FLUORIDE REMOVAL Fluorides removal plant Within the frame of the wastes pollution control upgrading programs, more stringent regulations now apply with reference to fluorides content. The maximum fluoride content in the treated water may be nowadays as low as 3ppm. In order to meet such design parameters the standard chemical physical process is not sufficient and a double step process has to be designed. PROCESS DESCRIPTION First step: physical-chemical treatment The target of this treatment step is to eliminate most of F- ions by precipitation of calcium fluoride. To this purpose, the treatment scheme is consisting of 4 sections: Acidification Precipitation Flocculation Clarification/Settling The composition of the clarified water is in equilibrium whit the precipitated Calcium Fluoride, thus being in the range of 15÷20 ppm F-. The involved reactions are the following Ca(OH)2 + 2H+ Ca2+ + 2H2O Ca2+ + 2F- CaF2 Al3+ + 3OH- Al(OH)3 Second step: polishing treatment Purpose of the second treatment step is to reduce the fluoride residual down to 3 ppm. To this purpose, the treatment scheme is consisting of 2 sections: Filtration Unifluor A adsorption Filtration The clarifier effluent is released to the filtration step consisting of pressure media filters. The purpose of this treatment stage is to remove any fine suspended matter from the treated water, to prevent any clogging effect on the following activated alumina fine filters. Fluoride removal by activated alumina filters The filtered stream is then fed, after pH conditioning, to the Unifluor A filters where the ultimate removal of Fluoride ions is finally achieved by an adsorption process. The term adsorption means the physical chemical process by which a porous solid (in our case Unifluor A, which is a particular type of activated alumina) is able to adsorb on its surface molecules of an element or of a compound (in our case F-) present in the fluid (water) which is percolated through it. This phenomenon is affected by the concentration of the compound to be removed both in the liquid and in the adsorbing material. Being the adsorption mainly a surface phenomenon, it is also of great importance the ratio surface/volume of the adsorbing material. All the wash and waste waters originated from filters backwash and Unifluor A regeneration are recycled back to the treatment so that the only by-product of the entire treatment is the sludge cake to be disposed off. top