Production of amorphous hydrated impure magnesium carbonate through ex situ carbonation

摘要

The evaluation of the feasibility of ex situ carbonation in landfills utilizing raw natural substances (namely serpentinites as Mg-source and the CO _2-rich fraction of biogas as C-source) was tested through a laboratory procedure comprising three steps. The first step is the acid attack of a serpentinite at 70°C, by means of HCl _2M, to get MgCl _2-rich solutions. Attacks of different durations were performed to evaluate the time needed. The second step is the neutralization of the MgCl _2-rich solution by addition of concentrated ammonia. The third (carbonation) step is mixing of the neutralized MgCl _2-rich solution with a solution of ammonium carbonate. This was produced in a landfill by absorption of CO _2 contained in biogas in a solution of ammonia. The neutralization of acid MgCl _2-rich solutions caused the precipitation of ferrihydrite with secondary ammonium carnallite and salammoniac, whereas abundant precipitation of Amorphous Hydrated Impure Magnesium Carbonate (AHIMC), sometimes with minor nesquehonite, occurred in the third step. This solid carbonate acts as a stable CO _2 sink up to 380°C. The geochemical behavior of some minor elements was also investigated during the experimental processes revealing that Al, Cr and Ni were removed during neutralization (second step), in contrast to Ca which remained in the circumneutral MgCl 2-rich solution and entered into the structure of AHIMC. During the carbonation step, precipitation of artinite, hydromagnesite, lansfordite, magnesite and nesquehonite was thermodynamically impossible as the aqueous phase was undersaturated with respect to these solid phases upon separation of AHIMC.The CO _2 sequestered through this multi-step procedure is 0.34ton for 1ton of serpentinite utilized. However, despite that the use of Mg-rich silicates is a suitable approach to ex situ carbonation, due to their huge availability worldwide, the CO _2 produced in making the chemicals and the overall energy balance of the process must be evaluated to assess its sustainability. Furthermore, this procedure could be of great interest in asbestos inertization and the AHIMC products could possibly be useful in industry.