Smelting reduction process

Smelting reduction processes are used to reduce iron ore to produce pig iron. The process combines coal gasification with melt reduction of iron ore, eliminating the need for coke production and ore preparation, thereby lowering energy requirement in comparison to a Blast furnace.

Smelting reduction ironmaking process was conceived in the late 1930s. History of development of smelting reduction processes dates back to 1950s. The first laboratory scale fundamental study was started in 1951 by Dancy. Major efforts on the development of the technology for ironmaking began only in the 1980s. Smelting reduction process is utilised in technologies like Direct Iron ore Smelting, COREX, AUSmelt, HISmelt and Plasmasmelt. Smelting reduction process is considered a third generational alternative for the modern-day iron and steelmaking process differentiating itself from the conventional blast furnace and and direct reduction processes.

The Process

Smelting reduction produces hot metal from iron ore using either of single-, two- or three-stage process.

In single stage process, hot metal is produced in a single reactor where both reduction and smelting takes place. Iron ore, coal and oxygen are fed into a single reactor and the gases that evolve from the liquid bath post-combustion are recirculated back into the reactor. Energy required for smelting is supplied by the combustion of coal with oxygen. this process is generally considered inefficient and economically unattractive compared to other smelting technologies. Romelt and AUSmelt processes are examples of single stage smelting reduction process.

Two-stage Smelting reduction process (Source: In Tech Open)

In two-stage process, the smelting reduction of iron ore takes place in two separate reactors: Reduction shaft and Melter gasifier. The raw materials constituting of lump ore, pellets and/or sinter are charged into the reduction furnace, where it is reduced to nearly 93% reduced iron by a counterflow of process gas. The reduced iron is then discharged into the melter gasifier, where final reduction and all metallurgical metal and slag reactions take place. Processes like Corex, HISmelt, DIOS, AISI-DOE, etc., use the two-stage process.

In three-stage process, in addition to the smelting unit and the pre-reduction unit, a separate gasifier is added to gasify coal producing carbon monoxide, hydrogen and methane. The additional gasification step in the process helps in reducing the temperature of the smelter off gases without the loss of energy.

Corex

The coal reduction process, known as COREX, developed by Voest-Alpine Industries (VAI) and DVAI, combines DRI from a shaft furnace with a final smelter-gasifier in a two-stage operation. The reducing gas needed for the shaft furnace is generated through partial coal combustion with oxygen in the fluidized bed of the smelter-gasifier. COREX consumes approximately one tonne of coal per tonne of hot metal, with 45% of energy used in ironmaking and the rest for export fuel gas. The hot metal's carbon and silicon contents resemble blast furnace hot metal, but sulfur content is higher due to coal sulfur entering the slag and hot metal, with organic sulfur gasifying and absorbed by the DRI and returned to the smelter-gasifier as iron sulfide.

HISmelt

HISmelt process has its origins in the early 1980s. In the HISMELT smelt reduction process, coal is injected through bottom tuyeres directly into a molten bath. Here, carbon quickly dissolves and combines with oxygen from incoming iron ore to produce carbon monoxide and iron. As this reaction is endothermic, requiring continuous heat input to sustain the process, additional heat is supplied by reacting the carbon monoxide released from the bath with oxygen from top injection of air. The resulting hot gases exit the vessel and are utilized in a fluidized bed to preheat and pre-reduce incoming ore, contributing to the overall efficiency of the system.

Fastmet

The Fastmet process, a solid reductant-based direct reduced iron method utilizing fine ore blended with pulverized coal, offers an alternative to gas-based processes. Developed by Midrex, it caters to minimill operations, providing European steelmakers with a viable source of virgin metal. Kobe Steel's demonstration plant, commissioned in 1995, showcased its potential.

In this process, a blend of pulverized coal and iron ore fines is pelletized. These pellets enter a doughnut-shaped rotary hearth furnace and undergo heating. At 1350 °C, reduction occurs within 8-10 minutes. The high heat enables pulverized coal to serve as the reductant, eliminating oxygen from the iron ore. The resulting DRI can be directly charged into an adjacent melting furnace or transformed into briquettes for merchant shipment.

DIOS

The Direct Iron Smelting Reduction or DIOS process, was developed through a collaboration among Japan Iron and Steel Federation (JISF), the Centre of Coal Utilization, and eight Japanese steelmakers. The process utilises three fluidized furnaces. Iron ore is preheated in the first fluidized bed reactor and is pre-reduced to 15-25% in the second reactor using cleaned offgas from the smelter. Dust, offgas and fines removed from the gases leaving the reactors are recirculated back into the smelter. Additionally, a small amount of coal fines, of the order of 50 kg per tonne of hot metal production, is injected into the smelter off-gas to cool it and provide extra CO and H2 for enhanced pre-reduction.

AUSmelt

In the AUSMELT smelt reduction process, lump ore or ore fines are continuously fed into a converter alongside lump coal and flux. Fine coal, oxygen, and air are injected through a top lance to enable submerged combustion. The degree of oxidation and reduction is controlled by fine tuning the fuel-to-air and coal ratios, along with the proportion of fine coal injected down the lance. All reactions take place within a single reactor.

Romelt

The Russian ROMELT process differs from others by omitting a pre-reduction step. Its smelter features a water-cooled roof and sidewalls in contact with slag, while conventional refractories are in contact with the metal. Air and oxygen are injected through two rows of tuyeres, while coal and ore are gravity-fed. While simple and robust, ROMELT consumes more energy than other smelting processes due to the absence of pre-reduction and the extensive use of water cooling.

Plasmamelt

The PLASMAMELT process operates within a coke-filled shaft furnace, with tuyeres strategically spaced around its lower section. The shaft is entirely filled with coke, while plasma generators and equipment for injecting metal oxides mixed with slag-forming materials and potential reductants are affixed to the tuyeres. Each tuyere creates a cavity within the coke column, where reduction and smelting occur. Periodically, slag and metal are tapped from the furnace's bottom. In iron ore smelting, the furnace's off-gas, primarily composed of carbon monoxide and hydrogen, can be employed for pre-reduction. In other applications, like reclaiming alloying metals from baghouse dust, the produced gas serves as a fuel source. If the raw material contains metals with high vapor pressures, such as zinc and lead, these metals exit the furnace with the off-gas. Subsequently, the gas undergoes condensation in a condenser to recover the metals.