HIsarna process of iron making


HIsarna process of iron making

HIsarna process is an ironmaking smelting reduction process where liquid iron is produced directly from iron ore fines and coal. This process eliminates prior processing of raw materials as needed by the blast furnace process. This process is an initiative of the ULCOS (ultra low CO2 steelmaking) consortium of European steelmakers. HIsarna is a combination of HIsmelt technology of Rio Tinto and the Isarna technology developed at Tata Ijmuiden. The process consists of pre-reduction of ore fines in cyclone converter furnace (CCF) of Isarna technology and bath smelting of iron in smelt reduction vessel (SRV) 0f HIsmelt process. The name of the process has been given by combining the names of the two technologies (‘HI’ from HIsmelt and ‘sarna’ from Isarna, a celtic word for iron). The process represents a new, potentially more efficient way of making iron. The technology is being developed in order to substantially reduce carbon emissions from the iron making process.

Initial developmental work

The very first attempt of applying cyclone technology in the reduction of the iron ore was attempted at Koninklijke Hoogovens in 1960s but the attempt was abandoned. Another serious attempt was made in 1986 but because economic crisis the project was put on the back burner until the 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 to 20 tons per hour of ore feed. The attempt was again halted in 1999 due to successful implementation high pulverized coal injections in the blast furnaces.

HIsmelt originally started By CRA (now Rio Tinto) in 1980s a 2 tons per hour HIsmelt pilot plant at Maxhutte, Germany followed by 8 tons per hour pilot plant in Kwinana, Western Australia in the 1990s. Later in the first decade of present century a commercial plant of 80 tons per hour 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 the European Union brought pressure on the steel industry to reduce its carbon footprint. The ULCOS consortium was founded as a result and in the period 2005 – 2007 the cyclone technology was selected as one of 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 the HIsarna process.

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 iron ore and the process gas. In both the stages the operating temperature is above the melting temperature. In stage 1 moten partly reduced ore is produced which runs downwards from the CCF into the SRV. (Fig 1). The two stage is highly integrated in physical sense and both the process stages are carried out in a single smelting vessel.

HIsarna process

Fig 1 HIsarna smelting vessel and two stage concept

The process seqquence of the HIsarna process is described in the following steps.

  1. Iron ore fines along with the oxygen gas are injected into the CCF portion of the smelting vessel, where the hot offgas from the SRV portion of the smelting vessel is burned. The heat thus
  2. generated is used to melt and partially reduce the ore. The 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 % to 20 %.
  3. 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 ero 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.
  4. Moten 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 % to 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 vesselin balance.
  5. 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 the lower region of the smelting vessel. The 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 as follows:

% PC = 100(%CO2+%H2O)/(%CO+%CO2+%H2+%H2O)

Hisarna pilot plant performance

The pilot plant was started on April 2011. In the first campaign the plant was operated from 18 th April to 11 th June 2011. There were four starts up. The first start up was not successful. The other 3 start ups were successful. The 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. The number of operating hours was below expectation. The second campaign has started on 16 th October 2012. It continued till end November 2012. The operational data is presently being analyzed.

Expected benefits of HIsarna process

As per present plans the industrial stage demonstration will be carried out during the period 2014 to 2018 and industrial implementation in 2020. The benefits expected from the process are given below.

  • 20 % savings in primary energy consumption.
  • 20 % reduction of CO2 emissions without CCS (carbon dioxide capture and storage in geological formations).
  • Well suited for CO2 storage (Nitrogen free off gas)
  • 80 % reduction of CO2 emissions with CCS.
  • Substantial 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
  • Steam coals and coals with high ash can be used.
  • Lower investment cost
  • Lower operational cost