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Turning waste from steel industry into a valuable low cost feedstock for energy intensive industry

Periodic Reporting for period 3 - RESLAG (Turning waste from steel industry into a valuable low cost feedstock for energy intensive industry)

Reporting period: 2018-09-01 to 2019-07-31

REslag project aims to the valorisation the main by-product of the steelmaking industry: the steel slag. REslag targeted to reach an optimized exploitation of natural resources proposing 4 innovative valorisation applications: as feedstock for the recovery of high added value metals (Pilot 1), as thermal energy storage (TES) material for heat recovery in the steel industry (Pilot 2) and for the concentrated solar power (CSP) sector (Pilot 3) and, as feedstock for the manufacturing of refractory materials (Pilot 4). This conceptual approach clearly contributes to the circular green economy in the steel sector.
Overall, the research carried out in the project has revealed the refractories production as the most promising application. Furthermore, it also showed appropriate thermal and mechanical stability for TES applications. Depending on the market development, the entire amount of annually produced European FeCr-slag could be consumed only in the refractory sector. Concerning the applications of slags as TES materials for CSP and industrial waste heat storage a rather small demand of up to 15 % is estimated.
The work performed in the REslag project is summarized below
WP2. Specifications and technical requirements of the process and of the final product: this activity was able to determine the exact conditions and relevant parameters of the real scenarios for each particular application.
WP3. Characterization and manufacturing of the slag pebbles: a complete characterization of the raw slag was carried out to determine its properties. With this data and the parameters released from WP2 and WP4, the specifications of the pebbles to fill Pilots 2 and 3 were fixed. Two different manufacturing routes were selected: mechanical conformation (Pilot 2) and sintering (Pilot 3). This WP was finalized with the manufacturing of 7 and 10 tons to fill Pilot 2 and 3, respectively.
WP4. Modelling and optimization of the processes and pebbles sizes for Pilot 2, 3 and 4. For Pilots 2 and 3, computational fluid dynamics and finite element analysis tools were implemented. In addition, modelling of the slag pebbles manufacturing processes was carried out to determine the optimized process parameters. Furthermore, HSC-Chemistry software and Thermo-Calc program were used to estimate compositions and mixing rules of the refractory bricks containing slag in its composition.
WP5. Design and construction of prototypes:
-Pilot 1: a pilot plant based on simulated moving bed technology was constructed at CEA laboratories in Grenoble (France) to demonstrate the recovery of high added value metals from slag.
-Pilot 2: a pilot plant was constructed at ArcelorMittal steelworks in Sestao (Spain) to demonstrate the potential of the steel slag as TES material for waste heat recovery applications.
-Pilot 3: two pilot plants were constructed to demonstrate the potential of steel slag as TES material for CSP applications. One of them was built at DLR facilities in Stuttgart (Germany) using air as heat transfer fluid (HTF) (Pilot 3a), while the second one was constructed at ENEA laboratories in Casaccia (Italy) using molten salts as HTF (Pilot 3b).
-Pilot 4: a pilot plant for the manufacturing of refractory bricks using steel slag as aggregate was constructed at Renotech laboratories in Turku (Finland).
WP6. Test and validation of prototypes:
-Pilot 1: a multistep process combining physical and chemical treatment was tested and optimized. It was proven to be highly efficient in rare earth recovery, achieving both high yield and purity in the recovered products.
-Pilot 2: the experimental campaign permitted the demonstration of the good thermal and mechanical properties of the slag particles over heating and cooling cycles. Furthermore, the heat exchanger design was identified as the key component for heat recovery applications from the electric arc furnace.
-Pilot 3a: the principle viability of using slag as a TES material for CSP applications using air as HTF was confirmed. Cyclic thermal and mechanical tests were performed to determine the resistance of the slag and inner insulation options on the one hand, and to determine the design of the TES on the other.
-Pilot 3b: the performed tests allowed to confirm that the sintered slag pebbles are compatible with the solar salt. Therefore, the sintered slags were proved to be suitable TES material for packed-bed TES systems in CSP plants using molten salts as HTF.
-Pilot 4: a valuable property data bank about slag containing refractory castables properties against commercial refractory materials were provided with the activities carried out in this pilot. The developed recipe formulas contain 80% slag (100% aggregates are slag based) with comparable properties to commercial reference.
WP7. Techno-economic viability and Environmental Impact. Replicability and up-scale: the life cycle assessment (LCA) and life cycle costing (LCC) analyses showed that the environmental benefit of REslag technologies can be summarized in 4 areas: reduction of waste generation, savings of primary raw materials, reduction in emissions and primary energy use and, reduction of EU energy dependence. Furthermore, a techno-economic analysis for the CSP technologies was also carried out for molten salt and air central receiver system using a thermocline hybrid energy storage system versus conventional two-tank storage. Finally, a business model for steel slag valorisation was also developed, through the launch of an ICT tool ( for mapping steel industries and doing a market screening for the different slag applications.
REslag faced the aforementioned environmental problems providing eco-innovative solutions for the valorisation of steel slag by means of 4 applications:
•Metal extraction of high added value metals. Most slags usually contain a quantity of valuable metals. This content could be an attracting resource of raw critical metals, quantified between 0.1 and 3%. A new methodology based on selective hydrometallurgy was developed to obtain an experimental demonstration of this technology.
•TES for steel industry. One of the main energy consuming applications corresponds to the heavy industrial activity. A particular example are the steelmakers. REslag aimed to improve the energy efficiency of this industry through the integration of low-cost TES system using steel slag as TES material. This innovative system will recover the waste heat from the off-gases of the electric arc furnace, around 15% of the primary energy.
•TES CSP applications. Current electricity production from CSP deploy a molten salt TES solution limited to a 290-550 °C temperature range. REslag project proposed an improved TES solution: the use of steel slag in temperatures above 600 °C. This concept represents a cost effective and technologically advanced TES alternative, which will enable CSP sector to compete with currently deployed fossil fuel plants.
•Raw material for new refractories. Slag shows very suitable properties to be used as aggregate in the manufacturing of refractory materials. At least 20% of virgin materials for castable refractories in the EU refractory industry can be substituted. In this frame, the energy consumption associated to the use of in-situ sintering castables, compared to fired refractories, can be decreased approximately in 1.3 MWh/ton, which decreases the corresponding CO2 emissions around 40%.