Leaching Kinetics of Willemite in Ammonia-Ammonium Chloride Solution
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RODUCTION
CURRENTLY, zinc is produced mostly from zinc sulfide ores because sulfides are easily separated and concentrated from gangue by conventional flotation techniques. However, the depletion of the sulfide ores has brought more emphasis on the extraction of zinc from oxidized zinc ores in various carbonate and silicate minerals, such as zincite (ZnO), smithsonite (ZnCO3), hydrozincite (Zn5(CO3)2(OH)6), willemite (Zn2SiO4), zincsilite (Zn3Si4O10(OH)2 4H2O), and hemimorphite (Zn4Si2O7(OH)2 H2O). Zinc extraction from the sulfide ores is mainly performed by the roasting-leaching-electrolysis (RLE) route, which is the combination of pyro- and hydrometallurgical operations. However, the processing of oxidized zinc ores containing silicates follows two different approaches. The acid leaching (AL)–solvent extraction–electrowinning process is one alternative,[1] whereas Souza[2] has integrated the other process to treat both zinc silicate and zinc sulfide concentrates, which consists of the leaching process for zinc silicate ores and the traditional RLE process for zinc sulfide concentrates. In both routes, zinc is extracted by AL from the concentrates or oxidized ores containing zinc silicates, and special care is taken to avoid the formation of silica gel. However, some disadvantages have been found in the AL of oxidized zinc ores as zinc carbonates and silicates are generally acid consumers. Oxidized zinc ores are also usually associated with other acid-consuming gangue ZHIYING DING, Doctor, ZHOULAN YIN, Professor, XIFEI WU, Postgraduate, and HUIPING HU and QIYUAN CHEN, Professors, are with the School of Chemistry and Chemical Engineering, Central South University, Hunan 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 8, 2010. Article published online February 23, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
minerals such as calcium carbonate. As a result, large volumes of carbonate-containing tailings with fairly reasonable zinc content are produced. Moreover, AbdelAal[3] and Feng et al.[4] found that Zn and Fe, Ca, Mg, and SiO2 were dissolved in sulfuric acid from oxidized zinc ores, which leads to difficulties for continued purification. Therefore, a more effective leaching route needs to be developed. Zinc leaching from oxidized ores can be addressed with alkaline solutions. Sodium hydroxide, lime, and ammonia are the most common alkaline reagents. Ammonia is an attractive lixiviant in hydrometallurgy and is commonly used in the extraction of cobalt, nickel, and copper because of the formation of stable metal ammine complexes.[5] Complexing ligands including ammonia attach to metal ions through a reactive nitrogen-containing group to form metal complexes with higher solubility in most cases. The major undesirable elements, such as iron and calcium in ores exhibit poor complexation ability with ammonia and are insoluble in alkaline solutions. Then the desired metals can be extracted and the undesirable components report to the residue. Meng and Han[5] reviewed the principles and application
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