An Improved Process for Obtaining Purified Silicic Acid and Fluo- silicic Acid from the Residual Mixture Obtained in the Purifica- tion of Graphite by Means of Dilute Fluoric Acid.
Year: 1898

page_text: Date of Application, 12th Nov., 1898-Accepted, 1st Apr., 1899
COMPLETE SPECIFICATION.
An Improved Process for Obtaining Purified Silicic Acid and Fluo-
silicic Acid from the Residual Mixture Obtained in the Purifica-
tion of Graphite by Means of Dilute Fluoric Acid.
I, SHOLTO DOUGLAS, of 6, Lessingstrasse, Berlin, in the Empire of Germany,
Mine Proprietor, do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascer-
tained in and by the following statement:-
The propositions already published in works relating thereto for the purifi-
cation of graphite by means of chemicals have in practice found little applica-
tion because the chemicals intended to be used are too high in price and are
converted into an almost valueless product so that the cost of chemical purifi-
cation of graphite rendered it unremunerative.
This invention has for its object to render one of the proposed
methods
of purifying graphite, capable of being carried out in practice by allowing a
remunerative utilization of the by-products.
The invention relates to those processes which recommend the purification of
graphite with fluorides, The fluorides of which it has hitherto been a question
are:
1. Gaseous fluoric acid,
2. Dilute fluoric acid,
3. Fluorine salts in combination with, or without. mineral acids.
As regards the first mentioned process, the practical purification with gaseous
fluoric acid is hardly capable of being carried out in practice in view of the
difficulties which the question of apparatus raises and further it is not possible
to free the graphite from a considerable part of its mineral impurities. This
method of purification is based on the conversion of these mineral constituents
of the graphite into volatile fluorides and their removal. Of the mineral con-
stituents only a portion of the silicic acid is as readily volatile as fluoride, whilst
the remainder of the silicic acid and also the fluorides of alumina, iron, lime
and other like admixtures either are not volatile at all or only volatile at very
high temperatures. The result of this method of purifying is therefore verv
defective and, in view of the prices obtainable for the purified graphite, is much
too expensive.
As regards the second method. the purification with dilute fluoric acid
appears at first the simplest way. On the one hand no insurmountable diffi-
culties are presented by the question of apparatus. and on the other hand the
effect of the purification is much more favourable because the resultant fluorides
are soluble partly in water, nartly in fluoric acid and partly in fluo-silicic acid.
Unfortunately the consumption of fluoric acid is too great and the value of
fluo-silicic acid, containing aluminium. iron, lime and the like. is so low that
the utilisation of this process cannot be entertained without a better means of
utilizing these waste acids.
As regards the third process, the purification of graphite by means of fluorine

page_text: salts in combination with, or without, mineral acids can be left out of the ques-
tion, as it is too expensive.
This invention has for its object a process for utilizing the mixture of soluble
fluorides resulting from the purification of graphite with dilute fluoric acid.
This mixture at a specific gravity of 1200 has about the following composition:
Water 75 32%, alumina 2.25% proto-oxide of iron 101%, fluo-silicic acid
20.55%, silicic acid 87%.
The above figures vary of course for different kinds of graphites which do not
always contain the mineral constituents in the same proportions.
According to the present invention this mixture or liquid containing the con-
stituents above mentioned is distilled and thereby divided into a distillate con-
taining an easily volatile fluo-silicate or rather silicic acid and into a heavy or
non-volatile residue which contains the fluoride of the alumina iron, lime and
the like. The distillate contains solid silicic acid and fluo-silicic acid.
The silicic acid and fluo-silicic acid obtained by the distillation is almost
chemically pure and may be sold or disposed of without further treatment in the
ordinary way. The residue consisting of a mixture of fluorides may either be
employed for the manufacture of fluoric acid by which at the same time sul-
phate of alumina is obtained or for making fluoride of aluminium or cryolite.
It is thus possible by the proposed process to convert almost the whole fluoride
contents of the fluoric acid employed into a saleable or valuable form. The
cost of the fluoric acid employed is reduced to a minimum by the recovery of the
fluoride contents and the possibility of purifying graphite by means of fluoric
acid in a practical efficient manner or in a manner which can be carried out
practically is brought much closer. Of course to render the carrying out of the
process possible an apparatus is requisite which allows of the distillation of the
crude fluoride solutions without too great expenditure for erection, wear and
tear and handling. It appears that apparatuses of copper or copper alloys are
the most suitable for this object.
Having now particularly described and ascertained the nature of my said in-
vention and in what manner the same is to be performed, I declare that what I
claim is:
1. A process for obtaining purified silicic acid and fluo-silieic acid from the
mixture of soluble fluorides resulting from the action of dilute fluoric acid on
impure graphite. consisting in the distillation of the mixture, substantially as
herein before set forth.
2. In the herein before described process for obtaining purified silicic acid
and fluo-silicic acid, the utilisation of the heavy or non-volatile residue remain-
ing after the distillation herein before mentioned, for making fluoric acid (and
if desired also sulphate of alumina) fluoride of aluminium or cryolite, substan-
tially as herein before set forth.
3. In the herein before described process for obtaining purified silicic acid
and fluo-silicic acid, the effecting of the said distillation in apparatuses made
of copper or copper alloys, substantially as described.
Dated this 11th day of November 1898
WM. P. THOMPSON & Co.,
Of 6, Lord Street, Liverpool, Agents for the Applicant.