AN IMPROVED PROCESS FOR MANUFACTURE OF BICHROMATE OF SODA
Year: 1883

page_text: LETTERS PATENT to Edmund Peel Potter and William Henry Higgin both of
Bolton in the county of Lancaster for an Invention of "AN IMPROVED
PROCESS FOR MANUFACTURE OF BICHROMATE OF SODA"
PROVISIONAL SPECIFICATION left by the said Edmund Peel Potter and
William Henry Higgin at the Office of the Commissioners of Patents on the
2nd February 1883.
EDMUND PEEL POTTER and WILLIAM HENRY HIGGIN both of Bolton in the
County of Lancaster "AN IMPROVED PROCESS FOR MANUFACTURE OF BICHROMATE
OF SODA"
Sodium having a lower combining equivalent than Potassium we reasoned that
sodium bichromate would be a better oxydizing agent than Potassium Bichromate,
but on wishing to try the experiment we were unable to procure the compound
anywhere or find any published method of isolating it. The ordinary process of
manufacturing bichromate of potash utterly failed when applied to the production
of its sister compound the sodic salt-at least in producing it in an isolated form:
We have however succeeded in isolating it so as to obtain it commercially pure by
the following process and have found it a much more powerful oxidizer than the
Potassium Salt.
We furnace chrome ore with lime or carbonate of lime and sodium sulphate or
lime or carb lime and sodium carbonate or other sodium salts in substantially
the same manner as in the Potassium Bichromate manufacture where lime and
sulphate of potash are used. The resulting mass is lixiviated and the liquor
treated with exactly sufficient Hydro chloric acid to change the chromate into a
bichromate. We then concentrate the liquor extracting the sodium chloride and
other sodium salts as their crystals form and continue the concentration until the
The
liquid changes to a thick paste. This paste is then dried at a low heat.
crystals fished out are washed in any convenient manner with prepared chrome
liquor before concentration. This liquor containing a considerable quantity of
Sodium Chloride fails to dissolve much of the latter salt but takes off most of
bichromate. When sufficiently saturated this liquor can be also concentrated in

page_text: the manner described. The above is a description of the process as experimentally
worked out by us but it can be largely varied. Thus in practice we may find it
convenient to get rid of the sulphate of soda by adding chloride of Calcium or of
Barium and may use a centrifugal machine in lieu of much washing.
We also recover from the washed fished salts, the chromium and sodium
compounds by means of decomposition with Sulphuric acid in an ordinary salt cake
furnace and using the resulting compound in the decomposition of fresh chrome
ore in the manner as before stated.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Edmund Peel Potter and William Henry Higgin in the Great
Seal Patent Office on the 2nd August 1883.
EDMUND PEEL POTTER and WILLIAM HENRY HIGGIN, both of Bolton, in the
county of Lancaster. "AN IMPROVED PROCESS FOR THE MANUFACTURE OF
BICHROMATE OF SODA
"
Sodium being a very much cheaper base and having a lower combining equivalent
than potassium, we reasoned that sodium bichromate would be a cheaper and
better oxydizing agent than potassium bichromate; but on wishing to try the
experiment, we were unable to procure the compound anywhere, or find any
published method of isolating it. The ordinary process of manufacturing
bichromate of potash utterly failed, when applied to production of its sister
compound, the sodic salt, at least in producing it in an isolated condition.
We have however succeeded in isolating it so as to obtain it in a commercially
useful form by the following process, and have found it, when used pure, a much
more powerful oxydizer than the potassium salt.
We furnace chrome ore with lime or carbonate of lime and a suitable sodium
salt, but preferably the sulphate, using in some cases a certain proportion of the
recovered sodium sulphate.
The furnacing is done in substantially the same manner, as in the potassium
bichromate manufacture, where lime and sulphate of potash are used, it is therefore
needless to describe it further as that process is well known.
The resulting material is lixiviated with water in the ordinary manner, and the
liquor obtained is treated with hydrochloric or other acid, in such quantity as to
exactly neutralize it; we have found that other acids may be used, but hydro-
chloric is much preferred to sulphuric or any other.
On neutralization the silica and alumina are precipitated, and we carefully
remove the same by settling and filtration.
We now proceed to concentrate the neutralized and purified liquor in under fired
pans of suitable construction, and find that as the boiling solution loses its water,
the chloride and residual sulphate of soda contained therein are thrown out in the
crystalline form, but that the chromates of soda remain in solution. The crystals

page_text: are removed by mechanical means, either by ordinary hand-fishing, or by other
means, and after draining are washed with neutralised vat liquor, before concen-
tration. This liquor containing a considerable quantity of these salts already, fails
to dissolve them, but being far from saturated with chromates takes up nearly all
the latter. The washings resulting are then concentrated as above, and practically
little or no neutralized liquor will go straight to the pan, but will all previously
be used to wash salts as above. It will thus be seen, that the action is an
accumulating one, by which gradually all the chromates are left in solution in the
pan, and all the other salts are obtained in a washed state nearly free from
chromates.
When sulphuric acid has been used to neutralise the vat liquor, these washed
fished salts containing a trace of chromates, are then used again, after drying, in
place of an equivalent quantity of fresh salt cake as above mentioned.
But when, as we prefer, hydrochloric acid is used to neutralise the vat liquor
then the washed fished salts, which consist in this case mainly of sodium chloride
with traces of chromates, are converted into sulphates by the ordinary salt-cake
process, and then used again in place of an equivalent quantity of fresh salt-cake
as above, thus recovering and utilising the last remaining chromium compounds.
The strong solution remaining in the pan is filtered, or otherwise freed from the
last crystals of chloride and sulphate of soda floating in it, and is then boiled
down to dryness and the salt perfectly dried, but not fused in a top fired mechanical
pan, or by other means, care being taken to keep the temperature low and an
excess of oxygen always in the gases in contact with the bichrome salt. The
bichromate of soda, obtained by the process as fully described above, contains
monochromate, but as the reactions of the salts are identical this is of no importance.
In cases however, where a pure specimen of bichromate is wanted for a special
purpose, the neutralized and purified vat liquor obtained as above is after removal
of alumina and silica, treated with a further quantity of acid (preferably hydro-
chloric) in exactly the quantity required to convert the whole of the monochromate
salt into bichromate, as calculated from analysis of the vat liquor before treatment
with acid, and the process is then conducted as above, but in this case the pans
used would have to be of special material, to avoid contaminating the acid liquor,
for which purpose lead would probably be the most available.
In practice however, we find the manufacture of the pure salt not so advantageous
as an impure article composed of about two thirds of bichromate, and one third
monochromate, with a very small but variable quantity of neutral salts of soda,
and at this point it has a neutral reaction and the same amount of oxydising
power as the potassium salt and can consequently be substituted for it in whole
or in part in equal quantities, weight for weight, a practical advantage. Whether
pure or impure as above described the action is the same only the quantity required
differing.
Finally, we believe that the reasons why no one has succeeded in obtaining
commercially useful bichromate of soda are these,-Firstly-Sulphuric acid has
been experimented with instead of hydrochloric acid, now the sulphate of soda
formed unites with a great quantity of water on crystallizing in the cold, and
hitherto the separation as far as we know has been attempted in the cold, and
hence the crystals of sulphate of soda enclosed and carried with them a very large
proportion of the bichromate present in the solution, on the other hand, hydro-
chloric acid forms common salt, which separates in the anhydrous condition and
we find practically carries with it a much less considerable fraction of the chromic
salts present, and moreover is much more easily washed, pressed et-cetera, owing to
the form of the crystals, which are square and hard, whilst the sulphate forms a
softer closer mass much more difficult to deal with. -Secondly-The whole
separation was attempted at one crystallization, the crystals not being fished out as
formed, and the "accumulating principle" was not applied.
Finally, The crystals obtained were useless for ordinary purposes, and it does
not appear to have occurred to the experimenters to use them after drying in

page_text: the furnace, in lieu of fresh sulphate, this would however be impracticable in this
case, owing to the extreme richness the mixing would arrive at, when as is well
known decomposition is apt to take place at the elevated temperatures of the
furnace.
Having now described our invention and the best mode known to us at present
of carrying it into effect, we wish it to be understood that what we believe to be
new and desire to protect under the herein before recited letters patent is.
First The process, as a whole, of manufacturing bichromate of soda, substantially
as herein described.
Second The process of separating the other salts of soda present from the chromic 1
salts, by the continuous boiling down of the chrome liquor, and removing
the crystals of those salts from the liquor as they form, which liquor is
afterwards evaporated at a low heat, and the salt obtained dried.
Third The process of recovering the chromium compounds from the separated
residual salts, by washing with prepared vat liquor, which is afterwards 1
concentrated as above.
Fourth The recovery from the washed fished and other residual salts, of the
chromium and sodium compounds, by means of decomposition with
sulphuric acid in an ordinary salt cake furnace, and using the resulting
compound in the decomposition of fresh chrome ore in manner as before 2
stated.
In witness whereof we the said Edmund Peel Potter, and William Henry
Higgin have hereunto set our hands and seals, this twenty seventh day of July
in the year of our Lord one thousand eight hundred and eighty three.
EDMTIND DEBL
TATTD