Acid decomposition and silver leaching with thiourea and oxalate from an industrial jarosite sample

Journal Pre-proof Acid decomposition and silver leaching with thiourea and oxalate from an industrial jarosite sample

D. Calla-Choque, G.T. Lapidus PII:

S0304-386X(19)30446-3

DOI:

https://doi.org/10.1016/j.hydromet.2020.105289

Reference:

HYDROM 105289

To appear in:

Hydrometallurgy

Received date:

17 May 2019

Revised date:

29 December 2019

Accepted date:

31 January 2020

Please cite this article as: D. Calla-Choque and G.T. Lapidus, Acid decomposition and silver leaching with thiourea and oxalate from an industrial jarosite sample, Hydrometallurgy(2019), https://doi.org/10.1016/j.hydromet.2020.105289

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© 2019 Published by Elsevier.

Journal Pre-proof Acid decomposition and silver leaching with thiourea and oxalate from an industrial jarosite sample D. Calla-Choque*, G. T. Lapidus Depto. Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana, Alcaldía de Iztapalapa, México City CP 09340, Mexico *[email protected] Abstract

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Jarosites are a waste product of most zinc processing plants, since these compounds are formed as a measure to eliminate iron and other impurities from zinc solutions in acidic media. However, they often occlude valuable elements, such as silver, in their structure. In the present study, an alternative is presented for the recovery of silver that is present in this type of waste. The effect of pH, temperature, oxalate concentration and solid/liquid ratio (S/L) on thiourea stability and the selective recovery of silver by acid decomposition and reductive leaching of jarosite were evaluated.

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The results show that increasing the temperature of the system favors the decomposition of the jarosite and the release of silver to the solution, obtaining recoveries of 86 and 100% silver at 50 and 60 °C, respectively, with 0.13 M thiourea, 0.2 M sodium oxalate, pH 2 and a S/L ratio of 50 g/L. At higher S/L ratios (500 g/L), an increase in the oxalate concentration to 1.2 M was necessary to achieve the decomposition of the jarosite, liberating both the iron and the occluded silver. Additionally, increasing the oxalate concentration is necessary to act as a complexing agent for the large amount of ferric and cupric ions released by the jarosite decomposition.

1. Introduction

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Keywords: Jarosite; reductive leaching; decomposition; oxalate; thiourea

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Jarosites are of interest in the zinc metallurgical industry because their formation facilitates the elimination of the iron in the leaching solution, while generating a stable and easily removable residue from the system. However, this type of waste tends to occlude important metals, such as silver, zinc and copper; the recovery of these could benefit the overall process economics. The stability of the jarosite has been studied by several authors, who show that this type of waste must be decomposed in order to recover the metals of interest. Decomposition can be carried out by thermal or hydrothermal methods (Das et al., 1996). Thermal decomposition entails disadvantages due to the necessity for high temperatures and the generation of toxic gases, such as SO2 (KerolliMustafa et al., 2016). On the other hand, hydrothermal decomposition could involve either of two alternatives routes (alkaline or acid). Alkaline decomposition is characterized by the removal of sulfate ions by the solution (Eq. 1), requiring temperatures above 90 °C (Das et al., 1995). The operation of this system requires an alkaline conditioning stage before decomposition at an elevated temperature. KFe3 (SO4 )2 (OH)6(s) + 4OH- → KOH + 3Fe(OH)3(s) + 2SO4 2-

(1)

The direct cyanidation of the jarosite achieves recoveries of less than 20%; these poor results were attributed to the refractory nature of jarosite. However, when it undergoes alkaline decomposition,

Journal Pre-proof cyanidation silver recoveries close to 80% can be attained (Gonzáles-Ibarra et al., 2016; Kasaini et al., 2008) Acid decomposition is an alternative for the recovery of metals present in the jarosite and for the formation of new products such as hematite or goethite. In addition, it has the great advantage over the previous alternatives because this type of waste is acidic nature, avoiding a stage of conditioning the medium. Acid decomposition is characterized by the release of ferric ions to the solution (Eq. 2) (Kunda et al., 1979). Jarosite decomposition is very dependent upon temperature (Reyes et al., 2017, Das et al., 1996). At 90 °C, silver dissolutions greater than 70% can be achieved (Kasaini et al., 2008; Calla-Choque et al., 2016) KFe3 (SO4 )2 (OH)6(s) + 6H+ → 3Fe3+ + K+ + 2SO4 2- + 6H2 O

Log K = -12.50

(2)

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Ag+ + 3CS(NH2 )2 → Ag(CS(NH2 )2 )3 +

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For the recovery of the metals of interest present in the jarosite, complexing agents must be employed; thiourea (CS(NH2 )2 , Tu) is a leaching agent used for the recovery of precious metals, such as gold and silver, in acid medium (Eq. 3). To dissolve these metals, it is often necessary to add an oxidant, to change their oxidation state or that of the structural anion (Li and Miller, 2006; Jing-ying et al., 2012). The oxidizing agents conventionally employed are hydrogen peroxide and ferric ion. However, ferric ions have the ability to oxidize thiourea to formamidine disulfide (CS(NH2 )(NH2 ))2 2+, FDS2+, Eq. 4), in addition to forming complexes with the thiourea (Eq. 5 and 6). Log K = 12.73

(3)

Log K = 11.83

(4)

CS(NH2 )2 + Fe3+ → (FeCS(NH2 )2 )3+

Log K = 2.19

(5)

2CS(NH2 )2 + Fe3+ → (Fe(CS(NH2 )2 )2 )3+

Log K = 8.44

(6)

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2CS(NH2 )2 + 2Fe3+ → 2Fe2+ + (CS(NH2 )(NH2 ))2 2+

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In some systems, the cupric ion (Cu2+) may be present due to the nature of the material to be treated or the operating conditions. Cupric ions also have the ability to oxidize and form complexes with thiourea as shown in Eq. 7.

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2Cu2+ + 4CS(NH2 )2 → 2Cu(CS(NH2 )2 )+ + CS(NH2 )(NH2 ))2 2+ log K = 24.04

(7)

If the presence of ferric and cupric ions is not controlled, these ions can become detrimental to the stability of the thiourea because they may increase the potential of the system, causing its oxidative destruction (Calla, 2017). According to the cited literature, acid decomposition and silver release are viable at temperatures of 90 °C to 250 °C. However, these conditions would generate a high consumption of leaching agents, such as thiourea, during the decomposition process; the release of ferric and cupric ions increase the solution redox potential (greater than 0.5 V vs SHE). Therefore, a ligand that can form complexes with these two ions is required to promote greater stability of the thiourea in the solution. In this sense, oxalate ion can form stable complexes with cupric ions (Eq. 7) and with ferric ions (Eq. 8 and 9). In addition, the reduction of ferric ions and the complexes formed with oxalate are promoted (Eq. 11 and 12) (Chandra et al., 2005) and oxalate generates a reducing medium, which improves thiourea stability.

Journal Pre-proof Cu2+ + 2 C2 O4 2- → Cu(C2 O4 )2 2-

log K = 10.23

(8)

Fe3+ + 2C2 O4 2- → Fe(C2 O4 )2 -

log K = 13.81

(9)

Fe3+ + 3C2 O4 2- → Fe(C2 O4 )3 3-

log K = 18.6

(10)

Fe(C2 O4 )3 3- + e- → Fe(C2 O4 )2 2- + C2 O4 2-

E0 = -0.025

(11)

Fe3+ + 2C2 O4 2- + e- → Fe(C2 O4 )2 2-

log K =16.25

(12)

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This investigation revalues the use of thiourea in acid medium that can be applied to complex minerals. Jarosite, upon decomposition, releases strong oxidants (principally iron) that cause chemical instability and oxidation of thiourea. With the use of oxalate, this degradative process can be minimized and the jarosite decomposition temperature decreased. Furthermore, this is a one-step leaching process, from which silver may be recovered by cementation. Simultaneous partial jarosite decomposition and silver leaching in an acid media has not been sufficiently studied, especially at high pulp densities.

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2. Experimental

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2.1 Materials and equipment

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All solutions were prepared with deionized water and reagent-grade chemicals: thiourea, sodium oxalate, potassium iodate.

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Fig. 1 shows the experimental equipment implemented where the tests were carried out in a Pyrex® beaker, covered four-necked ports to hold thermometer, pH and ORP electrode, and sampling points and submerged in a temperature-controlled water bath. The initial tests with 5 g in 100 mL used magnetic stirring with temperature control; However, when the solids were increased (5, 10, 20, 30 and 50 g in 100 mL), mechanical stirring (Caframo) at 350 RPM was employed to insure a homogeneous dispersion of the solid particles. The pH and the potential of the solution (ORP) were monitored throughout each experiment (HANNA Instruments: HI 4112); all ORP values are reported with reference to the hydrogen electrode (SHE).

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Fig. 1. Experimental equipment, 1) mechanical stirring, 2) propeller, 3) pH, 4) ATC temperature, 5) cover, 6) Pyrex® beaker, 7) ORP electrode, 8) constant temperature water bath, 9) magnetic stirring, 10) pH/mV meter with ATC.

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2.2 Characterization of the industrial jarosite

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The jarosite used was obtained from a Mexican hydrometallurgical zinc industry. The silver content in the jarosite was obtained by fire assay and the mineralogical species present in the sample were identified by X-ray diffraction (Philips, X-Pert PW3040). The morphology and elemental composition were analyzed in an electronic scanning microscope (Philips model XL30 ESEM) equipped with an EDS XE (X-ray energy scattering spectrometry) EDAX model Genesis model. The particle size distribution was determined by laser diffraction analysis (Coulter LS-100Q); These analyzes were carried out at CINVESTAV Saltillo. The metals present in the solutions were analyzed in an atomic absorption spectrometer (Varian SpectrAA220fs).

2.3 Experimental procedure All the leaching solutions contained a thiourea concentration of 10 g/L (0.13 M) with different concentrations of oxalate. The solution pH and temperature were varied and the silver recovery, degree of jarosite decomposition and thiourea stability were evaluated. These results are presented in two sections, according to the conditions used for pH, temperature, S/L ratio and oxalate concentration (Table 1). The initial tests were performed with magnetic stirring, using 5 g of jarosite (S/L ratio = 50 g/L) in 100 mL of 0.13 M thiourea and 0.2 M sodium oxalate, at different pH values (1, 2 and 3) and temperatures (25, 30, 40, 50 and 60 °C). The second group was carried out varying the amount of jarosite from 5 to 50 g (S/L ratio = 50 to 500 g/L) in the 100 mL of the above solution. Finally, experiments were performed with 50 g

Journal Pre-proof of jarosite in 100 mL (S/L ratio = 500 g/L), varying the oxalate concentration (0.2, 0.6, 1.2 M) at different temperatures (25, 40 and 60 °C). In all experiments, aliquots were taken at set time intervals to monitor the dissolved metal concentrations (Ag, Cu, Fe). The solution ORP was monitored during leaching, thiourea concentration was quantified at the end of each test by titration with a 0.05 M potassium iodate solution, using a starch solution as the indicator. The lead concentrations were also determined in the final leach liquor. Additionally, a metallurgical balance was performed to support the analytical techniques presented in the document and showed an error of less than 1%.

Table 1. Variables and composition of test performed. Thiourea (M)

Oxalate (M)

Temperature (°C)

Time (min)

50 g/L

1, 2, 3

0.13

0.2

25, 30, 40, 50, 60

120

50, 200, 500 g/L

1

0.13

500 g/L

1

0.13

0.2

50, 60

150

0.0, 0.2, 0.6, 1.2

25, 40, 60

150

Results and Discussion

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Low

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pH

S/L ratio

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3.1 Characterization of the industrial jarosite

The chemical composition of the jarosite, presented in Table 2, shows a silver content of 158 g/t, justifying a process for silver recovery.

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Table 2. Chemical composition of the industrial jarosite.

Composition

S

% wt

Ag

Fe

Cu

Pb

Cr

158

45.94 10.4 0.756 0.95 0.005

Cd

Sb

0.188 0.0022

Zn

As

4.48 0.012

In this table, the contents of lead, chromium, cadmium, antimony and arsenic are also displayed; of these five toxic metals, the highest concentration is lead (0.95%), followed by cadmium (0.188%), arsenic (0.012), chromium (0.005%), and finally antimony (0.0022%). The particle size distribution of the jarosite, analyzed by the laser diffraction technique, reveals a distribution of relatively fine particles, with an average particle diameter of 10.3 μm and a d80 of 24 μm. This characteristic of the material renders a mechanical reduction of the particle size unnecessary. X-ray diffraction analysis of the jarosite indicated that the material is composed mostly of natrojarosite and franklinite. Figure 2 shows the transverse morphology of the jarosite, where zones of high silver concentration (a1 and b1) can be observed in the internal part of the particle, which is

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surrounded by sodium jarosite (a2). It is important to remember that sodium, iron, sulfur and oxygen are structural elements of this type of sample, suggesting that silver is contained within the natrojarosite phase.

Fig. 2. Transverse morphology of the jarosite particles (a and b), and analysis by EDS in the zone enriched in silver (a1 and b1) and of the matrix (a2).

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This analysis is confirmed by elemental EDS mapping (Figure 3), where it is observed that the particle is surrounded by sodium, originating from the jarosite. Therefore, for the silver recovery process to occur, the decomposition of the surface layer of the particle composed of the sodium jarosite is necessary.

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Fig. 3. Elementary mapping by EDS carried out in the jarosite.

3.2 Decomposition and reductive leaching of industrial jarosite Effect of temperature at low Solid-Liquid ratios

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3.2.1

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Figure 4a shows the effect of temperature on the recovery of the silver from the industrial jarosite at pH 2. Silver dissolution increases with the temperature of the solution from 25 °C to 60 °C, achieving in 120 minutes silver recoveries of 37, 43, 50, 75 and 98% at temperatures of 25, 30, 40, 50 and 60 °C, respectively. According to Eq. 2, the degree of decomposition of jarosite can be evaluated by the release of the associated cations present in its structure. In this case, Figures 5a and 5b show that, despite the difference in temperature, the release of silver into the solution is directly related to the iron and copper concentrations, and that both are indicators of the decomposition of the jarosite, which is favored by increased temperature since both metals are present in the structure of the jarosite.

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Fig. 4. Silver recovery (a), dissolution of iron (a) and copper (b) from jarosite at pH 2 at different temperatures with 5 g of jarosite in 100 mL solution of 0.13 M thiourea and 0.2 M oxalate.

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Fig. 5. Release of Ag and Fe (a), Ag and Cu (b) from jarosite at different temperatures with 5 g of jarosite in 100 mL of 0.13 M thiourea and 0.2 M oxalate solution at pH 2. Figures 5a and 4b show that to obtain a recovery of silver above 70% (0.053 mM), the iron dissolution must be greater than 23% (5.3 g/L, 0.09 M approximately). Another important aspect is the dissolution of copper present in this type of samples; under these conditions (pH 2 and 60 °C), the copper dissolution is 37.5% (141.7 mg/L) (Figure 4c). It is important to mention that the copper and ferric ions released into the solution may be detrimental to this system since these ions can cause the oxidative degradation of thiourea. However, Figure 6 shows that thiourea degradation does not occur; this behavior is related to the potential of the system, which is less than 0.45 V vs SHE, the value above which thiourea is irreversibly oxidized (Deschênes et al., 1988; Calla-Choque et al., 2016).

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Fig. 6. % thiourea remaining and the solution ORP after completion of the leach (120 minutes), for different temperatures with 5 g of jarosite in 100 mL solution of 0.13 M thiourea and 0.2 M oxalate at pH 2.

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The ORP of the solution remains low due to the presence of the oxalate ion, which forms stable complexes with copper and iron (Eq. 8, 9 and 10). This behavior may be observed in Figure 7, where a clear predominance of the complexed species Fe(C 2 O4 )2 - is shown in the pH range considered in this study and at the highest concentration of the ferric ion observed in Figure 4b. Also, the presence of oxalate promotes a reducing environment in the system, preventing the potential from increasing, therefore, favoring the chemical stability of thiourea.

Fig. 7. Species distribution diagram for ferric iron with 0.2 M oxalate as a function of pH. Conditions 0.13 M of iron and 0.2 M of oxalate at 25 °C. Diagram generated with the MEDUSA© software suite (Puigdomenech, 2004).

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According to the results, Figure 8 shows schematically the most important steps that take place during the decomposition of sodium jarosite and the release of silver and ferric ions present in its structure, as well as the formation of complexes between these ions and the oxalate in acidic pH.

3.2.2

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Fig. 8. Schematic of the acid decomposition and silver leaching with thiourea and oxalate from industrial jarosite. Effect of pH

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To evaluate the effect of pH on the silver recovery, three pH values (1, 2 and 3) were tested at different temperatures (25, 30, 40, 50 and 60 °C). In this document, only the results obtained at 50 and 60 °C are reported since the silver extractions were inferior below these temperatures. Figure 9a shows the silver recovery at a temperature of 50 °C at different pH values (1, 2 and 3), It is interesting to note that as the pH increases from 1 to 3, the stability of the jarosite is favored, which leads to a lower recovery of the silver.

Fig. 9. Silver recovery, Eh and Copper dissolver at different pH values at 50 °C (a, b, c) and 60 °C (d, e, f) with 5 g of jarosite in 100 mL solution, 0.13 M thiourea and 0.2 M oxalate.

Journal Pre-proof Increasing the temperature to 60 °C favors the both the silver dissolution kinetics as well as the limit of extraction, e.g. at pH 1 a complete recovery of silver is achieved in the first 90 minutes and 86.4% and 98.3% silver at pH 3 and 2, respectively, after 120 minutes (Figure 9d). This behavior can be related to the degree of decomposition of jarosite at 60 °C and the effect of the oxalate ion present in the system. Consequently, the following reaction scheme is proposed. Jarosite decomposes in the presence of acid, favored by increased temperatures and acidity (Eq. 13 and 14); the ferric ions released can form complexes with the oxalate ion (Eq. 9). The ferric ions released can oxidize the thiourea to formamidine disulfide (Eq. 15, (CS(NH2 )(NH2 ))2 2+). Log K = -8.56 @ 25 °C (13)

AgFe3 (SO4 )2 (OH)6 + 6H+ → 3Fe3+ + Ag+ + 2SO4 2- + 6H2 O

Log K = -11.55 @ 25 °C (14)

2CS(NH2 )2 + 2Fe3+ → 2Fe2+ + (CS(NH2 )(NH2 ))2 2+

Log K = 11.83

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NaFe3 (SO4 )2 (OH)6 + 6H+ → 3Fe3+ + Na+ + 2SO4 2- + 6H2 O

(15)

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However, if oxalate is not available to complex the ferric ions, thiourea can be irreversibly oxidized, passing in a first stage to formamidine disulfide, then to cyanamide and elemental sulfur at a potential greater than 0.5 V.

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Table 3 and Figs 9b and 9e confirm the oxalate reducing nature because its presence stabilizes the potential of the system at a value lower than 0.48 V vs SHE at pH 1 and 60 °C despite the release of iron ions to the solution (Figs 9c and 9f). Even though this potential is near the potential for irreversible oxidation, in all cases, at least 67% of the original thiourea concentration remains after 120 minutes; this fact demonstrates the stability of the thiourea present in the solution.

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It is important to point out that without oxalate in the system, a recovery of silver of 90% can be achieved, but only at pH 1 and 90 °C; however, at these conditions the potential increases to a value greater than 0.50 V vs SHE, promoting a destruction of thiourea greater than 90% (Calla-Choque et al., 2016). With the oxalate, an increase in the recovery of silver and the stability of thiourea are enhanced (Figure 9a and Table 3).

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Table 3. ORP and %Tu remaining after completion of the leaching test (120 minutes) 50 and 60 °C with an initial concentration of 0.13 M thiourea and 0.2 M oxalate and 5 g of jarosite in 100 mL of solution (S/L ratio = 50 g/L).

pH 1

pH 2

pH 3

pH 1

pH 2

pH 3

Eh (V)

0.47

0.41

0.35

0.48

0.42

0.39

% Tu

74

99

100

67

99

100

50 °C

60 °C

Another important aspect that should be mentioned is for lead the dissolution percentage in all cases is less than 4.5%, due to the presence of sulfate and oxalate ions.

3.2.3

Effect of the Solid-Liquid ratio

In this section, the percentage of silver leaching from the jarosite in the thiourea-oxalate system is evaluated, varying the solid/liquid ratio (S/L); the ratios are 50, 200 and 500 g/L (5, 20 and 50 g of

Journal Pre-proof jarosite in 100 mL) employing a 0.13 M thiourea and 0.2 M sodium oxalate solution at pH 1 and 60 °C for 150 minutes.

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Fig. 10a shows that as the amount of jarosite in the system increases, the percentage of silver recovery decreases. With a S/L ratio of 50 g/L, a total dissolution is achieved in 60 minutes; however, the extraction diminishes to much lower levels at higher S/L ratios, achieving only 44% with a S/L ratio of 500 g/L. More importantly, the initial extraction velocity is very rapid, culminating in stagnation at the highest S/L ratio. This behavior indicates that there could be a depletion of the reagent or a solubility limitation for the silver in the medium.

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Fig. 10. Silver recovery (a), iron (b), copper (c) dissolution, potential (d & e) and % Tu (f) at different solid/liquid ratio at pH 1 and 60 °C in solutions of 0.13 M thiourea and 0.2 M sodium oxalate, e and f after 150 min.

Since the degree of release of the silver is related to the decomposition of the jarosite, as mentioned in the previous section, the decomposition behavior can be associated to the amount of iron simultaneously released from this type of material in the same experiments. According to the results shown in Fig. 10a, the decrease in the percentage of silver recovery is consistent with the percentage of iron dissolution (Fig. 10b), corresponding to 46% (0.19 M Fe) for S/L ratio of 50 g/L and 23% (0.39 M Fe) and 9.56% (0.39 M Fe) for the other two S/L ratios, respectively; and coincidentally, with the release of copper (Fig. 10c), corresponding to 36% (0.009 M Cu) and 24% (0.014 M Cu), for S/L ratios of 200 and 500 g/L, respectively. As was commented above, the presence of free/uncomplexed copper and iron in the system act as oxidizing agents in the system causing a decrease in available thiourea from 60% to 20% and 18% (Fig. 10f) for S/L ratios 50, 200 and 500 g/L, respectively at 150 minute; this decrease in free thiourea may be related to the increase in potential, greater than 0.5 V vs SHE, caused by the release of these oxidizing ions.

Journal Pre-proof This increase in the amount of iron and copper in the solution is of great importance since the oxalate concentration (0.2 M) is insufficient to complex the majority of these ions and the potential increases to above 0.5 V vs SHE (Figures 10d and 10e). Therefore, the oxidative decomposition of thiourea is unavoidable (Figure 10f). For lead, the dissolution percentage in all cases is less than 4%. Therefore, the oxalate concentration in the system should be increased in concordance with the S/L ratio to complex much of the iron and copper released. These complexes could prevent these two metal ions from degrading the thiourea. 3.2.3.1 Effect of the oxalate concentration

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Although temperature and pH are parameters of great importance in the decomposition of jarosite, the concentration of oxalate is an important parameter to be evaluated. Figure 11a shows the silver recovery in thiourea solutions without oxalate and with varying oxalate concentrations. As can be seen, there is no significant difference when oxalate is not present or in a low concentration (0.2 M) with a high solid/liquid ratio (500 g/L). However, if the oxalate concentration is increased to 0.6 and 1.2 M, silver recoveries of 78.9 and 91.7%, respectively, can be achieved after 150 minutes.

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Fig. 11. Silver recovery (a), iron (a) and copper (b) dissolution at different concentration of oxalate, 0.13 M thiourea at pH 1 and 50 g of jarosite in 100 mL solution (S/L ratio = 500 g/L) at 60 °C. At oxalate concentrations of 0.2, 0.6 and 1.2 M, the iron dissolution is 10.5, 15 and 21% from the sample. These percentages represent iron concentration of 21.8, 34.5 and 48.4 g/L (Figure 11b), corresponding to 0.39, 0.62 and 0.87 M, respectively. As expected at this degree of iron release increases the potential of the system increasing the % decomposition of thiourea. However, the differences in thiourea destruction and final solution ORP are small, considering the large disparities in the silver and iron extractions. Fig. 11b shows the percentage of copper released into the solution; although the degree of dissolution is less than 24, 30 and 32% with 0.2, 0.6 and 1.2 M oxalate, these percentages represent copper concentrations of 0.92 g/L (0.014 M), 1.1 g/L (0.018 M), and 1.2 g/L (0.019 M), which promote a greater degree of oxidation of the thiourea present in the solution (Table 4). According to Calla-Choque et al. (2016), when copper ions are present in thiourea solutions, a solid compound may form which can precipitate the silver present in the solution; in this case, since the oxalate can form complexes with the copper, the formation of the solid compound could be avoided, and silver stability is preserved (Eq. 8). In addition, the presence of oxalate promotes a reducing environment, avoiding the massive destruction of thiourea by oxidation.

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Table 4. Solution ORP and % free Tu remaining after 150 minutes of leaching with different sodium oxalate concentrations, at pH 1 and 50 g of jarosite in 100 mL solution, 0.13 M initial concentration thiourea at 60 °C.

0.2 0.6 1.2

% free Tu remaining 18 16 14

Eh (V) 0.56 0.57 0.57

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3.2.3.2 Effect of temperature at the high Solid-Liquid ratio

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Fig. 12 shows the silver recovery with a S/L ratio of 500 g/L at different temperatures (25, 40 and 60 °C), confirming the importance of temperature on the decomposition of jarosite. Despite the elevated oxalate concentration (1.2 M), only 30% of silver is released at room temperature. An increase in temperature to 40 °C and 60 °C achieved recoveries of 46.2 and 91.7% silver, respectively, in 150 minutes.

Fig. 12. Recovery of silver at different temperatures at pH 1 for 50 g of jarosite in 100 mL of 0.13 M thiourea and 1.2 M sodium oxalate solutions (S/L ratio = 500 g/L).

Finally, Table 5 shows that, under these conditions, iron extraction of 8.1% (18.7 g/L Fe = 0.33 M), 12.2% (27.9 g/L Fe = 0.50 M) and 21.1% (48.4 g/L Fe = 0.87 M) and of 0.37 (0.006 M), 0.63 (0.010 M) and 1.2 g/L (0.019 M) of copper are attained for a temperature of 25, 40 and 60 °C,

Journal Pre-proof respectively. These results confirm that as the recovery of silver increases with increasing temperature and that it is inevitably related to the release of copper and iron ions.

Table 5. Potential and %Tu remaining after 150 minutes at different temperatures at pH 1 and S/L ratio of 500 g/L (50 g of jarosite in 100 mL solution), 0.13 M initial concentration thiourea and 1.2 M sodium oxalate.

Cu (M) 0.006 0.010 0.019

Fe (M) 0.33 0.50 0.87

Eh (V) 0.41 0.49 0.57

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T (°C) 25 40 60

Tu remaining (%) Free Tu (a) Total Tu (b) 65.7 85.6 50.8 85.1 14.3 54.2

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Another aspect of great importance is the amount of thiourea remaining in the solution after the leaching process. The direct determination of thiourea with iodate quantifies the free thiourea available to form complexes with silver or other metals such as iron, copper, zinc or lead. However, if a strong reducing agent, such as metallic zinc, is added, formamidine disulfide (the first oxidation product of thiourea) is reduced back to thiourea and silver, iron and copper metals are precipitated, releasing the thiourea that had formed complexes during the leaching process. Therefore, with this reduction, the total thiourea left in the system can be determined and the degree of degradation can be quantified. These values are shown in Table 5, along with the final solution ORP, which confirm the fact that 45.8% of the original thiourea concentration was irreversibly oxidized at this high ORP (0.57 V) and temperature (60 °C), while much of the thiourea was conserved at the other two conditions.

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4. Conclusions

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A novel alternative is presented for the recovery of the silver from a refractory industrial jarosite. Since silver is occluded within the sodium jarosite, decomposition of the matrix is necessary. This decomposition is favored at low pH values (pH = 1) and by increasing the system temperature (60 °C). With the presence of sodium oxalate at a concentration of 0.2 M and a low solid/liquid ratio (50 g/L), 100% silver recovery was possible in 120 minutes at 60 °C in the pH range 1 – 2. These conditions promote the decomposition of jarosite and silver leaching, while maintaining the stability of thiourea. However, when the solid/liquid ratio is increased, a larger concentration of oxalate is necessary to act as a complexing agent for the large amount of ferric and cupric ions released by the jarosite decomposition. These ions are detrimental to thiourea stability. Conflicts of interest: None Acknowledgements D. Calla-Choque thanks the Universidad Autónoma Metropolitana for the postdoctoral scholarship.

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References Calla-Choque, D., (2017). “Study of the phenomena that limit the extraction of silver by acid decomposition and leaching with thiourea” Ph.D Thesis, CINVESTAV Saltillo, Mexico: 27-66. Calla-Choque, D., F. Nava-Alonso and J. C. Fuentes-Aceituno (2016). "Acid decomposition and thiourea leaching of silver from hazardous jarosite residues: Effect of some cations on the stability of the thiourea system." Journal of Hazardous Materials 317: 440-448. Chandra, I. and M. I. Jeffrey (2005). “A fundamental study of ferric oxalate for dissolving gold in thiosulfate solutions”. Hydrometallurgy, 77, 191-201.

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Das, G. K., S. Acharya, S. Anand and R. P. Das (1996). "Jarosites: A review." Mineral Processing and Extractive Metallurgy Review 16(3): 185-210.

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Das, G. K., S. Anand, S. Acharya and R. P. Das (1995). "Preparation and decomposition of ammoniojarosite at elevated temperatures in H 2 O-(NH4 )2 SO4 -H2 SO4 media." Hydrometallurgy 38(3): 263-276.

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Deschênes, G. and E. Ghali (1988). "Leaching of gold from a chalcopyrite concentrate by thiourea." Hydrometallurgy 20(2): 179-202.

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González-Ibarra A. A., Nava-Alonso F., Uribe-Salas A. and Castillo-Ventureño E. N. (2016). “Decomposition kinetics of industrial jarosite in alkaline media for the recovery of precious metals by cyanidation”, Canadian Metallurgical Quarterly 55 (4): 448-454

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Li, J. and J. D. Miller (2006). "A review of gold leaching in acid thiourea solutions." Mineral Processing and Extractive Metallurgy Review 27(3): 177-214.

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Jing-ying, L., X. Xiu-li and L. Wen-quan (2012). "Thiourea leaching gold and silver from the printed circuit boards of waste mobile phones." Waste Management 32(6): 1209-1212.

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Kasaini, H., K. Kasongo, N. Naude & J. Katabua (2008). “Enhanced leachability of gold and silver in cyanide media: Effect of alkaline pre-treatment of jarosite minerals”. Minerals Engineering, 21, 1075-1082. Kerolli-Mustafa, M., V. Mandić, L. Ćurković and J. Šipušić (2016). "Investigation of thermal decomposition of jarosite tailing waste." Journal of Thermal Analysis and Calorimetry 123(1): 421-430. Kunda, W. and H. Veltman (1979). "Decomposition of jarosite." Metallurgical and Materials Transactions B 10(3): 439-446. Puigdomenech, I. (2004). “Make Equilibrium Diagrams Using Sophisticated Algorithms” (MEDUSA). Inorganic Chemistry, Royal Institute of Technology, Sweden. Reyes, I. A., F. Patiño, M. U. Flores, T. Pandiyan, R. Cruz, E. J. Gutiérrez, M. Reyes & V. H. Flores (2017). “Dissolution rates of jarosite-type compounds in H2 SO4 medium: A kinetic analysis and its importance on the recovery of metal values from hydrometallurgical wastes”. Hydrometallurgy, 167, 16-29.

Journal Pre-proof Appendix. Metallurgical balance 25 °C

Assay

Vol (mL) Weight (g)

Solution

Distribution

Ag (ppm)

Cu (ppm)

Fe (ppm)

Ag (g/t)

Cu (%)

Fe (%)

Ag (%)

Cu (%)

Fe (%)

378

22,970

168

0.76

45.9

100

100

100

Feed

100

5

8.4

Pregnant Leach Solution

100

5

3.1

79

1,033

63

0.16

2.1

37.5

21.0

4.5

Residue

100

4.99

5.2

301

21,845

104

0.60

43.8

62.0

79.7

95.1

8.4

381

22,878

167

0.76

45.9

99.5

100.7

99.6

-0.5

0.7

-0.4

Calc Head

Error (%)

Solution

Vol (mL) Weight (g) Ag (ppm)

Solution Fe (ppm)

Ag (g/t)

Cu (%)

Fe (%)

Ag (%)

Cu (%)

Fe (%)

158.000

0.76

45.9

100

100

100

68.098

0.18

2.7

43.1

23.3

6.0

90.243

0.59

43.3

57.1

77.5

93.6

158.341

0.77

46.0

100.2

100.8

99.6

0.2

0.8

-0.4

100

5

7.9

378

22,970

Pregnant Leach Solution

100

5

3.4

88

1,369

Residue

100

4.97

4.5

293

21,509

7.9

381

Solution

Vol (mL) Weight (g) Ag (ppm)

100

5

Pregnant Leach Solution

100

5

Residue

100

4.94

Distribution

Fe (ppm)

Ag (g/t)

Cu (%)

Fe (%)

Ag (%)

Cu (%)

Fe (%)

378

22,970

158

0.76

45.9

100

100

100

4.0

100

2,131

79

0.20

4.3

50.0

26.4

9.3

3.9

281

20,747

79

0.57

42.0

50.2

74.3

90.3

7.9

381

22,878

158

0.77

46.3

100.2

100.7

99.6

0.2

0.7

-0.4

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Calc Head

7.9

Solution

Cu (ppm)

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Feed

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40 °C

22,878

-p

Error

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Feed

Calc Head

Distribution

Cu (ppm)

of

30 °C

Error

Vol (mL) Weight (g)

ur

50 °C

Ag (ppm)

100

Jo

Feed

Solution

Solution

Distribution

Cu (ppm)

Fe (ppm)

Ag (g/t)

Cu (%)

Fe (%)

Ag (%)

Cu (%)

Fe (%)

5

7.9

378

22,970

158

0.76

45.9

100

100

100

Pregnant Leach Solution

100

5

5.9

146

5,134

118

0.29

10.3

74.6

38.8

22.4

Residue

100

4.89

2.0

236

17,744

40

0.48

36.3

25.6

62.3

77.2

7.9

382

22,878

158

0.77

46.6

100.2

101.1

99.6

0.2

1.1

-0.4

Calc Head

Error

60 °C

Solution

Vol (mL) Weight (g) Ag (ppm)

Solution

Distribution

Cu (ppm)

Fe (ppm)

Ag (g/t)

Cu (%)

Fe (%)

Ag (%)

Cu (%)

Fe (%)

Feed

100

5

7.9

378

22,970

158

0.76

45.9

100

100

100

Pregnant Leach Solution

100

5

7.8

173

7,379

155.3

0.34

14.8

98.3

45.8

32.1

Residue

100

4.88

0.1

208

15,499

2.2

0.43

31.7

1.4

55.0

67.5

7.9

381

22,878

157.5

0.77

46.5

99.7

100.8

99.6

-0.3

0.8

-0.4

Calc Head

Error

Journal Pre-proof Highlights

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A novel alternative is presented for the silver recovery from a refractory industrial jarosite. Silver is occluded within the sodium jarosite, requiring decomposition of the matrix. Jarosite decomposition is favored at low pH values and moderate temperatures (60°C). Sodium oxalate promotes both jarosite decomposition and silver extraction with thiourea. High silver recoveries were possible at elevated S/L ratios, when sufficient oxalate was present.

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