processes Article
Separation of Ag(I) by Ion Exchange and Cementation from a Raffinate Containing Ag(I), Ni(II) and Zn(II) and Traces of Cu(II) and Sn(II) Wei Dong Xing 1 , Man Seung Lee 1, * and Seung Hoon Choi 2 1 2
*
Department of Advanced Material Science & Engineering, Institute of Rare Metal, Mokpo National University, Jeollanamdo 534-729, Korea;
[email protected] Department of Chemical and Biomolecular Engineering, Seonam University, Chonbuk 31556, Korea;
[email protected] Correspondence:
[email protected]; Tel.: +82-61-450-2492
Received: 10 July 2018; Accepted: 29 July 2018; Published: 1 August 2018
Abstract: Ion exchange and cementation experiments were done to separate silver(I) from a raffinate containing silver(I), nickel(II), and zinc(II) and small amounts of copper(II) and tin(II). The raffinate resulted from the recovery of gold(III), tin(II) and copper(II) by solvent extraction from a leaching solution of anode slime. Ion exchange with anionic resins was not effective in separating silver(I) because tin(II) and zinc(II) were selectively adsorbed into the anionic resins. It was possible to separate silver(I) by cementation with copper sheet. Treatment of the cemented silver with nitric acid solution increased the purity of silver(I) in the solution from 50.9% to 99.99%. Adjusting the pH of the AgNO3 solution to higher than 6, followed by adding ascorbic acid as a reducing agent, led to the synthesis of silver particles with micron size. Keywords: silver; ion exchange; cementation; copper sheet; silver particles
1. Introduction In recent years, resources recycling has become an important issue in the chemical and metallurgical industries. In order to recover pure metals or compounds with high purity from secondary resources, the valuable components together with some impurities are dissolved in the adequate leaching medium. Therefore, separation of the valuable component from the impurities is necessary to obtain a pure solution of the target component. There are several separation methods, such as solvent extraction, ion exchange, precipitation and cementation. In particular, solvent extraction and ion exchange are widely employed for the separation of chemically similar ions in the solution. In general, ion exchange is employed when the concentration of the impurities is less than 100 ppm [1]. Ion exchange is very useful for the removal of heavy metals from the solution with low metal concentration before discharge to the environment [2,3]. Cementation makes utilization of the difference in the reduction potential among the ions in the solution and can be represented as Equation (1). Since cementation has some advantages such as easy control, low energy consumption and low cost, it is widely employed for metal separation as well as metal production [4]. M1 n 1 + + (n1 /n2 )M2 0 = M1 0 + (n1 /n2 )M2 n 2 +
(1)
where M2 0 and M2 n 2 + are the metallic forms and ionic of reductive metals and M1 0 and M1 n 1 + are forms of noble metals. In our work on the recovery of noble metals present in the anode slimes from the electro-refining of tin, the components in the anode slimes were dissolved in hydrochloric acid solution in the Processes 2018, 6, 112; doi:10.3390/pr6080112
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presence of an oxidizing agent [5]. The leach liquor of the anode slimes contained gold(III), silver(I), copper(II), nickel(II), tin(II) and zinc(II). First, gold(III) was separated by solvent extraction with Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid, Cytec Industries, Woodland Park, NJ, USA) and then copper(II) was selectively extracted by LIX 63 (5,8-diethyl-7-hydroxydodecan-6-oxime, BASF Corporation, Tucson, AZ, USA) from the gold(III) free raffinate [6,7]. After the separation of gold(III) and copper(II), the raffinate contained silver(I), tin(II), nickel(II) and zinc(II) together with a small amount of copper(II). In order to recover the silver(I) present in the raffinate, ion exchange and cementation experiments were performed in this work. Some works have been reported on the cementation of silver(I) by employing metals such as zinc, copper, magnesium and iron from various solutions [8–12]. Employment of these metals leads to high recovery percentage of silver. However, these metals can be applied to the solution with weak acidity, and silver powders were thus obtained, easily covering the surface of reductive metals, resulting in a decrease in the purity of metals, and excessive consumption of reductive metals owing to the reaction with the acid in the solution. In this work, synthetic hydrochloric solutions were employed in the experiments. In a strong hydrochloric acid solution, soluble silver(I) exists as AgCln 1−n . Therefore, several anion exchange resins were employed for the ion exchange experiments and the ion exchange behavior was investigated by varying the concentration of the resins. In cementation experiments, copper sheet was employed and the cementation behavior was investigated by varying reaction temperature, stirring speed, reaction time and the area of copper sheet. An integrated process was proposed for the recovery of silver(I) from the hydrochloric acid solution by comparing ion exchange and cementation results. 2. Materials and Methods 2.1. Chemical Reagents A synthetic raffinate was prepared by dissolving a certain amount of AgCl (above 99.5%, Daejung Chemicals and Metals Co., Ltd., Shiheung, Korea), CuCl2 ·2H2O (97%, Daejung Chemicals and Metals Co., Ltd., Shiheung, Korea), SnCl2 ·H2O (97%, Daejung Chemicals and Metals Co., Ltd., Shiheung, Korea) as well as ZnCl2 (90%, Duksan Pure Chemicals Co., Ltd., Ansan-si, Korea) and NiCl2 ·6H2O (YAKURI Pure Chemicals Co., Ltd., Kyoto, Japan, 96%) into 5 M HCl solution. A small amount of H2O2 (30%, DaeJung Chemical and Metals Co., Ltd., Shiheung, Korea) was added in the synthetic solution to suppress the formation of some precipitates. The ion exchange resins used in this work are AG 1-X8 (Bio-Rad, Hercules, CA, USA), AG MP-1M (Bio-Rad), Diaion WA21J (Mitsubishi Chemical Corporation, Tokyo, Japan), Lewatit MP-64 (Lanxess Energizing Chemistry), Purolite A500 (Lenntech, Edmonton, Canada), and TEVA (Eichrom, Chicago, IL, USA). All the resins were directly used without further pretreatment. The physical properties of these resins are listed in Table 1. Table 1. The physical properties of the resins employed in this work. Resin
Ionic Forms
Functional Group
Bead Size (µm)
Capacity (meq/mL)
Density (g/mL)
Matrix
AG 1-X8
chloride
R-CH2 N+ (CH3 )3
45–106
1.2
0.75
/
AG MP-1M
chloride
R-CH2 N+ (CH3 )3
75–150
1
0.70
Macroporous
Diaion WA21J
free base
Polyamine
300–1180
2.0
1.07
Styrene-DVB, Porous
Lewatit MP-64
chloride
Tertiary/quarternary amine
300–1250
1.3
1.04
Macroporous
Purolite A500
chloride
Type 1 quaternary Ammonium
600–850
1.15
1.08
Macroporous polystyrene crosslinked with divinylbenzene
TEVA Resin
chloride
100–150
/
0.35
/
R3 -N+ CH3 Cl− /NO3 −
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2.2. Experimental Procedure 2.2.1. Ion Exchange Ion exchange experiments were carried out by using the screw bottle containing 20 mL of synthetic solution containing Ag(I), Cu(II), Ni(II), Sn(II) and Zn(II) with certain amounts of resins in a shaking incubator (HB-201SF, Hanbeak Scientific Co., Ltd., Bucheon-si, Korea). The mixtures were shaken at 200 rpm for 6 h at room temperature. After shaking, the loaded resin was separated from the effluent using a filter paper. The metal concentration in the effluent was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Spectro Arcos, Cleve, Germany). The metal concentration loaded on the resin was obtained by mass balance. 2.2.2. Cementation of Ag(I) by Copper SheetCementation experiments were carried out in a 100 mL beaker with magnetic stirrer on the hotplate (Daihan scientific Co., Ltd., Wonju-si, Korea). When the temperature reached the desired value, copper sheet was placed in the beaker lying on the wall with the cover to prevent the evaporation of the solution. The copper sheets were polished by 220 cc abrasive cloth before each experiment. A series of copper sheets with different width were cut by diamond cutter (METSAW-LS (RB 205), R&B Inc., Daejeon, Korea) based on the necessity of experiments. After finishing the cementation experiments, the solution was filtered through filter paper. The metal concentration in the solution was measured by ICP-OES (Inductively coupled plasma optical emission spectroscopy, Spectro Arcos, Cleve, Germany). Ag recovery percentage was calculated using Equation (2). The silver powders thus cemented were washed down with distilled water from the copper sheet. The powders were characterized by scanning electron microscopy (SEM, S-3500N Hitachi, Tokyo, Japan). Ag recovery percentage =
m0 − m1 × 100% m0
(2)
where m0 and m1 refer to the mass of silver(I) before and after cementation, respectively. 3. Results and Discussion 3.1. Speciation of the Metals in the Raffinate at 5 M HCl Table 2 shows the composition of the raffinate after the sequential extraction of gold(III) and copper(II) by Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid, Cytec Industries, Woodland Park, NJ, USA) and LIX 63 (5,8-diethyl-7-hydroxydodecan-6-oxime, BASF Corporation, Tucson, AZ, USA), respectively. The synthetic solution was prepared on the basis of the composition of the metals shown in Table 2. In ion exchange, the size and ionic charge of the reactants are very important. Therefore, the speciation of the metals present in the synthetic solution was investigated. In general, chloride ion has a strong tendency to form complexes with metal ion, and the degree of the complex formation depends on the concentration of chloride ion and the metal ion. In this work, the concentration of HCl in the synthetic solution was fixed at 5 M. According to the literature, silver(I) exists as AgCl2 − and AgCl3 2− in this solution [13]. When HCl concentration is lower than 4 M, most of copper(II) forms mononuclear complexes with chloride ion such as CuCl3 − and CuCl4 2− [3]. However, some of CuCl4 2− can be polymerized into Cu2 Cl6 2− when HCl concentration is higher than 5 M [3,14]. It has been reported that nickel(II) does not form anionic complex with chloride ion even in 10 M HCl solution and exists as NiCl+ in 5 M HCl solution [15]. The distribution of tin(II) in HCl solution is more complex than that of the above metals and the mole fraction of tin(II) complex is in the following order SnCl4 2− > SnCl3 − > SnCl2 (aq) in the 5 M HCl solution [16]. The predominant complex of zinc(II) in 5 M HCl solution is ZnCl4 2− [17]. The stability constants for the formation of complexes in the HCl solution are shown in Table 3 [18]. Based on the stability constant of these metals, the complexes of
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silver(I) and nickel(II) are more thermodynamically stable than those of other metals in the solution. Processes 2018, x FOR PEER REVIEW of 14 the In addition, the6,complexes with lower charge density has a stronger tendency to react 4with functional group in the resin than those with higher charge density [19]. the complexes with lower charge density has a stronger tendency to react with the functional group in the resin than those with higher charge density [19]. Table 2. The composition of the raffinate after selective extraction of gold(III) and copper(II) by Cyanex 272 and LIX2.63The at 5composition M HCl. Table of the raffinate after selective extraction of gold(III) and copper(II) by Cyanex 272 and LIX 63 at 5 M HCl.
Metals
Ag Metals C (mg/L) 40 C (mg/L)
Cu Ag Cu 4.5 40 4.5
Ni Ni 10 10
Sn Sn Zn 10 10 14
Zn 14
TableTable 3. The stability constants for of chloro-complexes chloro-complexes HCl solution. 3. The stability constants forthe theformation formation of in in HCl solution.
Metals Ag Ag Cu Cu NiNi SnSn ZnZn
Metals
Species Species AgCl, AgCl2−−, AgCl32−2−, AgCl43−3− AgCl, AgCl2 , AgCl3− , AgCl2− 4 2, CuCl3 , CuCl4 CuCl+,+ CuCl CuCl , CuCl2 , CuCl3 − , CuCl4 2− 2+, NiCl+ + Ni Ni2+ , NiCl ++ SnCl , SnCl SnCl , SnCl22, ,SnCl SnCl33−−, ,SnCl SnCl442−2− ZnCl+,+ ,ZnCl ZnCl22, ,ZnCl ZnCl33−−, ,ZnCl ZnCl442−2− ZnCl
Log βn (1~4) Log βn (1~4) (3.3, 5.3, 6.0, 3.6) (3.3, 5.3, 6.0, 3.6) (0.02, −0.71, −2.3) (0.02, −0.71, −2.3) 2.1 2.1 (1.5, 2.3, (1.5, 2.3,2.0, 2.0,1.5) 1.5) (0.43, 0.61,0.53, 0.53,0.2) 0.2) (0.43, 0.61,
References References [20,21] [20,21] [17] [17] [15][15] [20][20] [17][17]
3.2. Purification of Silver(I) by Ion Exchange 3.2. Purification of Silver(I) by Ion Exchange In order to investigate the adsorption behavior of the metal ions in the solution, ion exchange
In order to investigate the adsorption behavior of the metal ions in the solution, ion exchange experiments were performed. In these experiments, the concentration of the resins was varied from experiments were performed. In these experiments, the concentration of the resins was varied from 0.5 g/L to 8 g/L. Figure 1 shows the adsorption behavior of metals into AG 1-X8 from 5 M HCl 0.5 g/L to 8 g/L. Figure showswere the adsorption behaviorinto of metals into AG less 1-X8than from 5M solution. solution. Tin(II) and1zinc(II) selectively adsorbed AG 1-X8, while 20% of HCl silver(II) Tin(II)was andloaded. zinc(II)When wereAG selectively adsorbed into AG 1-X8, while less than 20% of silver(II) was loaded. 1-X8 concentration was higher than 2 g/L, tin(II) was completely adsorbed Wheninto AGthe 1-X8 concentration was higher thanincreased 2 g/L, tin(II) completely adsorbed into from the resin. resin. Zn(II) adsorption percentage to 60%was as resin concentration increased to 8 g/L. Copper(II) andincreased nickel(II) were negligibly adsorbed into AG 1-X8 in these experimental Zn(II)0.5 adsorption percentage to 60% as resin concentration increased from 0.5 to 8 g/L. conditions. Copper(II) and nickel(II) were negligibly adsorbed into AG 1-X8 in these experimental conditions. 100 Ag Cu Ni Sn Zn
Adsorption percentage, %
80
60
40
20
0 0
2
4
6
8
[AG 1-X8], g/L
Figure 1. Effect of 1-X8 AG 1-X8 concentration adsorption metalsinin5 5MMHCl HClsolution. solution. [AG [AG 1-X8] 1- = Figure 1. Effect of AG concentration on on thethe adsorption ofof metals X8] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm.
The chemical structure of AG MP-1M is similar to that of AG 1-X8. This is reflected in Figure 2
The chemical structure AGadsorption MP-1M is similar of to the that of AG This Comparing is reflected in which shows the variation of of the percentage metals into1-X8. AG MP-1M. Figure 2 which shows the variation of the adsorption percentage of the metals intoresins AG was MP-1M. Figures 1 and 2 indicates that the adsorption behavior of tin(II) and zinc(II) into these two Comparing 1 and 2 indicates that theofadsorption behavior tin(II) and zinc(II) these similar,Figures while the adsorption behavior silver(I) was slightlyofdifferent. Since AgClinto 32− is the two predominant species of Ag(I) in 5 M HCl solution [21], the adsorption of tin(II), zinc(II) and silver(I) resins was similar, while the adsorption behavior of silver(I) was slightly different. Since AgCl3 2− into these two resins might be represented as
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is the predominant species of Ag(I) in 5 M HCl solution [21], the adsorption of tin(II), zinc(II) and Processes 6, xtwo FOR PEER REVIEW 5 of 14 silver(I) into2018, these resins might be represented as 2−n
(n − 2)RCl + MeCl 2MeCln n+ + − 2)Cl − n n2−n==Rn Processes 2018, 6, x FOR PEER REVIEW (n − 2)RCl + MeCl Rn− − 2MeCl (n(n − 2)Cl 2−
−
5 of 14 (3)
(3)
(4) (3)
(4)
−
− 2RCl2RCl ++ AgCl 2Cl + AgCl R22AgCl AgCl33++2Cl 3 32−==R (n − 2)RCl MeCl n2−n = Rn − 2MeCln + (n − 2)Cl−
RCl refers to AG the AG 1-X8 AG MP-1Mand andMe Me refers refers to wherewhere RCl refers to the 1-X8 or or AG MP-1M totin(II) tin(II)and andzinc(II). zinc(II). 2RCl + AgCl32− = R2AgCl3 + 2Cl−
(4)
where RCl refers to the AG 1-X8 or AG MP-1M and Me refers to tin(II) and zinc(II).
Adsorption percentage, % Adsorption percentage, %
100
80 100
60 80 Ag Cu Ni Sn Ag Zn Cu Ni Sn Zn
40 60
20 40
200 0
2
4
6
8
6
8
[AG MP-1M], g/L
0 0
2
4
Figure 2. Effect AGMP-1M MP-1M concentration onon thethe adsorption of metals in 5 M in HCl5 solution. Figure 2. Effect of of AG concentration adsorption of metals M HCl [AG solution. [AG MP-1M], g/L MP-1M] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. [AG MP-1M] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. Figure 2. Effect of AG MP-1M concentration on the adsorption of metals in 5 M HCl solution. [AG Figure 3= 0.5–8 shows(g/L), the time adsorption behavior of =the MP-1M] = 6 h, shaking speed 200metal rpm. ions into Purolite A500. As resin Figure 3 showsincreased the adsorption behavior of the ions into PuroliteofA500. resin concentration concentration from 0.5 to 8 g/L, the metal adsorption percentage both As tin(II) and zinc(II) increased from 0.5 to 8 g/L, the adsorption percentage of both tin(II) and zinc(II) increased from increased from 53% to 93% and from 3% to 82%, respectively. However, silver(I), copper(II) and 53% Figure 3 shows the adsorption behavior of the metal ions into Purolite A500. As resin nickel(II) were hardly adsorbed into this resin in these experimental ranges. The adsorption reaction to 93% and from 3% to 82%, respectively. However, silver(I), copper(II) and nickel(II) were hardly concentration increased from 0.5 to 8 g/L, the adsorption percentage of both tin(II) and zinc(II) of tin(II) and zinc(II) into Purolite A500 can be represented as adsorbed into from this resin in 93% theseand experimental ranges. The adsorption reaction of copper(II) tin(II) andand zinc(II) increased 53% to from 3% to 82%, respectively. However, silver(I),
nickel(II)A500 were hardly into this resin nin experimental ranges. The adsorption reaction into Purolite can (n be−adsorbed represented as 2−nthese 2) RN(NH 3)3Cl + MeCl = [RN(NH 3)3]n−2MeCln + (n − 2)Cl− (5) of tin(II) and zinc(II) into Purolite A500 can be represented as
2−n − (n − 2) (n RN(NH MeCl ]nMeCl n n2−n==[RN(NH n + (n − 2)Cl 3 )3 Cl3)+3Cl −2 MeCl − 2) RN(NH + MeCl [RN(NH33))3]3n−2 n + (n − 2)Cl− 100
(5)
Adsorption percentage, % Adsorption percentage, %
100 80
80 60 Ag Cu Ni Sn Ag Zn Cu Ni Sn Zn
60 40
40 20
200 0
2
4
6
8
6
8
[Purolite A500], g/L
0 0
2
4
Figure 3. Effect of Purolite A500 concentration on the adsorption of metals in 5 M HCl solution. [Purolite A500], g/L
[Purolite A500] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. Figure 3. Effect of PuroliteA500 A500concentration concentration on of of metals in 5inM5HCl solution. Figure 3. Effect of Purolite on the theadsorption adsorption metals M HCl solution. Adsorption into= Diaion WA21J is =shown in Figure 4. As resin concentration increased from 0.5 [Purolite A500] 0.5–8 (g/L), time 6 h, shaking speed = 200 rpm. [Purolite A500] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. to 8 g/L, the adsorption percentage of tin(II) and zinc(II) increased almost linearly to 80% and 50%,
respectively. However, only 5% of silver(I) wasinadsorbed the resin when resinincreased concentration Adsorption into Diaion WA21J is shown Figure 4.into As resin concentration fromwas 0.5 to 8 g/L, the adsorption percentage of tin(II) and zinc(II) increased almost linearly to 80% and 50%, respectively. However, only 5% of silver(I) was adsorbed into the resin when resin concentration was
(5)
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Adsorption into Diaion WA21J is shown in Figure 4. As resin concentration increased from 0.5 to 8 g/L, the adsorption percentage of tin(II) and zinc(II) increased almost linearly to 80% and 50%, Processes 2018, 6, x FOR PEER REVIEW 6 of 14 respectively. However, only 5% of silver(I) was adsorbed into the resin when resin concentration was higher thanthan 2 g/L andand most ofof copper(II) remainedinin the solution. Figure 5 shows higher 2 g/L most copper(II)and and nickel(II) nickel(II) remained the solution. Figure 5 shows the the adsorption behavior of the metal ions into Lewaitit MP-64. Comparing Figures 4 and 5 indicates adsorption behavior of the metal ions into Lewaitit MP-64. Comparing Figures 4 and 5 indicates that that the adsorption behavior of the metal WA21Jand andLewaitit Lewaitit MP-64 similar to each the adsorption behavior of the metalions ionsinto intoDiaion Diaion WA21J MP-64 waswas similar to each The adsorption percentage tin(II)and and zinc(II) zinc(II) increased 41% to 92% andand fromfrom zero zero to to other.other. The adsorption percentage ofoftin(II) increasedfrom from 41% to 92% 78% as resin concentration increased from 0.5 to 8 g/L. Less than 5% of silver(I) was adsorbed into 78% as resin concentration increased from 0.5 to 8 g/L. Less than 5% of silver(I) was adsorbed into Lewaitit MP-64. adsorption reaction tin(II)and andzinc(II) zinc(II) into into Diaion MPLewaitit MP-64. TheThe adsorption reaction ofoftin(II) DiaionWA21J WA21Jand andLewaitit Lewaitit MP-64 64 can be represented as [22]. can be represented as [22]. n−n−2 2 = R4NMeCln + X− − R4NX + MeCl (6) R4 NX + MeCl (6) n n = 4 NMeCln + X 100
Ag Cu Ni Sn Zn
Adsorption percentage,%
80
60
40
20
0 0
2
4
6
8
Diaion WA21J, g/L
Figure 4. Effect of Diaion WA21Jconcentration concentration on of of metals in 5inM5HCl solution. Figure 4. Effect of Diaion WA21J on the theadsorption adsorption metals M HCl solution. [Diaion WA21J] = 0.5–8 6 h, shaking speed 200 rpm. [Diaion WA21J] = 0.5–8 (g/L),(g/L), time time = 6 h,= shaking speed = 200= rpm. 100
Ag Cu Ni Sn Zn
Adsorption percentage, %
80
60
40
20
0 0
2
4
6
8
Lewaitit MP-64, g/L
Figure 5. Effect of Lewaitit MP-64concentration concentration on of of metals in 5inM5HCl solution. Figure 5. Effect of Lewaitit MP-64 on the theadsorption adsorption metals M HCl solution. [Lewaitit MP-64] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm. [Lewaitit MP-64] = 0.5–8 (g/L), time = 6 h, shaking speed = 200 rpm.
Figure 6 shows the variation in the adsorption percentage of the metal ions with TEVA resin
Figure 6 shows the variation in theadsorbed adsorption the metal ions with TEVA concentration. Tin(II) was completely into percentage TEVA withinofthese experimental ranges. Theresin concentration. was completely adsorbed into TEVA experimental ranges. dependence Tin(II) of zinc(II) adsorption on TEVA concentration can bewithin dividedthese into two regions. When TEVA concentration increased from 0.5ontoTEVA 2 g/L, concentration the adsorption percentage of zinc(II) The dependence of zinc(II) adsorption can be divided intoincreased two regions. then the increase rate from with resin was reduced. Moreover,ofmost of allincreased the Whenrapidly TEVA and concentration increased 0.5 toconcentration 2 g/L, the adsorption percentage zinc(II) Sn(II) could be separated from other metals at 0.5 g/L of TEVA. A small amount of silver(I) was rapidly and then the increase rate with resin concentration was reduced. Moreover, most of all the
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Sn(II) could be separated from other metals at 0.5 g/L of TEVA. A small amount of silver(I) was adsorbed when when TEVA TEVA concentration while copper(II) copper(II) and and nickel(II) nickel(II) were were not not adsorbed adsorbed at at adsorbed concentration was was 88 g/L, g/L, while this condition. condition. The The adsorption adsorption reaction reaction of of tin(II) tin(II) and and zinc(II) zinc(II) into this into TEVA TEVA can can be be represented represented as as [23] [23] nn−n−2 2 = R3NCH3MeCln + (n − 2) X− − R3NCH 3X + MeCl = R3 NCH3 MeCln + (n − 2) X R3 NCH 3 X + MeCln
(7) (7)
where R3NCH 3X refers to TEVA resin and X is Cl−−or NO3−,− R = C8H17 and C10H21. where R 3 NCH3 X refers to TEVA resin and X is Cl or NO3 , R = C8 H17 and C10 H21 .
Adsorption percentage, %
100
80 Ag Cu Ni Sn Zn
60
40
20
0 0
2
4
6
8
[TEVA], g/L
Figure of TEVA resin concentration on the of metals in 5 M HCl Figure 6.6.Effect Effect of TEVA resin concentration onadsorption the adsorption of metals in 5solution. M HCl [TEVA] solution.
=[TEVA] 0.5–8 = (g/L), = time 6 h, shaking speedspeed = 200= rpm. 0.5–8time (g/L), = 6 h, shaking 200 rpm. From ion exchange experiments, it can be concluded that the anionic resins employed in this From ion exchange experiments, it can be concluded that the anionic resins employed in this work selectively adsorb tin(II) and zinc(II) from the synthetic solution with 5 M HCl. In most of the work selectively adsorb tin(II) and zinc(II) from the synthetic solution with 5 M HCl. In most of conditions, a small amount of silver(I) was adsorbed into these resins. Although it may be possible the conditions, a small amount of silver(I) was adsorbed into these resins. Although it may be to remove tin(II) and zinc(II) from the solution by ion exchange at high resin concentration, copossible to remove tin(II) and zinc(II) from the solution by ion exchange at high resin concentration, adsorption of silver(I) at these conditions makes the separation process complicated. Therefore, it is co-adsorption of silver(I) at these conditions makes the separation process complicated. Therefore, it is difficult to separate silver(I) from the solution by ion exchange with the resins employed in this work. difficult to separate silver(I) from the solution by ion exchange with the resins employed in this work. Since there are no nickel(II) anionic complexes in 5 M HCl solution, it is natural that nickel(II) Since there are no nickel(II) anionic complexes in 5 M HCl solution, it is natural that nickel(II) was was not at all adsorbed into the resins employed in this work. The selective adsorption of tin(II) and not at all adsorbed into the resins employed in this work. The selective adsorption of tin(II) and zinc(II) zinc(II) into the resins might be related to the charge density of the complexes of copper(II), silver(I), into the resins might be related to the charge density of the complexes of copper(II), silver(I), tin(II), tin(II), and zinc(II). The charge density of metal chloro-complexes is shown in Table 4. It is indicated and zinc(II). The charge density of metal chloro-complexes is shown in Table 4. It is indicated that that the charge density of metal complexes is in the following order: Ag < Sn < Zn< Cu < Ni. The metal the charge density of metal complexes is in the following order: Ag < Sn < Zn< Cu < Ni. The metal complexes with lower charge density have a stronger tendency to react with the resin than those with complexes with lower charge density have a stronger tendency to react with the resin than those with higher charge density [19]. Therefore, the adsorption behavior of other metal complexes might follow higher charge density [19]. Therefore, the adsorption behavior of other metal complexes might follow the above-mentioned order. Namely, tin chloro-complexes could be selectively adsorbed and then the above-mentioned order. Namely, tin chloro-complexes could be selectively adsorbed and then the zinc choro-complexes. However, the adsorption percentage of silver(I) was very low in our work the zinc choro-complexes. However, the adsorption percentage of silver(I) was very low in our work and this might be attributed to the characteristics of silver complexes. Silver(I) is classified as soft acid and this might be attributed to the characteristics of silver complexes. Silver(I) is classified as soft because it has high polarizability and low charge density [24]. Meanwhile, Cu(II) and Zn(II) are acid because it has high polarizability and low charge density [24]. Meanwhile, Cu(II) and Zn(II) are considered as borderline acids. Most of tertiary amine is hard base, while chloride ion is borderline considered as borderline acids. Most of tertiary amine is hard base, while chloride ion is borderline base. Table 5 shows the classification of the ions present in the solution according to HSAB (Hard soft base. Table 5 shows the classification of the ions present in the solution according to HSAB (Hard soft acid base). The soft acids are willing to react with soft bases, while hard acids are willing to react with acid base). The soft acids are willing to react with soft bases, while hard acids are willing to react with hard bases. Therefore, silver chloro-complexes do not have a strong tendency to react with tertiary hard bases. Therefore, silver chloro-complexes do not have a strong tendency to react with tertiary amine on the basis of HSAB and thus its adsorption percentage was low in our experiments. amine on the basis of HSAB and thus its adsorption percentage was low in our experiments.
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Table 4. The charge density of metal complexes in this work [25,26]. Ion
Ionic Radii (pm)
Charge Density (C/mm3 )
Cu2+ Ni2+ Ag+ Sn2+ Zn2+ Cl−
73 70 115 118 74 167
98.2 111.4 25.1 46.5 56.1 /
Table 5. Classification of ions present in the solution on the basis of hard soft acid base (HSAB) theory. Hard N3+ ,
Acid Base
Borderline
N+
Ni2+ ,
RNH2 , N2 H4
Cu2+ , Cl− ,
Zn2+ , N3
−
Soft Sn2+
Ag+
3.3. Purification of Silver(I) by Cementation with Copper Sheet According to the ion exchange results in Section 3.2, purification of silver(I) by ion exchange is not easy. The standard reduction potential of the metal ions in the solution is listed in Table 6. Among the metal ions in the synthetic solution, the reduction potential of silver(I) is the highest. Therefore, cementation experiments were done to make utilization of the difference in the standard reduction potential. For this purpose, the copper sheet was employed for the cementation of silver(I) from the synthetic solution with 5 M HCl. The cementation reaction can be described as Equation (8) and parts of copper might react with CuCl4 2− as represented in Equation (9) [27]. A series of cementation experiments was performed to obtain an optimum condition for silver recovery. 2AgCl2 − + Cu(s) = CuCl4 2− + 2Ag(s)
(8)
Cu(s) + CuCl4 2− = 2CuCl2 −
(9)
Table 6. The standard reduction electrode potential of the metal ions present in the solution at 25 ◦ C. Electrode
Reaction
E0 (V vs. SHE)
Ag+ /Ag Cu2+ /Cu Cu+ /Cu Ni2+ /Ni Sn2+ /Sn Zn2+ /Zn
Ag+ + e ↔ Ag0 Cu2+ + 2e ↔ Cu0 Cu+ + e ↔ Cu0 Ni2+ + 2e ↔ Ni0 Sn2+ + 2e ↔ Sn0 Zn2+ + 2e ↔ Zn0
0.80 0.34 0.52 −0.26 −0.14 −0.76
3.3.1. Cementation of Silver(I) Figure 7 shows the effect of reaction time on the recovery percentage of silver(I) and the amount of copper(II) dissolved during the reaction. The detailed experimental conditions were: 95 ◦ C, stirring speed of 200 rpm and copper sheet with 50 × 12.5 mm2 area. The recovery percentage of silver(I) increased rapidly as reaction time increased to 40 min and then remained constant with the further increase of reaction time to 60 min In cementation experiments, 50 mL of the synthetic solution was employed. In this case, the mass of silver ion in the 50 mL of the solution was about 2 mg. Equation (7) indicates that 0.6 mg of copper is enough to cement 2 mg of silver ion. However, the mass of dissolved copper was around 21 mg, which is much higher than the stoichiometric mass. This might be attributed to the dissolution of oxidized copper in HCl solution [28]. The mass of copper with the volume of 50 × 12.5 × 1 mm3 was around 11.2 g, which was enough for the silver cementation.
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Although Equation (7) can proceed to completion in terms of thermodynamics, about 88% of silver(I) Processes 2018, 6, x FOR PEER REVIEW 9 of 14 was cemented at 60 min reaction time. The reason why complete cementation of silver(I) did not occur in thesewas experiments is related to the employment of the copper sheet. In these of experiments, the copper cemented at 60 min reaction time. The reason why complete cementation silver(I) did not occur is related to the employment of the copper In these sheet with 50in×these 12.5experiments mm2 was employed. When 80% of silver(I) was sheet. cemented in experiments, these experiments, 2 was employed. When 80% of silver(I) was cemented in these theof copper sheet with 50 ×was 12.51.6 mmmg the mass cemented silver in the case of 50 mL solution. Since the density of silver experiments, 3the mass of cemented silver was 1.6 mg in the case of 50 mL solution. Since the density −4 2 metal is 10.5 g/cm , the thickness of silver layer thus cemented on the copper sheet is 2.4 × 10 m . of silver metal is 10.5 g/cm3, the thickness of silver layer thus cemented on the copper sheet is 2.4 × The cemented silver would cover the whole surface of copper employed the experiments and 10−4 m2. The cemented silver would cover the whole surfacesheet of copper sheet in employed in the thus prevent the contact ofprevent copperthe with the of silver ions inthe thesilver solution. experiments and thus contact copper with ions in the solution.
80
Ag recovery, %
100
Ag Cu Ni Sn Zn
80
60
60
40
40
20
20
0
Cu dissolved, mg
100
0 0
10
20
30
40
50
60
Reaction time, min Figure 7. of Effect of reaction time therecovery Ag recovery by cementation with sheet. Stirringspeed speed==200 rpm; Figure 7. Effect reaction time on theonAg by cementation with CuCu sheet. Stirring 200 rpm; temperature = 80 °C; time = 0–60 min; copper sheet = 50 × 12.5 mm2. ◦ 2 temperature = 80 C; time = 0–60 min; copper sheet = 50 × 12.5 mm .
In cementation, reaction temperature has a significant effect on the cementation kinetics and the effect reaction of reaction temperaturehas wasa investigated. In these the cementation In thus cementation, temperature significant effect on experiments, the cementation kinetics and thus reactions were done for 60 min at 200 rpm for each reaction temperature. According to Figure 8, the the effect of reaction temperature was investigated. In these experiments, the cementation reactions recovery percentage of silver(I) increased linearly with the increase of reaction temperature from 30 were done for 60 min at 200 rpm for each reaction temperature. According to Figure 8, the recovery to 65 °C and reached 93% at 80 °C and then decreased a little as reaction temperature increased to 95 ◦ percentage of silver(I) increased with thebase increase of reaction temperature from might 30 to be 65 C and °C. During the formation of linearly silver film, some metal ions such as tin(II) and nickel(II) ◦ C. During the reachedincorporated 93% at 80 ◦into C and then decreased a little as reaction temperature increased to 95 the rough surface of copper sheet together with cemented silver. Therefore, 80 °C formation silver as film, some base metal ions such as tin(II) and nickel(II) might be incorporated into wasof selected an optimum temperature. the rough surface of copper sheet together with cemented silver. Therefore, 80 ◦ C was selected as Processes 2018, 6, x FOR PEER REVIEW 10 of 14 an optimum temperature.
80
Ag recovery, %
100
Ag Cu Ni Sn Zn
80
60
60
40
40
20
20
0
Cu dissolved, mg
100
0 30
45
60
75
90
o
Temperature, C
Figure 8. Effect of temperature on the Ag recovery by cementation with Cu sheet. Stirring speed = 200
Figure 8. Effect of temperature on the Ag recovery by cementation with Cu sheet. Stirring speed = 200 rpm; rpm; temperature = 30–95 °C; time = 60 min; copper sheet = 50 × 12.5 mm2. temperature = 30–95 ◦ C; time = 60 min; copper sheet = 50 × 12.5 mm2 .
Stirring speed is also one of the important factors in cementation. In order to obtain proper stirring speed, the experiments were carried out by varying stirring speed from zero to 400 rpm. Reaction temperature and time were fixed at 80 °C and 60 min, respectively. Figure 9 shows the effect of stirring speed on the recovery percentage of silver(I). The recovery percentage of silver(I) rapidly increased as the stirring speed increased from zero to 200 rpm. After 200 rpm of stirring speed, the
20
20
0
0 30
45
60
75
90
o
Temperature, C
Processes 2018, 6, 112 Figure 8. Effect of temperature on the Ag recovery by cementation with Cu sheet. Stirring speed = 200 rpm; temperature = 30–95 °C; time = 60 min; copper sheet = 50 × 12.5 mm2.
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Stirring speed is also one of the important factors in cementation. In order to obtain proper is also one ofwere the important factors in cementation. order from to obtain stirringStirring speed, speed the experiments carried out by varying stirringInspeed zeroproper to 400 rpm. stirring speed, the experiments were carried out by◦ varying stirring speed from zero to 400 rpm. Reaction temperature and time were fixed at 80 C and 60 min, respectively. Figure 9 shows the Reaction temperature and time were fixed at 80 °C and 60 min, respectively. Figure 9 shows the effect effect of stirring speed on the recovery percentage of silver(I). The recovery percentage of silver(I) of stirring speed on the recovery percentage of silver(I). The recovery percentage of silver(I) rapidly rapidly increased as the stirring speed increased from zero After to 200 After 200speed, rpm of increased as the stirring speed increased from zero to 200 rpm. 200rpm. rpm of stirring thestirring speed, the recovery of silver(I) remained constant. Thesethat results indicate thatstep the mass recovery percentagepercentage of silver(I) remained constant. These results indicate the mass transfer transfer step plays a vital role during the cementation of silver(I) with copper sheet, which are plays a vital role during the cementation of silver(I) with copper sheet, which are in good agreementin good agreement the results reported with the with reported [29].results [29].
80
Ag recovery, %
100
Ag Cu Ni Sn Zn
80
60
60
40
40
20
20
0
Cu disolved, mg
100
0 0
100
200
300
400
Stirring speed, rpm
Figure 9. Effect of stirring speed on the Ag recovery by cementation with Cu sheet. Stirring speed = Figure 9. Effect of stirring speed on the Ag recovery by cementation with Cu sheet. Stirring speed = 0–400 rpm; temperature = 80 °C; time = 60 min; copper sheet = 50 × 12.5 mm2. 0–400 rpm; temperature = 80 ◦ C; time = 60 min; copper sheet = 50 × 12.5 mm2 .
The mass of copper sheet might be related to the recovery percentage of silver(I). Therefore, The mass ofsheets copper sheet be related to thewidth recovery percentage of the silver(I). several copper with the might same area but different were employed and results Therefore, are shown in Figure 10.with As the of copper sheet increased, the recovery percentage of silver(I) several copper sheets the width same area but different width were employed and the results are shown slowly increased. When theofwidth of the copper sheet was mm, around 93% of silver(I) was slowly in Figure 10. As the width copper sheet increased, the25recovery percentage of silver(I) recovered from the solution. increased. When the width of the copper sheet was 25 mm, around 93% of silver(I) was recovered 11 of 14
100
100
80
80 Ag Cu Ni Sn Zn
60
60
40
40
20
20
0
Cu dissolved, mg
Ag recovery, %
Processes 6, x FOR PEER REVIEW from 2018, the solution.
0 50 x 6
50 x 12.5
50 x 25
50 x 36
50 x 50
Cu sheet area, mm x mm
Figure 10. Effect of Cu sheet area on the Ag recovery by cementation with Cu sheet. Stirring speed = Figure 10. Effect of Cu sheet area on the Ag recovery by cementation with Cu sheet. Stirring speed = 200 rpm; 200temperature rpm; temperature 80 °C; 60 min; copper 5012.5, × 6, 50 50× × 25, 12.5, 502.× 50 = 80 ◦ C; =time = 60time min;=copper sheet = 50 sheet × 6, 50= × 5050 × ×36;25, 5050 × ×5036; mm mm2.
3.3.2. Synthesis of Silver Particles During the cementation of silver(I) by copper sheet, it is inevitable that trace amount of other metal ions such as copper(II), nickel(II), tin(II) and zinc(II) can be incorporated into the cemented silver. In order to recover the silver metal with high purity, treatment of the cemented silver is
50 x 6
50 x 12.5
50 x 25
50 x 36
50 x 50
Cu sheet area, mm x mm
Figure 10. Effect of Cu sheet area on the Ag recovery by cementation with Cu sheet. Stirring speed = 200 rpm; temperature = 80 °C; time = 60 min; copper sheet = 50 × 6, 50 × 12.5, 50 × 25, 50 × 36; 50 × 50 2. mm Processes 2018, 6, 112 11 of 14
3.3.2. Synthesis of Silver Particles 3.3.2. Synthesis of Silver Particles During the cementation of silver(I) by copper sheet, it is inevitable that trace amount of other the cementation of silver(I) by tin(II) copperand sheet, it is inevitable that trace amount of other metal metalDuring ions such as copper(II), nickel(II), zinc(II) can be incorporated into the cemented ions such as copper(II), nickel(II), tin(II) and zinc(II) can be incorporated into the cemented silver. In order silver. In order to recover the silver metal with high purity, treatment of the cemented silver is to recover the metal withthe high purity, treatment of washed the cemented silver is necessary. Forhydrochloric this purpose, necessary. Forsilver this purpose, cemented silver was several times by 2% (v/v) the cemented was washed several timesimage by 2%of(v/v) hydrochloric acid solution and distilled water. acid solution silver and distilled water. The SEM the dried silver powder after washing is shown The SEM image of the dried silver powder after washing is shown in Figure 11. The particle size and in Figure 11. The particle size and morphology of the silver metal was not homogeneous. This might morphology ofthe the non-smoothness silver metal was not homogeneous. This might beresults ascribed the non-smoothness of be ascribed to of copper sheet surface, which in to the heterogeneity of the copper sheet surface, which results in the heterogeneity of the cemented silver [8]. cemented silver [8].
Figure 11. SEM Figure 11. SEM images images of of the the HCl HCl and and water-washed water-washed silver silver particles particles on on the the surface surface of of copper copper sheet. sheet. [HCl] = 2% (v/v). [HCl] = 2% (v/v).
In the filtration and drying, a trace amount of silver powders might form the precipitate of AgCl In the filtration and drying, a trace amount of silver powders might form the precipitate of and be oxidized to Ag2O, which might be incorporated into the cemented silver. Therefore, the AgCl and be oxidized to Ag2 O, which might be incorporated into the cemented silver. Therefore, cemented silver powders were dissolved into 3 M nitric acid solution to dissolve silver metal and the cemented silver powders were dissolved into 3 M nitric acid solution to dissolve silver metal and then remove AgCl by filtration. Equation (9) represents the dissolution reaction of Ag2O in HNO3 then remove AgCl by filtration. Equation (9) represents the dissolution reaction of Ag2 O in HNO3 solution. The purity of silver in the AgNO3 solution thus obtained was 99.99%. Therefore, it can be solution. The purity of silver in the AgNO3 solution thus obtained was 99.99%. Therefore, it can said that cementation of the synthetic solution with copper sheet followed by HNO3 treatment be said that cementation of the synthetic solution with copper sheet followed by HNO3 treatment increased the purity of silver in the solution from 50.9% to 99.99%. increased the purity of silver in the solution from 50.9% to 99.99%. Ag2 O + 2HNO3 = 2AgNO3 + H2 O
(10)
In order to synthesize silver powders with micron size, ascorbic acid was added as a reducing agent to the AgNO3 solution thus obtained and the reduction reaction can be represented as Equation (11) [30,31]. Equation (11) indicates that the reduction of silver ion by ascorbic acid cannot occur in acidic solution. Therefore, solution pH of the AgNO3 solution was adjusted to 6, 8, and 10 by adding ammonia solution. Figure 12 shows the SEM images of the silver powders thus synthesized at three different solution pH values. C6 H8 O6 + 2Ag+ = 2Ag0 + C6 H8 O6 + 2H+ (11)
acidic solution. Therefore, solution pH of the AgNO3 solution was adjusted to 6, 8, and 10 by adding ammonia solution. Figure 12 shows the SEM images of the silver powders thus synthesized at three different solution pH values. C6H8O6 + 2Ag+ = 2Ag0 + C6H8O6 + 2H+ Processes 2018, 6, 112
(11) 12 of 14
Figure 12. SEM images of the silver powders reduced by ascorbic acid from the HNO3 solution with Figure 12. SEM images of the silver powders reduced by ascorbic acid from the HNO3 solution with several pH (6, 8, 10) at 30 ◦°C. several pH (6, 8, 10) at 30 C.
The shape of the silver particles synthesized at pH 6 consisted of polyhedral crystal blocks and The shape of the of silver at pHsolution 6 consisted polyhedral blocks and the average diameter the particles particles synthesized was 2 µm. When pH of was increasedcrystal to 8, the shape of the average diameter of the particles was 2 µm. When solution pH was increased to 8, the shape of the the silver particles was changed from polyhedral crystal block into lamellate with the average size silver particles wasAs changed blocktointo the average size lower lower than 2 µm. solutionfrom pH polyhedral was furthercrystal increased 10, lamellate the silver with lamellate disappeared and than 2 µm. As solution pH was further increased to 10, the silver lamellate disappeared and the shape the shape of the particles were changed into cubic and bar with average diameter of 2 µm. Therefore, ofcan thebe particles were changed intoacubic bar average diameter of 2 µm. reduced Therefore, can be it said that solution pH has great and effect onwith the shape of the silver particles by it ascorbic said that solution pH has a great effect on the shape of the silver particles reduced by ascorbic acid. acid. 3.4. Integrated Proposed Methods 3.4. Integrated Proposed Methods The anode slime resulted from the electro-refining of tin contained gold, silver, copper, nickel, The anode slime resulted from the electro-refining of tin contained gold, silver, copper, nickel, tin and zinc. Leaching of these anode slimes with hydrochloric acid solution in the presence of tin and zinc. Leaching of these anode slimes with hydrochloric acid solution in the presence of an an oxidizing agent led to the solution containing these metal ions [5]. Solvent extraction with Cyanex oxidizing agent led to the solution containing these metal ions [5]. Solvent extraction with Cyanex 272 followed by LIX 63 separated gold(III) and copper(II) and most of tin(II) [6,7]. From the raffinate 272 followed by LIX 63 separated gold(III) and copper(II) and most of tin(II) [6,7]. From the raffinate after the solvent extraction with the above extractants, silver(I) can be separated by cementation with after the solvent extraction with the above extractants, silver(I) can be separated by cementation with copper sheet. copper sheet. Ion exchange with the resins employed in this work could not separate silver(I) from the Ion exchange with the resins employed in this work could not separate silver(I) from the solution, but only both tin(II) and zinc(II) could be selectively adsorbed into the resins. However, solution, but only both tin(II) and zinc(II) could be selectively adsorbed into the resins. However, silver(I) cementation with copper sheet was successfully achieved from the solution. Therefore, silver(I) cementation with copper sheet was successfully achieved from the solution. Therefore, cementation can be applied to separate silver(I) from the hydrochloric acid solution containing cementation can be applied to separate silver(I) from the hydrochloric acid solution containing copper(II), nickel(II), tin(II) and zinc(II). copper(II), nickel(II), tin(II) and zinc(II). The dissolved Cu(II) during the cementation might be recovered by solvent extraction with LIX The dissolved Cu(II) during the cementation might be recovered by solvent extraction with LIX 63. In consequence, silver particles can be synthesized by adding ascorbic acid after changing solution 63. In consequence, silver particles can be synthesized by adding ascorbic acid after changing solution pH of the AgNO3 solution. pH of the AgNO3 solution. 4. Conclusions 4. Conclusions Gold(III), copper(II) and tin(II) can be separated from the hydrochloric acid leaching solution Gold(III), copper(II) andnickel(II), tin(II) cansilver(I), be separated from the acid leaching solution of of anode slimes containing and zinc(II) byhydrochloric solvent extraction. After the solvent anode slimes nickel(II),nickel(II), silver(I), silver(I) and zinc(II) by solvent extraction. solvent extraction, thecontaining raffinate contained and zinc(II) together with aAfter smallthe amount of copper(II) and tin(II). Ion exchange and cementation experiments were done in this work to separate silver(I) from this raffinate. In ion exchange with the anionic resins employed in this work, tin(II) and zinc(II) were selectively adsorbed into the resins, while the adsorption percentage of silver(II) was very low. Therefore, it was difficult to separate silver(I) by ion exchange with anionic resins employed in this work. Silver(I) was separated from the solution by cementation with copper sheet. Above 93% of silver was recovered at the optimum condition of 80 ◦ C, 200 rpm and 50 × 25 mm2 copper sheet in 60 min Dissolution of the cemented silver into nitric acid solution indicated that the purity of silver(I) was increased from 50.9% to 99.99% by cementation. Silver powders with micron size were synthesized by reduction of the AgNO3 solution with ascorbic acid at solution pH 6, 8 and 10.
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Author Contributions: M.S.L. designed the research and helped to analyze data. W.D.X. performed experiments. S.H.C. discussed the results and made the experimental plan. Funding: This work was funded by Korea Institute of Energy Technology Evaluation and Planning (KETEP). The grant number is 20165010100810. Acknowledgments: This work was supported by the Global Excellent Technology Innovation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20165010100810). Conflicts of Interest: The authors declare no conflict of interest.
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