Il ciclo dello zinco nel trattamento dei rottami ferrosi

The world zinc market is strictly connected to the iron market as 70% ca. of the produced zinc is associated with iron and steel goods. With the increasing of electric arc furnace (EAF) process in the production of steel, there is and will be an increase of ferrous scrap consumption. The scrap is more and more rich in Zn because of the use of galvanized products in goods with a not too long life, such as car bodies. The main connection between the Fe and the Zn cycles begins during the coating stage with an use of 50% ca. of the whole Zn production. At the manufacturing stage, another 20% ca. of Zn is assembled to steel structures as components in brass or in other alloys. At the end of the cycle of the commercial good, more than a half of the Zn content is lost by wearing and rusting or not collected in the recycling process; a second part is recovered through appropriate processes and sent to the non ferrous industry. A minor part follows the scrap inside the EAF where is separated from the liquid steel as a dust. Iron and zinc should be separated whether at the scrap stage, whether at the steelmaking stage. A new cycle begins with the galvanizing process with a Zn only in part recovered from the previous cycle. In 1996 the Italian Agency for Environmental Protection (ANPA) has initiated a study of the environment impact of the industrial sector of EAF steel-making. Within the framework of this study, the University of Rome "La Sapienza" developed a hydro-metallurgical treatment of the dust, consisting in leaching by sulphuric acid followed by solvent extraction (SX) and electrowinning (EW) steps in order to obtain electrolytic zinc. Experimental The EAF dusts tested in this work were provided as pellets coming from Carbon steel Italian manufacturers. The powders obtained after a crushing treatment were characterised by X-ray diffraction (XRD) and analysed by atomic absorption spectroscopy (AAS). The dusts were leached in different time and temperature conditions with aqueous sulphuric acid (0.1 to 2M) in order to optimize the Zn and Fe extraction yields. The SX process has been carried out, after batch preliminary tests, by means of a laboratory-scale mixer-settler apparatus using D2EHPA 1M in kerosene. The purified aqueous solution was fed to a traditional laboratory EW cell, where a pure cathodic deposit of Zn metal was obtained. Results and discussions The XRD analyses allowed us to identify the following main phases: zinc oxide (ZnO), zinc ferrite (ZnFe204), magnetite (FeFe-,O4), sylvite (KC1) and laurionite (PbClOH). The AAS analyses of the different dusts used in this research are reported in Table 1 as composition range of Zn,Fe,Mn and Pb. In fig. 1 the extraction yield of Fe, Zn and Mn leaching is reported. The leaching kinetics show always a similar trend, with an extraction yield rapidly growing in the first 30 min. With an appropriate choose of acid concentration (l,5M H2S04), temperature (30 °C) and solid/liquid ratio (1/20), is possible to obtain a very high Zn extraction yield with a lower extraction for Fe and Mn. Pb precipitates as sulphate in the solid residue, leaving a maximum content of 20 ppm in the solution. In fig. 2 the whole dust recovery flow sheet diagram is reported. After the leaching treatment, the SX stage allows to extract both Zn and Fe. Only Zn is stripped in the final stage, while Fe remains in the organic solution. When the Fe content in the organic solution reaches a stated level, an extraordinary stripping of Fe is provided. After a concentration stage, the aqueous Zn-rich and Fe-free solution is sent to EW cell where SHG Zn deposits are obtained with high current yield and low energy consumption. Conclusions The proposed hydrometallurgical treatment reaches Zn recovery yields up to 90% for Zn-rich dusts. The other main product is a sludge containing Pb sulphate and the unleached spinels.