## 1 Introduction
Therefore, if these products cannot access the market as secondary raw materials via the conventional recycling routes, novel applications need to be drafted to avoid landfilling and increase the sustainability of these processes. Figure 1: Production and handling of metallurgical slags from iron and steel mills in Sweden during the year 2015. Reproduced with permission from [5], Copyright Jernkontoret, 2018. The Use of Slag as a Substitute for the Time Products Used for the Neutralization of On-Site Produced Acidic Wastewater  
Neutralization of acidic wastewater with the use of slaked lime is a well-established and effective process [10] used by both OTK and SMT. Both companies produce acidic wastewater, formed by the rinsing of steel after their respective pickling processes. Lime derived from calcareous limestone is cheap and abundantly produced. It is also applied in other parts of the steelmaking process, thus creating no incentives to stop its use, translating into a situation of technological \"lock in\" [11], even though the necessity of recirculating slag highlights an opportunity to change the current technology. An inductively coupled plasma mass spectrometry (ICP-MS) analysis has been performed on different slags taken from different parts of the steelmaking processes, both at OTK and SMT [12]. The analysis revealed that all the slag samples presented a calcium content of 30 wt%-40 wt%. Calcium (Ca) in slag has already been identified as the main contributor for the neutralization of acidic solutions [13]; hence, it could be tested as a viable substitute for lime in the neutralization process at OTK and SMT, as well as in other stainless-steel plants. Figure 3 shows the current lime and slag routes at OTK and SMT. Slag, due to the lack of a suitable use, is produced in the steelmaking process and landfilled. Moreover, wastewater is collected after the pickling process and sent to the neutralization plant for treatment. There, with the use of lime, the pH is raised from \(\sim\)2 to neutral values, while almost all the metal particles dissolved in the water are absorbed by the same neutralizing agent. Solid residues, derived from the chemical reactions between the wastewater and lime (\"saturated lime\" in Figure 3 and from now on called \"sediments\"), are filtered away from the clean waters, pressed, dried, and ultimately landfilled. Furthermore, cleaned water is released into the municipal streams. There are two major differences between the processes used by the two companies. Before landfilling, metal extraction is performed on the SMT slag to recover as many alloying elements as possible, while OTK do not landfill their sediments, rather they processes them and recirculate them as slag formers in the melt shop. Once the current process has been analyzed, it is possible to theorize what changes need to be applied to ensure an internal recirculation of slag. As shown in Figure 4, slag is used instead of lime for the neutralization of the acidic wastewater. This has multiple advantages: first, there is a net reduction in the material input, which is a clear gain in terms of resource efficiency. Decoupling from the use of lime means not only an economical advantage for the companies, but also an increased sustainability of the overall process. Second, by using slag as a neutralizing agent it is possible to increase the concentration of valuable alloying elements in it. Although this could be a disadvantage, it could be turned into an asset if we consider the state of the art of slag recycling. The wastewater contains, among others, Cr, Ni, Mo, Fe, and F, which, if the neutralization processes work, end up in the slag. The high concentration of metals in the slag, which has previously been considered a liability for cement or asphalt applications due to the leaching phenomenon, can become an advantage if the  
Figure 2: Production of metallurgical slag from the iron and steel mills during the year 2015.