Granulation of Liquid Iron
Granulation of Liquid Iron
Granulation of liquid iron is a method of handling of excess production of hot metal in a blast furnace (BF) which cannot be consumed by steel making in the steel melting shop of an integrated iron and steel plant. It is a cost effective method of producing a solid product which is known as granulated iron (GI). GI has good chemical and physical properties like pig iron and can be used as a prime raw material for the purpose of steel making.
GI has a chemical composition identical to the liquid iron which is being granulated. There is no oxidation or slag entrapment in the GI and there is high metallic content. Fig 1 shows some pieces of GI.
Fig 1 Granulated iron
A GI plant takes care of any mismatch between the production at the iron making facilities and the requirement of liquid iron at the steel making facilities. It is logistically positioned in between the two facilities. Excess liquid iron from the BF is diverted to the GI plant for the production of GI. This eliminates reduction of hot blast volume at the BF while producing GI which can be used as internal feedstock as coolant in the BOF, or for external sales to be used by the cupolas, induction furnaces (IF) and electric arc furnaces (EAF).
GI plants can be constructed and operated with capacities matching with the BF outputs. They are alternative to the pig casting machines (PCM) but with considerable higher capacities. The capacities of even twin strand PCMs are limited due to the solidification time of the liquid iron in the pig moulds. The PCMs also requires frequent mechanical maintenance as a consequence of the complex design. The GI has identical properties to that of pig iron but with an additional advantage that it can be handled in bin systems.
The four basic steps of the granulation process for liquid iron are the following.
- Control of flow of liquid iron to the granulator
- Granulation by forming of droplets of liquid iron and their rapid quenching in water
- Discharge of solidified and cooled GI usually by air water ejector
- Dewatering of GI and transport to storage location
The equipment and the process of granulation
The equipment can be designed for granulation of large batches of liquid iron and at the production rate of a BF. The principle is based on heat transfer between the liquid iron and the cooling water. Heat released during the cooling and solidification of the liquid iron is transferred to the cooling water, which carries the heat out of the process.
The equipments used for granulation of iron are standard equipments.
For handling of liquid iron ladles at the granulation plant, a twin ladle turret, similar to the turret used in a continuous casting machine is used. The turret in combination with a tundish can ensure a smooth sequence operation at the granulation plant without any discontinuity between liquid iron ladles.
Tundishes used for granulation of iron are standard continuous casting tundishes with minor modifications. Tundishes are equipped either with stopper rod system or slide gate system. This enables stopping of granulation quickly in case of a problem. Standard system can be used for controlling the rate of granulation. The tundish and nozzle are normally preheated for around 20 to 30 minutes before the start of the granulating.
Liquid iron granulator is the heart of the process. It consists of a tank which contains water and has a liquid iron distributor. The main function of the liquid iron distributor is to split liquid iron stream to smaller particles and to distribute it evenly over the water surface. Splitting of the liquid iron stream not only provide larger surface area for faster cooling of liquid iron but also distributes the liquid iron over a larger surface area in the granulator. This also helps in avoiding heat concentration in smaller water volume and thus allows a high flow rate of the liquid iron. The liquid iron distributor is a critical equipment since it has to withstand thermal shocks and long term impact of the liquid iron stream.
The outer surface of the liquid iron droplets gets solidified during its movement path from the distributor to the water surface and before penetrating the surface of the water. The remaining inner part of the semi liquid droplet, now a granule, is quenched as it strikes the water surface and starts its movement in the water volume. At the time of impact of granules into the water they get deformed slightly but are prevented from splitting up, thus avoiding fines generation.
During the movement of iron granules through the water in the granulating tank, the heat of the iron gets transferred to the cooling water. The cooling water allows the granules to reach a temperature below 100 deg C.
For a granulation rate of 100 tons/hr the generated heat load that is transferred from liquid iron to cooling water is in the range of around 8 M cal/second. Water system is designed to take care of this magnitude of heat load. In the water system the heat is distributed to the water in a way to ensure that the heat concentration (heat / volume unit) is less than critical concentration for vapour explosions.
The granulation tank holds enough water volume necessary to accommodate the liquid iron droplets formed by splitting of liquid iron stream and by the impact momentum of the liquid iron stream on the water surface.
The design and construction of water tank facilitates the concentration and discharge of the pieces of cooled GI from the tank. Air -water ejector systems are normally employed for the discharge of cooled granulated iron.
The discharged solidified GI is dewatered and conveyed by the conveyor belt to the storage area where it is stored in a stockpile for dispatch.
The water cooling and handling system is carefully balanced so as to ensure that the large amount of heat added by the liquid iron is removed by the cooling water. The water system is normally a closed circuit process water system. The flow of cooling water in the granulation tank is counter current to the movement of liquid iron. During its flow in the granulated tank water takes the heat of the liquid iron and gets heated up. The heated water from the granulated tank is removed and sent back to the water handling system. The return hot process water is cooled either in a cooling tower or through heat exchangers.
Liquid iron granulation plants are usually completely automated which allows need of only small manpower to run the entire operation. The through put time for the granulation of iron is usually around 30 to 40 seconds and has a process yield of greater than 99 %. This is a good improvement when compared with the lower yields being obtained in the PCMs.
The following are the important characteristics of the process of granulating liquid iron.
- Short start up time to meet last minute decisions of diversion of liquid iron from the steel making facilities
- Rapid processing time from liquid iron to a cooled GI
- No change in the chemical analysis of iron due to the rapid quenching
- More than 99 % of the process yield
- Rugged process with high availability, and with limited maintenance because of the use of standard equipments
- Easy to operate
- High production capacity matching to the output of BF
- Production of prime iron product that needs no additional processing
- Low environmental impact
- Flexible layout and can be accommodated in the existing space available
- Low operating cost
- Reasonable investment cost
Granulated iron product
GI has consistent physical and chemical properties. It combines the high metal content of prime scrap with the low residuals contents of virgin iron sources. From a practical point of view, the high bulk density and physical shape is suitable for efficient material handling.
The chemical composition of GI is the same as that of the liquid iron. Typical analysis is 4 % to 4.5 % of carbon, 0.5 % to 0.6 % of silicon, and around 95 % to 95.5 % of iron. Tramp elements (copper, nickel, molybdenum, and tin) are up to a maximum of 0.05 %.
GI have compact and small shape of a flattened sphere which results in a high bulk density of around 4 tons/cum. The size of GI is in the range of 8 mm to 25 mm. GI has high angle of repose, which allows for effective transport and storage.
Some of the characteristics of GI are given below.
- Homogeneous composition
- Practically no oxide content
- High metallic yield during steel making
- Very good preheating properties and fast melting/dissolution when added to metallurgical process
- Has iron carbide in the matrix, which is beneficial for scrap replacement in EAF operations
- High bulk density
- Inert during shipping and storage
- Has size and shape which facilitates handling with conveyor belt, magnet, front–end loader, bin systems and scrap skip
- Has high physical strength and shape which eliminates break offs during handling and reduces dusting
- Shows no pyroforic behavior and hence it can be transported and handled without concerns of combustion. It is inert during shipping and storage
The simplicity of the process of solidifying and cooling liquid iron, in combination with a high capacity that meets a standard BF throughput, makes the process of granulating iron suitable for installation at the integrated iron and steel plants.