# Micro-Raman Spectroscopy Investigation of Crystalline Phases in EAF Slag
## 1 Introduction  
Steel production is one of the most widespread industrial activities in the world. The overall annual production of crude steel in 2017 was estimated at about 1690 Mt by the Worldstetel Association. The major producers are China, Japan and India, with about 832, 105 and 101 Mt, respectively, in 2017. In Europe, Italy plays a vital role, with a production of 24 Mt/year, being the second largest producer after Germany [1]. In Electric Arc Furnace (EAF), slags are formed on a steel bath for the effect of lime and injected oxygen. The slags are about 10-15% in weight of steel production [1, 2]. EAF slags are classified as black and white slags. Black slag has a lime content less than 40%, and results from the cold loading of scrap. White slag has a lime content higher than 40% and is generated during grinding when lime is added to remove sulphur and phosphorus. Black slags vary with the steel produced (EAF-C from carbon steel production and EAF-S from the production of stainless steel/high alloyed steel), and the chemical composition depends on the scraps, the quantity of oxygen adopted during the melting process and furnaces process [3]. Both slags are mainly formed of oxides FeO, CaO, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), MgO and other impurities in a wide range of percentages (10-40%, 22-60%, 6-34%, 3-14%, and 13-14%, respectively). Ca and Si come from the materials added to the liquid steel bath, while Mg originates from the attack of steel bath to the furnace firebrick. Other elements, such as Cr, Ti, Cu, are impurities present in the ferrous scrap [3, 4]. Crystals formation in slag is strongly affected by both the chemical composition of the molten slag and the cooling rate, which usually depend on the protocols of each steel plant. Significant variation of the slags is also due to the kind of steel produced [3]. For this reason, the identification of the crystalline phases is crucial, and is usually carried out by external laboratories. Thus, a technique that permits fast characterisation of the slag by unskilled personnel would be very beneficial for the quality control of steel production. In the scientific literature, the effects of the slagging procedures on the mineralogical and morphological structure of EAF slag have already been discussed. For example, the addition of SiO\({}_{2}\) combined with rapid cooling favours the formation of glassy phases [3]. The main crystalline phases, usually identified by XRD, are larnite (2 CaOSiO\({}_{2}\)), brownmillerite (Ca\({}_{2}\)(Al, Fe)\({}_{2}\)O\({}_{5}\)), wustite (FeO), calcium silicate, silicon aluminates, gehlenite (Ca\({}_{2}\)Al(AlSiO\({}_{7}\))), predigite (Ca\({}_{7}\)Mg(SiO\({}_{4}\))\({}_{4}\)), magnetite (Fe\({}_{3}\)O\({}_{4}\)), and magnesioferrite (MgFe\({}_{2}\)O\({}_{4}\)) [5; 6]. The main techniques adopted for investigating the structural and chemical composition of slags are: Powder X-ray Diffraction (PXRD) for the crystalline phase identification and Scanning Electron Microscopy (SEM) coupled to Energy Dispersive X-ray Spectrometry (EDXS) for the morphological and elemental analysis of the phases [7]. PXRD needs the samples to be pulverised and homogenised; thus, the patterns contain the signals of altogether the crystalline phases, making their identification sometimes tricky. Moreover, PXRD analysis does not give any information about the amorphous phases present in the sample. Raman spectroscopy is used in many applications, such as medicine, chemistry and remote sensing for the structural and chemical analysis [8]. Spectroscopic techniques are based on the interaction between electromagnetic radiation and matter and are based on the phenomenon of emission, absorption, fluorescence or scattering. Raman spectroscopy typically uses laser light in the visible field, near-infrared or near-ultraviolet fields to excite the vibro-rotational energy levels of molecules by generating scattered light [9].