Granulation of hot metal(HM) is a method for handling of excess HM produced in blast furnace (BF) which cannot be consumed in the steel melting shop (SMS). It is a cost effective method of producing prime product of granulated pig iron (GPI). Simplicity of the process of solidifying and cooling of HMin combination with high capacity that meets the BF output, makes this process suitable for installation at integrated steel plants.
HM is granulated by the ‘Granshot’ process. It was originally designed in early 1970s at the Hagfors works (Sweden) by UHT – Hagfors Teknologi AB(a Swedish engineering company). Both Granshot and GPI are the registered trade mark of the Swedish company UHT (Uvan Hagfors Teknologi).Since 2008, UHT and Danieli Corus have been working together on the implementation of Granshot systems. Presently Granshot plants for the production of GPI are working at six places. Granshotprocess has also been adapted on a large scale by ferro alloy industry with more than 35 installations globally.
Granshot plant takes care of any mismatch between the production of HM and its requirement at SMS. It is logistically positioned in between BF and SMS. Excess HM from BF is diverted to Granshot plant for GPI production. This eliminates reduction of hot blast volume at BF. Granshot plants are constructed and operated with capacities matching output ofBF.
GPI has identical properties to that of pig iron with an additional advantage that it can be handled with conveyors and bin systems.Itcan be used internally as coolant in basic oxygen furnace, or when sold externally can be used in cupola, induction furnace, or electric arc furnace. It has good chemical and physical properties and can be used as raw material in steelmaking. Chemical composition of GPI is thatof the HM being granulated. There is no oxidation and slag entrapment in GPI and it has high metallic content.
Equipment and the process of granulation
Equipment of Granshot process can be designed for granulation of large batches of HM at a rate of upto 250tons/hr per production line. HM granulation plants can be designed for granulation rate of upto 4 tons/min with HM ladle capacity of 200 tons. The principle is based on heat exchange between the HM and the cooling media(water). Heat released duringHM cooling and solidification is transferred to the cooling water which carries heat out of the system.
A twin ladle turret, similar to continuous casting (CC) machine turret, handlesHM ladles during granulation operation. Turret in combination with a tundish ensures smooth sequence operation without any discontinuity between ladles.
Tundishes used in Granshot process are standard CC tundishes with minor modifications. Tundish is equipped with stopper rod system enabling granulation to stop within seconds if problem occurs.Stopper rod is normally used for only start and stop. Rate of granulation is controlled by HM head control via load cells on tundish in combination with selection of outlet nozzle diameter. Upto 8 numbers of ladles in sequence with one tundish are usually being achieved.
A ceramic spray head is used to distribute the stream of HM evenly over the water surface. Spray head is critical equipment since it has to withstand thermal shocks and long term impact of the HM stream.
In the Granshot system having high granulation rates,HM is split up against an impact refractory disc placed in the centre of a cylindrical water tank. The disc oscillates vertically up and down in a controlled manner, distributing HM over a large surface area. The method reduces the power concentration in the water basin and hence allows a high metal flow rate.
Granulation tank holds water volume necessary to accommodate HM film and formed droplets distributed from the sprayhead by the impact momentum of HM stream.Tank has an upper cylindrical part and a lower conical part, which concentrates the iron granules to tank centre in the lower end.
Solidifiediron granules settle in granulation tank’s lower end and are transported out of the tank onto a dewatering screen by means of an air-water ejector system. After major amount of water has been removed in the dewatering system, conveyor belts transport GPI to itsstorage area.
For a granulation rate of 250 tons/hr, the generated heat load that is transferred from HM to water is in the range of 19-20 M cal/second. With this magnitude of heat load in a water system, heat has to be distributed to the water in a way securing that heat concentration (heat/volume unit) is less than critical concentration for vapour explosions.
Water cooling and handling system is carefully balanced so as to ensure that large amount of heat added by the HM is removed. From a closed circuit process water system, cooling water is pumped into the granulation tank’s lower section. As water moves upwards through the granulation tank, its temperature increases. At the tank top end, hot waterleaves through an overflow and returns to the water handling system. Return hotwater is cooled either in a cooling tower or through heat exchangers. Fig 1 shows GPI and schematic diagram of Granshot process.
Fig 1 GPI and schematic diagram of Granshot process
Four basic steps of Granshot process are (i) control of HM flow, (ii) granulationby forming of HM droplets and their rapid quenching, (iii) dischargeof solidified and cooled iron granules by ejector, and (iv) dewateringof granules and transport to storage.
When HM arrives at granulation plant, it is poured into the tundish. Tundish weighing system automatically controls pouring rate in order to maintain a constant ferro-static head, i.e. constant flow rate through the tundish nozzle outlet. As HM stream leaves the tundish and strikes refractory target positioned below the nozzle, it gets distributed evenly over the cooling water surface.
HM forms droplets of which the outer part solidifies during the flight before their penetrating the surface ofwater. Remaining inner part of the semi liquid droplet, now a granule, is quenched as it strikes water surface and starts its movement downwards in the water volume. At the time of impact of granules into the water theyget deformed slightly but are prevented from splitting up, thus avoiding fines generation.
As the granules go down by gravity through water in the granulating tank they transfer their heat to cooling water. Cooling water allowsgranules to reach a temperature below 100 deg C. After the granules reach the tank bottom they are discharged from the granulation tank and dewatered onto a vibrating dewatering screen. Finally, the product GPIis transported to storage area or fed into a rotary drier by conveyor belt.
Tundish and nozzle are preheated for around 20-30 minutes before the process is ready for use. The stand alone granulation unit is usually completely automated which needs only one operator. Throughput time is around 30 seconds and has a process yield of greater than 99%.
Other important characteristics of Granshot process are (i) short start up time (typically 20-30 minutes), (ii) no change in chemical analysis due to rapid quenching, (iii) rugged process with high availability and limited maintenance, (iv) easy to operate, (v) high production capacity matching to BF output, (vi) GPI needs no additional processing, (vii) low environmental impact, (viii) flexible layout, (xi) low operating costs, and (xii) reasonable investment cost.
GPI has consistent physical and chemical properties. It has high metal content and low residuals content. Its high bulk density and physical shape is suitable for efficient material handling.Typical analysis of GPI is 4-4.5% carbon, 0.5-0.6% silicon, and around 95-95.5% iron. Tramp elements are 0.05% maximum.
GPI has compact and small shape of a flattened sphere which results in a bulk density of around 4 tons/cum. Its size is in the range of 8-25 mm. Its high angle of reposeallows effective transport and storage.
Other Importantcharacteristics of GPI are (i) homogeneous composition, (ii) practically no oxide content, (iii) high metallic yield during steelmaking, (iv) very good preheating properties and fast melting/dissolution when added to metallurgical process, (v) has iron carbide in the matrix, which is beneficial for scrap replacement in EAF, (vi) inert during shipping and storage, (viii) itsshape (deformed spherical) facilitates handling with conveyor belt, magnet, front-end loader, bin systems, and skip, (ix) Low dust generation due to high physical strength and rounded shape which eliminates breakoffs during handling, (x) shows no pyrophoricbehaviour and hence it can be transported and handled without concerns of combustion.