Precious Metal Distributions Between Copper Matte and Slag at High PSO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ P_{{{\text{SO}}_{ 2} }} $$\end{document} in WEEE Reprocessing

The distributions of precious metals (gold, silver, platinum, and palladium) between copper matte and silica-saturated FeOx-SiO2/FeOx-SiO2-Al2O3/FeOx-SiO2-Al2O3-CaO slags were investigated at 1300 °C and PSO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ P_{{{\text{SO}}_{ 2} }} $$\end{document} = 0.5 atm. The experiments were carried out in silica crucibles under flowing CO-CO2-SO2-Ar gas atmosphere. The concentrations of precious metals in matte and slag were analyzed by Electron Probe X-ray Microanalysis and Laser Ablation-High-Resolution Inductively Coupled Plasma-Mass Spectrometry, respectively. The precious metal concentrations in matte and slag, as well as the distribution coefficients of precious metals between matte and slag, were displayed as a function of matte grade. The present results obtained at PSO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ P_{{{\text{SO}}_{ 2} }} $$\end{document} of 0.5 atm were compared with previous results at PSO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ P_{{{\text{SO}}_{ 2} }} $$\end{document} of 0.1 atm for revealing the effects of PSO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ P_{{{\text{SO}}_{ 2} }} $$\end{document} and selected slag modifiers (CaO and Al2O3) on precious metal distributions at copper matte smelting conditions. The present results also contribute experimental thermodynamic data of precious metal distributions in pyrometallurgical reprocessing of electronic waste via copper smelting processes.


I. INTRODUCTION
PRECIOUS metals, including gold (Au), silver (Ag), and platinum group metals, are widely used in the electronics industry and auto-catalyst industry, due to their properties of good electrical conductivity, high melting point, and corrosion resistance. [1,2] The current higher demand for precious metals and the depletion of natural resources result in an urgent need to recover precious metals from more complicated secondary materials, especially from waste electrical and electronic equipment (WEEE), [3] in which concentrations of precious metals are significantly higher than those of natural ores. [4,5] The most typical industrial methods for WEEE reprocessing are primary and secondary copper smelting, [6][7][8][9] followed by hydrometallurgical and electrochemical techniques. During the pyrometallurgical copper smelting processes, precious metals distribute principally between metal/matte/slag/gas phases, [10,11] and thus achieving partial recovery and purity of precious metals in the main product streams of matte and metal. [12] In order to maximize the recoveries of precious metals to the metal/matte phase, it is essential to investigate the distribution mechanism of the precious metals in the metal/matte-slag system.
The distributions of precious metals have been extensively investigated between copper matte and slag at P SO 2 lower than 0.2 atm. [13][14][15][16][25][26][27][28][29][30][31] However, the use of oxygen or oxygen-enriched air as the process gas in modern copper flash smelting produces a smaller amount of off-gas with a high SO 2 concentration (> 50 vol pct). [45,46] The mechanism of matte and slag formation in flash smelting generates SO 2 with partial pressures close to 1 atm. [47] Therefore, accurate information about the distributions of precious metals between matte and slag under a high P SO 2 is of practical importance for thermodynamically evaluating the behaviors of precious metals in copper smelting. The primary purpose of this study was to investigate the distribution behaviors of precious metals (Au, Ag, Pt, and Pd) between copper matte and silica-saturated FeO x -SiO 2 /FeO x -SiO 2 -Al 2 O 3 / FeO x -SiO 2 -Al 2 O 3 -CaO slags at 1300°C and P SO 2 of 0.5 atm, providing fundamental information for understanding and improving the recycling of precious metals from WEEE through primary copper smelting processes.

II. EXPERIMENTAL
Experiments were conducted in a similar way to our previous studies. [25,27,48,49] A high-temperature equilibration under controlled CO-CO 2 -SO 2 -Ar gas atmospheres followed by quenching in ice-water mixture and direct phase analyses by EPMA and LA-HR ICP-MS (HR-High Resolution) techniques was used to study the distributions of precious metals between copper matte and slag. A schematic image of the laboratory-scale furnace used is shown in Figure 1. All further details of the furnace and the gas flows used for achieving the desired atmosphere have been presented in our previous publication, which focused on the major element equilibrium in these same experiments. [49] The slag mixtures were produced using analytical high-purity powders of Fe 2 O 3 (99.998 wt pct, Alfa Aesar), SiO 2 (99.995 wt pct, Alfa Aesar), Al 2 O 3 (99.99 wt pct, Sigma-Aldrich), and CaO (99.9 wt pct, Sigma-Aldrich). The copper matte mixtures were prepared by Cu 2 S (99.5 wt pct) and FeS (99.9 wt pct), all from Alfa Aesar. Each metallic powder of Au (99.96 wt pct), Ag (99.95 wt pct), Pt (99.99 wt pct), and Pd (99.9 wt pct) was added into the copper matte mixture to a concentration of 1 wt pct. All precious metals were supplied by Alfa Aesar. The gas mixtures of CO (99.99 vol pct), CO 2 (99.999 vol pct), SO 2 (99.99 vol pct), and Ar (99.999 vol pct), supplied by Aga-Linde were introduced into the furnace for controlling the partial pressures of SO 2 , S 2 , and O 2 .
All gases were regulated by DFC26 digital mass flow controllers (Aalborg, USA). The partial pressure of SO 2 was fixed at 0.5 atm in all experiments. The gas atmosphere speciations at different target matte grades were calculated by MTDATA software [50,51] using the SGTE pure substance database. [51] In each experiment, around 0.1 g of copper matte with equal amounts of slag were pressed into a pellet and then placed into a bowl-shaped fused silica crucible. The samples were equilibrated under different CO-CO 2 -SO 2 -Ar gas atmospheres at experimental temperature for 4 hours. [25,48,49] The equilibrated samples were quenched into an ice-water mixture.
The quenched samples were dried, cut into half, embedded in epoxy resin (EpoFix, Struers, Denmark), and then ground and polished by metallographic methods. The polished surfaces were carbon coated using a LEICA EM SCD050 sputtering device (Leica Microsystems, Austria).
Microstructural and preliminary elemental analysis were carried out with Scanning Electron Microscope (SEM, Tescan MIRA 3, Brno, Czech Republic) equipped with an UltraDry Silicon Drift Energy Dispersive X-ray Spectrometer (EDS, Thermo Fisher Scientific, Waltham, MA, USA). The direct measurement of the concentrations of precious metals in matte, as well as the main compositions of matte and slag, were executed using a Cameca SX100 Electron Microprobe (Cameca SAS, Gennevilliers, France) coupled with five wavelength-dispersive spectrometers (WDS). The parameters were selected so that the accelerating voltage was 20 kV, beam current 60 nA, and beam diameter 100 lm and 50 to 100 lm for matte and slag, respectively. Small PGM-rich segregations (described below at the beginning of Section III) were distributed relatively evenly throughout the matte phase, and these were included in the EPMA analyses conducted with a 100-lm defocused beam. Some copper-rich veins were also found in the matte; these were avoided during the concentration quantifications.
The standards used and the X-ray lines analyzed were naturally occurring minerals quartz (Si Ka), almandine for (Al Ka), hematite (Fe Ka and O Ka), pentlandite (S Ka), diopside (Ca Ka), and synthetic pure metals Cu (Cu Ka), Au (Au La), Ag (Ag La), Pt (Pt La), and Pd (Pd La). PAP-ZAF online correction procedure was used for raw data processing. [52] Eight measuring points were taken from each phase for result averages and standard deviations.
LA-HR ICP-MS [53,54] was used to measure the precious metal concentrations in slags that were below the EPMA detection limits. The equipment and analysis procedures were exactly the same as in our previous publication dealing with the behavior of precious metals at lower sulfur dioxide partial pressure, [25] except for a slightly lower laser fluence (2.17 J/cm 2 ) on the sample surface. In this work, the slags of all samples were analyzed with LA-HR ICP-MS.
Fastscan mode, with a low resolution (M/DM = 300) for increased sensitivity, was applied for collecting the time-resolved analysis (TRA) signals. Signal processing was performed using the Glitter software. [55] The  concentrations of precious metals in slags measured by LA-HR ICP-MS were calculated using isotopes of 197 Au for gold, 104 Pd for palladium, an average of 107 Ag and 109 Ag for silver, and an average of 194 Pt, 195 Pt, and 196 Pt for platinum. Table II shows the elemental detection limits for EPMA and LA-HR ICP-MS. The TRA signals in different samples showed either completely homogeneous dissolution of the precious metals into the slag, or the presence of some nano-/ micronuggets containing higher precious metal concentrations. [56] Examples of both cases are shown in Figure 2. The occurrence of the nuggets in the slags did not follow any particular pattern: they were observed in some samples with and without Al 2 O 3 and CaO. In most cases, it was possible to select inclusion-free segments of the signal profiles for quantitative concentration calculations, as marked in Figure 2(b) with the light gray color.

III. RESULTS AND DISCUSSION
Typical microstructures of the copper matte equilibrated with different silica-saturated slags for target matte grade of 60 wt pct Cu are shown in Figure 3. The glassy slag phase was homogeneous; however, copper-rich veins and small segregations with higher precious metal concentrations were distributed randomly in the matte phase, most likely due to an insufficient quenching rate, as seen earlier. [25,48,49] The concentrations of precious metals in matte and slag with standard deviations (± 1r) are listed in Table III. The results for the major elements can be seen in our previous study. [49] A.

Concentrations of Precious Metals in Matte
The concentrations of precious metals in matte with standard deviations, measured by EPMA, are displayed as a function of matte grade in Figure 4. The results obtained at 1300°C and P SO 2 of 0.1 atm from our previous study, [25] shown as open symbols, are also plotted in the graphs for comparison with the results of few other studies. [27,28] The concentrations of all precious metals in matte, equilibrated with different slags, kept almost constant over the entire matte grade range studied, suggesting that the deportment of precious metals into the matte phase was not affected by slag modifiers. The results for silver and palladium obtained at P SO 2 of 0.5 atm are within error of the previous data achieved at P SO 2 of 0.1 atm [25] at the same matte grade. This indicates that the deportment of silver and palladium into the matte phase was not affected by the prevailing P SO 2 . However, the results for gold and platinum obtained at P SO 2 of 0.5 atm are approximately 0.1 and 0.2 wt pct higher than the previous results at 0.1 atm P SO 2 , respectively. The present results fit well with the observations by Avarmaa et al. [27] but are on the higher side of the results by Shishin et al. [28] Avarmaa et al. [27] reported that the silver concentration in matte was sensitive to the temperature and it decreased with increasing temperature. The higher volatility of silver at higher temperatures was regarded as the main factor for the decreasing concentration trend. [22,27] B.

Concentrations of Precious Metals in Slags
During recycling of WEEE through copper smelting routes, it is essential to minimize the losses of precious metals in the slag. Precious metal concentration data are helpful in analyzing and controlling these losses. The concentrations of precious metals in slags with standard deviations, measured by LA-HR ICP-MS, were plotted in Figure 5 against the matte grade. Figure 5(a) indicates that the concentrations of gold dissolved in all silica-saturated slags in equilibrium with copper matte were typically between 0.4 and 10 ppmw, and they decreased with increasing matte grade (i.e., with increasing oxygen partial pressure), as reported by Avarmaa et al. [26] and Shishin et al. [28] However, an opposite, increasing trend for gold solubility in sulfur-free iron silicate slag equilibrated with Au-Pd alloy was observed by Borisov and Palme, [57] reporting that the gold solubility increased from 1 to approximately 10 ppm with increasing oxygen partial pressure from 10 À8 to 10 À3 atm. Shishin et al. [28] measured the gold solubility in FeO x -SiO 2 slag in equilibrium with metallic gold at 1250°C to 1300°C. The values reported were lower than 4 ppmw, [28] which appears to be somewhat lower than the ones obtained in this study. The present results for gold in slags obtained at P SO 2 of 0.5 atm were almost identical to the observations at P SO 2 = 0.1 atm, suggesting that gold concentration in the slag was not sensitive to the prevailing SO 2 partial pressure. Gold losses in silica-saturated slags were, however, significantly suppressed by the addition of alumina and lime independent of P SO 2 .
Toguri and Santander [58] investigated the distribution of gold between copper-gold alloy and silica-saturated FeO x -SiO 2 slag at 1250°C to 1350°C and P SO 2 of 10 À10 to 10 À7 atm. The gold concentration in slag was    reported to be several hundreds of ppm, [58] which is significantly higher than the present results. In the study by Avarmaa et al., [21,29] a remarkably lower gold solubility (< 0.2 ppmw) was observed in alumina/ silica-saturated FeO x -SiO 2 -Al 2 O 3 slags in a sulfur-free system. Choi and Cho [59] measured the gold concentrations in silica-saturated FeO x -SiO 2 slags equilibrated with nickel-copper matte at 1300°C, and they reported that the concentrations of gold in these slags were approximately 100 to 800 ppmw, which are similar to the results by Toguri and Santander. [58] The impact of Al 2 O 3 , MgO, and CaO on decreasing the gold concentrations in slags were also observed in their study. [59] Altman and Kellogg [60] measured a concentration of gold in silica-saturated FeO x -SiO 2 slag at 1224°C to 1286°C of approximately 80 ppmw. Taylor and Jeffes [61] reported that the gold concentration in iron silicate slags at 1300°C to 1450°C and P O 2 10 À13 to 10 À5 atm was around 40 to 80 ppmw. In previous studies, [58][59][60][61] the concentrations of gold in slags were analyzed following physical separation of copper-gold alloy and slag. An incomplete separation of copper-gold alloy and slag most likely contributed to the higher gold concentration in slags in those studies. [58][59][60][61] Compared with the other three precious metals, silver exhibited a significantly higher concentration in silica-saturated iron silicate slags, varying between approximately 50 and 100 ppmw, as shown in Figure 5(b), which are on the higher side of the observations by Shishin et al. [28] The silver concentration in FeO x -SiO 2 slag in the present study was kept almost constant over the entire matte grade range investigated, fluctuating around 60 ppmw, similar to the results by Louey et al. [13] However, increasing trends were found for alumina-containing and alumina + lime-containing slags. A similar increasing trend was also reported by Avarmaa et al. [29] and Hidayat et al. [24] for alumina-saturated FeO x -SiO 2 -Al 2 O 3 slag and silica-saturated FeO x -SiO 2 slag in equilibrium with metallic copper at 1300°C, respectively. Avarmaa et al. [26] also reported that the concentration of silver in silica-saturated FeO x -SiO 2 slags, equilibrated with copper matte, was dependent on   [27] 1300 °C [27] 1350 °C [28] (b) 55  open symbols-our previous study [25] temperature and it decreased with increasing temperature. Shishin et al. [62] investigated the behavior of silver between metallic lead and silica-saturated FeO x -SiO 2 slag at 1000°C to 1200°C, and the silver concentration in the slag was reported to be approximately 45 to 47 ppmw, which was slightly lower than the present results for FeO x -SiO 2 slags in equilibrium with copper matte. The silver concentrations in slags obtained at 0.1 and 0.5 atm P SO 2 seemed not to be affected by the P SO 2 , similar to the results for gold, platinum, and palladium. The independence of silver concentrations in slags to P SO 2 was also reported by Roghani et al. [17][18][19][20] for magnesia-saturated FeO x -CaO/FeO x -SiO 2 -CaO/FeO x -SiO 2 -MgO/FeO x -SiO 2 -MgO-CaO slags at P SO 2 of 0.1, 0.5, and 1 atm. The use of magnesia crucibles in previous studies [14][15][16][17][18][19][20] resulted in MgO dissolution into the slags. The concentrations of precious metals were analyzed by wet chemical analysis techniques after physically separating the matte and slag, which may have caused erroneous results due to incomplete phase separation. The factors mentioned above lead to significant gaps and discrepancies between the present observations and the reported results by Roghani et al. [17][18][19][20] The concentrations of platinum and palladium in FeO x -SiO 2 and FeO x -SiO 2 -Al 2 O 3 slags have similar decreasing trends with increasing matte grade (i.e., increasing oxygen partial pressure), as reported by Avarmaa et al. [26] In the study by Yamaguchi [14,15] and Baba et al., [63] the concentrations of platinum and palladium in FeO x -SiO 2 slags, equilibrated with pure metallic platinum/palladium, increased from 3 to 7 ppmw and 7 to 20 ppmw, respectively, with increasing oxygen partial pressure from 10 À9 to 10 À6 atm at 1300°C . Those results are opposite to the present decreasing trends but agree well with the observation by Shuva et al. [64] Klemettinen et al. [65] reported that the platinum solubility in alumina spinel-saturated FeO x -SiO 2 -Al 2 O 3 slag equilibrated with pure platinum increased from around 15 to 80 ppbw when the P O 2 increased from 10 À10 to 10 À5 atm. The solubility data reported by open symbols-our previous study [25] Klemettinen et al. [65] were extremely low, even at the highest P O 2 of copper-making conditions, around 10 À5 atm. Murata and Yamaguchi [66] investigated the recovery of platinum and palladium from an Al 2 O 3 -CaO-SiO 2 slag using Cu or Cu 2 O. The concentrations of platinum in the slag were around 80 and 2 ppmw after being equilibrated with metallic Cu and Cu 2 O at 1450°C for 12 hours, respectively. Simultaneously, the results for palladium were reported to be 130 and 1.3 ppmw, respectively. The addition of alumina and lime decreased the platinum and palladium losses in iron silicate slags significantly in the present study, even down to 0.5 ppmw. Similarly, Wiraseranee et al. [67] reported that the solubility of platinum in Na 2 O-SiO 2based slags decreased with increasing contents of alumina, magnesia, ferric oxide, and copper oxide in the slags.

C. Distributions of Precious Metals Between Matte and Slag
The distribution coefficients of precious metals between copper matte and slag, L m/s (Me), were calculated based on Eq. [1]: where [wt pct Me] and (wt pct Me) refer to the average concentrations of precious metals in matte and slag, respectively. The obtained distribution coefficients for gold, silver, platinum, and palladium are displayed as a function of matte grade in Figure 6 together with selected literature data.
All precious metals were preferentially concentrated in the matte and their distributions were affected by the matte grade. Figure 6(a) indicates that the deportment of gold into the matte phase equilibrated with all slags were favored by increasing matte grade, which agrees well with the observations in our previous study at P SO 2 0.1 atm [25] and by Avarmaa et al. [26] The addition of alumina and lime increased the logarithmic distribution coefficient of gold by approximately 0.4 and 0.8, respectively, at the matte grade of 65 wt pct Cu. Similar positive effects of alumina and lime on increasing the deportment into the matte phase were also observed for platinum and palladium, however, the increasing L m/s trends as a function of matte grade were less obvious for the alumina + lime-containing slag. Chen et al. [68] determined the distribution coefficient of gold between metallic copper and silica-saturated FeO x -SiO 2 slag in the slag/matte/metal equilibrium system, reporting that the logarithmic value decreased with increasing the matte grade from 65 to 80 wt pct Cu, which is opposite to the tendency observed in the present study. Importantly, the distribution coefficients of the precious metals investigated seemed unaffected by the prevailing P SO 2 . Therefore, from the point of view of thermodynamics it can be concluded that the use of oxygen or oxygen-enriched air as the blowing gas should have no impact on the recovery of these precious metals in industrial copper flash smelting processes. Figure 6(b) shows that the distribution coefficient of silver between copper matte and FeO x -SiO 2 slag stayed almost constant over the entire matte grade range studied, close to that reported by Avarmaa et al. [26] However, opposite downward trends with increasing matte grade were observed for the matte/FeO x -SiO 2 -Al 2 O 3 and matte/FeO x -SiO 2 -Al 2 O 3 -CaO systems. The trend line of the distribution coefficients of silver between matte and FeO x -SiO 2 slag is on the higher side of that between matte and FeO x -SiO 2 -Al 2 O 3 slag but on the lower side of that between matte and FeO x -SiO 2 -Al 2 O 3 -CaO slag. Thus, the deportment of silver into the matte phase may be improved by lime addition, whereas alumina had no significant impact when compared with the results for FeO x -SiO 2 slag. Similar effects of alumina and lime on the deportment of silver into the metallic copper were also reported by Avarmaa. [69] The distribution coefficient of silver between matte and FeO x -SiO 2 -MgO slag reported by Roghani et al. [20] was slightly higher than the present results between matte and FeO x -SiO 2 -Al 2 O 3 slag. The L m/s Ag between matte and FeO x -SiO 2 -MgO-CaO slag obtained by Roghani et al. [19] was significantly higher than the present results between matte and FeO x -SiO 2 -Al 2 O 3 -CaO slag, indicating that magnesia has a larger impact than alumina on improving the recovery of silver into the matte phase. Kashima et al. [70] obtained distribution coefficients of silver between white metal and silica-saturated iron silicate slag at 1300°C and P SO 2 of 0.0007 to 0.20 atm. Similar to the results in this study the L m/s Ag were found to be independent of the P SO 2 . [17][18][19][20] The distribution coefficients of platinum and palladium between matte and FeO x -SiO 2 /FeO x -SiO 2 -Al 2 O 3 slag had similar increasing trends with increasing matte grade, [26] but the slope for platinum was steeper. Alumina addition improved the logarithmic distribution coefficients of platinum and palladium by approximately 0.7 and 0.6, respectively, at a matte grade of 65 wt pct Cu. However, the Log 10 [L m/s Pt] and Log 10 [L m/s Pd] for the matte/FeO x -SiO 2 -Al 2 O 3 -CaO slag system kept almost constant at 4.6 and 4.4, respectively. Yamaguchi [14,15] and Henao et al. [16] reported that the logarithmic distribution coefficients of platinum and palladium remained constant at around 3 in the matte grade range of 40 to 65 wt pct Cu, after which it started to decrease at higher matte grades. The incomplete phase separation of matte and slag may have led to the lower distribution coefficients in those studies. [14][15][16]

IV. CONCLUSIONS
The distribution equilibria of gold, silver, platinum, and palladium between copper mattes and silicasaturated FeO x -SiO 2 /FeO x -SiO 2 -Al 2 O 3 /FeO x -SiO 2 -Al 2 O 3 -CaO slags were determined at 1300°C and at a high P SO 2 of 0.5 atm using a high-temperature equilibration/quenching/EPMA/LA-HR ICP-MS technique. The present results help to deepen our understanding of the behaviors of precious metals in the copper flash smelting processes at high P SO 2 conditions. A comparison of the results obtained at P SO 2 of 0.1 and 0.5 atm shows that the concentrations of silver and palladium in matte at a fixed matte grade were not affected by the prevailing P SO 2 , however, the gold and platinum concentrations in matte were somewhat increased with higher P SO 2 . The precious metal concentrations in the slags were independent of P SO 2 . The concentrations of gold, platinum, and palladium in slags can be effectively decreased by alumina and lime additions, and by increasing the matte grade.
All the precious metals were found to highly distribute into the matte phase. Higher matte grades and addition of alumina and lime were proven to have positive effects on increasing the L m/s (Au), L m/s (Pt), and L m/s (Pd) and thus their recoveries to the sulfide matte. Consequently, it is essential to carefully control the slag composition and matte grade during the pyro-reprocessing of precious metals containing waste materials with the purpose of maximizing the recoveries of precious metals.  Matte grade/wt pct Cu (d) Fig. 6-Logarithmic distribution coefficients of precious metals between copper matte and silica-saturated iron silicate slags as a function of matte grade at 1300°C and P SO2 of 0.5 atm: (a) gold; (b) silver; (c) platinum; (d) palladium; colored symbols-present study; open symbols-our previous study [25] the