Hydrometallurgical Extraction of Lead in Brine Solution from a TSL Processed Zinc Plant Residue

Abstract

Brine leaching of a TSL processed zinc-plant residue (ZPR), containing 83.3% anglesite , has been investigated for lead recovery . A prior treatment of ZPR in acid solution did not show any significant effect of acid concentration in zinc dissolution. The study reveals that the solubility limits of PbCl₂ is as an important factor with respect to the brine concentration and pulp density in lixiviant. The dissociated sulfate ions hinder the formation of PbCl₂ on prolong leaching (above 60 min), while exhibiting the reverse solubility of lead as PbSO₄. A 3-step leaching process could yield >92% leaching efficiency when the parameters were maintained as: 10% pulp density in a 250 g/L NaCl solution, 80 °C temperature, and 60 min time. This leaves the insoluble ferrite, sulfides and oxy-sulfates of zinc and lead in the final residue as revealed by the XRD characterization .

Introduction

Lead is a toxic rather versatile and important metal due to its intrinsic association primarily with the automobile, energy storage, and nuclear industry [1]. The lead accumulator industry is the biggest consumer (~80% of total lead produced worldwide) of this non-ferrous metal in manufacturing of lead -acid batteries . Galena (PbS) is the major ore of lead along with minor contributions of other minerals like, anglesite (PbSO₄) and cerussite (PbCO₃) [2]. The production of lead from secondary resources by recycling is also gradually becoming the major source of lead worldwide, whose contribution in developed countries often exceeds that of the primary lead . Additionally, the occurrence of zinc-bearing ores (ZnS, ZnCO₃, and Zn₂SiO₄) is also deplete with the significant amount of lead , and becomes rich in lead after processing the ores for recovering the primary metal, zinc from its ore bodies [3]. The lead content in the secondary materials generated is often dependent on the route adopted for the primary ore processing (pyrometallurgy or, hydrometallurgy ). Approximately 80% of zinc is produced via roasting followed by sulfuric acid leaching and electrowinning route [4], the lead forms insoluble sulfate during the hot H₂SO₄ acid leaching . This lead sulfate remains in the zinc -plant residue (ZPR) which is a major stockpile in the zinc producing companies, and is available to recover the valuable metals [5]. Notably, the residue generated via the top submerged lancing (TSL ) technology in zinc production contains very large amount of lead . Recovery of lead from such residue by smelting is usual practice with a higher consumption of energy because of high operating temperature ~1300 K [6]. Hydrometallurgical processing of ZPR is mainly employed to low-contents of targeted metals, and no report exists for processing of residues with high lead contents therein. In most of the cases, lead accumulated by sulfuric acid leaching as lead sulfate concentrate is also sent to the lead -smelter for its final recovery [7].

Due to the low energy requirements and lesser environmental pollution caused, many researches have been carried out to optimize the recovery of metals from the secondary resources using different lixiviants. These include sulfuric/hydrochloric acid , alkaline solutions of ammonia or caustic, brine or other chloride solutions [7–18], however no attempt seems to be made for processing of the ZPR with very high lead content. Hence in this study, leaching behavior of a TSL processed zinc-plant residue of Young Poong Zinc (South Korea) has been investigated using the NaCl solution as a lixiviant. For this, the parameters like, NaCl concentration, pulp density, and time were varied and the optimal leaching condition for maximum dissolution of lead in chloride solution was obtained. The recovery obtained at each stage of leaching was balanced with lead mass distribution between the leach liquor and residue.

Experimental

The residue supplied by Young Poong Zinc (South Korea) was dried and used after grinding to collect the sample of particle size −315 μm. The wet-chemical analysis of the homogenized sample that was used in the study revealed the presence of 56.9% Pb, 3.9% Zn, 0.8% Fe, 0.9% Cu, 0.2% Al. The analysis of the X-ray diffraction (pattern given in Fig. 1) could only show anglesite mineral phase as the major constituent of the residue. A prior dissolution of zinc from ZPR using 0−4.0 M H₂SO₄ solutions in distilled water was performed with respect to different pulp densities at room temperature (~20 °C). For the leaching studies of lead , a known concentration of NaCl in brine solution taken into jacketed-jar was preheated to 80 (±2) °C. Thereafter a known amount of pre-washed ZPR was added to lixiviant while maintaining the desired pulp density at a constant stirring speed (300 rpm). The liquid samples were withdrawn at regular time intervals and appropriately diluted into the nitric acid solutions for analyzing metal contents using the inductively coupled plasma spectroscopy (ICP). Leach residue collected after filtration was used in next step leaching (wherever required) and for their characterization study. The NaCl crystals (purity 99.5%) and HNO₃ (purity 60–65%) were supplied by Junsei Chemical Co . Ltd. (Japan). All reagents were used without further purification and solutions were prepared in distilled water.

../images/468727_1_En_97_Chapter/468727_1_En_97_Fig1_HTML.gif — Fig. 1
XRD pattern of the ZPR sample used in this study

Results and Discussion

Prior Extraction of Soluble Zinc from ZPR

The TSL processed ZPR obtained after sulfuric acid leaching has lead and zinc sulfates as the main constituents. A selective leaching of zinc from the residue would be desirable so that it can go to the main leaching circuit of the zinc for recovery purpose. Therefore, firstly the extraction behavior of soluble zinc from the ZPR was investigated in 0−4.0 M H₂SO₄ solutions. With a purpose of investigating the maximum solubilization at each acid concentration, the pulp density was varied in the range of 10–40% while maintaining other conditions as: stirring speed, 300 rpm; temperature , 20 °C; and time, 120 min. Results of zinc extraction plotted as a function of pulp density, are given in Fig. 2 which indicate that zinc dissolution is almost independent of the acid concentration in solution. In fact a difference of only ~4% zinc was observed in the Zinc extraction efficiency in the entire range of 0−4.0 M H₂SO₄ under the best conditions. The results also showed a decline in the extraction percentage of zinc with increasing pulp density (Fig. 2), which can be ascribed to an effect of smaller contact area available to per unit weight of the sample at higher pulp density. A lower (10%) pulp density in the slurry yielded 56.5%, 58.6% and 60.5% zinc while using only water (no acid), 2.0 M H₂SO₄ and 4.0 M H₂SO₄, respectively; the extraction being 41.3%, 43.8% and 43.6%, respectively at the highest pulp density of 40%. The analysis of the results indicated the suitability of the pulp density of 10% of ZPR in water and this was maintained in the next set of experiments. The residue obtained was subsequently used for the brine leaching investigations.

../images/468727_1_En_97_Chapter/468727_1_En_97_Fig2_HTML.gif — Fig. 2
Effect of acid concentration on zinc extraction as a function of pulp density (variation of acid cocnetration, 0−4 M H₂SO₄; variation of pulp density, 10−40%; stirring speed, 300 rpm; temperature , 20 °C; and time, 120 min)

Effect of Pulp Density on Lead Extraction in Brine Solution

In order to examine the extraction behavior of lead in the brine solution while solubilizing the sulfate mineral, anglesite into chloride media , the effect of pulp density in the range of 10−20% of ZPR was investigated. For this, leaching conditions were maintained as: NaCl concentration 250 g/L; stirring speed 300 rpm; temperature 20 °C; and time 120 min. The experimental results given in Fig. 3 showed a decreasing trend for lead extraction percentage with respect to the increasing pulp density, which can be attributed to the availability of lower concentration of lixiviant per unit weight of the sample. The maximum lead dissolution of 43.6% could be achieved with 10% pulp density that decreased to only 20.4% while using a pulp density of 20% in the brine solution. Due to the higher leaching efficiency of lead , further studies were performed at 10% pulp density.

../images/468727_1_En_97_Chapter/468727_1_En_97_Fig3_HTML.gif — Fig. 3
Lead dissolution behaviour in brine solution as a function of pulp density (variation of pulp density, 10−20%; NaCl concentration, 250 g/L; stirring speed, 300 rpm; temperature , 80 °C; and time, 120 min)

Effect of NaCl Concentration on Lead Extraction

Lead is readily soluble in brine solution making its chloride complex contrary to forming the insoluble sulfate salt. Hence, the concentration of chloride ions in lead solubilization is a vital factor to be controlled, which was studied by varying the amount of NaCl in the range 50−300 g/L in brine solutions. Other parameters such as pulp density (10%), stirring speed (300 rpm), and temperature (80 °C) were maintained unless stated otherwise for 120 min of leaching . As the solubility of both NaCl and PbCl₂ increases with the increase in temperature , the leaching temperature study was maintained at 80 °C. The dissolution behavior of lead in brine solution as a function of time is given in Fig. 4, which clearly depicts the influence of NaCl on leaching efficiency by following the stepwise Eqs. (1–4) given below:

../images/468727_1_En_97_Chapter/468727_1_En_97_Fig4_HTML.gif — Fig. 4
Effect of NaCl concentration on lead extraction as a function of leaching time (variation of NaCl cocnetration, 50−300 g/L NaCl; pulp density, 10%; stirring speed, 300 rpm; temperature , 80 °C; and time, up to 120 min)

${\text{Pb}}^{2 + } + {\text{Cl}}^{ - } = {\text{PbCl}}^{ + }$

(1)

${\text{PbCl}}^{ + } + {\text{Cl}}^{ - } = {\text{PbCl}}_{2}$

(2)

${\text{PbCl}}_{2} + {\text{Cl}}^{ - } = {\text{PbCl}}_{3}^{ - }$

(3)

${\text{PbCl}}_{3}^{ - } + {\text{Cl}}^{ - } = {\text{PbCl}}_{4}^{2 - }$

(4)

As can be seen the extraction of lead increased with increasing concentration of NaCl in solution up to 250 g/L, thereafter it declined slightly. Also the leaching with ≤150 g/L NaCl was found to be totally independent to time, but at ≥200 g/L NaCl, the effect of time is visible on the leaching efficiency . The highest efficiency was achieved with 250 g/L NaCl in lixiviant and within 60 min of leaching time, however the maximum lead obtained in solution was below 50%. This limited efficiency of lead solubilization in the brine solution can be ascribed to the solubility limit of PbCl₂ under the studied conditions. A limited leaching above 60 min indicates the hindrance caused by the dissociated sulfate ions from anglesite (PbSO₄) minerals . With the maximum efficiency obtained with 250 g/L NaCl solution and 60 min time, the further leaching experiments were performed at the same condition.

Multi-step Leaching for the Maximum Lead Extraction

The abovementioned single step leaching data (after a prior water washing step), yielding only ~49% in leach liquor clearly showed that the desired efficiency of lead extraction could not be achieved. The XRD analysis of the leach residue (not given here) inferred no significant change in comparison to the XRD of initial sample, and hence multi-step leaching study was undertaken at the same condition optimized above. In each step, the leach liquor was analyzed to determine the total leaching efficiency , and leaching behavior was correlated with the phase change in the sample while examining the XRD of leach residues. This analysis was performed after water washing of the leach residues for a proper removal of metal and electrolytes. By analyzing the results of leaching efficiency in each step, a 3-step leaching was found to be sufficient to obtain the overall lead recovery of 92% from the ZPR. Stepwise results of the brine leaching are given in Table 1. The XRD patterns of the leach residue after the third step revealed the presence of refractory minerals like, ZnFe₂O₄, ZnS, PbS, FeS, Zn₃O(SO)₄ and Pb₃O₂SO₄ which are mainly responsible for the metals remaining undissolved in the brine solution.

Table 1

Stepwise extraction efficiency of lead from the ZPR leached in 250 g/L NaCl solution at 10% pulp density and 80 °C for 60 min

Leaching (with water washing)	Pb extraction (%)	Overall Pb extraction (%)
1st step	57.36	92.03
2nd step	27.84
3rd step	6.83

It needs to be mentioned here that only <4% of silver was analyzed in leach liquor (that too in only first step of leaching ) and no other metal get dissolved in the brine solution, showing the selectivity of this process. Furthermore, the processing of obtained leach liquor can be easily processed to recover the lead metal via cementation with metallic iron or aluminum . The applicability of cementation using (Fe⁰/Al⁰) will also be worthy by looking on the cost of the process and value of the metal recovered.

Conclusions

The simple brine leaching for lead extraction from ZPR has been investigated in the present study. The prior extraction of zinc in 0–4.0 M acid solutions was not influenced by the acid concentration in solution, and almost a similar amount of soluble zinc (56.5–60.5%, respectively) could be obtained from the ZPR. For brine leaching of the water washed ZPR, a low pulp density of 10% showed the highest lead extraction of 43.6% that also decreased with increase in the pulp density (20.4% lead extraction at 20% pulp density). The extraction of lead in the brine solutions of lower NaCl concentrations (50–150 g/L) was as low as <20%, thereafter it increased significantly up to ~48% at 250 g/L NaCl. This clearly indicates the major influence of chloride ions on the lead extraction which participated in metal complexation. A prolonged leaching however, adversely affected the lead extraction , which revealed the role played by the dissociated sulfate ions through the leaching of anglesite in brine solution. Thus optimized conditions for the maximum extraction of lead (250 g/L NaCl solution, 10% pulp density, and time 60 min) was also maintained in multi-step brine leaching , while yielding the overall Pb leaching efficiency of 92% in 3-steps. The XRD analysis of the final leach residue revealed the presence of refractory minerals of lead (Pb₃O₂SO₄ and PbS) and zinc (ZnFe₂O₄, Zn₃O(SO)₄ and ZnS) that could not be leached in the brine solutions, leaving ~8% Pb in final residue; the residue can be sent for roasting with the primary sphalerite .

Acknowledgements

The paper is based on the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM).

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