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 to produce 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 using the iron the metallurgical waste of a steel plant and iron ore fines in the blast furnace. 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 Principle of sintering

 The principle of sintering involves the heating of iron ore fines along with flux and coke fines or coal to produce a semi-molten mass that solidifies into porous pieces of sinter with the size and strength characteristics necessary for feeding into the blast furnace.

It is basically an agglomeration process achieved through combustion.

The Product Sinter

The product of the sintering process is called sinter and is having good following quality characteristics

  1. Chemical analysis
  2. Grain size distribution
  3. Reducibility
  4. Sinter strength

Typical properties of sinter are given in Tab 1

 

Tab 1  Typical properties of sinter

Item Unit Value
Fe % 56.5 to 57.5
Feo % 6.0-8.0
SiO2 % 4.0 to 5.0
Al2O3 % 1.8 to 2.5
CaO % 7.5 to 8.5
MgO % 1.6 to 2.0
Basicity (CaO/SiO2) 1.7 to 2.9
ISO Strength  (+6.3mm) % >75
RDI (-3 mm) % 27-31

The product sinter is shown in Fig. 1.

sinter

 Fig 1   Sinter

Advantages of adding flux to sinter

Sinters are classified into acid sinter, self-fluxing sinter and super fluxed sinter. Self-fluxing sinter brings the lime required to flux its acid components (SiO2, Al2O3). Super-fluxed sinter brings extra CaO to the blast furnace. For self-fluxing and super-fluxed sinter, the lime reduces the melting temperature of the blend and at relatively low temperature. Incase ofself-fluxing and super-fluxed sinter, the lime reduces the melting temperature of the blend 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 sinter
  • It reduces the melting temperature of the iron ore blend
  • It promotes the calcination reaction of the limestone (CaCO3 =CaO + CO2) outside of the blast furnace hence saving heat consumption in the blast furnace.

The 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 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 pallets assist in obtaining optimum permeability during the sintering process. These micro pellets are then conveyed to the sintering machine and charged.

A layer of controlled size sinter (bedding) is fed to the bottom of the sinter machine grates for the protection of the grates. After this the moistened micro pellets of the raw materials mix is fed and leveled.

After the material is leveled on the sinter machine, the surface of the charged material on the sinter machine is ignited using gas or oil burners. Air is drawn through the moving bed causing the fuel to burn. Sinter machine velocity and gas flow are controlled to ensure that “burn through” (i.e. the point at which the burning fuel layer reaches the base of the strand) occurs just prior to the sinter being discharged. During the machine movement the sintering of the material bed on the grate proceeds downward. 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.

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.

In the screening section the product sinter, bedding and return fines are separated. Return fines, not suitable for downstream processing, are conveyed to a bin for recycling in the sintering process.

Waste gases are treated for dust removal in a cyclone, electrostatic precipitator, wet scrubber or fabric filter.

The material flow of the sinter plant is shown in Fig 2.

 sinter process flow

 Fig 2 Material flow in a sinter plant

 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.

Sinter machines

Sinter machines are of two types i) circular and ii) Straight line. Straight line

Machines are also being known as Dwight and 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 Table 2.

Table 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 plant

Sqm

8000

9000

10000

12000

Land needed for building

Sqm

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 3

circular sinter machine

 Fig 3    Circular machine

 Straight line machines are normally used for large sinter plants. Present straight line machines are installed having widths from 2 meters to 5 meters and with effective sintering areas from 200 to 600 Sq m. The productivity of such machines typically ranges from 30 to 46t/Sqm/day. Capacities of such machines range from 190,000 tons per annum to 6.5 Mtpa. A straight machine is shown in Fig 4.

Straight line machine

 Fig 4   Straight line machine

 Important issues related to sinter And sinter plants

  1. Use of sinter reduces the coke rate and enhances the productivity in blast furnace
  2. Sintering process helps utilization of iron ore fines (0-10 mm) generated during iron ore mining operations
  3. Sintering process helps in recycling all the iron, fuel and flux bearing waste materials in the steel plant.
  4. Sintering process utilizes by product gases of the steel plant.
  5. Sinter can not be stored for a long time as it generate excessive fines during long storages
  6. Sinter generates excessive fines during multiple handling