Behind the scenes: Systematic research on DRI furnace technology – implications for the steel and cement industry ecosystem

The development path for Metso’s novel Outotec^® DRI Smelting Furnace (ESF) has included several steps. After a literature review and preliminary evaluations, Metso’s experts investigated DRI smelting at the laboratory scale (figure 2). For example, the distribution coefficients of iron between the metal and slag were researched for various slag compositions and the differences between natural gas and hydrogen reduced DRI were studied.

One interesting research topic was the carburizing of produced hot metal so that it would be directly suitable for further processing. This is extremely important in the case of hydrogen reduced DRI as its carbon content can be even zero, influencing the melting temperature and slag composition. We also studied the slag-refractory interactions on laboratory scale to enable the up-scaling to pilot and industrial case in a safe way.

The laboratory scale experiments have produced important data for the design of the industrial scale unit. The investment and operating costs and carbon footprint for different capacities and feed materials have been calculated. In addition, a CircoSmeltTM unit that integrates Metso’s CircoredTM reduction technology and Metso’s Outotec DRI smelting furnace has been developed. The integration of the DRI furnace to other direct reduction technologies proceeds in several customer projects.

Figure 3 illustrates the amount of slag and required energy for various feed materials. We have carefully evaluated how the process behaves in different operating environments, and what is an optimal solution for a steel mill and for producing independently granulated hot metal for sale. The circular economy options for the produced slag are under research, as slag produced with an electric arc furnace cannot replace blast furnace slag in cement production due to completely different slag chemistry and high share of non-reacting FeO in it. On the contrary, the slag produced with the DRI smelting furnace could substitute the blast furnace slag and yield to significant CO2 emission savings in the cement industry. This is one of the major ESF technology benefits that should be considered when steel producers are making decisions between EAF and ESF technologies replacing the current blast furnaces.  

Metso did a pilot study on the carburization of natural gas reduced DRI with a modified AC furnace. The focus of the pilot campaign was to study the use of a bank for carburization (figure 4). In the campaign, the carbon content of the produced hot metal could be increased to over 3% which was the target value. When the campaign was conducted in March 2023, hydrogen reduced DRI was not available in sufficiently large quantities. Nevertheless, the metallurgical behavior of the process could be studied with Metso’s existing circular furnace. The new, larger rectangular pilot furnace being built at the Pori Research Center will enable further studies and technology development (see figure 1). This new 3-in-line furnace is a smaller version of the industrial 6-in-line furnace.  

The name of the pilot furnace comes directly from the shape of its electrode assembly. The dimensions of the pilot furnace are roughly 3x2m and the feed rate is 1000 kg/h. The most important technical features are the same in the pilot and the industrial scale. The equipment will be used for Metso’s own development work, but also for testing different feed materials for customer needs.

The research topics for the pilot 3-in-line include: 

• Carburization of metal and impurities distributions between metal and slag 
• Properties of slag and its suitability to the cement industry 
• Investment (capex) and operating (opex) costs of the process 
• Durability of the refractory lining materials  
• Mechanical integrity of the square furnace body 
• Higher power density could allow larger capacities or a smaller furnace for a given capacity.  

On the brink of change 

The development of new more sustainable technologies has been rapid and technology providers are launching their solutions. Metso has developed the Outotec® DRI Smelting Furnace based on the know-how of several industrially proven furnace technologies. When the furnace is combined with a direct reduction unit, it provides substantial CO2 emission savings in the iron and steel industry. Technology development and validation is a complex multistep process, and the next step for Metso is the commissioning of the pilot-scale 3-in-line furnace and validating the technology towards larger capacities.