A mineral carbonation method based on steelmaking slags and ammonium salt solutions has been found to have economical and CO 2 emission reduction potential. Although the aqueous solution of ammonium salt can be recycled, some of the solvent is likely to escape in a form of ammonia vapor within the outgoing gas fl ow during the carbonation step. In this study, the escape of ammonia vapor was investigated. The loss of NH 3 was found to be quite insignifi cant and in addition, the total solution loss per step was very small. The CO 2 content of the ingoing gas fl ow affected the rate of the CaCO 3 precipitation; the smaller the CO 2 content, the longer the duration of the carbonation reaction. However, the CO 2 content did not seem to have a signifi cant effect on the degree of CaCO 3 precipitation, indicating that CO 2 capture in a separate process step should not be needed. CaCO 3 precipitated in a form of calcite, although two of the precipitates also contained aragonite. Furthermore, the precipitates were found to be of high quality with bright white color, and fi ne and powdery texture. Nonetheless, in order to obtain high-quality CaCO 3, the precipitate resulting from the carbonation step should be washed with an adequate amount of water.