Thiourea belongs to a broad class of compounds with the general structure (R'R2!'!) (R3R4N)C=S. It is a planar organic sulphide with a C=S bond distance of 1.60±0.1 A. Although the chemical structure of thiourea is similar to that of urea, except that the oxygen atom is replaced by a sulphur atom, the properties of thiourea differ significantly from those of urea. Thiourea crystals can be dissolved in water or acid solution, and can then react with Au/Ag to produce a stable cationic complex or aurous ion.

Thiourea leaching of Au and Ag from spent printed circuit boards was carried out by Xu and Li (2011), and the influence of leaching time, reaction temperature, thiourea concentration, Fe'+ concentration, and material particle size on Au leaching rates were investigated. The results showed that Au and Ag leaching rates can reach 90.9% and 59.8%, respectively, under optimum leaching conditions.

Li et al. (2012) examined the thiourea leaching of Au and Ag from spent printed circuit boards with 24 L'1 thiourea concentration and 0.6% of Fe,+ supplementation with 2 h of leaching time. The leaching efficiency of gold reached almost 89.7% along with 48.3% silver extraction within 2 h. In the absence of any ferric salt, it took 6 h to achieve the same extraction of gold in acidic thiourea solution. However, when the ferric ion concentration was higher than 0.01 M, gold recovery was lower. The iron present in printed circuit boards may also supply a part of iron during the leaching. In the case of silver leaching, there was no beneficial effect of ferric ions. Since gold has lower oxidation potential values than silver, it seems reasonable that the presence of an external oxidizing agent enhances gold dissolution. Birloaga et al. (2014) found that 69% of Au was extracted under conditions of 20 g L-1 thiourea, 6 g L'1 Fe,+ ions, and 10 g L1 H,S04, as well as 600 rpm. Furthermore, under the same reagent conditions, multistage crosscurrent leaching was used to reduce the consumption of thiourea and improve the efficiency of Au leaching from spent printed circuit boards.

Mechanism of Thiourea Leaching of Precious Metals from Urban Mine Source

The formation of gold complexes in thiourea solution can be better represented by the Eh-pH diagram (Figure 5.10) and probable electrochemical mechanism (Figure 5.11).

As can be seen from Figure 5.10, thiourea is unstable when decomposing in other than acid pH, hence the leaching of gold is most prominently performed in the pH range of 1-2 to form the only commonly existing cationic species of gold, Au[SC(NH2)2],+ (Munoz and Miller, 2000). The electrochemical nature of the reaction, which yields up to 99% of gold, can be presented as:

Eh-pH diagram of Au-SC(NH,),-H,0 system (conditions

FIGURE 5.10 Eh-pH diagram of Au-SC(NH,),-H,0 system (conditions: 5x 10 J M Au, 0.5M SC(NH,)2 at 25 C).

Probable electrochemical mechanism for gold-thiourea leaching from printed circuit boards

FIGURE 5.11 Probable electrochemical mechanism for gold-thiourea leaching from printed circuit boards.

The kinetics of Au dissolution in acidic thiourea solutions in the presence of a variety of oxidants, namely, H202, Fe2(S04)3, formamidine disulphide, oxygen, Na202, and Mn02 have been studied by many authors. Among them all, the Fe,+ ion is found to be the most effective, preferably in an acidic sulphate solution rather than chloride/nitrate media (Plaskin and Kozhukhova, 1960; Songina et al., 1971). The reaction for gold leaching in thiourea and ferric ion solutions can be written as:

In the presence of redox couple, Fe,+ and Fe2+, gold leaching in thiourea solution has been found to be up to four times faster than oxidation by air purging (Huyhua et ah, 1989). Deschenes and Ghali (1988) demonstrated the effect of ferric ion as an oxidizing agent during leaching of gold from chalcopyrite concentrate. Without any oxidizing agent, the thiourea solution provides an 80% gold extraction in 8 h. An addition of 2.0 g L'1 of this oxidant slightly improves the initial leaching kinetics but results in no real improvement in gold extraction. Increasing the concentration of oxidant to 5.0 g L'1 could increase leaching kinetics and result in extraction of > 93% gold in 7 h leaching.

The dissolution of precious metals from a spent printed circuit boards ash sample in acidic thiourea solutions with different concentrations of oxidant Fe2(S04)3 from 0 g L'1 to 7.5 g L'1 was also investigated by Batnasan et al. (2018), using a combined hydrometallurgical method consisting of high-pressure oxidative leaching and thiourea leaching. It was observed that increasing the oxidant concentration to 5 g L'1 resulted in an increase in the dissolution of Au and Pd. but a rapid decrease in Ag dissolution. The equations for dissolution of gold, silver, and palladium in acidic thiourea solutions in presence of Fe3+ ions can be generally described as follows;

However, a further increase in Fe,+ ion (8.0 g L1) decreases gold extraction. Studies revealed that a lower dosage of Fe,+ is not adequate to oxidize the metal ions, whereas a higher dosage oxidizes thiourea itself to S2 , S° and formamidine disulphide, which can suppress the leaching efficiency of gold. The oxidative degradation of thiourea into several degradation products can be written as:

The maximum dissolution of Au and Pd was found to be 68% and 7.1%, respectively, when the oxidant concentration was 5 g L1, whereas the Ag dissolution was 1.6%. It was observed that the trends with respect to silver dissolution in aqueous thiourea solutions in the presence of H,S04 (0.25 M to 4 M) and Fe,(S04)3 oxidant (0 g/L to

7.5 g L1) exhibited quite similar profiles. This observation is probably related to formation of an insoluble silver-thiourea complex and silver sulphide (Ag2S) under the oxidation conditions based on the following equations:

Formation of metal-thiourea complexes can be attributed to the variations in the pH and redox potential (Eh) of the thiourea solutions with the addition of oxidant, as high redox potential and acidity of the solution led to an increase of Au and Pd leaching, in contrast to a decrease in the leaching of Ag. It appears that optimization of the oxidant concentration during the thiourea leaching process is the main challenge to avoid thiourea loss/oxidation, because the dissolution of precious metals was strongly dependent on the oxidant concentration.

Effect of Various Process Parameters

Thiourea is thought to be a low-toxicity reagent that exhibits a faster gold leaching rate, with apparently less interference from the base metals (Pb, Co, Ni, Zn); moreover, it is used in acidic conditions rather the alkaline conditions of cyanide and thiosulphate (Yannopoulos, 1991). The problems in gold leaching due to the occurrence of base metals in spent printed circuit boards and the sorption loss of gold from leach solution with carbonaceous matter coming from the detached hydrocarbon of the spent printed circuit board resins can be handled in thiourea leaching. However, there are several factors affecting thiourea leaching of precious metals from secondary sources.

Effect of Temperature

Temperature plays an important role in enhancement of leaching kinetics; hence plenty of work has been carried out in this area. A calcine powder sample of printed circuit board (3470 g/t Au) leached in a solution of 0.5 M thiourea + 0.05 M H2S04 + 0.01 M Fe-,+ ions at a pulp density 2.85 g L'1 with varying temperatures (30-60°C) did not show much enhancement in gold extraction up to 45°C (with ~80% leaching), rather it decreased remarkably (~75% leaching) at 60"C . Spent printed circuit boards without calcination showed a similar trend in results. The maximum of 90% gold extraction efficiency was obtained at 25°C for 2 h leaching of a pulverized sample of 430 g/t Au with 0.31 M thiourea solution (with 6% Fe,+ ions at pH 1). This gold leaching behavior as a function of temperature and gold concentration in the charged sample can be elaborated with the complex electrochemical mechanism involved in the system. The reason behind the suitability of higher temperatures (up to 45 C) for an Au-concentrated sample of calcined printed circuit board than that (25°C) for the direct pulverized sample of lesser gold can be explained by the increase in gold concentration reducing the stability zone of the Au[SC(NH,)2]+2 complex. A decrease in gold extraction with a rise in temperature can be understood by thiourea decomposition, which results in the formation of colloidal sulphur in the presence of iron and the retardation of the reaction through passivation on the gold surfaces. The significant effect on gold leaching of increasing temperature can be corroborated by the chemically controlled reaction, also evidenced by the independence of the leaching rate from the stirring speed. This is found to be at variance with the reaction kinetics investigated using rotating discs by applying formamidine disulphide as an oxidant instead of ferric ions. The chemically controlled gold leaching from spent printed circuit boards is expected to be an electrochemical reaction (see Figure 5.1 l).The cathodic half-cell reaction for the reduction of formamidine disulphide (in Figure 5.11) make it seem as though the H+ ion might participate in the rate-limiting reaction. But it has been found that the pH does not change with decomposition of thiourea. indicating that the formamidine disulphide exists in a protonated form (as reaction-b) instead of a neutral molecule (Schulze 1984; Groenew'ald, 1976; Habashi, 1969).

The presence of ferric ions in the bulk thiosulphate solution catalyzes the electrochemical reactions to facilitate the oxidation of metallic gold to aurous (Au+) and silver to argentous (Ag+) ions, (for Au° to Au+).

Effect of Thiourea Concentration

Maintaining the appropriate dosage of thiourea in gold leaching is of prime importance. In general, for this kind of leaching system, 0.13M thiourea along with Fe,+ concentration up to 5 g L'1 at Eh and pH values nearly in the range of 400-450mV and 1-2, respectively, is optimal (Munoz and Miller, 2000). Some work has been reported with respect specifically to the leaching of gold from spent printed circuit boards. A pulverized sample of spent printed circuit board containing 430 g/t Au, 540 g/t Ag, 39.86% Cu, 0.457% Zn, and 0.396% Ni was leached in 0.26-0.36M thiourea solution along with 0.6% Fe3+ at pH 1.0 for 3 h duration (Li et al., 2012). At 0.26M thiourea, the leaching rate was the slowest, however it increased to the maximum (—90%) w'ith 0.36M thiourea in solution in the initial 2 h of leaching and then decreased significantly (to ~60% after 3 h) with prolonged leaching. A concentration of 0.31 M thiourea in lixiviant solution was optimized in this case to yield 89.7% Au along with 48.3% Ag for 2 h leaching time. A quite similar leaching trend was observed, with maximum leaching efficacy with 0.31 M thiourea, while below and above that concentration, there was a remarkable decline in gold leaching (Camelino et al, 2015). This behavior strongly supports the occurrence of passivation phenomena in leaching with the degradation products of thiourea (see Equations 5.58-5.61). At variance with these results, the optimized dosage of thiourea increased to 0.5 M when the calcined pow'der (size, 53-75pm) of spent printed circuit board (containing high amount of Au = 3470 g/t) was subjected to leaching (Gurung et al., 2011). At a pulp density of 2.85 g L1, the extraction efficiency was nearly 90% after 24 h of leaching w'ith 0.5 M thiourea in 0.05 M acidic solution. At concentrations lower than 0.5 M thiourea, gold extraction could not exceed 50%, even at a lower pulp density than 2.85 g L1. Going above 0.05M acid concentration did not improve the gold leaching yield.

Spent printed circuit board samples obtained after incineration and high-pressure oxidative leaching were examined by Batnasan et al. (2018) at various thiourea concentrations. The dissolution efficiency of the precious metals increased drastically with increase of the thiourea concentration from 1.25 g L-' to 12.5 gL1, then remained nearly unchanged on further increase of the thiourea concentration up to 20 g L1. The maximum dissolution efficiency of 91%, 81%, and 11.9% for Au. Ag, and Pd was achieved at thiourea concentration of 12.5 g L-1 due to the appropriate thiourea/ Fe2(S04), molar ratio under this condition. Use of appropriate concentrations of thiourea and formamidine disulphide results in an increase in the leaching of precious metals, which can be expressed as follows:

These results indicate that the formamidine disulphide (SCN2H,)2 produced can be beneficial to the leaching efficiency of precious metals under precisely optimized leaching conditions. It is noteworthy that, with the addition of thiourea, the pH of the leaching medium does not change remarkably, w'hereas the redox potential in the medium falls from 0.67 V to 0.43 V. The variations of the redox potential with addition of thiourea and oxidant show completely opposite trends, probably related to oxidation of thiourea to form a ferric-thiourea complex and formamidine disulphide, respectively.

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