The carbon-intensive blast furnace/basic oxygen furnace (BF/BOF) steelmaking route has to be replaced, and one way is to use hydrogen-based direct reduction in combination with an electric arc furnace (EAF). Most current pilot projects focus on using shaft furnaces for direct reduction with hydrogen. However, this method would result in the need to produce several hundred million tons of additional high-quality DR-grade pellets every year, with the associated impacts on pellet availability and prices. An alternative is hydrogen-based direct reduction using fine ore instead of pellets. Metso Outotec’s Circored technology is the only process for iron ore reduction based on 100% hydrogen that has proven its functionality and performance in an industrial-scale pilot plant. In this paper, the principles of the Circored fine ore direct reduction process and the results of the industrial-scale pilot plant operation are described, along with the possible role of the Circored process in the industry’s transition towards green steelmaking.
This paper was published in issue 5/22 of stahl + eisen. Authors are: Dr. Sebastian Lang, Max Köpf, Roberto Valery, Dr. Andreas Orth; Metso Outotec.
Metso Outotec has been involved in the direct reduction of iron ores ever since this process was first introduced, both as one of the inventors of the process based on the rotary kiln and as a successful licensee of the Midrex shaft furnace process. Circored is based on fluid bed (FB) technology, for which Metso Outotec has been the leading developer since decades. FB processes are state of the art for applications like coal combustion in circulating fluidized beds (CFB) with inherent SO2 absorption or alumina calcination. In these applications CFBs replace rotary kilns because of the significantly lower energy consumption and lower CAPEX and maintenance costs.
The main advantages of FB processes are excellent heat and mass transfer conditions, precise temperature control, short solids retention times leading to higher plant capacities, and lower investment and operating costs. FB technology can also be applied to the direct reduction of iron ore to eliminate the cost and energy-intensive agglomeration step of iron ore fines in the form of sintering or pelletizing.
The generic Circored process is based on the reduction behavior of iron ore, which is shown in Figure 1.
For the direct reduction of fine iron ore, Circored applies a two-stage reactor configuration with a circulating fluidized bed (CFB) followed by a bubbling fluidized bed (FB) downstream. A demonstration plant for hydrogen-based production of hot briquetted iron (HBI) using the Circored process commenced operations in 1999 in Trinidad. Over several months of successful operation more than 300,000 tons of high-quality carbon-free HBI were produced, and it became evident that the basic process concept was ideally suited for this application.
Below is a brief description of the Circored process, which is also visualized in the simplified process flow diagram in Figure 3.
Iron ore fines (0.1–2.0 mm) are dried and preheated in a CFB preheater to a temperature of approximately 850–900 °C before being fed into the first-stage CFB reactor. Figure 2 shows the suitable particle size distribution for fluidized bed applications. The preferred grain size for the process is 0.1–2.0 mm, though depending on decrepitation behavior, a grain size of up to 6 mm might also be acceptable. Ultrafine concentrate and in-plant fines can be microgranulated before being used in the process.
The initial reduction step is fast and controlled by the outer mass transfer of the reductant to the iron oxide particle. The CFB is the ideal reactor to achieve a prereduction degree of 65–80%, offering the following characteristics: high gas velocities of 4–6 m/s and high differential velocities between gases and solids leading to short solids retention times of 20–30 minutes, as well as an optimum radial and axial mixing of solids and gases ensuring uniform temperature distribution throughout the reactor.