Treatment of a leach liquor after leaching EAF dust

Zita Takacova, Frantisek Kukurugya, Tomas Vindt, Tomas Havlik, Peter Turek

The work deals with recovery of metallic zinc from leach liquor after leaching EAF dust. The leach liquor after leaching EAF dust in sulfuric acid solutions, containing Fe 3.34 g/L, Zn 4.17 g/L, Ca 0.6 g/L and other minor metals (Cd, Pb, Cu), was processed by iron precipitation, cementation of other accompanying metals and subsequent zinc electrowinning. Goethite and jarosite processes were used as iron precipitation step. Influence of a neutralizing agent (1 M NaOH, CaCO3 and lime milk), pH (2.5 – 4) and temperature (80 and 95 °C) on the process efficiency was investigated. The aim was to determine optimal conditions at which residual iron concentration after precipitation does not exceed 30 μg/ml; i.e. the limit for iron concentration in an electrolyte for zinc electrowinning. It was observed, that pH value and neutralizing agent have the most significant effect on the iron precipitation efficiency. Lime milk was determined as the most suitable neutralizing agent, where the residual iron concentration (80 °C, pH 4) was below 1 μg/ml, and representing precipitation efficiency of 99.98 %. After iron precipitation step, cementation of other impurities (Cd, Pb, and Cu) by zinc powder was carried out. The influence of pH and temperature on the cementation efficiency was investigated. The results showed that almost 100 % Cd and 90 % Pb can be removed by cementation. In zinc electrowinning step, the influence of temperature (20, 40 and 60 °C) and current density (333 – 1000 A/m2) on the current efficiency was studied. Maximum recovery of zinc (almost 99 %) was obtained at 40 °C and current density of 333 A/m2. At these conditions, zinc was recovered as a compact plate with a minimum of zinc being precipitated in dendritic form. Results of this study show possibility to recover high purity product by processing impure zinc containing solution coming from EAF dust leaching.