HIsarna Process for Ironmaking
HIsarna Process for Ironmaking
HIsarna process is a smelting reduction process for producing liquid iron directly from iron ore fines 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 carbon di-oxide steelmaking) consortium of European steelmakers and is a combination of HIsmelt technology of Rio Tinto and Isarna technology developed at Tata Steel Ijmuiden. It eliminates prior processing of raw materials as needed by the blast furnace process. The process consists of pre-reduction of iron ore fines 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). The process cuts both carbon and costs. HIsarna process takes place in a special reactor which has a narrow cyclone furnace on top of a wider convertor.
The combined process is around 20 % more energy efficient and produces less greenhouse gas emissions per ton of hot metal compared to current average blast furnace technology primarily because it does not require ore sintering and coke production. The hot metal produced can be processed into steel in a conventional basic oxygen furnace.
HIsarna process consists of a reactor in which iron ore 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 which leave the HIsarna reactor are mainly concentrated carbon di-oxide (CO2).
The overall HIsarna concept involves two-stage countercurrent contact between the iron ore and process gas. The process basically involves two stage counter current contact between the iron ore fines and the process gas. In both the 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 reactor (Fig 1).
Fig 1 HIsarna process reactor
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 tons per hour (tph) to 20 tph of ore feed. The attempt was again halted in 1999 because of the successful implementation of pulverized coal injection 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. In 2001, Rio Tinto partnered with Nucor Steel, Mitsubishi and Shougang Steel to build a commercial scale HIsmelt plant having a capacity around 800,000 tons of hot metal per year in Kwinana. The plant was constructed from 2003 to 2005, operated from 2005 to 2008, and produced around 400,000 tons of hot metal. The plant suffered from a series of problems with ancillary equipment which caused low availability and damaged the refractories. However, the production rate and availability improved steadily, and by the fourth quarter 2008, the plant was producing up to 1,800 tons of hot metal per day with 90 % availability. This production level matched the design rate of the plant when smelting hematite, and the process was considered proven.
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 to 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.
In the CCF, the pre-reduction and melting of fine ores takes place in a smelt cyclone. Iron ore and oxygen is injected into the CCF unit in the presence of hot smelter gas. The process originated from the ‘converted blast furnace’ (CBF) developed in the years 1986 to 1989. In the CBF process, lumpy ore is highly pre-reduced in a shaft furnace with final reduction and melting taking place in an iron bath in which fine coal is gasified. The process can avoid coke making but not ore agglomeration and related environmental problems. To further eliminate ore agglomeration in the process, the CCF has been developed, in which a melting cyclone is applied for pre-reduction and pre-melting of fine ore.
HIsarna process combines coal preheating and partial pyrolysis in a reactor, a melting cyclone for ore melting and a SRV for final ore reduction and iron production. The smelting cyclone and SRV are highly integrated and operated as a single smelting furnace. Fine ore and flux are fed into the smelting cyclone together with oxygen. The high purity oxygen is used to combust the SRV off-gas entering from the bottom of the cyclone. The combustion, which is preferably complete, generates a considerable amount of heat. This heat is used to melt the iron ore and heat it to the SRV temperature which is around 1450 deg C. HIsarna process is carried out in a smelting vessel (Fig 2) which is a combination of CCF and SRV.
Fig 2 HIsarna technology of ironmaking
HIsarna process does not need metallurgical coals, and can use more widely available (lower cost) thermal coals. In terms of iron ore, HIsarna process has the ability to reject phosphorous to slag. Around 90 % of the phosphorous reports to slag which is a direct result of its relatively oxidizing condition (slag contains around 5 % to 6 % FeO). Although the phosphorous tolerance is not a major issue in some parts of the world, it does open possible exploitation of certain iron ores which normally have been considered too high in phosphorous. A second possibility for non-conventional ores which can be used is the titani-ferrous magnetite ore. This iron ore is characterized by high titanium di-oxide levels and iron content of around 55 % to 60 %.
The iron ore containing high levels of alumina is not suitable for the blast furnace route, because of poor sinter properties and reduced blast furnace productivity. The HIsarna process has the capacity to operate with high alumina slag because the high FeO in the slag is a natural flux. These features place HIsarna in a very strong position with regard for the use of high alumina iron ores.
The sequential steps of the HIsarna process are described below.
Iron ore fines and pure oxygen are injected into CCF portion of the smelting vessel, where hot off gas from SRV portion of the smelting vessel is burned by the oxygen. The fines are separated from the gas by the centrifugal flow of the gas. Heat thus generated is used to melt and partially reduce the ore. The reduction reaction which takes place is given by the equation Fe2O3 (s) + 2CO (g) = 4 FeO (l) + 2 CO2 (g). Partially reduced molten ore runs downward under gravity into the SRV below. The cyclone product consists of the molten mixture of Fe3O4 and FeO. The expected temperature at this stage is around 1450 deg C and and the degree of pre-reduction is around 20 %.
At the top of the SRV, utilization of the post combustion (conversion of CO to CO2) heat is essential for the process. The heat of the post combustion is captured by the slag splash which circulate through the free board. The splash also protects the cooling panels from the post combustion flame.
Coal is injected at high velocity with a carrier gas (normally 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 deg C to1450 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. Dissolved carbon in the metal reacts with the oxygen of the ore and a significant amount of CO gas is formed. This reaction is represebnted by the equation FeO(l) + C(s) = Fe(l) + CO (g). This reaction is highly endothermic and takes place in the lower part of the vessel. A heat source is needed to keep this part of the vessel in balance. The iron oxides in the slag ae reduced at the slag / metal interface. Injected coal supplies the carbon and creates enough mixing. Due to this mixing, the FeO content of the slag is relativly low and the slag FeO level is typically around 5 % to 6 %.
CO gas from smelting, along with conveying gas (nitrogen) and the devolatilization 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 around 10 deg C. This allows heat to move downwards without compromising 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 deg C to1500 deg C and has a post combustion degree of around 50 %. Post combustion (PC) is defined by the equation % PC = 100(% CO2 + % H2O) / (% CO +% CO2 + % H2 +% H2O). The aim is to achieve almost 100 % post combustion at the top of the cyclone, in which case the off gas is to be highly concentrated nitrogen free CO2. This makes the process well suited for a combination with CO2 storage.
The products of the reactions separate into two molten layers (a top layer of slag and a bottom layer of hot metal. Both layers can be tapped individually, and the hot metal is sent for further processing in the steelmaking process.
The waste flue gas of the HIsarna process is nitrogen free since the process is oxygen based. The fully utilized gas has almost no remaining calorific value. The flue gas treatment of the process is shown in Fig 3. The treatment of flue gas can be without CCS (carbon di-oxide capture and sequestration) or with CCS.
Fig 3 Flow of exhaust gases without and with CCS
A HIsarna 8 tons of hot metal per hour (tHM/h) with a capacity of 60,000 tons of hot metal/year pilot plant has been successfully designed and developed at Tata Steel Ijmuiden and several campaigns have been run since 2011. The project has been jointly developed by Tata Steel and the mining company Rio Tinto. Further testing and development has been undertaken alongside additional partners who include Arcelormittal, ThyssenKrupp, Voestalpine, SSAB, LKAB, and Paul Wurth. In addition to the partner companies, the European Union has provided significant funding for the plant. Fig 4 shows the flowsheet of the pilot plant.
Fig 4 Flowsheet of HIsarna pilot plant
There were five campaigns. The first start up in the first campaign was not successful. The other four campaigns were successful. Before the start of each testing campaign, burners preheat the reactor of the HIsarna plant to a temperature of around 1,200 deg C. For the quick start-up of the process a first fill of hot metal was transported in a 50 ton ladle from the blast furnace to the HIsarna pilot plant. Once the metal was poured into the pilot plant, the maximum time slot was calculated based on the measured hot metal temperature and composition. Within this slot the process had to be started in order to avoid the risk of a ‘frozen hearth’.
First successful tap of the 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 2012 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 2013 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 2014 to 29 June 2014. The objective of sustained, stable production during several days was achieved in the end and tests of various kinds of raw materials was carried out.
The fifth campaign took place in October 2017. It was a six-month test campaign which was carried out proving that liquid iron can be produced for long running hours. For the 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 10 m (37 m 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. It is estimated that this campaign has costed around EUR 25 millions.
During the six month campaign, tests were done using steel scrap. The results showed that upto 53 % of material used in the process can be scrap. Then the concentration was on identifying the ideal raw material mix, looking for options to recycle steel slag, testing use of CO2 to inject raw materials, and checking whether CO2 can be captured and stored, which can result in reducing the emissions by 80 %. This was the final phase of the campaign. In this phase steel scrap and biomass were used and CO2 reduction of more than 50 % was achieved.
Following the success of this campaign, the next stage is intended to design, construct and test a larger-scale pilot plant with an estimated investment of EUR 300 millions. It is anticipated that this will have to go through several years of testing 2 to 3 times the size of the current pilot plant at Tata Steel Ijmuiden. In November 2018, it was announced that the new large-scale pilot plant will be built in Jamshedpur, India. The plant is planned to have a capacity to produce 400,000 tons of hot metal year. The next scale up is planned to have a capacity with a scale upto 1 million tons of hot metal per year eventually. The new plant does not signal the closure of the current pilot plant at Tata Steel Ijmuiden. Fig 5 shows the drawing of the incinerator and the scale up dimensions of the CCF-SRV reactor.
Fig 5 Incinerator drawing and scale up of CCF-SRV reactor
After the process is implemented on an industrial scale, HIsarna is claimed to produce at least 20 % lower CO2 emissions and use at least 20 % less energy compared to conventional steelmaking process. It is also ideally suited for CCS due to the absence of nitrogen in the gases, the compressibility of the gas due to sufficient CO2 content and the one-through gas flow nature. Taking into account CCS, upto 80 % CO2 reduction can be achieved compared to the conventional steelmaking process. Asides from energy and carbon savings, and hence cost reduction, HIsarna can eliminate 90 % of the process phosphorous to slag. This allows the use of cheaper, high-phosphorous iron ore which is not normally accepted in the conventional blast furnace process.
Important features and benefits of HIsarna process
CCF and SRV is a win-win combination of technologies. The important features and benefits of HIsarna process include (i) the heat to reduce and melt the iron ore is produced through oxygen, (ii) the technology eliminates the processes of converting iron ore into sinter or pellet and converting coking coal into coke, (iii) the technology can use raw materials of low quality, (iv) the carbon monoxide which is formed in SLV is used to create hot gases which are used to provide heat for the reaction occurring in CCF, (v) the technology is 20 % more energy efficient and 20 % less CO2 intensive, (vi) the CO2 produced is pure and it can be captured, stored and used and this can lead to further reduction in CO2 emissions (80 % reduction in carbon footprint), (vii) the technology can significantly improve steel production sustainability performance, (viii) The technology uses nitrogen as carrier gas to dissolve carbon into hot metal, (ix) the technology does not need energy-intensive and heavily-polluting processers, (x) the carbon collection of the process is highly efficient, (xi) it can use biomass or natural gas instead of coal and reduce the CO2 emissions, (xii) the investments and operating costs are also lower due to the use of wide range of feed stocks, (xiii) the process needs significantly lower capital investment costs and produces hot metal at significantly lower operational costs.
The attractiveness of the HIsarna process lies in fact that it combines environmental as well as economical benefits. The process provides easy ability to capture a high proportion (upto 80 %) of CO2 for geological storage. The process uses thermal coals instead of metallurgical coals and uses low-quality iron ore feed materials. There is 60 % to 80 % reduction in the emissions of dust, NOx, SOx, and CO.
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