Technologies for improvement in Sintering Process

Technologies for improvement in Sintering Process

Sintering of iron ore is a metallurgical process carried out on a sintering machine. It is basically a pre-treatment process step during iron making to produce charge material called sinter for the blast furnace. It is an agglomeration process achieved through combustion. In this process air is sucked at the sinter strand through a bed of raw mixture (also called sinter mix) of iron ore fines, limestone, dolomite, sand and quartzite fines (flux), solid fuel (coke breeze or anthracite) and metallurgical wastes (collected dusts, sludge and mill scale etc). The fuel particles on the top surface layer are first ignited in a furnace and as the strand move forward, the ignited or combustion front proceeds gradually downwards through the bed until the end is reached. . In last few decades several technologies have been developed which have not only resulted into vast improvements in the process of sintering but also have improved the quality of sinter. Majors of these technologies are given below.

Use of burnt lime as a replacement of limestone

Burnt lime (CaO) is an active binder since it gets hydrated {Ca(OH)2} as a result of hydrate reaction with water’ As a binder it promotes the quasi particle property in the sinter mix. It helps in increasing the micro fines input through iron ore fines. Due to better granulation of sinter mix it improves sinter productivity. In addition to the binding property, burnt lime also reduces the coke breeze rate due to reduced calcination of limestone during the sintering process. The use of burnt lime also reduces the crushing and screening load of harder raw limestone and hence saves in energy.

Intensive mixing and granulation system

The raw materials for sintering which contain iron ore fines, fluxing agents and waste materials are from different sources and have varying characteristics. They need to be blended to form a homogeneous mixture. Intensive mixing and granulation system enables an optimum preparation of the sinter mix by homogenizing the raw material feed and eliminates the need for blending yards. The system basically consists of high speed agitating mixer and a granulation drum. The system results in increased granulation rate, improvement in bed permeability, more equalized burn through zone and optimum burn through point control. With this system a more homogeneous sinter mix is prepared which reduces coke breeze consumption up to 5 % and increases the sinter productivity by up to 2 %. The system facilitates use of higher percentage of ultra fines in the sinter mix.

Twin layer charging

With a uniform charging of sinter mix on the sinter strand can lead to higher temperature causing fusion of the sinter mix. This will restrict downdraft air flow and sintering process. In twin layer charging, smaller grain size charge materials with higher concentration of coke breeze is charged in the top layer. Larger grain size material (ore and sinter return) with lower coke breeze concentration is charged in the bottom layer. This ensures proper passage of heat in the lower layers, high bed permeability and efficient use of fuel.

Improvements in sinter mix feeding equipment

Segregated blend loading of sinter mix results into big particles at the bottom and small particles at the top of the sinter mix bed on the pallets of sinter machine strand. The segregated blend loading helps in the permeability of the mix and hence helps in improving the machine productivity. There are several designs of the charging system for segregated loading. Some of them are (i) installation of an additional screen on the conventional sloping chute (ii) intensified sifting feeder (iii) segregated slit wire (iv) magnetic breaking feeder. Fig 1 shows charging systems without  segregated blend loading system and different types of the blend loading systems.

Sinter mix feeding arrangement

Fig 1 Segregated blend loading

 Multi slit burner in Ignition Furnace

While igniting the top of the sinter mix bed on the sinter machine in the ignition furnace, flame stability of the burner is essential. Multi slit burners help produce a single wide large stable flame which eliminates no flame areas and supplies minimum heat input for ignition. This in turn results into saving of energy input in the ignition hood. It has been reported in a Japanese plant that the total heat input for ignition with multi slit burners has been reduced by approximately 30 % compared with conventional burners. Outline of muti slit burner is in Fig 2.

multi slit burner

Fig 2 Outline of a multi slit burner

Stand support sintering

In the stand support sintering method the load of sinter cake in the upper portion of the sinter mix bed is supported by steel stands during the sintering process. The load of the sinter cake on the combustion-melting zone below it makes the sinter mix bed shrink (bed compaction), and thus significantly deteriorates the permeability of the bed. The support stands installed inside the sintering pallets begin to support the load of the sinter cake above at the time when the sinter mix bed portion around the tops of the stands begins to solidify after heating and melting. The sintering process of the lower portion of the bed proceeds thereafter under a reduced load, and a permeation network develops well in the portion to improve permeability. Due to stand support system the productivity of sinter machine increases significantly and the machine runs more stably.

Waste heat recovery

Heat recovery at the sinter plant is a means for improving the efficiency of sinter making process. Hot sinter is required to be cooled. The recovered heat from the sinter cooler is used to preheat the combustion air for the burners in the ignition furnace and to generate high pressure steam which can be used for generation of electricity. The system is also known as emission optimized sintering. Besides recovery of heat, the system helps in reduction in SOx, NOx and particulate emissions and improving the productivity, yield and cold strength of the sinter. Energy recovery to a level of 30 % is being achieved by this method.

Dust emission control

Increase of production in sinter machines leads to higher dust generation which means higher particulate emissions. These emissions are dust laden and contain a ide variety of organic and heavy metal hazardous air pollutants (HAPs). By sending the waste gas to electrostatic precipitators through negatively charged pipes, the particulate matter in the waste gas stream becomes negatively charged. Routing this stream past positively charged plates will then attract and collect the negatively charged particulate matter, thereby producing clean waste gas and increasing the quantity of steam recovery. Coarse dusts are removed in dry dust catchers and recycled. Use of ESP reduces the dust level of the off gases.

Selective waste gas recirculation system

Hot waste exhaust gases from the first and third sections of the sinter machine is mixed with off air of sinter cooler and ambient air and is recirculated back to the second section of the sinter machine. During the sintering process the sucked air volume is normally higher than required for complete combustion of the fuel in order to allow a high velocity of the flame front. Sinter waste gas therefore typically contains around 12 % to 15 % residual oxygen. It is also at a temperature which is well above the critical dew point. This is sufficient for recirculation to the sintering process after the addition of a small amount of supplementary air. A part of the waste gas is recycled back to a hood which covers a part of the sinter strand. The advantages of the system are (i) Reduced waste gas volume per unit sinter by around 50 % and reduced coke breeze consumption by 10 % to 15 % because of waste gas heat utilization and CO post combustion. Typical schematic diagram of selective waste gas recirculation system is given in Fig. 3.

selective waste gas recirculation
Fig 3 Typical schematic diagram of selective waste gas recirculation system

Automation system

Optimum process control conditions are achieved when perfect alignment of process parameters takes place. Integrated level 2 automation system helps a lot in this direction.  With this system, the standard deviation of quality parameters can be decreased by around 5 % to 10 %. This system also helps in reduction of coke breeze consumption can be reduced by around 3 % and productivity can be increased by around 3 % to 5 %.

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