## 1 Introduction  
Slag is produced as a by-product of pyrometallurgical metal production. The steel production of Sweden almost reached 4.7 million tonnes of crude steel in 2017; at the same time, around 1.3 million tonnes of slag was produced [1]. The application areas for slag vary: fertilizer, water purification, and different construction materials are some examples. In some areas, the properties of slag make it superior to natural resources, for example the porosity of slag in asphalt reduces road noise [2, 3]. By using slag, natural virgin resources can be preserved and unnecessary landfill is prevented. The properties of slag can be derived from the properties of the minerals in the slag. Different types of slag consist of different minerals, which make slag a versatile material. However, some of these properties, such as volume expansion [4], disintegration [5], and leaching of some elements, restrict how slag can be used. From the literature, it can be discerned that chromium is one of the main elements whose leaching should be restricted [6, 7]. The European Council Directive (2003/3/EC) limits chromium leaching to 0.5 mg/kg for the material to be classified as inert landfill material. In Sweden, 40% of the steel is made from scrap [8]. In scrap-based production, the elements in the scrap, such as chromium, partly distribute into the slag. Slag from scrap-based steelmaking can thereby contain chromium even if no alloys were added to the furnace. The first step in producing steel by scrap-based steelmaking uses an electric arc furnace (EAF). In this paper, low-alloy EAF slag from Ovako in Hofors was used as a starting material. Ovako in Hofors produced about 400,000 tonnes of ingot steel and with that about 40,000 tonnes of EAF slag in 2017; this corresponds to almost half of the low-alloy EAF slag in Sweden, which was 100 tonnes in 2015 [9]. Ovako Hofors regards EAF slag as a product and wants to guarantee that the slag is as environmentally friendly as possible, and has therefore continuously investigated the possibility of making a better slag with less chromium leaching [10; 11; 12; 13]. Chromium leaching has been reduced by changing the slag composition, but it can be further reduced to ensure it always maintains a level below the limit of inert material. The slag usually consists of larnite (\(\beta\)-Ca\({}_{2}\)SiO\({}_{4}\)), magnesiowustite (MeO), spinel (AB\({}_{2}\)O\({}_{4}\)), and brownmillerite (Ca\({}_{2}\)(AlFe)\({}_{2}\)O\({}_{5}\)), but occasionally merwinite (Ca\({}_{3}\)MgSi\({}_{2}\)O\({}_{8}\)) has been detected. The leaching of slag has been connected to the dissolution of the minerals in the slag [14; 15]. In order for a mineral to contribute to chromium leaching, two criteria must be fulfilled: the mineral has to contain chromium and dissolve or react in contact with water. The stoichiometric formula of a mineral does not have to include chromium for a mineral to contain chromium, as is the case for brownmillerite. The chromium may substitute for another atom or be an interstitial addition. Of the minerals in Ovako Hofors slag, merwinite is not the main mineral responsible for chromium leaching, as it is only present occasionally. Larnite dissolves [16] and has been reported to contain chromium [17]. However, chromium has not been detected in analyses of the investigated slag [18] and is therefore unlikely to cause the chromium leaching in this slag. The spinels are stable and do not react with water [6; 19] and can also be excluded. The dissolution of magnesiowustite depends on the FeO/MgO ratio. Magnesiowustite, with a FeO content of 60 wt% or more, does not react with water or leach chromium [20; 21]. Magnesiowustite primarily consists of FeO, MgO, MnO, and CrO\({}_{\rm v}\), but also contains small amounts of additional elements like calcium [22; 23]. The magnesiowustite in the studied slag is FeO rich most of the time and should therefore, in most cases, not dissolve in water and contribute to chromium leaching.