The experimental determination of hydromagnesite precipitation rates at 22.5-75°C

Natural hydromagnesite (Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O) dissolution and precipitation experiments were performed in closed-system reactors as a function of temperature from 22.5 to 75 degrees C and at 8.6 < pH < 10.7. The equilibrium constants for the reaction Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O + 6H(+) - 5Mg(2+) + 4HCO(3)(-) + 6H(2)O were determined by bracketing the final fluid compositions obtained from the dissolution and precipitation experiments. The resulting constants were found to be 10 (33.7 +/- 0.9), 10(30.5 +/- 0.5) and 10(26.5 +/- 0.5) at 22.5, 50 and 75 degrees C, respectively. Whereas dissolution rates were too fast to be determined from the experiments, precipitation rates were slower and quantified. The resulting BET surface area-normalized hydromagnesite precipitation rates increase by a factor of similar to 2 with pH decreasing from 10.7 to 8.6. Measured rates are approximately two orders of magnitude faster than corresponding forsterite dissolution rates, suggesting that the overall rates of the low-temperature carbonation of olivine are controlled by the relatively sluggish dissolution of the magnesium silicate mineral.