A special manufacturing process is making the production of iron much more environmentally friendly. The outstanding feature of the “Corex process” is that it uses conventional coal instead of coking coal, the customary ingredient.
Siemens is currently building the world’s largest Corex-based plant in China. Scheduled to begin operation in late 2007, this steel mill requires no coking plant, an element that typically accounts for a major share of emissions.
Corex is a smelting reduction process: Coal gasification, iron ore reduction, and liquefaction of the resulting iron are combined in one process. The gases produced can immediately be used for heating or for generating electricity in a gas and steam turbine power plant.
This plant burns uncoked coal, which means that the coking plant – one of the primary sources of emissions from an iron and steel plant – is now redundant. At the high gasification temperatures of the coal in the smelting reactor, organic compounds are completely reduced to their basic gaseous components and organic sulfur hydrides are converted to carbonated and/or hydrogenous gases. In a separate process in a melter gasifier, these vile smelling, and highly toxic, compounds are almost completely combined in sponge iron, aggregates, and slag.
In conventional pig iron production at iron works, coking coal is first mixed with charge consisting of pellets, sinter and lump ore, and then the mixture is put into a blast furnace. At the base of this giant, bell-shaped furnace injected hot air burns the coking coal up into carbon monoxide at 2,000 degrees Celsius. In the upper part of the furnace the soaring carbon monoxide withdraws the oxygen from the iron oxide in the iron ore. Because of the high temperature the reduced ore melts to liquid pig iron.
As a result of this traditional process with coking plant, powder metal facility and blast furnace, 1.4 kilograms of sulfur dioxide are created for each ton of pig iron. According to measurements taken by the TÜV Rheinland, with the Corex process this figure is reduced to only 40 grams, a reduction of 97%, and discharge of dust and nitrogen oxides is cut by more than 90 percent.
According to Sahoo, the advantages of the COREX process are:
It substantially reduces specific investment costs compared with conventional blast furnace steelmaking process; lowers the production costs 15% to 25% compared with a blast furnace; uses a wide range of iron ore and thermal coals; eliminates coking plants; hot metal quality is ideal for all steel applications; reduces CO2 generated per ton of iron production; and overall environmental compatibility.
One of the disadvantages of the process is caused by the strict particle size requirements for process, the process cannot tolerate coal particle sizes below 16 mm. A decrease in the mean particle size (MPS) reduces the permeability of the char bed, which results in gas channeling and a drop in the hot metal temperature and quality. The optimum MPS of coal used in the COREX process should be maintained between 20 and 30 mm.
Between the mine and the end user, lump coal is subjected to a number of mechanical actions with a resulting increase in the quantity of fines coal. Typically, the coal used in the process is sized at the mill at +16.0 mm with a top size as high as 70 to 80 mm. Fines, coal less than 16.0 mm, represents a reduction in the quantity of coal of the correct size for use in the COREX process and thus a higher net cost for the coal.
Compared to a blast furnace, COREX is a more complex process using 100 % oxygen and coal instead of coke.
China ’s booming economy and increasing urbanization are also leading to an upsurge of heavy industry. China’s metropolitan areas are now among the most polluted regions on earth, so new steel works are generally built with very environmentally friendly technology, and far from cities. Scheduled to become operational in the autumn of 2007, the new plant of the Baosteel subsidiary Baoshan in Luojing is using the Corex process.
When Baosteel decided to go with the environmentally-friendly Corex process, as opposed to the conventional blast furnace, they did so after exhaustive economic analysis and with stringent environmental legislation in mind. This is the first installation of the C-3000 COREX® module worldwide and has a pig iron capacity of 1.5 million tons per year; it is equivalent to a mid-sized blast furnace. The Corex export gas is fed into a modern gas and steam power plant for electricity generation; and into the iron and steel works to be used for heating. Baosteel is currently planning another Corex plant, again with a capacity of 1.5 million tons of pig iron.
Three plants, Pohang Iron and Steel Company in South Korea, Saldana in South Africa, and Jindal Vijayanagar Steel Limited in India are already successfully operating the process.