Surface treatment processes of stainless steel, such as pickling, produces acidic waste water consisting of Na⁺, Cl⁻, F⁻, NO⁻3, SO42-, PO43-, Fe3+, Cr6+ and Ni2+. At Sandvik ABs steel works in Sandviken, this waste water is treated and neutralized using slaked lime before being released into the lake Storsjön. The aim of this report was to make a literary review of previous work in using slag as a neutralizing agent for acidic waste water. Furthermore, to see if it’s possible to replace some or all of the slaked lime in the neutralization process with slag and to study what slag that might be suitable to use. Since the waste water contains HF acid, the focus of this report was on different materials used for fluoride adsorptions. The literary study showed that the fluoride adsorption process with BOF slag, quick lime and magnesium oxide as adsorbents were endothermic. Therefore, it benefitted from an increased temperature. Furthermore, the literature study showed that many materials follow a pseudo-second-order kinetic model as well as a Langmuir or Freundlich isotherm for a fluoride adsorption. A previous experiment showed that a mixture of BF slag and slaked lime had the best HF acid neutralization among the tested materials. A fluoride adsorption experiment was made at different temperatures (25 – 55 °C) by using a slag from an aluminium reduced steel melt and a slag from a silicon reduced steel melt from the AOD converter in Sandviken. The silicon reduced steel melt slag showed an increase of fluorides in the solution, due to the presence of calcium fluoride in the slag. This was observed for all temperatures. The aluminium reduced steel melt slag also increased the fluoride content in the solution at 25 and 30 °C. At higher temperatures the fluoride content in the solution decreased with 93.6 to 94.9 %. Na⁺, Cl⁻, F⁻, NO⁻3, SO42-, PO43-, Fe3+, Cr6+, and Ni2+

Fluoride, the most abundant form of fluorine, is an ion released into the environment, mainly via anthropogenic sources and erosion of mineral rocks. Although the element is well known for its health benefits on teeth and bones, it can as well be a harmful pollutant. In some areas on earth, the population can not obtain drinking water that is below the guideline limits of fluoride, which is set to 1.5 mg/L by the world health organization (WHO). Therefore there is a relevance to develop methods that can clean the waters from excess fluoride. The potential problems when it comes to finding these types of methods is that they can be expensive. However, materials generated as residues in industrial processes may be low in cost. In this thesis, silicon reduced AOD-slag, a material generated as a by-product from the production of stainless steel, was examined by its fluoride adsorption behavior and adsorption capacity. The concentrations of fluoride were measured with ion chromatography (IC) and the concentration of metals were analyzed with microwave plasma atomic emission spectroscopy (MP-AES). Additionally pH and conductivity were measured. The functionality of the material surface was analyzed with isotherm modeling, where the Sips isotherm model was tested. Moreover, optimization of the slag was performed by heat treating the material, as well as a sorption kinetics test on both optimized and original slags. The results from the analysis indicated that the material corresponds well to the Sips isotherm. Considering this result it is suggested that at low concentrations, the surface can be characterized as heterogeneous, with different binding energies at different available sites. At higher concentrations the Sips-model explains the surface to be saturated when a monolayer of fluoride is formed. Therefore the binding on the slag surface can be described to have an inner-sphere and covalent character. The metal analysis showed that calcium ions are released from the slag when in aqueous solutions. The presence of calcium in the liquid samples are believed to result in formations of solid calcium fluoride (CaF2), precipitated on the slag surface. Lastly, the maximum fluoride removal is believed to differ between different types of AOD-slag, where there as well are possibilities to optimize the material.