Accelerated Carbon
Technology (ACT)

About the reaction

The carbonation reaction primarily involves metal/mineral oxides, hydroxides and silicates, principally those of calcium and magnesium, reacting with CO₂ gas. If the reaction conditions are controlled, carbonation can be achieved in minutes instead of in the years taken for the natural reaction. Calcium and magnesium carbonates, on account of the environmental impact and stability, are the favoured reaction products, and are simply represented as Ca in the reactions below

CaO + CO₂ -> CaCO3

Ca(OH)₂ + CO₂ -> CaCO3 + H2O

The reaction pathways involved are more complicated than shown, as there are options to produce more than one valuable product from a carbonation step.

The first carbonation experiments involved the use of cement to bind heavily contaminated metalliferous wastes. Exposure of the cement to CO₂ shown as the bright grains in the SEM image right, rapidly de-calcifies the calcium silicates in the cement and precipitates the calcium as calcium carbonate needles in the pore space on the outside of the grains of cement. This process leaves the grey rims in the cement grains and the needles of carbonate lock together to harden the waste and lock up the metals.

11. SEM image

Exposure of the cement to CO₂ shown as the bright grains in the SEM image (above), rapidly de-calcifies the calcium silicates in the cement and precipitates the calcium as calcium carbonate needles (below).

11. Calcium Carbonate needles