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Article

Historical Lead Smelting Slag Harmlessness and Valuable Metals Recovery: A Co-Treatment of Lead Slag and Zinc-Bearing Material in Rotary Kiln

by
Xuekui Niu
1,2,
Minting Li
2,*,
Hongbin Wang
1,*,
Liping Ma
1,
Shuting Wang
3,
Tao Zhou
3 and
Wei Wang
3
1
Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
2
State key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China
3
Yunnan Research Academy of Eco-Environmental Sciences, Kunming 650091, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(20), 13647; https://doi.org/10.3390/su142013647
Submission received: 23 August 2022 / Revised: 11 September 2022 / Accepted: 23 September 2022 / Published: 21 October 2022
(This article belongs to the Section Hazards and Sustainability)
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Abstract

:
The harmless treatment of historical lead smelting slag (LSS) is of significance to ecological and environmental protection, but it is still challenging in terms of the economic feasibility of alone processing due to the low content of valuable metals. Here, we performed an industrialized test with a co-treatment of LSS and zinc oxide ore in a rotary kiln to evaluate the economic feasibility and solidification effect of harmful elements. The results revealed that more than 70% of Zn and Pb were recovered from LSS in the form of dust, while the nonvolatile part of Pb, Zn, and Cd were solidified in gangue as complex silicate phases. The nonvolatile part of As came into being Fe-As intermetallic compound which was encapsulated by gangue particles or was solidified in silicate phases. The entirely enclosed structure of water-quenched slag plays an important role in the stability of slag. The TCLP and SNAL leaching tests demonstrated the high stability of water-quenched slag. A zinc oxide ore addition of 20% was recommended for energy consumption and processing capacity. Our findings highlight that the valuable metals not only can be effectively recovered but also harmful elements are solidified in gangue, providing an economical and feasible technical route for the treatment of historical LSS.

1. Introduction

Located in Honghe state, Yunnan province in China, the Gejiu deposit is a super-large polymetallic deposit with silver, lead, zinc, tin, and copper [1,2]. This area has been mined from 202 B.C. to 220 A.D. intermittently for more than 2000 years [3]. Approximately 400 Mt of Pb–Zn ores with an average grade of 7% Pb+Zn were found [2]. Many small smelters processed galena (PbS) concentrate from the flotation plant. However, these small smelters were discontinued due to the high emission of SO2 in 2017. After many years of production, the smelters have produced approximately 30 million tons of lead smelting slags (LSS), among which 2.6 million tons with a cover area of 0.92 million square meters were determined as hazardous wastes. The smelters were used following a process of classical sinter roasting-blast furnace smelting to yield lead bullion [4]. The LSS produced from this process is a kind of high temperature melts with a very complicated chemical composition, consisting of FeO, SiO2, A12O3, CaO, MgO, ZnO, and so on, and being in the form of compounds, sosoloid and eutectic mixture [5,6]. The content of Pb+Zn is low at about 5~8%. The characterization and stability of both old and new LSS from all over the world have been widely studied [5], such as in France [7,8,9], the Czech Republic [10,11], Germany [6,12], South Africa [13], Belgium [14], the United Kingdom [15], Namibia [16], and China [17]. In general, the LSS composes of wüstite (FeO), kirschsteinite (CaFe(SiO4)), franklinite ((Zn,Fe,Mn)(Fe,Mn)2O4), spinel (MgAl2O4) and metallic lead [7,8,10].
The stability of the LSS depends on the raw material (galena concentrate and slagging constituent) and the process [6]. Many heavy metals in the LSS enter the atmosphere, water, and soil, leading to serious damage to the surrounding ecological environment [18,19]. The concentrations and distributions of heavy metals in the atmosphere [20], coastal sediment, and zooplankton [21,22,23,24,25,26], ballast water of commercial ships [22,23] and some other matrices have been monitored and analyzed, and the pollution situation regarding heavy metals is serious [27]. Many investigations have been given to the enrichment characteristics of heavy metals from soil [28] and plants [29] around the lead slag dump. The threat of involving lead slag containing heavy metals cannot be ignored. Potentially toxic elements, especially Pb and Cd, have been verified to affect living people near slag dumps or smelters [30,31].
At present, land stockpiling is a main approach for the LSS because of its low valuable metal content [32,33]. However, this approach resulted in the release of heavy metals into the environment, posing a great threat to humans and the environment [34,35]. In addition, stabilization was another method to treat the lead slag, including producing cement mix [36], glass-ceramics [37], and building materials [38,39] as a raw material. However, the presence of high amounts of toxic elements, such as Pb, Zn, and Sn, was not conducive to cement hydration and led to secondary pollution of the environment [40]. For the solidification method of the LSS, valuable metals such as Pb and Zn fail to be recycled.
Our previous research showed that the leaching concentrations of Pb, Zn, As, and Cd in the slag exceed the national standard limit values [41], which indicated that the LSS is dangerous solid waste. Hence, the slag must be treated harmlessly. Nevertheless, the slag contains a certain amount of valuable metals (zinc and lead). The valuable metals recovery should be considered on the basis of realizing the harmless treatment of the slag. Carbothermic reduction was considered to be an efficient method to recover heavy metals from hazardous wastes, in which zinc and lead oxides are reduced to volatilize by reductant. Zinc smelting slag [42], electric arc furnace dust (EAF) [43], copper smelting slag [44], and lead slag [45,46], were well treated with the carbothermic reduction method to volatile Pb/PbO(g) and Zn(g). Though lead and zinc in the LSS can be effectively recovered by carbothermic reduction, the economic feasibility of this method is low because of the low content of valuable metals in the LSS [46]. At present, there is no alone treatment method for lead smelting slag. In addition, the environmental risk of the slag extracted valuable metals needs to be evaluated. Then, it is necessary to develop an LSS treatment process characterized by high technical, economic, and environmental feasibility.
Therefore, in this work, a process of LSS harmlessness and valuable metals recovery in a rotary kiln with a co-treatment of LSS and zinc-bearing material using carbothermic reduction was developed. An industrial-scale experiment was carried out under different ratios of LSS to zinc-bearing materials. The migration and distribution behavior of heavy metals in the LSS were investigated by characterization of the phase transformation during carbothermic reduction, as well as the solidification effect of harmful elements and economic evaluation were discussed.

2. Materials and Methods

2.1. Lead Smelting Slag Sampling and Zinc-Bearing Material

In the Gejiu district, there are five large hazardous LSS heaps, as shown in Figure 1. Approximate weights of each heap are listed in Table 1. A sampling campaign was carried out in January 2021 according to the weight proportion of each slag heap (Table 1). The sample was collected from the bottom of each heap at different regions with a digging depth of 1.5 m using a digger. The samples taken from each slag heap were transported to material yard and were preliminarily blended according to the sampling weight using motor transport. Preliminary mixed sample was crushed (<5 cm) and further mixed well. The mixed sample with a moisture content of 12% was used as an industrial-scale experiment material. The mineralogical composition of mixed sample mainly composed of amorphous silicon dioxide, magnetite (Fe3O4), calcium silicate (CaSi2O3), lead silicate (Pb11Si3O17), iron aluminosilicate (Fe3Al2(SiO4)3), and ferrous sulfide (FeS). Zinc-bearing material was a low-grade zinc oxide ore. Anthracite fines (<5 mm) and gypsum from flue gas absorption process were used as a reductant and a slag form regulator, respectively.

2.2. Rotary Kiln Tests and Sampling

In this study, the equipment implemented in the industrial-scale experiment was a rotary kiln system, which was comprised of feeding device, rotary kiln (ϕ2.8 m × L44 m), settling chamber, surface cooler (900 m2), bag collector (1500 m2) and desulfurizing tower, as shown in Figure 2.
Zinc oxide ore was crushed to below 50 mm by the counter roll crusher (XPSF-Φ400 × 250). The mixed lead smelting slag sample was blended with crushed zinc oxide ore, anthracite fines (<5 mm), and gypsum using a grab to produce feed material (FM), in which the water content was controlled to 18 ± 1%, and then the FM was sent to feed material warehouse equipped with electronic weighing unit through belt conveyor. The materials in the warehouse were fed into the rotary kiln by belt and disk feeder for carbothermic reduction. After passing the settlement, surface cooler, bag dust collector, and desulfurization system comprising two-stage power wave washing and lime milk desulfurization in turn, the flue gas produced from reduction process reached the national discharge standard, while the dust containing Pb and Zn was obtained. Reduced slag (rotary kiln slag) was quenched by water in a closed annular water quenching tower and then was transported to slag yard for storage. The technological process is shown in Figure 3.
Five ratios of LSS to zinc oxide ore, which are 100% slag (1#), 80% slag + 20% ore (2#), 60% slag + 40% ore (3#), 40% slag + 60% ore (4#) and 20% slag + 80% ore (5#), were designed. The main parameters of reduction process were processing capacity of 120 t/d, reduction temperature of 1100−1200 °C, coal powder consumption of 0.46−0.53 ton per ton of FM, blasting air volume of 1050−1250 m3/min, kiln rolling speed of 0.7−0.95 r/min, and negative pressure of 60 Pa. The experiment of each ratio operated stably for 3 days. The samples, such as FM, settling chamber dust (SCD1), surface cooler dust (SCD2), bag collector dust (BCD), and water-quenched slag (WQS), were respectively collected every four hours with a weight of 2 kg during each ratio experiment. The samples taken at different times were severally mixed and sampled by quartering to generate a comprehensive sample, which was used to determine element content.

2.3. Leaching Tests

The toxicity characteristic leaching procedure (TCLP) developed by the United States Environment Protection Agency (USEPA) is useful in identifying the potential hazards of solid waste with a co-disposal assumption [47,48]. The TCLP test can simulate the effect of the eventual interaction of the slag with low-molecular-weight organic acids produced by the soil cover over the slag heap [5], which was carried out by using a leaching step under the solid size being less than 9.5 mm, the liquid/solid ratio of 20 mL/g, the agitation of 30 ± 2 r/min, the leaching time of 18 ± 2 h, and the temperature of 23 ± 2 °C [48]. The TCLP tests were conducted in a 20 mL polyvinyl bottle.
The sulfuric and nitric acids leaching test (SNAL) was applied to simulate leaching characteristics of solid waste under specified conditions of acid rain. This test was performed by extraction fluid of mixed solution (mass ratio of sulfuric acid to nitric acid being 2:1) at pH 3.20 ± 0.05 with a liquid/solid ratio of 10 mL/g [49].
The suspension was rotated in a rotary oscillator at 30 r/min for 18 h. In this work, The TCLP and SNAL tests were conducted to evaluate the stability of water-quenched slag (WQS) produced from rotary kiln.

2.4. Analysis Methods

The SEM-EDS (MLA250, FEI Quanta, Hillsboro, OR, USA) was performed to create a detailed mapping of the major elements of solid sample. Several analytical methods were used to determine the element’s content in solid sample. The contents of Pb, Zn, Cu, Cd, Cr, Ag, Be, Ba, As, Ni, Se, CaO, MgO, and Al2O3 were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The inductively coupled plasma optical emission spectrometry (ICP-OES) was used to analyze Fe, K, and Na in solid sample. Total S content was determined by combustion and infrared identifications (C/S analyzer by LECO). The concentrations of heavy metals in solution were determined by ICP-OES. The valuable metals recovery (ηMe) was determined as follows.
ηMe = [(m1 × w′1 + m2 × w′2 + m3 × w′3)/(mf × w′f)] × 100%
where m1, m2, and m3 are the weights of settlement dust, surface cooler dust and bag collector dust, respectively; and w′1, w′2, and w’3, respectively, are metals content in settlement dust, surface cooler dust and bag collector dust; mf is the weight of FM and w′f is the metals content in FM.
The formula of comprehensive energy consumption for processing one ton of LSS is listed below:
E c = ( ∑ i = 1 n ( E i × k i ) ) × a / m s
where Ec is the comprehensive energy consumption of processing one ton of LSS, n is the number of energy, Ei is the actual consumption of i-th energy during rotary kiln experiment (including water, electricity and coal), ki is the standard coal coefficient of the i-th energy, a is the input proportion of lead smelting slag in the FM, and ms is the processing capacity of lead smelting slag for each ratio experiment.

3. Results and Discussion

3.1. Main Chemical Compositions of Products

3.1.1. Water-Quenched Slag

For comparison, the major constituents of FM and WQS are listed in Table 2. The primary elements in the FM were Pb, Zn, Fe, Si, Ca, Al and Mg, and the content of Pb+Zn was above 3%. The most common trace elements in the LSS were Cu, Cl, F, In, Ge, and Ba, and the change in their content was not obvious with the increasing addition of oxidized ores. The content of Pb+Zn gradually increased with an increase in LSS to zinc oxide ore ratio. Cd concentrations in the FM enhanced as increasing zinc oxide ore addition, but As content showed the opposite rule compared with Cd. The results indicated that the increase of Cd content in the FM is due to the addition of zinc oxide ore, yet that of As content is due to the LSS addition.
After a carbothermic reduction in the rotary kiln, the Pb and Zn content in the WQS was markedly less than that in the FM. This was attributed to the high recovery of Pb and Zn during the reduction process. As content in the WQS was high because of low volatilization, being in the range of 1420~3800 mg/kg. A possible reason is the high content of Fe in the WQS (Chai et al., 2015) [34]. It is interesting to note that the Cd content in the WQS was low (3.3~34.9 mg/kg) because of the high volatilization of Cd. Lithophile elements such as Ba and Ti are incompatible elements with a large ion radius and are mainly oxidic-siliceous bound during the smelting process. Then, these elements are enriched in the WQS. Chalcophile elements such as As, Cd, Cu, Pb, and Zn are mainly sulfidic bound or become reduced to metals in the smelting process (Scheinert et al., 2009) [6].

3.1.2. Dust

The metal content in the dust obtained from the settling chamber, surface cooler, and bag collector is presented in Table 3. The major valuable metals in the dust were Pb and Zn. The content of Pb+Zn in the dust increased as the LSS-to-zinc-oxide-ore ratio increased, and in BCD, it was the highest, followed by SCD2 and SCD1. The Pb+Zn content in SCD2 and BCD was in the range of 20–45%, which can be used to extract lead and zinc. Nevertheless, As and Cd were enriched in the dust, especially in SCD2 and BCD. This was not conducive to zinc and lead extraction. Note that the As content in the dust decreased as increasing the addition of zinc oxide ore, while there were no significant differences for the Cd content in the dust.

3.2. Metals Distribution during Rotary Kiln Treatment

3.2.1. Valuable Metals Recovery

The volatilization recoveries of Pb, Zn, As, and Cd in the rotary kiln reduction process under the five ratios condition are presented in Figure 4. The metals recoveries were greatly affected by the ratio of LSS to zinc oxide ore. Apart from Cd, the recoveries of Pb, Zn, and As increased with an increase of LSS to zinc oxide ore ratio.
The reason may be that the reduction of Pb and Zn in the LSS was difficult, but the zinc oxide ore was easy to reduce. When the addition of zinc oxide ore increased from 0% to 80%, Pb recovery increased from 56.26% to 93.14%, Zn recovery increased from 63.38% to 95.99%, and As recovery increased from 9.24% to 40.07%. There was no obvious relationship between Cd recovery and the addition of zinc oxide ore, and its recovery was in the range of 32.45% to 69.24%.
It is pointed out that the recovery of Pb was less than that of Zn, though lead oxide was more easily reduced than zinc oxide. The reduction of ZnO can be well carried out when the reaction temperature is above 1273 K [50]. The research on the carbothermal reduction of zinc smelting slag indicated that the recoveries of zinc and lead were 94.5% and 97.6%, respectively, at 1050 °C for 6 h [42]. Lead and Zinc compounds in the LSS were transformed to the volatile Pb/PbO(g) and Zn(g) by carbothermal reduction and thus were recovered by volatilization [51]. Moreover, Fe recovery from LSS in one step smelting process has been proposed, and the heavy metals were enriched in flue dust [52,53]. These above-mentioned investigations indicated that the carbothermal reduction proved an effective way of heavy metals recovery from zinc and lead-bearing waste [54].

3.2.2. Metals Distribution

The distribution of heavy metals (Pb, Zn, As, and Cd) in the various materials produced from the rotary kiln test was shown in Figure 5, where the metal weight in desulfurized gypsum was counted as a loss.
The distributions of Pb and Zn in the WQS gradually decreased from 24.52% to 3.43%, and from 20.69% to 0.99%, respectively, with an increase in zinc oxide ore addition. An increasing trend for Pb and Zn distributions in the dust (SCD1, SCD2, and BCD) was observed. Moreover, Pb and Zn are mainly distributed in BCD, in which the distribution proportions of Pb and Zn were 53.81~78.04% and 59.42~90.31%, respectively.
As distribution, however, in the WQS was high and was in the range of 47.21%–72.48%, showing that the volatilization of arsenic was difficult during the rotary kiln process. Weak oxidation or reduction atmosphere is required for arsenic volatilization [55,56]. If the reducing atmosphere is strong, the As-Fe intermetallic compounds (FeAs or FeAs2) will form, reducing the arsenic volatilization. Nevertheless, the distribution behavior of Cd was significantly different from that of As. Cd distribution in the WQS was 0.4~6.21% for the zinc oxide ore addition of 20~80%. This was due to the high volatility of Cd compounds, for example, CdS and CdO, formed from the carbothermal reduction process. Unfortunately, the losses of Pb, As, and Zn were high, which may be caused by no complete collection of the dust during the 1# (100% slag) experiment that was carried out. Firstly, Cadmium loss can be attributed to many factors, such as sampling, chemical analysis, and dust collection.

3.3. Migration and Solidification of Pb, Zn, As, and Cd during Carbothermic Reduction

The morphology of FM and WQS for each ratio test is shown in Figure 6. The LSS and WQS presented a relatively enclosed structure. Particles of different compositions and sizes are connected with each other and aggregated into large particles. The distribution of particles was dispersive, which was ascribed to the homogenization of slag under high temperatures.
To further determine the particle chemical composition of different brightness in Figure 6, the SEM-EDS analysis (Figure 7) was carried out. The main phases in the LSS were matrix and metal sulfide (FeS, PbS, and ZnS, Figure 6a). There was metallic iron, metal sulfide, Fe3O4, arsenate, and matric phases in the WQS obtained from the different addition ratios of zinc oxide ore. It should be pointed out that the arsenate was wrapped by metal sulfide (Figure 7c)), and Fe3O4 formed from the carbothermic reduction process wrapped the metal sulfide and matric (Figure 7d). This may help to improve the stability of slag. The distribution of matric increased with increasing the addition of zinc oxide ore (Figure 7d–f).
Figure 8 presents a backscattering EPMA image of the polished WQS particles. Note that in Figure 8a, Pb is not associated with Si and Ca but is associated with Fe and S. These results clearly indicated that Pb in LSS formed into metal sulfide. Zn and Cd uniformly distribute in gangue components. However, As is closely related to basic components (such as PbO and MgO), forming arsenate. This may account for the low stability of arsenic in the LSS due to the acid-soluble characteristic of magnesium arsenate. After carbothermic reduction by adding zinc oxide ore, Pb, Fe, and S are closely linked, indicating that metal sulfide existed in the WQS. Zn, As, and Cd are related to Si and Ca, showing that they were solidified in the gangue. In the WQS, the relation between As and Fe decreased with a decrease in the LSS addition. When the LSS addition was below 60%, As was not associated with Fe but was related to Si and Ca. Thus, the certain addition of LSS contributed to the formation of Fe-As intermetallic compounds. This was beneficial to the stabilization of arsenic. In a word, after carbothermic reduction, Pb, Zn, and Cd were solidified in gangue, and As were solidified by forming Fe-As intermetallic compounds or complex silicate phases.

3.4. Solidification Characterization of Water Quenched Slag from Rotary Kiln

Leachable metal concentrations for LSS and different WQS as determined by TCLP and SNAL leaching tests are listed in Table 4. The results show that the concentrations of Pb, Zn, As and Cd in the LSS exceeded the respective threshold values [47,48]. The concentrations of Pb, Zn, As, and Cd leached by TCLP decreased in the water-quenched slags and decreased from 302.33, 899.15, 12.11, and 1.36 mg/L in the LSS to 0.11−0.17, 0.16−0.22, 0.83−0.95 and 0.014−0.017 mg/L in the WQS, respectively. According to the SNAL leaching tests, the leachable concentrations for these metals in the WQS declined significantly compared to that in the LSS and were below the acceptable threshold values [41]. The lower leachable concentrations of Pb, Zn, As, and Cd in the WQS may be due to the following reasons: (1) the total content of Pb, Zn, and Cd is low in the WQS because of the high recovery; (2) As is stabilized in the form of Fe-As (Figure 5b) or complex silicate phases (Figure 8c–f); (3) Fe has a strong buffering for acid solution to decrease the leaching of heavy (Chai et al., 2015) [34].
Hence, the carbothermic reduction was an efficient method to solidify heavy metals in hazardous wastes. The main environmental implication of the leaching test is that the WQS is stable, in the short term, in natural weather. The result is supported by the fact that Pb existed in the WQS as a metal sulfide particle and Zn, As, and Cd as complex phases, which are stable in most acidic solutions.

3.5. Economic Evaluation

3.5.1. Energy Consumption

Table 5 presents the consumption of materials during rotary kiln tests. The total capacities of LSS and zinc oxide ore for five ratio experiments were 1031.62 and 647.18 tons, respectively. The ratio of coal powder to FM was in the range of 46.69−53.12%. The consumption of electricity, water, and coal powder is shown in Table 6.
According to these data in Table 5 and Table 6 and Equation (2), coal powder, water, electricity, and comprehensive energy per ton of LSS were calculated and presented in Figure 9.
Coal powder consumption gradually decreased from 0.67 tons for 100% slag experiment to 0.51 tons for 20% slag+80% ore experiment per ton of LSS, but electricity consumption increased from 63.15 to 100.72 kW∙h per ton of LSS, with increasing addition of zinc oxide ore (Figure 7a). Water consumption was in the range of 1.25−3.83 tons per ton of LSS (Figure 7b). From energy consumption, the lower additions of LSS, the lower the energy consumption. However, increasing the addition of zinc oxide ore led to the processing ability reduction of LSS. Hence, the technical feasibility of rotary kiln co-treatment of LSS and zinc oxide ore should be comprehensively evaluated from processing ability and economy.

3.5.2. Actual Cost

The actual processing cost of LSS in the rotary kiln was calculated according to material costs, labor costs, energy costs, and fixed costs (including depreciation expenses, financial expenses, amortization of land use rights, and other expenses), as shown in Figure 10. It was greatly affected by the input cost of zinc oxide ore. The actual cost for the different ratios of LSS to zinc oxide ore were 610, 410, 1500, 620, and 910 RMB, respectively, but the processing ability of LSS decreased from 36,000 to 7200 t/a. Because material cost varied greatly according to the difference in zinc oxide ore type, zinc grade, addition, and unit price, there was uncertainty for practical industrial applications. Taking energy cost as a reference and only counting the water, electricity, and coal consumptions, the energy cost of processing a ton of LSS was in the range of 600 to 800 RMB for the five ratios experiments. A zinc oxide ore addition of 20% was recommended on the basis of energy cost and processing capacity of LSS.

4. Conclusions

In this work, the harmlessness treatment of historical lead smelting slag located in Gejiu, Yunnan province, China, and valuable metals recovery from this slag was investigated with a co-treatment of lead slag and zinc oxide ore in the rotary kiln. The valuable metals (Pb and Zn) are effectively recycled in the form of lead–zinc dust with a recovery of above 70%. The content of Pb and Zn in surface cooler dust and bag collector dust is in the range of 20∓45%, which can be used as a raw material for extracting lead and zinc. During the reduction process, arsenate (magnesium arsenate or lead arsenate) in the lead smelting slag was transformed into Fe-As intermetallic compounds encapsulated by gangue particles or complex silicate phases (gangue), and then arsenic is solidified in gangue components. The Cd content in water-quenched slag was low (below 35 mg/kg). On the basis of energy costs and the processing capacity of lead smelting slag, a zinc oxide ore addition of 20% was recommended. The results of TCLP and SNAL leaching tests confirmed that the water-quenched slag from the rotary kiln was stable in the short term. Although further research regarding the slag’s long-term stability is still needed, our works provide an economical and feasible technical process for the treatment of historical lead smelting slag.

Author Contributions

Conceptualization, X.N. and M.L.; methodology, H.W. and L.M.; software, S.W.; validation, T.Z.; formal analysis, W.W.; investigation, X.N.; resources, L.M.; data curation, W.W.; writing—original draft preparation, X.N.; writing—review and editing, M.L., H.W. and L.M.; project administration, X.N.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [the Natural Science Foundation of Yunnan Province] grant number [202101AT070091] and Analysis and Test Foud of Kunming University of Science and Technology grant number [2020T20120041]. And The APC was funded by [202101AT070091, 2020T20120041].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The lead smelting slags stockpiles in Gejiu, Yunnan province in China (The heap amount at points A–E in Figure 1 is shown in Table 1).
Figure 1. The lead smelting slags stockpiles in Gejiu, Yunnan province in China (The heap amount at points A–E in Figure 1 is shown in Table 1).
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Figure 2. The industrial process diagram of the rotary kiln system.
Figure 2. The industrial process diagram of the rotary kiln system.
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Figure 3. The technological process of industrial experiments.
Figure 3. The technological process of industrial experiments.
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Figure 4. Metals (Pb, Zn, As and Cd) recoveries during rotary kiln experiments.
Figure 4. Metals (Pb, Zn, As and Cd) recoveries during rotary kiln experiments.
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Figure 5. Distribution of Pb, Zn, As and Cd in the materials obtained from rotary kiln.
Figure 5. Distribution of Pb, Zn, As and Cd in the materials obtained from rotary kiln.
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Figure 6. Microstructures of FM and WQS. (a–e) being the FM, (a) 100% slag, (b) 80% slag + 20% ore, (c) 60% slag + 40% ore, (d) 40% slag + 60% ore, (e) 20% slag + 80% ore; (a′–e′) being the WQS corresponding to (a–e), respectively.
Figure 6. Microstructures of FM and WQS. (a–e) being the FM, (a) 100% slag, (b) 80% slag + 20% ore, (c) 60% slag + 40% ore, (d) 40% slag + 60% ore, (e) 20% slag + 80% ore; (a′–e′) being the WQS corresponding to (a–e), respectively.
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Figure 7. BSE image of LSS and WQS under different additions of zinc oxide ore. (a) lead smelting slag; (b–f) being water-quenched slag for 100% slag, 20% slag + 80% ore, 40% slag + 60% ore, 60% slag + 40% ore and 80% slag + 20% ore, respectively.
Figure 7. BSE image of LSS and WQS under different additions of zinc oxide ore. (a) lead smelting slag; (b–f) being water-quenched slag for 100% slag, 20% slag + 80% ore, 40% slag + 60% ore, 60% slag + 40% ore and 80% slag + 20% ore, respectively.
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Figure 8. Backscattering EPMA image of solid samples ((a) lead smelting slag; (b) “100% slag” water-quenched slag, (c) “80% slag + 20% ore” water-quenched slag, (d) “60% slag + 40% ore” water-quenched slag, (e) “40% slag + 60% ore” water-quenched slag, (f) “20% slag + 80% ore” water-quenched slag).
Figure 8. Backscattering EPMA image of solid samples ((a) lead smelting slag; (b) “100% slag” water-quenched slag, (c) “80% slag + 20% ore” water-quenched slag, (d) “60% slag + 40% ore” water-quenched slag, (e) “40% slag + 60% ore” water-quenched slag, (f) “20% slag + 80% ore” water-quenched slag).
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Figure 9. Coal powder, water, electricity and comprehensive energy comsumption per ton of lead smelting slag during rotary kiln experiments ((a) caol powder, water and electricity consumption; (b) comprehensive energy comsumption).
Figure 9. Coal powder, water, electricity and comprehensive energy comsumption per ton of lead smelting slag during rotary kiln experiments ((a) caol powder, water and electricity consumption; (b) comprehensive energy comsumption).
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Figure 10. Actual cost during rotary kiln experiments.
Figure 10. Actual cost during rotary kiln experiments.
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Table
Table 1. Storage and sampling weights of each slag heap in Gejiu district.
Table 1. Storage and sampling weights of each slag heap in Gejiu district.
SiteStorage Weight
(Ten Thousand Tons)		Weight Proportion of
Each Slag Heap (%)		Sampling Weight
(t)
A15.298.83106
B68.4039.51474
C77.0044.48533
D6.843.9547
E5.583.2238
Total173.11100.001198
Table
Table 2. Chemical composition of feed material and water quench slag under different ratios (%).
Table 2. Chemical composition of feed material and water quench slag under different ratios (%).
ElementExperiments with Different Ratios
Feed Material (FM)Water-Quenched Slag (WQS)
1#2#3#4#5#1#2#3#4#5#
Pb1.191.260.541.021.330.440.190.100.0750.068
Zn1.742.564.325.516.450.550.530.240.100.18
FeO38.7635.2831.6928.4325.7133.3030.8634.9726.6118.90
SiO215.4215.9612.1012.3810.3519.8219.4918.8719.3618.19
Al2O35.865.344.995.455.057.747.738.347.698.72
CaO7.887.036.618.286.639.979.399.568.9910.09
MgO1.041.191.642.272.141.151.321.721.532.35
F0.0240.0290.0340.0380.0350.030.030.0280.030.028
Cl0.0620.0880.0500.0260.0150.010.028<0.01<0.01<0.01
S4.023.294.592.652.363.754.343.894.082.63
In(g/t)38.1021.7028.4045.1032.5047.9037.3042.3056.0046.80
Ge(g/t)13.7017.7011.9017.4019.0010.1012.6010.6012.7011.00
* Hg3.7214.1014.0016.5015.700.1160.060.0270.0280.029
* Ag1.506.108.708.309.8020.9020.2019.1022.7017.80
* Ni7.7015.9020.6015.1021.2049.7040.6056.0034.1029.10
* Cr14.9015.8022.5017.2020.2040.6037.2056.8040.1026.50
* As4880380028501940176043203800231017101420
* Ba174147177132260504451503546442
* Be1.00ND0.901.500.501.50ND3.001.000.70
* Cd10130239247254527.7025.805.803.3034.90
* Cu3613693342752612170223025101410969
* Se6.407.805.804.206.605.8047.3018.6023.2015.70
* mg/kg.
Table
Table 3. Chemical composition of dust (%).
Table 3. Chemical composition of dust (%).
Dust TypeElementExperiments with Different Ratios
1# (100% Slag)2# (80% Slag
+ 20% Ore)		3# (60% Slag
+ 40% Ore)		4# (40% Slag
+ 60% Ore)		3# (20% Slag
+ 80% Ore)
Settling
chamber
dust (SCD1)		Pb2.213.083.193.932.58
Zn6.129.3510.9018.3017.70
* As93509400508035602700
* Cd792740736790780
Surface
cooler
dust (SCD2)		Pb0.624.074.595.624.64
Zn18.7021.227.225.1027.10
* As139009990774047704080
* Cd11601520164012501440
Bag
collector
dust (BCD)		Pb6.615.685.605.776.25
Zn38.936.637.1037.1035.10
* As110005720800057103800
* Cd18701820182018701660
* mg/kg.
Table
Table 4. Results of TCLP and SNAL leaching tests.
Table 4. Results of TCLP and SNAL leaching tests.
SampleTCLP (mg/L)SNAL (mg/L)
PbZnAsCdPbZnAsCd
Lead smelting slag (LSS)302.33899.1512.111.3650.77182.178.151.78
water-quenched
slag (WQS)		100% slag 0.110.190.920.0170.0840.0630.0710.0014
20% slag + 80% ore0.150.160.830.0140.0630.0500.140.0012
40% slag + 60% ore0.140.210.860.0180.0960.0590.150.0015
60% slag + 40% ore0.170.220.910.0150.0730.0350.160.0012
80% slag + 20% ore0.150.180.950.0140.0430.0230.17ND
Threshold value [U.S. EPA, 1986;
U.S. EPA, 1990; GB 5085.3−2007]		5.005.005.001.005.00100.005.001.00
Table
Table 5. Raw materials consumption during rotary kiln tests.
Table 5. Raw materials consumption during rotary kiln tests.
Ratios of Lead Smelting
Slag to Zinc Oxide Ore		Lead Smelting
Slag Handling
Capacity (t)
Zinc Oxide
Ore Addition
(t)		Gypsum
Addition
(t)		Coal Powder
Addition
(t)		Coal-Feed
Material Ratio
(%)
100% slag338.360.0088.42226.7253.12
80% slag + 20% ore263.7452.8653.36195.3052.79
60% slag + 40% ore226.70148.1445.28208.5849.65
40% slag + 60% ore146.94223.8228.94186.6046.69
20% slag + 80% ore55.88222.3617.66141.7447.90
Total capacity (t)1031.62647.18233.66958.9450.14 (Average)
Table
Table 6. Consumption of energy and energy consuming medium for different material ratios.
Table 6. Consumption of energy and energy consuming medium for different material ratios.
Energy and Energy Consuming
Medium Consumption		100% Slag80% Slag+
20% Ore		60% Slag+
40% Ore		40% Slag+
60% Ore		20% Slag+
80% Ore
* Electricity (kW·h)21367.522470.025147.527772.52814
* Water (t)1296.0588.01249.0460.0696.0
* Coal powder (t)226.7195.3208.6186.6141.7
* Standard coal coefficient of power, water and coal are 0.404 kgce/kW∙h, 0.2571 kgce/t and 0.9 kgce/kg.
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Niu, X.; Li, M.; Wang, H.; Ma, L.; Wang, S.; Zhou, T.; Wang, W. Historical Lead Smelting Slag Harmlessness and Valuable Metals Recovery: A Co-Treatment of Lead Slag and Zinc-Bearing Material in Rotary Kiln. Sustainability 2022, 14, 13647. https://doi.org/10.3390/su142013647

AMA Style

Niu X, Li M, Wang H, Ma L, Wang S, Zhou T, Wang W. Historical Lead Smelting Slag Harmlessness and Valuable Metals Recovery: A Co-Treatment of Lead Slag and Zinc-Bearing Material in Rotary Kiln. Sustainability. 2022; 14(20):13647. https://doi.org/10.3390/su142013647

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Niu, Xuekui, Minting Li, Hongbin Wang, Liping Ma, Shuting Wang, Tao Zhou, and Wei Wang. 2022. "Historical Lead Smelting Slag Harmlessness and Valuable Metals Recovery: A Co-Treatment of Lead Slag and Zinc-Bearing Material in Rotary Kiln" Sustainability 14, no. 20: 13647. https://doi.org/10.3390/su142013647

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