ROMELT PROCESS FOR IRONMAKING
Romelt process for ironmaking is a smelting reduction process for the production of hot metal. 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 under leadership of Vladimir Roments began working on designing of this process. The first patent in Russia was obtained in 1979.
A pilot production plant with a capacity 40,000 tons of hot metal per year was commissioned in 1985 at the Novolipetsk Iron and Steel Works (NLMK). The process was tested and mastered in 41 campaigns at this pilot plant between 1985 and 1998 when more than 40,000 tons of hot metal was produced.
The first industrial plant for hot metal production based on Romelt technology with a capacity of 200,000 tons per year is being built at Myanmar. The plant is based on the processing of iron ore (Fe content of up to 29 %) without its beneficiation from Pang Pet ore deposit and non-coking coal. The panoramic view of the Romelt plant at Myanmar is at Fig 1.
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 for the process. 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 route, and (vii) can be used for waste processing in which case the cost of hot metal is further reduced.
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 process flow sheet for the Romelt plant is shown in Fig 2.
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.
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 travel towards the furnace hearth because of gravity.
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.
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 to 1500 deg C. Non coking coal acts both as a reductant and as a heat source in this zone.
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.
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, it has a better balance of the chemical and energy aspects of the two reduction stages namely the solidphase and the liquid-phase. 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. Reduction 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.
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.
Disclaimer: The views, designs and information published herewith have been provided to Steel 360 by the author of this article, the purpose of which is solely educational so as to share a general understanding of the process. Steel-360 bares no responsibility of patent or copyright infringement with regard to the content of this article and neither does the magazine or its editors substantiate the authenticity or credibility of the information provided herewith.
Source: Steel 360 Magazine Aug’17 Issue