Contenimento delle emissioni di anidride carbonica da sorgenti puntiformi attraverso carbonatazione accelerata di rifiuti industriali

The steady increase of carbon dioxide (CO2) emissions to the atmosphere is considered the main responsible of global warming. Over the last years, the scientific community has developed the Carbon Capture and Storage (CCS) technologies, which represent a promising method for the reduction of emissions in the short-medium period. Among the CCS, the mineral carbon sequestration (or mineral carbonation) is able to reproduce the natural weathering processes in which CO2 reacts with alkali metals (hydr)oxides derived from minerals or industrial alkaline residues. Alkaline residues from both thermal processes and construction and demolition activities represent an alternative feedstock as they result more advantageous by a chemical-energy point of view. The main goal of the study was focused on the possibility of applying the accelerated carbonation process on three types of alkaline wastes: Waelz slag, Construction and Demolition Waste (CDW) and Cement Kiln Dust (CKD). Waelz slag were treated according to an aqueous direct carbonation route, with different liquid to solid ratios (L/S) (0-10 l/kg). The effects of operating parameters such as particle size (4-0.045 mm) and pressure (1-20 bar) on both the degree of carbonation and reaction kinetics were studied. Regarding the CDW, five different types of residues (CDW30-40, CDW10-20, CDW0-20, CDW0-8, CDWcem) were treated by using two specific L/S (0.4 and 5 l/kg), two particle sizes (4 and 0.045 mm) and a pressure of 5 bar. Previous results of carbonation experiments carried out on CKD residues were used to a final comparison between the different materials. Accelerated carbonation experiment results showed different degrees of CO2 uptakes and conversion yields. Among the operating parameters, the particle size and, in particular, the specific surface of Waelz slag showed the greatest influence on CO2 uptake. A milling treatment under a size of 0.045 mm allowed to achieve a sequestration capacity equal to 0.19 t CO2/t Waelz. Potentially, the carbonation of Waelz slag would have the potential for sequestering 16% of the plant CO2 emissions. In addition, the carbonation process resulted in a decrease of the alkaline-earth metals concentration in the leachate due to the formation of mineral carbonates, indicating possible positive effects on the slag environmental behavior. As far as the CDW are concerned, the process conditions must be carefully selected in order to achieve a significant CO2 uptake. By using a L/S of 5 l/kg and a particle size less than 0.045 mm, it was possible to obtain a maximum uptake of 5.11 % on the sample CDWcem. Despite allowing for a lower CO2 sequestration capacity, mineral carbon sequestration using CDW appears to be attractive due to the large quantity of waste available and the potential for the wastes to be converted into reusable products. Assuming an uptake of about 51 kgCO2/t CDW and an European production of CDW approximately equal to 0.85 Gt/y, the CO2 storage capacity would amount to about 1.16% of the European total CO2 emissions. As regards CKD residues, the sequestration capacity of the CO2 depended on the operating conditions, reaching a maximum value of 9.8% with a L/S = 2.5 l/kg, pressure of 3 bar, temperature of 50° C and a reaction time of 5 h. A preliminary environmental analysis on two different accelerated carbonation process routes of Waelz slag and CKD was conducted by performing a comparative life cycle assessment (LCA) study. In particular, four different scenarios of wet and slurry carbonation were built. The analysis highlighted that the most of the scenarios were able to provide an important contribution to the climate change mitigation. In conclusion, this study allowed to collect new information about the possibility of applying accelerated carbonation process on Waelz slag, CDW and CKD. The experimental results revealed that the mineralogical sequestration is a promising technique for CO2 storage.