The reaction with of CaF₂ with HCl is slow because the free energy of the displacement reaction is  +, so the equilibrium constant is small. Using DF_25^oC values from Perry’s, 6th Edn.,

CaF₂ + HCl(aq) = CaCl₂(aq) + HF(aq)

DF_Rxn = -195.36 –73.7(approx.) + 260.1(approx.) + 31.33 kcal/mol
           =  +22.37 kcal/mol

The addition of boric acid drives the reaction to the right by Le Chatelier’s Principle.  The boric acid reacts with and thereby removes the HF product.  Either the reaction
HBO₂ + 3 HF = BF₃ + 2 H₂O

or the formation of fluoboric acid HBF₄ are possible, but the thermodynamic data aren't in Perry’s so I won’t predict which is more likely.

To add a bit of information to the excellent explanation by Kenvlach: HCl is used because the line is of pvc make. Altough HCl is a stronger commercial acid than sulfuric as measured by its pKa, the industrial common way to produce HF is to treat fluorspar (calcium fluoride aka fluorite) with sulfuric acid obtaining gypsum as by-product. Not applicable in this case.

The fluorine anion in fluorite holds strongly to its calcium cation. Thus the reaction is slow. Besides, the solubility product of fluorite in water at 25^oC is very low, in the order of 3.5 x 10^-11.

Released HF is captured by combining with boric acid B(OH)₃, to form F₃B a gas that complexes swiftly to [BF₄]^- in solutions containing F^-. F₃B is toxic TLV: 0.3 ppm. I assume that F₃B may react with water to form other products.

Released HF can also be reacted with alumina trihydrate to produce solid F₃Al, with a solubility of 4.1 g/L water.

As a matter of record HBO₂ is metaboric acid.
Boric acid is B(OH)₃.

Using IUPAC conventions, boric acid is H₃BO₃

Old chemistry books speak of hydrates of boron oxide B₂O₃ (aka boric anhydride) with various molecules of water. With 3 H₂O we get orthoboric H₃BO₃. When heated, boric acid losses water becoming metaboric acid BO₂H = B₂O₃.H₂O.

Boric acid is essentially monomeric when highly diluted, but in solutions above 0.1 M may form polymeric species. The tetrahydroxyborate anion B(OH)₄^- is the conjugate base of boric acid. Together they act as pH buffer.

Out of curiosity, how do you get floride deposition.

All of the waterlines I've worked with produces Ca/Mg sulfate type deposits. Floride solubility (in most water systems) is so low that it is never a problem to begin with.

Semiconductor/chip manufacturers like Intel, AMD, Micron, IBM, Motorola, etc. use HF to clean the silicon substrates (it's the only acid that dissolves SiO₂.  I once did some consulting where such a firm had a problem meeting F^- effluent limit; the local POTW recycles water and had strict limits. Had them use lime Ca(OH)₂ to precipitate it as CaF₂.

Kev~,

No problem with that, it is just that the solubility of CaF2 in water (as orig. stated) is so low as to beg the question of having to remove it.

The semiconductor people rinse off the HF etchant with lots of high purity DI water. It's the fluoride that needs removal, ergo adding Ca to form CaF₂.

Kenvlach,

understand the usage of HF in the front-end.

the CaF2 deposition in the pipe strikes me as a matter of modifying the waste treatment process to better control the crytallization process. there has got to be a better way than re-creating soluable F-.

muth,
can you enlighten us as to the source of the CaF₂? If there was a substantial amount, it may be easier to just replace the PVC pipe.

hacksaw,
for the wastewater treatment case I mentioned, the CaF₂ was created in a neutralization tank & removed (along with a few heavy metals) by filter pressing.