Romelt Process for Ironmaking
Romelt Process for Ironmaking
Romelt process for ironmaking is a smelting reduction process for the production of hot metal (liquid iron). The process has been developed by The National University of Science & Technology ‘MISiS’, Russia (formerly known as Moscow Institute of Steel and Alloys). The development work of the process started in 1978 when a group of ‘MISiS’ scientists led by Vladimir Roments began working on designing of this process. The first patent in Russia was obtained in 1979.
A pilot production plant having a hearth area of 20 sq m and with a capacity 40,000 tons of hot metal per year was commissioned in 1985 at the Novolipetsk Iron and Steel Works (NLMK). The pilot plant was designed by Moscow Gipromez. The design of the reliable Vanyukov’s furnace was taken as the prototype for this new method of manufacturing hot metal. The process was tested and mastered at this pilot plant between 1985 and 1998. During this period forty-one campaigns were carried out, each of which included startup and slowdown, with full tapping of hot metal and slag from the furnace. More than 40,000 tons of hot metal was produced in the pilot plant during this period and used further in basic oxygen furnace (BOF) for steelmaking.
The first industrial plant for hot metal production based on Romelt technology is being built at Myanmar. The plant has been designed by Leningrad Gipromez and being supplied by Tyazpromexport, a subsidiary of Rostec. This plant has a capacity of 200,000 tons per year and is based on the processing of iron ore without its beneficiation from Pang Pet ore deposit. Pang Pet ore deposits have Fe content of up to 29 %. The plant will use non-coking coal from Kye Thee coal fields. The panoramic view of the Romelt plant at Myanmar is at Fig 1.
Fig1 Panoramic view of Romelt plant at Myanmar
Raw materials and special features of the process
The iron oxide feed to a Romelt process can be any iron containing material, e.g. iron ore fines and concentrates, blast furnace and BOF dusts and sludges, mill scale, iron bearing slags, scarfing wastes and turnings, and iron dusts etc. The wet non-coking coals of 15 % to 20 % volatile matter and around 8 % to 10 % ash can be used. The solid feeds (coal, iron oxides, and fluxes) are charged by gravity in the furnace.
The special features of the Romelt process include (i) flexibility to use wide range of iron bearing materials, (ii) no preparation needed for the raw materials, (iii) use of non-coking coal as fuel and as reducing agent, (iv) supporting production units such as coke-ovens and sintering plant are not required, (v) has capacity to generate sufficient power to meet overall plant requirements including oxygen plant, (vi) reduces the cost of hot metal as compared to blast furnace (BF) route, and (vii) can be used for waste processing in which case the cost of hot metal is further reduced.
Principle of the process
Romelt process is the single stage liquid-phase iron reduction process. In the process, the iron bearing materials are supplied to the slag bath by gravity and agitated by gas. They dissolve in slag. Iron oxides are reduced from slag with the help of coal carbon, which is also supplied by gravity and blown into the bath. In order to intensify heat and mass transfer slag bath bubbling with oxidizing gas, which is injected under the surface of the slag, is carried out. Gas containing CO and H2 evolve from the melted slag. The evolved gas is combusted at the top. The heat of this post combustion mostly provides thermal energy for the reactions taking place in the slag bath. A key element of effective process to take place is the active heat transfer between the zone of post-combustion and slag bath.
Romelt process employs single stage smelting reduction technology for the production of hot metal. The process utilizes non-coking coal for the reduction of iron oxides of iron ores and waste materials. The schematic of the process is shown in Fig 2.
Fig 2 Schematics of Romelt process
Iron-containing materials, coal, and flux are fed, using weigh hoppers, from relevant bins to the common conveyor. The charging into the furnace is carried out through the aperture in the furnace roof. Preliminary mixing of charge materials is not required since the materials after charging directly go into the slag bath due to its intensive agitation. Sluice arrangements, used in the units for other processes that operate under pressure, are not needed in the Romelt furnace. The working space of the Romelt furnace is under negative pressure of 1 mm to 5 mm water column which is ensured by induced draft fan. The schematic view of the smelting furnace of the Romelt furnace is shown in Fig 3.
Fig 3 Schematic view the Romelt furnace
The liquid slag bath is blown with either oxygen or an oxygen-air mixture through the lower tuyeres located below the slag layer. The tuyeres have simple structure and are reliable in operation. They ensure the required agitation of the slag bath. Non-coking coal present in the agitated liquid slag reduces iron oxides present in the iron bearing burden. Liquid iron produced by the reduction of iron oxides becomes enriched in carbon. Drops of liquid iron travels moves towards the furnace hearth because of gravity.
There are three zones in the smelting furnace. The first zone is the zone of the agitated slag. This is the zone where all the reactions take place. The second zone is the bottom of the hearth where produced hot metal gets collected. The third zone is the zone of calm slag and is situated between the first and second zone. The second and the third zone need to have sufficient capacities to accommodate the produced hot metal and slag.
Two lined chambers (sumps) are situated each at one of the end sides of the furnace. They are used for separate tapping of hot metal and liquid slag. The sumps are connected with the working space by channels of different heights. This ensures separate transportation of hot metal and liquid slag into the metal and slag sumps. There are tap holes for tapping of hot metal and liquid slag, which are located at different heights. This arrangement ensures continuous and free tapping of the liquid products (hot metal and slag) at the speed which matches the furnace capacity.
In the slag bath, the heat needed for the melting and reduction of the burden materials is higher than the heat available due to the burning of carbon of the non-coking coal into CO near the lower tuyeres. Thus, the main feature of the process is the post-combustion of CO, H2 and the volatile matter of the coal evolving from the bath by the oxygen blown through the upper tuyeres. Post-combustion of the gases to CO2 and H2O provides additional heat into the slag bath necessary for to maintain processing of raw materials.
The hearth and the lower part of the furnace bath, which contains permanently hot metal and calm liquid slag, are lined with refractory bricks. In this zone the refractory lining is under favourable conditions consisting of suitable temperature and non- oxidizing nature of the atmosphere. In the zone of agitated slag, the furnace walls are constructed with water cooled panels made of copper. Formation of the slag scull lining on them reduces the heat losses and removes the possibility of their wear. This also avoids wear of the lining in the places of the most aggressive attack of gas-slag metal emulsion. Above the slag bath, walls are made of water-cooled panels made of steel. The furnace roof is also water cooled.
After the post-combustion, gases at temperature of up to 1700 deg C flow through the water-cooled exhaust pipe into the waste heat boiler. There the gases are burned completely with natural air inflow and cooled to 250 deg C to 300 deg ?. Once the energy has been recovered and the gas is cooled, it is cleaned in the gas cleaning system and desulphurized before being discharged into the atmosphere through the chimney. Flue-dust generation from the Romelt furnace measured in the exhaust pipe is around 3 % on the average of the weight of the charged materials.
Behaviour of coal in the slag bath
Irrespective of the mechanism of reduction, coal is the only source of reducing agent in the process. There are no principal limitations on the range of the coal used for running the process under normal conditions. Any one of the coals with different content of fixed carbon, ash and volatile matter can be used as the reducing agent. However, the specific coal and oxygen consumptions depend very much on the composition of the coal used.
The unprepared wet coal in Romelt process is falling from above into the slag bath. The volatile matter is generated in slag bath and has a stimulating influence on the progress of the process. Both the material balance and heat balance of the process are dependent on how and in which form volatile matter is generated and the role it plays in taking place of the main process in the furnace. That is why the behaviour of volatile matter of coal is one of the most critical points for Romelt process irrespective of the grade of the coal being used.
Coal rate in Romelt process consists of the two parts namely (i) coal consumption needed for the reaction with the oxygen injected at the lower tuyeres to produce CO, and (ii) coal consumption needed for the reduction of oxides. Deficiency of coal can be the reason for the increase of the oxidizing potential of the slag bath, which can lead to the uncontrolled boiling of the same. However, the excessive coal rate in addition to the increasing the cost of hot metal production, also deteriorates the thermal conditions inside the Romelt furnace.
Generally it appears that the required quantity of coal depends only on the content of fixed carbon in the coal. However, in Romelt furnace volatile matter also participate partially in the processes which are taking place in the liquid slag bath. H2, CO and N2 of the volatile matter undergo no changes in the slag bath as these gases evolve from the coal to produce the gaseous phase. However, CH4 and CO2 of the volatile matter participate in the chemical reactions as per in the equations CH4 = C + 2H2 and CO2 + C = 2CO. If the quantity of CO2 is small and the same of methane (CH4) is substantial, then these chemical transformations lead to the availability of the additional quantity of carbon for reduction of oxides. Carbon produced by the decomposition of methane is fine-dispersed and highly active and improves the kinetics of reducing reactions.
In the Romelt process wet coal with moisture content of around 10 % to 12 % is used. In the furnace, this moisture gets evaporated and partial decomposed as per the equation H2O + C = CO + H2. This necessitates need of additional carbon for proceeding with the reaction of water decomposition for the production of water gas consisting of CO and H2.
All the three reactions namely decomposition of methane, reduction of CO2 to CO, and decomposition of water take place in the slag bath simultaneously.
Chemical and metallurgical aspects of the process
Bulk of the reduction process takes place in the agitated slag zone. Oxygen or a mixture of oxygen and air is blown through the bottom tuyeres to produce the highly agitated bath. The raw materials feed falls into the agitated slag where melting and reduction takes place. The slag bath is maintained at around 1400 deg C to 1500 deg C. Non coking coal acts both as a reductant and as a fuel source in this zone. The following reactions take place in the agitated slag zone.
- Reduction of iron oxides. x C + FeOx = x CO +Fe
- Gasification of carbon. 2 C + O2 = 2CO
- Cracking of volatile matter in coal. 2 CxHy = 2x C + y H2
- Reduction of water. H2O + C = CO + H2
The reduced iron forms small droplets which coalesce and separate from the slag moves to the hearth of the furnace below the calm slag zone because of its higher density. Interaction between the metal and the slag in the agitated and calm slag zones allows the metal to be refined through the partitioning of minor elements between the phases.
Gases generated in the bath, predominantly CO and H2, enter the combustion zone. Here the gases react with the oxygen blown in through the top tuyeres and liberates energy which is used for the smelting reactions. The reactions occurring in the combustion zone are as given below.
- Post combustion. CO + O2 = CO2 and 2 H2 + O2 = 2 H2O
- Combustion of volatile matter of coal. 4 CxHy + (4x+y) O2 = 4x CO2 + 2y H2O
Energy liberated from the combustion reactions is transferred back to the bath. The heat transfer is enhanced by the high degree of turbulence generated in the slag bath by the lower tuyeres. The off-gas is only partially combusted in the furnace which allows further recovery of energy in a conventional waste heat boiler system.
The Romelt process is based mainly on the liquid-phase reduction of iron. Hence the process has a better balance of the chemical and energy aspects of the two reduction stages namely the solid-phase and the liquid-phase. In the Romelt process, a large part of the heating and reduction is transferred to the liquid-phase stage.
The reduction of iron from its oxides in slag is carried out by coal particles and by carbon dissolved in metal inclusions in the slag. There are the following two ways which indicate the involvement of coal in the reduction of iron oxides in the furnace.
- Reduction occurring on the surface of gas bubbles which contain coal particles. The role of these particles is to regenerate the reducing atmosphere in the bubbles (the thermodynamic conditions which exist in the process make it difficult for gas bubbles which do not contain coal particles to reduce the iron oxides).
- Reduction occurring with the coal particles in direct random contact with the slag. Here, reduction takes place under conditions similar to those which exist when iron is reduced by a rotating carbon-bearing material and gas bubbles are forcibly removed from the material’s surface.
Reduction in the Romelt furnace takes place (i) when the coal particles are in direct contact with the slag (60 % to 80 %), (ii) when carbon is in direct contact with the metal drops (10 % to 15 %), and (iii) at the ‘gas–slag’ interface (10 % to 25 %). Typically, 85 % to 90 % of the iron is reduced with the direct participation of the coal particles. This differentiates the liquid-phase reduction which occurs in the Romelt process from other smelting reduction processes in which carbon dissolved in the metal plays a substantial role (DIOS) or the main role (Hlsmelt) in the reduction operation.
In the Romelt process, there are certain optimum values for the content of coal particles in the slag bath, although this parameter can vary within a broad range of values. The Romelt furnace cannot be overloaded or underloaded with coal. Charging of a suboptimal amount of coal leads to over-oxidation of the slag melt and its uncontrollable frothing. Thus, coal in excess of the calculated amount is often charged into the furnace to prevent over-oxidation, and this helps sometimes in the stabilization of the process. However, there is a limit for the excess amount of coal particles in slag. This limit can also cause disruptions in the process such as a decrease in the temperature of the slag bath, an increase in the content of iron oxides in the slag, a reduction in the degree of secondary combustion of the outgoing gases, and the release of more heat in the waste-heat boiler.
Feeding of additional oxygen into the furnace does not promote secondary combustion because the oxygen does not completely react with the coal floating on the surface of the slag. This dense layer of coal is formed as a result of overcharging of coal or undercharging of the oxide-bearing raw material. The presence of the layer suppresses spraying and adhesion of the slag to the walls, which adversely affects the transfer of heat from the primary gas combustion zone to the slag bath since the heat transfer takes place mainly through drops of slag and a slag film which flows down the walls in the secondary combustion zone.
It is seen that in a Romelt furnace, if the coal content of the surface layer of the slag is around 20 % to 30 %, the process transit to an undesirable regime in which coal blocks heat transfer from the secondary combustion zone to the bath. The occurrence of this regime depends not on the amount and composition of coal which has accumulated in the slag, but also on the rate of turbulent circulation of the slag since this turbulence determines the efficiency with which coal is mixed with other components of the slag melt.
Product characteristics and specific consumptions
Typical analysis of hot metal from the Romelt process consists of carbon – 4.5 %, silicon – 0.1 %, manganese – 0.08 %, sulphur – 0.05 %, and phosphorus – 0.1 %.
Typical analysis of slag from Romelt process consists of CaO – 39 %, MgO – 7 %, SiO2 – 36 %, Al2O3- 11 %, FeO – 3.0 %, MnO – 3 %, TiO2 – 0.1 %, and S – 0.04 %.
Typical specific consumption figures in per ton of hot metal are around 940 kg – 1200 kg for dry non-coking coal, and 750 N cum to 850 N cum for oxygen.
Advantages of Romelt process
The following are the advantages of the Romelt process.
- Low capital cost due to low pressure operation and use of conventional ancillary equipment.
- No requirement for coke or coking coals hence lower operational cost.
- Can process any iron containing materials including metallurgical wastes, without any pre-treatment.
- No requirement to agglomerate iron oxide.
- Has a high rate of iron recovery.
- Allows the establishment of effective small scale hot metal source for smaller plants.
- Environment friendly because of elimination of coke ovens and agglomeration (sintering and pelletizing) plants.