Optimization of Microwave-Assisted Removal of Lead from Anode

Article pubs.acs.org/IECR

Optimization of Microwave-Assisted Removal of Lead from Anode Slime in Triethanolamine Solutions Dilara Tokkan,† Turan Ç alban,‡,* Soner Kuşlu,‡ Sabri Ç olak,‡ and Bünyamin Dönmez‡ †

DSI The 8th Regional Directorate Quality Control Laboratory Department Office, Orgeneral Demircioğlu Street 25100 Erzurum, Turkey ‡ Engineering Faculty, Department of Chemical Engineering, Atatürk University, 25240 Erzurum, Turkey ABSTRACT: The aim of this work is to investigate the optimization of removal of lead from anode slime in triethanolamine solutions by the microwave effect using statistical design methods. Triethanolamine concentration, leaching temperature, solidto-liquid ratio, and leaching time has been selected as variables. A model has been obtained among relevant parameters by means of variance analysis by using the Matlab Computer Software Programme. The optimum conversion conditions for process are found to be a triethanolamine concentration of 3.5 M, leaching temperature of 313 K, solid-to-liquid ratio of 1/10, and leaching time of 150 min. Under the optimal conditions, the removal of lead is obtained as 87.67%. It is observed that the model obtained has nearly fitted the full second-order model, and the correlation coefficient calculated at 95% confidence level has a value of 0.97.

1. INTRODUCTION Industrial development has led to the generation of more and more industrial waste during production processes. Industrial sludge can normally be treated by extraction using acids or solvents. The extraction process is environmentally and economically attractive because it can dexotify industrial sludge and remove valuable metals for reuse. Various studies have investigated the effect of acids and solvents to remove metals from solid wastes, such as in extraction using nitric and sulfuric acids.1,2 However, some problems may arise during hydrometallurgical operations as extraction. This includes the low recovery of extracted metal, difficulties in solid−liquid separation, and effect of impurities on the case of purification. Anode slimes are collected from the bottom of the electrolytic cell during the copper refining. The value of the anode slimes is determined mainly by the noble metals content, but in certain processes the contributions of Ni, Se, and other impurities are significant. There are two different kinds of slimes depending on the sources obtained. The first one is produced during the processing of copper concentrate and contains a relatively high concentration of gold, silver, selenium, and tellurium. The secondary slime is produced during the processing of recycled scrap and contains a higher concentration of lead, copper, tin and silver than the first one. The anode slimes are characterized by high concentrations of silver, lead, copper, and tin. The slimes without the copper are then enriched in lead, tin, silver and gold.3 Several methods are used for the recovery of valuable metals from anode slimes. These methods can be differentiated into pyrometallurgical processing, which includes roasting in the presence of oxidizing agent, sulfate-roasting, and the soda/ash process, and hydrometallurgical processong in which anode slimes are worked using different leachants such as chlorine, nitric acid, and sulfuric acid.4−6 Anode slimes generally contain 1−25% Pb, and Pb is basically present as PbSO4. During the hydrometallurgical © 2012 American Chemical Society

processing, the waste lead precipitates mostly as lead sulfate from solutions due to its low solubility (4.3 mg/100 mL). Finally, the insoluble lead sulfate is mostly converted into soluble compounds by leachates of successive alkaline and acid leaching of industrial anode slimes. Copper refinery anode slimes can be advantageously deleaded by reacting the PbSO4 present in the anode slimes with Na2CO3 solutions and then leaching the resultant lead carbonate with acetic acid.7 Bulakhova and Ben’yash8 note that the reaction is essentially complete within 30 min at 20 °C and within 15 min at 50 °C. The rate depends slightly on the stoichiometric ratio of PbSO4/Na2CO3, and over 99% lead sulfate conversion is achievable at the higher temperature in the presence of excess Na2CO3. Talip et al.2 carried out a work that the optimum conversion conditions for the lead removal process by the Taguchi method are a solid−liquid ratio 0.05 of g/mL, a reaction period of 600 s, a reaction temperature of 50 °C, and a Na2CO3 concentration of 2 M. Under optimal conditions, the experimental results show that the conversion of lead sulfate at the 95% confidence level can be 97%, approximately. In another study, the copper anode slime was dissolved in concentrated H2SO4, and the optimum parameters were confirmed as reaction temperature of 210 °C, solid-toliquid ratio of 0.5, and the reaction period of 2 h.9 Microwave (MW) is a nonionizing electromagnetic energy. It is characterized by mutually perpendicular electric and magnetic fields, lies in the region of the electromagnetic spectrum between millimeter and radio waves, and is defined as those waves with wavelengths between 1 and 100 cm, corresponding to frequencies between 300 MHz and 300 GHz, respectively. MW causes movement of molecules and ions, will Received: Revised: Accepted: Published: 3903

March 14, 2011 February 13, 2012 February 21, 2012 February 21, 2012 dx.doi.org/10.1021/ie2005065 | Ind. Eng. Chem. Res. 2012, 51, 3903−3909

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attempt to improve the yield of extracted metal and to reduce process time, especially with increasing demand for more environmental friendly processes. Advantages of microwaves having a distinctive source of energy compared to conventional heating include low processing time, direct, selective, and volumetric heating, and a more controllable heating process. In contrast, microwave energy is transferred to materials by interaction of the electromagnetic fields at the moleculars level, and the dielectric properties ultimately determine the effect of the electromagnetic field on the material.10 Some researchers that focused especially on this subject are summarized as follows: Joret et al. studied the effect of microwaves on the dissolution rate of CeO2 and Co3O4 in nitric acid and explained that microwaves give rise to an apparent acceleration of the chemical reactions as a result of the superheating phenomenon.16 Shibata et al. studied the decomposition reaction of sodium hydrogen carbonate in water, and they found that activation energy of the reaction is reduced by microwave irradiation.17 Peng and Liu applied microwave energy in the leaching of sphalerite with acidic ferric chloride. Test results demonstrated that the leaching rate of zinc increased with microwave energy.18 Chao−Yin et al. investigated the removal of copper from industrial sludge by traditional and microwave acid extraction. Their experimental results showed that the most economical traditional extraction conditions were the use of 1 N sulfuric or nitric acid for 60 min at an S/L ratio of 1/20; however, at an S/L ratio of 1/6, the extraction time needed to achieve the same copper removal efficiency was increased to 36 h.1 Harahsheh et al. investigated the reality of nonthermal effects in microwave-assisted leaching systems. In this work, chalcopyrite and sphalerite were chosen as model materials because of their economic importance and the diversity of their heating behavior in a microwave field. Leaching of both chalcopyrite and sphalerite in ferric sulfate under microwave conditions has shown enhanced recoveries of metal values compared to that produced conventionally.14 This work focuses on the optimization of removal of lead in triethanolamin solutions from anode slime by microwave using statistical design methods. The effects of various parameters such as triethanolamine concentration, leaching temperature, solid-to-liquid ratio, and leaching time on the removal of lead from decopperized anode slime have been investigated. A model has been fitted among relevant parameters by means of variance analysis by using the Matlab computer software.

be reflected, transmitted, and absorbed, and heats throughout absorbing materials. The energy transfer is by dielectric loss in MW heating and is not by conduction or convection as in conventional heating. MW can affect molecules in two ways. The first effect is dipole rotation. Dipole rotation refers to the alignment with the electric field component of the radiation of molecules which have induced dipoles. When MW passes through a sample, the molecules of the samples having dipole moments will try to align themselves with it. At 2450 MHz, the field oscillates 4.9 × 109 times per second and sympathetic agitation of the molecules generates heat. The amount of the energy transferred, the loss tangent, is a function of both dipole moment and dielectric constant. This is not a linear function. Dielectric loss factor and dielectric constant of a sample are two important dielectric properties of a sample in MW heating. The high value of dissipation factor which is the ratio of dielectric loss factor to dielectric constant indicates the susceptibility of the sample to MW. The energy transfer is more efficient when the molecules are able to relax quickly. The most efficient transfer occurs when the relaxation time matches the frequency of the MW energy.11 Ionic conduction is another important microwave heating mechanism. Ionic conduction is the migration of dissolved ions with the oscillating electric field. Heat generation is due to frictional losses. In ionic conduction, the energy is transferred from the electric field causing ionic interactions that speed up the heating of solutions. Ionic conduction increases with temperature allowing ionic solutions to become stronger absorbers of MW as they are heated. Microwaves cause molecular motion by migration of ionic species and rotation of dipolar species. Microwave penetrates the material, vibrates the polar molecules at high frequencies, and produces energy in the form of heat.11,12 Microwave dielectric heating has attracted the attention of chemists. It causes the reduced time scales of chemical reactions. Indeed, there is no satisfactory explanation that has been put forward in order to explain the observed acceleration of the reaction rate and drastic reduction in the processing time. The reduced time is attributed to a specific “microwave effect”. A decrease in the activation energy observed in some researchers stimulates naturally the proposal of a “non-thermal” or “a-thermal” specific microwave effect13 which accelerates a reaction to a rate faster than would be expected on the basis of the bulk reaction temperature; an increase in the probability of contact between molecules or atoms by rapid rotation of dipoles induced by microwave field might cause a reduction of the activation energy. Meanwhile, some researchers claim that microwaves provide only a convenient way to transfer energy to a given system and give rise to an apparent acceleration of the chemical reactions as a result of a “superheating” phenomenon in the context of “hot spots” theory.13−15 The natural consequence of this theory is that the bulk temperature is no longer representative of the reaction conditions. Over the past few decades, microwave heating has been employed in various technological processes such as pretreatment of ores, leaching, roasting, drying, treating of slags, and wastes. MW is an increasingly used tool to enhance chemical process rates such as electrochemistry ultrasound and photochemistry and has been used in various areas of chemistry, such as, MW-assisted drying, solvent extraction, leaching, sample preparation, hydrolysis, digestion, and inorganic reactions, etc. Microwave-assisted leaching has been investigated in an

2. EXPERIMENTAL DESIGN The experimental design is used to investigate the effects of variable parameters on a process. The factorial experiment design method is a statistical method used to determine the best experimental conditions.20 In this method, the influences of all experimental variables, factors, and interactions on the responses are investigated. A frequently used factorial experiment design is known as the 2k factorial design, which is basically an experiment involving k factors, each of which has two levels (“low” and “high”). In addition, a zero-level is also included, a center, in which all variables are set at their mid value. The center experiments should always be included in factorial designs, for the following reasons: the risk of missing nonlinear relationships in the middle of the intervals is minimized and repetition allows for the determination of confidence intervals.21−24 The method used to compare the magnitude of estimated effects of factors with the magnitude of experimental error is 3904

dx.doi.org/10.1021/ie2005065 | Ind. Eng. Chem. Res. 2012, 51, 3903−3909

Industrial & Engineering Chemistry Research

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Table 1. The Chemical Analysis of the Decopperized Anode Slime chemical composition

Pb

Ag

Cu

Au

SO4−2

SiO2

Ni

Fe

Zn

Sn

Sb

As

humidity

others

decopperized slime (%)

29

2.15

0.33

0.13

28.70

1.72

0.03

0.16

0.28

15.9

17.1

0.93

0.78

2.86

Figure 1. X-ray diffractogram of the decopperized slime used in the study.

called analysis of variance. If the magnitude of a factor effect is large when compared with experimental error, it is decided that the changes in the selected response cannot occur by chance, and those changes in the response can be the effects of the factors. The factors causing a variation in the response are called significant. Also, it is controlled the effects of pure quadratic terms by means of the following statistic: moF(y1̅ − yo̅ )2 LOFcurv = mo + F

by defining

bo1 = bo −

Equation 6 may be written in usual from as Y = bo1 +

(1)

⎛ QF ⎞1/4 ⎜ ⎟ ⎝ 4 ⎠

(3)

N = F + 2n + mo

(4)

The second-order model is defined as follows so as to facilitate calculations: 4

Y = bo +

4

∑ biXi + ∑ bii(Xi2 − i=1

i=1

X12) +

4

4

∑ ∑ bijXiXj i = 1 j⟩1

(5)

where X12 =

1 N

N

∑ Xi2 = i=1

F + 2β2 N

4

4

i=1

i=1

4

4

∑ biXi + ∑ biiXi 2 + ∑ ∑ bijXiXj i = 1 j⟩1

(8)

3. MATERIAL AND METHODS The work planned to research and optimize the parameters influencing the removal of lead from the anode slime contains the following stages: (1) The removing of copper from the raw anode slime19 and (2) the removal of lead from decopperized anode slime in triethanolamine solutions by microwave effect (the present study). At the first stage, before the removal of lead, the copper in the anode slime supplied from Sarkuysan Copper Co. in Turkey was removed under appropriate conditions. On this occasion, the dissolution of the copper in the raw anode slime in H2SO4 solutions with/without oxygen was investigated, and the optimum conditions corresponding to a solubility of 99.67% were determined as follows:19 Blade number of 1, reaction temperature of 70 °C, O2 flow rate of 1.24 × 10−6 m3·s−1, stirring speed of 450 min−1, acid concentration of 5.43 wt %, solid-to-liquid ratio of 0.125 g/mL, reaction period of 3600 s, and roasting temperature of 300 °C. The decopperized anode slime was washed several times, filtered, and then dried at laboratory medium. The chemical composition of the homogeneously blended sample was determined by volumetric, standard gravimetric, and AAS methods. Chemical analysis of the decopperized anode slime used in the experiments may be seen in Table 1. An X−ray diffractogram illustrating the contents of the decopperized anode slime was given in Figure 1.

(2)

Q = (N1/2 − F1/2)2

(7)

i=1

Analysis of variance is curvature effect. Because of this, auxiliary experiments are carried out. Among various second-order designs, the orthogonal central composite design is the most popular, which required 2n auxiliary runs conducted at two new factor levels factorial design required −β and +β. β is calculated by the following relation: β=

4

∑ bij X12

(6) 3905

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generator at 2450 MHz, with adjustable power within the range 0−1 kW, an R26 standard rectangular waveguide, and an applicator. Three manually adjustable stub tuners inserted in the waveguide section and the tuning plunger of the applicator were used to maximize microwave absorption by minimizing the reflected power. A cylindrical Pyrex glass reactor was installed in the applicator at the position of the highest electric field, with an internal volume of 500 mL, an internal diameter of 110 mm, and a height of 130 mm. It was equipped with a reflux condenser to prevent evaporation during heating. An optical fiber connected to a transducer (PT-100) allowed the measurement of temperature inside the reactor with an accuracy of (1 °C). The magnitudes of the forward and reflected powers were measured by power meters through the directional coupler. The incident power minus the sum of the reflected and transmitted power gives the absorbed power. The forward and reflected powers and the temperature inside the reactor were recorded and monitored using a programmable controller (μscada control program).25,26

At the second stage, the optimum conditions on removal of lead from decopperized anode slime in triethanolamine solutions by microwave effect were determined using statistical design methods. The experiments were carried out using the above-mentioned methodology as described elsewhere. In the light of pre-experiments, four factors, namely, triethanolamine concentration, leaching temperature, solid-to-liquid ratio, and contact time, were chosen as independent process variables. The process variables and their levels for 24 factorial design are shown in Table 2. The 24 orthogonal factorial design and Table 2. Process Variables and Their Levels for 24 Factorial Design variable name TEA concentration (M) temperature (K) solid-to-liquid ratio (g·mL−1) contact time (min)

symbol for chosen variable

low level (−)

high level (+)

average level (0)

X1

2.5

3.5

3.0

X2 X3

303 1/10

313 1/5

308 3/20

X4

90

150

120

4. RESULT AND DISCUSSION Triethanolamine concentration, leaching temperature, solid-toliquid ratio, and leaching time were selected as process variables to investigate their effects on the process. The experiments for observing the effect of concentration of triethanolamine solutions on the removal process was studied by varying to 2.5, 3.0, and 3.5 M. The dissolution level of the process increases with increase in the concentration of triethanolamine solutions. The leaching temperature is a factor of great importance for the leaching process. The effect of reaction temperature was examined at 303, 308, and 313 K. The quantity of lead removed increases with increasing reaction temperature. The reaction rate constant is exponentially dependent on reaction temperature. The effect of solid/liquid ratio on the removal of lead was investigated by varying ratio to 1/10, 3/20 and 1/5 g/mL. Decreasing solid-to-liquid ratios favor the process. The rate decreases with increasing solid/ liquid ratio. This situation can be explained by the decrease in

central composite design for the microwave process were applied to estimate both main effects and second order effects; leaching experiments were performed in random order to avord systematic error. As usual, three central replicates were also employed to calculate pure experimental error. Microwave experiments were conducted as follows: The glass reactor was filled with 200 mL of triethanolamine solution, which was then heated to the desired temperature. Subsequently, a predetermined amount of anode slime was added to the solution, and the stirring operation was started at a stable speed. At the end of the reaction period, the contents of the reactor were immediately filtered, and the amounts of lead in the solution were analyzed by AAS. 3.1. Microwave Experimental System. The experimental microwave apparatus (Elta Ltd., Bursa, Turkey) designed in our laboratory can be seen in Figure 2. It consists of a microwave

Figure 2. Microwave experiment system. 3906

dx.doi.org/10.1021/ie2005065 | Ind. Eng. Chem. Res. 2012, 51, 3903−3909

Industrial & Engineering Chemistry Research

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Temperature has a significant effect at high levels (+1) of TEA concentration. A great dissolution is achieved under the following experimental conditions. The leaching reaction of anode slime in TEA solution is as follows:

the amount of solid colemanite particles per amount of reagent potassium hydrogen sulfate in the reaction mixture. The effect of contact time was studied by varying the ratio to 90, 120, and 180 min. The leaching tests of anode slime with TEA solution by using microwave were studied. The collected data were analyzed by a MATLAB compatible PC using the ANOVA computer software package to evalute the effect of each parameter on the leaching criteria. The process variables and their levels for 24 factorial design can be shown in Table 2. To minimize the effect of uncontrolled factors and time trends, all experiments were performed in a random order. Experimental design and results of the 24 factorial design, analysis of variance and experimental design for second-order model and dissolution yield can be shown in Tables 3, 4, and 5, respectively. These results indicate

PbSO4(s) + TEA(aq) → (PbTEA)+2 SO4−2(aq)

At the end of leaching experiments, a regression model for the leaching of anode slime under the best conditions was founded as full second-order model. The model is given as follows: ̂ = 64.84 + 8.09X1 + 5.78X2 − 4.19X3 + 3.28X 4 Yef − 0.44X12 − 1.05X2 2 − 3.43X32 − 1.91X 4 2 + 1.65X1X2 − 1.07X1X3 + 0.1X1X 4 − 0.33X2X3 + 0.63X2X 4 + 0.25X3X 4

Table 3. Experimental Design and Results of the 24 Factorial Design experiment no.

X1

X2

X3

X4

YPb

YPb (model)

16 12 2 6 13 9 1 5 4 8 10 11 3 7 14 15 1* 2* 3*

+ − + − + − + − + − + − + − + − 0 0 0

− − + + − − + + − − + + − − + + 0 0 0

+ + + + − − − − + + + + − − − − 0 0 0

− − − − − − − − + + + + + + + + 0 0 0

56.33 45.30 70.16 48.50 65.68 50.90 78.02 59.51 61.84 51.14 75.59 60.77 72.06 55.13 88.45 63.47 68.25 68.67 67.07

55.73 45.00 70.60 53.25 66.24 51.24 81.11 59.49 62.30 51.56 77.16 59.81 72.80 57.80 87.67 66.05 68.95 68.95 68.95

(9)

(10)

This full second model of 15 factors estimates the experimental data very well and the correlation coefficient (r2) of model obtained in this case is 0.97. However, a simpler model with fewer factors and with a high r2 value may also be established by means of variance analysis. For this purpose, the model established with effective parameters obtained by variance analysis at 95% confidence interval was given as follows: ̂ = 64.84 + 8.09X1 + 5.78X2 − 4.19X3 + 3.28X 4 Yee − 3.43X32 − 1.91X 4 2 + 1.65X1X2 − 1.07X1X3 (11)

where Yef represents full second-order model and Yee represents the model established with effective parameters obtained by variance analysis conducted at 95% confidence interval for the lead leaching efficiency from anode slime. The p-value is the probability of obtaining a test statistic at least as extreme as the one that was actually observed, assuming that the null hypothesis is true. One often ″rejects the null hypothesis″ when the p-value is less than the significance level α, which is often 0.05 or 0.01.22 This model in eq 5 is a simpler model with fewer factors since well-established, systematic errors are absent. Figure 3 shows the graphical representation of “size effect” of each parameter upon the microwave-assisted leaching of the lead from anode slime in aqueous media. From Figure 3, it can be

that the process is affected strongly from TEA concentration, temperature, time, solid-to-liquid ratio, and the interaction of TEA concentration, temperature, and solid-to-liquid ratio. Table 4. Analysis of Variancea

a

source of variation

sum of squares

d.f.

Mean squares

F ratio

P values

decision (α=0.05)

X1 X2 X3 X4 X1 X2 X1 X3 X1 X4 X2 X3 X2 X4 X3 X4 curvate lack of fit experimental error total

1111.22 463.97 252.17 182.12 43.76 18.19 0.14 1.69 6.33 0.99 71.34 23.88 1.38 2177.18

1 1 1 1 1 1 1 1 1 1 1 5 2 18

1111.22 463.97 252.17 182.12 43.76 18.19 0.14 1.69 6.33 0.99 71.34 4.77 0.69

1614.84 674.25 366.46 264.65 63.59 26.43 0.21 2.46 9.20 1.44 103.68 6.94