Nel presente lavoro viene presentato il sistema Slag-Rec per il trattamento della scoria EAF, che e stato realizzato presso MFL, Liezen (Austria) ed e attualmente in fase di installazione ed avviamento presso la acciaieria ASO, Ospitaletto (Brescia). Il sistema permette di realizzare la granulazione a secco della scoria, eliminando il contatto della stessa con acqua di raffreddamento, attuando allo stesso tempo un raffreddamento controllato e ripetibile in funzione del tipo di scoria da trattare.

In most industrialized countries slag deriving from iron and steel production is considered a potentially re-usable by-product, not a waste. Converting costs for dismissing in dumps into income from selling slag offers remarkable savings to steel producers having at the same time important benefits for the environment. Nevertheless not all the produced slag gets re-used, with noticeable differences among recycling percentages of the different kind of plants. Today almost all the slag deriving from blast furnaces (iron & steel making from iron ores) is mostly used in cement production and in cement conglomerates. On the other hand slag deriving from scrap melting in electric arc furnace steel works, the so called "black slag", is generally considered not suitable for use in cement conglomerates, but it can be used as an aggregate in road construction. The name black slag stems either from its dark color and also from the need to distinguish it from the "white slag", i.e. the secondary metallurgy slag from the ladle furnace, which has to be managed separately in view of the possible uses of the black slag. EAF slag which is not discharged in dumps has to be manipulated in order to be re-usable in road infrastructures, but treating methods are not always fully environment friendly. Slag is discontinuously discharged from electric-arc furnaces and is left to cool down to room temperature. Today EAF slag is gathered in large pots or it is simply poured on the ground, where it is eventually cooled with water, which is mainly employed in order to take under control excessive foaming that can continue also out of the furnace resulting in a friable, low density solid slag with a very extensive surface area. Solidification in blocks occurs in air, and the process is accelerated by water sprinkles. The obtained slag is not re-usable in blocks, it needs to be crushed and machined. Therefore, the existing methods to handle the EAF slag do not allow to take under control the slag cooling cycle. Slags which are not chilled with water can undergo volume expansion during time, due to their lime and magnesia content and to their non glassy structure. In order to avoid swelling, the free lime content has to be reduced to max. 2-3%, otherwise slag cannot be used for road construction, and it makes necessary further treatment. Slags with excessive free lime content should be stabilised by a seasoning period which can take months (depending on piece size). The actual process is not only needing a long time, but it is not even completely environment friendly since the moving operations produce dust, and the cooling or percolating water has a pH around 10 and may contain heavy metals. A possible way to obtain a directly usable slag could be represented by a granulation treatment with the actually available process by pouring it into a large mass of water. The main problems with water granulation are reported to be either the need of large quantities of water that must be treated to eliminate heavy metals, pH adjusted, and re-circulated, and the emission of dust. Handling of slag out of the EAF is therefore not a simply and safe task even when slag is intended for dumping. Actual slag handling practices pose in fact the following problems: . Slow or uncontrolled cooling rate . Explosion hazard clue to water sprinkles . Need for further handling, in adverse conditions (handling hot liquid slag, tilting slag pits to pour slag on the ground, etc.) also when slag is to be dumped . Crushing and milling to final size (dust emissions) if slag is to be re-used. Obviously, it not possible to transform EAF slag from a waste into a valuable co-product by applying exactly the same treatment cycles in use to prepare it for disposal into dumps, notwithstanding such treatment cycles generally have no problems in obtaining transformed slag that complies with regulatory restrictions; it is absolutely necessary to single out an appropriate range of chemical compositions that satisfy both the metallurgical process and the slag itself as a co-product, and to identify the critical steps in the transformation of liquid slag into a well defined, reliable and reproducible product with a given mineralogical structure and strictly controlled mechanical and environmental properties, and to establish the proper operating practice to maintain these critical steps under control. Chromium is one of the heavy metals that can be released by EAF slags during leach tests or in unbound uses in the environment; because of its harmful impact, especially Cr(VI) which is one of the pollutant of the environment and is also the most soluble of the rnultivalence chromium ions that can occur in EAF slags, chromium release is therefore of major concern for the potential uses of EA F slags. Leaching behaviour of EAF steel slags is the most important parameter to be considered for slag reuse. Especially chromium release deserves special attention because of its adverse environmental impact and its not definitely established relationship with slag treatment parameters. The most frequently considered parameter is rapid cooling, for example by water granulation, that can result in an amorphous slag, encapsulating metals and oxides, and thereby lowering the solubility of the heavy metals compared to rock material used for road making. The cooling rate is a very important parameter in the formation of Cr6+ because this cation develops at low temperature (below 1228 degrees C). Slags with a basicity factor (CaO+MgO)/(SiO2+Al2O3) higher than I may form glass when cooling rapidly, depending on chemical analyses of the smelt. However, the formation of glass in some investigated granulated slag samples has: not been sufficient to enclose heavy metals and prevent them from leaching. Besides glass formation, controlling cooling conditions can be a means of affecting mineral transformation and consequently the solubility of elements like chromium; rapid cooling should result in the prevention of leaching of chromium. Chemical compounds containing hexavalent chromium (Cr6+) that are not avoided by slow cooling are generally considered, far more toxic than those containing the trivalent form (Cr3+). According to results in the literature, Cr6+ is usually formed at lower temperatures and rapid cooling reduces its formation by limiting the reaction kinetics. ASO Siderurgica is the first to develop a complete solution for EAF black slag dry granulation to solve nearly all the problems connected with slag treatment out of EAFs. The system, as shown in the original schema of Fig. 1 or better illustrated in the drawing in Fig. 8, consists of one couple of side-by-side contra-rotating cylinders, rotating on their respective horizontal axes. Above them a refractory lined containing structure hosts the liquid slag when it comes directly from the furnace or at the end of a gathering channel, or poured from a transferring ladle. The water cooled cylinders rapidly cool down the slag in contact with their surfaces and disaggregate it while it passes through their gap. The main parameters that control the process are the rotation speed of the cylinders and the inter-axe distance; according to the slag characteristics these parameters can be adjusted in order to regulate the quantity of slag that is being processed per unit time and the slag mean temperature at the outlet of the gap between the cylinders. Below the cylinders, on the opposite sides of the slag trajectory at the gap outlet, the slag cooling is further controlled by air or air/water spray nozzles. The project was then further developed by means of simulation in order to assess the thickness of solid slag or the heating of the rotating cylinders. Results are reported in Figs. 3-6, and 9. Results show that with contact: times of 15 to 30 sec. between slag and cylinders surface the slag is completely solid and is further cooled below the melting temperature at the exit from the cylinders. The new system for EAF slag dry granulation has now been installed at ASO Siderurgica steelwork in Ospitaletto (Brescia) and has undergone several trials. From these initial experiments it can be concluded that the new SLAG-REC (R) machine would be practically applicable to all EAFs; it will contribute to save slag from dumps and make it re-usable mainly for road construction and for cement conglomerations, resulting in - less problems in handling hot slag for the steel works - less waste transported to dumps from EAFs, less use of natural gravel for road construction (saving up natural resources), no transport and dismissing costs of slag to dumps, - less water used in the overall electric arc furnace process, less emission of contaminant substances of slag to soil and atmosphere, - "cleaner" steelmaking industry - healthier environment.

The occurrence of exogenous non metallic inclusions in steel is sporadic, but because of their large size, they have the most deleterious effect on steel properties, becoming the major cause of failure of mechanical components in service. These inclusions are usually entrapped in steel during teeming and solidification and tend to concentrate in the regions of the steel section that solidify most rapidly or in zones from which their escape by flotation is in some way hampered, in the case of forging ingots this region corresponds to the ingot bottom. The majority of exogenous inclusions originates from mould fluxes entrapment, reoxidation processes, which occur when molten steel comes into contact with external sources of oxygen, such as casting atmosphere, and refractories wear. Considering the large amount of inclusions sources, it is sometimes very difficult for a steelmaker to assess what is the precise origin of macroinclusions, in order to improve the process quality. This investigation is also complicated by the fact that the chemical characterization of macroinclusions is not easy, because the inclusions have large size and multiphase composition, with a chemical analysis which call change from point to point. For these reasons a careful analysis of this matter is always needed.In the present work a precise characterization of macroinclusions which came from refractories, occurred in forging steel ingots, was carried out by means of SEM-EDS and non conventional X-Ray Microdiffraction technique. In order to explain the origin and the chemical evolution of these inclusions the analysis of ex-service refractory materials was carried out and a thermodynamical model taking in account the steel-refractory interactions was implemented.

Exogenous non metallic inclusions can be introduced into steel from many external sources. Typical examples are the particles of refractory material which detach from their parent material during ladle treatment, teeming and casting operations, remaining entrained in the steel. In the present work, the interactions between Al2O3-SiO2 refractories and steel during casting of ingots has been deeply investigated, permitting to explain the occurrence of ghelenite- and grossite-types inclusions into special grade steels. The experimental analyses were carried out on non-metallic inclusions and ex-service refractory materials by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Microdiffraction (mu XRD). The results were confirmed by thermodynamical considerations.