An efficient and environmentally friendly route for PbS crystal recovery from lead ash generated in tin removal section (LATR) via high-speed leaching and recrystallization method

: This paper mainly investigated on synthesis of a high purity PbS crystal directly from lead ash which was collected from Tin ash removal process (LATR). The LATR was firstly disposed by nitric acid leaching system to generate the lead nitrate solution. The PbS crystal would be prepared by mixing the lead nitrate solution with the sodium sulfide at the room temperature (25 °C). The effects of molar ratio of HNO 3 to Pb in the LATR on Pb leaching efficiency was investigated, demonstrating that the Pb leaching efficiency could attain to at a molar ratio of 1.5. Overall, this method proved to be an efficient and environmental friendly route for synthesis of high quality PbS crystal directly from the common lead containing waste from the lead ore or secondary smelting factory. determined 5100 (Keysight Technologies).

by the nitric acid, where the lead element would be entered into the filtrate (Pb in the filtrate), while the majority of impurity element would be left in the filter cake due to their attachment between the impurity compound and the residue. Secondly, the generated filtrate would be mixed and reacted with sodium sulfide to generate PbS crystal (namely the crystallization process). This study would have a positive significance to recovery of LATR recovery and preparation of high purity lead sulfide crystal.

Raw materials and Reactants
The LATR used in this experiment was collected from the tin element removing section (an essential process to refine the tin -lead alloy), applied by Hubei Jinyang Metallurgical Co., Ltd. The concentrations of main elements including Pb, Sn, As, Fe, Cu, and Al were determined and shown in Fig. 2(b), the detailed process for determination of the main elements was presented in Supporting Information. It was obvious that the major elements in the original LATR were Pb (81.4%) and Sn (7.2%), also with small amount of As, Fe, Cu, and Al. The XRD pattern of original LATR was shown in Fig.2 (c), also illustrating the main crystal composition in the LATR was PbO and SnO2. From the appearance ( Fig. 2(a)) of the original LATR, the collected LATR was yellow powder, mainly attributing to the existence of PbO crystal. As shown in the SEM image ( Fig. 2(d)), the LATR was with irregular sphere and rough porous surface. Mag of 1 kX. The nitric acid used in this study was of analytical purity, supplied by Yantai Shuangshuang Chemical Co., Ltd. The sodium sulfide was also of analytical purity, purchased by Tianjin Damao Chemical Reagent Co., Ltd. The standard solution of Al, Fe, and Cu was supplied by National Nonferrous Metals Research Institute.

Leaching of LATR
To study the leaching efficiency of LATR in nitric acid system, various concentrations of nitric acid (0.6, 0.8, 1.0, and 1.2 mol/L, in Table S1) were separately mixed up with 50 g of LATR in 1L beaker at 25 o C with a stirring speed of 400 rpm ( Figure S1).
During the leaching process, the solution sample (10 mL) was quickly withdrawn from the mixed solution in the beaker at setting leaching time of 0, 10, 20, 30, 60, 90, and 120 minutes, respectively. The withdrawn solution samples were then directly filtrated through the microporous membrane (0.45μm pore size). The concentration of lead in the filtrate was measured by the EDTA-2Na Testing Method (explained in SI). The concentrations of Sn, As, Cu, Fe, and Al were determined by ICP-OES 5100 (Keysight Technologies).
At the terminal of leaching time (120 minutes), the left mixed slurry was filtered to get the filtrate for further the next step. The residual filter cake was continuously washed with deionized water until the pH of washing water was near to 7, followed by dried in the oven under 55 o C for 24 hours. The concentrations of Pb, Sn, As, Cu, Fe, and Al in the filtrate and residual filter cake were measured to analyze the flow of main elements during the leaching process.
The leaching efficiency of lead and other elements were calculated by using Eq. (1).

Synthesis of PbS crystal
Various mass of sodium sulfide reagents was separately added into the stored filtrate solution in 250 mL tall beaker under 25 o C for 30 minutes (as shown in Figure  S1). The added amount of sodium sulfide reagents was calculated according to various molar ratio of Na2S to Pb in the filtrate as 0.75, 1, 1.25 and 1.5, respectively (as shown in Table S2).
At the reaction terminal of 30 minutes, the solution after reaction was quickly filtered, then the solid filter cake was washed by deionized water, finally dried in the oven under 55 o C. The conversion rate of Pb (wt%) at various molar ratio of Na2S to Pb in the filtrate could be calculated by using Eq. (2).
The conversion rate of lead (wt%) = g2×w2/(m2×v2)×100％ (2) In the Eq. (2), g2 is representing the mass of the synthesis product (g); w2 is representing the content of lead in the synthesized product (wt%); m2 is representing the concentration of lead in the stored filtrate solution (g/L); v2 is representing the volume of the stored filtrate solution (L).

Characterization of Original LATR and PbS crystal products
The original LATR and synthesized PbS crystal were both investigated by X-ray Powder Diffraction method (Philips, PANalytical B.V., Holland) with Cu Kα radiation (λ = 1.54 Å), while the XRD patterns were calculated through the X'pert Highscore Software.
The micromorphology for original LATR and prepared PbS crystal was investigated through Focused Ion Beam Scanning Electron Microscopy (Zeiss/Auriga FIB SEM).

Thermodynamics analysis of lead and other metal elements in the solution
The Potential-pH phase diagram and Fraction diagram for Pb, Sn, As, Cu, Fe, and Al were calculated by the MEDUSA software, which has been commonly used in thermodynamics analysis [25] [27].

Leaching of LATR
The leaching efficiency of lead from the LART with varying molar ratio of HNO3 to Pb in the LART was presented in Fig. 3. As shown in Fig. 3, the leaching efficiency of lead was increased quickly at leaching time ranging from beginning to 20 minutes. After the beginning 20 minutes, the leaching efficiency would slightly increase. This result was mainly attributed to it that the lead-containing components (mainly PbO in this study) would be easy to react with nitric acid in the mixed solution. Especially, the surface part of PbO would be firstly reacted with HNO3, and then the inner PbO be exposed and consumed by the HNO3 in the soluiton.
We could also know that more lead ion would be leached with increasing molar ratio of HNO3 to Pb in the LART (Fig.3). When the molar ratio was 1.5, the leaching efficiency of Pb at 120 min was only 67.5%, this results was mainly attributed to it that the added HNO3 (at the molar ratio of 1.5) could not entirely consume the whole PbO in the LART for the theoretical reaction molar ratio was 2. The leaching efficiency of lead was measured to be 76.7% and 82.9% at leaching time of 60 minutes, when the molar ratio was 2.5 and 3, respectively. The more nitric acid in the solution would be beneficial to react with PbO from the LART, for the easier contact between the solution and LART particls. . The Eh-pH and Fraction phase diagrams for lead in the leaching system were shown in Fig. 4. As shown, the pH and potential of the leaching system would influence the dominant phase of lead compound in the solution. When the pH was less than 4, and the ESHE was above -0.2V, the Pb 2+ was obviously dominated in the solution. With increasing pH, the dominated phase would be mainly transferred into Pb(OH)2. From this above analysis result, it was easy to find that it was not essential to add oxidant or reducing agent into the leaching system, for the Pb 2+ was dominated at a relatively large potential ranging from -0.2V to 7V. It could also be seen that the lead in the leaching system mainly existed in the dissolved state, further confirming the majority of lead was entering into the leaching solution. leaching system (Conditions: 25 °C, Designed with MEDUSA software). However, it should be clear that small amount of lead would still be left in the residual filter cake after leaching and filtration procedure. The distribution of Pb in the stored filtrate and residual filter cake was shown in Fig. 5(a). As shown in Fig. 5(a), the proportion of lead (wt%) in the filtrate was generally decreased with increasing molar ratio of HNO3 to Pb in the LART, that was attributed to it that more lead ions would be leached into the filtrate with increasing dosage of nitric acid. The sum percentage of lead in the filtrate and residual filter cake would all reach more than 88% under various molar ratios, indicating that our measurement for the distribution of lead during the leaching process should be relatively accurate. This leaching process should also be illustrated by Fig. 5(b). in the filter cake, respectively). The leaching efficiency of other metal impurities including As, Cu, Fe, and Al at various molar ratio of HNO3 to Pb in the LART was presented in Fig.6. The ratio of metal impurity entering into the filtrate at different molar ratio of HNO3 to Pb in the LATR at the terminal time of 120 min was shown in Table. S4. From Table. S4, the most abundant element -Sn would be hardly reacted with the nitric acid, for its leaching efficiency was only 0.0001% at molar ratio of 1.5 and 2. Thus, the leaching efficiency of Sn was not further studied in Fig.6. As shown in Fig. 6, the ratio of As entering into the filtrate was relatively higher, which would reach up to 16% at 120 minutes at the molar ratio of 3. The leaching efficiencies of As at the molar ratio of 2.5 and 3 were relatively kept constant. At a smaller molar ratio of 1.5 and 2, the ratios of As entering into the filtrate generally were decreased with increasing leaching time. The ratio of Cu entering into the filtrate generally decreased rapidly with leaching time ranging from leaching beginning to 120 min. The ratio of Cu entering into the filtrate was around 0.05~0.2% at leaching time after 20 min. The leaching efficiency of Fe generally increased with leaching time ranging from reaction beginning to 120 min, while the ratio of Fe entering into the filtrate was around 0.5~0.7% at terminal of leaching process. The leaching efficiency of Al would fluctuate between 0.6~1.3% during the leaching process. Under higher molar ratio of 3, the ratio of Al in the filtrate would be highest, indicating that more Al containing compound would be reacted with more nitric acid in the leaching system. The Potential-pH diagram of typical impurity element including Sn, As, Fe, and Al in the leaching system could be revealed in Fig. S2. As shown in Fig. S2(a), the dominant phase for Sn element was mainly SnO2 at pH ranging from 1 to 12 at potential of more than 0V, indicating that the Sn was merely measured in the filtrate (Table. S2). This result demonstrated that the SnO2 would be left in the residue after leaching process. The dominant phase of As in the leaching system was more complicated, for the dominant phase of As would easy to be varied at varying potential. The dominant phase of Fe would be transferred from Fe 2+ ions into Fe2O3 (solid) with pH ranging from 1 to 12 at potential of 0V. This Potential-pH diagram result would illustrate that the ratio of Fe entering into the filtrate would increase with increasing molar ratio of HNO3 to Pb in the LATR (Fig. 6). It was obvious that the dominate phase of Al was mainly Al 3+ ion in the solution, while the solid Al(OH)3 was dominant phase at pH of larger than 4.6. Thus, with more molar ratio of HNO3 to Pb in the LATR in the leaching system, the pH would decrease, which would result in more Al 3+ measured in the filtrate. The XRD patterns and appearance of filter residue was shown in in Figure S3. The XRD patterns at various molar ratios were illustrating that the PbO and SnO2 were main composition of the filter residue. This XRD analysis results were accordance with the Sn in the filtrate result (Fig. 6), for part of SnO2 was left in the residue. The appearance of the filter residue gradually changed from yellow into light yellow with increasing molar ratio of HNO3 to Pb in the LATR, which was attributed from that less PbO was left in the residue with increasing dosage of HNO3 in the leaching system. The weight of filter residue after leaching and filtration was shown in Tab. S3, indicating that the weight of filter residue decreased with the increasing molar ratio of HNO3 to Pb in the LATR. The decreasing of the filter residue was mainly resulting from that more PbO and element impurity in LATR have been reacted with more added HNO3.

Synthesis of PbS crystal
The conversion rate of lead in the store filtrate solution (wt%) under various molar ratio of Na2S to Pb in the filtrate was presented in Fig.7(a). As shown, the conversion rate of lead was increased with increasing molar ratio of Na2S to Pb in the filtrate ranging from 0.75 to 1.5. The reaction between the sodium sulfide and the lead nitrate in the filtrate was presented by Eq. 3, indicating that the theoretical molar ratio of sodium sulfide to lead nitrate was 1:1. With more added sodium sulfide, the lead ions in the stored filtrate would be easier to contact and react with the sulfur ion in the system, while more amount of PbS crystal would be generated, as illustrated by Fig. 7(b). The appearance of the synthetic PbS crystal product was shown in Fig.7 (I-IV), while the prepared PbS crystal products were all black powders, further confirming to the existence of PbS. From the appearance of products, not obvious appearance difference for PbS crystal was observed at different molar ratios.
Pb(NO3)2 + Na2S → PbS + 2NaNO3 (3)  Fig. 8(a). The XRD results were indicating that all of the generated powders were pure PbS crystal (PDF: 03-0614) at various molar ratios, indicating that the prepared PbS crystal was with high purity. The SEM images of PbS crystal synthesized at molar ratio of 0.75 and 1.5 were shown in Fig. 8(b1)-(b4). As shown, not obvious difference was observed between the products at different molar ratio of Na2S to Pb in the filtrate. The prepared PbS crystal was mainly rendering square blocks with varying dimension ranging among 20 and 100 nm, indicating the prepared PbS powders were mainly nano-sized materials. The crystallization process would be beneficial to the separation of other element from the lead containing crystal, due to their different crystallization performance as shown in Fig. 8(c) [28] [29]. Fig. 8. The material characterization of the prepared crystal products (Conditions: stirring speed of 300 rpm, temperature at 25°C): (a) the XRD patterns; (b) the SEM diagrams ((b1, b2) the molar ratio of Na2S to Pb in the filtrate=0.75, (b3, b4) the molar ratio of Na2S to Pb in the filtrate=1.5); (c) the proposed model for the separation of Pb ions from the impurities It could be revealed that most part of lead was transferred into the PbS crystal product, while only a smart part of lead was left in the filtrate. The lead distribution in the PbS crystal and filtrate was shown in Fig.S4. As shown in Fig.S4, the amount of lead in the synthetic product generally increased with an increasing molar ratio of Na2S to Pb in the filtrate, which was attributed from that more lead ions would contact and react with sufficient sulfur ions with the increasing dosage of sodium sulfide reagent. The percentage of lead in the synthetic product and filtrate would be summed for more than 88% under various molar ratios as presented in Fig.S4. The sum percentage of lead was maximum at molar ratio of Na2S to Pb in the filtrate was 1.5, which could attain to 94.5%.

Conclusion
The PbS crystal was successfully synthesized through this high-speed leaching and recrystallization method. The leaching efficiency of lead increased with an increasing molar ratio of HNO3 to Pb in the LATR, which would reach up to 82.9% at the molar ratio of 3. The main metal impurities in the leaching process was Sn, As, Cu, Fe, and Al. The ratio of As, Cu, Fe, and Al entering into the filtrate increased when the molar ratio of HNO3 to Pb in the LATR increasing from 1.5 to 3, while the ratio of Sn entering into the filtrate was only 0.002% at the molar ratio of 3. The yield of PbS crystal would increase with increasing molar ratio of Na2S to Pb in the filtrate, which could reach up to 93.1% at the molar ratio of Na2S to Pb in the filtrate as 1.5. As shown in the XRD patterns of the product, the product was relatively pure crystal of PbS. The method could provide a green route for the efficient recovery of lead from waste lead ash at room temperature, which would not generate lead particles and high energy consumption. In the future recycling process, the recovery of Sn and other precious metals should be further considered.   Figure S4.     S4. The molar ratio of Na2S to Pb in the filtrate influence on the percent of lead (wt%) in the product and filtrate. Here, the Filter and Product was representing the lead left in the filter and tranferred into the product, respectively.

EDTA-2Na Testing Method.
Pipette 3 mL of filtrate into a 250 mL erlenmeyer flask and dilute with water to about 90 mL. The pH of the solution was adjusted to 5 ~ 6 with 1 : 1 ammonia water, while 5 mL of 20% sodium acetate solution and 5 mL of 20% hexamethylenetetramine were used as buffer solutions to stabilize the pH. Add 3 drops of 0.5% xylenol orange indicator and shake well. At this time, the solution is purple -red. Titrate with the 0.05 mol/L EDTA-2Na standard solution after calibration until the solution changes from purple -red to bright yellow. Record the amount of EDTA-2Na standard solution as v4E, then the lead ion concentration was shown as Eq. (S1): m4 = c4×v4E×mr/v4 (S1) Where m4 is the concentration of the lead ion, g/L; c4 is the concentration of the EDTA-2Na, mol/L; v4E stands the dosage of EDTA-2Na, mL; mr is the relative molecular weight of lead, g/mol, 207.1; v4 is the filtrate volume, mL, take 3 mL at this experiment.

Determination of the impurity elements in the LATR.
LATR materilas (0.2 g) was placed in a beaker (50 mL), then adding 15.0 mL of aqua regia into the beaker. The slurry containg LATR materilas and aqua regia was heated at 200 o C until no obvious particle solid was observed in the slurry. The dilute nitric acid solution (3% wt) was added into the slurry until the volume reaching up to