Scale up Pf the Wet Route Carbonation of Steel Slag in a Rotary Kiln Pilot-Scale Reactor

BOF slags are alkaline residues that represent a potentially interesting feedstock for the application of mineral carbonation with the aim of storing CO₂ emissions as well as of valorizing the residues. Despite there are several works dealing with the carbonation of industrial residues, and BOF slags in particular, most of them rely on lab-scale data, whereas applications of this technology at pilot-scale are very scarce, if not missing in the case of the wet route.

This work was aimed at evaluating the performance of the wet carbonation process on BOF steel slags, carried out in a pilot-scale rotary drum reactor with a continuous steam flow, in order to store CO₂ in a solid and thermodynamically stable form.

The experiments were carried out using a gas mixture flow made of 60% N₂ and 40% CO₂ in order to simulate a or biogas composition. In this way, we tried to simulate the direct application of the process to an industrial stream, without any preliminary capture step. Operating temperatures were kept between 30 and 50°C.

The BOF slag used in the tests was characterized by an average size of 0.38 mm and a maximum size of 2.0 mm, whereas the main constituents were Ca (32.27%), Fe (18.2%), Si (6.51%) and Mg (6.37%), with an inorganic carbon content of 0.84%.

The carbonation treatment was performed in a pilot scale rotary drum reactor, with a size of about 1125 mm length and made up in steel. The rotary drum design consists in a cylindrical shell (tilt between 0 and 7°) rotating between two fixed flanges positioned at the two sides of the cylinder. The BOF slags were fed to the reactor via a hopper located upstream of the reactor; a continuous steam flow was also maintained in order to humidify the sample directly in the reactor, keeping the L/S ratio around 0.17 l/kg. The obtained product was cured for 28 days and characterized in terms of particle size distribution, humidity, CO₂ uptake and environmental behavior.

The CO₂ uptake measured immediately after the treatment by analyzing the TIC content of the collected solid samples was compared to the values calculated from a CO₂ mass balance performed on the basis of the measurements carried out on the inlet and outlet gas streams. Furthermore, the values of CO₂ uptake measured immediately at the end of the test were compared to the values measured after 28 d of curing.

The maximum value of CO₂ uptake was around 6% for all the tests, i.e. 60 g CO₂/100 g slag, and was not notably affected by the curing phase. Notably, in some samples a slight decrease of the CO₂ uptake was observed, which was temptatively attributed to the formation of unstable phases immediately after the carbonation, which decomposed during curing, releasing CO₂. The carbonation treatment was not observed to affect the environmental behavior of the material. In summary, the pilot-scale results demonstrated the feasibility of scaling-up the wet-route carbonation of BOF slags using a rotary drum.