Redsmelt process for ironmaking
Redsmelt process for ironmaking
Redsmelt is a new ironmaking process based a two reduction steps. These are (i) pre-reduction of iron bearing materials in a rotary hearth furnace (RHF), and (ii) smelting of the hot pre-reduced iron (DRI, direct reduced iron). Originally a submerged arc furnace (SAF) has been used for the second step. SAF has now been replaced by a coal and oxygen blown converter (oxy-coal reactor) known as ‘New Smelting Technology’ (NST). The RHF reduces green pellets made out of iron ore, reductant fines and binders to produce hot, metallized DRI which is charged to the NST for its smelting to hot metal.
Redsmelt process has been conceived to be consisting of a cost-effective and environmental-friendly technology. The important highlights of the process are as follows.
- The process does not need any prepared charge materials
- The process does not need electrical energy, since the DRI smelting is carried out using chemical energy
- The smelter is having high productivity resulting into limited investment cost
- The process can use practically all the residues generated during various processes of the steel plant (including sludges and oily mill scales), thus it solves the increasing issue of steel wastes treatment
- The off-gas coming from the smelting reactor is used as a fuel in the RHF, with optimization of the overall energy utilization. This results into effective reduction in energy consumption
A Redsmelt demonstration plant with two step smelting reduction process was built and tested in Piombino works (Italy) for the production of hot metal. The demonstration plant was commissioned in the year 2003. The two production steps in the demonstration plant have been based upon pre-reduction of iron-bearing materials in a RHF and smelting of the hot DRI in an oxy-coal converter. The plant has been designed to process up to 65,000 tons per year of feed materials (on a dry basis) with a rated hot metal output of 30,000 tons per year to 35,000 tons per year.
The Redsmelt process technology has been developed to meet the growing demand for a low cost environmental friendly ironmaking alternative to the traditional blast furnace route in large scale integrated steel plants. The plant with this process can be designed for a production capacity of 0.3 million tons per year to 1.0 million tons per year of hot metal. The process has been mainly designed for two basic applications namely (i) to convert iron-bearing by-products of the plant into valuable hot metal, and (ii) to produce hot metal at low-mid scale size (typically around 500,000 tons per year), in order to match with the mini-mill concept where hot metal is used as scrap substitute.
The concept of the Redsmelt process is shown in Fig 1.
Fig 1 Concept of the Redsmelt process
The process consists of several steps as given below.
Materials preparation – Finely ground iron-bearing materials and a carbon-based reductant, such as coal or petroleum coke are used for the preparation of green pellets. The pelletization process requires materials with granulometry as close as possible to the optimum which is 80 % below 100 micro meters and 100 % below 250 micro meters. A wet blend is prepared in a mixer where these materials are carefully dosed with the addition of water and a small quantity of binder (bentonite). The prepared mixture is pelletized on a disk pelletizer with the addition of more water. The produced green pellets are then screened to remove the under-size fraction which is recycled, while the sized product is loaded to a metallic belt dryer.
Green pellet dryer – The two purposes of the green pellet dryer are (i) to avoid sticking problems at the RHF feeding system, and (ii) to prevent decrepitation of green pellets in the RHF. The heat needed for drying of the green pellets is provided by the off-gas of the RHF. This also results into improvement of the overall energy efficiency of the process.
Rotary hearth furnace – The rotating annular hearth is placed in a furnace chamber covered by a suspended-type roof. The side walls, the roof and the hearth of the RHF are refractory lined to allow an operating temperature upto 1450 deg C. The dried green pellets are charged into the RHF through a vibrating feeder and distributed across the hearth as a uniform layer of around 20 mm (one to three pellets) around the whole width of the hearth.
Fuel gas and combustion air are introduced through several side burners which are grouped in three control zones. In each firing zone fuel and air flow rates are individually controlled by the control system, in order to obtain the desired temperature and gas composition (CO and O2). In zone 1 and 2, secondary air is introduced through separate air inlets for the combustion of CO released by the reduction process. Pellets, after charging, are heated up quickly to the reduction temperature. The total residence time ranges from 10 minutes to 18 minutes on the hearth of the RHF for reaching a final metallization degree of 70 % to 90 %. Depending on the properties of the different raw materials, the specific production of the DRI varies in the range of 60 kg/sqm hr to 100 kg/sqm hr.
The heat necessary for the process is provided by four different energy sources namely (i) combustion of the auxiliary fuel (CO-rich gas from the NST reactor vessel), (ii) combustion of the CO resulting from iron oxide reduction, (iii) combustion of volatiles released by the reductant (coal), and (iv) combustion of a fraction of the reductant itself (carbon burnout). The utilization of these energy sources is clearly in competition with the undesired phenomenon of the iron re-oxidation. The design of the RHF is specifically aimed at optimizing this complex gas-dynamic system. It includes special burners and air inlets, for the injection of secondary combustion air, capable to adjust the proper degree of turbulence in each zone and at each level of the furnace chamber. Another critical factor which is necessary for good RHF design is the need for an extremely accurate temperature control over the whole area of the hearth in order to obtain consistent mechanical and chemical properties of the produced pellets. The burner system is usually designed to meet all these targets and to guarantee the minimum NOx formation.
The produced DRI pellets are discharged via a water-cooled screw into a chute and then moved by a continuous metallic belt conveyor (designed for hot DRI transportation) to the smelting furnace. The metallic conveyor is made of heat resistant material and enclosed in a gas tight shaft. The off gas leaving the RHF and the dryer is discharged to the atmosphere after post combustion, air dilution, water injection and dedusting through a bag filter.
Part of the waste gas energy of the RHF is used to dry the green pellets. Waste gas energy is also used to preheat the combustion air and to provide heat for raw materials drying. In large-scale plants the waste gas energy can also be used to produce steam by a waste heat boiler.
Smelting furnace – In the original Redsmelt process concept, a submerged arc furnace has been included for the smelting and final reduction of the DRI. However in the Redsmelt demonstration at Piombino, NST smelting furnace has been used for the smelting and final reduction of the DRI.
The NST smelting furnace consists of a non-tilting vertical reactor vessel. Its bottom part (hearth) is equipped with a siphon taphole similar to those adopted in mini-blast furnaces or cupolas (slag and hot metal separation with a skimmer). Hot DRI is charged by gravity from the top by a water cooled chute, placed in the centre of the vessel. An air curtain around the lance tip minimizes carryover of DRI directly with the waste gas stream. The fluxes in lump sizes are charged via a separate feeding port. The cooling of the reactor vessel in the slag and metal-slag interface area is carried out by special copper cooling elements. The roof of the reactor vessel and the off-gas duct are made of water-walls with pipe-to-pipe welding.
The smelting reactor is equipped with two levels of side lances (three lances per level) to inject oxygen and coal. The position and the orientation of these lances are aimed at generating the proper chemical and fluid dynamic conditions for the process. Particularly the system is designed to improve the heat transfer between the upper oxidizing zone, where the post-combustion of CO gas occurs, and the reducing zone, where direct reduction of iron oxides and other endothermic reactions take place. The upper lances inject oxygen into the emulsion level to promote post-combustion in the transition zone while the lower lances inject oxygen and coal into the hot metal bath. With this arrangement the gas injection promotes a slag turbulence which is sufficient to convey the necessary heat energy from the exothermic (post-combustion) zone to the endothermic (smelting) zone where the FeO direct reduction takes place. Relatively coarse coal is utilized to reduce the carbon losses and improve the hot metal carburization. The hot metal produced is cast into pig iron either in sand moulds or a pig casting machine depending on the plant capacity.
The top of the smelter has a water-cooled roof to avoid refractory wear due to the high temperatures resulting from post combustion. After that a water-cooled duct collects and cools the smelter off-gas to the proper temperature for entering the quenching system. The off-gas is cooled and cleaned without combustion and is sent to a small gas-holder to stabilize its pressure and then utilized in the RHF as burner fuel.
Off-gas conditioning system – The off- gas conditioning line consists of a refractory lined post-combustion duct, gas quencher with water sprays, air dilution station, and bag filter. Because of the combination of the RHF and the NST off-gases into a common suction system, proper design of system is important especially the RHF off-gas duct.
Off-gas leaves the RHF at a temperature around 1100 deg C and is not completely oxidized. It is then conveyed to a refractory lined duct. Suitable nozzles for fresh air injection are located after the off-gas entrance in the refractory lined duct in order to burn compounds like CO, and to limit the temperature below the value at which fly ash starts to melt. The suitable conditions to reach the complete combustion of the off-gas are (i) free oxygen level higher than 3 %, (ii) high turbulence degree, and (iii) a residence time above 1 second. The air injection nozzles are distributed homogeneously along the duct in order to minimize the NOx formation.
In comparison to the off-gas leaving the RHF, the off gas coming from the smelter has a higher temperature (around 1700 deg C) and a lower post-combustion degree with consequent unburned compound content (CO+H2 is greater than 30 %). Smelter off-gas is conveyed to a water cooled duct, where the post-combustion air is injected. The combustion parameters (residence time, oxygen, turbulence and temperature) are the same as used for RHF off-gas treatment.
The RHF and smelter gases at a temperature not higher than 950 deg C are then conveyed to the same quencher, to reduce the fume temperature to around 320 deg C. The ‘spill-back’ type nozzles allow the complete nebulization of water droplets and a fast gas temperature reduction.
An emergency stack equipped with a bleeder valve (self-opening on emergency) is placed at the top of the quencher. The RHF primary, the smelter primary and the secondary dedusting air are finally sent to the dedusting plant.
The flow sheet of the Redsmelt process with submerged arc furnace is given in Fig 2.
Fig 2 Flow sheet of Redsmelt process with submerged arc furnace as smelting unit