Limitations, Challenges, and Environmental Impacts of Thiosulfate Leaching
Thiosulphate leach reactions are less favourable than gold cyanidation (Lee and Srivastava, 2016), hence high amounts are consumed to achieve the equivalent gold leaching rate. A thiosulphate leach solution requires a concentration of 5 to 20 g L'1 vs. 0.25 to 1 g L'1 cyanide in solution. Higher consumption of thiosulphate partially offsets its significantly lower cost, which is one-fifth of the cost of cyanidation. A poor affinity to adsorb the gold-thiosulfate complex onto carbon negates the use of conventional carbon-in-pulp (CIP) or carbon-in-leach (CIL) processes.
Thiosulphate may readily be oxidized or reduced according to the initial solution potential. Depending on the aqueous environment, thiosulphate can break down into sulphite, sulphate, trithionate, tetrathionate, sulphide, polythionates (SvOv2_) and/or polysulphides x (S*~). An important factor in thiosulphate stability is the pH of the solution, since thiosulphate rapidly decomposes in acidic media (Li et al„ 1995). Certain metal ions and reagents also cause the breakdown of thiosulphate, as shown in Equations 5.19-5.23 (Abbruzzese et al„ 1995; Briones and Lapidus, 1998; Williamson and Rimstidt, 1993;Tykodi, 1990; Xu and Schoonen, 1995).
Notably, the anions trithionate (S,062_) and tetrathionate (S4062-), which are not known to have any lixiviating activity (Aylmore, 2001), can interfere with resin- based recovery methods by displacing metal complexes from ion-exchange sites (Fleming. 1998; O'Malley, 2001). In addition to the above reactions, thiosulphate is also consumed by peroxides, phosphines, polysulphides, permanganates, chromates, the halogens (chlorine, bromine, and iodine), and their oxyanions. In addition, certain species of fungi, microfauna, and microflora can digest thiosulphate ions (Xu and Schoonen, 1995). The degradation of thiosulphate ions may be caused, or catalyzed, by the presence of certain metal ions. Fe,+ ion accelerates the decomposition of thiosulphate by intramolecular electron transfer. The deep purple [Fe(S,0,)]+ complex is formed, and decomposition occurs via reduction of the metal and concomitant oxidation and dimerization of the ligand to form the tetrathionate ion (Perez and Galaviz, 1987; Williamson and Rimstidt, 1993; Uri, 1947). Similarly, arsenic, antimony, and tin salts catalyze the formation of pentathionate from thiosulphates, while metallic copper, zinc, and aluminium result in the formation of sulphides (Bean, 1997; Xu and Schoonen, 1995).
The recyclability of thiosulphate solution strongly depends on the metal recovery technique employed, which may cause significant degradation of the liquor by oxidation, reduction, or contamination (Awadalla and Ritcey, 1991; Benedetti and Boulegue, 1991). Ammonia, however, can be stripped from metal-depleted liquor (tailings) by exploiting its significant volatility (Marchbank et al., 1996). By-products and tailings from thiosulphate processing should consist primarily of low-toxicity metal hydroxides, oxides, sulphates, polythionates, polysulphides and/ or insoluble sulphides, although pilot studies to date have not directly addressed waste management. There are several reversible reactions in which thiosulphate is either consumed or regenerated, some of which play a vital role in leaching by recycling various breakdown products and regeneration of thiosulphate ions, as shown in Equations 5.26-5.43, where each reaction formulates the thiosulphate as product (Bean, 1997; Byerley et al., 1975; Fleming, 1998; Hu and Gong, 1991; Perez and Galaviz, 1987; Roy and Trudinger, 1970; Xu and Schoonen, 1995; Zipperian et al., 1988):
Sulphite addition has been suggested by different researchers aiming at the regeneration of decomposed thiosulphate and lixivating refractory Mn02 (Flett et ah, 1983; Groudev et ah, 1996; Guerra and Dreisinger, 1999; Hemmati et ah, 1989; Johnson and Bhappu. 1969; Langhans et ah. 1992; Lulham and Lindsay, 1991). But the actual benefits of sulphite addition are questionable, due to the ready oxidation of sulphite by Cu2+, producing Cu+, sulphate and dithionate ions (Aylmore, 2001). Augmentation with excess sulphate to enhance thiosulphate stability (Gong et ah, 1993; Hemmati et ah, 1989; Hu and Gong, 1991), involving eight-electron redox reaction for the reduction of sulphate to thiosulfate, may not be feasible (as Equation 5.36). Apart from sulphide and sulphate, the breakdown products of thiosulphate are not known to form stable complexes with metal ions of interest (Aylmore, 2001; Smith and Martell, 1976). Metal sulphide complexes are generally sparingly soluble, while sulphate has negligible chelating ability, and complexes incorporating other polythion- ate (SxOy2-) ligands are overwhelmed by the abundant thiosulphate ions. A number of authors have reported the in-situ synthesis of thiosulphate ions from sulphoxy com- pounds/ions during controlled oxidative leaching (Chen et ah, 1996; Genik-Sas- Berezowsky et ah, 1978; Groves and Blackman, 1995). As a by-product of the destruction of a sulphide matrix, the oxidation of native sulphur or sulphides may be the cheapest source of lixiviant generation (Equations 5.44-5.47) (Bean, 1997; Chen et ah, 1996).
The reaction mechanism permitting this transformation appears to involve an attack on elemental sulphur by transitory polysulphide species (i.e., NaSJH) (Bean, 1997; Aylmore and Muir, 2001). Recovering harmful sulphurous matter in this fashion also has the advantage of minimizing the environmental impact of the operation.
The formation of decomposition products can also be avoided by oxidation of thiosulphate to sulphate prior to discharge, but the oxidation cost is higher than cyanide oxidation (Lee and Srivastava, 2016). Recycling of thiosulphate solution can be a possibility; the build-up of polythionates is another issue, making it necessary to use a minimal reagent. Moreover, ammonia also poses environmental issues both in gaseous and liquid form. The threshold limiting value (TLV) for gaseous ammonia in air is 14 mg/m’ (Gos and Rubo, 2000), whereas its toxicity in water is similar to chlorine; hence, strict precautions are needed for thiosulphate leaching.