CO_2 sequestration by carbonation of Finnish magnesium silicates

摘要

Carbon dioxide capture and storage (CCS) is considered as one of the main options for reducing atmospheric emissions of CO_2 from human activities. From Finland's perspective CCS does not provide an easy answer to decreasing of CO_2 emissions, since there do not seem to exist suitable geological formations for sequestering CO_2. Carbonation of Finnish magnesium silicates could provide a respectable reduction of CO_2 emissions in Finland, since Finland has a large abundance of mafic and ultramafic rocks, including serpentinites. It has been estimated that only in Eastern Finland there are about 121 km~2 of serpentinites. Serpentinites consist mainly of serpentine and are produced as by-products in several Finnish metal and industrial mineral mines. The total amount of hoisted rock in Finnish mines was about 31 Mt in 2004, of which about 11 Mt was from ultramafic deposits in general. The serpentinites of Outokumpu-Kainuu ultramafic rock belt alone could be sufficient for storing annually 10 Mt CO_2 for 200-300 years. Experiments with gas-solid carbonation of magnesium hydroxide have been performed using a pressurized thermogravimetric analyzer (PTGA). The results from the PTGA indicate that the carbonation reaction is controlled by product layer diffusion. The best carbonate conversion measured with the PTGA was 11% in 6 h at 510℃ and 40 bar gas pressure (99% CO_2, 1% H_2O), while a chemical analysis of the partially carbonated sample showed a carbonate conversion of 60%. Serpentinite from Hitura has been analyzed and leached in. aqueous solutions of acids and NaOH. All solutions except for those prepared from NaOH managed to extract part of the magnesium and iron in serpentinite in 1 h. Higher temperature improved the magnesium and iron extraction efficiency of an aqueous solution of HNO_3 significantly. It was possible to remove extracted silica from the solutions by filtration. Precipitation was formed by raising the pH of a filtered solution using NaOH while CO_2 was bubbled through the solution. The precipitate contained mostly nitratine and some magnesium oxide carbonate.