Evaluation of Activation Efficiency and Quality of Slag Carbonation

The capacity of some industrial waste to sequester carbon dioxide is well known and it is also noted that to achieve the carbonation reaction it is necessary to activate the precursors overcoming the initial energy barrier through processes of dissolution in an autoclave at high temperature and pressure.

The mechanisms which prevent the carbonation without activation are not the same for all minerals: for the hydroxylated phases carbonation proceeds in the moment in which the structures lose hydroxyls, while the anhydrous phases are partially dissolved in the solution and there transformed into carbonates.

To develop the direct carbonation, without autoclaving, our research group uses the mechanical activation of precursor minerals realized through an intense grinding made with friction mill (ring mills). We can estimate the timing of activation of mineral phases using a calculation model realized by us: it is sufficient to know the mechanical specifications of the milling system and the values of ΔH₀ of the main mineral phases present in the precursors. The intent is not to bring the minerals to a completely amorphous state, but rather to induce a large number of structural defects in order to increase the rate of dissolution of the carbonatable elements and at the same time to increase the surface defects which become points of aggregation of CO₂ in the formation of carbonates. If we know the amount of "sites of radical activity", we can estimate the number of structural defects correlating with the amount of aggregate CO₂. Therefore, knowing the mechanical parameters of the grinding machines, the thermodynamic parameters of the material and the amount of radicalic sites obtained by grinding, we can evaluate the rate of carbonation for each type of mineral.

The choice of materials for the laboratory tests was performed on the basis of the times of mechanical activation and the presence of alkali metals and alkaline-earth metals. Such materials are MS (secondary metallurgy) blast furnace clear slag, LD (Linz-Donawitz converter) dark slag, bottom ash (BA) and fly ash (FA): these materials, after the chemical analysis, were subjected to a sifting to separate the fraction between 300 and 800 microns and to a chemical analysis in X-ray fluorescence to determine the maximum degree of carbonation compatible with the chemical composition.

The mechanical activation was performed using a ring mill with controlled power. The precursor materials were ground for 1 minute and 5 minutes, with ratios of masses of grinding of 1: 400.

The level of mechanical activation was quantitatively measured by analyzing the radical activity: one of the most used and reliable methods is based on the use of the compound DPPH (2,2-diphenyl-1-picrylhydrazyl), a molecule which binds to radicalic sites of the material. The molecule exhibits an absorption spectrum in the visible with a characteristic peak at 536 nm, the intensity of which is quantitatively related to the number of passivated radicals. The mechanical activation was evaluated by measuring spectrophotometrically the number of radicals generated by grinding.

The tests of carbonation were carried out on 10 grams of material immersed in water (solid / liquid ratio equal to 1/10) in which carbon dioxide flowed (1 liter / minute) at room temperature and atmospheric pressure. Each sample was subjected to differential thermal analysis and X-ray diffraction analysis to evaluate the amount of captured CO₂ stabilized in the form of carbonates and also to evaluate the amount of water in the form of zeolitic water and hydroxyl groups presents in the materials before and after the carbonation reaction.

Through the spectrophotometric measurements of the radicals for all the precursors studied it is highlighted a good correlation between the number of active sites and the amount of CO₂ fixed as carbonates.

The analysis in X-ray diffraction and the thermal analysis have shown that the MS slag, the BA and the FA after 5 minutes of grinding reach a good degree of activation of the materials, corresponding to a very high level of amorphization, while the LD slag needs only one minute of activation after which the content of free radicals declines.  

The thermal analyses show that the MS slag activated up to 5 minutes is able to sequester CO₂ up to 20% by weight. The LD slag instead shows the maximum activation after 1 minute of grinding coming to capture 22% of CO₂. In the bottom ash and fly ash activation is more effective after 5 minutes of grinding and we get 44% of CO₂ captured.

Considering the content of carbonatable oxides present in the selected materials, the possible maximum levels of carbonation have been compared with the levels reached by the process studied and it was found that for the MS slag is reached 96% of the maximum possible value after 5 minutes of activation, for the LD slag 74% after only one minute of activation, for the bottom ash can be reached 97% of the maximum possible, while for the fly ash only 55% after 5 minutes.

The values of maximum activation, evaluated by the analysis of free radicals, is in perfect agreement with what was found in the evaluation of the level of carbonation.

The results show the possibility to achieve high levels of carbonation activating mechanically the materials, provided to control the level of activation by a quantitative method, such as that used by us.