Extraction and kinetic analysis of Pb and Sr from the leaching residue of zinc oxide ore

Pb and its compounds are used in several applications, including Pb batteries, pyrotechnics, X-ray absorptive coatings, and Pb glasses, but the heavy metal is a hazardous element in the environment because of its toxicity to organisms [1]. The pollution generated by Pb has become a global issue [2]. Anthropogenic discharge of Pb should be strictly restricted. Because of the gradual depletion of Pb sulfide ores and the heavy pollution brought about by pyrometallurgical processes, exploitation of Pb oxidized minerals, including low-grade Pb–Zn ore, lead–acid-battery paste, and its secondary smelting residues, using hydrometallurgical routes employing various agents, such as acids, chlorides, and NaOH, has been paid increased attention [3–7]. Sr shows the lowest abundance among alkaline earth metals, and Sr compounds, such as Sr(NO₃)₂ and SrCl₂, are usually prepared from SrCO₃ obtained by celestite mineral [8–11]. SrCO₃, as one of the more important Sr compounds currently available, has been widely used in a variety of fields, including cathode ray tubes for TV imaging, photocatalysts for degradation of organic pollutants, and advanced ceramic materials [8,12]. Well-known extraction processes for Sr include the black ash and direct conversion methods [8,12–14]. Unfortunately, the high energy consumption required by the black ash method, as well as the heavy pollution it causes, has led to a loss of interest in its use [10,12–14]. Metallothermic methods employing Al or Mg have also been carried out [15–16]. In the direct conversion method, reagents containing ${\rm{CO}}_3^{2-} $ are employed to convert celestite to SrCO₃ [8–9,17–18]. Mechanochemical and hydrothermal technologies are commonly adopted to enhance the conversion ratio of SrCO₃ [19–22]. Zoraga and Kahruman [9] studied the conversion kinetics of celestite concentrate in an equimolar NH₄HCO₃ and NH₄COONH₂ system and found that SrSO₄ dissolution is the rate-controlling step. Bingöl et al. [8] reported the production of SrCO₃ by dry mechanochemical processing and obtained a conversion rate of 98.1% under optimum conditions.

Due to rapid development in industrialization, the demand for Zn has gradually increased. China is the largest producer and consumer of Zn worldwide [23]. Abundant reserves of zinc oxide ore are available in Yunnan Province, which is located southwest of China [23]. Pyrometallurgical routes are unsuitable for treating zinc oxide ores because of observable decreases in Zn grade and high energy consumption [24–25]. Thus, hydrometallurgical processes, such as H₂SO₄ leaching, alkaline leaching, and solvent leaching, have been extensively studied [26–30]. He et al. [31] reported that pressure leaching of high silica Pb–Zn oxide ore by H₂SO₄ could extract 98% Zn from the ore under optimum operating parameters. Zhang et al. [32] attempted to extract Zn from a low-grade zinc oxide ore with high silica contents by H₂SO₄ leaching in a water-starved system followed by water leaching; in this work, 99.22% Zn extraction and 0.56% Si dissolution were observed. Researchers [33–34] reported an improved hydrometallurgical process in which zinc oxide ore was roasted by using (NH₄)₂SO₄ followed by water leached; here, extraction ratios of up to 98% Zn and 22% Fe were obtained.

Pb and Sr associated with Zn in zinc oxide ore are retained in zinc extracted residual as PbSO₄ and SrSO₄ when the ore is processed by acid and (NH₄)₂SO₄ leaching or roasting. The discharge and stockpiling of zinc extracted residual not only occupies large areas of land but also negatively affects the environment, especially when it releases Pb. The Pb and Sr in zinc extracted residual are valuable, but the grades of these metals are far below that of celestite concentrate. Thus, a reasonable and effective method to extract Pb and Sr from this residual must be developed. Extraction of Pb and Sr by NH₄HCO₃ conversion followed by HCl leaching is proposed in this work to promote the clean and comprehensive utilization of Zn oxide ore. Factors influencing the conversion process, including molar ratio of NH₄HCO₃ to zinc extracted residual, NH₄HCO₃ concentration, conversion temperature, conversion time, and stirring velocity, on the extraction ratios of Pb and Sr were investigated, and the conversion conditions were optimized by the orthogonal test. The relevant kinetic parameters were also determined according to the shrinking unreacted core model prior to describing the kinetic equations of the conversion process.

Dried zinc extracted residual was used as the raw material, and NH₄HCO₃ and HCl were used as conversion and leaching reagents in this work. The composition of the zinc extracted residual was chemically analyzed, and the results are listed in Table 1. The residual contained 7.40wt% Pb (7.97wt% PbO) and 5.55wt% Sr (6.56wt% SrO). Si, Pb, Sr, and Ba were enriched in the residual. The mineralogical characteristics and micro morphology of the Zn extracted residual were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the results are displayed in Fig. 1. The XRD pattern obtained indicates that the major mineral phases are quartz and hematite. No Zn phase was detected in the residual, thus confirming that most of the Zn had been effectively extracted. The SEM image shows that the particles are irregular in shape and feature a rough surface.

Figure 1.  XRD pattern (a) and SEM image (b) of the zinc extracted residual.

The required concentrations of pre-mixed NH₄HCO₃ solution varying from 1.0 to 2.8 mol·L⁻¹ were added to a beaker immersed in a constant temperature bath with a temperature accuracy of ±1°C and continuously mechanically stirred. When the desired temperature was reached and held steady, 30 g of the residual sample was added to the solution. The slurry was stirred for a certain amount of time and then separated by filtration. The filtrate containing (NH₄)₂SO₄ was recycled. The filter residue was leached with 150 mL of 0.75 mol·L⁻¹ HCl solution at 80°C for 60 min and then vacuum filtered. PbCl₂ was obtained by cooling the filtrate. The mother liquor was then purified by addition of dilute H₂SO₄ to eliminate Ca and Ba followed by 3wt% H₂O₂ and 5wt% NH₄OH to remove Fe and Al. Afterward, SrCO₃ was prepared by addition of NH₄HCO₃ to filtrate. The extraction ratios of Pb and Sr were determined by ethylene diamine tetraacetic acid (EDTA) titration. The technological flow chart is shown in Fig. 2.

Figure 2.  Technological flow chart of the extraction of Pb and Sr.

The extraction ratios of Pb and Sr were determined according to the following equations:

where 50 is the volume of the calibrated EDTA standard solution, mL; ρ_Pb is the extraction ratio of Pb, %; k_Pb is the coefficient of Pb acetate; f is the EDTA titer; v is the volume of Pb acetate consumed, mL; V_Pb is the volume of the sample solution, mL; $V_{\rm Pb}'$ is the total volume of the test solution, mL; G is the weight of the leaching residue, g; and M is the level of Pb in the leaching residue, wt%.

where ρ_Sr is the extraction ratio of Sr, %; V is the volume of EDTA consumed in the test, mL; V₁ is the volume of the sample solution, mL; V₂ is the total volume of the test solution, mL; C is the molar concentration of the standard EDTA solution, mol·L⁻¹; 0.1036 is the mass of SrO corresponding to 1.00 mL of standard EDTA, g; G is the weight of the leaching residue, g; M is the level of Sr in the leaching residue, wt%; and 88 and 104 are the molar weights of Sr and SrO, respectively.

The operating parameters of NH₄HCO₃ conversion were investigated, and the results are plotted in Fig. 3.

Figure 3.  Effects of the conversion parameters on the extraction ratios of Pb and Sr: (a) NH₄HCO₃ to zinc extracted residual molar ratio; (b) NH₄HCO₃ concentration; (c) conversion temperature; (d) conversion time; (e) stirring velocity.

The molar ratio of NH₄HCO₃ to zinc extracted residual is equivalent to the molar ratio of NH₄HCO₃ to sum of PbO and SrO in the residual. The influence of the molar ratio of NH₄HCO₃ to zinc extracted residual on the extraction ratios of Pb and Sr was investigated under an NH₄HCO₃ concentration of 1.6 mol·L⁻¹, conversion temperature of 65°C, conversion time of 180 min, and stirring velocity of 400 r·min⁻¹. The results presented in Fig. 3(a) show that increases in molar ratio have obvious effect on the extraction ratios of Pb and Sr, and the extraction percentages first increase and then decrease. The maximum extraction ratios of Pb and Sr were 84.70% and 87.28% at NH₄HCO₃ to zinc extracted residual molar ratios of 3:1 and 4:1, respectively. These results may be attributed to the low solubility of sulfates in the reaction system. While increasing the dosages of NH₄HCO₃ increases the driving force of conversion, an excess of this reagent results in increasing solution viscosity and bicarbonate ion concentration, which could inhibit the conversion reaction. Thus, a molar ratio of 3:1 was selected for the following experiments.

The effect of NH₄HCO₃ concentration on the extraction ratios of Pb and Sr was studied by varying the NH₄HCO₃ concentration in the range of 1.0–2.8 mol·L⁻¹ at an NH₄HCO₃ to zinc extracted residual molar ratio of 3:1, conversion temperature of 65°C, conversion time of 180 min, and stirring velocity of 400 r·min⁻¹. The results shown in Fig. 3(b) reveal that the extraction ratios of Pb and Sr first increase and then decrease with increasing NH₄HCO₃ concentration. An NH₄HCO₃ concentration of 1.6 mol·L⁻¹ was sufficient to achieve Pb and Sr extraction ratios of up to 84.65% and 84.57%, respectively. Adequate NH₄HCO₃ is necessary to the conversion process, but high NH₄HCO₃ concentrations can lead to increases in viscosity and bicarbonate ion concentration, which negatively affect conversion [8,10,35]. Thus, an NH₄HCO₃ concentration of 1.6 mol·L⁻¹ was selected for subsequent experiments.

The effect of leaching temperatures ranging from 25 to 85°C on the extraction ratios of Pb and Sr was investigated under an NH₄HCO₃ to zinc extracted residual molar ratio of 3:1, NH₄HCO₃ concentration of 1.6 mol·L⁻¹, conversion time of 180 min, and stirring velocity of 400 r·min⁻¹. The results shown in Fig. 3(c) indicate that the leaching temperature has a more significant effect on Pb extraction than on Sr extraction. The extraction ratio of Pb increased from 66.15% to 84.65% as the temperature increased from at 25 to 65°C; by comparison, Sr extraction only increased from 77.86% to 84.57%. Conversion reactions involve sulfate dissolution and carbonate deposition, and raising the temperature can improve the former. However, temperatures over 72°C can result in the rapid decomposition of NH₄HCO₃ [10,36]. Thus, 65°C was considered a suitable conversion temperature.

The effect of conversion times ranging from 60 to 240 min on the extraction ratios of Pb and Sr was studied under the conditions of an NH₄HCO₃ to zinc extracted residual molar ratio of 3:1, NH₄HCO₃ concentration of 1.6 mol·L⁻¹, temperature of 65°C, and stirring velocity of 400 r·min⁻¹. The results displayed in Fig. 3(d) show that the extraction ratios of Pb and Sr increase as the conversion time increases up to 180 min; thereafter, the extraction ratios decrease. A leaching time of 180 min was thus considered adequate to obtain Pb and Sr with maximum extraction ratios of 84.70% and 84.57%, respectively.

Investigation of the effect of stirring velocity on the extraction ratios of Pb and Sr was performed under an NH₄HCO₃ to zinc extracted residual molar ratio of 3:1, NH₄HCO₃ concentration of 1.6 mol·L⁻¹, temperature of 65°C, and conversion time of 180 min. The results presented in Fig. 3(e) reveal that the extraction ratios of Pb and Sr increase as the stirring velocity increases from 100 to 500 r·min⁻¹. Because stirring can improve the relative motion of liquid–solid systems and decrease the thickness of liquid films, the mass transfer rate is improved. The very slight increase in extraction ratio observed when the stirring velocity exceeds 400 r·min⁻¹ indicates that the resistance of the liquid film surrounding particle is not effective on the conversion reaction rate by using stirring velocity 400 r·min⁻¹ [10–11].

The orthogonal test was adopted to optimize the operating conditions based on the experimental results described above. An L9(3⁴) tabulation consisting of four factors and three levels was chosen, as shown in Table 2, and the results are displayed in Table 3.

Taking the average extraction ratio of Pb and Sr as the evaluation index, the order of factors influencing the conversion process to the greatest extent was NH₄HCO₃ to zinc extracted residual molar ratio > conversion temperature > conversion time > NH₄HCO₃ concentration. The optimized leaching conditions were as follows: NH₄HCO₃ to zinc extracted residual molar ratio of 3.5:1, conversion temperature of 65°C, conversion time of 210 min, NH₄HCO₃ concentration of 1.5 mol·L⁻¹, and stirring velocity of 400 r·min⁻¹. The extraction ratios of Pb and Sr under these optimized conditions were 85.15% and 87.08%, respectively.

The solubility product constants of PbSO₄, SrSO₄, PbCO₃, and SrCO₃ are ${K_{{\rm{sp}}}}_{{\rm{(PbS}}{{\rm{O}}_{\rm{4}}})}$ = 1.60 × 10⁻⁸, ${K_{{\rm{sp}}}}_{{\rm{(SrS}}{{\rm{O}}_{\rm{4}}})}$ = 2.81 × 10⁻⁷, ${K_{{\rm{sp}}}}_{{\rm{(PbC}}{{\rm{O}}_{\rm{3}}})}$ = 3.30 × 10⁻¹⁴, and ${K_{{\rm{sp}}}}_{{\rm{(SrC}}{{\rm{O}}_{\rm{3}}})}$ = 9.4 × 10⁻¹⁰, respectively [1,10]. Thus, phase transformations from PbSO₄ to PbCO₃ and from SrSO₄ to SrCO₃ are easily achievable. The phase transformation from CaSO₄ to CaCO₃ is also achievable because the solubility product constants of CaSO₄ and CaCO₃ are ${K_{{\rm{sp(}}}}_{{\rm{CaS}}{{\rm{O}}_{\rm{4}}})}$ = 4.93 × 10⁻⁵ and ${K_{{\rm{sp}}}}_{{\rm{(CaC}}{{\rm{O}}_{\rm{3}}})}$ = 0.87 × 10⁻⁸, respectively. However, because the solubility product constants of BaSO₄ and BaCO₃ are ${K_{{\rm{sp}}}}_{{\rm{(BaS}}{{\rm{O}}_{\rm{4}}})}$ = 1.08 × 10⁻¹⁰ and ${K_{{\rm{sp}}}}_{{\rm{(BaC}}{{\rm{O}}_{\rm{3}}})}$ = 0.81 × 10⁻¹⁰, respectively, the transformation from BaSO₄ to BaCO₃ is difficult to achieve. Eq. (6) may be expected to take place when the carbonate concentration is much higher than the sulfate concentration. Hence, after HCl leaching, solution purification to eliminate Ca²⁺ and Ba²⁺ is necessary to obtain PbCl₂ and SrCO₃ products.

The main reactions taking place during NH₄HCO₃ leaching are as follows:

The influence of conversion temperatures from 25 to 85°C on the extraction ratios of Pb and Sr in 1.5 mol·L⁻¹ NH₄HCO₃ solution was studied at an NH₄HCO₃ to zinc extracted residual molar ratio of 3.5:1 and stirring velocity of 400 r·min⁻¹, as shown in Figs. 4(a) and 4(b). The conversion reaction between sulfates and NH₄HCO₃ are typical liquid–solid reaction yielding solid products. If the particles are assumed to be spherical and have no obvious change in size, the conversion process can be analyzed by using the shrinking unreacted core model.

Figure 4.  Effect of temperature on the extraction ratios of Pb and Sr (a, b) and relationships (c, d) between 1 − 2α/3 − (1 − α)^2/3 and t at different temperatures and (e, f) between ln k and 1/T.

Heterogeneous liquid–solid reaction include four steps [37–39]: (1) outer diffusion of NH₄HCO₃ through the liquid boundary layer, (2) inner diffusion of NH₄HCO₃ through the solid product layer, (3) surface chemical reactions between sulfates and NH₄HCO₃, and (4) diffusion of the produced (NH₄)₂SO₄ through the solid and liquid layers to the leaching solution. The overall reaction rate is determined by the slowest controlling step in the leaching process. Because the reaction rate is independent of the liquid’s motion according to Fig. 3(e) and the solid product layer is formed during conversion, inner diffusion must be the dominant reaction. Therefore, the following expression of the shrinking unreacted core model can be used to describe the kinetics of the conversion reaction [37–38,40]:

where α refers to the extraction ratios of Pb and Sr, k_d is the apparent rate constant, and t is the reaction time.

The experimental data presented in Figs. 4(a) and 4(b) were analyzed by using the shrinking unreacted core model to determine the relevant kinetic parameters and rate-controlling step. Plots of 1 − 2α/3 − (1 − α)^2/3 against time t at different temperatures are plotted in Figs. 4(c) and 4(d). The lines obtained clearly show a linear relationship between 1 − 2α/3 − (1 − α)^2/3 and t, thus indicating that the conversion process is controlled by inner diffusion through the solid product layer in temperature range of 25–85°C. The corresponding k_d and correlation coefficients are listed in Table 4.

The Arrhenius equation describes the relationship between k and temperature T.

where k is the reaction rate constant; A is a pre-exponential factor; T is the thermodynamic temperature, K; E is the reaction activation energy, J∙mol⁻¹; and R is the molar gas constant, J∙mol⁻¹·K⁻¹.

Arrhenius plots of ln k versus 1/T are presented in Figs. 4(e) and 4(f). E and A can be determined from the slope and intercept of the line and were 13.85 kJ∙mol⁻¹ and 0.2027, respectively, for Pb and 13.67 kJ∙mol⁻¹ and 0.1285, respectively, for Sr.

E values in the range of 8–20 kJ∙mol⁻¹ confirm that the NH₄HCO₃ conversion process is controlled by inner diffusion through the product layer [37–38,40]. The kinetic equations for Pb and Sr conversion can be expressed by Eqs. (9) and (10), respectively.

The mineralogical characteristics and micro morphology of HCl leaching residue are displayed in Fig. 5. The XRD pattern shows that the main phases in the leaching residue include quartz, hematite, and baritite. No Pb or Sr minerals were detected, which means most of the Pb and Sr had been transformed into chlorides and entered the solution. The particles were nonuniform in shape and featured a rough surface.

Figure 5.  XRD pattern (a) and SEM image (d) of the HCl leaching residue.

NH₄HCO₃ conversion followed by HCl leaching to extract Pb and Sr from zinc extracted residual is technically feasible. The process is simple and easy to operate. The proposed strategy presents important significance for the comprehensive utilization of zinc oxide ore and environmental protection.

(1) The experimental results indicated that a molar ratio of NH₄HCO₃ to zinc extracted residual of 3.5:1, leaching temperature of 65°C, leaching time of 210 min, NH₄HCO₃ concentration of 1.5 mol·L⁻¹, and stirring velocity of 400 r·min⁻¹ could achieve Pb and Sr extraction ratios of 85.15% and 87.08%, respectively.

(2) Kinetic analysis indicated the conversion process follows the shrinking unreacted core model and is controlled by inner diffusion through the product layer in the temperature range of 25–85°C. The E values for Pb and Sr extraction were 13.85 and 13.67 kJ·mol⁻¹, respectively.

This work was financially supported by the National Natural Science Foundation of China (Nos. 51774070, 52004165, and 51574084).