U N I T 8
Prior to the 1940's only limited efforts were directed towards capturing and recovering fumes emitted from the cells. Until then the size of the individual plants was small, and therefore the total amount of fluorides that could be emitted was low compared to present day standards. Consequently, damage to vegetation surrounding smelters was rarely observed. However, with the subsequent increase in the size of smelters, damage to plants and livestock was observed in a few localities. This led to a growth in the knowledge of the effect of gaseous fluorides, which are now accepted as being much more toxic than the particulate fluoride emissions. This has influenced the design and location of smelters especially since the gaseous emissions are difficult, if not impossible, to totally prevent. Smelters invariably have accepted the responsibility of meeting environmental standards imposed on them, but as an added safeguard they are often situated in locations where the prevailing atmospheric conditions can normally be expected to transport stray emissions to areas where they can do no harm.
Operating procedures such as anode changing, anode setting, metal tapping, and alumina feeding make it impossible to operate cells continuously with a totally enclosed design. Thus gas collection can only be effected by using a hooding system that is designed for opening in sections for the specific cell operations.
Early attempts to capture the emissions varied from water spray systems located on the pot-room roof to exhausting the gases collected from the hooded cells through duct work, leading to a gas scrubbing system. Cells fitted with Soderberg anodes, have added problems due to the tars emitted and it is desirable to burn these volatile hydrocarbons so they do not interfere with any gas cleaning process.
Efficient capture of the fluoride emissions has the added economic advantage that it reduces damage caused in the potroom by corrosion and this has been another economic incentive towards total control of the emissions.
With the diminishing supply of natural cryolite, the losses of fluorine compounds from the cell assumed a new significance. Cleaning technology had to be developed to ensure useful fluoride by-product results, thus minimizing the loss of this valuable resource.
The waste gas composition depends on the carbon quality and the type of anode used, with the prebaked anode cells being fairly free from hydrocarbon emissions. Even with after-burners the Soderberg cell gases today still contain 1 to 3 kg of tar products per ton of aluminium produced. The rate of losses is dependent on cell type (being greater in Soderberg cells), the electrolyte used, the operating temperature and the current density. Because of the sulphur content in the anodes, oxides of sulphur are also emitted with the anode gases and these are equivalent to 3 to 15 kg of sulphur per ton of aluminium produced.
From the preceding it is obviously that not only is the industry faced with the technical difficulties in capturing the emissions, but it, is also dealing with a complex system for subsequent purification. Two different approaches have been developed for handling these emissions, these being the wet and dry scrubbing process. In the early period of gas scrubbing, uncertainties existed about the fluoride removal efficiency of the equipment used because of the absence of adequate sampling and analytical procedures. For example the sampling equipment rarely proved efficient enough to register the fine sub-micron fluoride fraction of particulate material. Subsequently with the improvement in micro-particle sampling wet scrubbers and cyclones have been shown to be less efficient than was originally claimed. A major change in emission control technology occurred about 1960 with the development of a completely dry gas cleaning system that was capable of high fluoride recovery efficiency. The first dry scrubbing system was based on the use of activated alumina, but subsequently the process has been modified to operate with metallurgical grade alumina.
Besides scrubbing emissions from cells, increasing attention is being directed towards cleaning flue gases from anode baking furnaces to lower further the total fluorine emissions. When baking anodes fluoride emissions arise from the electrolyte adhering to anode butts which are crushed and recycled during the prebaked anode formulation.
EXERCISES
1. Напишите транскрипцию и запомните произношение следующих слов:
capturing, recovering, individual, livestock, knowledge, responsibility, procedures, exhausting, desirable, cryolite, technology, hydrocarbon, equivalent, obviously, purification, uncertainties, adequate, micro-particle, improvement, cyclones, efficiency, modified, metallurgical, adhering, prebaked.
2. Найдите в тексте слова и выражения, соответствующие указанным значениям:
разрушение растительности, последующее увеличение, выделение твердого фторида, существующие атмосферные условия, подача глинозема, закрытая конструкция, система отвода конвертерных газов, свод огнеупорной печи, система очистки, летучий углеводород, эффективное улавливание, норма потерь, выбор оборудования, налипание к анодной массе.
3. Составьте вопросы к тексту, соответствующие заданным ответам.
Example: In a few localities.
Where was damage to plants and livestock observed?
1. Where they can do no harm.
2. By using a hooding system.
3. Operating procedures such as anode changing, anode setting, metal tapping, and alumina feeding.
4. These being the wet and dry scrubbing process.
5. Because of the absence of adequate sampling and analytical procedures.
6. About 1960.
7. On the use of activated alumina.
8. To lower further the total fluorine emissions.