Behavior of Sulfur Oxides in Nonferrous Metal Smelters and

Jun 26, 2019 - The Supporting Information is available free of charge on the ACS ... of SOX emission inventory; and S3, formation of SOX in the furnac...
0 downloads 0 Views 1MB Size
Download PDF
Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

pubs.acs.org/est

Behavior of Sulfur Oxides in Nonferrous Metal Smelters and Implications on Future Control and Emission Estimation Qingru Wu,†,‡ Xiaohui Sun,† Yingbin Su,§ Minneng Wen,† Guoliang Li,† Liwen Xu,† Zhijian Li,† Yujia Ren,† Jing Zou,† Haotian Zheng,† Yi Tang,† Lei Duan,†,‡ Shuxiao Wang,*,†,‡ and Qin Zhang*,§ †

Downloaded via MOUNT ALLISON UNIV on August 6, 2019 at 06:13:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China ‡ State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China § School of Metallurgy, Northeastern University, Shenyang 110819, China S Supporting Information *

ABSTRACT: This study jointly conducted a field investigation and thermodynamic model simulation in three nonferrous metal smelters (NFMS) to identify sulfur oxides (SOX) formation, transformation, and emissions in the flue gas. Most of SOX was released as sulfur dioxides (SO2) at the outlet of the furnace with the molar proportion of sulfur trioxides (SO3) of 1.0−4.1%. The formation of SO3 in smelters depended on temperature, material composition, and flue gas components. These factors were relatively certain once the production was designed. During the use of air pollution control devices (APCDs), SO3 removal alternated with its formation in the APCDs of the smelting/roasting step. Deep clean measures could not ensure standard emissions of SO3 in all smelters. Under the strict production and emission requirements, we recommended the combined effort of production design, parameter optimization, and deep clean measures to control SO3 pollution. In addition, recognizing the underestimation of the national inventory (10% at the most) due to the lack of emissions from NFMS, we suggested the attention on SOX emissions from sectors using high-sulfur raw materials in the pyroprocess. Besides, the higher potential of SO3 on secondary particle formation highlighted the distinction of SO3 and SO2 emissions in inventories for better evaluation of their environmental impact. device combinations (APCDs).15 Although air pollution control in nonferrous metal smelters has achieved great improvement,16 the environmental impact (e.g., acid deposition) due to historical accumulative emissions cannot be ignored. Recently, with the eruption of haze events in China, the contribution of gas precursors to the rapid growth of secondary particle matter is revealed.6 Sulfate radical (SO42−) is one significant composition of the atmospheric particle during the haze period,6,17 which is mainly transformed from gaseous pollutants.18−21 Compared to SO2, the emitted SO3 is much easier to react with water vapor and form SO42−, that is why we can see a blue colored plume at the outlet of the stack when SO3 concentration is greater than 5 ppm.22 Besides, SO3 can react with ammonia or chloride in the flue gas to form submicrometer aerosols and create a white, opaque plume.22 Thus, SO3 pollution control is also an important issue. In addition, due to the high concentration of SO3 in nonferrous metal smelters, SO3 can have negative effects on the

1. INTRODUCTION Air pollution has received increasing attention due to its characteristics of global transportation and severe health impacts.1−3 As the culprit of several air pollution incidents (e.g., haze, acid rain), the emissions of sulfur oxides (SOX) have raised a great deal of attention.4−6 Generally, there are two inorganic species of gaseous SOX, that is sulfur dioxides (SO2) and sulfur trioxides (SO3). Considering the large proportion of SO2 (compared to SO3) in the air and its long-term impact on acid rain and acid deposition, previous studies have paid attention to the emissions of SO2.5,7−10 However, most studies generally focused on SO2 emissions due to the use of energy and lacked the consideration of emissions from sectors using high-sulfur raw materials in the pyroprocess. Nonferrous ore concentrates generally have a relatively high sulfur content, with approximately 25%−32% in zinc sulfide concentrates, 14%− 20% in lead sulfide concentrates, and 18%−41% in copper sulfide concentrates,11−13 which are much higher than the content in coal.14 In addition, most of the refined zinc, lead, and copper are extracted from concentrates by using pyrometallurgical technologies. Thus, sulfur in the concentrates is released to flue gas, and the final SO2 emissions depend on the sulfur removal efficiency of air pollution control © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 9, 2019 June 21, 2019 June 26, 2019 June 26, 2019 DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 1. Schematic diagram of the studied production process.

(such as dust, SO3, arsenic, and fluorine) and dehydrated in the electrostatic demister (ESD). The cleaned flue gas was then used to make sulfuric acid in the double contact and double absorption towers (DCDA). An additional flue gas desulfurization tower (FGD) was installed, and another ESD was applied to get ultralow emissions of SO2 and particles. In the extraction step, the wet process (leaching and purifying) was used in the zinc smelter (smelter A), while the pyroprocess was applied in lead (smelter B) and copper smelters (smelter C). However, the flue gas after dedusting from this step was treated with the smelting flue gas in smelter C, whereas the flue gas after dedusting was discharged in smelter B. In the refining step, wet refining was used in zinc smelters. However, the leaching slag from the extraction step was used to produce zinc oxide with a reclaiming temperature of more than 1000 °C. In lead and copper smelters, products from the extraction step were used to extract refined metal. The flue gas from the above reclaiming/pyrorefining step was dedusted, and SOX was removed in FGD. 2.2. Sampling and Analysis Method. 2.2.1. Sampling Sites. The flue gas sampling sites were designed to be located at the inlets and outlets of APCDs. However, subject to test feasibility, the sampling ports did not completely cover all inlets and outlets of APCDs (Figure 1). Nevertheless, the sampling sites in these smelters have basically covered all commonly used APCDs in nonferrous metal smelters. 2.2.2. Sampling Methods. The EPA method 8A32 was applied to sample SOX in the flue gas with a modified procedure22 to eliminate measurement biases. In principle, flue gas was extracted from the pipeline by using a heated probe with a quartz tube at the temperature of 180 °C. The gas then passed through an inertial filter which was always maintained at the temperature greater than 260 °C by a cylindrical heating mantle. Particles in the flue gas were then captured by the filter. After the filter, a modified Grahm condenser was installed to collect SO3. The condenser was filled with water, and the temperature was maintained between 75 and 85 °C. After that, the flue gas was cleaned by a series of impingers to trap SO2 and then dehydrated by a silica gel drying tube before emitting to air from the pump. During the roasting/smelting step, the SO2 concentrations can vary in the range of less than one mg/Nm3 to several hundred g/Nm3. The minimum detectable limit of the method is 0.5 mg/m3. The upper limits are determined by the number of impingers (total H2O2 amount) and the sampling time, which can be calculated according to the theoretical calculations (eq 1)

performance and operation of furnace and APCDs, such as fouling and corrosion of equipment, efficiency loss in air preheater, and the increase of waste acid amount and stack opacity. Therefore, identifying the SOX behavior in nonferrous metal smelting is significant to evaluate the environmental impact of SOX and to control SOX corrosion during production. In this study, field experiments in three nonferrous metal smelters combining with the thermodynamic model simulation were carried out to identify SOX formation, transformation, and emissions in the flue gas. Based on this, a whole-process control strategy to control SO3 pollution was recommended, and SOX emission inventory from nonferrous metal smelters was compiled to focus attention on SOX emissions from sectors using high-sulfur materials in the pyroprocess.

2. METHODOLOGY 2.1. Studied Smelters. Zinc, lead, and copper are the dominant nonferrous metals with production accounting for more than 90% of the total production of nonferrous metals.23 In addition, pyrometallurgical processes are generally used for metal production in these three kinds of smelters11,12 and cause the emissions of air pollutants.24−30 In zinc smelters, the electrolytic process was the dominant production technology,24,26 with zinc production accounting for approximately 94.6% of total zinc production in 2015.31 As to lead production, approximately 61.6% of lead was produced from the rich-oxygen pool smelting process in 2015.31 In copper smelters, more than 99% of copper was produced by using the flash furnace smelting process and the rich-oxygen pool smelting process. To study SOX behavior in nonferrous metal smelters, we selected three nonferrous metal smelters by considering their metal production, technologies, and applied air pollution control devices (APCDs) (Table S1). The detailed processes and corresponding samples for each smelter were introduced in Figure S1. The schematic diagram was shown in Figure 1. The pyrometallurgical nonferrous metal production process generally consisted of three main steps, including roasting/ smelting, extraction, and refining/reclaiming.24 The roasting/ smelting step was the main node with large emissions of air pollutants. Thus, a series of APCDs were installed for environmental protection. The flue gas was dedusted by dry dust collectors (DC), including waste heat boiler (WHB) and electrostatic precipitator (ESP). The flue gas was further cleaned in the flue gas scrubber (FGS) to remove impurities B

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology n(SO2 ) =

ρmax, N (SO2 ) × Q s × ts

×

2.2.4. Quality Assurance and Quality Control. All bottles such as probe, quartz filter holder, and condenser were thoroughly cleaned and dehydrated in the laboratory before sampling. A leak test was performed for the SOX sampling system, and the flow of the sampling system was calibrated with brass calibration orifices. All filters before weighing (including blank and samplers) were conditioned in a constant temperature (20 ± 5 °C) and relative humidity (40 ± 2%RH) system for 48 h in order to conduct gravimetric analysis. During sampling, the impactors were preheated in the flue gas for 15 min to minimize the influence of moisture. Multiple dilutions of a 100 μg/mL certified SO42− standard solution (GSB 04-2081-2007, supplied by the National Steel Materials Testing Center) were used for the calibration of ICS2000. Each sample was analyzed 3 times, at least, to obtain parallel results, with a relative standard deviation of less than 10%. More than three parallel samplings under stable operating conditions were conducted to ensure the validity of the results. 2.3. Data Processing. 2.3.1. Calculation of Dew Point. Several formulas can be used to calculate the dew point of flue gas, such as the A. G. Okkes formula, formula, Muller curve, Verhoff and Banchero estimation formula, Halstead curve, and Haase and Borgmann. The evaluation and comparison of these equations in a previous study35 indicated that the formula as follows was the most suitable for wet flue gas

273 273 + T

64 ρ(H 2O2 ) × V (H 2O2 ) × N = = n(H 2O2 ) 34

(1)

where n(SO2) and n(H2O2) are the molar mass of SO2 and H2O2, mol; ρmax,N(SO2) is the upper concentration limit of SO2 under standard conditions, g/Nm3; Qs is the sampling flue gas flow, approximately 10 L/min; ts is the sampling time, min; T is the temperature of dry gas meter during sampling, approximately 25 °C; ρ(H2O2) is H2O2 concentration, 3%; V(H2O2) is the volume of H2O2 solution in the impinger, 100 mL; and N is the number of impingers. Thus, eq 1 can be abbreviated as eq 2. ρmax, N (SO2 ) =

616N ts

(2)

Generally, each sampling run lasted approximately 60 min, and two impingers with H2O2 solution were used. Thus, the upper limit of SO2 concentration was approximately 21 g/Nm3. SO2 concentrations in the flue gas after DCDA or FGD were generally lower than this upper limit. Thus, we adopted such a sampling train. At the sites before DCDA in the roasting/ smelting step and FGD in other steps, the number of impingers was arranged according to eq 2 to achieve a higher upper concentration limit. Simultaneously, the sampling time depended on SO3 sampling. When 2/3 of the condenser was covered with SO3 mist, the sampling run was stopped. Thus, we arranged the sampling train and sampling time according to the above two factors. Particles and water vapor content were measured simultaneously. The particles were sampled from flue gas by using a two-stage virtual impactor with Teflon or quartz filters. Teflon filters were used in sites where the temperature was lower than 300 °C, while quartz filters were used in higher temperature sites. The detailed sampling procedure was described in our previous study.33 Water vapor content was measured by using the same equipment for mercury sampling according to the EPA 29 method.34 However, the impingers train in this method was replaced with three impingers filled with silica gel, the weight of which was measured before sampling. The sampling time for particles and water vapor referred to that for SOX. 2.2.3. Sample Pretreatment and Analysis Methods. After sampling, the probe, quartz filter holder, and condenser for the SOX sampling system were rinsed with deionized water at least three times. The rinsed water was collected into a Glassic container and was added to reach a volume of 100 mL. The H2O2 absorption solutions were transferred into centrifuge tubes and sealed preserved. Before analysis, the rinsed water and solutions were pretreated with 0.45 μm filter so as to remove particles. The ion chromatography method was applied to tested SO42− concentrations in the liquid by using the ICS2000 (DIONEX) with a detection limit of 0.01 mg/L. The impingers filled with silica gel were weighed to get the water vapor content. The particle samples were placed in the filter boxes immediately after collection, and the filters were placed facing up until analysis. The filters were measured before and after sampling, and the PM10 concentration was calculated by considering the sampling flow and weight difference.33

Tdp = 186 + 20 L g VH 2O + 26 L g Vso3

(3)

where Tdp is the dew point of flue gas, °C, and VH2O and VSO3 are the contents of water vapor and SO3, respectively, %. Water vapor content and the calculated dew point for each site were listed in Table S2. 2.3.2. Simulation of SOX Transformation in the Flue Gas. SOX transformation in the flue gas was simulated by using the HSC chemical reaction and equilibrium software (http://www. hsc-chemistry.net/). This software is designed for various kinds of chemical reactions and equilibria calculations as well as process simulation, which is one of the most widely used softwares in nonferrous metallurgy with a versatile flowsheet simulation module. 2.3.3. Establishment of SOX Emission Inventory. The SOX emission inventory for one certain metal was compiled according to the following equation ÄÅ ÅÅ Å E = 10 ·∑ ∑ (UEFj ·ALj)·ÅÅÅÅ ∑ λx , i ,R/S·θi ,R/S·(1 − ηi ,R/S) ÅÅ i ,R/S x j ÅÅÇ −6

+

∑ λx , i ,E·pj ·θi ,E·(1 − ηi ,E) + ∑ i ,E

i ,R/R

ÉÑ ÑÑ Ñ λx , i ,R/R ·qj ·θi ,R/R ·(1 − ηi ,R/R )ÑÑÑÑ ÑÑ ÑÑÖ

(4)

where E is emissions amount, kt; x is the SOX type, SO2 or SO3; and j is the production technology, see Table S3. UEF is the unabated emission factor of SOX, kg/t, Table S4. We collected the UEF for SO2 from the guidance for the first national survey of pollution sources. Then we transferred to the data for SOX by considering the SOX speciation profile at the outlet of the furnace according to this study (Table S3). AL is metal productions, t.16,23 λ is the mass proportion of SOx at the exhausted gas after APCD combination, which was derived from this study, % (Table S4). θ is the application proportion of different APCD combinations at each production step, which was collected from Table S6 of Wu C

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology Table 1. SOX Concentrations in the Flue Gasa sampling sites site

description

furnace outlet no. 1 after WHB 1 no. 2 after ESP 1 no. 3 after FGS 1 no. 4 after ESD 1 no. 5 after DCDA no. 6 after FGD 1 no. 7 after ESD 2 no. 8 after WHB 2 no. 9 after ESP 2 no. 10 after ESP/FF no. 11 after FGD 2

smelter A

smelter B

smelter C

SO3 (mg/Nm3)

SO2 (mg/Nm3)

molar ratio of SO3 (%)

SO3 (mg/Nm3)

SO2 (mg/Nm3)

molar ratio of SO3 (%)

SO3 (mg/Nm3)

SO2 (mg/Nm3)

molar ratio of SO3 (%)

6076 2800

115157 83888

4.1 2.6

1533 NT

58315 NT

2.1 NT

1651 NT

130793 NT

1.0 NT

982 NT NT 81

79720 NT NT 500

1.0 NT NT 11.4

NT NT 5 564

NT NT 40000 241

NT NT 0.0 65.2

34 12 6 32

118186 6102 4327 215

0.0 0.2 0.1 10.6

11

86

9.3

NT

NT

NT

NT

NT

NT

3

70

2.7

40

68

32.0

1

15

4.1







5

82

4.7

NT

NT

NT







7

40

12.3

219

1328

11.7

71

21151

0.3

NT

NT

NT

48

639

5.7

4

70

4.1

2

39

3.9

0.3

34

0.8

− represents no sampling sites; NT represents that the concentrations were not tested due to no suitable sampling condition; bold font indicated exhausted gas; WHB − waste heat boilers; ESP − electrostatic precipitator; FGS − flue gas scrubber; ESD − electrostatic demister; DCDA − double contact and double absorption tower; FGD − flue gas desulfurization tower.

a

Figure 2. Formation of SOX in the furnace at reaction equilibrium state in smelter A (a) and smelter B (b).

et al., 2016.16 η is SOX removal efficiency, which was collected mainly based on our study (Table S4). R/S, E, and R/R refer to the roasting/smelting step, extraction step, and reclaiming/ recycling step, respectively. p and q are the proportion of the UEF for extraction and reclaiming/recycling in the UEF for roasting/smelting, respectively, %. We assumed 10% for p and 1% for q derived from this study and adjusted according to the experts’ estimation.

4.1%, 2.1%, and 1.0% in smelters A, B, and C, respectively (Table 1). Smelter A used sphalerite as raw materials where ZnS was the dominant composition. The ZnS was roasted into ZnSO4 or ZnO, and sulfur was transformed into SOX. The existing form of zinc compounds and SOX in the flue gas of a furnace mainly depended on the roasting temperature according to the simulation results at equilibrium status (Figure 2(a)). Before around 600 °C, ZnS tended to be oxidized to ZnSO4, and little SOX existed in the flue gas. With the increase of temperature, ZnSO4 became unstable and decomposed into ZnO and SO3. Simultaneously, the rapid growth of the SO2 equilibrium amount indicated a larger amount of SO3 reduction with increasing temperature. So the resulting level of SO3 in the flue gas reduced to a competition between ZnSO4 decomposition and SO3 reduction. At 600−940 °C, the equilibrium amount of SO3 increased with temperature, which indicated that the temperature increase promoted the net formation of SO3. After 940 °C, the increase of temperature counted against the formation of SO3. During the actual production process, the

3. RESULTS AND DISCUSSION 3.1. SOX Formation in the Furnace. Extraction of metals from sulfide ores during the pyrometallurgical process released the most sulfur from the ore concentrates to flue gas as SOX. Approximately 93.1%, 99.8%, and 86.3% of sulfur in the ores was released, respectively. This resulted in SOX concentrations of 121233, 59848, and 132444 mg/Nm3 at the outlet of the furnace in smelters A, B, and C, respectively. The average molar proportion of SO3 in total SOX reached 2.4%, with D

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology Table 2. SOX Removal Efficiencies Across APCD Combinationsa removal efficiencies of SOx (%) smelter A

smelter B

APCDs

SOX

SO3

SO2

WHB+ESP1 DCDA WHB+ESP+FGS1+ES D1+DCDA FGD1+ESD2 WHB+ESP+FGS1+ES D1+DCDA+FGD1+ES D2 FGD2

33.43

83.84

30.77

SOX

smelter C

SO3

SO2

SOX

SO3

SO2

av

standard value

97.94 −398.4 98.06

9.64 95.03 99.84

22.09 94.30 99.67

16.05 2.61 0.60

−11650 63.21

99.40 99.59

10.74 94.30 99.81

99.52

98.67

99.57

97.99 98.65

87.58

96.95

86.06

86.58

92.91

71.78

93.64

97.53

93.06

90.61

3.82

99.52

98.67

99.57

98.65

63.21

99.59

99.99

99.95

99.99

99.75

0.68

99.65

94.76

99.67

94.95

99.28

94.63

97.30

3.32

Abbreviations: APCD − air pollution control device; WHB − waste heat boilers; ESP − electrostatic precipitator; FGS − flue gas scrubber; ESD − electrostatic demister; DCDA − double contact and double absorption tower; FGD − flue gas desulfurization tower. a

roasting temperature maintained around 940 °C in smelter A when the net formation amount of SO3 at this temperature almost reached the peak. That was the main reason we observed the highest molar proportion of SO3 in total SOX in smelter A. Raw materials in smelter B were much more complicated, including PbS concentrates, lead slag, gold ore, silver ore, and returning dust. During the smelting process, the PbS was oxidized into PbO or PbSiO3, and sulfur in the raw materials was released into flue gas. The molar ratio of SO3 in total SOX at the equilibrium status decreased with an increasing smelting temperature (Figure 2(b)). A similar situation was observed in smelter C (Figure S2). During the actual smelting process, the smelting temperature for smelter B (around 1100 °C) was lower than that in smelter C (around 1265 °C). Besides, the higher ratio of oxygen/materials in smelter B (446 N m3/t) than that in smelter C (365 N m3/t) would inhibit the decomposition of SO3. Thus, the molar ratio of SO3 in total SOX in smelter B was higher than that in smelter C. The comparison between these three smelters indicated that the SO3 proportion at the outlet of the furnace was jointly impacted by the roasting/smelting temperature, material compositions, and flue gas components (e.g., O2 content). However, these factors were generally determined to meet the requirements of normal production. Our study indicated that the SO3 formation rate can be an index for production design. Take smelter A for example. A certain amount of ZnSO4 was required in addition to ZnO production in the roasting process so as to supplement acid loss in the leaching step. Thus, the roasting temperature was required to be in the range of 900− 1000 °C according to the thermodynamic equilibrium of the Zn−S−O−Fe system of this technology.36 However, at the actual roasting temperature (around 940 °C) in smelter A, the SO3 formation rate reached the peak. If SO3 inhibition would be required, the roasting temperature could be raised to the upper limit of the normal production temperature range (1000 °C for smelter A, for example). Considering the complicated reactions in the furnace, we suggested the SO3 formation rate as an index for production design. 3.2. SOX Transformation and Removal Across APCDs. SOX concentrations at different sampling sites were listed in Table 1. The APCD combination used in the roasting/ smelting step of the studied smelters was DC+FGS+ESD +DCDA+FGD+ESD. WHB+ESP was the most commonly used DC in the roasting flue gas of nonferrous metal smelters.

Although gaseous air pollutants such as SO2 were barely removed directly by dry dust removal devices, it did transform continuously across the pipeline and devices. That was why SO2 concentrations decreased in the WHB+ESP (Table 1). The sharp temperature decrease between the furnace outlet and the WHB outlet (300−350 °C) was one significant factor that promoted the decrease of SO2 concentrations according to the thermodynamic reaction process. Higher SO2 removal (Table 2) in the WHB+ESP of smelter A (30.77%) than in smelter C (9.64%) could be attributed to the following reasons. On one hand, a higher O2 concentration (6%) in the furnace of smelter A than smelter C (2.5%) (Table S1) promoted the homogeneous formation of SO3. On the other hand, the higher dust production rate (Table S1) in smelter A (30%) provided more surface for the heterogeneous oxidation of SO2 on the surface of dust than that in smelter C (6%). Although SO2 was transformed to SO3 in the flue gas, SO3 was removed from flue gas simultaneously in the pipeline and WHB+ESP. Approximately 83.84% and 97.94% of SO3 in the flue gas were removed in smelter A and smelter C, respectively (Table 1). Thus, the corresponding SO3 concentrations at the outlet of ESP were reduced to 982 and 34 mg/Nm3, respectively. SO3 removal in WHB+ESP generally included two dominant pathways. First, SO3 reacted with water vapor and formed the sulfuric acid vapor (eq 5). The vapor would condense on the surface of pipeline and devices under a suitable flue gas environment. This was the dominant factor of pipeline and devices corrosion in the nonferrous metal smelters. However, the dew points of sulfuric acid vapor at WHB+ESP (site no. 1 and no. 2) of both smelter A and smelter C were far lower than the flue gas temperature (Table S2). Thus, SO3 removal through eq 5 was limited. Second, SO3 reacted with metal oxides (e.g., ZnO, PbO, Al2O3, Fe2O3, CaO) and formed into metal sulfate on the dust, which was removed in WHB+ESP.37 SO3(g) + H 2O(g) → H 2SO4 (g)

(5)

SO3(g) + MO → MSO4

(6)

The acid plant consisted of the flue gas purification part (FGS +ESD) and the sulfuric acid making part (DCDA). During the flue gas purification part, impurities (e.g., SO3, dust, F, As) were cleaned by FGS, and fog was removed in ESD so as to maintain the activity of catalyst in the DCDA and to avoid the corrosion of devices. Thus, a large amount of SO3 in the flue gas was scrubbed by FGS, accounting for approximately 94% of E

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 3. SOX emission amounts in different smelters.

used to remove both SO2 and SO3. ESD was installed to remove H2O after FGD, which would also remove dissolved SO42−. Approximately 92.91%−97.53% of SO3 in the flue gas was absorbed in FGD+ESD and then recycled as sulfuric acid. SOX removal efficiencies of FGD+ESD were in the range of 86.58%−93.64%. To sum up, the average SOX removal efficiencies of DC +FGS+ESD+DCDA+FGD+ESD reached as high as 99.75 ± 0.68%, which was slightly higher than the combination without FGD+ESD. Nonferrous metal smelters also used dry dust removal devices to capture dust in extraction and the refining/ reclaiming step. New FGDs were installed in the pyrorefining and recycling process to further remove SO2, and the SOX removal efficiency of FGD at the refining/reclaiming step reached as high as 97.30 ± 3.32%. 3.3. SOX Emissions at the Outlets of Stacks. The concentrations of exhausted SO2 at all stack outlets were lower than the special emission limit of 100 mg/Nm,39,40 which reflected the strict requirement of SO2 emission control in China. Due to the high solubility of SO3 in the desulfurization solution, SO3 in most exhaust gas was lower than the limit of Emission Standard of Pollutants for Sulf uric Acid Industry (5 mg/Nm3).41 One exception was observed in the exhaust gas of the roasting step in smelter B, which reached 40 mg/Nm3. The excessive emission in smelter B could be attributed to the following reasons. On one hand, the SO3 concentrations after acid plants reached as high as 564 mg/Nm3, which could not be fully removed in FGD+ESD. On another hand, dissolved SO2 in the liquid could be oxidized to sulfate, which was also a source of SO3 in the exhausted gas. According to Figure 3, existing SOX emission amounts in the studied smelters were in the range of 0.1−0.3 t/d. Compared to the amount before the use of deep clean measures, the emissions were reduced by 98.9%, 77.1%, and 84.0% in smelters A, B, and C, respectively. In smelter A, a large quantity of SOX emissions was reduced in the reclaiming step by using FGD. Thus, the roasting step became the largest emission node in zinc smelters after the application of deep cleaning measures. In smelter B, the application of FGD+ESD at the roasting step reduced approximately 1.0 t/d of SOX emissions, which led to higher emission contribution from the extraction and pyrorefining step. However, the roasting step was still the dominant emission node. Comparing smelter A with smelter C, emissions from the pyrorefining step

total SO3 input into FGS according to the result of smelter C. ESD further removed sulfuric acid fog in the flue gas so as to reach the operation requirements of SO3 < 5 mg/Nm3 at the exit of ESD, and SO3 concentrations were only 4 and 5 mg/ Nm3 at the exits of ESD of smelter B and smelter C, respectively. The cleaned flue gas then passed through DCDA towers, and SO2 in the flue gas was used to make sulfuric acid. More than 99% of SO2 was transformed into SO3 under the catalytic oxidation effect of the vanadic oxide (V2O5) catalyst (eq 7). This catalyst was generally made by using the wet mixing method. The V2O5−K2SO4 mixture, diatomite, and water were poured into the mill line. The formed product was dehydrated and then roasted at 500−550 °C to prepare the catalyst. V2O5 was the dominant active ingredient, and K2SO4 was the assistant in this catalyst.38 SO3 was then absorbed by the condensed sulfuric acid. However, both SO3 concentrations after DCDA in smelter B and smelter C significantly increased, by 11650% and 398.4%, respectively (Table 2), which indicated lower absorption efficiency of SO3 in the studied smelters. Take smelter B for example. Under a normal operation situation, the absorption efficiency should be no less than 99.95%. However, if all reduced SO2 in the conversion tower was regarded to be transformed into SO3, the absorption efficiency of SO3 in the absorption towers was only 98.61% in smelter B. The increased SO3 concentrations after DCDA indicated that SOX emission control in nonferrous metal smelters also relied on the optimization of production parameters. The absorption efficiency of SO3 was impacted by various factors such as the equipment structure, gas velocity, sulfuric acid content of absorption chemical, sulfuric acid temperature, and the flue gas temperature. We noticed that the sulfuric acid content of the absorption chemical reached as high as 99.0%, which was higher than the standard value of 98.3% under a normal operation situation. This situation improved the partial pressure of SO3 in the absorption chemical and reduced the absorption efficiency of gaseous SO3. V2O5

SO2 (g) + 1/2O2 (g) ⎯⎯⎯→ SO3(g)

(7)

To meet the increasingly stringent pollutant discharge standards, pyrometallurgical nonferrous metal smelters have applied FGD and ESD after the DCDA at the roasting/ smelting step for the deep clean of SOX and H2O. H2O2 was F

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 4. SOX emissions from nonferrous metal smelters.

application of deep clean measures such as proper SO3 inhibitors (e.g., FeS) in dust collectors and FGD+ESD after acid plants. Although SOX pollution was a significant problem in nonferrous metal smelters, we were surprised as to the exclusion of this sector in the national and regional SOX emission inventories.5,46−48 According to our estimation, SOX emissions from nonferrous metal smelters due to the use of ore concentrates were also important for national inventory. SOX emissions continued to increase from 1978 to 2005 (Figure 4). The peak value was as high as 2.7 × 103 kt in 2005, which accounted for 10% of national SOX emissions.5,48,49 The accumulative SOX emissions from 1978 to 2014 reached 5.7 × 105 kt, which had caused a severe environmental impact. A similar situation might exist in other industries where highsulfur materials were used in the pyroprocess, such as the sulfuric acid production industry using brimstone as raw materials. In addition, annual SO3 emissions accounted for approximately 2%−5% in total SOX emission from nonferrous metal smelters. Although the SO3 emission amount was significantly smaller compared to SO2 emissions, SO3 has a much higher potential to generate secondary particle pollution.50 Thus, we suggested the addition of SOX emissions from sectors using high-sulfur materials in the pyroprocess to the national/regional inventories and the distinction of SO3 and SO2 emissions for better evaluation of their environmental impact. To sum up, this study identified SOX formation, transformation, and emissions in the flue gas of NFMS by jointly conducting field investigations and thermodynamic model simulations. Based on this, we recommended a whole-process SO3 control strategy by combining the effort of production design, parameters adjustment, and deep clean measures for SO3 pollution control. The monitoring of SOX emissions from NFMS also indicated potential SOX emissions from sectors using high-sulfur materials in the pyroprocess and the necessity to distinguish SO2 and SO3 emissions in the inventory. This study is useful as a guidance to conduct SO3 pollution control in NFMS. In the future, controlled experiments are required to elaborate the SOX removing mechanism and developing control technologies. In addition, potential emissions from other sectors using high-sulfur materials in the pyroprocess should be investigated to improve the national emission inventories.

dominated the whole SOX emissions whether deep cleaning measures was taken or not. The proportion shift of emissions from different steps before and after the application of deep cleaning measures indicated that although the roasting/ smelting step was the dominant sulfur released node, the improvement of APCD combinations focused the attention on SOX emissions from other steps when compiling the emission inventories. 3.4. Implications. The harm of SO3 on the performance and operation of furnace and APCDs has caused an increase in attention to nonferrous metallurgy. Large efforts have tried to inhibit SO3 formation by adjusting the production parameters, so as to avoid the fouling and corrosion of equipment, efficiency loss in air preheater, and the increase of waste acid amount.42−45 Guo et al. pointed out that accurate control of the air-leakage rates of feed opening and flue opening could reduce the SO3 formation by decreasing the oxygen content.42 Raw material dehydration could reduce the water vapor.44 The salinization ratio of dust in the flue gas was suggested to be around 90% so as to reduce SO3 production.45 These studies were generally conducted to solve a specific problem in a typical smelter. In our study, we conducted a whole-process on-site investigation of SOX formation, transformation, and emissions in three smelters and analyzed the impacting factors based on the thermodynamic model. We aimed to propose a whole-process control strategy as a guidance for SO3 pollution control in the nonferrous metal smelters. Our study recommended the combined effort of production design, parameters adjustment, and deep clean measures for SO3 pollution control so as to meet the requirement of both production and emissions. The factors impacting SO 3 formation could be adjusted in a relatively narrow range once the production technology was designed. Thus, the SO3 formation rate was recommended to be chosen as an evaluation index when designing the production technique for a new smelter. For existing smelters, parameters such as the smelting/roasting temperature could be adjusted in the range of normal production conditions for better environmental practices. In dust collectors and acid plants, the production parameters should be adjusted to meet the requirement of production. However, actual production conditions were impacted by various factors such as raw materials composition and stability of equipment. With the stricter requirement of environmental protection, we recommended additional G

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

■

Plan” on PM2.5 concentrations in Jing-Jin-Ji region during 2012− 2020. Sci. Total Environ. 2017, 580, 197−209. (11) Zhu, Z.; He, J. Modern metallurgy of copper; Science Press: Beijing, China, 2003. (12) Zhang, L. New metallurgic technologies for copper; Hunan science and technology publisher: Hunan, China, 2006. (13) Peng, R. Metallurgy of lead and zinc; Science Press: Beijing, 2003. (14) United States Geological Survey (USGS) Mercury content in coal mines in China; Reston, Virginia, United States, 2004. (15) Hylander, L. D.; Herbert, R. B. Global emission and production of mercury during the pyrometallurgical extraction of nonferrous sulfide ores. Environ. Sci. Technol. 2008, 42 (16), 5971−5977. (16) Wu, Q. R.; Wang, S. X.; Li, G. L.; Liang, S.; Lin, C.-J.; Wang, Y. F.; Cai, S. Y.; Liu, K. Y.; Hao, J. M. Temporal trend and spatial distribution of speciated atmospheric mercury emissions in China during 1978−2014. Environ. Sci. Technol. 2016, 50 (24), 13428− 13435. (17) Cheng, Y. F.; Zheng, G. J.; Wei, C.; Mu, Q.; Zheng, B.; Wang, Z. B.; Gao, M.; Zhang, Q.; He, K. B.; Carmichael, G.; Poschl, U.; Su, H. Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China. Sci. Adv. 2016, 2 (12), e1601530. (18) Yang, H. H.; Lee, K. T.; Hsieh, Y. S.; Luo, S. W.; Huang, R. J. Emission characteristics and chemical compositions of both filterable and condensable fine particulate from steel plants. Aerosol Air Qual. Res. 2015, 15 (4), 1672−1680. (19) Li, J. W.; Qi, Z. F.; Li, M.; Wu, D. L.; Zhou, C. Y.; Lu, S. Y.; Yan, J. H.; Li, X. D. Physical and chemical characteristics of condensable particulate matter from an ultralow-emission coal-fired power plant. Energy Fuels 2017, 31 (2), 1778−1785. (20) Wang, G.; Deng, J. G.; Ma, Z. Z.; Hao, J. M.; Jiang, J. K. Characteristics of filterable and condensable particulate matter emitted from two waste incineration power plants in China. Sci. Total Environ. 2018, 639, 695−704. (21) Yang, H. H.; Arafath, S. M.; Wang, Y. F.; Wu, J. Y.; Lee, K. T.; Hsieh, Y. S. Comparison of coal- and oil-fired boilers through the investigation of filterable and condensable PM2.5 sample analysis. Energy Fuels 2018, 32 (3), 2993−3002. (22) Cao, Y.; Zhou, H. C.; Jiang, W.; Chen, C. W.; Pan, W. P. Studies of the fate of sulfur trioxide in coal-fired utility boilers based on modified selected condensation methods. Environ. Sci. Technol. 2010, 44 (9), 3429−3434. (23) Nonferrous Metal Industry Association of China (NMIA) Yearbook of Nonferrous Metals Industry of China; NMIA: Beijing, China, 2006−2016. (24) Wu, Q. R.; Wang, S. X.; Zhang, L.; Song, J. X.; Yang, H.; Meng, Y. Update of mercury emissions from China’s primary zinc, lead and copper smelters, 2000−2010. Atmos. Chem. Phys. 2012, 12 (22), 11153−11163. (25) Wu, Q. R.; Wang, S. X.; Hui, M. L.; Wang, F. Y.; Zhang, L.; Duan, L.; Luo, Y. New insight into atmospheric mercury emissions from zinc smelters using mass flow analysis. Environ. Sci. Technol. 2015, 49 (6), 3532−3539. (26) Wu, Q. R.; Wang, S. X.; Zhang, L.; Hui, M. L.; Wang, F. Y.; Hao, J. M. Flow analysis of the mercury associated with nonferrous ore concentrates: Implications on mercury emissions and recovery in China. Environ. Sci. Technol. 2016, 50 (4), 1796−1803. (27) Wang, S. X.; Song, J. X.; Li, G. H.; Wu, Y.; Zhang, L.; Wan, Q.; Streets, D. G.; Chin, C. K.; Hao, J. M. Estimating mercury emissions from a zinc smelter in relation to China’s mercury control policies. Environ. Pollut. 2010, 158 (10), 3347−3353. (28) Engels, L. H. Dioxin and furan emissions from a smelting plant for nonferrous metals. Staub - Reinhalt. Luft 1995, 55 (11), 428−428. (29) Li, M. J.; Mi, Z. F.; Coffman, D.; Wei, Y. M. Assessing the policy impacts on non-ferrous metals industry’s CO2 reduction: Evidence from China. J. Cleaner Prod. 2018, 192, 252−261. (30) Du, Z. L.; Lin, B. Q. Analysis of carbon emissions reduction of China’s metallurgical industry. J. Cleaner Prod. 2018, 176, 1177−1184.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01600.

■

S1, smelter flowchart, sampling sites, and operation parameters; S2, parameters for the establishment of SOX emission inventory; and S3, formation of SOX in the furnace (PDF)

AUTHOR INFORMATION

Corresponding Authors

*S.W. Phone: +86 1062771466. Fax: +86 1062773597. E-mail: [email protected]. *Q.Z. Phone: +86 18602420020. Fax: +86 2423906316. Email: [email protected]. ORCID

Qingru Wu: 0000-0003-3381-4767 Xiaohui Sun: 0000-0002-5346-0305 Lei Duan: 0000-0001-9965-4618 Shuxiao Wang: 0000-0001-9727-1963 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by National Key Research and Development Program of China (2017YFC0210401).

■

REFERENCES

(1) Zhao, B.; Zheng, H. T.; Wang, S. X.; Smith, K. R.; Lu, X.; Aunan, K.; Gu, Y.; Wang, Y.; Ding, D.; Xing, J.; Fu, X.; Yang, X. D.; Liou, K. N.; Hao, J. M. Change in household fuels dominates the decrease in PM2.5 exposure and premature mortality in China in 2005−2015. Proc. Natl. Acad. Sci. U. S. A. 2018, 115 (49), 12401−12406. (2) Lin, J.; Pan, D.; Davis, S. J.; Zhang, Q.; He, K.; Wang, C.; Streets, D. G.; J, D.; Wuebbles, D. J.; Guan, D. China’s international trade and air pollution in the United States. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (5), 1736. (3) Kikuchi, R. Environmental management of sulfur trioxide emission: Impact of SO3 on human health. Environ. Manage. 2001, 27 (6), 837−844. (4) State Council of the People’s Republic of China (SC) Action plan of national air pollution prevention and control; SC: Beijing, China, 2013. (5) Wang, S. X.; Zhao, B.; Cai, S. Y.; Klimont, Z.; Nielsen, C. P.; Morikawa, T.; Woo, J. H.; Kim, Y.; Fu, X.; Xu, J. Y.; Hao, J. M.; He, K. B. Emission trends and mitigation options for air pollutants in East Asia. Atmos. Chem. Phys. 2014, 14 (13), 6571−6603. (6) Guo, S.; Hu, M.; Zamora, M. L.; Peng, J. F.; Shang, D. J.; Zheng, J.; Du, Z. F.; Wu, Z.; Shao, M.; Zeng, L. M.; Molina, M. J.; Zhang, R. Y. Elucidating severe urban haze formation in China. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (49), 17373−17378. (7) Zhang, Q. Q.; Wang, Y.; Ma, Q.; Yao, Y.; Xie, Y.; He, K. Regional differences in Chinese SO2 emission control efficiency and policy implications. Atmos. Chem. Phys. 2015, 15 (11), 6521−6533. (8) Liu, Q. L.; Wang, Q. Reexamine SO2 emissions embodied in China’s exports using multiregional input-output analysis. Ecol. Econ. 2015, 113, 39−50. (9) Shahzad, K.; Saleem, M.; Ghauri, M.; Akhtar, J.; Ali, N.; Akhtar, N. A. Emissions of NOx, SO2, and CO from co-combustion of wheat straw and coal under fast fluidized bed condition. Combust. Sci. Technol. 2015, 187 (7), 1079−1092. (10) Cai, S. Y.; Wang, Y. J.; Zhao, B.; Wang, S. X.; Chang, X.; Hao, J. M. The impact of the “Air Pollution Prevention and Control Action H

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology (31) Wu, Q.; Li, G.; Wang, S.; Liu, K.; Hao, J. Mitigation Options of Atmospheric Hg Emissions in China. Environ. Sci. Technol. 2018, 52 (21), 12368−12375. (32) North Carolina Association of Self-Insurers (NCASI) Method 8A- determination of sulfuric acid vapor or mist and sulfur dioxide emissions from kraft recovery furnaces; NCASI: Washington, DC, United States, 1996. (33) Jiang, J. K.; Deng, J. G.; Duan, L.; Zhang, Q.; Li, Z.; Chen, X. T.; Li, X. H.; Hao, J. M. Development of two stage virtual impactor for stationary source PM10 and PM2.5 sampling (in Chinese). Environ. Sci. 2014, 35 (10), 3639−3643. (34) United States Environmental Protection Agency (US EPA) US EPA Method 29: Determination of metals emissions from stationary sources; US EPA: Washington, DC, United States, 2000. (35) Tang, Z.; Jin, B.; Sun, K.; Zhong, Z. The contrast and evaluation of the calculation formulas of acid dew point temperature of the flue gas after the WFGD. Journal of power engineering 2005, 25, 18−21. (36) Zhai, X. J. Heavy Metal Metallurgy; Metallurgical industry publisher: Beijing, 2011. (37) Xiang, B. X.; Shen, W. F.; Zhang, M.; Yang, H. R.; Lu, J. F. Effects of different factors on sulfur trioxide formations in a coal-fired circulating fluidized bed boiler. Chem. Eng. Sci. 2017, 172, 262−277. (38) Chen, Z. Development and Application of High-active Low Temperature Vanadium Catalyst; South Central University: Changsha, Hunan, 2002. (39) Ministry of Environmental Protection (MEP) Emission standard of pollutants for lead and zinc industry (GB 25466-2010); MEP: Beijing, China, 2010. (40) Ministry of Environmental Protection (MEP) Emission standard of pollutants for copper, nickel, cobalt industry (GB 25467-2010); MEP: Beijing, China, 2010. (41) Ministry of Environmental protection (MEP) Emission Standard of Pollutants for Sulfuric Acid Industry (GB 26132-2010); China Standardization Publisher: Beijing, China, 2014. (42) Guo, X.; Yang, S.; Wang, Q.; Songsong, W. Formation and inhibition of SO3 in oxygen-enriched smelting flue gas. Chin. J. Nonferrous Met. 2018, 28 (10), 2077−2085. (43) Yuan, J.; Wang, X.; Li, K. Exploration and application of reducing SO3 content in the flue gas of sulphuric acid production from copper smelting. Sulphuric Acid Ind. 2018, 6, 21−27. (44) Qi, Y. The study on SO3 productivity rate in oxygen-rich flash smelting gas. Nonferrous Metals 2002, No. 1, 18−21. (45) Zhou, J. Analysis on dust accretion and SO3 formation in flash smelting furnace waste heat boiler. Nonferrous Metals 2003, No. 1, 17−20. (46) Zhang, Q.; Streets, D. G.; Carmichael, G. R.; He, K. B.; Huo, H.; Kannari, A.; Klimont, Z.; Park, I. S.; Reddy, S.; Fu, J. S.; Chen, D.; Duan, L.; Lei, Y.; Wang, L. T.; Yao, Z. L. Asian emissions in 2006 for the NASA INTEX-B mission. Atmos. Chem. Phys. 2009, 9 (14), 5131−5153. (47) Wang, Y.; Zhang, Q. Q.; He, K.; Zhang, Q.; Chai, L. Sulfatenitrate-ammonium aerosols over China: response to 2000−2015 emission changes of sulfur dioxide, nitrogen oxides, and ammonia. Atmos. Chem. Phys. 2013, 13 (5), 2635−2652. (48) Zheng, H.; Cai, S.; Wang, S.; Zhao, B.; Chang, X.; Hao, J. Development of a unit-based industrial emission inventory in the Beijing-Tianjin-Hebei region and resulting improvement in air quality modeling. Atmos. Chem. Phys. 2019, 19 (6), 3447−3462. (49) Yang, J.; Schreifels, J. Implementing SO2 emissions in China; Organisation for Economoic Co-operation and Development: Paris, France, 2003. (50) Mikkonen, S.; Romakkaniemi, S.; Smith, J. N.; Korhonen, H.; Petaja, T.; Plass-Duelmer, C.; Boy, M.; McMurry, P. H.; Lehtinen, K. E. J.; Joutsensaari, J.; Hamed, A.; Mauldin, R. L.; Birmili, W.; Spindler, G.; Arnold, F.; Kulmala, M.; Laaksonen, A. A statistical proxy for sulphuric acid concentration. Atmos. Chem. Phys. 2011, 11 (21), 11319−11334.

I

DOI: 10.1021/acs.est.9b01600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX