Cleaner extraction of lead from complex lead-containing wastes by reductive sulfur-fixing smelting with low

: A novel and cleaner process for lead and silver recycling from multiple lead-containing wastes, e.g., lead ash, lead sludge, lead slag, and ferric sludge, by reductive sulfur-ﬁxing smelting was proposed. In this process, coke and iron-containing wastes were employed as reductive agent and sulfur-ﬁxing agent, respectively. A Na 2 CO 3 -Na 2 SO 4 mixture was added as ﬂux. The feasibility of this process was detected from thermodynamic and experimental perspectives. The inﬂuence of Fe/SiO 2 and CaO/SiO 2 , composition of the molten salt, coke addition, smelting temperature, and smelting time on direct Pb recovery and sulfur-ﬁxation efﬁciency were investigated. The optimal process conditions were determined as follows: W Coke = 15% W Pb wastes , W Na 2 CO 3 / W Na 2 SO 4 = 0.7/0.3, Fe/SiO 2 = 1.10, CaO/SiO 2 = 0.30, smelting temperature 1200 ◦ C, and smelting time 2 h, where W represents weight. Under these optimum conditions, 92.4% Pb and 98.8% Ag were directly recovered in crude lead bullion in one step treatment, and total 98.6% sulfur was ﬁxed. The generation and emissions of SO 2 can be avoided. The main phases in ferrous matte obtained were FeS, NaFeS 2 , Fe 2 Zn 3 S 5 , and a little entrained Pb. The slag was a FeO-SiO 2 -CaO-Na 2 O quaternary melt.


Introduction
Today, large amounts of lead-containing wastes are produced in non-ferrous metallurgical industry [1][2][3], especially in Pb and Zn metallurgy fields, such as lead ash and lead slag generated in Pb/Zn smelting, lead anode slime, and lead sludge produced in the electrolytic refining. Other engineering fields [4][5][6] are also sources of Pb-bearing substances, including lead scrap and lead paste separated from spent lead-acid batteries [7]. In many countries, these lead-bearing wastes are classified as hazardous waste due to the high toxicity of lead [8]. They are greatly detrimental to environment and human health if left untreated and abandoned directly to the environment [9,10]. However, these wastes generally contain considerable amounts of precious metals, such as gold (Au) and silver (Ag). It also enormously challenges the circular economy if the valuable metals are not recycled. Various lead wastes are potential resources for extracting lead and precious metals

Materials
The lead-bearing wastes used in this study come from the processes of Chinese metallurgical industry. The feed mixture contained lead ash, lead sludge, lead slag, and ferric sludge. In order to have a steady assay of the feed, 23.9 g lead ash, 48.0 g lead sludge, 5.0 g lead slag, and 23.1 g ferric sludge were manually and carefully mixed to obtain a mixture containing 30% of Pb and 8% of S. Coke used as the reducing agent in this research had fixed carbon of 84.3%. The chemical compositions of the materials are given in Table 1. Figure 1 illustrates the X-ray diffraction analysis results (XRD, D/max 2550PC, Rigaku Co. Ltd, Tokyo, Japan). Other reagents, including Na 2 CO 3 and Na 2 SO 4 , flux Fe 2 O 3 , SiO 2 , and CaO, were analytical purity and supplied by Aladdin Industrial Corporation, China.   The XRD spectra shows that the main lead-bearing phases in the feed materials were sulfate (PbSO4), sulfide (PbS), oxide (Pb3O4 and PbO2), and silicate (PbSiO3). At the same time, zinc sulfate (ZnSO4), zinc oxysulfate (Zn3O(SO4)2), iron sulfate (Fe2(SO4)3), calcium sulfate (CaSO4), a complex lead-zinc mineral queitite (Pb4Zn2((SO4)(SiO4)(Si2O7)), iron silicate (FeSiO3), and ferric oxide (Fe2O3) were also detected in the materials. The feed material is thus a polymetallic complex material.  were also detected in the materials. The feed material is thus a polymetallic complex material. Figure 2 shows a detailed flowchart of this novel process. As an experimental procedure, 100 g lead-bearing waste mixture was mixed with coke powder, Na 2 CO 3 -Na 2 SO 4 mixture, and other fluxes (CaO, SiO 2 and Fe 2 O 3 if necessary). The mixture was placed in an alumina crucible. The crucible was pushed into the constant temperature zone of a tube furnace at the desired temperature. The schematic of the tube furnace is illustrated in Figure 3. In the smelting process, a series of reduction and sulfur-fixation reactions took place between the reactants. Metallic Pb was extracted from the raw materials and enriched in crude lead bullion. At the same time, the sulfur was fixed as ferrous matte, instead of emitting SO 2 . After the required smelting time in air, the crucible was taken out from the hot zone and cooled to room temperature. Next, the crucible was broken to carefully separate and weigh the smelting products obtained, crude lead, ferrous matte, and slag. The products were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Optima 3000, Perkin Elmer, Norwalk, CT, USA) and X-ray diffractometer.  Figure 2 shows a detailed flowchart of this novel process. As an experimental procedure, 100 g lead-bearing waste mixture was mixed with coke powder, Na2CO3-Na2SO4 mixture, and other fluxes (CaO, SiO2 and Fe2O3 if necessary). The mixture was placed in an alumina crucible. The crucible was pushed into the constant temperature zone of a tube furnace at the desired temperature. The schematic of the tube furnace is illustrated in Figure 3. In the smelting process, a series of reduction and sulfurfixation reactions took place between the reactants. Metallic Pb was extracted from the raw materials and enriched in crude lead bullion. At the same time, the sulfur was fixed as ferrous matte, instead of emitting SO2. After the required smelting time in air, the crucible was taken out from the hot zone and cooled to room temperature. Next, the crucible was broken to carefully separate and weigh the smelting products obtained, crude lead, ferrous matte, and slag. The products were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Optima 3000, Perkin Elmer, Norwalk, CT, USA) and X-ray diffractometer.    Figure 2 shows a detailed flowchart of this novel process. As an experimental procedure, 100 g lead-bearing waste mixture was mixed with coke powder, Na2CO3-Na2SO4 mixture, and other fluxes (CaO, SiO2 and Fe2O3 if necessary). The mixture was placed in an alumina crucible. The crucible was pushed into the constant temperature zone of a tube furnace at the desired temperature. The schematic of the tube furnace is illustrated in Figure 3. In the smelting process, a series of reduction and sulfurfixation reactions took place between the reactants. Metallic Pb was extracted from the raw materials and enriched in crude lead bullion. At the same time, the sulfur was fixed as ferrous matte, instead of emitting SO2. After the required smelting time in air, the crucible was taken out from the hot zone and cooled to room temperature. Next, the crucible was broken to carefully separate and weigh the smelting products obtained, crude lead, ferrous matte, and slag. The products were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Optima 3000, Perkin Elmer, Norwalk, CT, USA) and X-ray diffractometer.    The direct Pb recovery η and sulfur-fixing rate ξ were calculated using the following equations:

Methods
Direct Pb recovery η = Mass of lead in the crude lead Mass of lead in the initial feed materials (1) Sulfur − fixation rate ξ = Mass of sulfur in the matte and slag Mass of sulfur in the initial feed materials (2)

Thermodynamics Consideration
The main lead-bearing phases in the feed materials are lead sulfate (PbSO 4 ), lead sulfide (PbS), and lead oxide (PbO). In the smelting process, a series of reduction and sulfur-fixation reactions took place between the reactants at high temperature (1000-1300 • C), as shows in Table 2.
Minerals 2019, 9, x FOR PEER REVIEW 6 of 16 The Gibbs energy change ∆ of all above reactions was calculated using HSC Chemistry 9.2.6 and its database [31]. Figure 4 shows the ∆ vs. diagrams of Reactions (3)- (20). It illustrates that lead extraction reactions in the presence of sulfur-fixing agents and reductant are thermodynamically favorable in the temperature range of 900-1300 °C. Increase in temperature will promote the positive trends of the reactions. The reductive sulfur-fixing reactions in this smelting process are feasible. Metallic Pb can be extracted from PbSO4, PbS, and PbO, and enriched in crude lead bullion. At the same time, the sulfur in the initial raw materials was transferred to FeS, NaFeS2, ZnS, and ultimately fixed as ferrous matte, instead of emitting SO2.

Effect of Fe/SiO2
The effect of Fe/SiO2 on the direct Pb recovery and the sulfur-fixing rates was investigated in the range from Fe/SiO2 = 0.65 to 1.3 (w/w), with a fixed conditions of Wcoke = 10% Wraw materials, molten salt addition W Na 2 CO 3 Na 2 SO 4 = 18% Wraw materials and molten salt composition W Na 2 CO 3 / W Na 2 SO 4 = 0.3/0.7 (where W represents weight), CaO/SiO2 = 0.3, smelting at 1200 °C for 2 h. The results are illustrated in Figure 5. It can be observed that the direct Pb recovery increased gradually with increasing Fe/SiO2 and peaked at Fe/SiO2 = 1.1 where 85.3% Pb was directly recovered. However, a steady declining trend was observed when the Fe/SiO2 was above 1.1. Otherwise, the sulfur-fixing rate increased slowly from 89.5% to 95.5% as Fe/SiO2 increased from 0.65 to 1.3, and reached a maximum sulfur-

Effect of Fe/SiO 2
The effect of Fe/SiO 2 on the direct Pb recovery and the sulfur-fixing rates was investigated in the range from Fe/SiO 2 = 0.65 to 1.3 (w/w), with a fixed conditions of W coke = 10% W raw materials , molten salt addition W Na 2 CO 3 + Na 2 SO 4 = 18% W raw materials and molten salt composition W Na 2 CO 3 /W Na 2 SO 4 = 0.3/0.7 (where W represents weight), CaO/SiO 2 = 0.3, smelting at 1200 • C for 2 h. The results are illustrated in Figure 5. It can be observed that the direct Pb recovery increased gradually with increasing Fe/SiO 2 and peaked at Fe/SiO 2 = 1.1 where 85.3% Pb was directly recovered. However, a steady declining trend was observed when the Fe/SiO 2 was above 1.1. Otherwise, the sulfur-fixing rate increased slowly from 89.5% to 95.5% as Fe/SiO 2 increased from 0.65 to 1.3, and reached a maximum sulfur-fixing rate of 96.7% at Fe/SiO 2 = 1.25. This can be explained by the fact that a large fraction of iron oxide in the slag promotes the sulfur-fixing reactions. Figure 6 illustrates a liquidus surface diagram of the PbO-FeO-SiO 2 -CaO-Na 2 O system with fixed ratio of W Na 2 O /W SiO 2 = 0.5 and W CaO /W SiO 2 = 1/3. It reveals that the temperature of liquid phase boundaries deceased with the increasing FeO fraction. This indicates that a suitable FeO addition decreases the melting point of the slag; at the same time, the viscosity and fluidity will also be improved. As a result, the reduction and settling environment of lead particles are improved. However, it is also observed that excessive FeO fraction will also lead to a higher melt density and higher melting point in slag. It is unbeneficial to the settling and elimination of Pb droplets. At the same time, the blue liquidus lines gradually moved to PbO corner with the increasing FeO fraction. This indicates that the solubility of lead increase in the slag. Therefore, a moderate addition of iron helps to obtain a suitable FeO-SiO 2 -CaO-Na 2 O slag for effective lead recovery and sulfur-fixation.  Figure 6 illustrates a liquidus surface diagram of the PbO-FeO-SiO2-CaO-Na2O system with fixed ratio of W Na 2 /W SiO 2 = 0.5 and W CaO /W SiO 2 = 1/3. It reveals that the temperature of liquid phase boundaries deceased with the increasing FeO fraction. This indicates that a suitable FeO addition decreases the melting point of the slag; at the same time, the viscosity and fluidity will also be improved. As a result, the reduction and settling environment of lead particles are improved. However, it is also observed that excessive FeO fraction will also lead to a higher melt density and higher melting point in slag. It is unbeneficial to the settling and elimination of Pb droplets. At the same time, the blue liquidus lines gradually moved to PbO corner with the increasing FeO fraction. This indicates that the solubility of lead increase in the slag. Therefore, a moderate addition of iron helps to obtain a suitable FeO-SiO2-CaO-Na2O slag for effective lead recovery and sulfur-fixation. Figure 6. Liquidus contour diagram of the PbO-FeO-SiO2-CaO-Na2O system, the data was taken from MTDATA vers. 8.2 [32] MTOX database [33].   Figure 6 illustrates a liquidus surface diagram of the PbO-FeO-SiO2-CaO-Na2O system with fixed ratio of W Na 2 /W SiO 2 = 0.5 and W CaO /W SiO 2 = 1/3. It reveals that the temperature of liquid phase boundaries deceased with the increasing FeO fraction. This indicates that a suitable FeO addition decreases the melting point of the slag; at the same time, the viscosity and fluidity will also be improved. As a result, the reduction and settling environment of lead particles are improved. However, it is also observed that excessive FeO fraction will also lead to a higher melt density and higher melting point in slag. It is unbeneficial to the settling and elimination of Pb droplets. At the same time, the blue liquidus lines gradually moved to PbO corner with the increasing FeO fraction. This indicates that the solubility of lead increase in the slag. Therefore, a moderate addition of iron helps to obtain a suitable FeO-SiO2-CaO-Na2O slag for effective lead recovery and sulfur-fixation.  Figure 7 shows the effect of CaO/SiO 2 ratio of the silicate slag on the lead extraction and sulfur fixation. It is observed that the direct Pb recovery dropped sharply from 86.4% to 69.4% when CaO/SiO 2 was increased from 0.3 to 0.45. This indicates that the increasing CaO/SiO 2 was harmful to the settling of metallic lead. However, the change of CaO/SiO 2 had almost no effect on sulfur-fixing rate. It remained nearly constant in around 97%. Figure 8 illustrates a liquidus contour diagram of the CaO-FeO-SiO 2 -Na 2 O system in fixed W Na 2 O /W SiO 2 ratio of 0.5. It also indicates that increasing addition of CaO will lead to an increase in melting point of the slag. As a result, the viscosity and fluidity of slag will also deteriorate. The Pb recovery showed a diminishing trend at higher CaO additions. Figure 7 shows the effect of CaO/SiO2 ratio of the silicate slag on the lead extraction and sulfur fixation. It is observed that the direct Pb recovery dropped sharply from 86.4% to 69.4% when CaO/SiO2 was increased from 0.3 to 0.45. This indicates that the increasing CaO/SiO2 was harmful to the settling of metallic lead. However, the change of CaO/SiO2 had almost no effect on sulfur-fixing rate. It remained nearly constant in around 97%. Figure 8 illustrates a liquidus contour diagram of the CaO-FeO-SiO2-Na2O system in fixed W Na 2 /W SiO 2 ratio of 0.5. It also indicates that increasing addition of CaO will lead to an increase in melting point of the slag. As a result, the viscosity and fluidity of slag will also deteriorate. The Pb recovery showed a diminishing trend at higher CaO additions.  (W coke = 10%W raw materials , W Na 2 CO 3 + Na 2 SO 4 = 18%W raw materials , W Na 2 CO 3 /W Na 2 SO 4 = 0.3/0.7, Fe/SiO 2 = 1.1, 1200 • C, 2 h). Figure 7 shows the effect of CaO/SiO2 ratio of the silicate slag on the lead extraction and sulfur fixation. It is observed that the direct Pb recovery dropped sharply from 86.4% to 69.4% when CaO/SiO2 was increased from 0.3 to 0.45. This indicates that the increasing CaO/SiO2 was harmful to the settling of metallic lead. However, the change of CaO/SiO2 had almost no effect on sulfur-fixing rate. It remained nearly constant in around 97%. Figure 8 illustrates a liquidus contour diagram of the CaO-FeO-SiO2-Na2O system in fixed W Na 2 /W SiO 2 ratio of 0.5. It also indicates that increasing addition of CaO will lead to an increase in melting point of the slag. As a result, the viscosity and fluidity of slag will also deteriorate. The Pb recovery showed a diminishing trend at higher CaO additions.

Percentage [%]
Sulfur-fixing rate Direct Pb recovery rate

Effect of W Na
The relationship between W Na 2 CO 3 /W Na 2 SO 4 and Pb recovery and sulfur-fixing rates are presented in Figure 9. It can be seen that the direct Pb recovery increased steadily from 85.3% to 90.8% as the W Na 2 CO 3 /W Na 2 SO 4 increased from 0.3/0.7 to 0.9/0.1. However, the sulfur-fixing rate was constant on around 95.5%. Molten salt can simultaneously decrease the melting point, density, viscosity [34,35], and improve the fluidity of the slag [36]. Therefore, Na 2 CO 3 -Na 2 SO 4 mixture can provide a flexible reaction medium and significantly promote the smelting reactions. Figure 10 illustrates the phase diagram of the binary Na 2 CO 3 -Na 2 SO 4 system. It shows that the melting temperature of Na 2 CO 3 -Na 2 SO 4 mixture decreases with increasing Na 2 CO 3 fraction, and reaches the minimum in W Na 2 CO 3 /W Na 2 SO 4 = 0.54/0.45. Therefore, with increasing W Na 2 CO 3 /W Na 2 SO 4 ratio, the settling environment of lead particles was improved. More metallic Pb was settled and collected in crude lead. The relationship between W Na 2 CO 3 / W Na 2 SO 4 and Pb recovery and sulfur-fixing rates are presented in Figure 9. It can be seen that the direct Pb recovery increased steadily from 85.3% to 90.8% as the W Na 2 CO 3 / W Na 2 SO 4 increased from 0.3/0.7 to 0.9/0.1. However, the sulfur-fixing rate was constant on around 95.5%. Molten salt can simultaneously decrease the melting point, density, viscosity [34,35], and improve the fluidity of the slag [36]. Therefore, Na2CO3-Na2SO4 mixture can provide a flexible reaction medium and significantly promote the smelting reactions. Figure 10 illustrates the phase diagram of the binary Na2CO3-Na2SO4 system. It shows that the melting temperature of Na2CO3-Na2SO4 mixture decreases with increasing Na2CO3 fraction, and reaches the minimum in W Na 2 CO 3 /W Na 2 SO 4 = 0.54/0.45. Therefore, with increasing W Na 2 CO 3 / W Na 2 SO 4 ratio, the settling environment of lead particles was improved. More metallic Pb was settled and collected in crude lead.

Percentage [%]
Sulfur-fixing rate Direct Pb recovery rate Figure 9. Effect of Na 2 CO 3 /Na 2 SO 4 ratio on the Pb recovery and sulfur-fixing efficiency (W coke = 10%W raw materials , W Na 2 CO 3 + Na 2 SO 4 = 18%W raw materials , Fe/SiO 2 = 1.1, CaO/SiO 2 = 0.3, 1200 • C, 2 h). The relationship between W Na 2 CO 3 / W Na 2 SO 4 and Pb recovery and sulfur-fixing rates are presented in Figure 9. It can be seen that the direct Pb recovery increased steadily from 85.3% to 90.8% as the W Na 2 CO 3 / W Na 2 SO 4 increased from 0.3/0.7 to 0.9/0.1. However, the sulfur-fixing rate was constant on around 95.5%. Molten salt can simultaneously decrease the melting point, density, viscosity [34,35], and improve the fluidity of the slag [36]. Therefore, Na2CO3-Na2SO4 mixture can provide a flexible reaction medium and significantly promote the smelting reactions. Figure 10 illustrates the phase diagram of the binary Na2CO3-Na2SO4 system. It shows that the melting temperature of Na2CO3-Na2SO4 mixture decreases with increasing Na2CO3 fraction, and reaches the minimum in W Na 2 CO 3 /W Na 2 SO 4 = 0.54/0.45. Therefore, with increasing W Na 2 CO 3 / W Na 2 SO 4 ratio, the settling environment of lead particles was improved. More metallic Pb was settled and collected in crude lead.

Percentage [%]
Sulfur-fixing rate Direct Pb recovery rate Figure 10. Phase diagram of the Na 2 CO 3 -Na 2 SO 4 system; the data was taken from FactSage 7.2 and its FTsalt database [37]. Figure 11 illustrates the effect of coke addition on the smelting yields. When coke dosage was increased from 5% to 25%, Pb extraction was found to increase from 81.3% to 91.5%, and it reached a maximum of 92.4% at 20% coke addition. The sulfur-fixing rate increased from 91.7% to 94.8%, and peaked at 97.2% with 15% coke addition. However, a small decrease of sulfur-fixation was observed when the coke addition was more than 15%. Strong reductive atmosphere was beneficial to the reduction and enrichment of lead. It was also intended to promote iron oxide reduction. More Fe x O y was reduced to generate metallic Fe and then transferred into crude lead phase. Afterwards, iron oxide acting as sulfur-fixation agent correspondingly decreased. As a result, the slag composition was turned to a direction that was against the Pb recovery, as shown in Figure 6. Therefore, a slight decrease was observed in both the Pb recovery and sulfur-fixation beyond 15% coke addition. Figure 11 illustrates the effect of coke addition on the smelting yields. When coke dosage was increased from 5% to 25%, Pb extraction was found to increase from 81.3% to 91.5%, and it reached a maximum of 92.4% at 20% coke addition. The sulfur-fixing rate increased from 91.7% to 94.8%, and peaked at 97.2% with 15% coke addition. However, a small decrease of sulfur-fixation was observed when the coke addition was more than 15%. Strong reductive atmosphere was beneficial to the reduction and enrichment of lead. It was also intended to promote iron oxide reduction. More FexOy was reduced to generate metallic Fe and then transferred into crude lead phase. Afterwards, iron oxide acting as sulfur-fixation agent correspondingly decreased. As a result, the slag composition was turned to a direction that was against the Pb recovery, as shown in Figure 6. Therefore, a slight decrease was observed in both the Pb recovery and sulfur-fixation beyond 15% coke addition. Figure 11. Effect of coke dosage on the Pb recovery and sulfur-fixing efficiency (W Na 2 CO 3 Na 2 SO 4 = 18%WPb materials, W Na 2 CO 3 /W Na 2 SO 4 = 0.7/0.3, Fe/SiO2 = 1.1, CaO/SiO2 = 0.3, 1200 °C, 2 h).

Effect of Smelting Temperature
The direct Pb recovery and sulfur-fixing rates at different temperatures are shown in Figure 12. Below 1200 °C, Pb recovery and sulfur-fixing rates presented a sharp rising trend with increasing temperatures. The Pb recovery increased from 58.3% to 93.3% and sulfur-fixing rate increased at the same time from 75.9% to 96.2%. The phase diagram of the PbO-FeO-SiO2-CaO-Na2O system shown in Figure 6 also indicates that increasing temperature is beneficial to decrease the PbO content in the slag. However, direct Pb recovery steadily decreased when the temperature was higher than 1200 °C due to the intensified volatilization of metallic Pb, but sulfur-fixation continued to increase slightly. Therefore, a suitable smelting temperature was critical to achieve a high direct Pb recovery to the molten metal, and to avoid lead losses in the off-gas fumes. Sulfur-fixing rate Direct Pb recovery rate Figure 11. Effect of coke dosage on the Pb recovery and sulfur-fixing efficiency (W Na 2 CO 3 + Na 2 SO 4 = 18%W Pb materials , W Na 2 CO 3 /W Na 2 SO 4 = 0.7/0.3, Fe/SiO 2 = 1.1, CaO/SiO 2 = 0.3, 1200 • C, 2 h).

Effect of Smelting Temperature
The direct Pb recovery and sulfur-fixing rates at different temperatures are shown in Figure 12. Below 1200 • C, Pb recovery and sulfur-fixing rates presented a sharp rising trend with increasing temperatures. The Pb recovery increased from 58.3% to 93.3% and sulfur-fixing rate increased at the same time from 75.9% to 96.2%. The phase diagram of the PbO-FeO-SiO 2 -CaO-Na 2 O system shown in Figure 6 also indicates that increasing temperature is beneficial to decrease the PbO content in the slag. However, direct Pb recovery steadily decreased when the temperature was higher than 1200 • C due to the intensified volatilization of metallic Pb, but sulfur-fixation continued to increase slightly. Therefore, a suitable smelting temperature was critical to achieve a high direct Pb recovery to the molten metal, and to avoid lead losses in the off-gas fumes.

Percentage [%]
Sulfur-fixing rate Direct Pb recovery rate  Figure 13 shows the effect of smelting time on the Pb recovery and sulfur-fixation. It illustrates that extending the smelting time can promote the recovery of Pb and sulfur fixation. 93.3% Pb and 97.5% sulfur were recovered and immobilized within 2 h of reaction. After 3 h, Pb recovery and sulfur-immobilization rates were 92.6% and 98.3%, respectively. This indicates 2 h reaction time was enough for the settling and enrichment of the Pb product in the bullion. The further extension of smelting time would lead to a gradual decline in Pb recovery due to volatilization of metallic Pb. When the smelting time was increased from 2 h to 3 h, the sulfur-fixing rate showed a slight increase from 97.5% to 98.3%. Considering the volatilization of Pb and energy consumption, the optimum smelting time was suggested to be around 2 h. Figure 12. Effect of temperature on the Pb recovery and sulfur-fixing efficiency (Wcoke=15%WPb materials, W Na 2 CO 3 Na 2 SO 4 = 18%WPb materials, W Na 2 CO 3 / W Na 2 SO 4 = 0.7/0.3, Fe/SiO2 = 1.1, CaO/SiO2 = 0.3, 2 h). Figure 13 shows the effect of smelting time on the Pb recovery and sulfur-fixation. It illustrates that extending the smelting time can promote the recovery of Pb and sulfur fixation. 93.3% Pb and 97.5% sulfur were recovered and immobilized within 2 h of reaction. After 3 h, Pb recovery and sulfur-immobilization rates were 92.6% and 98.3%, respectively. This indicates 2 h reaction time was enough for the settling and enrichment of the Pb product in the bullion. The further extension of smelting time would lead to a gradual decline in Pb recovery due to volatilization of metallic Pb. When the smelting time was increased from 2 h to 3 h, the sulfur-fixing rate showed a slight increase from 97.5% to 98.3%. Considering the volatilization of Pb and energy consumption, the optimum smelting time was suggested to be around 2 h.

Bench-Pilot Experiments
Based on the above detailed experiments in the laboratory, the optimum processing conditions were obtained as: W coke /W raw materials = 15%, W Na 2 CO 3 + Na 2 SO 4 = 18% W raw materials , W Na 2 CO 3 /W Na 2 SO 4 = 0.7/0.3, Fe/SiO 2 = 1.1, CaO/SiO 2 = 0.3, smelting temperature 1200 • C, and smelting time 2 h. These conditions were then applied to carry out two bench-pilot experiments with 200g Pb-bearing materials mixture. The chemical compositions of different products obtained, crude Pb, ferrous matte and slag, were presented in Table 3. It was found that lead concentrations in the ferrous matte and slag were below 2.4% and 0.7%, respectively. Purity of the crude lead was higher than 96.5%, and its main impurity was metallic Fe. The optical macrograph and XRD results of smelting products are illustrated in Figure 14. It was observed that the product was separated to three layers: slag, ferrous matte, and crude lead. Slag floats on the surface of the ferrous matte, and metallic Pb subsides at the bottom due to their density differences, which guarantees a good separation and recovery during the smelting process. In addition, the main components in the solidified matte were FeS, NaFeS 2 , and Fe 2 Zn 3 S 5 . It indicated that sulfur was fixed in the matte in form of sulfide matte. A little entrained metallic Pb phase was also detected in it. The slag was a FeO-SiO 2 -CaO-Na 2 O quaternary and solidified as a vitreous slag system. The optical macrograph and XRD results of smelting products are illustrated in Figure 14. It was observed that the product was separated to three layers: slag, ferrous matte, and crude lead. Slag floats on the surface of the ferrous matte, and metallic Pb subsides at the bottom due to their density differences, which guarantees a good separation and recovery during the smelting process. In addition, the main components in the solidified matte were FeS, NaFeS2, and Fe2Zn3S5. It indicated that sulfur was fixed in the matte in form of sulfide matte. A little entrained metallic Pb phase was also detected in it. The slag was a FeO-SiO2-CaO-Na2O quaternary and solidified as a vitreous slag system. The distribution behaviors of Pb, Ag, Fe, and S in the bench-pilot experiments are summarized in Figure 15. As shown, 92.4% Pb could be directly enriched in the crude lead, and only 5.5% and 1.8% of Pb distributed into the ferrous matte and slag, respectively. 98.8% of Ag was also enriched in crude lead. 71.0% of iron was divided into ferrous matte as sulfide and the rest of 28.7% was in the FeO-SiO2-CaO-Na2O slag, respectively. Total sulfur-fixed rate was 98.6% to condensed phases, with 91.3% in matte, 7.3% in slag. At the same time, 0.2% sulfur was in crude lead. Only 1.2% distributed to the fume. Therefore, this novel reductive sulfur-fixing smelting process can significantly inhibit the generation of SO2 and largely decrease the SO2 emissions. The distribution behaviors of Pb, Ag, Fe, and S in the bench-pilot experiments are summarized in Figure 15. As shown, 92.4% Pb could be directly enriched in the crude lead, and only 5.5% and 1.8% of Pb distributed into the ferrous matte and slag, respectively. 98.8% of Ag was also enriched in crude lead. 71.0% of iron was divided into ferrous matte as sulfide and the rest of 28.7% was in the FeO-SiO 2 -CaO-Na 2 O slag, respectively. Total sulfur-fixed rate was 98.6% to condensed phases, with 91.3% in matte, 7.3% in slag. At the same time, 0.2% sulfur was in crude lead. Only 1.2% distributed to the fume. Therefore, this novel reductive sulfur-fixing smelting process can significantly inhibit the generation of SO 2 and largely decrease the SO 2 emissions. Recently, this novel Pb recycling process from polymetallic complex materials was industrially adopted [38] in several recycling plants in China, such as Guoda Nonferrous Metals Metallurgy Co., Ltd. [39] and Yongchang Precious Metals Co., Ltd. It has practically proved feasible. The industrial application results indicate that this process can be steadily executed on a continuous basis and offers advantages of low dust generation and high production capacity in currently available smelting Figure 15. The distribution behaviors of Pb, Ag, Fe, and S in crude lead, ferrous matte, slag, and off-gas fume in the bench-pilot tests.
Recently, this novel Pb recycling process from polymetallic complex materials was industrially adopted [38] in several recycling plants in China, such as Guoda Nonferrous Metals Metallurgy Co., Ltd. [39] and Yongchang Precious Metals Co., Ltd. It has practically proved feasible. The industrial application results indicate that this process can be steadily executed on a continuous basis and offers advantages of low dust generation and high production capacity in currently available smelting furnaces. The operating expenditure was reduced significantly. The off-gas emissions met the relevant Chinese standards. Furthermore, the matte produced can be sold directly for sulfuric acid manufacture and regenerated as sulfur-fixing agent. The slag is harmless and can be used as a raw material for cement production after water-quenching and granulation. Otherwise, in addition to LAB paste recycling, it also can be used for treating antimony- [28,30], bismuth- [40], zinc- [41], and copper-containing [42] wastes and residues. Sb and Bi are enriched and recovered in the bullion. Zinc and copper are primarily distributed to the sulfide matte. Direct recoveries of the metals, i.e., yields without subsequent slag cleaning and matte processing, reach more than 90%, and the sulfur-fixing rate is generally higher than 95%.

Conclusions
Cleaner extraction of lead from complex polymetallic lead-containing wastes using reductive sulfur-fixing smelting process was fundamentally and experimentally confirmed to be feasible. The optimum smelting conditions were determined in this study as: W coke = 15%W raw materials , W Na 2 CO 3 /W Na 2 SO 4 = 0.7/0.3, Fe/SiO 2 = 1.1, CaO/SiO 2 = 0.3, smelting temperature 1200 • C and smelting time 2 h. Under these conditions, three smelting products, crude lead, ferrous matte, and slag, were obtained. While 92.4% Pb was directly recovered as crude lead, 98.8% Ag was enriched in the crude lead. Lead concentration in the produced ferrous matte and slag were less than 2.4% and 0.7%, respectively. Purity of the crude lead was higher than 96.5%. Sulfur was fixed in the matte and slag as sulfide. The total sulfur-fixing rate was 98.3%, including 90.3% in the matte and 8.0% in the slag. Gaseous SO 2 generation and emissions could be essentially eliminated. This process is a promising high-efficiency technique that is cleaner and offers multiple application prospects in the fields of lead-, antimony-, bismuth-, zinc-, and copper-containing secondary materials, wastes, and residues recycling.