Claims:We Claim:
1. A green pellet for palletisation comprising:
an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6%dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture; and
at least one coating layer selected from (i) 1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof,
wherein the green pellets have the basicity in the range of 0.30 to 0.35, MgO in the range of 0.50 to 0.55%, carbon in the range of 1.3 to 1.4%, drop numbers in the range of 12 to 15, green compressive strength (GCS) in the range of 1.5 to 2.0kg/p and dry compressive strength (DCS) in the range of 5.0 to 5.5kg/p.

2. A green pelletfor pelletization as claimed in claim 1 comprising:
an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6% dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture,
a first coating layer of 1.5% to 2.5% magnetite fines on the surface of the inner core,
a second coating layer comprising of a combination of 0.2%to 0.6% limestone and 1.5% magnetite fines on the surface of the first coating layer
having the basicity in the range of 0.30 to 0.35, MgO in the range of 0.50 to 0.55%, carbon in the range of 1.3 to 1.4%, drop numbers in the range of 12 to 15, green compressive strength (GCS) in the range of 1.5 to 2.0kg/p and dry compressive strength (DCS) in the range of 5.0 to 5.5kg/p.

3. The green pellets for pelletization as claimed in anyone of claim 1 to 2, wherein the hematite comprises of beneficiation plant ground hematite concentrate and iron content is in the range of 61 to 62%

4. The green pellet for pelletization as claimed in anyone of the claims 1 to 3, wherein the mineralogical phases of hematite include 0.8 to 1.2% magnetite, 76 to 79% hematite, 5 to 7% goethite, 2 to 4% limonite, 4.5 to 6.5% quartzite, 4.2 to 6.4% kaolinite and 1 to 2.5% others from calcite, muscovite mica, biotite mica, zircon, silliminite, and garnet.

5. The green pellet for pelletization as claimed in anyone of the claims 1 to 6, wherein the mineralogical phases of magnetite fines include 55 to 57% magnetite, 30 to 33% hematite, 5.5 to 7.5%, quartzite, 2 to 4% kaolinite and 2 to 4.5% others from calcite, biotite, granet and schist.

6. The green pellet for pelletization as claimed in anyone of the claims 1 to 5, wherein the said first coating layer comprising of magnetite fines and said second coating layer comprising of mixed magnetite with limestone fines together includes 60.8 to 61.8% of iron, 5.35 to 5.50% of silica, 3.20 to 3.40% of alumina, 1.75 to 1.80% of calcium oxide, 0.5 to 0.55% of magnesium oxide and 0.25 to 0.35% of ferrous oxide.

7. The green pellet for pelletization as claimed in anyone of the claims 1 to 6, comprising of 64.0 to 66.0%magnetite fines having a grain size of -0.045µm, 62 to 64% hematite having a grain size of -0.045 mm, 64.0 to 66.5% limestone fines having a grain size of -0.045 µm, 63.8 to 66.2% dolomite having a grain size of -0.045 µm and 63.0 to 64.0%coke breeze having a grain size of -0.045 µm.

8. A method of pelletization in an induration furnace of the green pellets as claimed in anyone of the claims 1 to 7 comprises of following steps:
providing said green pellets comprising of an inner core containing 93.5 to 95.2% hematite , 1.3 to 1.7% limestone,2.0 to 2.6% dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture ; and atleast one coating layer selected from (i)1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof maintaining carbon from 1.3 to 1.4% and pellet basicity from 0.30 to 0.35 and MgO from 0.50 to 0.55% having green properties including drop no 12 to 15, GCS 1.5 to 2.0kg/p and DCS 5.0 to 5.5kg/p loaded on a travelling grate,

said green pellets transported through the drying zones, the updraft drying (UDD) and downdraft drying (DDD) zones, wherein the travelling time and temperature in the updraft drying zone is carried out for 8.60. to 8.86 minutes preferably about 8.73 minutes and 200 to 250oC respectively and the travelling time and temperature in the downdraft drying zone is carried out for 6.82. to 7.14minutes preferably 6.98 minutes and 250 to 350oC respectively,

green pellets are transported through the pre heat (PH) zone, wherein the travelling time and temperature maintained in the preheating zone is carried out for 2.44 to2.68 minutes preferably about 2.56 minutes and 350 to 800oC respectively,

green pellets are transported through the firing zone and after-firing zone, wherein the travelling time and temperature in the firing zone and after-firing zone is carried out for 16.40 to 16.68minutes preferably about 16.54 minutes and 800 to 1310 respectively,

green pellets are transported through the cooling zone, wherein the time and temperature maintained in the cooling zone is carried out for 15.40 to 15.80minutes preferably about 15.61 minutes and 900 to 90oC respectively, to form pellets which can be used for iron making.

9. The method of pelletization in an induration furnace by using the green pellets as claimed in claim 8, which is carried out such that the pellets surface hardness is achieved in the range of 1003 to 1138 Hv, cold crushing strength (CCS) is in the range of 277 to 283kg/p, tumbler index (TI) (+6.3mm), % is in the range of 95.8 to 96.2, abrasion index (AI) (0.5mm), % is in the range of 3.0 to 3.4, reduction degradation indices (RDI) (-6.3mm), % is in the range of 9.1 to 9.5 and ferrous oxide (FeO)% is in the range of. 0.29 to 0.3.

10. The pelllets obtained of green pellets having an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6% dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture; and
atleast one coating layer selected from (i) 1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof and having
surface hardness achieved in the range of 1003 to 1138 Hv, cold crushing strength (CCS) is in the range of 277 to 283kg/p, tumbler index (TI) (+6.3mm), % is in the range of 95.8 to 96.2, abrasion index (AI) (0.5mm), % is in the range of 3.0 to 3.4, reduction degradation indices (RDI) (-6.3mm), % is in the range of 9.1 to 9.5 and ferrous oxide (FeO)% is in the range of. 0.29 to 0.3.

Dated this the 3rd day of June, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
GREEN PELLETS HAVING SURFACE COATED WITH MAGENTITE FINES AND LIMESTONE AND A PROCESS OF PELLITIZATION USING THE SAME.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION

The present invention relates to green pellets which is used for making pellet by the process of pellitisation. More particularly, the present invention relates to hematite green pellets coated with magnetite fines and mixed magnetite and limestone fines on said pellets to improve overall pellet physical and metallurgical properties and to reduce fines generation during handling by improving the overall surface properties of the fired pellets.

BACKGROUND OF THE INVENTION

The heat hardening of hematite green pellets by oxidation is a process commonly used in iron ore pelletization process. The green pellets are fired in straight grate indurating machine using Corex gas at JSW Steel Ltd. The sufficient strength of the fired pellets is essential for DRI plant and to reduce the generation of fines during transportation from pellet plant to DRI plant. This will reduce the pellet plant production and affects the DRI plant performance.

The quality of hematite iron ore pellets required for DRI plant is one of the most important criteria for better performance of DRI plant. It was observed that magnetite pellets are better sintering properties and pellet quality compared to hematite pellets. The crystallization behaviour of iron particles during preheating and roasting of pellets made from mixed magnetite–hematite concentrates has been studied by many researchers. To enhance the induration of hematite pellets, both magnetite-addition and carbon-burdened methods are found to be the most favourable techniques in practice. During handling of the hematite pellets the fines generation is more. The investigation carried out by many researchers shown that the addition of magnetite concentrate to the hematite green mix and carbon loaded pellets are able to improve the compression strength of preheated and roasted pellets and reduces the fines generation. Many research’s worked on iron ore pellets produced from magnetite ore as well as mixed magnetite with hematite ore. Detailed research work has been carried out to further improve the quality of the pellet and to reduce the fines generation by coating the magnetite fines and mixed fines of magnetite& limestone on surface of produced hematite green pellets.

The available literatures on pellet making reveal that production of hematite pellets mixed with magnetite fines and its effect on iron ore pellet properties and coating of lime, limestone, serpentine, dolomite on iron ore pellets to reduce the sticking tendency during direct reduction at elevated temperatures. However, no research work is available on coating of hematite green pellets with natural magnetite fines and mixed magnetite and limestone fines to achieve superior quality of pellets. During our course of study, it is observed that coating of natural magnetite fines and mixed natural magnetite & limestone fines gives improved properties of the pellets. This work mainly focussing on coating of magnetite fines and mixed magnetite and limestone fines on hematite green pellets to improve overall pellet physical and metallurgical properties and to reduce fines generation during handling of fired pellets from pellet plant to iron making units and direct DRI plant by improving the overall surface properties of the fired pellets.

The technical paper titled Optimization of induration cycle for magnetite concentrate and its usage in pellet making process designed for hematite concentrate, T Umadevi et.al. Mining, Metallurgy and Exploration,38(1), pp 243-255 (2021), mentioned about the pelletization of magnetite concentrate as well as blend of hematite with magnetite concentrate. The strength of the magnetite concentrate roasted pellets was 325kg/p and strength of 5% magnetite and 95% hematite concentrate was 275kg/p. Laboratory as well as plant investigation has shown that the addition of magnetite to the hematite mix blend improved the strength of the pellets compared to hematite pellet alone.

The technical papertitled Induration of mechanisms of oxidised pellets prepared from mixed magnetite – hematite concentrates, G. H. Li et.al. Ironmaking and Steelmaking, 36(5), pp. 393-396 (2009), mentioned that addition of certain proportion of magnetite concentrate is also as effective way to enhance the roasting performance of hematite pellets with improved strength.

The technical papertitled Recrystallization of Fe2O3 during the induration of iron ore oxidation pellets, Guanghui Li et.al. www.intechopen.com, stated that to enhance the induration of hematite pellets with improved strength, both magnetite addition and carbon burdened methods are found to be the favourable techniques.

US patent No. 2960396A dated 15 November’1960 by F. D. DE VANEY et.al.describes in iron ore pelletization process, the magnetite addition is useful not only in the case of hematite (earthy or specular) but also in the cases of hydrous iron oxide such as limonite, boggy ores and ores containing iron carbonate (siderite) to achieve the better pellet properties.

OBJECT OF THE INVENTION

The main object of the present invention is thus directed to provide coated green pellets to have improved pellet properties and a method for making pellet through coating of magnetite fines and mixed magnetite and limestone fines to improve the physical and metallurgical properties of the fired pellets by improving surface properties of the produced hematite green pellet which obviates the drawbacks as mentioned above.

It is an object of the present invention to optimize the percentage of magnetite fines coating and mixed magnetite and limestone fines coating on hematite iron ore green pellet to achieve the better pellet properties for blast furnace iron making unit and DRI plant.

It is an object of the invention to provide a use of magnetite fines and magnetite & limestone fines as coating agent to reduce fines generation in the form of a high strength pellet.

In short, the purpose of the present invention is to propose a new method of green pellet making that can improve the fired pellet properties and reduce the fines generation.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a green pellet for pelletisation comprising:
an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6% dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture; and
at least one coating layer selected from (i) 1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof maintaining carbon from 1.3 to 1.4% and pellet basicity from 0.30 to 0.35 and MgO from 0.50 to 0.55% having green properties including drop no 12 to 15, GCS 1.5 to 2.0kg/p and DCS 5.0 to 5.5kg/p.

A further aspect of the present invention is directed to said green pellets wherein said green pellets having coating layer selected of 1.0 to 2.5 % magnetite fines are capable of generating fired pellets having surface hardness in the range of 900 to 1145 and microhardness of different phases including secondary hematite formed from magnetite>secondary hematite formed from hematite>magnetite phase in the range of secondary hematite 1138-1158; hematite 970-990 and magnetite 733-753.

A still further aspect of the present invention is directed to said green pellets wherein said green pellets having coating layer selected of 0.2%to 0.6% limestone and 1.5 % magnetite fines are capable of generating fired pellets having surface hardness in the range of 730to 1060and micro hardness of different phases including secondary hematite formed from magnetite>calcium ferrite >secondary hematite formed from hematite>magnetite in the range of secondary hematite 1158-1178, hematite 977-995, magnetite 740-760 and Ca ferrite 1050-1070.

A still further aspect of the present invention directed to a green pellet for pelletisation comprising an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6%dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture;
a first coating layer of 1.5% to 2.5% magnetite fines disposed on surface of the inner core;
a second coating layer comprising of a combination of 0.2%to 0.6% limestone and 1.5% magnetite fines disposed on surface of the first coating; said green pellets having the basicity in the range of 0.30 to 0.35, MgO in the range of 0.50 to 0.55%, carbon in the range of 1.3 to 1.4%, drop numbers in the range of 12 to 15, green compressive strength (GCS) in the range of 1.5 to 2.0kg/p and dry compressive strength (DCS) in the range of 5.0 to 5.5kg/p.

According to a further aspect of the present invention directed to said green pellets for pelletization wherein said hematite comprises of beneficiation plant ground hematite concentrate and iron content is in the range of 61 to 62%.

The green pellets for pelletization according to another aspect of the present invention wherein the mineralogical phases of hematite include 0.8 to 1.2% magnetite, 76 to 79% hematite, 5 to 7% goethite, 2 to 4% limonite, 4.5 to 6.5% quartzite, 4.2 to 6.4% kaolinite and 1 to 2.5% calcite, muscovite mica, biotite mica, zircon, silliminite, and garnet.

The green pellets for pelletization according to another aspect of the present invention wherein the mineralogical phases of magnetite fines include 55 to 57% magnetite, 30 to 33% hematite, 5.5 to 7.5%, quartzite, 2 to 4% kaolinite and 2 to 4.5% calcite, biotite, garnet and schist.

The green pellets for pelletization according to another aspect of the present invention wherein said first coating layer comprising of magnetite fines and said second coating layer comprising of mixed magnetite with limestone fines together includes 60.8 to 61.8% of iron, 5.35 to 5.50% of silica, 3.20 to 3.40% of alumina, 1.75 to 1.80% of calcium oxide, 0.5 to 0.55% of magnesium oxide and 0.25 to 0.35% of ferrous oxide.

The green pellets for pelletization according to another aspect of the present invention comprising of 64.0 to 66.0% magnetite fines having a grain size of -0.045µm, 62 to 64% hematite having a grain size of -0.045 mm, 64.0 to 66.5% limestone fines having a grain size of -0.045 µm, 63.8 to 66.2% dolomite having a grain size of -0.045 µm and 63.0 to 64.0%coke breeze having a grain size of -0.045 µm.

A still further aspect of the present invention is directed to a method of pelletization in an induration furnace of the green pellets comprises of following steps.
providing said green pellets comprising of an inner core containing 93.5 to 95.2% hematite , 1.3 to 1.7% limestone,2.0 to 2.6%dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture ; and at least one coating layer selected from (i)1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof maintaining carbon from 1.3 to 1.4% and pellet basicity from 0.30 to 0.35 and MgO from 0.50 to 0.55% having green properties including drop no 12 to 15, GCS 1.5 to 2.0kg/p and DCS 5.0 to 5.5kg/ploaded on a travelling grate.
said green pellets transported through the drying zones, the updraft drying (UDD) and downdraft drying (DDD) zones, wherein the travelling time and temperature in the updraft drying zone is carried out for 8.60 to 8.86 minutes preferably about 8.73 minutes and 200 to 250oC respectively and the travelling time and temperature in the downdraft drying zone is carried out for 6.82 to 7.14 minutes preferably about 6.98 minutes and 250 to 350oC respectively.
said green pellets are transported through the pre heat (PH) zone, wherein the travelling time and temperature maintained in the preheating zone is carried out for 2.44 to 2.68 minutes preferably about2.56 minutes and 350 to 800oC respectively.
said green pellets are transported through the firing zone and after-firing zone, wherein the travelling time and temperature in the firing zone and after-firing zone is carried out for 16.40 to 16.68 minutes preferably about16.54 minutes and 800 to 1310oCrespectively.
Said green pellets are transported through the cooling zone, wherein the time and temperature maintained in the cooling zone is carried out for 15.40 to 15.80 minutes preferably about15.61 minutes and 900 to 90oC respectively, to form pellets which can be used for iron making.

A further aspect of the present invention is directed to said method of pelletization in an induration furnace by using the green pellets which is carried out such that the pellets surface hardness isin the range of 1003 to 1138 Hv, cold crushing strength (CCS) is in the range of 277 to 283kg/p, tumbler index (TI) (+6.3mm), % is in the range of 95.8 to 96.2, abrasion index (AI) (0.5mm), % is in the range of 3.0 to 3.4, reduction degradation indices (RDI) (-6.3mm), % is in the range of 9.1 to 9.5 and ferrous oxide (FeO)% is in the range of. 0.29 to 0.3.

A still further aspect of the present invention is directed to pelllets obtained of green pellets having an inner core containing 93.5 to 95.2% hematite,1.3 to 1.7% limestone,2.0 to 2.6%dolomite,0.8 to 1.3% coke breeze,0.7 to 0.9% bentonite and 9 to 10% moisture; and
at least one coating layer selected from (i)1.0 to 2.5 % magnetite fines and (ii) 0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines and combinations thereof and having
surface hardness is achieved in the range of 1003 to 1138 Hv, cold crushing strength (CCS) is in the range of 277 to 283kg/p, tumbler index (TI) (+6.3mm), % is in the range of 95.8 to 96.2, abrasion index (AI) (0.5mm), % is in the range of 3.0 to 3.4, reduction degradation indices (RDI) (-6.3mm), % is in the range of 9.1 to 9.5 and ferrous oxide (FeO)% is in the range of. 0.29 to 0.3.

According to a further aspect of the present invention said pellets obtained of said green pellets having coating layer selected of 1.0 to 2.5 % magnetite fines having surface hardness in the range of 900 to 1145andmicro hardness of different phases including Secondary hematite formed from magnetite>secondary hematite formed from hematite>Magnetite phase in the range of Secondary hematite 1138-1158; Hematite 970-990 and Magnetite 733-753.

Yet another aspect of the present invention is directed to said pellets obtained of said green pellets having coating layer selected of0.2%to 0.6% limestone and 1.0 to 2.5 % magnetite fines are capable of generating fired pellets having surface hardness in the range of 730 to 1060and micro hardness of different phases including Secondary hematite formed from magnetite>calcium ferrite >secondary hematite formed from hematite>Magnetite in the range of secondary hematite 1158-1178; hematite 977-995magnetite 740-760 and Ca ferrite 1050-1070.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows Phase analysis of magnetite fines coated pellets.
Figure 2: shows Micrographs of pellets with magnetite fines coating.
Figure 3: shows Phase analysis of magnetite& limestone fines coated pellets (1.5% magnetite fines).
Figure 4: shows Micrographs of magnetite & limestone fines coated pellets (1.5% magnetite fines).

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

Present invention is directed to provide a process for production of superior quality of hematite iron ore pellets using feed mix consisting of hematite concentrate fines, fluxes like limestone and dolomite, bentonite, coke breeze, water addition to maintain moisture, and coating of hematite pellets with magnetite fines and mixed l magnetite s and limestone fines.

A process for the production of high strength hematite iron ore pellets has been developed using natural magnetite fines and mixed natural magnetite with limestone fines as surface catalyst on produced hematite pellets prepared with the addition of bentonite as binder, limestone and dolomite as flux to maintain basicity and pellet MgO, coke breeze fines to maintain the uniform heating of the pellets from core to shell, water for preparation of green pellets by maintaining optimum moisture. The optimum firing temperature was maintained during induration process to achieve the improved properties of the coated pellets with natural magnetite and magnetite mixed with limestone. The pelletization process comprises, grinding of beneficiation plant hematite concentrate, magnetite concentrate, fluxes like limestone & dolomite, coke breeze to achieve the desired fineness, production of hematite green pellets using balling disc by maintaining optimum moisture, further production of hematite green pellets using magnetite and mixed magnetite and limestone finesas surface catalyst, sizing of green pellets using screens to achieve -16+6 mm pellets, hardening of green pellets in rising hearth furnace by maintaining the optimum firing temperature to obtain superior pellet properties.

Optimization of coating of the magnetite fines and mixed magnetite and limestone fines to achieve superior quality of pellets after subjected to firing in straight grate induration furnace.

The green pellets, after sizing through a roller conveyor, are discharged onto the travelling grate induration machine on top of the hearth layer and between the side layers of the fired pellets. The hearth layer pellets protect the grate bars from high gas temperature. The travelling grate carries the pellets through the indurating furnace where they are subjected to the sequential zones of updraft drying, downdraft drying, preheating, firing & after firing and first & second cooling. The first stage of drying is updraft to prevent the condensation of the water and pellet deformation in the bottom layer of the pellet bed. For updraft drying the hotgas is recycled from the cooling zone 2. The drying is continued in a subsequent downdraft stage to remove free water of the top of the bed by relatively hot gases coming from the firing zone of the furnace. In these two zones, it is necessary to use drying air of sufficient temperature to cause rapid water evaporation yet not so high that the pellets generate cracks due to high internal stresses. In the preheating zone, the pellets are heated to about 500 to 1000oC by downdraft air flowing through the bed and the hot gas is recycled from the cooling zone 1. During this stage, pellets are completely dried and reactions such as removal of combined moisture, decomposition of carbonates, coke combustion and conversion of iron oxide to hematite take placeThe reactions from the preheating zone continue in the firing zone. Here pellet charge is heated to an optimum temperature for a controlled period. Strength of the pellets increases at this stage because of re-crystallization and formation of slag phase. The off-gas from the firing zone is recuperated to the drying zone.

The quality of iron ore pellet required for DRI plant is one of the most important parameter for DRI plant operation efficiency. The produced pellets for DRI plant should have optimum physical and physicochemical characteristics to withstand the number of physical and chemical transitions during its processing. Quality of magnetite pellets are superior compared to hematite pellets as per laboratory and plant experience. During induration process the secondary hematite phases are forming due to conversion of magnetite to hematite by oxidation of Fe2+to Fe3+. The results indicate that the strength of pellets is mainly provided by the crystalline connections between iron particles of original and secondary hematite particles formed from magnetite phase during preheating and firing. Secondary hematite phases will improve the fired properties of the pellets like Tumbler index, Abrasion index and cold crushing strength of the pellets. Based on the previous experience as per laboratory studies on magnetite pellets detailed laboratory trials have been planned to improve the hematite iron ore pellet properties by coating the natural magnetite fines and natural magnetite fines with limestone fines. In the laboratory trials the percentage of fines coating of the pellet was optimized to achieve the better quality of pellets. Table 1 and Table 2 shows the chemical analysis and size analysis of the raw material used for pelletization studies respectively. The pellet green mix proportion is shown in Table 3.

Table 1: Chemical analysis of the raw material

Table 2: Size analysis of the raw material

Table 3: Raw material mix proportion
Raw material %
Iron ore fines 93.5 to 95.2
Limestone 1.3 to 1.7
Dolomite 2.0 to 2.6
Coke breeze 0.8 to 1.3
Bentonite 0.7 to 0.9

Although many research works have been conducted on partial utilization of magnetite in hematite ore pellets and coating of fluxes on hematite pellets, further investigations on the new approach i.e. the effect of magnetite and mixed magnetite and limestone as coating agent on hematite pellets is required to know the improvement in quality of iron ore pellets. Detailed investigations have been carried out to optimize the coating of magnetite and and mixed magnetite with limestone fines on hematite iron ore green pellets to achieve better pellet properties. This study aims at studying the coating effect of magnetite and mixed magnetite with limestone fines on hematite green pellet by considering the pellet moisture addition, bentonite dosage, coke breeze addition, and fluxes like limestone and dolomite addition.

In present invention, pelletizing experiments have been carried out by collecting bentonite fines from pellet plant. Other raw materials like hematite concentrate, magnetite concentrate, limestone, dolomite and coke breeze were ground in a laboratory ball mill to achieve the fineness as shown in Table 2.The experimental plan for detailed studies is shown in Table 4. The phase analysis of magnetite and hematite was carried out on polished sections using optical microscope and image analyser software. The phase analysis is shown in Table 5.

Table 4: Experimental plan
Phase A Phase B
Description Magnetite Conc Magnetite + Limestone fines
Expt 1 0.0 0.0
Expt 2 0.5 Optimum A + 0.20
Expt 3 1.0 Optimum A + 0.30
Expt 4 1.5 Optimum A + 0.40
Expt 5 2.0 Optimum A + 0.50
Expt 6 2.5 Optimum A + 0.60

Table 5: Phase analysis of hematite and magnetite
Phases OBP-2 Concentrate (Hematite) Magnetite Concentrate
%
Magnetite 0.8 to 1.2 55 to 57
Hematite 76 to 79 30 to 33
Goethite 5 to 7 0.0
Limonite 2 to 4 0.0
Quartz 4.5 to 6.5 5.5 to 7.5
Kaolinite 4.2 to 6.4 2 to 4
Others 1 to 2.5 2 to 4.5

The green pellets were prepared in laboratory balling disc and the disc parameters are as follows:
• Disc diameter : 450mm
• Disc inclination : 45o
• Disc speed : 38rpm

9.0 to 10% green pellets moisture was maintained. The Green pellet carbon was maintained from 1.3 to 1.4% and pellet basicity was maintained from 0.30 to 0.35 and MgO was maintained from 0.50 to 0.55%.

Green pellets were prepared with ground material of beneficiation plant hematite concentrate mixed with bentonite, limestone, dolomite, coke breeze and water to maintain the require green pellet moisture and coating of produced hematite pellets with natural magnetite fines, and natural magnetite fines with limestone fines. The green properties like drop no 12 to 15, GCS 1.5 to 2.0kg/p and DCS 5.0 to 5.5kg/p were achieved.

For each experiment 20kg of green pellets were prepared in laboratory balling disc. The green pellets were filled in Inconel basket. The basket was kept in induration furnace. The firing temperature was varied from 1280 to 1310 oC based on coke breeze content. The firing cycle with temperature and time is shown in Table 6. Fired pellets were subjected to evaluations of chemical, physical and metallurgical properties.

Table 6: Firing cycle
Time, min Temp, 0C
Updraft drying 8.73 200-250
Down draft drying 6.98 250-350
Preheating 2.56 350-800
Firing 16.54 800 – 1280 to 1310
After firing 1280 to 1310 - 900
Cooling 15.61 900-300/300-80
50.42

Mineralogical and phase analysis of fired pellets have been carried out using optical microscope and image analyser software.

The produced fired pellets consist of Fe: 60.8 to 61.8%, silica:5.35 to 5.50, alumina: 3.20 to 3.40, CaO: 1.75 to 1.80, MgO: 0.5 to 0.55 and FeO: 0.25 to 0.35%.

The phase analysis and micrographs of the pellets coated with magnetite fines is shown in Figures 1 and 2 respectively. From phase analysis it was found that the magnetite and slag phase increased with increase in coating of magnetite fines. The pore phase decreased up to 1.5% magnetite fines as coating agent and after wards the pore phase increased with increase in magnetite fines.

The phase analysis and micrographs of the pellets coated with mixed magnetite (1.5%) & limestone fines (0 to 0.6%) are shown in Figure 3 and Figure 4 respectively. From phase analysis it was found that in mixed fines, with increase in limestone fines as coating agent hematite phase decreased and pore phase increased. Slag phase and magnetite phase decreased up to 0.2% limestone as coating agent and after wards slag phase increased with increase in coating of limestone fines. Ca-ferrite phase increased up to 0.2% limestone as coating agent and afterwards Ca-ferrite phase decreased with increase in coating of limestone fines.

Micro hardness of pellets coated with magnetite and mixed magnetite with limestone fines is shown in Table 7 and Table 8 respectively. Micro hardness of different phases of coated pellets is as follows: Secondary hematite formed from magnetite>secondary hematite formed from hematite>Magnetite phase. Hematite pellets coated with 1.5% magnetite fines showed maximum hardness of the pellet surface phases.

Table 7: Microhardness of pellets coated with natural magnetite fines

Micro-hardness of pellet coated with limestone fines along with 1.5% magnetite fines is shown in Table 9. Micro-hardness of different phases of coated pellets is as follows:Secondary hematite formed from magnetite>calcium ferrite >secondary hematite formed from hematite>Magnetite. With magnetite fines 1.5% and limestone fines 0.2% as coating agent showed maximum surface harness of the pellet phases.

Table 8: Micro-hardness of pellets coated with limestone fines (with magnetite fines 1.5%)

The physical and metallurgical properties of the fired pellets coated with natural magnetite fines on hematite pellets are shown in Table 9. Hematite green pellets coated with 1.5% natural magnetite fines showed better physical and metallurgical properties of the pellets.

Table 9: Physical and metallurgical properties of the pellets with natural magnetite fines coating

The physical and metallurgical properties of the fired pellets coated with mixed natural magnetite (1.5%) and limestone fines (0.2%) on hematite pellets are shown in Table 10. Hematite green pellets coated with mixed 1.5% natural magnetite fines and 0,2% limestone fines showed better physical and metallurgical properties of the pellets.

Table 10: Physical and metallurgical properties of the pellets with coating of mixed natural magnetite fines and limestone fines.

It is thus possible by way of the present invention to provide coated green pellets with enhanced green as well as fired properties wherein the green pellets have the basicity in the range of 0.30 to 0.35, MgO in the range of 0.50 to 0.55%, carbon in the range of 1.3 to 1.4%, drop numbers in the range of 12 to 15, green compressive strength (GCS) in the range of 1.5 to 2.0kg/p and dry compressive strength (DCS) in the range of 5.0 to 5.5kg/p.