IMPROVEMENTS IN THE MANUFACTURE OF SULPHURIC ACID
Year: 1883

page_text: LETTERS PATENT to Walter Weldon, of Rede Hall, Burstow, Surrey, for an
Invention of IMPROVEMENTS IN THE MANUFACTURE OF SULPHURIC ACID
communicated to him from abroad by George Lunge, Doctor of Philosophy,
Professor of Chemistry at the Polytechnikum, Zurich, Switzerland,
PROVISIONAL SPECIFICATION left by the said Walter Weldon at the
Office of the Commissioners of Patents on the 8th January 1883.
WALTER WELDON, of Rede Hall, Burstow, in the County of Surrey, IMPROVE-
MENTS IN THE MANUFACTURE OF STRONG SULPHURIC ACID, a Communication
from Abroad from George Lunge, Doctor of Philosophy, Professor of Chemistry at
the Polytechnikum, Zurich, Switzerland,
My correspondent's invention relates to the manufacture of monohydrated
sulphuric acid, SO, H₂, or such acid containing a very little water, but in any case
a much stronger acid than any which can be manufactured simply by boiling down
or distilling the ordinary commercial sulphuric acid. Tho "rectified oil of vitriol "
of commerce only contains from 93 to 96 per cent. of SO, H. Sometimes, but
comparatively rarely, acid of 97 per cent or a little more is made by direct
evaporation of ordinary oil of vitriol in platinum or glass vessels; any stronger
acid cannot possibly be obtained by evaporation, since the real monohydrate, that
is, an acid containing 100 per cent. of SO, H., is partially dissociated at a very
moderate heat and loses SO,, an acid of 98 to 98 per cent remaining behind.
The real monohydrate appears in the form of crystals fusing at 10° C.; but onco
fused they remain liquid at a much lower temperature. This pure monohydrate
was discovered by Marignac who prepared it from fuming or Nordhausen oil of
vitriol by heating this till nearly all the SO, was driven off and cooling the residue
considerably below 0° C. It has also been made by passing some sulphuric
anhydride (SO) into the strongest rectified oil of vitriol; or by mixing the latter
directly with the proper quantity of Nordhausen vitriol. No monohydrate has
ever been made on a manufacturing scale, because the high price of fuming oil of
vitriol would be an obstacle to this; and it was not thought possible to preparo

page_text: the monohydrate from the ordinary rectified oil of vitriol, or from acids of slightly
greater strength, which are stated by all observers to crystallize only at from
twenty to thirty degrees below zero Centigrade, or at an even lower temperature.
Now my correspondent has found that an acid of 98 per cent. or about 98 per
cent. can be easily brought to crystallize on a manufacturing scale by cooling it
rather below zero Centigrade, and that even the crystallization of weaker acids,
that is to say of acid of 97 or even of 96 per cent., is only prevented by the
phenomenon of "superfusion." This can be overcome by introducing into the
acid, cooled below zero Centigrade (preferably about ten degrees below zero
Centigrade) a few crystals of monohydrate with or without agitation. A similar
process had been applied by scientific observers to the monohydrate formed in one
of the above-mentioned ways, but never for any commercial purpose, nor has it
been previously known that the same method will answer with weaker acids, down
to 96 per cent, that is to say, at or slightly above the strength of ordinary
"rectified" oil of vitriol. This my correspondent has been the first to observe,
and he applies the observation practically by the present invention as follows. Ho
first prepares a very small quantity of crystallized monohydrate in any convenient
way. He finds that this can easily be done by making 98 per cent. acid either by
distilling rectified oil of vitriol in a glass retort, or by mixing it with fuming oil of
vitriol, and cooling this down to about ten degrees below zero Centigrade, when it
will almost completely solidify into a mass of crystals, which serve for starting the
crystallization in the large bulk of the acid to be treated by the present invention.
Where facilities exist for making such 98 per cent. acid on a manufacturing scale,
the just described process can serve for actually preparing monohydrate on the
large scale, but usually the acid from which monohydrate is to be prepared on a
commercial scale will be made by concentrating ordinary sulphuric acid rather
higher than usual, but well within the power of glass or platinum vessels, vis. to
97 per cent. or thereabouts. My correspondent prefers this strength because at a
lower one too much cooling is needed, and at a higher one the material of the
concentrating apparatus is too much strained, but my correspondent does not confine
Now tho
himself exclusively to employing the strength of 97 per cent.
concentrated acid is cooled down to the ordinary temperature, and the cooling is
carried further by artificial means of which there exists a variety which it is
unnecessary to enumerate. It may sometimes happen during very hard winters
that artificial cooling might be dispensed with. The more concentrated the acid
the less cooling there is required; for 97 per cent. acid about ten degrees bolow
freezing point (-10° C.) is a very suitable temperature, but my correspondent
does not confine his claim to that particular temperature. If the cooling is effected
by circulating a non-freezing liquid, itself cooled down below the required
temperature, in tubes or otherwise through the acid to be cooled down, the
aforesaid non-freezing liquid will issue from the apparatus still considerably below
zero Centigrade, and can be utilized for bringing down a fresh quantity of acid to
zero Centirgrade, [sic] or a little below, leaving the last refrigeration of the second
quantity of acid to a fresh quantity of cold liquid from the refrigerating
machine. Of course in any other system of artificial refrigeration a thorough
utilization of the cold produced can be equally obtained in a corresponding way.
When the acid has arrived at the proper temperature, a few crystals of solid acid,
previously prepared as above mentioned or otherwise, are introduced into the
cooled acid and the cooling is continued, preferably keeping the acid in motion all
the time, till crystallization sets in. It is possible to introduce the starting crystals
beforehand, but always at a few degrees below zero, so as to prevent their dissolving
in the liquid acid. In the current manufacture there will be usually enough
crystals left in the vessel from the last operation to dispense with the necessity
of employing fresh ones. Without any such crystals the point of solidification
would be very much lower and this would probably make the operation
unprofitable.
As soon as the-crystallization is complete, the liquid portion of the resulting

page_text: mass is drained from the crystals in any suitable way, with or without the aid of
suction, or of pressure, or of the centrifugal force; but in any case the mass ought
to be kept below zero Centigrade till the draining is complete. The crystals are
then taken out and will be found to be very nearly pure monohydrate, but they
will usually contain a very little water, according to the completeness or otherwise
of the draining operation. Of course they will soon melt at the ordinary
temperature. If absolutely pure monohydrate be required the liquid formed by
melting the crystals should be cooled down to about 0° C., when it will again set
to a crystalline mass which is once more submitted to the draining process, and
this operation may be again repeated. The purity of the crystals also depends
upon the temperature at which the draining takes place; by draining at too low
a temperature less mother-liquor is obtained and the crystals are less pure.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Walter Weldon in the Great Seal Patent Office on the
6th July 1883.
WALTER WELDON, of Rede Hall, Burstow, in the County of Surrey, "IMPROVE-
MENTS IN THE MANUFACTURE OF SULPHURIC ACID," a communication from
Abroad from George Lungo, Doctor of Philosophy, Professor of Chemistry at the
Polytechnikum, Zurich, Switzerland,
My correspondent's invention relates to the manufacture of monohydrated
sulphuric acid, SO, H,, or such acid containing a very little water, but in any case
a much stronger acid than any which can be manufactured simply by boiling down
or distilling the ordinary commercial sulphuric acid. The "rectified oil of vitriol"
of commerce only contains from 93 to 96 per cent. of SO, H. Sometimes, but
comparatively rarely, acid of 97 per cent. or a little more is made by direct
evaporation of ordinary oil of vitriol in platinum or glass vessels; any stronger
acid cannot possibly be obtained by evaporation, since the real monohydrate, that
is, an acid containing 100 per cent. of SO, H₂, is partially dissociated at a very
moderate heat and loses SO,. an acid of 98 to 98 per cent. remaining behind.
The real monohydrate appears in the form of crystals fusing at 10° C.; but once
fused they remain liquid at a much lower temperature. This pure monohydrato
was discovered by Marignac, who prepared it from fuming or Nordhausen oil of
vitriol by heating this till nearly all the SO, was driven off and cooling the residuo
considerably below 0° C. It has also been made by passing some sulphuric
anhydride (SO) into the strongest rectified oil of vitriol; or by mixing the latter
directly with the proper quantity of Nordhausen vitriol. No monohydrate has
ever been made on a manufacturing scale, because the high price of fuming oil of
vitriol would be an obstacle to this; and it was not thought possible to prepare
the monohydrate from the ordinary rectified oil of vitriol, or from acids of slightly
greater strength, which are stated by all observers to crystallize only at from
twenty to thirty degrees below zero Centigrade, or at an even lower temperature.
Now my correspondent has found that an acid of 98 per cent. or about 98 per
cent. can be easily brought to crystallize on a manufacturing scale by cooling it
rather below zero Centigrade, and that even the crystallization of weaker acids,
that is to say of acid of 97 or even of 96 per cent., is only prevented by the
phenomenon of "superfusion." This can be overcome by introducing into the acid,
cooled below zero Centigrade (preferably to about ten degrees below zero Centi-
grade) a few crystals of monohydrate with or without agitation. A similar process
had been applied by scientific observers to the monohydrate formed in one of the
above-mentioned ways, but never for any commercial purpose, nor has it been
previously known that the same method will answer with weaker acids, down to
96 per cent., that is to say, at or slightly above the strength of ordinary "rectified"
oil of vitriol. This my correspondent has been the first to observe, and he applies
the observation practically by the present invention as follows. He first prepares
a very small quantity of crystallized monohydrate in any convenient way. He
finds that this can easily be done by making 98 per cent. acid either by distilling
rectified oil of vitriol in a glass retort, or by mixing it with fuming oil of vitriol,
and cooling this down to about ten degrees below zero Centigrade, when it will
almost completely solidify into a mass of crystals, which serve for starting the
crystallization in the large bulk of the acid to be treated by the present invention.
Where facilities exist for making such 98 per cent, acid on a manufacturing scale,
the just described process can serve for actually preparing monohydrate on the
large scale, but usually the acid from which monohydrate is to be prepared on a
commercial scale will be made by concentrating ordinary sulphuric acid rather
higher than usual, but well within the power of glass or platinum vessels, viz., to
97 per cent. or thereabouts. My correspondent prefers this strength because at a
lower one too much cooling is needed, and at a higher one the material of the

page_text: concentrating apparatus is too much strained, but my correspondent does not
confine himself exclusively to employing the strength of 97 per cent. The concen-
trated acid should then be cooled down to the ordinary temperature, and the
cooling should then be carried further by artificial means of which there exists a
variety which it is unnecessary to enumerate. It may sometimes be during very
hard winters that artificial cooling might be dispensed with. The more concen-
trated the acid the less cooling will be required; for 97 per cent. acid about ten
degrees below freezing point (-10° C) is a very suitable temperature, but my
correspondent does not confine his claim to that particular temperature. If the
cooling is effected by circulating a non-freezing liquid, itself cooled down below the
required temperature, in tubes or otherwise through the acid to be cooled down, the
aforesaid non-freezing liquid will issue from the apparatus still considerably below
zero Centigrade, and can be utilized for bringing down a fresh quantity of acid to
zero Centigrade, or a little below, leaving the last refrigeration of the second
quantity of acid to a fresh quantity of cold liquid from the refrigerating machine.
Of course in any other system of artificial refrigeration a thorough utilization of
the cold produced can be equally obtained in a corresponding way.
When the acid has arrived at the proper temperature, a few crystals of solid
acid, previously prepared as above mentioned or otherwise, are introduced into the
cooled acid and the cooling is continued, preferably keeping the acid in motion all
the time, till crystallization sets in. It is possible to introduce the starting crystals
beforehand, but always at a few degrees below zero Centigrade, so as to prevent
their dissolving in the liquid acid. In the current manufacture there will be
usually enough crystals left in the vessel from the last operation to dispense with
the necessity of employing fresh ones. Without any such crystals the point of
solidification would be very much lower and this would probably make the operation
unprofitable.
As soon as the crystallization is complete, the liquid portion of the resulting
mass is drained from the crystals in any suitable way, with or without the aid of
suction, or of pressure, or of the centrifugal force; but in any case the mass ought
to be kept at a low temperature, preferably below zero Centigrade till the draining
is complete. The crystals are then taken out and will be found to be very nearly
pure monohydrate, but they will usually contain a very little water, according to
the completeness or otherwise of the draining operation. Of course they will soon
melt at the ordinary temperature. If absolutely pure monohydrate be required,
the liquid formed by melting the crystals should be cooled down to about zero
Centigrade, when it will again set to a crystalline mass which is once more
submitted to the draining process, and this operation may be again repeated. The
purity of the crystals also depends upon the temperature at which the draining
takes place; by draining at too low a temperature less mother-liquor is obtained
and the crystals are less pure. Some of the impurities of sulphuric acid such as
arsenic may also be got rid of in this way.
Having described and particularly ascertained the nature of the said invention
and in what manner the same is to be performed, I claim as the invention to be
secured by the hereinbefore in part recited Letters Patent the inanufacture of
monohydrated sulphuric acid, SO, H,, or such acid containing a very little water,
substantially in the manner described in the foregoing specification.
In witness whereof I, the said Walter Weldon, have hereunto set my hand
and seal this sixth day of July in the year of Our Lord one thousand eight
hundred and eighty three.