FIELD OF THE INVENTION
The present invention relates to a method of producing sinter and pellet from iron ore slimes having high water of hydration and alumina as impurities, with or without beneficiation.
BACKGROUND OF THE INVENTION
A significant amount of discarded waste product is generated during mechanized mining of iron ore and its processing like washing and beneficiation. This discarded waste by-product, which is generally 80-100% below 150 µm in size and comprises of 20-25 % of the run of mine (ROM) ores, is called as iron ore slimes and is generally stored in slime ponds/dams. This iron ore slime is a discarded product mainly because of two reasons which are particle size distribution and chemical composition, due to which these are not suitable for use in agglomeration processes like sinter making and pellet making. The major unwanted components of slimes are combined water (hydrated/crystallized water), silica and alumina which are as high as 10 wt.% or more in Indian iron ore slimes. Due to very high alumina to silica ratio, which is normally above one, poses serious problems in sintering and pelletizing of iron ore slimes and hence resulting in rejection of its usage in agglomeration processes and thus, iron making. In this work, a novel and innovative method is developed for using iron ore slimes in sinter and pellet making.
Sinter making and pellet making are high temperature iron ore agglomeration processes in which raw materials are treated to form suitable size agglomerates required for different iron making processes like Blast Furnace, Midrex, Corex etc.
In iron ore sintering, a mixture of different raw material constituents like iron ore fines, various fluxes like limestone fines, dolomite fines, pyroxenite fines,

calcined lime and solid fuel like anthracite coal or coke breeze etc. along with other metallurgical wastes are agglomerated into larger and porous solid lump. These large size agglomerates are then crushed and sized as per the requirement of the blast furnace. Raw material size suitable for this technology is 150 micron to 8 mm. Similarly, In iron ore pelletization, a mixture of powder of iron ore fines, limestone, dolomite, pyroxenite, burnt lime, coke breeze or anthracite coal and other metallurgical wastes etc. are agglomerated into 6-16 mm diameter balls. These balls are then indurated at high temperature in induration furnace in oxidizing environment and iron ore pellets are formed as final product. Powder size suitable for this technology is < 150 micron.
In sinter making, iron ore slimes cannot be used in their as-it-is form due to their fineness, the presence of higher amounts of combined water as well as due to alumina minerals. If these slimes are used in the sintering process, incipient fusion do not occur in a manner which is required for quality sinter production. This happens because of the reason that water being a compound having highest specific heat, absorbs significant part of heat input provided by solid fuel and therefore temperature required for sintering of constituent materials is not sufficient and hence quality of the product is not acceptable. In order to mitigate this, a significant increase in the amount of coke breeze is required which makes the whole process economically unattractive and environmentally unfriendly. Also, increase in coke breeze significantly affects the health of travelling grate machinery and ESP de-dusting system. In addition to high water of hydration, if material contains significantly high alumina than it also makes the assimilation of different constituents inadequate. Poor assimilation leads to poor quality sinter.
In iron ore pelletization process also, iron ore slimes cannot be used in their as-it-is form due to their fineness, the presence of higher amounts of combined water and alumina minerals. If a pellet is prepared with admixed

slimes in the pellet feed, it creates huge problem during its induration, especially, during drying and preheating. If the pellet with residual moisture (both hygroscopic and hydrates), enters high temperature zone, then sudden release of this moisture from the unfired pellet results in its disintegration. Even if any increase in the solid fuel is done to mitigate this phenomenon, the results remain the same. In order to mitigate the disintegration of pellets, only option available to operator is, therefore, to decrease the machine speed and hence low production. Even if a pellet plant decides to compensate high hydrated water with loss in production, then also, it has been observed that plant operation remains unstable because of various difficulties and challenges caused by material having high combined moisture in the area of grinding, filtering and green balling. In addition to high water of hydration, high alumina because of its poor assimilation properties further deteriorates pellet quality.
Till date, to use iron ores slimes and similar materials in sinter and pellet making and mitigating the detrimental effect of same in these processes, not much has been reported in earlier works. In order to make use of iron ore slimes in iron and steel industry, some work which are reported are mentioned here.
Binder containing lime-rich, lime-silica-alumina, for sinter - granulate production from iron ore slimes is described by WOLFFJOHANN ETS [LI] in Patent no CH534738 (A).
Process of mixing sinter of different basicity from iron-containing waste of metallurgy was described by KORSHIKOV G V et al in patent No. RU2221880 (C2).
Method of direct steel making from iron-containing materials in converter was described by NIKOLAEV A L et al in patent no RU2115743 (C1).

Mixture for production of highly basic sinter was described by LISIN V S et al in patent no RU2146297 (C1)
Above cited inventions either mentioned about alternate use of slimes such as either in alternate process of direct steel making for which new investments as well as new equipment and machinery set-up altogether will be required or in production of binders from slimes. For use in sinter making, above cited inventions mentioned about producing varying basicity and high basicity sinter to counter the negative effect of slimes in sintering. Since highly basic sinter will lead to higher slag rate in downstream process of iron making like blast furnace etc. therefore higher basicity sintering process becomes unattractive for using slimes.
OBJECTS OF THE INVENTION
The primary object of this invention is to propose a method by which iron ore slimes and similar materials can be used in sinter and pellet making process without adverse effect on productivity and physical quality of product.
Another object of the present invention is to propose a method in which cold bonded pellet or briquettes are developed using slimes and similar materials, suitable for charging at the bottom of the bed as hearth layer in sintering and as bottom and side layers in pellet making.
Yet another object of this invention is to propose a method in which drying of the cold bonded pellet or briquettes, prior to charging as hearth layer, can be done using un-utilized heat of waste process gas.
Still another object of the invention is to propose a method in which water of hydration is removed from cold bonded pellets or briquettes prepared from iron ore slimes and similar materials using sensible heat of both hot bottom sinter and process air.

A further object of the invention is to mitigate the moisture front formation at the bottom layer of wet sinter mix charge during sintering process and thus increase in the vertical sintering speed.
A still further object of the invention is to form alumina bearing phases which are easy to assimilate for forming bonds during recycling in sinter making and pellet making process.
SUMMARY OF THE INVENTION
This invention provides a method by which iron ore slimes can be used in sinter and pellet making without affecting the quality and productivity of the sinter and pellet plants.
At first stage, cold bonded agglomerates (pellets or briquettes) of iron ore slimes and similar materials, in the form of cold bonded pellets or briquettes are prepared. The cold bonded pellets or briquettes of the current invention are prepared using a mixture of iron ore slimes, 5-15 weight % CaO bearing materials. The Cao bearing material as per the current invention comprises LF slag, Lime, dolime and mixture thereof. The size of the cold bonded pellets or briquettes varies in the range of 10 to 40 mm, preferably, 10 to 25 mm.
These cold bonded agglomerates (pellets or briquettes) can be optionally cured/dried by waste process air heat. In second stage, these agglomerates (pellets or briquettes) are used as hearth and/or side layers. The heat of product (sinter and pellet) and heat of waste process air make the cold bonded slimes agglomerates suitable for use in agglomeration. First of these reaction is dehydration of combined water associated with these materials which starts occurring around 400 °C and since the temperature of these protective layers reaches as high as 1000 oC to 1200 °C in the process therefore second reaction of formation of calcium aluminate starts occurring. A part of the agglomerates is fused with main agglomerates and are high in strength and gets transferred to blast furnace. Other parts which are low in

strength reports to return fines but since these returns fines are free of combined water and contains low melting phases of alumina thus, processing of these materials becomes easy, since assimilation of these materials is easier as compared to original material, when they are recycled.
For sintering, direct usage of iron ore slimes and similar materials as raw material in the process is not preferred because of the reasons mentioned hereafter. First, the particle size distribution of slimes is not suitable for use in sinter making. Sinter prefers raw materials size in the range of 150 µm to 8 mm while slimes are almost 80 to 100% below 150 µm. Second, high water of hydration associated with slimes which require significant increase in solid fuel usage in sinter making. This kind of high combined water decreases productivity, increases fines, increases solid fuel consumption and thus slag content and are therefore economically unattractive for use in sinter making. The last major point is gangue associated with slimes, which in case of Indian iron ore slimes is majorly rich in alumina. Alumina associated with slimes is difficult to assimilate during sintering and thus requires higher fuel which is not suitable for complete economics and machine’s health related reasons.
For iron ore pelletization, the reasons are similar for disadvantages associated with direct usage of iron ore slimes and similar materials. First, the particle size distribution of slimes is not suitable for use in pellet making. Due to excess amount of -25 µm size fraction in iron ore slimes, the green balls formed are more densely packed and therefore oxidation is not proper during induration. Second is high water of hydration, which forces pellet makers to decrease machine speed and thus production. As a rule of thumb, each 1 percent increase in LOI i.e. hydrated water, results in 2% decrease in productivity. Iron ore slimes also pose difficulty in balling. Again the last major point is gangue associated with slimes, which in case of Indian iron ore slimes is majorly alumina based. Alumina associated with slimes is difficult to assimilate during induration and thus requires either higher solid fuel or higher burner temperature. Both of which are not suitable in pellet making for example, higher burner temperature is not preferred for machine’s health and higher solid

fuel for product quality as excess solid fuel results in duplex structure of pellets having high magnetite at core and hematite around it.
The present invention relates to developing a two stage method for using iron ore slimes and similar materials without showing their adverse effect in sinter and pellet making processes. First is development of cold bonded pellets or briquettes of iron ore slimes and similar materials in the size range of 10 to 40 mm, preferably 10 to 25 mm , suitable to sustain thermal shock and compressive loading in sinter and pellet making processes. The curing/drying of these cold agglomerates can be externally provided by waste process air heat. Secondly, these agglomerates are used as hearth and/or side layers, during which due to sensible heat of product (sinter and pellet) and sensible heat of waste process air, two major reactions occurs for making slimes suitable for use in agglomeration. These are basically decomposition of combined water associated with these materials which starts occurring around 400 °C and formation of calcium aluminate, mono calcium ferrite phases and dissolution of former in later between 1050-1150 °C. Part of these agglomerates either get fired, have high strength and thereafter transferred to blast furnace and part of these agglomerates are weaker and report in return fines due to poor firing but since these returns fines are free of combined water and contains low melting phases of alumina thus, processing of these materials becomes easy.
BRIEF DESCRIEPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Process Schematic of a sinter plant
Figure 2: Process Schematic of a pellet plant
Figure 3: Showing fired pellet as protection layer of metallic parts in pellet making furnace
Figure 4: Showing sinter as protection layer of metallic parts in sinter making equipment

Fig 5: Process flow chart for using slimes in sinter and pellet making
Fig 6 (a)- Preparation of cold agglomerates of slimes and similar materials
Fig 6 (b)- Charging of cold bonded agglomerates on sintering machine as hearth layer.
Fig 6(c) – Treatment of hearth layer cold bonded agglomerates from sensible heat of bottom sinter and process gas
Fig. 7 Comparison of sintering time with sinter Vs. cold bonded pellet as hearth
Fig 8 Image showing strongly and weakly fused hearth layer pellets to main sinter mass
Fig 9: Figure showing decrease in hydrated water content of hearth layer cold bonded pellet
DETAILED DESCRIPTION OF THE INVENTION
In this work, a two stage method for using iron ore slimes in sinter and pellet making is developed. In first step, cold agglomerates of iron ore slimes and similar materials, in the form of cold bonded pellets or briquettes, in the size range of 10 to 40 mm, preferably 10 to 25 mm, suitable for charging as hearth layer in sinter making and both hearth and side layer in pellet making operations, are developed. The curing/drying of these cold agglomerates is provided by waste process air heat. For making cold bonded agglomerate suitable for this process, calcined lime, dolime, or ground Ladle furnace slag in appropriate combination are used as binder. In second stage, these agglomerates are used as hearth and/or side layers, during which due to sensible heat of product (sinter and pellet) and sensible heat of process air, two major reactions occurs which makes these slimes suitable for use in agglomeration. First of these reaction is dehydration of combined water associated with these materials which starts occurring beyond 400 °C and

since the temperature of these protective layers reaches as high as 1000 oC to 1200 °C in the process therefore second reaction of formation of calcium aluminate, and mono-calcium ferrite starts occurring. In the temperature range of 1050-1150 calcium aluminate starts dissolving in mono-calcium ferrite phases. A Part of the agglomerates (mainly the top few layers of hearth layer) is fused with main agglomerates and are high in strength and gets transferred to blast furnace along with main agglomerates. Other parts which is low in strength reports in return fines but since these returns fines are now free of combined water and contains low melting phases of alumina, thus, processing of these materials becomes easy when they are recycled.
Schematic of sinter plant operation is shown in fig 1 and is described below. Separate stock piles of iron ore fines, fluxes, solid fuels, by-products etc. are stored in raw material yard. These raw materials are transported to proportioning unit wherein raw material are weighed and proportioned as per the required material balance. This proportioned material is conveyed to mixing and granulation unit where water and lime fines are added and mixing is done and wet granules are formed. The granulated mixture is then transported to feeding hopper of sinter plant straight grate furnace. Feeding hopper feeds the material on endless pallet car of sintering machine. Before feeding the mixture granules on the pallet cars, sinter of size (10-25 mm) are layered as hearth layer using hearth layer sinter bins. This bed of un-sintered mix having hearth layer at bottom and granules over it moves forward with the movement of endless car in forward direction. When the bed comes under ignition hood, top layer gets ignited and due to suction from bottom, flame front proceeds downwards and sintering occurs. Hearth layer in the process acts as a heat barrier between hot sinter and metallic grate bars.
On sintering completion, sinter machine is tilted and sinter cake falls on spike crusher and crushed sinter is transported to sinter cooler. Cooled sinter, thereafter, screened into three fraction, minus 5 mm which goes to

granulation unit for recycling, plus 5 mm which goes to blast furnace stock house and some amount of 10-25 mm size sinter which is recycled back in sinter plant for its use as hearth layer.
Schematic of pellet plant operation similar to sinter plant is shown in fig 2 and is described below.
Separate stock piles of iron ore fines, fluxes, solid fuels, by-products etc. are stored in raw material yard. These raw materials are transported to proportioning unit wherein raw material are weighed and proportioned as per the required material balance. This proportioned material is conveyed to drying unit and then grinding (ball mill) unit and thereafter to intensive mixing unit where water and bentonite powder is added to the grounded product coming from ball mill. This mixture is transported to balling unit (drum or disc pelletizer) where wet balls of 6-16 mm size are formed. These balls are then transported to induration unit of pellet plants and charged onto the endless chain of pallet cars. Before charging the wet balls on the pallet cars, fired pellets of size (9-16 mm) are layered as hearth and side layer using respective layer bins. This bed of wet balls having fired pellets as hearth layer and side layer moves forward with the movement of endless car in forward direction. When the bed passes through different zones of induration unit, wet balls get indurated .
On completion of the process, pallet machine is tilted and fired pellets are discharged as product. After screening undersize fraction (-6 mm) and certain amount of 9-16 mm pellets to be used as hearth layer, rest is transported to furnaces for iron making.
Fig 3 and 4 are transverse cross section of endless car of pellet and sinter making furnaces, respectively.
Fig 3 describes the arrangement of unfired pellet as feed for induration and fired pellet as layer for protecting metallic parts of the furnace. In fig 3,

unfired pellet (3) and fired pellet (2) and (4) are charged such that fired pellet are layered as side layer and hearth layer. These layer are charged in order to protect side wall (1), grate bars (7) and also center casting and wind box etc. from heat. This car moves in the induration unit in different zones with the use of friction roll (5) and gas is prevented from escaping using gas sealing (6).
Similarly in sintering, in fig 4, sinter of 10-25 mm size is charged as hearth layer (3) over the metallic grate bars (6). Un-sintered granulated wet mixture (2) is charged over it. This material is hold up inside the pallet car with the support of side wall (1). As the machine moves forward on friction rolls (4) sintering progresses downward as described earlier.
Process flow chart for using iron ore slimes and similar materials in sinter and pellet making is shown in figure 5 and described below.
Iron ore slimes and similar materials are mixed with CaO bearing materials like LF slag, Lime, dolime etc., suitable binder like bentonite and carbon bearing material. Enough water is added either to the mix or to the CaO bearing materials to make slaked solution. After subjection to intensive mixing cold bonded agglomerates are formed in the form of either of pellets, briquettes, extruded products etc. These are subjected to initial curing using heat of process waste gas. Cured agglomerates are layered as hearth layer in sintering and both hearth and side layer in pelletization. After layering cured agglomerates as hearth layer, wet granulated mixture is charged over it in sinter making and green pellets (wet pellets) are charged in pellet making process. During the process of sintering and pelletization, sensible heat of pellets and sinter as well as sensible heat of waste exit gas results in both the removal of hydrated water and formation of low melting point phases of alumina in the agglomerates. Because of the treatment of these hearth layer agglomerates by sensible heat of waste gas as well as bottom sinter and pellet near to hearth layer, a part of the agglomerate achieve sufficient

strength to be charged to Blast furnace etc. and other part though not sufficient in strength but becomes an easily recyclable material because of alumina not being present as raw alumina but in a calcium based phase which is low in melting point and easy to assimilate during recycling in any of the processes of sinter or pellet making.
Schematic of the process for using slimes and similar materials in sinter making is shown in fig 6a, 6b, 6c and described hereafter.
Iron ore slimes (1), preferred combination of LF slag (2), Dolime (3) and lime (4) etc. CaO bearing materials, suitable binder like bentonite and carbon bearing material and optimum water are mixed together in intensive mixing unit (5). This wet mixture is charged into wet agglomeration units like disc pelletizer (6) or briquetting (7) or any other cold agglomeration units like extrusion, drum pelletizer etc. Suitable size range of these agglomerates is 8-20 mm and preferable size range is 10-15 mm size. After production of these cold bonded wet agglomerates they can be subjected to atmospheric curing for 10-14 days and then conveyed to hearth layer bin (8) or they can directly conveyed to hearth layer bin (8) and cured inside the bin by using high CO2 and high temperature bearing waste exit gas of the process. From hearth layer bin (8) the cured cold bonded agglomerates, having sufficient strength to sustain load of bed above, are charged on the pallet car of straight travelling grate machine (12) to form the hearth layer (14). Simultaneously, at a small distance ahead of the hearth layer bin, feed mix bin (9) discharges the material onto drum feeder (10). Drum feeder discharges the feed onto segregation plate (11) which layers the feed onto the hearth layer (14). As the machine moves forward, a bed (13) comprising of un-sintered wet mixture granules over cured cold bonded agglomerates as hearth layer (14) is formed. As the machines moves forward, bed (13) comes under ignition hood (15). Ignition hood ignites the top layer of the bed (13) and a flame front

(17) is generated. Since the suction is applied from the bottom wind boxes (16), fuel in the lower level gets ignited while the earlier ignited zone gets sintered. This process of fuel getting ignited in lower level and sintering continues and hence flame front travel downwards as shown in fig 6 (c) wherein (18) represent sintered zone, (17) flame front zone and (20) un-sintered zone. When the flame front (17) reaches bottom of the pallet car where cold bonded agglomerates are layered as hearth layer, due to high temperature of flame front zone (~1350-1400 °C) some portion of the hearth layer (mainly top few layers) gets indurated and fused into the main agglomerate mass while the lower part of hearth layer reaches a temperature of ~1200 °C due to which dehydration or decomposition of hydrates associated with cold bonded hearth layer agglomerates occur. Also, since the temperature is greater than 1050 °C, calcium aluminate formation, calcium ferrite formation and dissolution of the former into the latter occurs. These bottom part hearth layer during discharge and transportation breaks into pieces and reports in return fines but since these fines now contains alumina in calcium based low melting point form therefore it becomes easy to recycles it as compared with ore having free alumina in iron oxide matrix. Also, it has been observed that as the flame front is generated and moves downwards, a moisture front also gets generated by evaporation of moisture in front of flame front and condensation of same a little below the flame front. This leads to increased moisture in the bottom layer zone which increases the plasticity of the bottom layer. Increased plasticity leads to compaction of bottom layer and hence flame front travels at very slow speed in the bottom zone as compared to middle and top layer of sinter bed and hence low productivity. Cold bonded agglomerates charged as hearth layer absorbs excess moisture of the bottom layer of bed while still maintaining its own voidage results in nullifying of moisture front at bottom and its deteriorating effect on productivity of the sinter plant. Similarly, charging of cold bonded

agglomerates results in similar advantage in pellet making as explained earlier.
Example:
Cold bonded pellets with 10-15 mm size were prepared using materials provided in table 1.

10% water was added to prepare the wet mix and then green pellets were prepared in drum type pelletizer. These pellets were then cured for 3 days by covering under a plastic and then were dried for another 7 days to gain strength. CCS of the cured pellets was measured. Average CCS of cured pellet was 6.37 kg/pellet. Accelerated curing can be done instead of atmospheric curing by using process gas of sintering furnace due to its higher temperature and CO2 content. Nullifying the effect of moisture front formation at the bottom part of sinter bed resulted in increase in production and is shown in fig 7. Completely indurated and Fused cold bonded pellets (from upper part of hearth layer) with bottom sinter and weakly or partially fused pellets with bottom sinter are shown in fig 8. LOI i.e. loss on ignition or hydrated water of lower part of hearth layer (weakly or non-fused pellets) mentioned in fig 8 is shown in fig 9. It shows significant decrease from 8.57 to 3.58. Since lots of pellet got rid of the hydrated moisture completely and were fused with the main sinter mass therefore weighted average value of LOI will be significantly lower to ~2% which is in the normal operating regimes of sinter plant units. CCS of the pellets fused with the bottom sinter was measured. Average CCS was found to be 143.5 kg/pellet.

Similar results and advantages are expected in pelletizing route of agglomeration. It is also to be understood that the invention is not limited by the specific example and embodiment described hereinabove, but includes such changes and modifications as may be apparent to one skilled in the art.

WE CLAIM:
1. A method to produce sinter or pellet from waste iron ore slimes in a
sintering/pelletization process, the process comprising:
placing a bed of cold bonded pellets or briquettes at the hearth layer in sintering machine in sinter making process and both hearth and side layer in pelletizing machine in pellet making process wherein the bed of cold bonded pellets or briquettes absorb heat of hot sinter/pellet and waste exit process gas.
2. The method as claimed in claim 1, wherein the cold bonded pellets or briquettes are un-fired.
3. The method as claimed in claim 1, wherein the bed height of cold bonded pellets or briquettes varies in the range of 20 to 100 mm.
4. The method as claimed in claim 1, wherein the cold bonded pellets or briquettes are prepared using a mixture of iron ore slimes, 5-15 weight % CaO bearing materials.
5. The method as claimed in claim 4, wherein CaO bearing material is
preferably selected from group comprising LF slag, Lime, dolime and
mixture thereof.
6. A method wherein as claimed in claim 1, wherein cold bonded pellets or briquettes are optionally given initial curing/drying using waste exit gas of sinter and pellet plant units before placing it on the hearth payer.
7. A method wherein as claimed in claim 1, wherein size of cold bonded pellets or briquettes varies in the range of 8 to 25 mm, preferably, 10 to 15 mm.

8. A method as claimed in claim 1, wherein Cold Compressive Strength (CCS) of cold bonded pellets or briquettes increases by 140 kg/pellet.