Conversion of CO2 into mineral carbonates using a regenerable bufferto control solution pH
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The barrier that is currently stalling the rapid conversion of magnesium silicate deposits into magnesium carbonate as method for storing CO2 is considered to be the difference in pH needed for magnesium dissolution from the silicate and magnesium precipitation as the carbonate, whereby rapid dissolution requires a low pH of around 1 while rapid precipitation requires a considerably higher pH of around 8. This paper investigates a novel concept which is to use a tertiary amine to bind with protons and raise the pH to around 8 and to then regenerate the amine through the use of heat due to the strength of the amine–proton bond decreasing with increasing temperature. This approach provides the low pH and high temperature that is needed for Mg dissolution and the high pH need for carbonate precipitation. The amine can be thought of as a regenerable buffer.Dissolution of Mg from serpentine has been found to be favourable with a solids to solution volume of more than 50 g/L to enable a low pH, and with temperatures close to the boiling point of the solution. The pH needed for magnesium carbonate precipitation was found to be approximately 8.2. Both triethylamine and tripropylamine were found to be capable of achieving this at 18 °C. Yields of around 20–40 wt.% carbonate were achieved using residence times of approximately 1 h. The pH swing for the tertiary amines was found to be approximately 2.5 pH units between 5 and 85 °C, suggesting that an amine capable of achieving a pH of 8.2 at low temperature generates a pH of 5.7 in solution when heated to 85 °C. Further work will examine whether the lower pH values needed for serpentine dissolution can be achieved by heating the protonated amine to higher temperatures.
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