Extraction of platinum and gold from copper anode slimes by a process of 1 chlorinating roasting followed by chlorinating leaching

: A novel process of chlorinating roasting followed by chlorinating leaching 14 to extract platinum and gold from copper anode slimes was proposed in this research. Results of thermodynamic analysis and experimental research show that the platinum is chlorinated into PtCl 2 while the gold exists in the form of metallic Au during the 17 roasting process. With the copper anode slime being directly leached using a traditional 18 process, the Pt recovery rate is low to 80.72%. After roasted with sodium chloride and 19 concentrated sulfuric acid in oxygen atmosphere, the recovery rate of Pt is increased to 20 a value around 95%. Moreover, with excessive addition of concentrated sulfuric acid, 21 more H 2 O (g) are generated and the formation of Cl 2 (g) is decreased due to the 22 transition from HCl (g) and Cl 2 (g), as a result of which the Pt recovery rate decreases. 23 In addition, this chlorinating roasting affects little on the Au recovery due to its difficulty to be chlorinated.


Introduction
28 Platinum (Pt) and gold (Au) are costly precious metals with wide applications in 29 the manufacturing of catalysts, electronic devices, space materials, biomedical devices, 30 jewelry and so on, due to their distinct properties, such as chemical inertness, corrosion 31 resistance, catalytic activity, thermoelectric stability and stable electrical properties [1-32 4] . However, they are limited availability in nature and both the abundance in the earth's 33 crust are below 10μg/g in addition to their mineral ores becoming depleted [5][6] . To meet 34 the future demand, it is necessary to process the secondary platinum and gold 35 containing materials, such as spent catalysts, electronic scraps, anode slimes and so on 36 [7][8] . 37 The copper anode slime, a byproduct of the copper electro-refining process, is a 38 valuable secondary resource containing valuable elements such as Cu, Se, Sb, Ag, Au 39 and platinum group metals [9][10][11] . Based on the different composition and mineralogy in 40 anode slimes, numerous approaches have been made to utilize it following the processes 1 of pyro-, pyrohydro-, hydropyro-and hydrometallurgical methods [12][13][14][15][16] . These 2 processes can be divided into in two steps of the separation and recovery of non-ferrous 3 metals first and then the recovery of target precious metals [17] . The leaching process is 4 mainly used to recover precious metals from the secondary slime using different 5 lixiviants, such as sulfuric acid, nitric acid, hydrochloric acid, chloride or iodide 6 solutions, etc. J. Hait found that the recovery rate of Au was low with sulfuric acid 7 leaching process without any additive [18] . Also in the thiosulfate leaching process, the 8 Au extraction rate was only 88% [19] . High leaching rates of Au and Pt could be reached 9 using aqua regia solution; however, it caused an environmental pollution [20] . With the 10 anode slime dissolved with the help of chlorine gas in aqueous medium, a high recovery 11 of gold of 94.4% could be obtained, but the chlorine gas causes serious corrosion of 12 equipment [21] . Compared with these methods, a chlorinating leaching process using 13 NaCl-H2SO4-NaClO3 mixed solution has a few advantages, including a higher 14 dissolution rate of gold, lower price of leaching reagents as well as the simplicity of the 15 process itself [22] . However, the leaching rate of platinum is low at the optimal condition 16 favoring Au leaching, which causes a considerable amount of platinum to be distributed 17 in the leaching residue and consequently the recovery rate of platinum difficult to be 18 increased.

19
In this paper, a novel process of chlorinating roasting followed by a chlorinating 20 leaching to recovery platinum and gold from copper anode slime was investigated.

21
Thermodynamic simulation of the chlorinating roasting procedure was discussed, and  The copper anode slime used in this work was obtained from a copper smelter 28 which located in Yunnan province of China. As presented in Table 1, the major elements 29 contained in this slime are Cu, Sn, Sb, Ag, As, S, O, and Te, and the contents of Pt and 30 Au are 93.76 g/t and 1812.15 g/t respectively. Fig.1 shows that this copper anode slime 31 is mainly composed phases of SnO2, Sn.918Sb.109O2, PbSO4, Cu2S, SbAsO5 and Ag2SO4, 32 while Pt and Au phases can't be detected due to their little contents. Generally, they 33 mainly occur in metallic state [23] . 34 The reagents used in the experiments were all analytical grade, including sodium 35 chloride, sodium chlorate, sodium hydroxide, concentrated sulfuric acid, gold powder 36 and platinum powder. Besides, deionized water was used to prepare solutions in all 37 leaching experiments.  The experimental apparatus and process flow sheet investigated for extractions of 4 platinum and gold from copper anode slimes are given in Fig.2. For the experimental 5 procedure, the copper anode slime was firstly dried at 100 °C for 24 h, ground to below 6 0.15 mm, and then mixed with a given amount of sodium chloride in a sample boat. 7 After the tube furnace being heated to a proper temperature, a given amount of identical condition (80 °C; 4h; 3mL/g liquid-solid ratio; 120 g/L sulfuric acid, was taken 16 as the leaching agent; 60 g/L sodium chloride solution, was used to provide Cl -, 18 g/L 17 sodium chlorate solution, was taken as the chlorinating agent). For the leaching 18 procedure, the roasted copper anode slime was firstly ground to below 0.074mm, put 19 into a 400 mL glass conical flask containing the leaching solution, and then the glass 20 conical flask was placed in an oscillation box equipped with a thermostatic water bath.

21
It is noteworthy that the sodium chlorate solution was added per half an hour apart to 22 ensure the high partial pressure of chlorine. After the leaching process finished, the 23 solution was filtrated, and then the filter residue was collected, dried and weighed for 24 analysis. was carried out using Cu Kα radiation with the scanning rate was 8°/min and the 2θ 12 was varying from 10 to 90°. The leaching rate of platinum and gold was calculated 13 according to: Here, εp and εa is the leaching rate of platinum and gold, respectively; mt and mr are the wa are the contents of platinum and gold in the leaching residue in mass%, respectively. 20 The thermodynamic data of species were given by FactSage 7.2 thermochemical 21 software. (2) in a solution of NaCl-H2SO4-NaClO3. The stability Eh-pH diagrams for systems of 1 Au-Cl-H2O and Pt-Cl-H2O at 80 °C are presented in Figs.3 (a) and (b) respectively. The 2 potential for Au leachable ranges from 0.96 V to 1.32 V with the pH from -2 to 5.45 3 seen from Fig.3 (a), and that for Pt ranges from 0.75 V to 1.58 V with the pH from -2 4 to 9.69 as shown in Fig.3 (b). To achieve these high potentials for obtaining high 5 leaching rates of Au and Pt, much NaClO3 should be employed in the leaching process. 6 However, it causes the utilization efficiency of chlorine to be decreased due to the more 7 emition of Cl2 during the leaching process.  The results in Table 3 indicate that the chlorination of Pt proceeds through a 21 progressive phase transformation process as the temperature rises as described in Eq.

22
(3) through reactions (4) to (6). Compared to that with 1% NaCl addition, the partial 23 pressure of Cl2 is higher at 5% NaCl added seen from Table 3, which increases the 24 phase transition temperature of both PtCl4 to PtCl3 and PtCl3 to PtCl2 as shown in Table   25 3 from the viewpoint of chemical equilibrium. In addition, with the temperature 26 increase, the partial pressure of Cl2 corresponding to the PtClx (x=2, 3, 4) stable region 27 increases seen from the predominance area diagram of Pt-Cl-O system at 50 °C, 250 °C 28 6 and 450 °C respectively as shown in Fig.4.    Table 4 shows that the chlorination of Au can only be carried out at a low 12 temperature, and then the AuCl3 will be transformed to Au completely at temperature 13 higher than 106 °C. Also as shown in Fig.5, the Au exists stably in a high partial 14 pressure of Cl2, and the temperature rise increases the upper limit of Cl2 partial pressure 15 for Au stable region. The thermodynamic analysis above supports a conclusion that the platinum is 3 more easily to be chlorinated than that for gold. Then an experiment was designed to 4 verify this, in which 10 g copper anode slimes mixed with 4 g concentrated sulfuric 5 acid, 0.5 g NaCl, powders of 0.5 g Pt and 0.5 g Au, were roasted at 450 °C for 180 min 6 in O2 atmosphere and the X-ray diffraction pattern of the corresponding roasted residue 7 is presented in Fig. 6. The presence of PtCl2 peak and Au peak in Fig.6 validates the 8 thermodynamic analysis. During the roasting process, the partial pressures of Cl2 and O2 determine the 13 chlorination behavior of Pt and Au [25] . The equilibrium composition of NaCl-H2SO4- Na2SO4 amounts increase along with the obvious reduction of SO3 (g) at 450°C. Thus, 18 the release of chlorine from NaCl might be carried out through reactions (7) -(9). Fig.7   19 (b) shows that with increasing H2SO4 amount, the Cl2 (g) amount firstly increases and 20 then decreases slightly while coupled with an always increase of HCl (g) amount, which 21 might be due to the translation between HCl (g) and Cl2 (g) according to Eq. (10). More 22 H2O (g) will be generated through reaction (7) with more H2SO4 added, and 23 consequently the generation of HCl (g) is promoted in reaction (10). Meanwhile, the 24 reaction (10) is a reversible exothermic reaction, due to which more Cl2 (g) will also be 25 transformed to HCl (g) with the temperature increase as shown in Fig.7 (c). The 26 increase in O2 (g) amount enhances Cl2 (g) formation obviously seen from Fig.7 (d), and the NaCl is almost transferred to Cl2 (g) and HCl (g) at O2 (g) amount of 0.2 mol. 1 Increasing O2 (g) amount further, the amounts of Cl2 (g) and HCl (g) changes slightly 2 while the partial pressures of them decrease. An appropriate NaCl amount, roasting 3 temperature and O2 (g) amount is important for the Cl2 (g) generation and Pt 4 chlorination.

13
This process consists of two main stages named chlorination roasting first and 14 chlorination leaching followed respectively, and the leaching procedure was carried out 15 with the same condition as described in Section of "2.2 Experimental procedures". The 16 parameters of roasting temperature, oxygen flow rate, addition amounts of concentrated 17 sulfuric acid and sodium chloride were focused to study the effect of the roasting 18 variables on leachability of gold and platinum, and the results are shown in Table 5.  First in the blank test, with the copper anode slime being directly treated through 3 a traditional leaching process using the parameters described in Section of "2.2 4 Experimental procedures", the Pt recovery rate is low to 80.72% causing the massive 5 waste of Pt resources. 6 In  Fig.7 (c). In addition, the massive volatilization of SO3 (g) caused by the quick 5 decomposition of concentrated sulfuric acid at higher temperatures also results in a 6 small portion of it available to participate in Cl2 (g) formation. Meanwhile, it is 7 noteworthy that the sintering degree is increased at higher temperatures seen from Fig.8,   8 which slows down the diffusion of SO3 (g) and O2 (g) to the surface of NaCl as well as 9 diffusion of the generated Cl2 (g) to the surface of platinum, thereby inhibiting the Pt Pt. In addition, though the Pt can be chlorinated to PtCl2 at 450 °C based on the result 12 in Fig.6, it cannot be detected in Fig.9 due to the small concentration of Pt in the roasted 13 residue. Effect of sodium chloride and concentrated sulfuric acid amounts 20 Roasted at 450 °C, the data in Table 5 shows that the Pt recovery rate increases 21 obviously from 93.85% to 98.37% with NaCl amount from 0 to 1%. Then with NaCl 22 amount beyond 1%, the sample will be sintered (Fig.10) and the gas-solid chlorination 23 of Pt will be hindered, as a result of which the Pt recovery rate decreases slightly. In 24 Fig.11, the foundation of Ag2SO4 peak in the X-ray diffraction pattern of the roasted 25 residue at 1.5% NaCl addition indicates the lower level of chlorination of the sample 26 compared to that with 1% NaCl added seen from Fig.8. In Fig.8, the Ag2SO4 in the 27 11 copper anode slime has been transformed to AgCl. Thus, the suitable sodium chloride 1 amount is 1 %. The concentrated sulfuric acid has a complicated effect on the Pt chlorination.