Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Patent
1982-11-17
1985-08-13
Smith, Alfred E.
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
250338, 250343, 356437, G01N 2135
Patent
active
045352410
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to improvements in and relating to a method and apparatus for the determination of concentration of gaseous hydrogen fluoride.
BACKGROUND ART
Gaseous hydrogen fluoride is evolved in several industrial processes, the most important of which are aluminium smelting and enrichment of uranium via uranium hexafluoride. As hydrogen fluoride is a serious atmospheric pollutant, it is important that the amount of hydrogen fluoride released into the atmosphere is monitored to control its release in such processes. Gaseous hydrogen fluoride may also be generated by power stations and in brickworks.
Whilst the following description is directed to monitoring hydrogen fluoride evolution in the aluminium smelting industry, it will be apparent that the method and apparatus of the present invention can be employed to detect and determine gaseous hydrogen fluoride in any medium with significant transmission at the appropriate frequencies.
Present proposals for aluminium smelting in the Hunter Valley of N.S.W. envisage a large increase in aluminium output ("Pollution control in the Hunter Valley with Particular Reference to Aluminium Smelting" N.S.W. State Pollution Control Commission (hereinafter referred to as SPCC) Report, August 1980).
In the production of aluminium, alumina powder is electrolysed in molten cryolyte (Na.sub.3 AlF.sub.6) to aluminium metal and oxygen which combines with the carbon of the carbon anodes to produce carbon monoxide and carbon dioxide. During the process about 20 kg of fluoride, mainly in the form of gaseous hydrogen fluoride is emitted per tonne of aluminium produced.
Aluminium production is carried out in long pot-rooms, which are typically 800.times.25 m and which each generally contain about 100 electrolytic cells. Hoods over the electrolytic cells draw off approximately 97.5% of the emitted gaseous hydrogen fluoride and exhaust it through dry scrubbers which retain 99.9% hydrogen fluoride, thence to stacks. The main source of fluoride pollution is gaseous hydrogen fluoride escaping from the cell hoods, particularly when anodes are changed. The gaseous hydrogen fluoride is released from the roof vents of the pot-room as described in the SPCC report, August 1980 supra. The maximum concentration allowed in the pot-rooms on the ground of industrial health is 2 mg.m.sup.-3. The SPCC have imposed an obligation on the smelter operators to monitor continuously the gaseous hydrogen fluoride concentration in the pot-rooms with levels in the range 40-400 .mu.g.m.sup.-3 as a suggested working range. Monitoring gaseous hydrogen fluoride conveniently and economically has heretofore posed many problems.
Present monitoring methods involve pumping air containing gaseous hydrogen fluoride through heated filters to remove fluoride particulates and then through solutions or lime beds. The calcium fluoride formed is assayed using a fluoride selective electrode. These methods are said to be inconvenient, not suitable for reliable automation, and not very accurate.
Whilst not required at present, it is proposed that the SPCC will require aluminum smelter operators to monitor continuously the scrubber stack emmissions, and the gaseous hydrogen fluoride concentrations at four positions inside and two positions outside the plants. Permissible gaseous hydrogen fluoride levels outside the pot-rooms may be in the range 1-10 .mu.g.m.sup.-3.
Atmospheric pollutants may be monitored by long path length optical absorption measurements as described in "Troposcopic Photochemical and Photophysical Processes", J. N. Pitts and B. J. Finlayson-Pitts; "Remote Sensing Using Tunable Lasers", K. W. Rothe and H. Walter; in "Tunable Lasers and their Applications," ed. A. Mooradian et al, Springer-Verlag 1976. When laser light sources are employed, ranges of up to 10 km are possible. With optical radar techniques using topographic reflectors or aerosol scattering it is possible to range gaseous absorbers up to a few kilometers from the light source.
The concentrations of gaseous hydrogen fluoride can be
REFERENCES:
patent: 2930893 (1960-03-01), Capenter et al.
patent: 3998557 (1976-12-01), Javan
patent: 4061918 (1977-12-01), Preier et al.
Laguna et al, "Direct Measurement of the Absorption Coefficient for the P.sub.1 (7) Transition in HF", J. App. Phys., 46 (11), Nov. 1975, pp. 5049-5050.
Australian Atomic Energy Commission
Fields Carolyn E.
Smith Alfred E.
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