After prereduction, the material is discharged into the secondary FB reactor, which is compartmentalized for better process control and to prevent back-mixing of solids. For the prolonged diffusion-controlled final reduction step, the bubbling FB reactor provides the ideal conditions for achieving reduction degrees of 93–95%, with low gas velocities of 0.5–0.6 m/s and longer solids retention times of 60–180 minutes depending on the reducibility of the ore at a relatively low reduction temperature. Operation at 4 bar(a) avoids the excessive equipment and ducting costs that would result from larger gas volumes at a lower pressure – an important consideration for a closed-loop system where gas is recycled within the process.
For the Circored process hydrogen was selected as the sole reducing agent. Its specific reduction reaction temperature characteristic lowers the temperature of the process to 620–650 °C and avoids particle sticking of the reduced material. Because of this, at the time of its development hydrogen gas was the enabler for a new process; today, with the steel industry seeking a viable pathway to decarbonization and emissions reduction, the potential use of blue or green hydrogen as the sole reductant has a much higher significance. One could say that the hydrogen-based Circored process was developed 20 years too early.
Key features of the Circored process
- Preheating the iron ore fines to 850–900 °C in a separate CFB reactor for calcining prior to charging in the primary CFB reduction stage. This method also allows the processing of magnetite ores, which can be difficult to reduce, as the preheating stage oxidizes the magnetite to hematite. In addition, the microgranules produced can be hardened to avoid the generation of ultrafine particles in the reduction stage.
- Prereduction in the CFB in about 20–30 minutes to yield a reduction degree of 65–80%.
- Final reduction in a compartmentalized FB reactor to yield a reduction degree of 93–95%.
- The use of hydrogen as the sole reductant enables low temperatures of 620–650 °C in the CFB and the FB; these low temperatures avoid particle sticking and allow to easily control the reaction.
- A zero-carbon process variant is possible with electric heating from renewables providing heat for the endothermic reduction reactions.
For better briquettability, the reduced fines, which are discharged from the FB reactor at a temperature of about 620–650 °C, are heated in a flash heater up to about 700–730 °C. At this step, preheated make-up hydrogen is used to heat the reduced fines and transport them to the top of the continuous discharge system.
Within the discharge system the hydrogen atmosphere is gradually replaced by nitrogen and the pressure lowered to atmospheric level, thereby ensuring safe conditions for briquetting. A minimum briquetting temperature of around 680 °C is required to obtain high-density HBI (> 5.0 g/cm3).
As the reduction of iron ore with hydrogen is an endothermic reaction, the energy must be supplied by heating both the ore and the process gases. In the past this was done by using natural gas and bleed gas. To achieve a totally carbon emission-free process, it is now planned to replace the combustion of natural gas with electric heating, with the electricity being generated from renewable sources.