Production of Pig Iron by Using the Romelt Process
In the iron and steel industry in the world there are currently two types of factories that produce ferrous metals. There are so-called integrated plants, which start from the extraction of iron ore from the mine to the final rolled products. There are also so-called small factories, which were developed greatly during the second half of the twentieth century. Where steel is produced by smelting scrap in electric arc furnaces.
Romelt plant at Myanmar
The amount of steel produced in small factories is about 2 million tons per year, which is much lower than the quantities produced in integrated plants. Therefore, steel producers in the world have turned to small factories because they do not need large investments with few associated facilities, use of minimum hours per ton of products, maintain environmental safety and reduce environmental pollution.
It is known that 90% of the iron produced in the world comes through the treatment of iron-containing materials (iron oxides) using blast furnaces, and the rest by other small units, such as Corex and Ausmelt, etc. However, this dominance of blast furnaces faces many problems such as coke shortage, quality of raw material specifications, environmental pollution reduction, and high investment cost of furnaces and auxiliary equipment. In addition, the iron and steel industry produces large quantities of soft waste as a byproduct. Where these wastes are disposed of by burying them underground after being sintered or pellets.
These materials include slag, sludge, rising dust from blast furnaces, sedimentation devices in base oxygen converters, electric arc furnaces and those resulting from the processes of iron ore, fine coke, fine coal, rolling scrap and small scrap parts.
Therefore, these restrictions on blast furnace technology and the resulting waste have forced iron-producing companies to look for alternative ways to produce pig iron to avoid dependence on coke, thereby limiting the process of production by eliminating coke production batteries and sintering units and limiting their impact on the environment, as well as the disposal of iron-containing waste and dust accumulations that have become a source of concern around the world by finding appropriate ways to recycle them.
In the late 1970s and early 1980s, concerted efforts were made in many parts of the world to obtain advanced alternative technology alongside blast furnaces to produce high quality pig iron at a competitive cost using coal and the use of massive reserves of low quality iron ore in terms of content which are not suitable for blast furnace technology. This alternative technology is of great importance in terms of reducing the large investments required to establish integrated iron production complexes, comply with environmental regulations, using coal reserves scattered around the world and take advantage of the large price difference between coke and coal.
Smelting Reduction (SR) is therefore an alternative approach to the production of pig iron without using of coke, replacing it with coal, as well as using of oxygen, and the electric power to reduce iron ore, pellets and soft materials for the production of pig iron.
There are many technologies that use the method of reduction of iron ore during the smelting process (SR):
1.Corex, 2. Romelt , 3. DIOS, 4. HIsmelt ,5. Ausmelt (AusIron), 6. AISI-DOE, 7. Redsmelt, 8. Iron dynamics, 9. Kawasaki SR (Star), 10. Fastmelt , 11. Finex, 12. Tecnored, 13. Inred, 14. Plasma-smel, 15. Cyclone converter.
With the exception of Corex, Finex and Romelt, other technologies have not yet been commercialized so far.
The Romelt process was invented for the production of the liquid phase of pig iron at Moscow Institute of Steel and alloys which underwent a lengthy commercial experiment on a pilot unit at the Novolibetsk complex in the Russian Federation.
It is summarized in the reduction of iron ores and other iron-containing materials which are in melting state for the production of pig iron. The process was developed by the National University of Science and Technology (formerly known as the Moscow Institute of Steel and Alloy), and the development work began in 1978 when a group of scientists led by Vladimir Romants began working on the design of this process. They are getting the first patent in Russia in 1979.
Production of pig iron in Romelt furnace
The furnace shall be charged with any iron-bearing materials such as iron ore, slag and dust produced from blast furnaces, converters, electric arc furnaces and oxides resulting from rolling and forging operations, and machining operation residues, and scraps from rolling and continuous casting units, etc. in addition to coal and limestone.
The special features of the Romelt process include:
(1) Flexibility in the use of a wide range of iron bearing materials;
(2) No need for iron ores;
(3) Using of coal as fuel and as a reducing agent;
(4) No need for other production units such as coke ovens and sintering units;
(5) Has the ability to generate sufficient power to meet the overall needs of the plant including the oxygen plant;
(6) Reduces the cost of producing pig iron compared to the blast furnace (BF);
(7) This process can be used for waste treatment in which case the cost of the hot metal is reduced.
The Romelt process of reducing molten is a continuous one-phase process used in the production of liquid pig iron from various raw materials that containing iron and waste with the use of coal. Figure 1, shows a general layout of a Romlet unit, where the furnace is fed with raw materials, coal and limestone using charging boxes connected to certain capacity bins that are connected by a belt conveyor. The burden materials delivered on the same conveyor do not require pre-mixing.
The materials of the burden fall in a container has the form of parallel rectangular or cylindrical shape which the hearth and the lower part of the furnace bath, which contains permanently metal and calm slag, are lined with refractory bricks. In this zone the refractory lining is under favorable conditions: at the suitable temperature and out of the oxidizing effect of the atmosphere. In the zone of agitated slag the furnace walls and ceiling are constructed of copper water-cooled panels.
The liquid slag bath is blown either by oxygen or by the oxygen and air mixture through the lower tuyeres under the slag layer. These tuyeres have a simple design that allows for the desired agitation process in the slag bath so that this slag contains coal can reduce iron oxides. Getting into the agitated slag that contains coal, iron-bearing materials are reduced. Iron produced by the reduction becomes enriched in carbon and the liquid iron droplets fall on the furnace hearth by gravity. The iron produced by iron oxide reduction is rich in carbon content.
Figure 1. General layout of the Romlet unit
There are three layers in the melting furnace from the bottom up. The first layer of the molten iron on the furnace hearth, and the second layer are assembled directly above, where the slag is calm between the molten iron and the lower tuyeres and the third layer where the slag is in a highly agitated state where all the chemical reactions are carried out.
On both sides of the oven there are two rooms lined with refractory bricks to separate the casting process of iron fumes from liquid slag. These two rooms are connected to the smelting area by two channels at different altitudes, ensuring separate transfer of iron from slag. There are two openings located at different elevations, one for pouring molten iron and the other for casting liquid slag.
This arrangement ensures the free casting of liquid products (metal and slag) at a speed that corresponds to the furnace capacity. In small-capacity furnaces, casting can take place at alternating intervals. The spent heat in the smelting and reduction process of the charge material is higher than the heat from carbon burning to carbon monoxide (CO) near the lower tuyeres.
Thus, the main feature of this process is what happens in the post-combustion (where carbon monoxide (CO) is oxidized to the second carbon (CO2) and hydrogen (H2) to water vapor (H2O) by oxygen blown through upper tuyeres. Exothermic reactions that occur in the post combustion zone provide additional heat in the slag bath that is necessary to maintain the continuity of the processes inside the furnace. In the post combustion zone, the temperature reaches 1700 ° C and hot gases flow through the water-cooled exhaust pipes to the depleted heat boiler. The heat is used to generate water vapor, which is used to generate electricity, and the excess energy is used for other purposes after covering the requirements of the Romelt furnace and its related units.
The remaining gases are cooled after exposure to normal air to a temperature of 250 to 300 ° C, which is cleaned in the gas cleaning net and discharged into the atmosphere through the chimney.
The chimney dust produced by the Romelt furnace is about 3% of the average weight of the charged material. In the case of a high sulfur content in the burden, sulfur absorber can be placed in the gas path of the chimney to meet SO2 emission standards. Emissions from the Romelt furnace are concentrated in one spot, unlike the sintering units and blast furnaces and other production processes. This is why Romelt is not only produce pig iron but also a power generator.
The Romelt furnace works under negative pressure from 1 mm to 5 mm (water column) due to the presence of a pull fan. Figure 3 shows the diagram of the Romelt furnace.
The Romelt furnace scheme:
1 – agitated slag, 2 – sump for slag, 3 – sump for hot metal, 4 – hearth with refractory lining,
5 – channels for slag and hot metal, 6 – feed tunnel, 7 – gas-escape branch pipe,
8 – lower tuyeres, 9 – upper tuyeres, 10 – calm slag, 11 – water-cooled panels.
The main advantages of the Romlet method:
1.One-phase process to reduce molten directly using iron-bearing materials and coal for the production of liquid iron.
2.Requires minimal processing of raw materials and there is no limit to the volumes of these materials and moisture content.
3.It has flexibility in the use of a wide range of iron-bearing materials, including raw blocks, pellets, etc.
4.Coke ovens and sintering plants are dispensed with.
5.It requires oxygen as a primary gas medium.
6.A simple design requires fewer operating units compared to other processes.
7.More environmentally acceptable compared to traditional methods. The furnace works under a little negative pressure, there is hardly any pollution or safety hazards, etc.8.Lower cost of capital due to low pressure operation and use of traditional auxiliary equipment.
9.Lack of requirements for lime stone and therefore low operating cost.
10.Treatment of iron-containing sediments and elimination of harmful heavy metal impurities from the production cycle of integrated steel plants. This will allow the evacuation of land occupied by sludge disposal sites, which contaminate soil, air and water that is considered environmentally friendly.
In recent years, some modifications have been made to the convetional Romelt furnace to improve the performance efficiency and smelting of any materials with less iron content, reduce the amount of coal and oxygen consumed and recover some valuable materials consumed during smelting. In this direction, the conventional furnace (one furnace in which the smelting and reduction process is done) has been modified into two furnaces, one of which melts the components of the burden into a slag with high iron content and transfers the slag to the other furnace through a runner between them.
By using the two furnaces, the incomplete combustion gases can be transferred from the reduction furnace to the melting furnace to be fully combusted. This process aims to increase the thermal input of the melting furnace and thereby reduce the amount of carbon consumed (see Figure 3).
In the Romelt double process, limestone can be used instead of lime, in addition to a significant reduction in the cost of coal and oxygen (20-30%).
Due to the lower contents of elements such as Si, Mn, P and S to limits approach its content in steel in produced pig iron by Romelt process, it is economically desirable to be used this pig iron without or with iron produced by direct reduction (DRI) in the electric arc furnaces for the production of high-strength steel and high quality and alloy steels…
The construction of Romelt complexes will make it possible to solve the problem of utilizing iron-containing waste, improve the environmental situation in different regions. The products of the Romelt process: inexpensive hot-metal (liquid iron), granulated slag and the electric power make this technology economically efficient.
Tebbin Institute For Metallurgical Studies – Egypt