An Improved Process of Manufacturing Bleaching Solutions by Electrolysis.
Year: 1893

page_text: Date of Application, 17th Mar., 1893-Accepted, 20th Jan., 1894
An Improved Process of Manufacturing Bleaching Solutions by
Electrolysis.
A communication from Dr. OSKAR KNOEFLER and FR. GEBAUER, of
Charlottenburg, in the Empire of Germany.
1. ALFRED JULIUS BOULT of 323 High Holborn in the County of Middlecer,
Chartered Patent Agent, do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:-
It is well-known that in the electrolysis of solutions of mineral salts, without the
employment of diaphragms, a solution of hypochlorite of soda is formed. Under
ordinary circumstances, however, these hypochlorite solutions are not sufficiently
strong, i.e. do not contain a sufficient proportion of active chlorine, to enable them
to be used directly for industrial bleaching purposes as a substitute for chloride of
lime, for the chemical known as "Eau de Javelle and other similar agents. This
well-known fact is accounted for by the circumstance that the hydrogen forming at
the cathode constantly keeps reducing the hypochlorite as it forms, while at the
same time the current itself converts the hypochlorite into chlorate, which is
incapable of exercising a bleaching action.
According to the German Patent No. 61708 this effect of the current is
counteracted, to such an extent that solutions containing from 04 to 0
of
active chlorine are obtained, by the addition of caustic lime to the solution of
mineral salt to be treated by electrolysis. By this means a solution of hypochlorite
of lime is obtained which, being a comparatively bad conductor of electricity better
resists the re-decomposing action of the current hereinbefore alluded to.
The said German Patent relates to the use of lime in the electrolysis of solutions
of mineral salts, a result similar to that obtained by us being thus arrived at, as
hypochlorite of calcium is formed which offers a stronger resistance to the influence
of the current and which cannot be easily reduced on account of the hydrogen.
A number of experiments have shown morcover that the same object may be
accomplished more simply and without any addition of lime by properly determining
the density of current, which may vary from say 300 to 800 amperes per square
metre of electrode surface. Thus the employment of lime, and all the expense and
annoyance attending it, are avoided, while an additional advantage is derived from
the fact that hypochlorite of soda is obtained instead of hypochlorite of lime, over
which besides other technical superiorities, it has that of being capable of a more
rapid and thorough bleaching action.
It has further been found that the comparatively large increase of temperature
produced by currents of from 300 to 800 amperes per square metre of electrode
surface, may be put to good advantage in the progress of the electrolysis. The
technical merit which the process thereby acquires chiefly lies in the fact that it
enables an ordinary unskilled operative to attend to and carry out the work and
to obtain a uniform solution or lye, always containing the same percentage of active
chlorine, without resorting to any chemical analysis.
In the plant which my correspondents have described in their British
Specification No. 20214 of 1892 and which is in perfect working order at present
the operation is conducted with a current of a density of 600 amperes per square
metre. The workman attending to the apparatus so regulates the inlet-valve for
supplying the salt solution to the several electric cells, that the supply of solution
arrives at such a speed (displacing the ready lye and forcing it through the
D

page_text: discharge-pipe at the same time) that the rise of temperature always takes place in
a constant and uniform manner.
The salt employed is rock or mineral salt (sodium chloride Na(1) the solution
preferably containing from 5 to 10 per cent, of the salt.
The quantity of actual bleaching-chlorine contained in the lye also remains
constantly the same. The respective rises of temperature corresponding to various
percentages of chlorine contained in the bleaching lye collected from the apparatus,
may be set down as follows;―
For 030 per cent. of active chlorine + 15º C.
For 0:10 per cent, of active chlorine + 21° (
For 0:50 per cent, of active chlorine + 28° C.
With this seale before him, the operative knows that assuming the temperature
of the salt-solution supplied to the apparatus to be + 12° C., the bleaching solution
produced will contain 0:10 per cent, of active chlorine, if he so adjust the inlet-valve
admitting the salt solution, that the ready bleaching lye is discharged at a
temperature of 12 + 21 = 33' ('. The degree of concentration of the mineral salt
solution which is considered most advantageous, is 10 per cent. It is not advisable
to carry concentration to a much higher degree, as this would involve to great a
loss of salt, while on the other hand, the solution should not be appreciably
weaker, lest the useful effect of the electrical current should be so far reduced as
to render too considerable a display of electrical energy, or in other words the
creation of too much mechanical power, necessary for each kilogramme of active
chlorine to be produced.
Having now particularly described and ascertained the nature of the said inven-
tion, as communicated to me by my foreign correspondents, and in what manner
the same is to be performed, I declare that what I claim is,
A process of manufacturing a bleaching solution consisting in subjecting a
solution of mineral salt without the employment of diaphragmns to electrolysis with
a density of current of from 300 to 800 ampères per square metre of electrode
surface the extent to which the lye is made richer in active chlorine being
determined by correspondingly regulating the rise of temperature while conversely
by maintaining a uniform rise of temperature an equally uniform percentage of
active chlorine may be secured in the lye produced substantially as specified.
Dated this 17th day of March 1803.
W. P. THOMPSON & BOULT,
Of London, Liverpool, Manchester, and Nottingham,
Agents for the Applicant.