 min preferably about 10 min.

16. Sintered iron ore pellets obtained by the process as claimed in anyone of claims 1 to 15 having superior sintering behavior, sinter quality and fines generation in comparison with the conventional process having 100% sinter grade fines as iron ore including: Tumbler index values (+6.3 mm) from 65.6 to 66.7 %.

Dated this the 23rd day of April, 2019

Anjan Sen
Of Anjan Sen & Associates

(Applicant’s Agent)

IN/PA-199