Experiences from the Circored plant in Trinidad
There were two main drivers for the development of the Circored process: the growing demand for a direct reduction process that directly utilizes iron ore fines to decrease HBI production cost by avoiding a costly agglomeration step, and the desire of Metso Outotec’s predecessor Lurgi Metallurgie to apply its vast experience in FB technology, with all its advantages, to direct reduction.
Table 2
After an extensive period of test work and process development the investigations showed that the use of pure hydrogen as a reducing agent in a two-reactor (CFB and FB) combination would ideally suit the direct reduction characteristics of iron ore fines. Shortly after finishing test work and a conceptual engineering study, a contract for the first Circored plant, to be built in Trinidad, was awarded by Cliffs and Associates Limited in 1996. The plant started operation in May 1999; after the discharge system was modified it reached its process design parameters in March 2001 and was operating at up to 105% of its design capacity.
Despite functioning normally, the plant was unfortunately idled after a short period of successful operation, which produced 300,000 tons of high-quality HBI. This was due to several changes in ownership and to economic and political reasons including the crisis in the steel market at that time. The final end for the plant was the loss of a natural gas supply, which also forced other iron ore direct reduction plants in the vicinity to close permanently.
The Trinidad plant fulfilled process expectations from the outset in terms of achieving a high and uniform degree of metallization of the product. The key performance figures from March 2001 are summarized in Table 2.
Key achievements of the Trinidad plant
- Over 300,000 tons of high- quality HBI were produced and subsequently processed in electric arc furnaces located in the US.
- Plant design HBI production of 63 t/h periodically exceeded.
- High HBI product quality with maximum metallization degrees greater than 95% and consistent briquette densities above 5.2 g/cm3
Perspectives
Following the design, erection, and operation of the first Circored plant in Trinidad, numerous enhancements have been investigated and developed:
- Plant capacity increased to 1.25 Mt/y per line, now considered the technical and economical optimum for an industrialscale plant.
- Replacing natural gas with green electricity for preheating to achieve complete carbon neutrality.
- Microgranulation: Metso Outotec has a patented and simple microgranulation process for processing ultrafine (< 50 μm) ores and scrubber dust. In this process, the ultrafine particles are agglomerated with a binder to microgranules of a particle size ranging between 0.3–2 mm. The process does not require any additional heat-hardening equipment as hardening of the granules takes place in the normal preheating section of the Circored plant.
- Direct feeding of hot DRI in an EAF to further improve energy efficiency.
- For low-grade iron ores, a combination of a single reduction stage Circored process (metallization degree of 75–85%) with smelting reduction in an electric smelter for hot metal production is feasible.
The Circored technology is very flexible in its production setup. Besides merchant cold HBI, which can easily be shipped, hot and cold DRI can be produced and directly linked to EAFs and BOFs as a substitute for hot metal and/or other virgin iron units. This guarantees the production of high-quality steel products that have traditionally been the strength of integrated steel plants.
Figure 5
From discussions with clients, it became evident that – at least for a transition period of existing steel plants – the feed material for the EAF is expected to be a mixture of different materials like HBI, DRI, and scrap. The carbon content can therefore be adjusted to the required level for EAF operation; this approach has been proven in industrial trials with DRI from the Circored process.
With the Circored process, Metso Outotec provides an alternative process route for the reduction of iron ores, using hydrogen as the sole reductant and fine ore instead of pellets as feed material. Circored is currently the only process for iron ore direct reduction based on pure hydrogen.
