in-vitro synthesis of gold nanoparticles using thiourea
IN-VITRO SYNTHESIS OF GOLD NANOPARTICLES USING THIOUREA LEACHATE OF ELECTRONIC WASTE S. DAS, AND Y.P. TING Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
SUMMARY: The generation of electronic waste (E-waste) has been growing at an alarming rate globally due to unrestrained consumption and the short life cycle of electronic and electrical devices. E-waste contains both metals that are precious (e.g. gold) and hazardous (e.g. copper and lead). Disposal of E-waste via landfill causes ground water pollution which has deleterious effects on human health and the environment. The recovery of precious metals from E-waste (typically via hydrometallurgical methods) provides a viable and economic secondary resource. In this study, we investigated the leaching efficiency of ultrasound-assisted thiourea for the recovery of gold from shredded electronic components obtained from assembled printed circuits boards. Cyanidation is commonly used for the extraction of gold but is a highly toxic process. Thiourea may also be used as an alternative lixiviant for the extraction of gold from electronic scrap materials (ESM) due to its low toxicity and faster leaching rate. The present study examined the effects of various operating parameters on the recovery of gold using thiourea. A non-linear regression analysis showed that the most significant factors were the concentration of thiourea, sodium sulphite and sulphuric acid, and ultrasound power, followed by leaching time and particle size. Initial ferric ion concentration had the least effect on gold recovery. Up to 98.5% gold recovery was obtained under the following optimal conditions: thiourea (12 g L-1); sulphuric acid (0.1M); sodium sulphite (0.5×10-4M); initial Fe3+ (2.5 g L-1); pulp density (2 g/L); particle size (45-100 micron); operating temperature (50 oC); ultrasonic frequency (37 kHz); ultrasound power (50 watts) and leaching time (45 mins). Various methods are commonly used for gold recovery from the leachate. In the current study, gold was recovered through in-vitro reduction of the thiourea leachate by the bacterium Delftia acidovorans in the form of gold nanoparticles (GNPs). The generation of GNPs by D. acidovorans using thiourea leachate of ESM is a green method of GNPs production which has not been reported.
1. INTRODUCTION The digital revolution has resulted in an increase in the number of electronic devices, like televisions, mobile phones, and computers [Natarajan and Ting 2014]. In addition, these electronic devices are discarded and replaced every few years due to the rapid development of technologies. This has resulted in an increase in the generation of electronic scrap material (ESM) at a rate of around 3 to 5 % annually in the EU, and 20 to 50 million tons worldwide [Tuncuk et al. 2012]. Electronic waste (E-waste) contains both metals that are precious (e.g. gold) and hazardous (e.g. copper and lead), and disposal via landfill causes ground water pollution which has deleterious effects on human health and the environment. For over 100 Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
years, cyanidation has been the conventional method of gold recovery from ores in the mining industry due to its simplicity, high dissolution of gold and low operating costs [Hilson et al. 2006]. However, contamination of cyanide in water-bodies through mining discharges has a detrimental effect on the environment, ecosystem, and human health. Recovery of gold using thiourea solution is an alternative and environment friendly approach. Thiourea dissolves gold in an acidic and oxidizing environment by forming coordination bonds with gold using its lone pairs from nitrogen and sulfur [Gurung et al. 2013]. Leaching of gold using thiourea is faster and more selective method compared to conventional cyanidation method [Gurung et al. 2013] and may be enhanced using ultrasound. Sonication creates acoustic cavitation in the reaction medium that results in the formation, growth, and implosive collapse of micro bubbles on the surfaces of the solid ESM particles which can break it into smaller sizes and improve mixing and heat transfer rate within the system [Narayana et al. 1997] The main objective of this project is to optimize the operating parameters of ultrasound assisted thiourea leaching of gold by means of response surface methodology, and to synthesise gold nanoparticles in-vitro from the leaching solution using a bacterium Delftia acidovorans.
2. MATERIAL AND METHODS 2.1 Electronic scrap material Electronic scrap material (ESM) was obtained from TES_AMM Singapore Pte. Ltd., Singapore. The ESM obtained from waste printed circuit boards of computers was ground to fine dust-like particles and sieved to a different particle sizes. 2.2. Thiourea leaching of gold from ESM The sonicator used for ultrasound assisted leaching is an Elmasonic P30H ultrasonic bath. The experiments were performed in stoppered 250 ml Erlenmeyer flasks. Appropriate amount of ESM, thiourea, sodium sulphite, sulphuric acid and ferric (III) sulphates was added to the conical flask. The conical flask was then immersed in the ultrasonic bath, and the ultrasound was turned on. After the leaching experiment, the reaction mixture was centrifuged and the supernatant was collected. The supernatant was then filtered using 45 µm syringe filter and the metal concentration was analysed using an Inductively Coupled Plasma - Mass Spectrometer (Agilent 7500a series ICP-MS). 2.3. Plackett-Burman design (PBD) PBD is an effective method for selecting important parameters of a system that involves many parameters in relatively fewer experimental runs [Asfaram et al. 2016]. In this study, PBD was implemented to evaluate the significance of ten variables influencing thiourea leaching of gold from ESM. The operating parameters considered for the study were as follows: thiourea concentration (g/L), sodium sulphite concentration (M), sulphuric acid concentration (M), initial Fe3+ ion concentration (g/L), pulp density (g/L), particle size (µm), operating temperature (oC), ultrasound frequency (kHz), ultrasound power (W), Leaching time (min). The response to the experimental design of 12 experimental runs was gold extraction (%). All experiments were carried out in duplicate, and the mean value was taken as the response for PBD software fitting (Design Expert 8.0.10). Assuming no interactions among the variables, a first-order polynomial model was applied for fitting PBD as follows: Y = β! + !" (1) !!! β! X !
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
where Y is the predicted response; b0 is the intercept; bi is the linear regression coefficient; and Xi is the coded independent variable. 2.4. Box-Behnken Design (BBD) The effect of four most important independent variables on the recovery of gold from ESM in thiourea leaching were investigated by BBD. Multivariate optimizations are commonly used in analytical studies due to their higher accuracy and effectiveness over univariate methods. Response surface methodology (RSM) is an example of multivariate optimizations commonly used in the Design of Experiment (DOE). BBD was used to design the experiments for this research. The empirical results obtained from the experiments as per the design matrix are fitted to a quadratic polynomial for modelling based on the following equation [Das et al. 2015a]: !
𝑌 = 𝛽! +
!
𝛽! 𝑥! + !!!
!
!
!
𝛽!! 𝑥! + !!!
𝛽!" 𝑥! 𝑥!
(2)
!!! !!!
where ‘Y’ is the predicted response by the model, β0, βi, βii and βij are constant regression coefficients of the model, xi, xii and xixj represent the linear, quadratic, and interactive terms of the coded independent factors, and ‘n’ represent the number of independent factors. In this research, Design Expert (8.0.10) software was used for the design of the thiourea leaching experiments for optimization using BBD. 2.5. Biomineralization study The leachate obtained after thiourea leaching was centrifuged to remove the toxic residual ESM. The supernatant was filtered for use in the biomineralization experiment after the pH was adjusted to 7 for bacterial growth. Two-step biomineralization approach was adopted for the invitro synthesis of gold nanoparticles from the thiourea leachate using D. acidovorans. In the two-step method, the bacterium was grown in LB medium to attain a maximum delftibactin concentration before the leachate was added, and GNP formed [Das et al. 2015b]. 2.6. Acid digestion The E-waste samples were digested using aqua regia, to evaluate its initial metal concentration [Natarajan and Ting 2014].
3. RESULTS AND DISCUSSION 3.1. Metal composition of ESM Acid digestion was carried out in triplicates to determine the initial metal concentration in the ESM sample provided by TES-AMM Singapore Pte. Ltd. The average concentration of metals extracted after acid digestion are shown in Table 1.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Concentration of metals extracted (mg/g sample) from ESM
Cu 282.06
Al 46.03
Concentration of metals extracted (mg /g sample) Pb Fe Zn Ni Ag 14.326 8.75 2.68 1.35 0.88
Sn 0.57
Au 0.42
The ESM has a high concentration of copper and other base metals compared to precious metals (gold and silver). These base metals can reduce the cyanidation efficiency for gold recovery by forming more stable water-soluble complexes than gold. A selective gold recovery method like thiourea requires less energy and resources, making the process sustainable, costeffective, and environmental friendly. 3.2. Screening of significant factors using PBD Utlrasound assisted thiourea leaching of gold from ESM is influenced by many operating parameters. In this study, PBD was used to investigate the effect degrees of various independent variables on gold recovery during thiourea leaching, and to screen and identify the important factors for further optimization [Li et al. 2004]. The factors considered for the study are: Thiourea concentration [A] (8-12 g/L), sulphuric acid concentration [B] (0.05-0.1 M), sodium sulphite concentration [C] (0.1×10-4 - 0.5×10-4M), initial Fe3+ concentration [D] (2-2.5 g/L), pulp density [E] (0.1-0.2 g/L), particle size [F] (45 – 150 µm), operating temperature [G] (30 – 50 oC), ultrasound frequency [H] (37-80 kHz), ultrasound power [I] (30-50 W), and leaching time [J] (3045 mins). This is the first study that screens all the operating parameters for significance using PBD approach in ultrasound assistated thiourea leaching of gold from ESM. The result of analysis of variance for the PBD matrix (data not shown) is tabulated in Table 2. For gold recovery in the thiourea leaching system, all the independent parameters, except particle size (F), and ultrasound frequency (H), presented positive effects (Equation 3). Ideally, a variable with confidence level above 95% (P < 0.05) is considered as a significant parameter [Das et al. 2015a]. ANOVA identified thiourea concentration, sulphuric acid concentration, sodium sulphite concentration and sonication power as significant parameters for the process, while initial ferric ion concentration had the least significance (P > 0.05). Taking this into account, thiourea concentration, sulphuric acid concentration, sodium sulphite concentration, and sonication power were considered for optimization using BBD response surface methodology, while keeping the remaining operating variables constant at their optimal operating level based on Equation 3. The first order model equation developed using PBD for the gold recovery from ESM by ultrasound assisted-thiourea leacning method can be described as follows: 𝐘 = 𝟏. 𝟐𝟓 + 𝟑. 𝟐𝟗 ∗ 𝐀 + 𝟐. 𝟎𝟒 ∗ 𝐁 + 𝟏. 𝟒𝟑 ∗ 𝐂 + 𝟎. 𝟏𝟓 ∗ 𝐄 − 𝟎. 𝟒𝟖 ∗ 𝐅 + 𝟎. 𝟕𝟕 ∗ 𝐆 − 𝟏. 𝟏𝟐 ∗ 𝐇 + 𝟏. 𝟐𝟑 ∗ 𝐈 + 𝟎. 𝟐𝟒 ∗ 𝐉 (3) The effect of independent parameters on the gold recovery and their statistical significance can also be illustrated using a Pareto chart [Li et al. 2004]. The degree of effect of each parameter on the gold recovery response during ultrasound assisted thiourea leaching was determined by comparing the t-value of effect (Figure 1). Two limit lines (Bonferroni limit line (4.1) and t-value limit line (2.36)) were calculated by the PBD matrix model to identify the extremely significant, significant, and insignificant coefficients of different factors. The design parameter is considered extremely significant if its t-value lies above both the limit lines; the factors are considered significant if their t-value remains between the two limit lines, and finally
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
the t-value of the factors that lie below the t-value limit line is of low significance to the model. In this study, thiourea concentration was extreamly significant, followed by sulphuric acid concentration, sodium sulphite concentration and ultrasonic power (Figure 1). The R2 value of 0.974 for the model indicated that the Pareto chart description was significant and reliable.
Pareto Chart 6
t-value of parameters
5
Bonferroni limit line (4.1) 4
3
t-value limit line (2.36) 2
1
0 A
B
C
I
H
G
F
J
E
D
Factors
Figure 1. Pareto chart of the effects of ten variables on gold recovery from ESM by ultrasound assisted thiourea leaching solution Variables with t-values higher than the critical value (2.36) are considered as statistically significant. The results of the less significant parameters were also investigated. Lower particle size result in a higher specific surface area, thus increasing the contact area between the ESM particles and leaching solution [Narayana et al. 1997]. Low sonication frequency results in higher cavitation effect which increases the overall leaching kinetics [Narayana et al. 1997]. Taking these into account, the following operating conditions for the optimization experiments were kept constant: initial Fe3+ concentration (2.5 g/L), pulp density (2 g/L), operating temperature (50 oC), ultrasound frequency (37kHz) and leaching time (45 mins). Table 2. Analysis of variance and regression analysis of Plackett–Burman design data for the prediction of significant gold extraction variable Source
SS
Df
Mean square
F-value
p-value Prob>F
Model
329.1
10
41.47
58.03
F
Model
643.29
14
91.2
188.38
SUMMARY: The generation of electronic waste (E-waste) has been growing at an alarming rate globally due to unrestrained consumption and the short life cycle of electronic and electrical devices. E-waste contains both metals that are precious (e.g. gold) and hazardous (e.g. copper and lead). Disposal of E-waste via landfill causes ground water pollution which has deleterious effects on human health and the environment. The recovery of precious metals from E-waste (typically via hydrometallurgical methods) provides a viable and economic secondary resource. In this study, we investigated the leaching efficiency of ultrasound-assisted thiourea for the recovery of gold from shredded electronic components obtained from assembled printed circuits boards. Cyanidation is commonly used for the extraction of gold but is a highly toxic process. Thiourea may also be used as an alternative lixiviant for the extraction of gold from electronic scrap materials (ESM) due to its low toxicity and faster leaching rate. The present study examined the effects of various operating parameters on the recovery of gold using thiourea. A non-linear regression analysis showed that the most significant factors were the concentration of thiourea, sodium sulphite and sulphuric acid, and ultrasound power, followed by leaching time and particle size. Initial ferric ion concentration had the least effect on gold recovery. Up to 98.5% gold recovery was obtained under the following optimal conditions: thiourea (12 g L-1); sulphuric acid (0.1M); sodium sulphite (0.5×10-4M); initial Fe3+ (2.5 g L-1); pulp density (2 g/L); particle size (45-100 micron); operating temperature (50 oC); ultrasonic frequency (37 kHz); ultrasound power (50 watts) and leaching time (45 mins). Various methods are commonly used for gold recovery from the leachate. In the current study, gold was recovered through in-vitro reduction of the thiourea leachate by the bacterium Delftia acidovorans in the form of gold nanoparticles (GNPs). The generation of GNPs by D. acidovorans using thiourea leachate of ESM is a green method of GNPs production which has not been reported.
1. INTRODUCTION The digital revolution has resulted in an increase in the number of electronic devices, like televisions, mobile phones, and computers [Natarajan and Ting 2014]. In addition, these electronic devices are discarded and replaced every few years due to the rapid development of technologies. This has resulted in an increase in the generation of electronic scrap material (ESM) at a rate of around 3 to 5 % annually in the EU, and 20 to 50 million tons worldwide [Tuncuk et al. 2012]. Electronic waste (E-waste) contains both metals that are precious (e.g. gold) and hazardous (e.g. copper and lead), and disposal via landfill causes ground water pollution which has deleterious effects on human health and the environment. For over 100 Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
years, cyanidation has been the conventional method of gold recovery from ores in the mining industry due to its simplicity, high dissolution of gold and low operating costs [Hilson et al. 2006]. However, contamination of cyanide in water-bodies through mining discharges has a detrimental effect on the environment, ecosystem, and human health. Recovery of gold using thiourea solution is an alternative and environment friendly approach. Thiourea dissolves gold in an acidic and oxidizing environment by forming coordination bonds with gold using its lone pairs from nitrogen and sulfur [Gurung et al. 2013]. Leaching of gold using thiourea is faster and more selective method compared to conventional cyanidation method [Gurung et al. 2013] and may be enhanced using ultrasound. Sonication creates acoustic cavitation in the reaction medium that results in the formation, growth, and implosive collapse of micro bubbles on the surfaces of the solid ESM particles which can break it into smaller sizes and improve mixing and heat transfer rate within the system [Narayana et al. 1997] The main objective of this project is to optimize the operating parameters of ultrasound assisted thiourea leaching of gold by means of response surface methodology, and to synthesise gold nanoparticles in-vitro from the leaching solution using a bacterium Delftia acidovorans.
2. MATERIAL AND METHODS 2.1 Electronic scrap material Electronic scrap material (ESM) was obtained from TES_AMM Singapore Pte. Ltd., Singapore. The ESM obtained from waste printed circuit boards of computers was ground to fine dust-like particles and sieved to a different particle sizes. 2.2. Thiourea leaching of gold from ESM The sonicator used for ultrasound assisted leaching is an Elmasonic P30H ultrasonic bath. The experiments were performed in stoppered 250 ml Erlenmeyer flasks. Appropriate amount of ESM, thiourea, sodium sulphite, sulphuric acid and ferric (III) sulphates was added to the conical flask. The conical flask was then immersed in the ultrasonic bath, and the ultrasound was turned on. After the leaching experiment, the reaction mixture was centrifuged and the supernatant was collected. The supernatant was then filtered using 45 µm syringe filter and the metal concentration was analysed using an Inductively Coupled Plasma - Mass Spectrometer (Agilent 7500a series ICP-MS). 2.3. Plackett-Burman design (PBD) PBD is an effective method for selecting important parameters of a system that involves many parameters in relatively fewer experimental runs [Asfaram et al. 2016]. In this study, PBD was implemented to evaluate the significance of ten variables influencing thiourea leaching of gold from ESM. The operating parameters considered for the study were as follows: thiourea concentration (g/L), sodium sulphite concentration (M), sulphuric acid concentration (M), initial Fe3+ ion concentration (g/L), pulp density (g/L), particle size (µm), operating temperature (oC), ultrasound frequency (kHz), ultrasound power (W), Leaching time (min). The response to the experimental design of 12 experimental runs was gold extraction (%). All experiments were carried out in duplicate, and the mean value was taken as the response for PBD software fitting (Design Expert 8.0.10). Assuming no interactions among the variables, a first-order polynomial model was applied for fitting PBD as follows: Y = β! + !" (1) !!! β! X !
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
where Y is the predicted response; b0 is the intercept; bi is the linear regression coefficient; and Xi is the coded independent variable. 2.4. Box-Behnken Design (BBD) The effect of four most important independent variables on the recovery of gold from ESM in thiourea leaching were investigated by BBD. Multivariate optimizations are commonly used in analytical studies due to their higher accuracy and effectiveness over univariate methods. Response surface methodology (RSM) is an example of multivariate optimizations commonly used in the Design of Experiment (DOE). BBD was used to design the experiments for this research. The empirical results obtained from the experiments as per the design matrix are fitted to a quadratic polynomial for modelling based on the following equation [Das et al. 2015a]: !
𝑌 = 𝛽! +
!
𝛽! 𝑥! + !!!
!
!
!
𝛽!! 𝑥! + !!!
𝛽!" 𝑥! 𝑥!
(2)
!!! !!!
where ‘Y’ is the predicted response by the model, β0, βi, βii and βij are constant regression coefficients of the model, xi, xii and xixj represent the linear, quadratic, and interactive terms of the coded independent factors, and ‘n’ represent the number of independent factors. In this research, Design Expert (8.0.10) software was used for the design of the thiourea leaching experiments for optimization using BBD. 2.5. Biomineralization study The leachate obtained after thiourea leaching was centrifuged to remove the toxic residual ESM. The supernatant was filtered for use in the biomineralization experiment after the pH was adjusted to 7 for bacterial growth. Two-step biomineralization approach was adopted for the invitro synthesis of gold nanoparticles from the thiourea leachate using D. acidovorans. In the two-step method, the bacterium was grown in LB medium to attain a maximum delftibactin concentration before the leachate was added, and GNP formed [Das et al. 2015b]. 2.6. Acid digestion The E-waste samples were digested using aqua regia, to evaluate its initial metal concentration [Natarajan and Ting 2014].
3. RESULTS AND DISCUSSION 3.1. Metal composition of ESM Acid digestion was carried out in triplicates to determine the initial metal concentration in the ESM sample provided by TES-AMM Singapore Pte. Ltd. The average concentration of metals extracted after acid digestion are shown in Table 1.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Concentration of metals extracted (mg/g sample) from ESM
Cu 282.06
Al 46.03
Concentration of metals extracted (mg /g sample) Pb Fe Zn Ni Ag 14.326 8.75 2.68 1.35 0.88
Sn 0.57
Au 0.42
The ESM has a high concentration of copper and other base metals compared to precious metals (gold and silver). These base metals can reduce the cyanidation efficiency for gold recovery by forming more stable water-soluble complexes than gold. A selective gold recovery method like thiourea requires less energy and resources, making the process sustainable, costeffective, and environmental friendly. 3.2. Screening of significant factors using PBD Utlrasound assisted thiourea leaching of gold from ESM is influenced by many operating parameters. In this study, PBD was used to investigate the effect degrees of various independent variables on gold recovery during thiourea leaching, and to screen and identify the important factors for further optimization [Li et al. 2004]. The factors considered for the study are: Thiourea concentration [A] (8-12 g/L), sulphuric acid concentration [B] (0.05-0.1 M), sodium sulphite concentration [C] (0.1×10-4 - 0.5×10-4M), initial Fe3+ concentration [D] (2-2.5 g/L), pulp density [E] (0.1-0.2 g/L), particle size [F] (45 – 150 µm), operating temperature [G] (30 – 50 oC), ultrasound frequency [H] (37-80 kHz), ultrasound power [I] (30-50 W), and leaching time [J] (3045 mins). This is the first study that screens all the operating parameters for significance using PBD approach in ultrasound assistated thiourea leaching of gold from ESM. The result of analysis of variance for the PBD matrix (data not shown) is tabulated in Table 2. For gold recovery in the thiourea leaching system, all the independent parameters, except particle size (F), and ultrasound frequency (H), presented positive effects (Equation 3). Ideally, a variable with confidence level above 95% (P < 0.05) is considered as a significant parameter [Das et al. 2015a]. ANOVA identified thiourea concentration, sulphuric acid concentration, sodium sulphite concentration and sonication power as significant parameters for the process, while initial ferric ion concentration had the least significance (P > 0.05). Taking this into account, thiourea concentration, sulphuric acid concentration, sodium sulphite concentration, and sonication power were considered for optimization using BBD response surface methodology, while keeping the remaining operating variables constant at their optimal operating level based on Equation 3. The first order model equation developed using PBD for the gold recovery from ESM by ultrasound assisted-thiourea leacning method can be described as follows: 𝐘 = 𝟏. 𝟐𝟓 + 𝟑. 𝟐𝟗 ∗ 𝐀 + 𝟐. 𝟎𝟒 ∗ 𝐁 + 𝟏. 𝟒𝟑 ∗ 𝐂 + 𝟎. 𝟏𝟓 ∗ 𝐄 − 𝟎. 𝟒𝟖 ∗ 𝐅 + 𝟎. 𝟕𝟕 ∗ 𝐆 − 𝟏. 𝟏𝟐 ∗ 𝐇 + 𝟏. 𝟐𝟑 ∗ 𝐈 + 𝟎. 𝟐𝟒 ∗ 𝐉 (3) The effect of independent parameters on the gold recovery and their statistical significance can also be illustrated using a Pareto chart [Li et al. 2004]. The degree of effect of each parameter on the gold recovery response during ultrasound assisted thiourea leaching was determined by comparing the t-value of effect (Figure 1). Two limit lines (Bonferroni limit line (4.1) and t-value limit line (2.36)) were calculated by the PBD matrix model to identify the extremely significant, significant, and insignificant coefficients of different factors. The design parameter is considered extremely significant if its t-value lies above both the limit lines; the factors are considered significant if their t-value remains between the two limit lines, and finally
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
the t-value of the factors that lie below the t-value limit line is of low significance to the model. In this study, thiourea concentration was extreamly significant, followed by sulphuric acid concentration, sodium sulphite concentration and ultrasonic power (Figure 1). The R2 value of 0.974 for the model indicated that the Pareto chart description was significant and reliable.
Pareto Chart 6
t-value of parameters
5
Bonferroni limit line (4.1) 4
3
t-value limit line (2.36) 2
1
0 A
B
C
I
H
G
F
J
E
D
Factors
Figure 1. Pareto chart of the effects of ten variables on gold recovery from ESM by ultrasound assisted thiourea leaching solution Variables with t-values higher than the critical value (2.36) are considered as statistically significant. The results of the less significant parameters were also investigated. Lower particle size result in a higher specific surface area, thus increasing the contact area between the ESM particles and leaching solution [Narayana et al. 1997]. Low sonication frequency results in higher cavitation effect which increases the overall leaching kinetics [Narayana et al. 1997]. Taking these into account, the following operating conditions for the optimization experiments were kept constant: initial Fe3+ concentration (2.5 g/L), pulp density (2 g/L), operating temperature (50 oC), ultrasound frequency (37kHz) and leaching time (45 mins). Table 2. Analysis of variance and regression analysis of Plackett–Burman design data for the prediction of significant gold extraction variable Source
SS
Df
Mean square
F-value
p-value Prob>F
Model
329.1
10
41.47
58.03
F
Model
643.29
14
91.2
188.38
