The N-Mg Process
The N-Mg process as we have called it has been extensively investigated by Ferrini et. al. [1] at the university of Rome and was inspired by Towe and Malone [2], two DOE scientists.
Ferrini and his colleagues have extensively examined a process whereby CO2 is flushed through a solution containing magnesium ions (in their case magnesium chloride) with a view to investigate a possible role for nesquehonite in a "CO2-sequestering process" and disposing of Mg-rich wastewater. Unbeknown to Ferrini and his colleagues the same carbonate is the precipitated binder in our Tec, Eco and Enviro cements.
Their result were encouraging, precipitation was fast and almost to completion as in the graph at Figure 1 in their precipitation paper [1]. Ferrini and his colleagues used ammonia to maintain the pH at the correct level.
Precipitation rate of Nesquehonite [1].
They then proceeded to test the stability of nesquehonite and found it quite stable in most conditions on the surface of the planet today. The material was also quite thermally stable only loosing water up to about 350 oC when CO2 loss started. In another paper [3] they make the very pertinant point that decompostion is to magnesite which is thermodynamically much more stable.
TG - DTG curves of nesquehonite
Although nesquehonite is thermodynamically stable under ambient conditions, possible leakage of CO2 could occur if it is exposed to rain. Teir et al. [4] reported stability of magnesium and calcium carbonate minerals in nitric acid solutions at various conditions of acidity. They demonstrated that no appreciable CO2 release will occur if a mixture of Mg hydrate + anhydrous carbonate is leached in solution of pH> 2. Their results also showed a higher stability of Mg carbonate than Ca carbonate in order to dispose CO2. Because the pH of acid rain is unlikely to be below ∼2.5 [5], the possible release of CO2 from sites of nesquehonite storage due to acid rain is probably not important.
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[1] Ferrini, V., De Vito, C. & Mignardi, S., 2009. Synthesis of nesquehonite by reaction of gaseous CO 2 with Mg chloride solution: Its potential role in the sequestration of carbon dioxide. Journal of Hazardous Materials, 168.
[2] Towe, M.K. & Malone, P.G., 1970. Precipitation of Metastable Carbonate Phases from Seawater. Nature, 226.
[3] Ballirano, P. et al., 2010. The thermal behaviour and structural stability of nesquehonite, MgCO3 • 3H2O, evaluated by in situ laboratory parallel-beam X-ray powder diffraction: New constraints on CO2 sequestration within minerals. Journal of Hazardous Materials, 178.
[4] Teir, S. et al., 2006. Stability of calcium carbonate and magnesium carbonate in rainwater and nitric acid solutions, Energy Convers. Manage. , 47, pp.3059 – 3068.
[5] Brownlow, A.H., 1996. Geochemistry, New Jersey: Prentice-Hall.