A novel conversion process for waste slag: synthesis of a hydrotalcite-like compound and zeolite from blast furnace slag and evaluation of adsorption capacities

The main components of blast furnace slag (BFS) are CaO, SiO2, A12O3, MgO and slight amounts of transition metals such as Fe, Ti and Mn. We successfully synthesized a hydrotalcite-like compound from BFS via a convenient chemical process: acid-leaching and precipitation. After the HCl acid-leaching process, BFS was separated into hydrated silica with 92 wt% SiO2 content and leaching solution including other components, which afforded a hydrotalcite-like compound after subsequent NaOH addition in high yield (6.2 g from 10.0 g of BFS). By means of XRD and chemical analysis, the product synthesized at 100 °C was identified to be a Ca-Al-based hydrocalumite compound with the stoichiometric molar ratio of Ca : Al : Cl = 2 : 1 : 1, incorporating other metal cations in its structure. The phosphate adsorption capacity of the raw slag was 1.5 mg P/g but increased to over 40 mg P/g when converted into the hydrotalcite-like compound, which was more than three times greater than that of conventional Mg-Al-based hydrotalcite. Furthermore, single-phase A- and X-type zeolites with high crystallinities and excellent water adsorption capacities (247 and 333 mg g−1, respectively) were successfully synthesized using the residual silica through a hydrothermal treatment for 6 h at 100 °C. This conversion process, which enables us to fabricate two different kinds of valuable materials from BFS at low cost and through convenient preparation steps, is surely beneficial from the viewpoint of economical use of BFS.