Urban Mining of Precious Metals with Thiosulfate and Thiourea as Lixiviant
URBAN MINING OF PRECIOUS METALS WITH THIOSULFATE AND THIOUREA: AN OVERVIEW
For precious metal leaching from spent printed circuit boards, the useful sulphur- based thio-compounds are thiosulfate and thiourea. Apart from the complications in solution chemistry exhibited by the instability of thio-compounds and many other reactions which simultaneously take place during gold leaching, these thio-compounds have chemical similarities to enable gold to be leached in an oxidizing environment (Ilyas and Lee, 2014).
THIOSULFATE LEACHING OF PRECIOUS METALS
Thiosulfate, with the chemical symbol, S2O3” and tetrahedral molecular shape, is an oxyanion of sulphur and can be derived by replacing one oxygen atom with a sulphur atom in a sulphate anion (Schmidt, 1962) as indicated in Figure 5.1.
The sulphur-to-sulphur (S—S) distance indicates a single bond, implying that the sulphur bears a significant negative charge and the S—О interactions have a more double-bond character. Thiosulphate can be produced from the reaction of elemental sulphur and sulphite at elevated temperature:
Under alkaline conditions, thiosulphate can be produced as a product of the reaction between sulphur or sulphide and hydroxide (Shieh et al„ 1965):
FIGURE 5.1 Tetrahedral structure of thiosulphate ion with bond angles.
The thiosulphate anion is a metastable donor of sulphur, and undergoes disproportionation to form sulphite and sulphur or an active sulphur species. It is sometimes regenerated from tri-, tetra-, or penta-thionates in an alkaline aqueous ammonia solution (Zhang and Dreisinger, 2002; Aylmore and Muir, 2001; Naito et al., 1970):
Thiosulphate is a divalent soft ligand (type A) which tends to form stable complexes with low-spin cl'° (Au+, Ag Cu+, Hg2+) and (f (Au,+, Pt2+, Pd2+) metal ions (Livingstone, 1965; Wilkinson and Gillard, 1987). Mostly, the thiosulphate ion acts as a unidentate ligand via the terminal sulphur atom, establishing strong о bonds with a metal ion which are stabilized by pn-dn back-bonding (back donation). Thiosulphate ligands may also act as a bidentate ligand through sulphur and an oxygen atom and as a bridging ligand via the terminal sulphur atom, usually resulting in an insoluble complex (Figure 5.2) (Gmelin, 1973; Livingstone, 1965; Ryabchikov, 1943; Zhao et ah. 1998).
FIGURE 5.2 Unidentate, bidentate and bridging complexes of thiosulphate with precious metals.
The recovery of gold using thiosulphate was first proposed early in the 1900s (White, 1900). The compatibility of an environmentally benign thiosulphate ligand to be complexed with gold along with the achievable faster kinetics, as with cyanidation, are some of the basic characteristics that make thiosulphate the prime alternative candidate in gold metallurgy. Especially in the case of secondary wastes containing carbonaceous material, the in-situ adsorption of anionic gold cyanide complex onto the carbonaceous matter can be avoided using thiosulphate as lixiviant. The affinity order of gold adsorption on the carbon surface is: SCN- > SC(NH2)2 > CN- S20,2-. Although thiosulphate exhibits the anionic species, possibly Au(S20,)- and Au(S20,)23, it shows less affinity for reductive adsorption of the thiosulphate complex than with the adsorption of cyanide and chloride complexes on the carbon surface, leading the way towards treat carbonaceous ores in an environmentally benign manner.
Mechanism of Thiosulphate Leaching of Precious Metals From Urban Mine Sources
Thiosulphate (S20,)2- has been widely accepted as the best suitable reagent and alternative to cyanide and aqua regia lixiviant for gold leaching from various primary and secondary sources. The chemistry of the gold-thiosulphate system is complex and needs an oxidizing atmosphere to keep reactions under control. Copper, which has self-catalytic behavior, is commonly used as an oxidizing agent. The compulsion to maintain the alkaline condition in the thiosulphate system to prevent its decomposition by acid is fulfilled in an ammoniacal medium; under such conditions copper can easily form ammine complexes to catalyze the reaction kinetics (Aylmore and Muir, 2001). In the case of gold extraction from spent printed circuit boards, the amount of copper and the reduced interference of foreign metals due to its inability to form soluble ammine complexes is another reason for its usefulness for thiosulphate leaching. Gold alloyed with nickel followed by nickel and copper layers can easily be
FIGURE 5.3 Eh-pH diagram of the Au-(S,0,)2_-H,0 system (conditions: 5xl0~J M Au, 1M S2032-, Ш NH,/NH4+ at 25 C).
FIGURE 5.4 Eh-pH diagram of the Cu—(S,0,)2_-H,0 system (conditions: 0.5 M Cu2+. 1M S2
FIGURE 5.5 Eh-pH diagram of the Ni—(S,0,)2'-H,0 system (conditions: 0.35 M Ni2 1M S2032'- 1M NH,/NH4+ at 25 C).
liberated to leach in ammoniacal solution as all three metals form their ammine complexes (Chen et al„ 1996; Srivastava et al., 2013). The potential-pH diagrams of Au, Cu, and Ni are given in Figures 5.3, 5.4 and 5.5, respectively.
There are so many entities for potential gold leaching in this complex system, including the simultaneous presence of thiosulphate and ammonia, the redox couple Cul+ and Cu2+ and the stability of thiosulphate itself under certain pH conditions, that understanding the aqueous chemistry is vitally important. The two gold-thiosulphate complexes are known to form as Au(S20,)‘ and Au(S203)23~, the latter complex being the more stable (Johnson and Davis, 1973). The plausible reaction with 02 used as oxidant can be written as follows:
But the above reaction has been found to exhibit slow kinetics due to passivation by sulphur deposited on the gold surfaces by the decomposition of thiosulfate(Pedraza et al., 1988). Introducing the ammonia prevents such passivation by preferential adsorption onto gold surfaces over the thiosulphate, and gold can be leached as follows (Chen et ah, 1996; Jiang et ah, 1993):
However, the formation of a gold-ammine complex is only possible at higher temperatures (Meng and Han. 1993), and >80C is economically not a good choice, with a high rate of ammonia decomposition at an elevated temperature (>60°C). This has been encountered in the catalytic action of copper ions in gold-thiosul- phate-ammonia leaching (Tyurin and Kakowski, 1960). The leaching of gold that occurs under the oxidizing environment of Cu2+ and Ni2+ has been found to be 18-20 times more beneficial with enhanced kinetics (Ter-Arakelyan 1984), also at a temperature <60’C (Tozawa et ah, 1981). Moreover, gold-thiosulphate-ammonia leaching in the presence of copper and nickel is an electrochemical reaction, in which the Cu(NH,)42+ is converted to Cu(NH3)2+ to support the formation of the oxidized product [Au(S20,)2]3~; the reverse oxidation of Cul+ to Cu2+ occurs in the presence of 02 (Byerley et ah, 1973). The reactions that take place, in this case, are as follows:
In contrast to the ore bodies, the liberation of gold from the spent printed circuit boards is depends to a great extent on how the alloyed nickel with the gold layer behaves in the ammonia-thiosulphate system. This makes the system much more complicated than the earlier complexed system of Au-Cu-NHx/NH4-S20, in solution; hence it has scarcely been described. The thermodynamic stability of the Cu2+- thiosulphate complex is higher than that of Cu(NH,)42+; for this reason , the Cu(NH,)42+ is reduced to a Cu2+-thiosulphate complex, causing oxidation of thiosulphate to degrade as tetrathionate. This condition does not arise in the presence of nickel, which also controls thiosulphate consumption during the whole process of gold leaching from spent printed circuit boards. Gold leaching can be catalyzed by the nickelous oxide under similar Eh-pH condition to the ammonia-thiosulphate system (Figure 5.5). The probable mechanism of these complicated electrochemical reactions and phenomena is presented in Figure 5.6.
FIGURE 5.6 Probable electrochemical mechanism for gold-thiosulphate leaching from spent printed circuit boards.
It can be seen that the thiosulphate ions react with Aul+ on the anodic surface of gold and enter the solution to form Au(S,03)23" which is catalyzed by the Cu2+- Cul+ ammine complexes. The reduction of Cu(NH,)42+ transfers two ammonia ligands to form the kinetically favoured diaminoaurate(I) complex, which subsequently exchanges the ligands with free thiosulphate ions to form the thermodynamically more stable auro-thiosulphate complex. At the same time, gold leaching is boosted by the oxidation reaction of the Ni2+-ammine complex to form Ni,04, and then the reduction of Ni,04 with oxidation of gold as the Au(NH3)2+ complex. The predominant cathodic reactions are dependent on the relative concentrations of the species.
Plenty of previously reported work suggests that the Au-Cu-NH3-S203 leaching process is diffusion controlled. But leaching of gold from spent printed circuit boards (in which Au is alloyed with Ni) has been found to be chemically controlled, indicating that other entities influence gold leaching, and the presence of nickel in the system as per the electrolytic mechanism shown in Figure 5.6 may account for this. It can easily be seen that together with gold, all the nickel was leached in ammoniacal thiosulphate solution, and only after this is the copper layer exposed to the lixiviant.
Moreover, the leaching rate drastically decreased (~20% leaching after 9 h) whed l5mM Cu2+ was used instead of 20 mM. Leaching of shredded printed circuit boards also showed reduced efficacy of gold leaching (30.3%),compared to 78.8% leaching of the spent printed circuit boards unit under the same condition of 0.1 M (NH4)2S203, 40mM CuS04, 40 g L'1 solid-liquid ratio and pH 10.0-10.5 (Tripathi et al„ 2012).
Effect of Various Process Parameters
A handful of research works have studied gold leaching from spent printed circuit boards in ammoniacal thiosulphate solutions. It is important to understand the influence of different factors.