Determination of the hydrofluoric acid concentration in the solutions

used for the chemical polishing of crystal.

Determination of the hydrofluoric acid concentration in the solutions

used for the chemical polishing of crystal.

E. BrientCristallerie de Baccarat, 20 rue des Cristalleries - 54120 Baccarat

Y. Pillet, M. VilasiLCSM UMR755, Faculté des Sciences et techniques UHP – Nancy I

Bd des Aiguillettes BP239 - 54506 Vandoeuvre

before after

polishing

INDUSTRIAL EXPECTATIONS

Savings in consumption of acids

Optimization of the polishing cycles : improvement of the capacity on the existing machines

Control of the « strength » of the bath to avoid quality problems as stuck salts or loss of acceptable dimensional tolerances

CONTEXT

AIM OF THE STUDY

Solution of HF + H2SO4

HF/H2SO4 ratio : a key factor for the quality of the process

Determination of the [F-] in a hyper-acidic medium (H2SO4)

Determination of [HF] to control ageing of polishing bath

Data should be achievable by a simple method

+ H2SiF6 (leaching of the glass)

The classical titration method consists on the following reaction:

La3+ + 3 F- => LaF3(solid)

acted at pH=5 and the determination of the equivalent volume is realized thanks to a potentiometric measurement

This technique cannot be used directly

In the industrial polishing baths, this reaction is hindered by the presence of H2SiF6 produced by the leaching of the glass

Laboratory experiments were performed on standard solutions containing : - pure H2SiF6

- pure HF- a mixture of H2SiF6 and HF

The titration of H2SiF6 is feasible according to the following reaction :

2 La3+ + SiF62- + 4H2O => 2LaF3(solid) + Si(OH)4 + 4H+

at pH= 5, H2SiF6 is existing as SiF62-

The titration of a mixture of H2SiF6 + HF determines the total amounts of F-I provided by the both acidic species

Potentiometric titration of a mixture of H2SiF6 + HF + H2SO4

2 mL of the mixture, stabilized at pH=5 , diluted in 25 mL H2O with La(NO3)3 (9.38x10-3 mole/L)

n(La3+) = (1/3) * {n(F -) + 6 * n(SiF62-)} = n/3 [F-I]

The determination of [H2SiF6] or [HF] in the samebath needs to use an other technique of titration

The easiest titration technique coming in mind is the acid/base method

The titration of standard solutions containing H2SiF6 was studied at the laboratory

H2SiF6 + 2 OH - SiF62- + 2 H2O

SiF62- + 4 OH - Si(OH)4 + 6 F –

Titration of 2 mL of H2SiF6 (10-1 mole/L)

diluted in 25 mL H2O with NaOH (9.38x10-3 mole/L)

[H2SiF6] is determinate by the volume difference

VmL = VEP2 – VEP1

VmL

Technique is therefore applicable to an industrial bath

Titration of a standard mixture of H2SO4 + HF + H2SiF6 gives a similar curve to the one obtained with pure H2SiF6 solution

[F-I] = [F -] + 6 [SiF62-]

Acido/BasicPotentiometric

VmL

VEP3

CONCLUSIONS

Chemical characterization of fluoride in hyperacidic medium can be done with an accuracy of about 10%.

This method is validated on industrial solutions for crystal polishingIts simplicity allows the application on the glass making site.

It should become a powerful tool to control the ageing of polishing baths

FIRST INDUSTRIAL POSITIVE CONSEQUENCE

The “virtuous circle”

Decrease of the ratio HF/H2SO4 (by additional H2SO4 and constant HF) corresponds to a lower value of H2SiF6 in the bath and lower necessary additions of HF to maintain a constant level of active HF in the bath

The hypothesis to be checked is the following one:

1. Ions SiF62- react better with the additional H+ than with

K+ to form H2SiF6 instead of K2SiF6

2. the following reaction permits to re-create active HF:

H2SiF6 + 2H2O => SiO2 + 6HF