Improvements in the Production or Treatment of Bichromate of Soda.
Year: 1898

page_text: PROVISIONAL SPECIFICATION.
Improvements in the Production or Treatment of Bichromate of Soda.
I, ANTON GOLDSCHMIDT, of Grenzallée 28, Bockum-Krefeld, Germany, Doctor
of Philosophy, and of the Firm of R. Wedekind & Co., Uerdingen am Rhine, do
hereby declare the nature of this invention to be as follows:-
In the method hitherto most usually employed in producing bichromate of soda,
the chrome-smelting or roasted material which is obtained, for example, by roast-
ing chrome ore such as chorme ironstone to which lime and soda has been added,
is exhausted with water, the solution being concentrated to a specific gravity of 1.5
and then mixed with a sufficient quantity of sulphuric acid for the purpose of
neutralizing the free sodic hydrate and converting the normal chromate into the
bichromate. In this manner a large portion of the sulphate is precipitated in an
anhydrous form, and is separated from the lyes by decanting, filtering, or the like.
By further concentration of the lyes, considerable quantities of sulphate are
produced from which, after its separation, the adhering liquid may be allowed to
drain, whereupon, when the specific gravity has reached 1.7, the hot filtered liquid
is crystallized slowly, or the said liquid is converted by further evaporation
and drying into an anhydrous article of commerce.
By the first-mentioned process it has only been possible with very careful
working to obtain a product comparatively free from sulphate of soda by reason of
the sulphate remaining in the mother liquor, whilst the products obtained by
complete evaporation contain up to 12 per cent. of sulphate; those that contain as
low as 2 to 3 per cent. being seldom produced.
Even in the process wherein the mono-chromate lye is evaporated only to 40
to 45 Baumé (1.38 to 1.45 specific gravity) and then has sulphuric acid added
thereto to form bichromate lye the separated sodium sulphate being removed
after the latter has cooled, it is impossible to obtain bichromate which is free from
sulphate of soda. In other words, if a mono-chromate solution is concentrated by
evaporation to a specific gravity of 1.5 and converted by means of sulphuric acid
into bichromate and this solution is then cooled for the purpose of separating the
sulphate, although considerable quantities of sulphate will still separate when this
solution is further concentrated to a specific gravity of 17, even when sulphate
is removed from the latter solution and the adhering liquid allowed to drain, there
is always still produced a bichromate that contains sulphate of soda.
Now the production of bichromate of soda in the dry state absolutely free from
sulphate of soda is of very great importance. For example in using the impure
material for the purpose of making chrome yellow, there is obtained a product
which contains lead sulphate, has a bad colour, and a bad covering power, and
in using the crystallized salt which is almost free from sulphuric acid, but still
contains two molecules of water, the user has to combat a very unpleasant running
of the goods. For this reason, therefore, consumers prefer to use for such purposes
the more expensive bichromate of potash which is free from sulphate of potash,
and is capable of resisting the action of the air.
Now this invention has for object to obviate the above described drawbacks, that

page_text: is to say, to produce bichromate of soda which is free from sulphate of soda, and
is also dry. The invention is based on the hitherto unknown fact that sodium
sulphate is absolutely insoluble in concentrated bichromate of soda lyes having a
specific gravity of 1795 (determined at 15° C.).
According thereto the solution which is made in the usual manner by leaching
the chrome-smelting or roasted material, is concentrated by evaporation without
an addition of sulphuric acid, until a considerable portion of the normal salt
has separated in a pasty condition, and a sample taken from the syrupy solution
above it becomes hard immediately on cooling. Then the mass is mixed with a
determined quantity of sulphuric acid, preferably of 60-66 Baumé whereby
bichromate of soda of the specific gravity of 1795 is formed, whilst all the sulphate,
which is insoluble in the former lye of this degree of concentration, separates and
is removed by immediate filtration from the boiling lye. It is then converted by
direct evaporation into solid salt, free from sulphate of soda.
The process may also be carried out by first mixing the lye in the usual manner
with sulphuric acid, removing the main body of the sulphate, and after evaporating
to a specific gravity of 1795, removing the sulphate remaining in the filtered lye.
Finally, the improved process may be employed for purifying bichromate of soda
that contains sulphate of soda, by treating it with a quantity of water or steam
sufficient to produce a solution of 1795 specific gravity at which degree of con-
centration the sulphate remains behind in an insoluble form.
F. WISE HOWORTH,
46, Lincoln's Inn Fields, London, W.C.,
Agent for the Applicant.
improvements in the Production or Treatment of Bichromate of Soda.
I, ANTON GOLDSCHMIDT, of Grenzallée 28, Bockum-Krefeld, Germany, Doctor
of Philosophy, and of the Firm of R. Wedekind & Co., Uerdingen am Rhein, 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:
-
In the method hitherto most usually employed in producing bichromate of soda,
the chrome-smelting or roasted material obtained, for example, by roasting chrome
ore (such as chrome ironstone) to which lime and soda have been added, is
exhausted with water, and the solution is then concentrated to a specific gravity
of 15, and finally mixed with a sufficient quantity of sulphuric acid for the
purpose of neutralizing the free sodic hydrate and converting the normal chromate
into the bichromate. In this manner a large portion of the sulphate is pre-
cipitated in an anhydrous form, and is separated from the lyes by decanting,
filtering, or the like
By further concentration of the lyes, considerable quantities of sulphate are
produced from which, after its separation, the adhering liquid may be allowed to
drain, whereupon, when the specific gravity has reached 17, the hot filtered liquid
is crystallised slowly, or the said liquid is converted by further evaporation and
drying into an anhydrous article of commerce.
By the first-mentioned process it has been possible only with very careful
working to obtain a product comparatively free from sulphate of soda, because the
sulphate remains in the mother liquor; on the other hand, the products obtained

page_text: by complete evaporation contain up to 12 per cent. of sulphate, those that contain
as low as 2 to 3 per cent. being seldom produced.
Even in the process wherein the mono-chromate lye is evaporated only to from
40° to 50° Baumé (1 38 to 145 specific gravity) and then has sulphuric acid added
to it to form bichromate lye, the separated sodium sulphate being removed after
the bichromate lye has cooled, it is impossible to obtain bichromate that is free
from sulphate of soda. In other words, if a monochromate solution is concentrated
by evaporation to a specific gravity of 15 and converted by means of sulphuric
acid into bichromate, and this solution is then cooled for the purpose of separating
the sulphate, although considerable quantities of sulphate will still separate when
this solution is further concentrated to a specific gravity of 17, even when sulphate
is removed from the latter solution and the adhering liquid allowed to drain
there is always still produced a bichromate that contains sulphate of soda.
Now the production of bichromate of soda both dry. (i.e., destitute both of
moisture and of water of crystallization) and also absolutely free from sulphate of
soda, is of very great importance. For example, in using the impure material
for the purpose of making chrome yellow, there is obtained a product which con-
tains lead sulphate, has a bad colour, and a poor covering power, and in using the
crystalized salt which is almost free from sulphuric acid, but whose molecule
still contains two molecules of water, the user has to combat the very unpleasant
deliquescence of the salt. For this reason, therefore, consumers prefer to usc
for such purposes the more expensive bichromate of potash, which is free from
sulphate of potash, and is capable of resisting the action of the air.
Now this invention has for object to obviate the herein before described draw-
backs that is to say, to produce bichromate of soda that will be free from
sulphate of soda and will also be dry in the sense hereinbefore stated. The
invention is based on the hitherto unknown fact that sodium sulphate is absolutely
insoluble in concentrated bichromate of soda lyes having a specific gravity of 1.795
(determined at 15° C.).
According thereto the solution, which is made in the usual manner by leaching
the chrome-smelting or roasted material, is concentrated by evaporation without
an addition of sulphuric acid, until a considerable portion of the normal salt has
separated in a pasty condition, and a sample taken from the syrupy solution above
it becomes hard immediately on cooling. Then the mass is mixed with a quantity
of concentrated sulphuric acid determined by calculation, preferably of 60°-66°
Baumé, so as to form a bichromate of soda lye of the specific gravity of 1795,
whereupon all the sulphate, which is insoluble in a lye of this degree of concen-
tration, separates and is removed by immediate filtration or the like from the
boiling lye. It is then converted by direct evaporation into solid salt, free from
sulphate of soda.
One may, however, proceed by first mixing the solution, with sulphuric acid and
then removing the main body of the sulphate in the usual way, and then, after
evaporating to a specific gravity of 1795, immediately removing, as by a second
filtration, the whole of the sulphate remaining in the filtered Ive.
Finally, the improved process may be employed for purifying solid bichromate
of soda that contains sulphate of soda by treating it with a quantity of water or
steam sufficient to produce a solution of the specific gravity of 1.795, so as to cause
the sulphate of soda, which is quite insoluble in such a solution, to separate out.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed I declare that what
I claim is
1. The production of dry bi-chromate of soda free from sulphate of soda by first
evaporating the mono-chromate solution prepared in the usual manner, to the
consistency of a paste, and then adding to it the calculated quantity of concen-
trated sulphuric acid so as to form a bichromate of soda lye of the specific
gravity of 1.795 in which the sulphate of soda, being insoluble therein, all separates

page_text: +
out; secondly, removing by filtration the whole of the sulphate of soda from the
boiling liquid; and, finally, evaporating the liquid to dryness.
2. The purification of bichromate lye containing sulphate and produced accord-
ing to the hitherto used process, by evaporating the lye to the specific gravity
of 1795 and then removing, as by a second filtration, the sulphate of soda, which
is completely separated out at this degree of concentration.
3. For purifying bichromate of soda contaminated by sulphate of soda, treating
it with sufficient water or steam to produce a solution of the specific gravity
of 1795 so as to cause the sulphate of soda, which is quite insoluble in such a
solution, to separate out completely.
4. The production (by means of the addition of sulphuric acid and subsequent
evaporation or vice versa) of, or treatment for purifying, bichromate of soda,
substantially as herein before described.
1000
F. WISE HOWORTH,
46, Lincoln's Inn Fields, London, W.C, Agent for the Applicant.