Seeded Precipitation of the Coarse Sodium Fluoride with High Purity for Efficient Removal of Fluoride Ion from Sodium Aluminate Solutions

Recovering fluoride from industrial alkaline solutions will benefit the environmental protection and improve product quality. This paper presented a novel approach to fluoride recovery by precipitating the coarse NaF from sodium aluminate solution. The equilibrium concentration of fluoride ion in the sodium aluminate solution was first presented as a function of caustic soda concentration and temperature. Coarse NaF and high NaF precipitation efficiency were obtained by the fine particle agglomeration under conditions of inhomogeneously distributed at weak agitation (IDWA) and the addition of activated seeds. Based on particle number calculated from the particle size distribution curves, the agglomeration rate was linearly dependent on the supersaturation of NaF in sodium aluminate solution. The agglomeration mechanism was described by the fresh ultrafine particles acting as a “binder” to agglomerate the fine particles (< 10 μm) into the coarse particles (> 30 μm). Therefore, 91.15% precipitation efficiency of NaF was achieved by adding the activated seeds, adopting the special schedule of decreasing temperature, and regulating the supersaturation under IDWA. Furthermore, about 65 μm NaF with more than 98% purity was received. These results provide a novel approach for recovering NaF from the sodium aluminate solution and benefit the green development of the alumina industry.

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