Circored and Circofer processes of ironmaking
Circored and Circofer processes of ironmaking
Circored and Circofer processes of ironmaking are fluidized bed based iron ore fines reduction processes. These processes completely avoid agglomeration process and make direct use of iron ore fines. Since the processes use non coking coal, necessity of coke oven battery is not there. Fluidized bed technology is ideally suited to energy-intensive processes like direct reduction because it enables high heat and mass transfer rates.
Both the Circored and the Circofer processes have been developed by Lurgi Metallurgie GmbH, Germany (now Outotec Oyj, Finland) for the production of direct reduced iron (DRI) from iron ore fines. For both processes, capacities above 1 million tons per annum are possible in a single production unit, resulting in improved economies of scale.
Circored process is hydrogen (H2) based process while the Circofer process is coal based. Circored has a two-stage configuration in order to achieve a high metallization of 90 % to 95 %, whereas Circofer has a single-stage configuration which can achieve pre-reduction up to a metallization of around 70 %. Circofer coal-based process produces pre-reduced feed material for smelting reduction reactors, such as AusIron, or electric smelting furnaces – the final product being hot metal or pig iron.
Circored process uses fluidized beds on a scale adopted by Outotec for other applications. Development of the process was initiated in the late 1970s with the pilot plant tests conducted at the ELRED plant of ASEA in Sweden. Tests were also carried out in the 3 tons per hour CFB reactor demonstration unit at Thyssen Stahl in Duisburg, Germany. These tests had focused on the treatment of steel plant wastes.
The first commercial Circored unit was built in 1998 by Cliffs and Associates Ltd. at Point Lisas Industrial Complex in Trinidad in 1998. The plant has a capacity of 500,000 tons per annum of HBI. The plant is presently not working.
The process is a natural gas based process for the production of DRI from iron ore fines. It is a two stage fluidized bed process. The first reduction stage is a circulating fluidized bed (CFB), the second reduction stage a bubbling fluidized bed (BFB) reactor. For the heating up of the fine grained DRI product to briquetting temperature, a flash heater is used.
The H2-based Circored process produces highly metalized DRI or hot briquetted iron (HBI) for direct feed into electric arc furnace (EAF) for the steelmaking. The process reduces energy consumption and emissions and offers a sustainable solution for the steelmaking. The special features of the process are (i) direct use of iron ore fines without prior agglomeration, such as pelletizing or sintering, and (ii) operation with pure hydrogen as reducing gas enables reduction with very low CO2 emission values, and allows the application of low reduction temperatures minimizing sticking tendencies. H2 is produced by means of natural gas reforming, but H2 from sources other than a steam reformer, e.g. from water electrolysis, can also be used.
The basic chemical reaction underlying the Circored process relies on an addition of H2 to the iron ore, which reacts to produce pure iron and water (Fe2O3 + 3H2 = 2Fe + 3H2O).
The CFB has been selected for achieving a pre-reduction degree in the range of 65 % to 70 %, due to (i) its high slip velocity between the gas and solids, (ii) allowing excellent heat and mass transfer and (iii) a short retention time in the reactor. High gas velocities allow for a smaller reactor size, thus saving on capital cost. The CFB also allows dust laden off-gas from the BFB to be used as a secondary gas, enhancing the overall gas utilization.
The BFB has been selected for the second stage of the process for achieving the final metallization of 93 % to 95 % due to its slower rate of reaction. The BFB is characterized by lower gas velocities and longer retention times up to 4 hours.
The Circored process operates at low reducing temperatures and uses natural gas to produce reducing gas by means of reforming. The process uses ore fines which have a particle size in the range of 0.03 mm to 1 mm and produces HBI.
The iron ore fines are first dried and pre-heated in a fluid bed preheater system at around 850 deg C to 900 deg C. The dried and pre-heated fines are then charged to a CFB reactor. The heat required is generated by the combustion of natural gas and air which is introduced into the CFB reactor. The fines undergo a fast pre-reduction step in a further CFB at 630 deg C to around 65 % to 70 % metallization. The CFB provides favorable heat and mass transfer conditions, which results in retention times of 20 minutes to 30 minutes. Gas velocities in CFB are 4 meters per second to 6 meters per second. High gas velocities are used in the CFB, which is designed with a relatively smaller diameter. Good lateral and vertical mixing of solids in the CFB ensures uniform temperature distribution in the reactor and uniform product chemistry. The CFB also allows dust laden off-gas from the BFB to be used as a secondary gas, enhancing the overall gas utilization.
A portion of the partially metallized fines are withdrawn from CFB and enter the BFB reactor. The BFB reactor is compartmentalized into several sections, and has gas velocities in the range of 0.5 metres per second to 0.6 meters per second. The BFB operates with relatively low gas velocities and provides retention times of 2 hours to 4 hours. The fines reach a final reduction in the second stage BFB reactor at 650 deg C. The reactions here are predominantly diffusion-controlled to produce product with a metallization of 93 % to 95 %. The off-gas leaving the top of the BFB reactor passes on to the CFB. The product leaves the BFB reactor at about 650 deg C, is then heated in a flash heater to achieve briquetting temperatures of around 700 deg C, and briquetted to produce HBI.
Circored uses H2, obtained from natural gas reforming, as its only reductant source (i.e. no CO is used) in the process, resulting in a product with no carbon. An operating temperature below 650 deg C is chosen for the process to avoid the sticking tendency of the reduced iron ore fines. The low operating temperature requires higher specific process gas volumes. Hence, an operating pressure of 4 kilograms per square centimeters is used to reduce the actual gas flow rates.
The compressed process gas is preheated to around 750 deg C in two gas fired pre-heat furnaces, to be injected into each stage of the process (i.e. the CFB and BFB). Off-gas from the BFBB is fed as secondary process gas to the CFB. The gas exiting the recycle cyclone of CFB is cooled in the process gas heat exchanger, passed through a multi-clone for the recovery of part of the dust particles, which is recycled into the flash heater. The off-gas is then scrubbed and quenched simultaneously for the final removal of dust and water produced during reduction. The gas is compressed and then re-heated in the process heat exchanger for re-injection into the process
For the recovery of the scrubber dust, a micro-granulation process is adopted. In this process of micro-granulation, the ultra-fine particles are agglomerated to micro-granules with the addition of a binder to an average size of about 350 micro meters. No additional heat hardening equipment is used as the hardening of the granules takes place in the preheating section of the Circored plant. Micro-granulation can also be applied in case ultra-fine ores, such as pellet feed, are to be processed.
The cooled and cleaned process gas is recompressed in a compressor and then preheated in gas fired heaters to a temperature of around 750 deg C before being reintroduced into the reduction reactor system. Fresh make-up H2, produced in a standard steam reformer equipped with a CO2 removal system, is added after the compression stage. The process gas is preheated before introduction to the reduction furnaces. Two-thirds of the fresh gas is used in the secondary BFB reactor, and one third in the CFB reactor.
The hot charging option can be incorporated for allowing the hot produced DRI to be fed directly without briquetting into the EAF.
The process flow sheet of the Circored process is at Fig 1 and the layout of 500,000 tons per annum plant is at Fig 2.
Fig 1 Process flow sheet of Circored process
Fig 2 Typical layout of the Circored plant
Typical process inputs for the Circored process per ton of HBI consist of (i) iron ore fines (67 % Fe) is 1470 kg, (ii) electric power is 100 kWh, (iii) natural gas is 2.75 Gcal, (iv) water is 0.6 cum, and (v) man hours is 0.23.
The advantages of the process include (i) direct use of low cost iron ore fines without prior agglomeration, such as pelletizing or sintering, (ii) operation with H2 as reductant enables reduction with very low CO2 emission values, and allows the application of low reduction temperatures minimizing sticking tendencies, (iii) good heat and mass transfer conditions in the CFB reactor, and (iv) low investment and operational costs. The following is the energy saving potential of the process.
- Gas usage of the process is low at 2.75 Gcal/ton.
- Electricity consumption per ton of liquid steel produced through Circored-HBI-EAF route is at 901 kWh/ton of steel.
Process related and total (including electricity) CO2 emissions of the process are reported to be 1.2 and 2.02 tons/tons of steel. The Circored-EAF route emits only about 50 % of the CO2 emitted by the conventional blast furnace-basic oxygen furnace route, assuming that H2 is generated by conventional steam reforming. If both, H2, generated by water electrolysis, and electrical power for the EAF, are based on renewable energy, CO2 emissions can be decreased by up to 90 %.
The Circofer process is similar to the Circored process. It reduces fine ores with coal in a CFB, in which the reducing gas used is produced by gasification of coal. The process has been designed with operating temperatures of around 950 deg C, without the production of any excess export gas. Because of the consistent utilization of the most diverse elements of the CFB technology, Circofer process distinguishes itself by exceptional heat and mass transfer, uniform temperature distribution over the entire reactor circuit and excellent heat and gas utilization.
A pilot plant for the Circofer process, with a capacity of 5 tons per day was installed in Frankfurt, Germany. The pilot plant provided a means of testing various iron ores and coals and allows simulation of process conditions to assist in developing design parameters for industrial scale operations. The DRI produced by Circofer process (greater than 93 % metallization) is mainly considered for use in mini steel plants and alloy steel plants for the production of special steels. However, it can also be used in integrated steel plants.
Circofer pre-reduction essentially consists of a CFB, where the iron ore is reduced by carbon monoxide (CO) and H2 gas generated from in-situ coal gasification. The off-gas from the CFB is used to pre-heat the iron ore in a two-stage suspension preheater. The iron ore is then further heated up by recycled char and through partial combustion of coal with oxygen in the heat generator. After the preheating stages the off-gas is further cooled in a waste heat boiler (generating steam for the CO2 scrubber) and finally cooled down in a venturi scrubber to remove the water vapour generated during the reduction. CO2 as remaining product from the reduction is removed in a CO2 scrubber and the CO and H2 process gas is returned to the reduction stage.
Coal, char and iron ore fines (0.1 mm to 1.0 mm) are the primary raw materials which are charged directly to the process. Coal of any variety with ash melting temperature greater than 1050 deg C and with volatile matter in the range of 10 % to 40 % can be used. However, it is desirable to have a coal with low ash content (less than 15 %) in order to keep the circulating load in the reactors, and in the case of direct charging into a smelter the slag volume, to a minimum.
Circofer process utilizes a combination of circulating and stationary fluidized beds. The fine ores are preheated in a two stage integrated preheating unit before being admitted to the first reduction stage. The first reduction stage is a CFB reactor, with an integrated heat generator in which the necessary energy is supplied to the system by partial oxidation of coal with oxygen. This partial coal combustion integrated with the CFB system not only produces the heat needed for the process but, specifically, supplies the char required as reducing carbon and anti-sticking agent. With this arrangement it is possible to conduct the CFB pre-reduction at high temperatures of a 950 deg C without prompting a sticking of the fine ores.
Iron ore fines and recycled char, from the magnetic separation of the product prior to briquetting, are pre-heated to around 800 deg C in a two stage CFB system utilizing the off-gas sensible heat from the process. The pre-heated materials are charged into the CFB reactor via a gasifier. Coal is charged directly into the gasifier which operates at a temperature of around 1000 deg C, where it is oxidized partially with the help of injected oxygen to generate the heat required for the process. The solids and gases enter the CFB where the iron oxide is reduced at a temperature of around 950 deg C to a metallization degree of around 70 %. Solids from the CFB are transferred to the FB (fluidized bed) reactor for the final reduction step to a metallization of around 93%. The metallized product, ash and excess char are discharged from the FB reactor, cooled to around 730 deg C and subjected to magnetic separation. The metallized product is hot briquetted at around 680 deg C. The non-magnetic char is recycled back to the process.
Typical composition of DRI from Circofer process shows Fe (total) – 92.7 %, metallic iron – 85.8 %, metallization – 92.6 %, % C – 1.32, % SiO2 – 1.25, % Al2O3 – 2.62, % CaO – 0.06.
After final reduction, the product is magnetically separated in hot conditions and can be processed further in different ways. One option consists in recycling most of the separated char (which means the nonmetallic fraction) and to briquette the high metallized product in hot condition in order to dispatch it from the plant as hot briquetted iron (HBI). Another option consists in the immediate melting of the hot DRI (AusIron). This route has an advantage in that it allows substantial energy savings by operating ‘in one heat’. In this process, the excess carbon can be used as an energy source for melting.
The flow sheet of Circofer process along with AusIron furnace is at Fig 3. The Circofer process is characterized by a closed gas circuit. The gas generated by coal gasification and ore reduction is first cooled, then dedusted before the reduction products water and CO2 can be scrubbed out. The gas with a high reduction potential thus obtained is compressed and heated before it is finally recycled to the process. The high reduction temperatures and the Boudouard reaction taking place allow high gas utilization and thus, a mode operation without producing export gas.
The gas flows in a closed gas circuit. The reactor off-gas is used in the ore pre-heater and then it is cooled to around 220 deg C in a waste heat boiler. The gas is dedusted in a bag filter, quenched and scrubbed to remove water vapour produced in the process prior to entering the CO2 removal unit. The CO2 removal unit uses steam raised in the waste heat boiler for striping of CO2 from the off-gas. The gas is then compressed and re-injected into both reactors and the gasifier.
Fig 3 Flow sheet of Circofer process with Ausiron furnace
The following are the important characteristics of the Circofer process.
- The process uses coal for the process
- Iron ore fines are used without agglomeration or other pre-treatment
- The process needs minimum of material preparation
- The process has got outstanding heat and mass transfer because of fluid bed technology
- The process has closed energy system with minimized primary energy demand
- The process has no excess energy
- The process has no hazardous waste
- The process is environment friendly with low emissions
- The process has flexibility to produce HBI a saleable product, or hot DRI can be transferred to a smelting furnace to produce hot metal
- Significantly increases the capacity of the downstream smelting (AusIron) furnace
- Decreases the power consumption of a downstream electric smelter.