Bio-oxidation of a high-sulfur refractory gold concentrate with a two-stage chemical-biological approach

Highlights

• Effect of two-stage approach on the oxidation efficiency of pyrite was studied.

• The formation of elemental sulfur on the surface of pyrite hindered the chemical oxidation.

• The biological oxidation was slightly influenced by the elemental sulfur at the initial stage.

• Partly disrupted surface and shrunken particle size improved the oxidation level.

Abstract

A two-stage chemical-biological oxidation approach, which applied a high temperature chemical oxidation stage prior to the bio-oxidation stage, was employed to accelerate the bio-oxidation rate of Axi gold concentrate with pyrite as the main sulfide phase. The extraction levels of Fe and S were 67.0% and 72.9% in the traditional one-stage bio-oxidation approach, while the values were elevated respectively to 78.0% and 79.6% with the two-stage approach. Accordingly, the extraction rate of gold was increased from 83.6% to 90.3% when subjected to the two-stage approach. The intensification mechanism of the two-stage approach was carried out by using Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM- EDX), X-ray photoelectron spectroscopy (XPS) and particle size analysis. The results indicated that pyrite on the surface of the concentrate was partly oxidized and particle size of the concentrate diminished after the first stage (chemical oxidation), which would facilitate the subsequent bio-oxidation process.

Introduction

In refractory gold ores, tiny gold particles may be highly disseminated and locked up within the grain boundaries or fractures of sulfide minerals such as pyrite and arsenopyrite (Deng et al., 2018b; Deng et al., 2018c; Zhang et al., 2016). Thus, decomposition of the sulfides is required to liberate the gold (Liu et al., 2017). Commercially, roasting process, pressure oxidation, chemical oxidation and biological oxidation are among the processes utilized in sulfide oxidation. As against the conventional processes, bio-oxidation, which makes use of iron and sulfur oxidizing microorganisms to catalyze the oxidation of sulfides, would prove to be more efficient, economically available and environmentally acceptable to treat such refractory ores (Guo et al., 2017; Mubarok et al., 2017). According to the indirect mechanism, the main oxidizing agent in the process of bio-oxidation is ferric ion [reaction (1)], the role of iron and sulfur oxidizing microorganisms is regenerating the ferric ion and oxidizing the S-layer formed on the surface of sulfide during the process of bio-oxidation [(2), (3)](Hu et al., 2017; Liu et al., 2017; Zhang et al., 2019). However, the bio-oxidation rate of sulfides which encapsulates gold is slow, forcing the mineral industry to seek improved processes for the treatment of refractory gold ores (Hu et al., 2017; Zhang et al., 2016). To achieve this goal, new promising highly efficient microbial communities or novel technological approaches are often investigated.

$$\mathrm{MS}+2{\mathit{Fe}}^{3+}\to M^{2+}+S^0+2{\mathit{Fe}}^{2+}$$

$$4{\mathit{Fe}}^{2+}+4H^{+}+O_2\stackrel{\mathit{microbes}}{\to }4{\mathit{Fe}}^{3+}+2H_{2}O$$

$$2S^0+3O_2+2H_{2}O\stackrel{\mathit{microbes}}{\to }2H_{2}S{O}_4$$

It has been proposed that the oxidation rate of sulfide by ferric ion increases as temperature rises (Li et al., 2009). Though the optimal temperature of extremely thermophiles is higher than the mesophiles and moderate thermophiles, the cell walls of extremely thermophiles are not strong enough as compared to the mesophiles and moderate thermophiles if the pulp density is higher than 5%, which results in the disruption of microbial cells due to shear and stress caused by stirring (Ciftci and Akcil, 2010; Ciftci and Akcil, 2013). The regeneration of ferric ion as oxidizing agent in the process of bio-oxidation would be hindered. Consequently, the extremely thermophiles are only available with low pulp density. Because the oxidation reactions of sulfides are often exothermic, the most promising operational temperature is 40–55 °C. Moderate thermophiles predominated under these conditions (He et al., 2012; Muravyov and Bulaev, 2013).

A two-stage oxidation approach, which consists of a high-temperature ferric leaching stage and a subsequent biological oxidation stage, was proposed by Fomchenko et al. (Fomchenko and Muravyov, 2014; Muravyov and Bulaev, 2013). In this approach, favorable conditions for the chemical oxidation and biological oxidation are created because of the separation of both stages. Thus, the recovery rate of gold was elevated from 71.7% in the one-stage bio-oxidation approach to 77.9% in the two-stage approach. Kaksonen et al. indicated that concentration of ferric sulfate could be as high as 30 g/L in the last bioreactor of a BIOX® process (Kaksonen et al., 2014). As a result, ferric sulfate, the oxidizing agent in the chemical oxidation stage, was often available.

Our previous study demonstrated that the two-stage approach could partly disrupt the surface of arsenopyrite and diminish the particle size of the Pingjiang gold concentrate (Wang et al., 2018). Subsequently, the oxidation rate of Pingjiang refractory gold concentrate with arsenopyrite as the major sulfide phase (20.3%) was enhanced. We found that the two-stage approach could also enhance the oxidation efficiency of Axi gold concentrate with pyrite (58.77%) as the major sulfide phase, which was more difficult to be oxidized from the perspective of chemical kinetics (Liu et al., 2015). However, the intensified mechanism of the two-stage approach on pyrite-bearing high‑sulfur refractory gold concentrate was still unknown, the effect of intermediate products after a chemical oxidation approach on the subsequent bio-oxidation approach in the process was unclear.

In this study, the potential application of a novel two-stage approach on a high‑sulfur refractory gold concentrate is investigated. In addition, combined X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscope–Energy Dispersive X-ray Spectroscopy (SEM-EDX) is utilized to illustrate the intensified mechanism of a two-step pretreatment approach in detail.