Abstract:

Calix’s Zero Emissions Steel TechnologY (ZESTY) offers an alternative process route engineered to overcome these limitations. Built on Calix’s patented indirectly heated Flash Calcination (CFC) technology, ZESTY enables flash hydrogen reduction of iron ore fines in a lean phase environment, with high metallisation, minimal hydrogen consumption, and lower processing temperatures than those of traditional ironmaking technologies.

Since 2021, Calix has completed over 130 pilot runs across nine hematite/goethite and magnetite ores. Test work at the Calix Technology Centre in Victoria, using the pilot electric CFC reactor modified for direct hydrogen reduction, generated critical process data to validate ore compatibility, optimise reduction parameters, informing the basis of design for the 30,000 tpa ZESTY H₂ DRI demonstration plant.

A key innovation of the ZESTY reactor is its indirect and electrically heated, vertical tube design. Ore fines, typically milled to <500 µm, are injected into the top of the reactor and descend through a precisely controlled hydrogen/steam atmosphere. Heat is supplied via external electrical furnace, which avoids hydrogen combustion and enables the process to approach the theoretical minimum hydrogen requirement. Short residence times of 40–120 seconds are sufficient to achieve high metallisation, as the combination of fine particle size, precise temperature control, and counter-flow hydrogen configuration ensures high reaction kinetics while mitigating the onset of sticking and accretion formation.

Pilot testing of hematite–goethite ores demonstrated clear temperature-dependent behaviour, with metallisation increasing sharply between 750 °C and 950 °C (Figure 1). Above approximately 950 °C, metallisation gains diminished and sticking became more evident, particularly at 1050 °C. As such, controlled operation within 950–1,000 °C was identified as the optimal regime for high metallisation while maintaining operational stability. All hematite–goethite ores processed at these conditions exceeded the 75 % metallisation threshold required for electric smelting furnace (ESF) entry.

Beyond reduction testing, downstream qualification of ZESTY H₂DRI included ESF smelting and hot briquetting trials. Briquetting conducted at TU Bergakademie Freiberg showed that increased furnace temperature and compaction pressure improved briquette strength and density to >4.5 t/m³, yielding products approaching IMO HBI shipping standards. ESF trials at the University of Newcastle demonstrated effective slag micro foaming, strong iron partitioning into the metal phase, and successful carburisation using reductant additions.

The ZESTY 30,000-ktpa demonstration project represents a major step toward TRL 7. The FEED study, completed in early 2024, delivered a fully specified design envelope, including heat transfer modelling, hydrogen utilisation strategies, DRI/HBI product handling systems, fines recirculation concepts, and a comprehensive risk mitigation framework. Key scale up risks, which include ultra-fines management, heat transfer limits, sticking avoidance, hydrogen utilisation and recycle, and safe hydrogen/DRI handling, remain central engineering priorities. Extended duration campaigns at Kwinana will validate full scale reactor performance, de risk commercial deployment, and accelerate the transition from TRL 5 to TRL 7.

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Mr. Seb Van Dorp

Biography