Early LasIR testing for HF analyzing

Three basic versions of instrument have been developed, -the LASIR-R for remote sensing, the LASIR-S for stack monitoring and the LASIR-P for point source monitoring. All three versions use a common control unit and laser housing. The control unit contains:

- The diode laser and its temperature and current control circuits
- The detector and its circuitry,
- The connectors to the launching and receiving optical fibers,
- The interface to the computer for automatic control and data acquisition and logging,
- A reference cell containing a known concentration of the gas

The manufacture of primary aluminium by electrolytic reduction releases HF gas. Hoods over the electrolytic cells (pots) collect much of the emitted gases which are led to scrubbers to remove HF before entering the stacks. Some of the gas, however, escapes into the pot rooms and need to be monitored for health and safety reasons. In addition the scrubbers are not always 00% efficient and some of the HF escapes into the atmosphere, representing an environmental concern as well as an economic loss. It is, therefore, desirable to have an HF analyzer both in the stacks and in the potrooms.

Two very preliminary tests were made with the LASIR-R HF analyzer for pot-room monitoring at two smelters in the USA. For proper pot room installation the telescope and retroreflector should be solidly mounted. Since these were only one day tests, designed to determine whether the sensitivity was sufficient to monitor pot-room concentrations, very rudimentary mountings were used. In June, 1995 , measurements were made between two pots (electrolytic cells) one of which was being worked upon during the test. Calibration was performed using an external cell containing known amount of HF. The higher concentrations at 17.07 corresponded to the actual opening of one of the pots.

These preliminary results indicate that the LASIR HF analyzer systems have adequate sensitivity and time response to measure HF both in stacks and pot-rooms of aluminum smelters.

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