Leaching of silver from solid waste using ultrasound assisted thiourea method

doi:10.1016/j.ultsonch.2003.10.007

Ultrasonics Sonochemistry

Volume 12, Issue 3, February 2005, Pages 237-242

https://doi.org/10.1016/j.ultsonch.2003.10.007 Get rights and content

Abstract

Thiourea leaching of precious metals such as gold and silver from ores has several advantages when compared with conventional cyanidation process. In recent years, the use of ultrasound in leaching processes is becoming increasingly popular in hydrometallurgy. This paper deals with combining these two techniques for silver leaching from solid waste of a cyanidation leach plant located in Kütahya, Turkey. The primary aim of this research is to assess the technical performance of the method. To achieve maximum leaching yield, eight process variables have been selected to estimate optimum process conditions by means of statistical factorial design and steepest ascent techniques. Laboratory-scale experiments showed that complete leaching of silver may be achieved by this process.

Introduction

Silver is one of the widely used metals in industry. The majority of silver is consumed in film processing industry with 40–50% of overall consumption, electrical industry follows with 20–30%, and ornaments and jewelry consist of 10% of overall consumption. In addition to the recovery of silver from scrap metals, silver is produced either directly from ores or as a side product during the productions of zinc, copper and lead.

Today, silver production is mainly performed by the conventional cyanidation process. Thiourea (NH₂CSNH₂), an alternative non polluting reagent for extracting precious metals, has shown promise for implementation in the metallurgical industry [1]. Laboratory testing has indicated that thiourea process for gold and silver extraction has several advantages over the conventional cyanidation; greater selectivity towards gold and silver, fast extraction kinetics, low environmental impact, and easier handling of reagent [2], [3], [4]. An additional advantage over cyanide exists when treating refractory sulphide ores by bio-oxidative pretreatment in highly acidic solutions; a neutralization prior to cyanidation is required, while thiourea leaching occurs in acidic solution that can be treated directly [5], [6].

The use of ultrasound for ore leaching becomes increasingly popular in hydrometallurgy. Sonochemical extraction techniques together with classical methods gave a faster [7], [8] and selective [9] extraction of metals. Orlov [10] and Chizhikov et al. [11] used ultrasound for Cu leaching from Copper ores in sulfuric acid. The application of ultrasound in extractive metallurgy has been reviewed by Polyukhin [12]. The effect of ultrasound on ammonium leaching of zinc from Galmei ore has been investigated by Slaczka [13]. The use of ultrasound in nickel extraction from lateritic nickel ore using a strain of Aspergillus niger was studied by Swamy et al. [14]. The kinetics of the ultrasound assisted dissolution of phosphate rock in HNO₃ is modeled by Tekin et al. [15], and in HCl by Tekin [16].

Recently the effects of ultrasound for improving chemical reactions have been reviewed [17]. In solid–liquid systems, Ultrasound increase the rate of dissolution principally by the cavitation effect leading to the appearance of many microcracks on the solid surface subjected to ultrasound. Ultrasound increases also the diffusion speed of soluble species in the liquid phase; this effect facilitates the leaching agents to reach more easily the bottom of capillaries [18]. Furthermore, if the solid reagent is in the form of a powder, ultrasound energy can cause particle rupture, with a consequent increase in surface area available for reaction. One might expect that the increase in surface area alone would be sufficient to explain any enhanced reactivity due to ultrasound [19].

The purpose of this work is to investigate the effect of ultrasonic energy on the leaching yield of silver content of a mining waste, and maximize the yield by means of response surface methodology. The effect of the ultrasound on the leaching kinetics and mechanism is planned to be investigated in a further study.

Section snippets

Experimental planning

A significant part of the industrial experimentation is devoted to finding functional relationship between independent variables and process response: $Y=f(X_{1},…,X_{n})$ Usually, at the beginning, a theoretical model is not of primordial importance, and an empirical one is sufficient for the final goal to estimate the optimum conditions by means of laboratory-scale experimentation. Efficient response surface methods have been developed and successfully used for this purpose [19]. If the experimenter

Results and discussion

In this three-phased leaching system, in addition to inherent process variables such as temperature, time, reactant concentrations, and ultrasound power, additional ones arise due to the presence of a solid phase; pulp density and particle diameter, and a gas phase; gas flowrate. Thus, eight process variables considered in this study, are presented in Table 2, with their low and high levels. Ultrasound power mentioned in this Table, correspond to the power supplied by the generator, which must

Conclusion

Laboratory-scale batch experiments showed that silver may be leached almost completely from the solid waste of a silver ore beneficiating plant, by means of ultrasound assisted thiourea leaching method. With 55 μm particle sized sample at 100 g l⁻¹ pulp density in a solution containing 7.4 g l⁻¹ thiourea and 28.7 g l⁻¹ H₂SO₄, for a leaching time of 24 min, and reaction temperature of 77 °C, the leaching yield is 98.6%. Ultrasound power absorbed by the leaching medium is approximately 80 W l⁻¹.

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Leaching of silver from solid waste using ultrasound assisted thiourea method

Abstract

Introduction

Section snippets

Experimental planning

Results and discussion

Conclusion

Hydrometallurgy

Miner. Eng

Ultrason. Sonochem

Hydrometallurgy

Innovation in gold metallurgy

Miner. Congr. J

Gold recovary from finely disseminated ore by use of cyanidation and thioureation

Potential application of thiourea in the processing of gold

J. S. Afr. Inst. Min. Metall

Gold recavory from pyrrhotite by bioleaching and ciyanidation

Factorial experiments in the study of a biooxidative pretreament for a conventional cyanidation of a Turkey arseical gold ore

Application of ultrasound in extractive metallurgy: sonochemical extraction of nickel

Metall. Trans

Leaching of metals from ores with ultrasound

Min. Proc. Ext. Met. Rev