At Alcoa, Tenn., the Aluminum Company of America operates a modern, center-worked, prebake, primary aluminum smelter. At this location, the company uses the latest technology for the Hall-Heroult smelting process. In this electrolytic process for the production of aluminum, the emission of gaseous fluoride is inevitable, resulting from the electrolyte-molten fluoride salts-in which the aluminum oxide is dissolved. Obviously, uncontrolledfluoride emissions are an environmental concern. Excessive levels of fluoride can cause visible effects on vegetation and contaminate foraging cattle. In the U.S., there are about 32 aluminum smelting operations. Alcoa has nine. All told. there are 12 primary producers including Reynolds. Kaiser, Anaconda, Intalco. Eastaico, and Conalco. But not only these aluminum processors are concerned with gaseous fluoride monitoring; other industrial activities include superphosphate producers of fertilizer, glass manufacturers, and steel manufacturers-all are sources of fluoride emissions. Several states, including Tennessee, Washington, and Texas, have adopted regulations to protect ambient air and grazing animals. Tennessee does not have vegetation standards. In addition, the Environmental Protection Agency has developed new source performance regulations for the primary aluminum reduction plants. These were published in the Federal Register, January 1976, and are presently the subject of a law suit. The EPA regulations limit fluoride emissions at the point of discharge-the smelter and its control equipment-to 1.9 Ibs fluoride per ton of aluminum produced, for prebaked cells.
Monitoring fluorides at an aluminum Aluminum Company of America checked its emissions by three techniques and compared the data
Controls and monitoring Chief chemist J. C. Vergho, at the Tennessee piant site, says that real-time, continuous ambient air monitoring is of interest to Alcoa. It provides useful data on the overall operation of a complex production system and its related control equipment. Vergho points out that Alcoa uses the Alcoa 398 process, a fluidized bed equipped with a baghouse, to control 5 5 0 . Environmental Science & Technology
fluoride emissions. The fluid bed sorbs gaseous fluoride emissions from the smelting cell; then particulate fluoride emissions are trapped in the baghouse. Engineers, using data from conventional emission monitoring test procedures, evaluated the efficiency of the control equipment. These data on emissions, ambient air, and vegetation provide historical data. But now, continuous real-time information on emissions can be provided. (Alcoa does not have, as yet, real-time ambient fluoride data.) Real-time implies that data reduction is accomplished by computer on an hourly averaged basis. The advent of automated ambient fluoride instrumentation, using the recently introduced Philips monitor, facilitates monitoring for several reasons: Several states (including Washington) require the reporting of ambient fluoride data in the vicinity of the emission point source. A reliable. real-time, automated analyzer in the'field can save laboratory man-hours. Ambient fluoride and meteorological data, coupled with computer modeling and field observations, aid in the assessment of the atmospheric dispersion and downwind impact of fluoride emissions upon receptors. As environmental impact assessment needs become more complex, the need for real-time data, rather than a past history of perspective, increases in importance. A field evaluation of the Philips new HF monitor PW9795 was performed at the Tennessee plant site, since it is Alcoa's policy to evaluate a new monitoring instrument prior to any purchase. In cooperation with Philips personnel, a field trial was conducted with the following criteria for monitoring: to locate the monitoring site downwind of the emission point source to sample from a common sampling intake, a 30 acfm PVC manifold to situate the instruments in a monitoring shelter maintained at 70 f 5 O F to reduce and evaluate the sample data and log maintenance requirements. In this program, three test methods
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AIcoa'S Vergho "adjusfing monitor"
Correlation of gaseous fluoride data 5.0
4.0
JY
..-B E
3.0
Y
0
E
O5
es"
0
0
1.0
0.0 I nterva I m b flouride
2.0 3.0 RAC tape monitor in ppb RAC Dual Tape Monitor No. of Observations
4.0
5.0
Philips HF Monitor
No. of Observations
0.0-0.5 0.5-1.5
58
70
25
1.5-2.5 2.5-3.5 3.5-4.5
8 9 4
11 12
were compared, two of which yielded data on particulate and gaseous fluoride and one on gaseous fluoride only. The methods
The fluoride dual-paper tape sampler, the Research Appliance Corp. Model 2356, is designated ASTM Method D 3266-73T, "Automated Separation and Collection of Particulate and Acidic Gaseous Fluoride in the Atmosphere". Two treated tapes, through which the sample stream is drawn at approximately 1 scfm, are indexed simultaneously at a preset interval. The first tape traps particulates. It is impregnated with citric acid and allows the passage of gaseous fluoride, which is then collected on the second tape impregnated with caustic. The tape-sampler process consists of field collection of the used tapes, subsequent laboratory analysis (particulate fluoride: ash, fuse, distill, and use of an appropriate readout; gaseous fluoride: pretreatment and selective ion readout or equivalent), and data reduction. For hydrogen fluoride the sensitivity and range bracket 2.4 to 90 ppb during 10 to 180 minute sample intervals. In the Philips HF Monitor, ambient air is continuously scrubbed by an absorption liquid, increasing the gaseous fluoride mass with time. This mass, which is related to the atmospheric HF concentration, averaged over the period since the
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4
start of measurement, is continuously measured potentiometrically, using a fluoride ion-selective electrode. The measuring periods, and the calibration intervals, may be independently selected from the various ranges. Also, a simple switch setting permits manual, automatic, or remote computer operation. On a 12-h measure cycle, the minimum detectable concentration is 0.12 ppb, with an upper range limit of 60 ppb. The manual filter and impinger train, ASTM Method D 3267-73Tr was used to provide a third comparative method. It consists of a citric-acid-impregnated cellulose filter for particulates, a plated impinger with 150 mL 1.0 N NaOH solution, and mist trap followed by a calibrated dry test meter and a vacuum pump. Laboratory analysis is similar to the tape sampler: ash, fuse, distill, and use of an appropriate readout for particulate fluoride electrode or equivalent is used to determine the gaseous fluoride in the impinger solution. A typical 12-h impinger train test can determine a minimum concentration of 0.1 ppb, with a relatively unlimited upper saturation range limit for ambient analysis. Results
During the period October 1-November 30, 1976, 104 direct comparisons were made between data collected by the Philips PW 9795 HF Monitor and the RAC
Fluoride Tape Sampler, Model 2356. Each data point represents a 12-h average in ppb of gaseous fluoride. Both monitors pulled the sample gas from a common positive pressure manifold. Statistical evaluation of the test results shows the Philips data to have a negative bias in the low range and a positive bias in the high range. In the range of interest,
