Study on continuous crystallization and purification of bismuth from Pb-Bi alloy

The crude bismuth obtained from the smelting of bismuth concentrate and bismuth-containing by-products contains 5%∼20% lead, which needs to be removed during the refining of crude bismuth. Due to the similar physical and chemical properties and metallurgical behavior of lead and bismuth, it is not easy to perform the separation by conventional methods. Although the conventional process of chlorinating crude bismuth to remove lead can meet the requirements of the refining process, the amount of chlorinated slag produced is large, and the safety of chlorine gas is difficult to control, making it difficult to achieve clean production. In this paper, single-factor experiments and semi-industrial experiments for the separation and purification of bismuth from Pb-Bi alloy were carried out by the continuous crystallization method, and the results showed that the bismuth content of Pb-Bi alloy increased from 90.24% to 98.98%, the direct yield of bismuth was 87.37%, the metal recovery rate was 99.84%, and the slag production rate was 0.62%. In comparison with existing processes and studies, the method is low-cost, clean, and efficient, has a wide treatment range, can be operated continuously, and can be easily industrialized and promoted. Theoretically, this method, combined with the process of vacuum distillation to treat Pb-Bi binary alloys with greater than 55.5% bismuth content, results in a 99.99% bismuth product.


Introduction
Lead (Pb) and bismuth (Bi) are located next to each other in the periodic table, have similar physicochemical properties, and are mostly associated with or co-occur in various complex minerals in nature as sulfides, oxides, or complexes [1][2][3][4]. Pb and Bi behave identically during metallurgy. In the Pb smelting system, the fire refining of Pb produces Ca-Mg-Bi slag, and the oxidation blowing of Pb anode sludge produces Bi oxide slag [5,6]. Other heavy non-ferrous metal smelting systems also produce soot or slag containing Pb and Bi; these Bi-containing by-products will enter the Bi smelting system as raw material for smelting Bi and Bi oxide slag due to their high Bi content [7][8][9]. Bi oxide slag, due to its high content, is melted together with Bi concentrate by reduction to produce crude Bi [10]. Ca-Mg-Bi slag, Cu converter soot, and other Bi-containing by-products have low Bi content, and further enrichment is required to obtain crude Bi by reduction smelting again. Generally, the Pb content in the crude Bi obtained by fire-method crude smelting is in the range of 5%∼20% [11].
Chlorination to remove Pb and Zn is the main means used in the crude Bi fire refining process, which involves passing chlorine gas into the liquid crude Bi after adding Zn and removing Ag [12], so that the Pb and Zn are converted into chlorinated slag, which floats to the surface of the melt, and the Pb and Zn are removed Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. from it after the slagging operation. The Pb content in the Bi liquid after the removal of Pb and Zn by chlorination is between 0.001 and 0.01%, and the removal effect is obvious, but the amount of chlorinated slag produced is large, the Pb slag obtained contains about 20% Bi [13], and the safety of chlorine gas is difficult to control [14].
Based on the above problems, metallurgists have carried out in-depth studies on the separation of Pb-Bi alloys. Zhang Cheng conducted a theoretical study on the gas-liquid phase equilibrium of Pb-Bi alloys and concluded that the vacuum distillation temperature interval of Pb-Bi alloys was less than 20 K. The interval separating the gas and liquid phase lines on the left and right sides of the azeotropic composition was not large, and it was theoretically extremely difficult to achieve the separation of Pb-Bi alloys by vacuum distillation [15]. Chen Feng treated a Pb-Bi alloy with a Pb content of 2.01%, and the Pb content of the volatiles was reduced from 2% to 0.01% by two-stage vacuum distillation at a temperature of 850°C, a residual pressure of 40 Pa, a material volume of 30 g, and a time of 40 min, which proved that the Pb-Bi alloy could be removed from the Pb by vacuum distillation, but the Pb content in the raw material must be below 2% [16]. Guo Weizhong enriched the Pb at the bottom of the crude Bi by using both rising and falling melt directional crystallization, and the Pb content was enriched from 2.12% to 11.2% [17]. Wen Xiaochun separated Pb-Bi alloys with 5% and 15% Pb content by supergravity melt precipitation at 130°C, with gravity coefficient of 600, separation time of 2 min, and holding time of 30 min, and the Bi content of the Bi-rich phase was 99.53% and 99.37%, respectively [18]. Ma Aijun used the electrolytic method to treat Pb-Bi alloy, controlling the concentration of Pb in the electrolyte PbSiF 6 at 100 g l −1 , the concentration of silicofluoric acid at 100 g l −1 , the current density at 80 A m −2 , the anode distance of 50 mm, and the electrolysis temperature at 35°C. The purity of cathode Pb was 99.7%, the recovery of Pb was more than 99.8%, and the enrichment of Bi in the anode sludge was 96% [19]. Zhao Zhupeng leached Pb-Bi alloy produced during the production of antimony white by the nitric acid method under the conditions of 25% nitric acid concentration, 90°C reaction temperature, 2 h reaction time, and a 5:1 liquid-solid ratio, where the recovery of Pb reached 99.2% and the recovery of Bi reached 99.2% when the dosage of sodium sulfate was 1.1 times the theoretical dosage [20]. The above studies suffer from low adaptability of raw materials, poor separation effects, and an inability to operate continuously.
The continuous crystallization method is an efficient and clean new purification technique that can effectively purify Bi from Pb-Bi binary alloys in the form of crystals. The method has the advantages of low cost, being clean and efficient, having a wide processing range, and being continuous working compared to the existing technology, which is a guideline for the purification of Bi from Pb-Bi binary alloys.

Experimental materials
The experimental raw material is a certain company 99.9% Pb ingot and 99.95% Bi ingot configuration of Pb-Bi alloy, with a Pb content of about 10%, divided into six configurations. Table 1 shows the specific composition of raw materials, where numbers 1-3 are the raw materials for single-factor experiments, and numbers 4-6 are the raw materials used in the best conditions verification experiments.

Experimental equipment
The equipment used for the experiments was a home-made liquid-solid separator, as shown in figure 1, containing an electric motor, a propeller, a crystallization depression tank, a melting and analysis furnace, a liquid pot, and a crystal pot. The main part is the crystallization depression tank, the bottom of which consists of a resistance heater and a thermocouple. The crystallization depression tank has six temperature zones, with the temperature gradually increasing from the tail to the head, forming a stable temperature gradient, with the highest temperature occurring at the head of the crystallization depression tank. An electric motor is connected to the propeller, which rotates the propeller and has a number of spiral blades evenly spaced on the propeller. The support body supports the crystallization depression, the motor, and the propeller at a certain angle to the ground, and the angle of inclination of the crystallization depression can be adjusted by adjusting the height of the support body.

Experimental process
Due to the limitations of the furnace capacity, the furnace can melt 65 kg of Pb-Bi binary alloy at a time, so the raw material for the small-scale experiments is 65 kg of Pb-Bi binary alloy. Prior to the start of the experiment, the raw material was added to the furnace and melted at 320°C while the liquid-solid separator was preheated, and the parameters were adjusted. The temperature was set to 195°C, 215°C, 235°C, 255°C, 275°C, and 295°C from the tail to the head of the crystalline recessed trough, respectively, at a speed of 1 r min −1 and an angle of 6°. The temperature settings were based on the phase diagram and the experience of the previous experiments, where water spraying at 195°C at the tail of the tank cools down the liquid but does not cause solidification, and 295°C at the head of the tank allows the Bi content of the crystals to be higher than 98%. After the actual parameters of the liquid-solid separator reached the above mentioned parameters, the experiment started.
(1) Initially, 20 kg of raw material was taken from the melt analysis furnace and added to the liquid-solid separator. After waiting for about 10 min so that the heat transfer to the melt inside the crystallization depression tank was completed and the temperature stabilized, water was sprayed at the end of the crystallization depression tank to cool it down, while the liquid phase at the end of the crystallization depression tank was slowly released into the liquid pot.
(2) About 5 kg of Pb-Bi alloy was added at the tail of the crystallization depression tank every 10 min while water was sprayed to cool it down, so that the concentration of the liquid phase inside the crystallization depression tank could be kept in equilibrium.
(3) After some time, Bi crystals formed at the trough's head and were dropped into the crystal pot, where the amount of water sprayed was adjusted based on the size of the liquid phase return flow inside the crystallization depression trough.
(4) There was a wait of about 10 min after adding all the raw materials to the inside of the crystallization depression tank to ensure that the crystals inside the crystallization depression tank were output, then the rotational speed was adjusted to 0, the temperature of each section was set to 350°C, and the residual materials were released inside the crystallization depression tank. The experiment was thus completed.

Analysis and characterization
Inductively coupled plasma-atomic emission spectrometry (ICP-AES, OPTIMA 8000, PerkinElmer Corporation, America) was used for the quantitative determination of the elemental contents in the obtained samples. The phases of the obtained samples were examined by x-ray diffraction (XRD, D8 VENTURE, Bruker AXS, Germany) employing a scan speed of 0.02°s −1 in the 2-theta range and using a Rigaku/D-Mac-3c x-ray diffractometer with Cu Kα radiation. The elemental distribution of the samples was analyzed using an scanning electron microscope (SEM, JSM-7800F, JEOL, Japan) at an accelerating voltage of 20 kV and a beam current of 2 × 10 -9 A.

Continuous crystallization process
The essence of continuous crystallization is a continuous solid and liquid phase transformation process that utilizes the temperature-composition relationship of the eutectic phase of the binary alloy. Within the eutectic phase of the binary alloy, the solid and liquid phase compositions change with temperature, as shown in figure 2, with point A being the highest point of solid phase composition, point B being the highest point of liquid phase composition, point C being the point of eutectic composition, t 0 being the temperature of the eutectic point, and the temperature gradually increasing from t 0 to t max . The continuous crystallization process begins, and the crystals gradually precipitate as the liquid cools. Under the propulsion of the propeller, the crystals move from t 0 to the regions of t 1 , t 2 KKt n , and t max ; under the action of gravity, the liquid produced by the melt precipitation flows towards the regions of t max , t n KKt 2 , and t 1 . The equilibrium between the crystal and the liquid is disrupted when the crystal passes through the region above the crystal's own temperature, and the crystal will melt out different components of the liquid l 1 , l 2 , KK, l n , respectively. The crystal melt analysis makes the crystal itself increase in purity, and the molten liquid flows back towards the low temperature region, crystallizing again different components of crystal s1, s2, KK, sn. These liquids, which run in the opposite direction to the crystals, not only exchange heat with the next level of crystals, but also mass. Crystals and liquids of the same composition establish a new equilibrium at a certain temperature, while crystals and liquids of different compositions maintain a dynamic equilibrium during the solid-liquid phase transition. The disruption and establishment of this crystal and liquid equilibrium allow for a continuous process of crystallization and fusion precipitation.

Balanced distribution factor
As can be seen from figure 3, the Pb-Bi alloy has a eutectic point C with a eutectic temperature of 124°C, and a eutectic composition of 44.5 wt% Pb + 55.5 wt.% Bi. To the right of the eutectic point, the higher the Bi content, the further the solid-liquid line is from the liquid-phase line, and the easier it is to achieve solid-liquid separation. Theoretically, for Pb-Bi binary alloys with greater than 55.5% Pb, 99.5% Bi content can be achieved by solid-liquid separation. The separation of the main metal from the magazine metal is based on the principle of unequal distribution of impurities between the solid-liquid phase [21][22][23] in order to achieve the purpose of impurity removal. The partitioning can be measured by the equilibrium partition coefficient K , 0 which can be defined as the ratio of the impurity concentration c s in the solid phase to the impurity concentration c l in the liquid phase and is given by the following equation.  For Pb-Bi binary alloys with greater than 55.5% Pb, the equilibrium partition coefficient of Pb in Bi is much less than 1, indicating that the Pb in the crystal is much lower than in the liquid. As long as the crystals and the liquid can be separated, the crystals can be purified.

Results and discussion
Bi was the target separating element for this experimental study, and the end product of the recovery of Bi from the Pb-Bi alloy was Bi crystals, the optimum parameters of which were obtained through a number of empirical experimental studies. In addition, the Bi direct yield and metal recovery rate were calculated by equations (6) and (7), and the slag rate for the whole experiment was calculated by equation (8).  band when the length of the crystallization recess is fixed. The effect of rotational speed on the elemental content and direct yield of Bi crystals was investigated at a trough head temperature of 295°C and an inclination angle of 6°, and the results are shown in figure 4. At a speed of 1 r min −1 , the crystals stay inside the crystalline depression for a long time, the crystals absorb more heat, and the crystals melt and precipitate a lot of liquid with high Pb content, which flows back towards the end of the tank, separating the liquid from the crystals, and the crystals are purified. The Bi crystals produced at the head of the trough contained 98.62% Bi and 1.38% Pb, but most of the crystals continued to melt and precipitate liquid as they moved towards the head of the trough, and the number of crystals was decreasing, so the yield of Bi crystals at the head of the trough was low and the Bi direct yield was only 12.23%. When the speed was increased from 1 r min −1 to 3 r min −1 , the Bi content and Pb content of the Bi crystals did not change much, but the crystal melting and refluxing inside the crystalline depression tank decreased, the crystal volume increased significantly, the output of Bi crystals at the head of the tank increased, and the Bi direct yield increased to 58.94%. The speed of rotation increased from 3 r min −1 to 6 r min −1 , the residence time of crystals inside the crystallization depression tank decreased, the crystals did not get enough heat for melt and precipitation reflux, the crystal Pb content increased from 1.30% to 4.92%, but the amount of crystals produced was greater than the liquid reflux, the Bi crystal yield increased, and the Bi direct yield increased to 69.42%. In the case of Bi crystals with a Bi content of 98% or more, the point with the highest direct yield, i.e., the speed of 3 r min −1 , was chosen as the best speed condition.

Effect of tank head temperature
The head of the tank is the end where the Bi crystals are obtained and is the last stage in the Bi crystallization process to remove the Pb. The temperature of the head of the tank affects the purity and yield of the final Bi crystals. Theoretically, the purity of the Bi crystals is at its highest when the temperature at the head of the tank reaches 271.5°C. Due to the presence of only crystals at the head of the trough, heat dissipation is fast, and the temperature difference between the upper and lower parts of the crystalline depression trough is large. The actual temperature of the upper crystals does not reach 271.5°C and tends to fluctuate, so the actual temperature of the lower part of the trough head needs to be higher than 271.5°C . The effect of the head temperature on the elemental content and direct yield of the Bi crystals was investigated at a rotational speed of 3 r min −1 and an inclination angle of 6°, and the results are shown in figure 5. During the increase of the bath head temperature from 295°C to 315°C, the Pb content in the Bi crystals decreased from 1.94% to 1.18%, the Bi content increased from 98.06% to 98.82% and the Bi direct yield increased from 59.64% to 61.09%. The overall elemental content and the Bi direct yield were high and did not change significantly. This is due to the heat equilibrium inside the crystallization depression; the heat released during the nucleation process is basically equal to the heat absorbed by the crystal melting and precipitation, and only a small portion of the output crystals are melted back, resulting in high purity and yield of Bi crystals and a naturally high direct yield. The temperature of the head of the tank was increased from 315°C to 345°C, and the actual temperature of both the upper and lower layers of the head of the tank exceeded 271.5°C. The Bi crystals were melted and refluxed in large quantities, and the yield decreased. The Bi direct yield decreased to 19.93%. The contact surface between the residual Bi crystals and air in the head of the tank increased, and the Pb mechanically trapped in the crystals was oxidized, the Bi content decreased to 93.98%, and the Pb content increased to 6.02%. In summary, 315°C was selected as the optimum bath head temperature condition.

Effect of inclination
From the equilibrium distribution coefficient, it can be seen that when the solid and liquid phases reach equilibrium, the crystal Pb content is much smaller than the liquid Pb content. In order to ensure the purity and yield of Bi crystals, the liquid return flow must be controlled. The effect of tilt angle on the elemental content and direct yield of Bi crystals was investigated at a speed of 3 r min −1 and a temperature of 315°C at the head of the tank, and the results are shown in figure 6. When the inclination was increased from 6°to 7°, the Bi content, Pb content, and the Bi direct yield did not change much. This is due to the small inclination; most of the liquid produced by crystal melting and precipitation only refluxed for a short distance before crystallising again, so the Pb content in the crystal was high, above 4%, but the amount of crystals inside the crystallisation depression tank was large, the Bi crystal yield at the head of the tank was high, and the Bi direct yield was high. After increasing the inclination from 7°to 8°, the Bi content of Bi crystals increased from 95.65 to 98.82% and the Pb content decreased from 4.35% to 1.18%, which was due to the increase in liquid return flow with the increase in inclination. The liquid with high Pb content flowed back and the Bi crystals were further purified, but the yield at the head of the tank was slightly reduced and the Bi direct yield remained basically the same. When the inclination was increased from 8°to 10°, the liquid reflux was obvious: the number of crystals inside the crystalline recessed trough decreased, the amount of oxidation in the Bi crystals produced from the trough head  increased, the Bi content decreased to 97.46%, the Pb content increased to 2.54%, and the Bi direct yield decreased from 61.09% to 34.82%. In summary, the highest Bi content and the Bi direct yield were obtained at an inclination of 8°.

Verification of optimal experimental conditions
Under the optimum experimental conditions of 3 r min −1 speed, 315°C head temperature, and an 8°tilt angle, the experiments were repeated three times, as shown in table 2 and the results showed that the Bi content in the Bi crystals was above 98.5% and the Bi direct yield was above 60%, indicating that the continuous crystallization and purification of Bi from Pb-Bi alloy was feasible. The reason for the low direct yield of Bi was that the experiment was intermittent, a large amount of raw material remained inside the crystallization recess after the output of Bi crystals, and the ratio of Bi crystals to residual material was small, which would affect the purity of Bi crystals if the output continued. The feedstock and Bi crystals from the validation experiments were observed by scanning electron microscopy (SEM) to compare the microscopic morphological states of the feedstock and Bi crystals. As shown in figure 7, the areas of Bi and Pb in the feedstock are highly overlapping, indicating that Bi and Pb have a strong bond and can easily form a stable alloy state. The results of the spot scan reveal that the Pb content is low at the locations where the surface of the Bi crystal particles is flat and smooth (figure 7(f)) and high at the locations where the surface of the Bi crystal is uneven ( figure 7(e)). In this regard, XRD analysis was carried out on the raw material and the Bi crystals from the three validation experiments. As shown in figure 8, figure 8(a)    the Bi crystals will be wrapped by oxides when they are produced. However, PbO and Bi 2 O 3 cannot be melted back, so the percentage of oxygen in the Bi crystals increases and the percentage of Bi in the Bi crystals decreases. A temperature gradient from 195 to 315°C existed from the tail to the head of the tank. Under the optimum experimental conditions of 3 r min −1 , 315°C at the head of the tank, and an inclination angle of 8°, the changes of crystal morphology and elemental content with temperature inside the crystalline depression tank were investigated. As shown in table 3, the Pb content decreased from 14.12% to 1.76%, and the Bi content increased from 85.88% to 98.24%. The Pb was enriched at the end of the trough, and the crystals were continuously purified as the temperature increased.
As shown in figure 9, it can be seen from the SEM that in the temperature region of 195°C, the crystalline depressed troughs have a high Pb content of crystals inside, with a small number of crystals mixed with the liquid and no obvious regular morphology. Under the action of the propeller, the liquid containing the crystals enters the temperature region of 215°C and 235°C, where there is continuous successful nucleation of crystals in the liquid, and the crystals that have completed nucleation continue to grow in the liquid. As the crystals grow into the 255°C temperature range, there is only a small amount of liquid at the bottom of the crystalline depressions, and most of the crystals are only in the process of fusion precipitation. The crystals slowly break free of the Pb, and the surface starts to become regular and shaped. As the crystals enter the temperature region of 275°C and 315°C, some of the crystals reach the melting point of Bi, and the outer layers of the crystals melt and flow backwards, taking with them a large amount of surface-wrapped Pb, giving the appearance of an increasingly regular rhombohedral structure.    Compared with the single factor experiment, the semi-industrial experiment could purify Bi to 98.98%, and the Bi direct yield increase from 61.09% to 87.37% ; the metal recovery was 99.84% and the slag yield was 0.62%.

Applicability and economy
As can be seen from table 6, the traditional chlorination process for Pb removal produces 13∼20% chlorination slag, and there is a risk of chlorine leakage. Compared to the traditional process, this method produces a low slag rate, is safe, environmentally friendly, clean, and efficient.
Compared to electrolysis and vacuum distillation, this method is more adaptable to raw materials and consumes less energy; compared to supergravity fusion analysis and directional crystallisation, this method is more adaptable to raw materials and can be operated continuously. In summary, the continuous crystallization method has the advantages of low cost, high efficiency, and wide treatment range for the treatment of Pb-Bi binary alloys with Bi content greater than 55.5%, and it is easy to realize the industrialization and promotion of this method.

Conclusion
Using theoretical analysis combined with experiments, the influence of various factors on the process of continuous crystallization for the separation and purification of bismuth from Pb-Bi alloys was studied, and the following conclusions were reached: (1) In theory, for Pb-Bi binary alloys with a bismuth content of greater than 55.5%, the continuous crystallization method can increase the bismuth content to 99.5%, with wide adaptability to the raw material.
(2) Through the single-factor experiment of continuous crystallization of Pb-Bi alloy to separate and purify bismuth, the best experimental conditions were determined: the rotational speed was 3 r min −1 , the temperature of the head of the tank was 315°C, the tilt angle was 8°, the bismuth content of bismuth crystals was increased from 90% to 98.82%, and the direct yield of bismuth was 61.09%.
(3) Compared with the single factor experiment, the semi-industrial experiment can increase the bismuth content of bismuth crystals to 98.98%, the bismuth direct yield from 61.09% to 87.37%, the metal recovery rate to 99.84%, and the slag yield to 0.62%. Compared with the traditional process and existing technology, the method is low cost, clean, and efficient; it has a wide treatment range; can be operated continuously, and can be easily industrialized and promoted.
(4) Theoretically, the method combined with the process of vacuum distillation to treat Pb-Bi binary alloys with greater than 55.5% bismuth content can yield 99.99% bismuth products.