A. Stramazzo

Thin-film versus slurry-phase carbonation of steel slag: CO2 uptake and effects on mineralogy

The results of direct aqueous accelerated carbonation of three types of steel manufacturing residues, including an electric arc furnace (EAF) slag and two basic oxygen furnace (BOF) slags, are reported. Batch accelerated carbonation tests were conducted at different temperatures and CO2 pressures applying the thin-film route (liquid to solid, L/S, ratio=0.3L/kg) or the slurry-phase route (L/S ratio=5L/kg). The CO2 uptake strongly depended on both the slag characteristics and the process route; maximum yields of 280 (EAF), 325 (BOF1) and 403 (BOF2) gCO2/kg slag were achieved in slurry phase at T=100°C and pCO2=10 bar. Differently from previous studies, additional carbonates (other than Ca-based phases) were retrieved in the carbonated BOF slags, indicating that also Mg-, Fe- and Mn-containing phases partially reacted with CO2 under the tested conditions. The results hence show that the effects of accelerated carbonation in terms of CO2 uptake capacity, yield of mineral conversion into carbonates and mineralogy of the treated product, strongly rely on several factors. These include, above all, the mineralogy of the original material and the operating conditions adopted, which thus need specific case-by-case optimization to maximize the CO2 sequestration yield.

Leaching modelling of slurry-phase carbonated steel slag

In the present work the influence of accelerated mineral carbonation on the leaching behaviour of basic oxygen furnace steel slag was investigated. The environmental behaviour of the material as evaluated through the release of major elements and toxic metals under varying pH conditions was the main focus of the study. Geochemical modelling of the eluates was used to derive a theoretical description of the underlying leaching phenomena for the carbonated material as compared to the original slag. Among the investigated elements, Ca and Si were most appreciably affected by carbonation. A very clear effect of carbonation on leaching was observed for silicate phases, and lower-Ca/Si-ratio minerals were found to control leaching in carbonated slag eluates as compared to the corresponding untreated slag sample as a result of Ca depletion from the residual slag particles. Clear evidence was also gained of solubility control for Ca, Mg and Mn by a number of carbonate minerals, indicating a significant involvement of the original slag constituents in the carbonation process. The release of toxic metals (Zn, V, Cr, Mo) was found to be variously affected by carbonation, owing to different mechanisms including pH changes, dissolution/precipitation of carbonates as well as sorption onto reactive mineral surfaces. The leaching test results were used to derive further considerations on the expected metal release levels on the basis of specific assumptions on the relevant pH domains for the untreated and carbonated slag.

Accelerated carbonation of steel slags using CO2 diluted sources: CO2 uptakes and energy requirements

This work presents the results of carbonation experiments performed on Basic Oxygen Furnace (BOF) steel slag samples employing gas mixtures containing 40 and 10% CO2 vol. simulating the gaseous effluents of gasification and combustion processes respectively, as well as 100% CO2 for comparison purposes. Two routes were tested, the slurry phase (L/S=5 l/kg, T=100 °C and Ptot=10 bar) and the thin film (L/S =0.3-0.4 l/kg, T=50 °C and Ptot=7-10 bar) routes. For each one, the CO2 uptake achieved as a function of the reaction time was analyzed and on this basis the energy requirements associated to each carbonation route and gas mixture composition were estimated considering to store the CO2 emissions of a medium size natural gas fired power plant (20 MW). For the slurry phase route, maximum CO2 uptakes ranged from around 8% at 10% CO2, to 21.1% (BOF-a) and 29.2% (BOF-b) at 40% CO2 and 32.5% (BOF-a) and 40.3% (BOF-b) at 100% CO2. For the thin film route, maximum uptakes of 13% (BOF-c) and 19.5% (BOF-d) at 40% CO2, and 17.8% (BOF-c) and 20.2% (BOF-d) at 100% were attained. The energy requirements of the two analyzed process routes appeared to depend chiefly on the CO2 uptake of the slag. For both process route, the minimum overall energy requirements were found for the tests with 40% CO2 flows (i.e. 1400-1600 MJ/t CO2 for the slurry phase and 2220-2550 MJ/t CO2 for the thin film route).

Preliminary results of the life+ project: CO-ordinated approach for sediment treatment and beneficial reuse in small harbours networks

a ISPRA (Italian Institute for Environmental Protection and Research). Via Vitaliano Brancati, 60 00144 Rome (Italy) b Università di Cagliari – Dipartimento di Geoingegneria e Tecnologie Ambientali. Piazza d’Armi, 1 – 09123 Cagliari (Italy) c ARPA Emilia Romagna (Agenzia Regionale Prevenzione e Ambiente). Largo Caduti del Lavoro, 6 40121 Bologna (Italy) d CNR (Consiglio Nazionale delle Ricerche) – Istituto di Geologia Ambientale e Geoingegneria. Piazza d’Armi, 19 – 09123 Cagliari (Italy) e Università degli Studi di Roma “La Sapienza” – Dipartimento di Ingegneria Civile, Edile ed Ambientale. Via Eudossiana, 18 – 00184 Rome (Italy) raffaella.pomi@uniroma1.it