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In this work, a chloride/hypochlorite leaching process was performed for zinc plant residues. Sodium chloride and calcium hypochlorite were used as leaching and oxidizing agents, respectively. Fractional factorial method has been used to test the main effects and the interactions among the factors was investigated. The statistical software named Design-Expert 7 (DX7) has been used to design experiments and subsequent analysis. Parameters and their levels were reaction time (t = 16 and 120 min), reaction temperature (T = 30 and 70 -C), solid-to-liquid ratio (S/L = 1/6 and 1/38), pH (pH = 0.5 and 2), and Ca(OCl)2 concentration (C = 0.6 and 3 g/L). Statistical analysis, ANOVA, was also employed to determine the relationship between experimental conditions and yield levels. The results showed that the reaction temperature and pH were significant parameters for both lead and silver extractions but the solid-to-liquid ratio had significant effect only on lead extraction. Increasing pH reduced leaching efficiency of lead and silver. However, increasing reaction temperature promoted the extraction of lead and silver. The ultimate optimum conditions from this study were t1:16 min, T2:70 -C, (S/L)2:1/38, pH1:0.5, and C1:0.6 g/L. Under these conditions extraction of lead and silver were 93.60% and 49.21%, respectively.
Keywords: Zinc plant residue; Chloride/hypochlorite leaching; Lead; Silver; Fractional factorial method.
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Growing environmental concerns and economic necessity are leading the mining and metallurgical industries around the world to invest in suitable techniques for processing the wastes generated and maximizing the recycling of resources. More than 80% of the world wide primary zinc is produced via a combined roast-leach-purification-electrowinning process. During the hydrometallurgical processing of zinc from the roaster, calcine lead and silver report to the leach residue [1, 2] which can be utilized for the recovery of these two metals. Also, 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]. 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 [7-10].
Different processing techniques are practiced to treat the leach residue to recover zinc as well as lead in the presence of high iron content . However, chloride leaching is the most recognized and widely used recovery method . Chloride leaching processes have been employed using either NaCl [1, 2, 11-15] , or MgCl2 and CaCl2 , or FeCl3 [12, 17] along with HCl. Recently, Behnajady and Moghaddam  have evaluated the chloride leaching of ZPR in NaCl-H2SO4 media. Based on their works, it was concluded that using H2SO4 is more cost-effective than HCl.
Due to the growing environmental concern of the use of cyanide for gold and silver processing, interest on the use of noncyanide lixiviants, especially with halides has been renewed recently [19, 20]. Leachants based on halides (chloride, bromide and iodide) have been used in the past to dissolve gold and silver into solution [21, 22]. Different combinations of oxidants (such as hypochlorite, hydrogen peroxide, bromine and iodine) can be used in conjunction with the complexants (chloride, bromide and iodide) to provide leaching conditions. One of the alternatives is a chloride solution with an appropriate oxidizing agent. Widely used oxidizing agent for gold and silver-chloride system is hypochlorite. Several workers have investigated chloride/hypochlorite solutions to leach gold and silver from several types of gold ores and concentrates [20, 23], but in our study chloride (complexant)-hypochlorite (oxidant) mixtures were used to leach lead and silver from a ZPR.
ZPRs are SO4-bearing residues and the presence of excessive levels of sulphate adversely affects the lead and silver extraction. To eliminate sulphate from the residues and to recover water-soluble zinc such as ZnSO4, a water washing step has been found to be very effective [1, 2, 11, 24].
The aim of the present study was the determination of the effects of the main factors involved in the chloride/hypochlorite leaching process for ZPRs. First the residue was leached with water. Then, washed residue was leached in NaCl-H2SO4-Ca(OCl)2 media to extract lead and silver. The planning of the experimental work was arranged using fractional factorial method in order to establish, in a very efficient manner, the main effects and the interactions among the factors were investigated. Accordingly, the effect of operating factors including reaction time (t), reaction temperature (T), solid-to-liquid ratio (S/L), pH (pH), and Ca(OCl)2 concentration (C) on the extraction efficiency of lead and silver were investigated.
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. Afterwards, 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 with mechanical stirrer at 600 rpm to improve the recovery in the subsequent stage of chloride/hypochlorite 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, lead, and silver using an atomic absorption spectrometer, and its mineralogical structure was identified by X-ray diffraction analysis.
2.3. Leaching with chloride/hypochlorite solutions
After water leaching, the ZPR was subjected to chloride/hypochlorite leaching to extract lead, and silver. Commercial grade salt (NaCl), as a cheap agent, was used to prepare chloride solution. Industrial grade H2SO4 was used for pH adjustment and technical grade Ca(OCl)2 was the oxidizing agent.
The leaching experiments were performed in a 2L beaker in a thermostatically controlled water bath equipped with mechanical agitator. After pouring 1 L of clear NaCl solution (300 g/L) to the beaker and setting the temperature at the desired value, a known quantity of residue was added and acidified by H2SO4, and finally Ca(OCl)2 was added while stirring the content of the beaker at 700 rpm. 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, and Ag by atomic absorption spectrometer (AAS).
2.4. Experimental design
In this study, a two-level fractional factorial design was employed. The 2k factorial design is particularly useful in the early stages of experimental work when many factors are likely to be investigated. In fact it is possible to arrange an orthogonal experimental plane in which both the main effect and the interaction among the factors investigated can be evaluated independently. As the number of factors increase in 2k factorial design, the numbers of required runs rapidly outgrow. Fractional factorial designs can often identify which factors are significant by running only a fraction (subset) of a full factorial experiment [25, 26].
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. On the basis of our previous experience in related works and those experimental conditions reported by other researchers for the leaching of similar residues and preliminary tests performed: reaction time (t), reaction temperature (T), solid-to-liquid ratio (S/L), pH (pH) and Ca(OCl)2 concentration (C) were chosen as the five factors to be investigated, and two levels are the exclusory set for each of our five factors. Five selected control factors in two levels applied in this study, have been listed in Table 1.
For the fractional factorial design and subsequent analysis, the statistical software named Design-Expert 7 (DX7) was used. Since we were studying 5 factors, the 25 full factorial design would require 32 experimental runs for all combinations of the levels of the factors were investigated. Sixteen experiments of the full factorial design have been considered instead of 32. Table 2 represents the selected experiments for this study. This design is a 1/2nd fraction, so every effect will be aliased with one other effects, most of which are ignored by default to avoid unnecessary screen clutter. The output indicates that each two-factor interaction will be confounded with one three-factor interaction, which are generally not important. The aliases structure for design indicates as below:
[A] = A [AB] = AB + CDE [BD] = BD + ACE
[B] = B [AC] = AC + BDE [BE] = BE + ACD
[C] = C [AD] = AD + BCE [CD] = CD + ABE
[D] = D [AE] = AE + BCD [CE] = CE + ABD
[E] = E [BC] = BC + ADE [DE] = DE + ABC
E = ABCD
Replicate tests were not done for the determination of the experimental error, for the F values in the ANOVA analysis, because it is possible to determine the error using interactions that are not significant. Self-contained experimental runs can be realized without replicate tests when a factorial design is applied to experimental data. In this way, the high interactions or the interactions with relatively low values of the effects can be used to estimate the experimental error. The process of ignoring a factor or interactions once it was deemed insignificant was called pooling. In this case, having used a fractional design, all the possible high interactions are saturated by main and two-interaction effects. However, by consideration of the relatively low values of some two-interactions, it is possible to use them to make an evaluation of experimental error.
3. Results and discussion
3.1. Characterization of the sample
Fig. 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 has been given in Table 3. The XRD analysis showed that the residue compounds were lead sulphate (PbSO4), calcium sulphate dihydrate (CaSO4.2H2O), zinc sulphate 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, 14.2% lead, and 0.106% silver.
3.2. Characterization of the washed sample
The particle size distribution of the washed residue is shown in Fig. 1b. As it can be seen from the particle size distribution curve, d80 was calculated as 105 μm. When the XRD pattern of washed residue was compared to that of the initial residue for their constituents, it was concluded that most of the zinc sulphate 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, 14.4% lead, and 0.11% silver. 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 sulphate in water.
3.4. Leaching in NaCl-H2SO4-Ca(OCl)2 media
Corresponding leaching efficiencies obtained under the candidate conditions based on fractional factorial design have been displayed in Table 4. The collected data were analyzed by software to evaluate the effect of each parameter on the optimization criteria. The maximum amount of lead and silver extraction were defined as optimization criterions. Statistical analysis of variance (ANOVA) was performed to check whether the process parameters were statistically significant or not. The F-value for each process parameter indicates which parameter has a significant effect on the leaching efficiencies and is simply a ratio of the squared deviations to the mean of the squared error . Usually, the larger F-value indicates the greater effect on the leaching efficiency.
The values of the ANOVA analysis for lead extraction after pooling have been given in Table 5. F- value for this condition with 95% confidence level is 6.61 . Therefore, the results of F-value from Table 5 show that the main effects including the pH (pH), reaction temperature (T), and solid-to-liquid ratio (S/L), and the interaction between T-S/L and S/L-C were significant for lead extraction responses within the levels and conditions tested. The F-value of these factors and interactions are greater than the extracted F-value from the table (95% confidence level). This means that the variance of these factors and interactions were significant compared to the variance of error. Another method to determine the significant factors is by P-values calculated using DX7 software. P values less than 0.05 indicate that the effect of model factors are significant within the 95% confidence level.
The F and P values of the model were calculated as 9.95 and 0.0102, respectively. Since the model P value is less than 0.05, the model is significant within the 95% confidence level. The model F-value (9.95) implies that the model is significant, and there is only 1.02% chance that a model F-value this large could occur due to noise.
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. Fig. 2 shows the effect of controllable factors on average lead extraction percentage.
According to the graphs illustrated in Fig. 2, it can be concluded that the reaction time (t) has 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 16 and 120 min but the extraction of lead increases slightly with increasing reaction time. However, the dissolution of lead increases significantly by increasing the reaction temperature (T) (lead chloride solubility increases with increasing temperature). The increase of the lead extraction with temperature reflects the endothermic heat of lead leaching. Because of the limited solubility of PbCl2, the increase in solid-to-liquid ratio (S/L) resulted in a decrease in lead extraction. 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.
As can be seen from graphs, the pH (pH) is a highly effective factor. Since activity of the chloride increases with decreasing pH, leaching of lead increases with declining pH. An increase in the activity of Cl− assists further dissolution of lead because the increasing chloride activity favours the formation of soluble lead chloride complexes. Also, the lead extraction decreases by increasing the Ca(OCl)2 concentration (C). This was likely due to the coprecipitation or adsorption of dissolved lead. Because the formation of calcium sulphate in a chloride leaching circuit adversely affects the metal extraction.
Ignoring interaction effects for the moment, notice that Fig. 2 shows an improvement at level 2 for reaction time (t), reaction temperature (T), and solid-to-liquid ratio (S/L) while level 2 effects for pH (pH) and Ca(OCl)2 concentration (C) cause a decrease in lead extraction. Hence the optimum levels for the factors based on the data are t2T2(S/L)2pH1C1. Coincidentally, trial number 8 was tested these conditions and produced the highest result for the extraction of lead (93.11%).
Fig. 3 shows the interaction effects of S/L-C and T-S/L on average lead extraction percentage. The intersection lines on the left represents the interaction between S/L and C. The second pair of lines represents the effect of S/L at fixed levels of T. Observe that the highest value for the pairs of lines corresponds to (S/L)2C1 and T2(S/L)2. Comparing (S/L)2C1 and T2(S/L)2 to the optimum condition for maximum lead extraction, it can be seen that (S/L)2C1 and T2(S/L)2 are included. Thus, the interactions S/L-C and T-S/L have no influence on the optimum and no further modification is needed.
The results of the ANOVA analysis for silver extraction after pooling have been given in Table 6. F- value for this condition with 95% confidence level is 5.99 . Therefore, the results of F-value from Table 6 show that the main effects including the pH (pH) and reaction temperature (T), and the interaction between T-pH, T-S/L, t-pH, and pH-C were significant for silver extraction responses within the levels and conditions tested. The main effects including the reaction time (t), solid-to-liquid ratio (S/L), and Ca(OCl)2 concentration (C) were not a significant term, but to present a hierarchic model they were included in the model. Model hierarchy maintains the relationships between the main and interaction effects.
The F and P values of the model were calculated as 27.97 and 0.0003, respectively. Since the model P value is less than 0.05, the model is significant within the 95% confidence interval. The model F-value (27.97) implies the model is significant, and there is only 0.03% chance that a model F-value this large could occur due to noise.
Fig. 4 shows the effect of controllable factors on average silver extraction percentage. According to the graphs, it can be seen that the reaction time (t), solid-to-liquid ratio (S/L), and Ca(OCl)2 concentration (C) have little effects on the leaching of silver from residue. On the other hand, the pH (pH) and reaction temperature (T) are highly effective factors. These two factors have same effects on lead and silver leaching (compare Figures 2 and 4). On the contrary, the increased leaching time (t) from 16 to 120 min, caused a decrease in silver extraction. This may be due to formation and precipitation of calcium sulphate or adsorption of complexes on solid surfaces [20, 23]. To minimize this undesirable adsorption of silver-chloro species, the leaching time must be limited to a few minutes only. Also, the changed solid-to-liquid ratio (S/L) from 1/6 to 1/38, caused a decrease in silver extraction. This may be attributed to the fixed pH in high pulp density and it can be concluded that silver solubility limitations cannot be a problem in the leaching of silver from ZPR. So chloride/hypochlorite leaching of silver from ZPR can be carried out in a higher pulp density which is more eco-friendly than a lower pulp density.
For maximizing silver extraction in the chloride/hypochlorite leaching of ZPR by ignoring interaction effects for the moment, the following conditions were chosen: t1T2(S/L)1pH1C2. The experiment corresponding to the optimum conditions for maximum silver extraction has not been carried out during the planned experimental work in Table 4. Therefore, the experiment corresponding to these optimum conditions performed and the corresponding Pb and Ag extractions were 80.82% and 50.56%, respectively. If the experimental plan given in Table 4 were to be studied carefully, it can be observed that there are good agreements between this and the experiments 4 (50.67%) and 7 (50.86%) results for silver extraction. Since differences between these experiments are related to insignificant factors, agreements among silver extraction results are logical. Noted that these experiments are the highest results for the extraction of silver (~50 %).
Fig. 5 shows the interaction effects of T-pH, T-S/L, t-pH, and pH-C on average silver extraction percentage. Observe that the highest value for the pairs of lines corresponds to T2pH1, T2(S/L)2, t2pH1, and pH1C2. On the one hand, comparing T2pH1 and pH1C2 to the optimum condition for silver extraction, it can be seen that T2pH1 and pH1C2 are included. So, the interactions T-pH and pH-C have no influence on the optimum and no further modification is needed. On the other hand, comparing T2(S/L)2 and t2pH1 to the optimum condition for silver extraction, it can be seen that T2(S/L)2 and t2pH1 are not included. If the experimental plan given in Table 4 were to be studied carefully it can be seen that the experiments corresponding to these conditions (t2pH1 and T2(S/L)2) have been carried out during the experimental work as experiments 4 and 7 in Table 4, respectively. As previously mentioned differences between these experiments are related to insignificant factors. Under economical considerations, it is desired that the reaction time and liquid-to-solid ratio should be kept low. For this reason, optimum levels for the reaction time (t) and solid-to-liquid ratio (S/L) were not changed from 1 to 2.
Thus, because the silver extraction is more important than the lead leaching (silver is precious metal) it is desired that those selected parameter levels to be near to the maximum extraction of silver. In the view of above, the following were selected as ultimate optimum conditions: t1: 16 min, T2: 70 -C, (S/L)2: 1/38, pH1: 0.5, and C1: 0.6 g/L. It can be seen that experiments corresponding to ultimate optimum conditions, which is obtained by combining the two series of optimum conditions in a logical manner, have not been performed during the experiments. So, a confirmation experiment was performed to verify the conclusions drawn based on statistical design. The confirmation leaching experiments were carried out twice at the same working conditions, and the experimental average results under these conditions were 93.60% and 49.21% for extraction of lead and silver, respectively. Thus, it is possible to increase lead and silver extraction percentage significantly using the proposed statistical technique.
The chloride/hypochlorite leaching of a ZPR was investigated in NaCl-H2SO4-Ca(OCl)2 media. The effect of process parameters including reaction time (t), reaction temperature (T), solid-to-liquid ratio (S/L), pH (pH), and Ca(OCl)2 concentration (C), each in two levels, was studied with the fractional factorial method. The percentage of lead and silver extraction were optimized separately. Meanwhile, the optimum conditions for simultaneous maximizing of these two responses were determined. 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 pH (pH), reaction temperature (T), and solid-to-liquid ratio (S/L), respectively. pH (pH) and reaction temperature (T) were also the most effective parameters on silver extraction.
(2) pH (pH) and reaction temperature (T) have same effects on lead and silver leaching. Increasing the pulp density decreases extraction of lead as a consequence of precipitation of PbCl2. However, this effect is different for silver and leaching of silver increases with increasing the pulp density.
(3) Quantity values of optimum conditions for lead extraction are 120 min for reaction time (t), 70 -C for reaction temperature (T), 1/38 for solid-to-liquid ratio (S/L), 0.5 for pH (denoted as pH) and, 0.6 g/L for Ca(OCl)2 concentration (C). Under these conditions, the extraction of lead in NaCl-H2SO4-Ca(OCl)2 media was ~93%.
(4) Quantity values of optimum conditions for silver extraction are 16 min for reaction time (t), 70 -C for reaction temperature (T), 1/6 for solid-to-liquid ratio (S/L), 0.5 for pH (denoted as pH) and, 3 g/L for Ca(OCl)2 concentration (C). Under these conditions, the leaching of silver in NaCl-H2SO4-Ca(OCl)2 media was ~50%.
(5) The total optimum leaching condition to maximize lead and silver extraction simultaneously were t=16 min, T=70 -C, (S/L)=1/38, pH=0.5, and C=0.6 g/L. In such a condition, extraction of lead and silver were 93.60% and 49.21%, respectively.
(6) It was shown that the leaching in NaCl-H2SO4-Ca(OCl)2 media is an efficient method to extract lead and silver from ZPR.