Calcium fluoride, scaling control

Calcium fluoride scale can form when the concentration of fluoride is as low as 0.1 ppm if the concentration of calcium is high. Scaling will occur when the ion product exceeds the solubility constant. Antisealants or sodium softening can be used to control calcium fluoride scale. 

Antisealants are usually fed alone for most applications. Acid feed is sometime used in conjunction with an antisealant to control LSI for calcium carbonate scale and to control calcium phosphate and calcium fluoride scales. Antisealants currently on the market are not generally effective at controlling calcium phosphate scale and have difficulty controlling calcium carbonate scale when the LSI is greater than about +2, depending... 

Alternatively, antisealants can be used to control calcium carbonate scale at LSI values as high as 2.0-2.5, depending on the specific antisealants. Calcium also forms scales with fluoride, sulfate, and phosphate. The LSI will not help predict these scales analysis of water quality, using the ion product and solubility constants, is required to determine the potential for scaling with calcium fluoride or calcium phosphate. Antisealants currently available can address calcium fluoride and calcium sulfate scale they do not address calcium phosphate scale (although newer antisealants will be available in the near future to address this scale). 

Calcium sulfate is a sparingly-soluble salt. As with barium and strontium sulfate, the potential to scale with calcium sulfate is high when the ion product exceeds 80% of the solubility constant. Antisealants or sodium softening to remove calcium can be used to control calcium sulfate scale. Calcium phosphate has become a common problem with the increase in treatment of municipal waste-water for reuse. Surface waters can also contain phosphate. Calcium phosphate compounds can also contain hydroxyl, chloride, fluoride, aluminum, and/ or iron. Several calcium phosphate compoimds have low solubility, as shown in Table 7.3. SolubiUty for calcium carbonate and barium sulfate are also shown by comparison. The potential for scaHng RO... 

NOTE Although the addition ofHF to HCl can be beneficial in helping to control the corrosion of steel (because fluoride ions form very stable self-limiting complexes with ferric ions), HF should not be used where significant hardness scales are present because calcium and magnesium fluorides (CaF2, MgF2) may be precipitated. 

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