The Role of Particle Size Distribution in CO2 Sequestration By Bof Slag

With regard to mineral carbonation as applied for CO₂ sequestration purposes, a wide variety of materials has been tested as the alkaline feedstock. These including minerals such as olivine, serpentine and wollastonite, as well as a number of residual streams from a variety of thermal processes (steel slag, alkaline ashes from combustion processes, municipal solid waste ashes, cement kiln dust and cement-based materials). Several studies have demonstrated that industrial residues are more reactive towards CO₂ compared to natural silicates, and accelerated carbonation of alkaline waste materials can achieve high CO₂ sequestration yields even when operated under mild operating conditions.

Although accelerated carbonation of industrial residues has been the subject of numerous studies conducted mainly over the past decade, a systematic and comprehensive assessment of both the individual and joint effects of the relevant operating parameters of the process is still missing in the scientific literature. In particular, the relevance of the particle size characteristics of the alkaline material subjected to accelerated carbonation has been mostly underrated so far. While it is commonly acknowledged that CO₂ uptake is inversely correlated with grain size of the material (or directly correlated with the specific surface area), to the authors’ knowledge no specific study has been conducted so far about the influence of particle size distribution of waste materials on the carbonation performance, while more emphasis is given to other typical operating variables of the carbonation process.

With this study, we aimed at investigating the effect of particle size distribution on the CO₂ sequestration yield of basic oxygen furnace (BOF) steelmaking slag. To this aim, different size classes of the slag were separately subjected to accelerated carbonation, and the CO₂ uptake performance, the mineralogical changes and the particle size variations upon carbonation were studied. The results showed that particle size played a very important role in determining the degree of CO₂ uptake by the alkaline material, and was largely more important than the elemental composition of the initial material. While differences in the total content of major elements by a factor of up to two did not reflect in altered carbonation performance for a given size fraction of the slag, changing the particle size distribution of the material dramatically affected the carbonation yield, which varied by up to two orders of magnitude. The CO₂ uptake ranged from a minimum of 0.47% to a maximum of 46.5%, while the observed range for the conversion yield was 0.9-72.9%.

Geochemical modelling of the slurry solutions at the end of the carbonation experiments indicated that a number of carbonate minerals involving the main potentially reactive elements were likely to form as a result of equilibrium with the solid phase. The main candidates for solubility control included CaCO₃´H₂O, amorphous MnCO₃, magnesite (MgCO₃), siderite (FeCO₃) and ZnCO₃. The presence of carbonate minerals containing not only Ca, but also Mn, Mg and Fe was also confirmed by XRD analyses. The nature of the carbonate minerals controlling the equilibrium between the solid and the liquid phase did not vary with the investigated size class (and thus with the carbonation efficiency attained), indicating that an increase in the surface area of solid particles enhanced the dissolution of the original minerals from the slag matrix and increased the amount of precipitated solids without changing their type.

The findings of the study suggest that the extent of dissolution of the original phases in the slag, which increases as surface area increases, largely determines the carbonation performance. A direct implication of this finding is the need of promoting the dissolution of the original matrix through either intensive size reduction pre-treatments or the application of chemical agents. Of course, an energetic assessment of the process overall, including the required pre-treatments, should be conducted in order to derive a net CO₂ balance and determine the actual contribution to carbon sequestration.