Determination of the Optimum Conditions for the Leaching of Lead

Article pubs.acs.org/IECR

Determination of the Optimum Conditions for the Leaching of Lead from Zinc Plant Residues in NaCl−H2SO4−Ca(OH)2 Media by the Taguchi Method Bahram Behnajady,† Javad Moghaddam,†,* Mohammad A. Behnajady,‡ and Fereshteh Rashchi§ †

Metallurgy and Materials Engineering Department, Advanced Material Research Centre, Sahand University of Technology, Tabriz, Iran ‡ Department of Chemistry, Faculty of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran § School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran S Supporting Information *

ABSTRACT: This research is part of a continuing effort to reduce environmental conflicts and occupational hazards of leadbearing zinc plant residues (ZPRs), and to break through this problem and recover lead of the wastes. The residue with an assay of 14.4% Pb was used in chloride leaching for lead recovery, and sulfate was controlled in the leaching stage by the addition of Ca(OH)2. In this paper, the effects of influential factors on extraction efficiency of Pb from ZPRs were investigated. Taguchi’s method based on orthogonal array design (OAD) has been used to arrange the experimental runs in order to maximize lead extraction from a ZPR. Orthogonal array (OA) L8(27) consisting of seven parameters, each with two levels, was employed to evaluate the effects of NaCl concentration (C = 300 and 400 g/L), stirring speed (R = 500 and 700 rpm), reaction temperature (T = 55 and 65 ◦C), reaction time (t = 1 and 8 h), liquid-to-solid ratio (L/S = 6 and 20), acidic pH (pHa = 2 and 3.5), and neutral pH (pHb = 4 and 5.5) on lead extraction percent. Statistical analysis, ANOVA, was also employed to determine the relationship between experimental conditions and yield levels. The results showed that the pulp density, and NaCl concentration were significant parameters, and increasing pulp density reduced leaching efficiency of lead. However, increasing NaCl concentration promoted the extraction of lead. The obtained optimum conditions from this study were C2, 400 g/L; R2, 700 rpm; T1, 55 °C; t1, 1 h; (L/S)2, 20; (pHa)2, 3.5; and (pHb)1, 4. But only two significant factors (C2, 400 g/L; and (L/S)2, 20) were used to estimate the performance at the optimum conditions. The calculated leaching percent (85.91%) was in reasonable agreement with the experimental results in optimum conditions. NaCl,7,8,13−17 or MgCl2 and CaCl2,18 or FeCl314,19 along with HCl. In water lead sulfate has a low solubility (Ksp = 10−8 at 25 °C), compared to lead chloride which has moderate solubility. The saturated solubility of PbC12 in water is 9.9 g/L g at 20 °C and 33.4 g/L g at 100 °C.20−24 The PbC12 solubility decreases rapidly with increasing chloride concentration due to the socalled common ion effect, but increases after passing through a minimum because the increasing chloride activity which favors the formation of soluble lead chloride complexes according to the following equations:18

1. INTRODUCTION The depletion of high-grade ores and primary sources push the scientific and technical communities to treat lean and complex ores as well as secondary metal resources for the recovery of valuable metals.1 The metal and metallurgical industry is associated with the generation of solid, liquid, and gaseous waste. The impact of complex and hazardous waste on ecology and environment is an alarming issue to the environmentalists.2 More than 80% of the worldwide primary zinc is produced via a combined roast-leach-purification-electrowinning process. Large quantities of iron waste are produced in the form of three main kinds of residues: (a) goethite (FeOOH), (b) jarosite (XFe3(SO4)2(OH6), or (c) hematite.3−6 During the hydrometallurgical processing of zinc from the roaster, calcine lead and silver report to the leach residue7,8 which can be utilized for the recovery of these two metals. The toxicity of the waste is mainly due to the presence of different metals such as lead, cadmium, arsenic, chromium, etc. The released residue during the process could be recycled for further processing.9−12 Different processing techniques are practiced to treat the leach residue to recover zinc as well as lead in the presence of high iron content.8 However, chloride leaching is the most recognized and widely used recovery method.13 Chloride leaching processes have been employed using either © 2012 American Chemical Society

Pb2 + + Cl− → PbCl+

K1 = 12.59

PbCl+ + Cl− → PbCl2 PbCl2 + Cl− → PbCl3−

K2 = 14.45 K3 = 3.89 × 10−1

PbCl3− + Cl− → PbCl 4 2 − Received: Revised: Accepted: Published: 3887

K 4 = 8.92 × 10−2

(1) (2) (3) (4)

November 11, 2011 February 16, 2012 February 19, 2012 February 21, 2012 dx.doi.org/10.1021/ie202571x | Ind. Eng. Chem. Res. 2012, 51, 3887−3894

Industrial & Engineering Chemistry Research

Article

with mechanical stirrer at 600 rpm to improve the recovery in the subsequent stage of chloride leaching. After filtration, the solution was analyzed by atomic absorption spectrometer for zinc and lead. The residue was dried, crushed by ball mill, sieved, and physically characterized to determine the particle size distribution. Then the residue was chemically analyzed for zinc, iron, and lead using an atomic absorption spectrometer, and its mineralogical structure was identified by X-ray diffraction analysis. 2.3. Leaching for Lead Extraction. After water leaching, the ZPR was subjected to chloride leaching to recover lead. Commercial grade salt (NaCl), as a cheap agent, was used to prepare chloride solution. Industrial grade HCl and H2SO4 were used for pH adjustment. Technical grade Ca(OH)2 was used for sulfate removal. The leaching experiments were performed in a 1-L beaker in a thermostatically controlled water bath equipped with mechanical agitator. After pouring 750 mL of clear NaCl solution with a known concentration to the beaker and setting the temperature at the desired value, a known quantity of residue was added and acidified by HCl or H2SO4, and finally Ca(OH)2 was added while stirring the content of the beaker at a certain speed. In all experiments the solution conditions (pH and temperature) were controlled using a pH controller (Metrohm-827 pH lab). After each trial, the leach slurry was filtered immediately and the leach solution was analyzed for Pb by atomic absorption spectrometer (AAS). 2.3.1. Preliminary Experiments. Preliminary experiments were done to determine leaching agent, acid type, and sulfate precipitation agent. The first two experiments (P1 and P2) were performed in the presence of NaCl alone to investigate the feasibility of lead extraction using this leaching agent. Then two other experiments (P3 and P4) were done in NaCl−HCl and NaCl−H2SO4 media to compare the effect of acid type on lead extraction from ZPR. HCl and H2SO4 were used to decrease pH from 5.5 to 2 in experiments P3 and P4. A final primary experiment (P5) was performed in NaCl-H2SO4− Ca(OH)2 media to investigate the effect of sulfate precipitating agent on the extraction of lead. In this experiment, like previous experiment (P4), pH was decreased to 2 using H2SO4, then, Ca(OH)2 was used to increase pH from 2 to 4. It should be mentioned that in the preliminary experiments the agitation rate, temperature, and pulp density were set at 700 rpm, 70 °C, and 25 g/L, respectively. All preliminary experiments were repeated three times in order to ascertain the reproducibility. Experimental parameters and their levels were determined in the light of preliminary tests and it was observed that particle size distribution could be neglected. Final pH was not a controllable factor during experiments. For these reasons, the final pH was not taken into consideration as a parameter. Indeed, final pH was a noise factor. 2.4. Taguchi Method. The most important stage in the design of an experiment lays in the selection of control factors, therefore as many factors as possible should be included and no significant variables must be identified at the earliest opportunity. Taguchi method creates an orthogonal array to accommodate these requirements. To seek optimum conditions for the lead extraction from the ZPR in NaCl−H2SO4− Ca(OH)2 media, the effect of some parameters on the process was investigated. According to the Taguchi parameter design methodology, one experimental design should be selected for the controllable factors.29

ZPRs are SO4-bearing residues and the presence of excessive levels of sulfate adversely affects the lead and silver extraction. To eliminate sulfate from the residues and to recover watersoluble zinc such as ZnSO4, a water washing step has been found to be very effective.7,8,13,25 A significant amount of sulfate is generated during chloride leaching. In most chloride leaching processes, the base metals are recovered from the pregnant solution which is eventually recycled to the leaching stage. Hence, the sulfate concentration of the leaching solution would increase with each leaching cycle carried out in the absence of a sulfate control process. The accumulation of sulfate in a chloride leaching circuit cannot be allowed to continue indefinitely, and some form of sulfate control must be provided. This control is especially important in lead or zinc−lead circuits where PbSO4 contamination of the PbCl2 intermediate product is a concern. Sulfate is controlled in the leaching stage by the in situ precipitation of either some form of calcium sulfate or a jarosite-type compound (MFe3(SO4)2(OH)6 where M = Na, K, NH4, H2O, etc.). Calcium sulfate occurs as gypsum (CaSO4·2H2O), bassanite or hemihydrate (CaSO4·1/2H2O) and anhydrite (CaSO4). Recently, two forms of hemihydrates have been reported, αhemihydrate (CaSO4·1/2H2O) and β-hemihydrate, which possibly have a composition of CaSO4·3/5H2O. Calcium sulfate has a limited solubility. The limited solubility is advantageous as it limits the accumulation of calcium and sulfate in the processing circuit. Also it is claimed that the ″gypsum″ facilitates the filtration and washing of the leach residue, because the filtration facility is affected by sulfate control in the chloride leaching processes.25−28 This study was designed to investigate the simultaneous chloride leaching of the ZPRs and sulfate controlling. At the first stage, the residue was leached with water to bring soluble zinc and sulfate into the solution. In the second stage for lead leaching, residual material from the first stage was leached using NaCl solution. Then, the leaching solution was acidified by H2SO4 and HCl, and finally, Ca(OH)2 has been added to leaching solutions to monitor its effect on the extraction efficiency of lead from washed ZPRs. The Taguchi’s approach was used to determine the most significantly affecting parameters and the optimum leaching conditions for maximizing lead extraction in NaCl-H2SO4− Ca(OH)2 media. Accordingly, the effect of operating factors including NaCl concentration (C), stirring speed (R), reaction temperature (T), reaction time (t), liquid-to-solid ratio (L/S), acidic pH (pHa) and neutral pH (pHb) on the extraction efficiency of lead were investigated using an L8 (27) orthogonal array.

2. EXPERIMENTAL PROCEDURE 2.1. Characterization of the Sample. ZPR was obtained from the Bafgh zinc smelting plant located at Yazd, Iran. Initially, the ZPR sample was dried, then it was crushed using a ball mill and sieved using a 100 mesh (149 μm) ASTM standard sieve. Noted that crushing and sieving were repeated until all particles became finer than 149 μm. Mineralogical structure of the residue was identified by X-ray diffraction (Bruker advanced-D8) analysis. Afterward, the residue was chemically analyzed using X-ray fluorescence (XRF) (Phillips, model PW2404) and atomic absorption spectrometer (AAS) (Perkin-Elmer, AA300). 2.2. Water Leaching. Prior to use, the residue was leached with tap water at 65 °C for 60 min at a pulp density of 250 g/L 3888

dx.doi.org/10.1021/ie202571x | Ind. Eng. Chem. Res. 2012, 51, 3887−3894


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On the basis of our previous experience in related works and the experimental conditions reported by other researchers for the leaching of similar residues7,8,13,15−17 and preliminary tests performed: NaCl concentration (C), stirring speed (R), reaction temperature (T), reaction time (t), liquid to solid ratio (L/S), acidic pH (pHa), and neutral pH (pHb) were chosen as the seven factors to be investigated. Two levels were the exclusory set for each of the seven factors. Seven selected control factors in two levels applied in this study, have been listed in Table 1. Table 1. Experimental Parameters and Their Levels for given level parametera

unit

1

2

C R T t L/S pHa pHb

g/L rpm °C h

300 500 55 1 6 2 4

400 700 75 8 20 3.5 5.5

a

Parameters: C, NaCl concentration; R, stirring speed; T, reaction temperature; t, reaction time; L/S, liquid to solid ratio; pHa, acidic pH; pHb, neutral pH. Figure 1. Particle size distribution of the (a) primary residue and (b) washed residue used in the experiments.

For the Taguchi design and subsequent analysis, the statistical softwares named Design-Expert 7 (DX7) and Qualitek-4 were used. The orthogonal array (OA), L8 (27) matrix, which denotes seven parameters, each with two levels, was chosen since it is the most suitable for the conditions being investigated (Table S1 in the Supporting Information represents the selected orthogonal array for this study). The interactive effect of parameters was not taken into consideration in the theoretical analysis because some preliminary tests reflected that this could be neglected. In the proposed method, possible interactions between variables were not in the matrix, and the focus was placed on the main effects of the seven most important factors. Furthermore, the validity of this assumption was checked by confirmation experiments conducted at the optimum conditions.

Table 2. Chemical Composition of ZPR (XRF Analysis of Residue)

a

3. RESULTS AND DISCUSSION 3.1. Characterization of the Sample. Figure 1a illustrates the particle size distribution of the residue. As it can be seen from the particle size distribution curve, d80 was calculated as 88 μm. The chemical composition (XRF analysis) of the residue is given in Table 2. The XRD analysis showed that the residue compounds were lead sulfate (PbSO4), calcium sulfate dihydrate (CaSO 4 ·2H 2 O), zinc sulfate heptahydrate (ZnSO4·7H2O), iron oxide (Fe2O3), jarosite (KFe3(SO4)2(OH)6), quartz (SiO2), zinc ferrite (ZnO·Fe2O3), and iron silicate (Fe2O3·SiO2). According to the atomic absorption analysis, the residue contains about 7% zinc, 8.8% iron, and 14.2% lead. 3.2. Characterization of the Washed Sample. The particle size distribution of the washed residue is shown in Figure 1b. As it can be seen from the particle size distribution curve, d80 was calculated as 105 μm. So, washed residue was directly used for the chloride leaching experiments without any particle size fractionation since some preliminary tests and previous researches showed that for particle sizes less than 200

component

amount (wt %)a

component

amount (wt %)a

SiO2 Al2O3 Fe2O3 CaO Na2O MgO K2O TiO2 MnO

15.08 2.48 16.90 4.70 1.31 1.80 0.37 0.17 0.77

BaO CuO PbO SrO ZnO Cl SO3 Ag2O

0.15 0.77 11.50 0.10 13.80 0.04 24.01 0.16

Semiquantitative.

μm, no observable change was achieved in lead recovery through chloride leaching of washed residue.13 When the XRD pattern of washed residue was compared with that of the initial residue for their constituents, it was concluded that most of the zinc sulfate heptahydrates (ZnSO4·7H2O) were taken into leach liquor during water washing. Since jarosite process was used to precipitate iron in this particular plant, iron hydroxide in the form of jarosite was detected in XRD pattern of washed residue. According to the atomic absorption analysis, after water washing, the residue contains about 3% zinc, 9.1% iron, and 14.4% lead. Also zinc and lead concentrations in the filtrate were in the range of 7500−8500 and 5−15 mg/L, respectively. This is consistent with a portion of the zinc in a water-soluble compound ZnSO4·7H2O, and the limited solubility of lead sulfate in water. 3.3. Preliminary Experiments. To determine the leaching agent, acid type and sulfate precipitating agent primary tests 3889



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Table 3. Preliminary Experiment Conditions and Results trial

reaction duration (min)

NaCl concn (g/L)

initial pH

P1 P2 P3 P4 P5

10 10 10 10 20

200 300 300 300 300

5.78 5.80 5.51 5.52 5.50

acid type

HCl H2SO4 H2SO4

acidic pH

additive type

2 2 2

Ca(OH)2

neutral pH

Pb extraction (%)

4

63.02 68.89 78.94 77.78 72.04

Table 4. L8 (27) Experimental Plan and Experimental Results experimental factors

response

experiment number

run

C

R

T

t

L/S

pHa

pHb

Pb extraction (%)

1 2 3 4 5 6 7 8

6 2 4 8 7 3 1 5

300 300 300 300 400 400 400 400

500 500 700 700 500 500 700 700

55 55 75 75 75 75 55 55

1 8 1 8 1 8 1 8

6 20 6 20 20 6 20 6

2 3.5 3.5 2 2 3.5 3.5 2

4 5.5 5.5 4 5.5 4 4 5.5

35.93 75.38 30.79 77.98 82.67 44 84.7 54.53

formation of soluble lead chloride complexes. Although lead dissolution in the case of HCl is higher than in the case of H2SO4, this difference between the performance of HCl and that of H2SO4 is too little. Thus, it can be concluded that at pH = 2 the beneficial effect of extra chloride ions added by HCl or detrimental effect of excess sulfate ions by adding H2SO4 are not significant. Therefore at this pH, HCl and H2SO4 almost have the same performance. On the other hand, by considering the amount of acid consumption to achieve a certain pH and considering the price of HCl and H2SO4, leaching by H2SO4 is more cost-effective than HCl. To reduce sodium chloride consumption and because of environmental hygiene, after stripping by cementation, crystallization, electrowinning, ion exchange or solvent extraction, the pregnant solution should be recycled. However, the accumulation of sulfate ions in a chloride leaching circuit adversely affects the lead and silver extraction. This problem seems to be aggravated when H2SO4 is used as chloride activator. Since, ZPR contains a high percent of sulfate compounds, its leaching yields a high percentage of sulfate ions in solution. Therefore, to recycle the solution, sulfate should be controlled in or after the leaching stage, and it does not depend on using either H2SO4 or HCl. 3.3.3. Determination of the Sulfate Precipitating Agent. In this study, sulfate removal has been done by the addition of Ca(OH)2 to the leaching medium to precipitate ″gypsum″, although the degree of hydration of the CaSO4 in this, and in most similar processes, is unknown. The calcium sulfate is removed with the leach residue and finally its effect has been investigated on the lead extraction. Experimental conditions and results obtained for lead extraction in NaCl-H2SO4− Ca(OH)2 media have been given in Table 3 as a result of trial P5. From the results, it can be seen that increasing pH from 2 to 4, caused a slight decrease in the lead extraction. This may be attributed to the coprecipitation or adsorption of dissolved lead or to the reduced reaction rate. As mentioned above, sulfate removal by Ca(OH)2 in the chloride leaching stage causes only a slight decrease in the lead extraction. As a result of these preliminary studies, it can be concluded that lead could be efficiently leached from ZPRs by using NaCl-H2SO4−Ca(OH)2 media. Thus, the lead extraction

were done. Experimental conditions and results of the various preliminary experiments have been summarized in Table 3. 3.3.1. Determination of the Leaching Agent. In the light of the preliminary chemical and physical characterization, and previous researcher’s reports, the most promising leaching agent has come out to be sodium chloride for lead. However, the effectiveness of NaCl as a source of chloride activity is limited by its solubility. In pure water, the solubility of NaCl increases from about 357 g/L at 0 °C to approximately 398 g/L at 100 °C.20 Initially, chloride leaching of residue was studied in the presence of NaCl alone to investigate the feasibility of lead extraction using this leaching agent. For this reason two trials (P1 and P2) were done with 200 and 300 g/L NaCl and results have been given in Table 3. In the presence of high chloride concentrations, PbCl2 is subsequently converted to PbCl3¯ and PbCl42− complexes with higher solubility.18 In the concentration of 200 g/L NaCl solution, the extraction of lead is not satisfactory and lead leaching increases with increasing NaCl concentration. Such findings for lead extraction were in good agreement with the other researchers.7,8,13,15−17 Since leaching with NaCl alone could not yield an acceptable value for lead extraction, the following experiments were performed in acidic solutions where the individual effects of acid concentration were evaluated. 3.3.2. Determination of the Acid Type. To enhance lead dissolution, the concentration of Cl− was increased, since increasing chloride activity favors the formation of soluble lead chloride complexes (eqs 3−4). Chloride activation can also be done by reducing pH. Therefore, at this stage, hydrochloric acid and sulfuric acid were used to acidify the leaching solution. The concentration of Cl− ions increases by the addition of HCl to the solution, and this is beneficial for chloride leaching. On the other hand, the concentration of SO42− ions in the solution increases using H2SO4 that can be a barrier for leaching reaction progress. Initial pulp pH with a 25 g/L pulp density was approximately 5.5−5.8. In these experiments, pH was adjusted to 2 using both hydrochloric acid and sulfuric acid. The results of trials P3 and P4 obtained for lead extraction have been given in Table 3. Extraction of lead increases with declining pH to 2, because activity of the chloride increases and consequently favors the 3890



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Table 5. Results of the Analysis of Variance (ANOVA) for the Lead Extraction parameter

sum of squares (S)

degree of freedom ( f)

variance (S/f)

C R T t L/S pHa pHb error total

262.43 12.55 28.50 39.60 3021.75 32.97 0.07 0.00 3397.88

1 1 1 1 1 1 1 0 7

262.43 12.55 28.50 39.60 3021.75 32.97 0.07

variance ratio (F value)

P value

contribution of factor % 7.72 0.37 0.84 1.17 88.93 0.97 0.00  100

Table 6. Results of the Pooled ANOVA for the Lead Extractiona parameter

sum of squares (S)

degree of freedom ( f)

variance (S/f)

variance ratio (F value)

P value

pure sum of squares

contribution of factor %

model C R T t L/S pHa pHb error total

3323.79 262.43 (12.55) (28.50) 39.60 3021.75 (32.97) (0.07) 74.09 3397.88

3 1 (1) (1) 1 1 (1) (1) 4 7

1107.93 262.43

59.81 14.17

0.0009 0.0197

3268.22 243.91

96.18 7.18

0.2175 0.0002

21.08 3003.23

0.62 88.39

a

pooled pooled 39.60 3021.75

2.14 163.14 pooled pooled

18.52

3.82 100

F = 7.7086 (at 95% confidence level).

of freedom, could not be calculated. Hence, it was impossible to calculate the F-ratio, defined as the variance of each factor dividing by Ve. To eliminate the zero f from the error term, a pooled ANOVA was applied. The process of ignoring a factor once it was deemed insignificant, was called pooling. Taguchi’s guideline for pooling, requires a start with the smallest main effect and successively includes larger effects, until the total pooled f equals approximately half of the total f. The larger f for the error term, as a result of pooling, increases the confidence level of the significant factors.29 The values of the ANOVA analysis for lead extraction after pooling of the stirring speed (R), reaction temperature (T), acidic pH (pHa), and neutral pH (pHb) have been given in Table 6. The F-value for this condition with 95% confidence level is 7.71.29 Therefore, the results of the F-value from Table 6 show that the liquid-to-solid ratio (L/S), and NaCl concentration (C) have meaningful effects on the extraction of lead, since the F-value of these factors are greater than the extracted F-value from the table for the 95% confidence level. This means that the variance of these factors is significant compared with the variance of error. On the other hand, the Fvalue for reaction time (t) is smaller than 7.71. This means that the variance of reaction time (t) factor is insignificant compared to the variance of error. Thus, this factor is not the statistically significant factor at least in the range under study. Another method to determine the significant factors is by Pvalues calculated using DX7 software. P values less than 0.05 indicate that the effect of model factors are significant within the 95% confidence interval. As mentioned above, in the case of lead extraction, only two factors including pulp density and NaCl concentration were significant model terms. The F and P values of the model were calculated as 59.81 and 0.0009 by DX7 software, respectively. Since the model P value is less than 0.05, the model is significant within the 95% confidence interval. The model F-value (59.81) implies the

and sulfate removal can be done simultaneously in the chloride leaching stage. In the next step Taguchi technique has been used to determine the most significantly affecting parameters and maximize the dissolution of lead in NaCl-H2SO4− Ca(OH)2 media. 3.4. Leaching in NaCl-H2SO4−Ca(OH)2 Media. Corresponding leaching efficiencies obtained under the proposed conditions based on L8 (27) matrix design have been shown in Table 4. The collected data were analyzed by softwares to evaluate the effect of each parameter on the optimization criteria. the maximum amount of lead extraction was defined as optimization criterion. Statistical analysis of variance (ANOVA) was performed to check whether the process parameters were statistically significant. The F-value for each process parameter indicates which parameter has a significant effect on the leaching efficiency and is simply a ratio of the squared deviations to the mean of the squared error.29 Usually, the larger F-value indicates the greater effect on the leaching efficiency. The results of ANOVA analysis for lead extraction have been given in Table 5. The percentage contribution of each factor to the leaching performance, which was obtained by the Qualitek4 software, has been shown in the mentioned table. The percentage contribution of the liquid to solid ratio (L/S) was the greatest (88.93%), and the NaCl concentration (C) was in the second place (7.72%). The percentage contribution reveals that the stirring speed (R), reaction temperature (T), reaction time (t), acidic pH (pHa), and neutral pH (pHb) are not statistically significant factors, at least in the range of this study. Therefore, high lead leaching could be achieved using an appropriate combination of the other operating parameters. On the other hand, the degree of freedom (f) for each factor was 1 and total f was 7, so the f for the error term was 0, and finally the variance for the error term (Ve), obtained by calculating error sum of squares and dividing by error degrees 3891



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It was verified that pulp density is conversely proportional to mean response. According to the graphs illustrated in Figure 3, it can be concluded that the pulp density is a highly effective parameter as the increase in liquid to solid ratio resulted in a sharp increase in lead extraction due to the limited solubility of PbCl2. In fact, lead chloride solubility even at elevated temperatures is not sufficiently high and lead solubility constraints limit the extraction of lead in most commercial chloride leaching processes; hence, the chloride leaching of ZPR is better carried out at low pulp density (high liquid to solid ratio). However, the dissolution of lead increases significantly by increasing the NaCl concentration, leaching with NaCl concentration above 400 g/L seems practically impossible due to saturation. The reaction between lead sulfate (PbSO4) in the residue and chloride ions in the solution can be depicted as

model is significant, and there is only 0.09% chance that a model F-value this large could occur due to noise. Meanwhile, Table 6 indicates that 96.18% of the total variation in the lead extraction was attributed to the studied experimental variables. A comparison between the ANOVA results (Table 5) and the pooled ANOVA results (Table 6), indicates that after pooling the factor with least variance, the percentage contributions of the remaining factors decreases slightly, however, the ranking of the factor effects still remains the same. From the statistical results obtained, it was shown that the developed model was adequate to predict the desired responses in the range of variables under study. Figure 2 shows the

PbSO4 + 2Cl− → PbCl2 + SO4 2 − (PbSO4 + 2NaCl → PbCl2 + Na2SO4 )

(5)

In the presence of higher chloride concentrations, formation of soluble lead chloride complexes (PbCl3¯ and PbCl42−) increases according to the eqs 3 and 4. As it can be seen from the graphs, reaction time (t) and acidic pH (pHa) have little effect on the lead extraction from residue. Since the extractions of metals can be improved at prolonged periods of leaching, it was intended to find such relations for this residue too. Thus, the leaching tests were carried out in 1 and 8 h. However, the extraction of lead increases slightly with increasing reaction time, and also as acidic pH (pHa) is reduced, lead extraction increases slightly, since by adding H2SO4 to the solution, which naturally reduces pH, the activity of Cl− ions increases. As seen in Figure 3 the extraction of lead does not change significantly with increasing stirring speed (R), reaction temperature (T), and neutral pH (pHb). For maximizing lead extraction in the leaching of ZPR the following vaules were chosen: C2, 400 g/L; R2, 700 rpm; T1, 55 °C; t2, 8 h; (L/S)2, 20; (pHa)1, 2; and (pHb)2, 5.5 [notice that R, T, pHa and pHb are pooled factors]. As previously mentioned, pulp density and NaCl concentration have significant effects on the lead extraction (Table 6) and higher extraction of lead from ZPR could be achieved by using low pulp density and the presence of enough concentrations of chloride ions in the leaching media (Figure 3), therefore, liquid-to-solid ratio and NaCl concentration were chosen in maximum quantities. Finally, only the significant factors (C2 = 400 g/L and (L/S)2 = 20) were used to

Figure 2. Predicted vs actual data for lead extraction percentage.

predicted values versus the experimental values for lead extraction. The value of correlation coefficient (R2) between the predicted and observed data is 0.9782. As it can be seen, the predicted values obtained were quite close to the experimental values, indicating that the developed model was successful in capturing the correlation between the leaching variables to the lead extraction. The average level response analysis is done by averaging the lead extraction percentage at each level of each factor and plotting the values in a graphical form. The average level responses from the plots, help in optimizing the objective function under study. The numerical values of the maximum points in these plots correspond to the best values of factors. Figure 3 shows the effect of controllable factors on average lead extraction percentage. The order of graphs in Figure 3 is according to the degree of the influence of parameters on the performance statistics.

Figure 3. Average effect of parameters for each level on lead extraction percentage. 3892



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estimate the performance at the optimum conditions, and, under these conditions, the predicted amount of lead extraction and confidence limits were 85.91% and 78.40%−93.41%, respectively. According to the experimental plan given in Table 4, the experiments corresponding to these conditions (C2 = 400 g/L and (L/S)2 = 20) for maximum lead extraction have been carried out in experiments 5 and 7, and the obtained lead extraction percentages (82.67 and 84.70) were accepted by confidence limits that were calculated to be 78.40%−93.41%. For economical considerations, it is desirable that the dissolved amount of lead be kept at maximum and the temperature T, reaction time t, acid, and Ca(OH)2 consumption be kept at a minimum. Therefore, the following were selected as ultimate optimum conditions for both economical point of view and maximum extraction of the lead: C2, 400 g/L; R2, 700 rpm; T1, 55 °C; t1, 1 h; (L/S)2, 20; (pHa)2, 3.5; and (pHb)1, 4. Finally, a confirmation experiment was performed to verify the conclusions drawn based on Taguchi. The confirmation leaching experiments were carried out twice at the same working conditions, and this combination of parameters as ultimate optimum conditions was also used in experiment 7 in Table 4. The experimental average result under these conditions was 85.1% for lead extraction. As it can be seen, there is a good agreement between the predicted and the experimental results, and these experimental results are within the 95% significance level confidence intervals. The experimental results confirmed the validity of the applied technique for optimizing the lead extraction parameters. Thus, it is possible to increase lead extraction percentage significantly using the proposed statistical technique.

combination of other factors (pulp density and NaCl concentration). (5) The total optimum leaching conditions for economical and maximum extraction of lead from ZPR were C2, 400 g/L; R2, 700 rpm; T1, 55 °C; t1, 1 h; (L/S)2, 20; (pHa)2, 3.5; and (pHb)1, 4. In such a condition, lead extraction of 85.9% was proposed as the optimum result by the statistical model. The average result of the confirmation leaching experiments was 85.1% which was in reasonable agreement with the predicted result in the confidence interval of 95%. This proved that interactive effects of parameters were indeed negligible. (6) The Taguchi method can successfully be applied to the simultaneous dissolution and sulfate removal of the ZPR for maximizing the lead extraction. The statistical model developed using the experimental results was effective to navigate the design space of the process.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 showing Taguchi L8 (27) orthogonal array. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +98412-3443802. Fax: +98-412-3443443. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS The main conclusions derived from the primary experiments are as follows: (1) Lead extraction increases by increasing NaCl concentration and reducing pH. (2) Acidic pH can be adjusted using H2SO4 and leaching by H2SO4 is more cost-effective than HCl. (3) Chloride leaching and sulfate removal can be done simultaneously. Based on the experimental results and their presented analysis, the following conclusions may be highlighted: (1) The most significant parameters affecting the lead extraction were pulp density and NaCl concentration, respectively. Because of the limited solubility of PbCl2, increasing the pulp density reduced the lead leaching efficiency. On the other hand, increasing NaCl concentration enlarged the lead extraction. (2) According to the percent contribution of each factor, indicated in the ANOVA table, the most effective factors are liquid to solid ratio (L/S), NaCl concentration (C), reaction time (t), acidic pH (pHa), reaction temperature (T), stirring speed (R), and neutral pH (pHb ), respectively. (3) In terms of maximizing lead extraction in the leaching of the ZPR, the following values were chosen: C2, 400 g/L; R2, 700 rpm; T1, 55 °C; t2, 8 h; (L/S)2, 20; (pHa)1, 2; and (pHb)2, 5.5 [notice that R, T, pHa and pHb are pooled factors]. (4) In optimum conditions of operating parameters, stirring speed (R), reaction temperature (T), reaction time (t), acidic pH (pHa), and neutral pH (pHb) were not statistically significant factors. This means that high extraction of lead could be achieved using optimum

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Determination of the Optimum Conditions for the Leaching of Lead

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