**1. Introduction**

The mining activity in Mexico has focused mainly on gold and silver ores, the processing of which entails grinding, froth flotation, and cyanidation stages. While the latter involves low operation costs, the reagent utilized is highly toxic, which has even been outlawed in several countries worldwide [1–3]. Moreover, particular operational challenges may arise, wherein the lasting cyanidation process can require up to 24 h [3]. This impairs the leaching efficiency since refractory minerals can be produced that encapsulate in small pyritic and quartz-type particles [4,5], and some cyanide-consuming minerals may also appear, hampering the suitable extraction of metallic contents [5–7].

To date, several studies have been carried out to find leaching reagents that can replace cyanide, falling into chloride, thiourea, and thiosulfate [8,9]. Chloride is corrosive and can cause hazardous working conditions, together with low selectivity during the extraction process [4,6,8]. Processing with

thiourea is expensive, and the background indicated that it is a potential carcinogen reagent [10,11]. However, leaching using thiosulfate is an economical and promising method for ore treatment. It offers high reaction selectivity during the process, reduced environmental risks, and low corrosive solutions, low price, offering also an efficient dissolution medium for refractory ores [12–19].

Several studies have displayed good performance on the use of thiosulfate instead of cyanide to extract gold and silver [20–23]. However, a particular challenge persists that is mostly related to the low stability of thiosulfate ions, where proper alternative considers the solution of copper–ammonium–thiosulfate. The Cu2<sup>+</sup> ions may oxidize the gold and silver, while the thiosulfate makes quite stable complexes with them, which might allow a suitable extraction from ores and waste. Also, the ammonia ions can also form a stable complex with copper ions, avoiding their precipitation [9]. Some studies have shown their performance in the presence of additives and electrolytes, considering the effect of ligands and oxidants reagents [24–27].

Recent studies have found that during the kinetic of silver leaching using thiosulfate, the overall process is controlled by the mass transfer of oxygen to the solid-liquid interface [28]. Additionally, other similar studies done for silver leaching have concluded that for different ranges of concentrations of ammonia and thiosulfate, silver complexes preferentially with thiosulfate [29–32]. On the other hand, the only company that at present is using the thiosulfate solutions for gold leaching is Barrick Gold Corporation (Elko, NV, USA), after an acidic or alkaline pressure oxidation pretreatment [33,34]. The carbonaceous gold ore cannot be treated with cyanide due to the "pre-robbing" phenomena, which does not occur during the leaching using thiosulfate solutions. The weak affinity of carbonaceous material for the gold thiosulfate complex forwards this stage [35]. Some authors have gotten extractions of 11% for gold and 21% for silver using both ammonium thiosulfate with the addition of H2O2 and cupric ions [36]. On the other hand, thiosulfate easily can be decomposed by some factors, like the Cu(II) content and the presence of different minerals like pyrite and hematite, promoting also high amounts of thiosulfate consumption during leaching process, generating diverse polythionates and at the end, S0, S2−, and SO3 <sup>2</sup>−, which can be deposited on the surface of mineral, passivating the dissolution of the metals of interest [9]. Therefore, this work aimed to analyze the dissolution kinetics of gold and silver from a sedimentary ore, where silver can be presented as metallic and/or sulfur, and gold is joint to carbonaceous material. Air-Na2S2O3 <sup>2</sup><sup>−</sup> solutions were used without adding cupric ions, which allowed evaluating how thiosulfate could extract these metallic values. The ore considered in this study might have some trace copper contents (less than 10 ppm, perhaps like chalcopyrite) that acted as an oxidizing reagent, improving the gold and silver leaching. Finally, the mechanisms that control the chemical reactions of both metals were discussed.

#### **2. Materials and Methods**

#### *2.1. Materials*

An ore, located at the northeast of the State of Hidalgo, Mexico, was collected selectively [37], taking 50,000 g of each sample in 4 different points of the mineralized zone of the outcrop. Samples collected were mixed and quartered, taking a representative sample to carry out the kinetics leaching study. The mineralogical characterization was executed to get accurate data of the phases present, for which an analysis of general phases was proposed through X-Ray diffraction (XRD). The samples were ground up to get an average particle size, less than 78 μm, and kept in an Equinox 2000 X-ray Diffractometer (INEL, Artenary, France, located at UAEH) with CoKα<sup>1</sup> radiation. The identification of present phases was executed using the COD Inorganics 2015 databases, which is included in the crystallography open database match software (v.1.10, Crystal Impact, Bonn, Germany).

The scanning electron microscopy (SEM) identified texture, particle sizes, and morphology of the detected phases. The semi-quantitative and punctual analysis was done by energy dispersive spectrometry of X-ray (EDS) (OXFORD Instruments, Oxford, UK). It determined the punctual and semi-quantitative composition of the previously identified particles, using a JEOL scanning electron microscope JSM-IT300 (JEOL Ltd., Tokyo, Japan, located at UAEH, Apan, Mexico)) and an OXFORD X-ray detector (OXFORD Instruments, Oxford, UK) with 30 kV of acceleration voltage. The analysis of samples was done using powders of the sample placed in uniform layers, where punctual semi-quantitative routines were done on scanning areas about 4.5 mm2.

An inductively coupled plasma spectrometry (ICP-MS) analysis was performed by Actlabs (Activation Laboratories Ltd., Ancaster, ON, Canada); this determined the average total rock composition of the mineralized phase, where the positive anomalies of the light rare earth and mineral contents of the platinum group (PGE) were found. In this case, the samples were fused and then diluted and analyzed by a Perkin Elmer Sciex ELAN 9000 ICP-MS spectrometer (Located at Actlabs, Ancaster, ON, Canada). Fused blank was run in triplicate every 10 samples, and then the instrument was recalibrated after every 44 samples.

An analysis by copelation determined the Au and Ag contents. This was executed in an oven EMISON brand, CL Series (Located at UAEH, Pachuca, Mexico). The sample preparation was done utilizing borax, PbO, bone ash, sodium carbonate as flux. The melting temperature was of 1273 K (1000 ◦C) during 90 min. In the slag separation, a button contained the values of Ag and Au. The release of these metals was executed in porcelain crucibles on a heating plate at 313 K (40 ◦C), adding 15% nitric acid to obtain a solution of silver nitrate. Then, the release of Au and Ag was done using regal water, adding 10% of hydrochloric acid in test tubes. The determination of Au and Ag contents was done using an ICP-OES Varian brand 735ES ICP (Located at UAEH, Pachuca, Mexico), where samples were analyzed with a minimum of 10 certified reference materials, all prepared with sodium peroxide fusion. Every 10 samples were analyzed by duplicate, and the blank was renewed after every 30 samples measurement. For the kinetics study, the samples were ground during 360 s at a working speed of 150 s−<sup>1</sup> with a ball charge of 10,230 g, a pulp charge of 2100 g, and a volume of 9.5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> m3 of water in the ball mill "Denver" (Located at the UAEH-Mexico). Then, wet sieve determined the particle size distribution. Finally, the samples were separated for the next stage of kinetics leaching.

#### *2.2. Experimental Procedure*

The experiments for the kinetics study were executed in a 0.001 m<sup>3</sup> flat bottom glass reactor mounted on a hot plate having a magnetic stirring system and coupled to a pH meter. The pH was monitored and adjusted continuously with NaOH solution at a concentration of 200 mol·m−3. A thermocouple attached to the hot plate controlled the temperature. All assays were done at an open atmosphere with vigorous mechanical stirring (500 s−1). The chemical reagents were of analytical grade, and distilled water was used for the preparation of solutions. Thus, leaching reagent was added like sodium thiosulfate from Sigma MEYER brand with an essay quality of [Na2S2O3·5H2O] of 99.5–100%.

For the leaching of silver, the kinetics leaching s using thiosulfate solutions was done using the following experimental conditions: concentration of sodium thiosulfate [Na2S2O3], 200 to 500 mol·m<sup>−</sup>3; temperature range, 298 to 328 K; mineral weight, 40 g·m3; pH range, 7 to 10; volume of reaction, 0.0005 m3; stirring rate, 500 s−1. This allowed comparing with previous results on mining waste [38]. For the leaching of gold, the kinetics study was executed under the following conditions: concentration of [Na2S2O3], 32 to 320 mol·m<sup>−</sup>3; temperature range, 298 to 323 K; mineral weight, 40 g·m<sup>−</sup>3, pH range, 9.5 to 11; volume of reaction, 0.0005 m3; stirring rate, 500 s−1. This followed recommendations stated previously [39,40].

The progress of all experiences was followed by sampling at pre-set times (0–14,400 s) throughout the experiment, and then the dissolved Au and/or Ag were analyzed by ICP and AAS. Mathematical calculations corrected variations in the mass balance in the sampling addition of a reagent.

#### **3. Results**

#### *3.1. Mineral Characterization*

The XRD analysis (Figure 1) showed that the mineral species involved were characteristics of a sedimentary exhalative ore [37]. This was principally composed of quartz, ilmenite, and monazite, with some contents of precious metals, such as Pt, Pd, Au, and Ag. Some light rare earths could be present (Table 1), and also trace contents of base metals sulfurs of Pb, Zn, Cu, chalcopyrite, and pyrite, associated with organic material (carbonaceous substance) with attractive contents of gold (5 g/ton) and silver (25 g/ton), determined by ICP, AAS, and cupellation test (Figure 2). All the above gave additional value to this ore.

**Figure 1.** XRD spectra of sedimentary ore, used for kinetic study of gold and silver leaching.

**Table 1.** Average chemical composition of mineral executed by ICP-OES/MS, XRF, SEM-EDS, and AAS.


The morphology obtained by SEM-SE showed irregular shape particles, typical of the mineral species mentioned above, that presented irregular faces in similar dimensional style, having sizes that vary from 30 to 354 μm. Figure 3 shows a general image of ore particles used in this study, also showing the size distribution of particles of this material and the anhedral type of morphology. A detailed zone from where an EDS was executed, revealing the presence of Au and Ag, is also shown in Figure 3. The particle size distribution is shown in Table 2, where the predominant particle (77.6%) had 44 μm of diameter.

**Figure 2.** Image of the button obtained by copelation and the corresponding XRD spectra.

**Figure 3.** The general image of the ore particle size distribution, and detailed zone with an SEM-EDS analysis (SEM-SE), similar for all samples used in this study.


**Table 2.** Ore particle size distribution.

## *3.2. Kinetic Study of Gold and Silver Leaching*

#### 3.2.1. Nature and Stoichiometry of Reactions

The experimental conditions for silver leaching were: concentration of [Na2S2O3], 500 mol·m<sup>−</sup>3; temperature, 298 K; pH, 9; mineral weight, 40 g·m−3; stirring rate 500 s−1; the volume of reaction, 0.0005 m3, reaching a maximum silver recovery of 80%. For gold leaching, the experimental conditions were the following: concentration of [Na2S2O3], 130 mol·m–3; temperature, 298 K; pH, 9.5; mineral weight, 40 g·m<sup>−</sup>3; stirring rate 500 s−1; the volume of reaction, 0.0005 m3, with a maximum recovery of

20%. Figure 4 shows a representation of the leached element (Ag or Au) versus time. For silver, there was no induction period, and the reaction started immediately, describing the progressive conversion until the end of the reaction, where it appeared a stabilization zone (Figure 4A). For gold leaching (Figure 4B), the graph showed a curve type "S" with a small period of induction, then a period of progressive conversion, a stabilization zone. The existence of a short induction period could be caused by the presence of pyrite, which could influence thiosulfate decomposition, leading to a slow gold dissolution.

**Figure 4.** Graph of the curve type "S", showing the period of progressive conversion for; (**A**) silver leaching and (**B**) gold leaching, in thiosulfate solutions.

The results showed in Figure 4 were managed according to the core model for diffusive (Equation (1)) and chemical control (Equation (3)) [41–43], determining which kinetics leaching model fit better.

$$\left[1 - \frac{2}{3}X - (1 - X)^{\frac{2}{3}}\right] = \left.k\_{\text{exp}} \times t\right|\tag{1}$$

where

$$k\_{\rm exp} = \frac{2V\_M \, D\_\rm c \, c\_A}{r\_0^2} \tag{2}$$

$$\begin{bmatrix} 1 - (1 - X)^{\frac{1}{5}} \end{bmatrix} = \begin{array}{c} k\_{\exp} \times t \end{array} \tag{3}$$

where

$$k\_{exp} = \frac{V\_M \, k\_q \, \mathcal{E}\_A^{\rm u}}{r\_0} \tag{4}$$

*X* is the reacted fraction of Ag or Au, *VM* is the molar volume of the mineral, *cA* is the concentration of leaching reactant (in this case thiosulfate), *De* is the diffusion coefficient through the product layer, *kq* is the kinetic coefficient, *r*<sup>0</sup> is the initial radius of particle (in average), *kexp* is the experimental constant, and, finally, *n* is the order of reaction.

Figure 5 shows the results for silver leaching, considering the fit of experimental data to (A) core model for diffusive control and (B) core model for chemical control. A better representation of the diffusive control model could be appreciated. The same behavior occurred for gold leaching in the thiosulfate solution, and then the kinetics study was analyzed according to the core model for diffusive control.

**Figure 5.** Treatment of silver leached data with kinetic models; (**A**) diffusive control and (**B**) chemical control.

3.2.2. Stoichiometry of Leaching of Gold and Silver

Mineralogical complexities prevented the determination of stoichiometry for gold and silver leaching system under study. However, a theoretical estimation based on the characterization results determined the presence of gold and silver in the sedimentary ore. These appeared in native form for both metals and like sulfur for the case of silver.

For the case of metallic gold:

$$4Au(s) + 8(\mathcal{S}\_2\mathcal{O}\_3)aq^{-2} + \mathcal{O}\_2\mathcal{g} + 2H\_2\mathcal{O}aq \to 4Au(\mathcal{S}\_2\mathcal{O}\_3)aq^{-2} + 4OH(aq)^- \tag{5}$$

For the metallic silver:

$$2\text{Ag}\_{(s)} + 4\text{(S}\_2\text{O}\_3\text{)}\_{(aq)}^{-2} + \text{O}\_{2(g)} + 2\text{H}\_2\text{O}\_{(aq)} \rightarrow 2\text{Ag}(\text{S}\_2\text{O}\_3\text{)}\_{(aq)}^{-3} + 4\text{OH}^-\_{(aq)}\tag{6}$$

When silver is like silver sulfide:

$$4\,\mathrm{g}\_2\mathrm{S}\_{(s)} + 4\,(\mathrm{S}\_2\mathrm{O}\_3)^{-2}\_{(aq)} + \,\mathrm{O}\_{2(g)} + 2\,\mathrm{H}\_2\mathrm{O}\_{(aq)} \to 2\,\mathrm{Ag}(\mathrm{S}\_2\mathrm{O}\_3)^{-3}\_{(aq)} + \,\mathrm{S}^{+2}\_{(s)}\mathrm{4OH}^-\_{(aq)}\tag{7}$$
