HIsarna process for iron making
HIsarna process for ironmaking
HIsarna process is a smelting reduction process for producing liquid iron directly from iron ore fines (IOF) and coal. It represents a new, potentially more efficient way of making iron and is being developed for substantial reduction of carbon emissions from the ironmaking process. It is an initiative of ULCOS (ultra low CO2 steelmaking) consortium of European steelmakers and is a combination of HIsmelt technology of Rio Tinto and Isarna technology developed at Tata Ijmuiden. It eliminates prior processing of raw materials as needed by the blast furnace process. The process consists of pre-reduction of IOF in cyclone converter furnace (CCF) of Isarna technology and bath smelting of iron in smelting reduction vessel (SRV) of HIsmelt process. The process name derives by combining the names of the two technologies (‘HI’ from HIsmelt and ‘sarna’ from Isarna, a celtic word for iron).
First attempt of applying cyclone technology for the reduction of iron ore was attempted at Koninklijke, Hoogovens in 1960s but was abandoned. Another serious attempt was made in 1986 but because of economic crisis, the project was put on hold until early 1990s. The project was revived when coke supply became scarce during mid 1990s. CCF technology was then developed at a pilot scale with capacities of 15-20 tons per hour (tph) of ore feed. The attempt was again halted in 1999 because of successful implementation of pulverized coal injections in the blast furnaces.
HIsmelt was originally started by CRA (now Rio Tinto) in 1980s in a 2 tph pilot plant at Maxhutte, Germany followed by 8 tph pilot plant in Kwinana, Western Australia in the 1990s. Later in the first decade of present century, a commercial plant of 80 tph (60,000 tons per annum) was commissioned and operated at Kwinana. This plant has since been closed down due to several reasons. However the core process worked well and a lot of experienced was gained when the process was scaled up.
In 2004, European Union brought pressure on the steel industry to reduce its carbon footprint and because of this ULCOS consortium was founded. During the period 2005-2007, cyclone technology was selected as one of the four high-potential technologies. A theoretical answer was found to the earlier problems of the post cyclone part of the cyclone furnace and ULCOS brought into the project the HIsmelt technology by an agreement with Rio Tinto so as to have a win-win technology combination. This led to an ULCOS supported pilot plant project in Europe. This combination of two technologies resulted into HIsarna process.
HIsarna process is carried out in a smelting vessel (Fig 1) which is a combination of CCF and SRV. The process basically involves two stage counter current contact between IOF and the process gas. In both stages the operating temperature is above melting temperature. In stage 1, molten partly reduced ore is produced which runs downwards from the CCF into the SRV. The two stages are highly integrated in physical sense and both the process stages are carried out in a single smelting vessel.
Fig 1 HIsarna smelting vessel and two stage concept
HIsarna process consists of a reactor in which IOF is injected at the top. The ore is liquefied in a high-temperature cyclone and drips to the bottom of the reactor where powder coal is injected. The powder coal reacts with the molten ore to produce liquid iron which is the base material to produce high quality steel. The gases that leave the HIsarna reactor are concentrated CO2.
HIsarna process has the following process steps.
- IOF and oxygen are injected into CCF portion of the smelting vessel, where hot offgas from SRV portion of the smelting vessel is burned. Heat thus generated is used to melt and partially reduce the ore.
- Partially reduced molten ore runs downward under gravity into the SRV below. The expected temperature at this stage is around 1450 deg C and and the degree of prereduction is around 10-20%.
- Coal is injected at high velocity with a carrier gas (generally nitrogen) into the bath. The primary process objective at this stage is to dissolve carbon which is used in the smelting step. Coal injection conditions are critical. The metal bath temperature is around 1400-1450 deg C with around zero silicon level in the metal. Other impurities such as manganese are also present at very low levels. Phosphorus and titanium partition largely to slag phase as oxides.
- Molten ore at this stage dissolves directly into the slag. The metal-slag mixing is generated by the coal injection plume. This metal slag mixing creates a large metal slag interfacial area for smelting. Slag FeO level is typically around 5-6%. Dissolved carbon in the metal reacts with the oxygen of the ore and a significant amount of CO gas is formed. This reaction is highly endothermic and takes place in the lower part of the vessel. A heat source is required to keep this part of the vessel in balance.
- CO gas from smelting, along with conveying gas (nitrogen) and the devolatilisation products of coal constitutes an upward moving flow of hot fuel gases. This upward movement of gases generates a large amount of splash, with metal and slag cycling through the upper portion of the smelting vessel as droplets. Oxygen is introduced into the upper section through lances and heat is generated by combustion. Heat is carried by these droplets from the upper region to lower region of the smelting vessel. Number of droplets passing through the hot combustion zone is so large that the average per pass temperature rise in each droplet is less than about 10 deg C. This allows heat to move downwards without compromizing the oxygen potential gradient in the system (relatively oxidizing at the top and strongly reducing at the bottom). Partly burnt gas leaving the SRV portion of the smelting vessel provides the necessary hot fuel gas for the CCF portion of the smelting vessel. This gas is typically at a temperature of around 1450-1500 deg C and has a post combustion degree of around 50%. Post combustion (PC) is defined by % PC = 100(%CO2+%H2O)/(%CO+%CO2+%H2+%H2O).
Pilot plant performance
Pilot plant was started during April 2011 and was operated from 18 April to 11 June 2011 in its first campaign. There were four starts up. The first start up was not successful. The other three were successful. First successful tap of liquid iron was done on 20 May 2011. Injection rate achieved was 60% of the capacity. Available data from the operation has shown that the process operated as expected but more operating hours are needed to confirm this. Numbers of operating hours were below expectation. However, the objective of showing that theory works in practice, i.e. producing liquid iron without preprocessing of raw materials was achieved.
Second campaign has run from 17 October to 4 December 2012. The objective of producing liquid iron for a longer, sustained period was achieved. Production at 80% of design capacity was achieved for periods of 8 to 12 hours. In the last run, full design capacity of 8 tph was reached.
The third campaign has run from 28 May to 28 June 2013. The objective of producing liquid iron for sustained periods and running tests with various kinds of raw materials was achieved. For the first time, steel was made from HIsarna liquid iron.
The fourth campaign has run from 13 May to 29 June 2014. The objective of sustained, stable production during several days on end and tests of various kinds of raw materials was achieved.
The fifth campaign took place in 2017. In preparation for this campaign, the installation has seen a significant overhaul. A completely new off-gas duct has been installed, increasing the height of the plant by more than 10m (37m highest point). Next to the pilot plant, a complete coal grinding and a drying and screening facility for ore and lime have been constructed. Closed conveyor belts have been installed to transport the raw materials from the storage facility to the installation injection points. The raw materials storage capacity has been doubled and a gas analysis laboratory has been added. The electronic monitoring system has been completely reprogrammed.
Before the start of each testing campaign burners preheat the reactor of the HIsarna plant to a temperature of about 1,200 deg C. Next, a layer of liquid iron is poured into the bottom of the melting vessel, to facilitate the start-up of the process.
Expected benefits of HIsarna process
Expected benefits of the process are given below.
- 20% savings in primary energy consumption
- 20% reduction of CO2 emissions without CCS (CO2 capture and storage in geological formations)
- Well suited for CO2 storage (Nitrogen free off gas)
- 80% reduction of CO2 emissions with CCS
- 60% t0 80% reductions in other emissions (dust, NOx, SOx, CO)
- There is possibility of biomass
- There is increased flexibility of raw material usage
- Ores with substantial P, Zn, alkalis and S content can be used
- Possibilities exist for using steam coals and high ash coals
- Lower investment and operational costs