Gold deportment and leaching study from a pressure oxidation residue of chalcopyrite concentrate

Highlights

• Gold deportment is studied in detail for a POX residue of copper concentrate.

• The iron-containing species are hematite, iron sulfate and jarosite after POX.

• Most gold associates with hematite as microscopic gold.

• Iron sulfate is an insignificant gold carrier, mainly resulting in high caustic consumption before leaching.

• Gold is extracted using hybrid leaching system of cyanide and glycine with low cyanide dosage and consumption.

Abstract

Pressure oxidation (POX) is an effective method for base metal extraction and a pretreatment of refractory gold ores. A residue from pressure leaching of chalcopyrite concentrate was collected and investigated for gold recovery and deportment of metals. A comprehensive mineralogical analysis and gold deportment study were conducted using techniques including X-ray diffraction (XRD), optical microscope, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX), and dynamic secondary ion mass spectrometry (D-SIMS). Gold leaching tests were performed using cyanide and glycine as lixiviants. The gold grade is 3.1 g/t, with 0.31% residual copper in the sample. Hematite was a dominant iron oxide (31.5%), and iron sulfate salts (45.4%) existed in various minerals phases. The gold deportment study revealed that 32.3% of the total gold was present as a visible gold, and the remainder was sub-microscopic gold. The visible gold was native gold with an average particle size of less than 20 μm. Colloidal gold, the dominant existence of invisible gold, was found in hematite, jarosite, and iron sulfate. Hematite was the major sub-microscopic gold carrier, forming a nonporous rimmed structure and locking gangue minerals randomly. Gold in arsenopyrite was found as solid-solution, accounting for 7.2%. Gold extraction of 84% was achieved by 500 mg/L NaCN, while only 35% of gold was extracted by 100 mg/L NaCN. A significant synergistic effect of cyanide and glycine was observed. The addition of glycine in cyanide leaching solution boosted the gold leaching kinetics and recovery. Gold recovery of 85% was observed with 100 mg/L NaCN and 0.3 M glycine in 24 h. In the hybrid leaching system, copper was stabilized by glycine and further utilized as a catalyst for gold leaching. To achieve higher gold extraction and lower lime consumption, a product with high sulfur oxidation degree and low iron sulfate content is preferable in pressure oxidation operation. Hematite is a stable product and does not impair gold leaching.

Introduction

Sulfidic refractory gold ore is an important resource of gold with the depletion of free-milling gold deposits. The finely disseminated or encapsulated gold particles in sulfide minerals matrix such as pyrite, arsenopyrite, and chalcopyrite are difficult to extract by cyanidation. Treatment of refractory gold ore is site-specific and highly dependent on the ore mineralogy. Physical separation techniques such as gravity separation and flotation are commonly used to achieve early gangue rejection and improve the gold grade for low grade gold-bearing sulfide ores (Faraz et al., 2014; Liu et al., 2016; Wang et al., 2019). The upgraded sulfide minerals are processed by chemical or biological oxidation to liberate gold from the associated sulfide prior to gold extraction (Fraser et al., 1991). Because of long residence time and limitation in treating high‑arsenic materials using microorganisms, chemical oxidation is preferable over biooxidation in industrial practice (Astudillo and Acevedo, 2008; Ahn et al., 2019). Conventionally, there are two chemical oxidation techniques, namely roasting and pressure oxidation (POX). Compared with oxidative roasting, pressure oxidation is carried out in an aqueous solution without SO₂ gas production and better fixation of arsenic. During the pressure oxidation, sulfide sulfur oxidized to sulfate, and arsenic can be stabilized in the form of ferric arsenate if the concentrate contains high arsenic content (Padilla et al., 2010; Thomas and Pearson, 2016).

Extensive studies are conducted concerning the pressure oxidation process of sulfide minerals. In acidic pressure oxidation with temperature over 175 °C, Eqs. (1), (2), (3) illustrate the reactions of pyrite, chalcopyrite, and arsenopyrite in an autoclave (McDonald and Muir, 2007; Thomas and Pearson, 2016).

$$4{\mathrm{FeS}}_2+15{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_2{\mathrm{SO}}_4$$

$$4\mathrm{Cu}{\mathrm{FeS}}_2+17{\mathrm{O}}_2+2{\mathrm{H}}_2{\mathrm{SO}}_4\to 4\mathrm{Cu}{\mathrm{SO}}_4+2{\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_2\mathrm{O}$$

$$2\text{FeAsS}+7{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_3{\mathrm{AsO}}_4$$

The subsequent reaction of Eq. (3) is that ferric sulfate reacts with arsenic acid to produce ferric arsenate that is stable at a temperature over 150 °C, shown as Eq. (4). On the other hand, the hydrolysis reactions of ferric sulfate are more frequent to produce hematite, hydronium jarosite and basic iron sulfate (BIS), illustrated in Eqs. (5), (6), (7) (McDonald and Muir, 2007; Fleming, 2009). These three are the main iron-bearing compounds in the solid discharge after pressure oxidation.

$${\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_3{\mathrm{AsO}}_4\to 2\text{FeAs}{\mathrm{O}}_4+3{\mathrm{H}}_2{\mathrm{SO}}_4$$

$${\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2{\mathrm{SO}}_4$$

$$3{\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+14{\mathrm{H}}_2\mathrm{O}\to 2\left({\mathrm{H}}_3\mathrm{O}\right){\mathrm{Fe}}_3{\left({\mathrm{SO}}_4\right)}_2{\left(\mathrm{OH}\right)}_6+5{\mathrm{H}}_2{\mathrm{SO}}_4$$

$${\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{Fe}\left(\mathrm{OH}\right)\mathrm{S}{\mathrm{O}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4$$

Hematite is the desired product because it is chemically stable. The presence of jarosite and basic iron sulfate is common yet problematic. During pressure oxidation, jarosite is likely to associate with silver as argentojarosite that needs to be decomposed for silver recovery (Choi et al., 2007; Gunaratnam et al., 2018). Basic iron sulfate is unstable in sulfuric acid at a temperature over 70 °C, and it can also react with caustic, resulting in excessive lime consumption during neutralization, as shown in Eqs. (8), (9) (Ji et al., 2006). To deal with it, hot cure, employing the idea shown in Eq. (8), is often adopted and removes it as soluble ferric sulfate.

$$2\mathrm{Fe}\left(\mathrm{OH}\right)\mathrm{S}{\mathrm{O}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{Fe}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_2\mathrm{O}$$

$$\mathrm{Fe}\left(\mathrm{OH}\right)\mathrm{S}{\mathrm{O}}_4+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+\mathrm{CaS}{\mathrm{O}}_4\bullet 2{\mathrm{H}}_2\mathrm{O}$$

Efforts have been made in controlling iron speciation during pressure oxidation of sulfide materials (Lu and Dreisinger, 2013; Fleuriault et al., 2016). It is not only vital in the subsequent leaching step but also in waste disposal. Conditions favorable for hematite formation can be realized by controlling the acid and oxidant content in the first two compartments of an autoclave. Additives such as the hematite seeds can promote hematite formation (Ji et al., 2006), and sulfate binding materials can eliminate the formation of jarosite and basic iron sulfate (Simmons and Gathje, 2003).

Besides assessing the mineralogy of solid autoclave discharge, understanding the gold association with different mineral phases is vital in the recovery prediction and flowsheet selection. The gold distribution and association is commonly interpreted using diagnostic leaching (Lorenzen and Tumilty, 1992; Edahbi et al., 2019). But it is more suitable for free-milling ores and cannot give a full understanding of the invisible gold (Cabri et al., 1991). The advanced technologies make it possible to analyze the gold mineralization in host rocks. Modern gold deportment studies involve the application of the scanning electron microscopic techniques such as Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) and Mineral Liberation Analyzer (MLA), which are especially suitable for samples containing relatively coarse gold grains (Goodall, 2008; Coetzee et al., 2011; Nazari et al., 2017). Secondary ion mass spectrometry (SIMS) is a superior technique to characterize sub-microscopic gold quantitively. It can also differentiate between solid solution and colloidal gold (Cook and Chryssoulis, 1990; Chryssoulis and McMullen, 2005; Bustos Rodriguez et al., 2008).

Many recent research activities on pressure oxidation of refractory gold ore focuse more on the control of the autoclave operation. There is a lack of a systematic study on the gold distribution in the solid discharge. Understanding the gold deportment in the residue will help develop a better prediction of the gold leaching behavior. In this paper, a comprehensive gold deportment study, including mineralogical characterization, visible gold, and sub-microscopic gold analysis, was conducted to reveal the gold distribution in various mineral phases after pressure leaching of copper concentrate.

For gold leaching, cyanidation is still the most economical and widely used lixiviant system, although different alternative lixiviants are well-developed. The amino acid, glycine, has been also studied by some researchers (Eksteen and Oraby, 2015; Oraby and Eksteen, 2015b; Eksteen et al., 2017a; Perea and Restrepo, 2018). The gold extraction using cyanide and glycine are expressed as Eqs. (10), (11) (Kudryk and Kellogg, 1954; Eksteen and Oraby, 2015).

$$4\mathrm{Au}+8\text{NaCN}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\text{NaAu}{\left(\mathrm{CN}\right)}_2+4\text{NaOH}$$

$$4\mathrm{Au}+8{\mathrm{NH}}_2{\mathrm{CH}}_2\text{COOH}+4\text{NaOH}+{\mathrm{O}}_2\to 4\mathrm{Na}\left[\mathrm{Au}{\left({\mathrm{NH}}_2{\mathrm{CH}}_2\mathrm{COO}\right)}_2\right]+6{\mathrm{H}}_2\mathrm{O}$$

Thus, the residue sample in this project was leached by using combinations of cyanide and glycine for gold extraction. The main goal is to establish a relationship between gold deportment in different mineral phases and gold leaching behavior from the pressure oxidation residue.