IMPROVEMENTS IN THE MANUFACTURE OF CHROMIC ACID OR TRIOXIDE OF CHROMIUM
Year: 1884

page_text: I WILLIAM AUGUSTUS ROWELL of Jesmond Gardens, Newcastle-upon-Tyne,
Chemist, do hereby declare the nature of my invention for "IMPROVEMENTS IN
THE MANUFACTURE OF CHROMIC ACID OR TRIOXIDE OF CHROMIUM."
The first operation in my process is the production of chromate of Strontia, and
for the sake of economy a certain quantity of chromate of Baryta.
I use a soluble chromate such as chromate of soda, lime or potash or mixtures of
any of these, and bring such chromate or chromates into solution.
This solution should neither contain sulphates or carbonates, nor should it
contain an acid chromate. Should it contain carbonates they are best neutralized
with a sufficient quantity of Muriatic Acid, if a bichromate is present it should be
converted into a normal chromate with soda or some other alkali. If sulphates
are present they must be allowed for in the process to be described.
-
To this solution as above described is added a solution of some soluble strontium
salt, in the proportion of the chemical equivalent of the sulphates and chromates
present, with some excess. I prefer the chloride on account of its cheapness and
ease of preparation, but the bydrate can be used in some cases, as can the nitrate
in all, but it is evident that these salts have no advantage over the chloride.
The addition of the solution of chloride of strontium, (which is best done in the
hot and with boiling and agitation,) results in the precipitation of most of the
chromic acid present in solution, as chromate of Strontium, and I prefer to have
my solutions of such strength as will result in the production of 1lb of chromate
of strontium for every gallon of supernatant liquor left after the above mentioned
precipitate has subsided thorougbly, considerable variation from this however is
possible as it mainly affects the amount of washing required by the precipitate.
The Chromate of Strontium is thoroughly freed from salts,.and the liquor from
which it has been precipitated, by repeated boiling and washing and is ready for
further treatment.
All the liquors which have been separated from the chromate of strontium, are
treated with chloride of Baryta, in excess of the chemical equivalent of the chromic
acid they still contain in solution, owing to a slight solubility chromate of strontia
possesses in water.
This results in the almost complete separation of the chromic acid as insoluble
chromate of Barium, which after washing thoroughly is ready for use as below.

page_text: The chromate of Baryta obtained as above is treated in the hot, preferably
boiled, with large excess of tolerably dilute sulphuric acid, which results in the
formation of sulphate of Baryta, and solution of the chromic acid-in the diluto
sulphuric acid.
This mixture of sulphuric and chromic acids, having been separated by decan-
tation or otherwise from the insoluble sulphate of Baryta, is treated hot, preferably
boiled, with an amount of chromate of strontium chemically equivalent to the
sulphuric acid present. This results in the formation of chromic acid in solution
and feebly soluble sulphate of Strontium.
The solution of chromic acid is soparated from the residue of sulphate of
Strontium, and evaporated to dryness, and it is advisable that a low heat be used
in this operation, as should the temperature rise much towards the end of the
evaporation chromium dioxide and even chromium sesqui-oxide is formed, oxygen
escaping,
The chromic-acid as formed contains a little sulphate of Strontia from most
which it may be freed, by re-solution in water, without boiling, settling, decanting
and again evaporating to dryness.
This acid may also be sold in solution.
Although I consider the above the most economical mode of conducting my
process, I sometimes vary it as follows:-
I precipitate the whole or nearly the whole of the chromic acid as chromate of
Baryta, and after thoroughly washing the precipitate, treat it in the same manner
as the chromate of Strontium, with sulphuric acid moderately dilute.. The result
here however is different, as I find that one equivalent of Sulphuric acid will not.
thoroughly decompose chromate of Baryta in one operation. I obtain therefore
sulphate of Barium mixed with some chromate of Baryta, and chromic acid, mixed
with some sulphuric acid, in solution. The mixture of Sulphate and chromate of
Baryta I treat with fresh sulphuric acid once or more as may be necessary and
thus obtain all the chromic acid in solution, using this sulphuric acid, which
contains some chromic acid, to act on fresh chromate of Baryta. To free the
chromic acid from the sulphuric acid, which, as, mentioned above, it contains,
I either treat it with successive quantities of chromate of Baryta or with chromate
of Strontia chemically equivalent to the sulphuric acid present. In either case
obtaining solution of chromic acid, which otherwise is treated as above described.
When I refer to soluble chromates, solutions of chromates etc. which are to be
operated on by my process, I include any solutions which may be obtained in
processes for the manufacture of chromate or Bichromate or Tor-chromate of Soda,
Potash, Lime etc, which contain chromic acid, fred or combined. Also any crude
solid containing or capable of yielding chromic acid in solution free or combined.
When I refer to "some soluble strontium salt" I do not include the use of saltsa
whose acid would re-act in a destructive way on the chromic acid to be precipitated
as would happen were a sulphide used. Chloride of Strontium I prefer, but there
might be advantages in using some salt such as the hydrate where the base
préviously combined with the chromic acid had a value as in the case of Bichro-
imate of Potash; where the use of Hydrato of Strontia and Hydrate of Baryta
would disponse with the use of any other alkali to render the Bichromate normal,
and would after precipitation of the chromates leave caustic Potash, a valuable
product, in solution.
When I speak of adding "solution of strontium salt" to solution of chromates,
I include the variation of this which would occur were the soluble chromates added ¿
solid to the solution of strontium salt or the soluble strontium salt added solid to
the solution of chromate.

page_text: COMPLETE SPECIFICATION.
i I, WILLIAM AUGUSTUS ROWELL of Jesmond Gardens, Newcastlo on Tyne,
Chemical Manager, do hereby declare the nature of my invention for "IMPROVE-
MENTS IN THE MANUFACTURE OF CHROMIC ACID OR TRIOXIDE OF CHROMIUM" and
in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement:-
1. This invention has for its object improvements in the manufacture of chromic
acid or trioxide of chromium.
The main principle on which I depend for the successful manufacturing of
chromic acid by iny process in the manner I prefer to adopt is as follows :—
It is of course well known that sulphuric acid attacks to some extent all
chromates, setting free chromic acid more or less, but all the processes now in use
or that have been used, labour under the disadvantage that either large excess of
sulphuric acid has to be used as occurs when this acid is made from Bichromate of
Potash; or else the sulphate, which results from the action of the sulphuric acid, is
soluble, more or less but always to a prejudicial degree, in the resulting chromic
acid of which the action of sulphuric acid on Calcium chromate is an example,
calcium sulphate dissolving in the chromic acid produced.
There are a few other processes for making chromic acid, but they are all either
from one cause or another too expensive for commercial use.
Now, I have discovered and I claim to be the first who has done so that
chromate of strontia is practically completely decomposed in one operation by one
equivalent of sulphuric acid, and that so complete is this, that 99% and
even 99% of the chromic acid present can be set free when 1 equivalent of -
chromate of strontia is boiled for a moderate time with 1 equivalent of sulphuric
acid with such reasonable precautions, as to fineness of division agitation as are
usual in chemical operations of a similar nature. Further I find that the sulphate
of strontia produced is almost completely insoluble in the chromic acid solution
and that any traces that do dissolve, generally 1 to 2% are easily separated by
redissolving the chromic acid after once boiling it to dryness, or solidifying in any
other manner.
The use of strontia therefore, a new feature in the manufacture of chromic acid
carries with it a series of advantages which render it particularly suited to the
purpose for which I apply it, its chromate being but feebly soluble, its sulphate
even less soluble, and yet the chromate being capable of being decomposed by one
equivalent of sulphuric acid to at least 99% of the whole equivalent.
The use of Baryta to precipitate any chromic acid unprecipitated by the strontia,
prevents loss, and the chromate of Baryta produced can be treated with large
excess of sulphuric acid, such excess being afterwards used by the chromate of
strontia.
It is evident that the properties of chromate of strontia mentioned above render
it mo muitable for nurifizing chromin said from auÌnhurie emid.
'

page_text: I have been unable to find any other technically available element which behavos
in the same manner towards sulphuric and chromic acid in every respect as
strontium, or is so suitable a base for the manufacture of chromic acid of
respectable purity and moderate price.
The first operation in my process is the production of chromate of strontia and
for the sake of economy a certain quantity of chromate of baryta.
I use a soluble chromate such as chromate of soda, lime or potash or mixtures of
any of these and bring such chromate or chromates into solution.
This crude solution should neither contain sulphates or carbonates nor should it
contain an acid chromate. Should it contain carbonates they are best neutralized
with a sufficient quantity of muriatic acid, if a bichromate is present it should bo
converted into a normal chromate with soda or some other alkali. If sulphates are
present they must bo allowed for in the process to be described.
To this solution as above described is added a solution of some soluble strontium
salt in the proportion of the chemical equivalent of the sulphates and chromates
presont with some excess. I prefer the chloride on account of its cheapness and
case of preparation but the hydrate can be used in some cases, as can the
nitrate in all, but it is evident that these salts have no advantage over the
chloride.
Chloride of strontium is cheaply prepared for this use by reducing, the sulphato.
with coal and dissolving the resulting sulphide in muriatic acid. Sulphuretted
hydrogen escapes.and can be used for making part of the necessary sulphuric acid
and chloride of strontium romains in "solution.
The addition of the solution of chloride of strontium to the chromate solution
(which is best done in the hot and with boiling and agitation) results in the
precipitation of most of the chromic acid present in solution as chromate of
strontium and I prefer to have my solutions of such strength as will result in the
production of 1 lb of chromate of strontium for every gallon of supernatant liquor
left after the above mentioned precipitate has subsided thoroughly considerable
variation from this however is possible as it mainly affects the amount of washing
required by the precipitate. The chromate of strontium is thoroughly freed from
salts and the liquor from which it has been precipitated by repeated boiling and
washing and is ready for further treatment.
All the liquors which have been separated from the chromate of strontium are
treated with chloride of baryta in excess of the chemical equivalent of the chromic
acid they still contain in solution owing to a slight solubility chromate of strontia
possesses in water.
This results in the almost complete separation of the chromic acid as insoluble
chromate of barium which after washing thoroughly is ready for use as bolow.
The chromate of baryta obtained as above is treated in the hot, preferably boiled,
with large excess of tolerably dilute sulphuric acid, one part concentrated acid to
fifteen parts water will be found a convenient proportion.. This treatment results
in the formation of sulphate of baryta and solution of the chromic acid in the
dilute sulphuric acid. This mixture of sulphuric and chromic acids having been
separated by decantation or otherwise from the insoluble sulphate of baryta is
treated bot, preferably boiled with an amount of, chromate of strontium chemically
equivalent to the sulphuric acid present. This results in the formation of chromic
acid in solution and feebly soluble sulphate of strontium, and I find that the
reaction is most complete when the sulphuric acid and chromate of strontin are
brought together as soon after the precipitation and washing of the latter as
possible,
The solution of chromio acid is separated from the residue of sulphate of
strontium and evaporated to dryness and it is advisable that a low heat be used
in this operation as should the temperature rise much towards the end of the
evaporation chromium dioxide and even chromium sesquioxide is formed, oxygen
oscaping.

page_text: The process described above yields directly chromic acid which is free (or nearly
free) from sulphuric acid because chromate of strontium can be practically
completely decomposed by an equivalent quantity of sulphuric acid; this is not
the case with chromate of barium which requires a large excess of sulphuric acid.
Again the process above described avoids loss of chromic acid in the waste liquors
notwithstanding the partial solubility of chromate of strontium because so much as
is dissolved is converted into insoluble chromate of barium which afterwards is
made to yield chromic acid.
The chromic acid as formed contains a little sulphate of strontia from most of
which it may be freed by redissolving it in water, without boiling, settling
decanting and again evaporating to dryness the clear liquor.
This acid may also be sold in solution,
In place of evaporating the solution of chromic acid to dryness, it may be.
concentrated to a considerable strength and then separated from solution by the
addition of commercial sulphuric acid.
This throws the chromic acid almost entirely out of solution. The sulphuric
acid is separated as completely as possible from the solid chromic acid by
decantation, draining, and if necessary by pressure applied in any suitable
apparatus for separating liquids from solids, such apparatus being constructed
where in contact with the acids of materials which are with difficulty attacked by
a mixture of chromic and sulphuric acids such as lead, stoneware etc.
The sulphuric acid is afterwards used for the decomposition of chromate of
strontium.
The water used in the manufacture of chromic acid according to my invention
should be free from lime if it is desired that the acid should be free from that
base.
Although I consider the above the most economical mode of conducting my
process I sometimes vary it as follows:-
The precipitation by chloride of Barium of the chromic acid which remains in
solution after the treatment with chloride of strontium may be omitted; this
variation will result in a loss of about five per cent of the chromic acid.
Or I precipitate the whole or nearly the whole of the chromic acid from the
original crude solution as chromate of baryta and after thoroughly washing the
precipitate, treat it in the same manner as the chromate of strontium with
sulphuric acid moderately dilute. The result here however is different, as I find
that one equivalent of sulphuric acid will not thoroughly decompose chromate of
baryta in one operation. I obtain therefore sulphate of barium mixed with some
chromate of baryta, and chromic acid, mixed with some sulphuric acid, in solution.
The mixture of sulphate and chromate of baryta I treat with fresh sulphuric acid
once or more as may be necessary and thus obtain the major part of the chromic
acid in solution, using this sulphuric acid which contains some chromic acid to act
on fresh chromate of baryta or at option on chromate of strontia To free the
chromic acid from the sulphuric acid which, as mentioned above, it contains
I treat it with chromate of strontia chemically equivalent to the sulphuric acid
present, obtaining solution of chromic acid which otherwise is treated as above
described.
When I refer to soluble chromates, solution of chromates etc. which are to be
operated on by my process I include any solutions which may be obtained in
processes for the manufacture of chromate or bichromate or ter-chromate of soda,
potash, lime etc which contain chromic acid, free or combined. Also crude solid
containing or capable of yielding chromic acid in solution free or combined.
When I refer to some soluble strontium salt I do not include the use of salts
whose acid would react in a destructive way on the chromic acid to be precipitated
as would happen were a sulphide used. Chloride of strontium I prefer, but there
might be advantages in using some salt such as the hydrate where the base
previously combined with the chromic acid had a value as in the case of bichromate
of potash, where the use of hydrate of strontia and hydrate of baryta would

page_text: dispenso with the use of any other alkali to render the bichromato normal and
would after precipitation of the chromates leave caustic potash, a valuable-product,
in solution..
When I speak of adding "solution of strontium salts" to solution of chromates
I include the variation of this which would occur wore the soluble chromates added 5
solid to the solution of strontium salts or the soluble strontium salt addod solid to
the solution of chromates.
.
When I speak of treating sulphuric acid with chromate of strontin or chromate
of strontia with sulphuric acid in" equivalent proportions " I wish to be understood
as meaning that I consider such the most advisable and only necessary amount of 1
either of these substances that must be added to one another. By using less
sulphuric acid than an equivalent quantity chromic acid is still obtained
(contaminated however with chromate of strontium) and by using more sulphuric
acid chromic acid is still obtained but contaminated with sulphuric acid, in the
first case either a fresh washing with acid (an unnecessary proceeding if my process
as described bo adhered to) or waste of chromic acid ensues. In any case I find
that this decomposition occurs whichever re-agent be in excess, in equivalent
proportions, in one operation.
When I prescribe the use of dilute acid I wish to be understood as meaning that
it is more convenient to use the acid in that form to enable the resulting chromic
acid to be separated from the bulky sulphates which are formed. Strong acid
equally decomposes the chromates as described.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is
First The manufacture of chromic acid or trioxide of chromium by producing
in a solution of a chromate a precipitate of chromate of strontium and afterwards
decomposing the chrómate of strontium with sulphuric acid substantially as
described.
Second. The manufacture of chromic acid or trioxide of chromium by producing
in a solution of a chroinate a precipitate of chromate of strontium; then completing
the precipitation of the chromate solution by means of barium; afterwards
decomposing this chromate of barium with excess of sulphuric acid and finally
applying the same acid to decompose chromate of strontium substantially as
described.
'
Third. In the manufacture of chromic acid or trioxide of chromium soparating
sulphuric acid from chromic acid by precipitating the former acid by means of
chromate of strontium substantially.as described.
Dated-tbis 25th day of June 1885.