National Mineral Resources University (Mining University), Saint Petersburg, Russian Federatation:

A. A. Pyaterneva, Post-Graduate, Department of Metallurgy
V. Yu. Bazhin, Professor, Dean of mineral raw materials processing faculty

Exhaust gases of aluminum reduction process in cells with prebaked anodes has a high content of fluoride, dust and hydrogen oxides. Under the modern ecological standards, aluminum plants are the ecological hazard and obey regulations on the maximum permissible emissions. The hazardous emission sources in the aluminum production are the crude products and consumables. The main sources are alumina, coal anode material (pre-baked coal anodes) and fluoride salts. During aluminum reduction process fluoride emissions have the greatest impact. Almost total fluoride emissions are entrapped in the system of dry gas treatment where alumina sand type is used as an active adsorbent. In transition to a technology with low cryolite ratio, it becomes even more important to stabilize composition of electrolyte since the operating mode with the low cryolite ratio is more sensitive to exposure and production impact. For that reason, the composition of electrolyte and other process parameters can suffer imbalance. Under higher amperage, fluoride emissions from the surface of electrolyte grows since aluminum fluoride partly adsorbed at the cryolite-alumina crust evaporates most intensively at low cryolite ratio and under temperature fluctuations. Main flow of volatile matters with fluoride goes with industrial gases (hydrogen oxide and dioxide) in the system of gas treatment where it flows through the bed of alumina sand type in adsorbingreactor. Fluorinated alumina is one of the items in the fluoride balance in aluminum electrolysis, and its change is governed by the change in specific consumption of all fluoride salts. The technology of dry cleaning of electrolysis gases is so far the most ecology-friendly process, and return of secondary alumina (after the dry gas treatment) in the electrolysis enables reduction in fluoride flow. This article discusses the issue of adsorption capacity of metallurgical alumina, which is closely related with the performance of dry gas treatment plants for removal of fluoride compounds from aluminum reduction process industrial gases. The authors have carried out laboratory bench testing of alumina sand type in the flow of exhaust gases from the surface of cryolite-alumina melt. As a result, new data of fluorides chemical adsorption process on the surface of alumina sand type are obtained and efficient operating mode of dry gas treatment system is identified.

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