Characterization of Airborne Heavy Metals within a Primary Zinc-Lead Smelting Works Roy
M. Harrison* and Clive R. Williams+
Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, England
Ian K. O'Neill RTZ Services Ltd., York House, Bond Street, Bristol B S I 3PE, England
w An investigation of airborne Pb, Zn, and Cd was conducted within the workplace environment at an important pyrometallurgical zinc-lead smelter. Particle size distributions for the metals were obtained by using cascade impactors, and chemical species were identified by X-ray powder diffraction spectrometry. The results are interpreted in terms of the various operations occurring within the works. The techniques employed are shown to be of value for the identification ,of emission sources in industrial atmospheres. Introduction Present industrial hygiene standards for toxic metals in air are based upon measurements of the total concentration of the metal in the atmosphere of the workplace ( I ) . Such standards ignore the important influence of particle size and chemical speciation upon the potential toxic hazard associated with the airborne metal ( 2 , 3 ) .The existence of detailed information on size distributions and chemical species is a valuable aid to the industrial hygienist, and this aspect of our results will be discussed elsewhere ( 4 ) . The present work demonstrates that experimental measurements of particle size and speciation may also enable tentative identification of the sources of airborne metals. Hence, an insight may be gained into the efficiency of specific engineering and process controls applied to limit metal releases within a works. While the techniques of determination of particle size distributions are well-known and have been applied widely in urban air ( 3 , 5), those of chemical speciation of airborne metals are less well established. In this work we have used X-ray powder diffraction (XRD), followingthe success of this method in elucidating the atmospheric chemistry of automotive lead compounds (6) and in characterizing lead smelter emissions (7).As indicated elsewhere (8)the major limitation of the XRD technique is that it will identify only major crystalline phases. Although this places limitations upon the value of the results obtained, particularly for highly aged samples (8),it still provides the best available information on the major species present in air. The present study has been carried out within a primary smelter a t Avonmouth, U.K., using the Imperial Smelting Blast Furnace Process. This is a pyrometallurgical method for producing zinc; large quantities of lead are obtained as a secondary product. Cadmium is inevitably present as a minor component of the concentrates fed to the process, and the metal is produced as a byproduct of the smelting and refining processes. + Present address: British Nuclear Fuels Limited! Springfields Works, Salwick, Preston, PR40XJ, England.
0013-936X/81/0915-1197$01.25/0
@ 1981 American Chemical Society
Description of Smelter The present smelting complex at Avonmouth, which was commissioned in December 1967, has an annual capacity of 100 000 tonnes of zinc and 45 000 tonnes of lead ( I O ) . The principal plant buildings are a raw materials store, a sinter plant, a crushing plant, a furnace (ISF), and a refinery (by refluxing). Figure 1 shows the flow of materials in the process ( I O ) . The principal raw materials are zinc-lead flotation concentrates and metallurgical coke. The ore concentrates are conveyed to proportioning bins in the sinter plant, from which a controlled mixture is fed to the sintering machine via a drum mixer. The feed to the sintering machine is ignited by downdraft; oxidation continues by updraft on the main section of the sintering strand. The cleaned offgas, containing 6-6.5% SOz, passes to a standard contact sulfuric acid plant. Oxidized sinter product passes to a crusher house where it is broken, crushed, and screened to provide a size suitable for charging to the furnace. The undersize material, -80% of the total, is recycled to the sintering process. Lump sinter and preheated coke are charged at the furnace top through two double-bell mechanisms. An air blast heated to 800 OC enters through water-cooled tuyeres near the base of the furnace, which operates above the boiling point of zinc but below that of lead. Zinc vapor passes from the top of the furnace to two lead-splash condensers, where it is shock-cooled and absorbed in a spray of molten lead. The lead-zinc solution so formed is cooled in launders to separate the zinc, which is removed by a series of quiescent baths and weirs. Cleaned low-calorific value (LCV) gas from the furnace is used to heat the blast and preheat the coke. Molten lead and slag are tapped into a forehearth a t the bottom of the furnace and are separated via an underflow syphon. The crude zinc is refined by refluxing; cadmium is mostly recovered in this section of the process. Cadmium solutions arising from the recovery of sintering fumes are processed in a cadmium plant on a separate part of the site. Lead bullion is decopperized by stirring with sawdust a t 400 "C to form a dross and then cast and sold to lead refiners. Experimental Section Sampling. Low-volume (1cfm) and high-volume (20 cfm) Andersen cascade impactors were operated at a variety of static locations within the works. Additionally, a 40 cfm HiVol sampler, without cascade impactor, was used a t certain sites. The collection substrates were Whatman EPM 1000 glass fiber, except for the high-volume Andersen sampler, where Gelman type A glass fiber was used. Samplers were positioned 1.2 m above floor level. Sampling periods were optimized such that none of the stages on the impactors was overloaded but that a sample was obtained Volume 15, Number 10, October 1981 1197
representing typical operations at a particular site. At a few sites, where sufficient material was present, samples of floor dust were collected by sweeping. All samples were stored in sealed polythene bags before analysis. Analysis. Size Distributions. Collection surfaces from the low-volume impactors were analyzed for lead, zinc, and cadmium by atomic absorption spectrophotometry following digestion in hot, concentrated nitric acid.
Compound Identification. Segments of high-volume impactor and Hi-Vol collection surfaces were attached to glass slides using amorphous rubber grease. For one sample a preconcentration was effected by stripping the filter into nhexane in an ultrasonic bath and filtering the resulting suspension onto a smaller area ( 1 2 ) . E loor dusts were sieved to obtain the size fraction smaller than 60 pm and sprinkled lightly onto slides coated with rubber grease.
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Figure 1. Imperial Smelting Process materials flowsheet 1198
Environmental Science & Technology
bullion
Coppir drors
Dross and load
GOB Zinc
)Loid
n % Cadmium from codmium plant
Powder diffraction patterns were obtained with a Philips PW 1720 X-ray generator and a P W 1050/70 diffractometer, using Cu Kcu radiation. The compounds represented by each diffraction pattern were identified by manual search of the Joint Committee on Powder Diffraction Standards (JCPDS) (12) powder diffraction files, using botk, d spacings and relative intensities.
Results and Discussion General Considerations. Size distributions for lead, zinc, and cadmium a t the various sites are shown in Figures 2-13. The graphs are plotted in the conventional manner, on log vs. probability axes. Mass-median aerodynamic diameters for the distributions are listed in Table I. In general, one might expect
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Figure 2. Size distributions for airborne lead, zinc, and cadmium at the sinter plant, head of conveyor from raw materials store, on (A) Aug 30, 1979, and on (B) Nov 15, 1979.
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1 10 Cumulative% smaller than given size
Cumulative
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Figure 5. Size distributions for airborne lead, zinc, and cadmium at the sinter plant, fines annex, on (A) Aug 22, 1979, and on (B) Nov 14, 1979.
Cumulative# smaller than given size Flgure 3. Size distributions for airborne lead, zinc, and cadmium at the sinter plant, first floor, proportioning bin house, on Nov 15, 1979.
B
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Figure 6. Size distributions for airborne lead, zinc, and cadmium at the ISF plant, furnace top, on (A) Aug 23, 1979, and on (B) Nov 13, 1979.
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Flgure 4. Size distributions for airborne lead, zinc, and cadmium at the sinter plant, ignition stove top, sintering machine, on (A) Aug 20-21, 1979, and on (B) Nov 14, 1979.
Figure 7. Size distributions for airborne lead, zinc, and cadmium at the ISF plant, condenser floor, on Nov 13-14, 1979. Volume 15, Number 10, October 1981
1199
particles originating directly from the combustion processes of sintering, smelting, and refluxing to appear in the “accumulation mode” range between ca. 0.1 and 2 pm, Particles from attrition, disintegration, and blowage of materials should be larger than this. However, from particle size measurements on zinc-lead smelter fume (9),it appears that, when particles originating as fume are confined in ventilation ducts, the number concentrations of these particles may be sufficiently high to promote agglomeration to particle sizes greater than 10 pm. Compounds identified in atmospheric and floor dust samples by XRD are listed in Tables 11-JV. Depending upon the particular part of the works, concentrations of cadmium in air were typically 2 or 3 orders of magnitude lower than those of lead and zinc, and while it is possible to report size distributions, the concentrations were too low to provide cadmium speciation in all but a few samples. It should be noted that the 60-pm screening of the floor dusts (Table IV) represents a
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direct physical sizing by sieving, as opposed to the aerodynamic classification of the atmospheric samples. Specific Sites. Sinter Plant. Within the proportioning bin house on the sinter plant, the size distributions for the three metals are similar (Figures 2 and 3) and are consistent with dust arising during the transport of the ore concentrates. Al-
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Figure 10. Size distributions for airborne lead, zinc, and cadmium at the ISF plant, bullion floor, on (A) Aug 24, 1979, and on (8) Nov 12, 1979.
8
Cumulative % smaller than given size
Figure 8. Size distributions for airborne lead, zinc, and cadmium at the ISF plant dross plant, on (A) Aug 28, 1979, and on (8) Nov 13-14, 1979.
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Figure 11. Size distributions for airborne lead, zinc, and cadmium at the refinery, fine cadmium column, melt bath, on (A) Aug 28-29, 1979, and on (B) Nov 15-16, 1979.
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Figure 9. Size distributions for airborne lead, zinc, and cadmium at the ISF plant, slagging floor, on (A) Aug 29-30, 1979, and on (B) Nov 16, 1979.
1200
Environmental Science & Technology
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Figure 12. Size distributions for airborne lead, zinc, and cadmium at the refinery, fine cadmium column, condenser sump, on Aug 27-28, 1979.
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Table 1. Mass-Median Aerodynamic Diameters (MMADs) for Size Distributions in Figures 2-13 MMAD, pm
Cd
Zn
Pb
Figure
5.4 >I 1 11 1.2 2.0 7.7
5.9 >11’ 11 6.0 8.5 10
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Figure 13. Size distributions for airborne lead, zinc, and cadmium at the cadmium plant on Aug 29-30, 1979.
though the distributions in Figure 2B are substantially coarser than those in Figure 2A, the total concentrations of the airborne metals were very similar on the two different occasions. Zinc and lead compounds found a t the head of the conveyor from the raw materials store (Tables I1 and IV) are typical of the raw materials, galena (PbS) and sphalerite (P-ZnS) being apparent. ZnO may exist as the mineral zincite within certain raw materials and may also be blown from the conveyor circuit which returns undersize material from the tip end of the sintering process to storage bins in the proportioning house. I t is notable that PbS04 is found in all size fractions of the aerosol. This compound may originate in two ways. Firstly, galena weathers gradually to the sulfate under moist conditions although this process is rather slow (13).Sphalerite weathers in a similar way; however, zinc sulfate could not be
Table II. High-Volume Impactor Samples with Phases Identified by XRD a site
head of conveyor from raw materials store
aerodynamlc dlam, pm
>7.0
P-zns PbS04 PbS ZnO
.
ignition stove top, sintering machine
fines annex
P-zns ZnO PbS04 PbS (Pb0.PbS04(tr)?) PbS04
b
3.3-7.0
Sinter Plant PbS04 PbS P-zns ZnO
