Experimental Study on Bismuth Removal from Lead with Auxiliary Calcium Magnesium

Abstract

The separation of lead from the impurity bismuth remains a significant challenge, with achieving effective separation being a critical bottleneck in the production of high-purity lead via the vacuum gasification method. This study focuses on lead as the primary subject of investigation, conducting both theoretical and experimental research on the auxiliary conversion of lead through vacuum gasification. The calculations of the Gibbs free energy indicate that, within the temperature range of 600 to 610 K, the impurity bismuth reacts completely with calcium and magnesium, resulting in the formation of the compound CaMg₂Bi₂. Under optimal experimental conditions, the bismuth compound CaMg₂Bi₂ is converted into BiCa₂. Notably, BiCa₂ is nonvolatile and remains in the crucible as a residue. The auxiliary calcium is entirely transformed into CaSe and CaTe, leading to a reduction in the calcium content of the volatile substances from 0.5% to 16 ppm. Similarly, the magnesium content in the volatiles decreases from 0.66% to 187 ppm. Ultimately, the bismuth content in the final product is reduced from 6 ppm to 1.4 ppm, achieving a removal rate of 76.6%, while the direct yield of metallic lead reaches 71%. This process effectively facilitates the separation of metallic lead from the bismuth impurities.

1. Introduction

Lead is a dense, malleable metal, characterized by the chemical symbol Pb. It possesses notable properties such as good ductility, corrosion resistance, and X-ray shielding capabilities, which contribute to its widespread application in various industries, including battery production, cable sheathing, automobile manufacturing, and the military sector [1,2,3,4,5]. High-purity lead serves as a fundamental raw material for advanced high-performance pure-lead batteries and horizontal batteries. Its significance is increasingly recognized in fields such as nuclear fast reactors and aerospace, marking it as a critical area of development in China. Vacuum gasification purification technology represents a metallurgical approach for the refinement, purification, and processing of metals within a vacuum-sealed system, operating at pressures below 0.1 MPa. This technology has found extensive application in alloy separation, the deep purification of metals, and the sustainable recovery of secondary resources [6,7,8,9,10]. Since the onset of the 21st century, the concept of green development has been emphasized by the state, posing challenges to traditional lead purification processes. The primary concerns regarding the conventional fire purification methods for lead include their outdated technology, significant energy consumption, and substantial environmental pollution, which result in considerable annual waste treatment costs. As the demand for high-quality materials continues to rise, vacuum gasification technology offers significant advantages in the production of high-purity lead materials by effectively minimizing material oxidation, thereby surpassing the efficacy of the traditional fire refining and the wet electrolysis purification methods. The research background concerning the removal of bismuth impurities from lead, utilizing calcium and magnesium, is both profound and significant. This technology emerged in the early 20th century within the domain of metal refining, with the primary objective of effectively eliminating bismuth contaminants from lead by incorporating calcium and magnesium elements. This process aims to enhance the purity and overall quality of lead. Lead is a widely utilized industrial metal, commonly employed in the manufacturing of batteries, production of paints, and the provision of radiation protection, among other applications. However, during the smelting of sulfide ores and the refining processes of lead, impurities such as bismuth frequently contaminate it. The presence of bismuth adversely affects the physical and chemical properties of lead, including its density, hardness, and electrical conductivity. Furthermore, it may negatively impact the subsequent processing and applications of lead, in ways such as a diminishing battery performance and compromising on the gloss and adhesion of coatings [11,12,13,14,15]. Consequently, the effective removal of bismuth impurities from lead remains a critical topic within the field of metal refining.

The removal of bismuth through the use of calcium and magnesium has been extensively investigated as an effective lead-refining technology since the early 20th century. This method is predicated on the ability of calcium and magnesium to form refractory compounds with bismuth in lead, which are insoluble in the lead solution, and can therefore be eliminated as scum. The identification and implementation of this principle have significantly advanced the field of lead-refining technology. Nevertheless, despite the notable successes of the calcium and magnesium bismuth removal method, foundational research remains inadequate. Specifically, further investigation is required to understand the effects of calcium and magnesium concentrations, temperature, and other critical factors on the efficacy of bismuth removal, as well as the underlying mechanisms involved in this process [16,17,18,19]. The experimental investigation into the removal of bismuth using calcium and magnesium not only elucidates the microscopic mechanisms underlying this process, but also offers theoretical insights and practical guidance for the optimization of the process parameters aimed at enhancing bismuth removal efficiency. Furthermore, the influence of temperature on bismuth removal was examined. The findings indicate that, within the temperature range of 1123 K to 1223 K, a system pressure of 2 to 5 Pa, and a holding time of 3 h, the removal rate of bismuth impurities from lead increases with rising temperature. Notably, when the temperature is elevated from 1223 K to 1323 K, there is a marked increase in the removal rate of bismuth impurities, accompanied by a significant reduction in its concentration in the volatile matter. The identification of this phenomenon holds considerable importance for the optimization of process parameters and the enhancement of bismuth removal efficiency.

Gibbs free energy, often referred to as the enthalpy of freedom, is a fundamental concept in the field of thermodynamics. It quantifies the capacity of a system to perform non-volume work under isothermal and isobaric conditions. In essence, it assesses the propensity and direction of a chemical reaction or physical process to occur spontaneously. The calculation of Gibbs free energy is instrumental not only in elucidating the spontaneity of reactions but also in informing chemical design, material development, and biological processes.

In practical applications, the accurate calculation of Gibbs free energy necessitates the prior determination of both the enthalpy change and the entropy change associated with the reaction. These values can typically be ascertained through experimental methods or by consulting the pertinent literature. For intricate chemical systems or multi-step reactions, a more precise approach involving thermodynamic cycles may be employed to evaluate the change in free energy at each individual step. It is crucial to acknowledge that, while Gibbs free energy serves as a valuable tool for predicting the spontaneity of reactions, it does not provide direct insights into the rates and kinetics of these reactions. Furthermore, in certain specific instances, it is essential to consider the impact of additional factors to achieve a more accurate assessment of the reaction’s direction [20,21,22]. Overall, the computation of Gibbs free energy offers significant assistance in addressing the practical issues.

In conclusion, the experimental investigation into the removal of bismuth from lead using the conjunction of calcium and magnesium holds significant theoretical implications and practical applications. Further exploration of the micro-mechanisms involved in bismuth removal through the use of calcium and magnesium, along with the optimization of process parameters, has the potential to enhance the purity and quality of lead metal. Additionally, this research may provide new impetus and support for advancements in the field of metal refining. Looking ahead, as scientific and technological advancements continue to progress alongside the development of metal refining techniques, it is anticipated that the method of bismuth removal utilizing calcium and magnesium will be widely adopted and promoted across various sectors.

2. Materials and Methods

2.1. Materials and Equipment

The source of experimental raw materials is shown in

Table 1. Source of experimental materials and purity.

Metal	Source	Purity
Lead	Henan Yuguang Gold&Lead Co., Ltd., Jiyuan, China	99.992%
Bismuth	Henan Yuguang Gold&Lead Co., Ltd., Jiyuan, China	99.999%
Calcium	Yifeng Metal Materials Co., Ltd., Dongguan, China	99.995%
Magnesium	Yifeng Metal Materials Co., Ltd., Dongguan, China	99.996%

. The primary equipment, manufacturers, and models utilized in the experiment are detailed in

Table 2. The manufacturer and equipment type of primary equipment.

Instrument	Manufacturer	Equipment Type
Electric blast drying oven	Yuyao Star Instrument Factory, Yuyao, China	XGQ-2000
Planetary ball mill	Nanjing University Instrument Factory, Nanjing, China	QM-BP
Powder block press	Tianjin Keqi High-tech Company, Tianjin, China	769YP-40C
Electronic balance	Ruian Yingheng Electric Appliance Co., Ltd., Ruian, China	JCS-31002C
Pit furnace	National Engineering Research Center of Vacuum Metallurgy, Kunming, China	Self-control
Argon	Kunming Stone Headman gas products Co., Ltd., Kunming, China	-
High-purity graphite crucible	National Engineering Research Center of Vacuum Metallurgy, Kunming, China	Self-control

. The vertical well furnace employed in this study comprises a furnace body, an argon gas protection, injection system, a vacuum pump, and a main control console. Specifically, the vertical shaft furnace body is constructed from stainless steel for both the body and base, while the heating system features copper electrodes and a heating element. The argon gas protection system encompasses two components: an argon gas cylinder and an argon gas circulation device. The gas cylinder continuously supplies protective gas to the quartz tube, whereas the circulation device ensures continuous inert gas coverage within the tube. In conjunction with the argon gas protection system, the vacuum system thoroughly purges the quartz tube to ensure complete coverage with the protective gas. The gas injection forced-stirring system achieves its stirring effect by modulating the flow rate of argon gas into the molten metal within the crucible. The control system encompasses the current regulation, voltage control, temperature monitoring, and pressure measurement.

2.2. Preparation of a Synthetic Sample

The raw lead is initially processed as a metallic cube, with the surface oxide film removed using a stainless-steel knife. The surface is then cleaned utilizing an ultrasonic cleaner. Following the cleaning process, the lead is wiped with alcohol and dried using a hair dryer. The lead is then cut into small pieces, and the resulting powder is ground in a vibration mill for three minutes per session, repeated four times, until it attains a fine particle size from 110 μm to 165 μm. The bismuth grains are subsequently placed in a vibrating mill, where they are ground for two minutes per session, also repeated four times. The proportioned fine powders of the raw materials are then mixed thoroughly and placed into a press block with a diameter of 30 mm. The resulting mixed metal block, measuring 30 mm in diameter and 35 mm in thickness, is placed into a graphite crucible, which is then situated within a quartz tube. This assembly is positioned in a shaft furnace, as illustrated in Figure 1. An argon gas protection system is connected, and ventilation is initiated. Once the airflow stabilizes, the vacuum pump is activated to evacuate the air from the quartz tube. After this, the vacuum pump is deactivated, and argon gas is reintroduced. When the pressure within the tube reaches standard atmospheric levels, the vacuum pump is activated once more to purge the air from the quartz tube, and this gas washing procedure is repeated three to five times. Argon gas is continuously introduced into the system with 0.2 L/min. Once the airflow stabilizes, the resistance heater is activated, and the crucible is heated for a duration of three hours to ensure complete mixing and uniformity of the alloy components. Subsequently, a quenching sample is extracted, and the alloy is reintroduced into the crucible for an additional one to two melting cycles. Ultimately, a stable 200 g lead–bismuth alloy is produced, as illustrated in Figure 2.

3. Results and Discussion

3.1. Theoretical Calculation Results

The optimal addition range for metallic calcium is between 0.10% and 0.13% of the total mass of metallic lead, while the optimal addition range for metallic magnesium is between 0.35% and 0.45% of the total mass of metallic lead. When the concentration of metallic calcium is below 0.1%, the removal efficiency of bismuth is less than 30%. Conversely, when the concentration of metallic calcium exceeds 0.13%, the removal efficiency of bismuth remains below 30%, indicating that it does not increase further. Similarly, when the concentration of magnesium is below 0.35%, the removal rate of impurity bismuth is less than 50%, but this rate increases with the mass fraction of auxiliary magnesium. However, when the concentration of magnesium surpasses 0.45%, there is no significant increase in the removal rate of bismuth impurities. The findings indicate that calcium and magnesium react with the bismuth impurities at temperatures ranging from 600 K to 650 K, resulting in the formation of light compounds that float on the surface of the lead solution. The primary product of the reaction between calcium, magnesium, and bismuth is likely to be CaMg₂Bi₂, as suggested by the solubility product. When the temperature reaches 660 K, the auxiliary calcium and magnesium elements react in significant quantities with lead, the primary metal, resulting in the consumption of lead and the production of a substantial amount of slag. At this temperature, metal bismuth remains unreacted. However, there is a lack of relevant theoretical calculations and characterization methods to accurately ascertain the products of the reactions involving calcium, magnesium, and bismuth. Based on the thermodynamic data [23], Gibbs free energy of the chemical reaction was calculated to determine the possibility of generating Bi₂Mg₃, Bi₂Ca₃ and CaMg₂Bi₂ from the conversion reaction of the calcium–magnesium adjuvant. The lower the free energy of the product, the more likely it is to form. The standard free energy of formation of Ca (s), Bi (s), Bi (l) and other substances at 600 K is shown in

Table 3. Standard free energy of formation of Ca (s), Bi (s), Bi (l), and other substances, at 600 K.

Substance	Standard Free Energy of Formation kJ/mol (600 K)
Ca (s)	–27.94
Mg (s)	–22.66
Bi (s)	–38.34
Bi (l)	–39.49
Bi2Mg3 (s)	–22.56
Bi2Ca3 (s)	–154.48
CaMg2Bi2 (s)	–118.16

. The formation equation for the adjuvant conversion reaction is presented as follows:

$$2\mathrm{B}\mathrm{i}\left(\mathrm{l}\right)+3\mathrm{M}\mathrm{g}\left(\mathrm{s}\right)={\mathrm{B}\mathrm{i}}_2{\mathrm{M}\mathrm{g}}_3\left(\mathrm{s}\right)$$

(1)

$$2\mathrm{B}\mathrm{i}\left(\mathrm{l}\right)+3\mathrm{C}\mathrm{a}\left(\mathrm{s}\right)={\mathrm{B}\mathrm{i}}_2{\mathrm{C}\mathrm{a}}_3\left(\mathrm{s}\right)$$

(2)

$$2\mathrm{B}\mathrm{i}\left(\mathrm{l}\right)+\mathrm{C}\mathrm{a}\left(\mathrm{s}\right)+2\mathrm{M}\mathrm{g}\left(\mathrm{s}\right)=\mathrm{C}\mathrm{a}{\mathrm{M}\mathrm{g}}_2{\mathrm{B}\mathrm{i}}_2\left(\mathrm{s}\right)$$

(3)

$$\mathrm{B}\mathrm{i}\left(\mathrm{s}\right)=\mathrm{B}\mathrm{i}\left(\mathrm{l}\right)$$

(4)

According to the manual of inorganic thermodynamics [24], at 600 K, the bismuth impurity undergoes a phase transition from solid to liquid. The formation energy of liquid bismuth, as referenced in the manual of inorganic thermodynamics, is –39.49 kJ/mol. By calculating the Gibbs free energy for the reactions represented in Equations (1) to (3) at 600 K, it is observed that the Gibbs free energy for the reaction in Equation (1) is −22.56 kJ/mol, while for Equation (2) it is –154.48 kJ/mol, and for Equation (3) it is –118.16 kJ/mol. Based on the Gibbs free energy values of these chemical reactions, it can be inferred that at 600 K, the auxiliary elements, calcium and magnesium, can react with the bismuth impurity to form compounds such as Bi₂Ca₃ and CaMg₂Bi₂. Notably, the likelihood of generating CaMg₂Bi₂ is significantly higher than that of producing Bi₂Mg₃.

3.2. Experimental Results

Under the conditions of complete melting and stirring at temperatures ranging from 683 K to 703 K, for a duration of 15 min, followed by a reaction hold at 608 K to 618 K for 3 h, with a stirring flow rate of 1 L/min and an argon shielding gas flow rate of 0.2 L/min, the bismuth impurity present in the lead–bismuth alloy reacts completely with the calcium and magnesium additives. This reaction results in the formation of CaMg₂Bi₂, which aligns with the theoretical calculations. The strengthening effect of the additives is significant. Additionally, within the temperature range of 1273 K to 1323 K, at a system pressure of 2 to 5 Pa and a holding time of 3 h, the impact of the gasification temperature on the separation of the bismuth impurities in the products of the lead–bismuth alloy was examined. The contents of various impurity elements (ppm) in the metal lead are shown in

Table 4. Contents of various impurity elements (ppm) in main metal, lead (99.99%).

Impurity	As	Fe	Cu	Sn	Ag	Bi
content	2	1	4	1	3	6

.

As illustrated in Figure 3, under the conditions of a vacuum gasification at a temperature of 1223 K and a holding time of 2 h, the only phases present in the residue of the adjuvant conversion product are BiCa₂ and Pb. The original intermetallic compound, CaMg₂Bi₂, was completely transformed. The metal bonds were disrupted during the high-temperature gasification process, leading to its partial dissociation into the BiCa₂ phase, which persisted in the residue as the holding time increased. Additionally, the auxiliary magnesium transitioned from the CaMg₂Bi₂ phase to an elemental magnesium phase. As the holding time increased, the high-temperature gasification process ultimately resulted in an enrichment of the condensate. Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) was employed to detect and analyze the products collected in the volatiles condensing dish, with the results presented in

Table 5. Impurity content of enhanced products of lead–bismuth alloy after 1223 K and 1323 K, 3 h high-temperature gasification.

Element (%)	Bi	Ca	Mg
Initial content	4.85	0.5	0.66
Product content (at 1223 K)	2.74	0.012	0.047
Product content (at 1323 K)	1.81	0.0045	0.0077

.

As illustrated in Table 5, at a gasification temperature of 1223 K, the concentration of the bismuth impurity in the product decreases from 4.85% to 2.74%. This observation aligns well with the X-ray diffraction pattern of the residue obtained via the high-temperature gasification of the enhanced product. An increase in the holding time leads to an intensified molecular–thermal motion of the enhanced product, resulting in the dissociation of the intermetallic compound CaMg₂Bi₂ into the BiCa₂ phase under vacuum conditions. Concurrently, the impurity magnesium is gradually vaporized and volatilized, becoming concentrated in the condensing cover. The concentrations of calcium and magnesium in the product decreased from 0.5% and 0.66% to 120 ppm and 470 ppm, respectively, yielding removal rates of 97.6% and 92.88%. The notably high calcium removal rate of 97.6% can be attributed to the fact that the BiCa₂ compound does not vaporize, but instead remains in the bottom graphite crucible, thereby facilitating the effective separation of calcium from metallic lead. Following the dissociation of CaMg₂Bi₂ into a singular substance, auxiliary magnesium was vaporized in substantial quantities and subsequently condensed in the upper condensing dish. As the duration of elevated temperature increased, the temperature within the furnace remained relatively constant. The auxiliary magnesium experienced a gas-liquid phase transition within the condensing cover and the condensing dish, undergoing a reciprocating cycle, ultimately solidifying in the condensing dish during the cooling phase.

Following the vacuum gasification conducted at a temperature of 1323 K for a duration of 3 h, only the BiCa₂ phase was identified in the residue of the lead–bismuth alloy. The original intermetallic compound, CaMg₂Bi₂, was completely transformed and fully dissociated into the BiCa₂ phase, which became increasingly prominent in the residue as the holding time was extended. Additionally, the auxiliary magnesium was found to be concentrated in the condensate as a result of high-temperature gasification, correlating with the increase in holding time. ICP-MS was employed to detect and analyze the products collected in the volatiles condensing dish, with the results presented in Table 5.

As illustrated in Table 5, at a gasification temperature of 1323 K, the concentration of the bismuth impurity in the product decreases from 4.85% to 1.81%. This reduction represents a significant increase in the removal rate compared to high temperature gasification at 1223 K. The primary explanation for this phenomenon is that with an increase in holding time, the molecular-thermal motion of the conversion products, derived from the lead–bismuth alloy adjuvant, is enhanced. Consequently, the intermetallic compound CaMg₂Bi₂ undergoes complete dissociation into the BiCa₂ and Mg phases. Additionally, as the holding time increases, the adjuvant magnesium gradually vaporizes and volatilizes, becoming concentrated in the condensation cap. The concentrations of calcium and magnesium introduced into the product decrease from 0.5% and 0.66% to 45 ppm and 77 ppm, respectively, resulting in removal rates of 99.1% and 98.8%. The primary reason for the observed phenomena is that as the gasification temperature increases from 1223 K to 1323 K, a significant quantity of the lead–bismuth alloy auxiliary conversion products volatilizes. During this process, the principal metal, lead, vaporizes alongside the auxiliary elements such as calcium and magnesium, subsequently accumulating in the condensing dish. As the duration of the temperature-holding period extends, the temperature within the furnace approaches equilibrium. Lead, calcium, and magnesium within the condensing cap and plate undergo cyclical gas-liquid phase changes until the conclusion of the insulation stage. In the cooling phase, lead, calcium, and magnesium transition from a gaseous to a liquid state. As the temperature continues to decrease, calcium and magnesium undergo further phase changes from liquid to solid, condensing on the condensation cover. In contrast, due to its distinct physical properties, lead remains in a liquid state and collects at the top of the condensation tray, thereby facilitating the efficient separation of lead, bismuth, calcium, and magnesium. The observed higher removal rate of auxiliary calcium, compared to auxiliary magnesium, can be attributed to the formation of the BiCa₂ compound, which, after prolonged exposure to high-temperature gasification, does not evaporate, but instead remains in the bottom of the graphite crucible. This phenomenon allows for a more effective separation of the principal metal, lead, from the auxiliary calcium. The impact of temperature on the removal of impurities from the lead–bismuth spray-enhanced products is illustrated in Figure 4, while Figure 5 presents a schematic representation of the vacuum vaporization of lead–bismuth alloy auxiliary conversion products at 1323 K.

With the substantial increase in the gasification temperature during the vacuum gasification, within the temperature range of 1223 K to 1323 K, and under a holding time of 3 h, the lead auxiliary conversion material was nearly entirely volatilized, resulting in the gasification of the product. CaMg₂Bi₂ underwent a complete dissociation, yielding BiCa₂ and magnesium as the resultant substances. Notably, BiCa₂ did not vaporize and became concentrated in the bottom residue. Only the impurity magnesium and lead, the primary metal, were simultaneously vaporized and volatilized to the condensing layer. The gaseous molecules of lead and magnesium underwent a phase transition upon contact with the upper condensing layer, rapidly transitioning from a gaseous to a liquid state and subsequently dripping from the upper condensing cover to the lower condensing pan. As the holding time increased, the temperature within the furnace approached equilibrium. At this juncture, the liquid lead and liquid magnesium present in the condensing pan were vaporized and re-condensed from the lower pan to the upper condensing cover, thereby perpetuating the cycle. Upon the conclusion of the insulation stage, as the cooling process within the furnace progressed into the condensation stage, the magnesium vapor that has accumulated on the upper condensing cover underwent direct solidification due to the significant drop in temperature. In contrast, the primary metal, lead, remained in a liquid state, owing to its relatively low melting point, and continued to flow to the lower condensation plate during the condensation phase. Consequently, the lead became increasingly concentrated in the condensation plate. The auxiliary magnesium was concentrated in the condensation cap, whereas the majority of the bismuth impurity was found in the bottom residue. A significant separation of lead from bismuth, calcium, and magnesium was accomplished through the process of enrichment conversion via vacuum gasification. Following a high-temperature gasification experiment conducted at a temperature of 1323 Kand, a system pressure ranging from 2 to 5 Pa, for a duration of 3 h, the concentration of bismuth impurity in the lead on the condensate disc decreased dramatically to 1.4 ppm. This resulted in a bismuth impurity removal rate of 76.6%, with the direct yield of the product reaching 71%.

4. Conclusions

This paper presents a novel approach to the purification of metallic lead, grounded in the principles of vacuum metallurgy, and addressing the significant challenges associated with the vacuum separation of metallic lead from the bismuth impurities. The proposed method involves the use of auxiliary conversion through vaporization and volatilization. The conversion reaction between auxiliary agents, specifically calcium and magnesium, and the impurity bismuth, is enhanced through the application of blowing and stirring techniques using argon. This process facilitates the transformation of bismuth impurities in metallic lead into complex compounds that exhibit characteristics distinct from lead. Subsequently, the bismuth is effectively separated via vacuum vaporization, resulting in the production of high-purity lead. This method holds considerable importance for the high-quality advancement of the lead industry, the energy sector, and the new materials industry.

The primary conclusion of this study is that the product of the reaction between the calcium and magnesium adjuvants and the bismuth impurity is CaMg₂Bi₂, according to the Gibbs free energy calculation, which undergoes a transformation into BiCa₂ under specific conditions, namely under a system pressure ranging from 2 to 5 Pa, a gasification temperature of 1323 K, and a holding duration of 3 h. BiCa₂ is characterized by its non-volatility, and it accumulates as residue within the crucible. A minor quantity of the bismuth impurity continues to vaporize, resulting in a reduction in its concentration from 4.85% to 1.81% in the volatile substance. The auxiliary calcium is entirely converted into CaSe and CaTe, leading to a decrease in the calcium content in the volatile substance from 0.5% to 16 ppm. Additionally, the magnesium content in the volatile substance diminishes from 0.66% to 187 ppm. Notably, the primary metal, lead, evaporates in substantial quantities during the vacuum gasification process, and is subsequently collected as a solid in the condensing dish. The bismuth content in the final product is reduced from 6 ppm to 1.4 ppm, while the yield of metal straightening is 71%, thereby facilitating the efficient separation of lead from impurity bismuth.