Optimisation of hybrid process to recover zinc from electric arc furnace dust using design of experiment approach

Recoverable metals like zinc and iron are present in the dust from electric arc furnaces (EAF), a byproduct of the steelmaking process. These metals are mainly present as zinc ferrite (ZnFe2O4), magnetite (Fe3O4), and zinc oxide (ZnO) compounds. ZnO is easily dissolved in either an acidic or an alkaline solution; however, ZnFe2O4 is more difficult to dissolve. Therefore, our earlier paper suggested the thermodynamic analysis of a preliminary caustic-roasting treatment to transform the ZnFe2O4 from EAF dust into more soluble products of ZnO and NaFeO2. Finally, the present study carried out a hybrid process that involved the caustic roasting of EAF dust and a caustic leaching of the roasted product. At first, the effects of caustic-roasting process parameters on zinc-ferrite conversion were studied using the Taguchi method. The implication of Design of Experiment (DoE)-Taguchi analysis and the analysis of variance (ANOVA) shows the optimum caustic-roasting conditions are: roasting temperature of 350 oC, NaOH/ EAF dust molar ratio of 4, and roasting time of 60 minutes. The effect of temperature, reaction time, NaOH concentration, and solid/liquid (S/L) ratio was also examined using the same method in the subsequent caustic-leaching process. Similarly, the analysis shows that maximum Zn dissolution is possible at a NaOH concentration of 6M, a leaching temperature of 40 oC, a liquid-to-solid ratio of 60 ml/g, and a leaching time of 60 minutes. After the hybrid process, the zinc recovery was 83%, while most iron oxides remained in the leach residue. Temperature and NaOH concentration are the most influencing factors during caustic roasting and leachingThe optimum caustic-roasting conditions are at 350 oC, 4 mol NaOH(s):EAFD(s) (S/S) ratio, and 60 min6M NaOH, 40 oC, 60 ml/g NaOH(L):EAFD(s) (L/S) ratio and 60 minutes maximise Zn dissolution during caustic-leachingThe zinc recovery was more than 83% after the hybrid process