Direct recovery of low-concentration Au(S2O3)3−2 from pregnant leach solution using an activated carbon-coated titanium electrode

Abstract

Thiosulfate gold extraction technology has gained considerable interest owing to its environmental compatibility and broad applicability to diverse ore types. However, the lack of efficient methods to recover Au(S2O3)3−2 from leaching solutions has hindered its industrial implementation. To address this challenge, this study used a thiosulfate leaching solution from quartz-type gold ores to design an activated carbon-coated titanium electrode (Ti@AC) with a porous surface structure. This electrode facilitated the direct reduction of low-concentration Au(S2O3)3−2 to metallic gold (Au0) in solution, achieving a gold recovery of 99.58%, surpassing other recovery methods from the leaching solution by 30%‒80%. After ten consecutive 200 L pilot-scale tests, the Ti@AC electrode demonstrated remarkable stability, consistently maintaining a recovery >98% and yielding 20.23 g of gold. The porous architecture of the activated carbon (AC) promoted the adsorption of low-concentration Au(S2O3)3−2, while its low charge transfer resistance facilitated the efficient conversion of Au(S2O3)3−2 to Au0. Moreover, the reduction reaction generated a concentration gradient near the cathode, promoting the diffusion of Au(S2O3)3−2 toward the electrode and ensuring an efficient recovery process. This study provides a feasible strategy for the direct reduction of low-concentration precious metal ions to monomers with high recovery and low costs, which is promising for industrial applications.

1. Introduction

Gold, a strategic resource critical to maintaining national financial and economic stability [1], is extensively utilized in vital sectors, including electronics, healthcare, aerospace, and catalytic chemistry [2–4]. Although cyanide-based extraction remains the predominant industrial gold recovery method because of its technical maturity [5–6], the use of cyanide poses significant environmental risks. Furthermore, the prevalence of cyanide-refractory ores, including carbonaceous deposits, continues to increase [7–8], exacerbating the limitations of conventional approaches. Consequently, environmentally benign and adaptable gold extraction technologies must be developed to ensure the sustainable development and technological modernization of the gold industry [9].

In recent years, cyanide alternatives, such as thioureas, halogens, thiocyanates, and thiosulfates, have been extensively investigated [10‒12]. Thiosulfate leaching is one of the most promising alternatives for industrial adoption because of its nontoxic nature, environmental compatibility, and efficiency in gold extraction from cyanide-refractory ores [13–14]. Current strategies for recovering low-concentration Au(S2O3)3−2 from thiosulfate leaching solutions include adsorption [15], cementation [16], solvent extraction [17], and electrodeposition [18]. However, Au(S2O3)3−2 exhibits a larger ionic radius and higher charge density than Au(CN)2−, resulting in poor adsorption efficiency on activated carbon (AC) even after surface modification [19–20]. Ion-exchange resins are limited by their low selectivity and interference from competing ions [21–22]. Cementation processes that utilize reactive metals, such as zinc and iron, to displace gold often suffer from copper co-deposition, leading to product contamination and substantial cementation reagent consumption [23]. Solvent extraction requires costly organic solvents and high-concentration Au(S2O3)3−2 solutions, which significantly increase recovery costs. Although electrodeposition is widely applied for metal recovery [24], its efficiency decreases in low-concentration Au(S2O3)3−2 solutions with a high impurity ion content.

The use of a pulsed voltage to accelerate metal recovery and reduce the product impurities that prevent the co-deposition of other metals during electrodeposition has been explored to enhance the electrodeposition method [25–26]. However, precise voltage control and the use of small Au(S2O3)3−2 reduction overpotentials are required, which necessitates a higher gold concentration in the solution and a prolonged recovery time. Another strategy involves the incorporation of an adsorbent or reducing material, such as MoS2 [27–28], on the electrode surface to reduce the electrodeposition voltage and achieve selective gold recovery [29]. However, the high cost associated with MoS2 synthesis renders this approach impractical for industrial applications. Research has also focused on the use of carbon materials on the electrode surface, which enables the recovery of Au(S2O3)3−2 from low-concentration simulated solutions [30–31]. However, using prepared carbon materials is costly, and its effectiveness in leaching