Ironmaking
Thermodynamic Simulation and Transformation Behaviours of Lead during Iron Ore Sintering
2021 Volume 61 Issue 11 Pages 2731-2736
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2021 Volume 61 Issue 11 Pages 2731-2736
Lead (Pb) increases the corrosion rate of refractory lining in blast furnaces. The iron ore sintering process plays a vital role in decreasing lead content of the feeding materials for blast furnaces. In this paper, lead compounds in the raw materials blend and the sinter product were leached out sequentially and classified into four categories. In the raw materials blend, PbS accounted for about 66.25% of lead compounds and 23.57% of lead was found as PbO. While in the sinter product, the main lead compounds were PbSO4, PbO·Fe2O3 and PbO. The transformation and removal behaviours of lead compounds were simulated via Factsage7.1 and tube furnace tests. It was confirmed Pb could be removed in forms of PbCl2, metallic Pb and PbO during iron ore sintering process. The lead removal rate in the industry was 49.77%. While, with the increasing carbon to 15%, over 93% of the lead was removed in lab. In addition, increasing the maximum sintering temperature and CaO content properly could also enhance the removal of lead.
The crude steel production in China has been experiencing a steady increase in recent 20 years and reached 1053 million tonnes in 2020.1) And over 90% of the crude steel is produced through BF-BOF route.2) The blast furnace iron-making (BF) and the basic oxygen furnace steel-making process (BOF), are the two main stages of the steel production.3) During the iron-making process, lead compounds are reduced easily into metallic Pb.4,5) On the one hand, Pb penetrates into gaps and fine pores of the refractory lining in the bottom part of the blast furnace, which leads to internal stress and cracks of the lining.6) On the other hand, Pb vapor condenses on the upper area of the blast furnace, leading to accretions formation.7) Therefore, Pb severely decreases the lifetime of refractories and is unfavourable in the blast furnace.8,9) Since 70%–75% of burden materials in the blast furnace is sintered iron ore fines, the iron ore sintering process is the vital process to decrease the lead content in burden materials of the blast furnace.10,11,12) However, the secondary iron-containing resources, which usually contain excess lead, are used in the iron ore sintering process for iron recovery and cutting down waste discharge.13,14,15,16) Especially, municipal solid waste incineration fly ash, which contains high level of heavy metals,17,18) has also been fed to the iron ore sintering process recently.19,20) Given the situation, it becomes more difficult to produce sinter product with qualified lead content for the blast furnace.
Long et al. and Das et al., studied the recovery of lead from the sinter dust and they found the lead was in form of PbO.21,22) While Sammut et al. considered the lead was associated with carbonates in the sinter dust and PbCO3 severely affects the environmental and human safety.23) Lead was also found in the sinter dust by Sheng et al. and Tsai et al.24,25) Fan et al., observed PbCl2 containing PM2.5 in the flue gas of the sintering process.26) Song et al., characterized the gas cleaning system residue of the sintering process. It was found the Pb content reached 4% in the residue.27) Kuo et al., indicated that Pb was one of the top five components of windbox dust during the sintering process.28) All these research indicate that lead can be removed into flue gas to form sinter dust during iron ore sintering process. However, the removal rate of lead was quite low during the conventional sintering process.29) To enhance the removal of lead and decrease the lead content in the sinter product, it is of great significance to investigate the phase transformation and removal behaviours of lead compounds during iron ore sintering process.
In this paper, the lead compounds in the raw materials blend and the sinter product of an industry are leached out sequentially, which is the first shot in the iron ore sintering process by far. The transformation and removal behaviours of four categories of lead compounds are simulated by Factsage7.1 and horizontal tube furnace tests. The removal paths of lead are analysed and concluded, which can be a clue for the practical sintering process.
The raw materials blend and the sinter product of an iron-making industry in south-eastern China were analysed via X-ray fluorescence spectrometry (XRF), chemical analysis and inductively coupled plasma atomic emission spectrometer (ICP-AES). The chemical composition and lead content of the raw materials blend and the sinter product are listed in Table 1. The total iron of the raw materials blend was 49.55% and increased to 55.32% in the sinter product. Meanwhile, the content of other main components, i.e., SiO2, CaO, MgO and Al2O3, also increased during iron ore sintering process. However, the lead content decreased from 0.0126% in the raw materials blend to 0.0073% in the sinter product.
| Sample | TFe | SiO2 | CaO | MgO | Al2O3 | Pb |
|---|---|---|---|---|---|---|
| Raw materials blend | 49.55 | 4.84 | 8.59 | 1.82 | 2.18 | 0.0126 |
| Sinter product | 55.32 | 5.79 | 9.72 | 2.07 | 2.47 | 0.0073 |
The raw materials blend and the sinter product from the industry were leached out sequentially to classify lead compounds into four categories, as shown in Fig. 1. In each step, the supernatant was diluted to the constant volume (100 mL) with deionized (DI) water in a plastic volumetric flask for the following ICP detection. The leaching residue was leached further in the next step. In the last step, the raw materials blend or the sinter product was totally dissolved. Finally, four types of lead-containing solutions in plastic volumetric flasks were measured by ICP-AES. In other words, water soluble PbCl2, ammonium acetate soluble PbCO3/PbSO4, acetic acid-ammonium acetate soluble PbO and aqua regia soluble PbS/PbO·Fe2O3, were leached out and measured quantitively. The ICP measuring results were transferred into weight percentage.
The sequential extraction procedure. 1. vibrating water bath, 2. conical flask, 3. deionized water, 4. sample, 5. plastic volumetric flask, 6. 2 M ammonium acetate solution, 7. water leaching residue, 8. acetic acid-ammonium acetate solution, 9. ammonium acetate leaching residue, 10. aqua regia solution, 11. acetic acid-ammonium acetate leaching residue, 12. polytetrafluoroethylene crucible.
As shown in Fig. 2, 0.2 g of the raw materials blend was digested by the mixture of 9 mL HCl, 3 mL HNO3, 3 mL HF and 5 mL DI water in a 50 mL polytetrafluoroethylene crucible on a 270°C heat plate for 2.5 hours. Then, the digested solution was diluted to the constant volume (100 mL) with DI water in a plastic volumetric flask. Finally, the solution in the volumetric flask was measured by ICP-AES. Similarly, 0.2 g of the sinter product was also digested and measured by ICP-AES.
The digestion of solid samples. 1. polytetrafluoroethylene crucible, 2. mixture of 9 mL HCl, 3 mL HNO3, 3 mL HF and 5 mL deionized water, 3. sample, 4. plastic volumetric flask.
The “Reaction” module of FactSage7.1 was applied to calculate the equilibrium constant and Gibbs energy of reactions. The “Predom” module was used to calculate the relationship between lead-containing phases and gas composition. During the calculation, the highest SO2 content of 0.05% and the lowest O2 of 8%, measured during the sintering process, were considered.
2.2.4. Tube Furnace ExperimentsChemically pure reagents Fe2O3, SiO2, CaO, PbO, MgO and Al2O3 were mixed according to the composition of the raw materials blend in Table 1. Then, the mixture was pressed into cylinders (diameter 1 cm, height 1 cm) and dried at 70°C for 24 hours. The dried cylinders were sintered in a horizontal tube furnace with air flow to study the influence of the maximum sintering temperature on the removal rate of lead. Similarly, chemically pure reagents Fe2O3, SiO2, CaO, PbO, MgO and Al2O3 with various carbon content were pressed into cylinders and sintered to study the effect of carbon ratio on the removal rate of lead.
The schematic of the horizontal tube furnace for sintering experiments is shown in Fig. 3. The inner diameter of the tube furnace was 50 mm and the air flow during the experiments was 6 L/min. Before sintering, an additional thermocouple was used to mark the position of temperature 300°C and 700°C in the furnace. During the sintering process, four dried cylinders were put in an alumina crucible. The crucible was heated for 3 minutes at 300°C and 700°C, respectively. Then the crucible was sintered for 4 minutes at the maximum temperature. After sintering, the crucible was pulled to the position of 700°C and held for 3 minutes. Finally, samples with the crucible were pulled out of the furnace and cooling down to the room temperature in air.
Schematic of sintering tests in the horizontal tube furnace.
The four categories of lead compounds, i.e., PbCl2, PbCO3/PbSO4, PbO and PbS/PbO·Fe2O3, were leached out and measured sequentially, as described in 2.2.1. The lead content as in each compound in the raw materials blend and the sinter product is shown in Table 2.
| Distribution of Pb | Raw materials blend | Sinter product |
|---|---|---|
| Percentage/wt% | Percentage/wt% | |
| Pb as in PbCl2 | 1.85 | 0.92 |
| Pb as in PbCO3/PbSO4 | 8.33 | 63.67 |
| Pb as in PbO | 23.57 | 16.13 |
| Pb as in PbS/PbO·2Fe2O3 | 66.25 | 19.28 |
| Total Pb | 100 | 100 |
Though PbS and PbO·2Fe2O3 are both aqua regia soluble compounds, the aqua regia soluble compound in the raw material blend was PbS because the raw materials blend were mainly natural ores and Pb tends to bind with sulphur in nature.30) In the raw materials blend, Pb as in PbS accounted for 66.25% of the total lead. And, 23.57% of the total lead was found in form of PbO. Pb as in PbCO3/PbSO4 and PbCl2 took a percentage of 8.33% and 1.85%, respectively. While in the sinter product, 63.67% of Pb was in form of PbSO4. Pb as in PbO and PbO·2Fe2O3 accounted for 16.13% and 19.28%, respectively. Besides, Pb as in PbCl2 decreased to 0.92%.
The lead removal rate was calculated as Eq. (1). In Eq. (1), η represents the lead removal rate. C1 represents the lead content in the sinter product and C2 represents the lead content in the raw materials blend. M1 and M2 represent the mass of the sinter product and the raw materials blend, respectively. The units of η, C and M are 1, wt% and gram.
| (1) |
As shown in Table 1, the lead content was 0.0126% and 0.0073% in the raw materials blend and the sinter product of the industry. The mass ratio of the sinter product and the raw materials blend (M1/M2) was 86.72%.16) According to Eq. (1), the lead removal rate in the industry was 49.77%.
3.2. Reaction of PbSThe Pb–S–O predominant area diagram is shown as Fig. 4. The shaded area `a` in the figure describes the atmosphere during the sintering process. As shown in Fig. 4, Pb is mainly in forms of PbSO4 and PbO·PbSO4 at 700°C. Therefore, PbS transforms into PbSO4 and PbO·PbSO4 in the preheating layer. PbO·PbSO4 is unstable and reacts with SO2, which generated by oxidation of sulphide ores, according to Eq. (2).
| (2) |
The predominant area diagram of Pb–S–O system at 700°C.
The PbCO3 decomposes into PbO when the temperature is higher than 318°C, as shown in Eq. (3). The PbSO4 also transforms into PbO by reacting with CaO when the temperature is higher than 900°C, as shown in Eqs. (4) and (5).
| (3) |
| (4) |
| (5) |
The indirect reduction of PbSO4 and FexOy is shown in Fig. 5. The CO/(CO+CO2) required for PbSO4 reduction decreases sharply with the increasing temperature. When the temperature is higher than 1170°C, the required CO/(CO+CO2) for PbSO4 reduction is lower than that for FexOy reduction. In addition, as the bright blue line indicating, CaO enhances the reduction of PbSO4 greatly. Therefore, part of PbSO4 transforms into PbO and Pb. However, nearly half of the PbSO4 enters the sinter product, as shown in Table 2.
Indirect reduction of PbSO4 and FexOy.
As described in 2.2.4, the chemical pure reagents Fe2O3, SiO2, CaO, MgO, Al2O3 and PbO were mixed and sintered to study the influence of the maximum sintering temperature on lead removal rate. The result is shown in Fig. 6. The lead removal rate increased from 31.76% to 45.40% with increasing the maximum sintering temperature from 1150°C to 1250°C. Further increasing the maximum sintering temperature to 1300°C and 1350°C led to a slight decrease of the lead removal rate. Therefore, Pb can be removed in forms of PbO and increasing the maximum sintering temperature properly is beneficial to PbO removal.
Effect of the maximum sintering temperature on the removal rate of lead in form of PbO.
The vapor pressure of Pb and PbO at various temperatures is shown in Fig. 7. The vapor pressure of Pb and PbO increases with the increasing temperature. The vapor pressure of PbO is 31986 Pa at 1350°C, which is higher than that of Pb (7092 Pa). However, PbO and PbSO4 are easily reduced into Pb during the sintering process. Consequently, the concentration of metallic Pb is high during the sintering process. Part of the metallic Pb can also be removed in the combustion zone where the temperature and CO concentration are high.
The vapor pressure of Pb and PbO at various temperatures.
Various amount of carbon and chemically pure reagents Fe2O3, SiO2, CaO, MgO, Al2O3 and PbO were mixed and sintered as described in 2.2.4. The result is shown in Fig. 8. Without carbon addition, the initial lead removal rate was 45.03% when the maximum sintering temperature was 1300°C. In this case, the Pb was purely removed in form of PbO. With increasing carbon ratio from 0% to 15%, the lead removal rate increased from 45.03% to 93.13%. When the carbon ratio was 15%, FexOy was reduced greatly to form metallic structure and PbO was also reduced into metallic Pb. Therefore, lead could also be removed in form of metallic Pb in strong reducing condition. The lead removal rate was about 93% when carbon ratio was between 15% and 25%.
Effect of carbon ratio on the removal rate of lead.
The residue metallic lead is oxidized by O2 as shown in Eq. (6). The temperature in the sinter layer is still high enough for PbO removal in a few minutes after the ending point of carbon combustion. As time went by, the temperature in the sinter layer decreases. Part of the residue PbO reacts with Fe2O3 as shown in Eq. (7) and enters the sinter product. The rest part of the residue PbO is entrapped into the sinter product directly.
| (6) |
| (7) |
The vapor pressure of PbCl2 at various temperature is shown in Fig. 9. The vapor pressure of PbCl2 increases sharply with increasing the temperature and reaches 101319.5 Pa at 953.25°C. As shown in Table 2, Pb as in PbCl2 declined from 1.85% to 0.92%. Thus, 75.06% of PbCl2 was removed directly by evaporation.
The vapor pressure of PbCl2 at various temperatures.
The phase transformation and removal behaviours of lead are summarized in Fig. 10. In the wet mixture layer (lower than 100°C), lead was in forms of PbCl2, PbCO3, PbS, PbO and PbSO4. While in the sinter product, lead was found in forms of PbO·2Fe2O3, PbSO4, PbO and PbCl2.
Transformation and removal behaviours of lead in four layers of the sintering process.
In the drying-preheating layer (temperature from 100 to 700°C), PbCO3 decomposed into PbO. And PbS was oxidized into PbSO4. In the combustion layer (temperature 700 – Tmax – 1200°C), Over half of PbSO4 transformed into PbO and Pb. Part of Pb and PbO, together with 75.06% of PbCl2 vaporized and entered the gas flow. When the PbCl2, Pb and PbO containing gas flow went through the underneath drying-preheating layer and wet mixture layer, PbO could condense as solid phase since the melting point of PbO is 888°C. Pb and PbCl2 could condense as solid phase or liquid phase because the melting point of Pb and PbCl2 is 327.5°C and 501°C, respectively. However, some of PbCl2, PbO and Pb could travel through all these underneath layers and enter flue gas.
With the combustion front moving forwards, the drying-preheating zone and wet mixture zone would be turned into combustion zone in sequence, the initial and trapped Pb, PbO, PbCl2 in these layers would vaporize and be removed into flue gas finally.
To be concluded, lead could be removed in forms of PbCl2, Pb and PbO during iron ore sintering process.
In this paper, four categories of lead compounds, i.e., water soluble PbCl2, ammonium acetate soluble PbCO3/PbSO4, acetic acid-ammonium acetate soluble PbO, aqua regia soluble PbS/PbO·Fe2O3 in the raw materials blend and the sinter product were classified and detected sequentially for the first time. The transformation and removal behaviours of lead compounds were simulated by Factsage7.1 and tube furnace tests. It was confirmed Pb can be removed in forms of PbCl2, Pb and PbO during the iron ore sintering process. The lead removal rate in the conventional sintering process is about 49.77%. Higher carbon ratio and temperature enhance the removal of lead. Increasing CaO content promotes PbSO4 transformation into Pb and PbO, which probably improve the lead removal rate.
The research was financially supported by the National Natural Science Foundation of China (51974371 and U1660206), and the National Key R&D Program of China (No. 2018YFC1900605).
