Hydrolysis of sodium silicofluoride

Author

Frank Arthur Graf, New Jersey Institute of Technology

Document Type

Thesis

Date of Award

6-30-1956

Degree Name

Master of Science in Chemical Engineering - (M.S.)

Department

Chemical Engineering

First Advisor

George C. Keeffe

Second Advisor

C. L. Mantell

Third Advisor

Michael Frederick

Abstract

The kinetics of an apparent solubility decrease of commercial sodium fluoride encountered in a piece of equipment designed to produce saturated sodium fluoride solution was Investigated, in order to understand why a consistently saturated solution was not possible to achieve with plain tap water as solvent.

The main impurity, about 1 percent sodium silicofluoride, coats the sodium fluoride crystals, resulting in an equilibrium shift during dissolution of the salt in water; and under certain conditions results in the formation of a protective gel. Therefore, in order to produce a constant strength sodium fluoride solution, this coated impurity must be decomposed under conditions which are unfavorable to the formation of the silica gel.

The investigation was concerned specifically with the kinetics of the hydrolysis of sodium silicofluoride in water and in saturated sodium fluoride solutions.

Two different mechanisms for the hydrolytic decomposition of the silicofluoride ion in water have been proposed by previous authors. Ryss and Slutskaya¹ and Kubelka and Pristonpil² postulate the reaction:

2 H₂O + SiF^-₆ ---> 4H+ 6F^- + SiO₂

A two step mechanism is offered by Hudleston and Bassett³ and also by Rees sad Hudleston, a slow reaction,

SiF₆- ---> SiF₄ + 2F^-

followed by a rapid reaction,

SiF₄ + 3 H₂O ---> 4HF + H₂SiO₃

This author has shown from experimental considerations of the primary and secondary salt effects, the effect of dielectric constant of the media on the reaction rate constant, end from theoretical calculations of the entropy factor that the following mechanism is more probable:

Na₂SiF₆ <--H₂O--> 2 MA+ + SiF₆- ionization step

SiF₆^* + xH₂O < -- in presence of alkali such as NaOH -- > SiF₆^* . xH₂O (Formation of an activated complex)

SiF₆^* . x H₂O + (2 + y) H₂O <--H₂O--> SiO₂ y H₂O + HF + 2F^- + x H₂O the rate determining step

followed by the rapid neutralization of the HF formed by the alkali (sodium hydroxide) that has been added

2 HF + 2(Na⁺ + OH-)---> -2(Na⁺ + F^-) + 2 H₂O

4(Na⁺ + OH-) + Na₂ SiF₆ -- H₂O --> SiO₂ + 6(Na⁺ + F^-) + 2H₂O

In this mechanism x and y are unknown due to the difficulty of determining their exact values.

A minimum alkalinity, pH of 10.2, is apparently necessary in order that the silica produced from the Nat SiF₆ present is not activated to form a gel. Equilibrium shift cannot explain entirely the required high alkaline conditions necessary in the effluent for complete dissolution of the commercial sodium fluoride in a continuous dissolution bed.

1. Ryss, I. G. and Slutskaya, M. M., J. Phys. Chem. (USSR). 14 (1940) p. 701.

2. Kulbelka, V. P. and Pristoupil, V., Z. Anorg. Allgem. Chem. 197, 393 (1931).

3. Hudleston, J. and Bassett, H., J. Chem. Soc. (1921) p. 403.

4. Rees, A. a, and Hudleston, J. L., J. Chem. Soc. (1921) p. 1334.

Recommended Citation

Graf, Frank Arthur, "Hydrolysis of sodium silicofluoride" (1956). Theses. 2407.
https://digitalcommons.njit.edu/theses/2407