IMPROVEMENTS IN CONVERTING SULPHIDE of SODIUM INTO CARBONATE OF SODA
Year: 1876

page_text: LETTERS PATENT to Walter Weldon, of Abbey Lodge, Merton, in the County
of Surrey, for the Invention of "IMPROVEMENTS IN CONVERTING SULPHIDE of
SODIUM INTO CARBONATE OF SODA"
Soaled the 24th November 1870, and dated the 28th August 1876.
PROVISIONAL SPECIFICATION left by the said Walter Weldon at the Office
of the Commissioners of Patents on the 28th August 1876.
WALTER WELDON, of Abbey Lodge, Merton, in the County of Surrey. "IMPROVE-
MENTS IN CONVERTING SULPHIDE OF SODIUM INTO CARBONATE OF SODA."
In certain other Specifications which will be deposited at the Patent Office on
the same day as this I have described certain Inventions relating to the manufacture
of sulphide of sodium, which I believe will render industrially practicable the pro-
duction of that compound as an intermediate step in the conversion of sulphate of
soda into carbonate. My present Invention relates to its decomposition by means
of gaseous carbonic acid in the wet way.
In all attempts hitherto made to manufacture soda by converting sulphate of
soda into sulphide of sodium, dissolving such sulphide in water, and then treating
the resulting solution by carbonic acid gas, the product directly so obtained has
been simply a solution of carbonate of soda requiring to be evaporated to dryness
to convert it into soda ash. If the sulphide of sodium employed has contained any
undecomposed sulphate or chloride, and it has usually contained both, these impu-
rities have been in solution with the carbonate of soda, and when the solution has
been boiled down have been contained in the resulting product. One object of my
present Invention is to avoid this operation of evaporation, and at the same time to
obtain an almost chemically pure soda ash, or other merchantable form of carbonate
of soda.
I prefer to accomplish this object by using a very strong solution of sulphide of
sodium, using it cold, and treating it by carbonic acid gas until I have obtained a
solid precipitate of bicarbonate of soda in such proportion as will be herein-after
set forth. I then separate this precipitate from its mother liquor, in which latter
will be contained all the soluble impurities which were associated with the sulphide
used, leaving the precipitate, when it has been thoroughly drained from the mother
liquor, almost cheinically pure bicarbonate. It can then either be sold as such, or

page_text: by a simple heating it can be converted into a purer soda ash than has ever hitherto
been manufactured. In the latter case the equivalent of carbonic acid driven off
during its calcination should be collected to be used for the treatment of more
sulphide of sodium.
In working as above described I prefer to use solutions containing from twelve
to twenty per cent. of sulphide of sodium, although I do not confine myself to these
strengths. If the treatment by carbonic acid of a solution containing, say, eighteen
per cent of sulphide be continued until the sulphide is completely decomposed I
find that about eighty per cent. of the resulting carbonato precipitates, the other
twenty per cent. being dissolved in the mother liquor. This mother liquor I of
course use to dissolve sulphide for the next operation.
To treat solution of sulphide of sodium by carbonic acid until it is completely
decomposed is not however an economical method of working, because as the
quantity of undecomposed sulphide of sodium remaining in the solution under
treatment diminishes the useful effect of the carbonic acid employed diminishes
also. It is a good plan therefore to separate such crystals as have been formed
before nearly so much as eighty per cent. of the sodium present has been precipi-
tated, and while there is yet an appreciable quantity of sulphide undecomposed, and
then to dissolve more sulphide in the mother liquor and recommence the treatment
by carbonic acid
I have said that I prefer to employ a cold solution of sulphide of sodium, and of
course for the production of bicarbonate of soda it is essential that neither the
solution of sulphide of sodium nor the carbonic acid applied to it should be at a
temperature even approaching 150° F., which is the temperature at which bi-
carbonate of soda decomposes. It will always be casy to obtain cold solution of
sulplude of sodium, but it may not always be so easy to obtain cold carbonic acid
gas. When for this or any other reason it is more convenient to work at a tem-
perature above that at which bicarbonate of soda decomposes it will be sesqui-
carbonate of soda which will be formed. The method of working may be the saine
however as when working in the cold, except that after treatment by the carbonic
acid the solution operated upon must be allowed to cool in order to the separation
from it of sesqui-carbonato in the solid state.
When the sulphide of sodium which is to be decomposed by carbonic acid in the
wet way, as aforesaid, is manufactured in accordance with the Inventions described
in my other Specifications already referred to the carbonic acid employed to
decompose the sulphide of sodium should be partly that produced in the manufac-
ture of the sulphide itself, and partly that given off in the calcination of the
bicarbonate or of the sesquicarbonate, as the case may be. The carbonic acid to
cominence with, and that required to make up any loss in working, as also the
additional quantity required when the product of the treatment of the sulphide of
sodium is sold as bicarbonate must be supplied from other sources.
The carbonic acid obtained, as above stated, will of course be obtained hot. If
it is to be used hot the furnaces in which it is produced may be connected directly
with the apparatus in which it is to be employed for the decomposition of sulphide
of sodium, as above set forth. I greatly prefer however that it should pass from
the furnaces in which it is produced into iron gasometers, in which it can remain
until it is cold. Even when it is to be used hot it should preferably be passed first
into gasometers, because the treatment of the solution of sulphide can then be so
much better regulated.

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 7th February
1877.
WALTER WELDON, of Abbey Lodge, Merton, in the County of Surrey. "IMPROVE-
MENTS IN CONVERTING SULPHLIDE OF SODIUM INTO CARBONATE OF SODA."
This Invention consists in a certain method or certain methods of procedure
when treating sulphide of sodium by carbonic acid gas in the wet way, by which
method or methods I am enabled at once to obtain a much purer alkali than has
been produced by any of those before me who have endeavoured to manufacture
soda by way of the intermediate production of sulphide of sodium, and to obtain
such alkali more economically than they have obtained their much inferior product.
By the said method or methods I obtain solid carbonate of soda from solution of
sulphide of sodium without that operation of evaporation which has always been
necessary to that end hitherto, and instead of having in my final product all the
impurities which were contained in the solution of sulphide of sodium used, as all
others before me have had, I leave those impurities behind in the mother liquor.
In carrying out my said Invention, in that form of it which I believe to be the most
advantageous, 1 make the solution of sulphide of sodium of such strength that when,
say, two-thirds or so of the sodium therein has been converted into bicarbonate the
greater part of such bicarbonate must necessarily be in the state not of solution but
of a solid precipitate. I have used successfully solutions containing less than twelve
per centum of Na, S, but stronger solutions are preferable, and the strength which
I prefer is that of a solution containing nearly or quite the maximum quantity of
sulphide which water can dissolve at ordinary temperatures Such a solution will
contain from about seventeen to about twenty per centum of Nag S, the proportion
varying with the temperature, and also with the quantity and the nature in respect
of solubility of whatever impurities may be present with the sulphide. I have this
solution at a temperature very appreciably below that at which bicarbonate of
soda in solution in water (lecomposes, which temperature is between 150 and 160
degrees Fahrenheit. I prefer indeed to have the solution little if any above the
temperature of the atmosphere, and I prefer to have the carbonic acid gas at about
the same temperature as the solution
By any method of treating a liquid by a current of gas, but preferably by the
method and by means of the apparatus which are described in the Specification of my
Patent, No. 3389 of the year 1876, I then treat the cold or nearly cold solution of
sulphide of sodium by the cold or nearly cold carbonic acid until there has preci-
pitated or crystallized out therefrom such a proportion as may be considered
desirable of solid bicarbonate of soda. By either decantation, filtration, or other
convenient means, as will be well understood, I then separate this precipitate from
its mother liquor, in which last are contained all the soluble impurities which were
associated with the sulphide used, so that the precipitate when it has been allowed
to thoroughly drain is almost chemically pure bicarbonate. Such traces of the mother
liquor as may remain with it after it has thoroughly drained may be removed by
washing it with a small quantity of a saturated solution of bicarbonate of soda It
can then after drying either be sold or employed as bicarbonate, or be treated, as
will be well understood, for its conversion into "soda ash" or anhydrous mono-
carbonate of soda by driving off from it by heat its combined water and its second
equivalent of carbonic acid When carbonic acid is so driven off from it such
carbonic acid should be applied to the treatment of more sulphide of sodium.
The treatment of the solution of sulphide of sodium by carbonic acid, as above
described, may be continued until the whole of the sulphide of sodium has been
decomposed, in which case at the end of the operation a proportion of the sodium
present, which will vary according to the strength of the solution of sulphide used, but
which may amount to eighty per centum thereof, or even more, will be in the solid
precipitate in the state of bicarbonate, the remainder of the sodium, in the state of

page_text: one or other carbonate of soda, together with all the soluble impurities originally
associated with the sulphide, being in solution in the mother liquor. This mother
liquor should be used to dissolve more sulphide of sodium, and should be thus re-
employed time after time until it has become too highly charged with impurities.
It must then be either thrown away or be treated for the utilization of all or part
of the various bodies it holds in solution, the method of treatment to this end
varying, as will be well understood, according to the nature of the said bodies in
each particular case.
To treat solution of sulphide of sodium by carbonic acid until the sulphide is
completely decomposed is not however a method of working which I recommend.
As decomposition of the sulphide in the solution goes on the quantity of un-
decomposed sulphide remaining in the solution of course diminishes, and as this
diminishes the useful effect of the ticatment of the solution by a given volume of
carbonic acid gas diminishes also, while the amount of mechanical force to be
exerted for its treatment thereby remains the same. An important part of the
present Invention, whereby I effect a very notable economy, therefore consists in
arresting the treatment by carbonic acid while there is as yet an appreciable
quantity of sulphide undecomposed. Instead of continuing the treatment by car-
bonic acid to the point of complete decomposition of the sulphide I stop and separate
from the mother liquor the bicarbonate which has precipitated or crystallized out!
some time before that point would be reached. I thus have a mother liquor which
contains undecomposed sulphide of sodium in addition to carbonate of soda and the
impurities aforesaid. I dissolve more sulphide in this liquor, and then treat it
again by carbonic acid. I proceed in this way time after time until too large a
quantity of impurities has accumulated in the mother liquor to permit of its being
used again with advantage. This time, however, before throwing the mother liquor
away, or treating it for the utilization of the bodies dissolved in it, I prefer to carry
the decomposition of the sulphide to completeness.
While I believe that it is most advantageous to perform the treatment of solution
of sulphide of sodium by carbonic acid gas at such moderate temperature as shall
permit of the product being obtained as a precipitate of bicarbonate of soda, as
above described, nevertheless when the carbonic acid to be employed is obtained
hot, and cannot conveniently be cooled, or when for any other reason it is desired
to perform the said treatment at a temperature at which bicarbonate of soda cannot
exist, I can still follow the methods of working which constitute my present
Invention. In such case the product will be sesquicarbonate of soda, which is very
much more soluble than the bicarbonate It is however much less soluble cold
than hot, so that although when solution of sulphide of sodium is treated by car-
bonic acid gas under such conditions of temperature that the product is sesqui-
carbonate, comparatively little thereof will separate out during the said treatment
itself; a considerable proportion will separate out on subsequently cooling When
therefore I am obliged to use either hot carbonic acid gas, or hot solution of
sulphide of sodiumn, or both, I proceed in just the same way as when working under
the conditions which yield bicarbonate, so far as the treatment of the solution of
sulphide by carbonic acid is concerned, except that I prefer to take advantage of
the greater solubility of sulphide of sodium lot than cold, and to make the solution
operated upon somewhat stronger than when working so as to obtain bicarbonate.
When a proportion of the sulphide used, which I prefer should not be below fifty
but not above eighty per centum thereof, has been transformed into sesquicarbonate,
I stop the treatment with carbonic acid, and I allow the solution to cool and remain
at rest until as much of its sesquicarbonate has crystallized out as will so separate
within a reasonable time. I then separate the crystals from the mother liquor,
and in this mother liquor I dissolve sulphide of sodium for another operation. In
the same way, as when working by the bicarbonate method, I use the mother liquor
over and over again until it has become too highly changed with unpurities. The
sesquicarbonate may be resolved by heat into monocarbonate, vapour of water, and
free carbonic acid just as the bicarbonato may be.

page_text: Having thus described the nature of my said Invention, and in what manner the
same is to be performed, I claim as the Invention to be secured to me by the herein-
before in part recited Letters Patent,—
Firstly. The method as above described of obtaining from solution of sulphide of
sodium a solid carbonate of soda without evaporation, and in such wise that it shall
be free or nearly so froin the impurities contained in the said solution of sulphide
of sodium, which method consists in so treating the solution of sulphide of sodium
by carbonic acid gas as to obtain a precipitate of bicarbonate of soda, leaving the
impurities in solution in the inother liquor.
And, Secondly. The method as above described of diminishing the cost of treating
solution of sulphide of sodium by carbonic acid gas, whether that treatment be
performed so that its product shall be bicarbonate of soda, or so that its product
shall be sesquicarbonate of soda, which method consists in keeping the solution of
sulphide stronger than it could otherwise be during a considerable part of the
operation by arresting the treatment by carbonic acid before the sulphide has been
completely decomposed, separating as much solid bicarbonate or sesquicarbonate as
can be separated conveniently, and then using the mother liquor to dissolve more
sulphide for another operation.
In witness whereof, I, the said Walter Weldon, have hereunto set my hand
and seal, this Twenty-ninth day of January, in the year of our Lord One
thousand eight hundred and seventy-seven.
(LS) WALTER WELDON.