Thermodynamic analysis of caustic roasting and investigation of caustic roasting-leaching parameters of electric arc furnace dust

Abstract

Electric Arc Furnace Dust (EAFD) is a byproduct of the steelmaking industry, which is one of the most significant and rapidly generated hazardous waste. This material includes recyclable elements such as Zn and Fe, exclusively in the form of zinc-ferrite (ZnFe2O4), zincite (ZnO), magnetite (Fe3O4) with some other minor compounds of Si, Mn, Mg, and Pb. A combination of pyro and hydrometallurgical route, also known as hybrid process, has acquired considerable attention to extract Fe and Zn from EAFD. In the current study, systematic thermodynamic caustic-roasting assessments of EAF dust using thermochemical FactSage software and caustic roasting-leaching experimental investigations were carried out to assess the suitability of this important hybrid process. Thermodynamic calculation showed that the major zinc-ferrite (ZnFe2O4) phase of EAFD can be potentially converted to ZnO and sodium ferrite (NaFeO2) intermediate phase while reacting with NaOH (>2 mol) at temperatures equal or higher than 100oC. The suitable condition was assessed to an EAFD to NaOH ratio of 1:3 molar fraction and a temperature, T>318oC at the normal atmospheric condition. Moreover, the selected experimental investigation was also showing a good agreement with the current thermodynamic assessment. L9 orthogonal array was utilized for investigation of roasting temperature, NaOH to EAF dust molar ratio and roasting time factors effect on caustic roasting of EAFD. Temperature of 350 oC; NaOH/ EAF dust molar ratio 4 and roasting time 60 minutes showed the maximum effects on conversion of zinc-ferrite (ZnFe2O4) to sodium ferrite (NaFeO2) where temperature is the most dominating factor. In case of caustic leaching of roasted EAFD, L9 orthogonal array was used for evaluating process parameters of leaching temperature, NaOH concentration, solid to liquid ratio and leaching time. Satisfactory result of Zn recovery had come during caustic leaching of roasted EAFD at 40 oC temperature; 6 M NaOH; 60 ml/g liquid to solid ratio and 60 minutes time. NaOH concentration is the most dominating factor during caustic leaching followed by leaching temperature.