Accepted Manuscript

Parameters optimization and kinetics of direct atmospheric

leaching of Angouran sphalerite

Saeid Karimi, Fereshteh Rashchi, Javad Moghaddam

PII: S0301-7516(17)30067-4
DOI: doi: 10.1016/j.minpro.2017.03.004
Reference: MINPRO 3032
To appear in: International Journal of Mineral Processing
Received date: 14 October 2016
Revised date: 13 March 2017
Accepted date: 16 March 2017

Please cite this article as: Saeid Karimi, Fereshteh Rashchi, Javad Moghaddam ,
Parameters optimization and kinetics of direct atmospheric leaching of Angouran
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 ACCEPTED MANUSCRIPT

Parameters optimization and kinetics of direct atmospheric leaching of

Angouran sphalerite

Saeid Karimi1, Fereshteh Rashchi1,∗, Javad Moghaddam2

1
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran

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2
Materials and Metallurgical Department, University of Zanjan, Zanjan, Iran

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Abstract

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In this research, a zinc sulfide (sphalerite) concentrate of Angouran mine was studied by direct
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atmospheric leaching process. This process is an alternative to the conventional method of roast-

leach-electrowinning (RLE) for zinc production by assisting ferric ions as powerful oxidant. The
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independent sphalerite leaching parameters investigated were ferric ions concentration (0.4–1.2
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M), temperature (40–80 °C), particle size (21–53 µm), sulfuric acid concentration (0.5–1.5 M)

and time (2–6 hours). Response surface methodology (RSM) was used to optimize the process
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parameters. The most influencing parameter was found to be temperature and the less effective
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was acid concentration. Based on the results, ferric ions illustrate a complex effect on the

recovery of zinc; In this regard, interaction of ferric ions with operational parameters was
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proposed. The optimum recovery for leaching of the zinc sulfide concentrate (e.g., 84.72%) was

obtained at ferric ions concentration of 1.2 M, temperature of 80 °C, mean particle size of 21 µm

and leaching time of 6 hours. The predicted percentage recovery of zinc at the optimum

condition was found to be 84.96% which was very close to the experimental value of 84.72%.

∗
Corresponding author. Tel.: +98 21 88012999; fax: +98 21 88006076. E-mail address: rashchi@ut.ac.ir.

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Kinetic investigation was carried out in the optimum condition that obtained by RSM. Kinetic

results showed that there were two stages in the sphalerite leaching. At the beginning of the

leaching process, kinetics of sphalerite leaching is fast, while after about an hour the overall rate

of leaching has decreased. The kinetic of leaching in the first stage is affected by both rate of

chemical reaction and rate of diffusion through the sulfur layer. In this stage, the contribution of

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chemical reaction gradually decreases by increasing the temperature. In the second stage, the

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leaching rate of sphalerite is controlled only by diffusion through the product layer.

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Keywords: Sphalerite concentrate, Direct atmospheric leaching, Optimization, Ferric ions,

Kinetics.
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1. Introduction
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The traditional method for the production of zinc from sphalerite concentrate is the roast–leach–

electrowinning (RLE) process. Sulfur dioxide (SO2) is produced in the roasting stage and the gas
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emission leads to overwhelming environmental barriers (Sahu et al., 2006). Hydrometallurgical

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processing, through which the sphalerite concentrates are directly leached in atmospheric

condition without roasting, is an alternative to the RLE process (Lampinen et al., 2015; Santos et
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al., 2010; Souza et al., 2007). Atmospheric leaching of sulfide minerals has some advantages

over RLE process and the most important among them is the production of elemental sulfur

instead of SO2. Furthermore, low grade materials can be used, so as to this route is economical.

However, a big challenge associated with atmospheric leaching of sphalerite process is mainly

found in the low kinetics. Of particular interest herein is a detailed kinetic study in the optimized

range of atmospheric leaching parameters.

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The sphalerite atmospheric leaching is influenced by different parameters such as ferric ions

concentration, temperature, particle size, sulfuric acid concentration and pulp density (Dutrizac,

2006; Lampinen et al., 2015; Markus et al., 2004; Santos et al., 2010; Souza et al., 2007).

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Existence of sulfuric acid is necessary for zinc sulfide concentrate leaching. Researchers have

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reported different consequences of consuming high concentrations of sulfuric acid in zinc

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concentrate leaching. Some researchers emphasize the positive impact of the increase of sulfuric

acid on the zinc recovery (Bobeck and Su, 1985; Souza et al., 2007). However, others reported

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that the acid concentration has no effect on the zinc recovery in some cases (Lampinen et al.,
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2015; Santos et al., 2010). Nevertheless, high acid sulfuric concentration should be used in

atmospheric leaching, because it protonates the species produced by the leaching and avoids the
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hydrolysis and precipitation of ferric ions during the atmospheric leaching process (Ruiz et al.,
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2011). Most of the previous researchers have agreed on the positive influence of temperature on

the zinc recovery (Bobeck and Su, 1985; Levchuk, 2010; Perez and Dutrizac, 1991; Souza et al.,
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2007). Besides, diffusion process is difficult through the by-production of elemental sulfur layer
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in atmospheric leaching at low temperature. On the other hand, electron exchange should occur

on the sphalerite surface during the leaching process; thus, electrical conductivity of elemental
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sulfur that forms on the particle surface is very important. Hence, lower zinc recovery is

attributed to the lower electrical conductivity of elemental sulfur at low temperatures (Munoz et

al., 1979; Parker et al., 1981). Many authors have emphasized the time impact on the zinc

recovery (Dutrizac, 2006; Lampinen et al., 2015; Markus et al., 2004; Santos et al., 2010; Souza

et al., 2007). Also, if ratio of solid to liquid is reduced, in constant concentration of ferric ions,

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the zinc recovery would be increased. According to Santos (Santos et al., 2010) and Massacci

(Massacci et al., 1998), the solid to liquid ratio is proposed to be 5% v/w. It is known that ferric

ions concentration is the most effectual parameter on the direct leaching process. It was reported

that an increase in the ferric ions concentration leads to an increase in zinc recovery (Dutrizac,

2006; Lampinen et al., 2015; Markus et al., 2004; Santos et al., 2010; Souza et al., 2007).

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However, to the best of our knowledge, the interactions between ferric ions and other operational

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parameters in this leaching system have not been studied yet. Optimization of sphalerite

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dissolution with a minimal number of experiments can be investigated by a design of experiment

(DOE) method to maximize the zinc recovery during the atmospheric leaching process.

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To optimize hydrometallurgical processes, response surface methodology (RSM) is one of the

best DOE methods because of two main reasons: first, it provides more favorable results in
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minimum number of experiments and second, it determines the interactions between the
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operational parameters (Dehghan et al., 2008; Karimi and Ataie, 2016). In a study using Taguchi

method, Dehghan et al. (Dehghan et al., 2008) optimized the leaching parameters of a low grade
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sphalerite concentrate. Since the leaching parameters are effectively dependent on each other,
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Dehghan et al. (Dehghan et al., 2008) have not considered the interactions between leaching

parameters and the derived model for the leaching process is not comprehensive. Using RSM,
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the interactions between the operational parameters are carefully considered. The central

composite design (CCD) has been used to collect the data for fitting the third order response

(Mirazimi et al., 2015; Nazari et al., 2014). In addition to the optimization of zinc recovery

through the atmospheric leaching process, advances in the knowledge of the chemical kinetics

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now permit in designing an appropriate production process, including sphalerite atmospheric

leaching.

Shrinking core model (SCM) has been extensively used for solid-liquid reactions, which is also

adopted for kinetic investigation in this study (Levenspiel, 1999). The kinetic of sphalerite

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leaching has been investigated by several authors (Lampinen et al., 2015; Markus et al., 2004;

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Souza et al., 2007; Xie et al., 2007). Researchers have reported activation energies for the

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leaching of sphalerite ranging from 63 to 97 kJ/mol (Markus et al., 2004), 40 to 70 kJ/mol (Perez

and Dutrizac, 1991), 32 to 59 kJ/mol (Salmi et al., 2010), 44 kJ/mol (Dutrizac, 2006) and 27.5

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kJ/mol (Souza et al., 2007). Moreover, most of these studies have emphasized that the zinc
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sulfide leaching is controlled by mixed mechanism (i.e., surface chemical reaction and diffusion

mechanism) or surface chemical reaction mechanism. Elemental sulfur is the main by-product of
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direct atmospheric leaching that covers the minerals surface; therefore, diffusion through this
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layer should be also considered. To the best of our knowledge, contribution of diffusion

mechanism has not been considered during the whole process of sphalerite leaching.
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In this paper, parameters influencing direct leaching of Angouran sphalerite concentrate are

determined by RSM method. The effectiveness of the operational parameters including ferric
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ions concentration, temperature, particle size, acid concentration and time on the zinc recovery

has been investigated. Determining the optimum conditions to maximize the zinc recovery was

also the subject of the current study. In addition, the leaching kinetics of sphalerite was

investigated using the shrinking core model.

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2. Experimental

2.1. Materials and analysis methods

All tests were performed with a sphalerite concentrate (approximately 73% ZnS) from Angouran

mine, Zanjan (Iran). Ore sample was analyzed by atomic absorption spectroscopy (AAS) using

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an (VARIAN 240, Australia) instrument. Table 1 presents the chemical composition of this

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concentrate sample. The zinc and iron content of the concentrate were of high and low grades,

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respectively. The phase analysis of the samples was investigated by x-ray diffraction (XRD)

using a Philips PW-3710 diffractometer equipped with a Cu Kα radiation source (λ = 0.154056

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nm).
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Table 1
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Fig. 1 shows the mineralogy of Angouran sphalerite concentrate. As it can be seen, the main

phase, sphalerite (ZnS), along with quartz (SiO2), pyrrhotite (FeS), smithsonite (ZnCO3) and
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galena (PbS) are present in the concentrate. The morphology and the chemical analysis of the
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sphalerite concentrate and the leach residue were investigated by high resolution scanning

electron microscopy (HRSEM) on a microscope (Hitachi S4160) equipped with an energy-

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dispersive X-ray spectroscopy (EDS) apparatus.

Fig. 1

Sulfuric acid (H2SO4) and iron (III) sulfate (Fe2(SO4)3) used in the leaching were analytical grade

from Merck Company, with a purity of 96–98% and 86–89%, respectively. The particle size
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distribution of as-received concentrate was 80% finer than 53 µm (D80 = 53 µm), which was

determined by a Laser Particle Size Analyzer (LPSA model: Cilas Laser 1064). Then, the as-

received concentrate was ground in a rod mill to obtain two other particle sizes (D80 = 37 µm and

D80 = 21 µm) for the leaching experiments. In all cases, the particle size distribution of the

sphalerite concentrate was measured using LPSA technique. Fig. 2 shows the three different size

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distributions of sphalerite concentrate, which is used in this study. As it is seen in Fig. 2a, the as-

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received concentrate has a particle size in the range of 1 to 112 µm. The particle size analysis of

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this fraction reveals the 80% cumulative volume cut point at 53 µm. The D80 = 37 µm

concentrate shown in Fig. 2b is in the size range of 0.3 to 100 µm. Fig. 2c shows that the smallest

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particle size is in the range of 0.1 to 56 µm. The particle size analysis of this fraction reveals the
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80% cumulative volume cut point at 21 µm.
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Fig. 2
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2.2 Design of experiments

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Design of experiments was used to develop an empirical model for obtaining the optimum

operating conditions for the leaching process. The optimization procedure was based on CCD

and RSM. In general, CCD requires a total of (2k + 2k + N0) runs where k is the number of

studied factors, 2k is the points from the factorial design, 2k is the face-centered points and N0 is

the number of experiments carried out at the center (Mirazimi et al., 2015). Duplication of the

center points was used to determine the experimental error. In our case, 25 factorial design was

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used and 4 (N0 = 4) central replicates were employed. As usual, the experiments were carried out

in a random order to minimize the effect of systematic errors. The effects of ferric ions

concentration, temperature, particle size, acid sulfuric concentration and reaction time on the

atmospheric leaching of sphalerite were determined. Levels of parameters were selected based

on previous researches reported on zinc recovery of sphalerite concentrate (Lampinen et al.,

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2015; Santos et al., 2010; Souza et al., 2007). The ferric ions concentration, temperature, particle

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size, sulfuric acid concentration and time parameters were varied within the range of 0.3–1.9 M,

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12.5–96 °C, 18–75 µm, 0.15–2.19 M and 0.5–8.8 hours, respectively (Table 2). The influence of

particle size on the zinc recovery was studied in the range of 18–75 µm. In order to determine

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the effects of particle size distribution on the zinc recovery, five different sizes were studied. In
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addition to three main grades of sphalerite concentrate (D80 = 21, 37 and 53 µm), two grades of

D80 = 18 and 75 µm were prepared by sieving and screening of the as-received sphalerite
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concentrate. DESIGN EXPERT 8.0.1 (DX8) trial version software (State-Ease Inc.,
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Minneapolis, MN, USA) was used for regression and graphical analyses.
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Table 2
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2.3. Leaching experiments

In a typical leaching test, after reaching the desired temperature of the leaching medium, 20 g of

the sample was leached in 400 mL solution at a specific ferric ions concentration, temperature,

particle size, sulfuric acid concentration and time (Table 3). The atmospheric direct leaching

experiments were performed in a Pyrex flask reactor equipped with a reflux condenser. The

reactor consisted of three necks, one for the condenser and another one for thermometer. The

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reaction mixture was agitated with a magnetic stirrer at a given speed of 700 rpm and heated on a

hot plate. After a specific leaching time, a Buckner funnel equipped with a glass filter, was used

to filter the solution. The slurry was filtered and the filtrate was analyzed by atomic absorption

spectroscopy (AAS).

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Table 3

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3. Results and Discussion

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3.1. Model fitting and analysis of variance (ANOVA)
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A cubic polynomial equation was derived to predict the zinc recovery as a function of
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independent variables and their interactions in terms of coded factors. The same procedure in

different leaching media has been reported earlier (Mirazimi et al., 2015; Nazari et al., 2014).
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The cubic polynomial equation is as follows:

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Zn Recovery= +47.37 + 2.61A+17.56B – 2.35C + 5.36E + 3.63AB + 2.20AE + (1)

+2.70ABE + 0.84A3
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where A, B, C and E are coded values of the tests variables, A: ferric ions concentration in M, B:
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temperature in °C, C: particle size in μm and E: time in hour. Several regression analyses were

engaged to evaluate the model coefficients and the response for the cubic polynomial equation.

The following equation was applied to transform a real value (Xi) into the corresponding coded

value:

(X i − X 0 )
Coded value = (2)
(X 1 − X 0 )

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in which Xi is the real value of leaching parameters (A, B, C, E), X1 is the real value in the

superior level (+1) and X0 is the real value in the center point.

The results of the ANOVA are shown in Table 4 collected by DX8 program. This standard

statistical software was applied to survey the significance and sufficiency of the model. In

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general, the F-value for a significant model should be larger than its standard value (Roy, 1990).

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The calculated F-value corresponding to the zinc recovery response model is 94.86 (Table 4) and

F-value of probability 5% is 2.20, indicating that the derived model is highly significant.

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Additionally, fitted polynomial equation is expressed via the regression coefficient, R2. The R2

value gives a good agreement between the experimental (R2=0.9535) and predicted (R2=0.9206)
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values of the fitted model. From F-values in Table 4, temperature is the most effective parameter.
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However, H2SO4 concentration has a minor influence on the zinc recovery; therefore, this

parameter is not employed in the proposed model. The negligible effect of high acid sulfuric
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concentration in zinc recovery through the sphalerite atmospheric leaching has been reported
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previously (Lampinen et al., 2015; Santos et al., 2010). In addition, the remaining sulfuric acid

concentration should be enough to keep ferric ions in solution for further sphalerite oxidation.
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Table 4
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3.2. Effects of parameters

3.2.1. Ferric ions concentration

Sphalerite leaching directly depends on the ferric ions concentration as shown by the following

reaction:

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ZnS + 2Fe2(SO4)3 → ZnSO4 + 2FeSO4 + S° (3)

When the ferric sulfate concentration is low, the zinc recovery will be low. According to Table 4,

small F value calculated for the ferric ions concentration (5.92) shows that this parameter is

significant in the selected range, as expected. The influence of rising ferric ions concentration on

zinc recovery through sphalerite atmospheric leaching can be clearly seen from Fig. 3, as the

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zinc recovery increases from 61% at 0.4 M to 85% at 1.2 M ferric concentration. Ferric ions act

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as an oxidant agent in atmospheric leaching; thus, increasing ferric concentration leads to a

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higher oxidation power and increases the zinc recovery through the atmospheric leaching

process. In previous studies, it is reported that sphalerite leaching improves as the ferric ions

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concentration increases (Lampinen et al., 2015; Santos et al., 2010; Souza et al., 2007).
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Fig. 3
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According to the derived model (Eq. (1)), although the ferric ions concentration has only

separate interactions with time and temperature, it interacts simultaneously with both parameters
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(i.e., ABE interaction). These facts reveal that the ferric ions concentration parameter possesses a
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complex influence on the zinc concentrate direct leaching. As continuing the leaching process,

zinc can be leached according to reaction (3), the elemental sulfur layer is formed on the
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sphalerite surface. As seen in SEM image of leached sphalerite particles (Fig. 4a), the zinc

sulfide particles are covered with porous elemental sulfur layer (Fig 4b and c). During the initial

stages of atmospheric leaching, the porous sulfur layer on the sphalerite surface is so thin that

does not hinder the sphalerite dissolution. As atmospheric leaching reaction proceeds, the

thickness of elemental sulfur layer on the sphalerite particles increases. For further zinc recovery,

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ferric ions should diffuse into the sulfur layer to reach sphalerite surface. The leaching products

(ferrous and zinc ions) should diffuse toward the solution through the sulfur layer (Nazari and

Asselin, 2009). As shown in Fig.4, if the sulfur layer on the sphalerites surface becomes too

thick, the diffusion process will be difficult which hinders the sphalerite dissolution. In addition,

it has been reported that temperature, dissolution current density, solution chemistry and redox

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potential parameters affect the mobility of ions through the leaching process. When an oxidant

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(such as ferric ions) has no movement to the fresh surface of the mineral, the leaching reaction

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will be hindered (Córdoba et al., 2008).

Fig. 4
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Further support for this model is given by the temperature dependence of the electrical
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conductivity of sulfur. According to Munoz et al. (1979), the electrical conductivity of elemental
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sulfur was dramatically increased by increasing temperature (i.e., electrical conductivity of sulfur

at 112 °C is 25000 greater than that at 20 °C). As a result, transportation of electrons through the
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sulfur layer is very difficult and this is a possible explanation for the slow kinetics observed at
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low temperatures.
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The ferric oxidizing performance is strongly sensitive to the temperature variation. As it can be

seen in Fig. 5, zinc recovery is not improved at 40 °C by increasing the ferric ions concentration.

Even though, zinc recovery significantly increases from 61% to 85% at 80 °C, which is proposed

as the optimum temperature. Fig. 5 demonstrates the best zinc recovery (85%) under

simultaneous increasing of ferric ions concentration and temperature. The kinetics of ferric to

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ferrous ions reduction in the presence of sphalerite concentrates has been studied by several

authors (Dutrizac, 2006; Markus et al., 2004). Markus et al. (2004) have reported that the rate of

reduction of ferric to ferrous is increased by increasing the temperature. Therefore, according to

reaction (3), at high temperatures the zinc oxidation will be increased. The ions exchange (ferric,

ferrous and zinc ions) from sphalerite particles towards the solution through sulfur layer is

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decelerated at low temperatures (Parker et al., 1981).

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Fig. 5

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Fig. 6a and b shows the interaction of ferric ions concentration and time on the zinc recovery of
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sphalerite atmospheric leaching at temperatures of 80 and 40 ºC, respectively. As it is seen in

Fig. 6a, increasing the ferric ions concentration and time at 80 ºC synergistically increase the
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zinc recovery from 58% at 0.4 M ferric ions concentration and 2 h leaching time to 85% at 1.2 M
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ferric ions concentration and 6 h leaching time. The slow kinetic in sphalerite leaching could be

attributed to decrease in diffusion rate of ferric ions through the sulfur layer. As leaching
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proceeds, the thickness of sulfur layer increases and this phenomenon changes the mechanism of
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the leaching reactions from surface chemical reaction or mixed mechanism to diffusion

mechanism. Whereas, increase in the ferric ions concentration is enough to progress the leaching
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reaction, longer time will be needed to increase the zinc recovery (Souza et al., 2007). The zinc

recoveries at 40 ºC do not exceed 40 % even after 6 h leaching time (Fig. 6b), which implies low

activation energy of the reaction and low diffusion rate of ferric ions within the sulfur layer.

Fig. 6

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3.2.2. Effect of temperature

In order to investigate the effect of temperature, leaching tests were done in a wide range of

temperature (40-80 °C). As can be seen in Fig. 7, temperature had a significant effect on the

leaching of zinc from the sphalerite concentrate. It is well known that a reaction which has high

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activation energy is more sensitive to temperature (Levenspiel, 1999). As a consequence, the

temperature is one of the main parameters in the leaching process. The strongest bond in the

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sphalerite crystal lattice (between zinc and sulfur) is responsible for the high values of activation

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energy. A high temperature should be applied in the leaching process to break down the bond in

the sphalerite crystal lattice (Córdoba et al., 2008).

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According to Table 4, F value for temperature shows that it is the most significant parameter in

selected ranges. Also, leaching recovery is relatively low at 40 °C but increases meaningfully
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when the temperature is increased to 80 °C. Detailed survey of the influence of temperature on
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the leaching of zinc concentrate shows that 80 °C is the optimum level. The obtained result for

the optimum temperature is in good agreement with the values that Santos et al. reported earlier
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(Santos et al., 2010). Similar results have also been reported in previous studies (Dutrizac, 2006;
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Lampinen et al., 2015; Salmi et al., 2010; Souza et al., 2007).

Fig. 7

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3.2.3. Effect of particle size

The effect of particle size on the zinc recovery is presented in Fig. 8. The size of zinc concentrate

particles is in the range of 21 μm to 53 μm. The zinc recovery improved by reducing particle size

from 79.8% at 53 μm to 85% at 21 μm. Therefore, a relatively small increase in solid surface

area causes a minor increase (ca. 5%) in the zinc recovery. This is in agreement with the other

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researchers’ results in recovery of zinc from sphalerite concentrates. (Dutrizac, 2006; Massacci

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et al., 1998; Souza et al., 2007). Souza et al. (2007) believed that with decreasing particle size,

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the small difference in zinc recovery is due to the presence of porosities and natural cracks on the

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sphalerite particles, which accelerate the diffusion process and increase the sphalerite leaching

rate. In addition to a small difference in mineral surface area, the slight difference in zinc
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recovery could be attributed to the negligible difference in the degree of liberation of the gangue

minerals such as alumina (Table 1). Fig. 9a and b shows the SEM images of sphalerite leach
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residue of experiments #20 and #24, respectively. Leach residue of experiment #24 (D80 = 53
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µm) has almost the same size and shape as the further ground concentrate (experiment #20, 21
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μm), which indicates some fraction of sphalerite concentrate cannot be easily ground.

Furthermore, EDS maps of aluminum for sphalerite leach residue of experiments #20 and #24
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are shown in Fig. 9c (the red patches). It is interesting that aluminum signal is detected on the
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surface of sphalerite particles, which implies the existence of alumina. It is clear that some

fraction of sphalerite particles are locked inside the alumina that is resistant to leaching under the

reaction conditions. Same phenomena had been observed in some other sphalerite particles,

which is not shown here. This is another reason for the constant zinc recovery when particle size

decreases.

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Fig. 8

Fig. 9

3.2.4. Effect of reaction time

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Fig. 10 represents the effect of reaction time on the zinc recovery of sphalerite atmospheric

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leaching. During the first 2 hours of leaching process, the zinc recovery increases rapidly to ca.

65% at 80 °C. However, the recovery of zinc gradually rises from 65% to 85% by increasing the

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reaction time from 2 to 6 hours, which indicates only 20% increment of zinc recovery during 4

hours of atmospheric leaching at 80 °C. It can be concluded that zinc sulfide leaching consists of
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two stages, where the first stage is fast and the second stage is slow. Other researchers also
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confirmed that the recovery of zinc changes by time through two-stages (Peng et al., 2005;

Santos et al., 2010; Souza et al., 2007). Santos et al. (2010) proposed that the sphalerite leaching
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process comprises two stages. The first stage occurs in 30 to 60 min of leaching reaction, and
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then the rate of the reaction decreases significantly in the second stage. The increase in sulfur

layer thickness with time results in the low rates of sphalerite dissolution. The presence of a thick
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sulfur layer on the sphalerite surface (Fig. 4) causes difficulty in diffusion process and hinders
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the sphalerite leaching. A part of downturn of zinc recovery is due to the diffusion from sulfur

layer and arrival to the fresh reaction surface (Souza et al., 2007). Lochmann and Pedlik

(Lochmann and Pedĺik, 1995) believed that the growth of elemental sulfur product on the surface

of particles is the main reason for low efficiency of leaching process. In addition,

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From Fig. 6b, recovery of zinc at 40 °C and 2 hours is about 27.6% and it gradually increases

and reaches to 36.0% after 6 hours leaching, which indicates only 8.4% increase during 4 hours

leaching. However, the difference in recoveries in 2 and 6 hours at 80 °C is 20% (Fig. 6a). From

Fig. 6b, the lowest zinc recovery of ca. 27.6% occurs at ferric ions concentration of 0.4 M and

reaction time of 2 hours. A simultaneous increase of ferric ions concentration, temperature and

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reaction time (ferric ions concentration: 1.2 M, temperature: 80 °C and reaction time: 6h) leads

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to highest zinc recovery of ca. 85% in this study (Fig. 6a). This result shows the strong

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interaction of ferric concentration, temperature and reaction time parameters (ABE), which

confirms the higher F-value for ABE factor as shown in Table 4.

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Fig. 10
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3.3. Evaluation of process optimization and confirmation test

In the numerical optimization, a minimum and a maximum level must be provided for each
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parameter. A multiple response of any combination of five parameters, namely ferric ions
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concentration, temperature, particle size, acid concentration and time was applied. Applying an

experimental design with ferric ions concentration of 1.2 M, temperature of 79.99 °C, particle
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size of 23.50 µm, acid concentration of 0.50 M within 6 hours results in a zinc recovery of

84.96%.

The condition of experiment number 20 (Table 3) is close to the optimum. To test the validity of

the optimized condition specified by the model, an experiment was carried out under the

parameters suggested. The conditions used in the confirmation experiment were as follows:

ferric ions concentration of 1.2 M, temperature of 80 °C, particle size of 21 µm, acid
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concentration of 0.5 M and leaching time of 6 hours. The zinc recovery reached to 84.72%. This

recovery is in a good agreement with the predicted value of the model with an error margin of

5%.

3.4. Characterization of the leach residue

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Fig. 11 shows SEM image of zinc sulfide concentrate (a) before and (b) after leaching under the

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optimized condition. The particles of the sphalerite concentrate before the leaching process

illustrate an irregular morphology with a smooth surface. However, the surface of the leach

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residue is rougher and more porous than that of the concentrate. Fig. 12 shows XRD pattern of
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the leach residue particles obtained at optimized condition. The major phases of the leach residue

are sphalerite (ZnS), lead sulfate (PbSO4) and elemental sulfur (S°). The peak intensities of ZnS
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decreased in comparison to the zinc concentrate. Galena (PbS) was oxidized by ferric ions and
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formed PbSO4 and elemental sulfur. Because of the low solubility of PbSO4 in sulfate solutions,

almost all the PbSO4 was introduced to the leach residue (Dutrizac and Chen, 1995). As
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mentioned above, sulfur is the main product in the direct leaching process and is produced
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according to Eq. (3). In addition, other sulfide substances can be produced through oxidization

with ferric ions forming a sulfate phase.

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Fig. 11

Fig. 12

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3.5. Kinetic

In the present work, the shrinking core model was employed to describe the leaching rate of zinc

from sphalerite particles. Three kinetic mechanisms have been proposed for sphalerite direct

leaching: diffusion (Hasani et al., 2016; Sokic et al., 2012), chemical reaction (Dutrizac, 2006;

Lampinen et al., 2015; Xie et al., 2007) and a mixed model including diffusion and chemical

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components (Bobeck and Su, 1985; da Silva, 2004; Souza et al., 2007; Weisener et al., 2004)

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which occur at different stages of leaching. By assuming different controlling stages various

kinetic models can be proposed.

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In order to find out whether the mechanism has been changed during leaching, the equation
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corresponding to one of the mechanisms should be considered for the whole period of leaching.
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If the trend changes (change in the slope of the line) at a point, it can be concluded that the

1
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mechanism has been changed at that point. Fig. 13 shows the plot of 1- (1 − X ) 3
(chemical

reaction control) versus the leaching time at temperature range of 50-80 °C and the optimum
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condition (ferric ions concentration of 1.2 M, particle size of 21 µm and acid concentration of 0.5
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M). It is clear in this figure that after ca. 1 h, the slope has changed significantly. This indicates

that the mechanism of sphalerite leaching has changed at this time. In other words, at the
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beginning of the leaching process, kinetics of sphalerite leaching is fast, while after about an

hour the overall rate of leaching has decreased. It is worth mentioning that using diffusion

control equation leads to the same results (figure not shown here). Additionally, this changing in

mechanism for the leaching system occurs later when temperature is higher; i.e., ca. 45 mins at

50 °C, ca. 60 mins at 60 and 70 °C and ca. 80 mins for 80 °C.

Fig. 13
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Considering the above discussion, two stages exist in the leaching process. At temperatures 50,

60, 70 and 80 °C, stage one ends at 45, 60, 60 and 80 mins, respectively. Stage two starts at 45,

60, 60 and 80 mins after leaching had been started for the temperatures 50, 60, 70 and 80 °C,

respectively. Assuming a constant size and a spherical geometry for the particles during

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leaching, using shrinking core model, the rate in each stage may be controlled by liquid film

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diffusion, product layer diffusion and/or chemical reaction (Levenspiel, 1999). In such a case,

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the following generalized formula can be used (Hasab et al., 2013):

1 − X i 13 1 − X i 23

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1
t − t 1 = τ F ( X i − X 1 ) + τ R (1 − ( ) ) + τ P (1 − 3( ) + 2 (1 − (1 − X 1 ) 3 ( X i − X 1 )) (4)
1− X1 1− X1
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where t is time of reaction, t1 is the start of time period, Xi is the leached fraction, X1 is the

leached fraction at time t1, and τ F , τ P , and τ R are the time for complete leaching by controlling
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mechanisms of liquid film diffusion, product layer diffusion and chemical reaction, respectively.
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The contribution of these mechanisms can be estimated based on positive amounts of τ F , τ P and
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τ R by fitting the following equation to experimental data considering a minimum value for φ

(Hasab et al., 2013):

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1 − X i 13 1 − X i 23 1
φ= ∑ [τ ( X i − X 1 ) + τ R (1 − ( ) ) + τ P (1 − 3( ) + 2 (1 − (1 − X 1 ) 3 ( X i − X 1 ))) − (t i − t1 )] 2 (4)
1− X1 1− X1
F
i

The calculated constants ( τ F , τ P and τ R ) for kinetics of zinc dissolution are shown in Table 5.

As seen in Table 5, in the first stage, the experimental data are best fitted by a mixed control

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kinetic model (chemical reaction and diffusion through the sulfur layer). As can be seen, τ F = 0

and film diffusion has no effect in the leaching system. According to previous researches

(Bobeck and Su, 1985; Souza et al., 2007), stirring the solution has no effect on solubility of zinc

in the direct leaching process. Based on the results of Dreisinger and Abed (Dreisinger and

Abed, 2002), resistance of the sulfur layer for the fluid mass transfer is much greater than the one

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for the fluid film surrounding the particles. The calculated value of τ F = 0 also confirms this

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matter.

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As mentioned before, elemental sulfur layer is formed on the sphalerite surface (Eq. (2)). This

layer significantly influences the kinetics of sphalerite leaching reaction by establishing a

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diffusion barrier (Bobeck and Su, 1985; Crundwell, 1987; da Silva, 2004; Souza et al., 2007;

Weisener et al., 2004). According to Table 5, by increment of temperature, the contribution of

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chemical reaction ( τ R ) and diffusion ( τ P ) in the first stage gradually decrease. Additionally, the
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ratio of τ P / τ R gradually increased at higher temperatures and thus effect of diffusion through
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the sulfur layer becomes greater than chemical reaction with rising temperature. The proposed

equation for leaching rate considers fraction of each mechanisms (chemical reaction and
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diffusion through the product layer). As time goes on, diffusion through the sulfur layer is
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predominated and its resistance against mass transfer becomes more significant in the second

stage. Therefore, it can be seen that in the second stage, the leaching rate of sphalerite is strongly

controlled by diffusion through the product layer. The contribution of chemical reaction ( τ R ) is

zero, thus the rate of reagent diffusion through the sulfur layer is very slow. Therefore, the

kinetics of sphalerite leaching is controlled only by diffusion mechanism in the second stage.

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Table 5

The calculated activation energy for first stage of sphalerite leaching is 23.91 kJ/mole. This is in

good agreement with results of Souza et al. (Souza et al., 2007). However, they proposed that the

rate of the first stage is controlled by chemical reaction and neglected the diffusion through

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sulfur layer. As mentioned above, kinetic mechanism of first stage is controlled by mixed

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mechanisms. As a result of this, the low activation energy obtained can be validated by

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contribution of diffusion mechanism in the first stage of sphalerite leaching. The activation

energy is about 12.30 kJ/mol in the second stage, which is consistent with the activation energies

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reported for the diffusion-controlled reactions (Levenspiel, 1999). The activation energies
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reported in the literature vary widely from 19.6 kJ/mol to 112 kJ/mol (Dutrizac, 2006; Lampinen

et al., 2015; Markus et al., 2004; Souza et al., 2007; Xie et al., 2007). Different amounts of
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activation energies shows that sphalerite with different origins and different conditions could
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influence on the leaching process and thus, on the leaching mechanism and activation energies.
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4. Conclusions
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Atmospheric leaching was carried out to leach a sphalerite concentrate from Angouran mine. A
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model was developed to describe the leaching process by response surface methodology. Using

this model, the most operational parameter was found to be temperature and the less effective

one was acid concentration. From the experimental results, the sulfuric acid consumption can be

reduced at high concentration of ferric sulfate.

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The results showed that ferric ions concentration has interactions with temperature and time.

Ferric ions, as a powerful oxidant, acts better at high temperatures (ca. 80 °C). This phenomenon

is due to increase in the diffusion rate and breaking up the zinc sulfide bonds at higher

temperatures. In addition, the conductivity of sulfur and movement of ions through the sulfur

layer is reduced by decreasing the temperature. Therefore, diffusion of ferric ions through the

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sulfur layer becomes slower and, as a result, a lower zinc recovery would be observed in this

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condition. The optimum condition for zinc recovery was determined to be 1.2 M ferric ions

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concentration, 80 °C temperature, 21 µm particle size, 0.5 M acid concentration and 6 hours.

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According to the kinetic results, it was found that two stages exist in the leaching process. At the
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beginning of the leaching process, kinetics of sphalerite leaching is fast, while after about an

hour the overall rate of leaching has decreased. The kinetic of leaching in the first stage is
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affected by both rate of chemical reaction and rate of diffusion through the sulfur layer. In this
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stage, the contribution of chemical reaction gradually decreases by increasing the temperature. In
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the second stage, the leaching rate of sphalerite is controlled only by diffusion through the

product layer. The activation energies for first and second stages were 23.91 kJ/mol and 12.30
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kJ/mol, respectively.
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Acknowledgments

The partial financial support for PhD studies of Mr. Saeid Karimi by Zanjan Zinc Khales Sazan

Industries Company is gratefully acknowledged.

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References

Bobeck, G.E., Su, H., 1985. The kinetics of dissolution of sphalerite in ferric chloride solution.
Metall. Trans. B 16, 413–424. doi:10.1007/bf02654839
Córdoba, E.M., Muñoz, J.A., Blázquez, M.L., González, F., Ballester, A., 2008. Leaching of
chalcopyrite with ferric ion. Part I: General aspects. Hydrometallurgy 93, 81–87.
doi:http://dx.doi.org/10.1016/j.hydromet.2008.04.015

T
Crundwell, F.K., 1987. Kinetics and mechanism of the oxidative dissolution of a zinc sulphide
concentrate in ferric sulphate solutions. Hydrometallurgy 19, 227–242.

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doi:http://dx.doi.org/10.1016/0304-386X(87)90007-7

CR
da Silva, G., 2004. Relative importance of diffusion and reaction control during the bacterial and
ferric sulphate leaching of zinc sulphide. Hydrometallurgy 73, 313–324.
doi:http://dx.doi.org/10.1016/j.hydromet.2003.12.004

US
Dehghan, R., Noaparast, M., Kolahdoozan, M., Mousavi, S.M., 2008. Statistical evaluation and
optimization of factors affecting the leaching performance of a sphalerite concentrate. Int. J.
Miner. Process. 89, 9–16. doi:http://dx.doi.org/10.1016/j.minpro.2008.07.003
AN
Dreisinger, D., Abed, N., 2002. A fundamental study of the reductive leaching of chalcopyrite
using metallic iron part I: kinetic analysis. Hydrometallurgy 66, 37–57.
M

doi:http://dx.doi.org/10.1016/S0304-386X(02)00079-8
Dutrizac, J.E., 2006. The dissolution of sphalerite in ferric sulfate media. Metall. Mater. Trans. B
ED

37, 161–171. doi:10.1007/bf02693145

Dutrizac, J.E., Chen, T.T., 1995. The leaching of galena in ferric sulfate media. Metall. Mater.
Trans. B 26, 219–227. doi:10.1007/bf02660962
PT

Hasab, M.G., Raygan, S., Rashchi, F., 2013. Chloride–hypochlorite leaching of gold from a
mechanically activated refractory sulfide concentrate. Hydrometallurgy 138, 59–64.
CE

Hasani, M., Koleini, S.M.J., Khodadadi, A., 2016. Kinetics of Sphalerite Leaching by Sodium
Nitrate in Sulfuric Acid. J. Min. Environ. 7, 1–12. doi:10.22044/jme.2016.454
AC

Karimi, S., Ataie, A., 2016. Characterization of mechanothermally processed nanostructured

ZnO. Int. J. Miner. Metall. Mater. 23, 588–594. doi:10.1007/s12613-016-1270-8
Lampinen, M., Laari, A., Turunen, I., 2015. Kinetic model for direct leaching of zinc sulfide
concentrates at high slurry and solute concentration. Hydrometallurgy 153, 160–169.
doi:http://dx.doi.org/10.1016/j.hydromet.2015.02.012
Levchuk, I., 2010. Role of oxidation-reduction cycle of iron in direct leaching of zinc
concentrate. Lappeenranta University of Technology.
Levenspiel, O., 1999. Chemical Reaction Engineering. Ind. Eng. Chem. Res. 38, 4140–4143.
doi:10.1021/ie990488g

24
 ACCEPTED MANUSCRIPT

Lochmann, J., Pedĺik, M., 1995. Kinetic anomalies of dissolution of sphalerite in ferric sulfate
solution. Hydrometallurgy 37, 89–96. doi:http://dx.doi.org/10.1016/0304-386X(94)00021-T
Markus, H., Fugleberg, S., Valtakari, D., Salmi, T., Murzin, D.Y., Lahtinen, M., 2004. Reduction
of ferric to ferrous with sphalerite concentrate, kinetic modelling. Hydrometallurgy 73,
269–282. doi:http://dx.doi.org/10.1016/j.hydromet.2003.11.002
Massacci, P., Recinella, M., Piga, L., 1998. Factorial experiments for selective leaching of zinc
sulphide in ferric sulphate media. Int. J. Miner. Process. 53, 213–224.
doi:http://dx.doi.org/10.1016/S0301-7516(98)00002-7

T
Mirazimi, S.M.J., Rashchi, F., Saba, M., 2015. A new approach for direct leaching of vanadium

IP
from LD converter slag. Chem. Eng. Res. Des. 94, 131–140.
doi:http://dx.doi.org/10.1016/j.cherd.2014.12.010

CR
Munoz, P.B., Miller, J.D., Wadsworth, M.E., 1979. Reaction mechanism for the acid ferric
sulfate leaching of chalcopyrite. Metall. Trans. B 10, 149–158. doi:10.1007/bf02652458

US
Nazari, E., Rashchi, F., Saba, M., Mirazimi, S.M.J., 2014. Simultaneous recovery of vanadium
and nickel from power plant fly-ash: Optimization of parameters using response surface
methodology. Waste Manag. 34, 2687–2696.
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doi:http://dx.doi.org/10.1016/j.wasman.2014.08.021
Nazari, G., Asselin, E., 2009. Morphology of chalcopyrite leaching in acidic ferric sulfate media.
Hydrometallurgy 96, 183–188. doi:http://dx.doi.org/10.1016/j.hydromet.2008.09.004
M

Parker, A.J., Paul, R.L., Power, G.P., 1981. Electrochemistry of the oxidative leaching of copper
ED

from chalcopyrite. J. Electroanal. Chem. Interfacial Electrochem. 118, 305–316.

doi:http://dx.doi.org/10.1016/S0022-0728(81)80549-9
Peng, P., Xie, H., Lu, L., 2005. Coupling leaching of sphalerite concentrate. Miner. Eng. 18,
PT

553–555. doi:http://dx.doi.org/10.1016/j.mineng.2004.08.012
Perez, I.P., Dutrizac, J.E., 1991. The effect of the iron content of sphalerite on its rate of
CE

dissolution in ferric sulphate and ferric chloride media. Hydrometallurgy 26, 211–232.
doi:http://dx.doi.org/10.1016/0304-386X(91)90032-H
AC

Roy, R.K., 1990. A Primer on the Taguchi Method. Van Nostrand Reinhold, New York.
Ruiz, M.C., Vera, M.V., Padilla, R., 2011. Mechanism of enargite pressure leaching in the
presence of pyrite. Hydrometallurgy 105, 290–295. doi:10.1016/j.hydromet.2010.11.002
Sahu, S.K., Sahu, K.K., Pandey, B.D., 2006. Leaching of zinc sulfide concentrate from the
ganesh-himal deposit of nepal. Metall. Mater. Trans. B 37, 541–549. doi:10.1007/s11663-
006-0037-4
Salmi, T., Grénman, H., Bernas, H., Wärnå, J., Murzin, D.Y., 2010. Mechanistic modelling of
kinetics and mass transfer for a solid–liquid system: Leaching of zinc with ferric iron.
Chem. Eng. Sci. 65, 4460–4471. doi:http://dx.doi.org/10.1016/j.ces.2010.04.004

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Santos, S.M.C., Machado, R.M., Correia, M.J.N., Reis, M.T.A., Ismael, M.R.C., Carvalho,
J.M.R., 2010. Ferric sulphate/chloride leaching of zinc and minor elements from a
sphalerite concentrate. Miner. Eng. 23, 606–615.
doi:http://dx.doi.org/10.1016/j.mineng.2010.02.005
Sokic, M., Markovic, B., Matkovic, V., Zivkovic, D., Strbac, N., Stojanovic, J., 2012. Kinetics
and mechanism of sphalerite leaching by sodium nitrate in sulphuric acid solution. J. Min.
Metall. Sect. B Metall. 48, 185–195. doi:10.2298/JMMB111130022S
Souza, A.D., Pina, P.S., Leão, V.A., Silva, C.A., Siqueira, P.F., 2007. The leaching kinetics of a

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zinc sulphide concentrate in acid ferric sulphate. Hydrometallurgy 89, 72–81.
doi:http://dx.doi.org/10.1016/j.hydromet.2007.05.008

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Weisener, C.G., Smart, R.S.C., Gerson, A.R., 2004. A comparison of the kinetics and

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mechanism of acid leaching of sphalerite containing low and high concentrations of iron.
Int. J. Miner. Process. 74, 239–249. doi:http://dx.doi.org/10.1016/j.minpro.2003.12.001
Xie, K., Yang, X., Wang, J., Yan, J., Shen, Q., 2007. Kinetic study on pressure leaching of high

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iron sphalerite concentrate. Trans. Nonferrous Met. Soc. China 17, 187–194.
doi:http://dx.doi.org/10.1016/S1003-6326(07)60070-3
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Fig. 1. XRD pattern of sphalerite concentrate.

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100 10 100 8
(b)
Qumulative values (%)

(a)

Qumulative values (%)

80 8 80
6

Volume (%)
Volume (%)
60 6 60
4
40 4 40
2

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20 2 20

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0 0 0 0
0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100

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D (Diameter/ µm) D (Diameter/ µm)

100 4

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Qumulative values (%)

(c)
80
3

Volume (%)
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60
2
40
M

1
20
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0 0
0.01 0.1 1 10 100
D (Diameter/ µm)
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Fig. 2. Particle size distribution of three different particle size distribution of sphalerite
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concentrate (a) D80 =53 µm, (b) D80 =37 µm and (c) D80 =21 µm.
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89 71.75
Recovery (%)
52.5

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16

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0.4 0.6 0.8 1.0 1.2
A: Ferric concentration (M)
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Fig. 3. Influence of ferric ions concentration in zinc recovery from sphalerite concentrate
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(temperature: 80 °C, particle size: 21 µm, sulfuric acid: 0.5 M, time: 6 h).
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(a)
S°

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10 µm

(b) (c) S

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Zn
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Fig. 4. (a) SEM image of leached sphalerite concentrate and (b) EDS maps of zinc and (c)

elemental sulfur at optimum conditions of atmospheric leaching (ferric ions concentration: 1.2 M
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, temperature: 80 °C, particle size: 21 µm, sulfuric acid: 0.5 M, time: 6 h).
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Recovery (%)

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Fig. 5. Interaction of ferric ions concentration with temperature on the zinc recovery of sphalerite
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atmospheric leaching (particle size: 21 µm, sulfuric acid: 0.5 M, time: 6 h).
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(a) (b)
Recovery (%)

Recovery (%)

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Fig. 6. Interaction of ferric ions concentration with time on the zinc recovery of sphalerite
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atmospheric leaching in two different temperature (a) 80 °C and (b) 40 °C (particle size: 21 µm,
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sulfuric acid: 0.5 M).

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Fig. 7. Effect of temperature on the zinc recovery of sphalerite atmospheric leaching (ferric ions

concentration: 1.2 M, particle size: 21 μm, sulfuric acid: 0.5 M, time: 6 h).
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Fig. 8. Effect of particle size in the range of 21 to 53 μm on the zinc recovery of sphalerite
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atmospheric leaching (ferric ions concentration: 1.2 M, temperature: 80 °C, sulfuric acid: 0.5 M,
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time: 6 h).
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(a) (b)

10 μm 10 μm

(c) (d)

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Fig. 9. SEM images (a) SEM images of sphalerite particle after leaching in condition of exp. #20
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and (b) exp. #24 and EDS map of aluminum (c) exp. #20, and (c) exp. #24 (see Table 3)
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89
71
Recovery (%)

52.5

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16

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2 3 4 5 6

time (h)
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Fig. 10. Effect of reaction time on the zinc recovery of sphalerite atmospheric leaching at
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optimum condition (ferric ions concentration: 1.2 M, particle size: 21μm, sulfuric acid: 0.5 M).
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Fig. 11. SEM images of (a) sphalerite concentrate of D80=21 µm and (b) leach residue of

sphalerite atmospheric leaching in optimum condition (ferric ions concentration: 1.2 M, time: 6
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h, temperature: 80 °C, sulfuric acid: 0.5 M, time: 6 h).

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Fig. 12. XRD pattern of leach residue obtained in optimum condition of sphalerite atmospheric

leaching (ferric ions concentration: 1.2 M, time: 6 h, temperature: 80 °C, sulfuric acid: 0.5 M,
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time: 6 h).
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1

Fig. 13. 1- (1 − X ) 3 versus leaching time at 50, 60, 70 and 80 °C.

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Table 1. Chemical analysis of the zinc concentrate determined by AAS analysis

Component Zn Fe Ni Cd Co Cu Mn Al Pb
Content (wt. %) 49.11 2.24 0.04 1.10 0.03 0.13 0.02 0.09 3.54

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Table 2. Parameters levels and values used in the experimental design.

Parameter (unit) –α* –1 0 +1 +α

A: Ferric concentration (M) 0.3 0.4 0.8 1.2 1.9
B: T (°C) 12.5 40 60 80 96
C: Particle size (μm) 18 21 37 53 75
D: H2SO4 (M) 0.15 0.5 1.0 1.5 2.19
E: t (hours) 0.5 2 4 6 8.8
*α = 2.378

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Table 3. Experimental design

Parameters Recovery (%)

Experiment Run
ferric conc., M T, °C particle size, μm H2SO4, M t, h experimental predicted
1 2 0.4 40 21 0.50 2.0 28.19 26.57
2 45 1.2 40 21 0.50 2.0 29.07 27.18
3 43 0.4 80 21 0.50 2.0 57.92 59.22
4 13 1.2 80 21 0.50 2.0 64.60 63.53
5 3 0.4 40 53 0.50 2.0 22.06 21.50
6 41 1.2 40 53 0.50 2.0 22.10 22.11

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7 22 0.4 80 53 0.50 2.0 54.44 54.15

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8 40 1.2 80 53 0.50 2.0 62.06 58.46
9 4 0.4 40 21 1.50 2.0 24.38 26.57
10 28 1.2 40 21 1.50 2.0 24.89 27.18

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11 1 0.4 80 21 1.50 2.0 59.72 59.22
12 36 1.2 80 21 1.50 2.0 61.54 63.53
13 21 0.4 40 53 1.50 2.0 21.36 21.50

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14 9 1.2 40 53 1.50 2.0 23.38 22.11
15 33 0.4 80 53 1.50 2.0 56.88 54.15
16 38 1.2 80 53 1.50 2.0 59.03 58.46
17 8 0.4 40 21 0.50 6.0 39.41 36.99
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18 18 1.2 40 21 0.50 6.0 37.58 35.59
19 32 0.4 80 21 0.50 6.0 53.63 61.29
20 15 1.2 80 21 0.50 6.0 82.70 85.21
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21 5 0.4 40 53 0.50 6.0 32.79 31.92

22 42 1.2 40 53 0.50 6.0 30.48 30.52
23 46 0.4 80 53 0.50 6.0 55.42 56.22
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24 12 1.2 80 53 0.50 6.0 79.01 80.14

25 23 0.4 40 21 1.50 6.0 33.24 36.99
26 6 1.2 40 21 1.50 6.0 34.17 35.59
27 30 0.4 80 21 1.50 6.0 58.75 61.29
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28 19 1.2 80 21 1.50 6.0 82.42 85.21

29 26 0.4 40 53 1.50 6.0 27.33 31.92
30 16 1.2 40 53 1.50 6.0 26.16 30.52
CE

31 17 0.4 80 53 1.50 6.0 64.58 56.22

32 37 1.2 80 53 1.50 6.0 84.93 80.14
33 27 0.3 60 37 1.00 4.0 43.09 39.36
AC

34 11 1.9 60 37 1.00 4.0 65.67 71.48

35 7 0.8 12.5 37 1.00 4.0 16.62 7.01
36 34 0.8 96 37 1.00 4.0 88.40 80.77
37 29 0.8 60 18 1.00 4.0 61.07 55.53
38 35 0.8 60 75 1.00 4.0 36.09 43.47
39 31 0.8 60 37 0.15 4.0 48.16 49.50
40 10 0.8 60 37 2.19 4.0 42.78 49.50
41 24 0.8 60 37 1.00 0.5 28.48 40.19
42 44 0.8 60 37 1.00 8.8 61.75 62.16
43 25 0.8 60 37 1.00 4.0 52.64 49.50
44 14 0.8 60 37 1.00 4.0 46.20 49.50
45 39 0.8 60 37 1.00 4.0 55.99 49.50
46 20 0.8 60 37 1.00 4.0 51.19 49.50

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Table 4. Results of the analysis of variance for zinc recovery.

Sum of Mean p-value

Source DF F Value
Squares Square Prob > F
Model 16043.72 8 2005.47 94.86 < 0.0001
A-Fe2(SO4)3 125.19 1 125.19 5.92 0.0199
B-T 13111.26 1 13111.26 620.16 < 0.0001
C-Particle Size 213.74 1 213.74 10.11 0.0030
E-time 1167.75 1 1167.75 55.23 < 0.0001
AB 421.23 1 421.23 19.92 < 0.0001

T
AE 154.75 1 154.75 7.32 0.0103

IP
ABE 233.54 1 233.54 11.05 0.0020
A3 73.41 1 73.41 3.47 0.0704

CR
Residual 782.25 37 21.14
Lack of Fit 732.60 34 21.55 1.30 0.4801
Pure Error 49.65 3 16.55 94.86 < 0.0001
Cor Total 16825.97 45 2005.47 0.0199

US
F95%(8, 37)=2.20 AN
M
ED
PT
CE
AC

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Table 5. Data resulted from Eq. (5) for sphalerite dissolution

Contribution of mechanisms τ P /τ R
Temperature time period (min) R2
τ F (min) τ P (min) τ R (min) Ratio
50 °C 0-45 0 1005.89 554.77 1.81 0.9996
45-360 0 5707.10 0 0.9865
60 °C 0-60 0 525.54 245.41 2.14 0.9994
60-480 0 2046.68 0 0.9653
70 °C 0-60 0 588.70 183.30 3.21 0.9997

T
60-480 0 1576.57 0 0.9654
80 °C 0-80 0 496.41 106.52 4.66 0.9989

IP
80-480 0 1024.43 0 0.9813

CR
US
AN
M
ED
PT
CE
AC

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