IMPROVE- MENTS IN THE MANUFACTURE OF CHROMATES AND BICHROMATES
Year: 1885

page_text: I JOHN JAMES HOOD of 21 Shepherds Bush Road in the County of Middlesex
Doctor of Scienco do hereby declare the nature of my invention for "IMPROVE-
MENTS IN THE MANUFACTURE OF CHROMATES AND BICHROMATES" and in what
manner the same is to be performed to be particularly described and ascertained
in and by the following statement:-
This process is for the manufacture of soluble chromates and bichromates such
as chromates and bichromates of sodium ammonium potassium magnesium and
calcium.
For the manufacture of Sodium and ammonium chromates, and bichromates, the
chrome ore is heated with potassium salts and with or without lime in the
customary manner for the fabrication of potassium chromate. The resulting mass
so obtained is lixiviated in the manner usual when potassium chromates are made
and the solution, consisting chiefly of potassium chromate, concentrated.
To this solution, for the manufacture of sodic chromates, sulphate of soda either
as crystals or in solution is added in quantity by preference sufficient to convert all
the potassium of the potassic chromate into sulphate as well as any calcium salts
that may be present, the whole is then concentrated by heating. The potassic
sulphate is taken out as it forms or the liquid run off the crystals of this salt. The
potassic sulphate is recrystallized or washed and drained by suitable means from
the adhering chromate the washings being returned to the original solution. The
potassic sulphate thus obtained is used for decomposing a fresh quantity of the ore.
The greater part of the potassic sulphate having been thus extracted the solution
of sodic chromate is allowed to cool, any further quantity of potassic sulphate that
separates being taken out and the solution allowed to stand when sodic chromate
crystallizes out more or less pure. Further purification from sulphate is effected if
necessary by recrystallizing the chromate.
To form sodic bichromate, acid sodic sulphate or sodic sulphate and sulphuric
acid may be used and the operations conducted on the solution obtained by
lixiviating the fritted mass as in the case of sodic chromato or the potassic chromate
may be converted into bichromate and sodic sulphate boiled with this salt but the
method preferred is as follows:―
To the saturated solution of sodic chromate obtained in the way above described
the necessary amount of sulphuric acid to combine with half the sodium is added,
the mixture concentrated allowed to cool and stand some hours undisturbed when
almost all the sodic sulphate separates out; this salt being but slightly soluble in

page_text: a strong solution of sodic bichromate. The crystals of sodic sulphate are drained
from any adhering bichromate, washed or not, and utilized in boiling with the
potassium chromate to form sodic chromate or for making the bichromate. The
saturated solution of sodic bichromate is further concentrated by heating and the
solution is allowed to crystallize or the solution is evaporated to dryness and the
sodic bichromate fused.
In manufacturing the ammonium chromates the process is slightly altered. To
the hot strong solution of potassic chromate a hot strong solution of ammonic
sulphate is added and the mixture allowed to cool. The potassic sulphate that
crystallizes out is separated from the solution and the adhering chromate extracted
either by washing or by suitable mechanical means or by recrystallizing. The
liquid from the potassic sulphate still containing some of this salt may be used for
making either ammonic chromate or bichromate. To make the former salt the
liquid is evaporated at a gentle heat, with occasional addition of ammonic hydrate
if necessary, till saturated, the potassic sulphate that crystallizes out being separated
from the liquid as it forms during the evaporation and the saturated solution of
ammonic chromate allowed to crystallize. To form ammonic bichromate from this
liquid it is simply boiled down the potassic sulphate being taken out as it
separates and the saturated solution of ammonic bichromate allowed to crystallize,
the ammonic gas that is given off during these evaporations being collected by
suitable means.
The preceding method of manufacturing the ammonium chromates from the
potassium chromate resulting from fritting the chrome ore with potassium salts
labours under the disadvantage that potassium bichromate is apt to be formed
simultaneously on heating the monochromate with ammonium salts. This is
avoided by using the sodium chromates for making the ammonium chromates
owing to the great solubility of both chromate and bichromate of sodium.
The following is the method preferred for making these ammonium salts.
The chrome ore is fritted with sodium salts and lixiviated in the customary,
manner for making sodium chromates. Ammonic chloride or ammonic sulphate is
added to this solution and concentrated by heating; the former salt being preferred.
The ammonic gas given off due to the decomposition of ammonic chromate into
bichromate is collected by suitable means. The sodic chloride or sulphate is fished
out or separated as it forms and the solution of aminonic bichromate allowed to
crystallize. This salt being purified from any adhering chloride or sulphate by
recrystallizing.
To avoid this evolution and loss of ammonia hydrochloric acid or sulphuric acid
is added in sufficient quantity to the sodium chromate solution to form sodic
bichromate. If sulphuric acid is employed the sodic sulphate may be separated -as
above described for the manufacture of sodic bichromate from the potassium
chromates or it may not be separated at this point but the requisite amount of
ammonic sulphate added to form ammonic bichromate the whole concentrated by
heating. The sodic sulphate is fished out as it forms by the customary process of
fishing and finally the solution of ammonic bichromate is allowed to crystallize.
Hydrochloric acid however is preferred. Sufficient of this acid is added to the
sodic chromate to form sodic bichromate together with the requisite amount of
ammonic chloride for forming aminonic bichromate. The mixture is evaporated
and the sodic chloride fished out or the solution of ammonic bichromate is run off
the crystals of sodic chloride and the ammonic bichromate allowed to crystallize.
The ammonic bichromate may be further purified by recrystallizing..
Ammonic bichromate is obtained by this process in large red crystals and almost
absolutely pure. Its high percentage of chromic acid and readiness with which it
crystallizes renders it an admirable medium for chromic acid. On ignition at a
gentle heat pure chromic sesquioxide is formed rendering this salt a good means of
inaking this oxide for green pigments etc.
Ammonic chromate is made by neutralizing the ammonic chromate obtained as
above with ammonic hydrate.

page_text: For the manufacture of magnesium chromates, sodic chromates are employed
using magnesic chloride or magnesic sulphate for the decomposition and the
method of procedure is as above described by heating the solution of the mixed
salts and fishing or separating the sodic sulphate or chloride by suitable means.
Calcium bichromate may also be made by this method of double decomposition
based on differences of solubilities. Calcic chloride and sodic bichromate or its
equivalent being used the sodic chloride being separated from the solution by
fishing or other means. This method is also suitable for the manufacture of
potassium chromates from the sodium chromates.
In the usual method for manufacturing potassic bichromate by fritting the
chrome ore with potassium salts there is an appreciable loss of the latter due. to
volatilization and these salts being more costly than the sodium salts if the latter
could be used for manufacturing potassium chromates this loss would be lessoned.
To make potassium bichromate in this way the chrome ore is fritted and the
resulting mass lixiviated in the usual manner for the fabrication of sodium
chromates. To the solution so obtained acid is added to form sodic bichromate or
its equivalent. If sulphuric acid be employed the sulphate of soda is separated
from the sodic bichromate solution as already described above or it may not be
separated at this stage. If bydrochloric acid be employed the sodic chloride that
separates is fished out or it need not be separated at this stage.
To the solution now containing sodic bichromate the requisite amount of a soluble
potassium salt is added to form potassic bichromate.
If hydrochloric acid has been used to convert the sodic chromate into bichromate
potassic chloride is employed and the solution now concentrated the sodic chloride
fished out as it forms including that portion of the salt that had not been fished
out after the addition of hydrochloric acid to convert the monochromate into
bichromate. If sulphuric acid has been used to make the sodic bichromate or its
equivalent a hot saturated solution of potassic sulphate is added; if the sodic
sulphate has not been separated in the first operation of making sodic bichromate
the whole of the sodic sulphate resulting from the two operations is now separated
as far as possible by fishing or other means and the solution is finally allowed to
crystallize when potassic bichromate separates in crystals. This salt being further
purified if necessary by recrystallization.
Instead of the above potassium salts potassic nitrate may be added to the
prepared solution of sodic bichromate or its equivalent obtained from the fritted
chrome ore in the usual. way. Potassic bichromate crystallizes out and the
solution of sodic nitrate is evaporated and this salt utilized in fusing fresh
chrome ore.
The methods of manufacturing the above chromates and bichromates are based
on double decomposition and differences of solubilities and I do not restrict myself
to the use of any particular salt in bringing about these double decompositions.
Whether the salts used be potassic chloride sulphate or nitrate or other potassium
salt in making potassic bichromate from the sodium chromate or bichromate or its
equivalent or ammonic sulphate or chloride in making the ammonium chromates from
the sodium chromates will depend upon their costliness other things being equal,
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
(1). The manufacture of chromate of sodium by the treatment of chromate of
potassium with sodic sulphate and the separation of the comparatively insoluble
potassic sulphate substantially as described.
(2). The manufacture of chromate of ammonium by the treatment in like manner
of chromate of potassium with ammonic sulphate.
(3). The manufacture in like manner of sodic and ammonic bichromates by the
5 treatment of bichromate of potassium with sodic sulphate or ammonic sulphate as
the case may be,

page_text: (4). The manufacture of chromates by a process consisting in heating chrome ore
with potassic sulphate; lixiviating the mass, adding sodic or ammonic sulphate to
the solution, separating the resulting potassic sulphate and again roasting the same
with chrome ore substantially as described.
(5). The process consisting in producing bichromate of soda by the action of
sulphuric acid on chromate of soda, separating the resulting sulphate of soda not
during concentration but after the solution has cooled and remained undisturbed
for some time and employing this sulphate of soda to decompose potassic chromate
substantially as described.
(6). The manufacture of aminonium chromate or bichromate from the respective
sodium salts by heating solutions of ammonic chloride or sulphate with sodium
chromate or bichromate or its equivalent substantially as described.
(7). The manufacture of potassium bichromate from sodium bichromate or its
equivalent by heating the solution of this salt with a soluble potassium salt
substantially as described.
(8). The manufacture of magnesium chromate or bichromate and of calcium
bichromate from sodium chromate or bichromate or its equivalent substantially as
described.