Pure calcium carbonate product from the carbonation of a steelmaking slag

The steel industry accounts for approximately 6-7% of the total anthropogenic CO2 emissions to the atmosphere. Significant CO2 emission reductions could be achieved by using steelmaking slags for carbon dioxide mineralization, (i.e mineral carbonation). Due to their high calcium content, steelmaking slags could be very suitable for this method that could store carbon dioxide safely for over 100 000 years. This option is especially important for a country like Finland, where there are no suitable geological formations for CO2 storage. If calcium could be extracted from the slags prior to carbonation, a pure, and thus also marketable, calcium carbonate could be produced. This could possibly replace some of the natural and synthetic CaCO3 used in industry, combining savings in natural resources with CO2 emissions reduction. By carbonating all of the steelmaking slag produced at Raahe Works, the largest steel-works in the Nordic countries, up to 260 kt of CO2 could be stored and 590 kt CaCO3 produced annually. If the produced precipitate met the quality standards of commercial precipitated calcium carbonate (PCC), process costs up to about 200 /t of CO2 could be allowed. The development work on the production of pure calcium carbonate from slag by mineral carbonation is presented in this paper. The study is based on experimental research. Selective extraction of calcium from steel converter slag was investigated using various solvents. Precipitation of calcium carbonate from dissolved calcium at atmospheric pressure was also investigated. Our results indicate that while acids are the most efficient solvents for extracting calcium from the steel converter slag, ammonium salt solutions extract calcium most selectively. For instance, a solution of 2 M ammonium acetate was able to extract ~ 70 % of the calcium, while the only other dissolved element was silicon (~5 %). When bubbling CO2 gas through the solution calcium carbonate precipitated. It appeared to be possible to produce a precipitate containing up to 99.8 % CaCO3 as calcite. The next task is to find out whether the produced precipitate meets the requirements for commercial PCC and which solvent gives the cheapest route to produce this precipitate.