Calcium fluoride, scaling

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... 

Hydrogen fluoride is the most important compound of fluorine. It is prepared in the laboratory, and on the large scale, by the reaction of calcium fluoride with concentrated sulphuric acid. ... 

Other Fluorides. Calcium fluoride is one of the oldest fluorinating agents. Ordinarily, it is used with sulfuric acid, which generates hydrogen fluoride. On a laboratory scale, this reagent is not very satisfactory because the solid cake of calcium sulfate is difficult to remove from the reactor flask. 

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). 

A small-scale technique only has been described, but the equipment was reasonably representative of a possible industrial process. Graphite, fused dolomite, pure magnesia or pure calcium fluoride have been used as reactor linings, inside a cylindrical steel casing, 18 in. deep by 8 in. diameter. Reactor lids consisted of either screw caps or bolted flanges and provision was made for argon purging and evacuation prior to initiation. 

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... 

Acid feed acid is added to reduce the LSI and calcium carbonate, calcium phosphate or calcuim fluoride scaling potential, if required. 

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. 

In addition to the preceding fluoride transport tests, laboratory-scale tests were conducted to investigate the possibility of containing or removing fluoride from the system to allow more economical materials of construction to be used in the design of the full-scale plant (AEA, 20011). A series of nine tests was to be conducted to obtain kinetic data on the use of calcium as an agent for fluoride removal from the GB simulant, fluorophosphoric acid. Data were to be obtained for the hydrolysis reaction under acidic, neutral, and alkaline conditions. 

Besides calcium carbonate, there are three other calcium-based compounds that will scale RO membranes. These compounds are calcium sulfate, phosphate, and fluoride. Although there are no specified feed water guidelines for these compounds, they are worth investigating. 

Additional considerations must be given to the potential for creating precipitation by-products if the CMP waste stream is mixed with other chemistries used in the manufacturing process that could accidentally be placed in the segregated drain. For example, if the CMP waste stream contains fluoride and city water is used to flush the piping system, then the calcium in the city water could react with the fluoride to produce a scale on the pipe, thus reducing the flow capacity of the pipe. Once precipitation starts, it will always be present since all seed crystals cannot be 100% eliminated from hundreds of feet of piping. 

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