Mass and energy flows in a fluoride scrubber when the fluoride is returned to single superphosphate

Abstract

The gaseous fluoride evolved during the production of single superphosphates may be disposed of by scrubbing it from the effluent gas streams with water sprays and using the resulting scrubber liquors as sources of dilution water for the concentrated sulphuric acid used in the superphosphate manufacturing processes.

Part of the returned fluoride is re-evolved which leads to higher fluoride loadings on the scrubbing systems which must therefore be relatively large and expensive installations. Accurate initial design is thus important to minimise the costs involved.

This paper presents a theoretical model of such a ‘zero effluent discharge’ system which will aid in this process. Parameters covered include the temperatures of the gases and liquids within the scrubbing system, the amount of fluoride re-evolved from the superphosphate and the vapour pressures of fluoride over the scrubber liquors.

The effects of increasing the number of scrubbing stages while keeping the total number of transfer units constant and varying the air flows within scrubbing systems have been calculated and the results are presented.