Highly efficient recovery of platinum, palladium, and rhodium from spent automotive catalysts via iron melting collection

Resources, Conservation & Recycling 155 (2020) 104644

Contents lists available at ScienceDirect

Resources, Conservation & Recycling journal homepage: www.elsevier.com/locate/resconrec

Full length article

Highly efficient recovery of platinum, palladium, and rhodium from spent automotive catalysts via iron melting collection

T

Yunji Dinga, Huandong Zhenga, Shengen Zhanga,*, Bo Liua, Boyu Wua, Zhuming Jianb a b

Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China Yunlong Country Platinum Cui Precious Metals Technology Co., Ltd., Dali 672711, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Enrichment Platinum group metals Iron collection Spent automotive catalysts Low temperature

Automobile catalysts sector is the largest consumption field of platinum group metals (PGMs) worldwide, and they are the most important secondary resources of Pt, Pd, and Rh once they reach the end of life. However, recycling PGMs from spent automotive catalysts is of great difficulty due to their complex physicochemical characteristics and low concentrations. A highly efficient technology for recovery of Pt, Pd, and Rh by iron melting collection was proposed. Owing to the same face-centred cubic structure and close atomic radius, iron powder is used as collector since Fe and PGMs can form continuous solid solution. PGMs are concentrated and formed Fe-PGMs alloy phase at a relatively low temperature of 1300–1400 °C through adjustment of slag compositions. The principles of slag design were put forward, and the effects of basicity, slag compositions and collector dosage on the recovery efficiency of PGMs were studied. Factsage 7.0 software (CRCT-Thermfact and GTT-Technologies) was used to calculate slag viscosity and simulate slag reaction equilibrium during the melting process. The recovery efficiency of PGMs was 99.25 % under the optimized conditions: mass ratio of CaO/ Na2O=35:20, CaF2 5 wt.%, Na2B4O7 8.5 wt.%, collector Fe 15 wt.%, and 5 wt.% C, where the concentration of Pt, Pd and Rh in slag phase were 2.398, 3.879, and 0.976 g/t, respectively. Meanwhile, the formation of ferrosilicon (FeSi2, FeSi) was avoided. 50 kg pilot-scale experiments also achieved over 99 % recovery efficiency of PGMs.

1. Introduction Platinum group metals (PGMs) are widely employed in the hightech, such as electronics, catalysts, and medicine, due to their outstanding physical and chemical properties (Bhogeswararao and Srinivas, 2015; Diaz et al., 2016). PGMs (Pt, Pd, and Rh) are essential active components in auto-catalysts with the aim of converting CO, CHx and NOx into CO2, H2O and N2, respectively (Nevalainen et al., 2018). PGMs particles are widely dispersed in the coating layer with the diameters of 1−10 nm. With the boost development of the car industry, automotive catalysts occpy the largest market share of PGMs, and the consumptions of Pt, Pd, and Rh in 2016 were over 45 %, 67 %, and 85 % of their global demand, respectively (USGS, 2019). However, the reserves of PGMs are extremely scarce and concentrated in only a few countries and regions. The world reserves of PGMs are only approximately 69,000 tons, and 99 % exist in South Africa, Canada, Russia, and America (Zhang et al., 2017). Hence, most countries extremely lack PGMs and face serious supply risk. Spent auto-catalysts have become the most significant source of PGMs in recent years due to the

⁎

increasing discard of cars (Osman et al., 2018; Sverdrup and Ragnarsdottir, 2016). However, spent automotive catalysts are hazardous solid waste, the negative environmental impacts would be generated if they were disposed improperly (Wei et al., 2019). Currently, spent auto-catalysts recycling has raised great attention for the consideration of environmental and economic benefits. PGMs can be recovered through various routes using the pyro and/or hydrometallurgical methods (Ding et al., 2019). The conventional hydrometallurgical processes always involve PGMs leaching, precipitation, solvent extraction, ion exchange and electrolysis (Jha et al., 2013). In order to achieve a high leaching efficiency of PGMs, the aqua regia, cyanide and mineral acids with the presence of H2O2, O2 or Cl2 are frequently employed (Saguru et al., 2018). Aqua regia and cyanide leaching system have been forbidden, as the generation of highly polluted NOx and HCN gases, as well as various harmful elements in wastewater (Baghalha et al., 2009; Benke and Gnot, 2002). The application of chlorination leaching is also limited since the generated chlorine is corrosive, toxic and the leaching efficiency of PGMs is low, especially for Rh and Pt. Low concentration (< 0.05 %) of Pt and Rh in

Corresponding author. E-mail address: [email protected] (S. Zhang).

https://doi.org/10.1016/j.resconrec.2019.104644 Received 9 October 2019; Received in revised form 10 December 2019; Accepted 11 December 2019 0921-3449/ © 2019 Elsevier B.V. All rights reserved.

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

2. Principle analysis

spent catalysts is one of the main reasons leading to their low recovery efficiencies. The pyrometallurgical process is a simplified and efficient route for the high enrichment of PGMs from spent automotive catalysts (Dong et al., 2015). The base metals including lead, copper, nickel sulfide and iron are usually used as collection reagents of PGMs (Kolliopoulos et al., 2014; He et al., 2016; Jones and Geldenhuys, 2011). Pt, Pd, and Rh are concentrated and entered into alloy phase at high temperature. The PGMs alloy and molten slags are separated for their large different density. Although collection and cupellation of lead are easy and mature with a high recovery rate of PGMs, it has the disadvantages of the long processing period, high consumption of energy, and emission of volatile lead. Zhang et al. (2019) proposed the capturing of Pd by molten copper at 1250–1400 °C and found 97 % of Pd were recovered under optimal condition. Copper collection method has been employed in large scale applications, such as in Tanaka Precious Metals in Japan and Umicore in Belgium (Hagelüken, 2006). Due to the close affinity of nickel matte for PGMs and unique chemical composition of copper and nickel ore, Jinchuan Group Co., Ltd, a PGMs production company in China, employed nickel matte for capturing PGMs from ores (Peng et al., 2018). However, environmental issues should be specially paid attention on through the melting collection by copper and nickel (Jones and Geldenhuys, 2011; Peng et al., 2017). As iron is non-toxic, economical and high affinity with PGMs, it is the most promising collector for concentrating PGMs from spent catalysts (Benson et al., 2000). The plasma furnace and electronic arc furnace (EAF) are mainly applied for the iron melting collection process. The recovery efficiencies of Pt, Pd, and Rh were over 98 %, 98 % and 97 %, respectively at the industrial scale by plasma melting technology (He et al., 2016). However, the operation temperature was ranged 1600–2000 °C, which resulted in the reduction of Si and the formation of Fe-Si alloy (Stoffner and Hobbs, 2018; Cichy, 1982). The Fe-Si alloy was hard to be dissolved and silicone was easy to be generated even if it was dissolved, leading to the low recovery efficiency of PGMs. Furthermore, it is challenging for refractory materials to maintain their properties at such high temperature. Table 1 shows the recovery efficiencies of PGMs by different kinds of melting methods and most of them remain the in experimental stage. Therefore, how to efficiently and economically concentrate PGMs through environmental technology is an urgent problem. Here in this work, we proposed an iron collection process for PGMs in spent automotive catalysts with consideration of both PGMs recovery efficiency and the environmental impacts. The melting point of slag and its viscosity decreased by optimizing slag composition, realizing highly efficient capture of PGMs at a relatively low temperature (1300–1400 °C) and avoiding the formation of Fe-Si alloy. The factors including basicity, the addition of CaF2, Na2O, Na2B4O7 and dosage of iron, on recovery of Pt, Pd, and Rh are studied. Finally, a pilot-scale of this process under optimum conditions is carried out to verify its feasibility. This study provides an efficient and environmental way of recovering PGMs from spent catalyst.

2.1. Thermomechanical analysis Although PGMs are chemically stable, they have a remarkable ability to adsorb O2 on their surfaces and can be oxidized under critical conditions (Trinh et al., 2017). The standard Gibbs free energy of PtO2, PdO, and Rh2O3 (shown in Fig. S1 in the supplementary material) reveal that Pt, Pd, and Rh can be oxidized spontaneously at about 500, 800–840, and 600 °C, respectively. The operation temperature of the catalytic converter can reach over 1000 °C, leading to the formation of PGMs oxides during their service period. These oxides are extremely stable and do not react with any acid, even aqua regia. Meanwhile, the oxides cannot be recovered through the iron melting method except being reduced into the metallic state. Some possible reactions involved in the reduction of PGMs during iron melting collection are given below, using PtO2 as an example. PtO2 → Pt + O2(g)

(1)

PtO2 + C → Pt + CO2(g)

(2)

PtO2 + 2Fe → Pt + 2FeO

(3)

PtO2 + 2FeO → Pt + Fe2O3

(4)

PtS + O2 → Pt + SO2(g)

(5)

PtS + C + 2O2 → Pt + SO2(g) + CO2(g)

(6)

The thermodynamics of the above reactions were calculated by Factsage 7.0 software, and the relationships between Gibbs free energy and temperature were shown in Fig. 1. The reduction reactions become easier with lower Gibbs free energy. However, the reactions are not available as ΔrGT > 0. As shown in Fig. 1(a), the oxides of PGMs form at low temperature and then decompose as temperature increases. The stabilities of PGMs oxides are Rh2O3 > PdO > PtO2 as their decomposition temperatures are 600, 800, and 1200 °C, respectively. The relationship between the Gibbs energies of reduction reactions of PGMs oxides with temperature is given in Fig. 1(b)–(d), indicating that PGMs oxides are easily reduced into the metallic state by C, Fe and FeO. The reductions of Rh2O3 are easiest since their Gibbs energy are minimum in the same condition. Fig. 1(e) and (f) show the relationship between Gibbs energies of possible reduction reactions of PGMs sulfides and temperature. The sulfides are easily transferred into metallic Pt, Pd, and Rh with the existence of O2 and C since the Gibbs energies are −250∼ −40 kJ/mol. Therefore, the compounds of PGMs can be reduced to elemental states during the iron melting collection period, which is beneficial to be collected in the molten iron phase.

2.2. Principles of slag design The high recovery efficiency requires low inclusion of PGMs in slag, avoiding the formation of Fe-Si alloy and highly separation of slag and Fe alloy. The reduction of SiO2 into Si can be prevented at relatively low

Table 1 The recovery efficiencies of PGMs by different kinds of melting methods. No.

Collector

Temperature

PGMs recovery efficiencies

Refs

1 2 3 4 5 6 7 8 9

Lead Copper Copper Copper Copper Iron plasma Melting Iron Ni2S3-CuS Ni2S3-CuS

1130 °C 1300 °C 1550 °C 1550 °C 1550 °C > 1600 °C 1560 °C 1200 °C 1050 °C

Pt/Pd > 98 %, Rh > 95 %, industrial scale PGMs > 95 %, industrial scale Pt 90 %, Pd, Rh 82 %, lab-scale PGMs > 84 %, lab-scale Pd > 97 % Lab, lab-scale Pt, Pd > 98 %, Rh > 97 %, industrial scale PGMs > 90 %, industrial scale Rh 94 %, lab-scale Pt 90 %, Pd 93 %, Rh 88 %, lab-scale

Ding et al. (2019) Hagelüken (2006) Kolliopoulos et al. (2014) Fornalczyk and Saternus (2013) Zhang et al. (2019) He et al. (2016) Benson et al. (2000) He et al. (2012) You et al. (2016)

2

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

Fig. 1. The standard Gibbs free energy of possible reactions during iron melting. (a) is the decomposition of PGMs-oxides, (b), (c) and (d) the reduction of PGMsoxides with C, Fe and FeO, respectively, (e) and (f) the reduction of PGMs- sulfides. Table 2 The main chemical composition of spent automotive catalysts by XRF (wt.%). Compositions

SiO2

Al2O3

MgO

ZrO2

CeO2

MnO

SO3

TiO2

Fe2O3

CaO

Others

Contents

28.82

42.31

10.6

4.93

3.49

1.14

1.21

0.63

1.36

0.68

4.83

compositions of slag. In this study, the chemical composition of spent automobile catalysts was shown in Table 2. XRD pattern (Fig. S2) and SEM-EDS (Fig. S3, Table S1 in the supplementary material) showed that spent catalysts mainly contained Mg2Al4Si5O15, Al2O3, Ce2Zr2O7.98, and

temperature (below 1400 °C) through the optimization design of slag. The factors including density difference between slag and Fe alloy, slag viscosity and its surface tension have significant influences on the separation of slag and Fe alloy, which are affected by chemical

3

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

1150 °C and hold for 60 min before casting. The obtained lead button was treated in a cupel at 850 °C to separate base metals and the PGMs alloy was recovered. The alloy was cut into small pieces and dissolved in aqua regia. The solution was concentrated to 0.5–1.0 ml and then diluted to 50 ml. Finally, ICP-OES was used to test the concentrations of Pt, Pd, and Rh. The recovery efficiency of PGMs was calculated by Eq. (7).

PGMs. The mass of Al2O3 and SiO2 in spent catalysts were over 70 %, and the mass ratio of Al2O3/SiO2 was 1.5. The CaO-Al2O3-SiO2 phase diagram (shown in Fig. S4) was used as the basis for slag design based on the principle of slag minimization. When the mass ratio of Al2O3/ SiO2 is 1.5, the melting point of slags is over 1400 °C. To reduce the melting temperature, the fluxing agents such as CaO, Na2CO3, Na2B4O7, CaF2 were added and formed lower melting temperature slags, which can reduce the viscosity as well.

R = (1 − 3. Experimental section

m1 × c1 ) × 100% m0 × c0

(7)

where R is the recovery efficiency of PGMs, m0 and m1 are the mass of spent catalysts and slag, respectively, c0 and c1 are the concentrations of PGMs in spent catalysts and slag. In this paper, the basicity (R) was defined as the mass ratio of CaO + MgO + Na2O and SiO2+Al2O3, while the enrichment coefficient of PGMs was the mass ratio of concentrations in iron alloy and spent catalysts.

3.1. Materials and reagents The employed spent automotive catalysts were provided by a PGMs recycling company from Yunlong Country Platinum Cui Precious Metals Technology Co., Ltd., Yunnan Province, China. The samples were crushed by Raymond mill and ground into 50−300 μm. The concentration of the PGMs in the samples was analyzed by ICP-OES and Pt, Pd, and Rh were 287.26, 1024.42 and 199.45 g/t, respectively. The fluxing agents (e.g. CaO, Na2CO3, Na2B4O7 and CaF2), reductant (carbon) and collector (iron powder) are of industrial purity.

4. Results and discussion 4.1. Effect of basicity

3.2. Experimental procedures

Basicity is one of the most important factors that determine the melting temperature and viscosity of slag. According to the above analysis, spent automobile catalysts mainly contain acidic oxides including cordierite and Al2O3. Therefore, to reduce melting temperature and viscosity of slag, fluxing agents such as CaO, Na2CO3, Na2B4O7 and CaF2 were used to adjust the basicity of slag. The added materials at different basicity were shown in Table 3. The effects of CaO and Na2CO3 were studied when the addition of Na2B4O7, CaF2 C and Fe were 5 wt.%, 5 wt.%, 5 wt.% and 15 wt.%, respectively. Fig. 2 shows the recovery efficiency of PGMs and slag viscosity at different basicity. The content of PGMs in slags decreased from 111.58 g/t to 26.34 g/t when the basicity increased from 0.65 to 1.0, and higher basicity showed a negative result. When the basicity was 1.0 (45 wt.% CaO and 15 wt.% Na2O), the recovery efficiency of PGMs increased to 97.33 % and the concentrations of Pt, Pd, and Rh in the slag were 6.69, 8.92 and 10.73 g/t, respectively. The enrichment coefficient was 7.07. Although the content of Pd in slags was higher than Pt and Rh, the recovery efficiencies were Pd > Pt > Rh, which was caused by the relative content of PGMs in spent catalysts. The increased basicity broke the structure of silicon-oxygen tetrahedron, reduced the viscosity of molten slag, enhanced its liquidity, decreased the inclusion of Fe alloy, and thus improved the recovery efficiency of PGMs. The viscosity of slag was calculated by Factsage 7.0 with the Viscosity module, and the results were shown in Fig. 2(b). Temperature shows a great effect on the viscosity, as well as basicity. The viscosity of slag decreased drastically with increasing temperature and higher basicity. For example, when R was 0.65, the viscosity was decreased from 1131.59 Pa s at 1000 °C to 0.23 Pa s at 1400 Pa s. When R was 1.0, the viscosity was 0.16 Pa s at 1400 °C. Higher basicity provided more amount of free O2−, which was beneficial to break up the glass network and therefore lowered the viscosity. The Equilib module in Factsage 7.0 software was used to describe the multi-phase reactions of molten slag in the iron collection system. The mass of spent catalysts was 100 g and all the fluxing agents were

All the smelting experiments were carried out in Yunlong Country Platinum Cui Precious Metals Technology Co., Ltd. through an intermediate frequency furnace (IFF), which was heated by induction coil (the picture of IFF was given in Fig. S5 in the supplementary material). The fluxing agents, collector (iron powder 50−200 g), and reductant (carbon powder 0−50 g) were mixed with 1000 g of spent catalysts. The fluxing agents were composed with 350−550 g of CaO, 171−242 g Na2CO3, 0−85 g of Na2B4O7 and 0−50 g of CaF2. Carbon powder was used as a reductant to avoid the oxidation of Fe. The mixed materials were put into a graphite crucible and then operated at the voltage of 300 V and current of 50 A (400–500 °C) for 20 min intending to dry the materials. After preheating, the voltage and current increased to 600 V and 90 A, respectively till the mixed materials were fully molten. Then decrease the voltage and current to 500 V and 75 A respectively and hold for 10 min. The graphite crucible was covered by firebrick during smelting. The smelting temperature was appropriately 1300–1400 °C. Finally, the slag and molten iron alloy were separated through casting. 3.3. Characterization As X-ray fluorescence (XRF) is suitable for the analysis of all-components of samples, and inductively coupled plasma optical emission spectrometer (ICP-OES) is usually used for the analysis of specified elements with trace amounts, the main chemical composition of spent automotive catalysts was characterized by XRF (XRF, DX-320 L, Shenzhen, China) and the contents of PGMs were tested by ICP-OES (Avio 200 & Optima 8000, PerkinElmer instruments, Waltham, MA, USA). X-ray diffraction (XRD, Rigaku D/max-2550 V, Tokyo, Japan) was utilized to determine the phase composition of spent catalysts and recovered Fe-PGMs. The Equilib and Viscosity modules of Factsage 7.0 software (CRCT-Thermfact and GTT-Technologies, Canada) were used to simulate the reaction process during iron smelting and calculate slag viscosity, respectively. The concentrations of Pt, Pd, and Rh in slag and spent catalysts were measured by the fire assay method. The detailed process was given below. 2.0000–2.5000 g of samples were put into 500 ml frosted bottle, as well as adding 15.0–20.0 g Na2CO3, 5.0–8.0 g CaO, 8.0–10.0 g ground glass, 3.0–5.0 g sodium borate, 15.0–20.0 g PbO, 3.0–5.0 g flour and 20.0 mg gold. The mixed reagents were poured into a 500 ml clay crucible with 20 g covering agent above the materials. The covering agent was composed of Na2CO3 and NaCl to avoid volatilization of PbO. Put the clay crucible into muffle furnace at 850 °C and then increased to

Table 3 The composition of added materials at different basicity (g).

4

Basicity

Spent catalysts

CaO

Na2CO3

Na2B4O7

CaF2

C

Fe

slag

0.65 0.80 0.85 1.0 1.05

1000 1000 1000 1000 1000

300 350 400 450 550

85 170 170 256 170

50 50 50 50 50

50 50 50 50 50

50 50 50 50 50

150 150 150 150 150

1293 1430 1453 1532 1598

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

Fig. 2. The effect of basicity on the concentration of PGMs. (a) The content of PGMs in slag and (b) recovery efficiency of PGMs.

Fig. 3. The simulation of slag reaction equilibrium when the basicity was (a) 0.65 and (b) 1.0.

5

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

slag were olivine, NCA2, and liquid slag when the temperature was below 900 °C. The content of olivine decreased and disappeared with temperature raising to 1150 °C, and bC2SA (mainly CaSO4) formed at the same time. The decomposition of olivine and NCA2 generated large amounts of CaO, SiO2, MgO, Na2O, and Al2O3, which promoted the precipitation of Ca2SiO4, Mg2SiO4 and spinel. The phase of bC2SA was composed of 94.91 % Ca2SiO4, 4.94 % Mg2SiO4 and 0.15 % Ca3B2O6 at 1150 °C. It began to decrease at 1150 °C and decomposed completely at 1350 °C. Compared with Fig. 3(b), the temperature of molten slag was about 1250 °C and 1350 °C with and without the addition of CaF2, respectively. When the melting temperature was more than 1400 °C, the fluorides such as NaF and MgF2 were formed with the addition of CaF2, which improved the fluidity of slag and recovery efficiency of PGMs.

calculated by the same ratio. All the inputs are converted into corresponding oxides. The simulated temperature was set as 800–1500 °C with an increment of 50 °C in the normal atmosphere. The phases transformation of slag were shown in Fig. 3 when the basicity was 0.65 and 1.0, respectively. When the basicity was 0.65, about 20 wt.% of molten slag was generated, as well as large amounts of Ca2Al3O7, CaMgSiO4, MgAlO2 and NaAlSiO4. The molten slag was mainly composed of Na2O-SiO2-CaO-Al2O3-B2O3. NaAlSiO4, CaMgSiO4 and Ca2Al3O7 gradually decomposed along with increasing temperature and completely vanished at 950, 1150 and 1200 °C. The mass of MgAlO2 increased gradually within the temperature of 800–1200 °C due to the decomposition of CaMgSiO4, which provided the source of MgO. As the decomposition of CaMgSiO4 completed, MgAlO2 gradually broke down and disappeared at 1500 °C. Fig. 3(b) showed the simulation result when the basicity was 1.0. Similar to Fig. 3(a), only appropriately 25 wt.% of molten slag formed at 800 °C and the solid slags were mainly composed of CaMgSiO4 and Na2CaAl4O8. Most of them were decomposed and entered into molten slag under the temperature of 1250 °C. The viscosity decreased sharply to 1.09 Pa·s at 1250 °C, and it was 0.16 Pa·s at 1400 °C. The chemical compositions of slag varied at different temperatures, as shown in Table S2. The mass of CaO, Na2O, MgO and MgF2 increased with higher temperature during the range of 1000–1500 °C as the melting or decomposition of high melting materials. The content of Al2O3 increased with temperature and kept constant at 1250 °C. Specifically, the content of CaO in the final slag had a slight increase compared with the original addition, while Al2O3 and fluoride decreased slightly. The reason was given shown as reaction (8). The reaction consumed part of CaF2 and Al2O3 and generated CaO, resulting in a slight increase if CaO. 3CaF2 + Al2O3 = 3CaO + 2AlF3

4.3. The effect of CaO/Na2O The effect of CaO/Na2O on the recovery efficiency of PGMs was studied when the basicity was 0.85 and 1.0, respectively under the conditions of 5 wt.% CaF2, 5 wt.% Na2B4O7, 15 wt.% Fe and 5 wt.% C. By increasing the ratio of Na2O at the same basicity, the concentration of PGMs in slags decreased visibly, as shown in Fig. 6. When the mass ratio of CaO/Na2O decreased from 45:10 to 35:20 (R = 0.85), the content of PGMs in slag reduced from 53.23 to 15.14 g/t. While R = 1.0, it decreased from 26.29 to 11.65 g/t as the mass ratio of CaO/ Na2O decreased from 45:15 to 40:20. The results also confirmed that basicity had a great influence on PGMs collection efficiency. Na2O with a relatively low melting point accelerated the dissolution of carriers of spent catalysts by forming Al2O3-SiO2-CaO-MgO-Na2O multi-phase slag with low melting temperature, good fluidity and low density. These properties contributed to the setting of Fe-PGMs particles and their separation with slag. However, with the increasing of Na2O, the viscosity of the slag rose up slightly. Compared with Na atom, Ca shows a weaker binding capacity for O atom due to its larger atomic radius, which therefore can provide more free O2−. And free O2− is used to break up silica tetrahedron. Hence, CaO has a greater effect on viscosity than Na2O with the same addition. For another, the addition of Na2O reduced the density of molten slag, decreased the buoyancy and viscous force of Fe-PGMs particles during settling, which enhanced the separation efficiency of alloy and slag. To avoid the corrosion of crucibles caused by CaF2 during the melting period, the effect of CaO/Na2O on PGMs recovery was studied without the addition of CaF2. As can be seen from Fig. 7, the concentrations of PGMs decreased obviously as the ratio of Na2O increasing. For example, the concentrations of Pt, Pd, and Rh were 19.31, 37.58 and 21.27 g/t, respectively when the mass ratio of CaO/Na2O was 55:10. When CaO/Na2O was 45:20, the content of PGMs in the slag was only 38.32 g/t, which dropped over 50 %. The viscosity showed that CaF2 had a positive effect on reducing viscosity when adding the same ratio of Na2O, as shown in Fig. 8(a). For instance, the viscosities of slag were 0.172 and 0.255 Pa·s, respectively with and without the addition of 5 wt.% CaF2 at 1400 °C when adding 20 wt.% Na2O. As shown in Fig. 8(b), the viscosity of slag decreased slightly with a lower mass ratio of CaO/Na2O when CaF2 was not added. For instance, when the temperature was 1400 °C, the viscosity was 0.222 and 0.255 Pa·s, respectively when the addition of Na2O was 10 wt.% and 20 wt.%, respectively. The decreased addition of CaO reduced the depolymerization Si-O tetrahedron, which resulted in higher viscosity. Meanwhile, the increased Na2O lowered the slag density and promoted the separation of slag and Fe alloy.

(8)

4.2. Effect of CaF2 The effect of CaF2 was discussed under the following conditions: R = 1.0, 15 wt.% Fe, CaO/Na2O = 50:15, 5 wt.% C. It can be found that the content of PGMs in the slag was 26.288 and 49.576 g/t with and without the addition of CaF2, as shown in Fig. 4. The addition of 5 wt.% CaF2 reduced about 40 % of PGMs mixed in the slag since CaF2 can reduce the viscosity, which promoted the setting of Fe-PGMs particles and decreased the inclusion of PMGs. The viscosity of slag with 5 wt.% and without CaF2 between the temperature 1200–1500 °C was shown in Fig. S6. When the temperature was below 1275 °C or above 1400 °C, CaF2 played a relatively small role in the viscosity. It had a great influence on the viscosity within 1275–1400 °C. For example, the viscosity decreased from 0.82 to 0.47 Pa·s by adding 5 wt.% CaF2 at 1300 °C. The phases transformation of slag were shown in Fig. 5 without the addition of CaF2 when the basicity was 1.0. The main components of

4.4. The effect of Na2B4O7 Na2B4O7 is one kind of important fluxing agents, which can form borate with CaO, SiO2, and Al2O3 with the performances of low melting point, viscosity and density. The effect of Na2B4O7 on PGMs recovery efficiency was studied with different basicity. Table S3 is the mass of

Fig. 4. The effect of CaF2 on the content of PGMs in slag when R = 1.0. 6

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

Fig. 5. The simulation of slag reaction equilibrium when the basicity was 1.0 without the addition of CaF2.

Fig. 6. Effect of CaO/Na2O on the concentration of PGMs in slag with adding 5 wt.% CaF2 when (a) R = 0.85 and (b) R = 1.0.

agents and the consumption of energy, we decreased 5 wt.% of CaO and increased 1.5 wt.% Na2B4O7, PGMs recovery efficiency decreased slightly to 98.43 %. However, their recovery efficiency increased obviously as Na2B4O7 increased to 8.5 wt.% and reached to 99.25 %, in which the concentrations of Pt, Pd, and Rh were only 2.40, 3.88 and 0.98 g/t. The enrichment coefficient was 6.75 in this case. The results indicated that Na2B4O7 has a greater impact on PGMs recovery efficiency than CaO within a certain range. As can be seen from Fig. 9(b), the viscosity of slag was effected by CaO, Na2O and Na2B4O7. The sample adding 5.0 wt.% Na2B4O7 has the minimum viscosity, which showed that Na2B4O7 had the biggest impact on reducing viscosity. However, when the basicity was 0.85 with the addition of 8.5 wt.% Na2B4O7, the viscosity of slag increased slightly to 0.20 compared with adding 6.5 wt.% Na2B4O7. The reason was that boron ions transferred to the closely packed tetrahedron structure of [BO4] as B2O3 increased. The higher addition of Na2B4O7 decreased the density of slag which promoted the separation of Fe-PGMs alloy and slag.

Fig. 7. Effect of CaO/Na2O on the concentration of PGMs in slag without adding CaF2 when R = 1.05.

input materials. PGMs recovery efficiency and contents in slag were shown in Fig. 9(a). It can be found that PGMs content in slag decreased with the addition of Na2B4O7. The concentrations of Pt, Pd, and Rh in slag were 17.43, 38.76 and 10.57 g/t, respectively when the basicity was 1.0 without adding Na2B4O7. By addition 5 wt.% of Na2B4O7, the concentration of PGMs decreased from 66.76 to 26.288 g/t, and Pt, Pd, and Rh was 8.916, 6.692 and 10.68 g/t respectively, indicating Na2B4O7 has obvious effects on PGMs enrichment. The concentrations of Pt, Pd, and Rh decreased to 5.56, 4.15 and 0.94 g/t, respectively by increasing Na2O from 15 wt.% to 20 wt.%. PGMs recovery efficiency was 98.94 % and the enrichment factor was 6.62. To reduce the addition of fluxing

4.5. Effect of iron dosage The effect of iron dosage on PGMs recovery efficiency was also studied under the following condition: 40 wt.% CaO, 20 wt.% Na2O, 5 wt.% Na2B4O7, 5 wt.% C. The results of adding 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% of Fe powder were shown in Table 4. The content of PGMs in the slag decreased with raising dosage of Fe. For instance, the concentrations of Pt, Pd, and Rh in slag decreased from 8.73, 19.18 7

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

Fig. 8. Effect of CaO/Na2O on slag viscosity when (a) with adding 5 wt.% CaF2 and (b) without adding CaF2.

Fig. 9. Effect of Na2B4O7 on (a) the concentration of PGMs in slag and (b) slag viscosity. Table 4 The recovery efficiencies of PGMs at different addition of collection agents. Fe (g)

50 100 150 200

PGMs in slag (g/t) Pt

Pd

Rh

8.73 7.42 4.26 3.08

19.18 11.08 5.15 3.85

5.37 4.63 1.24 1.12

m(Fealloy)/g

Recovery efficiency (%)

Enrichment coefficient

37 93 124 166

96.90 97.98 99.00 99.21

26.19 10.53 7.96 5.97

and 5.37 g/t to 3.08, 3.85 and 1.12 g/t as the dosage of Fe increased from 5 wt.% to 20 wt.%. More collectors enhanced the probability of contact and collection between Fe and PGMs, which led to higher recovery efficiency of PGMs. The recovery efficiencies of PGMs were 96.90 %, 99.00 % and 99.21 %, respectively when the dosage of Fe was 5 wt.%, 15 wt.% and 20 wt.% with corresponding enrichment factors of 26.19, 7.96 and 5.97 respectively. XRD patterns of Fe-PGMs alloys (Fig. 10) were in accordance with characteristic peaks of Fe (PDF#060696). The result indicated that there was no ferrosilicon in these alloys. Fe and PGMs can be separated efficiently via electrolysis. Therefore, with the consideration of reagents consumption during the following dissolution of Fe alloy and its heavy environmental pollution, the addition of collector (Fe) was confirmed to be 15 wt.% of spent catalysts.

Fig. 10. XRD pattern of Fe-PGMs alloys when adding 5−20 wt.% Fe.

4.6. Pilot-scale of iron melting PGMs recovery efficiency has achieved over 99 % through optimization of slag composition (R = 1.0, 35 wt.% CaO, 20 wt.% Na2O, 5 wt. % Na2B4O7, 5 wt.% CaF2, 5 wt.% C and 15 wt.% Fe). Pilot-scale experiments have been conducted to validate its industrial feasibility. The 8

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

Fig. 11. Iron collection experiments at a scale of 50 kg of spent catalysts.

Na2B4O7 can reduce the melting temperature, viscosity and density, promoting the separation of slag and Fe-PGMs alloy and decreasing the concentration of PGMs in slag. The Fe melting condition was determined to be CaO/Na2O = 35:20, CaF2 5 wt.%, Na2B4O7 8.5 wt.%, Fe 15 wt.% and C wt.5 %, the optimized yield was 2.398 g/t Pt, 3.879 g/t Pd and 0.976 g/t Rh in the slag. The recovery efficiency was over 99 % at pilot scale experiments.

Table 5 The concentrations of PGMs in slag at 50 kg scale experiments (g/t). No.

Pt

Pd

Rh

1 2 3 4

2.46 3.05 1.78 2.93

5.38 3.89 5.14 4.21

1.57 2.42 2.35 1.32

Author contributions

chemical composition of spent catalysts used in the large scale experiment were shown in Table S4. The concentrations of Pt, Pd, and Rh were 295.64, 949.79 and 199.23 g/t, respectively. 50 kg of spent catalysts was added in each experiment. Feed materials including spent catalyst, fluxing agents, collector and reductant were mixed and put into a large intermediate frequency furnace. The experiment temperature was within 1300–1400 °C. After all the materials were molten (about 60 min), hold for 15 min to set the Fe-PGMs to the bottom completely. Then slag and metals were poured out of the furnace and separated easily due to their large density difference, as shown in Fig. 11. The experiment was repeated four times in the same condition and the results were shown in Table 5. The concentrations of PGMs in slag were all below 10 g/t, and the recovery efficiencies of PGMs were over 99 %, indicating that this technology has the potential to be applied in industry.

Yunji Ding and Shengen Zhang conceived and designed the study, Yunji Ding, Huandong Zheng and Bo Liu performed the experiments. Yunji Ding wrote the paper. Shengen Zhang and Boyu Wu reviewed and edited the manuscript. Zhuming Jian provided the spent catalysts, equipment and characterized the concentration of PGMs. All authors read and approved the manuscript.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

5. Conclusions

This work was sponsored by the National Natural Science Foundation of China (Grants 51672024), Xijiang Innovation and Entrepreneurship Team, Fundamental Research Funds for the Central Universities (FRF-TP-19-003B1) and Integration of Green Key Process Systems MIIT. The authors would like to appreciate Prof. Yanping Bao from USTB for providing Fastsage 7.0 software.

This paper aimed to avoid the heavy metals pollution caused by lead/copper/nickel pyrometallurgical collection and the formation of insoluble ferrosilicon by the plasma iron melting method. An efficient iron melting method was proposed for the enrichment of platinum group metals at low temperature (< 1400 °C). The principle of spent catalysts carriers and slag design was put forward. The oxidations and sulfides of PGMs were found to be easily reduced to the metallic state during the melting period. The melting temperature was controlled within 1300–1400 °C through the optimization of slag type, avoiding the reduction of SiO2 and formation of ferrosilicon. The recovery efficiency was found to increase with the rising basicity of slag and reached the maximum as the basicity was 1.0. The addition of Na2O, CaF2 and

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.resconrec.2019. 104644. 9

Resources, Conservation & Recycling 155 (2020) 104644

Y. Ding, et al.

References

spent automotive catalysts. Open Access Lib. J. 1 (05), 1. Nevalainen, P., Kinnunen, N.M., Kirveslahti, A., Kallinen, K., Maunula, T., Keenan, M., Suvanto, M., 2018. Formation of NH3 and N2O in a modern natural gas three-way catalyst designed for heavy-duty vehicles: the effects of simulated exhaust gas composition and ageing. Appl. Catal. A Gen. 552, 30–37. Osman, A.I., Abu-Dahrieh, J.K., McLaren, M., Laffir, F., Rooney, D., 2018. Characterization of robust combustion catalyst from aluminum foil waste. Chemistry 3, 1545–1550. Peng, Z.W., Li, Z.Z., Lin, X.L., Tang, H.M., Ye, L., Ma, Y.T., Rao, M.J., Zhang, Y.B., Li, G.H., Jiang, T., 2017. Pyrometallurgical recovery of platinum group metals from spent catalysts. JOM 69 (9), 1553–1562. Peng, Z.W., Li, Z.Z., Lin, X.L., Ma, Y.T., Zhang, Y., Zhang, Y.B., Li, G.H., Jiang, T., 2018. Thermodynamic analysis of smelting of spent catalysts for recovery of platinum group metals. TMS Annual Meeting & Exhibition. Springer, Cham, pp. 215–223. Saguru, C., Ndlovu, S., Moropeng, D., 2018. A review of recent studies into hydrometallurgical methods for recovering PGMs from used catalytic converters. Hydrometallurgy 182, 44–56. Stoffner F, Hobbs C., 2018. Process for the production of a PGMs-enriched alloy. U.S. Patent Application 15/355,897. Sverdrup, H.U., Ragnarsdottir, K.V., 2016. A system dynamics model for platinum group metal supply, market price, depletion of extractable amounts, ore grade, recycling and stocks-in-use. Resour. Conserv. Recycl. 114, 130–152. Trinh, H.B., Lee, J.C., Srivastava, R.R., Kim, S., Ilyas, S., 2017. Eco-threat minimization in HCl leaching of PGMs from spent automobile catalysts by formic acid prereduction. ACS Sustain. Chem. Eng. 5 (8), 7302–7309. USGS, 2019. Platinum-Group Metals Statistics and Information. Available at 2019.10.10. https://www.usgs.gov/centers/nmic/platinum-group-metals-statistics-andinformation. Wei, X., Liu, C., Cao, H.W., Ning, P.G., Jin, W., Yang, Z.B., Wang, H.J., Sun, Z., 2019. Understanding the features of PGMs in spent ternary automobile catalysts for development of cleaner recovery technology. J. Clean. Prod. 239, 118031. You, G., Fang, W., Li, Q., Ma, Y., Yang, X.T., Yang, H., 2016. Study on enrichment method of platinum, palladium and rhodium in spent auto-catalysts. Metall. Anal. 36, 7–11. Zhang, L.G., Song, Q.M., Liu, Y., Xu, Z.M., 2019. Novel approach for recovery of palladium in spent catalyst from automobile by a capture technology of eutectic copper. J. Clean. Prod. 239, 118093. Zhang, S.G., Ding, Y.J., Liu, B., Chang, C.C., 2017. Supply and demand of some critical metals and present status of their recycling in WEEE. Waste Manag. 65, 113–127.

Baghalha, M., Khosravian Gh, H., Mortaheb, H.R., 2009. Kinetics of platinum extraction from spent reforming catalyst in aqua-regia solutions. Hydrometallurgy 95, 247–253. Benke, G., Gnot, W., 2002. The electrochemical dissolution of platinum. Hydrometallurgy 64 (3), 205–218. Benson, M., Bennett, C.R., Harry, J.E., Patel, M.K., Cross, M., 2000. The recovery mechanism of platinum group metals from catalytic converters in spent automotive exhaust systems. Resour. Conserv. Recycl. 31 (1), 1–7. Bhogeswararao, S., Srinivas, D., 2015. Catalytic conversion of furfural to industrial chemicals over supported Pt and Pd catalysts. J. Catal. 327, 65–77. Cichy P., 1982. Recovery of precious metals from spent alumina-containing catalysts. U.S. Patent 4,337,085. Diaz, L.A., Lister, T.E., Parkman, J.A., Clark, G.G., 2016. Comprehensive process for the recovery of value and critical materials from electronic waste. J. Clean. Prod. 125, 236–244. Ding, Y.J., Zhang, S.G., Liu, B., Zheng, H.D., Chang, C.C., Ekberg, C., 2019. Recovery of precious metals from electronic waste and spent catalysts: a review. Resour. Conserv. Recycl. 141, 284–298. Dong, H.G., Zhao, J.C., Chen, J.L., Wu, Y.D., Li, B.J., 2015. Recovery of platinum group metals from spent catalysts: a review. Int. J. Miner. Process. 145, 108–113. Fornalczyk, A., Saternus, M., 2013. Platinum recovery from used auto catalytic converters in electrorefining process. Metalurgija 52 (2), 219–222. Hagelüken, C., 2006. Recycling of electronic scrap at Umicore precious metals refining. Acta Metall. Slovaca 12, 111–120. He, X.T., Li, Y., Wu, X., Zhao, Y., Wang, H., Liu, W., 2016. Study on the process of enrichment platinum group metals by plasma melting technology. Precious Met. 37, 1–5 (Chinese). He, X.T., Wang, H., Wu, X.L., Li, Y., Zhao, Y., Han, S.L., Li, K., Guo, J.M., 2012. Study on the recovery of rhodium from spent organic rhodium catalysts of acetic acid industry using pyrometallurgical process. Precious Met. 33 (S1), 24–27 (Chinese). Jha, M.K., Lee, J.C., Kim, M.S., Jeong, J., Kim, B.S., Kumar, V., 2013. Hydrometallurgical recovery/recycling of platinum by the leaching of spent catalysts: a review. Hydrometallurgy 133, 23–32. Jones, R.T., Geldenhuys, I.J., 2011. The pros and cons of reductive matte smelting for PGMs. Miner. Eng. 24 (6), 495–498. Kolliopoulos, G., Balomenos, E., Giannopoulou, I., Yakoumis, I., Panias, D., 2014. Behavior of platinum group metals during their pyrometallurgical recovery from

10