1. Introduction
Chalcopyrite, as the most abundant source of sulfide copper in the Earth’s crust and one of the most refractory sulfide copper minerals, has been investigated intensively for copper extraction in past decades. Typically, copper from chalcopyrite is recovered by flotation followed by pyrometallurgical processes. However, the pyrometallurgical processes usually generate severe environmental pollutants such as furans, dioxins and highly acidic wastewater [
1], which has made the hydrometallurgical processing route more preferable [
2]. Additionally, smelting poses a problem when the concentrate contains trace amounts of mercury, arsenic or volatile radionuclides that may be present as the daughter products of uranium in the ore (e.g., chalcopyrite from some iron oxide–copper–gold (IOCG) deposits, such as that found in South Australia).
Chalcopyrite is usually considered refractory under atmospheric pressure leaching conditions. Many lixiviants have been studied for chalcopyrite leaching, such as sulfuric acid, nitric acid, ferric chloride, ferric sulfate, cyanide, ammonia and bioleaching [
3,
4,
5]. Sulfuric acid with ferric ions is the most widely accepted lixiviant as it is the cheapest lixiviant for leaching many base metals from their ores. However, gangue minerals can be dissolved by acids, and iron can contaminate the leachate due to the simultaneous dissolution of copper and iron in acid media [
2]. Additionally, the dissolution rate of chalcopyrite is generally slow under either acidic or alkaline conditions. In an acidic leaching system, it was speculated that the formation of a secondary product, e.g., a complex film of sulfide, polysulfide, elemental sulfide, ferric hydroxysulfate and jarosite, may passivate or form overlayers to impede the diffusion of reagents to the surface of chalcopyrite and/or the diffusion of ions away from the chalcopyrite surface [
4]. However, a critical review on the passivation mechanism of chalcopyrite leaching conducted by O’Connor and Eksteen [
6] stated that the leaching of chalcopyrite may actually be inhibited by its semi-conductor behavior, not passivation layers. Depending on the pH and the leach system, there remains uncertainty on the contribution of the various kinetic-retarding factors for chalcopyrite leaching.
Leaching in alkaline conditions appears to have some advantages, considering the known challenges using acidic media. Aqueous ammonia solutions and its derivatives are the most widely accepted and industrially applied lixiviant for extracting base metals, such as copper, nickel and cobalt, due to its low cost, easy formation of water-soluble metal complexes and ease of regeneration by evaporation [
7], and is a process that has been commercialized at an early stage. Ammoniacal leaching processes allow for the selectivity leaching of copper against undesirable elements such as iron and calcium due to the alkaline nature of ammonia. In oxygenated ammonia solutions, the dissolution of chalcopyrite forms SO
42− instead of S
0, a soluble ammine complex with copper and iron rejected as iron oxides, and the overall oxidative reaction between ammonia and chalcopyrite is shown in Equation (1) [
8].
An early study suggested that the insolubility of iron was due to the layer comprising several species of iron oxide and oxydryoxide being a passivation layer [
5]. Later, some researchers recognized that iron oxides are likely to play limited inhibiting roles due to the porous nature of iron oxides [
9,
10]. This is because it was observed that a high agitation speed can improve the leaching rate, while agitation has either no effect or reduces the leaching rate at acidic leaching environment. The leaching of chalcopyrite at alkaline conditions is not like that at acidic conditions, and the inhibition on the chalcopyrite surface in an alkaline solution is not fully understood [
6]. In a batch system, there are many limitations, such as the loss of reagents through volatilization, oxidation or product formation, approaching their solubility limits in the leach liquor.
Recently, alkaline glycine solutions as the lixiviant for the leaching and recovery of base metals and precious metals were intensively studied [
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28]. Some of them studied glycine as a complexing agent for chalcopyrite leaching [
13,
29,
30]. Through an analysis using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, it was found that a loosely held porous layer that developed on the surface of chalcopyrite and consisted mainly iron oxyhydroxides has no apparent passivation behavior in an alkaline glycine leaching system [
30]. A kinetic study conducted by Tanda, Eksteen, Oraby and O’Connor [
29] shows that after ultrafine grinding (100% passing 10 µm), a chalcopyrite mineral specimen comprising 67% chalcopyrite was leached by a 0.5 M glycine solution at pH 11.5, dissolved oxygen at 15 ppm and a temperature of 50 °C, with a copper extraction of about 90% after 96 h of leaching. This revealed that the leaching rate of chalcopyrite using glycine was markedly affected by the particle size and temperature, which means a high-energy intensity with commensurate costs. The anodic dissolution of chalcopyrite in alkaline glycine solutions may form sulfate or elemental sulfur, as described in Equations (2) and (3).
The solid content for chalcopyrite leaching was generally low in the literature, normally ranging from 0.75% to 2%, using glycine or ammonia as the lixiviant [
2,
29,
31]. A study conducted by Khezri et al. [
32] revealed that the leachability of chalcopyrite is extremely low using alkaline glycine solutions, and a decrease in copper extraction was observed if the pulp density >1%. The decline in extraction could be attributed to the precipitation of copper as copper sulfide and crystallized copper glycinate when the copper concentration was too high. A study that focused on extracting copper from waste-printed circuit boards (WPCBs) using alkaline glycine solutions has also shown that Cu extraction was low when the solid content increased to 10% [
14]. That is probably due to too high Cu concentration in the final leachate, which leads to cupric glycinate (
bis-glycinato copper (II)) crystallization. Glycine forms a range of coordination complexes with metals with potential cis- and trans-isomers as well as hydrates that influence the solubility of the complexes and the transition of kinetically favored isomers to thermodynamically favored isomers depending on a range of conditions [
33,
34].
Although shown by numerous authors that chalcopyrite can be leached by alkaline glycine solutions, it appears to require ultra-fine grinding, elevated temperature and prolonged leaching periods to reach good copper recovery, which makes for unappealing process economics. To further improve a glycine-based leaching system for chalcopyrite by avoiding ultra-fine grinding process and heating, this study proposed the use of ammonia solutions as the pH modifier (as a pH modifier is required) for the alkaline glycine system to leach chalcopyrite at ambient temperature and pressure. The effects of operating conditions, such as pH, glycine additions (Gly:Cu molar ratio) and the addition of ceramic beads as stirred mill media, on the copper extraction from chalcopyrite were investigated and are discussed in this research. The leachability of chalcopyrite at 10% solid/liquid ratios were studied. A kinetic study was undertaken using the shrinking core model to investigate the leaching behaviors of chalcopyrite in an ammonia–glycine system.