International Slag Valorisation SymposiumLeuven6-7/4/2009 - Third ...

obtained from pure CO2/N2 gas mixtures in the laboratory. Direct in-line mineral
carbonation thus seems to have potential.
Related to the industrial application of in-line ash carbonation, the concept of leading
the stack gas in counter-current over a layer of bottom ash on a moving belt has been
investigated with respect to the acceptable thickness of the bottom ash layer. 13) Figure 2
gives the pH results over time for a 10 cm thick layer of bottom ash of which the top
surface was contacted with 10% CO 2 at 50°C. After one week almost the complete
sample had reached pH 8.5-9 and was considered to be carbonated. A simple model
predicts the experimental results reasonably well. Further process improvement is
obviously necessary before industrial feasibility can be considered.
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1 st International Slag Valorisation Symposium│Leuven│6-7/4/2009
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Figure 2: Experimental (left) and modelling (right) results of accelerated carbonation of
a 10 cm bottom ask layer over time.
The reaction term was studied using a carbonation experimental set-up that was
developed at the department of Civil Engineering. In this set-up, lime paste is subjected
to high CO2 concentration to simulate an accelerated carbonation process. This
experimental study is based on measuring the CO2 concentration during the carbonation
reaction. Net CO2 uptake by calcium hydroxide is determined and based on this the
reaction rate is calculated. Using this set-up, the carbonation reaction was studied at
ambient temperatures with various lime binders 7) and with hydraulic binders 24) at
various CO2 gas concentrations (20%, 50% and 100%). This study has verified the lime
mortar carbonation model and has improved understanding of the factors influencing
the carbonation reaction rate in a porous system. The reaction rate is not proportional to
the CO2 gas concentration but is rather influenced by the material properties of lime as
the specific surface area. In the case of incinerator bottom ash, there is also no
advantage in increasing the CO2 gas concentration, which again illustrates that a 10%
CO2 stack gas is very suitable for mineral carbonation processes. 8,13)
Further insights have been gained on the carbonation reaction mechanism and on the
reaction rate with recent research using this carbonation experimental set-up coupled
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