Oxidative Pressure Leaching of Silver from Flotation Concentrates with Ammonium Thiocyanate Solution
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THE main product of the Huidong Lead-Zinc Mine in western China is zinc ingots, which are produced through a hot-acid leaching process. Residues from this process contain 700 g of silver per ton. In an attempt to recover value metals from the residues, Chen[1] established a sulfide flotation procedure that added a regulator, a collector, and a frother to produce flotation concentrates that bore about 1 pct silver. Because the silver flotation concentrates also contained more than 40 pct zinc and less than 2.0 pct lead, they could not be used as lead smelting concentrates. Liu[2] proposed a hydrometallurgical procedure, which comprised calcining, leaching zinc with hydrochloric acid, leaching silver with sodium sulfite, and reducing silver ions with hydrazine to treat the flotation concentrates. This procedure, however, was complex and had an adverse impact on the environment. In addition, because of the high content of elemental sulfur (17.49 pct wt) in the concentrates, the extraction percentage of silver was less than 70 pct with a cyanide, thiosulfate, thiourea, or thiocyanate solution at ambient pressure. The object of this research was to assess the feasibility of selective oxidative pressure leaching of silver from SHENG-HAI YANG and JIAN-GUANG YANG, Associate Professors, WEI LIU, Doctoral Candidate, GENG-TAO CHEN, Candidate for Master, and MO-TANG TANG, Professor, are with the School of Metallurgical Science and Engineering, and SHENG-HAI YANG, Associate Professor and GUAN-ZHOU QIU, Professor, are with the School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, P. R. China. Contact e-mail: [email protected]. Manuscript submitted June 15, 2008. Article published online October 15, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
flotation concentrates in an ammonium thiocyanate medium. Ammonium thiocyanate was selected as the leaching agent because of its many advantages, such as stability in weak acid solution, low toxicity, economic applicability, and so on. In addition, it can be complexed tightly with Ag+, Au+, Au3+, and Cu+, which allows ammonium thiocyanate to separate the abovementioned ions from other ions, such as Cd2+, Pb2+, Cu2+, and Zn2+. Thermodynamic analyses and experiments on gold dissolved in Fe3+-thiocyanate solution were undertaken by Barbosa-Filho et al.[3–5] and Broadhurst et al.[6] Thiocyanate was concluded to be an ‘‘effective lixiviant for gold in acidic conditions, yielding dissolution rates that are comparable to those obtained with thiourea while offering the advantage of much greater stability against oxidative decomposition.’’[5] More than 95 pct of the gold in refractory arsenopyrite concentrates was recovered in 0.4 M potassium thiocyanate (KSCN) solutions at pH 2–5 and a temperature of 20 °C for 4 to 5 hours by Kholmogorov et al.[7] The oxidative leaching of metallic silver and silver sulfide in a thiocyanate medium at ambient atmosphere has been studied by Zhuchkov et al.[8,9] The redox systems of thiocyanante solutions have been discus
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