Urban Mining of Precious Metals with Halide as Lixiviant

URBAN MINING OF PRECIOUS METALS WITH HALIDE AS LIXIVIANT: AN OVERVIEW

Chlorine was firstly applied to the recovery of gold in the 1800s, long before the introduction of cyanidation. In 1846, bromine was introduced as a solvent for gold, as its leaching kinetics are greatly enhanced by the presence of a protonic cation and an oxidizing agent (Filmer et al„ 1984; Kalocsai, 1984; von Michaelis, 1987). Although the cyanidation of gold diminished the potential of halogen in general, interest in halide(s) leaching emerged during the 1990s (Pesic et ah, 1992; Tran et ah, 2001).

The stability of gold-halide complexes is dependent on the Eh-pH of the solution, the composition (with respect to halide concentration), and the nature of the material to be processed (Sergent et ah, 1992; Tran et ah. 2001). A residual quantity of oxidant must maintain a high solution Eh. avoiding precipitation of metallic gold (Tran et ah, 2001).

Gold ions in the +1 and +3 oxidation states are В-type metal ions, therefore their complexation stability decreases as the electronegativity of the donor ligand increases, forming the stability order Г > В Г > СГ > F. According to the hard and soft acid-base (HSAB) theory, in the complexation of Au³⁺ with halides, hard donors will be more stable. Complexes of the type AuL,-^,+ with soft ligands will easily reduce to the +1 state.

All known AuL₄³⁺ complexes have the electronic configuration 4f^u5cP and exhibit low-spin diamagnetic properties. Compounds like AuCl, form dimers to satisfy the coordination number of four required by Au^,+. Chlorine is mainly generated in two ways: (i) by reactions of sodium hypochlorite with HC1, and (ii) production of anodic chlorine via electro-dissociation of concentrated HC1 at cathodic compartment. The reaction with sodium hypochlorite for chlorine takes place as:

Chlorine generation in an electrolytic cell at anodic compartment can be obtained as the reactions below:

Gaseous chlorine has high solubility in acidic water as (Snoeyink and Jenkins, 1979; Lee and Srivastava, 2016):

At a pH above 2.5, aqueous chlorine predominantly forms HOC1 as (Marsden and House, 2006):

Both acids, HC1 and HOC1, completely dissociate in aqueous solutions as:

Further, soluble chlorine can react with chloride ions to form trichloride ions under high acidic conditions (< pH 3):

Pt, Pd and Rh have great capacity to form halo-complexes with high acidity in halide solution. Pt is mainly present in the +2 and +4 oxidation states. In chloride solution, chloro-complexes of Pt are formed with coordination numbers 4 and 6 for +2 and +4 oxidation states, respectively. In the case of Pd. the oxidation states +2 and +4 are taken up in solid compounds. The chloro-complexes of Pd appear with the coordination numbers 4 and 6 for +2 and +4 oxidation states, respectively. The most stable oxidation state of Rh is +3. Rh has a strong tendency to form complex ions with coordination number 6 (Bard 1985). Chemical reactions involving Pt, Pd and Rh in HC1-H,0₂ and NaClO-HCl systems are described below (Harjanto et al., 2006; Bard, 1985; Pourbaix, 1974; Takeno, 2005).

MECHANISM FOR CHLORIDE LEACHING OF PRECIOUS METALS FROM URBAN MINE SOURCES

With chlorine species in aqueous solution, gold can form soluble chloride complexes as follows (Radulescu et ah, 2008):

The formation of soluble species (at < 6 pH) is also in line with the thermodynamic data plotted in Figure 3.1, for the Eh-pH diagram of the Au-C1₂-H₂0 system.

FIGURE 3.1 Eh-pH diagram of Au-Cu-Ni-Cl-H,0 system at 25 C (conditions: 5 x 10~^J MAu; 0.5M Cu^2t; 0.35 M Ni²⁺; 2 M CP).

Interestingly, a direct formation of auric chloride (AuC1₄ ) with Cl_2(aql and HOC1 differ with the reaction of Cl ions due to less oxidative power. In stepwise leaching of gold in the presence of Cl ions, an intermediate aurous complex forms on the surface as (Nicol, 1980; Nicol et al„ 1987):

Again, by the HSAB principle, the high electronegative Cl ions (a hard donor atom) tend to form a secondary intermediate compound of high valance in a second step with the following reaction (Finkelstein and Hancock, 1974):

In a subsequent step, AuCl, either oxidizes to form a stable auric complex (Nicol, 1980; Nicol et al„ 1987) or diffuses into solution as in the reaction below, forming the AuC1₂:

The copper and nickel often associated with gold sourced from electronic waste can also be leached in a chlorine environment:

Based on the above reactions, the plausible electrochemical mechanism for the chloro-leaching of gold (along with Ni and Cu which remain present in MP-PCBs) is presented in Figure 3.2.

The dissolution of large amounts of base metals in chloro-leaching has been found to be problematic. The comparatively lower concentration of gold in the leach liquor can present difficulties for subsequent downstream processing. However, by taking into account the solution chemistry of gold at an ORP value <350mV, the maximum amount of base metals can be leached out in an acidic chloride solution from the electronic wastes (spent printed circuit boards) by leaving gold in the residues. The Eh-pH diagram for an Au-Cu-Ni-Cl-H₂0 system (shown in Figure 3.1) also indicates plausible selectivity in leaching, as the copper and nickel form soluble species at lower potential than that required to form the chloro-complexes of gold. Kim et al.

FIGURE 3.2 Electrochemical mechanism of gold leaching in chlorine-based lixiviant. Source: Modified from Ilyas and Lee (2018).

(2016) investigated selectivity of gold from spent printed circuit boards under a controlled ORP system. In the first stage of leaching at 350mV ORP, the maximum amount of copper leaching (95%) could be obtained along with the minutely extracted gold (0.9 mg/L) in leach liquor. In the second stage of leaching performed at> 1 lOOmV ORP, the residues of the first stage of leaching yielded 93% gold with only 0.6 mg/L copper.

Studies conducted by various researchers indicate that palladium and platinum have a great capacity to form chloro-complexes in HC1 solution, appearing to have coordination numbers 4 and 6 for +2 and +4 oxidation states, respectively (Bard, 1985). The most stable oxidation state of Rh is +3. Rh has a strong tendency to form complex ions with coordination number 6 (Ilyas et al., 2020: Harjanto et al., 2006).

The formation of metal chloro-complexes is shown in Figures 3.3-3.5. The Eh-pH diagrams for Pt-Cl-FLO (Figure 3.3), Pd-Cl-FLO (Figure 3.4) and Rh-Cl-HLO (Figure 3.5) systems clearly illustrate the occurrence of metal species in two different oxidation states in the case of platinum and palladium and of one most stable oxidation state in the case of rhodium.

As these are noble metals, they are difficult to leach and their dissolution mainly depends upon lowering the redox potential in the presence of CL ions to form stable aqua chloro-complexes, which comes through HC1 dissociation as:

Reactions forming the chloro-complexes of an anodic nature take place as below:

FIGURE 3.3 Eh-pH diagram of Pt-Cl-H,0 system at 25 C (0.01 mole/L metal concentration, 6 mole/L CL).

FIGURE 3.4 Eh-pH diagram of Pd-Cl-H₂0 system at 25'C (0.01 mole/L metal concentration, 6 mole/L Cl").

FIGURE 3.5 Eh-pH diagram of Rh-Cl-H,0 system at 25 C (0.02 mole/L metal concentration. 5 mole/L CL).

The influence of Cl^- ions on aqua chloro-complexation can be understood by comparing the above-mentioned oxidation potential (Ei⁰) values with those in the absence of chloride ions (-0.98 V for Pd and -1.18 V for Pt) (Pourbaix, 1974). Thus, leaching can be promoted by decreasing the equilibrium potential and increasing the chloride concentration. On the other hand, in the presence of H,0₂, the cathodic reaction can be more progressively driven due to a higher redox potential by following the equations below:

Then, the reactions in the presence of Cl, with an enhanced mass transfer reaction can be written as:

FIGURE 3.6 Electrochemical mechanism of Pt, Pd and Rh leaching in chlorine-based lixiviant.

The electrochemical mechanism of Pt, Pd and Rh leaching in chlorine-based lixiviant is depicted in Figure 3.6.

INFLUENCE OF VARIOUS PROCESS PARAMETERS

Various process parameters such as lixiviant concentration, oxidant ratio, temperature, contact time and pulp density affect the halide leaching of precious metals from urban mine sources.

Effect of Acid Concentration

The basis of electrochemical kinetics for leaching gold in a chloride solution using dissolution chemistry have already been described (Finkelstein, 1972; Nicol, 1976; Avraamides 1982; Yen et ah, 1990; Tran et ah, 1992a ,b; Lee and Srivastava, 2016). More rapid weight loss of gold in various solutions of chloride- hypochlorite has been achieved than with the cyanidation process under similar parametric conditions (Tran et ah, 2001). The formation of a stable species AuClI strongly depends on the pH of the solution (< 3.0; as evident from the Eh-pH diagram in Figure 3.1) with high chloride/chlorine levels (> 100 g/L Cl ), elevated temperature and various particle sizes. The dissolved gold complex can reprecipitate by contact with a reductant such as sulphidic material, therefore the application of the chloride-chlorine systems is limited. Notably, the solubility of chlorine increases with respect to increasing acid concentration and forming various soluble species, such as aqueous Cl₂ and Cl, (Lee and Srivastava, 2016), creating a highly oxidative environment which can be helpful for processing material other than oxidized bodies. Additionally, the redox potential of the leaching system also needs to be maintained above 400 mV for faster kinetics and higher leaching yield of gold.

The leaching kinetics of gold in a chloride medium are proportional to the chlorine-chloride concentrations (Nicol, 1980). Leaching efficiency therefore increases with increased initial concentration of chloride and soluble chlorine in lixiviant solution with an enhanced temperature (preferably < 60°C). Gold leaching in chloride solution is much faster than the yield obtained in an alkaline cyanide solution. A rate of 0.008 g/m²/s gold leaching in cyanide solution was found to be much lower than the leaching rate of 0.3 g/mVs obtained in chloride solution by Putnam (1944). The high solubility of chlorine in water compared with oxygen (used in cyanidation) is a plausible reason for this. The presence of 3% NaCl in a chlorine solution has shown a significant effect on gold leaching, which may be due to the retarding effect of Cl ions on chlorine dissolution (Chao, 1968). Additionally, the amount of initial chlorine in solution increases the kinetics of gold leaching by shifting the reaction mechanism from diffusion control to a chemically controlled reaction (Lee and Srivastava, 2016).

As w'ell as the above parametric effects, the effect of roasting on chlorine leaching of a gold-bearing refractory concentrate has been studied (Birloaga et ah, 2013). An increase in roasting temperature has been found advantageous in improving the removal efficiency of Hg (~94%). Sulphur removal by roasting could significantly reduce chlorine consumption and yielded a far better leaching of gold (~93%) than when using only cyanidation (27%).

Ilyas et al. (2020) studied the effect of acid concentration (from 2.0 mol/L to

• 10.0 mol/L) on precious metal leaching from spent catalysts. The other parameters, such as pulp density (5%, wt./vol.), temperature (55 ± 2°C), time (2 h), and agitation speed (400 rpm), were maintained at constant. The results (shown in Figure 3.7) showed that the leaching of palladium and platinum increased with increasing concentrations of HC1. This is explained by thermodynamic considerations, due to a higher dissociation of CL ions at a higher acid concentration, which further allows a favourable dissolution of metals into HC1 solution. Leaching of palladium and platinum was found to increase from 8.6% to 61.3% and 6.9% to 34.8%, respectively within the studied range of acid variation (2.0—
• 10.0 mol/L). The higher leaching of palladium than platinum can be ascribed to the differences in redox potentials between the above equations. 8.0 mol/L HC1 was found by these researchers to extract more than 50% of the palladium.

A similar study conducted by Harjanto et al. (2006) with 1% H,0₂ and varied concentrations of HC1 (1 mol/L up to 10 mol/L) indicated that Pt, Pd, and Rh leaching increases with an increase in the HC1 concentration in the leaching solution with 1% H₂0₂. At any HC1 concentration, the order of PM leaching, from the highest, is Pd, Pt, and Rh. The leaching of Pt. Pd. and Rh at 11.6 mol/L HC1 was 95.5, 100, and 85.6%, respectively.

FIGURE 3.7 Leaching behavior of precious metals from exhausted DOC as a function of acid concentration (condition: pulp density. 5%; temperature. 55 C; time. 2 h; and agitation speed, 400 rpm).

Source: Adopted with permission from Ilyas et al. (2020).

Effect of H2О2 and NaCIO Concentration

The formation of the more stable chloro-complexes PtCl₆²- and PdCl₄²~ requires a higher redox potential, instead of the noted value of 368 mV with 8.0 mol/L HC1. Hence, H₂0₂ was added to the lixiviant solution to improve the conditions for metal complexation. The effect of H₂0₂ addition in terms of volume proportionate to 8.0 mol/L HC1 solution ranging from 0.5 vol% to 5.0 vol% was investigated by Ilyas et al. (2020). The addition of H₂0₂ creates a vigorous reaction and a favourable oxidative environment (as per a high redox potential) to form the chloro-complexes of palladium, platinum and rhodium (Bard, 1985). Consequently, the effect of the change in redox potential was reflected in the leaching behavior of precious metals. The results showed (Figure 3.8) that the leaching of palladium and platinum was increased by the addition of H₂0, and further improved when the dosage was increased from 0.5 vol% to 5.0 vol%. Leaching of palladium increased from 60% (with 0.5 vol% H₂0₂) to reach 90% (with 3.0 vol% H₂0₂), while platinum leaching improved from 38% to ~ 81% with the same proportion of H₂0₂ addition. A further increase in H₂0₂ dosage (above 3.0 vol%) did not much improve leaching of the precious metals, which accorded with the recorded Eh values of the system.

The effect of NaCIO addition was also examined for the NaC10-HCl-H₂0₂ system by Harjanto et al. (2006). Here NaCIO was selected as a promoter in a 5mol/L HC1- containing leaching system together with H₂0₂ (1%). As shown in Table 3.1, the addition of NaCIO (0-5 %), along with H₂0₂, to the leaching solution gives a Pt, Pd, and Rh dissolution in the range of 86.1-88.8%, 95.8-98.7% and 72.0-76.9%, respectively. The addition of 3% NaCIO yields a Pt, Pd and Rh dissolution of 87.7%, 98.7 and 76.9% respectively, which was higher than the HC1-H₂0₂ system (5 mol/L HC1, 1% H,0₂) but lower than the 7 mol/L HC1 and 1% H₂0₂-containing system. The

FIGURE 3.8 Leaching behavior of precious metals from exhausted DOC and changes in redox potential of the system as a function of H.O, addition (condition: pulp density, 5%; acid concentration, 8.0 mol/L HC1; temperature. 55 C; time, 2 h: and agitation speed. 400 rpm). Source: Adopted with permission from Ilyas et al. (2020).

TABLE 3.1

Consumption and products of leaching in the various chloride based leaching solutions

Source: Modified from Harjanto et al. (2006).

Detailed composition of the leaching solutions is as follows: (pretreated mass by hydrogen reduction was used throughout; NaClO-HCl-H.O, = 3 vol% NaClO-5 kmolm³ HCl-1 vol% H.O.; HC1-H,0,= 11.6 kmolnv³ HC1-1 vol% H,0,; NaClO-HCl = 3 vol% NaClO-5 kmolnv³ HCl).

results from leaching by the NaC10-HCl-H,0, system also suggest that the presence of NaCIO (up to 3%) is effective in increasing the dissolution of Pt, Pd and Rh by about 3-5%.

Effect of Temperature and Contact Time

In general, an increase in chlorine mass transfer rate by elevation of the temperature should increase gold-leaching efficiency (Nicol, 1980; Vinals et ah, 1995). But in contrast, the solubility of chlorine (as Cl₂(«t/)) decreases with an increase in the temperature (from 7 x Ю^-3 MC1₂ to 2.5 x 10-3 M Cl, at 20-60"C temperatures, respectively) due to the lower absorbability of gases at higher temperatures. In chlorine leaching of spent printed circuit boards, therefore, copper extraction is found to decrease with an increase in temperature. At the initial stage of gold leaching, the lowest concentration of residual chlorine was mainly responsible for slower kinetics for the reactions which accelerated after 40 min and after a 2 h prolonged leaching the gold extraction was found to be independent of the effect of temperature (Figure 3.9). Such behavior is accounted for by the favourable diffusion rate of hypo- chlorous species at higher temperatures. The diffusion coefficients of C₂(aq), HOC1 and OC1- also confirm the suitability of temperature at 50°C (Kim et ah, 2016; Lee and Srivastava, 2016).

While considering HC1 as leaching medium, the effect of leaching temperature and time was investigated by Harjanto et al. (2006), who concluded that an

FIGURE 3.9 Gold leaching rate at different temperatures and corresponding residual chlorine in HCl-leached solution (experimental conditions: 2.0 M HCI solution; electro-generation rate of Cl, at 714 A/m² current density; pulp density 17 g/L; temperature 25 C; particle size -3/+2 mm).

insignificant effect on leaching of precious metals was observed over 65'C and above lh of leaching time.

A detailed study by Ilyas et al. (2020) indicated that temperature (25-70°C, using

8.0 mol/L HC1 with 3.0 vol% H₂0₂ for 2 h duration) can significantly promote the solid-liquid mass transfer; however, a higher temperature may cause faster degradation of H₂0₂. Experimental results indicated a significant effect on leaching while temperature increased from 25 C to 55°C; thereafter, no remarkable change was observed. This commonly reveals the exothermic nature of the leaching process that might proceed through the change in reaction rate.

In order to establish dissolution kinetics and mechanism, leaching studies were carried out by the same group of researchers at different temperatures (25-70 C), with time variation from 10 min to 180 min. Results (Figure 3.10 a and b) show the progress of leaching with respect to elapsed time, which could indicate that dissolution of precious metals significantly increased with time and temperatures. Palladium efficacy of maximum 96% was obtained for 180 min of leaching at 70°C (Figure 3.10

• a) , whereas up to 90% was obtained for platinum in similar conditions (Figure 3.10
• b) . Interestingly, the leaching data at 55 C and 70 C were observed to be almost the same, decomposition of H,0₂ at a higher temperature (>55°C) is known to be fast. This phenomenon can be ascribed to the shifting of leach kinetics from diffusion control at low temperatures to chemical control at higher temperatures.

Effect of Pulp Density

The possibilities for maximum mass transfer into bulk solution from an exhausted diesel oxidative catalyst was investigated by Ilyas et al. (2020), who carried out leaching experiments at different pulp densities (from 5 to 20 wt./vol.%) at optimum conditions of: HC1 concentration. 8.0 M; H₂0₂ addition, 3 vol%; temperature, 55°C; time, 3 h; and agitation speed, 400 rpm. The results revealed that that the leaching

FIGURE 3.10 Leaching behavior of palladium (a) and platinum (b) from spent catalyst at different temperatures as a function of time (condition: pulp density. 5%; acid concentration, 8.0 mol/L HC1; H,0, addition dosage, 3%; and agitation speed, 400 rpm).

efficiency of precious metals was the same with 5% and 8% pulp density (>94% palladium and -90% platinum). Thereafter, increasing pulp density to 12% and above could result in decreased efficiency of precious metals in HC1 solution. Leaching was found to decline to 91.5% palladium and -86% platinum with a pulp density of 12%, with a much greater decline to 68.7% palladium and 60.4% platinum when the pulp density was increased up to 20%. This phenomenon of decreased leaching efficiency at higher pulp density can be ascribed to the lower availability of surface area per unit volume to the solution (Habashi, 1969).

Similar phenomena were observed by other researchers while performing experiments with spent catalytic converters at varied pulp densities (100 to 700 g/L), as in Table 3.1. It was observed that the dissolution of Pt and Rh decreases at higher pulp density. Only a slight decrease of Pd dissolution was observed during the leaching with pulp density higher than 100 g/L. The decrease of Rh dissolution is higher than that of Pt or Pd at pulp density higher than 100 g/L. This is understandable, since the amount of powder became excessive at high pulp density. One can assume that to some extent higher pulp density would reduce the agitation efficiency. In addition, leach lixiviant may not reach some PM particles at higher pulp densities, as these are shielded by the support materials (Graham et al., 2003; Harjanto et ah, 2006).