Direct Reduced Iron (DRI) refers to any process in which iron is extracted from iron ore at temperatures lower than the melting point, typically using direct reduction methods.
DRI gets its name from the chemical changes that occur when iron ore is heated in a furnace at high temperatures in the presence of hydrocarbon-rich gases, carbon monoxide, or elemental carbon.
The reducing gas is usually a mixture of hydrogen gas (H2) and carbon monoxide (CO), and the process operates at temperatures ranging from 800 to 1200 degrees Celsius to achieve thermal efficiency.
The direct reduction process is broadly divided into two categories: gas-based and coal-based. In both cases, the aim of this process is to remove oxygen from various forms of iron (such as iron ore, concentrate, fines, oxide scale, dust, etc.) to convert the raw material into metallic iron without melting it (below 1200 degrees Celsius).
Currently, DRI production is successfully conducted in various sectors worldwide. This method is suitable for countries in development stages due to lower initial investment and operational costs compared to integrated steel plants, especially where coal resources are limited.
Sponge iron(Direct Reduced Iron) has an iron content equivalent to pig iron (between 90 to 94 percent), making it a suitable raw material for use in small-scale production facilities and allowing for the production of alloys with higher purity using lower purity pig iron.
Hot Briquetted Iron (HBI) is a compacted form of DRI that facilitates transportation, handling, and storage.
It is worth noting that the DRI method has its challenges. For instance, Direct Reduced iron oxidizes rapidly if not protected from air and moisture due to its high sensitivity. Additionally, iron ore pellets have a risk of combustion because of their flammability properties. In contrast to pig iron, which is nearly pure iron, Direct Reduced Iron contains silicon impurities that need to be separated during the process. 
Heat Source:
Coke (essentially impure carbon) in the blast furnace reacts vigorously with hot air, which is the primary heat-generating reaction in the furnace.
Reaction 1: C + O2 = CO2
Iron reduction in both high and low temperature zones of the furnace involves carbon reacting with carbon dioxide to produce carbon monoxide.
Reaction 2: C + CO2 = 2CO
Carbon monoxide is a primary reducing agent in the blast furnace.
Reaction 3: Fe2O3 + 3CO = 2Fe + 3CO2
In the hotter parts of the furnace, carbon acts as the reducing agent.
Note that at higher temperatures, carbon monoxide rather than carbon dioxide is the by-product.
Reaction 4: Fe2O3 + 3C = 2Fe + 3CO
Sponge iron (Direct Reduced Iron) is produced using non-coking coal through the reduction process of iron ore in a rotary kiln. The reduction occurs at a predetermined temperature and under controlled atmospheric pressure. Raw materials such as iron ore, non-coking coal, and fluxing agents like limestone or dolomite, in specified sizes, are fed into the rotary kiln using volumetric and gravimetric feeders. Due to the slope and rotational movement of the rotary kiln, the raw materials gradually pass through the kiln and undergo reactions that convert them into sponge iron. Subsequently, the materials enter a rotary cooler where they are cooled down.
The temperature of the cooled product is approximately 80 to 100 degrees Celsius, at which point it is discharged from the cooler and then transferred to the separation and handling system. The product, consisting of sponge iron and magnetic materials such as coal, is separated into different sizes using screens and then segregated by a magnetic separator. The sponge iron is then transferred to a storage bin and dispatched.
Technical Examination of the Kiln
All reactions take place in the rotary cylindrical kiln, which rotates around its axis and has a gentle slope (less than 5 degrees) relative to the ground. Hematite or magnetite ores, in the form of lumps or pellets containing coal and limestone or dolomite, are introduced into the rotary kiln. As they move countercurrent to the gas flow, they are heated and reduced at a temperature of around 1100 degrees Celsius, achieving approximately 92% reduction and converting into sponge iron.
A distinctive feature of this method is the installation of pipes along the body of the rotary kiln and a burner at the sponge iron discharge point. These installations enable precise temperature control throughout the kiln.
Advantages of the Rotary Kiln Method:
The production of carbon-containing sponge iron, which is advantageous for steelmaking, permits the utilization of various types of iron ore in different forms such as fines, pellets, granules, and lumps.
- Reduction is carried out using a rotary kiln.
- Non-coking coal can be used as the reducing agent, and iron ore and dolomite as fluxing materials.
- Sponge iron, either cold or molten, is produced from feed materials such as iron ore pellets or lumps, beach sand, ilmenite (FeTiO3), carbon, and coal.
- This method is one of the successful designs for reducing energy consumption.
- The maximum capacity of the SL/RN method is 500 tons per day and at least 15,000 tons per month, making it suitable for the private sector and small-scale hematite iron ore mines. A major advantage of this project is the ability to utilize the heat from the kiln’s exhaust gases to generate 12 megawatts of electricity through a power plant unit.
