硫代硫酸盐的湿法冶金

Therefore, during the thiosulphate leaching of precious metals, some chemical reactions involving dissolution, oxidation and complexation occur. Only a few oxidising agents, such as copper ions, are adequate in ammoniacal solutions. For example, gaseous oxygen seems not to be efficient for metallic gold oxidation. The following reaction shows the role of copper(H) ions, present in the form of the tetraamine complex, in the oxidation of gold from the metallic state to aurous Au ÷ ion [7,8]: Au + 5S202- + Cu (NH3) 42 = Au ($203) 3- + 4NH3 + Cu ($203) 35+ (2)
2. Theoretical considerations
2.1. Leaching
The chemistry of the ammonia-thiosulphate system is very complicated due towenku.baidu.comthe simultaneous presence of complexing ligands such as ammonia and thiosulphate, the Cu(II)--Cu(I) redox couple and the possibility of oxidative decomposition reactions of thiosulphate involving the formation of additional sulphur compounds such as tetrathionate [5,6]. The redox equilibrium between the cuprous-cupric couple in ammoniacal solution is represented by the following reaction [ 1,7]: Cu(NH3)42+ + 3S2032- + e - = Cu($203) 5- +4NH3 (1)
For effective gold dissolution an oxidation potential of 150--200 mV with reference to standard hydrogen electrode (SHE) is required. The aurouthiosulphate complex is the most stable gold-bearing soluble species in the leaching system between pH 8.5-10.5, according to ammonia concentration. Above this value, when the pH becomes more alkaline, the predominant gold compound is represented by the gold(I) diammine complex [3,9] : Au ($203) ~- + 2NH3 = Au (NH 3) ~- + 2S20~(3)
Received 3 April 1995; accepted2 June 1995
Abstract Thiosulphate leaching of precious metals has been developed as an alternative and non-toxic technique to the conventional cyanidation. This process has advantages over cyanide in a decreased interference from foreign cations and a lower environmental impact. Ammoniacal thiosulphate solution allows the solubilization of gold as stable anionic complex. Leaching of gold occurs at appreciable dissolution rates. The aim of this work is devoted to a preliminary evaluation of the feasibility of thiosulphate leaching for the extraction of gold from precious metals ores on a laboratory scale. The experimental work has allowed the authors to point out the influence of temperature, thiosulphate concentration, ammonia concentration and copper sulphate concentration on the gold dissolution from an ore (51.6 g/t Au) that originated from the Dominican Republic. Gold was recovered from the leach liquors by adsorption onto activated carbon or by electrowinning. With the optimisation of the process parameters about 80% gold recovery has been attained, as in the case of conventional cyanidation.
1. Introduction Cyanide is the wide-spread leaching reagent for the recovery of precious metals, as it is cheap, well established and efficient for gold extraction. Considerable attention has been given to alternative leachants since the beginning of the 1980's. The main motivation is the fear of cyanide, especially in recent metallurgical operations starting in new precious metals mining districts. Despite the noteworthy safety regulations currently applied in cyanidation plants all around the world, real environmental risks and human toxicity hazards still remain [1]. *Corresponding author. 0304-386X/95/$09.50 © 1995Elsevier ScienceB.V. All fights reserved
hydrometallurgy
ELSEVIER Hydrometallurgy39 (1995) 265-276
Thiosulphate leaching for gold hydrometallurgy
C. Abbruzzese a, p. Fomari a, R. Massidda a, F. Vegli6 b, S. Ubaldini a,,
SSDIO304-386X(95)O0035-6
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C. Abbruzzese et al. / Hydrometallurgy 39 (1995) 265-276
Another reason for the renewed interest in non-cyanide leach reagents is the increased rate of dissolution for gold. Rapid leaching rates involve smaller leach tanks requiring lower capital costs and energy consumption [ 2 ]. However, non-cyanide reagents to date have not been widely employed for gold and silver recovery, but they may find application in future treatment operations when environmental contraints do not allow the customary practice of cyanidation [3]. The thiosulphate process may be considered as a non-toxic alternative to conventional cyanidation. Leaching by thiosulphate permits a decreasing interference from foreign cations and a results in a lower environmental impact [4]. Ammoniacal thiosulphate solution solubilises gold in the form of an anionic aurocomplex stable over a wide range of pH and Eh values. The presence of ammonia hinders the dissolution of iron oxides, silica, silicates and carbonates, the most common gangue minerals found in gold-bearing ores. Finally in the thiosulphate system, the oxidant necessary to oxidize metallic gold to gold(I) is present in solution as copper(II) ions [ 1--4].
a lstitutoper il Trattamento dei Minerali (ITM) del CNR, Via Bolognola 7, 00138 Roma, Italy Dip. di Chimica, Ingegneria Chimica e Materiali, Universitfi de L'AquUa, 67100 Monteluco di Roio, L'Aquila, Italy