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Session Chairperson: Michel Reverdy, Aluminium Pechiney, Pechiney-Balzac, France
CELL HOODING EFFICIENCY MEASUREMENTS FOR HS SØDERBERG CELLS: Alton T. Tabereaux, Jim Brown, Ivan Eldridge, Reynolds Metals Company, Manufacturing Technology Laboratory, 3326 East 2nd Street, Muscle Shoals, AL 35661
A method has been developed for measuring the gas capture efficiency of volatile gas emissions for the cell hooding system of HS Søderberg cells. No prior method for measuring the gas capture efficiency for HS Søderberg cells has been reported to date. This paper presents the newly developed procedures, techniques and results measured during plant tests for the gas capture efficiency of 92 kA HS Søderberg cells equipped with an improved cell hooding system to reduce plant emissions. The dependency of the gas capture efficiency upon the duct exhaust velocity was determined for the HS Søderberg cells using this method.
COS, CS2 AND SO2 EMISSIONS FROM PRE-BAKED HALL HEROULT CELLS: Frank M. Kimmerle, Luc Noël, Alcan International Ltd., Arvida Research and Development Centre, P.O. Box 1250, Jonquiere, QC, G7S-4K8, Canada; John T. Pisano, Unisearch Ass. Inc., 222 Snidercroft Rd., Concord, ONT, L4K 1B5, Canada
Measurements of CO2, CO, COS and CS2 emissions in collected gas samples by GC-MS and of SO2 by ion chromatography of scrubber solutions allowed us to complete a carbon and sulphur mass balance for P155 pre-bake electrolysis cells. COS and CS2 emissions were confirmed by continuous analysis using tunable diode laser absorption spectrometry of the flue cases from 64 cells over a two day period. While the volume mixing ratios of COS and CS2 of the flue gases agree with those reported recently for a German prebake smelter, the specific emissions were found to be considerably lower.
PERFLUOROCARBON (PFC) GENERATION IN LABORATORY-SCALE ALUMINUM REDUCTION CELLS: Steen Nissen, Donald R. Sadoway, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307
The generation of CF4 and C2F6 is being studied in a laboratory-scale aluminum reduction cell. During electrolysis in well behaved cells and in cells on anode effect, anode gases have been analyzed by gas chromatography (GC) on-line. Process parameters have been systematically varied to determine how PFC generation depends upon anode material (industrial pre-bake and Søderberg), bath ratio (1.45, 1.15 and 0.56), LiF concentration (0 and 2.7%), current density (0.7 A cm-2 and higher), and temperature (970 and 800°C). Measurements agree with those reported for industrial cells with respect to total PFC level as well as the ratio of CF4 to C2F6. The main factor determining PFC concentration appears to be the anodic overvoltage. The functional relationship between the rate of PFC generation and anodic overvoltage is derived from the current-overpotential equation. Research sponsor: U.S. Environmental Protection Agency, Atmospheric Pollution Prevention Division.
REDUCTION OF THE CF4 EMISSIONS FROM PRE-BAKED HALL HEROULT CELLS: Frank M. Kimmerle, Gilles Potvin, Alcan International Ltd., Arvida Research and Development Centre, P.O. Box 1250, Jonquiere, QC, G7S-4K8, Canada; John T. Pisano, Unisearch Ass. Inc., 222 Snidercroft Rd., Concord, ONT, L4K 1B5, Canada
It has been established that CF4 and C2F6 are given off from the Hall-Heroult electrolysis cells during so-called anode effects. The introduction of a novel alumina feeding and cell operation algorithm in the Grande Baie smelter operating with P155 pre-bake electrolysis cells, has dramatically reduced the anode effect duration and anode effect frequency. The implementation of the new cell control logic constituted a unique opportunity to compare data from some two hundred events before and after commissioning on identical cells. Using tunable diode laser absorption spectrometry we measured PFC emissions from individual anode effects with a temporal resolutions of as little as 8 seconds. This communication will describe the experimental set-up, present the decrease in PFC emissions attained, discuss the CF4/C2F6 ratios observed and compare the data observed with model predictions.
10:10 am BREAK
POTROOM COMPARISON TRIAL OF A LASER HF MEASURING INSTRUMENT AND AN INTENSIVE AIR SAMPLING ARRAY OVER A 400 M PATH: D.L. Death, J.E. Eberhardt, R.P. Read, C.A. Rogers, CSIRO Division of Minerals, Private Mail Bag #5, Menai, NSW, 2234 Australia; M. Atkinson, Comalco Research Center, P.O. Box 316, Thomastown, Vic. 3074, Australia; D. Collins, Alcoa of Australia Ltd., Point Henry Works, P.O. Box 460, Geelong, VIC. 3220, Australia; K. Whiteley, Tomago Aluminium Co. Ltd., P.O. Box 405, Raymond terrace NSW, 2324, Australia
The laser hydrogen fluoride measuring instrument described at TMS '95 was compared with an optimised eighty-fold array of air sampling cassettes over a 400m path in a potroom of the Tomago (NSW) aluminium smelter. Thirteen one-hour comparison trials were performed at average HF concentrations over the range 0.07-3.7 mg/Nm3. For the twelve concentrations up to 2.0 mg/Nm3 the cassette array average and the laser average correlated with a slope of 0.93 and a regression coefficient r2=0.98. For the nine points up to 1.5 mg/Nm3 the slope was 1.02 and the regression coefficient r2=0.99. As configured during the comparison trial the laser instrument did not correctly indicate the 3.7 mg/Nm3 HF concentration due to insufficient signal to noise ratio. The correlation between array and laser was considered very good. The technique is being commercialised. The contributions of Frank Fleer of AirWaterNoise Ltd to this work are gratefully acknowledged. This work was partly supported by the Australian Aluminium Council.
THE MEASUREMENT OF GASEOUS FLUORIDE IN THE ALUMINIUM INDUSTRY: John M. Jones, Anglesey Aluminium Metal Ltd., Penrhos Works, P.O. Box 4, Holyhead, Gwynedd, LL65-2UJ, United Kingdom
The introduction of integrated Pollution Control (IPC) by means of the Environmental Protection Act (EPA 90), has brought with it regulatory requirements for operators of industrial processes in the U.K. to undertake continuous monitoring of gaseous atmospheric releases. In response to these changes Anglesey Aluminium Metal Limited (AAM) has chosen to install Tunable Diode Laser Spectroscopy (TDLAS). This method, operating in the Middle Infra Red region (MIR), is gaining increased acceptance as the choice for trace gas measurement where sensitivity, specificity, and fast response are required. This paper focuses on this change to the gaseous fluoride monitoring activities at AAM with particular attention to stack monitoring.
IMPROVED EFFICIENCIES IN THE DRY SCRUBBING PROCESS: Geir Wedde, ABB Environmental, P.O. Box 6260 Etterstad, 0603 Oslo, Norway
An advanced process for removal and recovery of fluorides and particulates from the aluminium reduction pot gas has been developed and tested in pilot plant prior to full scale installation. The dry scrubbing process is arranged in two stages with a counter-current flow of the alumina allowing for the reactive fresh alumina at the tail end of the process. The benefits and features of this concept include high process flexibility with stable and improved performance on removal of fluorides, reduced pressure drop and low particulate emission by optimized filter design and operating performance, and improved compactness through design and equipment developments.
TGT RI - A NOVEL REACTOR / FILTER CONCEPT FOR DRY SCRUBBING: B. Cloutier, Procedair Industries Inc., 625 President Kennedy, Montreal, Quebec, Canada H3A 1K2; Ph. Dumortier, B. Caratge, Procedair SA, 25-27 Boulevard de la Paix, 78951 St. Germain en Laye Cedex, France
The fluorine emissions from modern PFPB pots have led to a requirement to further improve gas scrubbing efficiency. This improvement has been achieved without any detrimental effect on scaling or attrition of alumina. Economic factors create a further technical challenge of reducing capital cost and lowering energy consumption. A novel process called the TGT-RI which combines an innovative reactor concept integrated in a new generation of fabric filter fully responds to the new requirements of aluminium producers. Several years of exhaustive tests have been conducted on gases emitted by large pots to develop the TGT filter and then the TGT-RI concept of integrating the reactor within the filter unit. The paper will incorporate a comprehensive set of test results. A descriptive of the first industrial applications on pot gases and baking furnaces is presented.
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