IMPROVEMENTS IN THE MANUFACTURE OF BICHROMATE OF SODA
Year: 1885

page_text: I WILLIAM HENRY HIGGIN of Hall Chemical Works Little Lever near Bolton
in the County of Lancaster Bachelor of Science in the University of London
and Chemical Manufacturer do hereby declare the nature of my invention for
IMPROVEMENTS IN THE MANUFACTURE OF BICHROMATE OF SODA to be as
follows:-
This invention which relates to improvements in the manufacture of bichromate
of soda consists in a new and improved process by which that compound is prepared
and separated in a commercially useful form, from other substances with which it
is unavoidably mixed in the process of its manufacture.
The watery solution of chromate of soda obtained from the lixiviating tanks
by the ordinary method, is conveyed into settling tanks where it is thoroughly
mixed with the "acid washings" whose production is hereafter described; it is
then allowed to stand for some time undisturbed, when the impurities contained
both in the chromate liquor and in the "acid washings" settle to the bottom-The
clear purified liquor-which still has an alkaline reaction to red litmus paper, is
now-transferred to an evaporating pan heated in any convenient manner and is
there boiled down to dryness and the solid left is dried.
The dried mass produced consists mainly of monochromate of soda but will
contain more or less bichromate and other soda salts according to the quantity and
strength of the "acid washings" used as above-It is treated when cold with an
acid in such quantity as to either entirely, or very nearly but not quite entirely,
convert the whole of the chromate present into bichromate according as the
finished product is to be crystalized or calcined bichromate of soda-The soda
removed from combination with chromic acid forms soda compounds with the acid
used.
For the above process I find the hydrochloric acid of commerce containing about
thirty per centum of pure acid and seventy per centum of water to be the best and
practically most useful but other acids at suitable degrees of concentration may
also be used such as sulphuric or nitric.
The acid is added gradually to the dried mass, the whole being kept thoroughly
mixed preferaty mechanical means, as it is necessary to break up the solid into
fine powder as otherwise much remains unacted upon by the acid.

page_text: The chemical action causes the whole to heat and if necessary it is moderated by
stopping the addition of acid for a time.
The result of this process is a thick liquid which whilst still warm is filtered,
strained pressed or otherwise mechanically separated into a clear red coloured
liquid and a partially dried solid.
The clear red coloured liquid is a strong solution of bichromate of soda in water
(which water is that present in the acid used) sufficiently free from all other
compounds of soda and I now proceed to finish the bichromate in any of the well
known ways, either by crystallizing if crystals are required or by boiling down to
dryness and thoroughly drying if a " calcined" product is desired.
The partially dried solid from which the above liquid has been obtained as
described above, contains the soda compounds of the acid used for bichromating
with not more chromic acid than is equal to one fifth of its own weight of
bichromate of soda.
I place this in suitable tanks of stone or other material and wash it with acid of
the same kind as is used for bichromating preferably hydrochloric-This part of
the process is not new but is described in Potter and Higgins specification
No 555284 The last traces of chromic acid compounds are thus separated and the
washings consisting of a solution of chromic acid and other compounds form the
"acid washings" hereinbefore referred to and which are added to the fresh
chromate liquor from the lixiviating tanks as already described.

page_text: Improvements in the Manufacture of Bichromate of Soda.
I WILLIAM HENRY HIGGIN of The Hall Chemical Works Little Lever near
Bolton in the County of Lancaster Bachelor of Science in the University of
London and Chemical Manufacturer do hereby declare the nature of my invention
for IMPROVEMENTS IN THE MANUFACTURE OF BICHROMATE OF SODA and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :-
Bichromate of Soda has hitherto been produced and obtained in the process of
its manufacture from the monochromate mixed with and adulterated by other
compounds of soda (e.g. the chloride) which are formed at the same time and by
the same process as itself. It has been necessary to separate it from these
compounds by special processes.
By my improved process I obtain at once from the substance hereinafter
referred to as crude chromate a solution in water of the bichromate of any
desired degree of purity, short of absolute chemical freedom from other compounds.
The watery solution of monochromate of soda obtained from the lixiviating
tanks by the ordinary method is conveyed into settling tanks where it is
thoroughly mixed with the acid washings" whose production is hereafter
"
described. It is there allowed to stand for some time undisturbed when the
impurities, if any, contained either in the chromate liquor or in the "acid
washings" settle to the bottom. The clear purified liquor is then drawn off and
transferred to an evaporating pan, heated in any convenient manner, and is there
exposed to heat until the water it contains is driven off as steam.
The substance remaining in the pan consists mainly of monochromate of soda,
but it contains also more or less bichromate of soda and other soda salts, according
to the quantity and composition of the "acid washings" used as above. This
substance is hereinafter referred to as "crude chromate."
The "crude chromate" is removed from the pan and treated, when cold, with a
mineral acid of a certain strength in such quantity as either entirely or very nearly
but not quite entirely, to convert the whole of the monochromate present into
bichromate according as the finished product is to be crystallized or calcined
bichromate of soda: the soda removed from combination with chromic acid forms
soda compounds with the acid used. Water also is formed as may be seen from
the following reaction where hydrochloric acid is used; hydrochloric acid gives
bichromate of soda and common salt and water,' tlius
2 Na, Cr O, + 2 H Cl = Na₂ Cг₂ О7 + 2 Na Cl + H₂ O.
Now if the acid used be of the correct strength, that is, contains the right
proportion of water, I find that the bichromate of soda is dissolved in that water
and in the water formed by the above reaction whilst nearly the whole of the other
compound of soda is formed as a finely crystallized powder, very little being found
in the solution.
The strength of the acid used is governed by the degree of purity of the
bichromate (or freedom from the other compounds of soda) which it is desired to

page_text: "J
obtain, and also by the quantity of bichromate already present in the "crude
chromate." Other things remaining the same, if a less degree of purity will suffice,
then a weaker acid may be used, or if more bichromate be in the "crude chromate
then a proportionately weaker acid will give the same degree of purity. Moreover,
it may sometimes be convenient to have a certain amount of moisture in the
"crude chromate" when it will be necessary to use a correspondingly stronger
acid. In short all the conditions may be altered within certain limits, but to
obtain a bichromate of a certain degree of purity, there must always be one
particular relation preserved namely that of the total weight of water present at
the end of the reaction to that of the bichromate of soda. If that ratio be
disturbed the product will within certain limits be less pure if the proportion of
water is increased, more pure if it is diminished. For the manufacture of pure
crystallized or of very strong calcined bichromate of soda I find it most convenient
to always use the ordinary hydrochloric acid of commerce containing 28 to 30 per
centum of anhydrous acid and 72 to 70 per centum of water. This strength when
used with dry "crude chromate" allows a certain margin for slight variations
in the composition of the latter always yielding a solution of a high degree of
purity.
I add the acid gradually to the "crude chromate" so that it is at once taken up,
the whole being kept thoroughly mixed preferably by mechanical means since it is
necessary to break up the solid lumps into fine powder as otherwise much remains
unacted upon. The chemical reaction causes the whole to heat and it is advisable
to add the acid cautiously so as to duly moderate this heating.
The result of the foregoing process is a thick liquid which whilst still hot is
filtered, strained, pressed or otherwise mechanically separated into a clear red
colored liquid and a partially dried solid.
The clear red colored liquid is a strong solution of bichromate of soda in water
sufficiently free from all other compounds of soda and I now proceed to convert it
into the solid form for sale, in any of the well known ways either by concentrating
and crystallizing if crystals are required, or by boiling down to dryness and
thoroughly drying if a "calcined" product is desired.
The partially dried solid or mass of crystals from which the above liquor has
been obtained still retains a certain quantity of bichromate of soda; I place these
crystals in suitable tanks of stone or other material and wash them with acid of
the same kind as I use for bichromating-preferably hydrochloric. This part of
the process is not new but is described in Potter and Higgin's Patent No. 5552
of 1884. The last traces of chromic acid compounds are thus recovered in the
washings which consist of a solution of chromic acid and other compounds
constituting the "acid washings" herein before referred to, and which are added to
the fresh chromate liquor from the lixiviating tanks as already described.
Hydrochloric acid is used by me in carrying out my process as described above
because it is at present cheap and otherwise available. It is moreover preferable
to sulphuric or nitric acids because the residual compound it forms (common salt)
is very much more easily separated from the bichromate by mechanical means than
are those formed by the other acids. I distinctly state however that if at any
time or place it is advisable to use either of these other acids that the same can
be done and that iny improved process will yield bichromate of soda in connection
with them.
I find that commercial sulphuric acid even in it's weakest form ("chamber acid ")
does not contain sufficient water for the purpose of my invention; it forms a thick
paste with the "crude chromate" from which it is impossible to separate the
bichromate of soda as a watery solution. But the most advantageous degree of
strength to use for this acid is about 36 per centum of anhydrous acid and
64 per centum of water, that is sulphuric acid of about specific gravity 1.27.
Similarly I find with nitric acid that the ordinary commercial acid is too strong
but that the best strength to use it at is about 57 per centum of anhydrous
acid and 43 per centum of water that is acid of about 1.35 specific gravity.

page_text: Although these particular strengths of acid do not occur in commerce they are
readily prepared from the commercial acids by the addition of the requisite
quantity of water.
Having now fully described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what
I claim is
1. The Improvements in the process of manufacturing bichromate of soda which
consist in evaporating to dryness the solution containing monochromate of soda,
converting the monochromate of soda into bichromate by treating the solid product
so obtained by evaporation with an aqueous mineral acid of the required strength
and separating the solution of bichromate of soda so produced from the solid
compound of soda.
2. The Improvements in the process of manufacturing bichromate of soda which
consist in evaporating to dryness the solution containing monochromate of soda,
converting the monochromate of soda into bichromate by treating the solid product
so obtained by evaporation with an aqueous mineral acid of the required strength,
separating the solution of bichromate of soda so produced from the solid compound
of soda which contains a small proportion of the bichromate, treating this solid
compound with acid, and utilizing the acid washings so prepared, in the manner
bereinbeforo described.
"