The Sintering Process of Iron Ore Fines
The Sintering Process of Iron Ore Fines
Sintering plants are normally associated with the production of hot metal in blast furnaces in integrated steel pants. The process of sintering is basically a pre-treatment process step during iron making for the production of the charge material called sinter for the blast furnace from iron ore fines and also from metallurgical wastes (collected dusts, sludge and mill scale etc.).
The sintering technology was originally developed for the purpose of utilizing in the blast furnace the iron ore fines and the iron present in the metallurgical waste of a steel plant. But currently the focus has changed. Now the sintering process aims to produce a high quality burden for the blast furnace. Today sinter is the main metallic burden for a large blast furnace.
The sintering process is used to agglomerate a mix of iron ores (blend), return ﬁnes, ﬂuxes and coke, with a particle size of less than 10 mm, so that the resulting sinter, with a screened size of 10 mm to 30 mm, can withstand pressure and temperature conditions in the blast furnace.
Principle of sintering
Sintering is a thermal process (carried out at 1300 deg C to 1400 deg C) by which a mixture of iron ore, return fines, recycled products of the steel plant industry (such as mill scale, blast furnace dusts, etc.), slag forming elements, fluxes and coke fines are agglomerated in a sinter plant with the purpose of manufacturing a sintered product of a suitable chemical composition, quality (physical) and granulometry to be fed into the blast furnace, thus ensuring a homogenous and stable operation of the blast furnace . Prior to sintering, there is an important process called granulation. Granulation is the homogenization of the iron ore mixture in a rotating drum with 7 % to 8 % of water with the objective of obtaining of a pre-agglomerated product, which is then delivered as a layer over a continuously moving grate or strand for getting the sintered product. This process has a fundamental role since it ensures an adequate sinter bed permeability and hence good productivity of the sintering machine.
The flexibility of the sintering process permits conversion of a variety of materials, including iron ore fines, captured dusts, ore concentrates, and other iron-bearing materials of small particle size (e.g., mill scale) into a clinker-like agglomerate.
The process of sintering involves the heating of the pre-agglomerated product to produce a semi-molten mass which solidifies into porous pieces of sinter with the size and strength characteristics necessary for feeding into the blast furnace.
The product sinter
The product of the sintering process is called sinter and the quality characteristics of a good sinter include (i) chemical analysis, (ii) grain size distribution, (iii) reducibility, and (iv) sinter strength. Typical properties of sinter are given in Tab 1
Tab 1 Typical properties of sinter
|Fe||%||56.5 to 57.5|
|FeO||%||6.0 to 8.0|
|SiO2||%||4.0 to 5.0|
|Al2O3||%||1.0 to 2.5|
|CaO||%||7.5 to 8.5|
|MgO||%||1.6 to 2.0|
|2||Basicity (CaO/SiO2)||1.7 to 2.9|
|3||ISO Strength (+ 6.3 mm)||%||Greater than 75|
|4||Reduction degradation index (RDI)||(-3 mm) %||27 to 31|
|5||Reducibility index (RI)||(R60) %||55-75|
|6||Tumbler index||(-6.3 mm) %||65-75|
The sinter product of iron ore is shown in Fig 1.
Fig 1 Iron ore sinter
Types of sinter
Sinters are classified into acid sinter, self-fluxing sinter, and super fluxed sinter. Self-fluxing sinter has sufficient content of CaO (lime) in it which is required to flux its acid components (SiO2, and Al2O3). Super-fluxed sinter has additional content of CaO for fluxing of the acid components introduced in the blast furnace through other burden materials. In case of self-fluxing and super-fluxed sinter, the lime reduces the melting temperature of the sinter mix and at relatively low temperatures (1100 deg C to 1300 deg C) strong bonds are formed in the presence of FeO. The following are the advantages of adding flux to the sinter.
- It generates slag with the impurities present in the iron ores and solid fuels producing a suitable matrix for cohesion of the particles.
- It improves the physical and metallurgical properties of the sinter
- It reduces the melting temperature of the sinter mix.
- It reduces/eliminates the addition of limestone in the blast furnace thus saving the fuel needed for the calcination reaction of the limestone (CaCO3 =CaO + CO2) in the blast furnace hence reduces the coke rate in the blast furnace.
The sintering process
The process of sintering begins with the preparation of the raw materials consisting of iron ore fines, fluxes, in-plant metallurgical waste materials, fuel and return fines of the sinter plant. These materials are mixed in a rotating pelletizing drum and water is added in order to reach proper agglomeration of the raw materials mix. This agglomeration is in the form of micro-pellets. These micro pellets assist in obtaining optimum permeability during the sintering process. These micro pellets are then conveyed to the sintering machine and forms the upper layer of the charge mix.
The sintering process is a continuous process which is based on treating a charge mix (ore fines, return fines, and fluxes etc.) layer in presence of coke breeze to the action of a burner placed in the surface of the layer. In this way, heating takes place from the upper to the lower sections. The charge mix layer rests over a strand system and an exhausting system allows to the whole thickness to reach the suitable temperature for the partial melting of the mix, and the subsequent agglomeration.
In the Dwight-Lloyd sintering machine, the sintering grate is a continuous chain of large length and width, formed by the union of a series of pallet cars which make the sintering strand (Fig 2). Each pallet car passes below a charging hopper where it is charged firstly by material of coarse granulometry (10 mm to 20 mm) in a layer having thickness of 30 mm to 60 mm which forms the hearth layer composed mainly of return sinter. The hearth layer protects steel grates from over-heating during the sintering process.
Fig 2 Dwight Lloyd machine
A second layer of micro pellets is charged over the hearth layer and leveled. Then, the pallet car passes below an initializing furnace, where the combustible ignition takes place on the surface of the charge mix. At the same time, the mix is subjected to down draught suction through the sinter charge. Due to the down draught suction, air is drawn through the moving bed causing the fuel to burn.
The pallet car continues the process and the combustion progresses in the direction of the gas flow. In this way, the sintering process takes place. The combustion process does not happen simultaneously in the whole thickness of the bed. On the contrary, combustion happens as a horizontal layer which moves vertically through the bed. The thickness of this layer is a small fraction of the bed. Permeability of the bed is a quality requirement for the charge mix, and hence, the granulation process of the charge mix is an important step for the sintering process (permeability of the bed is improved due to granulation).
In the region above the combustion zone, very hot sintered product heats the air which passes through this layer. In this way, pre-heated air arrives to the combustion area. The heat of the air/gases previously heated is absorbed in these cold sections, causing preheating of the load and evaporation of the moisture of the charge mix. In this context, high temperatures which cause partial melting are reached, and the sintering process takes place.
The high thermal efficiency is caused by heat accumulation in a partial layer of the charge mix called sintering zone or flame front. The flame front progresses at a speed ranging from 10 mm to 30 mm/min towards the sintering grate. In a bed height of around 500 mm, the process normally takes around 25 minutes. Once the end of the strand is reached, the sintered material is discharged and subjected to cooling, crushing, and screening.
The sintering process is controlled by the ‘burn-through point’ (BTP) which is defined as the point where the temperature of the waste gas reaches its highest value. It is the point at which the flame front reaches the base of the bottom of the sinter bed. Sinter machine velocity and gas flow are controlled to ensure that the burn through point occurs just prior to the sinter being discharged. Burn-through point determination is very important in order to stabilize the process and to improve both quality and productivity.
At the end of the machine, the sintered material in the form of cake is discharged into the hot sinter crusher. Here the hot sinter cake is crushed to a pre-determined maximum particle size. From here the sinter is discharged onto sinter cooler which can be either straight line or circular cooler. After cooler the sinter is transferred to the screening section where it is divided normally into three granulometric fractions. The first fraction consists of 0 mm to 5 mm which is called return fines and is sent to the feeding hoppers. Sinter fraction with a granulometry range within 5 mm to 15 mm is used as hearth layer in the sinter strand. The balance amount of 5 mm to 15 mm fraction which is not used for hearth layer is mixed with the third granulometric fraction having a size range of 15 mm to 50 mm is sent to the blast furnace.
Return fines are unavoidably generated during the sintering process, and are recycled back into the sintering process. Return fines generally consists of around 30 % to 40 % of the iron bearing materials. Return fines from sintering sieving are a little easily assimilated than those from the blast furnace sieving because of the lower high-calcium ferrite content.
There are four zones which are identified in the sinter bed height. These are given below.
- Cold and wet zone – It includes the zone of the sinter bed with a temperature of less than 100 deg C. This area is formed by the charge mix to be sintered, with upper limit saturated in water/water vapour.
- Drying zone – It includes the sinter bed area with temperatures ranging between 100 deg C and 500 deg C. The evaporation of the sinter mix moisture and subsequent dehydration of hydroxides take place in this zone.
- Reaction zone – It includes the zone of the sinter bed with a temperature ranging between 500 deg C (coke ignition beginning) and 900 deg C (cooling period beginning). The maximum temperature which reaches in this zone is in the range of 1300 deg C to 1400 deg C. The main processes which happen in this zone are (i) coke combustion (exothermic), (ii) carbonates decomposition (endothermic), (iii) solid phase reactions, (iv) reduction and re-oxidation of iron oxides, and (v) reactions of formation of the sintered mass.
- Cooling zone – This zone is found immediately after the reaction zone. In this zone, cooling and re-crystallization of the sintered product take place. There is a superficial zone where the sinter layer is brittle than in the rest of the sinter bed.
The cause of sinter process fluctuations has been studied to lower consumption of carbon, thus lowering carbon dioxide emissions. Frequency analysis of plant data (exhaust gas temperature at one wind-box, mixture charging level and mixture moisture content) has indicated that feed mixture moisture variations are linked to fluctuations of gas exhaust temperature at the particular wind-box. Process control improvement by controlling the feed mixture moisture content in a narrow band of values enables lowering of carbon consumption and leads to lesser emissions of carbon dioxide.
Waste gas circuit is to be fully leak proof, not allowing air from atmosphere to be sucked by the system. This results into saving of power in the waste gas circuit. Waste gases are treated for dust removal in a cyclone, electrostatic precipitator, wet scrubber or fabric filter. Flowsheet of the sinter plant is shown in Fig 3.
Fig 3 Flowsheet of the sinter plant
Sinter machines are of two types namely (i) circular machines, and ii) straight line machines. Straight line machines are also being known as Dwight Lloyd machines. Dwight and Lloyd constructed the first continuous sinter plant in 1906.
Circular sinter machines are normally suitable for blast furnaces having useful volumes of 650 Cu m and less. The man parameters of some of the circular machines are given in Tab 2.
Tab 2 Main parameters of circular sinter machines
|Subject||Unit||12 sqm machine||17 sqm machine||25 sqm machine||33 sqm machine|
|Annual production||1000 tpa||172||253||404.5||556|
|Total power requirement||kW||700||1500||1750||2400|
|Land needed for the plant||sq m||8000||9000||10000||12000|
|Land needed for the building||sq m||800||3400||35000||3600|
Various features of the circular machines are as below.
- When compared with the straight line machines the capital investment costs are low and the construction periods are short.
- Sealing is better and air leakage is less in these machines since the wind boxes move synchronously with grates and since water sealing is adopted.
- Discharging system makes the size of cold sinter such that there is no need of an additional crusher.
- The circular machines are having high operational flexibility.
A circular sinter machine is shown in Fig 4.
Fig 4 Circular sintering machine
Straight line machines are normally used for high capacity sinter plants. The sintering areas of such machines are generally 50 sqm and above. Present straight line machines are installed having widths ranging from 2 m to 5 m and with effective sintering areas ranging from 200 sqm to 600 sqm. The productivities of such machines are typically in the range of 30 t/sqm/day to 46 t/sqm/day. Capacities of such machines range from 190,000 tons per annum to 6.5 Mtpa. A straight machine is shown in Fig 5.
Fig 5 Straight line sintering machine
Important issues related to sinter and sinter plants
The following are the important issues related to sinter and sinter plants.
- Use of sinter reduces the coke rate and enhances the productivity in blast furnace.
- Sintering process helps utilization of iron ore fines (0-10 mm) generated during iron ore mining operations.
- Sintering process helps in recycling all the iron, fuel and flux bearing waste materials in the steel plant.
- Sintering process utilizes by product gases of the steel plant.
- Sinter cannot be stored for a long time as it generates excessive fines during long storages.
- Sinter generates excessive fines/dust during multiple handling in the sinter plant.