Enhanced recovery of zinc and lead by slag composition optimization in rotary kiln

High-efficiency recovery of Zn and Pb from silicon-rich zinc leaching residues is realized in a rotary kiln. Characterizations by means of XRD, SEM, EDS and ICP reveal that the leaching residue contains 12.4 wt.% SiO2, 16.1 wt.% Zn, and 7.4 wt.% Pb. Thermodynamic analysis shows that metallic vapor of Zn and Pb can be easily generated from the zinc leaching residue at 1150-1250 degrees C inside the rotary kiln. Viscosities and melting points of 13 slag compositions were analyzed and three slag compositions (47wt.%SiO2-23wt.%CaO-30wt.%FeO, 40wt.%SiO228wt.%CaO-32wt.%FeO, and 40wt.%SiO2-30wt.%CaO-30wt.%FeO) possessed the desirable physical properties, with the melting point and viscosity in the range of 1150-1280 degrees C and 0.2-0.5 Pa center dot s, respectively. The industrial tests show that adopting the optimized slag composition can contribute to very high recovery rates of Zn and Pb (97.3% for Zn and 94.5% for Pb), corresponding to slags with very low average contents of Zn and Pb (0.51 wt.% Zn and 0.45 wt.% Pb). The National-Standard leaching tests of the water-quenched slags result in 1.82 mg/L Zn, similar to 0.01 mg/L Cu, 0.0004 mg/L As,similar to 0.01 mg/L Cd, 0.08 mg/L Pb, and similar to 0.02 mg/L Hg in the leachate, verifying the detoxification of the zinc leaching residue at the same time.