Optimization Study on the Leaching of High Iron-Bearing Zinc Calcine After Reduction Roasting
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C is of great importance for various applications in metallurgical, chemical, and textile industries and is mainly produced from zinc sulfide concentrates by conventional hydrometallurgical method.[1] By the process of zinc production, the sphalerite (ZnS) present in the concentrates is firstly converted into soluble oxide structure by an oxidative roasting and, then, the zinc calcine produced is subjected to a neutral and low acid leaching to generate a zinc sulfate solution for purification and subsequent electrolysis.[2–4] In general, approximately 10 pct of iron impurity accompanies with the concentrates, which tends to react with the zinc oxide to form zinc ferrite (ZnFe2O4) in the oxidative roasting process.[5–7] Zinc ferrite is stable and insoluble in dilute sulfuric acid solution, and thus its formation results in a generation of considerable amount of leach residues in the following leaching process.[8] The residues containing ZnFe2O4 pose a concern over not only environmental pollution but also economical loss.[9,10] On the other hand, with the consumption of low iron-bearing zinc sulfide concentrates, this problem is becoming more and more serious, because iron content of the concentrates used in zinc production is increasing gradually due to the fact that high iron-bearing zinc ores are abundant in the world, especially in China. JUNWEI HAN, WEI LIU, and YONGXING ZHENG, Doctors, HONGLIN LUO, Master, and WENQING QIN, Professor, are with the School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, P.R. China. Contact e-mails: [email protected]; [email protected] Manuscript submitted January 6, 2015. Article published online November 23, 2015. 686—VOLUME 47B, FEBRUARY 2016
Both pyrometallurgical and hydrometallurgical methods have been developed industrially to overcome the difficulty of zinc extraction from zinc ferrite. The most common pyrometallurgical method is Waelz process,[11,12] in which the residue obtained after low acid leaching is subjected to a reduction roasting using large amounts of coke or coal at above 1273 K (1000 °C) for the transformation of zinc ferrite into metallic zinc. Although 90 to 95 pct of zinc can be recovered from the leach residue, some fatal drawbacks including high energy cost and strict requirement on dust collection system make this technology, as well as other similar methods, being washed out gradually. The most widely used hydrometallurgical approach is to leach the neutral residue using hot and concentrated sulfuric acid instead of dilute sulfuric acid to destroy the ferrite structure and, therefore, increase the extraction of zinc and decrease the amount of leach residue.[13,14] By this method, the extraction of zinc can be improved significantly, but more than 80 pct of iron is also dissolved into the leachate, which must be precipitated as jarosite, goethite, or hematite prior to purification and electrolysis processes.[15–18] Although iron precipitation methods are a well-established technology, its residues have never found any ap
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