Granulation of Hot Metal
Granulation of hot metal is a method for handling of excess hot metal produced in a blast furnace 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 which fulfills all the basic requirements such as high capacity, low cost operation and prime product material known as granulated pig iron (GPI). GPI has good chemical and physical properties like pig iron and can be used as a prime raw material in the steelmaking operations.
The GPI has a chemical composition identical to the hot metal being granulated. There is no oxidation and slag entrapment in GPI and there is high metallic content. Fig 1 shows some pieces of GPI.
Fig 1 Granulated Pig Iron
The process of granulating hot metal is known is ‘Granshot’ process. It was originally designed in the early 1970s with the purpose of granulating liquid iron from a blast furnace at the Hagfors works (Sweden) of the company Uddeholm AB by UHT – Hagfors Teknologi AB which is 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 in integrated steel plants.
Presently the Granshot plants for the production of GPI are working at six places namely (i) Uddeholm, Sweden, (ii) SSAB Lulea, Sweden, (iii) Voest Alpine, Donawitz, (iv) Saldanha steel, South Africa, (v) SSAB Oxelosund, Sweden, and (Vi) Essar Steel, India. The Granshot metal granulation process has been adapted on a large scale by the ferro alloy industry with more than 35 installations globally.
A Granshot plant takes care of any mismatch between the production at the iron making facilities and the requirement of hot metal at the steel making facilities. It is logistically positioned in between the two facilities. Excess hot metal from the blast furnace (BF) is diverted to the Granshot plant which produces GPI. This eliminates reduction of hot blast volume at the BF while producing GPI 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).
Granshot plants can be constructed and operated with capacities matching with the blast furnace 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 GPI has identical properties to that of pig iron but with an additional advantage that it can be handled in bin systems.
The equipment and the process of granulation
The equipment is designed for granulation of large batches of hot metal at a rate of up to 250 tons/hr per production line. The granulation plants are designed for granulation rate of up to 4 tons/min with a hot metal ladle capacity of 200 tons. The principle is based on heat exchange between the liquid iron and the cooling media which is water. Heat released during the hot metal cooling and solidification is transferred to the cooling water, which carries the heat out of the system.
A twin ladle turret, similar to the turret used in a continuous casting machine, handles the ladles during the granulation operation. The turret in combination with a tundish ensures a smooth sequence operation without any discontinuity between ladles.
Tundishes used in the Granshot process are standard continuous casting tundishes with minor modifications. The tundish is equipped with a stopper rod system enabling the granulation to be stopped within seconds if problem occurs. The stopper rod is normally used for only start and stop. The rate of granulation is controlled by hot metal head control via load cells on the tundish in combination with selection of outlet nozzle diameter. Up to 8 numbers of ladles in sequence with one tundish are usually being achieved.
A ceramic spray head is used to distribute the stream of hot metal evenly over the water surface. The spray head is critical equipment since it has to withstand thermal shocks and long term impact of the hot metal stream.
In the Granshot system having high granulation rates, the hot metal 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 the hot metal over a large surface area. The method reduces the power concentration in the water basin and hence allows a high metal flow rate.
The granulation tank holds the water volume necessary to accommodate the hot metal film and formed droplets distributed from the spray head by the impact momentum of the hot metal stream.
The tank has an upper cylindrical part and a lower conical part, which concentrates the iron granules to the tank centre in the lower end.
The solidified iron granules settles in the granulation tank 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 transports the GPI to the off loading point where it is stored in a stockpile.
For a granulation rate of 250 tons/hr the generated heat load that is transferred from hot metal to water is in the range of around 19-20 M cal/second. With this magnitude of heat load in a water system the heat has to be distributed to the water in a way securing that the heat concentration (heat / volume unit) is less than critical concentration for vapour explosions.
The water cooling and handling system is carefully balanced so as to ensure that the large amount of heat added by the hot metal is removed. From a closed circuit process water system the cooling water is pumped into the granulation tank lower section. As the cooling water moves upwards through the granulation tank, its temperature increases. At the tank top end, the cooling water leaves through an overflow and returns to the water handling system. The return hot process water is cooled either in a cooling tower or through heat exchangers.
Schematic diagram of Granshot system is at Fig 2.
Fig 2 Schematic diagram of Granshot process
The four basic steps of the iron granulation process are the following.
Control of hot metal flow
Granulation by forming of hot metal droplets and rapid quenching
Discharge of solidified and cooled iron granules by ejector
Dewatering of granules and transport to storage location
When the hot metal arrives at the granulation plant, it is poured into the tundish. The tundish weighing system automatically controls the pouring rate in order to maintain a constant ferro-static head, i.e. constant flow rate through the tundish nozzle outlet. As the hot metal stream leaves the tundish and strikes the refractory target positioned below the nozzle, the hot metal is distributed evenly over the cooling water surface.
The hot metal forms droplets of which the outer part solidifies during the flight 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 downwards 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.
As the granules go down by gravity through the water in the granulating tank they transfer their heat to the cooling water. The cooling water allows the granules 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 which is the GPI is transported to the storage area or fed into a rotary drier by a conveyor belt. Rotary drier is a common practice in case of ferro alloy granulation.
The tundish and nozzle are preheated for around 20 to 30 minutes before the process is ready for use. The stand alone granulation unit is usually completely automated which allows only one person to run the entire operation. The through put time is around 30 seconds and has a process yield of greater than 99 %, which is a good improvement over yields obtained in the PCMs.
The following are the important characteristics of the Granshot process.
Short start up time (typically 20 to 30 minutes) to meet last minute decisions
Rapid processing time of 30 seconds from hot metal to a cooled granule
No change in the chemical analysis due to rapid quenching
More than 99 % process yield
Rugged process with high availability, and with limited maintenance
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 in order to meet existing space available
Low operating costs
Reasonable investment cost
The granulated product
The GPI 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 the granules is same as that of the hot metal. Typical analysis is 4- 4.5 % of carbon, 0.5 – 0.6 % of silicon, and around 95 – 95.5 % of iron. Tramp elements (copper, nickel, molybdenum, and tin) are up to a maximum of 0.05 %.
The iron granules have compact and small shape of a flattened sphere which results in a high bulk density of around 4 tons/cum. The size of granules is in the range of 8 mm to 25 mm. GPI has high angle of repose, which allows for effective transport and storage.
Some of the characteristics of GPI 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 shape (deformed spherical) which facilitates handling with conveyor belt, magnet, front–end loader, bin systems and scrap skip
Has high physical strength and rounded 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 Granshot process of solidifying and cooling hot metal, in combination with a high capacity that meets a standard BF throughput, makes this process suitable for installation at the integrated iron and steel plants. Granshot process has also been applied for granulating of liquid steel.