Leaching of Residual Steel Slag from a Mineral Carbonation Process

In an alternative process concept for producing precipitated calcium carbonate (PCC), calcium is extracted from steelmaking slag using ammonium salt solutions, from which PCC can subsequently be precipitated using CO₂. As the PCC precipitates the solvent is regenerated, and can be returned to the extraction reactor for reuse in the process. While PCC is a desired product of use in polymers and paper, a less desired product of the process is a “residual” slag, from which roughly half the original calcium content has been leached. In addition, the residual slag contains remnants of the ammonium salt solvent. The usability of the residual slag from the process can have a large effect on the economic feasibility of such a PCC process.

The objective of this study was to compare the composition and solubility of the residual slag generated by the PCC process to that of the raw, untreated slag. Leaching tests were carried out with three different stainless steel slags from the Outokumpu Tornio Works: ground, water-cooled Argon Oxygen Decarburization (AOD) slag, filler slag (the fine portion of AOD and ladle slag), and air-cooled AOD slag. The slags were leached in 4 M ammonium acetate solutions for 120 minutes to produce solids representing residual slags of the PCC process of each slag type. The solubility of each raw slag and residual slag in water was tested using standardized batch leaching tests (CEN EN 12457-3). The solutions were analysed using several different techniques (including ICP-MS, ICP-AES, CV-AFS, IS and IC) for measuring the various elements dissolved into the solutions. The resulting solution of the residual slag leaching in water consisted mainly of calcium, with a significant content of organic carbon (DOC). The organic carbon is most likely a remnant of the ammonium acetate solvent used. The processing does not appear to significantly affect the solubility of other elements. The results from this study implies that the PCC process would have to wash the residual slag to remove the remnants of organic carbon, which adds some costs to the process and increases the use of water.