COREX Process of Iron Making - its Merits and Demerits

COREX Process and Technology of Ironmaking

The COREX technology is a cost efficient, environmentally friendly and industrially accomplished alternative to the blast furnace route for the production of hot metal from iron ore and coal. By fulfilling more stringent ecological regulations by law, utilization of low-cost, highly available low grade raw materials including fines, the COREX process has been accepted as a commercially proven technology for present and future iron making process. The COREX process was created by Siemens VAI and was elevated to industrial maturity by PRIMETALS. Primetals Technologies Limited, is a London (UK) based engineering and plant construction company established in 2015 by a joint venture of Siemens VAI Metals Technologies and Japan’s Mitsubishi Hitachi Metals Machinery (MHMM).

In a nutshell, COREX is a coal based SR (smelting reduction) process of making hot metal or pig iron by direct use of non-coking coal. The outputs can be used either by integrated mills or EAF (electric arc furnace) mills. The process gasifies non-coking coal in a smelting reactor, which also produces liquid iron. The gasified coal is then fed into a shaft furnace to remove oxygen from iron ore lumps, pellets or sinter and finally, this direct reduced iron (DRI) is fed to the smelting reactor. Compared with the traditional iron making process via the blast furnace route, the COREX process differs since non coking coal can be directly used for ore reduction and melting work, eliminating the need for coking plants. The use of lump ore or pellets also dispenses with the need for sinter plants.

Read: Steel Plants with COREX Process in Operation: A Quick Review 

Some Merits and Demerits of COREX Process

=> Conducive to production of high end and special steel required for sophisticated industrial and scientific applications with minimum damage to the environment at various stages of steel making and mining.

=> Unlike the conventional Blast furnace route for production of hot metal, it can accept high alkali containing ores without any build up inside the reactor.

=> It takes only half an hour to stop the plant and only four hours to restart it.

=> Specific melting capacity is higher than that in Blast Furnace; productivity around 3.5 t/m³/d can be achieved.

=> In 2009 Siemens first completed a life cycle assessment for pig iron production, looking at both conventional production in a blast furnace and the more environmentally friendly COREX and FINEX processes. Siemens claimed the COREX and FINEX processes can substantially reduce pollutant emissions when compared to traditional steel production, with its blast furnace and coking and sintering facilities.

=> The iron content of the feed should not be less than 50% as otherwise the slag volume produced will be too high.

=> As is the case in blast furnaces, over 90% of the phosphorous input reports to the hot metal. So, the phosphorous content of ore and coal should be as low as possible.

=> There are also reports that the COREX process cannot be operated without some amount of coke along with non-coking coal - at least around 10% of coke is required in the total reductant charge.

=> As far as coal is concerned, the non-coking coals having too high volatile matter (VM) or too low fixed carbon (FC) cannot be used in corex process of iron making.

=> The heat transfer plays a crucial role in the overall efficiency of the COREX process. This being a two stage process, i.e. reduction and smelting taking place in two separate units, post combustion of the gas generated in the smelting unit provides the heat to melt the DRI produced in the reduction unit. This calls for high heat transfer efficiency.

=> Unless the net export gas from any Corex plant (extent of generation around 1650 Nm³/thm) can be utilized, the process will not be economical. Because of many peripheral requirements, the total cost of a Corex project can be relatively high.   

=> The export gas generated in Corex technology can be used as a fuel gas in the downstream facilities to generate electricity or for the production of direct reduced iron in a region that has almost no resources of natural gas.

=> COREX plant emissions contain only insignificant amounts of NOx, SO2, dust, phenols, sulphides, and ammonium. Emission values are already far below the maximum values allowed by future standards. Also, waste-water emissions from the COREX process are far lower than those in the conventional blast-furnace route. These environmental features are key reasons for the attractiveness of the COREX process.

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HYL III and SL/RN - The two widely accepted Direct Reduction (DR) Processes of ironmaking

10-Oct-2009

Direct Reduced Iron (DRI) is obtained by reducing lumps as well as fines of iron ore in solid state at a relatively low temperature of around 1000^OC. A large number of DR processes are available today. SL/RN and HYL are two such DR processes. While HYL is a batch-type gas based process and uses a countercurrent shaft-furnace, the SL/RN process utilizes rotary kiln to reduce lump ore, pellets and sand iron with coal. Here we will discuss about some key features, including advantages and disadvantages of these two DR processes.

HYL III

Fig: HYL III Process Scheme

The HYL process was developed in Mexico and was the forerunner of the HYL III direct reduction technology. In HYL I process, a mixture of gases containing about 89% of reducing compounds is used. Each reduction module in HYL plant consists of four units - three “in line” and the fourth in “turn around” mode. The principal change made over HYL I in HYL III was the modification of the four fixed bed reactors by a single moving bed reactor, utilizing the same gas reforming plant, auxiliary equipment and quenching towers.  Actually HYL III technology is characterized by its wide flexibility for adapting to special needs, depending on available reducing gases, energy use and melt-shop requirements. Use of spent gases from direct ironmaking processes, coal gasification, energy optimization in DR plants and technology developments aimed to improve EAF productivity have been the objective of HYL. Some distinctive features of HYL III process are:

Fig: HYL III - COREX Off-Gas Process Scheme

=> The H₂/CO of the reformed gas is 3, the temperature is about 930^OC, the inside pressure of the countercurrent shaft-furnace is 450 kilopascals and the energy required for the reduction is basically the same as in the MIDREX process.

=> The selective elimination of H₂O and CO₂ from the reducing gas circuit allows maximum recycle of the reducing gases to the reduction reactor. Hence, the reducing gas make-up and the process natural gas consumption are minimized.

=> The reducing gas generation and the reduction sections of a HYL III unit are independent from an operational point of view. This feature offers important flexibility for adapting to different reducing gas sources. The process schemes based on use of alternative reducing gases from different sources and other DR/ Ironmaking sources have been proven in HYL III plants. Such alternate sources of reducing gas can be -

• Coal gasification processes.
• Coke oven gas.
• Gases from Hydrocarbon gasification.
• Partially spent gases from another DR plant.
• COREX off-gases.

=> High pressure operation (4 atmospheres or more) enables the effective control of process conditions, with smaller equipment size for gas handling and lower energy requirements (9.0 - 10.0 GJ/t).

=> The process is much flexible as far as raw material use is concerned - while it operates best with 100% pellets, even 100% lump ore of a suitable type has been used, but it is suggested to use a mixture of pellets and lump ores.   

=> This technology offers the unique flexibility to produce three different product forms depending on the specific requirements of each user - Cold DRI, HBI and HYTEMP iron. Metallization can be controlled up to 95% and Carbon content 5.0%.

=> When combined with COREX off-gas as a source of reducing gas, the HYL III DR plant offers high productivity using available spent gas and benefits in steel production using HYTEMP® iron together with hot metal in EOF/BOF based steel mills.

=> The HYL III process features the flexibility of generating electric power, taking advantage of high pressure steam produced in the natural gas-steam reforming unit which can be used in a turbo generator or in a set of turbines, at a high generation capacity.

According a data of recent past, around 11 million tones of direct reduced iron (DRI) was produced in 2003 by this process in India, Grasim’s HYL plant at Raigad (Orissa) produced 0.75 million tones of HBI.     

SL/RN

SL/RN is the most widely accepted coal based DR process. It was jointly developed by Stelco, Lurgi Chemie, Republic Steel Company and National Lead Corporation in 1964. In this process, the materials charged into the kiln gravitate towards the discharge end during which they are progressively heated to the temperature of reduction of around 1000 - 1100^OC. The product discharged from the kiln is cooled in an extremely cooled rotary cooler around 100^OC before being subjected to magnetic separation to separate sponge iron from coal ash and char. Waste gases leaving the kiln at the inlet end pass through a dust chamber and a post combustion chamber, before being cooled and cleaned in electrostatic precipitators, scrubbers or bag filters. In SL/RN technology the clean gases can be used in waste heat boilers to recover the sensible heat and the steam generated can be utilized for heating purpose or for electric power generation. Some distinctive features of SL/RN process include:

=> Flexibility with regard to the type of iron bearing materials which can be used such as lump ore, pellets, ilmanite, iron sands and steel plant wastes.

=> Use of a wide variety of solid fuels ranging from anthracite to lignite and charcoal.

=> Improved heating of the charge by submerged air injection in pre-heating zone of the kiln. This process suffers, however, from relatively big heat loss and facility size.

=> SL/RN technology provides optimized coal injection facilities at the discharge end of the kiln.

=> Waste gas conditioning by controlled post combustion followed by power generation (the power generated is more than the requirement of the plant).

The original SL/RN process has been modified in a variety of ways, particularly in India where rotary kiln DR technology has been widely applied.