IMPROVEMENTS IN THE MANUFACTURE OF CAUSTIC SODA
Year: 1865

page_text: PROVISIONAL SPECIFICATION left by George Robinson at the Office of
the Commissioners of Patents, with his Petition, on the 31st October
1865.
I, GEORGE ROBINSON, of No. 26, Welbeck Street, Cavendish Square, in
the County of Middlesex, M.D., do hereby declare the nature of the said
Invention for "IMPROVEMENTS IN THE MANUFACTURE OF CAUSTIC SODA," to be as
follows:-
In the various processes employed for manufacturing caustic soda it is
found that the solutions of that substance, unless prepared by caustifying pure
carbonate of soda, contain certain impurities, viz., sulphide of sodium, hypo-
sulphite of soda, sulphite of soda, sulphate of soda, ferrocyanide of sodium,
iron in solution or suspension, chloride of sodium, and carbonate of soda. The
relative number and proportions of these impurities present in different
solutions vary to a considerable extent, but some always require removal.
In the ordinary modes of manufacturing caustic soda certain of these
impurities, viz., the carbonate of soda, chloride of sodium, and sulphate of
soda are deposited ("salted down") and may be removed by mechanical
means during the evaporation of part of the liquors, while the sulphide of
sodium, hyposulphite of soda, and sulphite of soda are converted by nitrate
of soda or other oxidizing agents into sulphate of soda, which is similarly

page_text: removed, and the same oxidizing agent or agents at the same time act upon
the soluble compounds of iron so as to cause them likewise to deposit.
With regard to the mechanical separation of the matters deposited during
the concentration of the liquors, I do not propose any alteration, but by my
Invention I obtain by chemical agencies only:-1st, a regulated and limited
oxidation of the sulphide of sodium present as an impurity in caustic soda;
2nd, the precipitation and removal from the impure solutions of caustic soda
of all the more objectionable impurities; 3rd, the utilization in various modes
of the precipitates so obtained.
1. It is well known that sulphide of sodium by exposure to air and water is
capable of being oxidized so far as to give rise to the formation of bypo-
sulphite of soda and caustic soda, and that the hyposulphite of soda so formed
on being treated with more powerful oxidizing agents, such as nitrate of
soda, may be converted into sulphate of soda, as previously mentioned. The
sulphate of soda thus expensively obtained as the ultimate product of
deoxidation of the sulphide of sodium is by the present mode of manufacture
allowed to deposit during the final concentration of the solution of caustic
soda, but the cost of the nitrate of soda employed, and the fact that the
resulting sulphate of soda is of comparatively little value, are serious objec-
tions to the plan now in use." I therefore propose to stop the oxidation of
the sulphide of sodium as soon as it is has been changed into hyposulphite of
soda, a point easily ascertained by its solution no longer blackening lead test
paper. This limited oxidation of the sulphide of sodium may be accomplished
in any convenient apparatus by exposing the impure caustic soda either in
the solid form or in solution to atmospheric air freed from carbonic acid, or
by treating it with various other oxidizing agents. I thus easily obtain in the
form of byposulphite of soda the sulphur of the sulphide of sodium which
is the chief source of the impurities ordinarily present in solutions of caustic
soda.
2. To the solution of caustic soda so far oxidized as to contain no more
sulphide of sodium, and having as its chief impurity byposulphite of soda, I
add a strong solution of caustic baryta, or chloride of barium, or nitrate or
other soluble salt of baryta, but preferably the chloride of barium as being
the cheapest, until a precipitate is no longer formed, or in such proportion as
previous trials may have shown to be sufficient to complete the purification,
it will then be found that the baryta has combined with and precipitated
the hyposulphurous, sulphurous, sulphuric, carbonic, and hydroferrocyanic
acids, or any of them that were in the solution of caustic soda.
An excess
of the precipitant must be carefully avoided, and then the solution will be

page_text: found to contain only caustic soda, together with chloride of sodium if chloride
of barium has been used as the precipitating agent, and traces of hyposulphite
of baryta. The whole of the latter from its slight solubility in water will
deposit and separate during the final concentration of the liquor, and the
chloride of sodium when present may also be removed as far as requisite
by "salting down." As soon as the precipitate produced by the baryta has
settled the clear solution is to be drawn off and concentrated until it solidifies
on cooling as caustic soda.
8. The precipitated matters obtained by means of baryta should be drained'
and washed, and may then be utilized in various ways according to the
requirements of the manufacturer, thus,-a, by treating the mixed precipi-
tates with sulphate of soda, the hyposulphite of baryta will be decomposed,
byposulphite of soda existing in the solution while sulphate of baryta remains
insoluble, on concentrating the clear solution crystals of hyposulphite of soda
may be obtained; b, artificial sulphate of baryta in a form available for
various useful purposes may readily be obtained from the precipitated
matters; c, the precipitated matters may be treated with muriatic acid
when from the action of the latter on the sulphite of baryta and hyposul-
phite of baryta the following useful products will be obtained, viz., sulphurous
acid in a gaseous state, chloride of barium in solution, and free sulphur as a
fine powder.