Leaching of silver contained in mining tailings, using sodium thiosulfate: A kinetic study

    Leaching of silver contained in mining tailings, using sodium thiosulfate: A kinetic study ´ Eleazar Salinas-Rodr´ıguez, Juan Hern´andez- Avila, Isauro Rivera-Landero, Eduardo Cerecedo-S´aenz, Ma. Isabel Reyes-Valderrama, Manuel CorreaCruz, Daniel Rubio-Mihi PII: DOI: Reference:

S0304-386X(15)30155-9 doi: 10.1016/j.hydromet.2015.12.001 HYDROM 4237

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

17 June 2015 24 November 2015 1 December 2015

´ Please cite this article as: Salinas-Rodr´ıguez, Eleazar, Hern´ andez-Avila, Juan, RiveraLandero, Isauro, Cerecedo-S´aenz, Eduardo, Reyes-Valderrama, Ma. Isabel, CorreaCruz, Manuel, Rubio-Mihi, Daniel, Leaching of silver contained in mining tailings, using sodium thiosulfate: A kinetic study, Hydrometallurgy (2015), doi: 10.1016/j.hydromet.2015.12.001

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ACCEPTED MANUSCRIPT Leaching of silver contained in mining tailings, using sodium thiosulfate: A kinetic study

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Eleazar Salinas-Rodríguez1,2*, Juan Hernández-Ávila1, Isauro Rivera-Landero1, Eduardo

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Cerecedo-Sáenz1, Ma. Isabel Reyes-Valderrama1, Manuel Correa-Cruz2 & Daniel Rubio-

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Mihi2

Universidad Autónoma del Estado de Hidalgo, Área Académica de Ciencias de la Tierra y Materiales.

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Carretera Pachuca – Tulancingo, Km 4.5 s/n, Mineral de la Reforma. Hgo., México., www.uaeh.edu.mx

Universidad Técnica de Esmeraldas – Luis Vargas Torres. Facultad de Ingeniería y Tecnologías. Ciudadela

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C.P. 42184.

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Universitaria - Nuevos Horizontes, Esmeraldas Ecuador. www.utelvt.edu.ec

* Corresponding author. [email protected], [email protected],

ACCEPTED MANUSCRIPT ABSTRACT Several years of mining in the Pachuca y Real del Monte mining district, has lead to the

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production of a great amount of mining tailings and is currently a great environmental

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problem. However, these residues contain metals of value and interest that cannot be recovered using harmful and aggressive reagents such as cyanide. Thiosulfate solutions

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represent a good alternative because of the advances made during the last few years of

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research on this subject. The kinetic study carried out indicates that the process involved in the leaching treatment of mining tailings for silver was only slightly affected by the stirring

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rate in the range studied; the reaction orders were 1 (oxygen partial pressure 0.2–1 atm), 0.074 (thiosulfate concentration 100-500 mol•m-3), 0.455 (pH 4-12) and 0.26 (Copper

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concentration 50-300 mol•m-3). The apparent activation energy of the process was 1.912

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kJ•mol-1 in the temperature range from 288 to 318 K. A change of control seemed to occur

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above 318 K, possibly due to the destabilization of oxygen in the inner portion of the solution at higher temperatures. According to the results, kinetic is controlled by a mass transfer of oxygen at the solid – liquid interface. In the presence of copper ions and oxygen, the reaction rate increases, so the process was carried out using a stoichiometric excess of oxygen. The process was applied to a mining tailing, containing both metallic and silver sulfides, having a similar behavior to that observed by previous authors using silver powder and silver plate.

Keywords: Silver recovery, leaching of tailings, kinetic study, controlled by diffusion.

ACCEPTED MANUSCRIPT 1. INTRODUCTION In Mexico, mining has been one of the most important economic activities since the pre-

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Hispanic era to the present day; traditionally, this activity has focused more on gold and silver ores. Processing of these types of ores has included technologies ranging from “The

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Benefit of Patio” and Pachuca tanks, to circuits of processing, which involve milling, froth

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flotation and cyaniding. In Hidalgo State (Mexico), during more than 458 years of mining,

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these technologies have lead to the generation of a great amount of tailings. Today these technologies produce almost 110 million tons of tailings of economic importance

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(Hernández et al., 2007; Salinas et al., 2006), because these types of residues contain valuable gold and silver (25 to 120 g of Ag per ton and 0.3 to 1.5 g of Au per ton)

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(Espinosa, 1984; Geyne & Fries, 1963; Hernández et al., 2007; Salinas et al., 2006).

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However, this type of waste presents some problems during its treatment, the most

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important being the presence of pyritic and quartz-type minerals where silver and gold contents are encapsulated in small particles (-75 µm) (Geyne & Fries, 1963). This causes difficulty in the extraction of such valuable metals. Furthermore, the presence of some elements and minerals that consume cyanide does not allow a suitable extraction of the valuable content in many cases. In the same way, the silver contained in the refractory ores, is a metallurgical problem for various conventional extraction processes (Abbruzzese et al., 1995; Geyne & Fries, 1963; Patiño & Ramírez, 1991,). To date, several studies on leaching have been carried out, and the most studied systems have been those based on cyanide use. Very few are based on thiosulfate media [Na2S2O32-] for silver recovery, particularly in the treatment of tailings in Hidalgo State (Mexico). The cyaniding system has been used the most for more than 100 years principally due to the high capacity of the CN- ions to form silver complexes. For this process, the oxidizing

ACCEPTED MANUSCRIPT agent used is the oxygen or air. However, this system has two disadvantages: the first one is related to its environmental impact, and the second is related to the presence of some

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species inert to the cyaniding process, known as refractory ores. It is well known that

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cyanide is toxic, and the contamination due to this reagent is high with alarming figures in terms of its impact on human health. Thus, currently its use worldwide is very restricted

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(Jeffrey et al., 2002). Another reason to search for an alternative to the use of cyanide as a

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leaching reagent is the need to increase the extent of dissolution of precious metals from the refractory ores where cyanide is less selective and causes low metallic recovery. Systems

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based on leaching using thiosulfates are considered to be non-toxic alternatives to the conventional processes such as cyaniding (Hernández, 2013; Rivera, 2003; Schmitz et al.,

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2001) and an efficient alternative dissolution medium for refractory ores (Rivera, 1994).

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One of the advantages of this method compared to the use of cyanide is the high selectivity

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for silver extraction from refractory ores. The low stability of thiosulfate ions is a great disadvantage of the process. Some previous works have studied the solution of copper – ammonium – thiosulfate as a leaching system of great potential, where Cu2+ ions oxidize the noble metals while the thiosulfate forms stable complexes with them. At the same time, ammonium ions form a stable complex with copper ions avoiding their precipitation process. In addition to the role of ligands and oxidants during the thiosulfate leaching of gold, the effect of additives and electrolytes on the dissolution of gold in thiosulfate solutions has also been studied (Senanayake, 2005; Cahndra and Jeffrey, 2004; Senanayake, 2012a,b). However, the chemistry and kinetics of such processes actually presents a challenge in the metallurgical industry. This is because recent studies have indicated that the use of thiosulfate as a leaching reagent could have important drawbacks, especially during silver leaching. Some researches (Rivera et al., 2015) have found that the

ACCEPTED MANUSCRIPT kinetics of leaching of silver using thiosulfate are controlled by the mass transfer of oxygen to the solid – liquid interface. Other studies have also been conducted on silver and silver

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sulfide leaching in a thiosulfate – ammonium – cupric ion system (Briones and Lapidus,

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1998; Deutsch and Dreisinger, 2013a) and in the presence of EDTA (Puente-Siller et al., 2013, 2014). All of these authors concluded that for high and low concentrations of

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ammonia and thiosulfate, silver combines preferentially with thiosulfate. Therefore, in this

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work, the kinetics of dissolution of silver contained in the tailings of the mining industry was studied using O2– Na2S2O32- as a silver dissolution medium instead of the conventional

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cyaniding process. The novelty of this work is related to the additional use of copper as an oxidizing reagent to improve silver leaching, and the system is now used to leach real

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minerals that were emitted as waste from the mining metallurgical industry.

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2. MATERIALS AND EXPERIMENTAL PROCEDURE 2.1.Materials For this study, the dump “Dos Carlos”, which is located in the urban zone of Pachuca city, Hidalgo (Mexico), was selected for sampling. Four representative samples, each of 50,000 g, were taken. The mineral used was previously subjected to a chemical and mineralogical characterization, by techniques such as X–ray diffraction (XRD), X–ray fluorescence (XRF), emission spectroscopy using inductively coupled plasma (ICP) and atomic absorption spectrometry (AAS). Then, the samples were ground for 480 seconds at a working speed of 71.87 s-1 (rpm) with a ball charge of 10230 g, a pulp charge of 2097.77 g and a volume of 9.3 x 10-4 m3 of water in the ball mill “Denver”. Then, samples were collected for the leaching study.

ACCEPTED MANUSCRIPT 2.2.Experimental Procedure All leaching experiments were carried out in a 0.001 m3 flat-bottom glass reactor mounted

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on a hot plate with magnetic stirring (750 s-1) and coupled to a pH meter. In this system, the pH was continuously measured and adjusted by the addition of controlled amounts of

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NaOH at a concentration of 200 mol • m-3. In the same way, ultra-high-purity oxygen was

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used to maintain a constant amount of oxygen dissolved in the solution and injected into the

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reactor. The flow was measured by a flow meter, and the partial O2 pressures used were 1 and 0.2 atm. This was done to maintain a controlled atmospheric pressure inside the

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solution. The temperature was controlled by a thermocouple attached to the hot plate. The kinetic study using thiosulfate was carried out under the following experimental

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conditions; concentration of [Na2S2O3], 100 to 500 mol • m-3; temperature range, 288 to

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333 K; mineral weight for each experiment, 40 g • m-3; pH range, 4 to 12; volume of

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reaction, 0.001 m3; stirring rate, 200 to 800 s-1; and concentration of Cu2+, 50 to 300 mol • m-3, which was added as cupric sulfate, 5-hydrate, reagent grade (J.T. Baker). The progress of the extraction reaction was monitored by sampling at pre-fixed times (from 0 to 14400 seconds) throughout the experiment and then analyzing the dissolved Ag by AAS. Variations in the mass balance due to sampling and the addition of reagent were corrected by simple mathematical calculations.

3. RESULTS AND DISCUSSION 3.1. Materials Characterization The characterization using XRD, XRF, SEM and EDS, confirmed that the majority of the mineral in mining tailings contains the following principal species: quartz, alumina,

ACCEPTED MANUSCRIPT hematite and potassium oxide, with minor percentages of calcium oxide, sodium oxide, lead, zinc, copper, silver and gold among others. Table 1 shows the chemical composition

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of this material, which was obtained using XRF, ICP and AAS.

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The morphology imaging showed particles of irregular shapes characteristic of quartz-type

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material, presenting irregular faces in an equal dimensional style with sizes varying from 177 to 37 µm. Figure 1 shows a general image of the particles distribution that represents

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this type of material.

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The distribution of particle sizes and the economic values (Ag and Au) is shown in Table 2. The size distribution results demonstrate that the mineral ores were treated with

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conventional metallurgical processes and hence confirm the presence of silver and gold in

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economically viable quantities. Due to the presence of relatively large sizes, with a standard

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size distribution of 80 mesh (177 µm) and an average size of mesh 200 (74 µm), these types of particles were not able to release silver and gold completely, as shown in Table 2. 3.2. Kinetic study of leaching 3.2.1. Stoichiometry of leaching Due to the complex mineralization in these dumps, it was quite difficult to determine the stoichiometry of the leaching system under study. Therefore, theoretical stoichiometry based on the results of characterization was used to estimate the presence of the silver in the mining tailings either as native silver or silver sulfide. For the case of metallic silver: 2Ag(s) + 4(S2O3)2-(aq) + 2H+(aq) + ½O2  2Ag(S2O3)23-(aq) + H2O(aq) For the case of the silver sulfide:

- (1)

ACCEPTED MANUSCRIPT Ag2S(s) + O2 + 4(S2O3)2-(aq) + 4H+(aq)  2Ag(S2O3)3-2(aq) + S2+ + 2H2O(aq)

- (2)

3.2.2. Effect of oxygen partial pressure

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For this part, two experiments were conducted at different oxygen partial pressures using the following experimental conditions: stirring rate, 600 s-1; temperature, 298 K; thiosulfate

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concentration, 500 mol • m-3; concentration of Cu2+, 200 mol • m-3; pH of 10 and 40 g • m-3

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of mineral. The oxygen partial pressures tested were PO2 = 0.2 atm and PO2 = 1.0 atm. Figure

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2 shows silver leaching versus time for these two experiments. According to the observations, the apparent order of the reaction is n = 1, indicating that the silver leaching

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rate is proportional to the oxygen partial pressure in the system. 3.2.3. Effect of the concentration of [Na2S2O3]

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The study of silver extraction in the Na2S2O3 - O2 system from dumps was investigated to

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determinate the influence of the [Na2S2O3], stirring rate, temperature, pH and [Cu]2+

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according the experimental conditions described in section 2.2. Figure 3 represents the fraction of Ag leached and evaluated using the decreasing core model (Patiño et al., 2007; Pérez et al., 2012; Salinas et al., 2001, Salinas et al., 2012; Shon & Wadsworth, 1986), which is [1 – ( 1 – XAg )1/3] for all of the concentrations of thiosulfates used. Straight lines whose slopes represent the experimental rate constant (kexp) are observed, and it can be seen that with an increasing concentration of thiosulfate, the reaction rate increases slightly. For this case, oxygen was injected to saturate the system 15 minutes before the beginning of each experiment, and it was maintained in stoichiometric excess during the progress of the reaction. According to the above discussion, oxygen acts more like an oxidant than Cu2+, which could be the reason for the poor effect of silver dissolution with respect to thiosulfate concentration, leading to a low order of the reaction.

ACCEPTED MANUSCRIPT In the same way, it was found that the maximum recovery of silver was of 77.9% at a concentration of 500 mol • m-3 of [Na2S2O3].

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The effect of [Na2S2O3] on the rate of silver leaching is shown in Figure 4. It can be seen

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that the order of the reaction is almost 0 for all of the concentration ranges of the reagent (n = 0.074). This indicates a poor effect of concentration on silver leaching. This phenomenon

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could be attributed to the use of low concentrations of thiosulfate and the use of excess

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oxygen in the system because the rate of silver leaching essentially depends of the complexing reagent and the partial pressure of oxygen. Thus, the rate of silver leaching

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could be increased at higher concentrations. However, as seen in Figure 4, in the range of thiosulfate concentrations used here, the rate of leaching remained stable and was not

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affected by the concentration itself. Thus, the rate of leaching will depend only on the

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concentration of the dissolved oxygen in the system and the diffusion of species after the

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reaction.

3.2.4. Effect of temperature The effect of temperature is shown in Figure 5. This shows a poor effect of temperature over the reaction rate of silver leaching in the range of 288 to 318 K; above 318 K, there appears a change in the effect of temperature, which is indicative of the apparent change of control in the global reaction. The above results could be possible due to the instability in the system of oxygen at high temperatures leading to a possible change from diffusive to chemical control in the reaction, as was demonstrated by Senanayake (2004, 2005), who used Cu2+ Fe2+ as an oxidant and observed that the temperature had a very important effect. However, it will be necessary to evaluate in more detail what happens at higher temperatures.

ACCEPTED MANUSCRIPT The activation energy of the system was calculated by plotting the natural logarithm of kexp, as a function of the reciprocal of temperature. The slope of the linear fit of the experimental

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data represents the activation energy (Ea) divided by the negative value of the universal gas constant, as shown in Figure 6. For this case, the energy calculated for this system had two

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values; the first one, in the temperature range from 288 to 318 K, was Ea = 1.912 kJ • mol-1,

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and then from 318 to 328 K, the energy of activation increased to 22.51 kJ • mol-1. Both

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activation energy values are indicative of diffusion control for silver leaching, and because the order of the reaction is known, we can assume that the process is independent of the

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concentration and temperature. Thus, the chemical reaction is fast, diffusion of the products

controls the overall process.

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from the particle surface to the inner depths of the solution is very slow, and this stage

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3.2.5. Effect of the stirring rate

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The results obtained during the study of the effect of stirring rate are shown in Figure 7. It can be seen that the results and the experimental conditions employed seem to fit well with the model of spherical particles with a decreasing core (Patiño et al., 2007; Pérez et al., 2012; Salinas et al., 2012; Salinas et al., 2001; Shon & Wadsworth, 1986), showing the corresponding kexp (in s-1) obtained for all stirring rate values. Thus, we can conclude that the stirring rate had no negative effect on the silver leaching process. This indicates that once the particles have reached a suspension condition, the chemical reaction starts without problems. Figure 8 shows that in the experimental range studied, the stirring rate appears to have a slight effect on the leaching of silver. However, the previous kinetics study has already shown that the rate is controlled by diffusion. It is possible that the stirring rate can have an effect on the silver leaching rate, but due to the experimental conditions employed, an effect was not observed.

ACCEPTED MANUSCRIPT 3.2.6. Effect of pH The pH effect is presented in Figure 9 for silver leaching under the experimental conditions

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studied as noted above. Little effect of this variable on the overall reaction rate was

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observed. The kexp dependence is presented with respect to the pH values, with a major rate of silver leaching at a pH of 12. Figure 10 confirms the influence of pH on the overall

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reaction rate, showing an order of reaction of n = 0.455. In this case, although the

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concentration of S2O32- strongly depends on the pH values, it is valid only below a pH of 4 and above a pH of 12, where thiosulfate degradation can occur, especially below a pH of 4

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with the subsequent formation of elemental sulfur (Rivera et al., 2015). For this reason, the study was carried out in the range of pH values from 4 to 12, where the thiosulfate is more

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stable and has no an important dependence on the pH.

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3.2.7. Effect of concentration of Cu2+

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Figure 11 shows the effect of the concentration of Cu2+ over the rate of silver leaching. It shows that with an increasing copper concentration, the rate of silver leaching also increases. Thus, a silver recovery of 96.43% was reached for the highest studied concentration of Cu2+. Conversely, Figure 12 shows the dependence of kexp versus log of the concentration of Cu2+. It shows that the order of the reaction is n = 0.26 for this variable, which may validate the use of Cu2+ during this process to achieve important and selective silver leaching. This could also be due to a reaction that occurs in parallel with silver oxidation, leading to the reduction from Cu2+ to Cu+ (equation 3) and promoting proper oxidation of silver. According to Rivera et al., (2015), in the presence of oxygen and copper, the reaction rate of leaching with thiosulfates increases up to a 30 % compared to the process where copper was not used. In the same way, the various reactions run in parallel that are principally related to thiosulfate decomposition could include the formation

ACCEPTED MANUSCRIPT of elemental sulfur (equations 4 and 5), as was proposed by Alymore and Muir, (2001). The maximum recovery of silver was of 96 % at 300 mol • m-3 of Cu2+ and a temperature of 298

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K. - (3)

3 S2O32- + H2O  2 SO42- + 4 S0 + 2 OH-

- (4)

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Ag0(s) + Cu2+(aq) + 5 S2O32-(aq)  Ag(S2O3)23-(aq) + Cu(S2O3)35-(aq)

- (5)

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S2O32-  SO32- + S0

4. CONCLUSIONS

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The leaching reaction of silver from the burrows, in the oxygen – thiosulfate – copper system involves two types of silver compounds that were adequately leached (metallic

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silver and silver sulfide).

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The order of the reaction with respect to the thiosulfate concentration was n = 0.074 in the

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range studied, showing a poor effect on the silver leaching rate. This could be because oxygen was injected to saturate the system during the 15 minutes prior to each experiment and was then maintained at a stoichiometric excess during the progress of the reaction. According the above results, oxygen acts more like an oxidant than Cu2+. This could be the reason for the poor effect of silver dissolution with respect to the thiosulfate concentration, leading to a low order of the reaction. The apparent activation energy in the temperature range of 288 to 328 K was 1.912 kJ • mol-1. The order of the reaction with respect to the oxygen partial pressure was 1. The kinetics are controlled by mass transfer of oxygen at the solid–liquid surface. Although the stirring rate appears to have a slight effect on the leaching of silver, because of the experimental conditions employed, this effect was not observed.

ACCEPTED MANUSCRIPT The reaction order with respect to pH was n = 0.455. Although the concentration of S2O32strongly depends on the pH, the dependence is only in the pH range of 4 to 12, where

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thiosulfate is more stable and hence, the dependence on the pH is not important.

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The reaction order with respect to the concentration of Cu2+ was n = 0.26. This showed an increase in the silver leaching rate. This is because in the presence of oxygen and copper,

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the reaction rate of leaching with thiosulfates increases up to a 30 % compared to the

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process in which copper was not used. In the same way, the various reactions that occurred in parallel and were principally related to thiosulfate decomposition, which include the

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formation of elemental sulfur and the reduction from Cu2+ to Cu+, promote proper oxidation of silver. The maximum recovery of silver was of 96 % at 300 mol • m -3 of Cu2+ and a

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temperature of 298 K.

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According to the results, this work shows a real possibility of using the oxygen-thiosulfate

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system for silver leaching with copper as an oxidizing reagent in this system to improve the kinetics conditions of the leaching process. This system is now used for leaching of silver involved in a real mineral, and in this case, the mineral is considered to be a waste from mining or the metallurgical industry, which represents a novelty.

ACKNOWLEDGMENTS The authors thank UAEH & FOMIX (CONACyT – Hidalgo state government) for financial support for the projects PAI 12A and 128491, respectively. Thanks also go to the Universidad Autónoma del Estado de Hidalgo, especially to the Researches Centre on Materials and Metallurgy, for the facilities provided during the execution of this work.

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Extractiva., Editorial Trillas, México.

Schmitz P.A., Duyvesteyn S., Johnson W.P., Enloe L., McMullen J., 2001. Ammoniacal

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thiosulfate and sodium cyanide leaching of pre-robbing Goldstrike ore carbonaceous

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matter. Hydrometallurgy, 60, 1, 25-40.

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Figure 1. Overall image of the average particle size of, ( a ) 80 mesh (180 m) and ( b ) 270

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mesh (53 m) (SEM, secondary electrons)

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Figure 2. Silver leaching. Effect of the oxigen partial pressure

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Figure 3. Silver leaching. Effect of the Na2S2O3 concentration

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Figure 4. Silver leaching. Effect of Na2S2O3 concentration (order of reaction)

Figure 5. Silver leaching. Effect of temperature

Figure 6. Silver leaching. Effect of temperature (activation energy)

Figure 7. Silver leaching. Effect of the stirring rate

Figure 8. Silver leaching. Effect of the stirring rate

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Figure 11. Silver leaching. Effect of Cu2+ concentration

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Figure 10. Silver leaching. Effect of pH (order of reaction)

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Figure 9. Silver leaching. Effect of pH

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Figure 12. Silver leaching. Effect of Cu2+ concentration (order of reaction)

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ACCEPTED MANUSCRIPT Table 1. Chemical composition of tailings (FRX, ICP and AAS) Pct Weight

Ag

0.0055 (55 g/ton.)

Au

0.000058 (0.58 g/ton.)

Fe Si Mn Ca Na K Al S Zn

2.690 56.00 0.046 0.200 0.300 2.320 6.095 11.45 0.045

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Cu Pb Cd As Sr Cr Ba P Ti Traces

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Element

0.010 0.031 0.017 0.043 0.463 0.037 0.330 0.140 0.278 19.499

ACCEPTED MANUSCRIPT Table 2. Sieve Analysis of the tailings from Dos Carlos Dumps Size

Retained

Accumulated

Accumulated

Ag

Au

#

(µm)

(%)

(-)

(+)

(g/ton)

(g/ton)

+ 80

177

16.80

83.20

16.80

18

0.20

-80 +100

-177 +149

19.18

64.02

35.98

34

0.35

-100 +140

-149 +105

17.99

46.03

53.97

39

0.40

-140 +200

-105 +74

15.58

30.45

69.55

59

0.60

-200 +270

-74 +53

10.12

20.33

79.67

51

0.55

-270 +400

-53 +37

4.50

15.83

84.17

60

0.65

-400

-37

15.83

---

100

147

1.20

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Mesh

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Graphical abstract

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We execute a kinetics study of silver leaching from tailings, using

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thiosulfate.

Reaction orders are 1 (oxygen), 0.07 (thiosulfate), 0.45 (pH) and 0.26 (copper).

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The activation energy in the range of temperatures 288-328 K, was 1.91 kJ•mol-1.

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The kinetics is controlled by, mass transfer of oxygen, in the solid–liquid surface.

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The presence of Cu2+ in the system increases silver leaching rate.

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