IMPROVEMENTS IN THE MANUFACTURE OF SODA FROM COMMON SALT."
Year: 1866

page_text: LETTERS PATENT to Walter Weldon, of Park Villa, West Hill, High-
gate, in the County of Middlesex, for the Invention of "IMPROVEMENTS
IN THE MANUFACTURE OF SODA FROM COMMON SALT."
PROVISIONAL SPECIFICATION left by the said Walter Weldon at the
Office of the Commissioners of Patents, with his Petition, 'on the 1st
March 1866.
"
I, WALTER WELDON, of Park Villa, West Hill, Highgate, in the County
of Middlesex. do hereby declare the nature of the said Invention for
"IMPROVEMENTS IN THE MANUFACTURE OF SODA FROM COMMON SALT" to be as
follows:-
The improvements which constitute my present Invention enable soda to be
manufactured from chloride of sodium or common salt by an entirely new
method, the essential feature of which new method consists in the inter-
mediate production of fluoride of sodium. These improvements are based
mainly upon the fact, which I believe to be at present entirely unknown to
chemists, and which has certainly not hitherto been taken advan-
tage of in the manufacture of soda, that hydrofluoric acid. will decompose both
chloride of sodium and sulphate of sodium, thereby producing fluoride
of sodium, and eliminating in the one case free hydrochloric acid, and

page_text: in the other free sulphoric acid. Another hitherto unknown fact
upon which my improvements partly depend is that fluoride of sodium, unlike
chloride of sodium, admits of being decomposed by steam at a
temperature below that at which caustic soda volatilizes. My present Inven-
tion therefore consists, first, in the decomposition of chloride of sodium by
hydrofluoric acid; secondly, in the decomposition of sulphate of
sodium by hydrofluoric acid; thirdly, in the decomposition of the result-
ing fluoride of sodium by means of steam; and,' fourthly, in a combina-
tion of processes whereby, when I do not effect that decomposition
by the means just mentioned. I decompose fluoride of sodium and then
recover, for use over and over again continually. the various reagents
employed in the formation and decomposition of that compound, so as to
enable me to obtain soda without throwing anything away, and therefore
at a cost for materials confined, except to the extent of the slight loss
which must inevitably result from repeated manipulation, exclusively to
that of salt and coal. The processes the combination of which is here
spoken of are not individually new, but depend upon reactions already
known to chemists, and these processes therefore constitute no part of my
Invention, except as combined with the processes which I have indicated
as constituting the first, second, and third parts of my Invention, and
which, both in themselves and in their combination with the other pro-
cesses referred to, are entirely novel.
I find that the decomposition by aqueous hydrofluoric acid of both
chloride of sodium and sulphate of sodium will take place at ordinary
temperatures without any application of heat, with production, as I have
said, of fluoride of sodium and elimination of hydrochloric acid or sulphuric
acid, as the case may be, the eliminated acid being obtained in cold
aqueous solution supernatant above the precipitated fluoride. (I do not,
however, confine myself to effecting the decomposition at ordinary tem-
peratures.) The resulting fluoride of sodium may be decomposed by the
reaction upon it of steam at a suitable temperature, with production of
caustic soda and reproduction of hydrofluoric acid; but I at present prefer
to decompose it by means of the oxide, hydrate, or carbonate of either
calcium, magnesium, or any other metal which has sufficient affinity for
fluorine, and forms with that element a compound sufficiently insoluble in
water to permit of the said metal separating the fluorine from fluoride of
sodium when its oxide, hydrate, or carbonate is treated therewith in pre-
sence of water, either by ebullition or otherwise. There is thus obtained a
solution of either caustic' soda or carbonate of sodium, and a precipitate of

page_text: either insoluble fluoride of calcium, insoluble fluoride of magnesium, or
insoluble or but sparingly soluble fluoride of some other metal, which
insoluble or but sparingly soluble fluoride can be used for the reproduction
of hydrofluoric acid. Like fluoride of sodium itself, the fluoride obtained
by decomposing that fluoride by the means just mentioned can be decom-
posed by steam, with regeneration simultaneously of hydrofluoric acid and
metallic oxide; but I at present prefer to decompose it by means of
sulphuric acid, and when I decompose the final fluoride by this means I
prefer to use for the decomposition of the fluoride of sodium the oxide,
hydrate, or carbonate of some metal whose sulphate can be employed
instead of sulphuric acid itself to decompose common salt with the result-
ing. production of sulphate of sodium, or can be used for the reproduction
of sulphuric acid, or applied to any other useful purpose.
One very convenient method of carrying out the Invention, the general
nature of which I have above described, commences with the production
of sulphate of sodium by heating chloride of sodium with sulphate of
magnesium, having associated with it at least one equivalent of water, in a
furnace connected with the coke tower such as is ordinarily used for the
condensation of hydrochloric acid. The chloride of sodium is thereby
decomposed, hydrochloric acid flying off just as in the first operation of
the Leblanc process, and sulphate of sodium mixed with magnesia remain-
ing behind.
The sulphate of sodium is separated from the admixed
magnesia by lixiviation, and is then decomposed by hydrofluoric acid,
fluoride of sodium being thereby produced and sulphuric acid set free.
The fluoride of sodium thus formed is boiled with the magnesia obtained
by the decomposition of the sulphate of magnesium used in the first opera-
tion of the process, and the results are a solution of caustic soda and a
precipitate of fluoride of magnesium. This fluoride of magnesium is finally
decomposed by means of the sulphuric acid set free in the third operation
of the process, with the production of sulphate of magnesium for use over
again in the first operation of the process, and hydrofluoric acid for use
over again in the third operation. The hydrofluoric acid, sulphuric acid,
and magnesia employed in this method of manufacturing soda are thus
used continually over and over again, so that, as regards materials, the
only cost of the soda is that of salt and fuel, together with that of such
very slight quantities of magnesia, sulphur, and fluor spar as may need to be
employed from time to time in order to make up loss by mechanical or
other waste. This method of manufacturing soda, besides requiring much
less labour and fuel than the method at present in use, therefore saves the

page_text: whole cost of the sulphuric acid employed in the manufacture as at present
conducted, the sulphuric acid being at present used only once, and a fresh
portion being required for every fresh portion of salt decomposed, whereas by
the method above described the same portion of sulphuric acid is made to serve,
excepting so far as that result is vitiated by the loss inseparable from frequent
manipulation, over and over again for ever.
A shorter method of proceeding consists in decomposing by hydrofluoric
acid chloride instead of sulphate of sodium, in then decomposing the result-
ing fluoride of sodium by ebullition with magnesia, and in finally decom-
posing the fluoride of magnesium thus obiained by sulphuric acid. Caustic
soda is thus obtained by two operations only, the first of which requires no
fuel at all, and the second but very little, while the third operation, which
also requires but very little fuel, continually yields hydrofluoric acid
wherewith to decompose a fresh equivalent of chloride of sodium and
sulphate of magnesium, which can be used either for any purpose to which
Epsom salts are ordinarily applied, or for the decomposition of chloride of
sodium in the first operation of the Leblanc process, or for the precipita-
tion of sulphate of barium from solution of the sulphate of that metal, or
may be decomposed by calcination with carbon, with production of
sulphurous acid, carbonic acid, and magnesia; or, where it may happen that
the sulphate of some other metal available for the decomposition of fluoride of
sodium shall be more useful than the sulphate of magnesium, the fluoride of
sodium may be decomposed by the oxide, hydrate, or carbonate of that metal
instead of by magnesia, the resulting fluoride decomposed by sulphuric acid,
and the resulting sulphate used as desired.
As regards the proportions to be employed of the various compounds con-
cerned in carrying out any Invention, I need only say that the only proportions
in which they will react upon each other, in whatever proportions they may
be brought into mutual contact, are those which are known to chemists as
"equivalent proportions," and which are 585 parts by weight of chloride of
sodium, 42 parts by weight of fluoride of sodium, 28 parts by weight
of oxide of calcium, 50 parts by weight of carbonate of calcium, 39 parts by
weight of fluoride of calcium, 20 parts by weight of oxide of magnesium,
42 parts by weight of carbonate of magnesium, 31 parts by weight
of fluoride of magnesium, as much aqueous hydrofluoric acid as contains 20
parts by weight of HF, and as much aqueous sulphuric acid as contains 49
parts by weight of HSO*.

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 31st August 1866.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, WALTER
WELDON, of Park Villa, West Hill, Highgate, in the County of Middlesex,
send greeting.
1
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the First day of March, in the year of our Lord One
thousand eight hundred and sixty-six, in the twenty-ninth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto me,
the said Walter Weldon, Her special licence that I, the said Walter Weldon,
my executors, administrators, and assigns, or such others as I, the
said Walter Weldon, my executors, administrators, and assigns, should at any
time agree with, and no others, from time to time and at all times there-
after during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention for
"IMPROVEMENTS IN THE MANUFACTURE OF SODA FROM COMMON SALT," upon the
condition (amongst others) that 1, the said Walter Weldon, my executors
or administrators, by an instrument in writing under my, or their, or
one of their hands and seals, should particularly describe and ascertain the
nature of the said Invention, and in what manner the same was to be
performed, and cause the same to be filed in the Great Seal Patent Office
within six calendar months next and immediately after the date of the said
Letters Patent.
NOW KNOW YE. that 1, the said Walter Weldon, do hereby declare
the nature of my said Invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:-
Invention enable
1. "The improvements which constitute my present
suda to be manufactured from common salt by a new method or process,
the esential feature of which new method or process consists in the inter-
mediate production of fluoride of sodium. The said improvements are
based mainly upon certain facts which I believe to have been first dis-
covered by myself, and to be at present quite unknown to chemists, and
which have certainly not hitherto been taken advantage of in the manu-
facture of soda; these facts being, firstly, that hydrofluoric acid will
decompose sulphate of sodium, with formation of fluoride of sodium and

page_text: elimination of sulphuric acid, the eliminated acid, though associated and
probably combined with an undecomposed portion of sulphate, and so
probably not being "free" in the strictest sense of the use of that word
that it can be used to
by chemists, being nevertheless so far "free"
decompose the compounds the decomposition of which by sulphuric acid
is required in carrying out some of the modifications of the new method
or process which constitutes my present Invention; secondly, that, hydro-
fluoric acid will decompose chloride of sodium more or less completely
with formation of fluoride of sodium and hydrochloric acid; thirdly, that
fluoride of sodium admits of being decomposed under the influence of
heat and in the presence of water, either in the state of liquid or of that
of vapour, not only by lime, as was previously known, but also, what
I believe has not been hitherto known, by magnesia, with formation of
caustic soda and fluoride of magnesium, and that carbonate of magne-
sium will under some circumstances, in presence of water, similarly
decompose, fluoride. of sodium, fluoride of magnesium, and carbonate of
sodium being formed; and fourthly, that the decomposition of fluoride of
sodium by means of steam is much more readily practicable than, the
decomposition of chloride of sodium by the same agent. By availing
myself of these hitherto unknown facts, and combining or not,
as, the
occasion may require, operations based on them with operations based on
other and previously known facts, I am. enabled to effect the transforma-
tion of salt into soda by a new method or process, by means of some of
the modifications of which new method or process I am enabled to manu-
facture soda without consuming any materials excepting salt and coal, all
the reagents employed being continually reproduced and obtained for use
over again, while other modification of it, though involving the consump-
tion of some of the reagents employed, nevertheless effect a very.consi-
derable economy as compared with the process of manufacturing soda at present
in use.
2. It is usually stated in the chemical books that when sulphate of
sodium is treated with hydrochloric acid one half the sulphate is converted
into bisulphate and the other half transformed into chloride,. the reaction
being-
2 Na SO+ HCl = NA CI+ Na H (SO4)2.
I believe that the reaction between sulphate of sodium and hydrofluoric
acid is exactly similar, two equivalents of sulphate of sodium and one equi-
valent of hydrofluoric acid yielding one equivalent of fluoride of sodium
and one equivalent of bisulphate of sodium. Whether, however, the

page_text: sulphuric acid which is eliminated by the hydrofluoric acid from the
portion of sulphate which is converted into fluoride be obtained, on the
completion of the reaction, in simple mixture with the undecomposed
portion of sulphate, or be united therewith in that comparatively loose
state of combination in which the second atom of sulphuric acid is held by
bisulphate of sodium, for the purposes of the present. Invention it is vir-
tually and practically "free," inasmuch as it is in a state in which it will
decompose, as I have said, the various compounds the decomposition of
which by sulphuric acid is required in the various modifications of the new
method of manufacturing soda which I base upon the newly discovered
facts stated above.
3. When steam at a high temperature is made to react upon fluoride of
sodium in or approaching a state of fusion the results are hydrate of sodium and
hydrofluoric acid in accordance with the equation-
Na F+Na O. HO-2 HO + HF.
The reaction will take place at temperatures very considerably below that
of fusion, but not so readily as at higher temperatures.
4. When strong aqueous hydrofluoric acid is added, at ordinary tem-
peratures, to a saturated solution of common salt the whole immediately
becomes milky looking, and there is gradually formed a precipitate which
consists of fluoride of sodium mixed with fluoride of calcium and fluoride
of magnesium if the salt contained any chloride of those metals, a quan-
tity of hydrochloric acid equivalent to the fluorides formed being pro-
duced at the same time. This hydrochloric acid dissolves as formed, and
remains in the solution. The transformation of salt into fluoride of sodium
by treating it with hydrofluoric acid in the manner just described is not
complete however, and, although a better result is got by passing hydro-
fluoric acid gas into the solution of chloride of sodium. I consider it pre-
ferable, on the whole, to first form sulphate of sodium and then
treat that compound with hydrofluoric acid, rather than to form fluoride of
sodium by acting with hydrofluoric acid directly upon chloride of sodium
itself. In some circumstances, however, the latter will be the most con-
venient method of forming the fluoride. When this direct method of
forming fluoride of sodium is adopted the hydrofluoric acid gas should be
cooled before being passed into the solution of chloride of sodium, and it
should be passed in so slowly as to prevent it developing heat enough to
interfere with the desired reaction.
5. When the plan which is adopted is that of transforming the chloride
of sodium into sulphate, and then the sulphate into fluoride, and then

page_text: decomposing the fluoride by means of steam, the first operation con-
sists, the first time, but not afterwards, in decomposing chloride of sodium by
means of sulphuric acid obtained by the ordinary process of manufacture
from sulphur or pyrites, just as in the first operation of the soda process at
present in use. After the first time, however, the first operation always
consists in the decomposition of an equivalent of chloride of sodium
by the bisulphate of sodium, if such it be, or the mixture of uncombined
sulphuric acid with normal sulphate of sodium, if it be that which is obtained,
together with an equivalent of fluoride of sodium, by treating two
equivalents of sulphate of sodium with one equivalent of hydrofluoric
acid. There are thus formed two equivalents of sulphate of sodium, and
the second operation consists in treating these with hydrofluoric acid, in
order to the formation of an equivalent of fluoride of sodium for conver-
sion into soda, and an equivalent of bisulphate of sodium or of uncom-
bined sulphuric acid mixed with normal sulphate of sodium, with which to
repeat the first operation. For convenience I will henceforth speak of
this last-mentioned product of the reaction between sulphate of sodium
and hydrofluoric acid as "bisulphate of sodium." The third operation con-
sists in decomposing by means of steam the fluoride of sodium obtained in the
second operation, thereby producing caustic soda and hydrofluoric acid where-
with to repeat the second operation. Caustic soda is thus obtained at no
expense except for salt, fuel, labour, and wear and tear, and without production
of any useless residue, the only reagents employed being sulphuric acid, hydro-
fluoric acid, and steam, and the two former being reproduced for use over and
over again continually.
"
6. The first of the three operations just described differs so little from
the first operation of the ordinary or Leblanc soda process that it does not
necessarily need any especial apparatus, but may be performed in any of
the "salt ovens at present ordinarily employed for the production of "salt
cake." The first effect of the action of sulphuric acid upon chloride of sodium
in the process at present in use consists in the production of
bisulphate of sodium, and the latter stage of the reaction which takes place
in the first operation of that process consists in the decomposition of the
remainder of the chloride of sodium by this bisulphate, so that the reaction
which takes place in the first of the three operations described in the
paragraph next preceding this is exactly the same as the latter phase of
the reaction which takes place in the first operation of the process
at present in general use. The only special precaution to be attended to,
therefore, in performing the first of the three operations in question is that

page_text: of heating the solution of bisulphate of sodium before putting it into the
oven or furnace in which the reaction between it and the chloride of sodium
which it is to decompose is to take place, and to collect the
hydrofluoric acid which will fly off during the heating.
Unless the
The vessel in
which the solution is heated, and which may conveniently be a leaden one,
should be connected with the apparatus in which the hydrofluoric acid
evolved in the third operation is condensed, or else with a special and
separate similar apparatus. The hydrofluoric acid which flies off during
this heating is due solely, when no excess of hydrofluoric acid has been used,
to dissolved fluoride of sodium, which the bisulphate under the influence
of heat decomposes into sulphate and hydrofluoric acid, but when an
excess of hydrofluoric acid has been used the hydrofluoric acid evolved
during the heating is partly due to the said excess of hydrofluoric acid
contained in the solution of bisulphate, and probably combined with some
fluoride of sodium, so as to form therewith acid fluoride.
precaution thus described were taken, not only would there be a loss of
hydrofluoric acid, but the hydrochloric acid evolved in the salt oven would
be contaminated with hydrofluoric acid, and would so be rendered more or
less unsuitable for subsequent utilization in the manufacture of chlorine
compounds, and moreover the material of the salt oven and of the coke
tower for condensing the hydrochloric acid would be subjected to seriously
injurious corrosion. The second of the three operations does not require
any fuel, the reaction between sulphate of sodium and hydrofluoric acid
taking place at ordinary temperatures, and the only apparatus required for
it is a vessel of some material which will withstand as completely as may
be the action of both bisulphate of sodium and hydrofluoric acid, and any
convenient appliance for stirring or agitating the contents of the vessel,
either by mechanical power or by manual labour as may be preferred.
The materials which seem to be best fitted for the purpose in question are
lead, gutta percha, vulcanized india-rubber, and wood thoroughly impreg-
nated with paraffin. The shape and arrangement of the vessel may be such
as may be most convenient in each particular case. The sulphate of sodium
and hydrofluoric acid having been placed within it, they should be agitated
together by any convenient means until the reaction between them is
complete. The completion of the reaction, which takes place very speedily,
will be known by the appearance of the mixture, the difference of appear-
ance
between fluoride of sodium and undecomposed and undissolved
sulphate being, although difficult to describe, so very marked that no
workman after one experience can possibly mistake one for the other. In

page_text: order that as little as possible of the fluoride of sodium formed may dissolve,
it is preferable so to arrange the strength of the hydrofluoric acid that the
quantity of water present shall not be greater than will just dissolve the
bisulphate produced. It is desirable not to use an excess of hydrofluoric
acid, because in that case acid fluoride of sodium, which is much more
soluble than the normal fluoride, is formed. The proper quantity to use,
when all the operations are conducted without loss or waste, is the quantity
evolved in the final operation of the process. If less than enough hydro-
fluoric acid be used there will be perceived in the mixture (unless an excess
of water be present, which should always be avoided) portions of undissolved
and unacted upon sulphate, readily distinguishable, as I have said, from the
fluoride formed. After the completion of the reaction between the
sulphate and the hydrofluoric acid, on the resulting mixture of fluoride of sodium
being left at rest the fluoride settles very readily, forming a precipitate but
slightly retentive of liquid and admitting of the supernatant solution
being decanted from it very completely and with great ease. After having
been decanted by any convenient means this solution is ready to
be heated as described above, and then employed to decompose a fresh equiva-
lent of chloride of sodium, with production of two or more equivalents of sul-
phate of sodium, and the separated fluoride, after being washed in the manner
to be described presently, is ready for the operation in which its decomposition
is effected by subjecting it to the action of steam.
7. In order to the transformation of salt into soda without any consumption
of reagents, when the fluoride of sodium produced by treating sulphate of
sodium with hydrofluoric acid is not
decomposed by means of
steam, the most convenient plan of proceeding commences with the formation
of sulphate of sodium by double decomposition between chloride of sodium
and sulphate of magnesium.
There are two ways of effecting this
decomposition, both of which have been, and one of which is now, practised
on a manufacturing scale, and this method of producing sulphate of sodium
consequently forms no part of my Invention except as combined with opera-
tions based on the newly discovered facts set forth in the first paragraph of
this Specification, and as regards the use for the purpose in question of sul-
phate of magnesium formed by the method which will be hereafter
described and of the particular kind of vessel and arrangement of
furnace which I recommend to be employed when the decomposition is effected
in the dry way. To effect the decomposition in the dry way, sulphate of
magnesium, having associated with it at least one atom of water,
must be intimately mixed with chloride of sodium, and the mixture heated

page_text: to bright redness, or nearly so. A lower temperature will do, especially if
steam be passed through the mixture, but this is the temperature I prefer.
Hydrochloric acid then flies off, and sulphate of sodium and oxide of mag-
nesium (otherwise magnesia) are formed in accordance with the equation-
Na Cl+Mg SO, HO_Na SO + Mg 0 + HCl.
more
a
When, however, the mixture just described is heated in an ordinary furnace,
not nearly an equivalent of sulphate of sodium is obtained for each equivalent
of sulphate of magnesium employed, and hence when this process has been
worked on a manufacturing scale it has been usual, I believe, to
heat the mixture of sulphate of magnesium and chloride of sodium in iron cylin-
ders. I prefer to heat in vessels having a very flat and narrow transverse
section, and to heat these vessels, especially when I make them of iron, by
means of a Siemens's furnace or any other furnace which enables a sufficiently
high temperature to be conveniently attained, and which admits of the main-
tenance in the furnace itself of a neutral or non-oxidising atmosphere, in which
iron vessels will not burn away. I recommend vessels having for
their transverse section an oblong parallelogram or else a very long.
than
oval, not much
one
foot in diameter
way by any
convenient number of feet the other, according to the size of the furnace and
the scale on which it is desired to conduct operations. I prefer that the
narrow diameter should measure even less than a foot rather than more, but
good results may be obtained with considerably wider vessels. I place these
vessels either vertically or in an inclined position, and arrange any convenient
number of them parallel to each other at a distance apart proportioned to their
sufficient space
size and to
furnace, leaving
the power of the
between one vessel and the next for the passage of enough of the flame and
hot gases of the furnace to quickly raise each vessel and its contents to a
bright red heat, and to keep it steadily at that temperature or at any lower
temperature at which it may be preferred to conduct the operation. I make
the upper part of each vessel communicate with a flue or other passage, by
which the hydrochloric acid gas evolved in it may be conducted to a coke
tower for condensation in the usual way, and I also furnish the upper part
of each vessel with a man-hole through which to charge the vessel with the
mixture of sulphate of magnesium and chloride of sodium, and with a hole or
holes through which to pass a rake with which from time to time
to stir the mixture. When the rake is not being used the hole or holes can
vessel is a door
be closed with clay. At the bottom of each
through which to withdraw the charge when the evolution of hydro-
chloric acid has ceased, and into each vessel near the bottom enter several

page_text: jets, by means of which steam, when preferred, can be passed through the
mixture. By using this apparatus and an excess of salt a full equivalent of
sulphate of sodium for each equivalent of sulphate of magnesium used may be
obtained, and the excess of salt remaining behind with the sulphate of sodium
(which last must necessarily be dissolved, even were there по
chloride of sodium remaining with it, in order to its separation from the mag-
nesia formed at the same time) may be separated by crystallization in the
manner well known to chemists. The sulphate of magnesium and chloride of
sodium should be mixed together very intimately before being put into the
vessels described, and it is well that the water of the sulphate of magnesium,
all but one atom, should be previously driven off. A good plan
is to dissolve the two salts in water, or to mix them with water into a
thick paste, evaporate to dryness, and then keep at a heat of from three
hundred to three hundred and fifty degrees Fahrenheit until the mass
consists of a mixture of chloride of sodium with sulphate of magnesium retain-
ing only one atom of water.
8. The double decomposition of chloride of sodium and sulphate of mag-
nesium in the wet way by the aid of artificial refrigeration is perhaps, however,
the more convenient plan. This plan is extensively practised on
the continent for the obtainment of sulphate of sodium and certain potassic
salts from the mother liquors of sea-water, the artificial refrigeration being
effected by means of what is known as "Carre's machine." Either this machine,
or Kirk's machine, or any other machine which will produce a sufficient
degree of cold with sufficient economy may be used for the produc-
tion from sulphate of magnesium and chloride of sodium of sulphate of
sodium for use in my process, when it is preferred to produce that sulphate
from those materials in the wet way. Carre's machine being patented and made
only by the agents of the Patentee, I need not describe its con-
struction. The case is similar with Kirk's machine. In order to turn any
such machine to account for the purpose in question sulphate of magnesium
and chloride of sodium should be dissolved together in water, and the solution
should then be passed through the machine so as to be cooled to
the point at which it will deposit the chief part of its sulphuric acid as sul-
phate of sodium. The degree of concentration of the solution which
answers best and the degree of cold to which the solution should be subjected
are well known to chemists and to the manufacturers who treat sea water by
what is known as Merle's modification of Balard's process, but I
may mention that perhaps the best degree of concentration is that at which
the solution has a specific gravity of about 12, and that perhaps

page_text: the temperature to which it is best to cool the solution is that of eighteen
degrees below zero on the centigrade scale. I may add that perhaps the
most accessible information with respect to Balard's process as modified by
Merle is contained in the Jury Report of the Chemical Department of the
International Exhibition of 1862. The solution of chloride of sodium and sul-
phate of magnesium having been cooled as described, and thereby caused to
deposit sulphate of sodium, the mother liquor which remains contains a quantity
of chloride of magnesium equivalent to the sulphate of sodium
formed, together with any chloride of sodium and sulphate of mag-
nesium left undecomposed, and probably a little dissolved sulphate of
sodium. The mother liquor must be evaporated to dryness, and the residue
heated sufficiently to decompose the hydrated chloride of magnesium of solu-
tion, of which the mother liquor chiefly consists, into magnesia and hydro-
chloric acid, which last then flies off in vapour, and must be conducted to a
coke tower for condensation in the ordinary way. This decomposition of
hydrated chloride of magnesium into hydrochloric acid and magnesia takes
place at a very moderate heat under the influence of a current of steam, but
when steam is not employed a higher temperature is necessary. I recommend
the operation to be performed in a muffle furnace, and the chloride to be fre-
quently stirred while the operation is in progress. There is
scarcely any variety of furnace, however, in which the decomposition
of hydrated chloride of magnesium into hydrochloric acid and mag-
nesia cannot be easily effected. The apparatus described in the
paragraph last preceding this, and therein recommended for use in the decom-
position of chloride of sodium by sulphate of magnesium in the dry way, yields
a very good result. A very good plan also is that adopted by those French
manufacturers who obtain chloride of magnesium by Balard's process,
and who decompose the said chloride of magnesium by heating it in clay
retorts through which a current of steam at a temperature of about three
hundred degrees centigrade is passed. The chloride of magnesium obtained
as one of the results of the decomposition of chloride of sodium by sulphate
of magnesium in the wet way, as described, having been decomposed, as indi-
cated, by heating it in whatever kind of furnace may be preferred, any sulphate
of magnesium and sulphate or chloride of sodium which may have been con-
tained with it in the mother liquor will remain behind with the
residue of magnesia, and may be dissolved out therefrom, and the
weak solution so obtained may be used instead of water to dissolve the next
charge of chloride of sodium and sulphate of magnesium. In this way the
decomposition of chloride of sodium and sulphate of magnesium into sulphate of

page_text: loss.
9. Sulphate of sodium having been produced by double decomposition
between sulphate of magnesium and chloride of sodium effected in
whatever way, the sulphate of sodium so obtained is treated with hydro-
fluoric acid, as described in the sixth paragraph of this Specification,
so as to produce fluoride of sodium and solution of bisulphate of sodium.
This solution of bisulphate having been decanted from the precipitate
of fluoride, I prefer to wash the latter in order to free it as far as possible
from adherent solution of bisulphate, and I employ for this purpose
water previously saturated with fluoride of sodium, and so prevent any
fluoride being dissolved during the washing. The saturated solution of
fluoride of sodium which I thus use as a wash-water can be used for
this purpose several successive times, and when it has come to contain too
large a proportion of bisulphate to be further useful as a wash-water it
should be evaporated to dryness, or nearly so, care being taken that the
hydrofluoric acid evolved during this evaporation be collected and con-
densed. The residue will consist of sulphate of sodium with perhaps
some bisulphato The washing process may be conducted by simply
adding the wash-water to the fluoride in the same vessel in which the latter
was formed after the solution of bisulphate formed at the same time has
been decanted away from it as completely as possible, agitating the fluoride
and the wash-water briskly together, then allowing the fluoride to settle
again, and when it has done so decanting the wash-water away from it;
or the fluoride to be washed may be spread upon cloths suitably supported,
and the wash-water be made to percolate through it.
10. The next operation consists in decomposing the fluoride by means of
magnesia, the magnesia used being preferably that obtained as one of the
results of the above-described double decomposition of sulphate of mag-
nesium and chloride of sodium followed by that of water and the chloride
of magnesium so formed. If it be desired to transform the fluoride into
caustic soda it must either be boiled with the magnesia or mixed intimately
therewith, and the mixture calcined in a current of steam. When the
fluoride and the magnesia are boiled together the vessel in which the
operation is performed may be made of iron, and when they are heated
together in a current of steam the furnace or vessel used may also be of
the same material. Very good results may be got by using in the latter
case an apparatus similar to that described in paragraph 7 as most suitable
for effecting the decomposition of chloride of sodium by sulphate of mag-

page_text: nesium in the dry way. It is desirable not to use an excess of magnesia,
but if an excess of magnesia have been used before the fluoride of mag-
nesium formed by the reaction between the magnesia and the fluoride of
sodium be submitted to the operation by which it is converted back into
sulphate wherewith to form more sulphate of sodium, the excess of mag-
nesia should be dissolved away from it by means of hydrochloric acid, the
resulting hydrated chloride being afterwards decomposed by heat into
hydrochloric acid and magnesia, as described above. When the decompo-
sition of the fluoride of sodium is effected by boiling with the magnesia,
when the decomposition has been carried as far as it will go, the fluoride
of magnesium must be allowed to settle from the resulting solution of
caustic soda, containing more or less undecomposed fluoride of sodium, and
the solution must then be decanted and concentrated in the usual way,
during which process of concentration the undecomposed portions of
fluoride will crystallize out, and may so be readily separated. When the
decomposition has been effected by heating in a current of steam the
resulting mass may be simply lixiviated with water, in order to the separa-
tion of the caustic soda from the insoluble fluoride of magnesium, or it may
be boiled with water, which last plan is on some accounts preferable. The re-
sulting solution of caustic soda must be treated in the same way as the solu-
tion obtained by simply boiling together fluoride of sodium and magnesia.
11. When it is desired that the ultimate product shall be, not caustic
soda, but carbonate of sodium, either the caustic soda obtained as just
described can be converted into carbonate by the injection of carbonic
acid gas into its solution, or the fluoride of sodium can be decomposed by
carbonate of magnesium instead of by magnesia. In order to the continual
use over and over again of the same magnesia, it must be converted into
carbonate each time if it is used in that form. This may be done very
readily by injecting carbonic acid gas under pressure into a closed vessel
containing a mixture of magnesia and water. The mixture should be
moderately heated, otherwise what would be formed would be simply
solution of bicarbonate of magnesium. A very good plan of decomposing
fluoride of sodium when it is not required to be converted into caustic
soda consists in putting water, magnesia, and fluoride of sodium together
into a vessel capable of withstanding an internal pressure of from seventy
to a hundred pounds per square inch, keeping the contents of the vessel
at any convenient temperature above that which is well known to chemists
as being the temperature at which bicarbonate of magnesium is decomposed
by heat alone, and pumping in carbonic acid gas as long as the gas con-

page_text: tinues to be absorbed. The results are insoluble fluoride of magnesium
and solution of carbonate of sodium. If the operation be performed in
the cold solid bicarbonate of sodium will be obtained, and may afterwards
be converted into neutral carbonate by heat. The process is hastened by
the contents of the vessel being mechanically agitated all the while. The
carbonic acid gas for use in this way may conveniently be obtained by
blowing air through a coke fire, passing the products of combustion after
cooling through a cold solution of carbonate of sodium, and when that is
thereby completely converted into bicarbonate applying heat, so as to drive
off carbonic acid gas free from admixture except with vapour of water. When
the carbonic acid is required simply to convert solution of caustic soda into
solution of carbonate, the whole of the products of combustion may be passed
directly from the fire into the solution of caustic soda without
previously cooling or separation of nitrogen. A good plan is to
put the caustic soda into a vessel having a false bottom furnished with
numerous holes, and to cause the products of combustion to pass first into
the space between the real bottom and this false one. They should be
under sufficient pressure to enable them to rise through the holes in the
false bottom, and thence in a vast number of minute streams through the
liquid above. The carbonic acid is then rapidly absorbed, nothing but
nitrogen passing off until the solution has become saturated with carbonic
acid. When this method is pursued, however, it is needless to first form
solution of caustic soda, it being sufficient to put fluoride of sodium and
magnesia together into the vessel described with sufficient water, and then
to inject the hot products of combustion as above described. These hot gases
cause the mixture to boil and rapidly effect the required decomposition, with
formation of solution of carbonate of sodium and insoluble fluoride of magnesium
as before.
12. The final operation of the series commencing with the production
of sulphate of sodium by double decomposition between chloride of sodium
and sulphate of magnesium consists in the decomposition of the fluoride of
magnesium formed, as just described, by means of the virtually free sul-
phuric acid of what I have called the solution of bisulphate of sodium,
formed as one of the results of the reaction between the said sulphate of
sodium and hydrofluoric acid. This solution of bisulphate may itself be
used to decompose the fluoride of magnesium, with production of hydro-
fluoric acid and either double sulphate of magnesium and sodium or a
mixture of normal sulphate of sodium with normal sulphate of magnesium;
or it may be separated into sulphate of sodium and free sulphuric acid, and

page_text: the fluoride of magnesium be treated with only the latter. This separation
may be effected by either of two methods, both of them well known to
chemists, one consisting in distilling off the second atom of sulphuric acid,
and the other and more economical one in adding water to the bisulphate
in such proportion that there shall be four parts of water of solution present
for each part of actual bisulphate of sodium, and then leaving the solution
to itself, whereupon normal sulphate of sodium crystallizes out, only free
When the solution of
sulphuric acid remaining in the mother liquor.
bisulphate is itself used to decompose the fluoride of magnesium, the
resulting double sulphate of magnesium and sodium, or mixture of normal
sulphate of magnesium with normal sulphate of sodium, whichever it may
be, can either be separated into sulphate of sodium and sulphate of mag-
nesium by boiling in solution, whereupon anhydrous sulphate of sodium
crystallizes out, or by any other convenient method of crystallization, or can
be used as it is first obtained for the decomposition of chloride of sodium
either in the wet way or in the dry way. When the double sulphate or mixture
of two sulphates, whichever it may be, is employed to decompose chloride
of sodium in the wet way, the temperature of the solution does not require
to be lowered quite so far as when only sulphate of magnesium and chloride
of sodium are used, and when the same double sulphate or mixture of two
sulphates is employed to decompose an equivalent of chloride of sodium in
the dry way the desired result is obtained more readily than when only the
sulphate of magnesium is employed. When the double sulphate or
mixture of two sulphates is used either in the wet way or in the dry way,
the result is two equivalents of sulphate of sodium, and so the right quantity
for treatment in the next operation with one equivalent of hydrofluoric
acid; when only sulphate of magnesium is used, the right quantity
of sulphate of sodium for treatment with an equivalent of hydrofluoric acid
is made up by adding to the equivalent obtained in the first operation the
equivalent separated either from the bisulphate obtained in the second
operation or from the double sulphate or mixture of sulphate obtained in the
final operation, as the case may be. The decomposition of the fluoride of
magnesium by the virtually free sulphuric acid of the solution of bisulphate,
whether that acid be first separated from the associated sulphate or not, must
be performed in a leaden or platinum still or other vessel, and the evolved
hydrofluoric acid must be condensed either by a series of leaden or gutta
percha "Wolfe's bottles," or, preferably, by a coke tower similar
to that which is ordinarily used for the condensation of hydrochloric
acid, except that it must be lined with either lead or gutta percha or some

page_text: other substance upon which hydrofluoric acid has not much action.
Similar
condensing apparatus must be used when the hydrofluoric acid is obtained
by the reaction between fluoride of sodium or other fluoride of steam.
Less condensing power is required, however, when the hydrofluoric acid is
obtained by means of sulphuric acid than when it is obtained by means of
steam, inasmuch as in the former case it flies off at a much lower temperature
than in the latter. The reaction between sulphuric acid and fluoride of
magnesium especially takes place at a very moderate temperature. The
still or other vessel in which the operation is performed requires
only very slight heating, which may conveniently be effected by means of a
sand-bath. When the hydrofluoric acid is used to act directly upon
chloride of sodium it does not require to be condensed, being in that case
most advantageously applicable in the gaseous condition. Before entering
the solution of chloride of sodium, however, the hydrofluoric acid gas must
be cooled as completely as possible, which may be done by passing
it through considerable lengths of pipe made of lead or gutta percha or other
material upon which hydrofluoric acid has not much action. It is a good plan
to cause these pipes to pass for a greater or less portion of their length through
cold water.
13. The series of operations described in paragraphs 7, 8, 9, 10, 11,
and 12, commencing with the production of sulphate of sodium, magnesia,
and hydrochloric acid by the mutual decomposition of water, sulphate of
magnesium, and chloride of sodium, followed by the decomposition of the
sulphate of sodium so formed by hydrofluoric acid, resulting in the forma-
tion of fluoride of sodium and solution of bisulphate of sodium, this being
succeeded by the decomposition of the fluoride of sodium by the magnesia
obtained in the first operation, and the series of operations concluding with
the reproduction for use over and over again of sulphate of magnesium
and hydrofluoric acid, like the series of operations described in paragraphs 5
and 6, transforms salt into soda without any consumption or necessary loss
of any of the reagents employed. I will now describe a series of operations
by which salt may be transformed into soda, by way of the intermediate
production of fluoride of sodium, with considerable economy as compared
with the method of proceeding at present in general use, though not with
reproduction of the reagents employed. The first two of this series of
operations are exactly the same as the first two of the series of operations
described in paragraphs 5 and 6. The third operation consists in
decomposing the fluoride of sodium contained in the second operation,
instead of by magnesia or carbonate of magnesium, by means of either

page_text: lime or carbonate of calcium, the operation being otherwise con-
ducted in substantially the same manner as the corresponding operation with
magnesia, and the fourth operation consists in the decomposition of
the resulting fluoride of calcium by sulphuric acid manufactured in the ordinary
way, with production of sulphate of calcium and reproduction of hydrofluoric
acid. As much sulphuric acid would thus be consumed as in the
ordinary soda process, but there would not be required either so
much fuel or so much lime as in that process, the salt would be transformed into
soda more speedily than by the ordinary process, and the residue, instead
of being useless and offensive as at present, would be a compound susceptible
of many valuable applications.
14. It will sometimes be advantageous where sulphate of magnesium is in
demand, either for medicinal use or for chemical applications, to proceed
exactly as described in the paragraph last preceding this, but to substitute
magnesia or carbonate of magnesium for the lime or carbonate of calcium
employed in the series of operations there described, so that the residue
shall be, instead of sulphate of calcium, sulphate of magnesium or "Epsom
salts." This method of proceeding, however, inasmuch as it involves the
consumption of an equivalent of magnesia or carbonate of magnesium for
each equivalent of soda obtained, can only be advantageous where sulphate
of magnesium can be turned to such account that its value shall exceed the
cost at that place of the equivalent of magnesia or carbonate of magnesium
essential to its production by the method indicated. The sulphate of mag-
nesium might be employed, and so its sulphuric acid be used at least a
second time, for the precipitation of sulphate of calcium, in a form suitable
for use by paper makers, from solution of chloride of calcium, or to preci-
pitate "permanent white" from solution of sulphide of barium. In the former
case the resulting chloride of magnesium might be subsequently decomposed
into magnesia and hydrochloric acid as above described, and in the
latter case
the resulting solution of sulphide of magnesium need only
be boiled to give off all its sulphur in the form of sulphuretted hydrogen,
magnesia being precipitated,and thus sulphuretted hydrogen might be decom-
posed by sulphurous acid or by oxide of iron, or might be burnt into sulphurous
acid in the usual way.
15. When preferred, still another method of proceeding, in order to the
transformation of salt into soda by way of the intermediate production of
fluoride of sodium. without consumption of any reagents, may be
based upon the known fact that sulphate of magnesium treated with charcoal
gives off all its sulphur as sulphurous acid. The operation may be per-

page_text: formed in earthen retorts, and the sulphurous acid evolved be passed into lead
chambers for transformation into sulphuric acid by the usual method. The
sulphuric acid so obtained may be used to decompose fluoride of magnesium,
and the residue of magnesia left in the retorts to decompose fluoride of sodium,
the fluoride of sodium being formed from sulphate produced by the method
described above in paragraphs 5 and 6.
16. When fluoride of sodium is obtained directly from chloride of
sodium by the action thereupon of hydrofluoric acid, the fluoride may be
treated by any of the methods described above, but when produced in the
way just mentioned I consider it most desirable that the fluoride should
be decomposed by means of steam. This is also, perhaps, the most desirable
mode of proceeding when the fluoride of sodium has been obtained
from sulphate. To decompose fluoride of sodium by that agent, I
pass the steam either through fluoride of sodium in a state of fusion, or
over and through fluoride of sodium heated to some temperature below
that of fusion, in the latter case stirring the fluoride constantly, so that all
the fluoride may be brought into contact with the steam. The lower part of
the vessel in which I perform this operation I make of iron, and the upper
part of iron carefully lined with gas-retort carbon, or any other convenient
form of graphitoidal or other carbon, so that the hydrofluoric acid cannot
attack the iron. The upper part of this vessel communicates with the leaden
or other flue or passage by which the hydrofluoric acid vapour is to be con-
veyed to the condensers or to the pipes for cooling it, as the case may be.
The process will proceed the more rapidly the more highly the steam is
superheated, and it is well, in order to prevent the steam from too greatly
lowering the internal temperature of the vessel, to inject also with the steam,
though preferably by a distinct jet or jets, air or carbonic oxide mixed with
nitrogen or any other convenient gas or gases heated to a little above the
point at which it is desired to keep the contents of the vessel. Those parts
of the iron or other tubes by means of which the steam and heated
air or other gas or gases are injected should be protected by means of a
coating of either platinum or carbon or of some fluoride unassailable or but
slightly assailable by steam. The vessel may be placed in a furnace, and
so be heated partly or wholly from the outside, or, which is what I greatly
prefer, it may be heated solely by the injection of hot air or other con-
venient gas or gases. When the fluoride of sodium is spread on the bottom
of the vessel, it is a good plan to direct the jets of air or gas and steam
downwards upon it from a little way above the surface of the layer of
fluoride. A good plan also is to fuse the fluoride wholly or partially, and

page_text: so cast or otherwise form it into solid blocks, and then to place
those blocks within the vessel upon supports of either iron, or carbon, or some
other suitable substance, and then to inject both steam and heated air or
gas from below. In this case the internal temperature of the vessel should
be kept just below the point at which the fluoride fuses. If that precaution
be taken there will collect at the bottom of the vessel almost chemically pure
caustic soda. When the fluoride of sodium is originally placed at the bottom
of the vessel, the caustic soda obtained is apt to be associated with more or
less undecomposed fluoride, which, however, can be readily separated by
treating the product with water and then concentrating by evaporation until
the fluoride crystallizes out.
17. It is well known that fluoride of calcium is susceptible of being decom-
posed by steam, and it may sometimes be advisable, instead of decomposing
fluoride of sodium by means of steam, to decompose it by means of lime or
magnesia, and then to decompose the resulting fluoride of calcium or magnesium,
as the case may be, by means of steam, on the plan just described for the
decomposition of fluoride of sodium by that agent.
18. Having now described the nature of my said Invention, and the
manner in which the same is to be performed, I claim as my Invention,
and desire to be secured to me by the herein in part recited Letters Patent,
the new method or process of transforming salt into soda, which consists
essentially in the intermediate production of fluoride of sodium, and in forming
and subsequently treating the said fluoride of sodium substantially as is above
described.
In witness whereof, I, the said Walter Weldon, have hereunto set my hand
and seal, this Thirty-first day of August, in the year of our Lord One
thousand eight hundred and sixty-six.
WALTER WELDON. (L.S.)