An Improved Process for Purifying Graphite.
Year: 1899

page_text: Date of Application, 21st Nov., 1899-Accepted, 20th Jan., 1900
An Improved Process for Purifying Graphite.
I, EMIL TEISLER, of Borsdorf, near Leipzig, in the Empire of Germany, Chemist,
do hereby declare the nature of this invention and in what manner the same is
to be performed, to be particularly described and ascertained in and by the
following statement:-
If hydro-fluoric acid be allowed to act on impure graphite, fluorides of the
mineral impurities contained in the graphite are formed. According to Dammer,
"Handbook of Inorganic Chemistry" Vol. 2, page 267, these mineral constituents
consist of silicic acid, alumina, iron, lime, magnesia and alkalies. The fluorides
formed are partly easily soluble, and partly insoluble, or only soluble with
difficulty. To the latter two groups belong the fluorides of lime and magnesia,
also the silicium fluoride of the alkalies. Moreover fluoride of aluminium only
dissolves in a large excess of acid, and even then often only incompletely, as is
shown by the existence of alumina in ashes of graphite exhausted with hydro-
fluoric acid. It is possible that in such cases the aluminium fluoride is combined
with the fluorides of the alkalies into cryolite which, as is well known, only
dissolves with difficulty in acids. The varying solubility of the oxides of alumina
of various origins in hydro-fluoric acid causes us to form the conclusion that the
alumina of graphite ashes has a varying solubility in hydro-fluoric acid. The
quantity of the insoluble fluorides is unequal in different substances, and varies
from 10% to 50% of the total impurities originally existing in the graphite..
These fluorides are dissolved by being decomposed by means of various mineral
acids or acid salts, thus fluoride of aluminium dissolves in hydrochloric and
sulphuric acids and solutions of acid sulphates; fluoride of magnesium, cryolite
and fluo-silicic alkalies in sulphuric acid. In practice, for the purification of
graphite, only sulphuric acid and bi-sulphate can come into question because of
technical and commercial reasons, as less difficulties in the way of apparatus have
to be overcome and all waste products may be easily employed or used up in
actual working. I therefore in my application only lay stress on the use of
sulphuric acid and acid alkali sulphate for the after treatment of graphite
preliminarily purified by means of fluoric acid.
It is also somewhat immaterial whether I employ dilute sulphuric acid, or
solutions of bi-sulphates, or concentrated sulphuric acid and deliquescent bi-
sulphate. The effect of the treatment is almost the same, and is only favorably
influenced by the presence of alkaline sulphates, probably owing to the possibility
of the formation of easily soluble double salts, such as sulphate of calcium and
soda. An excess of the solvent must exist, and this is already required in order
to obtain a good mixture of the sulphate with the graphite.
Whether it be better to employ dilute or concentrated acid depends on the
subsidiary circumstances, for instance whether it be desired to immediately
remove the hydro-fluoric acid released from out of the reaction mixture or not.
In the first case concentrated acid is employed, whereby the fluorides released
immediately escape in a gaseous form, whilst they remain dissolved otherwise in

page_text: the dilute sulphuric acid, and are then only expelled therefrom during the
concentration. It may appear self-evident that a subsequent treatment of the
graphite with sulphuric acid will diminish considerably the amount of ash
contained in several varieties of graphite whilst the preliminary treatment of
the same graphites is almost without effect. If sulphuric acid of 60% Beaume
be allowed to act at boiling temperature on burnt ferruginous clay which is com-
posed similarly to the ashes of graphite, hardly 2% of the substance of the clay is
dissolved. By means of hydro-fluoric acid however, over 80% is immediately
dissolved, considerable heat being developed, and by the succeeding further
treatment with dilute or concentrated sulphuric acid, a further 15% is dissolved.
The chemical constitution of the molecules of the clay must first be broken up,
and this is effected most thoroughly by means of hydro-fluoric acid.
The simultaneous treatment of graphite with hydro-fluoric and sulphuric acid,
or with fluorides and sulphuric acid, in excess, is inadvisable for both technical
and practical reasons, i.e. for technical reasons because there is no cheap material
for making vessels which can withstand the combined action of dilute hydro-
fluoric acid, sulphuric acid and fluo-silicic acid, and for practical reasons because
we are forced to employ such fluorides as yield with sulphuric acid soluble
sulphates.
The cheapest of these are the alkaline fluorides but these, with the aid of the
fluo-silicic acid present form the fluo-silico alkalies which are so difficult to
dissolve and decompose, and the technical effect of the same would also thus
be less.
The result of this explains why I lay stress on the subsequent or secondary
treatment with sulphuric acid or acid sulphates of graphite previously purified
with hydro-fluoric acid.
In my improved process the resultant salts are washed with water.
Having now particularly described and ascertained the nature of
invention and in what manner the same is to be performed, I declare that what
my said
I claim is
A process for removing the insoluble fluorides remaining in graphite and
which are formed in the purification of graphite with dilute hydro-fluoric acid.
characterised by the graphite being exposed to the action of sulphuric acid or
acid sulphates after being treated with hydro-fluoric acid, and the resultant
salts washed with water, substantially as hereinbefore set forth.
Dated this 20th. day of November 1899.
WM. P. THOMPSON & Co.,
Of 6, Lord Street, Liverpool, Agents for the Applicant.